Cytotoxicity Acquired by Conjugation of an Anti‐Thy_1.1 Monoclonal Antibody and the Ribosome‐Inactivating Protein, Gelonin

Gelonin, a plant protein which can powerfully reduce the protein‐synthetic capacity of ribosome preparations, was covalently coupled to anti‐Thy_1.1 antibody. The conjugate was prepared using N‐succinimidyl‐3‐(2‐pyridyldithio)propionate which generates a disulphide linkage between the component molecules. Two conjugate fractions were obtained with M_r of 180000 and > 200000. After its linkage to the antibody, gelonin suppressed those Thy_1.1‐bearing T lymphocytes from AKR mice which will respond to phytohaemagglutinin and concanavalin A in tissue culture. The [³H]leucine incorporation with the T‐cell mitogens was inhibited by 50% with the 180000‐M_r fraction at a concentration of 0.4 nM and with the > 200000‐M_r fraction of pM. Unconjugated gelonin induced comparable reductions in T‐cell responsiveness but at concentrations of 30 nM. The conjugates exerted little or no effect upon B lymphocytes or T lymphocytes from CBA mice (Thy_1.2+ve). Two Thy_1.1‐expressing AKR lymphoma cell lines, AKR‐A and BW5147, were found to be sensitive to the conjugates, albeit much less so than the normal T lymphocytes. The conjugates injected in vivo significantly prolonged the life of CBA mice bearing an AKR‐A lymphoma allograft. It is concluded that gelonin can, by its linkage to an antibody, be rendered cytotoxic with a potency to match or exceed those of the toxins abrin and ricin.