Endolysosomal processing of exogenous antigen into major histocompatibility complex class I-binding peptides

An alternative endolysosomal pathway has recently been suggested for the processing of MHC-I-binding peptides, and peptide/MHC-I complexes have been demonstrated in this compartment. However, it remains unclear where in the antigen-presenting cells such peptides are processed, in the endolysosomes themselves or in the proteasomal complex. Here, we have investigated this using monoclonal antibodies specific for the immunodominant SIINFEKL/Kb complex (25-D1) or for the carbohydrate part of Db- or Kb-binding glycopeptides in combination with inhibitors for classical and endolysosomal MHC-I-processing pathways. Alternative processing was detected in both wt and TAP1(-/-) immature DC (iDC) as the expression of SIINFEKL/Kb complexes on the surface of OVA-treated cells in the presence of Brefeldin A (BFA) or lactacystin and their absence in the presence of the lysosomotropic amines ammonium chloride, chloroquine and methylamine. Internalized Db- and Kb-binding glycopeptides, detected with high specificity using an anti-galabiose (Gal2) monoclonal antibody, were found to appear on the cell surface of BFA-treated cells after intracellular MHC-I-binding. Peptide exchange in Kb was demonstrated as the gradual appearance of SIINFEKL/Kb complexes on BFA-treated cells which earlier had been saturated with another Kb-binding peptide. Our data support the presence of a fully functional endolysosomal processing pathway in iDC guided by the chaperone function of MHC-I molecules.