REGULATION OF MHC CLASS II EXPRESSION IN TROPHOBLAST

滋养层细胞是独一无二的，因为它们在任何已知条件下都不表达主要组织相容性复合体(MHC)II类抗原。滋养层细胞表面缺乏MHC 11类抗原被认为是维持妊娠期间胎儿发育的关键。因此，哺乳动物的成功繁殖可能需要在滋养层细胞中严格抑制MHC II类基因的表达。II类抗原表达的调节是在转录水平上介导的，来自绒毛膜胎盘的滋养层细胞系。对小鼠IApha启动子的突变分析导致了一个新的滋养细胞特异性负调控元件(IAalphaNRE)的鉴定，该元件具有抑制II类转录的功能。随后的工作表明，人、小鼠和大鼠滋养层细胞株都缺乏II类反式激活因子(CIITA)，这是一种反式转录因子，在其他类型的细胞中对结构性转录和干扰素-γ诱导转录都是必不可少的。将CIITA表达载体导入人和大鼠滋养层细胞，可诱导细胞表面II类抗原的表达。基于这些数据，我的工作假设是，在滋养层细胞中，II类基因的表达可能受到两种互补机制(S)的调控：1)抑制CIITA合成，2)II类基因启动子中负调控元件的活性。此外，我认为CIITA基因在滋养层细胞中的转录可能受到只在滋养层细胞和早期胚胎细胞中起作用的顺式作用序列的抑制。这是我的第一个应用，其具体目的是：1)确定在滋养层和胚胎癌(EC)细胞中抑制CIITA基因转录所需的顺式作用DNA序列和调控因子；2)确定IAalphaNREs抑制滋养层细胞中II类基因转录的机制；以及3)检测调控新鲜人滋养层细胞中CIITA基因转录的转录因子的表达。这些研究将为了解滋养细胞表达第II类抗原对成功妊娠的生物学后果提供第一步。

Trophoblast cells are unique because they do not express major histocompatibility complex (MHC) class II antigens under any known conditions. The absence of MHC class 11 antigens from the surface of trophoblast cells is thought to be critical for maintenance of the fetus during pregnancy. Thus, successful reproduction of mammals may require that MHC class II gene expression be stringently repressed in trophoblast cells. The regulation of class II antigen expression is mediated at the level of transcription in trophoblast cell lines from species with hemochorial placentas. Mutation analysis of the mouse IAalpha promoter resulted in the identification of a novel trophoblast-specific negative regulatory element (IAalphaNRE) that functions to repress class II transcription. Subsequent work demonstrated that human, mouse and rat trophoblast cell lines all lack the class II transactivator (CIITA), a transacting transcription factor that is essential for both constitutive and IFNgamma-inducible transcription in other cell types. Transfection of human and rat trophoblast cells with CIITA expression vectors resulted in cell surface class II antigen expression. Based on these data, my working hypothesis is that expression of the class II genes may be regulated by two complementary mechanism(s) in trophoblast cells: 1) repression of CIITA synthesis, and 2) the activity of negative regulatory elements in the promoters of class II genes. Furthermore, I propose that CIITA gene transcription may be repressed in trophoblast cells by cis-acting sequences that function only in trophoblast and early embryonic cells. The specific aims of this, my first application, are to: 1) identify the cis-acting DNA sequences and transacting factors required for repression of CIITA gene transcription in trophoblast and embryonal carcinoma (EC) cells; 2) determine the mechanism by which the IAalphaNREs function to repress class II gene transcription in trophoblast cells; and 3) examine the expression of the transcription factors that regulate CIITA gene transcription in fresh human trophoblast. These studies will provide the first step in understanding the biological consequences of trophoblastic expression of class II antigens to successful pregnancy.