Holtz Renae, Choi Jason C, Petroff Margaret G, Piskurich Janet F, Murphy Shawn P
Department of Immunology, Roswell Park Cancer Institute, Buffalo, New York 14263, USA.
Biol Reprod. 2003 Sep;69(3):915-24. doi: 10.1095/biolreprod.103.017103. Epub 2003 May 14.
Trophoblast cells are unique because they do not express major histocompatibility complex (MHC) class II antigens, either constitutively or after exposure to interferon-gamma (IFN-gamma). The absence of MHC class II antigens on trophoblasts is thought to play a critical role in preventing rejection of the fetus by the maternal immune system. The inability of trophoblasts to express MHC class II genes is primarily due to lack of the class II transactivator (CIITA), a transacting factor that is required for constitutive and IFN-gamma-inducible MHC class II transcription. We, therefore, investigated the silencing of CIITA expression in trophoblasts. In transient transfection assays, transcription from the IFN-gamma-responsive CIITA type IV promoter was upregulated by IFN-gamma in trophoblasts, which suggests that CIITA is silenced by an epigenetic mechanism in these cells. Polymerase chain reaction analysis demonstrated that the CIITA type IV promoter is methylated in both the human choriocarcinoma cell lines JEG-3 and Jar and in 2fTGH fibrosarcoma cells, which are IFN-gamma inducible for CIITA. Conversely, methylation of the CIITA type IV promoter was not observed in human primary cytotrophoblasts isolated from term placentae or in mouse or rat trophoblast cell lines. Simultaneous treatment with IFN-gamma and the histone deacetylase inhibitor trichostatin A weakly activated CIITA transcription in mouse trophoblasts. Stable hybrids between human choriocarcinoma and fibrosarcoma cells and between mouse trophoblasts and fibroblasts expressed CIITA following treatment with IFN-gamma. These results suggest that silencing of CIITA transcription is recessive in trophoblasts and involves an epigenetic mechanism other than promoter methylation. The fact that CIITA is expressed in the stable hybrids implies that trophoblasts may be missing a factor that regulates chromatin structure at the CIITA promoter.
滋养层细胞很独特，因为它们无论是组成型表达还是在接触γ干扰素（IFN-γ）后，都不表达主要组织相容性复合体（MHC）II类抗原。滋养层细胞上缺乏MHC II类抗原被认为在防止母体免疫系统排斥胎儿方面起关键作用。滋养层细胞无法表达MHC II类基因主要是由于缺乏II类反式激活因子（CIITA），CIITA是一种反式作用因子，对于组成型和IFN-γ诱导的MHC II类转录是必需的。因此，我们研究了滋养层细胞中CIITA表达的沉默情况。在瞬时转染实验中，滋养层细胞中IFN-γ可上调来自IFN-γ应答性CIITA IV型启动子的转录，这表明CIITA在这些细胞中是通过表观遗传机制沉默的。聚合酶链反应分析表明，CIITA IV型启动子在人绒毛膜癌细胞系JEG-3和Jar以及在对CIITA有IFN-γ诱导性的2fTGH纤维肉瘤细胞中均发生甲基化。相反，在从足月胎盘分离的人原代细胞滋养层细胞或小鼠或大鼠滋养层细胞系中未观察到CIITA IV型启动子的甲基化。IFN-γ和组蛋白脱乙酰酶抑制剂曲古抑菌素A同时处理可在小鼠滋养层细胞中微弱激活CIITA转录。人绒毛膜癌细胞与纤维肉瘤细胞之间以及小鼠滋养层细胞与成纤维细胞之间的稳定杂种在用IFN-γ处理后表达CIITA。这些结果表明，CIITA转录的沉默在滋养层细胞中是隐性的，并且涉及除启动子甲基化以外的表观遗传机制。CIITA在稳定杂种中表达这一事实意味着滋养层细胞可能缺少一种调节CIITA启动子处染色质结构的因子。
