| Tuberculosis is caused by pathogenic mycobacteria, which infect and reside within host macrophages. Pathogenesis includes recruitment of macrophages to initial sites of infection, migration of infected macrophages into deeper tissues, and differentiation and aggregation of these macrophages to form a complex immune structure called granulomas. Granulomas have long been considered important host protective responses; nevertheless it has been recognized that, bacteria can persist indefinitely in them. Furthermore, outcomes of infection are varied, depending on both host and bacterial contributions to pathogenesis. To directly observe macrophage responses to mycobacterial infections, the Ramakrishnan laboratory developed the Mycobacterium marinum/zebrafish embryo model of infection. Mycobacterium marinum is a natural zebrafish pathogen, and a close relative of M. tuberculosis, the agent of human tuberculosis. M. marinum infection of zebrafish causes a persistent infection in the embryo, and recapitulates the stages of adult tuberculosis, including macrophage infection, dissemination into tissues, and aggregation of infected macrophages into granulomas. Zebrafish embryos are genetically tractable and optically transparent, allowing for both modulation of host immune determinants, and real time visualization of infection in whole animals. This work describes a dissection of the molecular pathways required for infected macrophage migration and granuloma formation. It also challenges the notion that granulomas are strictly a host protective immune response, and describes how mycobacteria intentionally direct granuloma formation, and in turn use this host response as a means of growth and dissemination. The mycobacterial virulence determinant RD1 encodes a specialized secretion system, ESX-1, that enhances macrophage aggregation into granulomas, a paradoxical result given that they are considered key host beneficial structures. Here I present the molecular and cellular details of RD1-induced granuloma formation. RD 1-secreted effectors induce host matrix metalloproteinase 9 (mmp9 ) in epithelial cells neighboring the infected macrophages. By specifically knocking down host mmp9, I have shown that its expression facilitates macrophage recruitment to form granulomas and bacterial expansion. These results provide direct evidence that early granulomas benefit mycobacteria and suggest new tuberculosis therapeutic strategies that target granuloma-promoting host susceptibility determinants such as mmp9. |
