Processing pathways for presentation of cytosolic antigen to MHC class II-restricted T cells

Antigens presented to CD4⁺ T cells derive primarily from exogenous proteins that are processed into peptides capable of binding tc class II major histocompatibility complex (MHC) molecules in an endocytic compartment^1-4. In contrast, antigens presented to CD⁸⁺ T cells derive mostly from proteins processed in the cytosol, and peptide loading onto class I MHC molecules in an early exocytic compartment is dependent on a transporter for antigen presentation encoded in the class II MHC region^5-11. Endogenous cytosolic antigen can also be presented by class II molecules^12,13. Here we show that, unlike class I-restricted recognition of antigen, HLA-DR1-restricted recognition of cytosolic antigen occurs in mutant cells without a transporter for antigen presentation. In contrast, DR1-restricted recognition of a short cytosolic peptide is dependent on such a transporter. Thus helper T-cell epitopes can be generated from cytosolic antigens by several mechanisms, one of which is distinct from the classical class I pathway.