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Phagocyte Antigen Presentation: Bridging Innate and Adaptive Immunity

Professional phagocytes (macrophages, dendritic cells, neutrophils) link innate detection to adaptive immunity through multiple communication channels:

1. MHC-Restricted Antigen Presentation

• Phagocytosed pathogens → proteolysis in phagolysosome → peptides loaded on MHC-II (exogenous) or cross-presented on MHC-I (endogenous pathway).
• Peptide-MHC complexes displayed on surface → recognized by CD4⁺ (MHC-II) or CD8⁺ (MHC-I) T cells via TCR.
• Co-stimulation (CD80/86-CD28) required for full T-cell activation.

2. Cytokine & Chemokine Secretion

• Pro-inflammatory: IL-1β, TNF-α, IL-6, IL-12 → activate endothelium, recruit neutrophils, drive Th1/Th17 differentiation.
• Chemokines: CCL2, CCL3, CXCL8 → gradient for leukocyte migration.
• Anti-inflammatory/resolution: IL-10, TGF-β (later phase).

3. Direct Cell-Cell Interactions

• Immunological synapse with T cells: sustained TCR signaling, co-stimulation, cytokine delivery.
• CD40-CD40L interaction → licenses DC for CD8⁺ T-cell priming.

4. Surface Marker Expression

• MHC-II, CD80/86, CD40, ICOS-L, PD-L1—indicate activation state and polarize T-cell response.

5. Extracellular Vesicles (Exosomes/Microvesicles)

Carry MHC-peptide complexes, miRNA, pathogen components → transfer to distant immune cells, amplifying or regulating response.

Outcome

Coordinated effector response: cytotoxic T cells, antibody-producing B cells, memory formation, and resolution.