HLA class I molecules bind peptides derived from proteins degraded in the cytoplasm and display them for surveillance by the immune system. The recognition of HLA class I molecules by natural killer (NK) cells generally inhibits the lytic process. To investigate the role of peptides in the interaction between HLA class I molecules and NK receptors, individual peptides bound to HLA-Cw*0304 were isolated and sequenced by tandem mass spectrometry. These peptides ranged in length from 8 to 11 residues and shared an alanine at position 2 and a C-terminal leucine. We found that, in the absence of exogenous peptides, the HLA-Cw*0304 transfectants were killed at levels comparable to untransfected RMA-S cells whereas protection from lysis required both HLA-Cw*0304 heavy chain expression and an exogenously added HLA-Cw*0304-binding peptide. Importantly, not only were HLA-Cw*0304-binding peptides required for protection, but the ability of individual peptides to provide protection differed widely. These studies indicate that the ability to distinguish between subsets of peptides may be a general feature of HLA class I recognition by NK cells. We screened an expression cDNA library with a radiolabeled C/EBP ( fusion protein and isolated three independent cDNAs encoding ATF-2, a bZIP protein that binds cAMP response elements (CRE). This interaction requires the respective bZIP domains, which form a typical bZIP heterodimer with altered DNA binding selectivity. C/EBP( and ATF-2 homodimers bind CRE sites, but ATF-2C/EBP heterodimers do not. Heterodimers bind an asymmetric sequence composed of one consensus half-site for each monomer, and may thus have a unique regulatory function. As predicted, co-transfection of ATF-2 with C/EBP results in a decreased activation of transcription driven from consensus C/EBP-binding sites. Both factors are expressed in liver, where immunoprecipitation experiments showed that ATF-2 co-precipitates with C/EBP. These results are consistent with the interpretation that C/EPB( and ATF-2 can associate in vivo. Moreover, the formation of ATF-2:C/EBP heterodimers suggests that cross-family dimerization with ATF-2 may be a general property for C/EBP family proteins. In a model of transplantation rejection, we tested whether a graft manipulated to secrete an immunomodulator could protect itself from immune destruction, thus waiving the need for administration of exogenous immunosuppressants to the recipient. An insulinoma cell line, NIT, having the nonobese diabetic (NOD) genotype but also expressing the SV40 large T antigen, was transfected with CTLA4Ig in an attempt to block the CD28/B7 costimulatory pathway between antigen-presenting cells and T lymphocytes near the site of the graft. The SV40 T antigen is potent at inducing graft rejection. NIT.CTLA4Ig and control transfectants were transplanted subcutaneously into young NOD mice to determine whether CTLA4Ig secretion would abet the survival of the insulinoma graft. The results indicated that blockade of the B7 costimulation pathway by graft manipulation can contribute to transplantation success.