BiTE antibodies are single-chain bispecific antibody constructs with dual specificity for CD3 on T cells and a surface antigen on target cells. By bridging cytotoxic T cells and cancer cells, BiTE antibodies are capable to temporarily mount a polyclonal T cell response that is not restricted by T cell receptor specificity, presence of MHC class I, generation and presentation of peptide antigen, or the need for T cell co-stimulation. Examples are the CD19/CD3-bispecific BiTE antibody blinatumomab, which has shown high response rates and durable remissions in leukemia and lymphoma patients, and EpCAM/CD3-bispecific BiTE antibody solitomab (MT110), which is in a dose-escalating phase 1 study in patients with solid tumors. CD8+ and CD4+ effector memory T cells were found to be the most potent effector cells for BiTE antibodies. Here, we have assessed whether highly purified T regulatory cells (T regs) can also serve as effector cells for BiTE antibody MT110, despite multiple reports suggesting that T regs rather suppress the anti-tumor activity of cytotoxic T cells. T regs were purified from human peripheral blood mononuclear cells (PBMC) using anti-CD25 magnetic beads and fluorescence-activated cell sorting (FACS) for expression of CD4 and CD45RA markers, followed by expansion in the presence of IL-2 (Hoffmann P. et al., Methods Mol. Biol. 677:15-30, 2011). Purified naïve CD4+/CD25high/CD45RA+/FOXP3+ T regs had a purity of 96-99.6%, and contained no detectable CD8+ T cells. In co-culture experiments, a small fraction of purified T regs up-regulated the expression of granzyme B and perforin when stimulated with MT110 in the presence of EpCAM+ target cells. As shown by chromium release assay and phase contrast microscopy, T regs caused a MT110 dose-dependent, redirected lysis of target cells. No target cell lysis was observed with a BiTE antibody solely binding to CD3 on T cells, and by T regs alone. We then tested the activity of T regs against spheroids formed in vitro from colorectal cancer stem cells. Only in the presence of MT110, T regs could significantly reduce the size and integrity of spheroids. In that respect, T regs had a similar activity as PBMC. Our study suggests that T regs, which often are associated with immune escape of tumors, can be effectively engaged for redirected cancer cell lysis by a BiTE antibody.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4841. doi:1538-7445.AM2012-4841