Distinct, gene-specific mechanisms mediate inhibition of macrophage responses to interferon gamma by Mycobacterium tuberculosis

Mycobacterium tuberculosis is able to survive the immune response due in part to inhibition of macrophage responses to IFN-gamma. Previous studies have shown that M. tuberculosis, mycobacterial lipoproteins, and the cell wall components phosphatidylinositol mannan and peptidoglycan can inhibit IFN-gamma-induced expression of CIITA and other genes involved in antigen presentation through TLR2-dependent and --independent mechanisms. To specifically address the role of TLR2 signaling in mediating this inhibition, macrophages were stimulated with the specific TLR2/1 ligand Pam3CSK4 and responses to IFN-gamma were assessed. TLR2 stimulation prior to IFN-gamma blocked expression of CIITA and the IFN-gamma-inducible chemokine CXCL11, but not NOS2. STAT1 binding to the promoters of the inhibited genes was unaffected by TLR2 stimulation, illustrating a gene-selective mechanism. Transcriptional inhibition also required new protein synthesis. TLR2 stimulation prevented RNA polymerase II from binding to the transcriptional start sites of the CXCL11 and CIITA promoters, and TATA binding protein was unable to bind the TATA box of the CXCL11 promoter, suggesting that assembly of the transcriptional preinitiation complex was hindered. IFN-gamma-induced histone acetylation at the CIITA promoter was reduced upon prior TLR2 stimulation, but was unaffected at the CXCL11 promoter. Additionally, NF-kappaB signaling was required for inhibition of CXCL11, but not CIITA. Similar results were seen when inhibition was mediated by live Mycobacterium tuberculosis. Taken together, these results indicate that TLR2-dependent inhibition of IFN-gamma-induced gene expression is mediated by distinct, gene-specific mechanisms that disrupt binding of the transcriptional machinery to the promoters.