Gelonin, a plant protein which can powerfully reduce the protein-synthetic capacity of ribosome preparations, was covalently coupled to anti-Thy1 .I antibody. The conjugate was prepared using N-succinimidyl-3-(2-pyridyldithi0)propionate which generates a disulphide linkage between the component molecules. Two conjugate fractions were obtained with M, of 180000 and > 200000.After its linkage to the antibody, gelonin suppressed those Thy1.1-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, fraction at a concentration of 0.4nM and with the > 200000-Mr fraction of pM. Unconjugated gelonin induced comparable reductions in T-cell responsiveness but at concentrations of 30 nM. The conjugates exerted little or n o effect upon B lymphocytes or T lyniphocytes from CBA mice (Thyl.z+ve). Two Thyl.l-expressing AKR lymphoma cell lines, AKR-A and BW5147, were found to be sensitive to the conjugates, albeit much less so than the norinal 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.Gelonin, a glycoprotein from the seeds of Gelonium nzulti-,florum, inhibits protein synthesis in rabbit reticulocyte lysates by damaging the 60-S subunit of the ribosomes. The inactivation of the ribosomes is irreversible, seems not to involve cofactors, and occurs with an efficiency that suggests lhat gelonin acts enzymically [l]. Several other plants have been found to contain proteins which can inactivate isolated eukaryotic ribosomes apparently by the same mechanism as does gelonin. The proteins can be divided into those with and those without the capacity to recognise and bind to carbohydrates on cell surfaces.The inhibitors which d o not bind to cells are single polypeptide chains and are more or less devoid of toxic effects upon mammalian cells. This group, to which gelonin belongs, also includes the pokeweed antiviral protein [2,3] and inhibitors from wheat germ [4,5], Momordica charuntia [6] and probably also from Croton tiglium and Jatropha curcus [7].The proteins which bind to cells d o so because the ribosomc-inhibitory subunit (the A chain) is linked by a disulphide bond to a second subunit (the B chain) which has the cell-binding property. These proteins are either monovalent or, as with the lectins, divalent with two A-B units associated noncovalently. They differ in toxicity both to intact animals and to cells in tissue culture. Some, such as ricin and abrin (reviewed in [S]), modeccin [9,10] and a toxic lectin from Viscum album [Ill, are extremely potent toxins whereas others are less toxic or non-toxic, such as Ricinus communis agglutinin [12], Ahrus precatorius agglutinin [8,13] and thc lectins from iz4. charantia, Vicia craccu and Crotulu...
