Major Histocompatibility Complex class II (MHC II) antigen presentation machinery (MHC II machinery)

The Major Histocompatibility Complex class II (MHC II) antigen presentation machinery is a multi-protein system essential for the adaptive immune system's ability to recognize foreign pathogens. This machinery functions primarily in professional antigen-presenting cells, such as dendritic cells, B cells, and macrophages, where it processes exogenous proteins into immunogenic peptides within endosomal and lysosomal compartments (PMID: 30104468). Key components include the MHC II heterodimers (HLA-DR, HLA-DQ, HLA-DP), the invariant chain (CD74) which chaperones the MHC II complex, and the specialized chaperone HLA-DM, which facilitates the loading of high-affinity peptides (UniProt: P01903, P04233). Once loaded, the MHC II-peptide complex is transported to the cell surface to activate CD4+ T helper cells, triggering a coordinated immune response (StatPearls: MHC Class II). Dysregulation of this pathway is a hallmark of many autoimmune diseases, such as rheumatoid arthritis, where self-peptides are inappropriately presented, while many tumors downregulate this machinery to evade immune detection (PMID: 31435310). Pharmacological intervention often involves drugs like hydroxychloroquine, which raises endosomal pH to inhibit the cathepsins required for antigen processing, thereby dampening the autoimmune response (PubMed: 25135138).

Other names

MHC class II pathwayExogenous antigen presentation pathwayHLA class II machineryAntigen processing and presentationMHCII presentation system

Modulation of endosomal pH to inhibit antigen proteolysis by cathepsins and interference with the HLA-DM-mediated exchange of CLIP for antigenic peptides on MHC class II molecules.