Activation of T Cells by Bispecific Single-Chain Construct MT103 (MEDI-538) Is Strictly Target Cell-Dependent

Background: Bispecific antibodies have been extensively studied in vitro and in vivo for their use in redirected tumor cell lysis. A particular challenge of bispecific antibody constructs recognizing the CD3 signaling complex is to achieve a controlled polyclonal activation of T-cells that, ideally, is entirely dependent on the presence of target cells. If this is not the case, systemic production of inflammatory cytokines and secondary endothelial reactions may occur as side effects, as are observed with the murine anti-human CD3e antibody OKT-3 (muromab, Orthoclone®). Here we present evidence that MT103 (or MEDI-538), a bispecific single chain antibody of the BiTE class that targets CD19 and CD3, induces T-cell activation exclusively in the presence of target cells. Material and methods: Peripheral blood mononuclear cells from healthy donors were prepared by Ficoll density centrifugation. PBMC were incubated for 24 hours with MT103 in presence or absence of specific target cells. Target cell lysis was determined by measurement of adenylate kinase activity released from lysed cells. De novo expression of activation markers CD69 and CD25 on T-cells was assessed by flow cytometry using directly conjugated monoclonal antibodies, and the concentration of cytokines in the supernatant was determined by a commercial FACS-based bead array. Results: MT103 was analyzed for conditional T-cell activation. In the presence of target-expressing cell lines, low picomolar concentrations of MT103 were sufficient to stimulate a high percentage of peripheral human T-cells to express cytokines and surface activation markers, to enter into the cell cycle and to induce redirected lysis of target cells. However, in the absence of target cells, the BiTE molecules no longer detectably activated human T-cells even at concentrations exceeding the ED50 for redirected lysis and conditional T-cell activation by more than five orders of magnitude. Conclusion: Our data show that T-cell activation by MT103 is highly conditional in that it is strictly dependent on the presence.