Recently, bispecific monoclonal antibodies (BsMAb) yielded promising results regarding the treatment of various malignant diseases. However, the serum levels that can be safely achieved in humans remain suboptimal in terms of saturation of the tumor-associated antigen (TAA), as the application of higher doses results in systemic cytokine release and transient leukopenia. Off-target activation of circulating immune cells, stimulated by the BsMAb, could explain most of the side effects observed

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... ter in-vivo application of BsMAb. In this work, immunoassays were conducted, which showed that BsMAb are capable of inducing T cell proliferation in the absence of target cells. Furthermore, it was demonstrated that human umbilical vein endothelial cells (HUVECs) as well as various lymphoid cell lines markedly amplify this effect. They are acting as stimulating bystander cells (SBCs). Several lymphoid and non-lymphoid cell lines did not exhibit this property, thus ruling out allogeneic effects. Functional assays were conducted to characterize blocking antibodies directed against costimulatory and adhesion molecules on both stimulating bystander cells and immune cells, that are capable of reducing off-target activation. Antibodies against CD54, CD2, and TNFα reduced off-target activation markedly, whereas antibodies directed against CD18 as well as prednisolone blocked it completely. Further experiments addressed the effect of blocking reagents on the induction of on- target activation and subsequent killing of tumor cells by T cells after stimulation with BsMAb. Prednisolone and aCD2 antibodies impaired tumor cell killing markedly, while only minor reduction was observed with aCD18 and aTNFα antibodies. Using an adhesion assay, it was shown that PBMCs adhere to a HUVEC layer to a higher extent when stimulated with BsMAb compared to unstimulated PBMCs. Antibodies directed against CD18 could greatly reduce this phenomenon. In summary, aCD18 and aTNFα antbodies appear to be promising agents for the prevention of cytokine rele [...]