Macrophage-Microglia Networks Drive M1 Microglia Polarization After Mycobacterium Infection

Central nervous system tuberculosis (CNS-TB) is caused by infection with Mycobacterium tuberculosis (Mtb). The inflammatory response following CNS-TB involves the activation of resident microglia and the infiltration of macrophages. However, it has not been clarified whether microglia can be polarized into the classically activated proinflammatory M1 phenotype or the alternatively activated anti-inflammatory M2 phenotype after Mtb infection. In this study, we found that BV2 treated with conditioned media from cultures of macrophages infected with Mycobacterium marinum (Mm) induced the expression of M1 phenotypic genes including iNOS, TNF-alpha, IL-1 beta, IL-6, CCL2, and CXCL10 but reduced that of M2 phenotypic genes such as Arginase 1, Ym1, and CD163. These results suggest that polarization of microglia is partly mediated through macrophage-microglia interactions as a priming signal. Overall, these results provide new insights into the modulatory mechanisms of microglial polarization, thereby possibly facilitating the development of new therapies for CNS-TB infection via the regulation of microglial polarization through signalling derived from macrophages infected with mycobacteria.