To study the regulation of MHC class I gene expression during embryonic development, we have characterized a number of clonal cell lines derived from somite stage mouse embryos that were established with or without infection by several transforming retroviruses in combination with murine leukemia viruses. Unlike embryonal carcinoma (EC) cells that have been used as a model for early embryos, the cell lines derived from somite stage embryos are negative for stage specific embryonic Ag-1 and do not appear to differentiate after retinoic acid treatment. Morphology varies from clone to clone and is distinct from that of F9 and other EC cells. In agreement with previous findings in in vivo embryos, expression of surface MHC class I antigen in 57 new clones is either undetectable or low (with variability). All of the clones respond to the addition of interferons and express MHC class I antigens at high levels, but the kinetics of mRNA accumulation vary considerably. To examine the basis of the generally low or absent MHC class I gene expression in these cells, we tested promoter activity of a MHC class I gene by CAT assay after transient DNA transfection. Regardless of the basal levels of mRNA or surface Ag, CAT activity directed by various portions of the 5' flanking region of the MHC class I gene was uniformly low. The cells showed neither the negative nor the positive regulation of MHC class I genes that had been noted respectively for EC cells and for cells expressing the Ag constitutively. The pattern seen in the new cell lines suggests that there is an intermediate stage in the developmental regulation of MHC class I gene expression that may operate during the middle to late stage of fetal development.

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