Expression and function of mouse and human CD8 (mCD8 and hCD8) α chain molecules In mouse T cell hybridomas were analyzed. The expression of cytolytic T lymphocyte-derived CD8 molecules was supprmsd In hybridomas established by fusing the BW5147 thymoma to a CD8+ cytolytic T lymphocyte clone, while expression of CD4 remained intact In BW × CD4+ helper T cell hybridomas. However, hybridomas established by tusing a cytolytlc T cell clone wtth BW5147 cell lines, transfected wlth either the mCD8α or hCD8α chaln, expressed the T cell-derived mCD8β chain as a CD8 heterodimer (mCD8α/mCD8α or hCD8α/mCD8β). These data suggest that negative regulatory mechanlsms for the mCD8α gene In BW thymoma failed to suppress mCD8β gene expression, indicating different regulatory mechanisms for the tightly linked mCDh and mCD8β genes. Analysis of the antlgen reactivity of the hybridomas revealed that the human CDBα chain felled to increase the mouse T cell receptor - class I MHC Interaction, even as a heterodimeric form with mCD8β8 molecules. However, both the human mCD8α homodimer and the heterodimeric form with mCD88 were found to be capable of suppressing the class 11-restricted T cell response.