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

Protection against exotoxin A (ETA) and Pseudomonas aeruginosa infection in mice with ETA-specific antipeptide antibodies

Pseudomonas aeruginosa is an opportunistic pathogen that causes serious and sometimes fatal infections in the compromised host, especially in patients with major trauma or thermal injuries. Exotoxin A (ETA) is the major and most lethal virulence factor produced by this ubiquitous microorganism. In a recent study (H. S. Elzaim, A. K. Chopra, J. W. Peterson, R. Goodheart, and J. P. Heggers, Infect. Immun. 66:2170-2179, 1998), we identified two major epitopes, one within the translocation domain (amino acid [aa] residues 289 to 333) of ETA and another within the enzymatic domain (aa 610 to 638), by using a panel of antipeptide antibodies. Synthetic peptides representing these two epitopes induced ETA-specific antibodies which were able to abrogate the cytotoxic activity of ETA, as measured by incorporation of [3H]leucine into 3T3 fibroblasts. In the present study, these antibodies were tested for the ability to provide protection against ETA and infection with a toxin-producing strain of P. aeruginosa in a mouse model. Antibodies to either of the synthetic peptides conferred protection against ETA. Also, when used for immunization, both peptides induced active immunity to ETA in mice. Antibodies to the peptide representing a region within the enzymatic domain of ETA, in combination with the antibiotic amikacin, enhanced the survival of mice infected with a toxin-producing strain of P. aeruginosa. Thus, antipeptide antibodies specific for ETA might be paired with antibiotic treatment for passive immunization of patients suffering from P. aeruginosa infection.

Generation of neutralizing antipeptide antibodies to the enzymatic domain of Pseudomonas aeruginosa exotoxin A

Burn patients suffer a break in the physical barrier (skin), which, when combined with their generalized state of immunodeficiency, creates an open window for opportunistic infections, mainly with Pseudomonas aeruginosa. Infection of the burn wound has always been a major factor in retardation of wound healing, and sepsis remains the leading cause of death in burn patients. Because studies have shown that topical treatment with antiexotoxin A (ETA) antibodies significantly increases survival in rats infected with toxin-producing strains of P. aeruginosa, we examined 11 synthetic peptides encompassing 12 to 45 amino acid (aa) residues, representing what were predicted by computer analysis to be the most hydrophilic and antigenic regions of ETA. These synthetic peptides were injected into rabbits for antibody production. Different groups of rabbits were immunized with a combination of peptides, with each combination representing one of the three distinct domains of ETA. Animals immunized with various peptide combinations produced peptide-specific antibodies that exhibited cross-reactivity to ETA. Two major epitopes were identified on the ETA molecule by experiments with peptide-specific antibodies in enzyme-linked immunosorbent assay and immunoprecipitation. One of these epitopes was located in the translocation domain (II) (aa 297 to 310), while the other was mapped to the last 13 aa residues at the carboxy-terminal end of the enzymatic domain (III) (aa 626 to 638). Of these two regions, the epitope in the enzymatic domain induced a much higher level of neutralizing antibodies that abrogated the cytotoxic activity of ETA in vitro. Antibodies to this epitope blocked the ADP-ribosyltransferase activity of ETA and appeared to interfere with binding of the substrate elongation factor 2 to the enzymatic active site of the ETA molecule. We conclude that polyclonal, as well as monoclonal, antibodies to short peptides, representing small regions of ETA, may have therapeutic potential in passive immunization or topical treatment of burn patients infected with toxin-producing strains of P. aeruginosa.

Pseudomonas aeruginosa antigens as potential vaccines

Pseudomonas aeruginosa is one of the most important opportunistic bacterial pathogens in humans and animals. This organism is ubiquitous and has high intrinsic resistance to antibiotics due to the low permeability of the outer membrane and the presence of numerous multiple drug efflux pumps. Various cell-associated and secreted antigens of P. aeruginosa have been the subject of vaccine development. Among pseudomonas antigens, the mucoid substance, which is an extracellular slime consisting predominantly of alginate, was found to be heterogenous in terms of size and immunogenicity. High molecular mass alginate components (30-300 kDa) appear to contain conserved epitopes while lower molecular mass alginate components (10-30 kDa) possess conserved epitopes in addition to unique epitopes. Surface-exposed antigens including O-antigens (O-specific polysaccharide of LPS) or H-antigens (flagellar antigens) have been used for serotyping due to their highly immunogenic nature. Chemical structures of repeating units of O-specific polysaccharides have been elucidated and these data allowed the identification of 31 chemotypes of P. aeruginosa. Conserved epitopes among all serotypes of P. aeruginosa are located in the core oligosaccharide and the lipid A region of LPS and immunogens containing these epitopes induce cross-protective immunity in mice against different P. aeruginosa immunotypes. To examine the protective properties of OM proteins, a vaccine containing P. aeruginosa OM proteins of molecular masses ranging from 20 to 100 kDa has been used in pre-clinical and clinical trials. This vaccine was efficacious in animal models against P. aeruginosa challenge and induced high levels of specific antibodies in human volunteers. Plasma from human volunteers containing anti-P. aeruginosa antibodies provided passive protection and helped the recovery of 87% of patients with severe forms of P. aeruginosa infection. Vaccines prepared from P. aeruginosa ribosomes induced protective immunity in mice, but the efficacy of ribosomal vaccines in humans is not yet known. A number of recent studies indicated the potential of some P. aeruginosa antigens that deserve attention as new vaccine candidates. The outer core of LPS was implicated to be a ligand for binding of P. aeruginosa to airway and ocular epithelial cells of animals. However, heterogeneity exists in this outer core region among different serotypes. Epitopes in the inner core are highly conserved and it has been demonstrated to be surface-accessible, and not masked by O-specific polysaccharide. The use of an in vivo selection/expression technology (IVET) by a group of researchers identified a number of P. aeruginosa proteins that are expressed in vivo and essential for virulence. Two of these in vivo-expressed proteins are FptA (ferripyochelin receptor protein) and a homologue of an LPS biosynthetic enzyme. Our laboratory has identified a highly conserved protein, WbpM, and P. aeruginosa with a deficiency in this protein produces only rough LPS and became serum sensitive. Results from these studies have provided the foundation for a variety of vaccine formulations.

Characterization of monoclonal antibody B7, which neutralizes the cytotoxicity of Pseudomonas aeruginosa exotoxin A

A nontoxic Pseudomonas aeruginosa exotoxin A (PE), which has the carboxyl-terminal 38 amino acid residues of native PE deleted, was used as an antigen to immunize BALB/c mice, which were then challenged with native PE in order to raise monoclonal antibodies (MAbs) that can neutralize PE cytotoxicity. A murine MAb against PE, designated MAb B7, was established. MAb B7 was characterized in terms of its ability to neutralize PE cytotoxicity, epitope mapping, inhibition of PE receptor binding, and influence on cellular processing of PE and ADP-ribosylation activities. We found that MAb B7 could neutralize PE cytotoxicity in cell culture and in BALB/c mice. The epitope recognized by MAb B7 was mapped to the carboxyl-terminal amino acid residues 575 to 595 of PE. Consistent with the results of epitope mapping, MAb B7 did not block PE receptor-binding activity or the cellular processing of PE but strongly inhibited the ADP-ribosylating activity of PE. In addition, MAb B7 retained strong binding to PE even at pH 4.0, indicating that the complex of MAb B7 and PE is stable in the phagolysosome. On the basis of these observations, the neutralization of PE cytotoxicity by MAb B7 could be due to its binding to the carboxyl terminus of PE. As a result, MAb B7 may interfere with the interaction of the carboxyl-end amino acid residues REDLK of PE with cellular factors. However, we could not rule out the possibility that MAb B7 directly blocks the ADP-ribosylation activity of PE in the cytosol.

Primate antibody response to immunotoxin: serological and computer-aided analysis of epitopes on a truncated form of Pseudomonas exotoxin

NLysPE38 is a 38-kDa derivative of Pseudomonas exotoxin (PE) in which domain Ia (amino acids 1 to 252) and part of domain Ib (365 to 380) are deleted and an 11-amino-acid N-terminal peptide is added. LMB-1 is an immunotoxin in which the monoclonal antibody B3 is coupled to NLysPE38 near its N terminus. LMB-7 is a single-chain immunotoxin in which the Fv fragment of B3 is fused to PE38. To identify the antigenic regions of PE38, 12 polyclonal serum samples from monkeys immunized with the immunotoxins LMB-1 (six monkeys) and LMB-7 (six monkeys) were tested for their reactivity to a panel of 120 synthetic, overlapping peptides representing the amino acid sequence of NLysPE38. The antibody responses to peptides were similar among the 12 serum specimens, identifying several major immunodominant B-cell epitopes. Predominant reactivity was seen in six locations: amino acids 272 to 287, 341 to 359, 504 to 516, 540 to 564, and 573 to 591 and the C-terminal amino acids 591 to 613. The sera did not react with approximately 75% of the peptides. Furthermore, a computer-aided analysis was done to predict the immunologically relevant areas and revealed the same antigenic regions defined by serum reactivity to peptides. Competition enzyme-linked immunosorbent assays and neutralization assays were performed with domain II, III, or III plus Ib of PE38 and confirmed the immunodominance of domain III. To analyze the role of specific amino acids in antibody binding, individual amino acids of PE38 with large accessible surface areas were altered by site-directed mutagenesis. These results also show that the predicted areas of immunogenicity agree with the reactivity of the anti-PE38 antibodies to peptides and to the mutants of PE.

Identification of a small epitope in domain Ib of Pseudomonas aeruginosa exotoxin A that elicits enzyme-neutralizing antibodies

A peptide corresponding to amino acids 392-404 of the amino acid sequence of Pseudomonas aeruginosa exotoxin A (the last 13 amino acids of domain Ib) was synthesized and coupled to thyroglobulin. The conjugate induced an antiserum in rabbits with high antibody titer against native toxin as measured by ELISA, and this antiserum was highly efficient in inhibiting the ADP-ribosyltransferase activity of exotoxin A. These data corroborate the potential importance of amino acids 400-404 in the enzymatic mechanism of exotoxin A.

Binding of monoclonal antibody specific for domain Ia/II of Pseudomonas aeruginosa exotoxin A at pH 4 strongly neutralizes exotoxin A-induced cytotoxicity in cell culture and in vivo

Mouse monoclonal antibodies (MAbs) against Pseudomonas aeruginosa exotoxin A (Ex-A) were established, and 4 of 20 MAbs were extensively studied for analysis of the structure-function relationship of Ex-A. IN vivo experiments demonstrated that MAb Ex-3C7 protected mice either injected with Ex-A or infected with Ex-A-producing P. aeruginosa from death caused by Ex-A at the highest rate, followed by MAbs Ex-4F2 and Ex-8H5, in that order. MAb Ex-2A10 failed to rescue the mice. MAb Ex-3C7 (immunoglobulin G1 [IgG1]) inhibited incorporation of Ex-A into target cells and strongly neutralized cytotoxicity in cell culture but did not inhibit an enzymatic activity of Ex-A, ADP-ribosyltransferase, at all. The MAb also bound Ex-A, even at a low pH of 4, and recognized amino acid residues 241 to 297 (domain Ia/II), suggesting that MAb Ex-3C7 can interfere with the conformational change and/or processing of Ex-A by keeping a complex of Ex-A and antibody stable at low pH in the phagolysosome. MAb Ex-4F2 (IgG1), which recognizes residues 550 to 590 (domain III), strongly inhibited Ex-A incorporation and neutralized cytotoxicity in cell culture but only weakly inhibited ADP-ribosyltransferase. MAb Ex-8H5 (IgG1), which recognizes residues 591 to 613 (domain III), also inhibited cytotoxicity in cell culture, but weakly. In contrast to the above three MAbs, MAb Ex-2A10 (IgG2b) greatly inhibited ADP-ribosyltransferase but showed no inhibition of Ex-A incorporation and no neutralizing activity against cell toxicity. A line of evidence indicates that (i) domain Ia/II plays an important role in the pathogenesis of Ex-A and (ii) MAbs that inhibit an intracellular postbinding process, such as conformational change, processing, and translocation of Ex-A in target cells, can display potent inhibitory activity against cytotoxicity in vivo, as well as in cell culture, and would be a good candidate for therapy of pseudomonal infections.

Production and characterization of human monoclonal antibody recognizing the N-terminal residues of Pseudomonas aeruginosa exotoxin A

Human cell lines producing monoclonal antibodies (MAbs) against Pseudomonas aeruginosa exotoxin A were established by EBV transformation followed by cell fusion. Monoclonal antibody FK-001, IgM (mu, kappa), was demonstrated to be specifically reactive with exotoxin A in ELISA and immunoblotting, by recognizing N-terminal 16 amino acid residues of exotoxin A as an epitope. This epitope region belongs to domain I which is required for the binding of exotoxin A to the receptor on target cells. FK-001 showed a partial neutralizing activity for cell toxicity caused by exotoxin A and appeared to be effective against exotoxin A-producing P. aeruginosa infection in mice. A line of evidence suggests that monoclonal antibody FK-001 neutralizes exotoxin A-induced cell toxicity by the interference of accessibility and/or binding of exotoxin A to animal cell receptors.

Identification of functional epitopes of Pseudomonas aeruginosa exotoxin A using synthetic peptides and subclone products

The structure-function relationship of P. aeruginosa exotoxin A (ETA) was examined using synthetic peptides and genetically engineered ETA deletion mutants. Antibodies directed against synthetic peptides have allowed the identification of three ETA epitopes, two within domain I and one within the last 33 amino acids of domain III. In addition two distinct neutralizing determinants have been identified by antibodies directed against subclone products. One was associated with the amino-terminal half of ETA, the proposed receptor binding region. The second was associated with the carboxy-terminal half of ETA, a region previously not associated with receptor-binding. The amino-terminal subclone also offers potential as an ETA vaccine, since it produces a stable, non-enzymatically active product, effective in inducing ETA neutralizing antibodies. Data derived from these studies were used in a re-evaluation of structure-function relationships between ETA and diphtheria toxin.

Biochemical and immunochemical studies of proteolytic fragments of exotoxin A from Pseudomonas aeruginosa

Limited proteolysis of Pseudomonas aeruginosa exotoxin A by four proteases (chymotrypsin, Staphylococcal serine proteinase, pepsin A and subtilisin) resulted in the formation of polypeptides having a molecular mass of approximately 25 kDa. They possessed both enzymatic activity and residual antigenicity. Their N-terminal sequence analysis showed that the different proteases cleaved exotoxin A in a very restricted area within domain Ib (amino acids 365-404). As a result, the polypeptides contained a large portion (13-34 amino acids) of domain Ib linked to the adjacent C-terminal domain III (amino acids 405-613). The major fragment derived from subtilisin cleavage, at a final yield of 35% (S-fragment; residues 392-613; 24201 Da; pI 4.7) possessed the same level of ADP-ribosyltransferase activity as uncleaved exotoxin A (by mass), and a 37-fold higher NAD-glycohydrolase activity. Polyclonal antibodies from rabbits against exotoxin A completely inhibited the ADP-ribosyltransferase activity of both exotoxin A and the S-fragment, but not the NAD-glycohydrolase activity of the S-fragment. Antibodies against the S-fragment neutralized the ADP-ribosyltransferase activity of exotoxin A. These data determine the primary proteolytic cleavage site of exotoxin A, suggest that some residues in the amino acid sequence 392-404 of exotoxin A seem to have a role in binding or positioning elongation factor 2 (EF-2) and show that antibodies recognize the EF-2-binding site but not the NAD(+)-binding site.

Production and characterization of monoclonal antibodies against the lethal factor component of Bacillus anthracis lethal toxin

The lethal toxin of Bacillus anthracis consists of two components, protective antigen and lethal factor. Protective antigen is cleaved after binding to cell receptors, yielding a receptor-bound fragment that binds lethal factor. Sixty-one monoclonal antibodies to the lethal factor protein have been characterized for specificity, antibody subtype, and ability to neutralize lethal toxin. Three monoclonal antibodies (10G3, 2E7, and 3F6) neutralized lethal toxin in Fisher 344 rats. However, in a macrophage cytolysis assay, monoclonal antibodies 10G3, 2E7, 10G4, 10D4, 13D10, and 1D8, but not 3F6, were found to neutralize lethal toxin. Binding studies showed that five of the monoclonal antibodies that neutralized lethal toxin in the macrophage assay (10G3, 2E7, 10G4, 10D4, and 13D10) did so by inhibiting the binding of lethal factor to the protective antigen fragment bound to cells. Monoclonal antibody 1D8, which was also able to neutralize lethal toxin activity after lethal factor was prebound to cell-bound protective antigen, only partially inhibited binding of lethal factor to protective antigen. Monoclonal antibody 3F6 did not inhibit the binding of lethal factor to protective antigen. A competitive-binding enzyme-linked immunosorbent assay showed that at least four different antigenic regions on lethal factor were recognized by these seven neutralizing hybridomas. The anomalous behavior of 3F6 suggests that it may induce a conformational change in lethal factor. Differences in neutralizing activity of monoclonal antibodies were related to their relative affinity and epitope specificity and the type of assay.

Production and characterization of monoclonal antibodies against the lethal factor component of Bacillus anthracis lethal toxin

The lethal toxin of Bacillus anthracis consists of two components, protective antigen and lethal factor. Protective antigen is cleaved after binding to cell receptors, yielding a receptor-bound fragment that binds lethal factor. Sixty-one monoclonal antibodies to the lethal factor protein have been characterized for specificity, antibody subtype, and ability to neutralize lethal toxin. Three monoclonal antibodies (10G3, 2E7, and 3F6) neutralized lethal toxin in Fisher 344 rats. However, in a macrophage cytolysis assay, monoclonal antibodies 10G3, 2E7, 10G4, 10D4, 13D10, and 1D8, but not 3F6, were found to neutralize lethal toxin. Binding studies showed that five of the monoclonal antibodies that neutralized lethal toxin in the macrophage assay (10G3, 2E7, 10G4, 10D4, and 13D10) did so by inhibiting the binding of lethal factor to the protective antigen fragment bound to cells. Monoclonal antibody 1D8, which was also able to neutralize lethal toxin activity after lethal factor was prebound to cell-bound protective antigen, only partially inhibited binding of lethal factor to protective antigen. Monoclonal antibody 3F6 did not inhibit the binding of lethal factor to protective antigen. A competitive-binding enzyme-linked immunosorbent assay showed that at least four different antigenic regions on lethal factor were recognized by these seven neutralizing hybridomas. The anomalous behavior of 3F6 suggests that it may induce a conformational change in lethal factor. Differences in neutralizing activity of monoclonal antibodies were related to their relative affinity and epitope specificity and the type of assay.

The cytotoxicity of Pseudomonas exotoxin A, inactivated by modification of the cell-binding domain I, is restored when conjugated to an erythroid cell-specific targeting agent

To be capable of selective killing of tumor cells, the non-selective Pseudomonas aeruginosa exotoxin A must have its cell-binding domain inactivated or removed and then be chemically linked to, or genetically fused with, a specific targeting agent. In the present study, epsilon-NH2 groups of lysine residues of the cell-binding domain of exotoxin A were extensively propionylated with N-succinimidyl-3-propionate (NSP). The NSP-treated exotoxin retained its cytocidal ADP-ribosyltransferase activity, but it could no longer bind to, and inhibit the proliferation of, Friend murine erythroleukemia cells. Cytotoxicity (i.e., the ability to inhibit proliferation) for the Friend erythroid cells was restored completely to the NSP-inactivated exotoxin by conjugating it to ADIF, an autocrine factor secreted by chicken erythroleukemia cells which selectively inhibits the differentiation of erythroid cells such as Friend erythroleukemia cells without inhibiting their proliferation.

A human-human hybridoma secreting anti-Pseudomonas aeruginosa exotoxin-A monoclonal antibody with highly potent neutralizing activity

A hybridoma secreting human monoclonal antibody (MAB) against Pseudomonas aeruginosa exotoxin A (PEA) was constructed by fusing Epstein-Barr virus-transformed peripheral blood lymphocytes with human B lymphoblastoid cell line TAW-925. The human-human hybridoma stably produced human IgG2 MAB at the rate of 0.4-0.5 microgram/ml per 10(6) cells per day for more than six months, and the MAB was capable of neutralizing the in vitro cytotoxic and in vivo lethal effects of PEA with approximately 100- and 70-fold, respectively, higher activity than serum polyclonal antibody preparations.