Cells of the Immune System and Hematopoiesis

Hematopoiesis and Immune Cell Development

• Definition of Hematopoiesis: The process of formation and development of red blood cells (erythrocytes) and white blood cells (leukocytes).

• Site of Hematopoiesis: In humans, the site of hematopoiesis changes throughout development:

  • Bone Marrow: The primary site for adult hematopoiesis.

• Pluripotent Hematopoietic Stem Cells (HSC): All blood cells arise from this single progenitor. The HSC differentiates into two main lineages:

  • Myeloid Progenitor Cell: Gives rise to megakaryocytes (platelets), erythrocytes, mast cells, and myeloblasts (basophils, neutrophils, eosinophils, monocytes/macrophages).

  • Lymphoid Progenitor Cell: Gives rise to T cells, B cells, Natural Killer (NK) cells, and certain dendritic cells.

• Stromal Cell Meshwork: Hematopoietic cells grow and mature on a meshwork of stromal cells.

  • Stromal Cells: These are self-renewing, non-hematopoietic cells that support the growth and differentiation of hematopoietic cells.

  • Examples: Fat cells (adipocytes), endothelial cells, fibroblasts, and bone-marrow mesenchymal cells.

  • Key Terms:

    • ASC: Adipose-derived Stromal Cell.

    • ECM: Extracellular Matrix.

    • HPC: Hematopoietic Progenitor Cell.

    • EVs: Extracellular Vesicles.

• Transcription Factors in Lineage Development:

  • GATA-1: Essential for the Erythroid lineage.

  • GATA-2: Essential for Erythroid, myeloid, and lymphoid lineages.

  • PU.1: Essential for Erythroid (maturational stages), myeloid (later stages), and lymphoid lineages.

  • BM11: Essential for Myeloid and lymphoid lineages.

  • Ikaros: Essential for the Lymphoid lineage.

  • Oct-2: Essential for B lymphoid lineage (specifically the differentiation of B cells into plasma cells).

Hematopoietic Homeostasis and Regulation

• Regulation Mechanisms: Hematopoietic homeostasis is maintained through complex mechanisms affecting all cell types:

  • Control of cytokine production by stromal cells.

  • Production of hematopoietic cytokines by other non-stromal cells.

  • Regulation of receptor expression in stem cells.

  • Removal of cells through the controlled induction of programmed cell death (apoptosis).

• Emergency Hematopoiesis: Occurs in response to severe bacterial infection. It is associated with the expansion of hematopoietic stem/progenitor cells (HSPCs) and their progeny, mediated by environmental cues like inflammatory cytokines and microbial products.

Mechanisms of Cell Death: Apoptosis vs. Necrosis

• Apoptosis (Programmed Cell Death): An active, programmed process of autonomous cellular dismantling that avoids eliciting inflammation.

• Necrosis: A passive, accidental cell death resulting from environmental perturbations with uncontrolled release of inflammatory cellular contents.

Comparative Features of Cell Death

• Morphology:

  • Apoptosis: Cell shrinks; chromatin condenses ($++$); exteriorization of phosphatidylserine ($++$); caspase activation ($++$); plasma membrane ruffles but remains intact until apoptotic bodies form.

  • Necrosis: Cell swells; vacuolization occurs (autophagosomes $+$); cell membrane ruptures with loss of integrity; organelle disintegration (swelling of mitochondria and ER).

• DNA Degradation:

  • Apoptosis: Controlled degradation resulting in a "ladder" pattern on an electrophoretic map (karyorrhexis/pyknosis).

  • Necrosis: Random DNA degradation resulting in a "smeared" electrophoretic map (karyolysis).

• Biological Impact:

  • Apoptosis: Beneficial process; phagocytized by phagocytes or adjacent cells; no inflammation.

  • Necrosis: Harmful to the organism; inflammation occurs due to leakage of cell contents; may be fatal if untreated.

Genetic Regulation of Apoptosis

• bcl-2: Prevents apoptosis (Inhibitor).

• bax: Opposes bcl-2 (Promoter).

• bcl-X (Long): Prevents apoptosis (Inhibitor).

• bcl-X (Short): Opposes bcl-X (Promoter).

• Caspases: Proteases that promote apoptosis.

• Fas: Induces apoptosis (Initiator).

Overview of Immune Cell Abundance and Function

• Total White Blood Cell (WBC) Count: Normal range is approximately $4,000 - 11,000\,\text{cells/mL}$.

• Distribution of Leukocytes in Peripheral Blood:

  • Neutrophils: $40-75\%$

  • Lymphocytes: $20-50\%$

  • Monocytes: $2-10\%$

  • Eosinophils: $1-6\%$

  • Basophils: <1\%

• Macrophage Summary:

  • Function: Phagocytosis and Antigen presentation to T cells.

  • Lifetime: Months to years.

  • Targets: Various pathogens.

• Natural Killer (NK) Cell Summary:

  • Proportion: $15\%$ of circulating lymphocytes.

  • Function: Killing virus-infected cells and tumor cells; tumor rejection.

  • Mechanism: Release of perforin and granzymes to induce apoptosis.

  • Lifetime: $7-10\,\text{days}$.

Lymphoid Cells: B Cells, T Cells, and NK Cells

B Lymphocytes (B Cells)

• Site of Maturation: Named after the Bursa of Fabricius (birds) or Bone Marrow (mammals).

• Markers: Synthesis and display of membrane-bound immunoglobulin (antibody) molecules. A mature B cell expresses approximately $1.5 \times 10^5$ antibody molecules.

• Other Surface Molecules: B220 (CD45), MHC II, CR1-2 (CD35, 21), CD32, B7-1, B7-2, and CD40.

• Plasma Cells: Fully differentiated B cells that specialize in secreting antibodies.

T Lymphocytes (T Cells)

• Site of Maturation: Thymus.

• Antigen Recognition: Use membrane-bound T-cell Receptors (TCR). Most T cells only recognize antigens bound to Major Histocompatibility Complex (MHC) molecules.

• Subpopulations:

  • CD8 Cytotoxic T Cells (CTL): Kill virus-infected cells and some intracellular bacteria.

  • CD4 TH1 Cells: Activate macrophages to kill microbes in vesicles (e.g., mycobacteria).

  • CD4 TH2 Cells: Provide help to B cells for antibody production, particularly switching to IgE; target helminth parasites.

  • CD4 Th17 Cells: Enhance neutrophil response and promote barrier integrity (skin/intestine); target fungi and extracellular bacteria.

  • TFH Cells: Reside in lymphoid follicles to help B cells with isotype switching and antibody production.

  • CD4 Regulatory T Cells (Treg): Suppress T-cell responses to maintain immune tolerance.

Natural Killer (NK) Cells

• Description: Large granular lymphocytes with cytotoxic activity.

• Recognition Mechanisms:

  1. NK cell receptor: Detects the reduction or absence of MHC I on target cells.

  2. CD16: A membrane receptor that binds the Fc portion of IgG.

• NK1-T Cells: A hybrid cell type possessing characteristics of both T cells and NK cells.

• Killer Mechanisms: Interaction between FasL (on NK cell) and Fas (on target) triggers a caspase cascade. They also utilize Perforin, Granzyme, and TNF/TNF receptors to induce apoptosis in virally infected targets.

The Myeloid Lineage: Granulocytes

Neutrophils

• Abundance: The most abundant white blood cell ($\sim 70\%$).

• Morphology: Multi-lobed nucleus; granulated cytoplasm that stains with both acidic and basic dyes. Immature forms with elongated nuclei are called "band forms."

• Kinetics: Circulate for $7-10\,\text{hours}$ before migrating to tissues. Large reserves are stored in bone marrow.

• Granules:

  • Primary (Azurophil) Granules: Contain Lysozyme, Myoperoxidase, Elastase, Cathepsin G, $H^+$ hydrolases, Defensins, and BPI.

  • Secondary (Specific) Granules: Contain Lysozyme, Cytochrome b558, Alkaline Phosphatase, Lactoferrin, and Vitamin $B_{12}$ binding protein.

• Functions: Phagocytosis, degranulation, and NETosis (Neutrophil Extracellular Traps). Neutrophils die in tissue after action and are cleaned up by macrophages (efferocytosis).

Eosinophils

• Morphology: Bi-lobed nucleus; phagocytic.

• Granule Contents: Released via exocytosis. Contains basic proteins (MBP, ECP, EDN, EPO), histaminase, and aryl sulphatase (to down-regulate inflammation).

• Eosinophilia Causes: Neoplasms, allergies, asthma, and parasitic infections.

Basophils

• Abundance: <1\% of leukocytes.

• Description: Non-phagocytic cells with lobed nuclei. Functionally similar to mast cells; contain inflammatory mediators.

Mast Cells

• Development: Released into blood as undifferentiated cells; differentiate upon entering tissues.

• Mechanism: IgE-induced granule release is central to allergic reactions.

• Mediators:

  • Pre-formed: Histamine (vasodilation, bronchoconstriction), Proteoglycan, Neutral Proteases, ECF (Eosinophil chemotaxis), NCF (Neutrophil chemotaxis).

  • Newly Synthesized: Leukotrienes C4/D (vasoactive, bronchoconstriction), Prostaglandins, Thromboxanes, and Cytokines (IL-3, 4, 5, 6, GM-CSF, TNF).

Mononuclear Phagocytes and Antigen Presentation

• Monocytes: Circulate in the blood for approximately $8\,\text{hours}$ before migrating into tissues to differentiate into macrophages.

• Macrophages:

  • Process: Use receptors to bind bacteria, leading to engulfment (phagosome) and fusion with lysosomes (phagolysosome) for degradation.

  • Respiratory Burst: Oxygen-dependent killing involving NADPH Oxidase, Myeloperoxidase, and Nitric Oxide Synthetase to produce reactive species like Superoxide dismutase.

  • Secreted Factors: IL-1 (fever), IL-6, TNF-$\alpha$ (inflammation), Complement proteins, Interferon alpha (IFN-$\alpha$ for antiviral state), and colony-stimulating factors (GM-CSF, G-CSF, M-CSF).

Dendritic Cells (DC)

• Types: Langerhans cells, Interstitial DCs, Monocyte-derived DCs, and Plasmacytoid-derived DCs.

• Function: The bridge between innate and adaptive immunity. They capture antigen in peripheral tissues, mature, and migrate to regional lymph nodes to activate naive T cells.

• Markers: Constitutively express high levels of MHC II and B7 co-stimulatory molecules.

Follicular Dendritic Cells (FDC)

• Origin: Non-hematopoietic origin.

• Distinction: They do not express MHC II and do not present antigen to TH cells.

• Location: Found in B-cell rich follicles of lymph nodes.

• Function: Express high levels of receptors for antibodies to bind antigen-antibody complexes for B cell inspection.