Rv3539 (PPE63) of Mycobacterium Tuberculosis Promotes Survival of Mycobacterium Smegmatis in Human Macrophages Cell Line via Cell Wall Modulation of Bacteria and Altering Host's Immune Response.

The modulation of host's immune response plays an important role in the intracellular survival of Mycobacterium tuberculosis. The intracellular pathogen counteracts environmental stresses with help of the expression of several genes. The M. tuberculosis genome encodes several immune-modulatory proteins including PE (proline-glutamic acid)/PPE (proline-proline-glutamic acid) superfamily proteins. It is unclear how the unique PE/PPE proteins superfamily contributes to survival under different stress and pathophysiology conditions. Previously, we showed that PPE63 (Rv3539) has C-terminal esterase extension and was localized as a membrane attached and in extracellular compartment. Therefore, the probability of these proteins interacting with the host to modulate the host immune response cannot be ruled out. The physiological role of PPE63 was characterized by expressing the PPE63 in the M. smegmatis, a non-pathogenic strain intrinsically deficient of PPE63. The recombinant M. smegmatis expressing PPE63 altered the colony morphology, lipid composition, and integrity of the cell wall. It provided resistance to multiple hostile environmental stress conditions and several antibiotics. MS_Rv3539 demonstrated higher infection and intracellular survival in comparison to the MS_Vec in the PMA-differentiated THP-1 cells. The decreased intracellular level of ROS, NO, and expression of iNOS was observed in THP-1 cells upon infection with MS_Rv3539 in comparison to MS_Vec. Further, the decrease in expression of pro-inflammatory cytokines like IL-6, TNF-α, and IL-1β and enhanced anti-inflammatory cytokines like IL-10, pointed toward its role in immune modulation. Overall this study suggested the role of Rv3539 in enhanced intracellular survival of M. smegmatis via cell wall modulation and altered immune response of host.