A Staphylococcus aureus capsular polysaccharide (CP) vaccine and CP-specific antibodies protect mice against bacterial challenge

Synthesis and evaluation of a conjugate vaccine composed of Staphylococcus aureus poly-N-acetyl-glucosamine and clumping factor A

The increasing frequency, severity and antimicrobial resistance of Staphylococcus aureus infections has made the development of immunotherapies against this pathogen more urgent than ever. Previous immunization attempts using monovalent antigens resulted in at best partial levels of protection against S. aureus infection. We therefore reasoned that synthesizing a bivalent conjugate vaccine composed of two widely expressed antigens of S. aureus would result in additive/synergetic activities by antibodies to each vaccine component and/or in increased strain coverage. For this we used reductive amination, to covalently link the S. aureus antigens clumping factor A (ClfA) and deacetylated poly-N-β-(1-6)-acetyl-glucosamine (dPNAG). Mice immunized with 1, 5 or 10 µg of the dPNAG-ClfA conjugate responded in a dose-dependent manner with IgG to dPNAG and ClfA, whereas mice immunized with a mixture of ClfA and dPNAG developed significantly lower antibody titers to ClfA and no antibodies to PNAG. The dPNAG-ClfA vaccine was also highly immunogenic in rabbits, rhesus monkeys and a goat. Moreover, affinity-purified, antibodies to ClfA from dPNAG-ClfA immune serum blocked the binding of three S. aureus strains to immobilized fibrinogen. In an opsonophagocytic assay (OPKA) goat antibodies to dPNAG-ClfA vaccine, in the presence of complement and polymorphonuclear cells, killed S. aureus Newman and, to a lower extent, S. aureus Newman ΔclfA. A PNAG-negative isogenic mutant was not killed. Moreover, PNAG antigen fully inhibited the killing of S. aureus Newman by antisera to dPNAG-ClfA vaccine. Finally, mice passively vaccinated with goat antisera to dPNAG-ClfA or dPNAG-diphtheria toxoid conjugate had comparable levels of reductions of bacteria in the blood 2 h after infection with three different S. aureus strains as compared to mice given normal goat serum. In conclusion, ClfA is an immunogenic carrier protein that elicited anti-adhesive antibodies that fail to augment the OPK and protective activities of antibodies to the PNAG cell surface polysaccharide.

Development of a multicomponent Staphylococcus aureus vaccine designed to counter multiple bacterial virulence factors

Staphylococcus aureus is a major cause of healthcare-associated infections and is responsible for a substantial burden of disease in hospitalized patients. Despite increasingly rigorous infection control guidelines, the prevalence and corresponding negative impact of S. aureus infections remain considerable. Difficulties in controlling S. aureus infections as well as the associated treatment costs are exacerbated by increasing rates of resistance to available antibiotics. Despite ongoing efforts over the past 20 years, no licensed S. aureus vaccine is currently available. However, learnings from past clinical failures of vaccine candidates and a better understanding of the immunopathology of S. aureus colonization and infection have aided in the design of new vaccine candidates based on multiple important bacterial pathogenesis mechanisms. This review outlines important considerations in designing a vaccine for the prevention of S. aureus disease in healthcare settings.

Abscess formation and alpha-hemolysin induced toxicity in a mouse model of Staphylococcus aureus peritoneal infection

Staphylococcus aureus is a frequent cause of skin infection and sepsis in humans. Preclinical vaccine studies with S. aureus have used a mouse model with intraperitoneal challenge and survival determination as a measure for efficacy. To appreciate the selection of protective antigens in this model, we sought to characterize the pathological attributes of S. aureus infection in the peritoneal cavity. Testing C57BL/6J and BALB/c mice, >10(9) CFU of S. aureus Newman were needed to produce a lethal outcome in 90% of animals infected via intraperitoneal injection. Both necropsy and histopathology revealed the presence of intraperitoneal abscesses in the vicinity of inoculation sites. Abscesses were comprised of fibrin as well as collagen deposits and immune cells with staphylococci replicating at the center of these lesions. Animals that succumbed to challenge harbored staphylococci in abscess lesions and in blood. The establishment of lethal infections, but not the development of intraperitoneal abscesses, was dependent on S. aureus expression of alpha-hemolysin (Hla). Active immunization with nontoxigenic Hla(H35L) or passive immunization with neutralizing monoclonal antibodies protected mice against early lethal events associated with intraperitoneal S. aureus infection but did not affect the establishment of abscess lesions. These results characterize a mouse model for the study of intraperitoneal abscess formation by S. aureus, a disease that occurs frequently in humans undergoing continuous ambulatory peritoneal dialysis for end-stage renal disease.

Novel structurally designed vaccine for S. aureus α-hemolysin: protection against bacteremia and pneumonia

Staphylococcus aureus (S. aureus) is a human pathogen associated with skin and soft tissue infections (SSTI) and life threatening sepsis and pneumonia. Efforts to develop effective vaccines against S. aureus have been largely unsuccessful, in part due to the variety of virulence factors produced by this organism. S. aureus alpha-hemolysin (Hla) is a pore-forming toxin expressed by most S. aureus strains and reported to play a key role in the pathogenesis of SSTI and pneumonia. Here we report a novel recombinant subunit vaccine candidate for Hla, rationally designed based on the heptameric crystal structure. This vaccine candidate, denoted AT-62aa, was tested in pneumonia and bacteremia infection models using S. aureus strain Newman and the pandemic strain USA300 (LAC). Significant protection from lethal bacteremia/sepsis and pneumonia was observed upon vaccination with AT-62aa along with a Glucopyranosyl Lipid Adjuvant-Stable Emulsion (GLA-SE) that is currently in clinical trials. Passive transfer of rabbit immunoglobulin against AT-62aa (AT62-IgG) protected mice against intraperitoneal and intranasal challenge with USA300 and produced significant reduction in bacterial burden in blood, spleen, kidney, and lungs. Our Hla-based vaccine is the first to be reported to reduce bacterial dissemination and to provide protection in a sepsis model of S. aureus infection. AT62-IgG and sera from vaccinated mice effectively neutralized the toxin in vitro and AT62-IgG inhibited the formation of Hla heptamers, suggesting antibody-mediated neutralization as the primary mechanism of action. This remarkable efficacy makes this Hla-based vaccine a prime candidate for inclusion in future multivalent S. aureus vaccine. Furthermore, identification of protective epitopes within AT-62aa could lead to novel immunotherapy for S. aureus infection.

Characterization of the mechanism of protection mediated by CS-D7, a monoclonal antibody to Staphylococcus aureus iron regulated surface determinant B (IsdB)

We previously reported the development of a human monoclonal antibody (CS-D7, IgG₁) with specificity and affinity for the iron regulated surface determinant B (IsdB) of Staphylococcus aureus. CS-D7 mediates opsonophagocytic killing in vitro and protection in a murine sepsis model. In light of recent data indicating that IsdB specific T cells (CD4+, Th17), not Ab, mediate protection after vaccination with IsdB, it is important to investigate the mechanism of protection mediated by CS-D7. The mAb was examined to determine if it blocked heme binding to IsdB in vitro. The mAb was not found to have heme blocking activity, nor did it prevent bacterial growth under in vivo conditions, in an implanted growth chamber. To assess the role of the mAb Fc a point mutation was introduced at aa 297 (CS-D7·N297A). This point mutation removes Fc effector functions. In vitro analysis of the mutein confirmed that it lacked measurable binding to FcγR, and that it did not fix complement. The mutein had dramatically reduced in vitro opsonic OP activity compared to CS-D7. Nonetheless, the mutein conferred protection equivalent to the wild type mAb in the murine sepsis model. Both wild type and mutein mAbs were efficacious in FcγR deletion mice (including both FcγRII^−/− mice and FcγRIII^−/− mice), indicating that these receptors were not essential for mAb mediated protection in vivo. Protection mediated by CS-D7 was lost in Balb/c mice depleted of C3 with cobra venom factor (CFV), was lost in mice depleted of superoxide dismutase (SOD) in P47phox deletion mice, and as previously reported, was absent in SCID mice (Joshi et al., 2012). Enhanced clearance of S. aureus in the liver of CS-D7 treated mice and enhanced production of IFN-γ, but not of IL17, may play a role in the mechanism of protection mediated by the mAb. CS-D7 apparently mediates survival in challenged mice through a mechanism involving complement, phagocytes, and lymphocytes, but which does not depend on interaction with FcγR, or on blocking heme uptake.

Immunization with Staphylococcus aureus iron regulated surface determinant B (IsdB) confers protection via Th17/IL17 pathway in a murine sepsis model

We have previously shown that IsdB, a conserved protein expressed by Staphylococcus aureus, induces a robust antibody response which correlates with protection in a murine challenge model. Here we investigate the role of cellular immunity in IsdB mediated protection using lymphocyte deficient SCID mice. As opposed to WT CB-17 mice the CB-17 SCID mice were not protected against a lethal challenge of S. aureus after active and passive immunizations with IsdB. Adoptive transfer of in vitro isolated lymphocyte subsets revealed that reconstituting mice with IsdB specific CD3+ or CD4+ T-cells conferred antigen specific protection while CD8⁺ T-cells, CD19⁺ B-cells and plasma cells (CD138^highB220^intCD19^lo) alone were not protective. A combination of CD3⁺ T-cells plus CD19⁺ B-cells conferred protection in CB-17 SCID mice, whereas bovine serum albumin (BSA) immune lymphocytes did not confer protection. Active immunization experiments indicated that IsdB immunized Jh mice (B-cell deficient) were protected against lethal challenge, while nude (T-cell deficient) mice were not. In vitro assays indicated that isolated IsdB specific splenocytes from immunized mice produced abundant IL-17A, much less IFN-γ and no detectable IL-4. IL-23 deficient mice were not protected from a lethal challenge by IsdB vaccination, pointing to a critical role for CD4⁺ Th17 in IsdB-mediated vaccination. Neutralizing IL-17A, but not IL-22 in vivo significantly increased mortality in IsdB immunized mice; whereas, neutralizing IFN-γ did not alter IsdB-mediated protection. These findings suggest that IL-17A producing Th17 cells play an essential role in IsdB vaccine-mediated defense against invasive S. aureus infection in mice.

Immunotherapeutic approaches against Staphylococcus aureus

Staphylococcus aureus is a major cause of life-threatening infections such as bacteremia and endocarditis. Unfortunately, many strains of this bacterial species have become resistant to certain antibiotics, including methicillin and amoxicillin. These strains are known as methicillin-resistant S. aureus (MRSA). Therefore, the prophylactic and therapeutic potential of antistaphylococcal vaccines is currently being explored with priority. In animal models, (passive) immunization with (antibodies directed against) certain S. aureus surface components, staphylococcal toxins and capsular polysaccharides protects against S. aureus colonization or infection. However, immunization studies performed in humans show less promising results. So far, not a single antistaphylococcal vaccine successfully passed clinical trials. This article focuses on the results that were obtained with immunotherapeutic approaches directed against S. aureus in animal and human studies. In addition, it is discussed whether effective immunization approaches against S. aureus are feasible in humans.

Progress in the development of effective vaccines to prevent selected gram-positive bacterial infections

Infections caused by virulent Gram-positive bacteria, such as Staphylococcus aureus, group B streptococci and group A streptococci, remain significant causes of morbidity and mortality despite progress in antimicrobial therapy. Despite significant advances in the understanding of the pathogenesis of infection caused by these organisms, there are only limited strategies to prevent infection. In this article, we review efforts to develop safe and effective vaccines that would prevent infections caused by these 3 pathogens.

Novel targeted immunotherapy approaches for staphylococcal infection

IMPORTANCE OF THE FIELD

Staphylococcus aureus is a leading human pathogen in the hospital and the community. Many S. aureus strains are resistant to antibiotics, making treatment of S. aureus infections often very complicated. In contrast to many other bacterial pathogens, a working vaccine has never been found for S. aureus despite considerable efforts in academia and pharmaceutical companies.

AREAS COVERED IN THIS REVIEW

The latest strategies aimed at finding a working vaccine against S. aureus, including active and passive immunization efforts in pre-clinical and clinical stages, and the molecular reasons for why it may be difficult to develop a vaccine are discussed.

WHAT THE READER WILL GAIN

In addition to receiving an overview of current efforts in S. aureus vaccine research, the reader will understand that vaccine development for S. aureus may be difficult owing to the facts that S. aureus is a commensal microorganism and produces toxins that lyse white blood cells, thereby undermining a vaccine's role as a facilitator of opsonophagocytosis.

TAKE HOME MESSAGE

As a result of failed clinical trials with monovalent traditional vaccines, recent developments include a shift towards the potential use of polyvalent formulas and therapeutic antibodies and more systematic selection of optimal antigens.

Genetic variation in Staphylococcus aureus surface and immune evasion genes is lineage associated: implications for vaccine design and host-pathogen interactions

Background

S. aureus is a coloniser and pathogen of humans and mammals. Whole genome sequences of 58 strains of S. aureus in the public domain and data from multi-strain microarrays were compared to assess variation in the sequence of proteins known or putatively interacting with host.

Results

These included 24 surface proteins implicated in adhesion (ClfA, ClfB, Cna, Eap, Ebh, EbpS, FnBPA, FnBPB, IsaB, IsdA, IsdB, IsdH, SasB, SasC, SasD, SasF, SasG, SasH, SasK, SdrC, SdrD, SdrE, Spa and SraP) and 13 secreted proteins implicated in immune response evasion (Coa, Ecb, Efb, Emp, EsaC, EsxA, EssC, FLIPr, FLIPr like, Sbi, SCIN-B, SCIN-C, VWbp) located on the stable core genome. Many surface protein genes were missing or truncated, unlike immune evasion genes, and several distinct variants were identified. Domain variants were lineage specific. Unrelated lineages often possess the same sequence variant domains proving that horizontal transfer and recombination has contributed to their evolution. Surprisingly, sequenced strains from four animal S. aureus strains had surface and immune evasion proteins remarkably similar to those found in human strains, yet putative targets of these proteins vary substantially between different hosts. This suggests these proteins are not essential for virulence. However, the most variant protein domains were the putative functional regions and there is biological evidence that variants can be functional, arguing they do play a role.

Conclusion

Surface and immune evasion genes are candidates for S. aureus vaccines, and their distribution and functionality is key. Vaccines should contain cocktails of antigens representing all variants or they will not protect against naturally occurring S. aureus populations.

Vaccine development in Staphylococcus aureus: taking the biofilm phenotype into consideration

Vaccine development against pathogenic bacteria is an imperative initiative as bacteria are gaining resistance to current antimicrobial therapies and few novel antibiotics are being developed. Candidate antigens for vaccine development can be identified by a multitude of high-throughput technologies that were accelerated by access to complete genomes. While considerable success has been achieved in vaccine development against bacterial pathogens, many species with multiple virulence factors and modes of infection have provided reasonable challenges in identifying protective antigens. In particular, vaccine candidates should be evaluated in the context of the complex disease properties, whether planktonic (e.g. sepsis and pneumonia) and/or biofilm associated (e.g. indwelling medical device infections). Because of the phenotypic differences between these modes of growth, those vaccine candidates chosen only for their efficacy in one disease state may fail against other infections. This review will summarize the history and types of bacterial vaccines and adjuvants as well as present an overview of modern antigen discovery and complications brought about by polymicrobial infections. Finally, we will also use one of the better studied microbial species that uses differential, multifactorial protein profiles to mediate an array of diseases, Staphylococcus aureus, to outline some of the more recently identified problematic issues in vaccine development in this biofilm-forming species.

Staphylococcus aureus AI-2 quorum sensing associates with the KdpDE two-component system to regulate capsular polysaccharide synthesis and virulence

Autoinducer 2 (AI-2) is widely recognized as a signal molecule for intra- and interspecies communication in Gram-negative bacteria, but its signaling function in Gram-positive bacteria, especially in Staphylococcus aureus, remains obscure. Here we reveal the role of LuxS in the regulation of capsular polysaccharide synthesis in S. aureus NCTC8325 and show that AI-2 can regulate gene expression and is involved in some physiological activities in S. aureus as a signaling molecule. Inactivation of luxS in S. aureus NCTC8325 resulted in higher levels of transcription of capsular polysaccharide synthesis genes. The survival rate of the luxS mutant was higher than that of the wild type in both human blood and U937 macrophages. In comparison to the luxS mutant, a culture supplemented with chemically synthesized 4,5-dihydroxy-2,3-pentanedione (DPD), the AI-2 precursor molecule, restored all the parental phenotypes, suggesting that AI-2 has a signaling function in S. aureus. Furthermore, we demonstrated that the LuxS/AI-2 signaling system regulates capsular polysaccharide production via a two-component system, KdpDE, whose function has not yet been clarified in S. aureus. This regulation occurred via the phosphorylation of KdpE binding to the cap promoter.

Antibody responses in patients with invasive Staphylococcus aureus infections

Correlation between antibody response and clinical outcome in Staphylococcus aureus bacteremia has yielded conflicting results. Immunization schedules have failed in clinical trials. Is the humoral response toward S. aureus of protective nature? A prospective study was performed in patients with invasive S. aureus (ISA) infections during the period 2003-2005. The antibody levels were determined at the beginning and at the end of treatment and one month later (n = 96, n = 71, and n = 51, respectively). As controls, 115 healthy individuals were used. A quantitative enzyme-linked immunosorbent assay (ELISA) against eight purified antigens was performed. Bacterial isolates were grouped as to the production of alpha-toxin, agr type, and pulsed-field gel electrophoresis (PFGE) type. Large variations were seen in the antibody levels. The levels in the second sample were the highest. A correlation between agr group, PFGE group, alpha-toxin production, and initial antibody levels was observed. Patients with fatal outcome displayed lower initial antibody levels to all antigens and significantly so in regard to teichoic acid, lipase, enterotoxin A, and scalded skin syndrome toxin. In episodes with complicated bacteremia, initial significantly low levels to teichoic acid and lipase were registered. Low initial antibody levels against several antigens were associated with increased mortality and complicated bacteremia in invasive S. aureus infections. Bacterial properties, strain, and toxin production affected the antibody response.

Efficacy of Rhodotorula glutinis and Spirulina platensis carotenoids in immunopotentiation of mice infected with Candida albicans SC5314 and Pseudomonas aeruginosa 35

Enhancement of the immune response leading to protection against bacterial and fungal infections was shown using different schedules of immunization with microbial pigments and a polysaccharide. The group of mice given carotenoids of Rhodotorula glutinis (preparation I) and polysaccharide of Spitulina platensis (IV) survived for 2 weeks after Pseudomonas aeruginosa infection. The groups of mice given carotenoids (I), polysaccharide (IV), I+IV and with the crude phycocyanin of S. platensis (III)+IV survived for 2 weeks after Candida albicans infection. All other groups recorded a maximum level of mortality reaching 2 mice per group either after immunization or post-infection. Adding the carotenoids, phycocyanin and polysaccharides to food as additives might therefore enhance the human immune response against microbial infections.

The influence of antibodies on Staphylococcus epidermidis adherence to polyvinylpyrrolidone-coated silicone elastomer in experimental biomaterial-associated infection in mice

Biomaterial-associated infection (BAI) is a major problem in modern medicine, and is often caused by Staphylococcus epidermidis. We aimed to raise monoclonal antibodies (mAbs) against major surface protein antigens of S. epidermidis, and to assess their possible protective activity in experimental BAI. Mice were vaccinated with a cell wall protein preparation of S. epidermidis. A highly immunodominant antigen was identified as Accumulation-associated protein (Aap). mAbs against Aap and against surface-exposed lipoteichoic acid (LTA) were used for passive immunization of mice in experimental biomaterial-associated infection. Neither anti-Aap nor anti-LTA mAbs showed protection. Either with or without antibodies, tissue surrounding the implants was more often culture positive than the implants themselves, but bacterial adherence to the implants was significantly increased in mice injected with anti-LTA. In vitro, anti-Aap and anti-LTA did show binding to S. epidermidis, but no opsonic activity was observed. We conclude that antibodies against S. epidermidis LTA or Aap showed no opsonic activity and did not protect mice against BAI. Moreover, the increase in binding to implanted biomaterial suggests that passive immunization may increase the risk for BAI.

Staphylococcal vaccines and immunotherapies

Staphylococcus aureus is an important pathogen in the hospital and in the community, and it is increasingly resistant to multiple antibiotics. A nonantimicrobial approach to controlling S aureus is needed. The most extensively tested vaccine against S aureus, which is a capsular polysaccharide-based vaccine known as StaphVAX, showed promise in an initial phase 3 trial, but was found to be ineffective in a confirmatory trial, leading to its development being halted. Likewise, a human IgG preparation known as INH-A21 (Veronate) with elevated levels of antibodies to the staphylococcal surface adhesins ClfA and SdrG made it into phase 3 testing, where it failed to show a clinical benefit. Several novel antigens are being tested for potential inclusion in a staphylococcal vaccine, including cell wall-anchored adhesin proteins and exotoxins. Given the multiple and sometimes redundant virulence factors of S aureus that enable it to be such a crafty pathogen, if a vaccine is to prove effective, it will have to be multicomponent, incorporating several surface proteins, toxoids, and surface polysaccharides.

Protection from Staphylococcus aureus mastitis associated with poly-N-acetyl beta-1,6 glucosamine specific antibody production using biofilm-embedded bacteria

Staphylococcus aureus vaccines based on bacterins surrounded by slime, surface polysaccharides coupled to protein carriers and polysaccharides embedded in liposomes administered together with non-biofilm bacterins confer protection against mastitis. However, it remains unknown whether protective antibodies are directed to slime-associated known exopolysaccharides and could be produced in the absence of bacterin immunizations. Here, a sheep mastitis vaccination study was carried out using bacterins, crude bacterial extracts or a purified exopolysaccharide from biofilm bacteria delivered in different vehicles. This polysaccharide reacted specifically with antibodies to poly-N-acetyl-beta-1,6-glucosamine (PNAG) and not with antibodies to other capsular antigens or bacterial components. Following intra-mammary challenge with biofilm-producing bacteria, antibody production against the polysaccharide, milk bacterial counts and mastitis lesions were determined. Bacterins from strong biofilm-producing bacteria triggered the highest production of antibodies to PNAG and conferred the highest protection against infection and mastitis, compared with weak biofilm-producing bacteria and non-cellular inocula. Thus, bacterins from strong biofilm bacteria, rather than purified polysaccharide, are proposed as a cost-efficient vaccination against S. aureus ruminant mastitis.

Antibodies to capsular polysaccharide and clumping factor A prevent mastitis and the emergence of unencapsulated and small-colony variants of Staphylococcus aureus in mice

The pathogenesis of Staphylococcus aureus infections is influenced by multiple virulence factors that are expressed under variable conditions, and this has complicated the design of an effective vaccine. Clinical trials that targeted the capsule or clumping factor A (ClfA) failed to protect the recipients against staphylococcal infections. We passively immunized lactating mice with rabbit antibodies to S. aureus capsular polysaccharide (CP) serotype 5 (CP5) or CP8 or with monoclonal antibodies to ClfA. Mice immunized with antibodies to CP5 or CP8 or with ClfA had significantly reduced tissue bacterial burdens 4 days after intramammary challenge with encapsulated S. aureus strains. After several passages in mice passively immunized with CP-specific antiserum, increasing numbers of stable unencapsulated variants of S. aureus were cultured from the infected mammary glands. Greater numbers of these unencapsulated S. aureus variants than of the corresponding encapsulated parental strains were internalized in vitro in MAC-T bovine cells. Furthermore, small-colony variants (SCVs) were recovered from the infected mammary glands after several passages in mice passively immunized with CP-specific antiserum. A combination of antibodies effectively sterilized mammary glands in a significant number of passively immunized mice. More importantly, passive immunization with antibodies to both CP and ClfA fully inhibited the emergence of unencapsulated "escape mutants" and significantly reduced the appearance of SCVs. A vaccine formulation comprising CP conjugates plus a surface-associated protein adhesin may be more effective than either antigen alone for prevention of S. aureus infections.

Targeted immunotherapy for staphylococcal infections : focus on anti-MSCRAMM antibodies

Staphylococcal infections represent an enormous burden to the public health system in the US and worldwide. While traditionally restricted to the hospital setting, highly virulent strains have recently emerged that may cause severe, even fatal, disease in healthy adults outside healthcare settings. This situation, together with the increasing resistance to many antibacterials in a wide variety of staphylococcal strains, requires that vaccine development for staphylococcal diseases be re-evaluated. Finding a vaccine for staphylococci is not trivial, as protective immunity to staphylococcal infections does not appear to exist at a significant degree, which may be partly due to the fact that our immune system is in constant contact with staphylococcal antigens and many strains are commensal organisms on human epithelia. Furthermore, the most virulent species, Staphylococcus aureus, produces protein A, a powerful means to evade acquired host defense. While two high-profile vaccine preparations have failed clinical trials within the last few years, promising results from novel approaches based on the combination of systematically selected antigens have been reported. These combinatory vaccines target microbial surface components recognizing adhesive matrix molecules (MSCRAMMs), a family of bacterial proteins that bind to human extracellular matrix components. In addition, polysaccharide and other nonprotein antigens may represent suitable vaccine targets on the staphylococcal cell surface.

Serotyping of Dutch Staphylococcus aureus strains from carriage and infection

International epidemiological studies have shown that clinical isolates of Staphylococcus aureus are usually capsulated with either type 5 or 8 capsular polysaccharides (CPs). Because all noncapsulated strains were found to be cross-reactive with polysaccharide 336 (336PS) antibodies, the noncapsulated strains were denoted as type 336PS. The capsular types of 162 Dutch methicillin-susceptible S. aureus strains derived from individuals living in the Rotterdam area were determined. The serotype distribution was 28.4% serotype 5, 53.7% type 8, and 17.9% type 336PS. Serotyping was in agreement with genotyping by amplified fragment length polymorphism (AFLP) and multi locus sequence typing (MLST). Among 49 nasal carriage isolates from healthy children 24.5% belonged to serotype 5, 67.3% were type 8 and 8.2% were type 336PS. For 28 adult patients on chronic ambulatory peritoneal dialysis (CAPD) the serotype incidences among carriage isolates obtained from the nose, catheter exit-site, and abdominal skin were 45.1%, 41.2% and 13.7%, respectively. Among S. aureus strains deriving from blood cultures, the serotype incidences were 17.7% serotype 5, 53.2% type 8, and 29.0% type 336PS. Apparently, type 336PS strains are more prevalent (P=0.017) among bacteraemia isolates as compared with the nasal carriage isolates obtained from healthy children and CAPD patients. In conclusion, all Dutch S. aureus isolates belonged to types 5, 8, or 336PS, which is in agreement with data from other countries. Thus, addition of the 336PS conjugate to a type 5- and type 8-CP protein conjugate vaccine would significantly extend the vaccine coverage.

Phase II, randomized, multicenter, double-blind, placebo-controlled trial of a polyclonal anti-Staphylococcus aureus capsular polysaccharide immune globulin in treatment of Staphylococcus aureus bacteremia

New treatment modalities are needed for the treatment of infections due to multidrug-resistant Staphylococcus aureus. S. aureus capsular polysaccharide immune globulin (Altastaph) is a polyclonal immune globulin preparation that is being developed as adjunctive therapy for persons with S. aureus infections complicated by bacteremia. In a phase II, multicenter, randomized, double-blind, placebo-controlled trial, 40 subjects with documented S. aureus bacteremia received standard therapy plus either Altastaph at 200 mg/kg of body weight in each of two infusions 24 h apart or placebo. During the 42-day observation period, antibody pharmacokinetics and safety were the primary characteristics studied. Information regarding the resolution of bacteremia and fever was also analyzed. Anti-type-5 and anti-type-8 capsular antibody levels peaked after the second infusion at 550 mug/ml and 419 mug/ml, respectively, and remained above 100 mug/ml at day 28. A total of 316 adverse events were noted in 39 of 40 subjects. Infusion-related adverse events in Altastaph recipients were infrequent and similar to those among recipients of commercial intravenously administered immunoglobulin G products. Five of 21 (23%) subjects in the Altastaph group died, whereas 2 of 18 (11%) subjects in the placebo group died (P = 0.42). Compared to the control patients, the Altastaph recipients had a shorter median time to the resolution of fever (2 days and 7 days, respectively; P = 0.09) and a shorter length of hospital stay (9 days and 14 days, respectively; P = 0.03). However, these findings are exploratory, and there were few differences in the other variables measured. High levels of opsonizing antibodies were maintained for the initial 4 weeks. Although the study was not powered to show efficacy, these preliminary findings and safety profile suggest that Altastaph may be an effective adjunct to antibiotics and warrants further investigation (ClinicalTrials.gov number NCT00063089).

Staphylococcus aureus protein A triggers T cell-independent B cell proliferation by sensitizing B cells for TLR2 ligands

B cells possess functional characteristics of innate immune cells, as they can present Ag to T cells and can be stimulated with microbial molecules such as TLR ligands. Because crude preparations of Staphylococcus aureus are frequently used as polyclonal B cell activators and contain potent TLR2 activity, the scope of this study was to analyze the impact of S. aureus-derived TLR2-active substances on human B cell activation. Peripheral B cells stimulated with chemically modified S. aureus cell wall preparations proliferated in response to stimulation with crude cell wall preparations but failed to be activated with pure peptidoglycan, indicating that cell wall molecules other than peptidoglycan are responsible for B cell proliferation. Subsequent analysis revealed that surface protein A (SpA), similar to BCR cross-linking with anti-human Ig, sensitizes B cells for the recognition of cell wall-associated TLR2-active lipopeptides (LP). In marked contrast to TLR7- and TLR9-triggered B cell stimulation, stimulation with TLR2-active LP and SpA or with crude cell wall preparations failed to induce IgM secretion, thereby revealing qualitative differences in TLR2 signaling compared with TLR7/9 signaling. Notably, combined stimulation with SpA plus TLR2 ligands induced vigorous proliferation of a defined B cell subset that expressed intracellular IgM in the presence of IL-2.

CONCLUSION

S. aureus triggers B cell activation via SpA-induced sensitization of B cells for TLR2-active LP. Combined SpA and TLR2-mediated B cell activation promotes B cell proliferation but fails to induce polyclonal IgM secretion as seen after TLR7 and TLR9 ligation.

Identification of antigenic components of Staphylococcus epidermidis expressed during human infection

A spectrum of in vivo-expressed Staphylococcus epidermidis antigens was identified by probing a bacteriophage lambda library of S. epidermidis genomic DNA with human serum from infected and uninfected individuals. This analysis resulted in identification of 53 antigen-encoding loci. Six antigenic polypeptides were expressed from these loci and purified. These polypeptides were the propeptide, mature amidase, and repeat sequence domains of the major autolysin AtlE, GehD (lipase), and two members of a conserved family of surface proteins (ScaA [AaE] and ScaB). AtlE, ScaA, and ScaB all exhibit human ligand binding capacity. Screening a bank of human serum samples revealed that there were significant increases in the amounts of reactive immunoglobulin G in infected individuals compared to the amounts in healthy individuals for the repeat sequence and mature amidase domains of AtlE, ScaB, and GehD. Vaccination of mice with recombinant antigens stimulated an immune response which in vitro opsonized S. epidermidis. In this study we identified prospective candidate antigens for prophylaxis or immunotherapy to control disease.

The role of gamma interferon in acquired host resistance against Staphylococcus aureus infection in mice

We investigated the expression of an acquired host resistance against Staphylococcus aureus infection in mice. When C57BL/6 mice were immunized with viable S. aureus and challenged with S. aureus eight weeks later, the elimination of S. aureus from the spleen and liver was enhanced in the immunized mice compared with the nonimmunized mice. When gamma interferon (IFN-gamma(-/-)) mice were immunized and challenged, the bacterial numbers in the organs of immunized mice were comparable to those in the nonimmunized mice, suggesting that IFN-gamma plays a critical role in an acquired host resistance against S. aureus infection. IFN-gamma(-/-) mice produced the lower level of anti-S. aureus immunoglobulin M (IgM) and IgG2a antibodies compared with C57BL/6 mice. To elucidate the role of IFN-gamma produced during a challenge with S. aureus, a single injection of anti-IFN-gamma monoclonal antibody to mice was carried out 1 h before challenge. An acquired resistance against S. aureus infection was inhibited by injecting with anti-IFN-gamma monoclonal antibody. However, anti-IFN-gamma monoclonal antibody treatment failed to modulate anti-S. aureus IgM, IgG1 or IgG2a responses in these animals. These results demonstrated that IFN-gamma is required for an acquired resistance against S. aureus infection in mice. However, IFN-gamma induced during the challenge failed to affect the secondary antibody responses.

The neutralizing effects of hyperimmune antibodies against extracellular fibrinogen-binding protein, Efb, from Staphylococcus aureus

Staphylococcus aureus is a significant cause of acute and chronic infection and boasts a diverse array of virulence factors. S. aureus produces and secretes a protein, extracellular fibrinogen (Fg)-binding protein (Efb), which contributes to virulence in wound infection. Efb binds to both Fg and platelets and inhibits platelet function in vitro and in vivo. In this study, we have characterized the antibody response against Efb. Antibodies generated in response to immunization with Efb can neutralize the biological effects of Efb. Hyperimmune sheep immunoglobulin (Ig)G against Efb blocked the binding of Efb to Fg and prevented Efb-mediated inhibition of platelet aggregation. Furthermore, these antibodies cross-reacted with coagulase and blocked coagulase activity in plasma. Immunization of mice with Efb resulted in the generation of high titre specific antibodies. When subjected to a foreign-body-associated wound infection, the vaccinated animals developed significantly less severe wound infection than the unvaccinated controls. Also, human IgG against Efb was prepared from commercial IgG pools; however, the monospecific human anti-Efb that was enriched was unable to neutralize Efb. We conclude that immunization with Efb is required in order to generate a protective antibody response to Efb from S. aureus.

A blinded, randomized, multicenter study of an intravenous Staphylococcus aureus immune globulin

OBJECTIVES

Very low birth weight (VLBW) infants are vulnerable to nosocomial infections and subsequent morbidity; including infections caused by Staphylococcus aureus: 85% of nosocomial S. aureus infections are caused by capsular polysaccharide (CPS) types 5 and 8. Altastaph is a polyclonal investigational human immunoglobulin G (IgG) with high levels of opsonizing S. aureus CPS types 5 and 8 IgG.

METHODS

A Phase 2 clinical trial to assess the safety and kinetics of Altastaph in VLBW infants. Neonates in this multicenter study were randomized to receive two identical 20 ml/kg i.v. infusions of either 0.45% NaCl placebo or 1000 mg Altastaph/kg. Each infant was followed for 28 days after the second infusion or until discharge. Serum S. aureus CPS types 5 and 8 IgG levels were measured preinfusion and at various times after each infusion.

RESULTS

Of 206 neonates, 158 received both infusions. Adverse events were similar in the two treatment groups. Six subjects (3% in each group) discontinued owing to an adverse event. Geometric mean anti-type 5 IgG levels were 402 and 642 mcg/ml 1 day following infusion of the first (day 0) and Second (day 14) doses, respectively, in neonates < or =1000 g and slightly higher in neonates 1001 to 1500 g. Trough levels before second infusion were 188 mcg/ml. Type 8 IgG levels were similar. Geometric mean IgG levels among placebo recipients were consistently <2 and <5 mcg/ml for types 5 and 8 in both weight groups. Three episodes of S. aureus bacteremia occurred in each arm.

CONCLUSIONS

Infusion of Altastaph in VLBW neonates resulted in high levels of specific S. aureus types 5 and 8 CPS IgG. The administration of this anti-staphylococcal hyperimmune globulin was well tolerated in this population.

The transmission and control of mastitis in dairy cows: A theoretical approach

A multi-species model that incorporates the transmission of both major and minor mastitis pathogens as well as the interaction between them via coinfection of a quarter is fitted to data from seven dairy herds. The results suggest that major and minor pathogens can interact, on occasion, in a counter-intuitive way with implications for the control of clinical mastitis. The key finding is that delaying culling of cows with major pathogen infections for more than 100 days post infection could result in a higher prevalence of major pathogen infections, whereas early culling would reduce the levels. A theoretical exploration of current and proposed control strategies is carried out, informed by parameters estimated from the model and data. The results at each stage suggest of areas of further research such as: field-testing of the hypotheses presented; the exploration of a stochastic formulation of the model; analysis of the raw repeated measures data; application of control theory to determine the most effective combination of control strategies; inclusion of economic factors into the modelling framework.

Use of a conjugate polysaccharide vaccine in the prevention of invasive staphylococcal disease: Is an additional vaccine needed or possible?

Staphylococcus aureus is a ubiquitous bacterial species that causes serious disease in certain settings. S. aureus disease is difficult to treat, and antibiotic-resistant strains have become common. A vaccine to protect against infection would therefore be beneficial. However, the virulence of S. aureus is determined by a number of different factors, which makes design of a widely effective vaccine difficult. Here, various bacterial virulence factors and attempts to develop vaccines based on these factors are briefly reviewed. In particular, the success of a Phase 3 clinical study of a vaccine directed at capsular polysaccharides types 5 and 8 is discussed.

Methicillin-resistant Staphylococcus aureus infections of the eye and orbit (an American Ophthalmological Society thesis)

PURPOSE

To ascertain if methicillin-resistant Staphylococcus aureus (MRSA) ophthalmic infections are increasing.

METHODS

A retrospective review of all patients with a culture positive for MRSA in the Parkland Health and Hospital System, the urban public healthcare system for Dallas County, Texas, for the years 2000 through 2004 was performed. Patients with ocular, orbital, and ocular adnexal infection were identified, and isolates were categorized as nosocomial or community-acquired (CA).

RESULTS

A total of 3,640 patients with a culture positive for MRSA were identified, with 1,088 patients (30%) considered to have acquired the isolate via nosocomial transmission and 2,552 patients (70%) considered to have CA-MRSA. Forty-nine patients (1.3%) had ophthalmic MRSA involvement. For both ophthalmic and nonophthalmic cases, the number of CA-MRSA patients increased each year, whereas the numbers of nosocomial patients remained fairly constant. Patients with ophthalmic MRSA tended to be younger than other MRSA patients (P = .023). The most common manifestation of ophthalmic MRSA infection was preseptal cellulitis and/or lid abscess followed by conjunctivitis, but sight-threatening infections, including corneal ulcers, endophthalmitis, orbital cellulitis, and blebitis, also occurred. Empirical antibiotic coverage was initially prescribed in 48 (98%) of ophthalmic cases and did not adequately cover for the MRSA isolate in 24 (50%).

CONCLUSIONS

CA-MRSA is becoming increasingly prevalent, and ophthalmologists will see more ophthalmic MRSA infections. Although ophthalmic CA-MRSA commonly presents as preseptal lid infection and conjunctivitis, sight-threatening infections also occur. Ophthalmologists must identify MRSA patients, adjust empirical treatment regimens where MRSA is endemic, and take steps to control emergence of resistant organisms in both inpatient and outpatient practices.

Revised structures for the capsular polysaccharides from Staphylococcus aureus Types 5 and 8, components of novel glycoconjugate vaccines

Glycoconjugate vaccines based on the capsular polysaccharides (CPSs) from Staphylococcus aureus serotypes 5 and 8 conjugated to genetically detoxified recombinant exoprotein A (rEPA) from Pseudomonas aeruginosa have been shown, in Phase 3 clinical trials, to elicit a strong bactericidal immune response in end-stage renal disease patients. Such vaccines have the potential to reduce morbidity and mortality due to methicillin-resistant Staphylococcus aureus (MRSA), a major cause of hospital-acquired infection. The serotype 5 and 8 polysaccharides have been fully characterized by NMR spectroscopy and full structural analyses carried out. Published structures were found incorrect and the revised structures of the repeat units of the two polysaccharides are: [carbohydrate structure: see text]. Resonances indicative of the presence of peptidoglycan were observed in the spectra of both CPSs, consistent with reports that the CPS is covalently linked to peptidoglycan.

Staphylococcus aureus capsular polysaccharide types 5 and 8 reduce killing by bovine neutrophils in vitro

Isogenic variants of Staphylococcus aureus strain Reynolds expressing either no capsule or capsular polysaccharide (CP) type 5 (CP5) or type 8 (CP8) were used to assess the effect of CP on bacterial killing and the respiratory burst of bovine neutrophils. The effects of antisera specific for CP5 and CP8 were also evaluated. The killing of live bacteria by isolated neutrophils was quantified in a bactericidal assay, while the respiratory burst after stimulation with live bacteria in whole blood was measured by flow cytometry. The expression of a CP5 or CP8 capsule protected the bacteria from being killed by bovine neutrophils in vitro (P <0.001), and the capsule-expressing variants did not stimulate respiratory burst activity in calf whole blood. The addition of serotype-specific antisera increased the killing of the capsule-expressing bacteria and enhanced their stimulating effect in the respiratory burst assay (P <0.01). When the S. aureus variants were grown under conditions known not to promote capsule expression, there were no significant differences between them. The present study demonstrates that the expression of S. aureus CP5 or CP8 confers resistance to opsonophagocytic killing and prevents the bacteria from inducing respiratory burst of bovine neutrophils in vitro and that these effects can be reversed by the addition of serotype-specific antisera.

Comparison of antibody repertoires against Staphylococcus aureus in healthy individuals and in acutely infected patients

The management of staphylococcal diseases is increasingly difficult with present medical approaches. Preventive and therapeutic vaccination is considered to be a promising alternative; however, little is known about immune correlates of protection and disease susceptibility. To better understand the immune recognition of Staphylococcus aureus by the human host, we studied the antistaphylococcal humoral responses in healthy people in comparison to those of patients with invasive diseases. In a series of enzyme-linked immunosorbent assay analyses performed using 19 recombinant staphylococcal cell surface and secreted proteins, we measured a wide range of antibody levels, finding a pronounced heterogeneity among individuals in both donor groups. The analysis revealed marked differences in the antibody repertoires of healthy individuals with or without S. aureus carriage, as well as in those of patients in the acute phase of infection. Most importantly, we identified antigenic proteins for which specific antibodies were missing or underrepresented in infected patients. In contrast to the well-described transient nature of disease-induced antistaphylococcal immune response, it was demonstrated that high-titer antistaphylococcal antibodies are stable for years in healthy individuals. In addition, we provide evidence obtained on the basis of opsonophagocytic and neutralizing activity in vitro assays that circulating antistaphylococcal serum antibodies in healthy donors are functional. In light of these data we suggest that proper serological analysis comparing the preexisting antibody repertoires of hospitalized patients with different outcomes for nosocomial staphylococcal infections could be extremely useful for the evaluation of candidate vaccine antigens in addition to protection data generated with animal models.

Immunotherapeutics for nosocomial infections

Nosocomial or hospital-acquired infections are associated with prolonged hospitalisation and increased healthcare costs and resource utilisation. Continued advances in sophisticated medical procedures, an increase in the number of immunocompromised patients and a steady rise in the prevalence of antibiotic-resistant organisms has renewed interest in the development of novel therapies that can treat nosocomial infections. This review focuses on novel immunological approaches to address this significant unmet medical need.

Capsular polysaccharide-protein conjugate vaccines and intravenous immunoglobulins

Capsular polysaccharides (CPs), present on the surface of most pathogenic bacteria, have been recognised as virulence factors. Antibodies specific to these polysaccharides can mediate the killing of these bacteria by phagocytes in the presence of complement. The conjugation of polysaccharides to carrier proteins enhances their immunogenicity and renders the immune response T-cell dependent. The currently licensed capsular polysaccharide vaccines and polysaccharide-protein conjugate vaccines under development for the prevention of bacterial infections will be discussed in this review. Use of these vaccines for active vaccination and for the vaccination of healthy plasma donors to produce hyperimmune iv. immunoglobulins for the passive immunisation of appropriate patient populations is also discussed.

Epidemiology of capsular and surface polysaccharide in Staphylococcus aureus infections complicated by bacteraemia

Staphylococcus aureus is a leading cause of serious hospital- and community-acquired infections. The discovery of serologically distinct capsular polysaccharides on the surface of clinical isolates has allowed the development of vaccines and passive protective immunity. We have studied patient characteristics, infection characteristics and the surface and capsular polysaccharide serotype distribution in patients with S. aureus infections complicated by bacteraemia admitted to VA hospitals in Maryland between 1995 and 2000. Nine hundred and ninety-three blood cultures from 331 patients were positive for S. aureus. Thirty-eight percent of patients had diabetes, 11% had end-stage renal failure, and 23% were injection drug users. Forty-two percent of infections were caused by methicillin-resistant strains (MRSA), and 60% were acquired during hospitalization. Serotyping of the first available isolate per patient (N=234 isolates) using polyclonal antibodies showed three major phenotypes--42%, type 8 (T8) capsule; 50%, type 5 (T5) capsule; and 8%, 336 polysaccharide. MRSA isolates were significantly more likely to be T5 than methicillin-susceptible isolates (66% vs. 39%, P<0.001). The proportion of T5 MRSA increased significantly (years 1-2: 41%; years 3-4: 65%; years 5-6: 90%, P<0.001). This large sample of patients with serious S. aureus infection confirms that capsular polysaccharides T5 and T8 cause most human infections, and together with serotype 336, account for nearly all those with bacteraemia.

Safety and immunogenicity of a booster dose of Staphylococcus aureus types 5 and 8 capsular polysaccharide conjugate vaccine (StaphVAX®) in hemodialysis patients

StaphVAX, an unadjuvanted, bivalent vaccine composed of Staphylococcus aureus (S. aureus) capsular polysaccharides (CPS) types 5 and 8 bound to the mutant non-toxic recombinant Pseudomonas aeruginosa exotoxin A (rEPA) conferred approximately 60% protection for 10 months against bacteremia caused by this pathogen in hemodialysis patients. A protective level of 80 microg/ml was estimated based upon geometric mean (GM) antibody levels at the end of the efficacy period. To extend the duration of protection conferred by StaphVAX in hemodialysis patients, recipients of the vaccine were reinjected in a randomized double-blinded, placebo-controlled study. Vaccinees received StaphVAX and a saline placebo injection 14 days apart according to the randomization schedule. The booster dose of StaphVAX was administered an average of 958 days (753-1167 days) after the first injection. There were no serious adverse reactions. Antibody levels at day 14, 28, 92, and 182 post-injection were measured by ELISA. Maximal levels of IgG anti-CPS were observed at the 28-day interval. For type 5, GM antibody levels increased from 73 microg/ml at day 0 to 162 microg/ml (P < 0.001) and for type 8 from 59 microg/ml to 133 microg/ml (P < 0.001). Anti-CPS antibody levels of approximately 80 microg/ml to type 5 and type 8 were achieved in 72.4 and 74.3% of vaccinees, respectively. There was excellent correlation between the level of anti-CPS and opsonic titer (r = 0.93). Moreover, the decline of anti-CPS antibody levels at six months was significantly less rapid than that observed from the first immunization (P < 0.001). We conclude that a booster immunization to maintain protective levels of specific antibodies for an extended period of time is feasible for patients at continuous risk for S. aureus bacteremia.

Staphylococcus aureus bound to complement receptor 1 on human erythrocytes by bispecific monoclonal antibodies is phagocytosed by acceptor macrophages

Antibiotic resistant strains of Staphylococcus aureus pose a growing threat to public health, and immunotherapy offers potential modalities to combat antibiotic resistance. We prepared bispecific monoclonal antibody complexes (heteropolymers, HP), specific for the primate erythrocyte complement C3b receptor (CR1) and type 5 capsular polysaccharide of the T5 isolate of S. aureus. Fluorescence microscopy and flow cytometry revealed that HP promote binding of S. aureus to human erythrocytes. Incubation of erythrocyte-bound, HP-opsonized S. aureus with human monocyte/macrophages or mouse macrophage cell lines led to transfer, internalization and killing of bacteria by macrophages with little erythrocyte loss. This reaction is similar to the process in which C3b-opsonized substrates, bound to erythrocyte CR1 by immune adherence, are transferred to acceptor phagocytes. Our results provide the basis for development of an in vivo paradigm focused on immunotherapeutic approaches for treatment of infections due to antibiotic resistant bacteria.

Flow cytometric measurement of neutrophil respiratory burst in whole bovine blood using live Staphylococcus aureus

A rapid and simple method for measurement of respiratory burst in neutrophil granulocytes in whole bovine blood is described. The respiratory burst was stimulated by live Staphylococcus aureus, and the production of reactive oxygen species quantified by the conversion of intracellular dihydrorhodamine 123 to the green fluorescent rhodamine 123, measured by flow cytometry. Assay conditions, including bacterial and dihydrorhodamine 123 concentrations and incubation time, were determined. Repeatability and precision of the method were assessed by testing parallel samples from clinically healthy dairy cows. In vitro and in vivo inhibition of respiratory burst was investigated, and labelling with a granulocyte marker antibody was performed. Stimulation with live S. aureus induced green fluorescence in the neutrophil granulocytes in a whole blood preparation. The fluorescence intensity increased with increasing bacterial concentration and increasing incubation time. Agreement analysis showed that the method gave repeatable results, and the intra-assay variability of the method was relatively low. The method is considered a useful technique for measurement of neutrophil respiratory burst in whole bovine blood.

Biologic properties and vaccine potential of the staphylococcal poly-N-acetyl glucosamine surface polysaccharide

Staphylococci have become the most common causes of nosocomial bacterial infections, and this fact, along with increasing problems associated with antimicrobial resistance, spurs the need for finding immunotherapeutic alternatives to prevent and possibly treat these infections. Most virulent, clinical isolates of both coagulase-negative staphylococci (CoNS) and Staphylococcus aureus carry the ica locus which encodes proteins that synthesize a polymer of beta-1-6 linked N-acetyl glucosamine residues (PNAG). Animal studies have shown purified PNAG can elicit protective immunity against both CoNS and S. aureus, suggesting its potential as a broadly protective vaccine for many clinically important strains of staphylococci.

Staphylococcus aureus types 5 and 8 capsular polysaccharide-protein conjugate vaccines

BACKGROUND

Staphylococcus aureus, the first or second most common pathogen isolated from patients, is capsulated; there are at least 12 capsular types, and types 5 and 8 comprise approximately 85% of blood. Types 5 and 8, composed of a trisaccharide repeat unit including a mannose uronic acid and 2 fucoses, are non-immunogenic. As protein conjugates, they induce opsonophagocytic antibodies that confer type-specific active and passive protection in mice.

METHODS

A phase II study of patients with end-stage renal disease showed that these conjugates induced approximately one third of the immunoglobulin G antibody of healthy individuals. Increasing the dose to 100 microg of polysaccharide induced levels similar to that in healthy individuals injected with 25 microg.

RESULTS

In a double-blinded randomized and controlled study of patients undergoing renal dialysis, the conjugates induced statistically significant protection against bacteremia for as long as 10 months after immunization. The estimated protective level was 80 microg Ab/mL. At re-injection approximately 2 years later, 83 of 83 recipients responded with protective levels.

CONCLUSIONS

Conjugate vaccine-induced antibodies to the types 5 and 8 capsular polysaccharide antibodies of S aureus prevent bacteremia caused by this pathogen. The extent and duration of conjugate-induced immunity can be extended by re-immunization approximately 1 year later. Studies of patients undergoing cardiovascular surgery who would be immunized with the staphylococcus conjugates when they are immunologically intact are planned.

Development of StaphVAX™, a polysaccharide conjugate vaccine against S. aureus infection: from the lab bench to phase III clinical trials

Staphylococcus aureus is the most common nosocomial pathogen and is responsible for approximately one-third of hospital-acquired bacteremias. The emergence of strains with multidrug resistance, including resistance to vancomycin, the antibiotic of last resort, presents the medical community with a major public health problem. Alternative therapies, including immunotherapy, have been in development for several decades. The discovery of S. aureus capsular polysaccharides from clinical isolates, and their importance to pathogenicity via antiphagocytic activity, opened a new window of opportunity for development of vaccines and immunotherapy against this pathogen. A conjugate vaccine, StaphVAX that includes the two most prevalent capsular polysaccharides, types 5 and 8, coupled to a carrier protein efficient in promoting a Th2 response, was developed. In a recent phase III clinical study in hemodialysis patients, StaphVAX was shown to prevent S. aureus bacteremia for up to 10 months following a single immunization. The history, epidemiology, serology, and development of StaphVAX, including preclinical and clinical studies demonstrating efficacy are described in this review.

Characterization of a protective monoclonal antibody recognizing Staphylococcus aureus MSCRAMM protein clumping factor A

The Staphylococcus aureus MSCRAMM (microbial surface components recognizing adhesive matrix molecules) protein clumping factor A (ClfA) has been shown to be a critical virulence factor in several experimental models of infection. This report describes the generation, characterization, and in vivo evaluation of a murine monoclonal antibody (MAb) against ClfA. Flow cytometric analysis revealed that MAb 12-9 recognized ClfA protein expressed by all of the clinical S. aureus strains obtained from a variety of sources. In assays measuring whole-cell S. aureus binding to human fibrinogen, MAb 12-9 inhibited S. aureus binding by over 90% and displaced up to 35% of the previously adherent S. aureus bacteria. Furthermore, a single infusion of MAb 12-9 was protective against an intravenous challenge with a methicillin-resistant strain of S. aureus in a murine sepsis model (P < 0.0001). These data suggest that anti-ClfA MAb 12-9 should be further investigated as a novel immunotherapy for the treatment and prevention of life-threatening S. aureus infections.

Staphylococcus aureus capsular polysaccharides

Serotype 5 and 8 capsular polysaccharides predominate among clinical isolates of Staphylococcus aureus. The results of experiments in animal models of infection have revealed that staphylococcal capsules are important in the pathogenesis of S. aureus infections. The capsule enhances staphylococcal virulence by impeding phagocytosis, resulting in bacterial persistence in the bloodstream of infected hosts. S. aureus capsules also promote abscess formation in rats. Although the capsule has been shown to modulate S. aureus adherence to endothelial surfaces in vitro, animal studies suggest that it also promotes bacterial colonization and persistence on mucosal surfaces. S. aureus capsular antigens are surface associated, limited in antigenic specificity, and highly conserved among clinical isolates. With the emergence of vancomycin-resistant S. aureus in the United States in 2002, new strategies are needed to combat staphylococcal infections. Purified serotype 5 and 8 capsular polysaccharides offer promise as target antigens for a vaccine to prevent staphylococcal infections, although the inclusion of other antigens is likely to be essential in the development of an effective S. aureus vaccine. The genetics and mechanisms of capsule biosynthesis are complex, and much work remains to enhance our understanding of capsule biosynthesis and its regulation.

Genome-based analysis of virulence genes in a non-biofilm-forming Staphylococcus epidermidis strain (ATCC 12228)

Staphylococcus epidermidis strains are diverse in their pathogenicity; some are invasive and cause serious nosocomial infections, whereas others are non-pathogenic commensal organisms. To analyse the implications of different virulence factors in Staphylococcus epidermidis infections, the complete genome of Staphylococcus epidermidis strain ATCC 12228, a non-biofilm forming, non-infection associated strain used for detection of residual antibiotics in food products, was sequenced. This strain showed low virulence by mouse and rat experimental infections. The genome consists of a single 2499 279 bp chromosome and six plasmids. The chromosomal G + C content is 32.1% and 2419 protein coding sequences (CDS) are predicted, among which 230 are putative novel genes. Compared to the virulence factors in Staphylococcus aureus, aside from delta-haemolysin and beta-haemolysin, other toxin genes were not found. In contrast, the majority of adhesin genes are intact in ATCC 12228. Most strikingly, the ica operon coding for the enzymes synthesizing interbacterial cellular polysaccharide is missing in ATCC 12228 and rearrangements of adjacent genes are shown. No mec genes, IS256, IS257, were found in ATCC 12228. It is suggested that the absence of the ica operon is a genetic marker in commensal Staphylococcus epidermidis strains which are less likely to become invasive.

Therapeutic approaches to superantigen-based diseases: a review

Superantigens secreted by the bacterial pathogen Staphyloccocus aureus are extremely potent toxins that overstimulate the host immune system by binding to the MHC class II and T cell receptors and activating a large population of T cells. Superantigen infection has been shown to be the causative agents in acute diseases, food poisoning and toxic shock syndrome, and in more chronic conditions such as inflammatory skin diseases. In addition to the toll on public health, S. aureus superantigens also represent a potential biothreat to our national security. To address these risks, a number of different therapeutic strategies have been developed that target different aspects of the pathogenic mechanism of S. aureus and superantigen infection. These therapies, which encompass strategies as diverse as production of neutralizing antibodies, inhibitory peptide/receptor design and blockage of superantigen gene transcription, are being tested for treatment of established S. aureus infections in pre- and post-exposure scenarios. In this review, we will describe these different strategies and their efficacies in inhibition of superantigen-induced effects in the host, and present the future outlook for successfully producing therapies for superantigen-based disease.

Availability of complement bound to Staphylococcus aureus to interact with membrane complement receptors influences efficiency of phagocytosis

Complement-mediated opsonization of encapsulated Staphylococcus aureus (CP+) of the predominant capsule types, 5 and 8, remains poorly understood. Our previous work showed that complement is important for mouse survival of CP+ type 5 bacteremia and that the type 5 capsule inhibits the binding of opsonic C3 fragments to the organism. The importance of complement-mediated opsonization of CP+ was tested by neutrophil phagocytosis assays. Complement-mediated opsonization of CP+ increased phagocytosis by 57% compared to opsonization in complement-inhibited serum. Agar-grown CP+, enhancing capsule expression, was phagocytosed only one-tenth as well as the capsule-negative organisms (CP-), supporting the belief that staphylococcal polysaccharide capsules impair phagocytosis. Despite relatively poor phagocytosis of CP+ compared to CP-, complement activation increased the phagocytosis of CP+ by 103%. Thus, complement in normal human serum may have an important role in opsonizing CP+, even when capsule expression is strong. The ability of bound C3 fragments to interact with complement receptor 1 (CD35) on the membrane of human erythrocytes was tested in an immune adherence assay. S. aureus capsule was able to mask C3 fragments on the organism from binding to complement receptor 1. The inhibition of C3 binding to CP+ and the masking of deposited C3 fragments caused by the presence of capsule was associated with markedly decreased phagocytosis. The addition of anti-capsule antibodies to normal human serum was found to markedly improve the recognition of deposited C3 fragments by complement receptor 1 even when the absolute number of C3 molecules bound to S. aureus was not increased.