Staphylococcus aureus toxin suppresses antigen-specific T cell responses

Linking S. aureus Immune Evasion Mechanisms to Staphylococcal Vaccine Failures

Vaccination arguably remains the only long-term strategy to limit the spread of S. aureus infections and its related antibiotic resistance. To date, however, all staphylococcal vaccines tested in clinical trials have failed. In this review, we propose that the failure of S. aureus vaccines is intricately linked to prior host exposure to S. aureus and the pathogen's capacity to evade adaptive immune defenses. We suggest that non-protective immune imprints created by previous exposure to S. aureus are preferentially recalled by SA vaccines, and IL-10 induced by S. aureus plays a unique role in shaping these non-protective anti-staphylococcal immune responses. We discuss how S. aureus modifies the host immune landscape, which thereby necessitates alternative approaches to develop successful staphylococcal vaccines.

Targeted Light-Induced Immunomodulatory Strategy for Implant-Associated Infections via Reversing Biofilm-Mediated Immunosuppression

The clinical treatment efficacy for implant-associated infections (IAIs), particularly those caused by Methicillin-resistant Staphylococcus aureus (MRSA), remains unsatisfactory, primarily due to the formation of biofilm barriers and the resulting immunosuppressive microenvironment, leading to the chronicity and recurrence of IAIs. To address this challenge, we propose a light-induced immune enhancement strategy, synthesizing BSA@MnO2@Ce6@Van (BMCV). The BMCV exhibits precise targeting and adhesion to the S. aureus biofilm-infected region, coupled with its capacity to catalyze oxygen generation from H2O2 in the hypoxic and acidic biofilm microenvironment (BME), promoting oxygen-dependent photodynamic therapy efficacy while ensuring continuous release of manganese ions. Notably, targeted BMCV can penetrate biofilms, producing ROS that degrade extracellular DNA, disrupting the biofilm structure and impairing its barrier function, making it vulnerable to infiltration and elimination by the immune system. Furthermore, light-induced reactive oxygen species (ROS) around the biofilm can lyse S. aureus, triggering bacterium-like immunogenic cell death (ICD), releasing abundant immune costimulatory factors, facilitating the recognition and maturation of antigen-presenting cells (APCs), and activating adaptive immunity. Additionally, manganese ions in the BME act as immunoadjuvants, further amplifying macrophage-mediated innate and adaptive immune responses and reversing the immunologically cold BME to an immunologically hot BME. We prove that our synthesized BMCV elicits a robust adaptive immune response in vivo, effectively clearing primary IAIs and inducing long-term immune memory to prevent recurrence. Our study introduces a potent light-induced immunomodulatory nanoplatform capable of reversing the biofilm-induced immunosuppressive microenvironment and disrupting biofilm-mediated protective barriers, offering a promising immunotherapeutic strategy for addressing challenging S. aureus IAIs.

Infection prevention-how can we prevent transmission of community-onset methicillin-resistant Staphylococcus aureus?

BACKGROUND

Staphylococcus aureus is a versatile organism, capable of existing as a commensal organism while also possessing pathogenic potential. The emergence of clinically and genetically distinct strains of methicillin-resistant S. aureus (MRSA), termed community-onset MRSA (CO-MRSA), resulted in an epidemic of invasive and skin and soft tissue infections (SSTI) in otherwise healthy individuals without traditional risk factors. Colonization with S. aureus is a risk factor for developing infection and also a source of transmission to close contacts. Outbreaks of S. aureus SSTI have been described in crowded settings and within households. Thus, preventive strategies are essential to interrupt recurrent infections.

OBJECTIVES

The objective of this narrative review is to provide a comprehensive, evidence-based approach to prevent transmission of CO-MRSA. We highlight key clinical trials that emphasize the importance of household and environmental S. aureus colonization in propagating household transmission. Finally, we highlight research priorities to prevent S. aureus infection.

SOURCES

We cite primary literature from peer-reviewed publications as sources for this review.

CONTENT

Our recommended approach to the management of individuals presenting with skin abscesses includes optimal treatment of the initial infection and hygiene education. Decolonization measures should be recommended for individuals with recurrent SSTIs or whose household members have SSTIs. Targeted decolonization with topical antimicrobials should be prescribed to all affected individuals within the household.

IMPLICATIONS

S. aureus infections result in substantial mortality and morbidity because of the high incidence of recurrent skin infections. Although current decolonization strategies are beneficial, interventions are often costly to families and effectiveness wanes over time. Results from a recently completed trial evaluating integrated periodic decolonization and household environmental hygiene will further add to our understanding of what constitutes a sustainable decolonization approach. In addition, novel preventive strategies are being developed such as S. aureus vaccines, lytic agents, probiotics, microbiota transplants, and phage therapy.

Virulence attributes of successful methicillin-resistant Staphylococcus aureus lineages

Methicillin-resistant Staphylococcus aureus (MRSA) is a leading cause of severe and often fatal infections. MRSA epidemics have occurred in waves, whereby a previously successful lineage has been replaced by a more fit and better adapted lineage. Selection pressures in both hospital and community settings are not uniform across the globe, which has resulted in geographically distinct epidemiology. This review focuses on the mechanisms that trigger the establishment and maintenance of current, dominant MRSA lineages across the globe. While the important role of antibiotic resistance will be mentioned throughout, factors which influence the capacity of S. aureus to colonize and cause disease within a host will be the primary focus of this review. We show that while MRSA possesses a diverse arsenal of toxins including alpha-toxin, the success of a lineage involves more than just producing toxins that damage the host. Success is often attributed to the acquisition or loss of genetic elements involved in colonization and niche adaptation such as the arginine catabolic mobile element, as well as the activity of regulatory systems, and shift metabolism accordingly (e.g., the accessory genome regulator, agr). Understanding exactly how specific MRSA clones cause prolonged epidemics may reveal targets for therapies, whereby both core (e.g., the alpha toxin) and acquired virulence factors (e.g., the Panton-Valentine leukocidin) may be nullified using anti-virulence strategies.

What are the missing pieces needed to stop antibiotic resistance?

As recognized by several international agencies, antibiotic resistance is nowadays one of the most relevant problems for human health. While this problem was alleviated with the introduction of new antibiotics into the market in the golden age of antimicrobial discovery, nowadays few antibiotics are in the pipeline. Under these circumstances, a deep understanding on the mechanisms of emergence, evolution and transmission of antibiotic resistance, as well as on the consequences for the bacterial physiology of acquiring resistance is needed to implement novel strategies, beyond the development of new antibiotics or the restriction in the use of current ones, to more efficiently treat infections. There are still several aspects in the field of antibiotic resistance that are not fully understood. In the current article, we make a non-exhaustive critical review of some of them that we consider of special relevance, in the aim of presenting a snapshot of the studies that still need to be done to tackle antibiotic resistance.

Toxin expression during Staphylococcus aureus infection imprints host immunity to inhibit vaccine efficacy

Staphylococcus aureus infections are a major public health issue, and a vaccine is urgently needed. Despite a considerable promise in preclinical models, all vaccines tested thus far have failed to protect humans against S. aureus. Unlike laboratory mice, humans are exposed to S. aureus throughout life. In the current study, we hypothesized that prior exposure to S. aureus "imprints" the immune response to inhibit vaccine-mediated protection. We established a mouse model in which S. aureus skin and soft tissue infection (SSTI) is followed by vaccination and secondary SSTI. Unlike naïve mice, S. aureus-sensitized mice were incompletely protected against secondary SSTI by vaccination with the inactivated α-hemolysin (Hla) mutant Hla_H35L. Inhibition of protection was specific for the Hla_H35L vaccine and required hla expression during primary SSTI. Surprisingly, inhibition occurred at the level of vaccine-elicited effector T cells; hla expression during primary infection limited the expansion of T cells and dendritic cells and impaired vaccine-specific T cell responses. Importantly, the T cell-stimulating adjuvant CAF01 rescued inhibition and restored vaccine-mediated protection. Together, these findings identify a potential mechanism for the failure of translation of promising S. aureus vaccines from mouse models to clinical practice and suggest a path forward to prevent these devastating infections.

A Dual-Action Molecule Suppresses S. aureus Infection as an Inhibitor Targeting Hla Pore Formation and TLR2 Signaling

Antibiotic resistance is the greatest challenge for the treatment of Staphylococcus aureus (S. aureus) infection under the global antibiotic resistance crisis. With the bottleneck period of the development of new antibiotics, novel alternative agents are urgently in need. In this study, the small molecule amentoflavone is identified as a dual-action inhibitor of Hla, a pore-forming virulence determinant particularly important for S. aureus pathogenicity and Toll-like receptor 2 (TLR2) signaling, which triggers inflammation response upon recognizing pathogen-associated molecular patterns. Amentoflavone treatment conferred effective protection against S. aureus pneumonia through this dual-action mechanism. Mechanically, amentoflavone effectively inhibited Hla pore formation, thereby reducing Hla-mediated cytotoxicity and tissue damage; at the same time, amentoflavone suppressed TLR2-mediated inflammatory response by blocking the interaction between TLR2 and its adapter myeloid differentiation primary response gene 88 (MyD88). Surprisingly, TLR2 signaling induced by synthetic bacterial TLR2 agonists and other heat-killed gram-positive bacteria is also blocked by amentoflavone. In summary, these results presented amentoflavone as a potential antibiotic alternative that curbed S. aureus infection by simultaneously suppressing host-damaging virulence determinants derived from bacteria and the detrimental effect of excessive inflammation derived from the host rather than bacteria viability.

Tissue specificity drives protective immunity against Staphylococcus aureus infection

Infections caused by Staphylococcus aureus range from mild to severe and frequently recur. Emerging evidence suggests that the site and severity of infection drive the potency of elicited immune responses and susceptibility to recurrent infection. In this study, we used tractable mouse models of S. aureus skin infection (SSTI) and pneumonia to determine the relative magnitude of elicited protective immunity. Surprisingly, despite both SSTI and pneumonia eliciting antibody and local effector T cell responses, only SSTI elicited protective antibody and memory T cell responses and subsequent protection against secondary SSTI and pneumonia. The failure of pneumonia to elicit protective immunity was attributed to an inability of S. aureus pneumonia to elicit toxin-specific antibodies that confer protection during secondary infection and was associated with a failure to expand antigen-specific memory T cells. Taken together, these findings emphasize the importance of understanding protective immunity in the context of the tissue-specificity.

Skeletal infections: microbial pathogenesis, immunity and clinical management

Osteomyelitis remains one of the greatest risks in orthopaedic surgery. Although many organisms are linked to skeletal infections, Staphylococcus aureus remains the most prevalent and devastating causative pathogen. Important discoveries have uncovered novel mechanisms of S. aureus pathogenesis and persistence within bone tissue, including implant-associated biofilms, abscesses and invasion of the osteocyte lacuno-canalicular network. However, little clinical progress has been made in the prevention and eradication of skeletal infection as treatment algorithms and outcomes have only incrementally changed over the past half century. In this Review, we discuss the mechanisms of persistence and immune evasion in S. aureus infection of the skeletal system as well as features of other osteomyelitis-causing pathogens in implant-associated and native bone infections. We also describe how the host fails to eradicate bacterial bone infections, and how this new information may lead to the development of novel interventions. Finally, we discuss the clinical management of skeletal infection, including osteomyelitis classification and strategies to treat skeletal infections with emerging technologies that could translate to the clinic in the future.

Integrating complex host-pathogen immune environments into S. aureus vaccine studies

Staphylococcus aureus (SA) is a leading cause of bacterial infection and antibiotic resistance globally. Therefore, development of an effective vaccine has been a major goal of the SA field for the past decades. With the wealth of understanding of pathogenesis, the failure of all SA vaccine trials has been a surprise. We argue that experimental SA vaccines have not worked because vaccines have been studied in naive laboratory animals, whereas clinical vaccine efficacy is tested in immune environments reprogrammed by SA. Here, we review the failed SA vaccines that have seemingly defied all principles of vaccinology. We describe major SA evasion strategies and suggest that they reshape the immune environment in a way that makes vaccines prone to failures. We propose that appropriate integration of concepts of host-pathogen interaction into vaccine study designs could lead to insight critical for the development of an effective SA vaccine.

Staphylococcus aureus specific lung resident memory CD4+ Th1 cells attenuate the severity of influenza virus induced secondary bacterial pneumonia

Staphylococcus aureus is a major cause of severe pulmonary infections. The evolution of multi-drug resistant strains limits antibiotic treatment options. To date, all candidate vaccines tested have failed, highlighting the need for an increased understanding of the immunological requirements for effective S. aureus immunity. Using an S. aureus strain engineered to express a trackable CD4⁺ T cell epitope and a murine model of S. aureus pneumonia, we show strategies that lodge Th1 polarised bacterium specific CD4⁺ tissue resident memory T cells (Trm) in the lung can significantly attenuate the severity of S. aureus pneumonia. This contrasts natural infection of mice that fails to lodge CD4⁺ Trm cells along the respiratory tract or provide protection against re-infection, despite initially generating Th17 bacterium specific CD4⁺ T cell responses. Interestingly, lack of CD4⁺ Trm formation after natural infection in mice appears to be reflected in humans, where the frequency of S. aureus reactive CD4⁺ Trm cells in lung tissue is also low. Our findings reveal the protective capacity of S. aureus specific respiratory tract CD4⁺ Th1 polarised Trm cells and highlight the potential for targeting these cells in vaccines that aim to prevent the development of S. aureus pneumonia.

Development of a Conserved Chimeric Vaccine for Induction of Strong Immune Response against Staphylococcus aureus Using Immunoinformatics Approaches

Staphylococcus aureus is one of the most notorious Gram-positive bacteria with a very high mortality rate. The WHO has listed S. aureus as one of the ESKAPE pathogens requiring urgent research and development efforts to fight against it. Yet there is a major layback in the advancement of effective vaccines against this multidrug-resistant pathogen. SdrD and SdrE proteins are attractive immunogen candidates as they are conserved among all the strains and contribute specifically to bacterial adherence to the host cells. Furthermore, these proteins are predicted to be highly antigenic and essential for pathogen survival. Therefore, in this study, using the immunoinformatics approach, a novel vaccine candidate was constructed using highly immunogenic conserved T-cell and B-cell epitopes along with specific linkers, adjuvants, and consequently modeled for docking with human Toll-like receptor 2. Additionally, physicochemical properties, secondary structure, disulphide engineering, and population coverage analysis were also analyzed for the vaccine. The constructed vaccine showed good results of worldwide population coverage and a promising immune response. For evaluation of the stability of the vaccine-TLR-2 docked complex, a molecular dynamics simulation was performed. The constructed vaccine was subjected to in silico immune simulations by C-ImmSim and Immune simulation significantly provided high levels of immunoglobulins, T-helper cells, T-cytotoxic cells, and INF-γ. Lastly, upon cloning, the vaccine protein was reverse transcribed into a DNA sequence and cloned into a pET28a (+) vector to ensure translational potency and microbial expression. The overall results of the study showed that the designed novel chimeric vaccine can simultaneously elicit humoral and cell-mediated immune responses and is a reliable construct for subsequent in vivo and in vitro studies against the pathogen.

Adenosine synthase A contributes to recurrent Staphylococcus aureus infection by dampening protective immunity

Background: Staphylococcus aureus is a common human pathogen capable of causing diverse illnesses with possible recurrent infections. Although recent studies have highlighted the role of cellular immunity in recurrent infections, the mechanism by which S. aureus evades host responses remains largely unexplored.

Methods: This study utilizes in vitro and in vivo infection experiments to investigate difference of pro-inflammatory responses and subsequent adaptive immune responses between adsA mutant and WT S. aureus strain infection.

Findings: We demonstrated that adenosine synthase A (AdsA), a potent S. aureus virulence factor, can alter Th17 responses by interfering with NLRP3 inflammasome-mediated IL-1β production. Specifically, S. aureus virulence factor AdsA dampens Th1/Th17 immunity by limiting the release of IL-1β and other Th polarizing cytokines. In particular, AdsA obstructs the release of IL-1β via the adenosine/A2aR/NLRP3 axis. Using a murine infection model, pharmacological inhibition of A2a receptor enhanced S. aureus-specific Th17 responses, whereas inhibition of NLRP3 and caspase-1 downregulated these responses. Our results showed that AdsA contributes to recurrent S. aureus infection by restraining protective Th1/Th17 responses.

Interpretation: Our study provides important mechanistic insights for therapeutic and vaccination strategies against S. aureus infections.

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.

Impaired T lymphocyte responses during childhood Staphylococcus aureus infection

BACKGROUND

Staphylococcus aureus infections are common throughout the lifespan, with recurrent infections occuring in nearly half of infected children. There is no licensed vaccine, underscoring the need to better understand how S. aureus evades protective immunity. Despite much study, the relative contributions of antibodies and T cells to protection against S. aureus infections in humans are not fully understood.

METHODS

We prospectively quantified S. aureus-specific antibody levels by ELISA and T cell responses by ELISpot in S. aureus-infected and healthy children.

RESULTS

S. aureus-specific antibody levels and T cell responses increased with age in healthy children, suggesting a coordinated development of anti-staphylococcal immunity. Antibody levels against leukotoxin E (LukE) and Panton-Valentine leukocidin (LukS-PV), but not α-hemolysin (Hla), were higher in younger infected children, compared with healthy children; these differences disappeared in older children. We observed a striking impairment of global and S. aureus-specific T cell function in children with invasive and non-invasive infection, suggesting that S. aureus-specific immune responses are dysregulated during childhood infection regardless of the infection phenotype.

CONCLUSIONS

These findings identify a potential mechanism by which S. aureus infection actively evades adaptive immune responses, thereby preventing the development of protective immunty and maintaining susceptibility to recurrent infection.

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.

Humanized Mice Exhibit Exacerbated Abscess Formation and Osteolysis During the Establishment of Implant-Associated Staphylococcus aureus Osteomyelitis

Staphylococcus aureus is the predominant pathogen causing osteomyelitis. Unfortunately, no immunotherapy exists to treat these very challenging and costly infections despite decades of research, and numerous vaccine failures in clinical trials. This lack of success can partially be attributed to an overreliance on murine models where the immune correlates of protection often diverge from that of humans. Moreover, S. aureus secretes numerous immunotoxins with unique tropism to human leukocytes, which compromises the targeting of immune cells in murine models. To study the response of human immune cells during chronic S. aureus bone infections, we engrafted non-obese diabetic (NOD)-scid IL2Rγ^null (NSG) mice with human hematopoietic stem cells (huNSG) and analyzed protection in an established model of implant-associated osteomyelitis. The results showed that huNSG mice have increases in weight loss, osteolysis, bacterial dissemination to internal organs, and numbers of Staphylococcal abscess communities (SACs), during the establishment of implant-associated MRSA osteomyelitis compared to NSG controls (p < 0.05). Flow cytometry and immunohistochemistry demonstrated greater human T cell numbers in infected versus uninfected huNSG mice (p < 0.05), and that T-bet⁺ human T cells clustered around the SACs, suggesting S. aureus-mediated activation and proliferation of human T cells in the infected bone. Collectively, these proof-of-concept studies underscore the utility of huNSG mice for studying an aggressive form of S. aureus osteomyelitis, which is more akin to that seen in humans. We have also established an experimental system to investigate the contribution of specific human T cells in controlling S. aureus infection and dissemination.

Targeting leukocidin-mediated immune evasion protects mice from Staphylococcus aureus bacteremia

Staphylococcus aureus is responsible for various diseases in humans, and recurrent infections are commonly observed. S. aureus produces an array of bicomponent pore-forming toxins that target and kill leukocytes, known collectively as the leukocidins. The contribution of these leukocidins to impair the development of anti–S. aureus adaptive immunity and facilitate reinfection is unclear. Using a murine model of recurrent bacteremia, we demonstrate that infection with a leukocidin mutant results in increased levels of anti–S. aureus antibodies compared with mice infected with the WT parental strain, indicating that leukocidins negatively impact the generation of anti–S. aureus antibodies in vivo. We hypothesized that neutralizing leukocidin-mediated immune subversion by vaccination may shift this host-pathogen interaction in favor of the host. Leukocidin-immunized mice produce potent leukocidin-neutralizing antibodies and robust Th1 and Th17 responses, which collectively protect against bloodstream infections. Altogether, these results demonstrate that blocking leukocidin-mediated immune evasion can promote host protection against S. aureus bloodstream infection.

Evasion of Immunological Memory by S. aureus Infection: Implications for Vaccine Design

Recurrent S. aureus infections are common, suggesting that natural immune responses are not protective. All candidate vaccines tested thus far have failed to protect against S. aureus infections, highlighting an urgent need to better understand the mechanisms by which the bacterium interacts with the host immune system to evade or prevent protective immunity. Although there is evidence in murine models that both cellular and humoral immune responses are important for protection against S. aureus, human studies suggest that T cells are critical in determining susceptibility to infection. This review will use an “anatomic” approach to systematically outline the steps necessary in generating a T cell-mediated immune response against S. aureus. Through the processes of bacterial uptake by antigen presenting cells, processing and presentation of antigens to T cells, and differentiation and proliferation of memory and effector T cell subsets, the ability of S. aureus to evade or inhibit each step of the T-cell mediated response will be reviewed. We hypothesize that these interactions result in the redirection of immune responses away from protective antigens, thereby precluding the establishment of “natural” memory and potentially inhibiting the efficacy of vaccination. It is anticipated that this approach will reveal important implications for future design of vaccines to prevent these infections.

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.

T Cell Immunity and the Quest for Protective Vaccines against Staphylococcus aureus Infection

Staphylococcus aureus is a wide-spread human pathogen, and one of the top causative agents of nosocomial infections. The prevalence of antibiotic-resistant S. aureus strains, which are associated with higher mortality and morbidity rates than antibiotic-susceptible strains, is increasing around the world. Vaccination would be an effective preventive measure against S. aureus infection, but to date, every vaccine developed has failed in clinical trials, despite inducing robust antibody responses. These results suggest that induction of humoral immunity does not suffice to confer protection against the infection. Evidence from studies in murine models and in patients with immune defects support a role of T cell-mediated immunity in protective responses against S. aureus. Here, we review the current understanding of the mechanisms underlying adaptive immunity to S. aureus infections and discuss these findings in light of the recent S. aureus vaccine trial failures. We make the case for the need to develop anti-S. aureus vaccines that can specifically elicit robust and durable protective memory T cell subsets.