B-219. Control of Listeria monocytogenes Infection by Murine Bone Marrow-Derived Dendritic Cells

M. M. Westcott1, J. E. Amis2, E. M. Hiltbold1;
1Wake Forest Univ. Sch. of Med., Winston-Salem, NC, 2Guilford Coll., Greensboro, NC.

L. monocytogenes is a facultative intracellular pathogen that must gain access to the mammalian cell cytosol in order to grow. We have recently demonstrated that murine bone marrow-derived dendritic cells (DC) are less permissive for bacterial replication than bone marrow-derived macrophages (BMM), due to an enhanced capacity to restrict bacterial exit to the cytosol. In order to understand this apparent control of L. monocytogenes infection by DC, we have utilized confocal microscopy to characterize the bacteria-containing phagosome at various time points after infection. Between 1.5 and 3 hrs after infection, when most bacteria were free in the BMM cytosol, large numbers of bacteria (approximately 50%) were found in DC vesicles that contained Lamp2 or the MHC II accessory molecule, H2-DM. The vesicles labeled with lysotracker red, indicating that they acidified to some extent, and were negative for the early endosomal marker Rab5a. Using MHC II and H2-DM staining pattern to assess DC maturation state, cells that trapped L. monocytogenes were typical immature DC (MHC II and H2-DM colocalized in intracellular vesicles,) or alternatively, were in the process of maturing (MHC II and H2-DM vesicular staining pattern reduced, with increased surface MHC II). Interestingly, a population of CD11c+ cells that expressed low levels of MHC II (intracellular and surface) and H2-DM, were more permissive for L. monocytogenes growth. DC with a mature phenotype (bright staining for surface MHC II), present at low frequency in day 7 DC cultures (< 10%) ingested fewer bacteria overall than immature DC, and tended to restrict bacteria to phagosomes. We conclude that DC have the capacity to inhibit escape of L. monocytogenes to the cytosol by an as yet undefined mechanism, and target phagosome-entrapped bacteria to a late endosomal/lysosomal MHC II+ compartment. These results highlight the phenotypic and functional heterogeneity characteristic of DC in vitro and in vivo, and suggest that the outcome of L. monocytogenes infection of this critical antigen presenting cell type ultimately depends on its developmental and/or maturation state at the time of infection.