Exploring the role of MPB70 and MPB83 in immune modulation by mycobacterium tuberculosis

Mycobacterium tuberculosis, the agent of tuberculosis (TB) in humans, and the animal TB pathogen Mycobacterium bovis display distinct host preferences, even though they are over 99.95% genetically identical. M. tuberculosis modulates the host’s immune system both to achieve disease pathology but also to suppress and evade immunity to persist latently in the host. It is known that M. tuberculosis has a range of molecules that enable it to modify the immune response by interacting and interfering with host cell signaling pathways; however, how the distinct host tropism of M. tuberculosis and M. bovis is achieved is unknown. We hypothesised that proteins that show differential expression between M. tuberculosis and M. bovis may play a role in host tropism through modulation of the immune response.MPB83 is a glycosylated lipoprotein embedded in the outer membrane of the bacterium, while MPB70 is an ortholog of MPB83 that is not post- translationally modified and is secreted from the cell. The function of these proteins is unknown but they have been shown to be expressed at higher levels in vitro in M. bovis than in M. tuberculosis. Intriguingly, the expression of the genes encoding these proteins is up-regulated by M. tuberculosis when the bacterium is engulfed by a macrophage, suggesting an important role in vivo. They have been shown to induce a T cell response in cattle reflected in enhanced IFN-γ production and T cell proliferation. It has also been reported that native and recombinant MPB83 binds to TLR1/2 to induce pro- inflammatory cytokine production.In order to explore the immune-modulatory function of these proteins on innate immune cells, dendritic cells and macrophages were stimulated with recombinant MPB70 and MPB83 proteins with cytokine production measured by ELISA and maturation marker expression analysed by flow cytometry. To examine the effect of MPB70 and MPB83 on a mixed cell population, they were incubated with murine splenocytes with ELISA and flow cytometry employed to analyse cytokine production and cellular proliferation.Our results demonstrated that recombinant MPB70 and MPB83, the latter lacking any post-translational modifications, did not induce pro-inflammatory cytokine production by antigen presenting cells. However, in a mixed cell population, both proteins triggered induction of IFN-γ production and CD4+ and CD8+ T cell proliferation. Blocking IL-12 was shown to inhibit this MPB70/MPB83-induced T cell activation. Our data therefore suggest that MPB70 and MPB83 may have a bearing on disease progression via the induction of IFN-γ. The data presented in this work therefore provides new insight into the potential role of MPB70 and MPB83 as host-tropic virulence factors in the Mycobacterium tuberculosis complex