An ineffective monoclonal antibody-ricin A chain conjugate is converted to a tumouricidal agent in vivo by subsequent systemic administration of ricin B chain

Addition of an ER retention signal to the ricin A chain increases the cytotoxicity of the holotoxin

With the exception of diphtheria toxin, which translocates from acidified endosomes, the intracellular organelle from which the catalytic moieties of several plant and bacterial toxins enter the target cell during endocytic uptake has not been identified. We have recently proposed that some toxins may travel the entire secretory pathway in reverse, moving from the cell surface to the lumen of the ER, before entering the cytosol. Several bacterial toxins have the ER retention sequence KDEL or a related analogue at their carboxyl termini, suggesting that the KDEL receptor may play a role in delivering these toxins to the ER. Here we provide further support for this possibility since the cytotoxicity of ricin, which lacks a KDEL sequence, can be significantly increased by adding KDEL to the C-terminus of its A chain.

Expression and functional properties of genetically engineered ricin B chain lacking galactose-binding activity

Ricin is a potent plant toxin consisting of two disulfide-bonded subunits. The A chain of ricin is an N-glycosidase which inactivates 28 S RNA and inhibits protein synthesis. The B chain is a galactose-specific lectin with two galactose-binding sites. The genes encoding preproricin and its A and B chains have been cloned and expressed. In addition, X-ray crystallographic studies have identified the galactose-contact residues in both the high- and low-affinity galactose-binding sites of the B chain. In this study, the high-affinity galactose-contact residue of the B chain was changed from Asn-255 to Ala-255 by oligonucleotide-directed mutagenesis. The resulting mutant was sequenced to confirm the presence of a single mutation and was expressed in Cos-M6 cells. Both wild-type and mutant recombinant B chain could be immunoprecipitated with a heterologous anti-B chain antibody and both could form A-B heterodimers. However, as compared to the wild-type, the mutant B chain lacked more than 99% of its lectin activity and cytotoxicity as an A-B dimer. In conclusion, altering the contact residue of the high-affinity galactose-binding site of ricin B chain from Asn-255 to Ala-255 abrogates more than 99% of its lectin activity and the cytotoxicity of the A-B heterodimer to ricin-sensitive cells.

Rationale for immunotoxin therapy of metastatic prostate carcinoma formatted as a multi-stage delivery system

A family of triple hybridomas secreting hybrid monoclonal antibodies has been developed in our laboratory. The hybrid monoclonal antibodies exhibit bimolecular specificity towards both antigenic determinants on the prostate carcinoma cell surface as well as toxin or toxic moieties (ricin A chain and pokeweed antiviral protein). These hybrid antibodies, when bound univalently to their cognate toxin, constitute the primary reagents responsible for selective in vitro prostate carcinoma cell kill; oncolytic impact is achieved by binding of the hybrid antibody-toxin complex (primary hybrid immunotoxin) to a prostate carcinoma cell surface-expressed antigen by the remaining univalent binding site of the hybrid antibody, allowing access of the toxin to the cytosol by internalization of the hybrid antibody-toxin complex. Internalization by endocytosis of a hybrid antibody-toxic subunit has been strikingly enhanced by the use of secondary monoclonal antibody reagents alone or in conjunction with other biomodifier reagents. For example, use of a second monoclonal antibody specific for ricin A chain to which ricin B chain (binding subunit) is chemically coupled results in selective and synergistic cell kill of targeted cancer cells. In vitro studies involving temporally staggered exposure of the cells to the individual components (primary hybrid antibody, toxin, and secondary antitoxin monoclonal antibody biomodifier) have been performed in a manner allowing maintenance of cytotoxic efficacy. It is concluded that sequential administration of these immunotherapeutic components, individually nontoxic, is a feasible strategy to develop an effective immunotherapeutic treatment of human prostate carcinoma.

Ricin B chain-containing immunotoxins prepared with heat-denatured B chain lacking galactose-binding ability potentiate the cytotoxicity of a cell-reactive ricin A chain immunotoxin

Ricin B chain incubated at 37 degrees C in the absence of lactose loses its ability to bind the galactose-containing protein, asialofetuin. Circular dichroism analysis of the B chain during thermal denaturation indicates that the loss of galactose-binding ability by the B chain correlates with limited unfolding of the molecule. As a result of this conformational change, disulfide bonds that are shielded from the solvent by the compact folded structure of the B chain become exposed and the chitobiosyl cores of both N-linked oligomannose chains become susceptible to cleavage by endoglycosidases. The heat-denatured B chain does not enhance the toxicity of a ricin A chain-containing rabbit anti-human immunoglobulin (RAHIg-A) to Daudi cells. However, when heat-denatured B chain is coupled to goat anti-rabbit immunoglobulin (GARIg), the resulting immunotoxin, GARIg-hdB, potentiates the killing of RAHIg-A-treated Daudi cells to an extent similar to that of an immunotoxin prepared with GARIg and native B chain. These results indicate that the native, galactose-binding structure of the B chain is not necessary to enhance the cytotoxicity of the cell-reactive A chain immunotoxin (IT-A) and suggests that regions of the B chain exposed by unfolding the molecule may mediate potentiation of cytotoxicity.

Cloning and expression of recombinant, functional ricin B chain

The cDNA encoding the B chain of the plant toxin ricin has been cloned and expressed in monkey kidney COS-M6 cells. The recombinant B chain was detected by labeling the transfected cells with [35S]methionine and [35S]-cysteine and demonstrating the secretion of a protein with a Mr of 30,000-32,000 that was not present in the medium of mock-transfected COS-M6 cells. This protein was specifically immunoprecipitated by an anti-ricin or anti-B-chain antibody and the amount of recombinant B chain secreted by the COS-M6 cells was determined by a radioimmunoassay. Virtually all of the recombinant B chain formed active ricin when mixed with native A chain; it could also bind to the galactose-containing glycoprotein asialofetuin as effectively as native B chain. These results indicate that the vast majority of recombinant B chains secreted into the medium of the COS-M6 cells retain biological function.