Immunodeficiency Notes

Immunodeficiency

Intended Learning Outcomes

• Classify immunodeficiencies.
• List Primary Immunodeficiencies (PID) and explain their mechanisms.
• Describe factors leading to secondary immunodeficiencies.
• Discuss organisms associated with specific immunodeficiency types.
• List treatments for immunodeficiency.

What is Immunodeficiency?

• Failure of body's defense mechanisms, leading to morbidity or mortality.
• Any part of the immune system can be deficient.
• Body becomes susceptible to infections with little or no resistance.
• Severity varies.
• Can be primary (genetic defect) or secondary (external agents).

Classification of Immunodeficiency

• Primary Immunodeficiencies (PID): Genetic defect.
• Secondary Immunodeficiencies: Effects of external agents or breakdown in other body systems affecting the immune system; much more common than primary.

10 Warning Signs of Primary Immunodeficiency

1. Four or more new ear infections within 1 year.
2. Two or more serious sinus infections within 1 year.
3. Two or more months on antibiotics with little effect.
4. Two or more pneumonias within 1 year.
5. Failure of an infant to gain weight or grow normally.
6. Recurrent, deep skin or organ abscesses.
7. Persistent thrush in mouth or fungal infection on skin.
8. Need for intravenous antibiotics to clear infections.
9. Two or more deep-seated infections including septicemia.
10. A family history of Primary Immunodeficiency (PI).

• Approximately 1 in 500 persons are affected by a known Primary Immunodeficiency.

The 4 Arms of the Immune System

• Innate (Nonspecific):
  • Cellular Components: Phagocytes.
  • Humoral Components: Complement.
• Adaptive (Specific):
  • Cellular Components: T cells (CD4+ T cell TH1 and TH2, CD8 T cells, TH17 cell, T-Regs).
  • Humoral Components: B cells producing IgM, IgA, IgG, IgE.

Primary Immunodeficiency: Deficiencies in:

1. Phagocytic: Susceptibility to fungi (Aspergillus) and low virulent bacteria.
2. Complement: Susceptibility to encapsulated bacteria.
3. Cellular (T cells): Susceptibility to viruses (Herpes, Varicella, CMV), fungi (Candida, Aspergillus), protozoa (Pneumocystis carnii), and intracellular bacteria (mycobacterium).
4. Humoral (B cells): Susceptibility to encapsulated organisms like Streptococcus pneumoniae.

Examples Affecting INNATE Immune Responses

Phagocytic Defects

• Chronic Granulomatous Disease (CGD): NADPH oxidase defect affecting phagocytic cells.
• Leukocyte Adhesion Deficiency: β2 integrin defect affecting neutrophils and monocytes.

Complement Deficiencies

• Defects in complement genes.

Chronic Granulomatous Disease (CGD)

• Oxidative burst defect due to NADPH oxidase defect in phagocytic cells.

Phagocytosis

1. Microbes bind to phagocyte receptors (Mac-1, Lectin receptor, integrin, Scavenger receptor).
2. Phagocyte membrane zips up around microbe, ingesting it into a phagosome.
3. Fusion of phagosome with lysosome to form a phagolysosome.
4. Killing of microbes by Reactive Oxygen Species (ROS), Nitric Oxide (NO), and lysosomal enzymes in phagolysosomes.

• Activation of phagocyte:
  • Arginine converted to Citrulline by iNOS, producing NO.
  • O_2 converted to ROS.

NADPH Oxidase & CGD

• In CGD, NADPH oxidase enzyme is nonfunctional.
• NADPH + O2 --(NADPH oxidase)--> NADP + O2^-
• Phagocytes clump when they can't kill pathogens efficiently, leading to granuloma formation.

Mutations in NADPH Oxidase Components Responsible for CGD

• Membrane:
  *Cytochrome b558:
  * gp91phox: Mutated in X-linked CGD. (Xp21.1)
  * p22phox: Mutated in autosomal recessive CGD. (16q24)

• Cytoplasm:

  • p47phox: Mutated in autosomal recessive CGD. (7q11.23)
  • p67phox: Mutated in autosomal recessive CGD. (1q25)
  • p40phox: Mutated in autosomal recessive CGD.

CGD Clinical Manifestations

• Soft tissue infections
• Lymphadenitis
• Liver/brain abscess
• Pneumonia
• Granulomas (GU/GI)
• Sepsis

Defective Respiratory Burst in Neutrophils of Patients with CGD

• Normal individuals and carriers show a normal respiratory burst (superoxide production) upon stimulation with phorbol ester, while patients with CGD show a significantly reduced or absent respiratory burst.

Nitroblue Tetrazolium (NBT) Test & Dihydrorhodamine (DHR) Test

• NBT Test:
  • CGD-negative NBT shows reduction of Nitro-blue Tetrazolium to Formazan.
• DHR Test (Flow Cytometry):
  • Measures the oxidative burst in neutrophils.
  • Dihydrorhodamine (DHR) oxidation to Rhodamine 123 (fluorescent) by Reactive oxygen species (ROS)

Interpretation
* X-linked normal: High DHR-FITC after stimulation.
* X-linked carrier: Reduced DHR-FITC after stimulation.
* Autosomal recessive: Very low DHR-FITC after stimulation.
* Diseased: Absent DHR-FITC.

X-Linked Immunodeficiencies

• X-Linked Recessive Inheritance: Carrier Mother.
• Probability:
  • Normal Daughter (25%)
  • Normal Son (25%)
  • Carrier Daughter (25%)
  • Affected Son (25%)

Leukocyte Adhesion Deficiency

• β2 integrin defect affecting neutrophils and monocytes (defects in leukocyte and endothelial adhesion molecules).

Leukocyte Adhesion Cascade

1. Rolling: Selectins (P-Selectin) bind to PGSL-1 on Neutrophils
2. Loose Adhesion:
3. Firm Adhesion/Diapedesis: Integrins (CD11/CD18) bind to ICAM-1 on Endothelium
4. Aggregation

Leucocyte Adhesion Defect (LAD)

• CD18 is required for holding on (adhesion).
• CD18 associates with CD11a (LFA-1), CD11b (CR-3), and CD11c (C3dg-R).
• Types:
  • LAD I: Defect in adhesion.
  • LAD II: Defect in rolling.
  • LAD III: Defect in activation

LAD Clinical Manifestations

• Delayed separation of umbilical stump.
• Omphalitis.

LAD Diagnosis

• Complete Blood Count (CBC): Leucocytosis.
• FLOW CYTOMETRY: Defective CD18, CD11 expression.
• Genetic diagnosis: Mutation in ITGB2 gene.

Examples Affecting ADAPTIVE Immune Responses

B-cell

• Selective IgA deficiency
• Hyper-IgM syndrome: CD40L or CD40 defect
• X-linked agammaglobulinaemia: Bruton’s tyrosine kinase (Btk) defect

T-cell

• Wiskott-Aldrich syndrome: WASP defect
• DiGeorge syndrome: TBX1 gene defect

Severe combined immunodeficiency (SCID)

• γC, RAG, ADA, PNP defects

Selective IgA Deficiency

• Low IgA with normal IgG and IgM levels.
• Recurrent ear infections, sinusitis, bronchitis, and pneumonia.
• Some patients have GIT infections and chronic diarrhea.
• Some patients are asymptomatic or have relatively mild illness.

Hyper-IgM Syndrome

• Class switching depends on interaction between CD40 on B cells and CD40L (CD154) on Th cells.
• Cytokines also play a role in class switching.

Activation and Class-switching of B-cells

1. APC presents antigen to T-helper cells.
2. B7 is expressed and interacts with CD28, activating T-helper cells.
3. Activated Th cells interact with B-cells via CD40 ligand, activating B-cells to proliferate, differentiate, and secrete antibodies
4. Th cells secrete cytokines that determine class switching

• Deficient expression of CD40L on activated T-helper cells.
• Deficient expression of CD40 on B cells.
• Normal T- and B-cells.
• Genetic testing.

Symptoms

• Severe pyogenic infections early in life.
• Chronic diarrhea.
• Opportunistic infections.

Diagnosis

• Ig levels: Increased IgM; Decreased IgA, IgG, and IgE
• Defect in class-switching confirmed with flow cytometry.

X-linked Agammaglobulinaemia (XLA)

• Failure of B cell to mature beyond pre-B stage in bone marrow due to mutation in Bruton's tyrosine kinase (Btk).
• Btk transduces signals from pre-BCR required for survival, proliferation, and maturation at and beyond pre-B cell stage.
• Affected patients remain healthy during the first 6–12 months due to maternal IgG.
• Typically, XLA patients suffer form recurrent otitis media, sinusitis, bronchitis, pneumonia and gastrointestinal infections.
• Bacterial infections are the hallmark of XLA.

Diagnosis:

• Immunoglobulins: Low to undetectable immunoglobulin serum levels.
• Flow cytometry: Complete absence of peripheral B cells, as defined by CD19 and CD20 expression. Detection of BTK expression by FCM
• Molecular diagnosis: Identification of the underlying gene defect.

Wiskott-Aldrich Syndrome (WAS)

• X-linked Recessive.
• Gene defect of WAS protein.
• B and T cell dysfunction.
• Triad of:
  • Thrombocytopenia
  • Eczema
  • Recurrent pyogenic infections
• WASP protein is responsible for cytoskeleton dependent responses including cell migration and signal transduction.

Treatment

• Stem cell or Bone Marrow transplant

Prognosis

• Average life expectancy 11 years

DiGeorge Syndrome

• Caused by abnormal migration and development of certain cells and tissues during fetal development.

Characteristics

• Low or absent T cells
• Parathyroid hypoplasia
• Cardiac defect.
• Facial features

CATCH-22 (DiGeorge Syndrome)

• Cardiac abnormalities
• Abnormal facies
• Thymic absence/abnormality, T cell abnormality
• Cleft palate
• Hypocalcemia
• Chromosome 22: Microdeletion in chromosome 22.

Other Associations

• Thymic hypoplasia
• Neonatal Seizure (Hypocalcemia or Tetany)
• Congenital heart defect

Severe Combined Immunodeficiency (SCID)

• Inherited immune system disorders characterized by BLOCK in T cells differentiation associated with abnormal development of other lineages B cells and NK cells.
• Patients are susceptible to infections including ear infections, pneumonia or bronchitis, oral thrush, and diarrhea.
• Due to recurrent infections, children with SCID do not grow and gain weight as expected.

Genetic Defects Responsible for SCID

Secondary Immunodeficiencies

Examples:

• Malnutrition: protein-calorie malnutrition & lack of dietary elements (e.g., iron, zinc).
• Loss of cellular/humoral components:
  • Lymphocytes passively lost into the intestine in intestinal lymphangiectasia
  • Proteins, especially antibodies, lost into the urine in nephrotic syndrome.
• Tumors: The direct effect of tumors of the immune system.
• Cytotoxic drugs/irradiation: Used for inhibiting tumor cell growth.
• Other diseases such as diabetes are associated with infections.
• Infections:
  • Malaria inhibit the development of immune responses
  • Human immunodeficiency virus (HIV) causes AIDS.

HIV & AIDS

• HIV takes out young adults, likened to 6 World Trade Center towers daily.

HIV/AIDS Statistics (2021)

HIV Entry into the Cell

• gp120 binds to CD4 receptor.
• gp41 mediates fusion.
• Co-receptors:
  • CCR5 (macrophage)
  • CXCR4 (T-cells)
• HIV-1 enters the cell by direct fusion.

HIV Life Cycle

1. Virion binding to CD4 and chemokine receptor.
2. Fusion of HIV membrane with cell membrane; entry of viral genome into cytoplasm.
3. Reverse transcriptase-mediated synthesis of proviral DNA.
4. Integration of provirus into cell genome.
5. Cytokine activation of cell; transcription of HIV genome; transport of spliced and unspliced RNAs to cytoplasm.
6. Synthesis of HIV proteins; assembly of virion core structure.
7. Expression of gp120/gp41 on cell surface; budding of mature virion.

Opportunistic Infections in AIDS

• Opportunistic infections characterize immunodeficiencies.
• In AIDS:
  • Virus: Cytomegalovirus
  • Bacteria: Mycobacteria tuberculosis
  • Fungi: Pneumocystis jirovecii
  • Protozoa: Toxoplasma gondii

Treatment of Immunodeficiencies

• Antibiotics
• Gammaglobulins for antibody deficiencies
• Cell replacement (bone-marrow / fetal liver and thymus grafts)
• Gene therapy – SCID (ADA, γC)

Gene Therapy for Immune Deficiency

1. Stem cells are removed from the patient.
2. A working copy of the gene is inserted into a virus.
3. The virus delivers the working gene to the patient's extracted stem cells.
4. The stem cells, now with the working gene, are returned to the patient.
5. Donor
6. Bone marrow
7. transplant
8. Recipient
9. Donor bone marrow cells repopulate
10. recipient bone marrow

Immunodeficiency – Summary

• Primary: Gene defect, rare
• Secondary: External cause, more common
• Both characterized by opportunistic infections