COMPARISON OF THE PERFORMANCE OF ANTI-CD7 AND ANTI-CD38 BISPECIFIC ANTIBODIES AND IMMUNOTOXINS FOR THE DELIVERY OF SAPORIN TO A HUMAN T-CELL ACUTE LYMPHOBLASTIC-LEUKEMIA CELL-LINE

We have investigated the cytotoxic performance of two different anti-C D7/anti-saporin BsAb's (HB2 x DB7-18 and Q1.1), three anti-CD38/anti-s aporin BsAb's (OKT10 x RabSap, OKT10 x DB7-18 and Q4.1) and an anti-CD 7 (HB2-Sap) and anti-CD38-saporin (OKT10-Sap) immunotoxin for deliveri ng the ribosome inactivating protein (rip) to the human T-cell acute l ymphoblastic leukemia cell line HSB-2. In the case of CD7 as target mo lecule the immunotoxin outperformed both anti-CD7 BsAb's being six tim es more effective than HB2 x DB7-18 and 98 times more so than Q1.1 at effectively inhibiting protein synthesis in a dose dependent manner. T he chemically constructed HB2 x DB7-18 BsAb was more effective at inhi biting protein synthesis and cell growth in target HSB-2 cells in a do se dependent manner than the quadroma produced BsAb Q1.1. Both BsAb de monstrated a prozone effect used at concentrations above 0.1 nM though this was more pronounced for Q1.1 than for HB2 x DB7-18. The prozone effect was partially though not completely reversed by increasing the concentration of saporin in the system. In the case of CD38 as target molecule the anti-CD38 IT OKT10-Sap performed poorly, never actually a chieving its IC50. Two BsAb's constructed with monoclonal anti-saporin Fab arms each recognizing a different epitope on the saporin molecule also performed poorly. In contrast the BsAb OKT10 x RabSap constructe d with Fab derived from a rabbit polyclonal anti-saporin antiserum per formed in a dose dependent manner achieving its IC50 at a concentratio n of 1.3 nM. This BsAb also exhibited a prozone effect. These results exemplify the importance of cross linking adjacent target molecules on the cell surface in order to achieve effective delivery of saporin to the cell interior.