Autocrine costimulation: tumor-specific CD28-mediated costimulation of T cells by in situ production of a bifunctional B7-anti-CEA diabody fusion protein.

T cells require two distinct signals for optimal activation, an antigen-specific signal, provided by engagement of the T-cell receptor (TCR) and a second costimulatory signal mediated by engagement of CD28 with members of the B7 family. Although infiltrating T cells are present in many malignancies, they appear to be mostly anergic and do not attack the tumor, presumably because of the absence of activation and/or costimulatory signals. Here we describe a novel strategy for the in situ activation of tumor-specific T cells. We genetically modified T cells to secrete a bifunctional fusion protein, comprising the extracellular portion of B7-1 fused to an anticarcinoembryonic antigen (CEA) diabody. In coculture with CEA(+) tumor cells autocrine and paracrine secretion of B7-alphaCEA provided a potent tumor-specific costimulatory signal to T cells in combination with a recombinant alphaCEAxalphaCD3 bispecific diabody. B7-alphaCEA was also found to strongly enhance survival and tumor-specific activation of T cells expressing an anti-CEA TCRzeta-based chimeric immune receptor (CIR) both when expressed in cis by the T cells themselves as well as in trans, when added to the culture medium. In the absence of costimulatory signals provided by the tumor, our strategy allows T cells to "arm themselves" by the production of tumor-specific costimulatory proteins. Sustained in situ production of such molecules, like the B7-diabody fusion protein may create a favorable local environment for the activation and proliferation of tumor-reactive T cells and increase the tumoricidal activity of immunotherapeutic approaches targeting the TCR pathway.