Localization of peptide/MHC class II complexes in macrophages following antigen processing of viable Streptococcus pyogenes

Abstract
The subcellular localization of peptide/MHC complexes was investigated during processing of the surface M5 protein from Streptococcus pyogenes. Bone marrow-derived macrophages were pulsed with viable S. pyogenes for 20 min followed by various periods of chase. T hybridoma cells detected complexes of one epitope, M517–31 with Ed on the surface of macrophages within 30 min of chase. In contrast, complexes with another epitope, M5308–319 with Ad peaked later. Intracellular localization of peptide/MHC-II complexes was studied by subcellular fractionation and detection of complexes in fractions by T hybridoma cells. M517–31/Ed complexes were detected in light membrane fractions containing plasma membrane and early endosomes by 10–30 min. M5308–319/Ad complexes were detected in these light membranes after 3 h of chase. Thus, the time course of M5308–319/Ad presentation was delayed relative to M517–31/Ed. However, neither type of complex was detected at any time in fractions containing phagosomes. Both species of peptide/MHC complexes localized to endocytic compartments, indicating a role for endosomes in presentation of antigens from phagocytosed bacteria.