Group B streptococcal conjugate vaccines

Effect of Side-Chain Functional Groups in the Immunogenicity of Bacterial Surface Glycans

Glycans on the surface of bacteria have diverse and essential biological functions and have widely been employed for treating various bacterial infectious diseases. Furthermore, these glycans comprise various functional groups, such as O-, N-, and carboxyl-modified, which significantly increase the diversity of glycan structures. These functional groups are not only crucial for glycans' structural identity but are also essential for their biological functions. Therefore, a clear understanding of the biological functions of these modified groups in corresponding bacterial glycans is crucial for their medical applications. Thus far, the activities of functional groups in some biomedical active carbohydrates have been elucidated. It has been shown that some functional groups are key constituents of biologically active bacterial glycans, while others are actually not essential and may even mask the functions of the glycans. This paper reviews the structures of naturally occurring side-chain functional groups in glycans located on the bacterial surface and their roles in immunological responses.

Proof of concept for a single-dose Group B Streptococcus vaccine based on capsular polysaccharide conjugated to Qβ virus-like particles

A maternal vaccine to protect neonates against Group B Streptococcus invasive infection is an unmet medical need. Such a vaccine should ideally be offered during the third trimester of pregnancy and induce strong immune responses after a single dose to maximize the time for placental transfer of protective antibodies. A key target antigen is the capsular polysaccharide, an anti-phagocytic virulence factor that elicits protective antibodies when conjugated to carrier proteins. The most prevalent polysaccharide serotypes conjugated to tetanus or diphtheria toxoids have been tested in humans as monovalent and multivalent formulations, showing excellent safety profiles and immunogenicity. However, responses were suboptimal in unprimed individuals after a single shot, the ideal schedule for vaccination during the third trimester of pregnancy. In the present study, we obtained and optimized self-assembling virus-like particles conjugated to Group B Streptococcus capsular polysaccharides. The resulting glyco-nanoparticles elicited strong immune responses in mice already after one immunization, providing pre-clinical proof of concept for a single-dose vaccine.

Immune responses against group B Streptococcus monovalent and pentavalent capsular polysaccharide tetanus toxoid conjugate vaccines in Balb/c mice

Immunization of pregnant women with Group B Streptococcus (GBS) capsular polysaccharide (CPS) conjugate vaccine (CV) could protect young infants against invasive GBS disease. We evaluated the immunogenicity of investigational five GBS monovalent (serotypes Ia, Ib, II, III, and V) CPS-tetanus toxoid (TT)-CV with adjuvant and GBS pentavalent CPS-TT-CV with adjuvant (GBS5-CV-adj) and without adjuvant (GBS5-CV-no-adj), in Balb/c mice. Aluminum phosphate was the adjuvant in the formulations, where included. The homotypic immunoglobulin G (IgG) geometric mean concentration (GMC) and opsonophagocytic activity (OPA) geometric mean titer (GMT) did not differ after the third dose of the GBS5-CV-adj vaccine compared with the monovalent counterparts for all five serotypes. The GBS5-CV-adj induced higher post-vaccination serotype-specific IgG GMCs and OPA GMTs compared to GBS5-CV-no_adj. The GBS5-CV with and without adjuvant should be considered for further development as a potential vaccine for pregnant women to protect their infants against invasive GBS disease.

Group B Streptococcus: Virulence Factors and Pathogenic Mechanism

Group B Streptococcus (GBS) or Streptococcus agalactiae is a major cause of neonatal mortality. When colonizing the lower genital tract of pregnant women, GBS may cause premature birth and stillbirth. If transmitted to the newborn, it may result in life-threatening illnesses, including sepsis, meningitis, and pneumonia. Moreover, through continuous evolution, GBS can use its original structure and unique factors to greatly improve its survival rate in the human body. This review discusses the key virulence factors that facilitate GBS invasion and colonization and their action mechanisms. A comprehensive understanding of the role of virulence factors in GBS infection is crucial to develop better treatment options and screen potential candidate molecules for the development of the vaccine.

Group B Streptococcus and Pregnancy: Critical Concepts and Management Nuances

Importance

Group B Streptococcus (GBS) is a common pathogen with an effective treatment. However, it remains a significant cause of neonatal sepsis, morbidity, and mortality. The screening and management of this infection are some of the first concepts learned during medical training in obstetrics. However, effective screening and evidence-based management of GBS are nuanced with many critical caveats.

Objective

The objectives of this review are to discuss the essential aspects of GBS screening and management and to highlight recent changes to recommendations and guidelines.

Evidence Acquisition

Original research articles, review articles, and guidelines on GBS were reviewed.

Results

The following recommendations are based on review of the evidence and professional society guidelines. Screening for GBS should occur between 36 weeks and the end of the 37th week. The culture swab should go 2 cm into the vagina and 1 cm into the anus. Patients can perform their own swabs as well. Penicillin allergy testing has been shown to be safe in pregnancy. Patients with GBS in the urine should be treated at term with antibiotic prophylaxis, independent of the colony count of the culture. Patients who are GBS-positive with preterm and prelabor rupture of membranes after 34 weeks are not candidates for expectant management, as this population has higher rates of neonatal infectious complications. Patients with a history of GBS colonization in prior pregnancy who are GBS-unknown in this current pregnancy and present with labor should receive intrapartum prophylaxis. Work on the GBS vaccine continues.

Conclusions

Although all of the efforts and focus on neonatal early-onset GBS infection have led to lower rates of disease, GBS still remains a major cause of neonatal morbidity and mortality requiring continued vigilance from obstetric providers.

Maternal vaccination with a type-III glycoconjugate protects mouse neonates against Group B Streptococcus intranasal infection

Group B Streptococcus (GBS) is generally an asymptomatic colonizer of human mucosa but it occasionally infects pregnant women and neonates through vertical transmission, causing disease during the first weeks of life with frequent and severe complications. Preclinical studies have shown that maternal vaccination with polysaccharide-based vaccines protects mothers and offspring from GBS mucosal colonization and consecutive infection. In these models, bacteria were inoculated in mouse either intravaginally in the last trimester of pregnancy or systemically in pups. Here, we investigated whether maternal vaccination with glycoconjugate vaccines may also prevent GBS-mediated colonization and disease in neonates using an infection route that more closely mimics inhalation or ingestion of bacteria during human delivery. To address this point, mice aged less than two days were intranasally challenged with epidemiologically relevant GBS strains. Bacteria were found to colonize nose and intestine, reaching in some cases lungs and blood during the first days of life. Bacteria were also found in vagina of a fraction of colonized female mice within the first month of life. GBS-specific IgG induced by maternal vaccination with a glycoconjugate vaccine formulation were found in blood and mucosal tissues of newborns. Finally, when intranasally challenged with GBS serotype III strains, pups delivered by vaccinated mothers were partially protected against mucosal colonization and deeper infection.

Invited review: A critical appraisal of mastitis vaccines for dairy cows

Infections of the mammary gland remain a frequent disease of dairy ruminants that negatively affect animal welfare, milk quality, farmer serenity, and farming profitability and cause an increase in use of antimicrobials. There is a need for efficacious vaccines to alleviate the burden of mastitis in dairy farming, but this need has not been satisfactorily fulfilled despite decades of research. A careful appraisal of past and current research on mastitis vaccines reveals the peculiarities but also the commonalities among mammary gland infections associated with the major mastitis pathogens Escherichia coli, Staphylococcus aureus, Streptococcus uberis, Streptococcus agalactiae, or Streptococcus dysgalactiae. A major pitfall is that the immune mechanisms of effective protection have not been fully identified. Until now, vaccine development has been directed toward the generation of antibodies. In this review, we drew up an inventory of the main approaches used to design vaccines that aim at the major pathogens for the mammary gland, and we critically appraised the current and tentative vaccines. In particular, we sought to relate efficacy to vaccine-induced defense mechanisms to shed light on some possible reasons for current vaccine shortcomings. Based on the lessons learned from past attempts and the recent results of current research, the design of effective vaccines may take a new turn in the years to come.

Sensitive quantitation of low level free polysaccharide in conjugate vaccines by size exclusion chromatography-reverse phase liquid chromatography with UV detection

The level of free polysaccharide is a critical quality attribute of polysaccharide-protein conjugate vaccines. The work presented describes a simple and sensitive method for the determination of low level free polysaccharides in multiple polysaccharide-protein conjugates. The method utilizes a reverse phase (RP) column to perform a size exclusion chromatography (SEC) separation of free polysaccharide and a reverse phase liquid chromatography (RPLC) separation of free protein and protein-polysaccharide conjugate. The use of phosphate buffer in the mobile phase enables the universal and sensitive detection of low level free polysaccharides at UV 200 nm. The method has been validated to monitor low level free polysaccharide (<1 %) in multiple polysaccharide-protein conjugates. The limit of quantitation is 2 μg/ml or 0.3 % free polysaccharide in 0.6 mg/ml polysaccharide-protein conjugate. The accuracy is in the range of 94.1.0-108.5 %.

Epitopes of Immunoreactive Proteins of Streptococcus Agalactiae: Enolase, Inosine 5'-Monophosphate Dehydrogenase and Molecular Chaperone GroEL

Three Streptococcus agalactiae (group B streptococci, GBS) immunoreactive proteins: enolase (47.4 kDa), inosine 5'-monophosphate dehydrogenase (IMPDH) (53 kDa) and molecular chaperone GroEL (57 kDa) were subjected to investigation. Enolase protein was described in our previous paper, whereas IMPDH and GroEL were presented for the first time. The aim of our paper was to provide mapping of specific epitopes, highly reactive with umbilical cord blood serum. Bioinformatic analyses allowed to select 32 most likely epitopes for enolase, 36 peptides for IMPDH and 41 immunoreactive peptides for molecular chaperone GroEL, which were synthesized by PEPSCAN. Ten peptides: two in enolase, one in IMPDH and seven in molecular chaperone GroEL have been identified as potentially highly selective epitopes that can be used as markers in rapid immunological diagnostic tests or constitute a component of an innovative vaccine against GBS infections.

Progress toward a group B streptococcal vaccine

Streptococcus agalactiae (group B Streptococcus, GBS) is a leading cause of severe invasive disease in neonate, elderly, and immunocompromised patients worldwide. Despite recent advances in the diagnosis and intrapartum antibiotic prophylaxis (IAP) of GBS infections, it remains one of the most common causes of neonatal morbidity and mortality, causing serious infections. Furthermore, recent studies reported an increasing number of GBS infections in pregnant women and elderly. Although IAP is effective, it has several limitations, including increasing antimicrobial resistance and late GBS infection after negative antenatal screening. Maternal immunization is the most promising and effective countermeasure against GBS infection in neonates. However, no vaccine is available to date, but two types of vaccines, protein subunit and capsular polysaccharide conjugate vaccines, were investigated in clinical trials. Here, we provide an overview of the GBS vaccine development status and recent advances in the development of immunoassays to evaluate the GBS vaccine clinical efficacy.

Immune Responses to Invasive Group B Streptococcal Disease in Adults

Antibodies to capsular polysaccharides and pilus proteins develop in recovering adults.

Immunization of nonpregnant adults could help prevent invasive group B Streptococcus (GBS) infections, but adult immune responses have not been investigated. We defined capsular polysaccharide (CPS) and pilus island (PI) surface antigen distribution and expression and immune responses to GBS infection in nonpregnant adults. Prospective surveillance from 7 hospitals in Houston, Texas, USA, identified 102 adults with GBS bacteremia; 43% had skin/soft tissue infection, 16% bacteremia without focus, and 12% osteomyelitis. CPS-specific IgG was determined by ELISA and pilus-specific IgG by multiplex immunoassay. CPS types were Ia (24.5%), Ib (12.7%), II (9.8%), III (16.7%), IV (13.7%), and V (12.7%); 9.8% were nontypeable by serologic methods. Pili, expressed by 89%, were most often PI-2a. CPS and pilus-specific IgG increased during convalescence among patients with strains expressing CPS or PI. All GBS expressed CPS or PI; 79% expressed both. Increased antibodies to CPS and PI during recovery suggests that GBS bacteremia in adults is potentially vaccine preventable.

An Overview of Structural Features of Antibacterial Glycoconjugate Vaccines That Influence Their Immunogenicity

Bacterial cell-surface-derived or mimicked carbohydrate moieties that act as protective antigens are used in the development of antibacterial glycoconjugate vaccines. The carbohydrate antigen must have a minimum length or size to maintain the conformational structure of the antigenic epitope(s). The presence or absence of O-acetate, phosphate, glycerol phosphate and pyruvate ketal plays a vital role in defining the immunogenicity of the carbohydrate antigen. The nature of the carrier protein, spacer and conjugation pattern used to develop the glycoconjugate vaccine also defines its overall spatial orientation which in turn affects its avidity and selectivity of interaction with the desired target(s). In addition, the ratio of carbohydrate to protein in glycoconjugate vaccines also makes an important contribution in determining the optimum immunological response. This Review article presents the importance of these variables in the development of antibacterial glycoconjugate vaccines and their effects on immune efficacy.

Development of Streptococcus agalactiae vaccines for tilapia

Vaccination is a widely accepted and effective method to prevent most pathogenic diseases in aquaculture. Various species of tilapia, especially Nile tilapia Oreochromis niloticus, are farmed worldwide because of their high consumer demand. Recently, the tilapia-breeding industry has been hampered by outbreaks of Streptococcus agalactiae infection, which cause high mortality and huge economic losses. Many researchers have attempted to develop effective S. agalactiae vaccines for tilapia. This review provides a summary of the different kinds of S. agalactiae vaccines for tilapia that have been developed recently. Among the various vaccine types, inactivated S. agalactiae vaccines showed superior protection efficiency when compared with live attenuated, recombinant and DNA vaccines. With respect to vaccination method, injecting the vaccine into tilapia provided the most effective immunoprotection. Freund's incomplete adjuvant appeared to be suitable for tilapia vaccines. Other factors, such as immunization duration and number, fish size and challenge dose, also influenced the vaccine efficacy.

Immunological Outcomes of Antibody Binding to Glycans Shared between Microorganisms and Mammals

Glycans constitute basic cellular components of living organisms across biological kingdoms, and glycan-binding Abs participate in many cellular interactions during immune defense against pathogenic organisms. Glycan epitopes are expressed as carbohydrate-only entities or as oligomers or polymers on proteins and lipids. Such epitopes on glycoproteins may be formed by posttranslational modifications or neoepitopes resulting from metabolic-catabolic processes and can be altered during inflammation. Pathogenic organisms can display host-like glycans to evade the host immune response. However, Abs to glycans, shared between microorganisms and the host, exist naturally. These Abs are able to not only protect against infectious disease, but also are involved in host housekeeping functions and can suppress allergic disease. Despite the reactivity of these Abs to glycans shared between microorganisms and host, diverse tolerance-inducing mechanisms permit the B cell precursors of these Ab-secreting cells to exist within the normal B cell repertoire.

Strategies to develop vaccines of pediatric interest

INTRODUCTION

The success of the vaccines available on the market has significantly increased interest in vaccine development. Areas covered: The main aim of this paper is to discuss the most important vaccines of pediatric interest that are currently being developed. New pneumococcal vaccines and vaccines against group B Streptococcus, Staphylococcus aureus and respiratory syncytial virus are analyzed in detail. Expert commentary: Advances in understanding human immunology, including human monoclonal antibody identification, sequencing technology, and the ability to solve atomic level structures of vaccine targets have provided tools to guide the rational design of future vaccines. It is likely that some of these vaccines will reach the market in the future and will thus partially contribute to the prevention of very severe diseases that significantly affect the morbidity and mortality of children. However, further studies in animals and several clinical trials in children must be performed before new vaccines become licensed.

Group B Streptococcus vaccine development: present status and future considerations, with emphasis on perspectives for low and middle income countries

Globally, group B Streptococcus (GBS) remains the leading cause of sepsis and meningitis in young infants, with its greatest burden in the first 90 days of life. Intrapartum antibiotic prophylaxis (IAP) for women at risk of transmitting GBS to their newborns has been effective in reducing, but not eliminating, the young infant GBS disease burden in many high income countries. However, identification of women at risk and administration of IAP is very difficult in many low and middle income country (LMIC) settings, and is not possible for home deliveries. Immunization of pregnant women with a GBS vaccine represents an alternate pathway to protecting newborns from GBS disease, through the transplacental antibody transfer to the fetus in utero. This approach to prevent GBS disease in young infants is currently under development, and is approaching late stage clinical evaluation. This manuscript includes a review of the natural history of the disease, global disease burden estimates, diagnosis and existing control options in different settings, the biological rationale for a vaccine including previous supportive studies, analysis of current candidates in development, possible correlates of protection and current status of immunogenicity assays. Future potential vaccine development pathways to licensure and use in LMICs, trial design and implementation options are discussed, with the objective to provide a basis for reflection, rather than recommendations.

Group B Streptococcus vaccine: state of the art

Group B Streptococcus (GBS) is cause of neonatal invasive diseases as well as of severe infections in the elderly and immune-compromised patients. Despite significant advances in the prevention and treatment of neonatal disease, sepsis and meningitis caused by GBS still represent a significant public health care concern globally and additional prevention and therapeutic strategies against infection are highly desirable. The introduction of national recommended guidelines in several countries to screen pregnant women for GBS carriage and the use of antibiotics during delivery significantly reduced disease occurring within the first hours of life (early-onset disease), but it has had no effect on the late-onset diseases occurring after the first week and is not feasible in most countries. Availability of an effective vaccine against GBS would provide an effective means of controlling GBS disease. This review provides an overview of the burden of invasive disease caused by GBS in infants and adults, and highlights the strategies for the development of an effective vaccine against GBS infections.

Streptococcus agalactiae in Brazil: serotype distribution, virulence determinants and antimicrobial susceptibility

Background

Group B Streptococcus (GBS) remains a major cause of neonatal sepsis and is also associated with invasive and noninvasive infections in pregnant women and non-pregnant adults, elderly and patients with underlying medical conditions. Ten capsular serotypes have been recognized, and determination of their distribution within a specific population or geographical region is important as they are major targets for the development of vaccine strategies. We have evaluated the characteristics of GBS isolates recovered from individuals with infections or colonization by this microorganism, living in different geographic regions of Brazil.

Methods

A total of 434 isolates were identified and serotyped by conventional phenotypic tests. The determination of antimicrobial susceptibility was performed by the disk diffusion method. Genes associated with resistance to erythromycin (ermA, ermB, mefA) and tetracycline (tetK, tetL, tetM, tetO) as well as virulence-associated genes (bac, bca, lmb, scpB) were investigated using PCR. Pulsed-field gel electrophoresis (PFGE) was used to examine the genetic diversity of macrolide-resistant and of a number of selected macrolide-susceptible isolates.

Results

Overall, serotypes Ia (27.6%), II (19.1%), Ib (18.7%) and V (13.6%) were the most predominant, followed by serotypes IV (8.1%) and III (6.7%). All the isolates were susceptible to the beta-lactam antimicrobials tested and 97% were resistant to tetracycline. Resistance to erythromycin and clindamycin were found in 4.1% and 3% of the isolates, respectively. Among the resistance genes investigated, tetM (99.3%) and tetO (1.8%) were detected among tetracycline-resistant isolates and ermA (39%) and ermB (27.6%) were found among macrolide-resistant isolates. The lmb and scpB virulence genes were detected in all isolates, while bac and bca were detected in 57 (13.1%) and 237 (54.6%) isolates, respectively. Molecular typing by PFGE showed that resistance to erythromycin was associated with a variety of clones.

Conclusion

These findings indicate that GBS isolates circulating in Brazil have a variety of phenotypic and genotypic characteristics, and suggest that macrolide-resistant isolates may arise by both clonal spread and independent acquisition of resistance genes.

Group B streptococcal epidemiology and vaccine needs in developed countries

Development of a group B streptococcal vaccine (GBS) vaccine is the most promising approach for the prevention of GBS infections in babies, given the potential adverse effects of intrapartum antibiotic prophylaxis as well as the need for effective prevention of both adult and late perinatal disease. There are numerous prevention strategies at this time but none are 100% effective in the eradication of neonatal early onset GBS disease and there are no preventative strategies for late onset disease. The need for a GBS vaccine is therefore, of utmost importance. Efforts applying genomics to GBS vaccine development have led to the identification of novel vaccine candidates. The publication of GBS whole genomes coupled with new technologies including multigenome screening and bioinformatics has also allowed researchers to overcome the serotype limitation of earlier vaccine preparations in the search of a universal effective vaccine against GBS. This review brings together the key arguments concerning the potential need of a GBS vaccine in developed countries and describes the current status with GBS epidemiology and microbiology in these countries.

Cost-effectiveness of a potential group B streptococcal vaccine program for pregnant women in South Africa

BACKGROUND

In low- and middle-income countries neonatal infections are important causes of infant mortality. Group B streptococcus (GBS) is a major pathogen. A GBS polysaccharide-protein conjugate vaccine, the only option that has the potential to prevent both early- and late-onset GBS disease, has completed Phase II trials. Screening-based intrapartum antibiotic prophylaxis (IAP) for pregnant women, an effective strategy in high-income countries, is often not practical in these settings. Risk factor-based IAP (RFB-IAP) for women with risk factors at delivery has had limited success in preventing neonatal infection. We evaluated the cost and health impacts of maternal GBS vaccination in South Africa.

METHODS AND FINDINGS

We developed a decision-analytic model for an annual cohort of pregnant women that simulates the natural history of GBS disease in their infants. We compared four strategies: doing nothing, maternal GBS vaccination, RFB-IAP, and vaccination plus RFB-IAP. Assuming vaccine efficacy varies from 50% to 90% against covered serotypes and 75% of pregnant women are vaccinated, GBS vaccination alone prevents 30-54% of infant GBS cases compared to doing nothing. For vaccine prices between $10 and $30, and mid-range efficacy, its cost ranges from $676 to $2390 per disability-adjusted life-year (DALY) averted ($US 2010), compared to doing nothing. RFB-IAP alone, compared to doing nothing, prevents 10% of infant GBS cases at a cost of $240/DALY. Vaccine plus RFB-IAP prevents 48% of cases at a cost of $664-2128/DALY.

CONCLUSIONS

Vaccination would substantially reduce the burden of infant GBS disease in South Africa and would be very cost-effective by WHO guidelines. RFB-IAP is also very cost-effective, but prevents only 10% of cases. Vaccination plus RFB-IAP is more effective and more costly than vaccination alone, and consistently very cost-effective.

Considerations in the pharmacologic treatment and prevention of neonatal sepsis

The management of neonatal sepsis is challenging owing to complex developmental and environmental factors that contribute to inter-individual variability in the pharmacokinetics and pharmacodynamics of many antimicrobial agents. In this review, we describe (i) the changing epidemiology of early- and late-onset neonatal sepsis; (ii) the pharmacologic considerations that influence the safety and efficacy of antibacterials, antifungals, and immunomodulatory adjuvants; and (iii) the recommended dosing regimens for pharmacologic agents commonly used in the treatment and prevention of neonatal sepsis. Neonatal sepsis is marked by high morbidity and mortality, such that prompt initiation of antimicrobial therapy is essential following culture collection. Before culture results are available, combination therapy with ampicillin and an aminoglycoside is recommended. When meningitis is suspected, ampicillin and cefotaxime may be considered. Following identification of the causative organism and in vitro susceptibility testing, antimicrobial therapy may be narrowed to provide targeted coverage. Therapeutic drug monitoring should be considered for neonates receiving vancomycin or aminoglycoside therapies. For neonates with invasive fungal infections, the development of new antifungal agents has significantly improved therapeutic outcomes in recent years. Liposomal amphotericin B has been found to be safe and efficacious in patients with renal impairment or toxicity caused by conventional amphotericin B. Antifungal prophylaxis with fluconazole has also been reported to dramatically reduce rates of neonatal invasive fungal infections and to improve long-term neurodevelopmental outcomes among treated children. Additionally, several large multicenter studies are currently investigating the safety and efficacy of oral lactoferrin as an immunoprophylactic agent for the prevention of neonatal sepsis.

Vaccination against Group B streptococcus

Streptococcus agalactiae (Group B streptococcus) is an important cause of disease in infants, pregnant women, the elderly and in immunosuppressed adults. An effective vaccine is likely to prevent the majority of infant disease (both early and late onset), as well as Group B streptococcus-related stillbirths and prematurity, to avoid the current real and theoretical limitations of intrapartum antibiotic prophylaxis, and to be cost effective. The optimal time to administer such a vaccine would be in the third trimester of pregnancy. The main limitations on the production of a Group B streptococcus vaccine are not technical or scientific, but regulatory and legal. A number of candidates including capsular conjugate vaccines using traditional carrier proteins such as tetanus toxoid and mutant diphtheria toxin CRM197, as well as Group B streptococcus-specific proteins such as C5a peptidase, protein vaccines using one or more Group B streptococcus surface proteins and mucosal vaccines, have the potential to be successful vaccines. The capsular conjugate vaccines using tetanus and CRM197 carrier proteins are the most advanced candidates, having already completed Phase II human studies including use in the target population of pregnant women (tetanus toxoid conjugate), however, no definitive protein conjugates have yet been trialed. However, unless the regulatory environment is changed specifically to allow the development of a Group B streptococcus vaccine, it is unlikely that one will ever reach the market.

Identification of high immunoreactive proteins from Streptococcus agalactiae isolates recognized by human serum antibodies

The aim of the studies was to identify immunogenic proteins of Streptococcus agalactiae (group B streptococcus; GBS) isolates. Investigation of the immunoreactivity with human sera allowed us to determine major immunogenic proteins which might be potential candidates for the development of vaccine. For the study, we have selected 60 genetically different, well-characterized GBS clinical isolates. The proteins immunoreactivity with 24 human sera from patients with GBS infections, carriers, and control group without GBS was detected by SDS-PAGE and Western blotting. As a result, some major immunogenic proteins were identified, of which four proteins with molecular masses of about 45 to 50 kDa, which exhibited the highest immunoreactivity features, were analyzed by LC-MS/MS. The proteins were identified by comparative analysis of peptides masses using MASCOT and statistical analysis. The results showed known molecules such as enolase (47.4 kDa), aldehyde dehydrogenase (50.6 kDa), and ones not previously described such as trigger factor (47 kDa) and elongation factor Tu (44 kDa). The preliminary results indicated that some GBS proteins that elicit protective immunity hold promise not only as components in a vaccine as antigens but also as carriers or adjuvants in polysaccharide conjugate vaccines, but more studies are needed.

A maternal vaccine against group B Streptococcus: past, present, and future

Group B Streptococcus (GBS) is a major cause of morbidity and mortality among neonates. Though there have been tremendous advances in prevention of invasive neonatal GBS disease through prophylactic antibiotic treatment of pregnant women, the incidence of neonatal disease has not changed significantly over the past several years. Vaccination of pregnant women is an important strategy that has the potential to improve further on existing protocols. In this review, we explore the history of the design of maternal GBS vaccines. We also discuss how recent applications of genomics and immunology to vaccine design promise to further enhance our ability to develop more effective vaccines against this important disease.

Group B Streptococcus induces a caspase-dependent apoptosis in fetal rat lung interstitium

Group B Streptococcus (GBS) is an important pathogen and is associated with sepsis and meningitis in neonates and infants. An ex vivo model that facilitates observations of GBS interactions with multiple host cell types over time was used to study its pathogenicity. GBS infections were associated with profound reductions in fetal lung; explant size, and airway branching. Elevated levels of apoptosis subsequent to GBS infections were observed by whole-mount confocal immunofluorescence using activated-caspase-3-antibodies and terminal deoxynucleotidyl transferase dUTP nick end-labeling (TUNEL) assays. The caspase inhibitor Z-VAD-FMK abolished the increase in TUNEL-positive cells associated with GBS infections, indicating that the GBS-induced apoptosis was caspase-dependent. Digital image analyses revealed that both GBS and the active form of caspase-3 were distributed primarily within the lung interstitium, suggesting that these tissues are important targets for GBS. Antibodies to the active form of caspase-3 colocalized with both macrophage- and erythroblast-markers, suggesting that these hematopoietic cells are vulnerable to GBS-mediated pathogenesis. These studies suggest that GBS infections profoundly alter lung morphology and caspase-dependent hematopoietic cell apoptosis within the lung interstitium play roles in GBS pathophysiology in this model.

Distribution of pilus islands of group B streptococcus associated with maternal colonization and invasive disease in South Africa

Group B streptococcus (GBS) is a leading cause of neonatal sepsis. Sortase-dependent pilus-like structures have been identified on the surface of GBS, and have been found to be important in the adhesion and attachment of GBS to host cells. Three pilus island alleles, PI-1, PI-2a and PI-2b, have been described, and their proteins are being explored as vaccine candidates. The pilus islands from 541 colonization isolates and 284 invasive isolates were characterized by PCR. All isolates carried at least one pilus island, and they were identified alone or in combinations at the following overall frequencies: PI-2a, 29.8 %; PI-2b, 0.2 %; PI-1+PI-2a, 24.8 %; and PI-1+PI-2b, 45.1 %. A combination of PI-1+PI-2a (28.7 vs 17.6 %) was more common among colonizing compared with invasive isolates. Conversely, a combination of PI-1+PI-2b (37.2 vs 60.2 %) was more frequently associated with invasive disease compared to colonization. There was a strong association between pilus islands when adjusted for serotype distribution, PI-2a was identified in 92.6 % of colonizing and 90.0 % of invasive serotype Ia isolates, whereas serotype III was associated with co-expression of a PI-1 and PI-2b among 84.6 % of colonizing and 96.5 % of invasive isolates. Based on this homogeneity of pilus island distribution, a pilus-based vaccine developed for Europe and the USA will have similar coverage in South Africa.

Developing vaccines in the era of genomics: a decade of reverse vaccinology

Vaccines have a significant impact on public health, and vaccinology in the era of genomics is taking advantage of new technologies to tackle diseases for which vaccine development has so far been unsuccessful. Almost all existing vaccines were developed based on traditional vaccinology methods, which relied on empirical screening of a few candidates at a time, based on known features of the pathogen. However, the ability to sequence a pathogen's genome provides access to its entire antigenic repertoire. As such, genomics has catalysed a shift in vaccine development towards sequence-based 'Reverse Vaccinology' approaches, which use high-throughput in silico screening of the entire genome of a pathogen to identify genes that encode proteins with the attributes of good vaccine targets. Furthermore, the increasing availability of genome sequences has led to the development and application of additional technologies to vaccine discovery, including comparative genomics, transcriptomics, proteomics, immunomics and structural genomics. Vaccine candidates identified from a pathogen's genome or proteome can then be expressed as recombinant proteins and tested in appropriate in vitro or in vivo models to assess immunogenicity and protection. The process of reverse vaccinology has been applied to several pathogens, including serogroup B Neisseria meningitidis, Streptococcus agalactiae, Streptococcus pyogenes, Streptococcus pneumoniae and pathogenic Escherichia coli, and has provided scores of new candidate antigens for preclinical and clinical investigation. As novel genome-based technologies continue to emerge, it is expected that new vaccines for unmet diseases will be within reach.

Evaluation of the brain-derived neurotrophic factor, nerve growth factor and memory in adult rats survivors of the neonatal meningitis by Streptococcus agalactiae

Streptococcus agalactiae (GBS) is a major cause of severe morbidity and mortality in neonates and young infants, causing sepsis, pneumonia and meningitis. The survivors from this meningitis can suffer serious long-term neurological consequences, such as, seizures, hearing loss, learning and memory impairments. Neurotrophins, such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) control the neuronal cell death during the brain development and play an important role in neuronal differentiation, survival and growth of neurons. Neonate Wistar rats, received either 10μL of sterile saline as a placebo or an equivalent volume of GBS suspension at a concentration of 1×10(6)cfu/mL. Sixty days after induction of meningitis, the animals underwent behavioral tests, after were killed and the hippocampus and cortex were retired for analyze of the BDNF and NGF levels. In the open-field demonstrated no difference in motor, exploratory activity and habituation memory between the groups. The step-down inhibitory avoidance, when we evaluated the long-term memory at 24h after training session, we found that the meningitis group had a decrease in aversive memory when compared with the long-term memory test of the sham group. BDNF levels decreased in hippocampus and cortex; however the NGF levels decreased only in hippocampus. These findings suggest that the meningitis model could be a good research tool for the study of the biological mechanisms involved in the behavioral alterations secondary to GBS meningitis.

Dominance of serotype Ia among group B Streptococci causing invasive infections in nonpregnant adults in Portugal

The population of group B streptococci (GBS) associated with invasive infections in nonpregnant adults from 2001 to 2008 was analyzed in isolates submitted from 24 hospital laboratories in Portugal (n = 225). The isolates were characterized by antimicrobial susceptibility, pulsed-field gel electrophoresis (PFGE), multilocus sequence typing (MLST), and surface protein gene profiling. GBS invasive cases were found more frequently among men in all age groups. In addition, serotype Ia was the most frequent in our collection, whereas serotype V is dominant elsewhere. Serotype Ia was represented mainly by a single PFGE cluster defined by sequence type 23 (ST23) and surface protein gene eps and by ST24 and bca, similarly to neonatal invasive infections in Portugal, indicating that the same genetic lineages can be responsible for both vaginal colonization and invasive disease in all age groups. In contrast, the hypervirulent serotype III/ST17 neonatal lineage was responsible for a minority of infections. Serotype V isolates were distributed into two genetic lineages, one defined by ST1 and surface protein gene alp3 and macrolide resistant, and another presenting with ST2 and eps and fully susceptible to all antimicrobials tested. The erm(TR) gene was the most frequently found among erythromycin-resistant isolates, while the bovine-associated tet(O) gene was found in a minority of tetracycline-resistant isolates. Our data emphasize the importance of local identification of the genetic lineages responsible for GBS invasive infections in nonpregnant adults. The dominance of serotype Ia in invasive disease in Portugal highlights the importance of this serotype in GBS pathogenesis.

The burden of invasive early-onset neonatal sepsis in the United States, 2005-2008

BACKGROUND

Sepsis in the first 3 days of life is a leading cause of morbidity and mortality among infants. Group B Streptococcus (GBS), historically the primary cause of early-onset sepsis (EOS), has declined through widespread use of intrapartum chemoprophylaxis. We estimated the national burden of invasive EOS cases and deaths in the era of GBS prevention.

METHODS

Population-based surveillance for invasive EOS was conducted in 4 of the Centers for Disease Control and Prevention's Active Bacterial Core surveillance sites from 2005 to 2008. We calculated incidence using state and national live birth files. Estimates of the national number of cases and deaths were calculated, standardizing by race and gestational age.

RESULTS

Active Bacterial Core surveillance identified 658 cases of EOS; 72 (10.9%) were fatal. Overall incidence remained stable during the 3 years (2005: 0.77 cases/1000 live births; 2008: 0.76 cases/1000 live births). GBS (∼ 38%) was the most commonly reported pathogen followed by Escherichia coli (∼ 24%). Black preterm infants had the highest incidence (5.14 cases/1000 live births) and case fatality (24.4%). Nonblack term infants had the lowest incidence (0.40 cases/1000 live births) and case fatality (1.6%). The estimated national annual burden of EOS was approximately 3320 cases (95% confidence interval [CI]: 3060-3580), including 390 deaths (95% CI: 300-490). Among preterm infants, 1570 cases (95% CI: 1400-1770; 47.3% of the overall) and 360 deaths (95% CI: 280-460; 92.3% of the overall) occurred annually.

CONCLUSIONS

The burden of invasive EOS remains substantial in the era of GBS prevention and disproportionately affects preterm and black infants. Identification of strategies to prevent preterm births is needed to reduce the neonatal sepsis burden.

Neonatal group B streptococcal disease: from pathogenesis to preventive strategies

Streptococcus agalactiae, or group B streptococcus (GBS), remains the leading cause of neonatal sepsis and meningitis, as early-onset or late-onset diseases (EOD, LOD). Where consensus guidelines to detect and treat intrapartum women with GBS colonization have been widely adopted, incidence of neonatal EOD has dramatically declined. In response to both successful impacts on the incidence of GBS-EOD and analyses of missed opportunities, the first American guidelines for prevention issued in the 1990s have since been adapted in several stages to improve their efficacy. In some countries in Europe, nationwide guidelines, whether screening-based or risk-based, for the prevention of neonatal GBS diseases have also been issued and adopted, with the expected impact on incidence of GBS-EOD. In spite of universal screening, in spite of the great progress that has been made, GBS-EOD continues to occur and the GBS burden remains a significant public health issue. Continuous efforts to improve screening for GBS status continue to be important and may be able to take advantage of new rapid diagnostic technologies. The current screening-based strategy for prevention is highly effective but imperfect. Given the challenges, limitations and potential complications of maternal intrapartum prophylaxis, a new approach is still needed. Maternal immunization against GBS is an attractive alternative for the prevention of not only neonatal diseases but also stillbirths and maternal diseases. Vaccines against GBS may become the most effective and sustainable long-term preventive strategy.

Structure-based approach to rationally design a chimeric protein for an effective vaccine against Group B Streptococcus infections

Structural vaccinology is an emerging strategy for the rational design of vaccine candidates. We successfully applied structural vaccinology to design a fully synthetic protein with multivalent protection activity. In Group B Streptococcus, cell-surface pili have aroused great interest because of their direct roles in virulence and importance as protective antigens. The backbone subunit of type 2a pilus (BP-2a) is present in six immunogenically different but structurally similar variants. We determined the 3D structure of one of the variants, and experimentally demonstrated that protective antibodies specifically recognize one of the four domains that comprise the protein. We therefore constructed a synthetic protein constituted by the protective domain of each one of the six variants and showed that the chimeric protein protects mice against the challenge with all of the type 2a pilus-carrying strains. This work demonstrates the power of structural vaccinology and will facilitate the development of an optimized, broadly protective pilus-based vaccine against Group B Streptococcus by combining the uniquely generated chimeric protein with protective pilin subunits from two other previously identified pilus types. In addition, this work describes a template procedure that can be followed to develop vaccines against other bacterial pathogens.

Serotype distribution and invasive potential of group B streptococcus isolates causing disease in infants and colonizing maternal-newborn dyads

BACKGROUND

Serotype-specific polysaccharide based group B streptococcus (GBS) vaccines are being developed. An understanding of the serotype epidemiology associated with maternal colonization and invasive disease in infants is necessary to determine the potential coverage of serotype-specific GBS vaccines.

METHODS

Colonizing GBS isolates were identified by vaginal swabbing of mothers during active labor and from skin of their newborns post-delivery. Invasive GBS isolates from infants were identified through laboratory-based surveillance. GBS serotyping was done by latex agglutination. Serologically non-typeable isolates were typed by a serotype-specific PCR method. The invasive potential of GBS serotypes associated with sepsis within seven days of birth was evaluated in association to maternal colonizing serotypes.

RESULTS

GBS was identified in 289 (52.4%) newborns born to 551 women with GBS-vaginal colonization and from 113 (5.6%) newborns born to 2,010 mothers in whom GBS was not cultured from vaginal swabs. The serotype distribution among vaginal-colonizing isolates was as follows: III (37.3%), Ia (30.1%), and II (11.3%), V (10.2%), Ib (6.7%) and IV (3.7%). There were no significant differences in serotype distribution between vaginal and newborn colonizing isolates (P = 0.77). Serotype distribution of invasive GBS isolates were significantly different to that of colonizing isolates (P<0.0001). Serotype III was the most common invasive serotype in newborns less than 7 days (57.7%) and in infants 7 to 90 days of age (84.3%; P<0.001). Relative to serotype III, other serotypes showed reduced invasive potential: Ia (0.49; 95%CI 0.31-0.77), II (0.30; 95%CI 0.13-0.67) and V (0.38; 95%CI 0.17-0.83).

CONCLUSION

In South Africa, an anti-GBS vaccine including serotypes Ia, Ib and III has the potential of preventing 74.1%, 85.4% and 98.2% of GBS associated with maternal vaginal-colonization, invasive disease in neonates less than 7 days and invasive disease in infants between 7-90 days of age, respectively.

Group B streptococcus serotype prevalence in reproductive-age women at a tertiary care military medical center relative to global serotype distribution

Background

Group B Streptococcus (GBS) serotype (Ia, Ib, II-IX) correlates with pathogen virulence and clinical prognosis. Epidemiological studies of seroprevalence are an important metric for determining the proportion of serotypes in a given population. The purpose of this study was to evaluate the prevalence of individual GBS serotypes at Madigan Healthcare System (Madigan), the largest military tertiary healthcare facility in the Pacific Northwestern United States, and to compare seroprevalences with international locations.

Methods

To determine serotype distribution at Madigan, we obtained GBS isolates from standard-of-care anogenital swabs from 207 women of indeterminate gravidity between ages 18-40 during a five month interval. Serotype was determined using a recently described molecular method of polymerase chain reaction by capsular polysaccharide synthesis (cps) genes associated with pathogen virulence.

Results

Serotypes Ia, III, and V were the most prevalent (28%, 27%, and 17%, respectively). A systematic review of global GBS seroprevalence, meta-analysis, and statistical comparison revealed strikingly similar serodistibution at Madigan relative to civilian-sector populations in Canada and the United States. Serotype Ia was the only serotype consistently higher in North American populations relative to other geographic regions (p < 0.005). The number of non-typeable isolates was significantly lower in the study (p < 0.005).

Conclusion

This study establishes PCR-based serotyping as a viable strategy for GBS epidemiological surveillance. Our results suggest that GBS seroprevalence remains stable in North America over the past two decades.

Neonatal sepsis and neutrophil insufficiencies

Sepsis has continuously been a leading cause of neonatal morbidity and mortality despite current advances in chemotherapy and patient intensive care facilities. Neonates are at high risk for developing bacterial infections due to quantitative and qualitative insufficiencies of innate immunity, particularly granulocyte lineage development and response to infection. Although antibiotics remain the mainstay of treatment, adjuvant therapies enhancing immune function have shown promise in treating sepsis in neonates. This article reviews current strategies for the clinical management of neonatal sepsis and analyzes mechanisms underlying insufficiencies of neutrophil defense in neonates with emphasis on new directions for adjuvant therapy development.

Vaccination with peptide mimotopes produces antibodies recognizing bacterial capsular polysaccharides

A phage display peptide library was screened using a panel of antibodies to the capsular polysaccharides of Streptococcus agalactiae and Neisseria meningitidis. Mimotopes NPDHPRVPTFMA (2-8), LIPFHKHPHHRG (3-2) and EQEIFTNITDRV (G3) showing the highest binding capacity and strongest ELISA reaction were selected for immunization experiments. These mimotopes were either synthesised as oligodeoxynucleotides for DNA immunization or MAP (multiple antigen peptide) for peptide immunization. Mimotope-DNA vaccination, particularly for G3, induced antibodies recognizing a number of target bacteria. This response was seen after the second boost injection and was significantly enhanced by the 3rd boost injection with a Th1-associated profile, which was dominated by IgG2a, followed by IgG1. Mimotope-MAP immunization also produced strong humoral immune responses to the bacteria. Antibodies from G3 DNA immunization reacted with the surface molecules of S. agalactiae, N. meningitidis and Escherichia coli K5 shown by indirect immunofluorescence staining, indicating a possible localization to the bacterial capsule. Antibodies produced both from DNA/MAP immunization reacted with purified bacterial capsular polysaccharides by ELISA and were of high avidity. We have further characterized peptide G3 by a 'tiling path' study to examine the effect of changing individual residues in the peptide in raising antibodies, which showed that the EIFTN motif in G3 was important in generating antibodies to several capsulated bacteria. We conclude that mimotope immunization with DNA or MAP potentially induces strong antibody responses against encapsulated bacteria. It is suggested that the antibody targets are polysaccharides, and these antibodies may cross react at least among closely related species of bacteria.

Clonal analysis of colonizing group B Streptococcus, serotype IV, an emerging pathogen in the United States

Colonizing group B Streptococcus (GBS) capsular polysaccharide (CPS) type IV isolates were recovered from vaginal and rectal samples obtained from 97 (8.4%) nonpregnant women of 1,160 women enrolled in a U.S. multicenter GBS vaccine study from 2004 to 2008. Since this rate was much higher than the rate of prevalence of 0.4 to 0.6% that we found in previous studies, the isolates were analyzed by using surface protein profile identification, pulsed-field gel electrophoresis (PFGE), and multilocus sequence typing (MLST) to characterize them and identify trends in DNA clonality and divergence. Of the 101 type IV isolates studied, 53 expressed alpha and group B protective surface (BPS) proteins, 27 expressed BPS only, 20 expressed alpha only, and 1 had no detectable surface proteins. The isolates spanned three PFGE macrorestriction profile groups, groups 37, 38, and 39, of which group 37 was predominant. The isolates in group 37 expressed the alpha and BPS proteins, while those in groups 38 and 39 expressed the alpha protein only, with two exceptions. MLST studies of selective isolates from the four protein profile groups showed that isolates expressing alpha,BPS or BPS only were of a new sequence type, sequence type 452, while those expressing alpha only or no proteins were mainly of a new sequence type, sequence type 459. Overall, our study revealed a limited diversity in surface proteins, MLST types, and DNA macrorestriction profiles for type IV GBS. There appeared to be an association between the MLST types and protein expression profiles. The increased prevalence of type IV GBS colonization suggested the possibility that this serotype may emerge as a GBS pathogen.

Group B streptococcal disease in infants: progress in prevention and continued challenges

The burden of early-onset disease caused by group B Streptococcus (GBS) has decreased dramatically in the United States over the past 20 years. Universal culture-based screening at 35 to 37 weeks gestational age and use of intrapartum antibiotic prophylaxis are the cornerstones of prevention measures that have led to this decline. GBS, however, remains the leading cause of early-onset neonatal sepsis in the United States. Revised guidelines for prevention of perinatal GBS are planned for issuance in 2010. This article discusses implementation challenges for clinicians caring for pregnant women and newborns and presents an updated algorithm for neonatal management.

Prevention of Group B Streptococcus early-onset disease: a toolkit by the California Perinatal Quality Care Collaborative

The California Perinatal Quality Care Collaborative (CPQCC) was organized in 1996 in an effort to improve perinatal outcomes in California. CPQCC has a robust infrastructure of quality improvement resources and expertise and uses a database of demographic and outcome data from neonatal intensive care units in California. In 2004, CPQCC developed a toolkit to provide an evidence-based framework and supporting documents for hospitals to use in systematically addressing persistent early-onset disease (EOD) because of Group B Streptococcus (GBS) in their centers. The CPQCC toolkit was based on the 2002 Centers for Disease Control guideline, 'Prevention of Perinatal Group B Streptococcal Disease.' This article presents an updated version of the CPQCC toolkit reflecting several population studies published since the 2002 guideline. Current epidemiological trends in incidence of EOD with GBS, changes in antibiotic sensitivity and the potential value of newer strategies are discussed.

Streptococcus adherence and colonization

Streptococci readily colonize mucosal tissues in the nasopharynx; the respiratory, gastrointestinal, and genitourinary tracts; and the skin. Each ecological niche presents a series of challenges to successful colonization with which streptococci have to contend. Some species exist in equilibrium with their host, neither stimulating nor submitting to immune defenses mounted against them. Most are either opportunistic or true pathogens responsible for diseases such as pharyngitis, tooth decay, necrotizing fasciitis, infective endocarditis, and meningitis. Part of the success of streptococci as colonizers is attributable to the spectrum of proteins expressed on their surfaces. Adhesins enable interactions with salivary, serum, and extracellular matrix components; host cells; and other microbes. This is the essential first step to colonization, the development of complex communities, and possible invasion of host tissues. The majority of streptococcal adhesins are anchored to the cell wall via a C-terminal LPxTz motif. Other proteins may be surface anchored through N-terminal lipid modifications, while the mechanism of cell wall associations for others remains unclear. Collectively, these surface-bound proteins provide Streptococcus species with a "coat of many colors," enabling multiple intimate contacts and interplays between the bacterial cell and the host. In vitro and in vivo studies have demonstrated direct roles for many streptococcal adhesins as colonization or virulence factors, making them attractive targets for therapeutic and preventive strategies against streptococcal infections. There is, therefore, much focus on applying increasingly advanced molecular techniques to determine the precise structures and functions of these proteins, and their regulatory pathways, so that more targeted approaches can be developed.

Selection, recombination, and virulence gene diversity among group B streptococcal genotypes

Transmission of group B Streptococcus (GBS) from mothers to neonates during childbirth is a leading cause of neonatal sepsis and meningitis. Although subtyping tools have identified specific GBS phylogenetic lineages that are important in neonatal disease, little is known about the genetic diversity of these lineages or the roles that recombination and selection play in the generation of emergent genotypes. Here, we examined genetic variation, selection, and recombination in seven multilocus sequence typing (MLST) loci from 94 invasive, colonizing, and bovine strains representing 38 GBS sequence types and performed DNA sequencing and PCR-based restriction fragment length polymorphism analysis of several putative virulence genes to identify gene content differences between genotypes. Despite the low level of diversity in the MLST loci, a neighbor net analysis revealed a variable range of genetic exchange among the seven clonal complexes (CCs) identified, suggesting that recombination is partly responsible for the diversity observed between genotypes. Recombination is also important for several virulence genes, as some gene alleles had evidence for lateral gene exchange across divergent genotypes. The CC-17 lineage, which is associated with neonatal disease, is relatively homogeneous and therefore appears to have diverged independently with an exclusive set of virulence characteristics. These data suggest that different GBS genetic backgrounds have distinct virulence gene profiles that may be important for disease pathogenesis. Such profiles could be used as markers for the rapid detection of strains with an increased propensity to cause neonatal disease and may be considered useful vaccine targets.