Delineating the functional role of the PPE50 (Rv3135) - PPE51 (Rv3136) gene cluster in the pathophysiology of Mycobacterium tuberculosis.

The extraordinary success of Mycobacterium tuberculosis (M. tb) has been attributed to its ability to modulate host immune responses, and its genome encodes multiple immunomodulatory factors, including several proteins of the multigenic PE_PPE family. To understand its role in M. tb pathophysiology we have characterised the PPE50 (Rv3135)-PPE51 (Rv3136) gene cluster, one of nine PPE-PPE clusters in the genome. We demonstrate here that this cluster is operonic, and that PPE50 and PPE51 interact - the first demonstration of PPE-PPE interaction. THP-1 macrophages infected with recombinant Mycobacterium smegmatis strains expressing PPE50 and PPE51 showed lower intracellular viability than the control, which correlated with an increase in transcript levels of iNOS2. Infected macrophages also exhibited an upregulation in levels of IL-10, indicating an immunomodulatory role for these proteins. Using pull-downs and signalling assays, we identified TLR1 to be the cognate receptor for PPE50 - all the phenotypes observed on infection of THP-1 macrophages were reversed on pre-treatment with an anti-TLR1 antibody, validating the functional outcome of PPE50-TLR1 interaction. Our data reveals a TLR1 dependent role for the PPE50-PPE51 cluster in promoting bacillary persistence, via CFU reduction and concomitant upregulation of the anti-inflammatory response - a two-pronged strategy to circumvent host immune surveillance.