What are the cells of innate immunity and their key cytokines/immunohistochemistry (IHC) markers?

Cells of Innate Immunity and Their Key Cytokines/IHC Markers

Primary Cellular Components

The innate immune system comprises neutrophils, monocytes/macrophages, dendritic cells (DCs), natural killer (NK) cells, and innate lymphoid cells (ILCs) as its principal cellular effectors. 1, 2

Neutrophils (Polymorphonuclear Cells)

• Primary function: First responders to inflammation sites, executing phagocytosis and oxidative burst 1
• Key markers: CD11b, L-selectin 1
• Cytokines produced: IL-8 (CXCL8), though primarily respond to rather than produce cytokines 1
• IHC considerations: Most abundant granulocytes; can be identified by polymorphonuclear morphology on H&E 1

Monocytes and Macrophages

• Primary function: Phagocytosis, antigen presentation, tissue-resident immune surveillance 1, 2
• Key surface markers: CD11b, HLA-DR 1
• Activation markers: Phospho-STAT3, C/EBPβ, S100A8/9 1
• Cytokines produced:
  • Pro-inflammatory: TNF, IL-1β, IL-6, IL-12p40/p70 1
  • Chemokines: MCP-1 (CCL2), IP-10 (CXCL10) 1
  • Anti-inflammatory: IL-10, TGFβ 1
• Functional molecules: ARG1, NOS2/iNOS (particularly in M-MDSCs), PGE2 1

Dendritic Cells (DCs)

• Primary function: Professional antigen-presenting cells bridging innate and adaptive immunity 1
• Key markers: CD11c, HLA-DR 1
• Cytokines produced: IL-12p40/p70, IL-18, type I interferons 1
• IHC note: Require specific markers for identification as morphology alone is insufficient 1

Natural Killer (NK) Cells

• Primary function: Cytotoxic killing of infected or transformed cells without prior sensitization 1, 2
• Key markers: CD56, CD16, NK receptors 1
• Cytokines produced: IFN-γ, TNF 1, 3
• IHC considerations: Part of innate lymphoid cell family; express NK receptors but lack antigen-specific receptors 4

Innate Lymphoid Cells (ILCs)

• Primary function: Tissue-resident sentinels producing rapid cytokine responses 4, 5
• Subtypes and markers:
  • ILC1: Produce IFN-γ, express T-bet 4
  • ILC2: Produce IL-5, IL-13; express GATA3 4
  • ILC3: Produce IL-17A, IL-22; express RORγt 4
• Key distinction: Lack antigen-specific receptors but respond rapidly to tissue signals 4

Key Cytokines in Innate Immunity

Pro-inflammatory Cytokines

• TNF (TNFSF2): Produced by microglia, astrocytes, neurons; receptors (TNFR1/TNFR2) on myeloid cells, glia, neurons, endothelial cells 1
• IL-1β: Produced by microglia, astrocytes, oligodendrocytes, neurons; receptors on endothelial cells and neurons 1
• IL-6: Produced by microglia, astrocytes, neurons; receptors on astrocytes, microglia, neurons 1
• IL-12p40/p70: Produced by dendritic cells, lymphocytes, microglia; receptors on microglia and neurons 1
• IL-18: Produced by microglia, dendritic cells; receptors on neurons 1

Type 1 Response Cytokines

• IFN-γ (IFNG): Produced by Th1 cells, immune cells, microglia, endothelial cells, astrocytes; receptors on microglia and neurons 1

Type 2 Response Cytokines

• IL-4: Produced by mast cells, microglia, Th2, neurons; receptors on endothelial cells, lymphocytes, all glia, neurons 1
• IL-13: Produced by Th2 and other immune cells; shares receptors with IL-4 (IL-13Rα1 on neurons) 1

Regulatory Cytokines

• IL-10: Produced by multiple immune cells, activated microglia, reactive astrocytes; receptors on microglia, astrocytes, oligodendrocytes, neurons 1
• TGFβ1/2/3: Produced by microglia, astrocytes, neurons (TGFβ2/3); receptors on astrocytes, microglia, endothelial cells, neurons 1

Chemokines

• IL-8 (CXCL8): Produced by microglia, possibly neurons; receptors on microglia, astrocytes, neurons 1
• MCP-1 (CCL2): Produced by macrophages, microglia, endothelial cells; receptors on astrocytes, microglia, endothelial cells, neurons 1
• IP-10 (CXCL10): Produced by monocytes, endothelial cells; receptors on lymphocytes, microglia, neurons 1
• RANTES (CCL5): Produced by microglia, astrocytes; receptors on glial cells and neurons 1

IHC Methodology Considerations

General Recommendations

• Cell identification: All mononuclear cells including lymphocytes and plasma cells should be scored; granulocytes and polymorphonuclear leukocytes are excluded from lymphocyte counts 1
• Immunohistochemistry limitations: While CD45, CD8, CD3 markers can improve accuracy, added value beyond morphology remains unclear for routine use outside research 1
• Stromal vs. intratumoral: For tissue infiltrates, stromal compartment assessment is more reproducible than intratumoral for diagnostic purposes 1

Critical Technical Points

• Antibody clone variability: Different clones of the same antibody may show different staining patterns, affecting interpretation 6
• ROI documentation: Clearly document region of interest selection methodology, number of ROIs analyzed, and inclusion/exclusion criteria 7
• Consistent parameters: Ensure consistent image acquisition parameters and document fixation details, processing techniques, and staining protocols 7

Complement System Markers

• C3, C5, C4, C1q: Produced by astrocytes, microglia, neurons; receptors on microglia, astrocytes, neurons 1

Common Pitfalls

Avoid relying solely on morphology for dendritic cell and macrophage identification, as these cells have irregular morphologies that reduce concordance in quantification studies 1. Machine learning approaches and digital image analysis are recommended for objective quantification rather than subjective scoring, which leads to inconsistencies 7. Ensure proper controls at each timepoint and document any variations in tissue processing that might affect results 7.