Introduction To The Phagocytic System

• The phagocytic system serves as the rapid effector arm of the innate immune response.
• It consists of two major cell types.
  • Granulocytes include neutrophils, eosinophils, and basophils.
  • Mononuclear phagocytes include circulating monocytes and tissue macrophages.
• Phagocytes primarily perform large-particle ingestion and microbial killing.
• They initiate acquired immunity by releasing chemotactic signals to attract dendritic cells.
• Primary defects in phagocyte function account for less than twenty percent of primary immunodeficiencies.
• Phagocyte defects classically present with deep tissue infections, adenitis, osteomyelitis, or cutaneous lesions.

Hematopoiesis And Phagocyte Kinetics

Neutrophil Maturation

• Hematopoietic progenitor cells reside in the bone marrow.
• Pluripotential stem cells give rise to common myeloid progenitor cells.
• These differentiate into committed single-lineage progenitors.
• Myelopoiesis is regulated by glycoproteins like granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF).
  • These growth factors stimulate cell division and induce transcription factors.
  • The transcription factor PU.1 is essential for myelopoiesis.
• Intramedullary granulocyte maturation involves nuclear condensation and granule accumulation.
  • Promyelocytes acquire peroxidase-positive azurophilic (primary) granules.
  • Myelocytes and metamyelocytes subsequently acquire specific (secondary) granules.
  • Tertiary granules and secretory vesicles develop in the final maturation stage.

Neutrophil And Monocyte Kinetics

The Phagocytic Response

Vascular Adherence And Transmigration

• Circulating neutrophils detect low levels of chemokines from sites of infection.
• Neutrophils loosely adhere to the endothelium through low-affinity receptors called selectins.
• They roll along the endothelium to form the marginated pool.
• Inflammatory effectors trigger changes in surface adhesion molecules.
• Neutrophils undergo qualitative and quantitative changes in beta-2 integrin adhesion receptors (CD11/CD18).
• Tight adhesion occurs between neutrophils and endothelial cells.
• The neutrophil transmigrates through the endothelium into the tissue.

Chemotaxis And Recognition

• The neutrophil senses a gradient of chemokines and migrates to the infection site.
• Migration involves rounds of receptor engagement, signal transduction, and actin microfilament remodeling.
• Neutrophils recognize pathogens via specific receptors.
  • Fc immunoglobulin receptors.
  • Complement receptors.
  • Toll-like receptors.

Ingestion And Phagosome Formation

• Neutrophils ingest microbes that are coated by opsonins.
  • Opsonins include immunoglobulins and complement components like C3b.
• Pathogens are engulfed into a closed vacuole termed the phagosome.

Intracellular Killing Mechanisms

Degranulation

• Neutrophil granule membranes fuse with the phagosome membrane.
• Fusion delivers potent antimicrobial proteins and small peptides into the phagosome.

Oxidative Burst (NADPH Oxidase Pathway)

• The nicotinamide adenine dinucleotide phosphate (NADPH)-dependent oxidase assembles at the phagosome membrane.
• Cytosolic components (p67phox, p47phox, p40phox, and Rac2) translocate to the membrane.
• They combine with the transmembrane flavocytochrome b558 (composed of gp91phox and p22phox).
• The active oxidase generates superoxide from molecular oxygen.
• Superoxide decomposes to form hydrogen peroxide and singlet oxygen.
• Myeloperoxidase from azurophil granules catalyzes the reaction of hydrogen peroxide with chloride ions.
• This reaction creates hypochlorous acid, a potent microbicidal agent.

Diagram Of Phagocytosis And Oxidative Burst

graph TD
    subgraph Vascular Phase
        A[Circulating Neutrophil] -->|Selectins| B[Rolling on Endothelium]
        B -->|Chemokines & Integrins CD11/CD18| C[Tight Adhesion]
        C --> D[Transmigration/Diapedesis]
    end
    subgraph Tissue Phase
        D -->|Actin Remodeling| E[Chemotaxis to Microbe]
        E -->|Fc & Complement Receptors| F[Recognition & Opsonization]
        F --> G[Ingestion into Phagosome]
    end
    subgraph Intracellular Killing
        G --> H[Degranulation of Primary/Secondary Granules]
        G --> I[NADPH Oxidase Assembly]
        I -->|O2 conversion| J[Superoxide Production]
        J -->|Superoxide Dismutase| K[Hydrogen Peroxide]
        K -->|Myeloperoxidase + Cl-| L[Hypochlorous Acid HOCl]
        L --> M[Microbial Destruction]
    end

Primary Immunodeficiencies Affecting Phagocytes

• Genetic defects can interrupt normal phagocyte physiology at multiple stages.

Defects In Neutrophil Production

• Severe congenital neutropenia is characterized by an arrest in myeloid maturation at the promyelocyte stage.
• It commonly results from pathogenic variants in the ELANE gene.
• Recessive forms arise from variants in HAX1 or G6PC3.
• Patients lack adequate peripheral neutrophils to combat pyogenic infections.

Defects In Adhesion And Chemotaxis

• Leukocyte adhesion deficiency type 1 results from an absence of CD11/CD18 beta-2 integrins.
  • Neutrophils cannot adhere firmly to intercellular adhesion molecules.
  • Patients exhibit striking neutrophilia but infections lack pus formation.
• Leukocyte adhesion deficiency type 2 is caused by a loss of fucosylation.
  • It affects the generation of sialyl Lewis X, which is critical for low-affinity rolling.
• Leukocyte adhesion deficiency type 3 is caused by pathogenic variants in KINDLIN3.
  • It results in impaired integrin activation and severe bleeding tendencies.

Defects In Microbicidal Activity

• Chronic granulomatous disease is caused by the failure to express functional NADPH oxidase components.
  • Pathogenic variants affect gp91phox, p22phox, p47phox, or p67phox.
  • Neutrophils phagocytose bacteria normally but fail to produce superoxide.
  • Patients suffer recurrent infections from catalase-positive organisms like Staphylococcus aureus and Aspergillus.
• Myeloperoxidase deficiency prevents the conversion of hydrogen peroxide to hypochlorous acid.
  • It is usually clinically silent but may present with disseminated candidiasis in diabetics.

Defects In Degranulation And Vesicular Trafficking

• Chédiak-Higashi syndrome involves an autosomal recessive defect in the LYST gene.
  • It causes disordered coalescence of lysosomal granules.
  • Neutrophils contain abnormally giant primary granules.
  • Secondary lysosomes have reduced contents of hydrolytic enzymes.
  • This results in impaired killing of microorganisms and progressive neuropathy.
• Specific granule deficiency arises from the functional loss of myeloid transcription factors.
  • It leads to an absence of secondary granules and their contents, impairing bactericidal activity.

Summary Of Selected Phagocyte Disorders