Hybridomas producing antibodies to the binding subunit of mistletoe lectin I are resistant to this toxin

The cytotoxic effect of mistletoe lectin I (MLI, viscumin) was tested on hybridomas producing monoclonal antibodies (MAb) against the binding B subunit (MLB) of the toxin. Hybridomas TB1 (IgG1), TB4 (IgM), and TB12 (IgG1) displayed resistance against the toxic action of MLI but not against a chimeric toxin composed of ricin 13 subunit and MLA. The sensitivity of other hybridomas (TB33 and TB35, IgG1) did not differ from the control (3B4 and 3D4). All cell lines exhibited about the same sensitivity to native ricin. The number of ML-binding sites on the cell was roughly equal for the most resistant TB12 and the controls, indicating that surface antibodies played no appreciable part: in toxin binding. In immunoenzyme and radioligand assays the MAb of nonresistant TB33 and TB35 recognized only MLI adsorbed on the plate but not in solution, whereas those of resistant TB1, TB4, and TB12 detected both. The TB12 MAb blocked MLI binding with asialofetuin bearing terminal galactose; further, incubation of the asialofetuin-MLI complex with TB12 MAI, caused toxin redistribution and formation of a new complex MAb-MLI. The data obtained indicate that (i) the B subunit is essential for a step in toxin transport: across main cell membranes, and (ii) the suppressive effect of TB12 MAb involves interaction with MLI in the Golgi complex.