Conjugates of folate and anti-T-cell-receptor antibodies specifically target folate-receptor-positive tumor cells for lysis

High-affinity folate receptors (FRs) are expressed at elevated levels on many human tumors. Bispecific antibodies that bind the FR and the T-cell receptor (TCR) mediate lysis of these tumor cells by cytotoxic T lymphocytes. In this report, conjugates that consist of folate covalently linked to anti- TCR antibodies are shown to be potent in mediating lysis of tumor cells that express either the α or β isoform of the FR. Intact antibodies with an average of five folates per molecule exhibited high affinity for FR⁺ tumor cells but did not bind to FR^- tumor cells. Lysis of FR⁺ cell lines could be detected at concentrations as low as 1 pM (≃0.1 ng/ml), which was 1/1000th the concentration required to detect binding to the FR⁺ cells. Various FR⁺ mouse tumor cell lines could be targeted with each of three different anti- TCR antibodies that were tested as conjugates. The antibodies included 1B2, a clonotypic antibody specific for the cytotoxic T cell clone 2C; KJ16, an anti-Vβ8 antibody; and 2C11, an anti-CD3 antibody. These antibodies differ in affinities by up to 100-fold, yet the cytolytic capabilities of the folate/antibody conjugates differed by no more than 10-fold. The reduced size (in comparison with bispecific antibodies) and high affinity of folate conjugates suggest that they may be useful as immunotherapeutic agents in targeting tumors that express folate receptors.