Dissociation of Innate Immune Responses in Microglia Infected with Listeria monocytogenes

Listeria monocytogenes target cell in the brain remains unclear. Our hypothesis is that microglia might be the cell target of this pathogen and the cause of several neuronal disorders. We used mouse purified microglia and bone-marrow macrophages, as well as microglial cells, BV2, and macrophage-like cells, J774. We also use L. monocytogenes pathogenic strain and mutants with gene deletions on virulence factors. Experiments include confocal microscopy, kinetic assays, phagosome and endosome isolation, flow cytometry analysis and microarrays. Using an in vitro murine hippocampal model that contained 95% neurons and 2% microglia we observed that virtually all bacteria were inside microglia. Infection kinetic analysis confirmed these results. Differences in bacterial intracellular replication and immune markers expression led us to think that microglia showed a lower bactericidal potential than macrophages. Gene expression analysis revealed two transcriptional patterns in microglia: one in common with macrophages and a microglia specific pattern. Two signals of innate immunity control L. monocytogenes infections, both signals related with nitric oxide and hydrogen peroxide. Lack of production of these compounds and the cytokine pattern in microglia, led us to think that both signals were dissociated in these cells. Finally, we tested how neurons reacted to infected microglia. When confronting murine HN9 neurons with supernatants obtained from infected cells, we observed lower apoptosis levels when using supernatants from microglia. All collected data allowed us to design a model for L. monocytogenes infection regulated by actA and hly genes. actA gene triggers early innate immune responses using a pathway which activates several cytokines involved in monocytes recruitment. On the other hand, hly gene represses late innate immune responses repressing IFN expression causing a decrease in nitric oxide and hydrogen peroxidase levels. While actA gene role is common to macrophages and microglia, hly gene repression is microglia specific. This dissociation of TNF and IFN-related responses seems to play a role in reducing neuronal damage during L. monocytogenes infections.