Correlates of immunity to Group A Streptococcus: a pathway to vaccine development

Recent approaches in computational modelling for controlling pathogen threats

In this review, we assess the status of computational modelling of pathogens. We focus on three disparate but interlinked research areas that produce models with very different spatial and temporal scope. First, we examine antimicrobial resistance (AMR). Many mechanisms of AMR are not well understood. As a result, it is hard to measure the current incidence of AMR, predict the future incidence, and design strategies to preserve existing antibiotic effectiveness. Next, we look at how to choose the finite number of bacterial strains that can be included in a vaccine. To do this, we need to understand what happens to vaccine and non-vaccine strains after vaccination programmes. Finally, we look at within-host modelling of antibody dynamics. The SARS-CoV-2 pandemic produced huge amounts of antibody data, prompting improvements in this area of modelling. We finish by discussing the challenges that persist in understanding these complex biological systems.

Invasive Group A Streptococcal Infections in Pediatric Critical Care: A Retrospective Cohort Study

BACKGROUND AND OBJECTIVE

The reported rising global rates of invasive group A Streptococcus (iGAS) infection raise concern for disease related increase in critical illness and fatalities. An enhanced understanding of various presentations to health care and clinical course could improve early recognition and therapy in children with iGAS. The objective of this study was to describe the epidemiology of iGAS infections among children admitted to critical care.

METHODS

A retrospective cohort study of children admitted to the PICU at The Hospital for Sick Children, in Toronto, Canada, between March 2022 and June 2023. Eligible patients were 0 to 18 years, with a diagnosis of iGAS infection. We describe the proportion of children admitted to the PICU with iGAS over the study period, their clinical characteristics, the frequency and timing of therapies, discharge versus baseline function, and PICU mortality.

RESULTS

Among the 1820 children admitted to the PICU, 29 (1.6%) patients had iGAS infection. Of these 29 patients, 80% (n = 23) survived to hospital discharge. Patients who survived generally had favorable functional outcomes. Despite the high severity of illness and mortality described in this cohort, 61% returned to their baseline functional status by hospital discharge.

CONCLUSIONS

This is the first report of critically ill children with iGAS in Canada during the increased incidence reported worldwide. We describe the clinical course of iGAS infection in children admitted to PICU with access to advanced extracorporeal interventions. Though there is a high mortality rate in this cohort, those who survive have favorable outcomes.

Serological Responses to Streptococcus pyogenes Vaccine Candidate Antigens Suggests That Streptococcus dysgalactiae Is the Predominant Cause of Lower Limb Cellulitis

Background

A future Streptococcus pyogenes (Strep A) vaccine will ideally prevent a significant burden of lower limb cellulitis; however, natural immune responses to proposed vaccine antigens following an episode of cellulitis remain uncharacterized.

Methods

We enrolled 63 patients with cellulitis and 26 with invasive beta hemolytic streptococci infection, using a multiplexed assay to measure immunoglobulin G against Strep A vaccine candidate antigens, including: streptolysin O (SLO), deoxyribonuclease B (DNB), group A carbohydrate (GAC), C5a peptidase (ScpA), cell envelope proteinase (SpyCEP), and adhesion and division protein (SpyAD). Responses in the invasive cohort were used to predict the infecting etiology in the cellulitis cohort.

Results

Of 41 patients with cellulitis and paired serological samples, 68.3% had evidence of beta hemolytic streptococci infection by conventional anti-SLO and/or anti-DNB criteria. A positive serological response to at least 1 of the tested antigens was seen in 78.0% of the cellulitis cohort. Individually, anti-SLO (58.5%), anti-SpyAD (46.3%), and anti-ScpA (39.0%) were the most common. Based on principal component analysis, increases in these 3 antibodies, without responses to DNB, GAC, and SpyCEP characterized Streptococcus dysgalactiae subspecies equisimilis (SDSE) infection.

Conclusions

SDSE appears to be the predominant cause of lower limb cellulitis. Effective Strep A vaccines incorporating antigens that provide additional cross protection against SDSE may prevent a significant burden of lower limb cellulitis.

Strep A: challenges, opportunities, vaccine-based solutions, and economics

This collection of articles focuses on Streptococcus pyogenes (Strep A) vaccine research and innovation, with a focus on emerging efforts to understand and estimate the full societal value of Strep A vaccination.

Disruption of IL-17-mediated immunosurveillance in the respiratory mucosa results in invasive Streptococcus pyogenes infection

Introduction

Streptococcus pyogenes is a Gram-positive pathogen that causes a significant global burden of skin pyoderma and pharyngitis. In some cases, infection can lead to severe invasive streptococcal diseases. Previous studies have shown that IL-17 deficiency in mice (IL-17-/-) can reduce S. pyogenes clearance from the mucosal surfaces. However, the effect of IL-17 on the development of severe invasive streptococcal disease has not yet been assessed.

Methods

Here, we modeled single or repeated non-lethal intranasal (IN) S. pyogenes M1 strain infections in immunocompetent and IL-17-/- mice to assess bacterial colonization following a final IN or skin challenge.

Results

Immunocompetent mice that received a single S. pyogenes infection showed long-lasting immunity to subsequent IN infection, and no bacteria were detected in the lymph nodes or spleens. However, in the absence of IL-17, a single IN infection resulted in dissemination of S. pyogenes to the lymphoid organs, which was accentuated by repeated IN infections. In contrast to what was observed in the respiratory mucosa, skin immunity did not correlate with the systemic levels of IL-17. Instead, it was found to be associated with the activation of germinal center responses and accumulation of neutrophils in the spleen.

Discussion

Our results demonstrated that IL-17 plays a critical role in preventing invasive disease following S. pyogenes infection of the respiratory tract.

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.

The full health, economic, and social benefits of prospective Strep A vaccination

Recent research has documented a wide range of health, economic, and social benefits conferred by vaccination, beyond the direct reductions in morbidity, mortality, and future healthcare costs traditionally captured in economic evaluations. In this paper, we describe the societal benefits that would likely stem from widespread administration of safe and effective vaccines against Streptococcus pyogenes (Strep A), which was estimated to be the fifth-leading cause of infectious disease deaths globally prior to the COVID-19 pandemic. We then estimate the global societal gains from prospective Strep A vaccination through a value-per-statistical-life approach. Estimated aggregate lifetime benefits for 30 global birth cohorts range from $1.7 to $5.1 trillion, depending on the age at which vaccination is administered and other factors. These results suggest that the benefits of Strep A vaccination would be large and justify substantial investment in the vaccines' development, manufacture, and delivery.

Naturally acquired functional antibody responses to group A Streptococcus differ between major strain types

Group A Streptococcus (GAS) is a leading human pathogen for which there is no licensed vaccine. Infections are most common in young children and the elderly suggesting immunity accumulates with exposure until immune senescence in older age. Though protection has been postulated to be strain type specific, based on the M-protein (emm-type), the antigenic basis of population-level immunity remains poorly understood. Naturally acquired GAS antibody responses were investigated using intravenous immunoglobulin (IVIG), which contains pooled immunoglobulins from thousands of healthy human donors, as a surrogate for population immunity. Functional opsonophagocytic killing assays were conducted with GAS strains (n = 6) representing the three major emm-pattern types (emm12, A-C pattern; emm53, D-pattern; and emm75, E-pattern). While IVIG induced opsonophagocytic killing of all GAS strains tested, specificity assays showed the profile of protective antibodies differed considerably between emm-types. Antibodies targeting the M-protein were a major component of the functional IVIG antibody response for emm12 and emm53 strains but not for emm75 strains. The striking differences in the contribution of M-protein specific antibodies to killing suggest naturally acquired immunity differs between strains from the major emm-patterns. This challenges the dogma that M-protein is the primary protective antigen across all GAS straintypes. IMPORTANCE Group A Streptococcus (GAS) is a globally important pathogen. With the surge of invasive GAS infections that have occurred in multiple countries, contemporaneous with the relaxation of COVID-19 pandemic restrictions, there is increased interest in the mechanisms underpinning GAS immunity. We utilized intravenous immunoglobulin (IVIG), pooled immunoglobulins from thousands of healthy donors, as a surrogate for population-level immunity to GAS, and explored the contribution of strain-specific (M-type specific) antibodies to GAS immunity using functional killing assays. This revealed striking differences between major strain types as to the contribution of strain specific antibodies to killing. For GAS strains belonging to the E pattern group, M-type specific antibodies do not mediate killing and immunity, which contrasts with strains belonging to pattern A-C and D groups. This challenges the historical dogma, originally proposed by Rebecca Lancefield in the 1950-1960s, that the M-protein is the major protective antigen across all GAS strain types.

Challenges in development of vaccines directed toward antimicrobial resistant bacterial species

Antimicrobial resistance (AMR) is considered by WHO one of the top ten public health threats. New control strategies involving concerted actions of both public and private sectors need to be developed. Vaccines play a major role in controlling the spread of AMR pathogens by decreasing transmission and limiting the use of antibiotics, reducing at the end the selective pressure for the emergence of new resistant strains. In this review, by using as example some of the most serious AMR pathogens, we highlighted the major hurdles from a research and development point of view. New approaches to better understand the immunological mechanisms of response to both natural infections and vaccines that aimed to identify correlates of protection, together with the application of new technologies for vaccine design and delivery are discussed as potential solutions.

Polymeric epitope-based vaccine induces protective immunity against group A Streptococcus

Group A Streptococcus (Strep A) is a life-threatening human pathogen with no licensed vaccine. Here, we used a biopolymer particle (BP) approach to display repeats of Strep A vaccine candidate peptides p*17 and K4S2 derived from M and non-M protein, respectively. BPs densely displaying both peptides (BP-p*17-S2) were successfully assembled in one-step inside an engineered endotoxin-free Escherichia coli strain. Purified BP-p*17-S2 showed a spherical core-shell morphology with a biopolymer core and peptide shell. Upon formulation with aluminum hydroxide as adjuvant, BP-p*17-S2 exhibited a mean diameter of 2.9 µm and a positive surface charge of 22 mV. No cytotoxicity was detected when tested against HEK-293 cells. Stability studies showed that BP-p*17-S2 is ambient-temperature stable. Immunized mice showed no adverse reactions, while producing high titers of peptide specific antibodies and cytokines. This immune response could be correlated with protective immunity in an animal model of infection, i.e. intranasal challenge of mice with Strep A, where a significant reduction of >100-fold of pathogen burden in nose-associated lymphoid tissue, lung, and spleen was obtained. The cost-effective scalable manufacture of ambient-temperature stable BPs coated with Strep A peptides combined with their immunogenic properties offer an attractive alternative strategy to current Strep A vaccine development.

Global economic burden per episode for multiple diseases caused by group A Streptococcus

Considering the lack of existing evidence on economic burden for diseases caused by group A Streptococcus, we estimated the economic burden per episode for selected diseases. Each cost component of direct medical costs (DMCs), direct non-medical costs (DNMCs), and indirect costs (ICs) was separately extrapolated and aggregated to estimate the economic burden per episode by income group as classified by the World Bank. Adjustment factors for DMC and DNMC were generated to overcome related data insufficiencies. To address uncertainty surrounding input parameters, a probabilistic multivariate sensitivity was carried out. The average economic burden per episode ranged from $22 to $392 for pharyngitis, $25 to $2,903 for impetigo, $47 to $2,725 for cellulitis, $662 to $34,330 for invasive and toxin-mediated infections, $231 to $6,332 for acute rheumatic fever (ARF), $449 to $11,717 for rheumatic heart disease (RHD), and $949 to $39,560 for severe RHD across income groups. The economic burden for multiple Group A Streptococcus diseases underscores an urgent need to develop effective prevention strategies including vaccines.

Serum Immune Responses to Group A Streptococcal Antigens following Pharyngeal Acquisitions among Children in Cape Town, South Africa

There is limited information on the human immune response following infection with group A Streptococcus (Strep A). Animal studies have shown, in addition to the M protein, that shared Strep A antigens elicit protective immunity. This study aimed to investigate the kinetics of antibody responses against a panel of Strep A antigens in a cohort of school-aged children in Cape Town, South Africa. Participants provided serial throat cultures and serum samples at two-monthly follow-up visits. Strep A recovered were emm-typed, and serum samples were analyzed by enzyme-linked immunosorbent assay (ELISA) to assess immune responses to thirty-five Strep A antigens (10-shared and 25-M peptides). Serologic evaluations were performed on serial serum samples from 42 selected participants (from 256 enrolled) based on the number of follow-up visits, the frequency of visits, and throat culture results. Among these, there were 44 Strep A acquisitions, 36 of which were successfully emm-typed. Participants were grouped into three clinical event groups based on culture results and immune responses. A preceding infection was most convincingly represented by a Strep A-positive culture with an immune response to at least one shared antigen and M peptide (11 events) or a Strep A-negative culture with antibody responses to shared antigens and M peptides (9 events). More than a third of participants demonstrated no immune response despite a positive culture. This study provided important information regarding the complexity and variability of human immune responses following pharyngeal acquisition of Strep A, as well as demonstrating the immunogenicity of Strep A antigens currently under consideration as potential vaccine candidates. IMPORTANCE There is currently limited information regarding the human immune response to group A streptococcal throat infection. An understanding of the kinetics and specificity of antibody responses against a panel of Group A Streptococcus (GAS) antigens will serve to refine diagnostic approaches and contribute to vaccine efforts, which together will serve to reduce the burden of rheumatic heart disease, a major source of morbidity and mortality especially in the developing world. This study, utilizing an antibody-specific assay, uncovered three patterns of response profiles following GAS infection, among 256 children presenting with sore throat to local clinics. Overall, the response profiles were complex and variable. Of note, a preceding infection was most convincingly represented by a GAS-positive culture with an immune response to at least one shared antigen and M peptide. Also, more than a third of participants demonstrated no immune response despite a positive culture. All antigens tested were immunogenic, providing guidance for future vaccine development.

The Streptococcus pyogenes vaccine landscape

Recent efforts have re-invigorated the Streptococcus pyogenes (Group A Streptococcus) vaccine development field, though scientific, regulatory and commercial barriers persist, and the vaccine pipeline remains sparse. There is an ongoing need to accelerate all aspects of development to address the large global burden of disease caused by the pathogen. Building on over 100 years of S. pyogenes vaccine development, there are currently eight candidates on a product development track, including four M protein-based candidates and four candidates designed around non-M protein antigens. These candidates have demonstrated proof of concept for protection against S. pyogenes in preclinical models, one has demonstrated safety and immunogenicity in a Phase 1 trial and at least four others are poised to soon enter clinical trials. To maintain momentum, the Strep A Vaccine Global Consortium (SAVAC) was established to bring together experts to accelerate global S. pyogenes vaccine development. This article highlights the past, present and future of S. pyogenes vaccine development and emphasizes key priorities, and the role of SAVAC, in advancing the field.