Microglia response to Listeria monocytogenes infection

OBJECTIVES
Listeria monocytogenesis an intracellular bacterial pathogen with tropism for the nervous tissue,causing several damages as meningitis, encephalitis and brain abscess.L. 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 as they produce pro and anti-inflammatory factors.

METHODS
We used mouse purified microglia and bone-marrow macrophages, as well as the microglial cells, BV2, and the macrophage-like cells, J774. We also use L. monocytogenes pathogenic strain and mutants with gene deletions on three virulence factors (hly, actA and plcA-plcB). Experiments include confocal microscopy, kinetic infection assays, phagosome and endosome isolation, flow cytometry analysis of immune markers and cytokines levels, microarrays to evaluate transcriptional expression and protein compositional analysis.

RESULTS
Using confocal microscopy with 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, as bacterial replication was similar in primary microglia and BV2 cells but different than replication in bone-marrow macrophages and J774 cells. 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, genes regulated by tumour necrosis factors (TNF) and those ones regulated by interferons (IFN), both signals related with nitric oxide and hydrogen peroxide, the main listericidal mechanisms in microglia and macrophages. 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.

CONCLUSION
All collected data allowed us to design a model for L. monocytogenes infection in microglia and regulated by actA and hly genes. actA gene triggers early innate immune responses using a pathway which activates NF-kB, TNF-alpha and other 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