Specificity and function of murine monoclonal antibodies and immunization-induced human polyclonal antibodies to lipopolysaccharide subtypes of Pseudomonas aeruginosa serogroup 06

Chemical Biology Tools for Modulating and Visualizing Gram-Negative Bacterial Surface Polysaccharides

Bacterial cells present a wide diversity of saccharides that decorate the cell surface and help mediate interactions with the environment. Many Gram-negative cells express O-antigens, which are long sugar polymers that makeup the distal portion of lipopolysaccharide (LPS) that constitutes the surface of the outer membrane. This review highlights chemical biology tools that have been developed in recent years to facilitate the modulation of O-antigen synthesis and composition, as well as related bacterial polysaccharide pathways, and the detection of unique glycan sequences. Advances in the biochemistry and structural biology of O-antigen biosynthetic machinery are also described, which provide guidance for the design of novel chemical and biomolecular probes. Many of the tools noted here have not yet been utilized in biological systems and offer researchers the opportunity to investigate the complex sugar architecture of Gram-negative cells.

Assessment of Pasteurella multocida A Lipopolysaccharide, as an Adhesin in an In Vitro Model of Rabbit Respiratory Epithelium

The role of the P. multocida lipopolysaccharide (LPS) as a putative adhesin during the early stages of infection with this bacterium in the respiratory epithelium of rabbits was investigated. By light microscopy and double enzyme labeling of nasal septa tissues, the amount of bacteria attached to the respiratory epithelium and the amount of LPS present in goblet cells at different experimental times were estimated. Transmission electron microscopy (TEM) and LPS labeling with colloidal gold particles were also used to determine the exact location of LPS in the cells. Septa that were challenged with LPS of P. multocida and 30 minutes later with P. multocida showed more adherent bacteria and more severe lesions than the other treatments. Free LPS was observed in the lumen of the nasal septum, forming bilamellar structures and adhering to the cilia, microvilli, cytoplasmic membrane, and cytoplasm of epithelial ciliated and goblet cells. The above findings suggest that P. multocida LPS plays an important role in the process of bacterial adhesion and that it has the ability of being internalized into host cells.

Monoclonal antibody S60-4-14 reveals diagnostic potential in the identification of Pseudomonas aeruginosa in lung tissues of cystic fibrosis patients

The lipopolysaccharide (LPS) of Pseudomonas aeruginosa has been identified to contain an inner-core structure expressing a Pseudomonas-specific epitope. This target structure is characterized by a highly phosphorylated and 7-O-carbamoyl-l-glycero-alpha-d-manno-heptopyranose (CmHep) and was found to be present in all human-pathogenic Pseudomonas species of the Palleroni (RNA)-classification I scheme. We raised and selected the monoclonal antibody S60-4-14 (mAb S60-4-14, subtype IgG1) from mice immunized with heat-killed Pseudomonas bacteria. The epitope of this mAb was found to reside in the inner-core structure of P. aeruginosa and, hence, successfully evaluated for the immunohistochemical detection of P. aeruginosa in formalin- or HOPE-fixed (Hepes-glutamic acid buffer-mediated organic solvent protection effect) and paraffin-embedded human lung tissue slices. Lung specimens, mainly from explanted lungs of cystic fibrosis (CF) patients, as well as P. aeruginosa isolates from patients suffering from CF and patients with extrapulmonar Pseudomonas infections were investigated by PCR, immunohistochemistry, and Western blot analysis with mAb S60-4-14. The results revealed an unequivocal coincidence of PCR and immunohistochemistry. Together with the Western blot results mAb S60-4-14 displays a potential diagnostic tool for the specific identification of P. aeruginosa in infected lungs of CF.

Application of vaccine technology to prevention of Pseudomonas aeruginosa infections

Development of an effective vaccine against the multiple presentations of Pseudomonas aeruginosa infection, including nosocomial pneumonia, bloodstream infections, chronic lung infections in cystic fibrosis patients and potentially sight-threatening keratitis in users of contact lenses, is a high priority. As with vaccine development for any pathogen, key information about the most effective immunologic effectors of immunity and target antigens needs to be established. For P. aeruginosa, although there is a role for cell-mediated immunity in animals following active vaccination, the bulk of the data indicate that opsonically-active antibodies provide the most effective mediators of acquired immunity. Major target antigens include the lipopolysaccharide O-polysaccharides, cell-surface alginate, flagella, components of the Type III secretion apparatus and outer membrane proteins with a potentially additive effect achieved by including immune effectors to toxins and proteases. A variety of active vaccination approaches have the potential for efficacy such as vaccination with purified or recombinant antigens incorporating multiple epitopes, conjugate vaccines incorporating proteins and carbohydrate antigens, and live attenuated vaccines, including heterologous antigen delivery systems expressing immunogenic P. aeruginosa antigens. A diverse range of passive immunotherapeutic approaches are also candidates for effective immunity, with a variety of human monoclonal antibodies described over the years with good preclinical efficacy and some early Phase I and II studies in humans. Finding an effective active and/or passive vaccination strategy for P. aeruginosa infections could be realized in the next 5 to 10 years, but will require that advances are made in the understanding of antigen expression and immune effectors that work in different human tissues and clinical settings, and also require a means to validate that clinical outcomes achieved in Phase III trials represent meaningful advances in management and treatment of P. aeruginosa infections.

The role and application of enterococci in food and health

The genus Enterococcus is the most controversial group of lactic acid bacteria. Studies on the microbiota of many traditional cheeses in the Mediterranean countries have indicated that enterococci play an important role in the ripening of these cheeses, probably through proteolysis, lipolysis, and citrate breakdown, hence contributing to their typical taste and flavour. Enterococci are also present in other fermented foods, such as sausages and olives. However, their role in these products has not been fully elucidated. Furthermore, the production of bacteriocins by enterococci is well documented. Moreover, enterococci are nowadays used as probiotics. At the same time, however, enterococci have been associated with a number of human infections. Several virulence factors have been described and the number of vancomycin-resistant enterococci is increasing. The controversial nature of enterococci has prompted an enormous increase in scientific papers and reviews in recent years, where researchers have been divided into two groups, namely pro and contra enterococci. To the authors' impression, the negative traits have been focused on very extensively. The aim of the present review is to give a balanced overview of both beneficial and virulence features of this divisive group of microorganisms, because it is only acquaintance with both sides that may allow their safe exploitation as starter cultures or co-cultures.

Naturally acquired antibodies against four Enterococcus faecalis capsular polysaccharides in healthy human sera

Healthy human sera (HHS) contain naturally acquired enterococcal antibodies which promote neutrophil-mediated killing. The target antigens remain unknown. The present study used a capsular polysaccharide (CPS)-enzyme-linked immunosorbent assay (ELISA) to investigate whether the HHS antibodies of 12 healthy donors bound to the CPS of four E. faecalis serotypes (CPS-A to CPS-D) and then employed an opsonic-killing assay to determine if these antibodies mediated phagocyte-dependent killing. All HHS contained immunoglobulin G (IgG) and IgM antibodies directed against capsular polysaccharides of the four serotypes. Absorption of the sera with homologous and heterologous strains showed a majority of antibodies to be cross-reactive among the prototype strains. The susceptibility of the four prototype strains to opsonic killing varied. Opsonic killing of CPS-A and CPS-B strains was significantly higher than killing of CPS-C and CPS-D strains. Absorption studies revealed that the opsonic killing of HHS was only partially type specific, with cross-reactivity between CPS-A and CPS-B strains and between CPS-C and CPS-D strains. These data indicate that healthy individuals possess opsonic antibodies specific for CPS-A and CPS-B but only low titers of opsonic antibodies against CPS-C and CPS-D. Titers of opsonic antibodies did not correlate with antibody titers measured by ELISA. Whether this lack of correlation is due to the low frequency of opsonic antibodies or to increased resistance to the opsonophagocytic killing of some serotypes remains to be determined.

Opsonophagocytic assay as a potentially useful tool for assessing safety of enterococcal preparations

Enterococci are commonly used in foods and food additives. A number of putative virulence factors are usually evaluated to assure that the strains used are not harmful. We propose an additional test to assess the safety of these bacteria by testing the susceptibility to opsonophagocytic killing. One probiotic Enterococcus faecalis strain was compared to a collection of 27 clinical isolates and our results indicate that 89% of the clinical strains were less susceptible to killing mediated by normal rabbit sera. Opsonophagocytic killing is the best in vitro surrogate for a protective immune response against bacteria, and the susceptibility of bacteria against normal rabbit sera indicates that these strains may not be able to survive in the bloodstream of the host. Further studies comparing a larger collection of pathogenic strains with commensal isolates are necessary to confirm these findings.

Promises and pitfalls of Pseudomonas aeruginosa lipopolysaccharide as a vaccine antigen

Antibodies directed to the Pseudomonas aeruginosa lipopolysaccharide (LPS) O-antigens have clearly shown to mediate the most effective immunity to infection caused by LPS-smooth strains. Such strains are major causes of disease in immunocompromised hosts such as burn or cancer patients, individuals in intensive care units, and those who utilize extended-wear contact lenses. Yet producing an effective vaccine composed of non-toxic, immunogenic polysaccharides has been challenging. The chemical diversity among the different O-antigens representative of the 20 major serotypes, plus additional diversity among some O-antigens representing variant subtype antigens, translates into a large degree of serologic variability that increases the complexity of O-antigen specific vaccines. Further complications come from the poor immunogenicity of the major protective epitope expressed by some O-antigens, and a large degree of diversity in animal responses that preclude predicting the optimal vaccine formulation from such studies. Nonetheless human trials over the years of vaccines eliciting O-antigen immunity have been encouraging, though no vaccine has yet been fully evaluated and found to be clinically efficacious. Newer vaccine approaches such as using polysaccharide-protein conjugates and passive therapy with monoclonal or polyclonal immune sera offer some additional means to try and produce an effective immunotherapeutic reagent for this problematic pathogen.

Functional properties of isotype-switched immunoglobulin M (IgM) and IgG monoclonal antibodies to Pseudomonas aeruginosa lipopolysaccharide

Controversy exists regarding isotype-related differences in the antibacterial and protective properties of lipopolysaccharide (LPS)-specific antibodies of the immunoglobulin M (IgM) class and various IgG subclasses. To clarify this issue, a murine hybridoma secreting IgM monoclonal antibody (MAb) specific for the O polysaccharide of Pseudomonas aeruginosa serogroup O6 LPS was class switched, by sib selection, to produce an IgG3 MAb with identical specificity and variable region heavy and light chain nucleotide sequences. This IgG3-secreting cell line was further switched to the production of O-specific, variable region-identical IgG1, IgG2b, and IgG2a MAbs. Functional comparisons of these LPS-specific IgM and IgG MAb isotypes revealed similar LPS binding, opsonic, and protective activities. Relatively minor isotype-related differences in levels of efficiency of MAb-mediated, complement-dependent opsonophagocytic killing (IgM > IgG2a > IgG3 > IgG2b > > IgG1) were not associated with corresponding differences in in vivo functions. These findings, in conjunction with previously published data, support a cautious approach to generic conclusions regarding the immunotherapeutic superiority of LPS-specific antibodies belonging to either the IgM or IgG class or to a particular IgG subclass.

Role of O-antigen variation in the immune response

The O antigen is an extremely variable surface polysaccharide of Gram-negative bacteria. This variation is thought to allow the various clones of a species each to present a surface that offers a selective advantage in the niche occupied by that clone. The interactions between O antigen and the immune system are central to determining the selective advantage of each clone.

Clearance of Pseudomonas aeruginosa from the murine gastrointestinal tract is effectively mediated by O-antigen-specific circulating antibodies

The colonization of mucosal surfaces by Pseudomonas aeruginosa can lead to local or disseminated disease. Secretory immunoglobulin A (IgA) has been assumed to be responsible for preventing mucosal colonization by interfering with the binding of bacterial ligands to epithelial surface receptors. However, the efficacy of this mechanism of immunity derives little actual support from in vivo experiments. In an investigation of the role of local and systemic immunization strategies in reducing colonization of the gastrointestinal tract of mice by P. aeruginosa, the bacterial antigens that were potential targets for immune effectors promoting mucosal clearance were identified. Levels of gastrointestinal colonization were reduced when immunity to homologous O antigens, but not that to pili or flagella, was elicited. Oral vaccination with attenuated Salmonella typhimurium expressing P. aeruginosa serogroup O11 antigen elicited mucosal and serum IgA antibodies and serum IgG antibodies specific for the recombinant antigen. Oral challenge of immunized mice with P. aeruginosa serogroup O11 demonstrated protection against gastrointestinal colonization. Intraperitoneal immunization with a serogroup O11 high-molecular-weight O-polysaccharide antigen elicited only serum IgG and IgM antibodies yet was as effective as oral vaccination in protecting mice against gastrointestinal colonization. This finding was confirmed by the demonstration that intraperitoneal immunization with purified lipopolysaccharide was also protective against mucosal surface colonization. These results call into question the need for local immune effectors, particularly secretory IgA, directed at bacterial ligands for epithelial surface components, in protecting a mucosal surface from bacterial challenge.

Biologic activities of antibodies to the neutral-polysaccharide component of the Pseudomonas aeruginosa lipopolysaccharide are blocked by O side chains and mucoid exopolysaccharide (alginate)

Virulent strains of Pseudomonas aeruginosa are either of a nonmucoid, lipopolysaccharide (LPS)-smooth or mucoid, LPS-rough phenotype, and immunity to these different variants is efficiently mediated by antibodies specific to O antigens or mucoid exopolysaccharide (also called alginate), respectively. In addition to O side chains and core polysaccharide components, the LPS of P. aeruginosa also contains neutral-polysaccharide components that express antigenic determinants common to many clinical isolates. We evaluated antibodies specific to neutral polysaccharides for the ability to mediate opsonic killing and protective immunity. Antibodies to these antigens mediated opsonic killing of poorly virulent nonmucoid LPS-rough isolates but not of isogenic strains with either a LPS-smooth or a mucoid phenotype. Antibodies to neutral-polysaccharide antigens also failed to protect neutropenic mice from challenge with modest doses of LPS-smooth P. aeruginosa strains (< 10(3) CFU per mouse), whereas O-antigen-specific antibodies were highly protective. Antibodies to neutral polysaccharides deposited significantly (P = 0.002) more C3 onto LPS-rough strains than did antibodies to O side chains, but this situation was reversed when isogenic LPS-smooth strains were tested. Given that protective immunity against P. aeruginosa must be directed against either nonmucoid LPS-smooth strains or mucoid LPS-rough strains, it appears that antibodies specific to neutral-polysaccharide antigens do not protect against P. aeruginosa infection. Lack of protection is likely due to the ability of both O side chains and mucoid exopolysaccharide (alginate) to interfere with the opsonic killing activity of neutral-polysaccharide-specific antibodies.

Immunogenic and antigenic properties of a heptavalent high-molecular-weight O-polysaccharide vaccine derived from Pseudomonas aeruginosa

We investigated the chemical and immunologic properties of a heptavalent vaccine composed of high-molecular-weight polymers of the lipopolysaccharide (LPS) O polysaccharides representative of the most common clinical isolates of Pseudomonas aeruginosa. We also evaluated the serum antibody response to nonvaccine strains of P. aeruginosa, including strains expressing structural variants (subtype strains) of the O side chain of the vaccine strains. The polyvalent vaccine, prepared under conditions suitable for human use, contained low levels of contaminants and passed preclinical safety and toxicity tests required for human use. Chemical analyses indicated that individual polysaccharides were composed of both O-side chain and core sugars. Following immunization of C3H/HeN mice and New Zealand White rabbits, antibody titers against vaccine components increased between 32- and 200-fold. Antibodies reactive with LPS isolated from smooth and rough nonvaccine strains were also elicited. Analysis of the opsonic activity against the known LPS subtype variants of the vaccine strains revealed a variable pattern of killing, which ranged from opsonic killing of > or = 69% of bacterial cells representing all subtype variants within a serogroup to opsonization of only a minority of the subtype variant strains. Mouse and rabbit immune sera showed different patterns of opsonic activity against subtype strains, indicating that different epitopes on these antigens are immunodominant in the representatives of these two animal species tested. The polyvalent vaccine was effective at eliciting antibodies to vaccine components in mice and rabbits, but it remains to be determined if the current heptavalent formulation contains sufficient components to provoke human antibodies reactive with a majority of clinical strains of P. aeruginosa.