Early Secreted Antigen ESAT-6 of Mycobacterium tuberculosis Promotes Protective T Helper 17 Cell Responses in a Toll-Like Receptor-2-dependent Manner

Abstract

Despite its relatively poor efficacy, Bacillus Calmette-Guérin (BCG) has been used as a tuberculosis (TB) vaccine since its development in 1921. BCG induces robust T helper 1 (Th1) immune responses but, for many individuals, this is not sufficient for host resistance against Mycobacterium tuberculosis ( M. tb) infection. Here we provide evidence that early secreted antigenic target protein 6 (ESAT-6), expressed by the virulent M. tb strain H37Rv but not by BCG, promotes vaccine-enhancing Th17 cell responses. These activities of ESAT-6 were dependent on TLR-2/MyD88 signalling and involved IL-6 and TGF-β production by dendritic cells. Thus, animals that were previously infected with H37Rv or recombinant BCG containing the RD1 region (BCG::RD1) exhibited improved protection upon re-challenge with virulent H37Rv compared with mice previously infected with BCG or RD1-deficient H37Rv (H37RvΔRD1). However, TLR-2 knockout (TLR-2 ^-/-) animals neither showed Th17 responses nor exhibited improved protection in response to immunization with H37Rv. Furthermore, H37Rv and BCG::RD1 infection had little effect on the expression of the anti-inflammatory microRNA-146a (miR146a) in dendritic cells (DCs), whereas BCG and H37RvΔRD1 profoundly induced its expression in DCs. Consistent with these findings, ESAT-6 had no effect on miR146a expression in uninfected DCs, but dramatically inhibited its upregulation in BCG-infected or LPS-treated DCs. Collectively, our findings indicate that, in addition to Th1 immunity induced by BCG, RD1/ESAT-6-induced Th17 immune responses are essential for optimal vaccine efficacy.

Author Summary

Tuberculosis is a global health problem, with one-third of the global population infected with tubercle bacteria. Numerous studies have shown that Th1 cell responses are indispensable for protective immunity against TB. However, while the vaccine strain BCG induces sufficient Th1 cell response, this response does not appear to be sufficient for immune protection in many individuals. Here, we provide evidence for the first time that Th17 cell responses in the lung play a critical role for enhanced protection against TB. Surprisingly, the virulent M. tb strain H37Rv induced Th17 cell responses in the lung. Consequently, antibiotic-treated animals that were previously infected with H37Rv, as compared with similarly treated BCG-infected mice, generated improved protective immune responses against infection with virulent M. tb. We also provide evidence that the ESAT-6 protein, which is absent in BCG but present in H37Rv, induces IL-6 and TGF-β in dendritic cells in a TLR-2 and MyD88-dependent manner, which generates an environment that is conducive for the differentiation of Th17 cells in the lung. Our findings indicate that, in addition to Th1 cells, Th17 cells play a critical role in conferring optimal protection against TB.