The role of T-cell subsets in the response to anti-CD3 monoclonal antibodies.

Administration of monoclonal antibodies (mAb) to CD3 elicits an immune response to the mAb and an acute toxic syndrome that has been attributed to the release of cytokines from activated T cells. To clarify the cellular basis for these effects, we used anti-lymphocyte mAb to deplete selected T-cell subsets from BALB/c mice prior to administration of anti-CD3. In our first series of experiments, anti-CD4 repeatedly blocked the immune response to anti-CD3, but did not prevent severe toxicity. This observation suggested that other T-cell subsets might contribute to anti-CD3 induced toxicity. Therefore, we treated mice with mAb to CD8 as well as mAb to CD4 prior to administration of anti-CD3. Despite depletion of > 95% of CD8+ and CD4+ T cells, toxicity was not suppressed. This finding cast doubt on the belief that toxicity is due to activation of either CD4+ or CD8+ T cells by anti-CD3. Therefore, we assessed the role of thymocytes (which are not deleted by the mAb) and gamma delta + T cells. Thymectomy did not prevent toxicity in CD4/CD8-depleted mice, demonstrating that thymocytes are not responsible for toxicity. Anti-alpha beta TCR mAb produced a toxic reaction similar to anti-CD3 whereas anti-gamma delta TCR mAb did not, suggesting that gamma delta+ T cells are not the source of toxic cytokines. In addition, we proved that anti-CD3-induced toxicity was not due to direct effects on macrophages or to other nonspecific factors associated with the hamster mAb. These findings imply that a few residual mature T cells in mice treated with mAb to CD4 and CD8 are sufficient for the full expression of the anti-CD3-induced toxic syndrome. To confirm that both CD4+ and CD8+ T cells can mediate toxicity, we showed that:(i) SCID mice, which normally do not develop anti-CD3-induced toxicity, can be rendered susceptible by reconstitution with purified CD4+ T cells; and (ii) CD4-knockout mice that lack CD4+ T cells but have normal CD8+ T cells are susceptible to anti-CD3-induced toxicity. These findings establish that both CD4+ and CD8+ cells contribute to the toxic effects of anti-CD3, and that relatively few cells are required to mediate the full effect.