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 FcγRII 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.