LYSIS OF MURINE B-LYMPHOMA-CELLS BY TRANSGENIC PHAGOCYTES VIA A HUMANFC-GAMMA-RI X MURINE MHC CLASS-II BISPECIFIC ANTIBODY

The class I IgG receptor (Fc gamma RI) on cytotoxic effector cells has been reported to initiate destruction of tumour cells by effector cel ls in vitro. We are aiming at developing an immunocompetent model to e valuate the cytotoxic capacity of human Fc gamma RI for the rejection of tumour cells in vivo. Therefore, we recently generated a transgenic mouse strain expressing human Fc gamma RI on monocytes, macrophages, and neutrophils. In these mice, the human receptor is up-regulated by granulocyte-colony-stimulating factor (G-CSF) and is able to trigger c ellular responses. Subsequently, in the present study the B cell lymph oma IIA1.6 cell line is selected as a tumour target, and a human Fc ga mma RI-directed antitumour bispecific antibody (bsAb) is constructed a nd characterized. Fab' fragments of mAb 22, which bind hFc gamma RI at an epitope that is distinct from the ligand binding site, were chemic ally linked to Fab' fragments of rat anti-(mMHC class II antigens) mAb M5/114, yielding bsAb 22xM5/114. This bsAb was able to bind simultane ously to hFc gamma RI and mMHC class Il antigens in a dose-dependent f ashion. Binding of 22xM5/114 to Fc gamma RI was not inhibited in the p resence of human IgG. It is important to note that, MHC-class-II-expre ssing IIA1.6 lymphoma cells were lysed by whole blood from G-CSF-treat ed transgenic mice in the presence of bsAb 22xM5/114. No lysis by whol e blood from non-transgenic mice or from transgenic animals that had n ot received G-CSF was observed. These results indicate that human Fc g amma RI is able to mediate lysis of murine IIA1.6 lymphoma cells by tr ansgenic effector cells via bsAb 22xM5/114. A trial with transgenic mi ce, evaluating the efficacy of these hFc gamma RI-directed bsAb in com bination with G-CSF for treatment of IIA1.6 B cell lymphoma, is curren tly in progress.