Haemato-immunological responses and effectiveness of feed-based bivalent vaccine against Streptococcus iniae and Aeromonas hydrophila infections in hybrid red tilapia (Oreochromis mossambicus × O. niloticus).

BACKGROUND: Streptococcosis and Motile Aeromonad Septicemia (MAS) are important diseases of tilapia, Oreochromis spp. and causes huge economic losses in aquaculture globally. The feed-based vaccination may be an alternative to minimize major infectious diseases in tilapia. Thus, this study aims to evaluate the haemato-immunological responses and effectiveness of a newly developed feed-based killed bivalent vaccine against Streptococcus iniae and Aeromonas hydrophila in hybrid red tilapia. A total of 495 hybrid red tilapia of 61.23 ± 4.95 g were distributed into 5 groups (each with triplicate). The fish were immunized orally through bivalent (combined S. iniae and A. hydrophila) spray vaccine (BS group), bivalent formulate vaccine (BF group), monovalent S. iniae vaccine (MS group), monovalent A. hydrophila vaccine (MA group) and unvaccinated as a control group. The vaccine was orally administered on days 0, 14 and 42 applied feed-based bacterin at 5% body weight. The blood and spleen samples were collected from all groups on 7, 21 and 49 days post-vaccination, and also 96 h post-infection to assess their haemato-immune responses. RESULTS: Compared with the unvaccinated group, leukocyte, lymphocytes, monocytes, granulocytes counts in vaccinated groups were significantly (P < 0.05) increased on 21, 49 days post-vaccination and also 96 h post-infection, while erythrocytes, haemoglobin and haematocrit in vaccinated groups were significantly (P < 0.05) enhanced only 96 h post-infection. Additionally, the lysozyme and phagocytic activity and, serum antibody (IgM) were significantly higher (P < 0.05) against S. iniae and A. hydrophila in vaccinated groups compared to the unvaccinated group in the pre- and post-infection. Results from the challenge through co-infection with S. iniae and A. hydrophila showed the relative percent survival (RPS) in BF group was 76.67 ± 4.71%, which had the capacity to induce significant protection (P < 0.05) compared to others groups. CONCLUSIONS: This study demonstrates the bivalent formulate (BF) group could elicit significant non-specific and specific immunological responses with higher protection in hybrid red tilapia. In addition, this newly developed feed-based bivalent vaccination can be a promising technique for effective and large scale fish immunization in the aquaculture industry.

Comparison of protective efficacy between two DNA vaccines encoding DnaK and GroEL against fish nocardiosis.

Fish nocardiosis is a chronic granulomatous bacterial disease mainly caused by three pathogenic bacteria, including Nocardia seriolae, N. asteroids and N. salmonicida. Molecular chaperone DnaK and GroEL were identified to be the common antigens of the three pathogenic Nocardia species in our previous studies. To evaluate the immune protective effect of two DNA vaccines encoding DnaK or GroEL against fish nocardiosis, hybrid snakehead were vaccinated and the immune responses induced by these two vaccines were comparatively analyzed. The results suggested it needed at least 7 d to transport DnaK or GroEL gene from injected muscle to head kidney, spleen and liver and stimulate host's immune system for later protection after immunization by DNA vaccines. Additionally, non-specific immunity parameters (serum lysozyme (LYZ), peroxidase (POD), acid phosphatase (ACP), alkaline phosphatase (AKP) and superoxide dismutase (SOD) activities), specific antibody (IgM) production and immune-related genes (MHCIα, MHCIIα, CD4, CD8α, IL-1ß and TNFα) were used to evaluate the immune responses induced in vaccinated hybrid snakehead. It proved that all the above-mentioned immune activities were significantly enhanced after immunization with these two DNA vaccines. The protective efficacy of pcDNA-DnaK and pcDNA-GroEL DNA vaccines, in terms of relative percentage survival (RPS), were 53.01% and 80.71% respectively. It demonstrated that these two DNA vaccines could increase the survival rate of hybrid snakehead against fish nocardiosis, albeit with variations in immunoprotective effects. Taken together, these results indicated that both pcDNA-DnaK and pcDNA-GroEL DNA vaccines could boost the innate, humoral and cellular immune response in hybrid snakehead and show highly protective efficacy against fish nocardiosis, suggesting that DnaK and GroEL were promising vaccine candidates. These findings will promote the development of DNA vaccines against fish nocardiosis in aquaculture.

Pneumococcal Capsules and Their Types: Past, Present, and Future.

Streptococcus pneumoniae (the pneumococcus) is an important human pathogen. Its virulence is largely due to its polysaccharide capsule, which shields it from the host immune system, and because of this, the capsule has been extensively studied. Studies of the capsule led to the identification of DNA as the genetic material, identification of many different capsular serotypes, and identification of the serotype-specific nature of protection by adaptive immunity. Recent studies have led to the determination of capsular polysaccharide structures for many serotypes using advanced analytical technologies, complete elucidation of genetic basis for the capsular types, and the development of highly effective pneumococcal conjugate vaccines. Conjugate vaccine use has altered the serotype distribution by either serotype replacement or switching, and this has increased the need to serotype pneumococci. Due to great advances in molecular technologies and our understanding of the pneumococcal genome, molecular approaches have become powerful tools to predict pneumococcal serotypes. In addition, more-precise and -efficient serotyping methods that directly detect polysaccharide structures are emerging. These improvements in our capabilities will greatly enhance future investigations of pneumococcal epidemiology and diseases and the biology of colonization and innate immunity to pneumococcal capsules.

Tissue-Resident T Cells as the Central Paradigm of Chlamydia Immunity.

For almost 2 decades, results from Chlamydia pathogenesis investigations have been conceptualized using a cytokine polarization narrative. Recent viral immunity studies identifying protective tissue-resident memory T cells (Trm) suggest an alternative paradigm based on localized immune networks. As Chlamydia vaccines enter the preclinical pipeline and, in the case of an attenuated trachoma vaccine, are given to human subjects, it may be useful to ask whether cytokine polarization is the appropriate framework for understanding and evaluating vaccine efficacy. In this review, we revisit C. trachomatis pathogenesis data from mice and humans using a Trm narrative and note a comfortable concordance with the Chlamydia pathogenesis literature.

Efficacy of Mycoplasma hyopneumoniae vaccination before and at weaning against experimental challenge infection in pigs.

BACKGROUND: Commercial bacterins are widely used at weaning to control Mycoplasma hyopneumoniae infections in pigs. However, it is not known whether the efficacy of vaccinating against M. hyopneumoniae can be influenced by the weaning process when vaccination is applied at the day of weaning. The present study assessed the efficacy of a single M. hyopneumoniae vaccination (Ingelvac MycoFLEX®) three days before weaning (V1) or at weaning (V2) against experimental challenge infection. Four weeks after vaccination, groups V1 and V2 (n = 20 pigs each) and a non-vaccinated, positive control group (PCG) (n = 20) were endotracheally inoculated with a virulent M. hyopneumoniae field strain. Five pigs were used as a negative control group. All pigs were euthanized 5 weeks after challenge. The main parameters investigated included macroscopic and histopathological lung lesions at necropsy, immunofluorescence (IF) staining and quantitative real-time PCR (qPCR) on broncho-alveolar lavage (BAL) fluid for quantifying M. hyopneumoniae. RESULTS: The average macroscopic lung lesion scores in groups V1, V2 and PCG were 0.54, 0.88 and 1.04, respectively (P > 0.05). The average lymphohistiocytic infiltration scores in groups V1, V2 and PCG were 2.95, 3.16 and 3.61, respectively (P < 0.05). The average IF scores were: V1 = 1.13, V2 = 1.19 and PCG = 1.25 (P > 0.05), the qPCR values were: V1 = 10(2.94), V2 = 10(2.76) and PCG = 10(3.23) (P > 0.05). All pigs of the negative control group remained negative throughout the study. CONCLUSIONS: Both vaccinated groups had lower numbers of macroscopic and histopathological lung lesions, and lower numbers of M. hyopneumoniae organisms in the BAL fluid compared to the PCG. However, no firm conclusions could be made on whether weaning negatively influences the efficacy of M. hyopneumoniae vaccination, since significant differences between the treatment groups were only obtained for the histopathological lung lesions. This could be attributed to the fact that milder macroscopic lung lesions were produced in the inoculated pigs, when compared to previous trials conducted by the same group. Further research under field conditions is warranted to assess possible differences between the two vaccination strategies.

Development and validation of glycoprotein-based native-subunit vaccine for fish against Aeromonas hydrophila.

Aeromonas hydrophila is known to be causative agent of an infection named as Bacterial haemorrhagic septicaemia or red pest in freshwater fish. The aim of this study was to develop and validate the glycoprotein-based fish vaccine against Aeromonas hydrophila. For this aim, after identification and characterization of A. hydrophila isolates from fish farms, one A. hydrophila isolate was selected as vaccine strain. Antigenic glycoproteins of this vaccine strain were determined by Western blotting and glycan detection kit. The connection types of these glycoproteins were examined by glycoprotein differentiation kit. Two glycoproteins, molecular weights of 19 and 38 kDa, with SNA connection type were selected for use in vaccination trials. After their purification by SNA-specific lectin and size-exclusion chromatography, protection studies with purified proteins were performed. For challenge trials, four experimental fish groups were designated: Group I (with montanide), Group II (with montanide and ginseng), Group III [with Al(OH)3 ] and Group IV [with Al(OH)3 and ginseng]. The survival ratings of fish were determined, and protection was calculated as 21.56%, 29.41%, 69.83% and 78.88% in groups I, II, III and IV, respectively. In conclusion, A. hydrophila glycoproteins with Al(OH)3 and ginseng could be used as a safe and effective vaccine for fish.

Development of functional and molecular correlates of vaccine-induced protection for a model intracellular pathogen, F. tularensis LVS.

In contrast with common human infections for which vaccine efficacy can be evaluated directly in field studies, alternative strategies are needed to evaluate efficacy for slowly developing or sporadic diseases like tularemia. For diseases such as these caused by intracellular bacteria, serological measures of antibodies are generally not predictive. Here, we used vaccines varying in efficacy to explore development of clinically useful correlates of protection for intracellular bacteria, using Francisella tularensis as an experimental model. F. tularensis is an intracellular bacterium classified as Category A bioterrorism agent which causes tularemia. The primary vaccine candidate in the U.S., called Live Vaccine Strain (LVS), has been the subject of ongoing clinical studies; however, safety and efficacy are not well established, and LVS is not licensed by the U.S. FDA. Using a mouse model, we compared the in vivo efficacy of a panel of qualitatively different Francisella vaccine candidates, the in vitro functional activity of immune lymphocytes derived from vaccinated mice, and relative gene expression in immune lymphocytes. Integrated analyses showed that the hierarchy of protection in vivo engendered by qualitatively different vaccines was reflected by the degree of lymphocytes' in vitro activity in controlling the intramacrophage growth of Francisella. Thus, this assay may be a functional correlate. Further, the strength of protection was significantly related to the degree of up-regulation of expression of a panel of genes in cells recovered from the assay. These included IFN-γ, IL-6, IL-12Rß2, T-bet, SOCS-1, and IL-18bp. Taken together, the results indicate that an in vitro assay that detects control of bacterial growth, and/or a selected panel of mediators, may ultimately be developed to predict the outcome of vaccine efficacy and to complement clinical trials. The overall approach may be applicable to intracellular pathogens in general.

Development of an in vitro assay based on humoral immunity for quality control of oil-adjuvant Pseudotuberculosis vaccine in Yellowtail Seriola quinqueradiata.

Photobacterium damselae subsp. piscicida is an infectious pathogen that causes Pseudotuberculosis in Yellowtail fish. In Japan, several oil-adjuvant vaccines for Pseudotuberculosis have been approved for control of infectious diseases in aquaculture. Before distribution of an approved fish vaccine, an artificial challenge test for quality control is performed by the manufacturer and National Veterinary Assay Laboratory under Pharmaceutical Law of Japan to confirm potency. In this study, artificial challenge tests with a range of five diluted or undiluted approved vaccines was performed to determine the relationship between antigen levels and vaccine efficacy. Immunization of fish with the undiluted vaccine prevented Pseudotuberculosis. Results of artificial challenge tests demonstrated vaccine efficiency was dose dependent. Agglutination assays using immune sera were performed to determine agglutination titers, which were also dose dependent. These results suggest a link between survival rate in the artificial challenge tests and agglutination titers. Western blotting analysis identified a specific protein approximately 37 kDa in size in vaccinated fish. We confirmed antibodies were produced in vaccinated fish by immunoreactions with the approved vaccine. An agglutination assay based on humoral immunoreactions would be a useful alternative to the artificial challenge test for quality control of vaccines for aquaculture.

International regulatory requirements for Leptospira vaccine potency testing. Roundtable: current requirements and opportunity for harmonization.

Progress continues to be made in the ongoing efforts to replace, reduce, or refine the use of laboratory animals for Leptospira vaccine potency testing in certain markets/regions. Leptospira-containing vaccines, as with many veterinary vaccines, are manufactured and distributed both on a regional basis by local manufacturers and internationally by large multinational firms. Three general scenarios exist for the international testing and distribution of veterinary vaccines including: 1) the importing country recognizes the country of origin's testing and batch release data with no additional testing; 2) the importing country requires the manufacturer to conduct a specific potency assay based on the current importing market's regulations for the importing country or 3) the importing country requires retesting of the product in country prior to distribution. Scenarios 2 and 3 both have the potential to significantly increase the usage of laboratory animals for what may be considered redundant testing. Specific requirements for the importation of Leptospira vaccines in the United States, Europe, and Mexico were presented as well as efforts to reduce the use of laboratory animal testing through the availability of internationally recognized tests.

Opportunities and strategies to further reduce animal use for Leptospira vaccine potency testing.

Hamsters are routinely infected with virulent Leptospira for two purposes in the regulation of biologics: the performance of Codified potency tests and maintenance of challenge culture for the Codified potency tests. Options for reducing animal use in these processes were explored in a plenary lecture at the "International Workshop on Alternative Methods for Leptospira Vaccine Potency Testing: State of the Science and the Way Forward" held at the Center for Veterinary Biologics in September 2012. The use of validated in vitro potency assays such as those developed by the U.S. Department of Agriculture for Leptospira (L.) canicola, Leptospira grippotyphosa, Leptospira pomona, and Leptospira icterohaemorrhagiae rather than the Codified hamster vaccination-challenge assay was encouraged. Alternatives such as reduced animal numbers in the hamster vaccination-challenge testing were considered for problematic situations. Specifically, the merits of sharing challenge controls, reducing group sizes, and eliminating animals for concurrent challenge dose titration were assessed. Options for maintaining virulent, stable cultures without serial passage through hamsters or with decreased hamster use were also discussed. The maintenance of virulent Leptospira without the use of live animals is especially difficult since a reliable means to maintain virulence after multiple in vitro passages has not yet been identified.

Development and validation of a serological potency test for the release of Leptospira vaccines--requirements in the European Union.

Both European Pharmacopoeia Monograph 01/2008:0447 "Canine Leptospirosis vaccine (inactivated)" and the more recent Monograph 01/2008:1939 "Bovine Leptospirosis vaccine (inactivated)" explicitly allow for a sero-response test to assess batch potency. Test setup and requirements for in vivo and in vitro validation are described. Furthermore, the two main strategies to assess batch potency and their specific demands are addressed.

Product-specific validation of a serological potency test for release of Leptospira vaccines in the European Union.

Historically in the European Union, all Leptospira vaccines were released using the European Pharmacopoeia (Ph. Eur.) hamster potency assay. Recently, there has been a shift toward alternatives that offer either refinement of testing or replacement of animals for product release. This is being driven by animal welfare concerns but also by a drive to have more consistent, cheaper, and faster batch release tests. This publication discusses one such example of a multicomponent canine vaccine that includes three Leptospira serovars and has recently been registered in the European Union. The potency release test is a refinement because it uses rabbit serology rather than hamster challenge. This publication covers the principles of the test method, challenges faced during its development and registration, and discussion about benefits and limitations of this method. It concludes with a view of how the use of serology testing could fit into an overall strategy to move to fully in vitro testing by adopting a consistency approach.

Safety and immunogenicity of bacterial and tobacco plant cell line derived recombinant native and mutant Escherichia coli heat-labile toxin in chickens.

The safety and immunogenicity of the mammalian mucosal adjuvants, Escherichia coli wild-type heat-labile holotoxin (LT) and E. coli mutant LT (LTA-K63/LTB), were examined in 1-day-old chicks and 10-day-old to 21-day-old broilers. Biologically active, E. coli recombinant wild-type LT and recombinant LTA-K63/LTB produced in a transgenic Nicotiana tabacum (NT-1) tobacco cell line (SLT102) were tested for safety and antigenicity following various routes of administration. Safety was assessed by clinical signs, body weight gain, gross organ pathology and wet organ weight, and histopathology. Antigenicity was assessed using LT-B-specific serum IgG enzyme-linked immunosorbent assay. Parenteral administration of E. coli recombinant wild-type LT did not have any discernible effect on bird health and was well tolerated at levels up to 400 µg per dose. Recombinant, SLT102-derived mutant LT derived from SLT102 cells retained in vitro ganglioside binding and was safe and antigenic following repeated mucosal administration to birds. The highest systemic LT-B-specific IgG titres were detected in birds that received three on-feed doses of SLT102-derived mutant LT. Among the various SLT102-derived mutant LT preparations tested, whole, wet cells or whole cell lysates were the most antigenic. These results demonstrate for the first time that E. coli-derived recombinant, wild-type LT holotoxin is well tolerated following multiple administrations to young birds at body weight doses previously reported to be enteropathogenic and toxic in mammalian species. Moreover, these data also demonstrate the feasibility of using recombinant wild-type and mutant LT produced in transgenic NT-1 tobacco cells as safe and potent vaccine adjuvants in poultry.

In vitro properties of a Listeria monocytogenes bacteriophage-resistant mutant predict its efficacy as a live oral vaccine strain.

A Listeria monocytogenes glcV mutation precludes the binding of certain listerial phages and produces a profound attenuation characterized by the absence of detectable mutants in the livers and spleens of orally inoculated mice. In vitro, we found that the mutant formed plaques on mouse enterocyte monolayers as efficiently as the parent but the plaques formed were smaller. Intracellular growth rate determinations and examination of infected enterocytes by light and fluorescence microscopy established that the mutant was impaired not in intracellular growth rate but in cell-to-cell spreading. Because this property is shared by other immunogenic mutants (e.g., actA mutants), our glcV mutant was tested for vaccine efficacy. Oral immunization with the mutant and subsequent oral challenge (22 days postvaccination) with the parent revealed a ca. 10,000-fold increase in protection afforded by the mutant compared to sham-vaccinated controls. The glcV mutant did not stimulate innate immunity under the dose and route employed for vaccination, and an infectivity index time course experiment revealed pronounced mutant persistence in Peyer's patches. The immunogenicity of the glcV mutant compared to an isogenic actA mutant reference strain was next tested in an experiment with a challenge given 52 days postvaccination. Both mutant strains showed scant vital organ infectivity and high levels of protection similar to those seen using the glcV mutant in the 22-day postvaccination challenge. Our results indicate that oral administration of a profoundly attenuated listerial mutant can safely elicit solid protective immunity.

Robustness testing in pharmaceutical freeze-drying: inter-relation of process conditions and product quality attributes studied for a vaccine formulation.

CONTEXT: The recent US Food and Drug Administration (FDA) legislation has introduced the evaluation of the Design Space of critical process parameters in manufacturing processes. In freeze-drying, a "formulation" is expected to be robust when minor deviations of the product temperature do not negatively affect the final product quality attributes. OBJECTIVE: To evaluate "formulation" robustness by investigating the effect of elevated product temperature on product quality using a bacterial vaccine solution. MATERIALS AND METHODS: The vaccine solution was characterized by freeze-dry microscopy to determine the critical formulation temperature. A conservative cycle was developed using the SMART™ mode of a Lyostar II freeze dryer. Product temperature was elevated to imitate intermediate and aggressive cycle conditions. The final product was analyzed using X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), Karl Fischer, and modulated differential scanning calorimetry (MDSC), and the life cell count (LCC) during accelerated stability testing. RESULTS: The cakes processed at intermediate and aggressive conditions displayed larger pores with microcollapse of walls and stronger loss in LCC than the conservatively processed product, especially during stability testing. For all process conditions, a loss of the majority of cells was observed during storage. CONCLUSION: For freeze-drying of life bacterial vaccine solutions, the product temperature profile during primary drying appeared to be inter-related to product quality attributes.

Efficacy of whole-cell killed bacterial vaccines in preventing pneumonia and death during the 1918 influenza pandemic.

BACKGROUND: Most deaths in the 1918 influenza pandemic were caused by secondary bacterial pneumonia. METHODS: We performed a systematic review and reanalysis of studies of bacterial vaccine efficacy (VE) in preventing pneumonia and mortality among patients with influenza during the 1918 pandemic. RESULTS: A meta-analysis of 6 civilian studies of mixed killed bacterial vaccines containing pneumococci identified significant heterogeneity among studies and estimated VE at 34% (95% confidence interval [CI], 19%-47%) in preventing pneumonia and 42% (95% CI, 18%-59%) in reducing case fatality rates among patients with influenza, using random-effects models. Using fixed-effect models, the pooled VE from 3 military studies was 59% (95% CI, 43%-70%) for pneumonia and 70% (95% CI, 50%-82%) for case fatality. Military studies showed less heterogeneity and may provide more accurate results than civilian studies, given the potential biases in the included studies. Findings of 1 military study using hemolytic streptococci also suggested that there was significant protection. CONCLUSIONS: Despite significant methodological problems, the systematic biases in these studies do not exclude the possibilities that whole-cell inactivated pneumococcal vaccines may confer cross-protection to multiple pneumococcal serotypes and that bacterial vaccines may play a role in preventing influenza-associated pneumonia.

Identification of in vivo expressed proteins in live attenuated lipopolysaccharide mutant that mediates heterologous protection against Leptospira spp.

Leptospirosis vaccines that elicit broad protection against a range of pathogenic Leptospira spp. would overcome a major drawback of currently licensed bacterin vaccines. Live attenuated vaccine produced from a lipopolysaccharide (LPS) mutant strain of L. interrogans serovar Manilae M1352 (Live M1352) stimulated better protective efficacy than heat killed M1352 (HK M1352) against a heterologous challenge with L. interrogans serovar Pomona. To identify antigens of Live M1352 potentially responsible for cross protection, in vivo-induced antigen technology (IVIAT), a powerful tool to identify in vivo-induced (ivi) genes expressed during infection, was employed in this study. Pooled sera from hamsters immunized with Live M1352 were sequentially adsorbed with various preparations of in vitro grown M1352. The pre-adsorbed sera were used to screen a genomic expression library of M1352. Nineteen strongly reactive clones were selected for DNA sequencing. These ivi genes are conserved in most Leptospira strains. Four selected genes including LIMLP_04965 (tolB), LIMLP_01535, LIMLP_06785 (fliI), and LIMLP_14930 were confirmed for their upregulated expression in kidneys of infected hamsters by RT-qPCR, suggesting their role in leptospiral infection. These ivi proteins represent potential targets for vaccine candidates that warrant further investigation for their protective efficacy.

COVID-19 impact on infant and adolescent vaccine supplies.

Vaccine production is quadrupling rapidly, creating supply chain challenges.

An in vitro growth inhibition test for measuring the potency of Leptospira spp. Sejroe group vaccine in buffaloes.

Leptospira spp. serovars Hardjo and Wollfi from Sejroe serogroup have been detected in livestock in Brazil, where the main control procedures rely on vaccination. The potency of two commercial vaccines available in this country was monitored by microagglutination test-MAT and in vitro growth inhibition test-GIT in serum samples from 33 female buffaloes divided into: G1-unvaccinated control; G2-vaccinated with Leptobac-6 containing serovars Hardjo and Wolffi and G3-vaccinated with Triangle-9 containing serovar Hardjo. G2 and G3 animals were vaccinated on day zero, and received a booster and two revaccinations on days 30, 210 and 390 and G1 animals received phosphate buffered saline. Serum samples were collected at 15-day intervals between days 0 and 60; and at 30-day intervals between days 60 and 540 and were tested by MAT and GIT with serovars Hardjo and Wolffi. G1 remained negative throughout the experiment. Both vaccines were able to induce agglutinating and growth inhibition antibodies. Six months after the last revaccination, all animals tested negative by MAT, but still were positive by GIT until the end of experimental period. GIT could be a good tool to evaluate the potency and to monitor antibodies responses of vaccines of Sejroe group serovars.

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.