Non-clinical immunological comparison of a Next-Generation 24-valent pneumococcal conjugate vaccine (VAX-24) using site-specific carrier protein conjugation to the current standard of care (PCV13 and PPV23)

Streptococcus pneumoniae serotype distribution in low- and middle-income countries of South Asia: Do we need to revisit the pneumococcal vaccine strategy?

S. pneumoniae serotypes responsible for pneumococcal disease differ with respect to disease severity, invasiveness, antimicrobial susceptibility, geographies, immunization history, age groups, and with time. Although PCVs have blunted the pneumococcal disease burden, they are plagued with numerous challenges, especially the emergence of NVTs. In this review, we show that there are diverse serotypes, especially NVTs, responsible for causing pneumococcal diseases in LMICs of South Asia across different studies conducted between 2012 and 2024. We propose that pharmaceutical/biotech companies should tailor/customize the PCVs as per the region-specific serotype prevalence based on surveillance data. Furthermore, protein-based vaccines, or WCVs, have been explored and can serve as viable alternatives to address the limitations associated with PCVs. However, robust studies are warranted in different geographies to demonstrate its efficacy and safety in clinical trials as well as the real-world effectiveness of these promising candidates.

Distribution of pneumococcal serotypes causing invasive and non-invasive diseases in children in Mexico after introduction of PCV13 (2012-2023). Results from the GIVEBPVac group

BACKGROUND

The introduction of pneumococcal conjugate vaccines (PCVs) since 2000 has altered the epidemiology of invasive (IPD) and non-invasive pneumococcal diseases (NIPD). This study aims to analyze trends in pneumococcal serotype distribution among children in Mexico, focusing on the period following the introduction of PCV13, and assess the potential impact of future vaccines.

MATERIAL AND METHODS

Pneumococcal isolates were collected from hospitalized children in participating hospitals from January 2012 to December 2023. Serotype distribution was analyzed in children under <5 years and those aged ≥5 to 17.9 years. The average annual change (AAC) in serotype proportions was calculated, and trends were analyzed using the Cochran-Armitage test.

RESULTS

Serotype 19A was the most frequent PCV13 serotype, followed by serotypes 3 and 19F, in both age groups. Serotype 33F, included in PCV15 and PCV20, was absent in children aged ≥5-17.9 years. Among PCV20 serotypes, serotype 15B was the most common, and serotype 17F, covered by PCV24, showed a significant increase in the older age group (p = 0.037). No significant trends in the increase or decrease of individual serotypes were found, except for serotypes 17F and serotype 34, which increased in both age groups. A decrease in serotypes covered by PCV13 (excluding serotypes 3, 19A, and 19F) was observed in both age groups (p = 0.04, 0.002). A significant increase in non-PCV13 serotypes occurred in children aged ≥5-17.9 years (p = 0.023).

CONCLUSIONS

After a decade of the introduction of PCV13 in Mexico, 10 of the 13 vaccine serotypes have not been detected in the past six years. However, serotypes 3, 19A, and 19F persist at high frequencies as causes of IPD and NIPD in children. Ongoing robust surveillance is critical for identifying emerging pneumococcal serotypes, selecting appropriate vaccines for each country, and developing next-generation vaccine formulations.

Cost-Effectiveness and Public Health Impact of 24-Valent Pneumococcal Conjugate Vaccine Compared With the Recommended Pneumococcal Vaccines in Older Adults

INTRODUCTION

The potential impact of an in-development 24-valent pneumococcal conjugate vaccine compared with that of currently recommended vaccines in older adults is unclear. Similar to most currently available pneumococcal conjugate vaccines, 24-valent pneumococcal conjugate vaccine's formulation is based on childhood pneumococcal disease epidemiology. Decision analysis techniques were used to estimate 24-valent pneumococcal conjugate vaccine cost-effectiveness and public health effects in U.S. older adults.

METHODS

A Markov model compared 24-valent pneumococcal conjugate vaccine with currently recommended U.S. pneumococcal vaccination strategies in older adults (aged ≥65 years) and with no vaccination. Age-, race-, and chronic medical condition-specific pneumococcal illness risks and serotype-specific disease risks were obtained from Centers for Disease Control and Prevention data. Vaccine effectiveness was estimated using Delphi panel and clinical trial data. Vaccination and pneumococcal illness costs were from U.S.

DATABASES

Scenario analyses examined indirect effects of childhood pneumococcal vaccination on adult disease. Data were collected and analyses were performed in 2024.

RESULTS

The 24-valent pneumococcal conjugate vaccine prevented fewer pneumococcal disease cases and deaths than the recently recommended 21-valent pneumococcal conjugate vaccine, which is formulated on the basis of adult pneumococcal disease serotypes. In cost-effectiveness analyses, 21-valent pneumococcal conjugate vaccine was economically favorable compared with 24-valent pneumococcal conjugate vaccine and all other vaccination strategies, both without and with consideration of potential childhood vaccination indirect effects. These findings were robust and consistent with those in deterministic and probabilistic sensitivity analyses.

CONCLUSIONS

In older adults, 24-valent pneumococcal conjugate vaccine was clinically and economically unfavorable compared with 21-valent pneumococcal conjugate vaccine, which covers more adult disease-causing pneumococcal serotypes and is less susceptible to childhood vaccination indirect effects.

Bioengineering lipid-based synthetic cells for therapeutic protein delivery

Synthetic cells (SCs) offer a promising approach for therapeutic protein delivery, combining principles from synthetic biology and drug delivery. Engineered to mimic natural cells, SCs provide biocompatibility and versatility, with precise control over their architecture and composition. Protein production is essential in living cells, and SCs aim to replicate this process using compartmentalized cell-free protein synthesis systems within lipid bilayers. Lipid bilayers serve as favored membranes in SC design due to their similarity to the biological cell membrane. Moreover, engineering lipidic membranes enable tissue-specific targeting and immune evasion, while stimulus-responsive SCs allow for triggered protein production and release. This Review explores lipid-based SCs as platforms for therapeutic protein delivery, discussing their design principles, functional attributes, and translational challenges and potential.

Health-economic burden attributable to novel serotypes in candidate 24- and 31-valent pneumococcal conjugate vaccines

INTRODUCTION

Next-generation pneumococcal vaccines currently in clinical trials include 24- and 31-valent pneumococcal conjugate vaccines (PCV24, PCV31), which aim to prevent upper-respiratory carriage and disease involving the targeted serotypes. We aimed to estimate the comprehensive health-economic burden associated with acute respiratory infections (ARIs) and invasive pneumococcal disease (IPD) attributable to PCV24- and PCV31-additional (non-PCV20) serotypes in the United States.

MATERIAL AND METHODS

We multiplied all-cause incidence rate estimates for acute otitis media (AOM), sinusitis, and non-bacteremic pneumonia by estimates of the proportions of each of these conditions attributable to pneumococci and the proportions of pneumococcal infections involving PCV24- and PCV31-additional serotypes. We estimated serotype-specific IPD incidence rates using US Active Bacterial Core surveillance data. We accounted for direct medical and non-medical costs associated with each condition to estimate resulting health-economic burden. Non-medical costs included missed work and lost quality-adjusted life years due to death and disability.

RESULTS

The health-economic burden of PCV24-additional serotypes totaled $1.3 ($1.1-1.7) billion annually in medical and non-medical costs, comprised of $0.9 ($0.7-1.2) billion due to ARIs and $0.4 ($0.3-0.5) billion due to IPD. For PCV31-additional serotypes, medical and non-medical costs totaled $7.5 ($6.6-8.6) billion annually, with $5.5 ($4.7-6.6) billion due to ARIs and $1.9 ($1.8-2.1) billion due to IPD. The largest single driver of costs was non-bacteremic pneumonia, particularly in adults aged 50-64 and ≥65 years.

CONCLUSIONS

Additional serotypes in PCV24 and PCV31, especially those included in PCV31, account for substantial health-economic burden in the United States.

Development of cell-free platforms for discovering, characterizing, and engineering post-translational modifications

Post-translational modifications (PTMs) are important for the stability and function of many therapeutic proteins and peptides. Current methods for studying and engineering PTM installing proteins often suffer from low-throughput experimental techniques. Here we describe a generalizable, in vitro workflow coupling cell-free protein synthesis (CFPS) with AlphaLISA for the rapid expression and testing of PTM installing proteins. We apply our workflow to two representative classes of peptide and protein therapeutics: ribosomally synthesized and post-translationally modified peptides (RiPPs) and conjugate vaccines. First, we demonstrate how our workflow can be used to characterize the binding activity of RiPP recognition elements, an important first step in RiPP biosynthesis, and be integrated into a biodiscovery pipeline for computationally predicted RiPP products. Then, we adapt our workflow to study and engineer oligosaccharyltransferases (OSTs) involved in conjugate vaccine production, enabling the identification of mutant OSTs and sites within a carrier protein that enable high efficiency production of conjugate vaccines. In total, we expect that our workflow will accelerate design-build-test cycles for engineering PTMs.

Analysis of immunogenicity and purification methods in conjugated polysaccharide vaccines: a new approach in fighting pathogenic bacteria

Carbohydrates are commonly found in conjunction with lipids or proteins, resulting in the formation of glycoconjugates such as glycoproteins, glycolipids, and proteoglycans. These glycoconjugates are essential in various biological activities, including inflammation, cell-cell recognition, bacterial infections, and immune response. Nonetheless, the isolation of naturally occurring glycoconjugates presents challenges due to their typically heterogeneous nature, resulting in variations between batches in structure and function, impeding a comprehensive understanding of their mechanisms of action. Consequently, there is a strong need for the efficient synthesis of artificial glycoconjugates with precisely described compositions and consistent biological properties. The chemical and enzymatic approaches discussed in this paper present numerous research opportunities to develop customised glycoconjugate vaccines.

Non-invasive Streptococcus pneumoniae infections are associated with different serotypes than invasive infections, Belgium, 2020 to 2023

BackgroundDespite widely implemented pneumococcal vaccination programmes, Streptococcus pneumoniae remains a global risk for human health. Streptococcus pneumoniae can cause invasive (IPD) or non-invasive pneumococcal disease (NIPD). Surveillance is mainly focusing on IPD, assessing the full impact of pneumococcal vaccination programmes on pneumococcal disease is challenging.AimWe aimed to prospectively investigate serotype distribution and antimicrobial resistance (AMR) of S. pneumoniae isolates from patients with NIPD and compare with data on IPD isolates and with a 2007-2008 dataset on NIPD.MethodsBetween September 2020 and April 2023, we collected isolates and patient data from patients with NIPD from 23 clinical laboratories in Belgium. Capsular typing was performed by a validated Fourier-Transform Infrared spectroscopic method, and AMR was assessed with broth microdilution, using the European Committee on Antimicrobial Susceptibility Testing (EUCAST) clinical breakpoints.ResultsWe received S. pneumoniae isolates from 1,008 patients with lower respiratory tract infections (n = 760), otitis media (n = 190) and sinusitis (n = 58). Serotype 3 was the most prevalent serotype among the NIPD isolates. Serotypes not included in the 20-valent pneumococcal conjugate vaccine (PCV20) were significantly more common among the NIPD than among the IPD isolates. Antimicrobial resistance levels were significantly higher among the NIPD isolates (n = 539; 2020-2022) compared with the IPD isolates (n = 2,344; 2021-2022). Resistance to several β-lactam antimicrobials had increased significantly compared with 15 years before.ConclusionsThe NIPD isolates were strongly associated with non-vaccine serotypes and with increased AMR levels. This underlines the importance of continued NIPD surveillance for informed policy making on vaccination programmes.

CT584 Is Not a Protective Vaccine Antigen against Respiratory Chlamydial Challenge in Mice

Background:Chlamydia trachomatis is the most prevalent bacterial sexually transmitted pathogen in humans worldwide. Since chlamydial infection is largely asymptomatic with the potential for serious complications, a preventative vaccine is likely the most viable long-term answer to this public health threat. Cell-free protein synthesis (CFPS) utilizes the cellular protein manufacturing machinery decoupled from the requirement for maintaining cellular viability, offering the potential for flexible, rapid, and decentralized production of recombinant protein vaccine antigens. Methods: Here, we use CFPS to produce the full-length putative chlamydial type three secretion system (T3SS) needle-tip protein, CT584, for evaluation as a vaccine antigen in mouse models. High-speed atomic force microscopy (HS-AFM) (RIBM, Tsukuba, Japan) imaging and computer simulations confirm that CFPS-produced CT584 retains a native-like structure prior to immunization. Female mice were primed with CT584 adjuvanted with CpG-1826 intranasally (i.n.) or CpG-1826 + Montanide ISA 720 intramuscularly (i.m.), followed four weeks later by an i.m. boost before respiratory challenge with 104 inclusion forming units (IFU) of Chlamydia muridarum. Results: Immunization with CT584 generated robust antibody responses but weak cell-mediated immunity and failed to protect against i.n. challenge as demonstrated by body weight loss, increased lung weights, and the presence of high numbers of IFUs in the lungs. Conclusion: While CT584 was not a protective vaccine candidate, the speed and flexibility with which CFPS can be used to produce other potential chlamydial antigens make it an attractive technique for antigen production.

Vaccine process technology-A decade of progress

In the past decade, new approaches to the discovery and development of vaccines have transformed the field. Advances during the COVID-19 pandemic allowed the production of billions of vaccine doses per year using novel platforms such as messenger RNA and viral vectors. Improvements in the analytical toolbox, equipment, and bioprocess technology have made it possible to achieve both unprecedented speed in vaccine development and scale of vaccine manufacturing. Macromolecular structure-function characterization technologies, combined with improved modeling and data analysis, enable quantitative evaluation of vaccine formulations at single-particle resolution and guided design of vaccine drug substances and drug products. These advances play a major role in precise assessment of critical quality attributes of vaccines delivered by newer platforms. Innovations in label-free and immunoassay technologies aid in the characterization of antigenic sites and the development of robust in vitro potency assays. These methods, along with molecular techniques such as next-generation sequencing, will accelerate characterization and release of vaccines delivered by all platforms. Process analytical technologies for real-time monitoring and optimization of process steps enable the implementation of quality-by-design principles and faster release of vaccine products. In the next decade, the field of vaccine discovery and development will continue to advance, bringing together new technologies, methods, and platforms to improve human health.

Current trends in development and manufacturing of higher-valent pneumococcal polysaccharide conjugate vaccine and its challenges

Pneumococcal conjugate vaccines (PCVs) have been developed to protect against pneumococcal diseases caused by the more than 100 serotypes of the bacterium Streptococcus pneumoniae. PCVs primarily prevent pneumococcal infections such as sepsis, bacteraemia, meningitis, otitis media, pneumonia, septicaemia, and sinusitis among infants, adults, elderly, and immunocompromised individuals. The current available PCVs only cover a limited number of serotypes, and there is an immense need for developing higher-valent PCVs that can protect against non-vaccine serotypes to overcome challenges like serotype replacement and antibiotic resistance. The main challenges for developing higher valent PCVs are the complexity of the manufacturing process comprising polysaccharide fermentation, purification, modification or sizing of multiple polysaccharides and conjugation between polysaccharides and carrier proteins, the stability of the conjugates, and the immunogenicity of the vaccine. Different manufacturing processes have been explored to produce higher valent PCVs using different serotypes of S. pneumoniae and conjugation with different carrier proteins. The global coverage of higher valent PCVs are still low, mainly due to the high cost and limited supply of the vaccine. This review focuses on the existing and emerging manufacturing processes and challenges associated with higher-valent pneumococcal PCV development.

Evaluation in mice of cell-free produced CT584 as a Chlamydia vaccine antigen

Chlamydia trachomatis is the most prevalent bacterial sexually transmitted pathogen worldwide. Since chlamydial infection is largely asymptomatic with the potential for serious complications, a preventative vaccine is likely the most viable long-term answer to this public health threat. Cell-free protein synthesis (CFPS) utilizes the cellular protein manufacturing machinery decoupled from the requirement for maintaining cellular viability, offering the potential for flexible, rapid, and de-centralized production of recombinant protein vaccine antigens. Here, we use CFPS to produce the putative chlamydial type three secretion system (T3SS) needle-tip protein, CT584, for use as a vaccine antigen in mouse models. High-speed atomic force microscopy (HS-AFM) imaging and computer simulations confirm that CFPS-produced CT584 retains a native-like structure prior to immunization. Female mice were primed with CT584 adjuvanted with CpG-1826 intranasally (i.n.) or CpG-1826 + Montanide ISA 720 intramuscularly (i.m.), followed four-weeks later by an i.m. boost before respiratory challenge with 104 inclusion forming units (IFU) of Chlamydia muridarum. Immunization with CT584 generated robust antibody responses but weak cell mediated immunity and failed to protect against i.n. challenge as demonstrated by body weight loss, increased lungs' weights and the presence of high numbers of IFUs in the lungs. While CT584 alone may not be the ideal vaccine candidate, the speed and flexibility with which CFPS can be used to produce other potential chlamydial antigens makes it an attractive technique for antigen production.

Immunogenicity of Current and Next-Generation Pneumococcal Conjugate Vaccines in Children: Current Challenges and Upcoming Opportunities

Global use of pneumococcal conjugate vaccines (PCVs) with increasingly broader serotype coverage has helped to reduce the burden of pneumococcal disease in children and adults. In clinical studies comparing PCVs, higher-valency PCVs have met noninferiority criteria (based on immunoglobulin G geometric mean concentrations and response rates) for most shared serotypes. A numeric trend of declining immunogenicity against shared serotypes with higher-valency PCVs has also been observed; however, the clinical relevance is uncertain, warranting additional research to evaluate the effectiveness of new vaccines. Novel conjugation processes, carriers, adjuvants, and vaccine platforms are approaches that could help maintain or improve immunogenicity and subsequent vaccine effectiveness while achieving broader protection with increasing valency in pneumococcal vaccines.

Evaluating the safety, tolerability, and immunogenicity of a 24-valent pneumococcal conjugate vaccine (VAX-24) in healthy adults aged 18 to 64 years: a phase 1/2, double-masked, dose-finding, active-controlled, randomised clinical trial

BACKGROUND

Despite substantial reductions in pneumococcal disease with the availability of pneumococcal conjugate vaccines, a significant burden of pneumococcal disease remains due to the diversity of serotypes combined with serotype replacement. We developed a new vaccine candidate, VAX-24 (24-valent pneumococcal conjugate vaccine), using cell-free protein synthesis to produce a variant of cross-reactive material 197 (eCRM) as the carrier protein, increasing serotype coverage while minimising carrier suppression. The aim of this clinical trial was to assess the safety, tolerability, and immunogenicity of three different doses of VAX-24 compared to pneumococcal 20-valent conjugate vaccine (PCV20).

METHODS

This was a phase 1/2, randomised, double-masked study of VAX-24 versus PCV20 conducted in the USA. Key inclusion criteria included being a male or female aged 18 to 64 years in good health; key exclusion criteria included previous history of pneumococcal disease, receipt of a licensed or investigational pneumococcal vaccine, or immunosuppressive therapy. Participants were randomly allocated in a 1:1:1:1 ratio by permuted block to receive one dose of VAX-24 (1·1 μg of each antigen, 2·2 μg of each antigen, or 2·2 μg of 17 antigens mixed with 4·4 μg of seven antigens), or PCV20. The safety population included all participants with safety data. The immunogenicity population was as per-treatment in phase 2. Primary outcome measures included solicited and unsolicited adverse events. Secondary outcomes included serotype-specific opsonophagocytic activity (OPA) geometric mean titres (GMT), and IgG geometric mean concentrations (GMC) were measured 1 month postvaccination. Traditional non-inferiority criteria included OPA geometric mean ratio (GMR), with a lower bound of the two sided 95% CI of greater than 0·5 for shared serotypes. This completed trial is registered at ClinicalTrials.gov, NCT05266456.

FINDINGS

Safety profiles were comparable among the treatment groups, with 170 of 209 participants (81%, 95% CI 75·2-86·2) to 178 of 207 participants (86%, 80·5-90·4) reporting at least one solicited adverse event among the three VAX-24 groups. 24 of 207 participants (12%, 7·6-16·8) to 32 of 209 of participants (15%, 10·7-20·9) experiened an unsolicited treatment emergent adverse event within 1 month postvaccination. VAX-24 2·2 μg met traditional OPA GMR non-inferiority criteria for all 20 shared serotypes; 16 serotypes elicited GMR point estimates greater than 1·0, and four reached the lower bound of the two-sided 95% CI greater than 1·0.

INTERPRETATION

VAX-24 had a safety profile similar to PCV20 at all doses, with the 2·2 μg dose showing increased serotype coverage with decreased carrier suppression.

FUNDING

Vaxcyte.

[After the COVID-19 pandemic-Which new vaccinations for adults are available or coming soon?]

The accumulation of respiratory infections in the winter months repeatedly highlights the relevance of prevention through vaccination, even beyond a pandemic. Current developments in this field are therefore highly relevant, particularly for older people who are more susceptible to infections due to immune senescence and comorbidities. The Standing Committee on Vaccination (STIKO) has responded accordingly by recommending the 20-valent pneumococcal conjugate vaccine PCV20 for standard and indication vaccination of adults. Furthermore, new vaccines against respiratory syncytial virus (RSV) infections are available for which the STIKO has not yet issued a recommendation. The development of other more effective and more immunogenic vac2cines is being driven in particular by new technologies, such as mRNA or vector vaccines. Various higher valent pneumococcal vaccine candidates and, for example, universal influenza vaccines are also already in development.

Glycovaccinology: The design and engineering of carbohydrate-based vaccine components

Vaccines remain one of the most important pillars in preventative medicine, providing protection against a wide array of diseases by inducing humoral and/or cellular immunity. Of the many possible candidate antigens for subunit vaccine development, carbohydrates are particularly appealing because of their ubiquitous presence on the surface of all living cells, viruses, and parasites as well as their known interactions with both innate and adaptive immune cells. Indeed, several licensed vaccines leverage bacterial cell-surface carbohydrates as antigens for inducing antigen-specific plasma cells secreting protective antibodies and the development of memory T and B cells. Carbohydrates have also garnered attention in other aspects of vaccine development, for example, as adjuvants that enhance the immune response by either activating innate immune responses or targeting specific immune cells. Additionally, carbohydrates can function as immunomodulators that dampen undesired humoral immune responses to entire protein antigens or specific, conserved regions on antigenic proteins. In this review, we highlight how the interplay between carbohydrates and the adaptive and innate arms of the immune response is guiding the development of glycans as vaccine components that act as antigens, adjuvants, and immunomodulators. We also discuss how advances in the field of synthetic glycobiology are enabling the design, engineering, and production of this new generation of carbohydrate-containing vaccine formulations with the potential to prevent infectious diseases, malignancies, and complex immune disorders.

Comparison of pneumococcal immunogenicity elicited by the PCV13 and PCV15 vaccines in adults 18 through 49 years of age

AIM

Pneumococcal conjugate vaccines (PCV13, PCV15, PCV20) effectively target the capsular polysaccharides of the most common disease-causing Streptococcus pneumoniae serotypes. In this short communication, we analyzed healthy participants who received PCV13 and PCV15 vaccines as part of a recently concluded exploratory clinical trial and report antibody responses to the 13 shared serotypes (1, 3, 4, 5, 6A, 6B, 7F, 9V, 14, 18C, 19A, 19F, and 23F) as well as functional OPA responses to serotype 3.

METHODS

Sera from 87 adult participants (18 through 49 years of age) randomized to receive either PCV13 or PCV15 were collected (n = 46 or n = 41, respectively), from 17 study centers in the US. IgG concentrations of the 13 shared serotypes and serotype 3-specific OPA titers were analyzed before and 1 month after vaccination using internally validated assays.

RESULTS

At 1 month after vaccination, IgG GMCs of the 13 shared serotypes in PCV13 were similar to those for PCV15. Specifically, serotype 3 OPA GMTs and 95% CIs were similar 1 month after vaccination for PCV13 (62.9 [48.9, 80.9]) and PCV15 (71.1 [50.9, 99.2]).

CONCLUSION

In healthy participants who received either PCV13 or PCV15, similar serotype-specific responses were observed between all shared serotypes when a uniform validated internal assay was used. Of note, data from this study suggest that both vaccines induce similar functional antibody responses against pneumococcal serotype 3.

Pilus of Streptococcus pneumoniae: structure, function and vaccine potential

The pilus is an extracellular structural part that can be detected in some Streptococcus pneumoniae (S. pneumoniae) isolates (type I pili are found in approximately 30% of strains, while type II pili are found in approximately 20%). It is anchored to the cell wall by LPXTG-like motifs on the peptidoglycan. Two kinds of pili have been discovered, namely, pilus-1 and pilus-2. The former is encoded by pilus islet 1 (PI-1) and is a polymer formed by the protein subunits RrgA, RrgB and RrgC. The latter is encoded by pilus islet 2 (PI-2) and is a polymer composed mainly of the structural protein PitB. Although pili are not necessary for the survival of S. pneumoniae, they serve as the structural basis and as virulence factors that mediate the adhesion of bacteria to host cells and play a direct role in promoting the adhesion, colonization and pathogenesis of S. pneumoniae. In addition, as candidate antigens for protein vaccines, pili have promising potential for use in vaccines with combined immunization strategies. Given the current understanding of the pili of S. pneumoniae regarding the genes, proteins, structure, biological function and epidemiological relationship with serotypes, combined with the immunoprotective efficacy of pilins as protein candidates for vaccines, we here systematically describe the research status and prospects of S. pneumoniae pili and provide new ideas for subsequent vaccine research and development.

Emerging vaccine strategies against the incessant pneumococcal disease

The incidence of invasive pneumococcal disease (IPD) caused by infection with the pathogen Streptococcus pneumoniae (Spn) has been on a downward trend for decades due to worldwide vaccination programs. Despite the clinical successes observed, the Center for Disease Control (CDC) reports that the continued global burden of S. pneumoniae will be in the millions each year, with a case-fatality rate hovering around 5%. Thus, it is a top priority to continue developing new Spn vaccination strategies to harness immunological insight and increase the magnitude of protection provided. As emphasized by the World Health Organization (WHO), it is also crucial to broaden the implementation of vaccines that are already obtainable in the clinical setting. This review focuses on the immune mechanisms triggered by existing pneumococcal vaccines and provides an overview of the current and upcoming clinical strategies being employed. We highlight the associated challenges of serotype selectivity and using pneumococcal-derived proteins as alternative vaccine antigens.

Worldwide Index of Serotype-Specific Pneumococcal Antibody Responses (WISSPAR): A curated database of clinical trial data

The Worldwide Index of Serotype Specific Pneumococcal Antibody Responses (WISSPAR; https://wisspar.com), is a centralized, online platform housing data on immunogenicity from clinical trials of pneumococcal vaccines. The data on WISSPAR are primarily curated from outcomes tables from clinical trials and are made available in a searchable format that can be readily used for downstream analyses. The WISSPAR database includes trials covering numerous vaccine products, manufacturers, dosing schedules, age groups, immunocompromised groups, and geographic regions. Customizable data visualization tools are embedded within the site, or the data can be exported for further analyses. Users can also browse summary information about the clinical trials and their results. WISSPAR provides a platform for analysts and policy makers to efficiently gather, compare, and collate clinical trial data about pneumococcal vaccines.

Improving the soluble expression of difficult-to-express proteins in prokaryotic expression system via protein engineering and synthetic biology strategies

Solubility and folding stability are key concerns for difficult-to-express proteins (DEPs) restricted by amino acid sequences and superarchitecture, resolved by the precise distribution of amino acids and molecular interactions as well as the assistance of the expression system. Therefore, an increasing number of tools are available to achieve efficient expression of DEPs, including directed evolution, solubilization partners, chaperones, and affluent expression hosts, among others. Furthermore, genome editing tools, such as transposons and CRISPR Cas9/dCas9, have been developed and expanded to construct engineered expression hosts capable of efficient expression ability of soluble proteins. Accounting for the accumulated knowledge of the pivotal factors in the solubility and folding stability of proteins, this review focuses on advanced technologies and tools of protein engineering, protein quality control systems, and the redesign of expression platforms in prokaryotic expression systems, as well as advances of the cell-free expression technologies for membrane proteins production.

Strengths and weaknesses of pneumococcal conjugate vaccines

Multivalent vaccines addressing an increasing number of Streptococcus pneumoniae types (7-, 10-, 13-, 15-, 20-valent) have been licensed over the last 22 years. The use of polysaccharide-protein conjugate vaccines has been pivotal in reducing the incidence of invasive pneumococcal disease despite the emergence of non-vaccine serotypes. Notwithstanding its undoubtable success, some weaknesses have called for continuous improvement of pneumococcal vaccination. For instance, despite their inclusion in pneumococcal conjugate vaccines, there are challenges associated with some serotypes. In particular, Streptococcus pneumoniae type 3 remains a major cause of invasive pneumococcal disease in several countries.Here a deep revision of the strengths and weaknesses of the licensed pneumococcal conjugate vaccines and other vaccine candidates currently in clinical development is reported.

A Phase II/III, Multicenter, Observer-blinded, Randomized, Non-inferiority and Safety, study of typhoid conjugate vaccine (EuTCV) compared to Typbar-TCV® in healthy 6 Months-45 years aged participants

The typhoid conjugate vaccine (TCV) ensures a long-lasting protective immune response, requires fewer doses and is fit for children under 2 years of age. From Phase I study, EuTCV displayed considerable immunogenicity and reliable safety, thus endorsing further examination in Phase II/III trials. Therefore, a clinical Phase II/III study (NCT04830371) was conducted to evaluate its efficacy in healthy Filipino participants aged 6 months to 45 years through administration of the test vaccine (Arm A, B, and C) or comparator vaccine Typbar-TCV® (Arm D). Sera samples were collected pre-vaccination (Visit 1) and post-vaccination (Visit 4, Day 28) to assess the immunogenicity of EuTCV and Typbar-TCV®. During the study, participants were regularly monitored through scheduled visits to the clinic to report any adverse events associated with the vaccine. For vaccine safety, the proportion of solicited and unsolicited Treatment-Emergent Adverse Events was all comparable between EuTCV and Typbar-TCV® groups. A single dose of EuTCV produced seroconversion in 99.4% of treated participants, with seroconversion rates non-inferior to that of Typbar-TCV®. Batch-to-batch consistency was concluded based on the 90% Confidence Interval of the geometric mean ratio (EuTCV Arm A, B, and C) at Week 4, lying within the equivalence margin of 0.5 to 2.0 for all batches. Results from this Phase II/III clinical trial of EuTCV in healthy volunteers show comparable safety and considerable immunogenicity, compared to Typbar-TCV®, meeting the objectives of this pivotal study. ClinicalTrials.gov registration number: NCT04830371.

A Review of Protein- and Peptide-Based Chemical Conjugates: Past, Present, and Future

Over the past few decades, the complexity of molecular entities being advanced for therapeutic purposes has continued to evolve. A main propellent fueling innovation is the perpetual mandate within the pharmaceutical industry to meet the needs of novel disease areas and/or delivery challenges. As new mechanisms of action are uncovered, and as our understanding of existing mechanisms grows, the properties that are required and/or leveraged to enable therapeutic development continue to expand. One rapidly evolving area of interest is that of chemically enhanced peptide and protein therapeutics. While a variety of conjugate molecules such as antibody-drug conjugates, peptide/protein-PEG conjugates, and protein conjugate vaccines are already well established, others, such as antibody-oligonucleotide conjugates and peptide/protein conjugates using non-PEG polymers, are newer to clinical development. This review will evaluate the current development landscape of protein-based chemical conjugates with special attention to considerations such as modulation of pharmacokinetics, safety/tolerability, and entry into difficult to access targets, as well as bioavailability. Furthermore, for the purpose of this review, the types of molecules discussed are divided into two categories: (1) therapeutics that are enhanced by protein or peptide bioconjugation, and (2) protein and peptide therapeutics that require chemical modifications. Overall, the breadth of novel peptide- or protein-based therapeutics moving through the pipeline each year supports a path forward for the pursuit of even more complex therapeutic strategies.

Cell-free protein synthesis systems for vaccine design and production

Vaccines are vital for protection against existing and emergent diseases. Current vaccine production strategies are limited by long production times, risky viral material, weak immunogenicity, and poor stability, ultimately restricting the safe or rapid production of vaccines for widespread utilization. Cell-free protein synthesis (CFPS) systems, which use extracted transcriptional and translational machinery from cells, are promising tools for vaccine production because they can rapidly produce proteins without the constraints of living cells, have a highly optimizable open system, and can be used for on-demand biomanufacturing. Here, we review how CFPS systems have been explored for the production of subunit, conjugate, virus-like particle (VLP), and membrane-augmented vaccines and as a tool in vaccine design. We also discuss efforts to address potential limitations with CFPS such as the presence of endotoxins, poor protein folding, reaction stability, and glycosylation to enable promising future vaccine design and production.

Glycoconjugate vaccines against antimicrobial resistant pathogens

INTRODUCTION

Antimicrobial resistance (AMR) is responsible for the death of millions worldwide and stands as a major threat to our healthcare systems, which are heavily reliant on antibiotics to fight bacterial infections. The development of vaccines against the main pathogens involved is urgently required as prevention remains essential against the rise of AMR.

AREAS COVERED

A systematic research review was conducted on MEDLINE database focusing on the six AMR pathogens defined as ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Escherichia coli), which are considered critical or high priority pathogens by the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC). The analysis was intersecated with the terms carbohydrate, glycoconjugate, bioconjugate, glyconanoparticle, and multiple presenting antigen system vaccines.

EXPERT OPINION

Glycoconjugate vaccines have been successful in preventing meningitis and pneumoniae, and there are high expectations that they will play a key role in fighting AMR. We herein discuss the recent technological, preclinical, and clinical advances, as well as the challenges associated with the development of carbohydrate-based vaccines against leading AMR bacteria, with focus on the ESKAPE pathogens. The need of innovative clinical and regulatory approaches to tackle these targets is also highlighted.

Synthetic Glycans to Improve Current Glycoconjugate Vaccines and Fight Antimicrobial Resistance

Antimicrobial resistance (AMR) is emerging as the next potential pandemic. Different microorganisms, including the bacteria Acinetobacter baumannii, Clostridioides difficile, Escherichia coli, Enterococcus faecium, Klebsiella pneumoniae, Neisseria gonorrhoeae, Pseudomonas aeruginosa, non-typhoidal Salmonella, and Staphylococcus aureus, and the fungus Candida auris, have been identified by the WHO and CDC as urgent or serious AMR threats. Others, such as group A and B Streptococci, are classified as concerning threats. Glycoconjugate vaccines have been demonstrated to be an efficacious and cost-effective measure to combat infections against Haemophilus influenzae, Neisseria meningitis, Streptococcus pneumoniae, and, more recently, Salmonella typhi. Recent times have seen enormous progress in methodologies for the assembly of complex glycans and glycoconjugates, with developments in synthetic, chemoenzymatic, and glycoengineering methodologies. This review analyzes the advancement of glycoconjugate vaccines based on synthetic carbohydrates to improve existing vaccines and identify novel candidates to combat AMR. Through this literature survey we built an overview of structure-immunogenicity relationships from available data and identify gaps and areas for further research to better exploit the peculiar role of carbohydrates as vaccine targets and create the next generation of synthetic carbohydrate-based vaccines.

Addition of Lauryldimethylamine N-Oxide (LDAO) to a Copper-Free Click Chemistry Reaction Improves the Conjugation Efficiency of a Cell-Free Generated CRM197 Variant to Clinically Important Streptococcus pneumoniae Serotypes

Strain-promoted azide-alkyne cycloaddition (SPAAC) reactions like click chemistry have the potential to be highly scalable, robust, and cost-effective methods for generating small- and large-molecule conjugates for a variety of applications. However, despite method improvements, the rates of copper-based click chemistry reactions continue to be much faster than the rates of copper-free click chemistry reactions, which makes broader deployment of click chemistry challenging from a safety and compatibility standpoint. In this study, we used a zwitterionic detergent, namely, lauryldimethylamine N-oxide (LDAO), in a copper-free click chemistry reaction to investigate its impact on the generation of conjugate vaccines (CVs). For this, we utilized an Xpress cell-free protein synthesis (CFPS) platform to generate a proprietary variant of CRM197 (eCRM) containing non-native amino acids (nnAA) with azide-containing side chains as a carrier protein for conjugation to several clinically relevant dibenzocyclooctyne (DBCO)-derivatized S. pneumoniae serotypes (types 3, 5, 18C, and 19A). For conjugation, we performed copper-free click chemistry in the presence and absence of LDAO. Our results show that the addition of LDAO significantly enhanced the reaction kinetics to generate larger conjugates, which were similarly immunogenic and equally stable to conjugates generated without LDAO. Most importantly, the addition of LDAO substantially improved the efficiency of the conjugation process. Thus, our results for the first time show that the addition of a zwitterionic surfactant to a copper-free click chemistry reaction can significantly accelerate the reaction kinetics along with improving the efficiency of the conjugation process.

Classical- and bioconjugate vaccines: comparison of the structural properties and immunological response

Glycoconjugate vaccines have been effectively used in humans for about 40 years. The glycoconjugates have substituted plain polysaccharide vaccines that have many limitations, especially in infants. The covalent linking of protein to carbohydrates has allowed to overcome T-cell-dependent type-2 response of sugars. Glycoconjugates can show improved responses (over plain saccharides) also in elderly and immunocompromised (and depending on the endpoint also in immunocompetent adults), but infants represent the main target of these vaccines because of their unique immune system. Differently from the plain polysaccharide vaccines, the glycoconjugates are also able to induce Immunoglobulin G (IgG) response in infants. Recently, vaccines containing conjugates directly expressed in Escherichia coli (bioconjugates) have been tested in the clinic against Shigella dysenteriae type 1, uropathogenic E. coli, and Streptococcus pneumoniae. Here, we report an overall comparison of classical- and bioconjugate vaccines in terms of the structural properties and the immunological response elicited.

The remarkable history of pneumococcal vaccination: an ongoing challenge

Although it varies with age and geographical distribution, the global burden of infection with Streptococcus pneumoniae (pneumococcus) remains considerable. The elderly, and younger adults with comorbid conditions, are at particularly high risk of pneumococcal infection, and this risk will increase as the population ages. Vaccination should be the backbone of our current strategies to deal with this infection.

Main body: This manuscript reviews the history of the development of pneumococcal vaccines, and the impact of different vaccines and vaccination strategies over the past 111 years. It documents the early years of vaccine development in the gold mines of South Africa, when vaccination with killed pneumococci was shown to be effective, even before the recognition that different pneumococci were antigenically distinct. The development of type-specific vaccines, still with whole killed pneumococci, showed a high degree of efficacy. The identification of the importance of the pneumococcal capsule heralded the era of vaccination with capsular polysaccharides, although with the advent of penicillin, interest in pneumococcal vaccine development waned. The efforts of Austrian and his colleagues, who documented that despite penicillin therapy, patients still died from pneumococcal infection in the first 96 h, ultimately led to the licensing first of a 14-valent pneumococcal polysaccharide in 1977 followed by the 23-valent pneumococcal polysaccharide in 1983. The principal problem with these, as with other polysaccharide vaccines, was that that they failed to immunize infants and toddlers, who were at highest risk for pneumococcal disease. This was overcome by chemical linking or conjugation of the polysaccharide molecules to an immunogenic carrier protein. Thus began the era of pneumococcal conjugate vaccine (PCV), starting with PCV7, progressing to PCV10 and PCV13, and, most recently, PCV15 and PCV20. However, these vaccines remain serotype specific, posing the challenge of new serotypes replacing vaccine types. Current research addresses serotype-independent vaccines which, so far, has been a challenging and elusive endeavor.

Conclusion: While there has been enormous progress in the development of pneumococcal vaccines during the past century, attempts to develop a vaccine that will retain its efficacy for most pneumococcal serotypes are ongoing.

Cross reacting material (CRM197) as a carrier protein for carbohydrate conjugate vaccines targeted at bacterial and fungal pathogens

This paper gives an overview of conjugate glycovaccines which contain recombinant diphtheria toxoid CRM197 as a carrier protein. A special focus is given to synthetic methods used for preparation of neoglycoconjugates of CRM197 with oligosaccharide epitopes of cell surface carbohydrates of pathogenic bacteria and fungi. Syntheses of commercial vaccines and laboratory specimen on the basis of CRM197 are outlined briefly.

Non-Native Amino Acid Click Chemistry-Based Technology for Site-Specific Polysaccharide Conjugation to a Bacterial Protein Serving as Both Carrier and Vaccine Antigen

Surface-expressed bacterial polysaccharides are important vaccine antigens but must be conjugated to a carrier protein for efficient antigen presentation and development of strong memory B cell and antibody responses, especially in young children. The commonly used protein carriers include tetanus toxoid (TT), diphtheria toxoid (DT), and its derivative CRM197, but carrier-induced epitopic suppression and bystander interference may limit the expanded use of the same carriers in the pediatric immunization schedule. Recent efforts to develop a vaccine against the major human pathogen group A Streptococcus (GAS) have sought to combine two promising vaccine antigens-the universally conserved group A cell wall carbohydrate (GAC) with the secreted toxin antigen streptolysin O (SLO) as a protein carrier; however, standard reductive amination procedures appeared to destroy function epitopes of the protein, markedly diminishing functional antibody responses. Here, we couple a cell-free protein synthesis (CFPS) platform, allowing the incorporation of non-natural amino acids into a C-terminally truncated SLO toxoid for the precise conjugation to the polyrhamnose backbone of GAC. The combined immunogen generated functional antibodies against both conserved GAS virulence factors and provided protection against systemic GAS challenges. CFPS may represent a scalable method for generating pathogen-specific carrier proteins for multivalent subunit vaccine development.

Dynamic changes in clinical characteristics and serotype distribution of invasive pneumococcal disease among adults in Japan after introduction of the pediatric 13-valent pneumococcal conjugate vaccine in 2013-2019

Nationwide population-based surveillance for invasive pneumococcal disease (IPD) is being conducted in few Asian countries. We aimed to evaluate the clinical characteristics and serotype distribution among Japanese adult patients with IPD after introduction of the pediatric 13-valent pneumococcal conjugate vaccine (PCV13) in 2013. IPD surveillance was conducted among adults between 2013 and 2019, and 1,995 patients were analyzed by time period (early, 2013-2015; middle, 2016-2017; late, 2018-2019). We found that the period of 2018-2019 was independently associated with a lower risk of fatal outcome, compared with the period of 2013-2015. The proportion of those with serotype PCV13-nonPCV7 decreased significantly in patients aged 15-64 years and in those aged ≥ 65 years within 3 years after the introduction of pediatric PCV13. By contrast, the proportion of those with nonvaccine serotype increased significantly in those aged ≥ 65 years, but not in those aged 15-64 years. No significant change was found in the proportion of 23-valent polysaccharide pneumococcal vaccine (PPSV23)-nonPCV13 in both of adults aged 15-64 years and ≥ 65 years. The proportions of PCV15-, PCV20- and PCV24-covered serotypes were 38%, 56% and 58% in adult patients with IPD aged ≥ 65 years during the late period. Our data on the serotype distribution support an indirect effect from pediatric PCV13 use among adults, and afford a basis for estimates of protection against IPD by vaccination with newly developed PCVs in older adults in Japan.

Pneumococcal meningitis in adults in 2014-2018 after introduction of pediatric 13-valent pneumococcal conjugate vaccine in Japan

We assessed the impact of the pediatric 13-valent pneumococcal conjugate vaccine (PCV13) on pneumococcal meningitis in adults in Japan in 2014–2018 by comparing epidemiological characteristics of adults with invasive pneumococcal disease with (n = 222) and without (n = 1258) meningitis. The annual incidence of pneumococcal meningitis in 2016–2018 was 0.20–0.26 cases/100,000 population. Age (p < 0.001) and case fatality rate (p = 0.003) were significantly lower in patients with meningitis than in those without meningitis. The odds of developing meningitis were higher in asplenic/hyposplenic or splenectomized patients (adjusted odds ratio [aOR] 2.29, 95% CI 1.27–4.14), for serotypes 10A (aOR 3.26, 95% CI 2.10–5.06) or 23A (aOR 3.91, 95% CI 2.47–6.19), but lower for those aged ≥ 65 years (aOR 0.59, 95% CI 0.44–0.81). PCV13 had an indirect effect on nonmeningitis, but its impact on meningitis was limited because of an increase in non-PCV13 serotypes. Of meningitis isolates, 78 (35.1%) and 3 (1.4%) were penicillin G- or ceftriaxone-resistant, respectively. We also confirmed an association of the pbp1bA641C mutation with meningitis (aOR 2.92, 95% CI 1.51–5.65).

Pneumococcal Vaccines: Past Findings, Present Work, and Future Strategies

The importance of Streptococcus pneumoniae has been well established. These bacteria can colonize infants and adults without symptoms, but in some cases can spread, invade other tissues and cause disease with high morbidity and mortality. The development of pneumococcal conjugate vaccines (PCV) caused an enormous impact in invasive pneumococcal disease and protected unvaccinated people by herd effect. However, serotype replacement is a well-known phenomenon that has occurred after the introduction of the 7-valent pneumococcal conjugate vaccine (PCV7) and has also been reported for other PCVs. Therefore, it is possible that serotype replacement will continue to occur even with higher valence formulations, but the development of serotype-independent vaccines might overcome this problem. Alternative vaccines are under development in order to improve cost effectiveness, either using proteins or the pneumococcal whole cell. These approaches can be used as a stand-alone strategy or together with polysaccharide vaccines. Looking ahead, the next generation of pneumococcal vaccines can be impacted by the new technologies recently approved for human use, such as mRNA vaccines and viral vectors. In this paper, we will review the advantages and disadvantages of the addition of new polysaccharides in the current PCVs, mainly for low- and middle-income countries, and we will also address future perspectives.

Glycan Array Evaluation of Synthetic Epitopes between the Capsular Polysaccharides from Streptococcus pneumoniae 19F and 19A

Vaccination represents the most effective way to prevent invasive pneumococcal diseases. The glycoconjugate vaccines licensed so far are obtained from capsular polysaccharides (CPSs) of the most virulent serotypes. Protection is largely limited to the specific vaccine serotypes, and the continuous need for broader coverage to control the outbreak of emerging serotypes is pushing the development of new vaccine candidates. Indeed, the development of efficacious vaccine formulation is complicated by the high number of bacterial serotypes with different CPSs. In this context, to simplify vaccine composition, we propose the design of new saccharide fragments containing chemical structures shared by different serotypes as cross-reactive and potentially cross-protective common antigens. In particular, we focused on Streptococcus pneumoniae (Sp) 19A and 19F. The CPS repeating units of Sp 19F and 19A are very similar and share a common structure, the disaccharide ManNAc-β-(1→4)-Glc (A-B). Herein, we describe the synthesis of a small library of compounds containing different combinations of the common 19F/19A disaccharide. The six new compounds were tested with a glycan array to evaluate their recognition by antibodies in reference group 19 antisera and factor reference antisera (reacting against 19F or 19A). The disaccharide A-B, phosphorylated at the upstream end, emerged as a hit from the glycan array screening because it is strongly recognized by the group 19 antisera and by the 19F and 19A factor antisera, with similar intensity compared with the CPSs used as controls. Our data give a strong indication that the phosphorylated disaccharide A-B can be considered a common epitope among different Sp 19 serotypes.

Invasive pneumococcal disease caused by serotypes 22F and 33F in Canada: the SAVE study 2011-2018

A 15-valent conjugate vaccine that provides protection against Streptococcus pneumoniae serotypes 22F and 33F is in development. Here we report on the prevalence, antimicrobial susceptibility, and clonal structure of these serotypes in Canada. From 2011 to 2018, the SAVE study collected 11,044 invasive S. pneumoniae isolates. Of these, 9.3% (1024/11,044) and 3.8% (416/11,044) were 22F and 33F, respectively. Serotype 22F isolates were susceptible to most antimicrobials tested except clarithromycin, where susceptibility significantly decreased over time (2011: 80.4%, 2018: 52.9%, P < 0.0001). Only 1.6% of serotype 22F isolates were multidrug-resistant (MDR), while 96% of typed strains were clonal cluster (CC) 433. Serotype 33F isolates demonstrated low susceptibility to clarithromycin and trimethoprim/sulfamethoxazole (22.4% and 24.6%, respectively) and 4.8% MDR. Most serotype 33F isolates were CC100, CC673 and CC717. CC100 prevalence increased significantly over time (2011: 50.0%, 2018: 84.8%, P < 0.006). Continued surveillance of these serotypes is crucial to identify further changes in prevalence, antimicrobial susceptibility, and clonal spread.