The role of T cell activation in anti-CD3 x antitumor bispecific antibody therapy.

Anti-CD3 x antitumor bispecific Ab can retarget T cell mediated lysis in an MHC-independent fashion and prevent tumor growth in animal models. Two bispecific Ab preparations that differ in the presence or absence of Fc were compared in the 38C13 immunocompetent murine lymphoma model to evaluate how functional Fc and T cell activation impact on response to bispecific Ab therapy. Bispecific (bs) IgG contained functional Fc and was purified from hybrid-hybridoma Ab product. Bsf(ab')2 lacked functional Fc, and was genetically constructed using the leucine zipper technique. In vitro, bsF(ab')2 induced tumor cell lysis by activated T cells more effectively than bsIgG. However, bsF(ab')2 failed to induce T cell activation in the absence of tumor cells, and did so more slowly than bsIgG when tumor cells were present. In vivo, bsIgG induced nonspecific T cell activation whereas bsF(ab')2 did not. In therapy experiments, bsIgG inhibited tumor growth in mice although a single dose of bsF(ab')2 had minimal antitumor effect. BsF(ab')2 was capable of preventing tumor growth and improving survival when mice were also treated with T cell activators (IL-2 or staphylococcal enterotoxin B), or given repeated bsF(ab')2 doses. We conclude that therapeutic response to bispecific Ab was not dependent on functional Fc, but did require T cell activation. The use of bifunctional constructs that lack functional Fc therefore allows for separate manipulation of T cell retargeting and T cell activation and deserves further evaluation as a potential immunotherapy for malignancy.