Regulation of MHC class II gene expression at the fetal-maternal interface

MHC class II molecules function in the initiation of an immune response by presenting antigens to CD4+ T helper cells. Class II molecules are constitutively expressed on antigen presenting cells (APCs), but expression can be induced on many cell types by exposure to IFN-γ. One cell type that lacks MHC class II expression, either constitutively or after IFN-γ exposure, is the trophoblast. Trophoblast cells form the embryonic portion of the placenta during pregnancy. Inhibition of MHC class II gene expression in these cells is thought to be one mechanism of maternal tolerance to the feto-placental unit, which expresses both maternally and paternally derived genes.; MHC class II genes are coordinately regulated at the transcriptional level. The class II transactivator (CIITA) functions as a master regulator of class II gene expression. Expression of CIITA is constitutive in APCs and is induced by IFN-γ. CIITA expression is under the control of four separate promoters. Promoter III primarily directs constitutive expression, whereas promoter IV drives IFN-γ-inducible expression. The studies described in this thesis examine the mechanism of MHC class II gene suppression in trophoblast cells and its importance for fetal survival.; The lack of MHC class II gene expression following exposure to IFN-γ was found to result from inhibition of induction of CIITA expression. Transfection of CIITA into two trophoblast-derived cell lines restored class II gene expression. This lack of expression of CIITA is not due to a lesion in the IFN-γ signal transduction pathway, but rather to hypermethylation of CIITA promoter IV. Treatment of the cell lines with a methylation inhibitor restored CIITA-inducibility by IFN-γ. Bisulphite sequencing of the sites of methylation in promoter IV showed that several CpG dinucleotides are methylated in the trophoblast cell lines, but not in CIITA-expressing cells. This methylation was shown to correlate with histone hypoacetylation and a lack of transcription factor binding. Development of a mouse model in which CIITA expression can be induced during pregnancy will allow the in vivo importance of MHC class II gene suppression in the placenta to be determined.