
==== Front
Hum Vaccin Immunother
Hum Vaccin Immunother
Human Vaccines & Immunotherapeutics
2164-5515
2164-554X
Taylor & Francis

39262177
10.1080/21645515.2024.2399915
2399915
Version of Record
Article Commentary
Novel Vaccines
Syphilis vaccine development: Aligning vaccine design with manufacturing requirements
S. WAUGH AND C. E. CAMERON
HUMAN VACCINES & IMMUNOTHERAPEUTICS
https://orcid.org/0009-0009-1154-1850
Waugh Sean a
https://orcid.org/0000-0002-8786-4359
Cameron Caroline E. a b
a Department of Biochemistry and Microbiology, University of Victoria , Victoria, Canada
b Department of Medicine, Division of Allergy and Infectious Disease, University of Washington , Seattle, WA, USA
CONTACT Caroline E. Cameron caroc@uvic.ca Department of Biochemistry and Microbiology, University of Victoria, Victoria BC V8P 5C2, Canada.
11 9 2024
2024
11 9 2024
20 1 2399915Integra09 9 2024
Integra09 9 2024
24 7 2024
20 8 2024
30 8 2024
© 2024 The Author(s). Published with license by Taylor & Francis Group, LLC.
2024
The Author(s)
https://creativecommons.org/licenses/by-nc/4.0/ This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) or with their consent.

ABSTRACT

Syphilis, caused by Treponema pallidum subsp. pallidum, is a global health concern with increasing rates worldwide. Current prevention strategies, including screen-and-treat approaches, are not sufficient to resolve rising infection rates, emphasizing the need for a vaccine. Developing a syphilis vaccine necessitates a range of cross-disciplinary considerations, including essential disease-specific protection, technical requirements, economic feasibility, manufacturing constraints, public acceptance, equitable vaccine access, alignment with global public vaccination programs, and identification of essential populations to be vaccinated to achieve herd immunity. Central to syphilis vaccine development is prioritization of global vaccine availability, including access in low- to middle-income settings. Various vaccine platforms, including subunit, virus-like particle (VLP), mRNA, and outer membrane vesicle (OMV) vaccines, present both advantages and challenges. The proactive consideration of both manufacturing feasibility and efficacy throughout the pre-clinical research and development stages is essential for producing an efficacious, inexpensive, and scalable syphilis vaccine to address the growing global health burden caused by this disease.

KEYWORDS

Syphilis
vaccine
Treponema pallidum
vaccine manufacturing
National Institute of Allergy and Infectious Disease National Institutes of Health 10.13039/100000002 52345 Primary research reported in this commentary was supported by grants [R37AI051334]; [U19AI144133] and [1U01AI182035] from the National Institute of Allergy and Infectious Disease, National Institutes of Health and award [52345] from Open Philanthropy to CEC. SW is the recipient of a CIHR Canada Graduate Scholarship-Doctoral [CGS-D].
==== Body
pmcIntroduction

Syphilis, caused by the extracellular bacterium Treponema pallidum subsp. pallidum, is a multi-stage infection that persists for a lifetime without antibiotic treatment.1 Syphilis remains prevalent in low- and middle-income countries; in 2019 there were an estimated 49.71 million cases of syphilis globally.2 In high-income countries, including the US, Canada and Europe, syphilis rates have been rising at an alarming rate over the last decade, with the highest increase in cases observed in populations of men who have sex with men (MSM), transgender women, and cisgender women.3–7 Treponema pallidum can cross the placental barrier and cause congenital syphilis, which is estimated to affect 1 million pregnancies annually and result in approximately 661,000 cases of congenital syphilis and 355,000 adverse birth outcomes.8 These numbers may be an underestimation since accurate determination of the burden of congenital syphilis is challenging, due to country-specific variation in access to antenatal screening, syphilis testing during pregnancy, and reliable surveillance data.9 To address rising congenital syphilis rates, in 2021 the WHO launched a global initiative to eliminate mother-to-child transmission of syphilis,10 with the aim of reducing global syphilis incidence by 90% by 2030.11

Infectious and congenital syphilis rates are highest in low- and middle-income countries (LMICs).2 As a result, countries with a low sociodemographic index exhibit the highest age-standardized rates (ASR) and disability-adjusted life years (DALYs) due to syphilis, underscoring health disparities in both disease prevalence and disease impact on individuals.2 In addition to the deleterious physical health outcomes from syphilis, it is essential to consider the psychosocial, quality of life, and economic impacts a syphilis diagnosis can have on individuals. Accordingly, achieving a successful reduction in the incidence of syphilis is predicted to have the highest impact on reducing DALYs of all the curable STIs.12,13

Increasing syphilis rates suggest that screen-and-treat public health strategies alone may not effectively reduce disease incidence, underscoring the importance of developing a syphilis vaccine. Modeling predicts a syphilis vaccine with 80% efficacy would significantly reduce both infectious and congenital syphilis over 20 years,14 and when paired with public health initiatives geared to raise disease awareness and reduce disease incidence the goal of syphilis elimination may be possible. Given that syphilis rates and associated DALYs are highest in LMICs, and that access to screening and effective treatment can be challenging in resource-limited settings, it is crucial to prioritize the design of a syphilis vaccine that is suitable for use in LMIC settings. Developing an effective syphilis vaccine to address global need requires a careful assessment of needed product performance characteristics (PPCs) aligned with requirements for delivering effective protection against infection and disease (within an individual) and spread (within the population), balanced with feasibility of manufacturing. By designing a vaccine where ideal vaccine PPCs and manufacturing requirements are considered early in the design pipeline and revisited at all stages of development, vaccine researchers can proactively avoid concerns from industry, regulatory, and decision-making partners regarding manufacturing cost, feasibility, performance, and suitability for use in remote or resource-limited settings. This approach ensures maximum market potential, increases the chances of industry partner support, and enhances global vaccine equity and access. Here, we outline the optimal molecular and biophysical properties, as well as important economic and manufacturing considerations, for a syphilis vaccine and consider the suitability of various platforms including subunit-, virus-like particle (VLP)-, mRNA-, and outer membrane vesicle (OMV)-based vaccines.

The status of syphilis vaccine development

Treponema pallidum presents unique biological challenges for vaccine design, including the minimal complement of outer membrane proteins that are expressed on the host-interacting pathogen surface, and the extensive antigenic variation that is found in select outer membrane proteins, including the T. pallidum repeat (Tpr) protein family.1,15–17 Individuals are also susceptible to reinfection with heterologous strains.18,19 Although details regarding the immune correlates of protection are limited, current evidence indicates that protection is dependent, at least in part, upon the generation of a delayed-type hypersensitivity-like TH1 immune response that activates interferon-gamma (IFN-γ)-secreting T-cells, and antibodies that opsonize and neutralize T. pallidum organisms.20,21

Syphilis vaccine development has been designated by vaccine development oversight committees as being between the basic and pre-clinical stages on the vaccine development continuum.22 The host interfacing outer membrane proteins present on the T. pallidum surface have long been known to be important targets for vaccine design; computational analyses performed over two decades ago identified a complement of candidate outer membrane proteins in T. pallidum,23 and more recent analyses targeting the “OMPeome“ have advanced this line of investigation.24 Current high priority syphilis vaccine candidates that are in the pre-clinical assessment stage target known or suspected T. pallidum outer membrane proteins and, to date, all have been pursued as protein subunit vaccines. These include proteins such as Tp0751,25 Tp0136,26 Tp0326,27 Tp0633,28 Tp0856,29 and Tpr family members including TprK (Tp0897).30,31 An important consideration from a manufacturing standpoint is that only two of these vaccine candidates have been successfully produced as soluble and stable recombinant proteins (Tp0751 and Tp0136); the other vaccine targets being pursued are beta barrel-containing integral membrane proteins, which require complex re-folding and additional quality control (QC) measures that are not conducive with vaccine manufacturing requirements. Since current syphilis vaccine candidates being pursued have elicited partial protection at best,25,30,31 it is apparent that an effective syphilis vaccine will require induction of an immune response against multiple T. pallidum proteins, an expensive endeavor from a manufacturing standpoint.

Overall, the complexity of T. pallidum biology, unknown correlates of protection, limited vaccine-targetable proteins, and inherent need to induce immune responses to multiple proteins pose significant challenges for syphilis vaccine design. To overcome these limitations and ensure compatibility with manufacturing requirements, researchers are focusing on characterizing the immunogenicity and protective capacity of regions of the proteins that correspond to surface-exposed extracellular loops, identifying peptides containing T- and B-cell epitopes to ensure effective T. pallidum neutralization and clearance,29,32–37 and pursuing platforms that can incorporate these essential regions from multiple T. pallidum proteins into a single chimeric polyvalent vaccine candidate.38–40

Product performance characteristics (PPCs) for a syphilis vaccine

Previous commentaries have outlined the investment case for development of a syphilis vaccine and the economic impact of vaccine implementation.41–43 The World Health Organization (WHO), in collaboration with the National Institute for Allergy and Infectious Diseases (NIAID), Centers for Disease Control and Prevention (CDC), and STI experts, have proposed a roadmap for advancing STI vaccine development.22,44 Prior reviews have comprehensively documented the challenges, safety requirements, and needed correlates of protection associated with development of a syphilis vaccine.20,41,45–47 The current commentary focuses upon aligning the critical requirements for development of an effective syphilis vaccine with industry requirements for vaccine manufacturing. For the syphilis vaccine development pipeline, meaningful and early engagement with industry, regulatory, and advisory agencies will maximize the chances of development of vaccine candidates that offer protection against all stages of disease in a format that aligns with standard industry manufacturing requirements.

Syphilis disease progression includes development of a characteristic chancre at the initial site of infection (primary disease stage), followed by a disseminated rash and general malaise (secondary disease stage). In the absence of treatment, the infection becomes latent and persists for an individual’s lifetime. Approximately one-third of individuals infected with T. pallidum develop symptoms associated with tertiary disease, including gumma and central nervous system and cardiovascular involvement. Asymptomatic infections and varied disease presentations can also occur.1,19 An ideal syphilis vaccine would provide protection against chancre and secondary lesion formation, as well as bacterial shedding at other body sites, to prevent infection transmission between individuals. Also necessary for a vaccine is protection against bacterial dissemination across the placental, endothelial and blood-brain barriers, thus preventing congenital syphilis and disease symptom development within an individual. Although achieving complete protection against population spread and disease symptoms via vaccination is desired, it is understood to be a challenging goal due to the complexity of T. pallidum infection. A more achievable vaccination goal may be establishment of partial protection against chancre and secondary lesion development and treponemal shedding, which would presumably result in decreased syphilis transmission at the population level and attenuated symptom development at the individual level. However, induction of partial protection against infection, while expected to decrease T. pallidum burden within an individual, may still place infected individuals at risk of developing tissue and organ damage as well as transmitting congenital infection to their developing fetus. For this reason, simultaneous with syphilis vaccine development it will be imperative to develop a direct syphilis diagnostic test that can accurately detect active infection in a vaccinated individual and successfully differentiate from previous infection(s).

To provide broad protection against clinical T. pallidum strains, and to protect against reinfection with heterologous strains, vaccine candidates should target T. pallidum proteins or peptides that are invariant and shared across all circulating strains of T. pallidum. Further, candidate vaccine constructs should mimic the endogenous state of T. pallidum proteins to ensure an effective immune response is generated.39 Additional vaccine considerations include the need for a vaccine that can be safely administered to pregnant individuals at all stages of gestation, individuals who are HIV+ and/or taking PrEP/PEP,41,48 and individuals who have had a previous T. pallidum infection. Induction of long-term protection through vaccination would be ideal, optimally striving for protection that lasts 10–15 years to match the WHO objectives for STI vaccines, including Neisseria gonorrhoeae and Herpes Simplex Virus vaccines.49,50

The process of vaccine manufacturing is expensive, long-term, and technically challenging. Thus, vaccine candidates that show promise in pre-clinical studies frequently fail at the manufacturing stage.51 To maximize the likelihood of a syphilis vaccine candidate progressing through manufacturing, implementation, and market introduction phases, it is essential that researchers consider vaccine candidate features that align with manufacturing requirements (including ease of production, scalability, and long shelf-life) and global and public expectations for performance (including reasonable dosing schedule, induction of long-term protection, convenient route of immunization, ease of storage, and low cost). Further, early consideration of strategies for cost-effective vaccine production includes designing vaccines that can utilize existing vaccine manufacturing infrastructure and are compatible with current Good Manufacturing Processes (cGMP)52 production. By designing a vaccine that avoids the need for complicated manufacturing strategies, can be produced using existing manufacturing systems, and prioritizes optimal production characteristics from the early stages of development, researchers reduce the probability of late-phase vaccine product failure and increase the likelihood that a vaccine can be produced at scale.51

Current vaccine platforms

Protein subunit vaccines

Protein subunit vaccines, which consist of recombinantly produced, pathogen-originating proteins, are a well-established and highly effective vaccine platform. Subunit vaccines offer advantages such as consistency, safety, and established production infrastructure.53 To produce a subunit vaccine for syphilis, researchers must identify infection-relevant proteins that can be recombinantly expressed at large scale and are stable, soluble, and do not require downstream processing steps such as refolding in order to be representative of the natural state of the protein found in the pathogen. Since each protein contained within a subunit vaccine requires an independent production line, production costs are commensurate with the number of antigens.51 To minimize these constraints, an ideal syphilis vaccine candidate will be a single antigen that is soluble and retains native epitope conformation without requiring additional refolding or other laboratory-based, non-automated manipulations. Further, protein vaccine candidates should be able to be produced with reproducible quantity and quality by commonly used biomaterial production organisms. For example, subunit vaccine production pipelines using Escherichia coli, Saccharomyces cerevisiae, or Pichia pastoris are generally less expensive due to lower material costs and well-established manufacturing facilities and procedures.53,54 However, use of these organisms may not be compatible with complex vaccine product requirements, including vaccines dependent upon the incorporation of T. pallidum-specific post-translational modifications that mimic the natural state of the protein(s), highlighting the importance of considering vaccine design constraints early and throughout the development pipeline.39

According to the WHO vaccine preferred product characteristics,55 subunit vaccines should be stable at standard cold-chain temperatures (2° to 8°C), tolerate a freeze-thaw cycle, retain stability for several hours after removal from cold storage, and tolerate temperatures up to 40°C for short durations.55 To address this need, the stability and efficacy of candidate subunit vaccines following long-term storage should be evaluated to ensure consistency between production and delivery.

Due to the complex pathogenesis of T. pallidum infection, and the fact that individual subunit vaccine trials to date for syphilis vaccine development have resulted in partial protection at best,25,30,31 it is expected that an effective syphilis vaccine will need to comprise multiple prioritized T. pallidum vaccine candidates. To avoid increased manufacturing costs associated with producing multiple antigens, one strategy being pursued by researchers is to develop protein scaffolds that can be decorated with B- and T-cell epitopes derived from multiple T. pallidum proteins.40 By pairing with an adjuvant, these scaffolds can elicit specific and robust immune responses toward each included epitope, inducing high titers of neutralizing and/or opsonic antibodies and a robust cell-mediated immune response. Optimally, these scaffolds would originate from T. pallidum proteins, thus ensuring limited off-target immune responses. One such endogenous scaffold that has shown promise at providing partial protection against T. pallidum dissemination within the body25,30 and is amendable to incorporation of heterologous epitopes into flexible loop regions is the T. pallidum protein Tp0751.38,40 The ability to graft functionally and/or immunologically relevant T. pallidum protein epitopes onto a soluble and stable T. pallidum-derived scaffold will address many of the constraints associated with subunit vaccines. Overall, the innovative and flexible design of contemporary T. pallidum protein scaffolds, paired with the abundance of existing infrastructure for subunit vaccine production, position this vaccine platform as a leading formulation for developing an efficacious and cost-effective syphilis vaccine.24,29,38,40,41,45

Virus-like particle vaccines

Virus-like particles (VLPs) are a vaccine technology used in several commercially available vaccines, including vaccines against human papilloma virus (Cervarix®, Gardasil® and Gardasil9®), hepatitis B virus (Sci-B-Vac™), and malaria (Mosquirix™).56 For this technology custom epitopes are presented on viral capsid proteins that self-assemble into an ultrastructure that resembles or mimics that of a virus.57 The immune system recognizes repeated viral patterns, making VLPs innately immunogenic, thereby bypassing the requirement for an adjuvant.58,59 These repetitive viral structures allow for greatly increased epitope presentation which enables B-cell crosslinking, and thus may require fewer doses to achieve desired immunity thresholds.57,59 VLPs can be enveloped, or non-enveloped, and range from 1 to 3 protein capsid layers. VLPs naturally accommodate the presentation of multiple epitopes, which meets the need for a syphilis vaccine to target multiple T. pallidum proteins.

Drawbacks to VLP production include that it is significantly more expensive than traditional recombinant protein production, epitopes that can be presented on VLPs have a size restriction, and epitopes may misfold during presentation or interfere with VLP assembly.59 VLPs also require further processing and purification steps, often involving specialized equipment and techniques such as density gradients and chromatography for particle assembly and purification. Additional analysis techniques are therefore also necessary to ensure product quality and purity.60 The need for specialized equipment, skilled personnel, and the increased complexity of the product all contribute to higher production costs, hinder scalability, and reduce the number of existing production facilities that can produce VLPs. Stability, which has been documented to be a concern for VLP vaccines,58,59 would need to be tested to ensure that storage and shelf-life are practical for delivery and use in resource-limited settings, and are compatible with cold-chain restrictions. Given these technical and economic constraints, a syphilis-based VLP vaccine may pose feasibility concerns for production and use in LMIC settings.

mRNA vaccines

mRNA vaccines consist of a core mRNA strand that encodes one or more vaccine antigens, which are then intracellularly translated into protein(s) in vaccinated individuals. mRNA vaccines can also be self-replicating, where mRNA strands include an RNA-dependent RNA polymerase which replicate the mRNA strand, thereby lowering the mRNA required per dose.61 Naked mRNA requires packaging in a delivery vehicle, such as a lipid nanoparticle, to cross cell membranes. mRNA vaccines typically require an adjuvant to elicit specific immune responses; however, mRNA vaccines may also self-adjuvant based on the characteristics of mRNA or of delivery components such as the lipid nanoparticle.62 Since all mRNA products share the same physical and chemical properties, manufacturing only differs by the specific nucleotide sequences.63 This feature allows production to be versatile and responsive to current vaccine needs, and the manufacturing process to be standardized to enable production of multiple different mRNA products by a single manufacturing facility.63 Further benefits of mRNA vaccines include fast production time (i.e. completed within hours) and cell-free production, thus avoiding impurities derived from production organisms.64

Although recent advances have been made in mRNA vaccine technology, manufacturing challenges for mRNA vaccines persist. These challenges include the high costs and shortages of essential mRNA production reagents, particularly enzymes.64 Additionally, mRNA purification and downstream processing, such as encapsulation in a lipid nanoparticle, are costly and difficult to scale.64 mRNA vaccines also rely on strict cold- or ultra-cold-chain temperatures.64 The future development of continuous manufacturing processes, and innovation in mRNA structure, encapsulation, and delivery, will increase accessibility and affordability of this technology.64,65

With the flexibility of mRNA manufacturing facilities and COVID 19-related investment in global infrastructure, mRNA vaccine manufacturers are able to produce vaccines on-demand, a potential benefit for a disease such as syphilis that may experience fluctuations in the demand for a vaccine.63,64 Manufacturing costs will also decrease as the technology further matures, and current limitations and cost-barriers are overcome.64 Therefore, mRNA vaccine platforms show promise for syphilis vaccine development, addressing both economic and antigen design constraints. Similar to other vaccine formulations, research on syphilis vaccines utilizing mRNA technology must evaluate stability, immunogenicity, and continued efficacy after long-term storage. The duration of protection from mRNA vaccines is not well-established, and likely varies depending on antigen properties.61 Hence, investigations into syphilis mRNA vaccines should include evaluation of protection duration, antibody titers, and other relevant metrics during pre-clinical testing. Like other vaccine platforms, aligning research and development with product feasibility for use in LMIC settings enhances the likelihood of bringing a successful vaccine to market.

Outer membrane vesicle vaccines

Outer membrane vesicle (OMV) vaccines are an emerging platform for the development of vaccines against infectious pathogens.66 OMVs are released from bacteria, and thus retain both the lipid and protein structures from the bacterial membrane, but are non-replicative and noninfectious.66 Since OMVs are derived from bacterial membranes, they can naturally elicit an immune response and do not require adjuvants.66,67 However, OMVs also contain toxic membrane components such as LPS and pathogen associated molecular patterns (PAMPs), often requiring the removal of these components to ensure vaccine safety.66,68 Further, OMVs need to be purified from any cytoplasmic bacterial proteins released during cell lysis.66 The success of the OMV vaccines against Neisseria meningitidis serogroup B69 positions OMV vaccines as an attractive platform for the development of vaccines against infectious pathogens.

OMV vaccines can be produced by harvesting OMVs released from bacteria, termed natural OMV (nOMV), though this technique typically results in low yields. To address low nOMV yields, OMV vaccines can be produced using detergent extraction (dOMV) or by genetically modified bacteria that release increased levels of OMVs, termed mutant-derived OMV (mdOMV) or GMMA (Generalized Modules for Membrane Antigens).66 OMV vaccine manufacturing generally involves growth of bacterial strains, harvesting or induction of OMVs, and purification of OMVs using affinity purification, size exclusion chromatography, hydrostatic filtration dialysis, or differential centrifugation.67 As with other vaccine platforms, temperature sensitivity, cold chain limitations, and long-term potency are concerns for OMV vaccines. However, recent assessments demonstrate that OMV vaccines show promising stability and potency following long-term storage, with comparable or superior stability relative to other vaccine platforms.70 Other recent advances in OMV production include the development of continuous N. meningitidis OMV vaccine production, which is estimated to increase OMV yields 9-fold and significantly reduce production costs.71 Collectively these technology advances position OMV vaccines as an attractive platform for syphilis vaccine design.

Electron microscopy images suggest that T. pallidum may naturally produce OMVs though membrane blebbing.72 However, detection and isolation of T. pallidum OMVs is challenging due to low yields and technical limitations associated with T. pallidum growth. An alternative approach is to develop genetically engineered strains of N. meningitidis or other OMV production organisms to heterologously express T. pallidum vaccine candidates. An OMV vaccine production approach could address requirements associated with a syphilis vaccine, including the need to incorporate multiple T. pallidum proteins, accommodate the hydrophobic or amphipathic nature of many T. pallidum OMP vaccine candidates, and present T. pallidum proteins in a conformation that mimics the natural folding state of the proteins.39 Overall, the success of recent OMV vaccines,66,67,69 improved continuous OMV vaccine production71 and OMV stability and long-term potency,70 and decreased OMV vaccine manufacturing costs identify this platform as a leading candidate for a syphilis vaccine.

Conclusion

Syphilis is a growing global public health threat despite increased surveillance and treatment initiatives, which emphasizes the need for a syphilis vaccine to complement public health approaches to disease prevention. To ensure successful development of a syphilis vaccine, researchers and other stakeholders must align pre-clinical vaccine design with economic and technical feasibility, with prioritization of syphilis vaccine design that is compatible with delivery in LMIC settings. Key factors influencing feasibility include cost-effective production that achieves cGMP standards, and compatibility with the logistical requirements of vaccine delivery and cold-chain limitations in LMICs. By keeping these considerations in mind, researchers can develop products that have reasonable manufacturing costs and reduce the risk of product failure at early clinical stages due to manufacturing difficulties. Here we outline the optimal product performance characteristics of a syphilis vaccine, and assess the benefits, limitations, and costs for subunit, VLP, mRNA, and OMV vaccine platforms. Although the most effective platform for syphilis vaccine development is currently unknown, by proactively addressing the benefits and constraints associated with each platform during the design phase, researchers can maximize the likelihood of developing an effective vaccine that is feasible to produce and distribute on a global scale. This proactive approach is critical in the effort to eliminate syphilis and to foster equitable healthcare access globally.

Sean Waugh is a PhD candidate studying with Dr. Caroline Cameron at the University of Victoria (UVic) in Victoria, B.C., Canada. UVic is located on the unceded territories of the Lək̓ʷəŋən (Songhees and Esquimalt) Peoples, on whose traditional lands the university stands, and where the Lək̓ʷəŋən and W̱SÁNEĆ Peoples’ historical relationships with the land continue to this day. Sean’s research focuses on characterizing host-pathogen interactions, the host molecular response to Treponema pallidum, and using these discoveries to inform syphilis vaccine development.

Dr. Caroline Cameron is a Professor in the Department of Biochemistry and Microbiology at the University of Victoria and an Affiliate Professor in the Division of Infectious Diseases (Department of Medicine) at the University of Washington. Dr. Cameron also serves as President of the Canadian branch of the International Union against Sexually Transmitted Infections and President of the International Society for Sexually Transmitted Diseases Research Foundation. Dr. Cameron’s research program, which focuses on diagnostic and vaccine development for infectious and congenital syphilis, has been recognized with an NIH MERIT Award and a CIHR Canada Research Chair.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Author contributions

SW: Conceptualization, Formal analysis, Writing – original draft, Writing – review & editing.

CC: Conceptualization, Writing – review & editing, Funding acquisition, Project administration, Supervision, Writing – original draft.
==== Refs
References

1. LaFond RE, Lukehart SA. Biological basis for syphilis. Clin Microbiol Rev. 2006;19 (1 ):29–8. doi:10.1128/CMR.19.1.29-49.2006.16418521
2. Chen T, Wan B, Wang M, Lin S, Wu Y, Huang J. Evaluating the global, regional, and national impact of syphilis: results from the global burden of disease study 2019. Sci Rep. 2023;13 (1 ):11386. doi:10.1038/s41598-023-38294-4.37452074
3. Aho J, Lybeck C, Tetteh A, Issa C, Kouyoumdjian F, Wong J, Anderson A, Popovic N. Rising syphilis rates in Canada, 2011–2020. Can Commun Dis Rep. 2022;48 (23 ):52–60. doi:10.14745/ccdr.v48i23a01.35341093
4. Spiteri G, Unemo M, Mårdh O, Amato-Gauci AJ. The resurgence of syphilis in high-income countries in the 2000s: a focus on Europe. Epidemiol And Infect. 2019;147 :e143. doi:10.1017/S0950268819000281.30869043
5. Tsuboi M, Evans J, Davies EP, Rowley J, Korenromp EL, Clayton T, Taylor MM, Mabey D, Chico RM. Prevalence of syphilis among men who have sex with men: a global systematic review and meta-analysis from 2000–20. Lancet Glob Health. 2021;9 (8 ):1110–1118. doi:10.1016/S2214-109X(21)00221-7.
6. Tuddenham S, Hamill MM, Ghanem KG. Diagnosis and treatment of sexually transmitted infections: a review. JAMA. 2022;327 (2 ):161–172. doi:10.1001/jama.2021.23487.35015033
7. Peeling RW, Mabey D, Kamb ML, Chen X-S, Radolf JD, Benzaken AS. Syphilis. Nat Rev Dis Primers. 2017;3 (1 ):1–21. doi:10.1038/nrdp.2017.73.
8. Korenromp EL, Rowley J, Alonso M, Mello MB, Wijesooriya NS, Mahiané SG, Ishikawa N, Le L-V, Newman-Owiredu M, Nagelkerke N, et al. Global burden of maternal and congenital syphilis and associated adverse birth outcomes—estimates for 2016 and progress since 2012. PLOS ONE. 2019;14 (2 ):e0211720. doi:10.1371/journal.pone.0211720.30811406
9. Gilmour LS, Walls T. Congenital syphilis: a review of global epidemiology. Clin Microbiol Rev. 2023;36 (2 ):e00126–22. doi:10.1128/cmr.00126-22.36920205
10. Global guidance on criteria and processes for validation- elimination of mother-to-child transmission of HIV, syphilis and hepatitis B virus. [accessed 2024 Mar 4]. https://www.who.int/publications-detail-redirect/9789240039360.
11. Global health sector strategy on Sexually Transmitted Infections, 2016-2021. [accessed 2024 Mar 4]. https://www.who.int/publications-detail-redirect/WHO-RHR-16.09.
12. Murray CJL, Vos T, Lozano R, Naghavi M, Flaxman AD, Michaud C, Ezzati M, Shibuya K, Salomon JA, Abdalla S, et al. Disability-adjusted life years (DALYs) for 291 diseases and injuries in 21 regions, 1990–2010: a systematic analysis for the global burden of disease study 2010. Lancet. 2012;380 (9859 ):2197–2223. doi:10.1016/S0140-6736(12)61689-4.23245608
13. Xu C, Shi Y, Yu X, Chang R, Wang H, Chen H, Wang R, Liu Y, Liu S, Cai Y, et al. The syndemic condition of psychosocial problems related to depression among sexually transmitted infections patients. PeerJ. 2021;9 :e12022. doi:10.7717/peerj.12022.34616597
14. Champredon D, Cameron CE, Smieja M, Dushoff J. Epidemiological impact of a syphilis vaccine: a simulation study. Epidemiol And Infect. 2016;144 (15 ):3244–3252. doi:10.1017/S0950268816001643.27477823
15. Lieberman NAP, Avendaño CC, Bakhash SAKM, Nunley E, Xie H, Giacani L, Berzkalns A, Soge OO, Reid TB, Golden MR, et al. Genomic epidemiology of Treponema pallidum and circulation of strains with diminished TprK antigen variation capability in Seattle, 2021–2022. J Infect Dis. 2023;229 (3 ):866–875. doi:10.1093/infdis/jiad368.
16. Giacani L, Molini BJ, Kim EY, Godornes BC, Leader BT, Tantalo LC, Centurion-Lara A, Lukehart SA. Antigenic variation in Treponema pallidum: TprK sequence diversity accumulates in response to immune pressure during experimental syphilis. J Immunol. 2010;184 (7 ):3822–3829. doi:10.4049/jimmunol.0902788.20190145
17. Reid TB, Molini BJ, Fernandez MC, Lukehart SA, Morrison RP. Antigenic variation of TprK facilitates development of secondary syphilis. Infect Immun. 2014;82 (12 ):4959–4967. doi:10.1128/iai.02236-14.25225245
18. Morgan CA, Lukehart SA, Van Voorhis WC. Protection against syphilis correlates with specificity of antibodies to the variable regions of Treponema pallidum repeat protein K. Infect Immun. 2003;71 (10 ):5605–5612. doi:10.1128/IAI.71.10.5605-5612.2003.14500480
19. Kenyon C, Osbak KK, Crucitti T, Kestens L. Syphilis reinfection is associated with an attenuated immune profile in the same individual: a prospective observational cohort study. BMC Infect Dis. 2018;18 (1 ):479. doi:10.1186/s12879-018-3399-8.30253745
20. Cameron CE, Lukehart SA. Current status of syphilis vaccine development: need, challenges, prospects. Vaccine. 2014;32 (14 ):1602–1609. doi:10.1016/j.vaccine.2013.09.053.24135571
21. Carlson JA, Dabiri G, Cribier B, Sell S. The immunopathobiology of syphilis: the manifestations and course of syphilis are determined by the level of delayed-type hypersensitivity. Am J Dermatopathol. 2011;33 (5 ):433–460. doi:10.1097/DAD.0b013e3181e8b587.21694502
22. Gottlieb SL, Deal CD, Giersing B, Rees H, Bolan G, Johnston C, Timms P, Gray-Owen SD, Jerse AE, Cameron CE, et al. The global roadmap for advancing development of vaccines against sexually transmitted infections: update and next steps. Vaccine. 2016;34 (26 ):2939–2947. doi:10.1016/j.vaccine.2016.03.111.27105564
23. Cameron CE. Identification of a Treponema pallidum laminin-binding protein. Infect Immun. 2003;71 (5 ):2525–2533. doi:10.1128/IAI.71.5.2525-2533.2003.12704124
24. Hawley KL, Montezuma-Rusca JM, Delgado KN, Singh N, Uversky VN, Caimano MJ, Radolf JD, Luthra A. Structural modeling of the Treponema pallidum outer membrane protein repertoire: a road map for deconvolution of syphilis pathogenesis and development of a syphilis vaccine. J Bacteriol. 2021;203 (15 ):10–1128. doi:10.1128/jb.00082-21.
25. Lithgow KV, Hof R, Wetherell C, Phillips D, Houston S, Cameron CE. A defined syphilis vaccine candidate inhibits dissemination of Treponema pallidum subspecies pallidum. Nat Commun. 2017;8 (1 ):14273. doi:10.1038/ncomms14273.28145405
26. Brinkman MB, McGill MA, Pettersson J, Rogers A, Matějková P, Šmajs D, Weinstock GM, Norris SJ, Palzkill T. A novel Treponema pallidum antigen, TP0136, is an outer membrane protein that binds human fibronectin. Infect Immun. 2008;76 (5 ):1848–1857. doi:10.1128/IAI.01424-07.18332212
27. Cameron CE, Lukehart SA, Castro C, Molini B, Godornes C, Van Voorhis WC. Opsonic potential, protective capacity, and sequence conservation of the Treponema pallidum subspecies pallidum Tp92. J Infect Dis. 2000;181 (4 ):1401–1413. doi:10.1086/315399.10762571
28. Xu M, Xie Y, Zheng K, Luo H, Tan M, Zhao F, Zeng T, Wu Y. Two potential syphilis vaccine candidates inhibit dissemination of Treponema pallidum. Front Immunol. 2021;12 :12. doi:10.3389/fimmu.2021.759474.
29. Delgado KN, Montezuma-Rusca JM, Orbe IC, Caimano MJ, La Vake CJ, Luthra A, Hennelly CM, Nindo FN, Meyer JW, Jones LD, et al. Extracellular loops of the Treponema pallidum FadL orthologs TP0856 and TP0858 elicit IgG antibodies and IgG±specific B-cells in the rabbit model of experimental syphilis. mBio. 2022;13 (4 ):e0163922. doi:10.1128/mbio.01639-22.35862766
30. Lukehart SA, Molini B, Gomez A, Godornes C, Hof R, Fernandez MC, Pitner RA, Gray SA, Carter D, Giacani L, et al. Immunization with a tri-antigen syphilis vaccine significantly attenuates chancre development, reduces bacterial load, and inhibits dissemination of Treponema pallidum. Vaccine. 2022;40 (52 ):7676–7692. doi:10.1016/j.vaccine.2022.11.002.36376214
31. Morgan CA, Lukehart SA, Voorhis WCV. Immunization with the N-Terminal portion of Treponema pallidum repeat protein K attenuates syphilitic lesion development in the rabbit model. Infect Immun. 2002;70 (12 ):6811–6816. doi:10.1128/IAI.70.12.6811-6816.2002.12438357
32. Ferguson MR, Delgado KN, McBride S, Orbe IC, La Vake CJ, Caimano MJ, Mendez Q, Moraes TF, Schryvers AB, Moody MA, et al. Use of epivolve phage display to generate a monoclonal antibody with opsonic activity directed against a subdominant epitope on extracellular loop 4 of Treponema pallidum BamA (TP0326). Front Immunol. 2023;14 :1222267. doi:10.3389/fimmu.2023.1222267.37675118
33. Liu D, Chen R, Wang Y-J, Li W, Liu L-L, Lin L-R, Yang T-C, Tong M-L. Insights into the protective immune response by immunization with full-length recombinant TprK protein: cellular and humoral responses. NPJ Vaccines. 2023;8 (1 ):146. doi:10.1038/s41541-023-00748-1.37773233
34. Molini B, Fernandez MC, Godornes C, Vorobieva A, Lukehart SA, Giacani L. B-Cell epitope mapping of TprC and TprD variants of Treponema pallidum subspecies informs vaccine development for human treponematoses. Front Immunol. 2022;13 :13. doi:10.3389/fimmu.2022.862491.
35. Parker ML, Houston S, Pětrošová H, Lithgow KV, Hof R, Wetherell C, Kao W-C, Lin Y-P, Moriarty TJ, Ebady R, et al. The structure of Treponema pallidum Tp0751 (Pallilysin) reveals a non-canonical lipocalin fold that mediates adhesion to extracellular matrix components and interactions with host cells. PloS Pathog. 2016;12 (9 ):e1005919. doi:10.1371/journal.ppat.1005919.27683203
36. Li Q-L, Li W, Zheng X-Q, Ye W-M, Xu Q-Y, Ke W-J, Yang T-C. Screening the B- and T-cell epitope map of TP0136 and exploring their effect in a Treponema pallidum rabbit model. Biomed Pharmacother. 2023;167 :115628. doi:10.1016/j.biopha.2023.115628.37804809
37. Reid TB, Godornes C, Campbell VL, Laing KJ, Tantalo LC, Gomez A, Pholsena TN, Lieberman NAP, Krause TM, Cegielski VI, et al. Treponema pallidum periplasmic and membrane proteins are recognized by circulating and skin CD4+ T cells. J Infect Dis. [2024 June 27]:jiae245. doi:10.1093/infdis/jiae245.
38. Gomez A, Thompson L, Haimour A, Geppert A, Schovanek E, Houston S, Mateyko B, Waugh S, Ranasinghe A, Lukehart SA, et al. OS9.4 syphilis vaccine development: generating a stable and efficacious multi- epitope vaccine chimera through protein engineering. Sex Transm Dis. 2024;51 (1S ):43–4. doi:10.1097/OLQ.0000000000001886.
39. Goodyear MC, Cameron CE. How proteomics can inform vaccine design for sexually transmitted infections. Sex Transm Dis. 2024 June 11. doi:10.1097/OLQ.0000000000001986.
40. Liu A, Giacani L, Hawley KL, Cameron CE, Seña A, Konda K, Radolf JD, Klausner JD. New pathways in syphilis vaccine development. Sex Transm Dis. 2024 Apr. 11 :10. doi:10.1097/OLQ.0000000000002050.
41. Cameron CE. Syphilis vaccine development: requirements, challenges, and opportunities. Sexual Trans Dis. 2018;45 (9S ):17–19. doi:10.1097/OLQ.0000000000000831.
42. Investment case for eliminating mother-to-child transmission of syphilis. [accessed 2024 Mar 7]. https://www.who.int/publications-detail-redirect/9789241504348.
43. Garnett GP. The theoretical impact and cost-effectiveness of vaccines that protect against sexually transmitted infections and disease. Vaccine. 2014;32 (14 ):1536–1542. (Sexually transmitted infections: Vaccine development for global health). doi:10.1016/j.vaccine.2013.11.007.24606635
44. Gottlieb SL, Johnston C. Future prospects for new vaccines against sexually transmitted infections. Curr Opin Infect Dis. 2017;30 (1 ):77–86. doi:10.1097/QCO.0000000000000343.27922851
45. Kojima N, Konda KA, Klausner JD. Notes on syphilis vaccine development. Front Immunol. 2022;13 :13. doi:10.3389/fimmu.2022.952284.
46. Ávila-Nieto C, Pedreño-López N, Mitjà O, Clotet B, Blanco J, Carrillo J. Syphilis vaccine: challenges, controversies and opportunities. Front Immunol. 2023;14 :14. doi:10.3389/fimmu.2023.1126170.
47. Lithgow KV, Cameron CE. Vaccine development for syphilis. Expert Rev Vaccines. 2017;16 (1 ):37–44. doi:10.1080/14760584.2016.1203262.27328030
48. Molina J-M, Charreau I, Chidiac C, Pialoux G, Cua E, Delaugerre C, Capitant C, Rojas-Castro D, Fonsart J, Bercot B, et al. Post-exposure prophylaxis with doxycycline to prevent sexually transmitted infections in men who have sex with men: an open-label randomised substudy of the ANRS IPERGAY trial. Lancet Infect Dis. 2018;18 (3 ):308–317. doi:10.1016/S1473-3099(17)30725-9.29229440
49. WHO preferred product characteristics for gonococcal vaccines. [accessed 2024 Mar 28]. https://www.who.int/publications-detail-redirect/9789240039827.
50. WHO preferred product characteristics for herpes simplex virus vaccines. [accessed 2024 Mar 28]. https://www.who.int/publications-detail-redirect/9789241515580.
51. Plotkin S, Robinson JM, Cunningham G, Iqbal R, Larsen S. The complexity and cost of vaccine manufacturing – an overview. Vaccine. 2017;35 (33 ):4064–4071. doi:10.1016/j.vaccine.2017.06.003.28647170
52. Covarrubias CE, Rivera TA, Soto CA, Deeks T, Kalergis AM. Current GMP standards for the production of vaccines and antibodies: an overview. Front Public Health. 2022;10 :1021905. doi:10.3389/fpubh.2022.1021905.36743162
53. Wang M, Jiang S, Wang Y. Recent advances in the production of recombinant subunit vaccines in Pichia pastoris. Bioengineered. 2016;7 (3 ):155–165. doi:10.1080/21655979.2016.1191707.27246656
54. Huang C-J, Lin H, Yang X. Industrial production of recombinant therapeutics in Escherichia coli and its recent advancements. J Ind Microbiol And Biotechnol. 2012;39 (3 ):383–399. doi:10.1007/s10295-011-1082-9.22252444
55. Preferred Product Characteristics and Target Product Profiles. [accessed 2024 Mar 7]. https://www.who.int/teams/immunization-vaccines-and-biologicals/product-and-delivery-research/ppcs.
56. Mohsen MO, Zha L, Cabral-Miranda G, Bachmann MF. Major findings and recent advances in virus-like particle (vlp)-based vaccines. Semin Immunol. 2017;34 :123–132. doi:10.1016/j.smim.2017.08.014.28887001
57. Mohsen MO, Bachmann MF. Virus-like particle vaccinology, from bench to bedside. Cell Mol Immunol. 2022;19 (9 ):993–1011. doi:10.1038/s41423-022-00897-8.35962190
58. Nooraei S, Bahrulolum H, Hoseini ZS, Katalani C, Hajizade A, Easton AJ, Ahmadian G. Virus-like particles: preparation, immunogenicity and their roles as nanovaccines and drug nanocarriers. J Nanobiotechnol. 2021;19 (1 ):59. doi:10.1186/s12951-021-00806-7.
59. Tariq H, Batool S, Asif S, Ali M, Abbasi BH. Virus-like particles: revolutionary platforms for developing vaccines against emerging infectious diseases. Front Microbiol. 2022;12 :790121. doi:10.3389/fmicb.2021.790121.35046918
60. Moleirinho MG, Silva RJS, Alves PM, Carrondo MJT, Peixoto C. Current challenges in biotherapeutic particles manufacturing. Expert Opin Biol Ther. 2020;20 (5 ):451–465. doi:10.1080/14712598.2020.1693541.31773998
61. Chaudhary N, Weissman D, Whitehead KA. mRNA vaccines for infectious diseases: principles, delivery and clinical translation. Nat Rev Drug Discov. 2021;20 (11 ):817–838. doi:10.1038/s41573-021-00283-5.34433919
62. Xie C, Yao R, Xia X. The advances of adjuvants in mRNA vaccines. NPJ Vaccines. 2023;8 (1 ):1–6. doi:10.1038/s41541-023-00760-5.36650164
63. Kis Z, Kontoravdi C, Dey AK, Shattock R, Shah N. Rapid development and deployment of high-volume vaccines for pandemic response. J Adv Manuf & Process. 2020;2 (3 ):e10060. doi:10.1002/amp2.10060.33977274
64. Rosa SS, Prazeres DMF, Azevedo AM, Marques MPC. mRNA vaccines manufacturing: challenges and bottlenecks. Vaccine. 2021;39 (16 ):2190–2200. doi:10.1016/j.vaccine.2021.03.038.33771389
65. Zeng C, Zhang C, Walker PG, Dong Y. Formulation and delivery technologies for mRNA vaccines. Curr Top Microbiol Immunol. 2022;440 :71–110. doi:10.1007/82_2020_217.32483657
66. Micoli F, MacLennan CA. Outer membrane vesicle vaccines. Semin Immunol. 2020;50 :101433. (Challenges in Vaccinology). doi:10.1016/j.smim.2020.101433.33309166
67. Zhu Z, Antenucci F, Villumsen KR, Bojesen AM. Bacterial outer membrane vesicles as a versatile tool in vaccine research and the fight against antimicrobial resistance. mBio. 2021;12 (4 ):e0170721. doi:10.1128/mBio.01707-21.34372691
68. Mancini F, Rossi O, Necchi F, Micoli F. OMV vaccines and the role of TLR agonists in immune response. Int J Mol Sci. 2020;21 (12 ):4416. doi:10.3390/ijms21124416.32575921
69. O’Ryan M, Stoddard J, Toneatto D, Wassil J, Dull PM. A multi-component meningococcal serogroup B vaccine (4CMenB): the clinical development program. Drugs. 2014;74 (1 ):15–30. doi:10.1007/s40265-013-0155-7.24338083
70. Palmieri E, Arato V, Oldrini D, Ricchetti B, Aruta MG, Pansegrau W, Marchi S, Giusti F, Ferlenghi I, Rossi O, et al. Stability of outer membrane vesicles-based vaccines, identifying the most appropriate methods to detect changes in vaccine potency. Vaccines. 2021;9 (3 ):229. doi:10.3390/vaccines9030229.33800727
71. Gerritzen MJH, Stangowez L, van de Waterbeemd B, Martens DE, Wijffels RH, Stork M. Continuous production of Neisseria meningitidis outer membrane vesicles. Appl Microbiol Biotechnol. 2019;103 (23 ):9401–9410. doi:10.1007/s00253-019-10163-z.31676919
72. Izard J, Renken C, Hsieh C-E, Desrosiers DC, Dunham-Ems S, La Vake C, Gebhardt LL, Limberger RJ, Cox DL, Marko M, et al. Cryo-electron tomography elucidates the molecular architecture of Treponema pallidum, the syphilis spirochete. J Bacteriol. 2009;191 (24 ):7566–7580. doi:10.1128/JB.01031-09.19820083
