During infection of macrophages, prolonged signaling by Mycobacterium tuberculosis (Mtb) or its 19-kDa lipoprotein (LpqH; Rv3763) inhibits IFN-γ-induced expression of several immune function genes, including class II transactivator (CIITA), which regulates class II MHC. Mtb does not inhibit early IFN-γ signaling events, e.g., Stat1α activation. This study analyzed downstream mechanisms that regulate the transcription of MHC2TA, the gene encoding CIITA. Chromatin immunoprecipitation showed that IFN-γ induced acetylation of histones H3 and H4 at the CIITA promoter IV (pIV). In contrast, IFN-γ-dependent histone acetylation at CIITA pIV was inhibited by Mtb or 19-kDa lipoprotein. Mtb 19-kDa lipoprotein also inhibited IFN-γ-dependent recruitment of Brahma-related gene 1, a chromatin remodeling protein, to CIITA pIV. Mtb 19-kDa lipoprotein did not inhibit histone acetylation in TLR2−/− macrophages. Furthermore, 19-kDa lipoprotein did not inhibit CIITA expression or IFN-γ-dependent histone acetylation of CIITA pIV in macrophages treated with inhibitors of MAPKs p38 or ERK. Thus, CIITA expression was inhibited by TLR2-induced MAPK signaling that caused histone hypoacetylation at CIITA pIV and suppression of CIITA transcription. Chromatin remodeling at MHC2TA is a novel target of inhibition by Mtb. These mechanisms may diminish class II MHC expression by infected macrophages, contributing to immune evasion by Mtb.

Infection of macrophages with

Sequence analysis of the genome of the strict intracellular pathogen Chlamydia trachomatis revealed the presence of a SET domain containing protein, proteins that primarily function as histone methyltransferases. In these studies, we demonstrated secretion of this protein via a type III secretion mechanism. During infection, the protein is translocated to the host cell nucleus and associates with chromatin. We therefore named the protein nuclear effector (NUE). Expression of NUE in mammalian cells by transfection reconstituted nuclear targeting and chromatin association. In vitro methylation assays confirmed NUE is a histone methyltransferase that targets histones H2B, H3 and H4 and itself (automethylation). Mutants deficient in automethylation demonstrated diminished activity towards histones suggesting automethylation functions to enhance enzymatic activity. Thus, NUE is secreted by Chlamydia, translocates to the host cell nucleus and has enzymatic activity towards eukaryotic substrates. This work is the first description of a bacterial effector that directly targets mammalian histones.

Woodbury disclosed that they are a government support contractor employed by Aveshka Inc., which receives funds from the U.S. government under contract to provide technical and programmatic support for HHS-BARDA. Dr. MaCurdy's institution received funding and research support from Centers for Medicare & Medicaid Services.