With 10.4 million incident cases and 1.7 million deaths reported in 2016, tuberculosis (TB) continues to remain one of the world's deadliest communicable diseases, and constitutes a significant burden on resource-limited countries. An estimated 13% of new TB cases are also known to be HIV-positive. The causative agent, Mycobacterium tuberculosis (MTB), persists within host macrophages, where it uses a plethora of complex strategies to alter host immune responses in order to facilitate its long-term survival. HIV and MTB appear to potentiate one another, accelerating the deterioration of host immune functions. The continued search for bacterially secreted protein virulence factors that gain access to macrophages and modulate immune responses has overlooked `moonlighting or multitasking' MTB proteins that have normal cellular functions within the pathogen, but serve as virulence factors once secreted. Our recent studies indicate that PpiA, a cyclophilin-like peptidyl-prolyl isomerase secreted by MTB, functions as a moonlighting protein that alters host immune responses in vitro and in vivo. Using gain-of function and loss-of-function PpiA mutants of MTB, we found that excess PpiA secreted by intracellular MTB reduced pro-inflammatory cytokine responses in vitro, and led to increased bacterial burden and disease severity in a mouse model of TB, associated with suppression of pro-inflammatory cytokine response. PpiA therefore acts as a bacterial virulence factor which inhibits key intracellular host responses required for pathogen control. PpiA is also a human cyclophilin analogue displaying close structural similarity, strongly suggesting that it could be deployed by MTB as an effector mimic against the host cyclophilins, particularly during MTB/HIV co-infection, thus protecting HIV cDNA from recognition by cGAS, and hence from subsequent antiviral Type I IFN response. Further contribution could also come from its ability to bind IFITM1, an anti-viral protein produced in response to HIV infection. PpiA produced by MTB could therefore significantly alter the host response and outcome of MTB mono-infection and MTB/ HIV co-infection, suggesting a molecular basis for the well-known synergy between the pathogens. Identifying host binding partners of PpiA, and evaluating its functional role during MTB infection and MTB/HIV co-infection will therefore facilitate the recognition of novel therapeutic targets.
Our aims therefore are to identify macrophage host binding partner(s) for MTB PpiA (Aim 1), and to evaluate the role of PpiA and its binding proteins in modulating host immune responses in vitro to MTB mono-infection and MTB/HIV co- infection (Aim 2).
We recently found that a virulence factor secreted by the tuberculosis-causing pathogen Mycobacterium tuberculosis (MTB), namely a cyclophilin-like peptidyl-prolyl isomerase PpiA, modifies immune response pathways in the host. Similar pathways are also activated during HIV infection, suggesting that the factor could play important roles in defining the outcome of tuberculosis infection and of HIV co-infection. Using in vitro studies, our proposed research will attempt to further identify host proteins that bind to PpiA to bring about alterations in host immune responses during MTB infection and MTB/HIV co-infection, with a goal towards therapeutic approaches that are beneficial to the host.
