REGULATION OF MHC CLASS II EXPRESSION IN TROPHOBLAST

滋养层细胞是独特的，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物 (MHC) II 类抗原。 滋养层细胞表面缺乏 MHC 11 类抗原被认为对于妊娠期间胎儿的维持至关重要。 因此，哺乳动物的成功繁殖可能需要严格抑制滋养层细胞中的 MHC II 类基因表达。 II 类抗原表达的调节是在具有血绒质胎盘物种的滋养层细胞系的转录水平上介导的。 对小鼠 IAalpha 启动子的突变分析发现了一种新型滋养层特异性负调控元件 (IAalphaNRE)，其功能是抑制 II 类转录。 随后的工作表明，人类、小鼠和大鼠的滋养层细胞系均缺乏 II 类反式激活因子 (CIITA)，这是一种反式转录因子，对于其他细胞类型的组成型转录和 IFNγ 诱导型转录都至关重要。用 CIITA 表达载体转染人和大鼠滋养层细胞导致细胞表面 II 类抗原表达。 基于这些数据，我的工作假设是，II类基因的表达可能受到滋养层细胞中两种互补机制的调节：1）CIITA合成的抑制，以及2）II类基因启动子中负调控元件的活性。 此外，我提出 CIITA 基因转录可能在滋养层细胞中被仅在滋养层和早期胚胎细胞中发挥作用的顺式作用序列所抑制。 我的第一个应用的具体目标是：1）鉴定滋养层细胞和胚胎癌（EC）细胞中抑制 CIITA 基因转录所需的顺式作用 DNA 序列和反式作用因子； 2)确定IAalphaNREs在滋养层细胞中抑制II类基因转录的机制； 3)检查新鲜人滋养层中调节CIITA基因转录的转录因子的表达。 这些研究将为了解滋养层表达 II 类抗原对成功妊娠的生物学影响迈出第一步。

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.