Mycobacterium tuberculosis EsxL inhibits MHC-II expression by promoting hypermethylation in class-II transactivator loci in macrophages.

Mycobacterium tuberculosis is known to modulate the host immune responses to facilitate its persistence inside the host cells. One of the key mechanisms includes repression of class-II transactivator (CIITA) and MHC-II expression in infected macrophages. However, the precise mechanism of CIITA and MHC-II down-regulation is not well studied. M. tuberculosis 6-kDa early secretory antigenic target (ESAT-6) is a known potent virulence and antigenic determinant. The M. tuberculosis genome encodes 23 such ESAT-6 family proteins. We herein report that M. tuberculosis and M. bovis bacillus Calmette-Guérin infection down-regulated the expression of CIITA/MHC-II by inducing hypermethylation in histone H3 lysine 9 (H3K9me2/3). Further, we showed that M. tuberculosis ESAT-6 family protein EsxL, encoded by Rv1198, is responsible for the down-regulation of CIITA/MHC-II by inducing H3K9me2/3. We further report that M. tuberculosis esxL induced the expression of nitric-oxide synthase, NO production, and p38 MAPK pathway, which in turn was responsible for the increased H3K9me2/3 in CIITA via up-regulation of euchromatic histone-lysine N-methyltransferase 2 (G9a). In contrast, inhibition of nitric-oxide synthase, p38 MAPK, and G9a abrogated H3K9me2/3, resulting in increased CIITA expression. A chromatin immunoprecipitation assay confirmed that hypermethylation at the promoter IV region of CIITA is mainly responsible for CIITA down-regulation and subsequent antigen presentation. We found that co-culture of macrophages infected with esxL-expressing M. smegmatis and mouse splenocytes led to down-regulation of IL-2, a key cytokine involved in T-cell proliferation. In summary, we demonstrate that M. tuberculosis EsxL inhibits antigen presentation by enhancing H3K9me2/3 at the CIITA promoter, thereby repressing its expression through NO and p38 MAPK activation.