Considering the ‘Alternatives’ for Next-Generation Anti-Staphylococcus aureus Vaccine Development

IL-10 inhibition during immunization improves vaccine-induced protection against Staphylococcus aureus infection

Staphylococcus aureus is a major human pathogen. An effective anti-S. aureus vaccine remains elusive as the correlates of protection are ill-defined. Targeting specific T cell populations is an important strategy for improving anti-S. aureus vaccine efficacy. Potential bottlenecks that remain are S. aureus-induced immunosuppression and the impact this might have on vaccine-induced immunity. S. aureus induces IL-10, which impedes effector T cell responses, facilitating persistence during both colonization and infection. Thus, it was hypothesized that transient targeting of IL-10 might represent an innovative way to improve vaccine efficacy. In this study, IL-10 expression was elevated in the nares of persistent carriers of S. aureus, and this was associated with reduced systemic S. aureus-specific Th1 responses. This suggests that systemic responses are remodeled because of commensal exposure to S. aureus, which negatively implicates vaccine function. To provide proof of concept that targeting immunosuppressive responses during immunization may be a useful approach to improve vaccine efficacy, we immunized mice with T cell-activating vaccines in combination with IL-10-neutralizing antibodies. Blocking IL-10 during vaccination enhanced effector T cell responses and improved bacterial clearance during subsequent systemic and subcutaneous infection. Taken together, these results reveal a potentially novel strategy for improving anti-S. aureus vaccine efficacy.

Examining the causal relationship between circulating immune cells and the susceptibility to osteomyelitis: A Mendelian randomization study

BACKGROUND

Osteomyelitis is considered as a deleterious inflammatory condition affecting the bone, primarily attributed to pathogenic infection. However, the underlying factors predisposing individuals to osteomyelitis remain incompletely elucidated. The immune system plays a multifaceted role in the progression of this condition, yet previous observational studies and randomized controlled trials investigating the association between circulating immune cell counts and osteomyelitis have been constrained. In order to address this knowledge gap, we conducted a Mendelian randomization (MR) analysis to evaluate the impact of diverse immune cell counts on the risk of developing osteomyelitis.

METHODS

In our study, we utilized single nucleotide polymorphisms (SNPs) that have been strongly linked to circulating immune cells or specific lymphocyte subtypes, as identified in large-scale genome-wide association studies (GWAS). These SNPs served as instrumental variables (IVs) for our MR analysis. We employed a more relaxed clumping threshold to conduct MR analysis on several related lymphocyte subtypes. To estimate causal effects, we utilized the Wald ratio, as well as the random-effects inverse variance weighted (IVW) and weighted median (WM) methods. To enhance the credibility of our results, we performed F-statistic calculations and a series of sensitivity analyses.

RESULTS

Our findings revealed a significant correlation between the absolute count of circulating lymphocytes and the risk of osteomyelitis [odds ratio(OR) 1.20；95 % confidence interval (CI), 1.08-1.32；P = 0.0005]. Furthermore, we identified a causal relationship between the absolute count of CD8+ T cells and susceptibility to osteomyelitis (OR 1.16; 95 % CI, 1.04-1.30; P = 0.0098). Importantly, these findings remained robust across a wide range of sensitivity analyses.

CONCLUSION

Through our MR analysis, we have provided evidence supporting a causal relationship between genetic predisposition to higher circulating immune cell counts and an increased risk of osteomyelitis. Specifically, our findings highlight the association between elevated CD8+ T cell counts and a heightened susceptibility to osteomyelitis. These results offer valuable insights for the future exploration of immunotherapy approaches in the management of osteomyelitis.

Distinct T cell signatures are associated with Staphylococcus aureus skin infection in pediatric atopic dermatitis

Atopic dermatitis (AD) is an inflammatory skin condition with a childhood prevalence of up to 25%. Microbial dysbiosis is characteristic of AD, with Staphylococcus aureus the most frequent pathogen associated with disease flares and increasingly implicated in disease pathogenesis. Therapeutics to mitigate the effects of S. aureus have had limited efficacy and S. aureus-associated temporal disease flares are synonymous with AD. An alternative approach is an anti-S. aureus vaccine, tailored to AD. Experimental vaccines have highlighted the importance of T cells in conferring protective anti-S. aureus responses; however, correlates of T cell immunity against S. aureus in AD have not been identified. We identify a systemic and cutaneous immunological signature associated with S. aureus skin infection (ADS.aureus) in a pediatric AD cohort, using a combined Bayesian multinomial analysis. ADS.aureus was most highly associated with elevated cutaneous chemokines IP10 and TARC, which preferentially direct Th1 and Th2 cells to skin. Systemic CD4+ and CD8+ T cells, except for Th2 cells, were suppressed in ADS.aureus, particularly circulating Th1, memory IL-10+ T cells, and skin-homing memory Th17 cells. Systemic γδ T cell expansion in ADS.aureus was also observed. This study suggests that augmentation of protective T cell subsets is a potential therapeutic strategy in the management of S. aureus in AD.

Staphylococcus aureus vaccine strategy: Promise and challenges

Staphylococcus aureus (S. aureus) is a leading and crucial infectious threat to global public health due to the widespread emergence of antibiotic-resistant strains such as Methicillin-Resistant S. aureus (MRSA). MRSA infects immunocompromised patients and healthy individuals and has rapidly spread from the healthcare setting to the outside community. The development of flawless vaccines become a medical need worldwide against multi-drug resistant S. aureus. Therefore, protection by an immune-based strategy may provide valuable measures to contain the spread of invasive S. aureus infections. Several vaccine candidates have been prepared which are either in the preclinical phase or in the early clinical phase, whereas several candidates have failed to show a protective efficacy in human subjects. Currently, research is focusing on identifying novel vaccine formulations able to elicit potent humoral and cellular immune responses. Several approaches have also been made to the development of monoclonal or polyclonal antibodies for passive immunization to protect against S. aureus infections. In recent years, a multi-epitope vaccine has emerged as a novel platform for subunit vaccine design by using computational approaches. Therefore, in this review, we have summarized and discussed the mechanistic overview of different strategies used to develop potential vaccine candidates and passive interventions which are in different stages of clinical trials to fight multi-drug resistant S. aureus infections.

Oral Vaccination with Engineered Probiotic Limosilactobacillus reuteri Has Protective Effects against Localized and Systemic Staphylococcus aureus Infection

Staphylococcus aureus is a Gram-positive bacterium responsible for most hospital-acquired (nosocomial) and community-acquired infections worldwide. The only therapeutic strategy against S. aureus-induced infections, to date, is antibiotic treatment. A protective vaccine is urgently needed in view of the emergence of antibiotic-resistant strains associated with high-mortality cases; however, no such vaccine is currently available. In our previous work, the feasibility of implementing a Lactobacillus delivery system for development of S. aureus oral vaccine was first discussed. Here, we describe systematic screening and evaluation of protective effects of engineered Lactobacillus against S. aureus infection in terms of different delivery vehicle strains and S. aureus antigens and in localized and systemic infection models. Limosilactobacillus reuteri WXD171 was selected as the delivery vehicle strain based on its tolerance of the gastrointestinal environment, adhesion ability, and antimicrobial activities in vitro and in vivo. We designed, constructed, and evaluated engineered L. reuteri strains expressing various S. aureus antigens. Among these, engineered L. reuteri WXD171-IsdB displayed effective protection against S. aureus-induced localized infection (pneumonia and skin infection) and, furthermore, a substantial survival benefit in systemic infection (sepsis). WXD171-IsdB induced mucosal responses in gut-associated lymphoid tissues, as evidenced by increased production of secretory IgA and interleukin 17A (IL-17A) and proliferation of lymphocytes derived from Peyer's patches. The probiotic L. reuteri-based oral vaccine appears to have strong potential as a prophylactic agent against S. aureus infections. Our findings regarding utilization of Lactobacillus delivery system in S. aureus vaccine development support the usefulness of this live vaccination strategy and its potential application in next-generation vaccine development. IMPORTANCE We systematically screened and evaluated protective effects of engineered Lactobacillus against S. aureus infection in terms of differing delivery vehicle strains and S. aureus antigens and in localized and systemic infection models. Engineered L. reuteri was developed and showed strong protective effects against both types of S. aureus-induced infection. Our findings regarding the utilization of a Lactobacillus delivery system in S. aureus vaccine development support the usefulness of this live vaccination strategy and its potential application in next-generation vaccine development.

Mucosa-Associated Invariant T Cell Hypersensitivity to Staphylococcus aureus Leukocidin ED and Its Modulation by Activation

Mucosa-associated invariant T (MAIT) cells recognize bacterial riboflavin metabolite Ags presented by MHC class Ib-related protein (MR1) and play important roles in immune control of microbes that synthesize riboflavin. This includes the pathobiont Staphylococcus aureus, which can also express a range of virulence factors, including the secreted toxin leukocidin ED (LukED). In this study, we found that human MAIT cells are hypersensitive to LukED-mediated lysis and lost on exposure to the toxin, leaving a T cell population devoid of MAIT cells. The cytolytic effect of LukED on MAIT cells was rapid and occurred at toxin concentrations lower than those required for toxicity against conventional T cells. Furthermore, this coincided with high MAIT cell expression of CCR5, and loss of these cells was efficiently inhibited by the CCR5 inhibitor maraviroc. Interestingly, exposure and preactivation of MAIT cells with IL-12 and IL-18, or activation via TCR triggering, partially protected from LukED toxicity. Furthermore, analysis of NK cells indicated that LukED targeted the mature cytotoxic CD57⁺ NK cell subset in a CCR5-independent manner. Overall, these results indicate that LukED efficiently eliminates immune cells that can respond rapidly to S. aureus in an innate fashion without the need for clonal expansion, and that MAIT cells are exceptionally vulnerable to this toxin. Thus, the findings support a model where LukED secretion may allow S. aureus to avoid recognition by the rapid cell-mediated responses mediated by MAIT cells and NK cells.

Human MAIT Cells Respond to Staphylococcus aureus with Enhanced Anti-Bacterial Activity

Mucosal-Associated Invariant T (MAIT) cells have been shown to play protective roles during infection with diverse pathogens through their propensity for rapid innate-like cytokine production and cytotoxicity. Among the potential applications for MAIT cells is to defend against Staphylococcus aureus, a pathogen of serious clinical significance. However, it is unknown how MAIT cell responses to S. aureus are elicited, nor has it been investigated whether MAIT cell cytotoxicity is mobilized against intracellular S. aureus. In this study, we investigate the capacity of human MAIT cells to respond directly to S. aureus. MAIT cells co-cultured with dendritic cells (DCs) infected with S. aureus rapidly upregulate CD69, express IFNγ and Granzyme B and degranulate. DC secretion of IL-12, but not IL-18, was implicated in this immune response, while TCR binding of MR1 is required to commence cytokine production. MAIT cell cytotoxicity resulted in apoptosis of S. aureus-infected cells, and reduced intracellular persistence of S. aureus. These findings implicate these unconventional T cells in important, rapid anti-S. aureus responses that may be of great relevance to the ongoing development of novel anti-S. aureus treatments.

Staphylococcus aureus adaptive evolution: Recent insights on how immune evasion, immunometabolic subversion and host genetics impact vaccine development

Despite meritorious attempts, a S. aureus vaccine that prevents infection or mitigates severity has not yet achieved efficacy endpoints in prospective, randomized clinical trials. This experience underscores the complexity of host-S. aureus interactions, which appear to be greater than many other bacterial pathogens against which successful vaccines have been developed. It is increasingly evident that S. aureus employs strategic countermeasures to evade or exploit human immune responses. From entering host cells to persist in stealthy intracellular reservoirs, to sensing the environmental milieu and leveraging bacterial or host metabolic products to reprogram host immune responses, S. aureus poses considerable challenges for the development of effective vaccines. The fact that this pathogen causes distinct types of infections and can undergo transient genetic, transcriptional or metabolic adaptations in vivo that do not occur in vitro compounds challenges in vaccine development. Notably, the metabolic versatility of both bacterial and host immune cells as they compete for available substrates within specific tissues inevitably impacts the variable repertoire of gene products that may or may not be vaccine antigens. In this respect, S. aureus has chameleon phenotypes that have alluded vaccine strategies thus far. Nonetheless, a number of recent studies have also revealed important new insights into pathogenesis vulnerabilities of S. aureus. A more detailed understanding of host protective immune defenses versus S. aureus adaptive immune evasion mechanisms may offer breakthroughs in the development of effective vaccines, but at present this goal remains a very high bar. Coupled with the recent advances in human genetics and epigenetics, newer vaccine technologies may enable such a goal. If so, future vaccines that protect against or mitigate the severity of S. aureus infections are likely to emerge at the intersection of precision and personalized medicine. For now, the development of S. aureus vaccines or alternative therapies that reduce mortality and morbidity must continue to be pursued.

Oral Delivery of Novel Recombinant Lactobacillus Elicit High Protection against Staphylococcus aureus Pulmonary and Skin Infections

Staphylococcus aureus is a leading cause of nosocomial and community-associated infection worldwide; however, there is no licensed vaccine available. S. aureus initiates infection via the mucosa; therefore, a mucosal vaccine is likely to be a promising approach against S. aureus infection. Lactobacilli, a non-pathogenic bacterium, has gained increasing interest as a mucosal delivery vehicle. Hence, we attempted to develop an oral S. aureus vaccine based on lactobacilli to cushion the stress of drug resistance and vaccine needs. In this study, we designed, constructed, and evaluated recombinant Lactobacillus strains synthesizing S. aureus nontoxic mutated α-hemolysins (Hla_H35L). The results from animal clinical trials showed that recombinant Lactobacillus can persist for at least 72 h and can stably express heterologous protein in vivo. Recombinant L. plantarum WXD234 (pNZ8148-Hla) could induce robust mucosal immunity in the GALT, as evidenced by a significant increase in IgA and IL-17 production and the strong proliferation of T-lymphocytes derived from Peyer’s patches. WXD234 (pNZ8148-Hla) conferred up to 83% protection against S. aureus pulmonary infection and significantly reduced the abscess size in a S. aureus skin infection model. Of particular interest is the sharp reduction of the protective effect offered by WXD234 (pNZ8148-Hla) vaccination in γδ T cell-deficient or IL-17-deficient mice. In conclusion, for the first time, genetically engineered Lactobacillus WXD234 (pNZ8148-Hla) as an oral vaccine induced superior mucosal immunity, which was associated with high protection against pulmonary and skin infections caused by S. aureus. Taken together, our findings suggest the great potential for a delivery system based on lactobacilli and provide experimental data for the development of mucosal vaccines for S. aureus.

Vaccine Composition Formulated with a Novel Lactobacillus-Derived Exopolysaccharides Adjuvant Provided High Protection against Staphylococcus aureus

A vaccine that effectively targets methicillin-resistant Staphylococcus aureus (MRSA) is urgently needed, and has been the focus of studies by numerous research groups, but with limited success to date. Recently, our team found that exopolysaccharides derived from probiotic Lactobacilluscasei strain WXD030 as an adjuvant-formulated OVA could upregulate IFN-γ and IL-17 expression in CD4⁺ T cells. In this study, we developed a vaccine (termed rMntC-EPS) composed of S. aureus antigen MntC and Lactobacillus casei exopolysaccharides, which conferred high levels of protection against S. aureus infection. Methods: Six–eight-week-old female mice were vaccinated with purified rMntC-EPS30. The immune protection function of rMntC-EPS30 was assessed by the protective effect of rMntC-EPS30 to S. aureus-induced pulmonary and cutaneous infection in mice, bacterial loads and H&E in injury site, and ELISA for inflammation-related cytokines. The protective mechanism of rMntC-EPS30 was assessed by ELISA for IgG in serum, cytokines in the spleen and lungs of vaccinated mice. In addition, flow cytometry was used for analyzing cellular immune response induced by rMntC-EPS30. For confirmation of our findings, three kinds of mice were used in this study: IL-17A knockout mice, IFN-γ knockout mice and TCRγ/δ knockout mice. Results: rMntC-EPS30 conferred up to 90% protection against S. aureus pulmonary infection and significantly reduced the abscess size in the S. aureus cutaneous model, with clearance of the pathogen. The rMntC-EPS vaccine could induce superior humoral immunity as well as significantly increase IL-17A and IFN-γ production. In addition, we found that rMntC-EPS vaccination induced robust Th 17/γδ T 17 primary and recall responses. Interestingly, this protective effect was distinctly reduced in the IL-17A knockout mice but not in IFN-γ knockout mice. Moreover, in TCRγ/δ knockout mice, rMntC-EPS vaccination neither increased IL-17A secretion nor provided effective protection against S. aureus infection. Conclusions: These data demonstrated that the rMntC formulated with a novel Lactobacillus-derived Exopolysaccharides adjuvant provided high protection against Staphylococcus aureus. The rMntC-EPS vaccine induced γδ T cells and IL-17A might play substantial roles in anti-S. aureus immunity. Our findings provided direct evidence that rMntC-EPS vaccine is a promising candidate for future clinical application against S. aureus-induced pulmonary and cutaneous infection.

Staphylococcus aureus Vaccine Research and Development: The Past, Present and Future, Including Novel Therapeutic Strategies

Staphylococcus aureus is one of the most important human pathogens worldwide. Its high antibiotic resistance profile reinforces the need for new interventions like vaccines in addition to new antibiotics. Vaccine development efforts against S. aureus have failed so far however, the findings from these human clinical and non-clinical studies provide potential insight for such failures. Currently, research is focusing on identifying novel vaccine formulations able to elicit potent humoral and cellular immune responses. Translational science studies are attempting to discover correlates of protection using animal models as well as in vitro and ex vivo models assessing efficacy of vaccine candidates. Several new vaccine candidates are being tested in human clinical trials in a variety of target populations. In addition to vaccines, bacteriophages, monoclonal antibodies, centyrins and new classes of antibiotics are being developed. Some of these have been tested in humans with encouraging results. The complexity of the diseases and the range of the target populations affected by this pathogen will require a multipronged approach using different interventions, which will be discussed in this review.

Antigen-Adjuvant Interactions in Vaccines by Taylor Dispersion Analysis: Size Characterization and Binding Parameters

Vaccine adjuvants are immunostimulatory substances used to improve and modulate the immune response induced by antigens. A better understanding of the antigen-adjuvant interactions is necessary to develop future effective vaccine. In this study, Taylor dispersion analysis (TDA) was successfully implemented to characterize the interactions between a polymeric adjuvant (poly(acrylic acid), SPA09) and a vaccine antigen in development for the treatment of Staphylococcus aureus. TDA allowed one to rapidly determine both (i) the size of the antigen-adjuvant complexes under physiological conditions and (ii) the percentage of free antigen in the adjuvant/antigen mixture at equilibrium and finally get the interaction parameters (stoichiometry and binding constant). The complex sizes obtained by TDA were compared to the results obtained by transmission electron microscopy, and the binding parameters were compared to results previously obtained by frontal analysis continuous capillary electrophoresis.

Vaccination with Detoxified Leukocidin AB Reduces Bacterial Load in a Staphylococcus aureus Minipig Deep Surgical Wound Infection Model

Vaccines against Staphylococcus aureus have eluded researchers for >3 decades while the burden of staphylococcal diseases has increased. Early vaccine attempts mainly used rodents to characterize preclinical efficacy, and all subsequently failed in human clinical efficacy trials. More recently, leukocidin AB (LukAB) has gained interest as a vaccine antigen. We developed a minipig deep surgical wound infection model offering 3 independent efficacy readouts: bacterial load at the superficial and at the deep-seated surgical site, and dissemination of bacteria. Due to similarities with humans, minipigs are an attractive option to study novel vaccine candidates. With this model, we characterized the efficacy of a LukAB toxoid as vaccine candidate. Compared to control animals, a 3-log reduction of bacteria at the deep-seated surgical site was observed in LukAB-treated minipigs and dissemination of bacteria was dramatically reduced. Therefore, LukAB toxoids may be a useful addition to S. aureus vaccines and warrant further study.

Immunopathogenesis of Craniotomy Infection and Niche-Specific Immune Responses to Biofilm

Bacterial infections in the central nervous system (CNS) can be life threatening and often impair neurological function. Biofilm infection is a complication following craniotomy, a neurosurgical procedure that involves the removal and replacement of a skull fragment (bone flap) to access the brain for surgical intervention. The incidence of infection following craniotomy ranges from 1% to 3% with approximately half caused by Staphylococcus aureus (S. aureus). These infections present a significant therapeutic challenge due to the antibiotic tolerance of biofilm and unique immune properties of the CNS. Previous studies have revealed a critical role for innate immune responses during S. aureus craniotomy infection. Experiments using knockout mouse models have highlighted the importance of the pattern recognition receptor Toll-like receptor 2 (TLR2) and its adaptor protein MyD88 for preventing S. aureus outgrowth during craniotomy biofilm infection. However, neither molecule affected bacterial burden in a mouse model of S. aureus brain abscess highlighting the distinctions between immune regulation of biofilm vs. planktonic infection in the CNS. Furthermore, the immune responses elicited during S. aureus craniotomy infection are distinct from biofilm infection in the periphery, emphasizing the critical role for niche-specific factors in dictating S. aureus biofilm-leukocyte crosstalk. In this review, we discuss the current knowledge concerning innate immunity to S. aureus craniotomy biofilm infection, compare this to S. aureus biofilm infection in the periphery, and discuss the importance of anatomical location in dictating how biofilm influences inflammatory responses and its impact on bacterial clearance.

Targeting Skin-Resident Memory T Cells via Vaccination to Combat Staphylococcus aureus Infections

Tissue-resident memory T cells are important in adaptive immunity against many infections, rendering these cells attractive potential targets in vaccine development. Genetic and experimental evidence highlights the importance of cellular immunity in protection from Staphylococcus aureus skin infections, yet skin-resident memory T cells are, thus far, an untested component of immunity during such infections. Novel methods of generating and sampling vaccine-induced skin memory T cells are paralleled by discoveries of global, skin-wide immunosurveillance. We propose skin-resident memory CD4⁺ T cells as a potential missing link in the search for correlates of protection during S. aureus infections. A better appreciation of their phenotypes and functions could accelerate the development of preventive vaccines against this highly virulent and antibiotic-resistant pathogen.

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.

Type 3 immunity: a perspective for the defense of the mammary gland against infections

Type 3 immunity encompasses innate and adaptive immune responses mediated by cells that produce the signature cytokines IL-17A and IL-17F. This class of effector immunity is particularly adept at controlling infections by pyogenic extracellular bacteria at epithelial barriers. Since mastitis results from infections by bacteria such as streptococci, staphylococci and coliform bacteria that cause neutrophilic inflammation, type 3 immunity can be expected to be mobilized at the mammary gland. In effect, the main defenses of this organ are provided by epithelial cells and neutrophils, which are the main terminal effectors of type 3 immunity. In addition to theoretical grounds, there is observational and experimental evidence that supports a role for type 3 immunity in the mammary gland, such as the production of IL-17A, IL-17F, and IL-22 in milk and mammary tissue during infection, although their respective sources remain to be fully identified. Moreover, mouse mastitis models have shown a positive effect of IL-17A on the course of mastitis. A lot remains to be uncovered before we can safely harness type 3 immunity to reinforce mammary gland defenses through innate immune training or vaccination. However, this is a promising way to find new means of improving mammary gland defenses against infection.

Development of a vaccine against Staphylococcus aureus invasive infections: Evidence based on human immunity, genetics and bacterial evasion mechanisms

Invasive Staphylococcus aureus infections are a leading cause of morbidity and mortality in both hospital and community settings, especially with the widespread emergence of virulent and multi-drug resistant methicillin-resistant S. aureus strains. There is an urgent and unmet clinical need for non-antibiotic immune-based approaches to treat these infections as the increasing antibiotic resistance is creating a serious threat to public health. However, all vaccination attempts aimed at preventing S. aureus invasive infections have failed in human trials, especially all vaccines aimed at generating high titers of opsonic antibodies against S. aureus surface antigens to facilitate antibody-mediated bacterial clearance. In this review, we summarize the data from humans regarding the immune responses that protect against invasive S. aureus infections as well as host genetic factors and bacterial evasion mechanisms, which are important to consider for the future development of effective and successful vaccines and immunotherapies against invasive S. aureus infections in humans. The evidence presented form the basis for a hypothesis that staphylococcal toxins (including superantigens and pore-forming toxins) are important virulence factors, and targeting the neutralization of these toxins are more likely to provide a therapeutic benefit in contrast to prior vaccine attempts to generate antibodies to facilitate opsonophagocytosis.

T Cell Immunity to Bacterial Pathogens: Mechanisms of Immune Control and Bacterial Evasion

The human body frequently encounters harmful bacterial pathogens and employs immune defense mechanisms designed to counteract such pathogenic assault. In the adaptive immune system, major histocompatibility complex (MHC)-restricted αβ T cells, along with unconventional αβ or γδ T cells, respond to bacterial antigens to orchestrate persisting protective immune responses and generate immunological memory. Research in the past ten years accelerated our knowledge of how T cells recognize bacterial antigens and how many bacterial species have evolved mechanisms to evade host antimicrobial immune responses. Such escape mechanisms act to corrupt the crosstalk between innate and adaptive immunity, potentially tipping the balance of host immune responses toward pathological rather than protective. This review examines the latest developments in our knowledge of how T cell immunity responds to bacterial pathogens and evaluates some of the mechanisms that pathogenic bacteria use to evade such T cell immunosurveillance, to promote virulence and survival in the host.

Staphylococcus aureus Alpha-Toxin Limits Type 1 While Fostering Type 3 Immune Responses

Staphylococcus aureus can cause life-threatening diseases, and hospital- as well as community-associated antibiotic-resistant strains are an emerging global public health problem. Therefore, prophylactic vaccines or immune-based therapies are considered as alternative treatment opportunities. To develop such novel treatment approaches, a better understanding of the bacterial virulence and immune evasion mechanisms and their potential effects on immune-based therapies is essential. One important staphylococcal virulence factor is alpha-toxin, which is able to disrupt the epithelial barrier in order to establish infection. In addition, alpha-toxin has been reported to modulate other cell types including immune cells. Since CD4⁺ T cell-mediated immunity is required for protection against S. aureus infection, we were interested in the ability of alpha-toxin to directly modulate CD4⁺ T cells. To address this, murine naïve CD4⁺ T cells were differentiated in vitro into effector T cell subsets in the presence of alpha-toxin. Interestingly, alpha-toxin induced death of Th1-polarized cells, while cells polarized under Th17 conditions showed a high resistance toward increasing concentrations of this toxin. These effects could neither be explained by differential expression of the cellular alpha-toxin receptor ADAM10 nor by differential activation of caspases, but might result from an increased susceptibility of Th1 cells toward Ca²⁺-mediated activation-induced cell death. In accordance with the in vitro findings, an alpha-toxin-dependent decrease of Th1 and concomitant increase of Th17 cells was observed in vivo during S. aureus bacteremia. Interestingly, corresponding subsets of innate lymphoid cells and γδ T cells were similarly affected, suggesting a more general effect of alpha-toxin on the modulation of type 1 and type 3 immune responses. In conclusion, we have identified a novel alpha-toxin-dependent immunomodulatory strategy of S. aureus, which can directly act on CD4⁺ T cells and might be exploited for the development of novel immune-based therapeutic approaches to treat infections with antibiotic-resistant S. aureus strains.

Challenges for Clinical Development of Vaccines for Prevention of Hospital-Acquired Bacterial Infections

Increasing antibiotic resistance in bacteria causing endogenous infections has entailed a need for innovative approaches to therapy and prophylaxis of these infections and raised a new interest in vaccines for prevention of colonization and infection by typically antibiotic resistant pathogens. Nevertheless, there has been a long history of failures in late stage clinical development of this type of vaccines, which remains not fully understood. This article provides an overview on present and past vaccine developments targeting nosocomial bacterial pathogens; it further highlights the specific challenges associated with demonstrating clinical efficacy of these vaccines and the facts to be considered in future study designs. Notably, these vaccines are mainly applied to subjects with preexistent immunity to the target pathogen, transient or chronic immunosuppression and ill-defined microbiome status. Unpredictable attack rates and changing epidemiology as well as highly variable genetic and immunological strain characteristics complicate the development. In views of the clinical need, re-thinking of the study designs and expectations seems warranted: first of all, vaccine development needs to be footed on a clear rationale for choosing the immunological mechanism of action and the optimal time point for vaccination, e.g., (1) prevention (or reduction) of colonization vs. prevention of infection and (2) boosting of a preexistent immune response vs. altering the quality of the immune response. Furthermore, there are different, probably redundant, immunological and microbiological defense mechanisms that provide protection from infection. Their interplay is not well-understood but as a consequence their effect might superimpose vaccine-mediated resolution of infection and lead to failure to demonstrate efficacy. This implies that improved characterization of patient subpopulations within the trial population should be obtained by pro- and retrospective analyses of trial data on subject level. Statistical and systems biology approaches could help to define immune and microbiological biomarkers that discern populations that benefit from vaccination from those where vaccines might not be effective.

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.

Exopolysaccharides from Lactobacillus kiferi as adjuvant enhanced the immuno-protective against Staphylococcus aureus infection

Exopolysaccharides from lactic acid bacteria (LAB) have gained more attention due to their health benefits. Most research on LAB EPS focuses on antitumor and antioxidant activities. To our knowledge, the immunoadjuvant activity of LAB EPS has not been thoroughly studied. In this study, the EPS produced by Lactobacillus kiferi WXD029 were purified by ethanol precipitation and column chromatography fractionation. The molecular weight of the EPS was 3.423 × 10⁵ Da and was mainly composed of Glu, GlcN, and GalN in a molar ratio of 3.1:1:1. In vitro, EPS could significantly enhance the proliferation and phagocytic activity as well as induce the production of NO, TNF-α, IL-1β, and IL-6 in RAW264.7 cells. In vivo, the EPS adjuvant could increase the titers of S.aureus antigen-specific antibodies and markedly enhanced T cell proliferation. Notably, EPS adjuvant also induced a strong potential Th1, Th2 and Th17-cell mixture responses. Furthermore, immunization with S.aureus antigen plus EPS adjuvant induced a protective effect when compared with S.aureus antigen alone in murine bacteremia, pneumonia and mastitis model. Collectively, these results suggest that EPS derived from probiotic Lactobacillus kiferi strain is promising as an efficient adjuvant candidate for the prevention of S. aureus infections.

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.

Conjugation of Synthetic Trisaccharide of Staphylococcus aureus Type 8 Capsular Polysaccharide Elicits Antibodies Recognizing Intact Bacterium

Staphylococcus aureus causes a wide range of life-threatening diseases. One of the powerful approaches for prevention and treatment is to develop an efficient vaccine as antibiotic resistance greatly increases. S. aureus type 8 capsular polysaccharide (CP8) has shown great potential in vaccine development. An understanding of the immunogenicity of CP8 trisaccharide repeating unit is valuable for epitope-focused vaccine design and cost-efficient vaccine production. We report the chemical synthesis of conjugation-ready CP8 trisaccharide 1 bearing an amine linker, which effectively served for immunological evaluation. The trisaccharide 1-CRM197 conjugate elicited a robust immunoglobulin G (IgG) immune response in mice. Both serum antibodies and prepared monoclonal antibodies recognized S. aureus strain, demonstrating that synthetic trisaccharide 1 can be an efficient antigen for vaccine development.

Iron Metabolism at the Interface between Host and Pathogen: From Nutritional Immunity to Antibacterial Development

Nutritional immunity is a form of innate immunity widespread in both vertebrates and invertebrates. The term refers to a rich repertoire of mechanisms set up by the host to inhibit bacterial proliferation by sequestering trace minerals (mainly iron, but also zinc and manganese). This strategy, selected by evolution, represents an effective front-line defense against pathogens and has thus inspired the exploitation of iron restriction in the development of innovative antimicrobials or enhancers of antimicrobial therapy. This review focuses on the mechanisms of nutritional immunity, the strategies adopted by opportunistic human pathogen Staphylococcus aureus to circumvent it, and the impact of deletion mutants on the fitness, infectivity, and persistence inside the host. This information finally converges in an overview of the current development of inhibitors targeting the different stages of iron uptake, an as-yet unexploited target in the field of antistaphylococcal drug discovery.

Rapid and Broad Immune Efficacy of a Recombinant Five-Antigen Vaccine against Staphylococcus Aureus Infection in Animal Models

Staphylococcus aureus (S. aureus) is a leading cause of both healthcare-and community-associated infections globally, which result in severe disease and readily developing antibiotic resistance. Developing an efficacious vaccine against S. aureus is urgently required. In the present study, we selected five conserved antigens, including the secreted factors α-hemolysin (Hla), staphylococcal enterotoxin B (SEB) and the three surface proteins staphylococcal protein A (SpA), iron surface determinant B N2 domain (IsdB-N2) and manganese transport protein C (MntC). They were all well-characterized virulence factor of S. aureus and developed a recombinant five-antigen S. aureus vaccine (rFSAV), rFSAV provided consistent protection in S. aureus lethal sepsis and pneumonia mouse models, and it showed broad immune protection when challenged with a panel of epidemiologically relevant S. aureus strains. Meanwhile, rFSAV immunized mice were able to induce comprehensive cellular and humoral immune responses to reduce bacterial loads, inflammatory cytokine expression, inflammatory cell infiltration and decrease pathology after challenge with a sub-lethal dose of S. aureus. Moreover, the importance of specific antibodies in protection was demonstrated by antibody function tests in vitro and in vivo. Altogether, our data demonstrate that rFSAV is a potentially promising vaccine candidate for defensing against S. aureus infection.

Mechanisms of Immune Evasion and Bone Tissue Colonization That Make Staphylococcus aureus the Primary Pathogen in Osteomyelitis

PURPOSE OF REVIEW

Staphylococcus aureus is the primary pathogen responsible for osteomyelitis, which remains a major healthcare burden. To understand its dominance, here we review the unique pathogenic mechanisms utilized by S. aureus that enable it to cause incurable osteomyelitis.

RECENT FINDINGS

Using an arsenal of toxins and virulence proteins, S. aureus kills and usurps immune cells during infection, to produce non-neutralizing pathogenic antibodies that thwart adaptive immunity. S. aureus also has specific mechanisms for distinct biofilm formation on implants, necrotic bone tissue, bone marrow, and within the osteocyte lacuno-canicular networks (OLCN) of live bone. In vitro studies have also demonstrated potential for intracellular colonization of osteocytes, osteoblasts, and osteoclasts. S. aureus has evolved a multitude of virulence mechanisms to achieve life-long infection of the bone, most notably colonization of OLCN. Targeting S. aureus proteins involved in these pathways could provide new targets for antibiotics and immunotherapies.

Broad and Effective Protection against Staphylococcus aureus Is Elicited by a Multivalent Vaccine Formulated with Novel Antigens

The demand for a prophylactic vaccine against methicillin-resistant Staphylococcus aureus (MRSA) has motivated numerous dedicated research groups to design and develop such a vaccine. In this study, we have developed a multivalent vaccine, Sta-V5, composed of five conserved antigens involved in three important virulence mechanisms. This prototype vaccine conferred up to 100% protection against multiple epidemiologically relevant S. aureus isolates in five different murine disease models. The vaccine not only elicits functional antibodies that mediate opsonophagocytic killing of S. aureus but also mounts robust antigen-specific T-cell responses. In addition, our data implied that γδ T cells contribute to the protection induced by Sta-V5 in a murine skin infection model.IMPORTANCE Staphylococcus aureus infections, especially MRSA infections, are becoming a major global health issue and are resulting in mortality rates that are increasing every year. However, an effective vaccine is lacking due to the complexity of the infection process of S. aureus In this study, we found that the addition of two novel protein components to three well-studied vaccine candidates significantly improved the efficacy of the combined vaccine. Furthermore, the five-component vaccine not only elicits a robust antibody response but also induces cytokine secretion by T cells, making it a promising vaccine candidate to fill the void.

Staphylococcus aureus Orchestrates Type 2 Airway Diseases

Staphylococcus aureus persistently colonizes the nostrils of one-third of the population but colonizes the sinus mucosa in up to 90% of patients with nasal polyps, implying a possible role in airway disease. Recent findings give new mechanistic insights into the ability of S. aureus to trigger type 2 inflammatory responses in the upper and lower airways. This novel concept of a S. aureus-driven chronic airway inflammatory disease suggests a new understanding of disease triggers. This article reviews the role of S. aureus in chronic inflammatory airway diseases and discusses possible therapeutic approaches to target S. aureus.

Recent advances in Staphylococcus aureus infection: focus on vaccine development

Staphylococcus aureus normally colonizes the nasal cavity and pharynx. After breaching the normal habitat, the organism is able to cause a number of infections at any site of the body. The development of antibiotic resistance has created a global challenge for treating infections. Therefore, protection by vaccines may provide valuable measures. Currently, several vaccine candidates have been prepared which are either in preclinical phase or in early clinical phase, whereas several candidates have failed to show a protective efficacy in human subjects. Approaches have also been made in the development of monoclonal or polyclonal antibodies for passive immunization to protect from S. aureus infections. Therefore, in this review we have summarized the findings of recently published scientific literature to make a concise report.