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Department of Microbiology and Immunology, Emory University School of Medicine, Atlanta, GA 30322
B cell to plasma cell maturation is marked by the loss of MHC class II expression. This loss is due to the silencing of the MHC class II transcriptional coactivator CIITA. In this study, experiments to identify the molecular mechanism responsible for CIITA silencing were conducted. CIITA is expressed from four promoters in humans, of which promoter III (pIII) controls the majority of B cell-mediated expression. Chromatin immunoprecipitation assays were used to establish the histone code for pIII and determine the differences between B cells and plasma cells. Specific histone modifications associated with accessible promoters and transcriptionally active genes were observed at pIII in B cells but not in plasma cells. A reciprocal exchange of histone H3 lysine 9 acetylation to methylation was also observed between B cells and plasma cells. The lack of histone acetylation correlated with an absence of transcription factor binding to pIII, particularly that of Sp1, PU.1, CREB, and E47. Intriguingly, changes in chromatin architecture of the 13-kb region encompassing all CIITA promoters showed a remarkable deficit in histone H3 and H4 acetylation in plasma cells, suggesting that the mechanism of silencing is global. When primary B cells were differentiated ex vivo, most of the histone marks associated with pIII activation and expression were lost within 24 h. The results demonstrate that CIITA silencing occurs by controlling chromatin accessibility through a multistep mechanism that includes the loss of histone acetylation and transcription factor binding, and the acquisition of repressive histone methylation marks.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
1 This work was supported by a National Research Service Award F31 GM 20675 (to M.G.) and Grant AI34000 (to J.M.B.) from the National Institutes of Health.
2 Address correspondence and reprint requests to Dr. Jeremy M. Boss, Department of Microbiology and Immunology, Room 3131, Rollins Research Center, Emory University School of Medicine, 1510 Clifton Road, Atlanta, GA 30322. E-mail address: boss{at}microbio.emory.edu
3 Abbreviations used in this paper: MHC-II, MHC class II; Blimp, B late-induced maturation protein; ChIP, chromatin immunoprecipitation; HDAC, histone deacetylase; RFX, regulatory factor X; ARE, activation response element.
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