Purification of Pseudomonas aeruginosa exotoxin by affinity chromatography

Assessment of the Effects of Single-Domain Anti-Idiotypic Distribution Enhancers on the Disposition of Trastuzumab and on the Efficacy of a PE24-Trastuzumab Immunotoxin

BACKGROUND/OBJECTIVES

Antibody-based therapies often exhibit limited distribution within solid tumors due to the "binding-site barrier" (BSB). Our group has developed and validated the use of anti-idiotypic distribution enhancers (AIDEs), which transiently block antibody binding, improving intra-tumoral distribution and efficacy. This study evaluated 1HE and LG1, model anti-trastuzumab AIDEs, in combination with trastuzumab-PE24, a highly potent immunotoxin.

METHODS

The effects of 1HE on the whole-body disposition of radiolabeled trastuzumab were assessed in NCI-N87 tumor-bearing mice. Mechanistic pharmacokinetic/pharmacodynamic (PK/PD) modeling was employed to explore how AIDE binding kinetics influence antibody intra-tumoral distribution and immunotoxin potency. Trastuzumab-PE24 was developed by site-specific conjugation, enabled by self-splicing split intein, with cytotoxicity tested on various cell lines in vitro. The impact of 1HE and LG1 coadministration on trastuzumab-PE24 efficacy was evaluated in NCI-N87 xenograft-bearing mice.

RESULTS

1HE coadministration decreased trastuzumab tumor maximum concentration, reducing tumor terminal slope by 8% and overall tumor exposure by 2.6%, without negatively affecting selectivity. Modeling predicted the optimal AIDE dissociation rate constant for trastuzumab-PE24 to be between 0.015 and 0.3 h-1. The coadministration of trastuzumab-PE24 with 1HE and LG1 improved anti-tumor efficacy and extended median survival to 60 days (p = 0.0002).

CONCLUSIONS

AIDE coadministration led to minimal negative impacts on overall tumor exposure, consistent with model simulations. AIDE coadministration improved the efficacy of trastuzumab-PE24 in NCI-N87 xenografts. Modeling further predicted that repeated AIDE administration with trastuzumab-PE24 could induce complete tumor regression. These findings highlight the advantages of the AIDE strategy, particularly when coadministered with highly potent immunotoxins.

Targeted Delivery of Endosomal Escape Peptides to Enhance Immunotoxin Potency and Anti-cancer Efficacy

This work describes use of anti-carcinoembryonic antigen antibodies (10H6, T84.66) for targeted delivery of an endosomal escape peptide (H6CM18) and gelonin, a type I ribosome inactivating protein. The viability of colorectal cancer cells (LS174T, LoVo) was assessed following treatment with gelonin or gelonin immunotoxins, with or without co-treatment with T84.66-H6CM18. Fluorescent microscopy was used to visualize the escape of immunoconjugates from endosomes of treated cells, and efficacy and toxicity were assessed in vivo in xenograft tumor-bearing mice following single- and multiple-dose regimens. Application of 25 pM T84.66-H6CM18 combined with T84.66-gelonin increased gelonin potency by ~ 1,000-fold and by ~ 6,000-fold in LS174T and LoVo cells. Intravenous 10H6-gelonin at 1.0 mg/kg was well tolerated by LS174T tumor-bearing mice, while 10 and 25 mg/kg doses led to signs of toxicity. Single-dose administration of PBS, gelonin conjugated to T84.66 or 10H6, T84.66-H6CM18, or gelonin immunotoxins co-administered with T84.66-H6CM18 were evaluated. The combinations of T84.66-gelonin + 1.0 mg/kg T84.66-H6CM18 and 10H6-gelonin + 0.1 mg/kg T84.66-H6CM18 led to significant delays in LS174T growth. Use of a multiple-dose regimen allowed further anti-tumor effects, significantly extending median survival time by 33% and by 69%, for mice receiving 1 mg/kg 10H6-gelonin + 0.1 mg/kg T84.66-H6CM18 (p = 0.0072) and 1 mg/kg 10H6-gelonin + 1 mg/kg T84.66-H6CM18 (p = 0.0017). Combined administration of gelonin immunoconjugates with antibody-targeted endosomal escape peptides increased the delivery of gelonin to the cytoplasm of targeted cells, increased gelonin cell killing in vitro by 1,000-6,000 fold, and significantly increased in vivo efficacy.

Pseudomonas aeruginosa exotoxin A: alterations of biological and biochemical properties resulting from mutation of glutamic acid 553 to aspartic acid

Glutamic acid 553 of Pseudomonas aeruginosa exotoxin A (ETA) was identified earlier as a putative active-site residue by photoaffinity labeling with NAD. Here ETA-E553D, a cloned form of the toxin in which Glu-553 has been replaced by aspartic acid, was purified from Escherichia coli extracts and characterized. Cytotoxicity of the mutant toxin for mouse L-M cells was less than 1/400,000 that of the wild type. The mutation caused a 3200-fold reduction in NAD:elongation factor 2 ADP-ribosyltransferase activity, as estimated by assays with an active fragment derived from the toxin by digestion with thermolysin. NAD glycohydrolase activity was reduced somewhat less, by a factor of 50, and photoaffinity labeling with NAD by a factor of 2. We detected less than 2-fold change in the values of KM for NAD or elongation factor 2 and no change in KD for NAD, as determined by quenching of protein fluorescence. The drastic reduction of ADP-ribosyltransferase activity therefore results primarily from an effect of the mutation on kcat, implying that Glu-553 plays an important and possibly direct role in catalyzing this reaction. The effects of the E553D mutation are similar to those of the E148D mutation in diphtheria toxin, supporting the notion that these two Glu residues perform the same function in their respective toxins.

Functionally distinct monoclonal antibodies reactive with enzymatically active and binding domains of Pseudomonas aeruginosa toxin A

Monoclonal antibodies (MAbs) are described which react with two discrete structural domains of Pseudomonas aeruginosa toxin A and which have two distinct functional profiles. The MAbs designated T3-1C7 and T4-1F2 reacted with a 46,000-dalton peptide similar to the putative B or binding fragment of toxin A. These antibodies neutralized the cytotoxic and lethal properties of toxin but had no effect on its ADP-ribosyl transferase activity. T4-1F2 interfered with the binding of toxin A to membrane receptors on mouse fibroblasts (L cells), although the epitope for the antibody appears to be distinct from the actual receptor binding site. The MAb designated T2-1H2 reacted with intact toxin A and with a cloned, enzymatically active carboxy-terminal polypeptide similar to the toxin A fragment. This MAb neutralized the ADP-ribosyl transferase activity of activated holotoxin and of the cloned peptide, but inhibited neither binding of toxin to membrane receptors nor its cytotoxic and lethal actions. The complementary specificity and function of these MAbs confirm the functional specialization of discrete structural domains within the toxin A molecule. Our findings suggest the greater antitoxic potential of antibodies that block binding, compared with those which inhibit the enzymatic activity of toxin A.

Reaction of antibody in sera from cystic fibrosis patients with non-toxic forms of Pseudomonas aeruginosa exotoxin A

Sera from 48 cystic fibrosis patients from two hospitals were screened for antibody against rods, non-toxic macromolecular structures which share antigenic determinants with Pseudomonas aeruginosa exotoxin A. A solid-phase radioimmunoassay employing (125I)-staphylococcal protein A was used to detect anti-rod IgG. Antibodies recognizing rods, exotoxin A, or both antigens, were demonstrated using a competitive radioimmunoassay in cystic fibrosis patient sera, and in sera from animals immunized with exotoxin A, rods, or infected with Pseudomonas aeruginosa. Anti-rod titers of cystic fibrosis patients (1.07 to 14 X control serum levels) inversely correlated with aggregate clinical evaluation scores, and in most instances, with X-ray scores. Since rods are non-toxic and cross-reactive with exotoxin A, they may represent therapeutically useful antigens for producing immunity to exotoxin A.

Proteinases of Pseudomonas aeruginosa evoke mucin release by tracheal epithelium

We have determined the potential of exoproducts from pathogenic bacteria to stimulate the release of high molecular weight mucins from goblet cells of airway epithelium in a rabbit tracheal explant system. Culture supernatants from proteolytic strains of Pseudomonas aeruginosa and Serratia marcescens, but not supernatants from a number of non-proteolytic strains, released mucins from goblet cells. Highly purified elastase and alkaline proteinase from P. aeruginosa stimulated goblet cell mucin release in a dose-dependent fashion. Lipopolysaccharide, exotoxin A, and alginate of P. aeruginosa did not possess mucin release properties. Proteolytic activity was required for mucin release by P. aeruginosa elastase, but such release in goblet cells was not mediated by cyclic AMP. Morphologic studies suggested rapid release of mucins from goblet cells was response to elastase by a process resembling apocrine secretion. Several nonbacterial proteinases mimicked the effect of Pseudomonas proteases. These studies provide support for the hypothesis that bacterial and other play a role in the pathogenesis of mucus hypersecretion in acute and chronic lung infections.

A macromolecular structure produced by Pseudomonas aeruginosa is recognized by antibody to exotoxin A

Organized particulate structures (rods) identified in purified preparations of exotoxin A from culture supernatants of Pseudomonas aeruginosa PA103 were found to be immunochemically cross-reactive with exotoxin A. The rods were visualized by electron microscopy after negative staining as hollow tubes or sheaths (45 by 15 nm). Purified rods were not toxic and not enzymatically active in the ADP-ribosylation assay. Antigenic cross-reactivity between exotoxin A and rods was demonstrated by using monoclonal antibodies directed against either rods or a toxoid of exotoxin A. Hybridoma clones derived from mice immunized with rods or toxoid reacted with both antigens in the enzyme-linked immunosorbent assay. Rods could be dissociated by boiling and resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis into three subunit polypeptides with molecular weights of 70,000, 45,000, and 27,000. Two of the three subunit polypeptides reacted both with antirod and antitoxin monoclonal antibodies after electrophoretic transfer of sodium dodecyl sulfate-polyacrylamide gel electrophoresis-separated proteins to nitrocellulose filters. The results indicate that rods and exotoxin A share common antigenic determinants.

In vitro inhibition of lymphocyte proliferation by Pseudomonas aeruginosa phenazine pigments

Human lymphocyte proliferation is inhibited in vitro in the presence of killed Pseudomonas aeruginosa or cell-free P. aeruginosa culture supernatants. A comparison of culture supernatants obtained under similar conditions from Staphylococcus aureus, Escherichia coli, P. aeruginosa, and Pseudomonas cepacia strains demonstrated that all P. aeruginosa supernatants were strongly inhibitory, whereas supernatants from other bacteria were mildly inhibitory or not inhibitory at all. These P. aeruginosa inhibitors prevent proliferative responses of resting cells upon mitogen activation and decrease [3H]thymidine uptake when added to human lymphocytes undergoing active proliferation in culture. The inhibitory effect is reversible and not due to cytotoxicity. Most of the inhibitory activity present in crude supernatants was detected in ultrafiltrates of molecular weights below 2,000. Purified P. aeruginosa pyocyanine, a low-molecular-weight phenazine pigment present in culture supernatant, was strongly inhibitory for lymphocyte proliferation. Extraction of pyocyanine and phenazine pigments from inhibitory P. aeruginosa supernatants eliminated their inhibitory activity. Inhibitors were recovered from reverse-phase chromatographic cartridges by both chloroform and methanol elution, indicating that pyocyanine and other phenazine pigments present in P. aeruginosa supernatants are responsible for the inhibition of lymphocyte proliferation. In addition to the identification of phenazine pigments as lymphocyte proliferation inhibitors, several criteria ruled out major contributions of P. aeruginosa polysaccharide, exotoxin A, and proteases to this phenomenon. P. aeruginosa strains selected for very low protease production or for very low exotoxin A production produced supernatants as inhibitory for lymphocyte proliferation as supernatants obtained from clinical P. aeruginosa isolates. Purified P. aeruginosa lipopolysaccharide and protease preparations failed to induce reversible lymphocyte proliferation inhibition. Finally, heat inactivation of P. aeruginosa supernatants at 100 degrees C for 60 min inactivates exotoxin A and proteases but produced only a moderate decrease of the inhibitory activity for lymphocyte proliferation.

Clinical significance of serum antibody responses to exotoxin A and type-specific lipopolysaccharides in patients with Pseudomonas aeruginosa infections

Serum antibodies to Pseudomonas aeruginosa exotoxin A and immunotype-specific lipopolysaccharides were evaluated as diagnostic and prognostic markers in patients with Pseudomonas disease. Hemagglutination titers to exotoxin A were 1:1,024 or higher and/or showed a fourfold acute-to-convalescent increase in 17 of 25 (68 percent) patients infected with Pseudomonas compared with only one of seven (15 percent) colonized (p = 0.01) and two of 24 (8 percent) culture-negative patients (p less than 0.001). By comparison, hemagglutination titers to the lipopolysaccharide of patients' Pseudomonas isolates were 1:1,024 or higher or showed a fourfold increase in only four of 17 (24 percent) infected patients and in none of six (0 percent) colonized patients (p = 0.96). Serial antibody titers to exotoxin A provided serologic confirmation of invasive disease, distinguished infection from colonization, and, in the case of decreasing titers, indicated successful therapy. It is concluded that serum antibodies to exotoxin A are useful serologic markers for the clinical assessment of Pseudomonas infections in man.

Pseudomonas aeruginosa exotoxin A inhibits proliferation of human bone marrow progenitor cells in vitro

Pseudomonas aeruginosa exotoxin A, a potent inhibitor of eukaryotic protein synthesis, is produced in vivo during human infection. We tested the hypothesis that exotoxin A may be responsible for the leukopenia which sometimes accompanies pseudomonas disease by examining the in vitro toxicity of exotoxin A for human bone marrow granulocyte-macrophage progenitor cells (colony-forming units in culture [CFU-c] in the soft agar cloning system. Colony formation by freshly obtained marrow cells from five normal subjects was inhibited by exotoxin A in a concentration-dependent manner. The mean 50 and 100% inhibitory concentrations of toxin were 1.4 x 10(-10) and 1.4 x 10(-8) M, respectively, and significant inhibition was observed at a toxin concentration as low as 1.4 x 10(-13) M in two subjects. The inhibitory effect of exotoxin A on colony formation was specifically neutralized by antiserum to exotoxin A. Although mouse CFU-c were somewhat less sensitive to exotoxin A in vitro compared with human CFU-c, exotoxin A produced significant leukopenia in vivo in mice. These data suggest a possible mechanism for the leukopenia which sometimes occurs in human pseudomonas disease.

Pseudomonas aeruginosa mutants altered in their sensitivity to the effect of iron on toxin A or elastase yields

Iron affects yields of toxin A, alkaline protease, elastase, pyochelin, and pyoverdin in Pseudomonas aeruginosa. Mutants of P. aeruginosa PAO1 resistant to the effect of iron on toxin (toxC) or elastase (elaC) yields were isolated. Two types of mutants were isolated: iron transport and iron regulatory mutants. The toxC regulatory mutants produced toxin A in medium containing iron; however, yields of elastase and alkaline protease remained sensitive to regulation by iron. The elaC regulatory mutants were resistant to the effect of iron on elastase yields, but toxin A and alkaline protease yields were decreased by iron, analogous to the parent strain. These data suggest that toxin A, elastase, and alkaline protease yields can be independently regulated by iron.

Protection against experimental Pseudomonas aeruginosa infection in mice by active immunization with exotoxin A toxoids

The immunoprophylactic effect of chemically inactivated Pseudomonas aeruginosa exotoxin A in experimental pseudomonas infections was studied. Exotoxin A toxoids were prepared by Formalin (f-TXD) or glutaraldehyde (g-TXD) treatment. Immunization of mice with three or four doses (10 micrograms each) of f-TXD and the synthetic adjuvant N-acetylmuramyl-L-alanyl-D-isoglutamine (50 micrograms) induced high levels of antiexotoxin A antibodies as measured by passive hemagglutination assay and enzyme-linked immunosorbent assay. Immunization with toxoid alone did not elicit antitoxin. A significant increase in survival time and survival rate (P less than 0.01) was seen in immunized (f-TXD) and in burned and infected mice (50 to 85%) as compared with control mice immunized with formalinized bovine serum albumin (6 to 20%). Virtually 100% survival was obtained when preinfection immunization weas combined with single-dose gentamicin treatment within 24 h of infection. Immunization with g-TXD increased survival time (P less than 0.01) but did not consistently increase survival rate, and the results were not as satisfactory as those with formalinized exotoxin. The data presented indicate that active immunization with formalinized exotoxin A toxoid and adjuvant induced protective immunity to various degrees against infections in mice and could be potentially useful in prophylaxis of P. aeruginosa infections.

Toxin A-deficient mutants of Pseudomonas aeruginosa PA103: isolation and characterization

An immunological assay utilizing double-diffusion principles was developed for identification of Pseudomonas aeruginosa mutants deficient in toxin A. Mutations were chemically induced, and mutants were isolated from P. aeruginosa strain PA103. Quantitative assays, both enzymatic and immunological, indicated that five mutants produced toxin A in vitro at levels of 0.3% or less of parental strain levels. Characterization indicated that the mutants fell into four classes and suggested that multiple genes are involved in the regulation of toxin A yields. Classes 1 to 3 produced less than 1% of parental levels of extracellular toxin A. Class 1 mutants are apparently specific for toxin A. Class 2 mutants are pleotropic and produced toxin A, protease, and other extracellular proteins at reduced yields. Class 3 mutants are pleotropic and in addition have relatively high levels of cell-bound toxin A. Class 4 mutants produce toxin A at levels greater than 1% of parental yields. Of 16 toxin A-deficient mutants examined, only 1 was a class 1 mutant. This mutant (PA103-29) was shown to be identical to the parental strain in all respects tested except for its marked deficiency in toxin A. The suitability of this class 1 mutant for use in virulence studies is discussed.

Production of exoenzyme S during Pseudomonas aeruginosa infections of burned mice

Antisera which distinguished between Pseudomonas aeruginosa exoenzyme S and toxin A neutralized the adenosine diphosphate ribosyl transferase activity of the homologous, but not the heterologous, enzyme. Skin extracts and sera from burned mice infected with the exoenzyme S-producing strain P. aeruginosa 388 contained adenosine diphosphate ribosyl transferase activity that was not found in skin extracts or sera from uninfected mice. On the basis of immunological reactivity and enzymatic properties, the adenosine diphosphate ribosyl transferase activity present in skin extracts and sera from P. aeruginosa 388-infected mice was identified as exoenzyme S. Active elongation factor 2 levels in tissues from strain 388-infected mice were normal at 24 h postinfection, indicating that strain 388 does not produce detectable amounts of toxin A in vivo. An unexpected finding in this investigation was the presence of exoenzyme S-inactivating activity in the sera from some nonimmunized animals.

Enzyme-linked immunosorbent assay for Pseudomonas aeruginosa exotoxin A

An enzyme-linked immunosorbent assay (ELISA) is described for Pseudomonas aeruginosa exotoxin A. A double antibody sandwich method was used, employing polyvinyl microtiter plates as the solid phase, a primary coat of monospecific rabbit antitoxin serum, an outer layer composed of a horseradish peroxidase-sheep antitoxin immunoglobulin G conjugate, and an ortho-phenylene-diamine substrate. Absorbance (optical density) of hydrolyzed end product was read spectrophotometrically at 492 nm. ELISA detected as little as 30 pg (0.3 ng/ml) of purified toxin, and absorbance was linear over a 20-fold or greater concentration range. Toxin was demonstrated in culture filtrates from 42 of 48 (88%) consecutive clinical P. aeruginosa isolates compared with 37 of 48 (77%) positive by hemagglutination inhibition. Results of the two assays correlated closely (r = 0.82, P less than 0.001). Specificity was confirmed by neutralizability of ELISA activity with monospecific antitoxin. ELISA was thus a sensitive, specific, and quantifiable technique for the assay of P. aeruginosa exotoxin A in both purified and crude culture materials.

Protective activity of antibodies to exotoxin A and lipopolysaccharide at the onset of Pseudomonas aeruginosa septicemia in man

Serum antibodies to exotoxin A and type-specific lipopolysaccharide were measured by passive hemagglutination in 52 patients with Pseudomonas aeruginosa septicemia. Their comparative protective activities were evaluated by relating the titers of each at the onset of bacteremia to subsequent outcome. High acute serum antitoxin and antilipopolysaccharide titers (log2 reciprocal mean titers greater than 5) were associated with survival (76% of 17 with high vs. 46% of 24 with low antitoxin titers, P = 0.05; 85% of 13 with high vs. 48% of 29 with low antilipopolysaccharide titers, P = 0.03). In contrast, neither antibody titer was significantly associated (P less than or equal to 0.05) with patients' age or sex, severity of underlying disease, presence of leukopenia, steroid or immunosuppressive therapy. Despite a correlation between acute titers of the two antibodies (r = 0.33, P = 0.06), they appeared to protect independently and additively. Whereas 75% of 8 patients with high antitoxin titers and only 38% of 16 with low titers survived with low antilipopolysaccharide titers (P = 0.10), 100% (6/6), 73% (8/11), and 38% (6/16) survived, respectively, when both, one, or neither antibody was present in high titer (P = 0.01). Furthermore, the association between high acute serum antitoxin titers and survival was more pronounced in patients with rapidly fatal underlying disease (P = 0.06) and leukopenia (P = 0.12) than in more favorable prognostic and immune categories. These data indicate that serum antibodies to exotoxin A and lipopolysaccharide are found in most patients with P. aeruginosa septicemia and both are protective. Both antibodies may have therapeutic or prophylactic potential, whereas serum antiexotoxin A antibodies may be particularly beneficial in compromised hosts.