Salmonella infection of bone marrow-derived macrophages and dendritic cells: influence on antigen presentation and initiating an immune response

Traditionally macrophages (MΦ) have been considered to be the key type of antigen presenting cells (APC) to combat bacterial infections by phagocytosing and destroying bacteria and presenting bacteria-derived antigens to T cells. However, data in recent years have demonstrated that dendritic cells (DC), at their immature stage of differentiation, are capable of phagocytosing particulate antigens including bacteria. Thus, DC may also be important APC for initiating an immune response to bacterial infections. Our studies focus on studying how DC and MΦ process antigens derived from bacteria with no known mechanism of phagosomal escape (i.e. Salmonella typhimurium) for T cell stimulation as well as what role these APC types have in Salmonella infection in vivo. Using an in vitro antigen processing and presentation assay with bone marrow-derived (BM) APC showed that, in addition to peritoneal elicited MΦ and BMMΦ, BMDC can phagocytose and process Escherichia coli and S. typhimurium for peptide presentation on major histocompatibility complex (MHC) class I (MHC-I) and class II MHC-II. These studies showed that both elicited peritoneal MΦ and BMMΦ use an alternate MHC-I presentation pathway that does not require the transporter associated with antigen processing (TAP) or the proteasome and involves peptide loading onto a preformed pool of post-Golgi MHC-I molecules. In contrast, DC process E. coli and S. typhimurium for peptide presentation on MHC-I using the cytosolic MHC-I presentation pathway that requires TAP, the proteasome and uses newly synthesized MHC-I molecules. We further investigated the interaction of Salmonella with BMDC and BMMΦ by analyzing surface molecule expression and cytokine secretion following S. typhimurium infection of BMDC and BMMΦ. These data reveal that Salmonella co-incubation with BMDC as well as BMMΦ results in upregulation of MHC-I and MHC-II as well as several co-stimulatory molecules including CD80 and CD86. Salmonella infection of BMDC or BMMΦ also results in secretion of cytokines including IL-6 and IL-12. Finally, injecting mice with BMDC that have been loaded in vitro with S. typhimurium primes naïve CD4⁺ and CD8⁺ T cells to Salmonella-encoded antigens. Taken together, our data suggest that DC may be an important type of APC that contributes to the immune response to Salmonella.