Processing of Exogenous Antigens for Presentation by Class I MHC Molecules Involves Post-Golgi Peptide Exchange Influenced by Peptide-MHC Complex Stability and Acidic pH

摘要

Vacuolar alternate class I MHC (MHC-I) Ag processing allows presentation of exogenous Ag by MHC-I molecules with binding of antigenic peptides to post-Golgi MHC-I molecules. We investigated the role of previously bound peptides and their dissociation in generating peptide-receptive MHC-I molecules. TAP1-knockout macrophages were incubated overnight with an initial exog-enous peptide, producing a large cohort of peptide-K~b complexes that could influence subsequent peptide dissociation/exchange. Initial incubation with FAPGNYPAL, KVVRFDKL, or RGYVYQGL enhanced rather than reduced subsequent binding and presentation of a readout peptide (SIINFEKL or FAPGNYPAL) to T cells. Thus, Kb molecules may be stabilized by an initial (stabilizing) peptide, enhancing their ability to bind readout peptide and implicating peptide dissociation/exchange. In contrast, incubation with SIINFEKL as stabilizing peptide reduced presentation of readout peptide. SIINFEKL-K~b complexes were more stable than other peptide-K~b complexes, which may limit their contribution to peptide exchange. Stabilizing peptides (FAPGNYPAL, KVVRFDKL, or RGYVYQGL) enhanced alternate MHC-I processing of HB101.Crl-OVA (Escherichia coli expressing an OV A fusion protein), indicating that alternate MHC-I Ag processing involves peptide dissociation/exchange. Sta-bilizing peptide enhanced processing of HB101.Crl-OVA more than presentation of exogenous OV A peptide (SIINFEKL), sug-gesting that peptide dissociation/exchange may be enhanced in the acidic phagosomal processing environment. Furthermore, exposure of cells to acidic pH increased subsequent binding and presentation of readout peptide. Thus, peptide dissociation/ exchange contributes to alternate MHC-I Ag processing and may be influenced by both stability of peptide-MHC-I complexes and pH.