Pseudomonas aeruginosa immunotype 5 polysaccharide-toxin A conjugate vaccine

Recent advances in lipopolysaccharide-based glycoconjugate vaccines

INTRODUCTION

The public health burden caused by pathogenic Gram-negative bacteria is increasingly prominent due to antimicrobial resistance. The surface carbohydrates are potential antigens for vaccines against Gram-negative bacteria. The enhanced immunogenicity of the O-specific polysaccharide (O-SP) moiety of LPS when coupled to a carrier protein may protect against bacterial pathogens. However, because of the toxic lipid A moiety and relatively high costs of O-SP isolation, LPS has not been a popular vaccine antigen until recently.

AREAS COVERED

In this review, we discuss the rationales for developing LPS-based glycoconjugate vaccines, principles of glycoconjugate-induced immunity, and highlight the recent developments and challenges faced by LPS-based glycoconjugate vaccines.

EXPERT OPINION

Advances in LPS harvesting, LPS chemical synthesis, and newer carrier proteins in the past decade have propelled LPS-based glycoconjugate vaccines toward further development, through to clinical evaluation. The development of LPS-based glycoconjugates offers a new horizon for vaccine prevention of Gram-negative bacterial infection.

Vaccines for multidrug resistant Gram negative bacteria: lessons from the past for guiding future success

Antimicrobial resistance is a major threat to global public health. Vaccination is an effective approach for preventing bacterial infections, however it has not been successfully applied to infections caused by some of the most problematic multidrug resistant pathogens. In this review, the potential for vaccines to contribute to reducing the burden of disease of infections caused by multidrug resistant Gram negative bacteria is presented. Technical, logistical and societal hurdles that have limited successful vaccine development for these infections in the past are identified, and recent advances that can contribute to overcoming these challenges are assessed. A synthesis of vaccine technologies that have been employed in the development of vaccines for key multidrug resistant Gram negative bacteria is included, and emerging technologies that may contribute to future successes are discussed. Finally, a comprehensive review of vaccine development efforts over the last 40 years for three of the most worrisome multidrug resistant Gram negative pathogens, Acinetobacter baumannii, Klebsiella pneumoniae and Pseudomonas aeruginosa is presented, with a focus on recent and ongoing studies. Finally, future directions for the vaccine development field are highlighted.

Understanding Pseudomonas aeruginosa-Host Interactions: The Ongoing Quest for an Efficacious Vaccine

Pseudomonas aeruginosa is a leading cause of chronic respiratory infections in people with cystic fibrosis (CF), bronchiectasis or chronic obstructive pulmonary disease (COPD), and acute infections in immunocompromised individuals. The adaptability of this opportunistic pathogen has hampered the development of antimicrobial therapies, and consequently, it remains a major threat to public health. Due to its antimicrobial resistance, vaccines represent an alternative strategy to tackle the pathogen, yet despite over 50 years of research on anti-Pseudomonas vaccines, no vaccine has been licensed. Nevertheless, there have been many advances in this field, including a better understanding of the host immune response and the biology of P. aeruginosa. Multiple antigens and adjuvants have been investigated with varying results. Although the most effective protective response remains to be established, it is clear that a polarised Th2 response is sub-optimal, and a mixed Th1/Th2 or Th1/Th17 response appears beneficial. This comprehensive review collates the current understanding of the complexities of P. aeruginosa-host interactions and its implication in vaccine design, with a view to understanding the current state of Pseudomonal vaccine development and the direction of future efforts. It highlights the importance of the incorporation of appropriate adjuvants to the protective antigen to yield optimal protection.

Polysaccharide conjugate vaccine protein carriers as a "neglected valency" - Potential and limitations

The development of vaccines against polysaccharide-encapsulated pathogens (e.g. Haemophilus influenzae type b, pneumococci, meningococci) is challenging because polysaccharides do not elicit a strong and long-lasting immune response (i.e. T-cell independent). This can be overcome by conjugating the polysaccharide to a protein carrier (e.g. tetanus toxoid, cross-reacting material 197 [CRM]), which vastly improves the immune response and induces memory to the polysaccharide (T-cell dependent). Although it is well documented that protein carriers additionally induce an immune response against themselves, this potential "additional valency" has so far not been recognized. The only exception is for the protein D carrier (derived from non-typeable Haemophilus influenzae [NTHi]) used in a pneumococcal conjugate vaccine, which may have a beneficial impact on NTHi acute otitis media. In this review, we describe the immunogenicity of various protein carriers and discuss their potential dual function: as providers of T-cell helper epitopes and as protective antigens. If this "additional valency" could be proven to be protective, it may be possible to consider its potential effect on the number of required immunizations. We also describe the potential for positive or negative interference between conjugate vaccines using the same protein carriers, the resulting desire for novel carriers, and information on potential new carriers. The range of conjugate vaccines is ever expanding, with different carriers and methods of conjugation. We propose that new conjugate vaccine trials should assess immunogenicity to both the polysaccharide and carrier. Ultimately, this so-far "neglected valency" could be an exploitable characteristic of polysaccharide conjugate vaccines.

Passive protection of diabetic rats with antisera specific for the polysaccharide portion of the lipopolysaccharide isolated from Pseudomonas pseudomallei

Polyclonal and monoclonal antisera raised to tetanus toxoid-conjugated polysaccharide of lipopolysaccharide (lps) and purified lps of Pseudomonas pseudomallei that reacted with a collection of 41 strains of this bacterium from 23 patients are described. The common antigen recognized by these sera was within the polysaccharide component of the lps of the cells. The sera were specific for P pseudomallei in that none of 37 strains of other bacteria, including 20 Gram-negative and three Gram-positive species, were recognized, although cross-reaction occurred using the anticonjugate serum with some strains of Pseudomonas cepacia serotype A, a closely related bacterium. Passive protection studies using a diabetic rat model of P pseudomallei infection showed that partially purified rabbit polyclonal and mouse monoclonal antisera were protective when the median lethal dose was raised by four to five orders of magnitude. The wide distribution of the polysaccharide antigen among isolates of P pseudomallei used in this study and the protective role of antibody to the conjugated polysaccharide antigen suggest potential as a vaccine.

Pseudomonas immunotherapy: a historical overview

The historic development of vaccines to be used as immunotherapy for Pseudomonas aeruginosa infections, in various patient populations, is reviewed. Commentary is offered concerning the relevance of each approach in light of our current understanding of the pathological process of these infections.

Anti-ETA IgG neutralizes the effects of Pseudomonas aeruginosa exotoxin A

BACKGROUND

The opportunistic pathogen Pseudomonas aeruginosa causes severe infections in immunocompromised hosts. Among P. aeruginosa-infected burn patients, mortality may reach as high as 50%. Due to their immunocompromised status, burn patients may benefit from passive immunotherapy against infection. As a potential multivalent immunoglobulin therapy, specific polyclonal antibodies against four P. aeruginosa virulence factors, including exotoxin A (ETA), were prepared.

MATERIALS AND METHODS AND RESULTS

In this study, we analyzed the ability of ETA antibody (ETA-Ab) to neutralize the in vivo effects of ETA. Adult mice injected with purified ETA suffered 100% mortality. The cytosolic DNA of their hepatocytes was fragmented, indicating ETA induction of apoptosis. In addition, multiprobe RNase protection assays showed that ETA upregulates the expression of the genes for proinflammatory cytokines as well as apoptosis genes in the livers of ETA-injected mice. Treatment with ETA-Ab prior to ETA injection prevented mortality, ETA-induced hepatocyte DNA fragmentation, and upregulation of the cytokine and apoptosis-related genes. The role of ETA during P. aeruginosa infection of the burn wound was examined by determining the in vivo virulence of P. aeruginosa PA103 and its isogenic, ETA-deficient mutant PA103Omega::toxA using the thermally injured mouse model. The lethality, local spread, and systemic spread of PA103Omega::toxA were significantly reduced compared to PA103.

CONCLUSION

These results suggest that (1) ETA induces apoptosis in hepatocytes, (2) specific cytokines are produced in response to ETA, (3) ETA-Ab neutralizes these effects, and (4) ETA contributes to the spread of P. aeruginosa during burn wound infection.

Dual-function vaccine for Pseudomonas aeruginosa: characterization of chimeric exotoxin A-pilin protein

Pseudomonas aeruginosa is the major infectious agent of concern for cystic fibrosis patients. Strategies to prevent colonization by this bacterium and/or neutralize its virulence factors are clearly needed. Here we characterize a dual-function vaccine designed to generate antibodies to reduce bacterial adherence and to neutralize the cytotoxic activity of exotoxin A. To construct the vaccine, key sequences from type IV pilin were inserted into a vector encoding a nontoxic (active-site deletion) version of exotoxin A. The chimeric protein, termed PE64Delta553pil, was expressed in Escherichia coli, refolded to a near-native conformation, and then characterized by various biochemical and immunological assays. PE64Delta553pil bound specifically to asialo-GM1, and, when injected into rabbits, produced antibodies that reduced bacterial adherence and neutralized the cell-killing activity of exotoxin A. Results support further evaluation of this chimeric protein as a vaccine to prevent Pseudomonas colonization in susceptible individuals.

A nontoxic Pseudomonas exotoxin A induces active immunity and passive protective antibody against Pseudomonas exotoxin A intoxication

Pseudomonas exotoxin A (PE) is one of the most potent cytotoxic agents produced by Pseudomonas aeruginosa. In this study, we examined the possibility of using PE with a deletion of 38 carboxyl-terminal amino acid residues, designated PE(Delta576-613), for active immunization against PE-mediated disease. We first examined the toxic effects of PE and PE(Delta576-613) on 5- and 9-week-old ICR mice. The results show that the subcutaneous administration of PE(Delta576-613) at a dose of 250 microg was still nontoxic to 5- and 9-week-old ICR mice, while native PE was lethal at a dose of 0.5 and 1 microg, respectively. PE(Delta576-613) was then used to immunize ICR mice. The minimum dose of PE(Delta576-613) that could effectively induce anti-PE antibodies in 5- and 9-week-old ICR mice was found to be 250 ng. However, immunization with 250 ng PE(Delta576-613) failed to protect the immunized mice from a lethal dose of PE. The effective immunization dose of PE(Delta576-613) that could protect mice against a 2 microg PE challenge was found to be 15 microg. In addition, sera obtained from PE(Delta576-613)-immunized ICR mice were able to neutralize PE intoxication and effectively protect mice from PE. Thus, PE(Delta576-613) may be used as an alternative route to new PE vaccine development.

Immunogenicity of purified lipopolysaccharide or protein-oligosaccharide conjugates of Pasteurella multocida type 6:B in mice

Purified lipopolysaccharide (LPS) of Pasteurella multocida type 6:B, while toxic at higher doses, was protective at lower dose levels against experimentally-induced pasteurellosis in mice. However, the observed protection was abrogated if such LPS was digested with proteinase K prior to use in immunisation. The O-antigen polysaccharide side-chain (OS) of LPS did not appear to contribute to the observed protection as judged by the fact that immunisation of mice with purified OS or OS-protein conjugates, all of which were nontoxic, failed to confer protection against challenge with homologous virulent organisms. This was despite generation of significant levels of OS-specific antibodies, predominantly either of the IgM or IgG isotypes, in immunised mice.

Potential of preventing Pseudomonas aeruginosa lung infections in cystic fibrosis patients: experimental studies in animals

In patients with cystic fibrosis (CF), respiratory tract infections caused by Staphylococcus aureus and Haemophilus influenzae are followed by Pseudomonas aeruginosa with increasing age. Chronic endobronchial lung infection with P. aeruginosa is the leading cause of morbidity and mortality. In Danish CF patients we noted that both onset of initial colonization and chronic lung infection with P. aeruginosa peaked during the winter months which is the season for respiratory virus infections. Virus may therefore pave the way for P. aeruginosa. We established a chronic P. aeruginosa lung infection in rats by embedding mucoid bacteria in seaweed alginate and installing the beads intratracheally into the lower part of the left lung. Although the rats did not suffer from CF, the antibody responses and the pathologic changes of the lungs mimicked the findings in CF patients. By using this model in normal and athymic rats we showed that the T-cell response during the "natural" course of the infection played no major role. In a model of acute P. aeruginosa pneumonia we found that the macroscopic inflammatory response of the lungs was immense and that the natural capacity to clear P. aeruginosa was very efficient and could not be improved by immunization, although high serum levels of IgM, IgG and IgA antibodies to P. aeruginosa alginate, LPS, exotoxin A and sonicate were induced. We developed a method for collecting and measuring IgA in saliva and noted that mucosal IgA antibodies were induced by vaccination; they did not significantly prevent inflammation, however. In the chronic rat model we succeeded to improve the survival significantly and to change the inflammatory response subsequent to vaccination from an acute type inflammation dominated by polymorphonuclear leukocytes (PMNs) as in CF patients to a chronic type inflammation dominated by mononuclear leukocytes. Furthermore, we found that rats immunized with an alginate containing vaccine had a significantly earlier cellular shift to a chronic type inflammation as well as a significant reduction in the severity of the macroscopic inflammation compared to two other vaccine groups and to nonimmunized controls. Similar results were obtained in rats treated with the TH1 cytokine, interferon-gamma (IFN-gamma). Several authors have shown that the lung tissue damage during chronic infection in CF patients is caused by a type III hypersensitivity reaction leading to release of elastase by PMNs surrounding the bacterial microcolonies. The cellular shift we have induced by vaccination and by IFN-gamma treatment therefore offers a possible new strategy for improving the clinical course in chronically infected CF patients.

Effect of conjugation methodology, carrier protein, and adjuvants on the immune response to Staphylococcus aureus capsular polysaccharides

Conjugate vaccines were prepared with S. aureus type 8 capsular polysaccharide (CP) using three carrier proteins: Pseudomonas aeruginosa exotoxin A (ETA), a non-toxic recombinant ETA (rEPA), and diphtheria toxoid (DTd). Adipic acid dihydrazide (ADH) or N-succinimidyl 3-(2-pyridyldithio) propionate (SPDP) was used as a spacer to link the CP to carrier protein. All conjugates gave a high immune response with a boost after the second immunization. Conjugates prepared with ADH gave higher antibody titers than conjugates prepared with SPDP. IgG1 was the primary subclass elicited by all conjugates regardless of the carrier protein or the conjugation method used to prepare the vaccines. The non-immunogenic CP and the conjugates were formulated with either monophosphoryl lipid A (MPL), QS21, or in Novasomes and evaluated in mice. While the adjuvants failed to improve the immunogenicity of the nonconjugated CP, a more than fivefold increase in the antibody levels was observed when these adjuvants were used with the conjugates. Significant rises in IgG2b and IgG3 were observed with all formulations. The enhancement of the immunogenicity and the IgG subclass shift, as seen with some adjuvants, may prove to be important in immunocompromised patients.

Human immunodeficiency virus-1 principal neutralizing domain peptide-toxin A conjugate vaccine

To enhance the potential efficacy of peptide-based vaccines for human immunodeficiency virus-1 (HIV-1), a principal neutralizing domain (PND) peptide (KRIHIGPGRAFYT) (HIV-1MN) was covalently coupled to Pseudomonas aeruginosa toxin A (TA). Immunization of guinea-pigs with this conjugate vaccine, in the absence of an adjuvant, engendered a high-affinity antibody response to the homologous HIV-1MN PND peptide and to analogous peptides from variant strains of HIV-1. A substantial proportion of such antibodies was directed to the conserved GPGRAF motif. Anti-PND peptide antibodies were capable of neutralizing the homologous strain, HIV-1MN, in addition to two heterologous (RF, IIIB) variants, as determined either by inhibition of syncytia formation or by suppression of p24 antigen production in cultured cells. Therefore, the method of conjugation used preserved critical neutralizing epitopes expressed by the PND peptide. Monovalent or polyvalent PND-TA conjugates, which meet all safety criteria for human use, are a promising approach towards the development of an acquired immunodeficiency syndrome (AIDS) vaccine.

Escherichia hermannii (ATCC 33651) polysaccharide-protein conjugates: comparison of two conjugation methods for the induction of humoral responses in mice

Escherichia hermannii (ATCC 33651) LPS O-polysaccharide was covalently linked to a carrier (bovine serum albumin) to form conjugates either directly or with a spacer arm (adipic acid dihydrazide). The immunogenicity of both conjugates at three different doses was tested in mice. Antibodies to the conjugate were produced and were shown to react with free lipopolysaccharide. The directly-coupled conjugate was found to be more immunogenic than the indirect one (i.e. lower dose necessary for a similar response). The antibody response elicited by the directly coupled conjugate (1 microgram/animal) began at 21 days and was sustained for at least 4 months. The mouse model described here may be applicable to the testing of other conjugates composed of bacterial cell wall polysaccharides and LPS O-chains.

A Pseudomonas aeruginosa alginate-exotoxin A conjugate that elicits anti-alginate and exotoxin A-neutralizing antibodies

Pseudomonas aeruginosa alginate was covalently coupled to exotoxin A by reductive amination using adipic acid dihydrazide as spacer. The conjugate was composed of 25% alginate and 75% exotoxin A and possessed an average molecular mass higher than 700 kDa as determined by polyacrylamide gel electrophoresis. The conjugate had virtually no ADP-ribosyltransferase activity and a reduced cytotoxicity for TSA8 murine cells, derived from Friend erythroleukemia cells, as indicated by a greater than 50-fold increased LD50. Anti-conjugate antibodies recognized exotoxin A and alginate. A booster injection resulted in markedly increased antibody ELISA titers to both exotoxin A and alginate. The antibodies neutralized the exotoxin A toxicity.

Analysis of Pseudomonas exotoxin activation and conformational changes by using monoclonal antibodies as probes

Pseudomonas exotoxin (PE) is a protein toxin composed of three structural domains. In its native form, the toxin is a 66,000-Mr proenzyme that must be activated to express full ADP-ribosylating activity. To study the process of activation and accompanying conformational changes, we have isolated 10 monoclonal antibodies to a 40,000-Mr fragment of the toxin (PE40) that exhibits full enzyme activity but lacks the toxin's cell-binding domain and contains amino acids 253 to 613 (comprising domains II, Ib, and III). By using mutant PE molecules in which all of domain I and portions of domains II, Ib, and III were deleted, the locations of the epitopes for each of the antibodies were determined. Eight of these monoclonal antibodies were further characterized. Of these eight, all reacted with soluble PE40 and an interleukin-2-PE40 conjugate, but only two reacted strongly with native soluble PE. However, all eight reacted with PE after it had been immobilized on nitrocellulose or after it had been activated to express full ADP-ribosylating activity. Antibodies were also assessed for their ability to neutralize the cytotoxic activity of either PE or interleukin-2-PE40. These antibodies should be useful as probes for monitoring the activation and processing of PE that occur during endocytosis and in determining the location of epitopes that are important for toxin activity.

Synthesis and characterization of a Pseudomonas aeruginosa alginate-toxin A conjugate vaccine

Alginate from Pseudomonas aeruginosa 3064 was depolymerized by controlled heating in dilute acid. The resulting depolymerized alginate (Mr less than 60,000) was covalently coupled to toxin A with adipic acid dihydrazide as a spacer molecule and carbodiimide as a linker. The resulting conjugate was composed of toxin A and depolymerized alginate at a ratio of 4:1 and possessed an Mr of 260,000. The conjugate was nontoxic and nonpyrogenic. While native alginate (Mr greater than 640,000) given in a range of doses was poorly immunogenic in mice and rabbits, the conjugate induced high levels of antibody which bound to native alginate. Rabbits, but not mice, also produced an antitoxin immunoglobulin antibody response. Alginate derived from three other strains of P. aeruginosa competed with the homologous 3064 alginate for binding to anticonjugate antibody. This indicates that the conjugate elicits an antibody response able to recognize heterologous alginates. The serum from rabbits immunized with the conjugate was effective at promoting the uptake and killing of mucoid strains of P. aeruginosa by human polymorphonuclear leukocytes. In contrast, immunization with native alginate did not engender an opsonic antibody response. Rabbit anticonjugate antibody also neutralized the cytotoxic potential of toxin A.

Qualitative analysis of antibody binding. An in vitro assay for the evaluation and development of vaccines

We describe a rapid in vitro assay for the evaluation of in vivo properties of conjugate vaccines. Using human and murine monoclonal antibodies (mAb) specific for lipopolysaccharides (LPS), isolated from Pseudomonas aeruginosa, we determined in a competitive binding assay the amount of LPS or conjugate vaccine which was required to inhibit the antibody binding to LPS by 50% (I50 values). Furthermore, utilizing a murine burn wound sepsis model, we determined the potential of the same conjugates to induce protection in vivo against infection with the corresponding bacteria. Protective mAb have approximately 100-fold lower I50 values for preparations which are highly effective in inducing protection than for preparations which are ineffective. Furthermore, in the case of potent preparations it was noted that protective mAb exhibit similar I50 values for the conjugates and for the corresponding LPS. These results suggest that the fast and easily interpretable in vitro assay described may significantly facilitate the development and optimization of vaccines.

Synthesis and immunologic properties in mice of vaccines composed of Staphylococcus aureus type 5 and type 8 capsular polysaccharides conjugated to Pseudomonas aeruginosa exotoxin A

Epidemiological, serological and in vitro phagocytosis experiments provide evidence that the newly discovered type 5 and type 8 capsular polysaccharides (CPs) are both virulence factors and protective antigens for bacteremia caused by Staphylococcus aureus. Neither type 5 nor type 8 CP elicited serum antibodies when injected into mice. These two CPs were bound to Pseudomonas aeruginosa exotoxin A (ETA) to form conjugates by using the synthetic scheme devised for the CP (Vi) of Salmonella typhi and of pneumococcus type 12F (A. Fattom, W. F. Vann, S. C. Szu, A. Sutton, X. Li, D. Bryla, G. Schiffman, J. B. Robbins, and R. Schneerson, Infect. Immun. 56:2292-2298, 1988; S. C. Szu, A. L. Stone, J. D. Robbins, R. Schneerson, and J. B. Robbins, J. Exp. Med. 166:1510-1524, 1987). Both S. aureus CP-ETA conjugates elicited a rise in CP antibodies. As components of conjugates, both S. aureus CPs acquired T-cell-dependent properties, as shown by their ability to respond to carrier priming and to stimulate booster responses. The conjugate-induced antibodies facilitated type-specific opsonization of S. aureus by human polymorphonuclear leukocytes. The conjugates also induced ETA antibodies which neutralized the native toxin in vitro. Clinical studies of these two conjugates for active or passive immunization of patients at risk for S. aureus bacteremia are planned.

Octavalent Pseudomonas aeruginosa O-polysaccharide-toxin A conjugate vaccine

An octavalent Pseudomonas aeruginosa conjugate vaccine was synthesized by covalently coupling the O-polysaccharide (O-PS) moiety derived from lipopolysaccharides of Habs serotypes 1, 2, 3, 4, 5, 6, 11 and 12 to toxin A. Adipic acid dihydrazide was used as a spacer molecule to facilitate conjugation. The vaccine was composed of 37% (w/w) O-PS and 63% toxin A, devoid of enzymatic activity characteristic of toxin A, non-toxic for mice and guinea pigs, and non-pyrogenic. The vaccine elicited a significant rise in immunoglobulin G antibody levels to all serotypes of lipopolysaccharide contained in the vaccine and to toxin A. Serotypes 6, 10 and 11 were most immunogenic in mice whereas serotypes 1 and 5 engendered the lowest antibody response. Antitoxin A antibody was able to neutralize the cytotoxicity of toxin A. Immunization of mice with the vaccine conferred significant protection against subsequent challenge with all P. aeruginosa serotype strains contained in the vaccine.

Cloning and characterization of cDNAs coding for the heavy and light chains of a monoclonal antibody specific for Pseudomonas aeruginosa outer membrane protein I

A set of seven monoclonal antibodies (MAb) directed against outer membrane proteins of Pseudomonas aeruginosa has been examined by Western blot analysis, indirect immunofluorescence tests and subclass typing. The hybridoma cell line secreting MAb 6A4, which reacts with outer membrane protein I, belongs to the IgG2a subclass and crossreacts with the 17 P. aeruginosa serotypes as listed in the International Antigenic Typing System, was selected as source for the preparation of poly(A)+RNA which in turn was used as template for cDNA synthesis and cloning. Full length cDNA clones of the gamma heavy chain as well as the kappa light chain were obtained and characterized by nucleotide sequence analysis. The complete cDNA sequences coding for the heavy and light chains will be the prerequisite for the construction and heterologous expression of a chimeric human-mouse monoclonal antibody which might be used in therapy of P. aeruginosa infections.

Immunization with a Pseudomonas aeruginosa immunotype 5 O polysaccharide-toxin A conjugate vaccine: effect of a booster dose on antibody levels in humans

Healthy adult volunteers were vaccinated on day 0 and 28 and at 15 months with a Pseudomonas aeruginosa immunotype 5 O polysaccharide-toxin A conjugate vaccine. Immunization resulted in mild, transient local reactions in less than 20% of the subjects. Maximal immunoglobulin G (IgG) antibody titers to both toxin A and lipopolysaccharide (LPS) as determined by enzyme-linked immunosorbent assay were seen at day 42, at which time 50% of the vaccinees showed a fourfold or greater rise in toxin A-neutralizing titers. By 15 months postvaccination, both antitoxin A and anti-LPS IgG antibodies had markedly declined. A booster dose of vaccine administered at 15 months evoked a vigorous anti-toxin A IgG antibody response with 100% of the volunteers showing a fourfold or greater rise in neutralizing antibody titer compared with preimmunization levels. In contrast, there was no significant elevation of anti-LPS IgG antibody levels. At 24 months postimmunization, only anti-toxin A antibody levels were significantly higher than preimmunization levels.

Tolerability and immunogenicity of a polyvalent Pseudomonas aeruginosa extract vaccine in human volunteers

A polyvalent 16 serotype Pseudomonas extract vaccine was administered to normal volunteers to ascertain tolerability, immunogenicity and immunisation schedule. Four groups of nine volunteers were immunised with 0.8 X 10(9) bacterial equivalents (BE), 1.2 X 10(9) BE, 1.6 X 10(9) BE or placebo, respectively, on days 0, 14 and 21. A further six volunteers were immunised with 1.6 X 10(9) BE on days 0 and 28. Tolerability was excellent, slight side effects were unrelated to vaccine concentration and decreased with the number of injections. All concentrations of vaccine gave a significantly increased titre 14 days after the first immunisation. Reimmunisation did not increase the titre, which reached a plateau at day 14. Individuals with high pre-immunisation-titres produced very high post-immunisation-titres with a conversion factor of 6.5 whilst those with low pre-immunisation-titres had a higher conversion factor of 13 but produced lower final titres. ELISA titre did not always correlate with biological activity (mouse protection assay) suggesting that effective protective antibodies are only part of the total specific antibody measured by ELISA with the polyvalent whole-cell vaccine antigen.

Structural and immunochemical aspects of Brucella abortus endotoxins

Smooth lipopolysaccharide (sLPS) of Brucella abortus, which is the most immunodominant component among the antigens of B. abortus isolated, has been used for diagnosis for decades. High yields of sLPS can be prepared by a modification of the procedures of Moreno et al. (J. Bacteriol. 138:361-369, 1979). Washed B. abortus cells can be disrupted by 21 freeze-quick thaw cycles and ultrasonication to separate non-membrane-bound material; then phenol extraction is performed 3 times and the phenol fraction is washed with H2O intensively. The membrane-bound sLPS can be fractionated into 3 to 5 groups according to the extent of dialysis and centrifugation. These membrane bound sLPS fractions show marked individual differences in their precipitin profile and chemical composition. Their protein content varies from 16% to 42% as determined by dye binding test and 17 to 60% by Lowry phenol method using bovine serum albumin as the standard, which indicates that these proteins associated with LPS may play important roles in the immunochemical interactions, solubility, and the heterogeneity of B. abortus lipopolysaccharides. Compared to previously published methods, a higher yield of sLPS, ranging from 3.6% to 7.7% of dried bacteria, is obtained. Group f5A, which has a standard bell shaped curve in the precipitin assay, is one of the major fractions in all three strains (1119.3, 19, 2308). The protein free sLPS (less than 1% of Lowry reactive component) can be prepared by pronase digestion. The immunochemical reactivity remains about the same before and after this treatment. The O-chains of the major fraction (f5A) of B. abortus (Strains 2308 and 19) membrane bound smooth lipopolysaccharide (sLPS) are obtained by hydrolysis of f5A native sLPS in 1% acetic acid at 100 degrees C for 2 hours. After hydrolysis, the O-chains are separated from the lipid A protein complex by centrifugation, and from small fragments by ultrafiltration of a molecular weight cut-off (MWCO) of 1.0 x 10(3). These carbohydrate haptens can be identified by precipitin-inhibition assay and further fractionated by both membrane filtration and dialysis. The size distributions of carbohydrate haptens of the endotoxins (f5A) ranged from several oligosaccharides up to 1.0 x 10(4) MWCO. Three major fractions of MWCO 8.0-10.0 x 10(3), 3.5-5.0 x 10(3), and less than 1.0 x 10(3) for both strains 2308 and 19 contain more than 85% of the total immunreactive materials.(ABSTRACT TRUNCATED AT 400 WORDS)

Sequence and transcriptional start site of the Pseudomonas aeruginosa outer membrane porin protein F gene

Porin F is one of the major proteins of the outer membrane of Pseudomonas aeruginosa. It forms water-filled pores of variable size. Porin F is a candidate for a vaccine against P. aeruginosa because it antigenically cross-reacts in all serotype strains of the International Antigenic Typing Scheme. We have isolated the gene for porin F from a lambda EMBL3 bacteriophage library by using oligodeoxynucleotide hybridization probes and have determined its nucleotide sequence. Different peptide sequences obtained from isolated porin F confirmed the deduced protein sequence. The mature protein consists of 326 amino acid residues and has a molecular weight of 35,250. The precursor contains an N-terminal signal peptide of 24 amino acid residues. S1 protection and primer extension experiments, together with Northern (RNA) blots, indicate that the mRNA coding for porin F is monocistronic with short untranslated regions of about 58 bases at the 5' end and about 47 bases at the 3' end. The sequences in the -10 and -35 regions upstream of the transcriptional start site are closely related to the Escherichia coli promoter consensus sequences, which explains why the porin F gene is expressed in E. coli under the control of its own promoter. The amino acid sequence of porin F is not homologous to the different E. coli porins OmpF, OmpC, LamB, and PhoE. On the other hand, a highly homologous region of 30 amino acids between the OmpA proteins of different enteric bacteria and porin F of P. aeruginosa was detected. The core region of the homology to E. coli OmpA had 11 of 12 amino acid residues in common.

Prospects for the prevention and control of gram-negative nosocomial infections

Nosocomial infections of bacterial origin continue to be a leading cause of morbidity and mortality among virtually all hospitalized patient populations. It is estimated that nearly 5% of all hospitalized individuals will acquire an infection during their stay. Approximately 3% of nosocomial infections will contribute to an eventual fatal outcome. Preeminent among nosocomial pathogens are the aerobic Gram-negative bacilli, with Escherichia coli, Pseudomonas aeruginosa and Klebsiella spp. being the causative agents for the majority of life-threatening infections. This review describes the current situation and future prospects for combatting these infections.

Vaccine potential of Pseudomonas aeruginosa O-polysaccharide-toxin A conjugates

Serologically reactive O-polysaccharide from nine serotypes of Pseudomonas aeruginosa were covalently linked to toxin A via reductive amination, with adipic acid dihydrazide serving as a spacer molecule. The conjugates were composed of toxin A/O-polysaccharide ratios ranging from 1.17:1 to 3:1. All possessed an average Mr of greater than 10(6), were devoid of ADP ribosyltransferase activity associated with toxin A, and were nontoxic for mice and guinea pigs. The conjugates were stable from toxic reversion when stored at 37 degrees C for 28 days. The conjugation condition used preserved a substantial proportion of critical epitopes on the toxin A molecule as shown by the ability of toxin A-neutralizing monoclonal antibodies to react with the various conjugates. All nine conjugates were capable of evoking an antitoxin A and an antilipopolysaccharide immunoglobulin G (IgG) response in mice and rabbits. Rabbit antitoxin A IgG was capable of neutralizing the cytotoxic effect of toxin A, whereas mice immunized with any of the conjugates were protected against toxin A intoxication. Rabbit anti-conjugate IgG, when passively transferred to mice, was highly effective at preventing fatal P. aeruginosa burn wound sepsis.

Safety and immunogenicity of a Pseudomonas aeruginosa O-polysaccharide toxin A conjugate vaccine in humans

Lipid A-free polysaccharide (PS) isolated from Pseudomonas aeruginosa immunotype 5 lipopolysaccharide (LPS) was covalently coupled to toxin A via reductive amination. The PS-toxin A conjugate was comprised of 29.8% PS and 70.2% toxin A, possessed a molecular weight of greater than 1 X 10(6), was nontoxic for animals and was nonpyrogenic for rabbits at a dose of 50 micrograms/kg body wt when administered intravenously. The conjugate evoked only mild, transient reactions upon subcutaneous administration to human volunteers. Vaccination engendered immunoglobulin G (IgG) antibody, which neutralized the cytotoxic effect of toxin A and promoted the uptake and killing of P. aeruginosa in the presence of human polymorphonuclear leukocytes. Passively transferred IgG isolated from the serum of immunized donors was far more effective at preventing fatal P. aeruginosa burn wound sepsis than paired preimmunization serum. These studies establish the potential usefulness of such a PS-toxin A conjugate as a vaccine against P. aeruginosa.

[Significance of Pseudomonas aeruginosa virulence factors in acute and chronic Pseudomonas aeruginosa infections]

The virulence of Pseudomonas aeruginosa is multifactorial and caused by several extracellular enzymes and other substances. The importance of these virulence factors for the pathogenesis of human P. aeruginosa infections is dependent on the type of infection. For acute, systemic infections in immunocompromised patients, exotoxin A, alkaline protease and elastase are essential virulence factors. In localized infections (e.g., cystic fibrosis) they seem to be of minor importance, since they are neutralized by specific antibodies in immune complexes, and in the case of exotoxin A cleavage by proteinases from polymorphonuclear leukocytes occurs. The rhamnolipid of P. aeruginosa which has been detected in sputa of patients with cystic fibrosis evades the host's immune response and has to be regarded as a potential virulence factor together with the phenazine pigments, also in chronic P. aeruginosa infections.