Chapter 22: Phagocyte System
Definition and Classification of Phagocytes
Phagocytes are bone marrow-derived leukocytes that play a crucial role in the immune response by engulfing and digesting particulate matter;
they are equipped to seek out, ingest, and kill microorganisms
Functions include cytokine synthesis, secretion of inflammatory mediators, and digestion of debris.
Link innate immunity to adaptive immunity and are vital for injury resolution and wound repair.
They are classified into two main groups: both of which are essential for innate immunity and the initiation of acquired immune responses.
Granulocytes include neutrophils, eosinophils, basophils;
Neutrophils - rapid effector cells of the innate immune system
Mononuclear phagocytes - monocytes, and tissue macrophages; function as resident cells in certain tissues (e.g lungs, liver, spleen and peritoneum)
Phagocyte Distribution and Structure
Phagocytes are distributed throughout the body and have distinct structural features that enable their functions. Granulocytes circulate in the bloodstream, while mononuclear phagocytes often reside in specific tissues.
PU.1 - important for the development of early myeloid precursors; essential for subsequent differentiation of monocyte/macrophage lineage
C/EBPa - early steps in the differentiation of granulocytes
C/EBPe- required for terminal maturation beyond the metamyelocyte stage
Cytokines that promote proliferation and differentiation:
IL-3, IL-6, GM-CSF, GCSF, M-CSF - neutrophils and monocytes from primitive precursor cells
Apoptotic neutrophils - taken up by macrophages in tissues; deliver negative feedback via T-helper type (Th17) lymphocytes that results in reduction of GCSF production
Emergency granulopoieis - Activated macrophages - IL-1, IL-6, TNF —> activates stromal cells and T lymphocytes to produce additional amounts of CSFs and increase production of myeloid cells
IL-3 and IL-5 - primary cytokine inducing human basophil growth and differentiation
Gfi-1 - transcriptional repressor which modulates myeloid differentiation; key repressor of miR21 and miR196b promoting monocytic differentiation and block granulocyte differentiation
Retinoic acid receptors - induce differentiation of myeloid leukemia cell lines and leukemic promyelocytes in APML
Micro-ribonucleic acids (miRs) - participate in several levels of myelopoiesis; expressed at different stages during myelopoiesis
* Neutrophils are rapid responders that circulate until they encounter chemotactic signals.
* Mononuclear phagocytes serve as resident cells in tissues like the lung, liver, spleen, and peritoneum.
* Both types of phagocytes play roles in surveillance and response to microbial invasion or tissue injury.
Quantitative Granulocyte and Mononuclear Phagocyte Disorders
Disorders can arise from either an excess or deficiency of phagocytes, leading to various clinical states. These disorders can significantly impact the immune response.
* Neutropenia refers to a low neutrophil count, increasing infection risk.
* Neutrophilia indicates an elevated neutrophil count, often due to infection or inflammation.
* Eosinophil disorders include conditions associated with altered eosinophil numbers.
* Basophilia and basophilopenia reflect abnormal basophil levels.
* Monocytosis and monocytopenia involve changes in monocyte counts.
Disorders of Phagocyte Function
Phagocyte function can be impaired by various disorders, affecting their ability to respond to infections. These disorders can be intrinsic or secondary to other diseases.
* Disorders of adhesion affect the ability of phagocytes to stick to blood vessels.
* Chemotaxis disorders impair the movement of phagocytes toward infection sites.
* Opsonization and ingestion disorders hinder the recognition and uptake of pathogens.
* Neutrophil granule disorders affect the storage and release of antimicrobial substances.
* Impaired oxidative metabolism can reduce the effectiveness of phagocytes.
* Cytokine-related disorders can lead to overall impaired phagocyte function.
* Evaluation of patients with recurrent infections is crucial for diagnosing these disorders.
Regulation of Myelopoiesis and Granulocyte Production
Myelopoiesis is a complex process involving the differentiation of hematopoietic stem cells into granulocytes and monocytes, regulated by various transcription factors and cytokines. Key factors such as PU.1 and C/EBP families play crucial roles in this differentiation process.
* Granulocytes and monocytes are produced in the bone marrow through a regulated process.
* Hematopoietic stem cells (HSCs) give rise to lineage-restricted progenitor cells.
* Transcription factors PU.1 and C/EBP are essential for myelopoiesis.
* Cytokines like IL-3, IL-6, GM-CSF, M-CSF, and G-CSF promote neutrophil and monocyte differentiation.
* Emergency granulopoiesis occurs during infections, involving macrophage activation and cytokine release.
* MicroRNAs (miRs) also regulate myelopoiesis and lineage decisions.
Neutrophil Life Cycle and Maturation
Neutrophils undergo a life cycle divided into bone marrow, circulating, and tissue phases, with a lifespan of approximately 14 days in the marrow and 1-2 days in tissues. Their maturation involves several stages, each characterized by specific granule formation and transcriptional changes.
* Release is regulated by chemokines expressed on the cells and their ligands expressed by stromal cells
CXCR4 and its ligand CXCL12 - retain cells
CXXR2 and its ligands CXCL1 and CXCL2 - promote neutrophil release
* The life span of circulating neutrophils is estimated at 6-10 hours.
* Neutrophils migrate to tissues where they live for 1-2 days before apoptosis.
Neutrophil extracellular traps (NETS) - consists of DNA and antibiotic proteins from nucleus, granules and cytosol
Netosis - traps organisms
* Maturation stages include myeloblast, promyelocyte, myelocyte, metamyelocyte, band, and segmented neutrophil.
Myeloblasts - earliest morphologically recognizable granulocyte precursors in the marrow; large, oval nucleus, several prominent nucleoli, and few or no granules in a gray blue cytoplasm in Wright-stained preparations
Promyelocyte - formation of azurophilic or primary granules and contain myeloperoxidase (MPO), bactericidal peptides and lysosomal enzymes
Myelocyte - formation of peroxidase-negative specific, or secondary granules containing lactoferrin
Metamyelocyte, band, and segmented neutrophil - progressive nuclear condensation, accumulation of glycogen, and accumulation of tertiary, gelatinase-rich granules and secretory vesicles; endocytic vesicles marked by albumin and Complement Receptor 1 (CR1) (CD35)
Mature neutrophil - multilobed, polymorphic nucleus, highly condensed chromatin and a yellow-pink cytoplasm containing numerous granules and clumps of glycogen
Mean lobe count: less than 3
Neutrophil Granule Composition and Function
Neutrophils contain various granules that store enzymes and proteins essential for their immune functions, classified into azurophilic, specific, and gelatinase granules. These granules play critical roles in microbial killing and inflammatory responses.
* Neutrophil granules include azurophilic (primary), specific (secondary), and gelatinase (tertiary) granules
* Azurophilic granules contain myeloperoxidase (MPO), cathepsin G (restricted to myeloblasts and promyelocytes) , bactericidal proteins and lysozyme
MPO - enzyme in the oxygen-dependent killing pathway; green heme enzymes - mature neutrophils (pus) or myeloid leukemia cells in the bone marrow or extramedullary tumors (chloromas)
Neutral serine proteases and other digestive enzymes characteristic of lysosomes
* Specific granules contain lactoferrin, transcobalamin I (myelocytes and metamyelocytes) and lysozyme
Lactoferrin - iron-binding protein that has direct bactericidal activity
Antibiotic substances - lysozyme, lipocalin 2 (neutrophil gelatinase-associated lipocalin; NGAL) - a bacterial siderophore-binding protein, and the metalloproteases collagenase and gelatinase
Membrane contains a major proportion of the neutrophil’s supply of flavocytochrome b558, electron carrier of respiratory burst oxidase
Pool of receptors for adhesive proteins, TNF, and chemotactic formyl peptides
* Gelatinase granules - first detected in bands and bone marrow neutrophils; are rich in metalloproteinases
Contain membrane associated metalloproteinase leukolysin (MMP-25)
Formed late in neutrophil differentiation and are smaller and more easily mobilized for exocytosis
* Secretory vesicles - Leukocyte B-integrin subunit CD11b - first detectable in myelocytes and increases throughout the later stages
Formed in bands and mature neutrophils by endocytosis of the plasma membrane
Important store of leukolysin and adhesive protein Mac I (CD11b/CD18)
* gp91 phox - subunit of the respiratory burst oxidase complex is expressed relatively late in neutrophil maturation; not detected until metamyelocyte stage
* Diapedesis and exposure to cytokine induce neutrophil expression of mRNA transcripts for IL-1, IL-6, TNF-a, GM-CSF, M-CSF, and IL-8 - promote recruitment and activation of both phagocyte and lymphocyte populations in the inflammatory response
* Granule biosynthesis is tightly regulated during neutrophil maturation.
Abnormalities in Neutrophil Morphology
*Granule fusion - result in vacuolization and toxic granulation (prominent azurophilic granules)
Chédiak-Higashi and Alder-Reilly anomalies
large azurophilic granules representing defective membrane targeting of proteins in secretory lysosomes but normal specific granules; autosomal recessive traits
*Döhle bodies - seen in normal neutrophils at times of infection; represent strands of RER that are retained from a more immature stage and stain bluish because of high content of RNA and ribosomes
May-Hegglin Anomaly and Fechtner and Sebastian Syndromes
autosomal dominant; döhle bodies in granulocytes and monocytes, leukopenia, giant platelets and variable thrombocytopenia
Mutations in the gene encoding non-muscle myosin heavy chain 9
*Hypersegmentation - sign of vitamin B12 or folate deficiency; reduced expression of L-selectin (CD62L)
mature approaching senescence; capable of suppressing T-cell activation during inflammation
Mean of 4 lobes
*Nuclear Hyposegmentation
Pelger-Huët Anomaly - AD trait, mutation in the gene encoding the laminin B receptor, an integral protein of the nuclear envelope; typically the nucleus is bilobed (pince-nez) but has mature, coarse and densely clumped chromatin
Specific granule deficiency (SGD) - pink staining specific granules are absent
Neutrophil Granule biosynthesis and Classification
*Cathepsin G defensins and azurocidin - antimicrobial and chemotactic for monocytes and T cells which helps amplify the inflammatory response and link innate to adaptive immunity.
Neutrophil Cell Surface Receptors
*Primary function of mature neutrophil is to move rapidly into tissue sites to destroy invading microbes and clear inflammatory debris and respond to inflammatory stimuli
*Pattern recognition molecules such as Toll-like receptors (TLRs) and formyl-peptide receptors reflect the neutrophil’s role in the innate immune response
*lipid rafts - microdomains in the plasma, enriched in cholesterol, glycosphingolipids and glycosylphosphatidylinositol (GPI)-anchored proteins and CD11b/CD18 - pattern recognition structures by virtue of their glycosphingolipids such as lactosylceramide (CDw17), lipopolysaccharide (LPS)-binding GPI-anchored protein CD14 and carbohydrate-binding domain of CD11b
RECEPTORS IN NEUTROPHILS
G-protein linked - fMLP, C5a, PAF, LTB4, IL-8, chemokines
seven-transmembrane-spanning domains (serpentine); linked to heterotrimeric GTP-binding proteins
Membrane tyrosine kinases - PDGF
integral membrane protein, intrinsic tyrosine kinase activity; ligation leads to receptor dimerization and cross (“auto”) phosphorylation
Tyrosine kinase linked - FcyRIIa, GM-CSF
FcyRII is a member of the immunoglobulin family of receptors
The GM-CSF recepptor is an 84-kD transmembrane protein related to receptors for IL-2 and IL-6
Ligation of receptor activates cytosolic tyrosine kinases
GPI linked - FcyRIIIb, DAF, CD14
Receptors with no transmembrane or intracellular domains. May associate with a partner receptor to mediate signal transduction
Adhesion molecules - β2 Integrins, L-selectin
β Integrins are heterodimers with relatively long cytoplasmic tails
L-selectin has na extracellular lectin-binding domain and a very short cytoplasmic tail
Ligation results in potentiation of the oxidative burst and phagocytosis in adherent cells, calcium signaling, actin cytoskeletal changes, and upregulation of gene expression
Ceramide linked - TNF
Single-membrane-spanning glycoproteins; ligation activates membrane-bound sphingomyelinase with generation of ceramide, which in turn activates a protein kinase
Neutrophil Subsets
*Two Subsets of circulating neutrophils
Presence of CD177
CD177 - surface receptor for proteinase 3; associated with Wegener granulomatosis
Absence of CD177
OLFM4 - specific granule protein with anti-bacterial properties
Eosinophil Development and Function
Eosinophils, arising from committed progenitor cells, CFC-Eo, play roles in immune responses, particularly against helminthic infections and allergies. Their life span in blood is around 24 hours, while tissue eosinophils may survive for several weeks.
* Eosinophils constitute about 0.3% of nucleated bone marrow cells.
* Key cytokines for eosinophil production include IL-3, IL-5, and GM-CSF
*IL-5, IL-13, chemokines (such as eotaxins [CCL11] regulated on activation, normal T cell expressed and secreted [RANTES; CCL5]) and leukotrienes [LTB4] - eosinophil regulation, differentiation, chemotaxis and functional activation
* Eosinophils have distinctive granules that stain strongly with acid dyes - prominent by the myelocyte stage
* They can undergo piece-meal degranulation (PMD), or graded release of granule content
tubulovesicular system
*Mature eosinophil
nucleus is bilobed; cytoplasm with prominent and morphologically distinctive granules that stain strongly with acid aniline dyes
Capacity for chemotaxis, phagocytosis,degranulation, and the synthesis of reactive oxidants and arachidonate metabolites
MBP - major basic protein; induces histamine release from basophils and mast cells; autocrine degranulation of eosinophils
*Eosinophil peroxidase plays a role in antihelminthic function and utilizes bromate to generate hypobromous acid from hydrogen peroxide
*Primary granules - contains lysophospholipase, the Charcot-Leyden Crystal (CLC) protein that can polymerize to form the bipyramidal hexagons that are the CLCs that are found in areas of eosinophil degeneration
CLC lysophospholipase - catalyzes hydrolysis and inactivation of lysophospholipids generated by phospholipase A2, thus preventing the generation of proinflammatory arachidonic acid metabolites; 5% of total protein in eosinophils
*MBP, eosinophil peroxidase, and eosinophil neurotoxin - toxic to tissues, including the heart, lungs and brain
Basophils and Mast Cells: Similarities and Differences
Basophils and mast cells share a common progenitor but differ in their maturation and tissue distribution. Both cell types are involved in hypersensitivity reactions and release granules containing histamine and other mediators.
* Basophils account for approximately 0.5% of circulating leukocytes and 0.3% of nucleated marrow cells
full differentiation occurs in the bone marrow over 7 days before their release into the bloodstream
Bilobed and smaller than mast cells
Synthesize IL-4 and IL-13, linking innate and adaptive immunity;
Heparin have poor anticoagulant activity
* Mast cells mature in tissues and are not found in circulation.
lack receptors for IL-2, IL-3 and CD11b/CD18 that are present in basophils
Secrete mitogenic or inflammatory cytokines - IL-1, IL-3, IL-4, IL-5, IL-6, chemokines, GM-CSF and TNF-a - for leukocyte recruitment and adhesion
C-kit receptor for stem cell is present on mast cells but absent from basophils —> main survival and developmental factor for mast cells
IL-3, IL-4 and IL-9 - supportive for mast cells development
Mature mast cells do not circulate in the blood
* Both cell types contain large metachromatic granules high content of sulfated glycosaminoglycans; rich in heparin-type and chondroitin sulfate-type glucosaminoglycan responsible for packing the cationic proteins, histamine and kallikrein
*High affinity receptor for the Fc portion of IgE - an important trigger for release of granule contents and production of arachidonic acid metabolites in anaphylactic degranulation on the plasma membrane and key effector cells in hypersensitivity reactions
Mononuclear Phagocytes: Monocytes and Macrophages
Monocytes differentiate from granulocyte-monocyte progenitors (GMP) and migrate into tissues to become macrophages, which play vital roles in immune responses and tissue homeostasis. They exhibit diverse functions, including phagocytosis and antigen presentation.
* Monocytes spend about 24 hours in the marrow before entering circulation and spend several days in the intravascular compartment
* They can differentiate into macrophages, which have a longer lifespan.
Production and differentiation are regulated by IL-3, IL-6, GM-CSF and more lineage specific cytokine M-CSF
Convoluted suface, grey-blue cytoplasm, indented or kidney-shaped , foamy nucleus
Contain secretory vesicles
Classic monocytes: high expression of CD14 (the LPS receptor) and a low expression of CD16 (low affinity IgG)
Nonclassical monocytes: high expression of CD16
Intermediate type: high expression of CD14 and intermediate expression of CD16 - possible precursors of dendritic cells
* Monocytes upon leaving the circulation become larger and take on appearance of tissue macrophages
nucleus is typically oval with more prominent nucleoli, and the cytoplasm stains blue because of an increase in RNA content
Distinguished histochemically by fluoride-inhibitable nonspecific esterase and a variety of antibodies such as anti-CD68
Capable of sensing chemotactic gradients, migrating to inflamed sites, ingesting microorganisms and killing them using a variety of cytocidal products
Histiocytes - resident tissue macrophages
* Mononuclear phagocytes - central role in the adaptive immune response by presenting antigens to lymphocytes, elaborate growth factors and cytokines important for lymphocyte function, wound repair and hematopoiesis; scavenger and homeostatic pathways
*They express various surface markers and can be activated by inflammatory signals.
* Spleen - germinal centers associated with lymphocytes
macrophages located in the red pulp and sinuses serve as a clearance function
*Liver - kuppfer cells
* Lymph nodes - most abundant in the medullary zone close to efferent lymphatic and blood capillaries
*Lungs - reside both in the interstitium of alveolar sacs and free with the air spaces
Hemosiderin-laden alveolar macrophages - recurrent pulmonary hemorrhage in idiopathic hemosiderosis or Goodpasture Syndrome
*Bone Marrow - abundant within hematopoietic islands and on the walls of the marrow sinuses
clearance function in normal or pathologic states of ineffective hematopoiesis; support hematopoiesis by modulating mesenchymal stem cell and osteoblasts to express retention signals (CXCL12)
Dendritic Cells: Key Antigen-Presenting Cells
Dendritic cells are specialized antigen-presenting cells that play a crucial role in initiating T-cell responses. They develop from a common progenitor with monocytes and are found in various tissues.
* Dendritic cells are located throughout the body, except the brain.
* They are essential for T-cell activation and the adaptive immune response.
*Langerhans cells of the epidermis - antigen presenting cells, self-sustained and localize to skin during embryonic development
* Plasmacytoid dendritic cells respond rapidly to viral infections by producing type I interferons.
NETs linked to its activation and the pathogenesis of lupus
Osteoclasts: Specialized Bone Resorbing Cells
Osteoclasts are multinucleated cells derived from monocyte-macrophage lineage that resorb bone and cartilage. Defects in their function can lead to osteopetrosis, a condition characterized by defective bone resorption.
* Osteoclasts are derived from granulocyte-macrophage progenitor cells.
* They play a critical role in bone remodeling and mineral resorption.
*Infantile (malignant) Osteopetrosis - AR disorder with progressive obliteration of the marrow space.
Caused by mutations in genes encoding a vacuolar proton pump or the chloride channel 7
Function of Phagocytes
Phagocytes perform various functions essential for the immune response, including migration, recognition, and destruction of pathogens. They also produce mediators that facilitate inflammation and healing.
*Phagocytic leukocytes- central role in the acute phases of the inflammatory response
Initiate an adaptive immune response; essential for normal repair of tissue injury
*Classic signs of inflammatory respose - redness, swelling with heat and pain
* Humoral mediators are of inflammatory response.
Proinflammatory cytokines:
IL-1 and TNF-a - can cause fever and muscle breakdown; cachexia associated with chronic infection and malignancy
Synthesis in the liver is induced by IL-6 and secretion stimulated by IL-1
IFN-y - enhances responsiveness of phagocytes to inflammatory stimuli
Anti-inflammatory cytokines
IL-4, IL-10, and TGF-B
Lipid Mediators
Both inflammatory and anti-inflammatory
Lipoxygenase activity induces production of antiinflammatory lipoxins instead of proinflammatory leukotrienes
Resolvins D1 and D2 - inhibit neutrophil transmigration by downregulating surface proteins involved in transepithelial migration, by inducing nitric oxide (NO) production in endothelial cells
Enhances production of antiinflammatory cytokines and inhibiting production of proinflammatory cytokines of macrophages
Vasodilation and increased vascular permeability - early responses to an inflammatory insult
Histamine released by activated basophils and tissue mast cells leads to vasodilation
Platelet activating factor (PAF) - secreted by activated macrophages, mast cells, and endothelial cells induces platelet degranulation and release of histamine and serotonin
Potent vasodilators: Prostaglandin E and NO
Increase vascular permeability: histamine, serotonin, PAF, and leukotrienes (LT) C4, D4, and E4 and bradykinin
* Chemokines, Small Lipids and other chemoattractants
ensures that leukocytes will be attracted to the site of injury or infection
Induce activation of many other phagocyte function
Upregulation and increased affinity of leukocyte integrin adhesion receptors to promote firm attachment to the endothelium, degranulation, and activation of the phagocyte respiratory burst
Phospholipid PAF - triggers platelet activation and granule release; potent chemoattractant for neutrophils and eosinophils
Stimulates the phospholipase A2-mediated cleavage of membrane phospholipids to generate arachidonic acid —> eicosanoid metabolites including leukotriene B4
*Chemokines - phagocyte chemoattractants; orchestrate the sequential influx of neutrophils, monocytes, and lymphocytes into an inflamed tissue site
regulation of lymphoid homeostasis, hematopoiesis, and angiogenesis
SDF-1 (CXCl2) - provides a key retention signal for neutrophils in the marrow through its interaction with the CXCR4 receptor
Mutations in CXCR4 receptor - WHIM syndrome
*Phagocytes adhere to and migrate into tissues during an immune response.
* They recognize pathogens through opsonization and perform phagocytosis.
* Cytocidal activity includes the digestion of pathogens and debris.
* Specialized functions exist for mononuclear phagocytes, eosinophils, and basophils.
Adhesion and Migration into Tissues
The migration of leukocytes from the bloodstream to sites of inflammation was first described by Cohnheim in 1867, marking a significant advancement in understanding inflammation. Cohnheim's work laid the foundation for later discoveries regarding the molecular mechanisms involved in leukocyte adhesion and migration.
* Cohnheim utilized intravital microscopy to study microvasculature in frogs.
* He proposed that inflammatory stimuli cause molecular changes in blood vessels, enhancing leukocyte adherence.
* Diapedesis - leukocytes must attach to the vascular endothelium , migrate between adjacent endothelial cells and penetrate the basement membrane
Mechanisms of Leukocyte Adhesion and Migration
Leukocyte migration involves a series of steps including rolling, firm adhesion, diapedesis, and penetration of the basement membrane, facilitated by various adhesion molecules. These processes are crucial for leukocytes to reach inflamed tissues effectively.
* Initial step: rolling - mediated by low-affinity interactions between neutrophil and the endothelium.
*tethering - transient adherence mediated by the upregulation of selectin expression on endothelial cells
*Selectins are adhesion molecules that play a critical role in the initial rolling of leukocytes along the endothelium during inflammation. They facilitate the transient interactions necessary for leukocyte adhesion.
P-selectin - initial steps of neutrophil adhesion to the endothelium stored in the Weibe-Palade bodies and a-granules of endothelial cells and platelets; activated by histamine, thrombin and other inflammatory molecules
E-selectin - expressed on endothelial cells at low levels; binds to different ligands on neutrophils to capture neutrophils by mediating tethering, rolling and slowing of neutrophil velocity
L-selectin - expressed on surface of neutrophils, mononuclear phagocytes, and lymphocytes; modulate leukocytes adhesion during inflammation; shed upon activation
*Rolling neutrophils - flattened, adherent morphology and attach firmly to the vessel wall mediated by leukocyte integrin adhesion receptors binding to intracellular adhesion molecules (ICAMs)
* Firm adhesion is primarily mediated by integrins binding to intracellular adhesion molecules (ICAMs).
* Diapedesis involves leukocytes squeezing between endothelial cells and requires chemotactic gradients.
Integrins and Their Function in Adhesion
Integrins are essential for the firm adhesion of leukocytes to the endothelium and play a significant role in the inflammatory response. They mediate strong adhesive interactions necessary for leukocyte migration.
*Neutrophil β2 and β1 integrins - regulate neutrophil retention and release
*Leukocyte β2 integrins play a critical role in mediating adhesive interactions in inflammation, including attachment of leukocytes to endothelial cells and are opsonic receptors for complement fragment C3bi-coated particles.
* Four types of leukocyte β2 integrins exist, each with distinct α subunits and functions. (CD11a, CD11b, CD11c, CD11d)
*Lymphocyte function antigen 1 (LFA-1) - expressed on the surface of all leukocytes including lymphocytes
*Mac-1 and p150,95 - expressed by granulocytes, mononuclear phagocytes, some activated T lymphocytes and large granular lymphocytes
Mac-1 - most prominent β2 integrin on neutrophils; adβ2 is expressed particularly in tissue macrophages
* Mutations in β2 subunit result in inherited defect in phagocyte function, leukocyte adhesion deficiency type I (LAD I) - all β2 integrins are absent
Intercellular Adhesion Molecules (ICAMs)
ICAMs are important counter-receptors for integrins that facilitate leukocyte adhesion to endothelial cells during inflammation. Their expression is upregulated in response to inflammatory cytokines.
* ICAM-1 and ICAM-2 are crucial for neutrophil binding to the endothelium.
ICAM-1 endothelial cells expression increases response to inflammatory cytokines which promote increased cell-cell interactions with leukocytes at inflamed sites
* VCAM-1 - counter-receptor for β1 integrin, VLA-4 and is involved in monocyte and eosinophil adhesion during inflammation.
* ICAMs contain multiple immunoglobulin-like domains and are widely distributed on various cell types; can induce vascular permeability, activate PECAM-1 for enhanced adhesivity, and support of neutrophil transendothelial migration
*Final steps in emigration involves squeezing between adjacent endothelial cells (diapedesis) and penetrating the basement membrane
Chemotactic gradient is required to induce the directional migration of neutrophils
Adhesive interactions between the β2 integrins and endothelial cell ICAM-1 are essential for neutrophil diapedesis
VCAM-1 and E-selectin mediate the transmigration of monocytes and neutrophils
Chemotaxis and Neutrophil Migration
Chemotaxis is the directed movement of leukocytes along a concentration gradient, essential for effective immune responses. Neutrophils exhibit polarized movement and utilize actin dynamics for migration.
* Defects in neutrophil cellular motility or other steps in chemotaxis result in decreased resistance to bacterial and fungal infections
*Migrating neutrophils has polarized appearance, extend pseudopodia or lamellipodia, thin structures rich in actin filaments and lacking intracellular organelles, at the leading edge.
requires formation of uropod at the “tail” of the leukocyte, which detaches from the underlying matrix and retracts the rear of the cell as it moves forward
* Rho GTPases establish chemoattractant-induced polarization and migration.
Regulates actin remodelling and activated by ligand binding to chemoattractant receptors
* Defects in chemotaxis can lead to increased susceptibility to infections.
Recognition, Opsonization and Phagocytosis
Phagocytosis involves the recognition and ingestion of pathogens, facilitated by opsonins that enhance the process. Opsonization is critical for effective immune responses against microbes.
* Key humoral opsonins include complement fragments (C3b, C3bi) and specific immunoglobulins (IgG, IgM)
* Phagocytes have receptors for opsonins, enhancing their ability to recognize and ingest targets.
* Deficiencies in opsonization can lead to increased infection risk.
Inflammasomes
Inflammasomes are complexes that activate pro-inflammatory cytokines IL-1β and IL-18, playing a key role in the inflammatory response. They are activated by various stimuli and are crucial for immune signaling.
* Inflammasomes process proIL-1β and proIL-18 for secretion.
* Activation involves different subunits depending on the stimulus.
* Mutations in inflammasome components can lead to inflammatory diseases.
Complement Receptors System and Phagocyte Interaction
The complement system generates opsonins that enhance phagocytosis and immune responses.80 Phagocytes express various receptors for complement fragments, facilitating their function.
* C3b and C3bi are key opsonins recognized by phagocyte receptors.
* CR1 and CR3 are important receptors for C3 fragments on phagocytes.
* Deficiencies in complement receptors can impair immune responses.
Fc Receptors and Antibody-Mediated Responses
Fc receptors (FcRs) on phagocytes bind to antibodies, facilitating the opsonization and clearance of pathogens.77 Different classes of FcRs mediate various immune functions.82
* FcγRs are crucial for recognizing IgG and initiating phagocytosis.
* Cross-linking of FcRs triggers signaling pathways that enhance immune responses.
* Polymorphisms in FcRs can influence susceptibility to infections and autoimmune diseases.
Specialized Functions of Mononuclear Phagocytes
*Activated macrophages play a key role in the ingestion and killing of intracellular parasites, such as mycobacteria, Listeria, Leishmania, Toxoplasma and some fungi
IFN-y - one of the principal macrophage-activating factors, secreted by T lymphocytes as well as by macrophages and neutrophils
Endotoxin - bacterial LPS derived from gram negative bacteria trigger of macrophage activation through pathways involving CD14 and TLRs
M1 macrophages - classically activated; induced or supported by IFN-y or IFN-β in combination with TNF-a
producer of proinflammatory cytokines and potent activators of T-helper cells (Th1 and Th17) - TNF-a, IL-1, IL-6, IL-12, and IL-23
M2 phenotype - antiinflammatory functions and promote healing
key inducers - IL-4 and IL-13; IL-10, IL-21 and GM-CSF
Primarily produced by eosinophil and basophils and CD4+ type 2 helper T cells (Th2 cells)
Antiinflammatory cytokines: IL-10 and TNF-β
Myeloid derived suppressor cells (MDSCs)
inhibition of antitumor activity of T cells
Ability to deplete T cells of arginase
Wound Repair
early phase - influx of neutrophils, followed by migration of monocytes that differentiate into activated macrophages —> appearance of T lymphocytes
Proliferating fibroblasts secrete collagen and other matrix protein important for wound closure and tissue remodeling, and migrating keratinocytes regenerate the epithelial surface
Macrophages ingest and dispose of apoptotic and necrotic cells
Neutrophil Granule Contents and Antimicrobial Activity
Neutrophils contain granules filled with antimicrobial substances that are released during immune responses. These granules play a vital role in the destruction of pathogens.
* Azurophilic granules contain enzymes like myeloperoxidase and defensins.
* Specific granules contain proteins like lactoferrin and lysozyme that aid in microbial killing.
* Oxygen-independent and oxygen-dependent pathways work together to eliminate pathogens.
Role of Eosinophils and Basophils in Inflammation
Eosinophils and basophils contribute to the inflammatory response, particularly in allergic reactions and defense against parasites. They interact with other immune cells to modulate responses.
*Eosinophils and mast cells are often situated beneath epithelial surfaces exposed to environmental antigens such as respiratory and GI tracts
* Eosinophils are effective defense against helminths and release cytotoxic granules.
bind to surface of both larval and adult helminths and inflict damage through release of cationic granule proteins
*PAF and leukotriene C4 - induce smooth muscle contraction and promote secretion of mucous; potent activators of eosinophils
Release of eosinophil granule contents may contribute to localized tissue damage
Purified eosinophil major basic protein (MBP) - cause cytopathic damages in tracheal epithelium
*Eosinophils may also perform immunosuppressive function in immediate hypersensitivity reactions
Release prostaglandin E1/E2 to suppress basophil degranulation —> release histamine —> oxidation of slow-reacting substance of anaphylaxis
Release of phospholipase D to inactive mast cell PAF
Release of MBP for binding of mast cell heparin
Release of plasminogen to reduce local thrombus formation
* Basophils and mast cells are involved in immediate hypersensitivity reactions.
IgE activated; release eosinophilic chemotactic factor of anaphylaxis which recruit eosinophils to the site
* Both cell types release mediators that influence inflammation and immune responses.
Inflammatory Response and Tissue Injury
The inflammatory response, while protective, can lead to significant tissue damage through the release of harmful agents by activated phagocytes. This section discusses various pathologic inflammatory reactions and the role of neutrophils and macrophages in tissue injury and chronic inflammatory diseases.
* Inflammatory response can cause tissue damage due to proteases, oxygen radicals, and cytokines released by phagocytes.
* Neutrophil elastase is linked to the pathogenesis of emphysema in smokers and α1-antitrypsin deficiency
* Neutrophil granule proteases contribute to joint destruction in rheumatoid arthritis
*systemic inflammatory response syndrome.
* Activated neutrophils can lead to adult respiratory distress syndrome and lung injury during procedures like hemodialysis.
* Macrophages play a role in atherosclerosis and can influence plaque development and rupture.
* Oxidative products from phagocytes are mutagenic, increasing cancer risk in chronic inflammatory states like ulcerative colitis.
Neutropenia: Definition and Classification
Neutropenia is characterized by a reduced number of circulating neutrophils, with varying clinical significance based on the severity of the condition. This section categorizes neutropenia into intrinsic and extrinsic causes, highlighting genetic defects and acquired factors.
* Neutropenia is defined as an absolute neutrophil count (ANC) below normal levels, with specific thresholds based on age and race.
* Normal ANC for whites is 1000 cells/μL in infants and 1500 cells/μL in adults; lower limits for African descent are noted.
* Neutropenia can arise from production disturbances, altered distribution, increased destruction, or a combination of these factors.
* Classifications include intrinsic defects (e.g., genetic disorders) and extrinsic factors (e.g., infections, drugs).
*Mild neutropenia - 1000-1500 cells/μL; Moderate neutropenia - 500-1000 cells/μL
* Severe neutropenia is defined as ANC below 500 cells/μL, with agranulocytosis reserved for counts below 200 cells/μL.
Symptoms of Neutropenia
Neutropenia primarily increases susceptibility to bacterial and fungal infections, with specific clinical manifestations depending on the severity and underlying cause. This section outlines common infections and the variability in susceptibility among patients.
* Increased risk of bacterial and fungal infections, particularly with significant neutropenia - halllmark of neutropenia
* Most common types of pyogenic infections - cellulitis, furunculosis, superficial or deep cutaneous abscesses, pneumonia and septicemia
* Stomatitis, gingivitis and periodonitis - may be presenting signs of neutropenia that develop into deep problems
* Endogenous bacteria are the most common cause of infections; most commonly isolated organisms: Staphylococcus aureus and gram-negative bacteria.
* Symptoms of local infections may be less evident in neutropenic patients.
* Some patients with chronic neutropenia may not experience serious infections despite low neutrophil counts
normal to increased numbers of circulating monocytes, an alternative phagocyte
recruitment of monocytes to inflammatory sites is delayed relative to neutrophils; not as efficient as neutrophils in ingesting bacteria
* Monocytes can provide marginal protection, but their recruitment is delayed compared to neutrophils.
Intrinsic Defects Leading to Neutropenia
This section discusses various intrinsic genetic defects that result in neutropenia, including reticular dysgenesis, severe congenital neutropenia, and cyclic neutropenia. Each disorder is characterized by specific clinical features and genetic mutations.
* Reticular dysgenesis is one of the rarest and more severe form of severe combined immunodeficiency with severe leukopenia and high mortality in infancy.
selective failure of stem cells committed to myeloid and lymphoid development
Defective cellular and humoral immunity functions
Absence of lymph nodes, tonsils, Peyer’s patches and splenic follicles — as well as sensorineural hearing loss
Erythroid and megakaryocyte development is normal
AR; biallelic mutations in the AK2 gene encoding the mitochondrial energy metabolism enzyme adenylate kinase 2
* Severe congenital neutropenia (SCN) is characterized by ANC below 200 cells/μL, with infections often caused by S. aureus and E. coli.
AD, AR, or X-linked inheritance associated with severe neutropenia
Refers to entire disorder regardless of inheritance or genotype
Kostmann disease - AR subtype caused by mutations in the HAX1 antiapoptotic gene and G6PC3
AD SCN - mutations in the Gfi1 gene which encodes a transcriptional repressor that inhibits multiple genes involved in neutrophil maturation, including ELANE
ANC below 200 cells/μL documented on the 1st day of life
Episodes of fever, skin infections (including omphalitis), stomatitis, pneumonia, and perirectal abscesses typically appear during the 1st months of life
Usually caused by S. Aureus, Escherichia coli, and Pseudomonas species
Risk of developing MDS and AML
Peripheral blood: agranulocytosis, moderate to marked monocytosis, mild to moderate eosinophilia, anemia and thrombocytosis
G-CSF production appears to be normal if not elevated
Antineutrophil antibodies may be present
Bone marrow: myeloid “maturation arrest” at the myelocyte stage of development, with normal to increased numbers of promyelocytes, but few if with any more mature forms; promyelocytes may show dysplastic morphology: large size, atypical nuclei, and vacuolated cytoplasm
Marrow eosinophilia and monocytosis is common
Cellularity is usually normal or slightly decreased; megakaryocytes are normal
* SCN can be caused by mutations in the ELANE gene, accounting for 40% to 80% of cases.
encodes neutrophil elastase; responsible for the related but distinct disorder, CyN
* Cyclic neutropenia (CyN) features periodic oscillations in neutrophil counts, with nadirs below 200 cells/μL every 21 days±3 days.
Both SCN and CyN show increased apoptosis of neutrophil precursors, linked to mutations in neutrophil elastase.
During the neutropenic nadir - malaise, fever, oral and genital ulcers, gingivitis, periodontitis, and pharyngitis with local lymph node enlargement; may begin during 1st yr of life but may not present until adulthood
Most serious complications - intestinal perforation with peritonitis, mastoiditis, and pneumonia; clostridial sepsis - most common cause of death
Cyclic hematopoiesis - oscillations in reticulocyte and platelet counts
Diagnosis: neutrophil counts should be monitored twice, or at least once a week for to 8 weeks
Confirmed by sequencing of ELANE gene
Fevers, upper respiratory symptoms, and cervical lymphadenopathy occurring during neutropenic episodes require no specific therapy
Oral and dental hygiene are important to minimize periodontal disease and ameliorate discomfort of mouth sores
GCSF for CyN with ANC nadirs consistently below 500 cells per μL at 2 to 4 μg/kg/day daily or on alternate days
* X-linked SCN - gain of function mutation in WAS gene in Wiskott-Aldrich syndrome
Treatment and Management of Neutropenia
Management of neutropenia involves addressing underlying causes and utilizing therapies such as granulocyte colony-stimulating factor (G-CSF) to improve neutrophil counts. This section highlights treatment efficacy and potential complications.
* G-CSF is the standard treatment for SCN, improving ANC and reducing infection rates in over 95% of patients.
* Recommended starting dose of G-CSF is 5 μg/kg/day, with adjustments based on response.
* Side effects of G-CSF include bone pain, headache, and splenomegaly; monitoring for bone density is advised; vasculitis or glomerulonephritis
* Long-term G-CSF therapy carries a risk of developing thrombocytopenia concurrent with conversion to myelodysplastic syndrome (MDS) or acute myelogenous leukemia (AML).
* Most serious complications: conversion to MDS/AML and bacterial sepsis
* Regular monitoring of blood counts and bone marrow examinations are essential for patients on G-CSF therapy.
Rare Genetic Syndromes Associated with Neutropenia
This section covers rare genetic syndromes that present with neutropenia, including Shwachman-Diamond syndrome, WHIM syndrome, and others. Each syndrome has distinct clinical features and genetic underpinnings.
* Myelokathexis and WHIM Syndrome - uncommon form of moderate to severe chronic neutropenia
granulocyte hyperplasia in the bone marrow
Contains degenerating neutrophils with cytoplasmic vacuoles, prominent granules, and nuclear hypersegmentation with very thin filaments connecting pyknotic-appearing nuclear lobes
Eosinophils - abnormal morphology, lymphocytes, monocytes, and basophils appear normal
Recurrent warts and hypogammaglobulinemia (low IgG and occasionally IgM and IgA)
Recurrent sinopulmonary infections and other bacterial infections
WHIM syndrome is characterized by neutropenia, recurrent infections, and hypogammaglobulinemia, linked to mutations in the CXCR4 gene.
Partially corrected by GCSF or GM-CSF therapy, responses seen within 4 to 8 hours of administration
Ig levels are normal; Specific therapy aimed at the primary molecular defect using CXCR4 antagonist plerixafor to correct the leukopenia
* Shwachman-Diamond syndrome includes triad of neutropenia, pancreatic insufficiency, and skeletal abnormalities, with a risk of MDS/AML.
AR trait; mutation in the SBDS gene
Pancreatic acini are largely replaced by fatty tissue with relative sparing of the pancreatic ducts and islets
Growth failure and short stature area usually noted and puberty is often delayed
Shows symptoms in infancy with eczema, skin infections, and otitis media; pneumonia, osteomyelitis and sepsis
Levels of of serum trypsinogen, pancreatic isoamylase and stool elastase are generally low
CT scan - pancreatic lipomatosis; hepatomegaly or elevated serum liver enzyme
Skeletal abnormalities - short stature and bone dysplasia
Neutropenia - most common hematologic manifestation; defects in chemotaxis or in numbers or function of B, T, and NK cells that may increase susceptibility to pyogenic infection
Leukocytes may show telomere shortening
Tx: pancreatic enzyme replacement; GCSF for neutropenia
Complications may require HSCT; chrom abnormality mc isochrome i(7q) followed by del(20q) precedes conversion to MDS
* Albinism-Neutropenia Syndrome
AR; defects in the biogenesis or trafficking of lysosomes and related late endosomal organelles
Defectst in formation of melanosomes (partial albinism), abnormal platelet function and immunological defects involving number and function of neutrophils, B lymphocytes and cytotoxic T lymphocytes
High risk for HLH - defects in T and NK cells
CHS
partial oculocutaneous albinism, giant lysosomes in granulocytes and neuropathy
Moderate neutropenia due to ineffective granulopoiesis
Hermansky-Pudlak Syndrome
AR; AP3B1 gene
Oculocutaneous albinism associated with platelet defects
Type 2 - includes neutropenia and decreased numbers of NK cells
Griscelli sydrome type 2
Mutations in RAB27A - encodes a small GTPase key effector of granule exocytosis
Hypomelanosis with varying degrees of neurological impairment and immunodeficiency
Neutropenia, hypogammaglobulinemia, partial albinism and predisposition to HLH
Cohen Syndrome
AR; COH1 termed VPS13B
Neutropenia, pigmentary retinopathy, microcephaly, intellectual deficiency, and facial dysmorphism
Yeast protein that functions in vesicular sorting and intracellular protein trafficking
* Familial Benign Neutropenia - AD; nonspecific descriptive diagnosis indicative of mild neutropenia with no tendency to increased infection
* Neutropenia Associated with Immune Dysfunction
X-linked agammaglobulinemia
Hyper-IgM immunodeficiency syndrome - secondary to formation of autoantibodies
Tx: Immunoglobulin replacement therapy
Autoimmune Lymphoproliferative Syndrome (ALPS)
Defects in lymphocyte apoptosis
Lymphadenopathy, splenomegaly, and a variety of autoimmune disorders - immune thrombocytopenia, anemia, and neutropenia
Tx: immunosuppression with corticosteroids or mycophenolate mofetil
Cartilage Hair Hypoplasia
AR; mutation in RMRP
short limbed dwarfism, fine hair, moderate neutropenia and impaired cell-mediated immunity
Tx: GCSF therapy; BMT
Schimke immuno-osseous dysplasia
Rare AR; SMARCAL1 mutation
spondyloepiphyseal dysplasia, steroid-resistant nephritic syndrome, lymphopenia with defective cellular immunity, neutropenia
Life limiting complication: cerebral and generalized atherosclerosis
Tx: HSCT, GCSF
Poikiloderma with neutropenia, Clericuzio type
AR genodermatosis; USB1 gene
Early onset poikiloderma, pachyonychia, palmoplantar hyperkeratosis, and skeletal defects
Variable degrees of neutropenia and predisposition to malignancy
*Neutropenia associated with Metabolic disorder
Barth Syndrome
X-linked; mutations in TAZ gene involved in maintenance of mitochondrial cardiolipin
Dilated cardiomyopathy, growth retardation, and 3-methylglutaconic aciduria
Neutropenia precede development of cardiac abnormalities
Bone Marrow - maturation arrest at the myelocyte stage, with myeloblasts and promyelocytes demonstrating abnormal mitochondria
Glycogen storage disease type Ib
Mutations in glucose-6-phosphate transporter SLC37A4 gene
Tx: GCSF
Pearson Syndrome - macrocytic anemia accompanied with neutropenia or thrombocytopenia; involves exocrine pancreas, liver and kidney
BMA - normal cellularity with vacuolization of erythroid and myeloid precursorsm hemosiderosis and ringed sideroblasts
Related to large deletions in mitochondrial DNA leads to impaired hemtopoiesis
Metabolic abnormalities - acidosis, ketosis, hyperammonemia and hypoglycemia
Screening of urine organic acids
*Neutropenia Caused by Extrinsic Factors
Causes and Mechanisms of Neutropenia
Infection
* Viral infections, such as influenza and measles, are common causes of transient neutropenia in children, typically lasting 3 to 6 days. (Acute viremia)
* Transient and chronic neutropenia
Epstein-Barr virus - splenomegaly, direct infection of progenitors, and antineutrophil antibodies
Cytomegalovirus - anti CMV therapies cause neutropenia in the setting of perinatal infection or immune suppression (HSCT or solid organ transplant)
Parvovirus B19 and Hepatitis A and B
HIV infection - from antiretroviral drugs, cellular immune dysfunction, ineffective hematopoiesis, antineutrophil antibodies, B12 or Folate deficiency, and hypersplenism
* Bacteremia and endotoxemia - excessive destruction of neutrophils with depletion of bone marrow reserve pool
occur after phagocytosis of microbes, from the release of metabolites of arachidonic acid, or from activation of complement system
Generation of chemoattractant C5a induces neutrophil aggregation and leads to formation of leukoemboli, which adhere to and damage endothelial surfaces in the pulmonary capillary bed
* Nutritional deficiencies, such as vitamin B12 or folate deficiency, can lead to ineffective granulopoiesis and associated anemia.
Drug-Induced Neutropenia
* Drug-induced neutropenia is often idiosyncratic, with a higher incidence in older adults and women, and can result in agranulocytosis with neutrophil counts below 200 cells/μL.
* Almost any drug can cause neutropenia, with common culprits including anti-inflammatories (aminopyrine and sulfasalazine), antimicrobial agents (Cotrimoxazole), antithyroid drugs and antipsychotics (clozapine and phenothiazine); deferiprone and rituximab
* The most serious form, agranulocytosis, can lead to high morbidity and mortality rates, particularly in elderly patients.
* Mechanisms include toxic suppression of neutrophil formation or immune-mediated destruction of neutrophils.
*neutropenia resulting from myelosuppression is usually insidious and may be asymptomatic; oral mucositis may be the first clinical sign before infectious complication
* G-CSF therapy may hasten recovery but lacks compelling evidence for routine use.
Autoimmune Neutropenia in Children
* Autoimmune neutropenia can occur as an isolated condition or secondary to other autoimmune diseases, with a peak incidence in infants and young children.
*Primary AIN - low circulating neutrophil counts are the only hematological finding
peak incidence occurs in infants and and young children; benign disorder that remits spontaneously
Monocytosis is common.
BMA normal to increased cellularity with myeloid hyperplasia and normal to increased numbers of mature neutrophils, diminished neutrophils or maturation arrest at earlier stages of differentiation
Mild splenomegaly, pyogenic infections not always related to degree of neutropenia
Neutrophil-specific cell surface antigens identified as targets of autoantibodies in primary AIN: Human neutrophil antigen (HNA) 1a and HNA-2(CD117)
Secondary AIN associated with pan FcyRIIIb antibodies
Peripheral destruction of antibody-coated neutrophils, may be augmented by the deposition of C3; phagocytosis of neutrophils in the spleen
Impairment of phagocytosis, respiratory burst activity, and adhesion by neutrophil-directed antibodies
Detected by direct or indirect granulocyte immunofluorescence tests or by granulocyte agglutination tests (less sensitive; but may be useful in Felty syndrome)
Tx: judicious use of antibiotics for infections; prophylaxis administration of Cotrimoxazole
Definitive therapy: GCSF at low dose of 1-2 μg/kg/day
Incidence of bone pain is higher, not associated with development of MDS/AML
*Chronic Benign Neutropenia and AIN of Childhood
typically occurs in children under 3 years, with a median age of diagnosis between 8 to 11 months.
Absolute neutrophil count (ANC) at diagnosis is often around 200 cells/μL, with normal hemoglobin and mild thrombocytopenia.
Infections are generally mild and respond well to standard antibiotics, with serious infections being rare
Spontaneous remission occurs in almost all patients, usually within 20 months; with increasing age, spontaneous remission becomes less likely
* Neonatal Immune Neutropenia
Neonatal immune neutropenia can be isoimmune from maternal immune responses to fetal alloantigens or maternal autoimmune as result of passive transfer of maternal antibodies leading to significant neutropenia in newborns.
Maternal IgG antibodies can cross the placenta, causing neutropenia that may last from weeks to 6 months.
Neutrophil antibodies in the serum of the mother and infant are often directed to the neutrophil-specific HNA antigen system - HNA-1 antigen most often responsible are isotypes of the neutrophil Fcyreceptor IIIb
Affected infants may be asymptomatic or develop omphalitis, cutaneous infections, pneumonia or sepsis and meningitis
Treatment includes parenteral antibiotics and G-CSF for profound neutropenia or serious infections.
* Nutritional Deficiencies and Neutropenia
Nutritional deficiencies, particularly of vitamin B12, folate, and protein-calorie malnutrition, can lead to neutropenia and associated cytopenias. These deficiencies often present with additional hematological abnormalities.
* Megaloblastic marrow pathology is observed in vitamin B12 or folate deficiencies, leading to ineffective granulopoiesis.
* Severe protein-calorie malnutrition, as seen in conditions like anorexia nervosa, can also result in neutropenia.
* Copper deficiency has been linked to neutropenia and marrow megaloblastosis.
Evaluation of Neutropenia
* Persistent neutropenia should have wbc and diff count obtained at least twice weekly for 6-8 weeks to evaluate for periodicity of CyN
* Direct and indirect antiglobulin tests to evaluate for presence of red cell autoantibodies
* Quantitative serum immunoglobulins (IgG, IgA, and IgM, but not IgE)
* HIV testing indicated in chronic cases
* Bone marrow aspiration and biopsy with cytogenetics and cytogenetic screening for monosomy 7
* ANC before and 4 to 6 hours after a single dose of glucocorticosteroid (prednisone 1 to 2 mg/kg) measures mobilization of bone marrow reserve pool of mature neutrophils - increase to normal or only moderately low indicates chronic benign or idiopathic neutropenia
Management Strategies for Neutropenia
The management of neutropenia is tailored to the underlying cause and severity, with a focus on preventing infections and addressing the specific etiology.
* Patients with severe neutropenia should receive prompt broad-spectrum antibiotics upon fever presentation.
* G-CSF is recommended for patients with severe chronic neutropenia and high infection risk.
* Prophylactic antibiotics may be beneficial for patients with recurrent infections.
* Good dental hygiene is crucial to prevent infections in patients with chronic neutropenia.
Neutrophilia
alteration in the total number of blood neutrophils in excess of about 7500 cells/μL in children and adults
Result of a disturbance of the normal equilibrium involving neutrophil bone marrow production and movement in and out of the marrow and circulation
Acute Neutrophilia - occurs rapidly within minutes in response to exercise or epinephrine-induced reactions attributed to mobilization of the marginating pool of neutrophils into the circulating pool
Slower onset of acute neutrophilia occur after glucocorticoid administration or with inflammation or infection associated with the generation of endotoxin, TNF, IL-1 and IL-8 and GCSF
Maximal response within 4 to 24 hours after exposure is the result of release of neutrophils from the marrow storage compartment into the circulation
Chronic Neutrophilia - prolonged administration of glucocorticoids, persistent inflammatory reactions, infection, chronic blood loss or chronic anxiety
Marked neutrophilia - hallmark of functional disorders of neutrophils that are caused by impaired adhesion or motility
Sustained moderate neutrophilia occur after surgical or functional asplenia because of decreased clearance of circulating neutrophils
Also occurs in familial cold urticaria and periodic fever syndromes
Congenital Primary Neutrophilia
AD hereditary neutrophilia; mutation in CSF3R gene
ANCs of 14,000 to 164,000 cells/μL along with hepatosplenomegaly, increased leukocytes alkaline phosphatase, and Gaucher-type bone marrow histiocytes
Leukemoid Reactions
leukocytes greater than 50 × 1000 normal cells/μL
Peripheral blood shows small proportion of immature myeloid cells, including occasional myeloblasts and promyelocytes
Most commonly triggered by pyogenic infections - S. Aureus, S. Pneumoniae
Transient myeloproliferative disorder - infants with DS; marked leukoerythroblastosis with circulating blast cells
Eosinophilia and Its Causes
Eosinophilia can arise from various conditions, including allergic reactions, infections, and malignancies, and is characterized by an elevated eosinophil count.
* occurs after repetitive or prolonged antigen exposure, especially when the antigens are deposited in the tissues and elicit hypersensitivity reactions whether of immediate (IgE-mediated) or delayed (T-lymphocyte-mediated) type
* Stimulation of eosinophilia is T-lymphocyte dependent
* Allergy is the most common cause of eosinophilia in children, with counts exceeding 20,000 cells/μL during acute reactions.
*Atopic dermatitis, eczema, pemphigus, acute urticaria, and toxic epidermal necrolysis
* Parasitic infections, particularly helminths, are significant causes of eosinophilia outside the United States.
* Eosinophilia can also be associated with drug reactions, autoimmune diseases, and certain malignancies.
* Marked eosinophilia can produce organ damage - heart, lungs and GI tract by infiltration and deposition of toxic granule proteins
* Steroid therapy generally produces a rapid decline in eosinophil production and circulating cell numbers
* Steroid-sparing targeted therapies such as monoclonal antibodies directed at IL-5 or IL-5R (mepolizumab, reslizumab, and benralizumab) or T cells (e.g alemtuzumab)
Hypereosinophilic Syndrome
* Defined by persistent eosinophilia and organ involvement, often leading to significant morbidity.
* Criteria: 1. Absolute eosinophil count of 1500 cells/μL or more for over 6 months, (or fatal termination within 6 mos); 2. Lack of other diagnoses to explain secondary eosinophilia; and 3. Signs and symptoms of organ involvement by infiltrating eosinophils or damage.
* Non-specific findings of fever, weight loss, and fatigue
* Cardiac complications are a major cause of morbidity and mortality in HES such as endocardial fibrosis and formation of mural thrombi with infiltrating eosinophils (Loeffler endocarditis)
* Hepatosplenomegaly, pulmonary infiltrates, and skin involvement with urticarial or nodular lesions are common
* Genetic rearrangements, particularly involving PDGFRA, a platelet derived growth factor receptor a, forms fusion products that generate constitutively active tyrosine kinase molecules .
* Most common fusion partner is the Fip1-like 1 gene, FIP1L1, from an interstitial chromosome 4q12 deletion
* PDGFR fusion products are common in clonal HES and can guide treatment with targeted therapies like imatinib.
Eosinopenia
2 mechanisms: primary elevation of adrenal corticosteroids or epinephrine; and acute inflammation or stress, acting in part through secondary release of adrenal corticosteroids and/or epinephrine
* Glucocorticosteroids suppress the transcription of a number of genes involved in eosinophil production and trafficking, including IL-3, IL-4, IL-5 and GM-CSF
* AML M4Eo characterized by myelomonocytic blasts with eosinophilia and inv(16) cytogenetics
Basophilia and Basophilopenia
* associated with hypersensitivity reactions of the immediate type; role in innate immunity, particularly to helminths and other parasites
*elevated in allergic responses; infections with helminthes, viruses, and mycobacteria; and chronic inflammatory diseases
* increased in myeloproliferative disorders such as CML; basophilia exceeding 30% often heralds a poor prognosis
* May benefit from antihistamines
* May accompany AML usually in association with 6p or 12p chromosomal abnormalities
Monocytosis and Monocytopenia
* Monocytosis is characterized by an elevated monocyte count of more than 500 cells/μL and can indicate various underlying conditions, including infections and malignancies - hallmark of JMML
* Common causes include bacterial infections, tuberculosis, and malignancies such as leukemia and lymphoma.
* Monocytopenia can occur after glucocorticoid administration and in infections associated with endotoxemia.
* The recognition of monocytopenia is crucial for diagnosing specific immunological disorders, such as MonoMAC syndrome.
B- and NK-cell deficiencies, mild neutropenia, susceptibility to severe and chronic nontuberculous mycobacterial, fungal and viral infections
Predisposition to MDS/AML and autoimmunity; pulmonary alveolar proteinosis; and lymphedema
Derives from hypomorphic and regulatory mutations that diminish expression of the GATA-2 transcription factor
Mutations in the IRF8 gene resulting in monocytopenia, absent or deficient dendritic cells and immunodeficiency presenting as disseminated BCG disease
Disorders of Phagocyte Function
Phagocyte function abnormalities can lead to inherited and acquired disorders, resulting in recurrent bacterial and fungal infections. These disorders are classified based on the specific phagocyte functions that are impaired.
DISORDERS OF ADHESION
Leukocyte Adhesion Deficiency Type I
LAD I is a rare autosomal recessive disorder caused by mutations in the CD18 gene, leading to impaired phagocyte adhesion and function. Patients typically experience severe recurrent infections without pus formation and exhibit persistent granulocytosis.
* Caused by mutations in the β2 integrin CD18 gene; expression of β2 integrins on leukocyte cell surfaces is reduced or absent
* hallmark - occurrence of repeated, often severe bacterial and fungal infections without the accumulation of pus despite a persistent granulocytosis
* Severe form - early infancy neutrophilia, omphalitis and delayed separation of the umbilical cord
* Patients show persistent granulocytosis (12,000-100,000 cells/mm³) and delayed umbilical cord separation.
* Recurrent necrotic and indolent infections of skin, mucous membranes and GI tract including perirectal abscesses which often heal poorly - aggressive form of gingivitis and periodontitis is characteristic
* Characterized by severe bacterial and fungal infections, particularly from S. aureus and gram-negative bacteria.
* Neutrophils neither adhered to plastic surfaces nor underwent an oxidative burst when exposed to serum-opsonized particles fail to migrate to inflammation sites due to deficient Mac-1 expression.
* Levels of all leukocyte β2 integrin were absent or severely deficient; neutrophils most significantly affected
* Neutrophil intravascular survival is prolonged
* Neutrophil adhesion and emigration in the pulmonary bed, which can be mediated by CD11/CD18-independent mechanisms
* Other leukocytes, monocytes, eosinophils and lymphocytes, express β1 integrin, VLA-4 and are able to utilize adhesion molecule to emigrate into inflammatory sites throughout the body
* Diagnosis confirmed by flow cytometry to assess cell surface expression of any of the β2 integrin a (CD11) subunits shared CD18 subunit and showing reduced β2 integrin expression.
* Treatment depends on the clinical severity of the disorder; includes aggressive management of infections and prophylactic treatment of periodontal disease. Potential bone marrow transplantation for severe cases.
* Gene therapy to correct CD18 expression
Leukocyte Adhesion Deficiency Type II
LAD II is a rare autosomal recessive disorder associated with defective selectin-mediated adhesion due to a deficiency of Sialyl-Lewis X ligands. Patients experience recurrent infections similar to LAD I but with less severity and additional developmental issues.
* Caused by mutations in the SLC35C1 gene encoding the Golgi GDP-fucose transporter 1, affecting fucose metabolism.
* Patients present with recurrent cellulitis, otitis media, periodontitis, and pneumonia without pus formation despite leukocyte counts of 30,000 to 150,000
* Normal levels of CD18 and less serious infection; short stature, distinctive facial appearance with a flat face and depressed nasal bridge, and severe mental retardation
* Neutrophils have normal CD18 levels but cannot bind to selectin receptors.
* Diagnosis involves flow cytometry for leukocyte CD15s (SLeX) and blood typing for the Bombay (hh) phenotype.
Leukocyte Adhesion Deficiency Type III
LAD III is characterized by severe recurrent infections and leukocytosis similar to LAD I and a bleeding tendency with abnormal platelet aggregation due to defects in integrin activation. It is caused by mutations in the FERMT3 gene and absence of its Kindlin 3 protein, affecting the activation of β integrins.
* Integrin expression is normal or near normal; defect in the inside-out activation of hematopoietic cells integrins to a high-affinity binding state
* Patients exhibit normal integrin expression but impaired activation.
* Symptoms include severe infections and bleeding diathesis.
* Diagnosis involves identifying normal β2 integrin expression with functional assays.
* Supportive care includes prophylactic antibiotics and potential early bone marrow transplantation.
DISORDERS OF CHEMOTAXIS
Hyperimmunoglobulin E Syndrome
HIES is a complex disorder marked by elevated serum IgE levels (often greater than 2000 IU/mL) and recurrent staphylococcal infections of the skin and lower respiratory tract, pneumatoceles, chronic pruritic dermatitis, and skeletal and dental abnormalities. It results from mutations in the STAT3 gene, leading to impaired neutrophil chemotaxis.
* Job syndrome - hyperextensible joints and “cold” abscesses that lacked the usual characteristics of inflammation
* Characterized by "cold" abscesses, chronic dermatitis, and recurrent pneumonia and pneumatoceles
* AD and sporadic; Mutations in STAT3 disrupt TH17 cell differentiation, affecting immune responses.
Proinflammatory IL-6 and antiinflammatory cytokines are regulated by STAT3
Keratinocytes and bronchial epithelial cells are dependent on TH17 cytokines to secrete defensins and chemokines
Skeletal defects - hallmark of HIES may reflect defective STAT3 signaling in osteoblasts and osteoclasts
Inactivation of IL-11 signaling - dental abnormalities and craniosynostosis
* AR forms result from mutations in DOCK8 or less commonly TYK2, more profound impairments in lymphocyte function
Associated with increased risk of malignancies, including non-Hodgkin lymphoma and cancers of the vulva, liver and lungs
Additional defects in IL-12 and IFN-a signaling
DOCK8 regulates cytoskeletal rearrangements involved in migration, adhesion and growth, lead to defective CD4+ T cell priming because of impaired dendritic cells migration and impair STAT3 activation in B cells
* Patients often have elevated IgE levels (>2500 IU/mL) and peripheral eosinophilia
*DNA testing - definitive diagnosis
* Treatment includes prophylactic antibiotics (dicloxacillin or cotrimoxazole), antifungal prophylaxis and monitoring for complications.
Neutrophil Actin Dysfunction
Neutrophil actin dysfunction is a rare disorder leading to impaired chemotaxis and phagocytosis due to defects in actin polymerization. Patients experience recurrent infections caused by S. Aureus and cutaneous-cecal fistula complicated by Streptococcus faecalis sepsis despite neutrophilia, sites of infections were devoid of neutrophils and healed slowly
* First reported case involved a male infant with severe skin infections.
* Neutrophils show markedly diminished chemotaxis and decreased capacity to ingest serum-opsonized particles; decreased actin polymerization and abnormal motility behaviors.
* Underlying defect involves neutrophil actin or an ABP
* Treatment may involve bone marrow transplantation, which can resolve symptoms.
Localized Aggressive Periodontitis
LAP is characterized by severe localized alveolar bone loss around molars and incisors, often linked to genetic factors. It is associated with impaired neutrophil chemotaxis in a significant percentage of patients.
* Onset typically occurs around puberty with severe periodontal disease.178
* Approximately 70% of patients exhibit defective neutrophil chemotaxis.
* Twofold decrease in the total number of receptors for formyl peptides and C5a
* Diagnosis is based on clinical presentation and exclusion of other neutrophil disorders.
Neonatal Neutrophils
Neonates are at increased risk for infections due to impaired neutrophil function, including reduced chemotaxis and adhesion, phagocytosis and bacterial killing. These deficiencies are more pronounced in premature infants.
* Neonatal neutrophils show approximately 50% reduced chemotactic ability compared to adults.
* Defects include decreased Mac-1 expression and impaired actin polymerization.
* Impaired formation of NETs; depressed chemotactic ability of neonatal neutrophils and their adhesion
* Increased susceptibility to infections like sepsis and pneumonia is observed.
Other Disorders of Neutrophil Chemotaxis
Various genetic mutations can lead to impaired neutrophil chemotaxis, resulting in recurrent infections. These disorders often present with normal integrin expression but functional defects.
* Dominant-negative mutations in Rac2 lead to severe neutrophil dysfunction.
* Patients may experience recurrent infections and neutrophilia without pus formation.
* Diagnosis involves assessing neutrophil function and genetic testing for mutations.
Chediak-Higashi Syndrome
Chediak-Higashi Syndrome (CHS) is a rare autosomal recessive disorder characterized by defects in granule morphogenesis, leading to immunodeficiency and neurological complications. Patients often experience recurrent infections, partial oculocutaneous albinism, and may progress to hemophagocytic lymphohistiocytosis (HLH).
* Incidence: More than 200 cases described.
* Inheritance: Autosomal recessive.
* Molecular defect: Mutations in the CHS1 gene in long arm of chromosome 1 affecting lysosomal trafficking.
* Pathogenesis: Giant granules in neutrophils lead to ineffective granulopoiesis, neutropenia, and impaired chemotaxis.
neutrophils are markedly deficient in neutral proteases, including two azurophil granule enzymes, cathepsin G and elastase
Platelets have decreased number of dense granules, and a storage pool deficiency of ADP and serotonin - defect in platelet aggregation and an increased bleeding time
* Clinical manifestations: partial oculocutaneous albinism, includes recurrent bacterial infections (commonly S. aureus), gingivitis, periodontitis, and neurological issues like muscle weakness and ataxia and mild bleeding diathesis
* Susceptible to severe sunburns and photosensitivity in bright light
* Laboratory evaluation: Giant granules in periheral blood and granulocytes and bone marrow myeloid progenitor cells; lymphohistiocytic infiltrates in accelerated phase.
* Therapy: Prophylactic antibiotics and treatment of infection, ascorbic acid (200mg/day), and bone marrow transplantation are key treatments.
* Steroids, cyclosporine, etoposide for HLH
* Prognosis: High mortality risk from infections or HLH without transplantation.
Specific Granule Deficiency
Specific Granule Deficiency (SGD) is an extremely rare congenital disorder of neutrophil function, leading to recurrent bacterial infections and structural abnormalities in neutrophils. The condition is characterized by absent specific granules and bilobed nuclei, with a significant impact on host defense mechanisms.
* Inheritance: Primarily autosomal recessive, with some cases reported as autosomal dominant.
* Molecular defect: Deficiencies in proteins of azurophilic and specific granules and secretory vesicles linked to mutations in the C/EBPε transcription factor.
3 cellular compartments are affected: nucleus, specific granules and azurophilic granules
Nucleus- kidney shaped bilobed configuration that is flawed by a series of microlobulations and clefts apparent by electron microscopy
Specific granules are not actually absent but present as empty, elongated vesicles that retain their characteristic trilamellar membrane structure and positive staining for complex carbohydrates
Azurophilic granules are also strikingly abnormaL, though normal amount of MPO, they are severely deficient in defensins
Severe deficiency in alkaline phosphatase activity, which is normally localized to secretory vesicles
Abnormal granule formation in eosinophils; 3 eosinophil-specific granules proteins are deficient - eosinophil cationic protein, MBP, and eosinophil-derived neurotoxin
* Pathogenesis: Recurrent infections arise from deficiencies in microbicidal proteins and abnormal chemotaxis.
neutrophils adhere normally to plastic surfaces but exhibit slightly diminished sticking to nylon fibers and endothelial cells
Severe deficiency of key bactericidal proteins such as lactoferrin and the defensins render the cell less efficient in killing bacteria and Candida
* Clinical manifestations: Patients experience recurrent pyogenic infections, often indolent, affecting skin, ears, lungs, and lymph nodes.
*Key features - indolent, smoldering cutaneous infections punctuated by episodes (sometimes prolonged) of severe infections involving the lungs, ears, lymph nodes, and deeper structures of the skin
* Lung abscesses and mastoiditis; S. Aureus, P. Aeruginosa, Proteus species and other enteric gram negative bacteria, and C. Albicans
* Laboratory evaluation: Absent or empty specific granule vesicles in neutrophils, bilobed nuclei resembling Pelger-Huët anomaly, and severe deficiency of key proteins like lactoferrin, Vit B12 - binding proteins, defensins and alkaline phosphatase
DNA sequencing of the C/EBPε gene has not revealed abnormalities
* Differential diagnosis: Includes acquired specific granule deficiency due to conditions like thermal burns.
* Therapy: Prophylactic and parenteral antibiotics are essential for managing infections.
* Prognosis: With appropriate management, patients can survive into adulthood.
DISORDERS OF OXIDATIVE METABOLISM
Deficiencies in the NADPH oxidase
G6PD
MPO
glutathione reductase
glutathione synthetase
Chronic Granulomatous Disease
Chronic Granulomatous Disease (CGD) is an inherited disorder characterized by the inability of phagocytes to produce superoxide, leading to increased susceptibility to infections and chronic granuloma formation. The condition is primarily caused by mutations in the NADPH oxidase complex, with varying inheritance patterns and clinical manifestations.
* Most common inherited phagocyte function disorder, incidence ~1 in 200,000.201; X-linked recessive, AR
* Caused by mutations in NADPH oxidase subunits (gp91phox, p22phox, p47phox, p67phox, p40phox).
superoxide production by activated phagocytes is below 2% of normal or undetectable
Variant CGD - respiratory burst of 2%-10% of normal
Other aspect of phagocyte function are normal - adherence, ingestion, and degranulation
*Mutations in flavocytochrome b
neutrophils lacked a low-potential cytochrome b corresponding to a flavocytochrome comprised of 2 subunits
gp91phox, encoded by CYBB gene mapped to Xp21.1, X-linked form of CGD, which harbors both flavin and heme groups to function as the redox center of the NADPH oxidase
p22phox, corresponding CYBA gene that is affected in a subgroup of AR CGD
* Patients experience severe purulent bacterial and fungal infections caused by organisms not ordinarily pathogens, particularly from catalase-positive organisms.
* Distinctive hallmark - propensity to develop chronic inflammatory granulomas that can be widespread tissue distribution; reflects an indolent infection because of inadequate phagocyte killing
major sites of infection - lungs, skin, GI tract and the lymph nodes
Hematogenous seeding can lead to liver abscesses or osteomyelitis
* Common pathogens include S. aureus, Aspergillus species, and Burkholderia cepacia.
* Clinical features include recurrent infections, granulomas, and chronic inflammatory conditions.
vacccination with BCG can result in severe localized or less commonly disseminated BCG infection, infections with other atypical mycobacteria
Granulomas - distinctive feature; contain a mixture of lymphocytes, and inflammatory macrophages, foamy lipoid cytoplasm that has a characteristic brown color suggestive of CGD
* Diagnosis is established by demonstrating an absent or greatly diminished neutrophil respiratory burst
involves measuring neutrophil respiratory burst activity, often using the simplest most accurate technique of Nitroblue Tetrazolium (NBT) test.
Stimulated neutrophils and monocytes are strongly stained with NBT formazan deposits whereas cells from a patient with classic X-linked CGD show complete abscence of staining
False positive in patients with MPO deficiency
* Management includes prophylactic antibiotics, antifungals, and recombinant human IFN-γ therapy.
Receive all routine vaccinations including live vaccines on schedule and influenza vaccine yearly
Daily Itraconazole as prophylaxis for fungal infections
Gene replacement therapy - partial correction of oxidase activity in as few as 10% of circulating neutrophils will have substantial clinical benefits
* Most common cause of death: pneumonia and/or sepsis caused by Aspergillus species of B. Cepacia
Glucose-6-Phosphate Dehydrogenase Deficiency
Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency is a rare inherited disorder that affects the respiratory burst function of neutrophils, leading to increased susceptibility to infections. The condition is characterized by severely diminished G6PD activity, resulting in a clinical picture similar to CGD.
* 1st 2 reactions of the hexose monophosphate shunt, G6PD and 6-phosphogluconate dehydrogenase
G6PD is the 1st enzyme in this pathway; its absence results in greatly diminished shunt activity thus a decrease in the availability of NADPH
Organisms causing infection are predominately catalase-positive bacteria
* Extremely rare condition, primarily X-linked inheritance.
* Severe deficiency of <5% of normal G6PD activity in neutrophils before respiratory burst is adversely affected
* Clinical manifestations include chronic granulomatous disease-like syndrome and congenital nonspherocytic hemolytic anemia (CNSHA).
* Diagnosis should be considered in any patient with congenital nonspherocytic hemolytic anemia in whom the erythrocyte G6PD level is unusually low or the frequency of infections is high
* Diagnosis is established by measuring the level of G6PD activity in neutrophil homogenates; neutrophil function tests show a diminished respiratory burst (5% to 30% of normal)
* Laboratory evaluation shows diminished respiratory burst and elevated reticulocyte count.
* Treatment includes prophylactic antibiotics (Cotrimoxazole) and aggressive parenteral antibiotics as management of infections
* Transfusion support for severe anemia
Myeloperoxidase Deficiency Overview
Myeloperoxidase (MPO) deficiency is the most common inherited disorder of phagocytes, often asymptomatic but can lead to increased susceptibility to certain infections. The condition is characterized by a deficiency in the enzyme MPO, which is crucial for amplifying the antimicrobial activity of hydrogen peroxide.
* Incidence of complete MPO deficiency is approximately 1 in 4000 individuals.
* Inherited in an autosomal recessive manner, with variable expression; localized in chrom 17 at q22-q23
* MPO deficiency is seen in neutrophils and monocytes; Peroxidase levels in eosinophils, on the other hand, are normal
* MPO - pivotal role in amplifying the toxicity of H2O2 generated by the respiratory burst, and catalyzes the formation of HOCl from chloride and hydrogen peroxide —> capable of activating latent metalloproteinases
decreased susceptibility to neutrophil-mediated vascular dysfunction
* MPO deficient cells can be seen in AML M2, M3, and M4; secondary MPO deficiency can occur in pregnancy, iron deficiency, lead poisoning , ceroid lipofuscinosis, and Hodgkin disease
* Patients typically do not show increased infection rates, except in rare cases with diabetes; markedly abnormal in vitro killing of C. Albicans and hyphal forms of A. Fumigatus
* Laboratory findings include absence of neutrophil peroxidase and delayed bacterial killing.
* Treatment is generally not required unless infections occur, particularly candidiasis.
Disorders of Glutathione Metabolism
Deficiencies in glutathione metabolism, including glutathione reductase and synthetase deficiencies, can lead to impaired respiratory burst function in neutrophils. These rare disorders are characterized by abnormal glutathione levels and can result in chronic infections and metabolic complications.
* Glutathione peroxidase protects neutrophil proteins from harmful effects of H2O2 generated in the course of the respiratory burst by degrading it to water
reduced glutathione (GSH), the intracellular levels of which are maintained by recycling oxidized glutathione back to GSH by means of glutathione reductase
* Glutathione reductase deficiency is extremely rare, with only a few families reported.
AR inheritance; large deletion affecting the enzyme’s dimerization domain
Diminshed glutathione reductase levels in neutrophils (10%-15% of normal) and erythrocytes
No history of recurring infection
Glutathione reductase level diminished; premature cessation of O2 production of neutrophils
No treatment required
* Glutathione synthetase deficiency leads to metabolic acidosis and intermittent neutropenia.
Moderate to severe deficiency of glutathione synthetase activity but have a normal respiratory burst, in vitro bacterial killing is decreased
Metabolic acidosis from elevated levels of 5-oxoproline as well as progressive neurologic symptoms, otitis media, hemolysis with oxidant stress, intermittent neutropenia
Diagnostic test - measurement of glutathione synthetase activity and glutathione levels in skin fibroblasts and red blood cells
Treatment may involve vitamin E and vitamin C for hemolysis and managing metabolic acidosis.
Inherited Defects in Cytokine Response
Inherited defects in cytokine production and response can lead to recurrent infections, particularly with atypical mycobacteria and certain bacteria. These rare disorders are associated with mutations affecting the immune response pathways, particularly those involving interferon-gamma.
* HIES caused by defects in cytokine signaling
* Disorders include Mendelian susceptibility to mycobacterial diseases (MSMD) and defects in IL-17 signaling.
* Patients with MSMD have recurrent infections due to defects in the IFN-γ pathway.
IFN-γ secreted by NK and T cells in response to IL-12 and IL-23 released by macrophages and dendritic cells
AR mutations in p40 subunit , mutations in the CYBB gene
* IL-17 signaling defects lead to chronic mucocutaneous candidiasis, cutaneous Staphylococcus infections and dermatophyte infections.
dominant negative mutations in STAT3 associated with AD HIES and recessive mutation in CARD9
Impaired killing of Candida but not for Aspergillus
* Genetic mutations can affect various components of the immune response, including receptors and signaling proteins.
NF-KB signaling pathway; AR mutations in IRAK 4, a kinase in the toll and IL-1 receptor signaling pathway
* Management focuses on treating infections and may involve immunotherapy in some cases.
Diagnostic Challenges in Recurrent Infections
Recurrent infections can indicate underlying phagocyte disorders, which often present similar clinical manifestations to lymphocyte function disorders. Identifying patients who require thorough evaluation is crucial due to the low yield of identifiable defects in routine studies.
* Patients with recurrent infections may have phagocyte defects if they exhibit at least one of the following:
High frequency of bacterial and fungal infections.
Infections at unusual sites (e.g., hepatic or brain abscess).
Infections with atypical pathogens (e.g., Aspergillus pneumonia).
Infections of exceptional severity.
Childhood periodontal disease.
* A thorough clinical history and physical examination are essential for diagnosis.
Evaluation Algorithm for Phagocyte Disorders
An organized algorithm can assist physicians in evaluating patients suspected of having phagocyte disorders. This algorithm, when combined with clinical assessments, aids in establishing a diagnosis and treatment plan.
* Initial evaluation includes:
* Comprehensive history and physical examination.
* Family history and laboratory tests (leukocyte, platelet counts).
* Cultures to identify infections.
* If initial tests are normal, further evaluations for immunoglobulin and complement deficiencies are warranted.
* Specific tests for phagocyte function, such as the Nitroblue tetrazolium (NBT) test, are crucial for diagnosis.
Specific Phagocyte Disorders and Their Indicators
Certain phagocyte disorders have distinct clinical and laboratory indicators that can guide diagnosis. Recognizing these indicators is vital for timely intervention.
* Conditions to consider include:
* Chronic Granulomatous Disease (CGD): Decreased or absent superoxide production.
* Chédiak-Higashi Syndrome: Abnormal granules and partial albinism.
* Leukocyte Adhesion Deficiency (LAD): Absent or markedly decreased CD11/CD18.
* Specific tests for these disorders include:
* Myeloperoxidase stain for myeloperoxidase deficiency.
* Flow cytometry to measure surface glycoproteins.
Future Directions in Phagocyte Function Research
Advancements in genome and exome sequencing are expected to enhance the understanding of inherited phagocyte function disorders. This could lead to the identification of new disorders and improve diagnostic capabilities.
* Anticipated developments include:
* Definition of new inherited disorders related to phagocyte function.
* Improved diagnostic accuracy through genetic testing.
* Ongoing research is essential for understanding the complexities of phagocyte disorders and their implications for patient care.