The adaptive immune system relies fundamentally upon Major Histocompatibility Complex (MHC) glycoproteins to survey the intracellular and extracellular proteomes. T lymphocytes are blind to native, folded antigens; they recognize exclusively degraded, linearized peptide fragments presented within the peptide-binding grooves of MHC Class I and Class II molecules.
1. Structural Architecture of MHC Molecules
- MHC Class I: A heterodimer composed of a polymorphic, membrane-anchored (alpha)-heavy chain ((alpha_1, alpha_2, alpha_3) domains) non-covalently complexed with the non-polymorphic (beta_2)-microglobulin ((beta_2 ext{m})). The peptide-binding cleft (formed by (alpha_1) and (alpha_2) (alpha)-helices atop an 8-stranded (beta)-pleated sheet) is closed at both ends, accommodating peptides of strictly 8 to 10 amino acids in length. Expressed on virtually all nucleated cells to present endogenous antigens to ( ext{CD8}^+) cytotoxic T cells.
- MHC Class II: An obligate heterodimer of two polymorphic transmembrane chains ((alpha_1, alpha_2) and (beta_1, beta_2)). The peptide groove is open at both termini, allowing peptides of 13 to 25+ amino acids to extend beyond the groove boundaries. Expressed selectively on Professional Antigen-Presenting Cells (APCs: dendritic cells, macrophages, B cells) to present exogenous antigens to ( ext{CD4}^+) helper T cells.
2. The MHC Class I Pathway (Endogenous / Cytosolic Surveillance)
Every nucleated cell continuously surveys its internal cytosol for viral, bacterial, or oncogenic proteins:
- Ubiquitination & Immunoproteasome Degradation: Cytosolic proteins are tagged with polyubiquitin chains. In inflamed tissues exposed to ( ext{IFN-}gamma), standard constitutive (26 ext{S}) proteasomes are replaced by immunoproteasomes (incorporating catalytic subunits LMP2, LMP7, and MECL-1), preferentially cleaving proteins after hydrophobic or basic residues optimal for MHC-I anchor motifs.
- Translocation via TAP: Peptides are pumped from the cytosol into the lumen of the Endoplasmic Reticulum (ER) by the Transporter associated with Antigen Processing (TAP1/TAP2) heterodimer, an ATP-binding cassette (ABC) transporter.
- The Peptide Loading Complex (PLC): In the ER lumen, newly synthesized MHC-I (alpha)-(beta_2 ext{m}) heterodimers are held in a receptive, partially folded state by a chaperoning machine composed of Calreticulin, ERp57, and Tapasin (which physically bridges MHC-I directly to TAP). Tapasin functions as a peptide editor, ensuring that only high-affinity peptides are stably loaded.
- Exocytosis: High-affinity peptide binding stabilizes the heterotrimer, triggering its release from the PLC and transport through the Golgi apparatus to the plasma membrane.
3. The MHC Class II Pathway (Exogenous / Endosomal Surveillance)
APCs internalize extracellular antigens (bacteria, toxins, dead cells) via receptor-mediated endocytosis, phagocytosis, or macropinocytosis:
- Invariant Chain (( ext{Ii}) / CD74) Chaperone: In the ER, newly synthesized MHC-II (alphabeta) heterodimers assemble with a trimeric Invariant Chain. A specific segment of ( ext{Ii}), the CLIP (Class II-associated Invariant Chain Peptide), sits directly inside the peptide-binding cleft, preventing premature ER peptides from binding.
- Endosomal Routing & Proteolysis: The ( ext{Ii}) tail directs the complex through the trans-Golgi into the endosomal/lysosomal pathway (the MIIC compartment). As endosomes acidify, acid-dependent cathepsins (e.g., Cathepsin S and L) degrade the bulk of the invariant chain, leaving only the small CLIP fragment lodged in the groove.
- Peptide Exchange Catalyzed by HLA-DM: Concurrently, internalized exogenous proteins are chopped into peptides by cathepsins. In the MIIC compartment, the non-classical MHC molecule HLA-DM acts as a molecular catalyst: it binds to MHC-II, inducing a conformational change that ejects CLIP, allowing foreign antigenic peptides to compete. Only peptides with stable thermodynamic affinity remain bound.
- Surface Presentation: The stable MHC-II-peptide complex is trafficked to the cell surface for presentation to ( ext{CD4}^+) T cells.
4. Cross-Presentation: Bridging the Divide
Certain specialized dendritic cell subsets (notably classical dendritic cells type 1, ( ext{cDC1}), expressing CD8(alpha) in mice or CD141 in humans) possess the unique capability of cross-presentation: routing internalized exogenous viral or tumor antigens into the MHC Class I pathway. This permits naïve ( ext{CD8}^+) T cell priming against viruses that do not directly infect dendritic cells.
