A multivalent T-antigen-based vaccine for Group A Streptococcus

Recombinational exchange of M-fibril and T-pilus genes generates extensive cell surface diversity in the global group A Streptococcus population

Among genes present in all group A streptococci (GAS), those encoding M-fibril and T-pilus proteins display the highest levels of sequence diversity, giving rise to the two primary serological typing schemes historically used to define strain. A new genotyping scheme for the pilin adhesin and backbone genes is developed and, when combined with emm typing, provides an account of the global GAS strain population. Cluster analysis based on nucleotide sequence similarity assigns most T-serotypes to discrete pilin backbone sequence clusters, yet the established T-types correspond to only half the clusters. The major pilin adhesin and backbone sequence clusters yield 98 unique combinations, defined as "pilin types." Numerous horizontal transfer events that involve pilin or emm genes generate extensive antigenic and functional diversity on the bacterial cell surface and lead to the emergence of new strains. Inferred pilin genotypes applied to a meta-analysis of global population-based collections of pharyngitis and impetigo isolates reveal highly significant associations between pilin genotypes and GAS infection at distinct ecological niches, consistent with a role for pilin gene products in adaptive evolution. Integration of emm and pilin typing into open-access online tools (pubmlst.org) ensures broad utility for end-users wanting to determine the architecture of M-fibril and T-pilus genes from genome assemblies.IMPORTANCEPrecision in defining the variant forms of infectious agents is critical to understanding their population biology and the epidemiology of associated diseases. Group A Streptococcus (GAS) is a global pathogen that causes a wide range of diseases and displays a highly diverse cell surface due to the antigenic heterogeneity of M-fibril and T-pilus proteins which also act as virulence factors of varied functions. emm genotyping is well-established and highly utilized, but there is no counterpart for pilin genes. A global GAS collection provides the basis for a comprehensive pilin typing scheme, and online tools for determining emm and pilin genotypes are developed. Application of these tools reveals the expansion of structural-functional diversity among GAS via horizontal gene transfer, as evidenced by unique combinations of surface protein genes. Pilin and emm genotype correlations with superficial throat vs skin infection provide new insights on the molecular determinants underlying key ecological and epidemiological trends.

Group A Streptococcus adaptation to diverse niches: lessons from transcriptomic studies

Group A Streptococcus (GAS) is a major human pathogen, causing diseases ranging from mild superficial infections of the skin and pharyngeal epithelium to severe systemic and invasive diseases. Moreover, post infection auto-immune sequelae arise by a yet not fully understood mechanism. The ability of GAS to cause a wide variety of infections is linked to the expression of a large set of virulence factors and their transcriptional regulation in response to various physiological environments. The use of transcriptomics, among others -omics technologies, in addition to traditional molecular methods, has led to a better understanding of GAS pathogenesis and host adaptation mechanisms. This review focusing on bacterial transcriptomic provides new insight into gene-expression patterns in vitro, ex vivo and in vivo with an emphasis on metabolic shifts, virulence genes expression and transcriptional regulators role.

Conserved molecular chaperone PrsA stimulates protective immunity against group A Streptococcus

Group A Streptococcus (GAS) is a significant human pathogen that poses a global health concern. However, the development of a GAS vaccine has been challenging due to the multitude of diverse M-types and the risk of triggering cross-reactive immune responses. Our previous research has identified a critical role of PrsA1 and PrsA2, surface post-translational molecular chaperone proteins, in maintaining GAS proteome homeostasis and virulence traits. In this study, we aimed to further explore the potential of PrsA1 and PrsA2 as vaccine candidates for preventing GAS infection. We found that PrsA1 and PrsA2 are highly conserved among GAS isolates, demonstrating minimal amino acid variation. Antibodies specifically targeting PrsA1/A2 showed no cross-reactivity with human heart proteins and effectively enhanced neutrophil opsonophagocytic killing of various GAS serotypes. Additionally, passive transfer of PrsA1/A2-specific antibodies conferred protective immunity in infected mice. Compared to alum, immunization with CFA-adjuvanted PrsA1/A2 induced higher levels of Th1-associated IgG isotypes and complement activation and provided approximately 70% protection against invasive GAS challenge. These findings highlight the potential of PrsA1 and PrsA2 as universal vaccine candidates for the development of an effective GAS vaccine.

A worldwide population of Streptococcus pyogenes strains circulating among school-aged children in Auckland, New Zealand: a genomic epidemiology analysis

Background

Acute rheumatic fever (ARF) is a serious post-infectious sequala of Group A Streptococcus (GAS, Streptococcus pyogenes). In New Zealand (NZ) ARF is a major cause of health inequity. This study describes the genomic analysis of GAS isolates associated with childhood skin and throat infections in Auckland NZ.

Methods

Isolates (n = 469) collected between March 2018 and October 2019 from the throats and skin of children (5-14 years) underwent whole genomic sequencing. Equal representation across three ethnic groups was ensured through sample quotas with isolates obtained from Indigenous Māori (n = 157, 33%), NZ European/Other (n = 149, 32%) and Pacific Peoples children (n = 163, 35%). Using in silico techniques isolates were classified, assessed for diversity, and examined for distribution differences between groups. Comparisons were also made with GAS strains identified globally.

Findings

Genomic analysis revealed a diverse population consisting of 65 distinct sequence clusters. These sequence clusters spanned 49 emm-types, with 11 emm-types comprised of several, distinct sequence clusters. There is evidence of multiple global introductions of different lineages into the population, as well as local clonal expansion. The M1UK lineage comprised 35% of all emm1 isolates.

Interpretation

The GAS population was characterized by a high diversity of strains, resembling patterns observed in low- and middle-income countries. However, strains associated with outbreaks and antimicrobial resistance commonly found in high-income countries were also observed. This unique combination poses challenges for vaccine development, disease management and control.

Funding

The work was supported by the Health Research Council of New Zealand (HRC), award number 16/005.

Different Group A Streptococcus pili lead to varying proinflammatory cytokine responses and virulence

The human pathogen Streptococcus pyogenes, or Group A Streptococcus (GAS), is associated with a variety of diseases ranging from mild skin and soft tissue infections to invasive diseases and immune sequelae such as rheumatic heart disease. We have recently reported that one of the virulence factors of this pathogen, the pilus, has inflammatory properties and strongly stimulates the innate immune system. Here we used a range of nonpathogenic Lactococcus lactis gain-of-function mutants, each expressing one of the major pilus types of GAS, to compare the immune responses generated by various types of fully assembled pili. In vitro assays indicated variability in the inflammatory response induced by different pili, with the fibronectin-binding, collagen-binding, T antigen (FCT)-1-type pilus from GAS serotype M6/T6 inducing significantly stronger cytokine secretion than other pili. Furthermore, we established that the same trend of pili-mediated immune response could be modeled in Galleria mellonella larvae, which possess a similar innate immune system to vertebrates. Counterintuitively, across the panel of pili types examined in this study, we observed a negative correlation between the intensity of the immune response demonstrated in our experiments and the disease severity observed clinically in the GAS strains associated with each pilus type. This observation suggests that pili-mediated inflammation is more likely to promote bacterial clearance instead of causing disruptive damages that intensify pathogenesis. This also indicates that pili may not be the main contributor to the inflammatory symptoms seen in GAS diseases. Rather, the immune-potentiating properties of the pilus components could potentially be exploited as a vaccine adjuvant.

Research opportunities for the primordial prevention of rheumatic fever and rheumatic heart disease-streptococcal vaccine development: a national heart, lung and blood institute workshop report

Streptococcus pyogenes, also known as group A streptococcus (StrepA), is a bacterium that causes a range of human diseases, including pharyngitis, impetigo, invasive infections, and post-infection immune sequelae such as rheumatic fever and rheumatic heart disease. StrepA infections cause some of the highest burden of disease and death in mostly young populations in low-resource settings. Despite decades of effort, there is still no licensed StrepA vaccine, which if developed, could be a cost-effective way to reduce the incidence of disease. Several challenges, including technical and regulatory hurdles, safety concerns and a lack of investment have hindered StrepA vaccine development. Barriers to developing a StrepA vaccine must be overcome in the future by prioritising key areas of research including greater understanding of StrepA immunobiology and autoimmunity risk, better animal models that mimic human disease, expanding the StrepA vaccine pipeline and supporting vaccine clinical trials. The development of a StrepA vaccine is a complex and challenging process that requires significant resources and investment. Given the global burden of StrepA infections and the potential for a vaccine to save lives and livelihoods, StrepA vaccine development is an area of research that deserves considerable support. This report summarises the findings of the Primordial Prevention Working Group-VAX, which was convened in November 2021 by the National Heart, Lung, and Blood Institute. The focus of this report is to identify research gaps within the current StrepA vaccine landscape and find opportunities and develop priorities to promote the rapid and successful advancement of StrepA vaccines.

Identification of an immunodominant region on a group A Streptococcus T-antigen reveals temperature-dependent motion in pili

Group A Streptococcus (GAS) is a globally important pathogen causing a broad range of human diseases. GAS pili are elongated proteins with a backbone comprised repeating T-antigen subunits, which extend from the cell surface and have important roles in adhesion and establishing infection. No GAS vaccines are currently available, but T-antigen-based candidates are in pre-clinical development. This study investigated antibody-T-antigen interactions to gain molecular insight into functional antibody responses to GAS pili. Large, chimeric mouse/human Fab-phage libraries generated from mice vaccinated with the complete T18.1 pilus were screened against recombinant T18.1, a representative two-domain T-antigen. Of the two Fab identified for further characterization, one (designated E3) was cross-reactive and also recognized T3.2 and T13, while the other (H3) was type-specific reacting with only T18.1/T18.2 within a T-antigen panel representative of the major GAS T-types. The epitopes for the two Fab, determined by x-ray crystallography and peptide tiling, overlapped and mapped to the N-terminal region of the T18.1 N-domain. This region is predicted to be buried in the polymerized pilus by the C-domain of the next T-antigen subunit. However, flow cytometry and opsonophagocytic assays showed that these epitopes were accessible in the polymerized pilus at 37°C, though not at lower temperature. This suggests that there is motion within the pilus at physiological temperature, with structural analysis of a covalently linked T18.1 dimer indicating "knee-joint" like bending occurs between T-antigen subunits to expose this immunodominant region. This temperature dependent, mechanistic flexing provides new insight into how antibodies interact with T-antigens during infection.

Streptococcus pyogenes: Pathogenesis and the Current Status of Vaccines

Streptococcus pyogenes (group A Streptococcus; GAS), a Gram-positive coccal bacterium, poses a significant global disease burden, especially in low- and middle-income countries. Its manifestations can range from pharyngitis and skin infection to severe and aggressive diseases, such as necrotizing fasciitis and streptococcal toxic shock syndrome. At present, although GAS is still sensitive to penicillin, there are cases of treatment failure for GAS pharyngitis, and antibiotic therapy does not universally prevent subsequent disease. In addition to strengthening global molecular epidemiological surveillance and monitoring of antibiotic resistance, developing a safe and effective licensed vaccine against GAS would be the most effective way to broadly address GAS-related diseases. Over the past decades, the development of GAS vaccines has been stalled, mainly because of the wide genetic heterogeneity of GAS and the diverse autoimmune responses to GAS. With outbreaks of scarlet fever in various countries in recent years, accelerating the development of a safe and effective vaccine remains a high priority. When developing a GAS vaccine, many factors need to be considered, including the selection of antigen epitopes, avoidance of self-response, and vaccine coverage. Given the challenges in GAS vaccine development, this review describes the important virulence factors that induce disease by GAS infection and how this has influenced the progression of vaccine development efforts, focusing on several candidate vaccines that are further along in development.

Pathogenesis, epidemiology and control of Group A Streptococcus infection

Streptococcus pyogenes (Group A Streptococcus; GAS) is exquisitely adapted to the human host, resulting in asymptomatic infection, pharyngitis, pyoderma, scarlet fever or invasive diseases, with potential for triggering post-infection immune sequelae. GAS deploys a range of virulence determinants to allow colonization, dissemination within the host and transmission, disrupting both innate and adaptive immune responses to infection. Fluctuating global GAS epidemiology is characterized by the emergence of new GAS clones, often associated with the acquisition of new virulence or antimicrobial determinants that are better adapted to the infection niche or averting host immunity. The recent identification of clinical GAS isolates with reduced penicillin sensitivity and increasing macrolide resistance threatens both frontline and penicillin-adjunctive antibiotic treatment. The World Health Organization (WHO) has developed a GAS research and technology road map and has outlined preferred vaccine characteristics, stimulating renewed interest in the development of safe and effective GAS vaccines.

Characterization of M-Type-Specific Pilus Expression in Group A Streptococcus

Our ability to characterize how a pathogen infects and causes disease, and consequently our ability to devise approaches to prevent or attenuate such infections, is inhibited by the finding that isolates of a given pathogen often show phenotypic variability, for example, in their ability to adhere to host cells through modulation of cell surface adhesins. Such variability is observed between isolates of group A Streptococcus (GAS), and this study investigates the molecular basis for why some GAS isolates produce pili, cell wall-anchored adhesins, in lower abundance than other isolates do.

Pilus proteins from Streptococcus pyogenes stimulate innate immune responses through Toll‐like receptor 2

The group A Streptococcus (GAS) pilus is a long, flexible, hair‐like structure anchored to the cell surface that facilitates the adherence of GAS to host cells, thus playing a critical role in initiating infections. Because of its important role in GAS virulence, the pilus has become an attractive target for vaccine development. While current research mainly focuses on pilus function and its potential as a vaccine component, there is a lack of knowledge on how the host immune system recognizes and responds to this abundant surface structure. Here we show that both assembled GAS pili and individual pilus proteins induce a potent release of the proinflammatory cytokines tumor necrosis factor and interleukin‐8. We further show that the surface‐exposed backbone pilin and ancillary pilin 1 subunits are Toll‐like receptor 2 (TLR2) agonists. Using reporter cell lines coexpressing human TLR2 in combination with either TLR1 or TLR6, we determined that activation was mediated by the TLR2/TLR6 heterodimer. Finally, we used solid‐phase and flow cytometry binding assays to illustrate a direct interaction between the pilus subunits and TLR2. These results provide further support for the suitability of the pilus as a vaccine component and opens potential avenues for using GAS pili as an adjuvant or immune‐modulation agent.

Streptococcal Pharyngitis and Rheumatic Fever

Streptococcus pyogenes (Group A Streptococcus) causes a variety of diseases, from benign self-limiting infections of the skin or throat to lethal infections of soft tissue accompanied by multi-organ failure. GAS is one of significant species among Gram-positive pathogens which is responsible for several suppurative infections and non-suppurative sequelae. They also cause pharyngitis, streptococcal toxic shock syndrome (STSS), necrotizing fasciitis and other diseases. Currently, global burden of RF / RHD is undervalued. In 2010, RF and RHD were estimated as 15.6 million cases and deaths around 200,000 annually. Laboratory diagnosis includes cultural techniques, serology, PYR test, Bacitracin susceptibility test and antibiotic resistance testing helps in differentiating the Streptococcus pyogenes from other groups of Streptococci. Most of the Acute Rheumatic Fever cases gets missed or does not present in the initial stage rather it has been developed into advanced Rheumatic Heart Disease condition. Modified Jones criteria in 2015 will be helpful especially to the low risk population as it is challenging because of limited access to primary health care, diagnosis of streptococcal disease. In addition to this revised criteria, diagnosis still relies on clinical diagnostic algorithm. Vaccines based on M protein and T antigens are continuing to evolve with different results. Ongoing vaccine development is still challenging for the GAS research community, it will make a positive and lasting impact on the peoples globally.

Progress in the Development of Structure-Based Vaccines

The immune response elicited by vaccines against microorganisms makes it the most successful medical interventions against infectious diseases. Conventional vaccines have limitations in inducing immunity against many types of pathogenic microorganism. The genetic diversity of microorganisms, coupled with the high degree of sequence variability in antigenic proteins, presents a challenge to developing broadly effective conventional vaccines. Atomic-resolution structure determination is crucial for understanding antigenic protein function. Cryo-electron microscopy, nuclear magnetic resonance spectroscopy coupled with bioinformatics provide three-dimensional structure of the antigenic proteins and provide a wealth of information about the organization of individual atoms and their chemical makeup. The atomic detail information of proteins offers enormous potential to rationally engineer proteins to enhance their properties and act as effective immunogens to induce immunity. The observation that whole protein antigens are not necessarily essential for inducing immunity has led to the emergence "structural vaccinology." Structure-based vaccines are designed on the rationale that protective epitopes should be sufficient to induce immune responses and provide protection against pathogens. In 2013 we published a review on structure-based vaccines (Thomas and Luxon. Expert Rev Vaccines 12 1301-11, 2013). This review states the progress in development of structure-based vaccines since the first review.