Exogenous Antigen Processing Pathway for MHC Class II

The exogenous antigen processing pathway is crucial for presenting antigens on MHC class II molecules, primarily to CD4 T cells.
In this pathway, specialized antigen presenting cells (APCs), such as dendritic cells, capture antigens from the extracellular environment.
- These antigens are transported to endosomes where they are degraded into peptide fragments.

APCs include specialized cells such as dendritic cells, macrophages, and B cells, each with distinct endocytic capabilities.

Dendritic cells can perform various forms of endocytosis, allowing them to nonspecifically sample extracellular environments.
They can also specifically uptake tagged antigens via receptor-mediated endocytosis and phagocytosis.

Macrophages mainly act as phagocytes to degrade pathogens.
They are less likely to nonspecifically sample the extracellular environment but are efficient in receptor-mediated endocytosis and phagocytosis.

B cells communicate with CD4 T cells to receive help in driving antibody production.
They primarily utilize receptor-mediated endocytosis, governed by their B cell receptors (BCR), maintaining specificity in antigen presentation.

APCs use several processes to uptake antigens from the extracellular environment:
- Macropinocytosis: Engulfment of large volumes of extracellular fluid, driven by cytoskeletal rearrangement (actin-dependent).
- Micropinocytosis: Similar to macropinocytosis but involves smaller volumes and the formation of pits in the membrane via clathrin.
- Receptor-Mediated Endocytosis: Specific uptake of molecules recognized by cell surface receptors, involving clathrin cage formation to engulf small particles.
- Phagocytosis: Uptake of larger particles (e.g., whole pathogens) through receptor-mediated processes, also actin-dependent.

Once antigens are captured, they are delivered to endosomes, which undergo a transition from early to late endosomes and eventually to lysosomes.
- Each endosomal stage has a progressively lower pH and increased hydrolytic activity, crucial for antigen processing.
The components involved include:
- Proteolytic Enzymes: Found in endosomes such as cathepsins (B, D, S, L), which cleave proteins into peptide fragments.
- Acidification: ATPase pumps hydrogen ions into endosomes, lowering the pH and enhancing proteolytic enzyme activity.
- GILT (interferon gamma-inducible lysosomal thiol reductase): Breaks disulfide bonds, facilitating further protein denaturation and peptide liberation.

MHC Class II molecules are synthesized in the endoplasmic reticulum (ER).
- The alpha and beta chains of MHC class II are translated, while the invariant chain is also synthesized to prevent premature loading of peptides.
Invariant Chain Role: Forms a trimeric structure occupying the peptide binding cleft, preventing access to peptides from the ER.

The MHC class II-invariant chain complex is exported from the ER via the Golgi apparatus.
- The N-terminus of the invariant chain acts as a molecular homing beacon directing the complex towards endosomes.
The fusion of the MHC class II vesicle with the endosomes creates an antigen processing compartment containing peptide fragments and proteolytic enzymes.

The invariant chain must first be cleaved to allow peptide entry into the MHC class II binding pocket.
- Steps involved in invariant chain cleavage:
- Proteolytic enzyme cleaves the C-terminus of the invariant chain, leaving the CLIP sequence bound to MHC class II.
- Cathepsin S removes the N-terminal part of the invariant chain, allowing peptide loading.
Role of HLA DM: Associates with MHC class II to open the peptide binding cleft, facilitating stable peptide binding until a suitable peptide is identified.
Once a peptide is bound securely, HLA DM dissociates, and the stabilized MHC-peptide complex is transported to the cell surface to present to CD4 T cells.

Antigens are captured by APCs through various endocytic methods and delivered to endosomes.
In these compartments, proteolytic processes occur, leading to peptide fragment generation.
Meanwhile, MHC class II is synthesized in the ER, prevented from loading self-peptides by the invariant chain.
The complex is guided to the endosomes, where peptides are loaded onto MHC class II, which is then exported to cell surface for T cell presentation.

This concludes the overview of the exogenous antigen processing pathway, focusing on the critical roles of APCs, endocytosis, processing steps, and MHC class II assembly and peptide loading.