Immune Response Modifiers of Biological Origin Chapter 15 IMMUNOTOXINS

Immunotoxins are covalent conjugates of monoclonal antibodies directed against cellspecific antigens and potent toxins of bacterial or plant origin. Bacterial toxins that have been used to prepare immunotoxins include diphtheria toxin and Pseudomonas aeruginosa exotoxin A (reviewed in Blakey et al., 1988a; Wawrzynczak and Davies, 1990). Such immunotoxins may have limited clinical use in patients, however, due to the presence of preexisting neutralizing antibodies to those toxins either as a result of previous bacterial infection or immunization. The majority of work has thus concentrated on the use of toxins or ribosomal-inactivating proteins (RIPs) from plants. Toxins include ricin and abrin from the seeds of Ricinus communis and Abrus precatorius, respectively. They consist of two polypeptide chains, A and B, each having a molecular weight of approximately 30 kDa, which are linked by a single disulfide bond. Individually, the A and B chains are virtually devoid of cytotoxic action, both being required to exert potent cytotoxic effects. The B-chain subunit contains two binding sites that recognize galactose containing glycoproteins and glycolipids present on the surface of virtually all cell types (Robertus, 1988). Following binding via the B chain, the toxin is taken into the cell by receptor-mediated endocytosis, and the A chain, by an ill-defined mechanism, crosses a cell membrane to reach the cytosol. The A chain then inactivates the 60S ribosomal subunit, thereby terminating protein synthesis and killing the cell. Endo and Tsurugi (1987) have shown that the A chains are specific N-glycosidases that remove an adenine residue from adenosine 4324 in the 28S rRNA of the 60S ribosomal complex. The extreme potency of these toxins results from the fact that entry of very few molecules of A chain into the cytoplasm, perhaps just one, appears sufficient to kill the cell (Eiklid et al., 1980). Several plants have been found to contain A-chain-like proteins called RIPs (reviewed in Stirpe and Barbieri, 1986). Examples of RIPs include gelonin, saporin, and pokeweed antiviral protein from the seeds of Gelonium multiflorum, Saponaria offidnalis, and Phytolacca americana, respectively. As with the isolated A chains of the toxins, these RIPs are not themselves toxins because they lack a B chain by which to bind to cells. They all inactivate