HOST T-CELL PRIMARY ALLOSENSITIZATION TO MHC CLASS-I-EXPRESSING AND CLASS-II-EXPRESSING HUMAN CARDIAC MYOCYTES REQUIRES THE PRESENCE OF A 2ND SIGNAL

Normal FHCMs, or transformed cell lines derived from FHCMs, such as W1 , even after induction of MHC antigens by pretreatment with IFN-gamma, failed to induce proliferation of allogeneic human PBMCs in vitro. To test the hypothesis that antigen-specific T-cell activation and proli feration require not only the binding of the TCR with its ligand, the MHC molecule, but also a second signal that involves the interaction o f T-cell surface molecules with their natural ligands on the stimulati ng cells, a mAb against CD28 was used. Cocultures of allogeneic PBMCs with IFN-gamma-pretreated irradiated FHCMs or the W1 cell line in micr otiter plates containing immobilized anti-CD28 mAb induced marked stim ulator cell MHC class-II-specific proliferative responses. The W1 cell line and FHCMs failed to express detectable levels of the BB1/B7 mole cule (the natural ligand for CD28) as determined by flow microfluorome try or mRNA levels coding for BB1/B7 as determined by RT-PCR. These da ta suggest that one of the probable reasons for the failure of MHC-exp ressing cardiac myocytes to induce allogeneic activation is the absenc e of costimulatory signals.