REGULATION OF TL ANTIGEN EXPRESSION - ANALYSIS OF THE T18(D) PROMOTERREGION AND RESPONSES TO IFN-GAMMA

Type I (alpha/beta) and type II (gamma) IFN enhance MHC class I gene e xpression through an IFN-responsive element (IRE) present in the 5' fl anking region of the class I-a genes. Comparison of the 5' sequences b etween classical class I-a genes and T region class I-b genes reveals little homology except for presence of a potential IRE. We have found that cell surface expression of thymus leukemia Ag (TL) was up-regulat ed by IFN-gamma to a greater extent than H-2K,D in all TL+ T cell line s tested. In contrast, IFN-alpha/beta, which significantly increased H -2K and H-2D Ag expression, had only minor effects on TL expression. R esting peripheral T cells, which were considered to be TL- from previo us studies, were found to express TL at a low level as determined by f low cytometry, immunoprecipitation, as well as polymerase chain reacti on; the level of expression also could be elevated by IFN-gamma. To ex amine the control of TL gene transcription and its regulation by IFN-g amma, varying lengths of the T18d 5' flanking region were analyzed in chloramphenicol acetyl transferase assays. By deletion analysis, promo ter activity and IFN-gamma responsiveness were localized to an 86-bp f ragment that contains the IRE. Both responses were localized further t o a 32-bp fragment that contained the IRE at its 3' end. RNase protect ion assays revealed two major transcription initiation sites, one imme diately 5' of the IRE and another approximately 60 bp downstream. Furt hermore, polymerase chain reaction analysis of mRNA from resting T cel ls, thymocytes, and T cell tumor lines confirmed the RNase protection data. Thus, transcription of T18d initiates much further upstream than the classical class I genes, can utilize an unusual promoter element, and can be elevated by IFN-gamma.