Cell-type specificity of interferon-γ-mediated HLA class I gene transcription in human hematopoietic tumor cells

Major histocompatibility complex class I gene expression plays a central role in cellular immunity and tumor surveillance. A substantial proportion of spontaneous tumors are class I-deficient and numerous experiments have suggested that alterations in class I expression may alter oncogenicity and, as a result, have potential therapeutic impact. Interferons (IFNs) are able to upregulate class I expression by mechanisms that remain to be elucidated, but which appear to be IFN- and cell-type specific. We have characterized in detail the in vivo class I transcriptional response to IFN-γ in two human hematopoietic tumor cell lines, the class I-deficient K562 cell line and the class I-positive Ramos cell line. In each, IFN-γ induces a rapid increase in class I transcription, which is sustained in Ramos cells, but transient in K562 cells. In each, stimulation by IFIM-γ is dependent on ongoing protein synthesis, suggesting the requirement for production of a "primary response" protein. These data suggest that more than one type of IFN-γ-induced signal is operative in the transcriptional response to IFN-γ. Cycloheximide alone is also capable of inducing a rapid increase in class I transcription in both cell types, suggesting that constitutive attenuation of class I transcription may be a common phenomenon, and that IFN-γ may act, in part, by interfering with such attenuation.