LT adjuvant modulates epitope specificity and improves the efficacy of murine antibodies elicited by sublingual vaccination with the N-terminal domain of Streptococcus mutans P1

The Evolving Microbiome of Dental Caries

Dental caries is a significant oral and public health problem worldwide, especially in low-income populations. The risk of dental caries increases with frequent intake of dietary carbohydrates, including sugars, leading to increased acidity and disruption of the symbiotic diverse and complex microbial community of health. Excess acid production leads to a dysbiotic shift in the bacterial biofilm composition, demineralization of tooth structure, and cavities. Highly acidic and acid-tolerant species associated with caries include Streptococcus mutans, Lactobacillus, Actinomyces, Bifidobacterium, and Scardovia species. The differences in microbiotas depend on tooth site, extent of carious lesions, and rate of disease progression. Metagenomics and metatranscriptomics not only reveal the structure and genetic potential of the caries-associated microbiome, but, more importantly, capture the genetic makeup of the metabolically active microbiome in lesion sites. Due to its multifactorial nature, caries has been difficult to prevent. The use of topical fluoride has had a significant impact on reducing caries in clinical settings, but the approach is costly; the results are less sustainable for high-caries-risk individuals, especially children. Developing treatment regimens that specifically target S. mutans and other acidogenic bacteria, such as using nanoparticles, show promise in altering the cariogenic microbiome, thereby combatting the disease.

Mucosal Immunization Has Benefits over Traditional Subcutaneous Immunization with Group A Streptococcus Antigens in a Pilot Study in a Mouse Model

Group A Streptococcus (GAS) is a major human pathogen for which there is no licensed vaccine. To protect against infection, a strong systemic and mucosal immune response is likely to be necessary to prevent initial colonization and any events that might lead to invasive disease. A broad immune response will be necessary to target the varied GAS serotypes and disease presentations. To this end, we designed a representative panel of recombinant proteins to cover the stages of GAS infection and investigated whether mucosal and systemic immunity could be stimulated by these protein antigens. We immunized mice sublingually, intranasally and subcutaneously, then measured IgG and IgA antibody levels and functional activity through in vitro assays. Our results show that both sublingual and intranasal immunization in the presence of adjuvant induced both systemic IgG and mucosal IgA. Meanwhile, subcutaneous immunization generated only a serum IgG response. The antibodies mediated binding and killing of GAS cells and blocked binding of GAS to HaCaT cells, particularly following intranasal and subcutaneous immunizations. Further, antigen-specific assays revealed that immune sera inhibited cleavage of IL-8 by SpyCEP and IgG by Mac/IdeS. These results demonstrate that mucosal immunization can induce effective systemic and mucosal antibody responses. This finding warrants further investigation and optimization of humoral and cellular responses as a viable alternative to subcutaneous immunization for urgently needed GAS vaccines.

A Nanoparticle-Based Anticaries Vaccine Enhances the Persistent Immune Response To Inhibit Streptococcus mutans and Prevent Caries

Caries vaccines have been identified as a good strategy for the prevention of caries through the mechanism of inoculation against Streptococcus mutans, which is the main etiological bacterium causing caries. Protein antigen c (PAc) of S. mutans has been administered as an anticaries vaccine but shows relatively weak immunogenicity to elicit a low-level immune response. Here, we report a zeolitic imidazolate framework-8 nanoparticle (ZIF-8 NP)-based adjuvant with good biocompatibility, pH responsiveness, and high loading performance for PAc that was used as an anticaries vaccine. In this study, we prepared a ZIF-8@PAc anticaries vaccine and investigated the immune responses and anticaries efficacy induced by this vaccine in vitro and in vivo. ZIF-8 NPs substantially improved the internalization of PAc in lysosomes for further processing and presentation to T lymphocytes. In addition, significantly higher IgG antibody titers, cytokine levels, splenocyte proliferation indices, and percentages of mature dendritic cells (DCs) and central memory T cells were detected in mice subcutaneously immunized with ZIF-8@PAc than in mice subcutaneously immunized with PAc alone. Finally, rats were immunized with ZIF-8@PAc, and ZIF-8@PAc elicited a strong immune response to inhibit colonization by S. mutans and improve prophylactic efficacy against caries. Based on the results, ZIF-8 NPs are promising as an adjuvant for anticaries vaccine development. IMPORTANCE Streptococcus mutans is the main etiologic bacterium of dental caries, whose protein antigen c (PAc) has been administered as an anticaries vaccine. However, the immunogenicity of PAc is relatively weak. To improve the immunogenicity of PAc, ZIF-8 NP was used as an adjuvant, and the immune responses and protective effect elicited by ZIF-8@PAc anticaries vaccine were evaluated in vitro and in vivo. The findings will help in prevention of dental caries and provide new insight for the development of anticaries vaccine in the future.

Streptococcus mutans glutamate binding protein (GlnH) as antigen target for a mucosal anti-caries vaccine

BACKGROUND

In recent years, several studies have demonstrated that bacterial ABC transporters present relevant antigen targets for the development of vaccines against bacteria such as Streptococcus pneumoniae and Enterococcus faecalis. In Streptococcus mutans, the glutamate transporter operon (glnH), encoding an ABC transporter, is associated with acid tolerance and represents an important virulence-associated factor for the development of dental caries.

RESULTS

In this study, we generated a recombinant form of the S. mutans GlnH protein (rGlnH) in Bacillus subtilis. Mice immunized with this protein antigen elicited strong antigen-specific antibody responses after sublingual administration of a vaccine formulation containing a mucosal adjuvant, a non-toxic derivative of the heat-labile toxin (LTK63) originally produced by enterotoxigenic Escherichia coli (ETEC) strains. Serum anti-rGlnH antibodies reduced adhesion of S. mutans to the oral cavity of naïve mice. Moreover, mice actively immunized with rGlnH were partially protected from oral colonization after exposure to the S. mutans NG8 strain.

CONCLUSIONS

Our results indicate that S. mutans rGlnH is a potential target antigen capable of inducing specific and protective antibody responses after immunization. Overall, these observations raise the prospect of the development of mucosal anti-caries vaccines.

Molecular mechanisms of inhibiting glucosyltransferases for biofilm formation in Streptococcus mutans

Glucosyltransferases (Gtfs) play critical roles in the etiology and pathogenesis of Streptococcus mutans (S. mutans)- mediated dental caries including early childhood caries. Gtfs enhance the biofilm formation and promotes colonization of cariogenic bacteria by generating biofilm extracellular polysaccharides (EPSs), the key virulence property in the cariogenic process. Therefore, Gtfs have become an appealing target for effective therapeutic interventions that inhibit cariogenic biofilms. Importantly, targeting Gtfs selectively impairs the S. mutans virulence without affecting S. mutans existence or the existence of other species in the oral cavity. Over the past decade, numerous Gtfs inhibitory molecules have been identified, mainly including natural and synthetic compounds and their derivatives, antibodies, and metal ions. These therapeutic agents exert their inhibitory role in inhibiting the expression gtf genes and the activities and secretion of Gtfs enzymes with a wide range of sensitivity and effectiveness. Understanding molecular mechanisms of inhibiting Gtfs will contribute to instructing drug combination strategies, which is more effective for inhibiting Gtfs than one drug or class of drugs. This review highlights our current understanding of Gtfs activities and their potential utility, and discusses challenges and opportunities for future exploration of Gtfs as a therapeutic target.

Immunodominance of Epitopes and Protective Efficacy of HI Antigen Are Differentially Altered Using Different Adjuvants in a Mouse Model of Staphylococcus aureus Bacteremia

HI, a fusion protein that consists of the alpha-toxin (Hla) and the N2 domain of iron surface determinant B (IsdB), is one of the antigens in the previously reported S. aureus vaccine rFSAV and has already entered phase II clinical trials. Previous studies revealed that HI is highly immunogenic in both mice and healthy volunteers, and the humoral immune response plays key roles in HI-mediated protection. In this study, we further investigated the protective efficacy of immunization with HI plus four different adjuvants in a mouse bacteremia model. Results showed that HI-mediated protection was altered in response to different adjuvants. Using antisera from immunized mice, we identified seven B-cell immunodominant epitopes on Hla and IsdB, including 6 novel epitopes (Hla_1-18, Hla_84-101, Hla_186-203, IsdB_342-359, IsdB_366-383, and IsdB_384-401). The immunodominance of B-cell epitopes, total IgG titers and the levels of IFN-γ and IL-17A from mice immunized with HI plus different adjuvants were different from each other, which may explain the difference in protective immunity observed in each immunized group. Thus, our results indicate that adjuvants largely affected the immunodominance of epitopes and the protective efficacy of HI, which may guide further adjuvant screening for vaccine development and optimization.

The Multifaceted Nature of Streptococcal Antigen I/II Proteins in Colonization and Disease Pathogenesis

Streptococci are Gram-positive bacteria that belong to the natural microbiota of humans and animals. Certain streptococcal species are known as opportunistic pathogens with the potential to cause severe invasive disease. Antigen I/II (AgI/II) family proteins are sortase anchored cell surface adhesins that are nearly ubiquitous across streptococci and contribute to many streptococcal diseases, including dental caries, respiratory tract infections, and meningitis. They appear to be multifunctional adhesins with affinities to various host substrata, acting to mediate attachment to host surfaces and stimulate immune responses from the colonized host. Here we will review the literature including recent work that has demonstrated the multifaceted nature of AgI/II family proteins, focusing on their overlapping and distinct functions and their important contribution to streptococcal colonization and disease.

Roles of membrane vesicles from Streptococcus mutans for the induction of antibodies to glucosyltransferase in mucosal immunity

Glucosyltransferase (Gtf) B and GtfC from Streptococcus mutans are key enzymes for the development of biofilm-associated diseases such as dental caries. Gtfs are involved in membrane vesicles (MVs) and function in the formation of biofilms by initial colonizers such as Streptococcus mitis and Streptococcus oralis on the tooth surface. Therefore, MVs may be important virulence factors and targets for the prevention of biofilm-associated disease. To clarify how GtfB encoded by gtfB and GtfC encoded by gtfC associate with MVs and whether MVs are effective as a mucosal immunogen to induce the production of antibodies against Gtfs, MVs from S. mutans UA159 wild-type (WT), gtfB^-, gtfC^- and gtfB^-C^- were extracted from culture supernatants by ultracentrifugation and observed by scanning electron microscopy. Compared with GtfB, GtfC was mainly contained in MVs and regulated the size and aggregation of MVs, and the biofilm formation of S. mutans. The intranasal immunization of BALB/c mice with MVs plus a TLR3 agonist, poly(I-C), was performed 2 or 3 times for 5 weeks, with an interval of 2 or 3 weeks. MVs from all strains caused anti-MV IgA and IgG antibody production. In quality analysis of these antibodies, the IgA and IgG antibodies produced by immunization with MVs from WT and gtfB^- strains reacted with Gtfs in the saliva, nasal wash and serum but those produced by immunization with MVs from gtfC^- and gtfB^-C^- strains did not. S. mutans MVs mainly formed by GtfC are an intriguing immunogen for the production of anti-Gtf antibodies in mucosal immunogenicity.

Precision Reengineering of the Oral Microbiome for Caries Management

Technological advancements have revolutionized our understanding of the complexity and importance of the human microbiome. This progress has also emphasized the need for precision therapeutics, as it has underscored the dilemmas, such as dysbiosis and increasing antibiotic resistance, associated with current, broad-spectrum treatment modalities. Dental caries remains the most common chronic disease worldwide, accompanied by a tremendous financial and social burden, despite widespread and efficacious fluoride and hygienic regimens. Over the past several decades, various precision approaches to combat dental caries, including vaccines, probiotics, and antimicrobial compounds, have been pursued. Despite the distinct overall conceptual strengths of each approach, for various reasons, there are currently no approved precision antibiotic therapeutics to prevent dental caries. Specifically targeted antimicrobial peptides (STAMPs) are synthetic molecules that combine the antibiotic moiety of a traditional antimicrobial peptide with a targeting domain to provide specificity against a particular organism. Conjoining the killing domain from the antimicrobial, novispirin G10, and a targeting domain derived from the Streptococcus mutans pheromone, CSP, the STAMP C16G2 was designed to provide targeted killing of S. mutans, widely considered the keystone species in dental caries pathogenesis. C16G2 was able to selectively eliminate S. mutans from complex ecosystems while leaving closely related, yet health-associated, oral species unharmed. This remodeling of the dental plaque community is expected to have significant advantages compared to conventional broad-spectrum mouthwashes, as the intact, surviving community is apt to prevent reinfection by pathogens. Following successful phase I clinical trials that evaluated the safety and basic microbiology of C16G2 treatments, the phase II trials of several C16G2 formulations are currently in progress. C16G2 represents an exciting advance in precision therapeutics, and the STAMP platform provides vast opportunities for both the development of additional therapeutics and the overall study of microbial ecology.

Dental caries vaccine: are we there yet?

Dental caries, caused by Streptococcus mutans, is a common infection. Caries vaccine has been under investigation for the last 40 years. Many in vitro and in vivo studies and some human clinical trials have determined many pertinent aspects regarding vaccine development. The virulence determinants of Strep. mutans, such as Ag I/II, responsible for adherence to surfaces, glucosyltransferase, responsible for the production of glucan, and the glucan-binding protein, responsible for the attachment of glucan to surfaces, have been known to elicit an antigen-specific immune response. It is also known that more than one antigen or a functional part of the genome responsible for these virulence determinants provide a better host response compared with the monogenic vaccine or complete genome of a specific antigen. To enhance the host response, the use of adjuvants has been studied and the routes of antigen administration have been investigated. In recent years, some promising vaccines such as pGJA-P/VAX, LT derivative/Pi_39-512 , KFD2-rPAc and SBR/GBR-CMV-nirB have been developed and tested in animals. New virulence targets need to be explored. Multicentre collaborative studies and human clinical trials are required and some interest from funders and public health experts should be generated to overcome this hurdle. SIGNIFICANCE AND IMPACT OF THE STUDY: Dental caries is an irreversible, multifactorial opportunistic infection. The treatment is costly, making it a public health problem. Despite many years of promising laboratory research, animal studies and clinical trials, there is no commercially available vaccine today. The research objectives have become more refined from lessons learnt over the years. Multigenic DNA/recombinant vaccines, using the best proved adjuvants with a delivery system for the nasal or sublingual route, should be developed and researched with multicentre collaborative efforts. In addition, new vaccine targets can be identified. To overcome the economic hurdle, funders and public health interest should be stimulated.

Exploiting the Oral Microbiome to Prevent Tooth Decay: Has Evolution Already Provided the Best Tools?

To compete in the relatively exposed oral cavity, resident microbes must avoid being replaced by newcomers. This selective constraint, coupled with pressure on the host to cultivate a beneficial microbiome, has rendered a commensal oral microbiota that displays colonization resistance, protecting the human host from invasive species, including pathogens. Rapid increases in carbohydrate consumption have disrupted the evolved homeostasis between the oral microbiota and dental health, reflected by the high prevalence of dental caries. Development of novel modalities to prevent caries has been the subject of a breadth of research. This mini review provides highlights of these endeavors and discusses the rationale and pitfalls behind the major avenues of approach. Despite efficacy, fluoride and other broad-spectrum interventions are unlikely to further reduce the incidence of dental caries. The most promising methodologies in development are those that exploit the exclusive nature of the healthy oral microbiome. Probiotics derived from the dental plaque of healthy individuals sharply antagonize cariogenic species, such as Streptococcus mutans. Meanwhile, targeted antimicrobials allow for the killing of specific pathogens, allowing reestablishment of a healthy microbiome, presumably with its protective effects. The oral microbiota manufactures a massive array of small molecules, some of which are correlated with health and are likely to antagonize pathogens. The prohibitive cost associated with sufficiently rigorous clinical trials, and the status of dental caries as a non-life-threatening condition will likely continue to impede the advancement of new therapeutics to market. Nevertheless, there is room for optimism, as it appears evolution may have already provided the best tools.

The Biology of Streptococcus mutans

As a major etiological agent of human dental caries, Streptococcus mutans resides primarily in biofilms that form on the tooth surfaces, also known as dental plaque. In addition to caries, S. mutans is responsible for cases of infective endocarditis with a subset of strains being indirectly implicated with the onset of additional extraoral pathologies. During the past 4 decades, functional studies of S. mutans have focused on understanding the molecular mechanisms the organism employs to form robust biofilms on tooth surfaces, to rapidly metabolize a wide variety of carbohydrates obtained from the host diet, and to survive numerous (and frequent) environmental challenges encountered in oral biofilms. In these areas of research, S. mutans has served as a model organism for ground-breaking new discoveries that have, at times, challenged long-standing dogmas based on bacterial paradigms such as Escherichia coli and Bacillus subtilis. In addition to sections dedicated to carbohydrate metabolism, biofilm formation, and stress responses, this article discusses newer developments in S. mutans biology research, namely, how S. mutans interspecies and cross-kingdom interactions dictate the development and pathogenic potential of oral biofilms and how next-generation sequencing technologies have led to a much better understanding of the physiology and diversity of S. mutans as a species.