The PPE18 Protein of Mycobacterium tuberculosis Inhibits NF-κB/rel-Mediated Proinflammatory Cytokine Production by Upregulating and Phosphorylating Suppressor of Cytokine Signaling 3 Protein

Mycobacterium tuberculosis bacteria are known to suppress proinflammatory cytokines like IL-12 and TNF-alpha for a biased Th2 response that favors a successful infection and its subsequent intracellular survival. However, the signaling pathways targeted by the bacilli to inhibit production of these cytokines are not fully understood. In this study, we demonstrate that the PPE18 protein of M. tuberculosis inhibits LPS-induced IL-12 and TNF-alpha production by blocking nuclear translocation of p50, p65 NF-kappa B, and c-rel transcription factors. We found that PPE18 upregulates the expression as well as tyrosine phosphorylation of suppressor of cytokine signaling 3 (SOCS3), and the phosphorylated SOCS3 physically interacts with I kappa B alpha-NF-kappa B/rel complex, inhibiting phosphorylation of I kappa B alpha at the serine 32/36 residues by I kappa B kinase-beta, and thereby prevents nuclear translocation of the NF-kappa B/rel subunits in LPS-activated macrophages. Specific knockdown of SOCS3 by small interfering RNA enhanced I kappa B alpha phosphorylation, leading to increased nuclear levels of NF-kappa B/rel transcription factors vis-a-vis IL-12 p40 and TNF-alpha production in macrophages cotreated with PPE18 and LPS. The PPE18 protein did not affect the I kappa B kinase-beta activity. Our study describes a novel mechanism by which phosphorylated SOCS3 inhibits NF-kappa B activation by masking the phosphorylation site of I kappa B alpha. Also, this study highlights the possible mechanisms by which the M. tuberculosis suppresses production of proinflammatory cytokines using PPE18. The Journal of Immunology, 2011, 186: 5413-5424.