The PPE25 (Rv1787)-PE18 (Rv1788)-PPE26 (Rv1789) gene cluster is involved in immune evasion by Mycobacterium tuberculosis

Mycobacterium tuberculosis (M. tb) the causative agent of human tuberculosis, encodes multiple virulence factors to subvert host immune responses. One such class of proteins is encoded by the multigenic PE_PPE family which accounts for 10% of its coding potential. A number of these genes occur in clusters, of which the PPE25(Rv1787)-PE18(Rv1788)-PPE26(Rv1789) locus alone, is organised in a PPE-PE-PPE arrangement. We establish here that this cluster is co-operonic in M. tb, and identify for the first time, PPE25::PPE26 as the sole interacting protein pair encoded by this cluster. Recombinant M. smegmatis strains expressing PPE25, PE18, and PPE26, exhibited enhanced survival in THP-1 macrophages, with infected cells displaying increased levels of the anti-inflammatory cytokine IL-10, and reduced levels of the pro-inflammatory cytokine IL-12. Macrophages infected with recombinant M. smegmatis expressing PPE26 showed 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 recombinant expressing the entire PPE25-PE18-PPE26 operon showed no change in intra-macrophage CFUs, suggestive of an inhibitory role for the PPE25::PPE26 complex in CFU enhancement. Taken together, our findings implicate the PPE25-PE18-PPE26 cluster in playing an immune evasion role in the pathophysiology of M. tb.