A NON-ACTIVATING HUMANIZED ANTI-CD3 MONOCLONAL-ANTIBODY RETAINS IMMUNOSUPPRESSIVE PROPERTIES IN-VIVO

OKT3, a mouse anti-human CD3 mAb, is a potent immunosuppressive agent used in clinical transplantation to prevent or treat allograft rejecti on. Associated with this therapy is the systemic release of several cy tokines that result in a series of adverse side effects. This release of cytokines is dependent on the cross-linking mediated by OKT3 betwee n T cells and the Fc gamma R-bearing cells. To generate an anti-human CD3 mAb with reduced activating properties as compared with OKT3, we h ave transferred the complementary determining regions of OKT3 onto hum an IgG frameworks and then performed point mutations that reduce the a ffinity of the ''humanized'' anti-CD3 mAbs for Fc gamma Rs. Initial, i n vitro, studies showed that whereas OKT3 and the parental humanized a nti-CD3 mAbs activated T cells similarly, a humanized Fc variant faile d to do so. Both the Fc variant and the activating anti-CD3 mAbs induc ed comparable modulation of the TCR and suppression of cytolytic T cel l activity, in vitro. In the current study, we exploited an experiment al model in which human splenocytes from cadaveric organ donors were i noculated into severe combined immunodeficient mice (hu-SPL-SCID mice) to test the activating and immunosuppressive properties of these anti -human CD3 mAbs in vivo. Unlike injection of OKT3 or of the parental h umanized mAb, administration of the Fc variant did not result in T cel l activation in vivo, as evidenced by the lack of induction of surface markers of activation, and of systemic human cytokines, including IL- 2. Importantly, similar prolongation of human allograft survival was a chieved with all anti-CD3 mAbs, indicating that the nonactivating anti -CD3 mAbs retained significant immunosuppressive properties in vivo. T hus, the use of an Fc variant in clinical transplantation should resul t in fewer side effects than observed with OKT3, while maintaining its clinical efficacy.