Functional characterization of a novel anti-B7 monoclonal antibody

For optimal activation of T cells, binding of their T cell receptor to antigenic peptides in the context of major histocompatibility complex molecules on antigen-presenting cells (APC) is not sufficient. Accessory signals, provided by accessory cells, are needed to induce proliferation and clonal expansion of normal T cells. It has been shown previously that the B7 molecule, present on the cell surface of activated APC, can provide the second signal by binding to the CD28 molecule on T cells. Here we describe a novel anti-B7 (mAb), B7-24. This mAb binds to a functionally important epitope of the B7 molecule. Fab fragments of B7-24 can almost completely block anti-CD3-induced, B7-dependent T cell proliferation when tested in a model system where purified T cells are co-cultured with 3T6 cells expressing the human Fc gamma RII and human B7, in the presence of anti-CD3 mAb. In contrast, mAb B7-24 is not able to inhibit T cell proliferation in primary mixed lymphocyte reactions where purified T cells are co-cultured with Epstein-Barr virus-transformed B cells. These findings suggest that other cell surface molecules allow for maximal proliferation of T cells in mixed lymphocyte reactions, even when the interaction between B7 and CD28 is blocked by B7-24.