PROS1 and neoplasm: On the other hand, the observation that a large proportion of peptides purified from MHC class II are derived from cytosolic self-proteins (e.g., metabolic enzymes, cytoskeletal proteins, and tumor antigens) indicates that MHC class II may also present endogenous peptides for CD4+ T cell recognition, which has potential relevance to autoimmunity and tumor immunity (62, 63).