METHYLATION STATUS AND TRANSCRIPTIONAL EXPRESSION OF THE MHC CLASS-I LOCI IN HUMAN TROPHOBLAST CELLS FROM TERM PLACENTA

Of the various molecular regulatory mechanisms that may be used by hum an trophoblast cells to down-regulate expression of HLA class I genes, we chose to investigate the methylation of DNA, generally associated with inhibition of transcription. We analyzed the methylation status o f different HLA class I loci in villous and extravillous cytotrophobla st cells and in vitro-differentiated syncytiotrophoblast, purified fro m human term placenta, as well as in the human trophoblast-derived JAR and JEG-3 cell lines. We then compared methylation status and transcr iptional activity. An inverse relationship was established between JAR and JEG-3: HLA-A, -B, and -G are methylated and repressed in JAR, whe reas in JEG-3, HLA-A is methylated and repressed but HLA-B and -G are partially methylated and transcribed. HLA-E is unmethylated and transc ribed in both cell lines. Apart from HLA-E, which is always unmethylat ed and transcribed, no such relationship exists for the other class I loci in trophoblast cells. Whereas nonclassical HLA-C and classical HL A-A and -B class I genes are undermethylated in both cytotrophoblast a nd syncytiotrophoblast, they are clearly transcribed in the former but minimally transcribed in the latter subpopulation. Thus, the down-reg ulation of class I gene expression in the in vitro-differentiated sync ytiotrophoblast is unlikely to be caused by DNA methylation. Furthermo re, there is no detectable expression of any class I molecule at the c ell surface of either trophoblast cell subpopulation, suggesting a neg ative control on translation and/or on the secretory pathway to the pl asma membrane.