Early secreted antigen ESAT-6 of Mycobacterium tuberculosis promotes protective T helper 17 cell responses in a toll-like receptor-2-dependent manner.

Samit Chatterjee,Luc Van Kaer,Ved Prakash Dwivedi,Pawan Sharma,Imran Siddiqui,Gobardhan Das,Debprasad Chattopadhyay,Yogesh Singh,William R Bishai Jr

doi:10.1371/journal.ppat.1002378

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.

Highlights

Tuberculosis (TB) remains a major health problem, with an estimated one third of the world’s population infected with Mycobacterium tuberculosis, the causative agent of TB, resulting in,3 million deaths annually
Antibiotic-treated animals that were previously infected with H37Rv, as compared with treated BCGinfected mice, generated improved protective immune responses against infection with virulent M. tb
We provide evidence that the Early secreted antigenic target protein 6 (ESAT-6) protein, which is absent in Bacillus Calmette-Guerin (BCG) but present in H37Rv, induces IL-6 and TGF-b in dendritic cells in a toll-like receptor-2 (TLR-2) and MyD88-dependent manner, which generates an environment that is conducive for the differentiation of Th17 cells in the lung

Summary

Introduction

Tuberculosis (TB) remains a major health problem, with an estimated one third of the world’s population infected with Mycobacterium tuberculosis, the causative agent of TB, resulting in ,3 million deaths annually. Secreted antigenic target protein 6 (ESAT-6) is one of the most prominent antigens expressed by Mycobacterium tuberculosis (M. tb), but not by BCG [6,7]. ESAT-6specific T cells are frequently found in TB patients as well as in infected animals [8,9,10]. T cell receptor transgenic T cells specific for ESAT-6 exhibit significant protection against TB [12]. Consistent with this, a recombinant BCG strain that contains region of difference 1 (RD1), which includes ESAT-6, exhibited improved protection against TB [13]. The mechanism by which ESAT-6 vaccination induces protective immune responses against TB remains to be investigated

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