Non-opsonic recognition of Mycobacterium tuberculosis by phagocytes

The interactions between Mycobacterium tuberculosis and host phagocytes such as macrophages and dendritic cells are central to both immunity and pathogenesis. Many receptors have been implicated in recognition and binding of M. tuberculosis such as the mannose receptor, dendritic-cell-specific intercellular adhesion molecule-3 grabbing nonintegrin, dectin-1 and complement receptor 3 as well as Toll-like receptors, scavenger receptors and CD14. While in vitro studies have demonstrated clear roles for particular receptor(s), in vivo work in receptor-deficient animals often revealed only a minor, or no role, in infection with M. tuberculosis. The initial encounter of phagocytic cells with myco- bacteria appears to be complex and depends on various parameters. It seems likely that infection with M. tuberculosis does not occur via a single receptor-mediated pathway. Rather, multiple receptors play different roles in M. tuberculosis infection, and the overall effect depends on the expression and availability of a particular receptor on a particular cell type and its triggered downstream responses. Moreover, the role of membrane cholesterol for M. tuberculosis interactions with phagocytes adds to the complexity of mycobacterial recognition and response. This review summarizes current knowledge on non-opsonic receptors involved in binding of mycobacteria and discusses the contribution of individual receptors to the recognition process.