Bacterial antigen delivery systems: phagocytic processing of bacterial antigens for MHC-I and MHC-II presentation to T cells.

Using an in vitro model system we have studied parameters of both bacteria and antigen presenting cells that influence peptide presentation by murine major histocompatibility complex class II (MHC-II) and class I (MHC-I) molecules. To study MHC-II presentation, the HEL (52-61) epitope, which binds the murine MHC-II molecule I-Ak, was expressed as the cytoplasmic Crl-HEL fusion protein in S. typhimurium. When murine peritoneal macrophages mediated phagocytic processing of S. typhimurium expressing Crl-HEL, HEL (52-61) was processed and presented on I-Ak more efficiently from heat-killed S. typhimurium than from viable bacteria, and from a rough LPS strain compared to its isogenic smooth LPS counterpart, most likely due to enhanced phagocytosis of the rough LPS strain. Macrophages also processed phoP S. typhimurium strains with greater efficiency for peptide presentation by I-Ak than wild type bacteria while Salmonella constitutively expressing phoP were processed for peptide presentation by I-Ak less efficiently than wild type Salmonella. We have also shown that macrophage phagocytosis of E. coli or S. typhimurium results in presentation of bacterial antigens by MHC-I molecules. To investigate the role of post-Golgi MHC-I molecules in this presentation pathway, peritoneal macrophages from TAP1-/- mice, which are deficient in presenting endogenous antigens on MHC-I and lack significant surface MHC-I expression, were co-incubated with bacteria containing the 257-264 epitope from ovalbumin [OVA(257-264)], which binds the murine class I molecule Kb. Peritoneal macrophages from TAP1-/-/ mice could process bacteria expressing the OVA epitope for recognition by epitope-specific T hybridoma cells. This processing and presentation was reduced in efficiency between three to 100 fold compared to C57BL/6 macrophages, depending on the protein harbouring the OVA (257-264) epitope (Crl-OVA or native OVA). This suggests that the protein context of the OVA (257-264) epitope influences the extent of TAP-independent processing for MHC-I presentation. In addition, we show that murine bone marrow-derived dendritic cells can phagocytose and process viable gram negative bacteria for peptide presentation on MHC-I and MHC-II; inhibition studies showed that acidic compartments in dendritic cells are required for this presentation. These results suggest that dendritic cells may be potential antigen presenting cells used in eliciting specific immune responses against bacteria.