MHC class II peptide-loading pathway (MHCII pathway)

The MHC class II peptide-loading pathway is a fundamental biological process in professional antigen-presenting cells (APCs), including dendritic cells, B cells, and macrophages, responsible for presenting exogenous antigens to CD4+ T cells. The process begins in the endoplasmic reticulum, where MHC class II alpha and beta chains assemble with the invariant chain (CD74), which blocks the peptide-binding groove and directs the complex to late endosomal compartments. Within these acidic compartments, the invariant chain is sequentially cleaved by proteases such as Cathepsin S, leaving a small fragment called CLIP in the groove. The non-classical MHC molecule HLA-DM then catalyzes the exchange of CLIP for high-affinity antigenic peptides derived from internalized pathogens or cellular debris. Once loaded, the peptide-MHC II complexes are transported to the cell surface to initiate adaptive immune responses. Dysregulation of this pathway is a hallmark of various pathologies; for instance, the presentation of self-peptides can lead to autoimmune disorders like rheumatoid arthritis and type 1 diabetes, while its downregulation in tumors facilitates immune evasion. Pharmacological modulation of the pathway, such as through the use of hydroxychloroquine to alter endosomal pH or specific Cathepsin S inhibitors, represents a key strategy for treating inflammatory and autoimmune conditions.

Other names

MHC class II antigen presentation pathwayExogenous antigen presentation pathwayMHCII processing and presentationMHC class II-restricted antigen presentation

Drugs targeting this pathway act through several mechanisms: hydroxychloroquine and chloroquine inhibit endosomal acidification, thereby preventing the pH-dependent activity of proteases and the loading of peptides onto MHC II molecules. Specific inhibitors of Cathepsin S (e.g., RO5459072) prevent the proteolysis of the invariant chain, which is necessary for clearing the binding groove for antigenic peptides. Monoclonal antibodies like milatuzumab target the invariant chain (CD74) directly. Additionally, interferon gamma acts as a transcriptional inducer of the pathway components via the Class II transactivator (CIITA).