Bone marrow-derived dendritic cells can process bacteria for MHC-I and MHC-II presentation to T cells.

Dendritic cells can engulf particulate Ags and induce T cell proliferative responses after pulsing with particulate Ag. However, their capacity to process viable Gram-negative bacteria for presentation by MHC-I and MHC-II has not been shown. We therefore characterized the ability of murine bone marrow-derived dendritic cells to process Escherichia coli and Salmonella typhimurium, expressing defined epitopes for presentation by MHC-I and MHC-II molecules. The I-Ak-restricted 46-61 epitope from hen egg white lysozyme (HEL(46-61)) or the Kb-restricted 257-264 epitope from chicken egg OVA (OVA(257-264)) was expressed as fusion proteins in the bacterial cytoplasm as the Crl-HEL and Crl-OVA fusion proteins, respectively. Bacteria expressing Crl-HEL or Crl-OVA, or beads coated with HEL or OVA, were coincubated with murine bone marrow-derived dendritic cells, and Ag processing and presentation were quantitated using T cell hybridomas. The data show that granulocyte-macrophage CSF-stimulated dendritic cells can process live intact Gram-negative bacteria for peptide presentation by MHC-I and MHC-II. Cytochalasin D inhibition studies revealed that processing for both MHC-I and MHC-II presentation required cytoskeletal rearrangements. Processing for MHC-I and MHC-II presentation was inhibited by ammonium chloride, suggesting that acidic compartments were required. Thus, granulocyte-macrophage CSF-stimulated murine bone marrow dendritic cells are capable of processing exogenous particulate Ags, including bacteria with no known mechanism for phagosomal escape, for peptide presentation by both MHC-I and MHC-II. These data suggest that dendritic cells may be important in priming both CD4+ and CD8+ T cells to bacterial Ags.