Blockade of the galactose‐binding sites of ricin by its linkage to antibody: Specific cytotoxic effects of the conjugates

A method is described for preparing specific cytotoxic agents by linking intact ricin to antibodies in a manner that produces obstruction of the galactose‐binding sites on the B chain of the toxin and so diminishes the capacity of the conjugate to bind non‐specifically to cells. The conjugates were synthesised by reacting iodoacetylated ricin with thiolated immunoglobulin and the components of conjugate with reduced galactose‐binding capacity were separated by affinity chromatography on Sepharose (a β‐galactosyl matrix) and asialofetuin‐Sepharose. Fluorescence‐activated cell sorter (FACS) analyses revealed that the fraction of a monoclonal anti‐Thy1.1‐ricin conjugate that passed through a Sepharose column had markedly diminished capacity to bind non‐specifically to Thy1.2‐expressing CBA thymocytes and EL4 lymphoma cells. The fraction of conjugate that passed through an asialofetuin‐Sepharose column displayed no detectable non‐specific binding. Both fractions of conjugate were potent cytotoxic agents for Thy1.1‐expressing AKR‐A lymphoma cells in tissue culture. They reduced the [³H]leucine incorporation of the cells by 50% at a concentration of 2–5 pM. Comparable inhibition of EL4 cells was only achieved with 3000–7500‐fold greater concentrations of conjugate. By contrast, the fraction of anti‐Thy1.1‐ricin that retained Sepharose‐binding capacity showed marked non‐specific binding and toxicity to EL4 cells. A conjugate with diminished galactose‐binding capacity was also prepared from the W3/25 monoclonal antibody which recognises an antigen upon helper T‐lymphocytes in the rat. It elicited powerful and specific toxic effects upon W3/25 antigen‐expressing rat T‐leukaemia cells. This finding is of particular importance because isolated ricin A‐chain disulphide‐linked to W3/25 antibody is not cytotoxic. The property of the B‐chain in intact ricin conjugates that facilitates delivery of the A‐chain to the cytosol thus appears to be independent of galactose recognition. It is concluded that the ‘blocked’ ricin conjugates combine the advantages of high potency, which is often lacking in antibody‐A‐chain conjugates, with high specificity, which previously was lacking in intact ricin conjugates.