Streptococcus mutans is a well-recognized cariogenic factor for dental caries in humans [53]. This bacterium has the ability to produce biofilm and promote the adhesion and attachment of other bacteria and leading to cause several serious infections such as bacteremia, atheromatous plaques of atherosclerosis, and infective endocarditis [54–56]. S. mutans inhabits primarily in biofilms on the tooth enamel, known as dental plaque [1]. The biofilm produced by S. mutans supports resistance to different antimicrobial agents, and human immune clearance [57–60]. Thus, preventing the infection caused by this bacterium still becomes one of the most challenging issues in oral dental practice.
With the continuous emergence of antibiotic resistance in S. mutans, there needs an alternative strategy to prevent the infection, especially dental caries. Several studies had reported the application of several antimicrobial agents, and monovalent vaccines, however, these agents have limitations in triggering biofilm formation and inducing long last immune responses [61–63]. In this regard, peptide-based vaccines are an alternative approach, nevertheless, these monovalent vaccines provide short immune responses, and their development process is labor-intensive, expensive, and time-consuming [64, 65]. Due to advances in immunoinformatics, the introduction of multi-valent (multiepitope) based vaccines are considered over the conventional vaccine, since these multiepitope vaccines have cost-effectiveness, improved safety, and the ability to induce both native and acquired immune responses [66–68] as well as coverage diverse serotypes and geographic prevalence [17]. Currently, studies on a multiepitope vaccine against the S. mutans have been limited and no advanced treatment is available for dental caries and it has long-lasting prevention [69]. Therefore, to address these challenges, we proposed a multiepitope vaccine by fusing five highly conserved antigenic proteins of PBP2X, PBP2b, MurG, ATP-F, and AGPAT using immunoinformatics approach to contrive a multiepitope vaccine of S. mutans.
In this study, the subtractive genomic approach of S. mutans core-genome identified class B penicillin-binding proteins (PBP2X and PBP2b), MurG, ATP-F, and AGPAT. It was reported that PBP2X and PBP2b are essential in S. pneumonia for the septal and peripheral peptidoglycan synthesis, respectively [70, 71] and considered important targets for the beta-lactam antibiotics [72]. Previous studies suggested that these proteins have high immunogenic properties and could be used as vaccine candidates in Neisseria meningitidis, and Staphylococcus aureus [73, 74]. MurG is an essential bacterial glycosyltransferase that is involved in the biosynthesis of peptidoglycan which is a target for the design of new antibiotics [75]. The ATP synthase b (ATP-F) was found to have antigenic potential (Table S2) and the study conducted by Modhammand Nahian Rahman also suggested that epitopes of this protein were non-allergenic, non-toxic, and immunogenic properties that have the ability to trigger a host defense mechanism [76]. 1-acylglycerol-3-phosphate O-acyltransferase (AGPAT) is involved in the de-novo synthesis of triacylglycerol (TAG) and glycerophospholipids and thus is important for lipid storage/transport and membrane biogenesis (Röttig and Steinbüchel, 2013). AGPAT has not been reported as a vaccine candidate, however, it was suggested that this protein elicits an immune response due to its high antigenic properties [77]. The information on these five proteins as vaccine candidates were limited, however, these proteins owned the high antigenicity (Table S2) defined by Vaxign [30] as well as AntigenPro [31] and were chosen as vaccine candidates for the construction of multiepitope vaccine.
Several bioinformatic tools have been employed on these selected five vaccine candidate proteins to identify linear B and T-cell epitopes that could trigger both cellular and humoral immunity [23] and incorporate them into the multiepitope vaccine construction. The T-cell epitopes are essential for adaptive immune response and are sufficient to cooperate with MHC molecules [78]. The CTL epitopes with antigenicity aid in the recognition of foreign antigen fragments on MHC-I molecules and neutralized the target cells while in case of HTL epitopes prediction, several epitopes have been selected that has a binding affinity to HLA-DRB1*01:01, HLA-DRB1*01:03, HLA-DRB1*03:01, HLA-DRB1*04:01, HLA-DRB1*04:02, HLA-DRB1*04:04, and HLA-DRB1*07:01. Geographical differences impact the development of vaccines because they influence the variety of HLA allele expression throughout the world [79].
The HTL epitopes are to be selected based on their ability to produce INF-ꝩ, TNF, and IL-10 cytokines which play key functions against pathogens [80]. According to Kang et al., 2018, INF-ꝩ plays a critical role in the activation of both innate and adaptive immune responses [81]. In addition to helping diagnose diseases, B-cell epitopes are important in the creation of epitope vaccines. The MHC-II-processed epitopes are delivered to T cells by these cells and recognized by B-cell receptors. T-cell receptors were able to recognize its properties as a result. According to Naveed et al., 2022, B cells can also develop into memory cells and plasma cells which generate antibodies [82]. The development of an immunotherapeutic and preventive vaccination depends on the prediction of these epitopes.
According to Apostólico et al., 2016, an adjuvant is crucial for enhancing and directing the adaptive immune response to vaccination antigens [84]. The 50S ribosomal L7/L12 protein was employed in this investigation as an adjuvant, and it has been reported that this protein has been demonstrated to encourage dendritic cells, CD4+, CD8+, and INF-producing cells to mature after the simulation of naive T-cells [85]. The predicted CTL, HTL, and B-cells epitopes were linked together using AAY, GPGPG, and KK linkers, respectively. These linkers are flexible and hydrophilic amino acids that help to prevent domain disruption [86]. To improve the immunogenic qualities of the multiepitope vaccine, an EAAAK, a stiff linker, was fused between the adjuvant and the epitope sequences [87]. The multiepitope vaccine that was created demonstrated improved antigenicity and stability scores.
To facilitate subsequent experimental evaluations of the vaccination and the effective setup of in-vitro and in-vivo experiments, the final multiepitope vaccine was assessed for its physiochemical properties. The purpose multiepitope vaccine has a molecular weight of 49.7 kDa, which is considered a good vaccine candidate and could be easier to clone and express in an expression system [88]. The theoretical pI of the multiepitope vaccine was estimated as 8.20, suggesting that the vaccine is basic. The constructed multiepitope vaccine has a negative GRAVY value (-0.030) indicating that the vaccine is hydrophilic and has a high degree of solubility, suggesting that the designed vaccine will interact better with water molecules [19]. Considering that this multiepitope vaccine's half-lives in mammalian, yeast, and bacterial cells are 30, 20, and 10 hours correspondingly, it is possible to infer that it could expose the immune system for a longer length of time and elicit more immunological responses [32, 89].
The 3D structure of the multiepitope vaccine generated by AlphaFold v.2 was then refined by the GalaxyRefine server. In the initial 3D vaccine model, the Ramachandra plot revealed that 87.5% of residues were in favor regions, which was then improved to 97.1% residues in favor regions after structure refinement. Also, the Z-score was the designed multiepitope vaccine calculated by the ProSA server was improved from − 7.66 to -7.72 after structural refinement. The Z-score with more negative values implies that the refined 3D model of a multiepitope vaccine has high structural quality [90]. Discontinuous B-cell epitope plays an important role in humoral immune responses by the production of antibodies [91]. In this study, the ElliPro server identified 11 discontinuous B-cell epitopes in the refined 3D model of the multiepitope vaccine, revealing that the designed multiepitope vaccine has the capability to induce large amounts of antibody production. Enhancing protein stability is of vital importance in numerous biological and therapeutic applications [79]. To increase the protein's thermostability, four residue pair of mutations in the ALA250-ALA262, ALA325-SER345, GLY335-VAL338, and Leu435-ALA439 residue pairs were made, along with disulfide engineering.
In addition, the immunological simulation outcomes showed that the vaccine formulation induced an antibody cascade against S. mutans that closely mimicked the typical immune response to pathogen infections. After vaccinations, secondary and tertiary immune responses were found to be stronger than initial immune responses, with significant numbers of antibodies being generated and antigens being eliminated [79]. This study found that the multiepitope vaccine was effective in boosting both cell-mediated and humoral immunity, as evidenced by an increase in B-cells (memory B-cell and plasma B-cell) and T-cells (cytotoxic and helper T-cell) for up to 350 days after immunization. The amount of IFN-ꝩ, a pro-inflammatory protein, increased in this study before falling after the third dose. It was clear that there were very few anti-inflammatory cytokines present.
Additionally, to examine the capability of the constructed multiepitope vaccine to bind with TLRs (Toll-Like Receptors) on immune cells, the TLR-2, TLR-3, and TLR-4 were docked with the proposed multiepitope vaccine. TLRs on immune cells identify pathogens that have penetrated the mucosal barrier and trigger the adaptive immune response [92]. This docking experiment demonstrated that the TLR-4 has the lowest binding energy of -1397.7 kcal/mol and revealed a binding affinity of -20.3 ΔG (kcal mol-1), indicating the highest binding affinity with the lowest dissociation constant (1.20E-15 kd (M). According to a previous study, during bacterial infection, TLR-4 is activated by toxins or lipopolysaccharides and causes pro-inflammatory reactions that aid in the elimination of invasive pathogens [93]. TLR-3 appears to be a preventive factor against infections since it detects viral load [94]. TLR2, on the other hand, has been demonstrated to have a protective role during infection by inducing a potent pro-inflammatory response that is thought to be helpful for bacterial clearance [95, 96]. The docking investigation revealed that the TLR-2 and TLR-3 had a high affinity for interacting with the designed multiepitope vaccine which possesses a low dissociation constant. These findings revealed that the vaccine could potentially elicit an immunological response and remove the burdens of S. mutans infections.
Furthermore, the docking of the multiepitope vaccine construct with MHC-I, and MHC-II molecules showed reliable interaction with low energy scores. These molecules play critical roles in pathogen clearance [95]. MHC-I molecules convey intracellular pathogen-derived peptides to cytotoxic CD8 + T lymphocytes, causing infected cells to be eliminated. MHC-II molecules activate immune responses and coordinate pathogen clearance by presenting extracellular pathogen-derived peptides to CD4 + T cells [97]. These MHC molecules are essential to simulate together for effective immune recognition and response against infections. According to this docking result analysis, the multiepitope vaccine has shown reasonable docking energy with these immune receptors, suggesting that they may be able to trigger the subsequent immunological responses [98]. Using a molecular dynamics simulation of the docked complex, it was possible to determine the stability and mobility of the immune receptor-multiepitope vaccination complex in the mimic biological environment. The TLR-3-vaccine complex and the TLR-4-vaccine complex both have high eigenvalue values, according to the molecular dynamic analysis. Compared to the TLR-4-vaccine complex, the TLR-3-vaccine complex showed higher eigenvalues, indicating a lower degree of deformation and improved stability [99]. When compared to other protein complexes, the TLR-4-vaccine complex has a low eigenvalue, which could indicate that it vibrates at a low frequency or is not very flexible. A low eigenvalue often indicates that the protein complex is stiffer and less flexible in terms of stability (Sayed et al. 2020). In some circumstances, rigidity in protein complexes might be advantageous since it may signify a clearly defined and stable structure [73].
The E. coli cell culture technique is frequently used for mass production of recombinant proteins. To ensure efficient expression in the host, codon optimization for E. coli strain K12 was prepared. A codon adaptability index (CAI) of 0.99 and a GC content of 49.13% were discovered to guarantee a better level of protein production in E. coli expression systems [100, 101]. Also, it was suggested that for target organism expression, a CAI higher than 0.890 and a GC content between 30 and 70% are optimal [102].