ThePPE25 (Rv1787)- PE18 (Rv1788)- PPE26 (Rv1789)gene cluster is involved in immune evasion byMycobacterium tuberculosis

Abstract

ABSTRACTMycobacterium tuberculosis (M. tb)the causative agent of human tuberculosis, is one of the most successful pathogens known to man. The key to its success is the multiple factors it encodes to subvert host immune responses. One such class of virulence factors is encoded by the multigenic PE_PPE family which accounts for 10% of the coding potential of theM. tbgenome, and is primarily found in pathogenic mycobacteria. A number of these genes occur in clusters, of which thePPE25 (Rv1787)-PE18 (Rv1788)-PPE26 (Rv1789)locus is the only one organised in aPPE-PE-PPEarrangement. We establish here that this cluster is co-operonic inM. tb, and in a pairwise interaction screen, identify PPE25::PPE26 as the only interacting protein pair encoded by this cluster. In the THP-1 macrophage infection model, recombinantM. smegmatisstrains expressingPPE25, PE18, andPPE26, exhibited enhanced survival compared to the control. Consistent with this finding, was the decrease in inducible nitric oxide synthase (iNOS2) transcript levels in these macrophages. We also observed a significant increase in levels of the anti-inflammatory cytokineIL-10, and a reduction in levels of the pro-inflammatory cytokineIL-12upon infection. Macrophages infected with recombinantM. smegmatisexpressingPPE26showed increased phosphorylation of the MAP kinase p38, consistent with the known TLR2 binding activity of PPE26. In contrast to strains expressing the individual cluster genes, the recombinantM. smegmatisstrain expressing the entirePPE25-PE18-PPE26operon showed no change in intra-macrophage CFUs in comparison to the control strain, which is suggestive of an inhibitory role for the PPE25-PPE26 complex in CFU enhancement. Taken together, our findings implicate thePPE25-PE18-PPE26cluster in playing an immune evasion role in the pathophysiology ofM. tb.