Macrophage as an Antigen-Presenting Cell (APC)

SLIDE 1 - Title

SLIDE 2 - Introduction: What is an APC?

1. Macrophages
2. Dendritic cells
3. B lymphocytes

SLIDE 3 - Origin and Distribution of Macrophages

• Macrophages arise from monocytes in the blood, which in turn derive from bone marrow progenitors
• Monocytes circulate for ~3 days, then migrate into tissues where they differentiate into tissue-resident macrophages
• Different tissue macrophages have different names but the same APC function:

SLIDE 4 - Key Surface Molecules for APC Function

SLIDE 5 - Step-by-Step Antigen Processing and Presentation (MHC Class II Pathway)

Step 1: Antigen Uptake

• The macrophage engulfs extracellular antigens (bacteria, foreign proteins) by:
  • Phagocytosis (large particles, opsonized bacteria)
  • Macropinocytosis (bulk fluid uptake)
  • Receptor-mediated endocytosis (via Fc receptors, complement receptors, mannose receptors)

Step 2: Phagosome Formation

• The ingested material is enclosed in a phagosome (membrane-bound vesicle)
• At this point the pH is near neutral

Step 3: Acidification and Lysosomal Fusion

• The phagosome fuses with lysosomes to form a phagolysosome
• A V-type ATPase pumps H+ ions in, dropping the pH to ~4.5-5
• This activates lysosomal cathepsins and other proteases
• The enzyme GILT (IFN-gamma-induced lysosomal thiol reductase) breaks disulfide bonds, further exposing protein for proteolysis
• The antigen is degraded into peptide fragments of 13-25 amino acids

Step 4: MHC Class II Biosynthesis and Transport

• MHC class II (alpha + beta chains) is synthesized in the ER
• The invariant chain (Ii, CD74) binds the peptide-binding groove using its CLIP segment, preventing premature peptide loading and directing the MHC II complex to endosomal vesicles
• The MHC II - Ii complex travels from ER → Golgi → late endosome

Step 5: CLIP Removal and Peptide Loading

• In the acidified late endosome, cathepsin S (and other proteases) cleave most of the invariant chain, leaving only CLIP (class II-associated invariant chain peptide) in the groove
• The accessory molecule HLA-DM catalyzes the exchange of CLIP for a high-affinity antigenic peptide
• Only peptides that bind stably are retained

Step 6: Surface Display

• The stable peptide:MHC II complex moves to the cell surface
• Here it is "read" by the TCR of CD4+ helper T cells

SLIDE 6 - Diagram of the MHC Class II Pathway

Extracellular Antigen (bacteria/protein)
           ↓  Phagocytosis / Endocytosis
       Phagosome (neutral pH)
           ↓  Lysosome fusion + V-ATPase
    Phagolysosome (acidic pH ~5)
      Proteases + GILT active
           ↓
      Peptide fragments (13-25 aa)
           ↓
     Late endosome + MHC II-Ii complex
     (HLA-DM removes CLIP)
           ↓
    Peptide loads onto MHC II groove
           ↓
  Peptide:MHC II complex → Cell Surface
           ↓
   Recognized by CD4+ T cell TCR

SLIDE 7 - Two-Signal Model for T Cell Activation

Signal 1: Antigen-Specific (TCR Signal)

• TCR on CD4+ T cell binds peptide:MHC II complex on macrophage
• This alone causes T cell anergy (unresponsiveness), not activation

Signal 2: Costimulatory

• CD80/CD86 (B7) on macrophage binds CD28 on T cell
• This provides the second signal needed for full T cell activation, proliferation, and cytokine production

Additional Signals: Cytokines (Signal 3)

• Macrophages secrete IL-12 → drives differentiation toward Th1 phenotype
• IL-1, IL-6, TNF-alpha amplify the inflammatory response
• These cytokines polarize the T cell response toward the most effective response for the type of pathogen

"The macrophage plays an important role in immune responses by partially degrading antigens and presenting their fragments on MHC II molecules on the macrophage surface to helper CD4+ T lymphocytes for recognition." - Histology: A Text and Atlas

SLIDE 8 - MHC Class II vs MHC Class I Presentation by Macrophages

SLIDE 9 - Macrophage Activation by T Cells (The Feedback Loop)

1. CD4+ Th1 cells recognize peptide:MHC II on macrophage
2. T cell secretes IFN-gamma - the most potent macrophage-activating cytokine
3. T cell expresses CD40 ligand (CD40L) which binds CD40 on macrophage
4. This "classical activation" (M1) triggers:
   • Upregulation of MHC II and B7 molecules (more APC function)
   • Induction of iNOS → nitric oxide (NO) production
   • Increased reactive oxygen species (ROS) via NADPH oxidase
   • Enhanced phagocytosis and intracellular killing
   • Secretion of pro-inflammatory cytokines: TNF-α, IL-1, IL-12, IL-6, IL-18, IL-23

SLIDE 10 - Macrophages vs. Dendritic Cells as APCs

SLIDE 11 - Macrophage APC Function in Disease States

Tuberculosis (M. tuberculosis)

• Mycobacteria survive inside macrophage phagosomes by inhibiting phagolysosome fusion and preventing acidification
• Despite this, peptides still reach MHC II and are presented to Th1 cells
• Th1 cells release IFN-gamma → granuloma formation
• Ongoing APC function is critical for containment

HIV Infection

• HIV infects macrophages (via CD4 + CCR5)
• Impairs MHC II surface expression, reducing APC function
• Loss of macrophage APC capacity contributes to immune evasion

Autoimmunity (e.g., Rheumatoid Arthritis, SLE)

• Macrophages present self-antigens on MHC II → activate autoreactive T cells
• Excess IL-1, TNF-alpha from activated macrophages drives tissue damage
• Target of many therapies (anti-TNF, IL-1 blockers, hydroxychloroquine which impairs lysosomal acidification and thus antigen processing)

Tumor Microenvironment

• Tumor-associated macrophages (TAMs) are often switched to M2 (alternatively activated) phenotype
• M2 macrophages have reduced APC function and suppress T cell responses
• This is a key mechanism of tumor immune escape

SLIDE 12 - M1 vs M2 Macrophage Polarization and APC Function

SLIDE 13 - Summary: Key Points

1. Macrophages are professional APCs that bridge innate and adaptive immunity
2. They capture antigen by phagocytosis and process it through the phagolysosomal pathway
3. Antigenic peptides (13-25 aa) are loaded onto MHC class II molecules via the invariant chain/CLIP/HLA-DM system
4. The peptide:MHC II complex is displayed on the surface for CD4+ T cell recognition
5. T cell activation requires two signals: TCR-MHC II interaction (Signal 1) + CD28-B7 costimulation (Signal 2)
6. Macrophage-secreted IL-12, TNF, IL-1 provide Signal 3 (cytokine polarization)
7. Activated Th1 cells feedback to macrophages via IFN-gamma and CD40L, amplifying killing
8. Macrophage APC function is impaired in M. tuberculosis, HIV, tumor microenvironments
9. Dendritic cells are more potent for naive T cell priming; macrophages are critical for effector and memory T cell restimulation

SLIDE 14 - References

• Mescher M.F. - Junqueira's Basic Histology / Histology: A Text and Atlas - Monocyte-macrophage as APC, MHC II presentation
• Murphy K., Weaver C. - Janeway's Immunobiology 10e - MHC class II antigen processing pathway, invariant chain, HLA-DM, TH1-macrophage interaction
• Roitt I. - Roitt's Essential Immunology - Cell-mediated immunity, professional APCs
• Rheumatology 2-Volume Set (Elsevier 2022) - Macrophage cytokine production and APC function
• Medical Microbiology 9e - Macrophage as APC in bacterial/viral infections