The pathway for MHCII-mediated presentation of endogenous proteins involves peptide transport to the endo-lysosomal compartment.

Antigen-presenting cells (APCs) are expected to present peptides from endocytosed proteins via major histocompatibility complex (MHC) class II (MHCII) molecules to T cells. However, a large proportion of peptides purified from MHCII molecules are derived from cytosolic self-proteins making the pathway of cytosolic peptide loading onto MHCII of critical relevance in the regulation of immune self-tolerance. We show that peptides derived from cytoplasmic proteins either introduced or expressed in the cytoplasm are first detectable as MHCII-peptide complexes in LAMP-1(+) lysosomes, prior to their delivery to the cell surface. These peptide-MHC complexes are formed in a variety of APCs, including peritoneal macrophages, dendritic cells, and B cells, and are able to activate T cells. This process requires invariant chain (Ii)-dependent sorting of MHCII to the lysosome and the activity of the molecular chaperone H-2M. This pathway is independent of the ER resident peptide transporter complex TAP and does not take place by cross-presentation from neighbouring cells. In conjunction with our earlier results showing that these peptides are derived by cytosolic processing via the proteasome, these observations provide evidence for a ubiquitous route for peptide transport into the lysosome for the efficient presentation of endogenous and cytoplasmic proteins to CD4 T cells.