Raphael Hirsch

Anti-CD3 F(ab')2 fragments are immunosuppressive in vivo without evoking either the strong humoral response or morbidity associated with whole mAb.

The use of anti-CD3 monoclonal antibodies to treat organ allograft rejection has been complicated by (1) the morbidity associated with the initial dose, (2) the humoral response of the patients against the mAbs, and (3) the generalized immunosuppression induced by the mAbs. We investigated the potential of F(ab)2 fragments of the anti-murine-CD3 mAb, 145-2C11, to avoid these complications in the murine model. Both whole mAb and F(ab)2 fragments induced T cell depletion. However, injection of F(ab)2 fragments of anti-CD3 mAb did not cause T cell activation, and did not induce the morbidity and mortality observed following injection of whole mAb. The humoral response against the F(ab)2 fragments was significantly reduced compared with the response against the whole mAb. Furthermore, repeated administration of F(ab)2 fragments of anti-CD3 mAb resulted in prolongation of allogeneic skin graft survival superior to that seen following treatment with a single dose of whole mAb. Finally, while T cells from mice treated with whole mAb displayed profound suppression of in vitro CTL generation, T cells from mice treated with F(ab)2 fragments had significant in vitro CTL function. These results suggest that the use of F(ab)2 fragments of anti-CD3 mAb may offer significant advantages over whole mAb for the induction and maintenance of immunosuppression.

Suppression of the humoral response to anti-CD3 monoclonal antibody.

Anti-CD3 monoclonal antibodies are used clinically to treat organ allograft rejection. Their administration can result in reversal of rejection even in episodes resistant to other modes of therapy. A major limitation to their use has been the humoral response of the patients against the mAbs, resulting in loss of therapeutic efficacy. We have established an animal model for anti-CD3 treatment using the antimurine CD3 mAb, 145-2C11. Exposure of mice to this mAb, like exposure of humans to its antihuman analog OKT3, results in suppression of graft rejection but also stimulates a strong humoral response that abrogates the efficacy of further treatments. Administration of an additional dose of anti-CD3 mAb did not prolong skin graft survival--and, in some instances, resulted in a lethal anaphylactic reaction. In an attempt to suppress the humoral response against the anti-CD3 mAb, anti-CD4 mAb was administered prior to the anti-CD3 mAb treatment. Pretreatment of mice with anti-CD4 mAb (GK1.5) almost completely suppressed the humoral response to anti-CD3 mAb, and permitted readministration of the anti-CD3 mAB without loss of efficacy as assessed by prolongation of skin graft survival. The data suggest that the use of anti-CD4 mAb to suppress the humoral response against anti-CD3 mAb should be attempted clinically, as it might permit repeated courses of anti-CD3 administration, thus significantly improving the efficacy of these agents in the therapy of organ allograft rejection.

Immunopotentiation of anti-viral and anti-tumor immune responses using anti-T cell receptor antibodies and mitogens.

Although the immunosuppressive properties of anti-CD3 mAbs are now widely recognized, we have accumulated data characterizing the T cell activating properties of these antibodies. While in some situations these activating properties may be viewed as unwanted side-effects (for instance OKT3-mediated T cell activation may be responsible for some of the first dose toxicity seen with patients receiving OKT3 for suppression of allograft rejection), we have shown that anti-CD3 mAb therapy can augment host immune responses and provide protection against some tumors and viral infections. Importantly, this augmented response allows the development of long term, specific immunity. Because the immunosuppressive and activating properties of anti-CD3 mAbs are so closely overlapping, we have sought to identify other agents that are capable of activating T cell subsets selectively. We have found that SEB activates T cell subsets selectively in vivo and that this activation can be exploited to prevent the outgrowth of a malignant murine tumor. Studies currently in progress, including phenotypic and functional analysis of TILs and in vivo T cell subset depletions, should result in a more precise understanding of how SEB-induced T cell activation inhibits tumor growth.