Targeted nasal vaccination provides antibody-independent protection against Staphylococcus aureus

Particle Nanoarchitectonics for Nanomedicine and Nanotherapeutic Drugs with Special Emphasis on Nasal Drugs and Aging

Aging is a multifunctional physiological manifestation. The nasal cavity is considered a major site for easy and cost-effective drug and vaccine administration, due to high permeability, low enzymatic activity, and the presence of a high number of immunocompetent cells. This review article primarily focuses on aging genetics, physical parameters, and the use of nanoparticles as delivery systems of drugs and vaccines via the nasal cavity. Studies have identified various genes involved in centenarian and average-aged people. VEGF is a key mediator involved in angiogenesis. Different therapeutic approaches induce vascular function and angiogenesis. FOLR1 gene codes for folate receptor alpha protein that helps in regulating the transport of vitamin B folate, 5-methyltetrahydrofolate and folate analogs inside the cell. This gene also aids in slowing the aging process down by cellular regeneration and promotes healthy aging by reducing aging symptoms. It has been found through the literature that GATA 6, Yamanaka factors, and FOLR1 work in synchronization to induce healthy and delayed aging. The role and applications of genes including CBS, CISD, SIRT 1, and SIRT 6 play a significant role in aging.

B and T cell epitope-based peptides predicted from clumping factor protein of Staphylococcus aureus as vaccine targets

Staphylococcus aureus infection is emerging as a global threat because of the highly debilitating nature of the associated disease's unprecedented magnitude of its spread and growing global resistance to antimicrobial medicines. Recently WHO has categorized these bacteria under the high global priority pathogen list and is one of the six nosocomial pathogens termed as ESKAPE pathogens which have emerged as a serious threat to public health worldwide. The development of a specific vaccine can stimulate an optimal antibody response, thus providing immunity against it. Therefore, in the present study efforts have been made to identify potential vaccine candidates from the Clumping factor surface proteins (ClfA and ClfB) of S. aureus. Employing the immunoinformatics approach, fourteen antigenic peptides including T-cell, B-cell epitopes were identified which were non-toxic, non-allergenic, high antigenicity, strong binding efficiency with commonly occurring MHC alleles. Consequently, a multi-epitope vaccine chimera was designed by connecting these epitopes with suitable linkers an adjuvant to enhance immunogenicity. Further, homology modeling and molecular docking were performed to construct the three-dimensional structure of the vaccine and study the interaction between the modeled structure and immune receptor (TLR-2) present on lymphocyte cells. Consequently, molecular dynamics simulation for 100 ns period confirmed the stability of the interaction and reliability of the structure for further analysis. Finally, codon optimization and in silico cloning were employed to ensure the successful expression of the vaccine candidate. As the targeted protein is highly antigenic and conserved, hence the designed novel vaccine construct holds potential against emerging multi-drug-resistant organisms.

Identification of CD4+ T cell epitopes from Staphylococcus aureus secretome using immunoinformatic prediction and molecular docking

One major reason for the lack of clinical success of Staphylococcus aureus vaccine candidates is the inability of the antigens to develop a CD4⁺ T cell-mediated immune response. Hence, it is important to identify CD4⁺ T cell antigens from S. aureus. CD4⁺ T cells are activated following the presentation of epitopes derived from exogenous proteins on HLA class II molecules. Fifty-nine secretory proteins of S. aureus were analyzed computationally for the presence of HLA class II binding peptides. Fifteen-mer peptides were generated, and their binding to 26 HLA class II alleles was predicted. The structural feasibility of the peptides binding to HLA-II was studied using molecular docking. Of the 16,724 peptides generated, 6991 (41.8%) were predicted to bind to any one of the alleles with an IC₅₀ value below 50 nM. Comparative sequence analysis revealed that only 545 of the strong binding peptides are non-self in the human system. Approximately 50% of the binding peptides were monoallele-specific. Moreover, approximately 95% of the predicted strong binding non-self peptides interacted with the binding groove of at least one HLA class II molecule with a glide score better than -10 kcal/mol. On the basis of the analysis of the strength of binding, non-self presentation in the human host, propensity to bind to a higher number of alleles, and energetically favorable interactions with HLA molecules, a set of 11 CD4⁺ T cell epitopes that can be used as vaccine candidates was identified.

A comparative investigation on the role and interaction of EsxA and EsxB in host immune response

Staphylococcus aureus (S. aureus) is a frequent and major cause of bovine mastitis; it poses a tremendous economic burden to dairy industries of numerous countries. Early-secretion antigen-6 secretion system (ESS) has been viewed as an essential virulence and pathogenic factor of S. aureus. EsxA and EsxB are small acidic proteins secreted by ESS and identified as potential T-cell antigens of S. aureus. Unlike those of Mycobacterium tuberculosis (M. tuberculosis), the EsxA and EsxB of S. aureus do not form a dimer. Instead, EsxA dimerizes with itself or EsaC. Therefore, the interaction of EsxA and EsxB remains incompletely understood. In this study, to explore their interactions, EsxA and EsxB were expressed and used for immunization, alone or in combination, of murine infection models. Both components can interact with each other. Through the analysis of the immune response by immunological method, EsxB could significantly enhance the EsxA-specific IgG2a antibody level and increase the proliferation proportion of CD8⁺ T cells. These results indicate that when vaccinated with EsxA, EsxB can play a critical role in stimulating T helper 1 immunity by activating IgG2a and CD8⁺ T cells. We further show that vaccination with the combination of EsxA and EsxB resulted in enhanced stimulation of TLR-4 and improved protection against S. aureus. The findings may help us better understand the role of EsxB in the virulence and pathogenesis of S. aureus.

Molecular and cellular cues governing nanomaterial-mucosae interactions: from nanomedicine to nanotoxicology

Mucosal tissues constitute the largest interface between the body and the surrounding environment and they regulate the access of molecules, supramolecular structures, particulate matter, and pathogens into it. All mucosae are characterized by an outer mucus layer that protects the underlying cells from physicochemical, biological and mechanical insults, a mono-layered or stratified epithelium that forms tight junctions and controls the selective transport of solutes across it and associated lymphoid tissues that play a sentinel role. Mucus is a gel-like material comprised mainly of the glycoprotein mucin and water and it displays both hydrophilic and hydrophobic domains, a net negative charge, and high porosity and pore interconnectivity, providing an efficient barrier for the absorption of therapeutic agents. To prolong the residence time, absorption and bioavailability of a broad spectrum of active compounds upon mucosal administration, mucus-penetrating and mucoadhesive particles have been designed by tuning the chemical composition, the size, the density, and the surface properties. The benefits of utilizing nanomaterials that interact intimately with mucosae by different mechanisms in the nanomedicine field have been extensively reported. To ensure the safety of these nanosystems, their compatibility is evaluated in vitro and in vivo in preclinical and clinical trials. Conversely, there is a growing concern about the toxicity of nanomaterials dispersed in air and water effluents that unintentionally come into contact with the airways and the gastrointestinal tract. Thus, deep understanding of the key nanomaterial properties that govern the interplay with mucus and tissues is crucial for the rational design of more efficient drug delivery nanosystems (nanomedicine) and to anticipate the fate and side-effects of nanoparticulate matter upon acute or chronic exposure (nanotoxicology). This review initially overviews the complex structural features of mucosal tissues, including the structure of mucus, the epithelial barrier, the mucosal-associated lymphatic tissues and microbiota. Then, the most relevant investigations attempting to identify and validate the key particle features that govern nanomaterial-mucosa interactions and that are relevant in both nanomedicine and nanotoxicology are discussed in a holistic manner. Finally, the most popular experimental techniques and the incipient use of mathematical and computational models to characterize these interactions are described.

Staphylococcus aureus Protein A Induces Human Regulatory T Cells Through Interaction With Antigen-Presenting Cells

Despite continuous exposure and development of specific immunity, Staphylococcus aureus (Sa) remains one of the leading causes of severe infections worldwide. Although innate immune defense mechanisms are well understood, the role of the T cell response has not been fully elucidated. Here, we demonstrate that Sa and one of its major virulence factors protein A (SpA) induce human regulatory T cells (Tregs), key players in immune tolerance. In human PBMC and MoDC/T cell cocultures CD4⁺CD25⁺CD127^dim Tregs were induced upon stimulation with Sa and to a lower extent with SpA alone. Treg induction was strongly, but not exclusively, dependent on SpA, and independent of antigen presentation or T cell epitope recognition. Lastly, soluble factors in the supernatant of SpA-stimulated MoDC were sufficient to trigger Treg formation, while supernatants of MoDC/T cell cocultures containing Sa-triggered Tregs displayed T cell suppressive activity. In summary, our findings identify a new immunosuppressory function of SpA, which leads to release of soluble, Treg-inducing factors and might be relevant to establish colonization.

Harnessing antifungal immunity in pursuit of a Staphylococcus aureus vaccine strategy

Staphylococcus aureus (S. aureus) is one of the most common bacterial infections worldwide, and antibiotic resistant strains such as Methicillin-Resistant S. aureus (MRSA) are a major threat and burden to public health. MRSA not only infects immunocompromised patients but also healthy individuals and has rapidly spread from the healthcare setting to the outside community. However, all vaccines tested in clinical trials to date have failed. Immunocompromised individuals such as patients with HIV or decreased levels of CD4+ T cells are highly susceptible to S. aureus infections, and they are also at increased risk of developing fungal infections. We therefore wondered whether stimulation of antifungal immunity might promote the type of immune responses needed for effective host defense against S. aureus. Here we show that vaccination of mice with a fungal β-glucan particle (GP) loaded with S. aureus antigens provides protective immunity to S. aureus. We generated glucan particles loaded with the four S. aureus proteins ClfA, IsdA, MntC, and SdrE, creating the 4X-SA-GP vaccine. Vaccination of mice with three doses of 4X-SA-GP promoted protection in a systemic model of S. aureus infection with a significant reduction in the bacterial burden in the spleen and kidneys. 4X-SA-GP vaccination induced antigen-specific Th1 and Th17 CD4+ T cell and antibody responses and provided long-term protection. This work suggests that the GP vaccine system has potential as a novel approach to developing vaccines for S. aureus.

Bacterial extracellular vesicle-coated multi-antigenic nanovaccines protect against drug-resistant Staphylococcus aureus infection by modulating antigen processing and presentation pathways

Background: Vaccination provides an alternative to antibiotics in addressing drug-resistant Staphylococcus aureus (S. aureus) infection. However, vaccine potency is often limited by a lack of antigenic breadth and a demand on the generation of antibody responses alone.

Methods: In this study, bacterial extracellular vesicles (EVs) coating indocyanine green (ICG)-loaded magnetic mesoporous silica nanoparticles (MSN) were constructed as multi-antigenic vaccines (EV/ICG/MSN) with the ability to modulate antigen presentation pathways in dendritic cells (DCs) to induce cellular immune responses.

Results: Exposing the EV/ICG/MSNs to a laser could promote DC maturation and enhance the proteasome-dependent antigen presentation pathway by facilitating endolysosomal escape, improving proteasome activity, and elevating MHC-I expression. Immunization by EV/ICG/MSNs with laser irradiation in vivo triggered improved CD8⁺ T cell responses while maintaining CD4⁺ T cell responses and humoral immunity. In addition, in vivo tracking data revealed that the vaccine could be efficiently transported from the injection site into lymph nodes. Skin infection experiments showed that the vaccine not only prevented and treated superficial infection but also decreased bacterial invasiveness, thus strongly suggesting that EV/ICG/MSNs were effective in preventing complications resulting from the introduction of S. aureus infections.

Conclusion: This multi-antigenic nanovaccine-based modulation of antigen presentation pathways provides an effective strategy against drug-resistant S. aureus infection.

Engineering drug delivery systems to overcome mucosal barriers for immunotherapy and vaccination

Mucosal surfaces protect our bodies from pathogens and external irritants using a system of biological barriers. Overcoming these barriers is a significant drug delivery challenge, particularly for immunotherapies that aim to modulate the local immune response. Reaching local lymphoid tissues and draining lymph nodes (LNs) requires crossing the mucus mesh, mucosal epithelium, and either targeting M cells covering lymphoid tissues or utilizing lymphatic transport that shuttles molecules and particulates from the periphery to the LN. We first highlight the barrier properties of mucus and mucosal epithelium, and the function of the mucosal immune system. We then dive into existing drug delivery technologies that have been engineered to overcome each of these barriers. We particularly focus on novel strategies for targeting lymphoid tissues, which has been shown to enhance immunotherapies and vaccinations, via directly targeting LNs, lymphatic vessels, and M cells that transport samples of mucosal content to the lymphoid tissues.

Mucosal vaccines: Strategies and challenges

Mucosal immunization has potential benefits over conventional parenteral immunization, eliciting immune defense in both mucosal and systemic tissue for protecting from pathogen invasion at mucosal surfaces. To provide a first line of protection at these entry ports, mucosal vaccines have been developed and hold a significant promise for reducing the burden of infectious diseases. However, until very recently, only limited mucosal vaccines are available. This review summarizes recent advances in selected aspects regarding mucosal vaccination, including appropriate administration routes, reasonable formulations, antigen-sampling and immune responses of mucosal immunity, and the strategies used to improve mucosal vaccine efficacy. Finally, the challenges of developing successful mucosal vaccines and the potential solutions are discussed.

Staphylococcus aureus-specific IgA antibody in milk suppresses the multiplication of S. aureus in infected bovine udder

Background

Bovine mastitis caused by Staphylococcus aureus (S. aureus) is extremely difficult to control and new methods for its prevention and management are required. Nasal vaccines may prevent initial bovine mastitis infection caused by S. aureus. However, limited information is available regarding induction of mucosal immune response through nasal immunization with antigen and its suppression of S. aureus multiplication during bovine mastitis. This study sought to investigate whether induction of immunoglobulin A (IgA) in milk by nasal immunization could suppress multiplication of S. aureus in the bovine udder.

Results

Nasal immunization with formalin-killed S. aureus conjugated with a cationic cholesteryl-group-bearing pullulan-nanogel was performed. Anti-S. aureus-specific IgA antibodies were significantly more abundant in the milk of immunized cows than in non-immunized animals (P < 0.05). S. aureus counts in the quarter were negative in both non-immunized and nasal-immunized cows 1 week after mock infusion. In S. aureus-infused quarters, S. aureus multiplication was significantly suppressed in immunized compared with non-immunized cows (P < 0.05). Furthermore, a significant negative correlation was found between S. aureus-specific IgA antibodies and S. aureus counts in infused quarters of both non-immunized and nasal-immunized cows (r = − 0.811, P < 0.01).

Conclusion

In conclusion, the present study demonstrates that S. aureus-specific IgA antibodies in milk successfully suppressed the multiplication of S. aureus in infected bovine udders. Although the exact mechanism explaining such suppressive effect remains to be elucidated, nasal vaccines that can induce humoral immunity may help prevent initial infection with S. aureus and the onset of bovine mastitis.

Electronic supplementary material

The online version of this article (10.1186/s12917-019-2025-3) contains supplementary material, which is available to authorized users.

Would hemodialysis patients benefit from a Staphylococcus aureus vaccine?

Staphylococcus aureus bloodstream infection can have potentially catastrophic consequences for patients on hemodialysis. Consequently, an effective vaccine to prevent S aureus infection would have a significant influence on morbidity and mortality in this group. To date, however, efforts to develop a vaccine have been unsuccessful. Previous antibody-inducing vaccine candidates did not prevent or attenuate S aureus infection in clinical trials. Recent advances have helped to elucidate the role of specific T-cell subsets, notably T-helper cell 1 and T-helper cell 17, in the immune response to S aureus. These cells are essential for coordinating an effective phagocytic response via cytokine production, indirectly leading to destruction of the organism. It is now widely accepted that next-generation S aureus vaccines must also induce effective T-cell-mediated immunity. However, there remains a gap in our knowledge: how will an S aureus vaccine drive these responses in those patients most at risk? Given that patients on hemodialysis are an immunocompromised population, in particular with specific T-cell defects, including defects in T-helper cell subsets, this is likely to affect their ability to respond to an S aureus vaccine. We urgently need a better understanding of T-cell-mediated immunity in this cohort if an efficacious vaccine is ever to be realized for these patients.

Advances and Opportunities in Nanoparticle- and Nanomaterial-Based Vaccines against Bacterial Infections

As the dawn of the postantibiotic era we approach, antibacterial vaccines are becoming increasingly important for managing bacterial infection and reducing the need for antibiotics. Despite the success of vaccination, vaccines remain unavailable for many pressing microbial diseases, including tuberculosis, chlamydia, and staphylococcus infections. Amid continuing research efforts in antibacterial vaccine development, the advancement of nanomaterial engineering has brought forth new opportunities in vaccine designs. With increasing knowledge in antibacterial immunity and immunologic adjuvants, innovative nanoparticles are designed to elicit the appropriate immune responses for effective antimicrobial defense. Rationally designed nanoparticles are demonstrated to overcome delivery barriers to shape the adaptive immunity. This article reviews the advances in nanoparticle- and nanomaterial-based antibacterial vaccines and summarizes the development of nanoparticulate adjuvants for immune potentiation against microbial pathogens. In addition, challenges and progress in ongoing antibacterial vaccine development are discussed to highlight the opportunities for future vaccine designs.

Peptide and protein nanoparticle conjugates: versatile platforms for biomedical applications

Peptide- and protein-nanoparticle conjugates have emerged as powerful tools for biomedical applications, enabling the treatment, diagnosis, and prevention of disease. In this review, we focus on the key roles played by peptides and proteins in improving, controlling, and defining the performance of nanotechnologies. Within this framework, we provide a comprehensive overview of the key sequences and structures utilised to provide biological and physical stability to nano-constructs, direct particles to their target and influence their cellular and tissue distribution, induce and control biological responses, and form polypeptide self-assembled nanoparticles. In doing so, we highlight the great advances made by the field, as well as the challenges still faced in achieving the clinical translation of peptide- and protein-functionalised nano-drug delivery vehicles, imaging species, and active therapeutics.

Live attenuated Salmonella typhimurium vaccines delivering SaEsxA and SaEsxB via type III secretion system confer protection against Staphylococcus aureus infection

BACKGROUND

Staphylococcus aureus (S. aureus) causes a wide range of infectious diseases in human and animals. The emergence of antibiotic-resistant strains demands novel strategies for prophylactic vaccine development. In this study, live attenuated S. enterica subsp. enterica serotype Typhimurium (S. Typhimurium) vaccine against S. aureus infection was developed, in which Salmonella Pathogenesis Island-1 Type 3 Secretion System (SPI-1 T3SS) was employed to deliver SaEsxA and SaEsxB, two of ESAT-6-like (Early Secreted Antigenic Target-6) virulence factors of S. aureus.

METHODS

Antigens SaEsxA and SaEsxB were fused with the N-terminal secretion and translocation domain of SPI-1 effector SipA. And cytosolic delivery of Staphylococcal antigens into macrophages was examined by western blot. BALB/c mice were orally immunized with S. Typhimurium-SaEsxA and S. Typhimurium-SaEsxB vaccines. Antigen-specific humoral and Th1/Th17 immune responses were examined by ELISA and ELISPOT assays 7-9 days after the 2nd booster. For ELISPOT assays, the statistical significance was determined by Student's t test. The vaccine efficacy was evaluated by lethal challenge with two S. aureus clinical isolates Newman strain and USA 300 strain. Statistical significance was determined by Log rank (Mantel-Cox) analysis. And a P value of < 0.05 was considered statistically significant.

RESULTS

Oral administration of S. Typhimurium-SaEsxA and S. Typhimurium-SaEsxB vaccines induced antigen-specific humoral and Th1/Th17 immune responses, which increased the survival rate for vaccinated mice when challenged with S. aureus strains.

CONCLUSIONS

The newly developed S. Typhimurium-based vaccines delivering SaEsxA and SaEsxB by SPI-1 T3SS could confer protection against S. aureus infection. This study provides evidence that translocation of foreign antigens via Salmonella SPI-1 T3SS into the cytosol of antigen presenting cells (APCs) could induce potent immune responses against pathogens.

The Staphylococcus aureus Cell Wall-Anchored Protein Clumping Factor A Is an Important T Cell Antigen

Staphylococcus aureus has become increasingly resistant to antibiotics, and vaccines offer a potential solution to this epidemic of antimicrobial resistance. Targeting of specific T cell subsets is now considered crucial for next-generation anti-S. aureus vaccines; however, there is a paucity of information regarding T cell antigens of S. aureus. This study highlights the importance of cell wall-anchored proteins as human CD4⁺ T cell activators capable of driving antigen-specific Th1 and Th17 cell activation. Clumping factor A (ClfA), which contains N1, N2, and N3 binding domains, was found to be a potent human T cell activator. We further investigated which subdomains of ClfA were involved in T cell activation and found that the full-length ClfA N123 and N23 were potent Th1 and Th17 activators. Interestingly, the N1 subdomain was capable of exclusively activating Th1 cells. Furthermore, when these subdomains were used in a model vaccine, N23 and N1 offered Th1- and Th17-mediated systemic protection in mice upon intraperitoneal challenge. Overall, however, full-length ClfA N123 is required for maximal protection both locally and systemically.

Nanogel-based nasal vaccines for infectious and lifestyle-related diseases

Because the mucosa is the major entry route for most pathogens, the development of mucosal vaccines is a rational approach for protecting against these undesired agents. Mucosal administration of vaccine antigen is useful for non-infectious chronic diseases as well, because of its advantages over injection routes, including comparable efficacy in the induction of systemic immune responses, less pain, and no risk of adverse events at the injection site. However, because it is difficult to effectively induce and regulate antigen-specific mucosal and systemic immune responses when antigen alone is mucosally administered, an appropriate form of mucosal delivery vehicle must be used. Antigen delivery systems involving nanogels, which act as artificial chaperones and mucosal adhesives, are a promising approach to overcoming this problem. Here, we introduce current perspectives regarding the development of nanogel-based nasal vaccines for both infectious and lifestyle-related diseases.

Nasal nanovaccines

Nasal administration of vaccines is convenient for the potential stimulation of mucosal and systemic immune protection. Moreover the easy accessibility of the intranasal route renders it optimal for pandemic vaccination. Nanoparticles have been identified as ideal delivery systems and adjuvants for vaccine application. Heterogeneous protocols have been used for animal studies. This complicates the understanding of the formulation influence on the immune response and the comparison of the different nanoparticles approaches developed. Moreover anatomical and immunological differences between rodents and humans provide an additional hurdle in the rational development of nasal nanovaccines. This review will give a comprehensive expertise of the state of the art in nasal nanovaccines in animals and humans focusing on the nanomaterial used.

Nano-therapeutics: A revolution in infection control in post antibiotic era

With the arrival of antibiotics 70 years ago, meant a paradigm shift in overcoming infectious diseases. For decades, drugs have been used to treat different infections. However, with time bacteria have become resistant to multiple antibiotics, making some diseases difficult to fight. Nanoparticles (NPs) as antibacterial agents appear to have potential to overcome such problems and to revolutionize the diagnosis and treatment of bacterial infections. Therefore, there is significant interest in the use of NPs to treat variety of infections, particularly caused by multidrug-resistant (MDR) strains. This review begins with illustration of types of NPs followed by the literature of current research addressing mechanisms of NPs antibacterial activity, steps involved in NP mediated drug delivery as well as areas where NPs use has potential to improve the treatment, like NP enabled vaccination. Besides, recently emerged innovative NP platforms have been highlighted and their progress made in each area has been reviewed.

Recent developments in nanocarrier-aided mucosal vaccination

To date, most of the licensed vaccines for mucosal delivery are based on live-attenuated viruses which carry the risk of regaining their pathogenicity. Therefore, the development of efficient nonviral vectors allowing the induction of potent humoral and cell-mediated immunity is regarded as an imperative scientific challenge as well as a commercial breakthrough for the pharma industries. For a successful translation to the clinic, such nanocarriers should protect the antigens from mucosal enzymes, facilitate antigen uptake by microfold cells and allow the copresentation of robust, safe for human use, mucosal adjuvants to antigen-presenting cells. Finally, the developed formulations should exhibit accuracy regarding the administered dose, a major drawback of mucosal vaccines in comparison with parenteral ones.

Biomimetic strategies for targeted nanoparticle delivery

Nanoparticle‐based drug delivery and imaging platforms have become increasingly popular over the past several decades. Among different design parameters that can affect their performance, the incorporation of targeting functionality onto nanoparticle surfaces has been a widely studied subject. Targeted formulations have the ability to improve efficacy and function by positively modulating tissue localization. Many methods exist for creating targeted nanoformulations, including the use of custom biomolecules such as antibodies or aptamers. More recently, a great amount of focus has been placed on biomimetic targeting strategies that leverage targeting interactions found directly in nature. Such strategies, which have been painstakingly selected over time by the process of evolution to maximize functionality, oftentimes enable scientists to forgo the specialized discovery processes associated with many traditional ligands and help to accelerate development of novel nanoparticle formulations. In this review, we categorize and discuss in‐depth recent works in this growing field of bioinspired research.

Intranasal vaccination with adjuvant-free S. aureus antigens effectively protects mice against experimental sepsis

Staphylococcus aureus (S. aureus) is a Gram-positive coccal bacterium comprising part of the human skin, nares and gastrointestinal tract normal microbiota. It is also an important cause of nosocomial/community-acquired infections in humans and animals, which can cause a diverse array of infections, including sepsis, which is a progressive systemic inflammation response syndrome that is frequently fatal. The emergence of drug-resistant strains and the high toxicity of the treatments used for these infections point out the need to develop an effective, inexpensive and safe vaccine that can be used prophylactically. In this work, we used an experimental sepsis model to evaluate the effectiveness of whole antigens from S. aureus (SaAg) given by the intranasal route to induce protective immunity against S. aureus infection in mice. BALB/c mice were vaccinated via intranasal or intramuscular route with two doses of SaAg, followed by biocompatibility and immunogenicity evaluations. Vaccinated animals did not show any adverse effects associated with the vaccine, as determined by transaminase and creatinine measurements. Intranasal, but not intramuscular vaccination with SaAg led to a significant reduction in IL-10 production and was associated with increased level of IFN-γ and NO. SaAg intranasal vaccination was able to prime cellular and humoral immune responses and inducing a higher proliferation index and increased production of specific IgG1/IgG2, which contributed to decrease the bacterial load in both liver and the spleen and improve survival during sepsis. These findings present the first evidence of the effectiveness of whole Ag intranasal-based vaccine administration, which expands the vaccination possibilities against S. aureus infection.

The T Cell Response to Staphylococcus aureus

Staphylococcus aureus (S. aureus) is a dangerous pathogen and a leading cause of both nosocomial and community acquired bacterial infection worldwide. However, on the other hand, we are all exposed to this bacterium, often within the first hours of life, and usually manage to establish equilibrium and coexist with it. What does the adaptive immune system contribute toward lifelong control of S. aureus? Will it become possible to raise or enhance protective immune memory by vaccination? While in the past the S. aureus-specific antibody response has dominated this discussion, the research community is now coming to appreciate the role that the cellular arm of adaptive immunity, the T cells, plays. There are numerous T cell subsets, each with differing functions, which together have the ability to orchestrate the immune response to S. aureus and hence to tip the balance between protection and pathology. This review summarizes the state of the art in this dynamic field of research.

The Role of Staphylococcus aureus Virulence Factors in Skin Infection and Their Potential as Vaccine Antigens

Staphylococcus aureus (S. aureus) causes the vast majority of skin and soft tissue infections (SSTIs) in humans. S. aureus has become increasingly resistant to antibiotics and there is an urgent need for new strategies to tackle S. aureus infections. Vaccines offer a potential solution to this epidemic of antimicrobial resistance. However, the development of next generation efficacious anti-S. aureus vaccines necessitates a greater understanding of the protective immune response against S. aureus infection. In particular, it will be important to ascertain if distinct immune mechanisms are required to confer protection at distinct anatomical sites. Recent discoveries have highlighted that interleukin-17-producing T cells play a particularly important role in the immune response to S. aureus skin infection and suggest that vaccine strategies to specifically target these types of T cells may be beneficial in the treatment of S. aureus SSTIs. S. aureus expresses a large number of cell wall-anchored (CWA) proteins, which are covalently attached to the cell wall peptidoglycan. The virulence potential of many CWA proteins has been demonstrated in infection models; however, there is a paucity of information regarding their roles during SSTIs. In this review, we highlight potential candidate antigens for vaccines targeted at protection against SSTIs.

Memory Th1 Cells Are Protective in Invasive Staphylococcus aureus Infection

Mechanisms of protective immunity to Staphylococcus aureus infection in humans remain elusive. While the importance of cellular immunity has been shown in mice, T cell responses in humans have not been characterised. Using a murine model of recurrent S. aureus peritonitis, we demonstrated that prior exposure to S. aureus enhanced IFNγ responses upon subsequent infection, while adoptive transfer of S. aureus antigen-specific Th1 cells was protective in naïve mice. Translating these findings, we found that S. aureus antigen-specific Th1 cells were also significantly expanded during human S. aureus bloodstream infection (BSI). These Th1 cells were CD45RO⁺, indicative of a memory phenotype. Thus, exposure to S. aureus induces memory Th1 cells in mice and humans, identifying Th1 cells as potential S. aureus vaccine targets. Consequently, we developed a model vaccine comprising staphylococcal clumping factor A, which we demonstrate to be an effective human T cell antigen, combined with the Th1-driving adjuvant CpG. This novel Th1-inducing vaccine conferred significant protection during S. aureus infection in mice. This study notably advances our understanding of S. aureus cellular immunity, and demonstrates for the first time that a correlate of S. aureus protective immunity identified in mice may be relevant in humans.

Staphylococcus aureus is a leading cause of skin, soft tissue and bone infections and, most seriously, bloodstream infection. When S. aureus does get into the bloodstream, it is more likely to kill than any other bacterial infection, despite all the treatments modern medicine has to offer. It has thus far developed resistance to all antibiotics licensed to treat it. Thus, there is an urgent need to develop a vaccine against S. aureus. However, such a vaccine remains elusive. This is largely due to the fact that we have a very limited understanding of how our immune system fights this infection. Here, we examine how certain T cells of the mouse immune system effectively recognise and respond to S. aureus, and show that transferring these cells to other mice improves their ability to clear infection. We then demonstrate that a vaccine which drives these specific T cells also improves clearance of infection. Until recently, it was not known if human T cells could recognise and respond to S. aureus. Here we show, for the first time, that these cells are expanded in patients with S. aureus bloodstream infection, and suggest that they should be targeted in anti-S. aureus vaccines.

Delivery strategies to enhance oral vaccination against enteric infections

While the majority of human pathogens infect the body through mucosal sites, most licensed vaccines are injectable. In fact the only mucosal vaccine that has been widely used globally for infant and childhood vaccination programs is the oral polio vaccine (OPV) developed by Albert Sabin in the 1950s. While oral vaccines against Cholera, rotavirus and Salmonella typhi have also been licensed, the development of additional non-living oral vaccines against these and other enteric pathogens has been slow and challenging. Mucosal vaccines can elicit protective immunity at the gut mucosa, in part via antigen-specific secretory immunoglobulin A (SIgA). However, despite their advantages over the injectable route, oral vaccines face many hurdles. A key challenge lies in design of delivery strategies that can protect antigens from degradation in the stomach and intestine, incorporate appropriate immune-stimulatory adjuvants and control release at the appropriate gastrointestinal site. A number of systems including micro and nanoparticles, lipid-based strategies and enteric capsules have significant potential either alone or in advanced combined formulations to enhance intestinal immune responses. In this review we will outline the opportunities, challenges and potential delivery solutions to facilitate the development of improved oral vaccines for infectious enteric diseases.

Active targeted drug delivery for microbes using nano-carriers

Although vaccines and antibiotics could kill or inhibit microbes, many infectious diseases remain difficult to treat because of acquired resistance and adverse side effects. Nano-carriers-based technology has made significant progress for a long time and is introducing a new paradigm in drug delivery. However, it still has some challenges like lack of specificity toward targeting the infectious site. Nanocarriers utilized targeting ligands on their surface called 'active target' provide the promising way to solve the problems like accelerating drug delivery to infectious areas and preventing toxicity or side-effects. In this mini review, we demonstrate the recent studies using the active targeted strategy to kill or inhibit microbes. The four common nano-carriers (e.g. liposomes, nanoparticles, dendrimers and carbon nanotubes) delivering encapsulated drugs are introduced.

Recombinant ESAT-6-like proteins provoke protective immune responses against invasive Staphylococcus aureus disease in a murine model

Staphylococcus aureus is a common pathogen found in the community and in hospitals. Most notably, methicillin-resistant S. aureus is resistant to many antibiotics, which is a growing public health concern. The emergence of drug-resistant strains has prompted the search for alternative treatments, such as immunotherapeutic approaches. To date, most clinical trials of vaccines or of passive immunization against S. aureus have ended in failure. In this study, we investigated two ESAT-6-like proteins secreted by S. aureus, S. aureus EsxA (SaEsxA) and SaEsxB, as possible targets for a vaccine. Mice vaccinated with these purified proteins elicited high titers of anti-SaEsxA and anti-SaEsxB antibodies, but these antibodies could not prevent S. aureus infection. On the other hand, recombinant SaEsxA (rSaEsxA) and rSaEsxB could induce Th1- and Th17-biased immune responses in mice. Mice immunized with rSaEsxA and rSaEsxB had significantly improved survival rates when challenged with S. aureus compared with the controls. These findings indicate that SaEsxA and SaEsxB are two promising Th1 and Th17 candidate antigens which could be developed into multivalent and serotype-independent vaccines against S. aureus infection.

Sortase A induces Th17-mediated and antibody-independent immunity to heterologous serotypes of group A streptococci

Group A streptococci (GAS) are associated with a variety of mucosal and invasive human infections. Recurrent infections by highly heterologous serotypes indicate that cross-serotype immunity is critical for prevention of GAS infections; however, mechanisms underlying serotype-independent protection are poorly understood. Here we report that intranasal vaccination of mice with Sortase A (SrtA), a conserved cell wall bound protein, reduced colonization of nasal-associated lymphoid tissue (NALT) by heterologous serotypes of GAS. Vaccination significantly increased CD4⁺ IL-17A⁺ cells in NALT and depletion of IL-17A by neutralizing antibody prevented GAS clearance from NALT which was dependent on immunization with SrtA. Vaccination also induced high levels of SrtA-specific antibodies; however, immunized, B cell-deficient mice cleared streptococcal challenges as efficiently as wild type mice, indicating that the cross-serotype protection is Th17-biased and antibody-independent. Furthermore, efficient GAS clearance from NALT was associated with a rapid neutrophil influx into NALT of immunized mice. These results suggest that serotype independent immune protection against GAS mucosal infection can be achieved by intranasal vaccination with SrtA and enhanced neutrophil function is critical for anti-GAS defense and might be a target for prevention of GAS infections.

Staphylococcus aureus infection of mice expands a population of memory γδ T cells that are protective against subsequent infection

The development of vaccines against Staphylococcus aureus has consistently failed in clinical trials, likely due to inefficient induction of cellular immunity. T cell-derived IL-17 is one of the few known correlates of antistaphylococcoal immunity, conferring protection against S. aureus infections through its ability to promote phagocytic cell effector functions. A comprehensive understanding of the discrete T cell subsets critical for site-specific IL-17-mediated bacterial clearance will therefore be necessary to inform the development of vaccines that efficiently target cellular immunity. In this study, we have identified a population of CD44+ CD27- memory γδ T cells, expanded upon infection of C57BL/6 mice with S. aureus, which produce high levels of IL-17 and mediate enhanced bacterial clearance upon reinfection with the bacterium. These cells are comprised largely of the Vγ4+ subset and accumulate at the site of infection subsequent to an initial Vγ1.1+ and Vγ2+ T cell response. Moreover, these Vγ4+ T cells are retained in the peritoneum and draining mediastinal lymph nodes for a prolonged period following bacterial clearance. In contrast to its critical requirement for γδ T cell activation during the primary infection, IL-1 signaling was dispensable for activation and expansion of memory γδ T cells upon re-exposure to S. aureus. Our findings demonstrate that a γδ T cell memory response can be induced upon exposure to S. aureus, in a fashion analogous to that associated with classical αβ T cells, and suggest that induction of IL-17-expressing γδ T cells may be an important property of a protective vaccine against S. aureus.