Somatic cell mutants resistant to ricin, diphtheria toxin, and to immunotoxins

The conjugation of an AQP1-directed immunotoxin in the study of site-directed therapy within the CNS

PURPOSE

The water channel, aquaporin (AQP)1, is highly specific to the choroid plexus (CP) epithelium within the brain. It is therefore a potential target through which therapeutic agents could be selectively directed to the CP. Here we describe the conjugation of a monoclonal antibody (mAb), raised against an extra-cellular domain of AQP1, to the A chain of ricin (RCA). This reagent should allow study of a highly specific chemical lesion of the CP.

METHODS

A published method was used to couple the anti-AQP1 mAb to the A chain of ricin using an SPDP cross-linker. The conjugate was purified by Superdex S-200 chromatography. Column fractions were analysed by SDS-PAGE and Western blotting. Fractions containing conjugate were assayed for cytotoxicity on rat CP cells in culture, using a WST-1 viability assay to assess cell death.

RESULTS

SDS-PAGE and Western blotting demonstrated separation of conjugated anti-AQP1 mAb from unconjugated ricin by the Superdex column. Cultured rat CP cells were killed with an IC(50) of 1.81 μg/ml when exposed to the anti-AQP1 mAb-RCA conjugation reaction products. Selectivity of this reagent was demonstrated by the higher IC(50) exhibited by non-AQP1-expressing primary fibroblast cultures and cultures exposed to the reaction products from the non-specific IgG-RCA conjugation attempt.

CONCLUSION

We have been able to demonstrate selective cytotoxicity in vitro of an anti-AQP1 mAb-RCA conjugate. We hope that this work will generate further interest in the use of this site-specific conjugate to target the CP in conditions such as hydrocephalus, tumours that arise from here, and other AQP1-expressing tumours.

Somatic cell mutants resistant to retrovirus replication: intracellular blocks during the early stages of infection

To identify cellular functions involved in the early phase of the retroviral life cycle, somatic cell mutants were isolated after selection for resistance to infection. Rat2 fibroblasts were treated with chemical mutagens, and individual virus-resistant clones were recovered after selection for resistance to infection. Two clones were characterized in detail. Both mutant lines were resistant to infection by both ecotropic and amphotropic murine viruses, as well as by human immunodeficiency virus type 1 pseudotypes. One clone showed a strong block to reverse transcription of the retroviral RNA, including formation of the earliest DNA products. The second clone showed normal levels of viral DNA synthesis but did not allow formation of the circular DNAs normally found in the nucleus. Cell fractionation showed that the viral preintegration complex was present in a form that could not be extracted under conditions that readily extracted the complex from wild-type cells. The results suggest that the DNA was trapped in a nonproductive state and excluded from the nucleus of the infected cell. The properties of these two mutant lines suggest that host gene products play important roles both before and after reverse transcription.

Conjugation of Blocked Ricin to an Anti-CD19 Monoclonal Antibody Increases Antibody-Induced Cell Calcium Mobilization and CD19 Internalization

CD19 (B4) is a signal transduction molecule restricted to the B-cell lineage and the target of the immunotoxin anti-B4–blocked ricin (anti-B4–bR), which is composed of the monoclonal antibody (MoAb) anti-B4 and the modified plant toxin blocked ricin. To explore the influence of conjugation of blocked ricin to anti-B4 on functional activation of CD19, we investigated the effects of anti-B4–bR, and that of unconjugated anti-B4, on intracellular calcium mobilization and ligand/receptor internalization. The data showed that anti-B4–bR was more potent than anti-B4 in triggering cell calcium mobilization. Two other immunotoxins that bind to the B-cell surface, anti-CD20–bR and anti-CD38–bR, were devoid of the calcium increasing effect of anti-B4–bR. Furthermore, anti-B4 conjugated to ricin A-chain was also without effect in Namalwa cells, indicating that the ricin B-chain component was required for anti-B4–bR effect. Anti-B4–bR-induced calcium mobilization was inhibited in the presence of lactose, yet the calcium response induced by cross-linking anti-B4–bR with a second step antibody was not affected. The extent of CD19 modulation induced by anti-B4–bR was higher than that induced by anti-B4, and lactose dampened the effect of the immunotoxin down to that of the MoAb. Moreover, the number of internalized immunotoxin molecules was higher than that of unconjugated MoAb. Although a mechanism involving dimerization of the immunotoxin cannot be excluded, our findings suggest that the residual binding activity of the blocked ricin B-chain to cell surface molecules plays an important role in the greater calcium fluxes and greater internalization rate of anti-B4–bR, and is of functional significance in the mechanism of intoxication of cells by the immunotoxin.

Conjugation of Blocked Ricin to an Anti-CD19 Monoclonal Antibody Increases Antibody-Induced Cell Calcium Mobilization and CD19 Internalization

CD19 (B4) is a signal transduction molecule restricted to the B-cell lineage and the target of the immunotoxin anti-B4–blocked ricin (anti-B4–bR), which is composed of the monoclonal antibody (MoAb) anti-B4 and the modified plant toxin blocked ricin. To explore the influence of conjugation of blocked ricin to anti-B4 on functional activation of CD19, we investigated the effects of anti-B4–bR, and that of unconjugated anti-B4, on intracellular calcium mobilization and ligand/receptor internalization. The data showed that anti-B4–bR was more potent than anti-B4 in triggering cell calcium mobilization. Two other immunotoxins that bind to the B-cell surface, anti-CD20–bR and anti-CD38–bR, were devoid of the calcium increasing effect of anti-B4–bR. Furthermore, anti-B4 conjugated to ricin A-chain was also without effect in Namalwa cells, indicating that the ricin B-chain component was required for anti-B4–bR effect. Anti-B4–bR-induced calcium mobilization was inhibited in the presence of lactose, yet the calcium response induced by cross-linking anti-B4–bR with a second step antibody was not affected. The extent of CD19 modulation induced by anti-B4–bR was higher than that induced by anti-B4, and lactose dampened the effect of the immunotoxin down to that of the MoAb. Moreover, the number of internalized immunotoxin molecules was higher than that of unconjugated MoAb. Although a mechanism involving dimerization of the immunotoxin cannot be excluded, our findings suggest that the residual binding activity of the blocked ricin B-chain to cell surface molecules plays an important role in the greater calcium fluxes and greater internalization rate of anti-B4–bR, and is of functional significance in the mechanism of intoxication of cells by the immunotoxin.

Cytotoxic effects of ricin without an interchain disulfide bond: genetic modification and chemical crosslinking studies

Ricin is a toxic glycoprotein made of two polypeptide chains (A and B) linked by a disulfide bond. Ricin binds to cells by the B chain and is then internalized. The interchain disulfide bond is believed to be reduced in endosomes, and the A chain is then subsequently translocated to cytoplasm where it inactivates ribosomes. To understand the role of the disulfide bond in ricin toxicity, we prepared two types of ricin molecules. First, cysteine 259 of the A chain was mutated to an alanine residue. The mutant A chain was then reassociated with the native B chain to determine whether ricin is biologically active in the absence of an interchain disulfide bond. Reassociated mutant ricin showed a 40-fold reduction in biological activity. Binding studies using a hydrophobic fluorescence probe indicated that the associated complex was stable only at neutral pH and became highly unstable at a lower pH characteristic of the endosomal milieu. In the second construct, the interchain disulfide bond was replaced with a non-reducible bond by chemical derivatization. Interestingly, the non-reducible ricin molecule was equally cytotoxic as native ricin. These results show: (i) that the interchain disulfide bond is necessary to hold the A chain and the B chain together at endosomal pH, and (ii) that intact ricin may be transported to the cytoplasm where proteolysis or hydrolysis may occur to release the biologically active moiety.

A cytotoxicity assay utilizing a fluorescent dye that determines accurate surviving fractions of cells

A cytotoxicity assay has been developed based on the measurement of the proliferative activity of surviving cells as quantified by a cell-incorporated fluorescent dye, 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). The BCECF proliferative assay is fast (the results are obtained within 3-4 days depending on the cell line), accurate, not labor-intensive, does not require the use of radioisotopes or toxic compounds, and is amenable to automation. The BCECF proliferative assay was compared with two other indirect cytotoxicity tests, a trypan blue exclusion test and a BCECF viability test. Neither of these two latter assays reflected in any way the killing of cells by ricin. In contrast, using the BCECF proliferation assay, an optimal period of cell culturing after exposure to a toxin could be found so that the cytotoxicity values produced agreed with the surviving fractions of cells measured in a direct cytotoxicity assay. Under non-optimal conditions, the assay reflected the cell kill only qualitatively. Although it is common practice to conduct indirect cytotoxicity tests without validating them with a direct assay, our experiments demonstrate that the values obtained in such non-optimized indirect cytotoxicity tests may not parallel the cell kill and may, therefore, be meaningless.

High-level expression and simplified purification of recombinant ricin A chain

Ricin toxin is a glycoprotein which catalytically inactivates eukaryotic ribosomes by depurination of a single adenosine residue from the 28S ribosomal RNA. The enzymatic activity is present in the A chain of the toxin molecule, whereas the B chain contains two binding sites for galactose. Since it is highly potent in inhibiting protein synthesis, the A chain is used to prepare cytotoxic conjugates effective against tumor cells. Such chimeric proteins are highly selective and have a wide range of clinical applications. Extensive preclinical studies on these conjugates require large amounts of purified A chain. Native ricin A chain is heterogeneous, since plants produce a number of isoforms of ricin toxin. Purified, native preparations often contain two types of ricin A chain which differ in the extent of glycosylation. By cloning and expressing the gene of A chain, one could obtain homogeneous toxin molecules devoid of carbohydrates. In addition, structural changes in the toxin polypeptide could be introduced by in vitro mutagenesis, which can improve the pharmacological properties and antitumor activity. Earlier methods of expression strategies using Escherichia coli have yielded only moderate levels of expression. In the present study, the coding region of ricin A chain was cloned into pET3b, a high-level expression vector under the control of the T7 promoter. Recombinant ricin A chain produced by this construct has an additional 14 amino acid residues at the NH2 terminus. Subsequently, a NdeI site was created at the 5' end of the gene by oligonucleotide-directed mutagenesis. The modified fragment was then introduced into pET3b vector to produce toxin polypeptide identical to the native sequence.(ABSTRACT TRUNCATED AT 250 WORDS)

High sensitivity of cultured human trophoblasts to ribosome-inactivating proteins

Many plant proteins possessing abortifacient activities were identified as ribosome-inactivating proteins (RIPs). The effect of several ribosome-inactivating proteins (saporin 6, dianthin 32, pokeweed antiviral protein from seeds, gelonin, bryodin-R, and momordin) on primary cultures of human trophoblasts and human embryonal fibroblasts and on choriocarcinoma (JAR and BeWo) and ovarian carcinoma (TG) cell lines was studied. Protein synthesis of human trophoblasts and BeWo cells was lowered by RIPs more than that of other cells. The trophoblastic receptors for estradiol were not affected by treatment of the cells with momordin. The binding and uptake of saporin 6 and momordin by BeWo and HeLa cells were not correlated to cell toxicity.

Resistance of myeloid leukaemia cell lines to ricin A-chain immunotoxins

Nineteen monoclonal antibodies that recognize antigens on myeloid leukaemia cells were screened upon HL60, KG1, U937 and K562 cells for their ability to form effective ricin A-chain immunotoxins. The screening was performed using an indirect assay in which the cells were treated firstly with the test antibody and then with a Fab' immunotoxin directed against mouse immunoglobulin. Only two antibodies, MEM75 and 120-2A3, both directed against the transferrin receptor (TfR) were predicted to form immunotoxins that would inhibit protein synthesis by the cells by 50% at a concentration (IC50) of 10(-8) M or less. This prediction was subsequently confirmed using several of the antibodies directly conjugated to ricin A-chain. By contrast, the same immunotoxins were highly toxic to non-myeloid cells which shared the target antigens. A comparison was made between the rates of endocytosis and degradation by HL60 cells of an anti-TfR immunotoxin 120-2A3.dgA, that was effective at killing myeloid cells, and a CD33 immunotoxin, p67-7.dgA, that bound to myeloid cells but did not kill them. The difference in potency of the two immunotoxins on HL60 cells was not due to deficient uptake of p67-7.dgA but was probably due to the more rapid intracellular degradation of p67-7.dgA. Fast and effective degradation in lysosomes, if a general finding, could explain the poor susceptibility of myeloid cells to ricin A-chain immunotoxins.

Cytotoxicity of gelonin and its conjugates with antibodies is determined by the extent of their endocytosis

Conjugates of the single-chain ribosome-inactivating protein gelonin with ligands that bind to cell surface molecules vary greatly in their cytotoxicity. Conjugates that are not endocytosed after binding to cells exhibit low cytotoxicity similar to that of free gelonin, while conjugates that are endocytosed demonstrate enhanced cytotoxicity relative to free gelonin. However, the number of internalized gelonin molecules needed to intoxicate cells to the same degree has been found to be similar for all conjugates and for free gelonin. The intracellular concentration of gelonin has to be between 2,000-10,000 molecules/cells to achieve a surviving fraction of 0.37. Our studies revealed the presence of three distinct categories of cell surface molecules, those that are efficient in mediating endocytosis of immunotoxins, those that are only moderately efficient, and those that seem not to cause internalization of bound immunotoxins.

Prospects for the treatment of B cell tumors using idiotypic vaccination

Manipulation of the immune system in order specifically to attack autologous tumor cell has been a distant goal, largely due to the poor definition of tumor-associated antigens. By focussing on B cell lymphomas which express a molecularly defined clonal marker, the idiotypic immunoglobulin, it has been possible to generate autologous anti-idiotypic responses which suppress individual tumors. Studies of the components of these responses are providing insight into host effector mechanisms which can be activated against tumors, and also into the many strategies adopted by the target cells in order to avoid such attack. Promising results in various animal models where tumor bearers can be treated by idiotypic immunization have led to the point where application of this approach to the treatment of certain categories of human B cell lymphoma can be considered.

Killing of human myelomonocytic leukemia and lymphocytic cell lines by Actinobacillus actinomycetemcomitans leukotoxin

The purified leukotoxin of Actinobacillus actinomycetemcomitans kills human leukemic cell lines (e.g., HL-60, U937, and KG-1) and human T- and B-cell lines (e.g., JURKAT, MOLT-4, Daudi, and Raji) in a dose- and time-dependent manner. The 50% effective doses for these cell lines are similar to those established for human polymorphonuclear leukocytes and monocytes. In contrast, other human and nonhuman tumor cell lines are not susceptible to the leukotoxin. These human leukemia and lymphoid cell lines will serve as useful model systems with which to study the molecular specificity and mechanism(s) of action of the actinobacillus leukotoxin.

Purification and biochemical characterization of immunotoxins

Written on a plastic bottle of liquid soap in one of our bathrooms (JML) is the phrase 'Absolute cleanliness is next to Godliness!'. Perhaps absolute purity for ITs does not rank so highly, but the availability of purified ITs that contain no nonconjugated antibody or toxin, and no material of very high Mr (aggregates) is essential for a proper comparison of the biological activities of the component proteins with their nonconjugated counterparts. Purified and biochemically well-defined ITs make it possible to perform quantitative binding assays, to evaluate receptor-mediated endocytosis and to do cytotoxicity tests in vitro and efficacy studies in vivo, without fear that competition by nonconjugated antibody is affecting the experimental result. This chapter illustrates some of the methods that in combination can be used to purify ITs, including affinity chromatography, gel filtration, and ion-exchange chromatography with buffers of carefully defined composition, using examples from our laboratory to illustrate the procedures. For further details concerning these methods, we would refer the reader to the excellent book by Scopes [40], Purification of Proteins: Principles and Practice, which contains much useful information of both a technical and a practical nature regarding methods for purification of proteins.

Immunotoxins containing single-chain ribosome-inactivating proteins

We have summarized what is currently known about the distribution, biological role, and the mechanism of action of the single chain ribosome-inactivating proteins and described the purification of one of them, gelonin, as an example. ITs have been made with several of these proteins and, depending upon the antibody used for conjugation, these immunoconjugates can show specific in vitro cytotoxicity which is similar to that shown by equivalent ITs prepared with ricin A chain. The most potent of these conjugates have shown antitumor efficacy in a variety of animal tumor models, including both syngeneic rodent tumors and xenografts in nude or immunosuppressed mice. An important point needs to be addressed, however, before concluding that ITs containing single chain toxins will be clinically useful. A major problem with this approach is that it is likely that both the antibody and the toxin components of these conjugates will be immunogenic. Both antitoxin and antixenogenic immunoglobulin responses have been shown to occur in animals after infusion of IT, although it has not yet been clearly demonstrated that such antibody responses adversely effect the pharmacokinetics or the efficacy of immunoconjugates. Thus, preliminary enthusiasm over the efficacy of these new reagents must be tempered with the knowledge that their use in the clinic may be limited by the host immune responses or other as yet undefined factors. The fact that there are many immunologically distinct single chain ribosome-inactivating proteins does suggest one way of evading the antitoxin response, by a sequential treatment with a panel of immunoconjugates, each containing a different single chain toxin.