BLOCKADE OF MULTIPLE COSTIMULATORY RECEPTORS INDUCES HYPORESPONSIVENESS - INHIBITION OF CD2 PLUS CD28 PATHWAYS

T-lymphocyte activation requires engagement of the T cell receptor wit h antigen-major histocompatibility complex, and simultaneous ligation of costimulatory pathways via the lymphocyte receptors CD2 and CD28/CT LA4. Anti-CD2 monoclonal antibody (mAb) blocks the interaction of the antigen-presenting cell receptor CD48 with its ligand CD2, whereas CTL A4Ig binds with high affinity to the antigen-presenting cell ligands B 7-1 and B7-2, blocking their interaction with CD28/CTLA4. We tested th e immunosuppressive effects of simultaneously blocking both costimulat ory pathways, Using donor C57BL/6J (H2(b)) hearts transplanted to CBA/ J (H2(k)) recipients, anti-CDS mAb plus CTLA4Ig administered at the ti me of transplantation prolonged cardiac allograft mean survival time t o >120 days compared with untreated controls (12.2+/-0.5 days, p<0.01) , anti-CD2 mAb alone (24.8+.-1.0 days, P<0.01), or CTLA4Ig alone (55.0 +/-2.0 days, P<0.01). Retransplantation of these recipients with donor -specific and third-party grafts demonstrated that hyporesponsiveness and tolerance were achieved. In vitro stimulation of lymphocytes from tolerant recipients with donor-specific alloantigen resulted in normal cytotoxic T lymphocyte and mixed lymphocyte reaction responses, showi ng that clonal deletion or anergy did not occur, but that graft adapta tion or suppression likely helped to maintain long-term graft survival . In vitro combinations of anti-CDS mAb and CTLA4Ig suppressed the gen eration of allogeneic cytotoxic T lymphocytes (58%) and the mixed lymp hocyte reaction (36%); CTLA4Ig was more effective in this regard and t he two agents were not synergistic, Anti-CDB mAb and CTLA4Ig suppresse d mitogen-driven proliferation in differential fashions, suggesting th at they affected independent signaling pathways. Anti-CD2 mAb and CTLA 4Ig also inhibited interleukin (IL)-2, IL-4, and IL-2 receptor (CD25). These data indicate that anti-CD2 mAb plus CTLA4Ig induces hyporespon siveness and tolerance. The mechanism is likely related to the initial disruption of independent pathways of T-lymphocyte activation leading to antigen-specific long-term graft survival.