Distinct Strategies Employed by Dendritic Cells and Macrophages in Restricting Mycobacterium tuberculosis Infection: Different Philosophies but Same Desire

Dendritic cells (DCs) and macrophages (M phi s) are professional antigen-presenting cells (APCs) that can efficiently phagocytose Mycobacterium tuberculosis (Mtb), the etiological agent of tuberculosis (TB). It is quite interesting to mention here that DCs and M phi s use distinct strategies to combat and eliminate Mtb. Similarly, Mtb employs different mechanisms to counteract the action of DCs and M phi s. M phi s are evolved with specialized, innate, defensive machinery to restrict growth of Mtb at the initial phase of infection. However, DCs are more endowed toward initiating adaptive immunity by activating naive T cells. During encounter with Mtb, DCs and M phi s deliver discrete functions via triggering through different pattern recognition receptors (PRRs) expressed by these APCs. Mtb-infected DCs and M phi s show differential expression of genes encoding cytokines, chemokines, costimulatory molecules, and adhesion molecules. Interestingly, Mtb impairs the immune defensive machinery by exploiting various PRRs. Remarkably, selective signaling through PRRs by Mtb abrogates the bactericidal activity of M phi s, but subverts differentiation of monocytes to DCs. In this article, we highlight the role of PRRs in inducing distinct immune response by DCs and M phi s against Mtb. Concurrently, we also discuss smart strategies exploited by Mtb to impair the function of host DCs and M phi s.