Antibody formation against the cytotoxic proteins abrin and ricin in humans and mice.

Antibody formation may limit the therapeutic use of cancerostatic proteins. To study the significance of antibody formation against abrin and ricin, highly sensitive ELISA procedures for determination of anti-abrin and anti-ricin were developed. In mice treated weekly with therapeutic doses of ricin, antibodies appeared after 2-3 weeks and then rose rapidly, whereas after abrin treatment the antibody formation was slower. Ricin A-chain was found to be more immunogenic than either intact ricin or human serum albumin (HSA). Cyclophosphamide inhibited the antibody response to both abrin and ricin and a combination of cyclophosphamide and prednisolone totally inhibited both anti-abrin and anti-ricin formation during the 6-week observation period. In mice treated weekly with HSA, abrin treatment strongly reduced the anti-HSA formation, showing that abrin has an immunosuppressive effect which appeared to be stronger than that of cyclophosphamide. The existence of circulating antigen-antibody complexes could be demonstrated in the sera of toxin-treated mice by precipitation with polyethyleneglycol, whenever antibodies were detectable with ELISA. The life-span of animals given lethal ricin doses was appreciably enhanced in animals having antibody levels in excess of 10-20 ng/ml. In cancer patients treated i.v. every second week with therapeutic toxin doses, the 10-20 ng/ml levels of anti-ricin and anti-abrin were reached 6-8 weeks and 7-10 weeks after the first injection of ricin and abrin, respectively. The data indicate that the effective therapeutic use of abrin and ricin as single agents may be limited to these time frames, but that the period of effective use may be substantially prolonged if the toxins are given together with conventional cytostatic agents having immuno-suppressive activity.