Receptor-mediated endocytosis of antigens overcomes the requirement for HLA-DM in class II-restricted antigen presentation.

HLA-DM- APC are unable to present soluble Ags to T cells in the context of class II DR molecules. This defect in DM- APC can be overcome by receptor-mediated delivery of Ag into cells. Ag conjugated to ligands for cell surface receptors, such as transferrin or goat anti-human Ig, was processed and presented by DM- T2.DR4 cells. Intracellular processing of Ag conjugates was required, as receptor cross-linking alone did not restore presentation by DM- APC. Ag conjugates targeted by transferrin receptors to endosomes or via surface Ig to endosomal and lysosomal compartments, were each efficiently presented by DM- cells. These Ag conjugates were predominantly localized in light density endosomes in T2.DR4 cells. This study demonstrates that the facilitated uptake and sorting of exogenous Ag by cell surface receptors allow efficient class II-restricted presentation even in the absence of HLA-DM.