Immunotoxin-mediated inhibition of chronic lymphocytic leukemia cell proliferation in humans

We evaluated the cytotoxicity of the immunotoxin OKT1-SAP on fresh B-chronic lymphocytic leukemia (B-CLL) cells from 31 consecutive patients. OKT1-SAP comprised the OKT1 (CD5) monoclonal antibody disulfide linked to saporin-6 (SAP) ribosome-inactivating protein from the plant Saponaria officinalis. The effect of OKT1-SAP on target CD5-positive B-CLL cells was estimated using an in vitro proliferation inhibition assay in which control or OKT1-SAP-treated B-CLL cells were induced to proliferate by sequential stimulation with insolubilized anti-C3b receptor CB04 (CD35) antibody and low molecular weight B-cell growth factor. In 90% of patients, OKT1-SAP specifically suppressed B-CLL cell proliferation in a dose-related manner (50% inhibitory concentration, 4.0-6.8 nM). Taken together the findings reported in this article provide information relevant to the clinical development of immunotoxins because: (a) the in vitro conditions under which B-CLL cell proliferation is inhibited by OKT1-SAP are achievable in vivo without nonspecific toxicity according to our previous toxicology and pharmacokinetics studies in primates; and (b) the B-CLL cell proliferation inhibition assay described here provides a basis for future comparative studies.

High-pressure-liquid-chromatographic and fluorimetric methods for the determination of adenine released from ribosomes by ricin and gelonin

The high fluorescence of adenine-containing compounds after reaction with chloroacetaldehyde was used to measure the adenine released from rat liver and Artemia salina ribosomes by the action of ricin A chain and gelonin, two ribosome-inactivating proteins (RIPs) that share the same mechanism of action, consisting in the hydrolysis of the N-glycosidic bond of A-4324 of 28 S rRNA. Two methods were employed: (i) h.p.l.c. of the chloroacetaldehyde-reactive material released by RIPs; h.p.l.c. associated with a fluorescence detector allows the identification of adenine and its dosage at quantities as low as 2 ng; (ii) the direct fluorimetric measurement of the material that had reacted with chloroacetaldehyde. The amount of adenine released increases when ribosomes are pretreated in conditions that lead to their dissociation into subunits. Adenine protects ribosomes from the inhibition by ricin A-chain. When ribosomes were incubated with ricin A-chain in the presence of [14C]adenine no incorporation of radioisotope in ribosomes was observed, indicating that neither exchange nor reversal reactions occurred. A binding of [14C]adenine to ricin A chain was not detected by equilibrium dialysis.

Biological and chemical characterization of basic FGF-saporin mitotoxin

Basic fibroblast growth factor (FGF) and saporin-6, a ribosome-inactivating protein, were chemically conjugated and characterized as a cytotoxin to cells expressing the basic FGF receptor. Structural and Western blot analysis of the conjugate showed that it contained saporin and basic FGF in equimolar amounts. The conjugate inhibited protein synthesis in a cell-free system and had potent cytotoxic activity (ID50 = 25pM) for cells expressing the basic FGF receptor. It is equipotent with basic FGF in radioreceptor assays and elutes from heparin Sepharose columns with 2M NaCl. The activity of the mitotoxin can be inhibited by competition with an excess of basic FGF but not nerve growth factor. The possibility that this mitotoxin can be used as an anti-angiogenic factor in paradigms that involve basic FGF is discussed.

Amino acid sequence of Mirabilis antiviral protein, total synthesis of its gene and expression in Escherichia coli

We have determined the complete amino acid sequence of Mirabilis antiviral protein (MAP). MAP is composed of 250 amino acids having a combined molecular weight of 27,833 and contains 23 lysine residues and 7 arginine residues. The amino acid sequence of MAP has 24% homology with the Ricin D-A chain. To carry out systematic structure-function studies of MAP, we have accomplished the total synthesis of its gene. We designed a synthetic MAP gene containing 12 unique restriction sites that were on the average 65 base pairs apart. Thirty synthetic oligonucleotides were enzymatically joined to form DNA duplexes. These were strategically synthesized to have EcoRI and HindIII cohesive ends and were cloned in pUC19. Nine blocks of the synthetic fragments were assembled in pUC19 to form the MAP gene consisting of 759 base pairs. The correctness of the connecting reactions was confirmed by step-wise sequencing of each assembled fragment as well as the total gene. When expressed under control of the tac promoter in Escherichia coli, the synthetic gene gave a protein similar to the native MAP. This was confirmed by an enzyme-linked immunosorbent assay and Western blotting analysis.

Marginal zone macrophages and their role in the immune response against T-independent type 2 antigens: modulation of the cells with specific antibody

Marginal zone macrophages are strategically positioned in the marginal zone of the spleen and are thought to play an important role in the initiation of the immune response to T-independent type 2 responses. The cells are characterized by high phagocytic activity and by the selective uptake of neutral polysaccharides. In the mouse marginal zone macrophages react specifically with the monoclonal antibody ER-TR9. Injection of the antibody resulted in a complete abrogation of the uptake of neutral polysaccharides by the cells in vivo, although the cells were still capable of taking up latex and carbon particles. The complete blockade of the polysaccharide uptake did not result in an altered humoral immune response against this antigen. When the antibody ER-TR9 was coupled to the toxin gelonin a complete elimination of the marginal zone macrophages could be established in vivo. However, complete elimination did not result in changes of the immune responses against 2,4,6-trinitrophenylated Ficoll, suggesting that the marginal zone macrophages are either not involved in this type of response, or that their function can be taken over by other cells. The possible role of these cells and the importance of the spleen in the immune response against bacterial antigens is discussed.

Immunotoxin mediated killing of clonable T-lymphocytes infiltrating an irreversibly rejected human renal allograft

We isolated and treated in vitro with a novel CD5-specific saporin immunotoxin, referred to as OKT1-SAP, the cells infiltrating an irreversibly rejected renal allograft from a patient who rejected while on cyclosporine plus steroids and then failed to respond to multiple courses of high-dose steroids, intravenous OKT3 antibody, and local irradiation to the graft. We report here that under experimental conditions achievable in vivo the immunotoxin OKT1-SAP was capable of eliminating in vitro more than 95% of clonable T-lymphocytes infiltrating the rejected allograft of this patient despite their resistance to previous aggressive immunosuppression. To our knowledge, this is the first report of an immunotoxin-mediated suppression of the clonogenic growth of rejected renal allograft infiltrating T-lymphocytes.

Immunotoxins containing saporin 6 and monoclonal antibodies recognizing plasma cell-associated antigens: effects on target cells and on normal myeloid precursors (CFU-GM)

Monoclonal antibodies 8A and 62B1, recognizing plasma cell-associated antigens, were covalently linked to saporin 6, a ribosome-inactivating protein similar to the A-chain of ricin. Both immunotoxins were tested on target human cell lines U266 and Raji, on non-target K562 cell line and on myeloid CFU-GM progenitors. The cloning efficiency and viability of target cells were strongly reduced by 8A-saporin 6 and 62B1-saporin 6 immunotoxins, with an ID50 up to 200,000-fold lower than free saporin 6, whilst the K562 non-target cell line was unaffected. Normal human myeloid precursors (CFU-GM) were inhibited by immunotoxins only to a limited extent. An application of this model for autologous bone marrow transplantation in multiple myeloma patients is proposed. Since no eradication of cloning target cells was achieved by a single immunotoxin, mixtures made with different antibodies could help to reach this goal.

Hormonotoxins. Preparation and characterization of ovine luteinizing hormone-gelonin conjugate

With the aim of targeting toxins to selected cells in the gonad, we have prepared conjugates of ovine luteinizing hormone (oLH) with a single chain ribosome-inactivating protein called gelonin. The two proteins were thiolated by using N-succinimidyl-3-(2-pyridyldithio)propionate and subsequently reacted under appropriate conditions to form oLH-S-S-gelonin complex. A complete biochemical analysis of thiolated oLH and oLH-gelonin conjugates has been performed. The linkage of the hormone to the toxin probably occurred through a single amino group in the alpha-subunit, with the beta-subunit remaining free. Modification of a single amino group on the alpha-subunit reduced receptor binding and immunological reactivity of the thiolated oLH, but subsequent complexing with the toxin-gelonin did not seriously compromise these activities. oLH and gelonin were calculated to be present in a 1:1 ratio in the hormonotoxin preparation. The conjugate retained significant steroidogenic activity in rat granulosa cells. Upon reaction with mouse tumor Leydig cells (MA-10 cells), the toxin component of the complex became internalized to a sufficient degree to effectively inhibit protein synthesis. The studies provide a rational basis for the design and study of large hormonotoxins.

B-cell restricted saporin immunotoxins: activity against B-cell lines and chronic lymphocytic leukemia cells

B cell-restricted immunotoxins were constructed by conjugating anti-B monoclonal antibodies to saporin, the major ribosome inactivating protein from the seeds of the plant Saponaria officinalis. HD37-SAP is directed against CD19, the broadest B cell-specific determinant. HD39-SAP and HD6-SAP recognize two different epitopes on the CD22 molecule, an antigen present on the cell surface of B cells at late stages of differentiation. All three immunotoxins inhibited DNA synthesis and protein synthesis in target B lymphoma cells with a dose-related effect, in short incubation times and in the absence of potentiators. A clonogenic assay demonstrated that all immunotoxins could eliminate more than two logs of clonogenic malignant B cells with a two-hour incubation at concentrations not toxic to cells not bearing target antigens. The immunotoxin activity was evaluated by DNA synthesis inhibition in fresh B-chronic lymphocytic leukemia cells (B-CLL) stimulated to proliferate by incubation with an antibody specific for the receptor of C3b complement component (CR1) plus B cell growth factor. B-CLL cell DNA synthesis was actively inhibited by treatment at low immunotoxin concentration without need of potentiators. Immunotoxins exerted their effect also in whole blood of CLL patients under conditions achievable in vivo. We conclude that B cell-restricted immunotoxins HD37-SAP, HD39-SAP, and HD6-SAP are good candidates for in vivo therapy of B-cell malignancies.

Selective killing of squamous carcinoma cells by an immunotoxin that recognizes the EGF receptor

We have conjugated a murine monoclonal antibody (B4G7) against the human epidermal growth factor (EGF) receptor to gelonin, a 60S ribosome inactivating protein, via N-succinimidyl-3-(2-pyridyldithio)propionate (SPDP) and 2-iminothiolane. The B4G7-gelonin conjugate bound to the cell surface in proportion to the number of EGF receptors and competed with B4G7 antibody for binding to EGF receptors. The conjugate killed EGF receptor-hyperproducing squamous carcinoma cells (A431, NA, Ca9-22, TE5), and to some extent, human fibroblasts (HFO). It did not kill EGF receptor-deficient small-cell lung cancer cells (H69) and mouse fibroblasts (Swiss/3T3). Free B4G7, gelonin or a mixture of B4G7 and gelonin did not kill A431 cells. The number of EGF receptors was correlated to cytotoxicity at 10(-8) M of the conjugate, and the data were fitted to the regression equation: y = -35.83 log x +233.4 (correlation coefficient = -0.9995). These results suggest that the B4G7-gelonin conjugate may be a useful weapon for targeting therapy to squamous-cell carcinomas.

Hormonotoxins. I. Strategy for synthesis of ovine luteinizing hormone--gelonin conjugate bearing the toxin in the beta-subunit

The amino groups in the beta-subunit of ovine luteinizing hormone (oLH) were modified by thiolation using N-succinimidyl-3-(2-pyridyldithio) propionate so that it may be coupled in a disulfide linkage to similarly modified ribosome inactivating protein, gelonin. The modified beta-subunit was able to hybridize with free LH alpha-subunit and the complex retained full biological activity. However, when gelonin was coupled to the beta-subunit, the resulting conformational changes masked or eliminated the sites necessary for intersubunit recognition of the free alpha-subunit. This has important implications for the design in the synthesis of gonadotropin-toxin/drug conjugates.

Comparison of the pharmacokinetics and hepatotoxic effects of saporin and ricin A-chain immunotoxins on murine liver parenchymal cells

Immunotoxins containing the ribosome-inactivating protein, saporin, are very effective antitumor agents but are highly toxic to mice. They induce severe necrotic lesions in the liver parenchyma of the recipients. Such extensive damage to the liver parenchyma is not observed with ricin A-chain immunotoxins even at 5-fold higher dosage. The hepatotoxicity of the saporin immunotoxins was found in the present study to arise from a combination of two effects. First, saporin and saporin immunotoxins were 30- and 6-fold more toxic to primary cultures of mouse liver parenchymal cells than were ricin A-chain and ricin A-chain immunotoxins, respectively. This was despite the fact that the cells bound 4- to 5-fold less saporin or saporin immunotoxins than ricin A-chain or ricin A-chain immunotoxins. The binding of ricin A-chain and its immunotoxin to the cells was mediated through the carbohydrate residues present on the A-chain whereas saporin is not glycosylated and thus must bind to other sites on the cell surface which result in transport of saporin relatively efficiently to the cytosol. The second reason for the hepatotoxic action of the saporin immunotoxin was that it had a longer blood half-life (t 1/2 alpha = 1.1 h; t 1/2 beta = 17.1 h) than the ricin A-chain immunotoxin (t 1/2 = 0.52 h; t 1/2 beta = 9.7 h). Analyses using a two-compartment pharmacokinetic model showed that the two immunotoxins broke down in vivo to give free antibody at a similar rate (t 1/2 = 10-12 h) but that the ricin A-chain immunotoxin was eliminated 11 times more rapidly than the saporin immunotoxin by routes other than breakdown. It was calculated that, in mice given a median lethal dose of saporin immunotoxin, the blood levels of immunotoxin remained above the concentration that killed 50% of parenchymal cells in vitro for more than 48 h. In mice given a median lethal dose of ricin A-chain immunotoxin, the blood levels fell below the concentration that was toxic to parenchymal cells in vitro within 4 h. The longer blood half-life of the saporin immunotoxin may also explain our previous finding that it had antitumor activity superior to that of a ricin A-chain immunotoxin in mice.

Bispecific F(ab' gamma)2 antibody for the delivery of saporin in the treatment of lymphoma

This work describes the successful use of bispecific F(ab' gamma)2 antibody (Ab) in combination with a ribosome-inactivating protein (RIP), saporin, for the treatment of neoplastic disease in vivo. A total of three thioether-linked F(ab' gamma)2 heterodimers were prepared, each having dual specificities for saporin and the guinea pig lymphoblastic leukemia, L2C. In all three cases specificity for the L2C cells was provided by a high affinity mouse anti-idiotype (anti-Id) mAb, whereas the antisaporin activity came from either one of two mouse mAb or an affinity-purified rabbit polyclonal Ab. In vitro studies, measuring protein synthesis, showed that all three derivatives were extremely efficient at delivering saporin to L2C cells, to the extent that addition of the rabbit Fab' gamma-containing bispecific Ab to cell culture at 1 microgram/ml increased the toxicity of saporin (50% inhibiting concentration) by close to 90,000-fold. Similarly, in leukemic guinea pigs, a small dose of saporin (10 micrograms) which by itself showed no therapeutic effect, was able to completely eradicate Id-positive tumor when given in combination with an excess of bispecific Ab. Although tumors did eventually emerge in most of these animals, immunofluorescence analysis showed that in almost all instances the escaping cells were Id- variants of the L2C. Experiments to define the optimal treatment regimen in this model showed that, although the administration of saporin and bispecific Ab at separate sites could be therapeutically effective, mixing the Ab and saporin to form immune complexes before injection did generally enhance their performance. A molar surplus of bispecific Ab in these mixtures both extended the metabolic survival of the saporin and enhanced the therapeutic performance, with molar ratios above 3:1 generally being required for optimum treatment when using saporin at 10 micrograms. Derivatives containing polyclonal antisaporin were more efficient than those containing mAb, yielding optimum therapeutic results with a molar ratio of 1.5:1 when combined with 10 micrograms saporin. These findings have shown that bispecific F(ab' gamma)2 Ab, as well as being straightforward to prepare, can also function as an extremely efficient vector for delivering cytotoxic agents such as ribosome-inactivating protein to unwanted cells in vivo.

Evaluation of antihuman T lymphocyte saporin immunotoxins potentially useful in human transplantation

We have synthesized 3 immunotoxins (ITs) by covalently coupling the saporin-6 hemitoxin (SAP) to OKT11, SOT3, and SOT1a murine monoclonal antibodies that recognize human T lymphocyte CD2, CD3, and CD5 surface antigens, respectively. The resulting ITs, referred to as OKT11-SAP, SOT3-SAP, and SOT1a-SAP, are equally effective in inhibiting eukaryotic protein synthesis in a cell-free system, and all 3 ITs bind to human T lymphocytes in an almost comparable manner. However, these reagents differ markedly in their ability to kill target T lymphocytes as assessed by measuring the inhibition of DNA synthesis and growth of clonable T lymphocytes in response to mitogenic and allogeneic stimuli. Whereas the anti-CD2 IT, OKT11-SAP, shows moderate cytotoxicity against T lymphocytes, the anti-CD3 IT, SOT3-SAP, and the anti-CD5 IT, SOT1a-SAP, are highly effective in eliminating the same target cells. The concentrations inhibiting 50% (IC50) of T lymphocyte DNA synthesis are 60 nM, 4.5 nM, and 1.4 nM for OKT11-SAP, SOT3-SAP, and SOT1a-SAP, respectively. Among 3 tested lysosomotropic amines, i.e., ammonium chloride, chloroquine, and amantadine, the latter only moderately potentiates the cytotoxicity of SOT1a-SAP (IC50 0.36 nM). We show that the conditions under which T lymphocyte killing is accomplished require less than 10 min exposure of T lymphocytes to the ITs, in the absence of adjuvant molecules artificially added to the incubation medium and at physiologic culture pH. These experimental characteristics of unprecedented closeness to a physiologic in-vivo model are likely to reflect the biophysical properties of the SAP moiety of the ITs. We conclude that clinical studies are warranted to define the advantage of using SAP ITs over previously described immunoconjugates.

Selective cytotoxic activity of immunotoxins composed of a monoclonal anti-Thy 1.1 antibody and the ribosome-inactivating proteins bryodin and momordin

The ribosome-inactivating proteins, bryodin, from Bryonia dioica, and momordin, from Momordica charantia, were coupled by a disulphide bond to a monoclonal anti-Thy 1.1 antibody (OX7). Both immunotoxins were specifically cytotoxic to the Thy 1.1-expressing mouse lymphoma cell line AKR-A in vitro. The OX7-bryodin immunotoxins were the more powerfully toxic and reduced protein synthesis in AKR-A cells by 50% at a concentration of 1-4 x 10(-11) M as compared with 1 x 10(-9) M for the OX7-momordin immunotoxins. Neither of the immunotoxins was toxic to mouse lymphoma EL4 cells, which lack the Thy 1.1 antigen, at concentrations up to 3 x 10(-8) M. Further, bryodin and momordin immunotoxins made from an antibody (R10) of irrelevant specificity were without effect on AKR-A cells.

Trichokirin, a ribosome-inactivating protein from the seeds of Trichosanthes kirilowii Maximowicz. Purification, partial characterization and use for preparation of immunotoxins

A protein, here named trichokirin, was extracted from the seeds of Trichosanthes kirilowii and purified by ion-exchange and gel-filtration chromatography. Trichokirin is a basic glycoprotein of apparent relative molecular mass of 27,000 with a strong ribosome-inactivating activity. Alignment of the trichokirin, trichosanthin and momordin N-terminal sequences shows a substantial degree of homology. Trichokirin was conjugated to a monoclonal antibody directed against the Thy 1.2 antigen with the cleavable dimethyl 3,3'-dithiobispropionimidate cross-linking reagent. This immunotoxin selectively killed leukemia cells expressing the Thy 1.2 antigen. The addition of ammonium chloride, which increases the cytotoxicity of ricin A-chain immunotoxins, blocks that of the trichokirin immunotoxin, suggesting that they enter cells by different mechanisms. In vivo studies showed that the pharmacokinetic properties of the trichokirin immunotoxin could be more advantageous than those of the ricin A-chain immunotoxins for in vivo applications.

Ribosome-inactivating proteins from plants inhibit ribosome activity of Trypanosoma and Leishmania

Ribosomes from Trypanosoma brucei rhodesiense and from Leishmania infantum were isolated and optimal conditions for in vitro translation were established. The effect of ribosome-inactivating proteins extracted from several plants was then assessed in order to identify those suitable for the preparation of immunotoxins against these organisms. Ribosomes from both species were inactivated by some ribosome-inactivating proteins (dianthins, saporins, pokeweed antiviral proteins, and the ribosome-inactivating chain of abrin). The similarity of the effects on the ribosomes from the two species examined indicates that ribosome-inactivating proteins should also be effective in a similar way on ribosomes from other species of Trypanosoma and Leishmania.

Synthesis and characterization of an antihuman T-lymphocyte saporin immunotoxin (OKT1-SAP) with in vivo stability into nonhuman primates

The authors conjugated, by a disulphide bond, the antihuman T-lymphocyte (CD5) monoclonal antibody (MoAb) OKT1 to the saporin-6 (SAP) ribosome-inactivating protein of the plant Saponaria officinalis. The resulting OKT1-SAP immunotoxin bound to CD5-expressing target cells and under standard culture conditions specifically suppressed mitogen-induced-T-lymphocyte DNA and protein synthesis in a dose-related manner. T-lymphocyte killing was achieved by five-minute exposure of the target cells to OKT1-SAP. The concentration inhibiting 50% (IC50) of T-lymphocyte DNA synthesis was 0.32 nmol/L. The potency of OKT1-SAP was moderately enhanced by amantadine (IC50 0.08 nmol/L) but not by ammonium chloride or chloroquine. Whole blood components did not interfere with the efficacy of OKT1-SAP, as in vitro treatment of fresh whole blood resulted in effective elimination of clonable peripheral blood T-lymphocytes assessed by a limiting dilution assay. Because these characteristics of T-lymphocyte killing by OKT1-SAP (ie, rapidity of action, potency also without potentiators) and lack of inhibition by whole blood components may be relevant for the use of an immunotoxin as a therapeutic agent in humans, the authors evaluated the stability in vivo and the circulatory clearance of OKT1-SAP in cynomolgus monkeys. Following a single intravenous (IV) injection of nontoxic dosages (0.16 to 1.3 mg/kg), an initial rapid decline (t1/2 alpha = 1.0 to 4.1 hours) was followed by a long-lasting slower decrease (t1/2 beta = 11.6 to 20.6 hours) of OKT1-SAP plasma concentrations as detected by double-antibody solid phase enzyme-linked immunosorbent assay (ELISA) assay. Not only did OKT1-SAP remain intact immunologically but it also retained its biological activity, as measured by the ability of plasma samples from monkeys given immunotoxin to inhibit DNA synthesis in human T-lymphocytes. Taken together the findings presented in this article indicate the feasibility of using OKT1-SAP as a therapeutic tool and provide information that will facilitate the rational use of immunotoxins as a treatment modality in humans.

In vivo administration of lymphocyte-specific monoclonal antibodies in nonhuman primates. IV. Cytotoxic effect of an anti-T11-gelonin immunotoxin

The cytotoxic effect of a lymphocyte-specific immunotoxin formed by disulfide conjugation of an anti-T11 monoclonal antibody with the ribosome-inactivating protein gelonin was assessed in vitro on peripheral blood T cells and in vivo on splenic and lymph node T cells of macaque monkeys. This immunotoxin was cytotoxic to proliferating peripheral blood T cells in vitro as measured by both direct and indirect assays. Two sequential intravenous infusions into macaque monkeys achieved plasma concentrations of immunotoxin far in excess of those shown to be cytotoxic for cultured T cells and coated all T cells in lymph nodes and spleen with intact immunotoxin for four days. However, the cytotoxic effect of the immunotoxin on T cells in vivo was considerably less than that predicted by the in vitro studies. Further experiments suggested that the state of activation of the targeted T cell population in vivo, or the appearance of anti-immunotoxin antibodies, which occurred in all infused monkeys, might attenuate immunotoxin-mediated cell killing in vivo. These studies illustrate the significant differences between the action of immunotoxin conjugates in vitro, and those seen when these conjugates are utilized as therapeutic agents in vivo.

Activity of a monoclonal antibody-saporin-6 conjugate against B-lymphoma cells

A monoclonal antibody reactive with the immunoglobulin heavy chain (TEC IgM) has been conjugated to saporin-6 (SAP), which is the major ribosome-inactivating protein from the seeds of the plant Saponaria officinalis. Studies with Burkitt's lymphoma cell line Bjab 113 demonstrate that this immunotoxin is capable of killing 3 logs (99.9%) of clonogenic lymphoma cells after a 2-hour incubation. The presence of human bone marrow inhibits the activity of the conjugate. However, full potency of TEC IgM-SAP immunotoxin is restored by adding 1 mM amantadine to the incubation medium. The reaction is highly specific and is inhibited by the presence of excess anti-mu-antibody or human serum. Clonal growth of other Burkitt's lymphoma cell lines is inhibited to a lesser extent by the immunotoxin. The presence of surface IgM on the different cell lines is directly correlated to target cell killing by TEC IgM-SAP. Isolation of Bjab 113 clones surviving treatment demonstrates that only a minority are truly resistant and that the others randomly escape the treatment. The highly potent and specific activity of this conjugate in the presence of bone marrow buffy coat and its exceptionally rapid onset of action make this conjugate a good candidate for the ex vivo elimination of neoplastic cells from the bone marrow of non-Hodgkin's lymphoma patients.

Dissociation of the adrenocorticotropin secretory responses to corticotropin-releasing factor (CRF) and vasopressin or oxytocin by using a specific cytotoxic analog of CRF

Control of ACTH secretion in the pituitary in the absence of target cells for CRF, the most potent ACTH secretagogue, was studied in dissociated bovine anterior pituitary cells treated with a potent selective cytotoxin. The cytotoxin is a conjugate of the CRF analog [Nle21,38, Arg36]rat (r) CRF and the plant toxin gelonin. Dissociated bovine anterior pituitary cells were pretreated with vehicle, 2 nM ovine CRF, 2 nM cytotoxic conjugate, or unconjugated [Nle21,38,Arg36]rCRF and gelonin in amounts equivalent to that of 2 nM cytotoxic conjugate for 12 h, then extensively washed and cultured for 3 days before acute secretion experiments. Unstimulated ACTH secretion was similar in all groups. ACTH secretion in response to CRF was attenuated by pretreatment with the cytotoxic conjugate; CRF (2.5 nM)-stimulated secretion was 7.0, 6.3, and 2.8 times the unstimulated rate in cells pretreated with vehicle, 2 nM CRF, or 2 nM cytotoxic conjugate, respectively. Likewise, the ACTH secretory response to a cAMP analog was attenuated by pretreatment with the conjugate; 8-bromo-cAMP (10 mM)-stimulated secretion was 6.8, 7.1, and 3.3 times the unstimulated rate in cells pretreated with vehicle, CRF, or conjugate, respectively. In contrast, the ACTH responses to vasopressin (VP) or oxytocin (OR) remained intact. VP stimulated the ACTH secretion rate by 4.2, 4.0, and 3.5 times, respectively, in the three groups. OT stimulated the ACTH secretion rate by 2.7, 2.6, and 2.3 times in the three groups. Pretreatment with the conjugate attenuated the response to CRF and VP in combination by the same amount as it attenuated the response to CRF alone. The ACTH secretory responses in cells pretreated with unconjugated [Nle21,38,Arg36]rCRF and gelonin were not different from responses in cells pretreated with vehicle. These results suggest that there is a separate mechanism or cell type for OT- and VP-stimulated ACTH secretion distinct from that responsible for the action of CRF on pituitary cells.

Introduction of definite amounts of nonpermeant molecules into living cells after electropermeabilization: direct access to the cytosol

The possibility of introducing definite amounts of nonpermeant molecules into electropermeabilized living cells has been approached by quantifying the amounts of Lucifer Yellow (LY; a 457-Da highly fluorescent molecule) and Phytolacca americana (Pokeweed) antiviral protein (PAP; a 30,000-Da ribosome-inactivating protein) retained by the cells after closure of the electric-field-induced transient structures of permeation. Without the electropermeabilization, these two molecules enter the cell only in very small amounts by fluid-phase pinocytosis. Under our experimental conditions, using the NIH 3T3 cells, the intracellular LY concentration can reach a value equivalent to the extracellular concentration and can be regulated by controlling the external concentration. We describe the use of LY in a rapid and efficient test for the determination of the best electrical-shock conditions of other cell lines. After electropermeabilization, PAP is 2 X 10(5) times more cytotoxic. Its toxicity can be detected at external concentrations (10(-11) M) corresponding to less than 10 internalized molecules per electropermeabilized cell. Therefore, after electropermeabilization, the nonpermeant molecules have a direct access to the cytosol and the biological effect of nonpermeant substances can be revealed.

An immunotoxin containing a rat IgM monoclonal antibody (Campath 1) and saporin 6: effect on T lymphocytes and hemopoietic cells

The elimination of the cells responsible for graft-versus-host disease in allogeneic bone marrow transplantation has been attempted with a variety of methods, including the use of the ribosome-inactivating toxin ricin bound to monoclonal antibodies acting as carriers. However the high nonspecific toxicity of these immunotoxins containing the whole toxin greatly limited clinical application. Toxicity can be reduced using the A-chain of ricin or other ribosome-inactivating proteins (RIPs) which are devoid of a B-chain with lectin properties. We used saporin 6 purified from Saponaria officinalis seeds, which was conjugated with the rat IgM monoclonal antibody Campath 1 specific for mature T and B lymphocytes as well as for monocytes. The immunotoxin retained both RIP and antibody activity, inhibiting protein synthesis both in a cell-free system and in cells bearing the Campath 1 antigen; it also abolished methyl 3H-thymidine uptake in phytohemagglutinin-stimulated T lymphocytes. Myeloid progenitors were largely spared as shown by myeloid stem cell (CFU-GM) growth which was scarcely affected. Toxicity of the immunotoxin to cell lines not expressing the antigen recognized by Campath 1 monoclonal antibody was not greater than the toxicity due to free saporin 6, while the immunotoxin was more toxic to mice than free saporin.

Hepatotoxicity of immunotoxins made with saporin, a ribosome-inactivating protein from Saponaria officinalis

Immunotoxins were prepared by conjugating saporin, a ribosome-inactivating protein from Saponaria officinalis, to a monoclonal antibody against the Thy1.1 antigen, or to its F(ab')2 fragment. The immunotoxins were eight- to 16-fold more toxic to mice than free saporin. Injection of the immunotoxins induced necrosis of the liver and spleen, whereas free saporin caused necrosis of the epithelium of the kidney tubules. The cytoplasm of the hepatic parenchymal cells was affected by the immunotoxins, lesions being apparent in the rough endoplasmic reticulum and, later, in the mitochondria. These changes were associated with a reduced capacity to synthesise proteins both in the intact liver and by isolated liver microsomes. Studies of the in vivo distribution showed that 90% of the free saporin was removed from the bloodstream, mainly by the kidneys, within 10 min of injection. By contrast, the immunotoxins persisted in the blood for several hours and the only organ in which they consistently accumulated was the liver. The hepatotoxic effect of the immunotoxins was not due to their binding to liver cells via the antigen-binding sites or the Fc-piece of the antibody moiety, nor was it due to hepatic recognition of carbohydrate in the immunotoxin. It is concluded that free saporin, although capable of entering liver cells, is filtered so rapidly by the kidney that liver damage does not occur to a significant extent. Filtered saporin, however, is reabsorbed by renal tubules, whose epithelial cells are damaged. The antibody-saporin conjugate is too large to filter at the glomerulus and so has greater opportunity to penetrate into and to damage the hepatic parenchymal cell.