Group A streptococcal vaccines

Streptococcus pyogenes genes that promote pharyngitis in primates

Streptococcus pyogenes (group A streptococcus; GAS) causes 600 million cases of pharyngitis annually worldwide. There is no licensed human GAS vaccine despite a century of research. Although the human oropharynx is the primary site of GAS infection, the pathogenic genes and molecular processes used to colonize, cause disease, and persist in the upper respiratory tract are poorly understood. Using dense transposon mutant libraries made with serotype M1 and M28 GAS strains and transposon-directed insertion sequencing, we performed genome-wide screens in the nonhuman primate (NHP) oropharynx. We identified many potentially novel GAS fitness genes, including a common set of 115 genes that contribute to fitness in both genetically distinct GAS strains during experimental NHP pharyngitis. Targeted deletion of 4 identified fitness genes/operons confirmed that our newly identified targets are critical for GAS virulence during experimental pharyngitis. Our screens discovered many surface-exposed or secreted proteins - substrates for vaccine research - that potentially contribute to GAS pharyngitis, including lipoprotein HitA. Pooled human immune globulin reacted with purified HitA, suggesting that humans produce antibodies against this lipoprotein. Our findings provide new information about GAS fitness in the upper respiratory tract that may assist in translational research, including developing novel vaccines.

Safety and immunogenicity of a 30-valent M protein-based group a streptococcal vaccine in healthy adult volunteers: A randomized, controlled phase I study

BACKGROUND

Streptococcus pyogenes (group A Streptococcus, Strep A) is a widespread pathogen that continues to pose a significant threat to human health. The development of a Strep A vaccine remains an unmet global health need. One of the major vaccine strategies is the use of M protein, which is a primary virulence determinant and protective antigen. Multivalent recombinant M protein vaccines are being developed with N-terminal M peptides that contain opsonic epitopes but do not contain human tissue cross-reactive epitopes.

METHODS

We completed a Phase I trial of a recombinant 30-valent M protein-based Strep A vaccine (Strep A vaccine, StreptAnova™) comprised of four recombinant proteins containing N-terminal peptides from 30 M proteins of common pharyngitis and invasive and/or rheumatogenic serotypes, adjuvanted with aluminum hydroxide. The trial was observer-blinded and randomized in a 2:1 ratio for intramuscular administration of Strep A vaccine or an alum-based comparator in healthy adult volunteers, at 0, 30 and 180 days. Primary outcome measures were assessments of safety, including assays for antibodies that cross-reacted with host tissues, and immunogenicity assessed by ELISA with the individual vaccine peptides and by opsonophagocytic killing (OPK) assays in human blood.

RESULTS

Twenty-three Strep A-vaccinated participants and 13 controls completed the study. The Strep A vaccine was well-tolerated and there was no clinical evidence of autoimmunity and no laboratory evidence of tissue cross-reactive antibodies. The vaccine was immunogenic and elicited significant increases in geometric mean antibody levels to 24 of the 30 component M antigens by ELISA. Vaccine-induced OPK activity was observed against selected M types of Strep A in vaccinated participants that seroconverted to specific M peptides.

CONCLUSION

The Strep A vaccine was well tolerated and immunogenic in healthy adults, providing strong support for further clinical development. [ClinicalTrials.gov NCT02564237].

New 30-valent M protein-based vaccine evokes cross-opsonic antibodies against non-vaccine serotypes of group A streptococci

Our previous studies have shown that recombinant multivalent vaccines containing amino-terminal M protein fragments from as many as 26 different serotypes of group A streptococci (GAS) evoked opsonic antibodies in animals and humans. In the present study, we constructed a new 30-valent vaccine containing M protein peptides from GAS serotypes prevalent in North America and Europe. The vaccine was immunogenic in rabbits and evoked bactericidal antibodies against all 30 vaccine serotypes of GAS. In addition, the vaccine antisera also contained significant levels of bactericidal antibodies against 24 of 40 non-vaccine serotypes of GAS. These results indicate that the potential efficacy of the new multivalent vaccine may be greater than predicted based on the "type-specific" M peptides represented.

The NH(2)-terminal region of Streptococcus pyogenes M5 protein confers protection against degradation by proteases and enhances mucosal colonization of mice

BACKGROUND

The NH(2)-terminal sequence of the M protein from group A streptococci defines the serotype of the organism and contains epitopes that evoke bactericidal antibodies.

METHODS

To identify additional roles for this region of the M protein, we constructed a mutant of M5 group A streptococci expressing an M protein with a deletion of amino acid residues 3-22 (DeltaNH(2)).

RESULTS

M5 streptococci and the DeltaNH(2) mutant were resistant to phagocytosis and were similarly virulent in mice. However, DeltaNH(2) was significantly less hydrophobic, contained less lipoteichoic acid on its surface, and demonstrated reduced adherence to epithelial cells. These differences were abolished when organisms were grown in the presence of protease inhibitors. Treatment with cysteine proteases or with human saliva resulted in the release of M protein from the DeltaNH(2) mutant at a significantly greater rate than observed with the wild-type M5 strain. Compared with the parent strain, the DeltaNH(2) strain also showed a significant reduction in its ability to colonize the upper respiratory mucosa of mice.

CONCLUSIONS

The NH(2) terminus of M5 protein has an important role in protecting the surface protein from proteolytic cleavage, thus preserving its function as an anchor for lipoteichoic acid, which is a primary mediator of adherence to epithelial cells and colonization.

Prospects for a group A streptococcal vaccine: rationale, feasibility, and obstacles--report of a National Institute of Allergy and Infectious Diseases workshop

Infections due to group A streptococci (GAS) represent a public health problem of major proportions in both developing and developed countries. Currently available methods of prevention are either inadequate or ineffective, as attested to by the morbidity and mortality associated with this ubiquitous pathogen worldwide. Advances in molecular biology have shed new light on the pathogenesis of GAS infections and have identified a number of virulence factors as potential vaccine targets. Therefore, the National Institute of Allergy and Infectious Diseases convened an expert workshop in March 2004 to review the available data and to explore the microbiologic, immunologic, epidemiologic, and economic issues involved in development and implementation of a safe and effective GAS vaccine. Participants included scientists and clinicians involved in GAS research, as well as representatives of United States federal agencies (Centers for Disease Control and Prevention, Food and Drug Administration, Department of Defense, and National Institute of Allergy and Infectious Diseases), the World Health Organization, and the pharmaceutical industry. This report summarizes the deliberations of the workshop.

Group A Streptococcus gene expression in humans and cynomolgus macaques with acute pharyngitis

The molecular mechanisms used by group A Streptococcus (GAS) to survive on the host mucosal surface and cause acute pharyngitis are poorly understood. To provide new information about GAS host-pathogen interactions, we used real-time reverse transcription-PCR (RT-PCR) to analyze transcripts of 17 GAS genes in throat swab specimens taken from 18 pediatric patients with pharyngitis. The expression of known and putative virulence genes and regulatory genes (including genes in seven two-component regulatory systems) was studied. Several known and previously uncharacterized GAS virulence gene regulators were highly expressed compared to the constitutively expressed control gene proS. To examine in vivo gene transcription in a controlled setting, three cynomolgus macaques were infected with strain MGAS5005, an organism that is genetically representative of most serotype M1 strains recovered from pharyngitis and invasive disease episodes in North America and Western Europe. These three animals developed clinical signs and symptoms of GAS pharyngitis and seroconverted to several GAS extracellular proteins. Real-time RT-PCR analysis of throat swab material collected at intervals throughout a 12-day infection protocol indicated that expression profiles of a subset of GAS genes accurately reflected the profiles observed in the human pediatric patients. The results of our study demonstrate that analysis of in vivo GAS gene expression is feasible in throat swab specimens obtained from infected human and nonhuman primates. In addition, we conclude that the cynomolgus macaque is a useful nonhuman primate model for the study of molecular events contributing to acute pharyngitis caused by GAS.

Postgenomic analysis of four novel antigens of group a streptococcus: growth phase-dependent gene transcription and human serologic response

Analysis of three group A Streptococcus genomes (serotypes M1, M3, and M18) recently identified four previously undescribed genes that encode extracellular proteins. Each of these genes encode proteins with an LPXTG amino acid motif that covalently links many virulence factors produced by gram-positive bacteria to the cell surface. Western immunoblot analysis of serum samples obtained from 80 patients with invasive infections, noninvasive soft tissue infections, pharyngitis, and rheumatic fever indicated that these four proteins are expressed in vivo. However, the level of gene transcript and the time of maximal gene transcription varied in representative serotype M1, M3, and M18 strains. Surface expression of two proteins was confirmed by flow cytometry. Studies using a mouse infection model suggest that antibodies specific for one of the proteins (Spy0843) may contribute to a protective host immune response against a serotype M1 infection. These results are additional evidence that postgenomic strategies provide new ways to identify and investigate novel bacterial proteins that may participate in host-pathogen interactions or serve as targets for therapeutics research.

Antibodies against a synthetic peptide of SagA neutralize the cytolytic activity of streptolysin S from group A streptococci

Virtually all group A streptococci (GAS) produce streptolysin S (SLS), a cytolytic toxin that is responsible for the beta-hemolysis surrounding colonies of the organisms grown on blood agar. SLS is an important virulence determinant of GAS, and recent studies have identified a nine-gene locus that is responsible for synthesis and transport of the toxin. SLS is not immunogenic; thus, no neutralizing antibodies are evoked during the course of natural infection. In the present study, we show that a synthetic peptide containing amino acid residues 10 to 30 of the putative SLS (SagA) propeptide [SLS(10-30)] coupled to keyhole limpet hemocyanin evoked antibodies in rabbits that completely neutralized the hemolytic activity of the toxin in vitro. Inhibition of hemolysis was reversed by preincubation of the immune serum with soluble, unconjugated peptide, indicating the specificity of the antibodies. In addition, antibodies that were affinity purified over an SLS(10-30) peptide column completely inhibited SLS-mediated hemolysis. The SLS(10-30) antisera did not opsonize group A streptococci; however, when combined with type-specific M protein antisera, the SLS antibodies significantly enhanced phagocytosis mediated by M protein antibodies. Thus, we have shown for the first time that it is possible to raise neutralizing antibodies against one of the most potent bacterial cytolytic toxins known. Our data also provide convincing evidence that the sagA gene actually encodes the SLS peptide of GAS. The synthetic peptide may prove to be an important component of vaccines designed to prevent GAS infections.

Fba, a novel fibronectin-binding protein from Streptococcus pyogenes, promotes bacterial entry into epithelial cells, and the fba gene is positively transcribed under the Mga regulator

In infection by Streptococcus pyogenes, fibronectin (Fn)-binding proteins play important roles as adhesins and invasins. Here, we present a novel Fn-binding protein of S. pyogenes that exhibits a low similarity to other Fn-binding proteins reported. After searching the Oklahoma Streptococcal Genome Sequencing Database for open reading frames (ORFs) with an LPXTG motif, nine ORFs were found among those recognized as putative surface proteins, and one of them was designated as Fba. The fba gene was found in M types 1, 2, 4, 22, 28 and 49 of S. pyogenes, but not in other serotypes or groups of streptococci. Fba, a 37.8 kDa protein, possesses three or four proline-rich repeat domains and exhibits a high homology to FnBPA, the Fn-binding protein of Staphylococcus aureus. Recombinant Fba exhibited a strong binding ability to Fn. In addition, Fba-deficient mutants showed diminished invasive capabilities to HEp-2 cells and low mortality in mice following skin infection. The fba gene was located downstream of the mga regulon and analysis using an mga-inactivated mutant revealed that it was transcribed under the control of the Mga regulator. These results indicate that Fba is a novel protein and one of the important virulence factors of S. pyogenes.

Rheumatic fever in the 21st century

In the first half of the twentieth century, the group A streptococcus (GAS) was established as the sole etiologic agent of acute rheumatic fever (ARF). In the century's latter half, the clinical importance of variation in the virulence of strains of GAS has become clearer. Although still obscure, the pathogenesis of ARF requires primary infection of the throat by highly virulent GAS strains. These contain very large hyaluronate capsules and M protein molecules. The latter contain epitopes that are cross-reactive with host tissues and also contain superantigenic toxic moieties. In settings where ARF has become rare, GAS pharyngitis continues to be common, although it is caused by GAS strains of relatively lower virulence. These strains, however, colonize the throat avidly and stubbornly. Molecularly distinct pyoderma strains may cause acute glomerulonephritis, but they are not rheumatogenic, even though they may secondarily colonize and infect the throat. Guidelines for the diagnosis, treatment, and prevention of GAS pharyngitis and ARF are reviewed with particular reference to the prevalence of the latter in the community.

Multilocus analysis of extracellular putative virulence proteins made by group A Streptococcus: population genetics, human serologic response, and gene transcription

Species of pathogenic microbes are composed of an array of evolutionarily distinct chromosomal genotypes characterized by diversity in gene content and sequence (allelic variation). The occurrence of substantial genetic diversity has hindered progress in developing a comprehensive understanding of the molecular basis of virulence and new therapeutics such as vaccines. To provide new information that bears on these issues, 11 genes encoding extracellular proteins in the human bacterial pathogen group A Streptococcus identified by analysis of four genomes were studied. Eight of the 11 genes encode proteins with a LPXTG(L) motif that covalently links Gram-positive virulence factors to the bacterial cell surface. Sequence analysis of the 11 genes in 37 geographically and phylogenetically diverse group A Streptococcus strains cultured from patients with different infection types found that recent horizontal gene transfer has contributed substantially to chromosomal diversity. Regions of the inferred proteins likely to interact with the host were identified by molecular population genetic analysis, and Western immunoblot analysis with sera from infected patients confirmed that they were antigenic. Real-time reverse transcriptase-PCR (TaqMan) assays found that transcription of six of the 11 genes was substantially up-regulated in the stationary phase. In addition, transcription of many genes was influenced by the covR and mga trans-acting gene regulatory loci. Multilocus investigation of putative virulence genes by the integrated approach described herein provides an important strategy to aid microbial pathogenesis research and rapidly identify new targets for therapeutics research.

emm and sof gene sequence variation in relation to serological typing of opacity-factor-positive group A streptococci

Approximately 40-60% of group A streptococcal (GAS) isolates are capable of opacifying sera, due to the expression of the sof (serum opacity factor) gene. The emm (M protein gene) and sof 5' sequences were obtained from a diverse set of GAS reference strains and clinical isolates, and correlated with M serotyping and anti-opacity-factor testing results. Attempts to amplify sof from strains with M serotypes or emm types historically associated with the opacity-factor-negative phenotype were negative, except for emm12 strains, which were found to contain a highly conserved sof sequence. There was a strong correlation of certain M serotypes with specific emm sequences regardless of strain background, and likewise a strong association of specific anti-opacity-factor (AOF) types to sof gene sequence types. In several examples, M type identity, or partial identity shared between strains with differing emm types, was correlated with short, highly conserved 5' emm sequences likely to encode M-type-specific epitopes. Additionally, each of three pairs of historically distinct M type reference strains found to share the same 5' emm sequence, were also found to share M serotype specificity. Based upon sof sequence comparisons between strains of the same and of differing AOF types, an approximately 450 residue domain was determined likely to contain key epitopes required for AOF type specificity. Analysis of two Sof sequences that were not highly homologous, yet shared a common AOF type, further implicated a 107 aa portion of this 450-residue domain in putatively containing AOF-specific epitopes. Taken together, the serological data suggest that AOF-specific epitopes for all Sof proteins may reside within a region corresponding to this 107-residue sequence. The presence of specific, hypervariable emm/sof pairs within multiple isolates appears likely to be a reliable indicator of their overall genetic relatedness, and to be very useful for accurate subtyping of GAS isolates by an approach that has relevance to decades of past M-type-based epidemiological data.

Current and Future Use of Vaccines for Viral and Bacterial Respiratory Tract Infections

Viral and bacterial respiratory infections remain the number one cause of infectious disease-related deaths around the world. In the past, vaccines were often created by repeatedly passing laboratory cultures to develop attenuated strains or simply by inactivating live cultures of pathogens. A variety of new and innovative technologies are being applied to develop vaccines against the more elusive pathogens. A variety of protein conjugates have been used to greatly enhance the immunogenicity of Haemophilus influenzae type B vaccine, and are now being employed for new pneumococcal and meningococcal vaccines. Live attenuated vaccine strains of respiratory syncytial virus and influenza, which induce protective immunity through localized replication in the nasopharynx, may soon be available for routine use. Future innovations may include genetic vaccines that introduce DNA into host cells to produce specific protective antigens, along with a desired cytokine response to induce a protective immune response.