Identification and assessment of new vaccine candidates for group A streptococcal infections

Recent Advances in the Development of Peptide Vaccines and Their Delivery Systems Against Group A Streptococcus

Group A Streptococcus (GAS) infection can cause a variety of diseases in humans, ranging from common sore throats and skin infections, to more invasive diseases and life-threatening post-infectious diseases, such as rheumatic fever and rheumatic heart disease. Although research has been ongoing since 1923, vaccines against GAS are still not available to the public. Traditional approaches taken to develop vaccines for GAS failed due to poor efficacy and safety. Fortunately, headway has been made and modern subunit vaccines that administer minimal bacterial components provide an opportunity to finally overcome previous hurdles in GAS vaccine development. This review details the major antigens and strategies used for GAS vaccine development. The combination of antigen selection, peptide epitope modification and delivery systems have resulted in the discovery of promising peptide vaccines against GAS; these are currently in preclinical and clinical studies.

Identifying protective Streptococcus pyogenes vaccine antigens recognized by both B and T cells in human adults and children

No commercial vaccine exists against Group A streptococci (GAS; Streptococcus pyogenes) and only little is known about anti-GAS protective immunity. In our effort to discover new protective vaccine candidates, we selected 21 antigens based on an in silico evaluation. These were all well-conserved among different GAS strains, upregulated in host-pathogen interaction studies, and predicted to be extracellular or associated with the surface of the bacteria. The antigens were tested for both antibody recognition and T cell responses in human adults and children. The antigenicity of a selected group of antigens was further validated using a high-density peptide array technology that also identified the linear epitopes. Based on immunological recognition, four targets were selected and tested for protective capabilities in an experimental GAS infection model in mice. Shown for the first time, three of these targets (spy0469, spy1228 and spy1801) conferred significant protection whereas one (spy1643) did not.

Group A streptococcal vaccines: facts versus fantasy

PURPOSE OF REVIEW

This review provides an overview of progress of the development of group A streptococcal (GAS) vaccines with a focus on recent advances.

RECENT FINDINGS

Historically, GAS vaccine development has focused on the N-terminus of the M protein, which ultimately led to successful phase I/II clinical trials of a 26-valent recombinant M protein vaccine in 2004-2005. More recently, interest in antigens conserved among most, if not all, group A streptococci has increased. However, no vaccines containing these antigens have reached clinical trials. Three strategies have been used to develop conserved antigen vaccine candidates: use of the conserved region of the M protein; use of well described virulence factors as antigens, including streptococcal C5a peptidase, streptococcal carbohydrate, fibronectin-binding proteins, cysteine protease and streptococcal pili; and use of reverse vaccinology to identify novel antigens.

SUMMARY

Several vaccine candidates against GAS infection are in varying stages of preclinical and clinical development. Although there is great hope that one of these vaccine candidates will reach licensure in the next decade, only one, the multivalent N-terminal vaccine, has entered clinical trials in the last 30 years. Although strong advocacy for GAS vaccine development is important, there remains an urgent need to institute available public health control measures against GAS diseases globally, particularly in developing countries.

Future diagnostic and therapeutic approaches in surgical infections

Despite ongoing efforts to standardize therapy and improve management, the morbidity and mortality associated with surgical infections remain high. Continued innovation is required to improve outcomes further, particularly in the face of the increasing prevalence of multidrug resistant organisms. Although they remain in the experimental stages, a number of recent advances have the potential to have significant impact on the management and outcomes of surgical infections. These include novel diagnostic strategies, antimicrobials targeting microbial virulence factors, novel vaccines, and risk stratification based on genetic profiling.

Advances in potential M-protein peptide-based vaccines for preventing rheumatic fever and rheumatic heart disease

Rheumatic fever (RF) and rheumatic heart disease (RHD) are postinfectious complications of an infection (or repeated infection) with the Gram-positive bacterium, Streptococcus pyogenes (also known as group A streptococcus, GAS). RF and RHD are global problems and affect many indigenous populations of developed countries and many developing countries. However, RF and RHD are only part of a larger spectrum of diseases caused by this organism. The development of a vaccine against GAS has primarily targeted the abundant cell-surface protein called the M-protein. This review focuses on different M-protein-based-subunit vaccine approaches and the different delivery technologies used to administer these vaccine candidates in preclinical studies.

Bioinformatics and multiepitope DNA immunization to design rational snake antivenom

BACKGROUND

Snake venom is a potentially lethal and complex mixture of hundreds of functionally diverse proteins that are difficult to purify and hence difficult to characterize. These difficulties have inhibited the development of toxin-targeted therapy, and conventional antivenom is still generated from the sera of horses or sheep immunized with whole venom. Although life-saving, antivenoms contain an immunoglobulin pool of unknown antigen specificity and known redundancy, which necessitates the delivery of large volumes of heterologous immunoglobulin to the envenomed victim, thus increasing the risk of anaphylactoid and serum sickness adverse effects. Here we exploit recent molecular sequence analysis and DNA immunization tools to design more rational toxin-targeted antivenom.

METHODS AND FINDINGS

We developed a novel bioinformatic strategy that identified sequences encoding immunogenic and structurally significant epitopes from an expressed sequence tag database of a venom gland cDNA library of Echis ocellatus, the most medically important viper in Africa. Focusing upon snake venom metalloproteinases (SVMPs) that are responsible for the severe and frequently lethal hemorrhage in envenomed victims, we identified seven epitopes that we predicted would be represented in all isomers of this multimeric toxin and that we engineered into a single synthetic multiepitope DNA immunogen (epitope string). We compared the specificity and toxin-neutralizing efficacy of antiserum raised against the string to antisera raised against a single SVMP toxin (or domains) or antiserum raised by conventional (whole venom) immunization protocols. The SVMP string antiserum, as predicted in silico, contained antibody specificities to numerous SVMPs in E. ocellatus venom and venoms of several other African vipers. More significantly, the antiserum cross-specifically neutralized hemorrhage induced by E. ocellatus and Cerastes cerastes cerastes venoms.

CONCLUSIONS

These data provide valuable sequence and structure/function information of viper venom hemorrhagins but, more importantly, a new opportunity to design toxin-specific antivenoms-the first major conceptual change in antivenom design after more than a century of production. Furthermore, this approach may be adapted to immunotherapy design in other cases where targets are numerous, diverse, and poorly characterized such as those generated by hypermutation or antigenic variation.

Group A streptococcal infections of the skin: molecular advances but limited therapeutic progress

PURPOSE OF REVIEW

With the sequencing of several Streptococcus pyogenes (group A Streptococcus) genomes have come major advances in understanding the pathogenesis of group A Streptococcus-associated diseases. This review focuses on group A Streptococcus skin infections and summarizes data published in the English language medical literature in 2004 and 2005.

RECENT FINDINGS

Group A Streptococcus shows enormous and evolving molecular diversity driven by horizontal transmission between group A Streptococcus strains and between group A Streptococcus and other streptococci. Acquisition of prophages accounts for much of the diversity, conferring both virulence through phage-associated virulence factors and increased bacterial survival against host defences. Studies of group A Streptococcus isolates outside the US also question the generalizability of classic group A Streptococcus M serotype associations with specific disease entities such as acute rheumatic fever and necrotizing fasciitis. The distinction between throat and skin group A Streptococcus has become blurred. Although there have been few advances in treatment of group A Streptococcus skin infections, developments towards group A Streptococcus vaccines are promising.

SUMMARY

The diversity of group A Streptococcus remains a challenge for vaccine development. As acute rheumatic fever and streptococcal pyoderma occur predominantly in disadvantaged populations, international funding support will be necessary for any group A Streptococcus vaccine to have a sustained impact on the global burden of disease.

Identification of a surface protein of Streptococcus suis and evaluation of its immunogenic and protective capacity in pigs

A Streptococcus suis surface protein reacting with convalescent-phase sera from pigs clinically infected by S. suis type 2 was identified. The apparent 110-kDa protein, designated Sao, exhibits typical features of membrane-anchored surface proteins of gram-positive bacteria, such as a signal sequence and an LPVTG membrane anchor motif. In spite of high identity with the partially sequenced genomes of S. suis Canadian strain 89/1591 and European strain P1/7, Sao does not share significant homology with other known sequences. However, a conserved avirulence domain that is often found in plant pathogens has been detected. Electron microscopy using an Sao-specific antiserum has confirmed the surface location of the Sao protein on S. suis. The Sao-specific antibody reacts with cell lysates of 28 of 33 S. suis serotypes and 25 of 26 serotype 2 isolates in immunoblots, suggesting its high conservation in S. suis species. The immunization of piglets with recombinant Sao elicits a significant humoral antibody response. However, the antibody response is not reflected in protection of pigs that are intratracheally challenged with a virulent strain in our conventional vaccination model.

Characterization and identification of vaccine candidate proteins through analysis of the group A Streptococcus surface proteome

We describe a proteomic approach for identifying bacterial surface-exposed proteins quickly and reliably for their use as vaccine candidates. Whole cells are treated with proteases to selectively digest protruding proteins that are subsequently identified by mass spectrometry analysis of the released peptides. When applied to the sequenced M1_SF370 group A Streptococcus strain, 68 PSORT-predicted surface-associated proteins were identified, including most of the protective antigens described in the literature. The number of surface-exposed proteins varied from strain to strain, most likely as a consequence of different capsule content. The surface-exposed proteins of the highly virulent M23_DSM2071 strain included 17 proteins, 15 in common with M1_SF370. When 14 of the 17 proteins were expressed in E. coli and tested in the mouse for their capacity to confer protection against a lethal dose of M23_DSM2071, one new protective antigen (Spy0416) was identified. This strategy overcomes the difficulties so far encountered in surface protein characterization and has great potential in vaccine discovery.

Application of Microbial Genomic Science to Advanced Therapeutics

Since the publication of the first complete microbial genome sequence of Haemophilus influenzae in 1995, more than 200 additional microbial genome sequences have become available in the public domain. Approximately 40% of these represent important human pathogens. Comparative in silico methods, along with large-scale approaches such as transcriptomics and proteomics, are beginning to reveal insights into new virulence genes, pathogen-host interactions, and the molecular basis of host specificity. Sequence data are also starting to accumulate from multiple isolates or strains of a single pathogen, and this type of data has proven to be quite valuable in providing new insights into the genetic variability that is present in a particular species as well as in facilitating correlations between genotype and phenotype. Ultimately, a major goal of genome-enabled infectious disease research is the development of novel diagnostics, therapeutics, and vaccines.