Selective killing of tumor cells in vitro by an immunotoxin composed of ricin and monoclonal antibody against Ia antigen.

A monoclonal antibody (MAb-HB55) directed against a HLA class II antigen (Ia), was purified by DE-52 chromatography. The purified MAb contained less than 5% impure protein detected by SDS-PAGE. Ricin was conjugated with the MAb via a disulfide bond to construct an immunotoxin (ricin-HB55). The concentration causing fifty percent growth inhibition (IC50) was 2 x 10(-11)M for the Raji cells. In contrast, the IC50 for Molt-4 and K562 cells was 100 times higher than that for the Raji cells. B lymphocytes separated from peripheral blood lymphocytes by nylon wool were selectively killed by the conjugate, but T lymphocytes were not apparently affected. Our results demonstrate that the immunotoxin is selectively cytotoxic to Raji cells and B-cells, and may have potential for purging malignant cells from bone marrow of patients with B-cell leukemia and lymphoma.