TrAbs were produced by quadroma technology as described [16, 25]. Quadroma-derived supernatants were purified by protein A chromatography applying sequential pH elution followed by a cationic exchange chromatography purification step. Surek [17, 19, 25, 26] is a trAb that was generated by fusion of the parental hybridomas 17A2 (anti-mouse CD3, rat IgG2b) and Me361 (anti-GD2, mouse IgG2a) . BiLu  comprises the same anti-CD3 specificity and additionally includes a mouse IgG2a binding arm recognizing human EpCAM, which was derived from the clone C215 . For production of HB304 (anti-mouse CTLA-4), the hamster hybridoma clone UC10-4F10-11  was used. Cell lines were cultivated in chemically defined protein-free medium. All antibodies were manufactured by Trion Research GmbH.