<i>Mycobacterium tuberculosis</i> early secreted antigenic target of 6 kDa (ESAT-6) induces IL-6 production by macrophages via STAT3 (MPF3P.820)

Abstract Early secreted antigenic target of 6 kDa (ESAT-6) of Mycobacterium tuberculosis (Mtb) is a major virulence factor. Mtb-induced proinflammatory cytokine IL-6 inhibits macrophage responses to IFN-γ and contributes to the survival of Mtb. To explore the role of ESAT-6 in Mtb infection, we studied if ESAT-6 induces IL-6 production by macrophages. Incubation of mouse bone marrow-derived monocytes (BMDM) with ESAT-6 induced IL-6 production in dose and time dependent manner. However, culture filtrate protein of 10 kDa (CFP10) and antigen 85A (Ag85A) of Mtb did not induce IL-6 production, suggesting a specific effect of ESAT-6 on IL-6 production. Likewise, ESAT-6 but not CFP10 and Ag85A induced IL-6 secretion by human peripheral blood derived monocytes. Polymyxin B inhibited LPS but not ESAT-6 induced IL-6 secretion by macrophages refuting the possible role of LPS contamination in ESAT-6. ESAT-6 but not Pam3CSK4, a TLR2 agonist, stimulated IL-6 production by TLR2 knockout BMDM, indicating that this effect of ESAT-6 is TLR2 independent. To explore the signaling pathway of this, we examined the role of STAT3 since ESAT-6 induced activation of STAT3 in BMDM. Preincubation of cells with Stattic and S31-201, STAT3 inhibitors, but not rapamycin and DMSO (vehicle) inhibited ESAT-6 induced IL-6 expression by BMDM both at mRNA and protein level in a dose dependent manner without affecting cell viability. Thus, our results suggest that ESAT-6 induces IL-6 production in macrophages via STAT3.