Monoclonal Antibodies Against Ricin: Effects on Toxin Function

Monoclonal antibodies against ricin toxin were produced and some of their properties investigated. Antibodies 196 C12 and 197 C7 raised against A-chain reacted with a CnBr fragment probably comprised between amino acid 254 and 262. Antibodies 193 A9, 196 A3, and 191 B7 recognized a 6-7 kD CnBr peptide. A second set of antibodies was raised against whole inactivated ricin. Most of them bound in a solid phase radioimmunobinding assay only to ricin and few had a low activity against purified A-chain. Different effects were noted on toxin action in cultured leukemic cells. If cells were preincubated with ricin followed by antibodies, MAb 207 E5 and 216 B3 had a strong enhancing effect on toxin action. If antibodies and toxin were mixed and then added to sensitive cells, antibody 207 E5 gave a strong protection while 216 B3 maintained its enhancing activity. The effect of antibody 216 B3 was further investigated by quantitative cloning experiments which showed that toxin had a fivefold enhancement in its activity by a preincubation with this antibody. Binding of fluoresceinated ricin to leukemic target cells was inhibited by a preincubation with antibody 207 E5 while antibody 216 B3 had no effect.