Triggering T cells by otherwise inert hybrid anti-CD3/antitumor antibodies requires encounter with the specific target cell.

We used a purified bispecific antibody (Ab) against CD3 and an ovarian carcinoma (OVCA) antigen to ask whether the binding of a monovalent ligand to CD3 can induce triggering of T cells. In the presence of OVCA cells, this Ab bridges the CD3 complex to the target cell and triggers proliferation and cytotoxicity in T cells. In the absence of target cells, however, this monovalent Ab, even when bound to T cells at high levels, fails to induce any increase in cytosolic Ca2+, nor does it induce responsiveness to IL-2 or modulation of the CD3 complex. Because it is inert when bound monovalently, this hybrid Ab can be used to arm in vitro CTL clones, which then retain the capacity to kill the specific tumor for up to 2 d.