Major histocompatibility complex class I-restricted antigen processing and presentation.

Major histocompatibility complex (MHC) class I molecules present antigenic peptides to CD8-expressing cytotoxic T lymphocytes (CTLs). This antigen recognition system is critically important for immune surveillance against viruses and tumors. Most class I-binding peptides are generated in the cytosol, as side products from the degradation of misfolded proteins by proteasomes. A subset of the resulting peptides are translocated across the endoplasmic reticulum (ER) membrane by a dedicated peptide transporter, and these peptides are then loaded onto peptide-receptive class I molecules in the ER. The stable assembly of class I molecules with peptides is controlled by a variety of accessory proteins, including chaperones with general housekeeping functions and factors with dedicated roles in class I assembly. Peptide-filled class I molecules are then delivered to the cell surface for recognition by CTLs. This highly regulated process permits the host to rapidly counter invading pathogens with strong and sustained CTL responses and, at the same time, avoid misguided attacks. Here, how the class I antigen processing machinery accomplishes this daunting task is reviewed.