The Effects of Continuous In Vivo Administration of Nisin on Staphylococcus aureus Infection and Immune Response in Mice

Mice were intraperitoneally infected with 2 × 10⁸ cfu Staphylococcus aureus Xen 36 and treated with 2,130 AU (arbitrary units) nisin (equivalent to 27.7 μg pure nisin), a class Ia lantibiotic, over 7 days. The metabolic activity of S. aureus Xen 36, concluded from changes in cell bioluminescence, declined for the first 3.5 h, but increased over the next 24 h and remained at this level for the remainder of the 7-day trial. Similar results were obtained with heat-inactivated (25 min at 121 °C) nisin, suggesting that the decline in metabolic activity of S. aureus Xen 36 cannot be attributed to the bacteriostatic activity of nisin. The decline in lymphocyte numbers in infected mice was of smaller magnitude after treatment with active nisin compared to inactive nisin, suggesting that active nisin limited the apoptosis of lymphocytes. The drastic increase in neutrophil versus lymphocyte (N:L) ratio observed in the presence of active nisin suggested that the decline in metabolic activity of S. aureus Xen 36 was due to an immune response triggered by the infection. Nisin, active or inactive, stimulated the activity of cytokines interleukin-6, interleukin-10 and tumour necrosis factor. However, the overall immune response triggered by both forms of nisin was too minute to trigger an abnormally high antigenic immune reaction.     

Lactobacilli and Bifidobacteria Promote Immune Homeostasis by Modulating Innate Immune Responses to Human Rotavirus in Neonatal Gnotobiotic Pigs

The effects of co-colonization with Lactobacillus rhamnosus GG (LGG) and Bifidobacterium lactis Bb12 (Bb12) on 3-dose vaccination with attenuated HRV and challenge with virulent human rotavirus (VirHRV) were assessed in 4 groups of gnotobiotic (Gn) pigs: Pro+Vac (probiotic-colonized/vaccinated), Vac (vaccinated), Pro (probiotic-colonized, non-vaccinated) and Control (non-colonized, non-vaccinated). Subsets of pigs were euthanized pre- [post-challenge day (PCD) 0] and post (PCD7)-VirHRV challenge to assess diarrhea, fecal HRV shedding and dendritic cell/innate immune responses. Post-challenge, Pro+Vac and Vac groups were completely protected from diarrhea; protection rates against HRV shedding were 100% and 83%, respectively. Diarrhea and HRV shedding were reduced in Pro compared to Control pigs following VirHRV challenge. Diarrhea scores and virus shedding were significantly higher in Controls, compared to all other groups, coincident with significantly higher serum interferon-alpha levels post-challenge. LGG+Bb12 colonization ±vaccine promoted immunomaturation as reflected by increased frequencies of CD4, SWC3a, CD11R1, MHCII expressing mononuclear cells (MNCs) and conventional dendritic cells in intestinal tissues and blood post-challenge. Colonization decreased frequencies of toll-like receptors (TLR) 2 and TLR4 expressing MNCs from vaccinated pigs (Pro+Vac) pre-challenge and increased frequencies of TLR3 expressing MNCs from Pro pigs post-challenge, suggesting that probiotics likely exert anti-inflammatory (TLR2 and 4 down-regulation) and antiviral (TLR3 up-regulation by HRV dsRNA) actions via TLR signaling. Probiotic colonization alone (Pro) increased frequencies of intestinal and systemic apoptotic MNCs pre-challenge, thereby regulating immune hyperreactivity and tolerance. However, these frequencies were decreased in intestinal and systemic tissues post-challenge, moderating HRV-induced apoptosis. Additionally, post-challenge, Pro+Vac and Pro groups had significantly decreased MNC proliferation, suggesting that probiotics control excessive lymphoproliferative reactions upon VirHRV challenge. We conclude that in the neonatal Gn pig disease model, selected probiotics contribute to immunomaturation, regulate immune homeostasis and modulate vaccine and virulent HRV effects, thereby moderating HRV diarrhea.     

Characterization of the Humoral Immune Response during Staphylococcus aureus Bacteremia and Global Gene Expression by Staphylococcus aureus in Human Blood

Attempts to develop an efficient anti-staphylococcal vaccine in humans have so far been unsuccessful. Therefore, more knowledge of the antigens that are expressed by Staphylococcus aureus in human blood and induce an immune response in patients is required. In this study we further characterize the serial levels of IgG and IgA antibodies against 56 staphylococcal antigens in multiple serum samples of 21 patients with a S. aureus bacteremia, compare peak IgG levels between patients and 30 non-infected controls, and analyze the expression of 3626 genes by two genetically distinct isolates in human blood. The serum antibody levels were measured using a bead-based flow cytometry technique (xMAP®, Luminex corporation). Gene expression levels were analyzed using a microarray (BµG@s microarray). The initial levels and time taken to reach peak IgG and IgA antibody levels were heterogeneous in bacteremia patients. The antigen SA0688 was associated with the highest median initial-to-peak antibody fold-increase for IgG (5.05-fold) and the second highest increase for IgA (2.07-fold). Peak IgG levels against 27 antigens, including the antigen SA0688, were significantly elevated in bacteremia patients versus controls (P≤0.05). Expression of diverse genes, including SA0688, was ubiquitously high in both isolates at all time points during incubation in blood. However, only a limited number of genes were specifically up- or downregulated in both isolates when cultured in blood, compared to the start of incubation in blood or during incubation in BHI broth. In conclusion, most staphylococcal antigens tested in this study, including many known virulence factors, do not induce uniform increases in the antibody levels in bacteremia patients. In addition, the expression of these antigens by S. aureus is not significantly altered by incubation in human blood over time. One immunogenic and ubiquitously expressed antigen is the putative iron-regulated ABC transporter SA0688.     

Dietary Supplementation with Lactobacilli Improves Emergency Granulopoiesis in Protein-Malnourished Mice and Enhances Respiratory Innate Immune Response

This work studied the effect of protein malnutrition on the hemato-immune response to the respiratory challenge with Streptococcus pneumoniae and evaluated whether the dietary recovery with a probiotic strain has a beneficial effect in that response. Three important conclusions can be inferred from the results presented in this work: a) protein-malnutrition significantly impairs the emergency myelopoiesis induced by the generation of the innate immune response against pneumococcal infection; b) repletion of malnourished mice with treatments including nasally or orally administered Lactobacillus rhamnosus CRL1505 are able to significantly accelerate the recovery of granulopoiesis and improve innate immunity and; c) the immunological mechanisms involved in the protective effect of immunobiotics vary according to the route of administration. The study demonstrated that dietary recovery of malnourished mice with oral or nasal administration of L. rhamnosus CRL1505 improves emergency granulopoiesis and that CXCR4/CXCR12 signaling would be involved in this effect. Then, the results summarized here are a starting point for future research and open up broad prospects for future applications of probiotics in the recovery of immunocompromised malnourished hosts.     

Toward an Alternative Therapeutic Approach for Skin Infections: Antagonistic Activity of Lactobacilli Against Antibiotic-Resistant Staphylococcus aureus and Pseudomonas aeruginosa

The wide spread of antimicrobial resistance has urged the need of alternative therapeutic approach. In this context, probiotic lactobacilli have been reported for the prevention and treatment of many gastrointestinal and urogenital infections. However, very little is known about their antagonistic activity against skin pathogens. Accordingly, the present study aimed to investigate the potential of lactobacilli to interfere with pathogenesis features of two antibiotic-resistant skin pathogens, namely methicillin-resistant Staphylococcus aureus and multiple-resistant Pseudomonas aeruginosa. A total of 49 lactobacilli were recovered, identified and tested for their antagonistic activities against the aforementioned pathogens. Of these, eight isolates were capable of blocking the adherence of pathogens to mammalian cells independent of the skin pathogen tested or model adopted. Moreover, three Lactobacillus isolates (LRA4, LC2 and LR5) effectively prevented the pathogen internalization into epithelial cells in addition to potentiating phagocyte-mediated pathogen killing. Interestingly, the lactobacilli LC2, LF9 and LRA4 markedly inhibited the growth of P. aeruginosa and S. aureus isolates in coculture experiments. Besides, the lactobacilli LRA4, LC2, LR5 and LF9 have counteracted pathogen cytotoxicity. Taken together, the present study revealed some inhibitory activities of lactobacilli against two antibiotic-resistant skin pathogens. Moreover, it revealed two lactobacilli, namely LC2 and LRA4, with antagonistic capacity against different virulence determinants of skin pathogens. These lactobacilli are considered promising probiotic candidates that may represent an alternative therapeutic approach for skin infections.     

口服乳酸杆菌对实验动物免疫功能及肠道正常菌群的影响

本文报告C_(57)BL/6小鼠口服乳酸杆菌后,脾细胞和胸腺细胞的增殖反应,腹腔巨噬细胞的C_3b受体活性及其对L_(929)细胞的细胞毒性作用都明显增强。停用乳酸杆菌10天后,以上免疫指标又恢复到正常水平。其次,Wister大鼠口服乳酸杆菌后,检查粪便菌群中几种厌氧菌和需氧菌的活菌数目,结果表明对厌氧菌群的生长有扶持作用,对需氧菌群的生长则起限制作用,这提示有利于宿主调整肠道正常菌群的平衡。     

Antibody Response to Staphylococcus aureus in Rabbits: Sequence of Immunoglobulin Synthesis and Its Correlation with Passive Protection in Mice

Antibody in hyperimmune rabbit antisera specific for Staphylococcus aureus teichoic acid was shown to be associated with the IgM fraction. Treatment of such sera with mercaptoethanol destroyed its activity in passive mouse protection tests, whereas absorption with antirabbit IgG had no effect. Antibody response in normal rabbits immunized by a single or by three daily injections of a killed vaccine of S. aureus was followed by a sensitive passive hemagglutination test. Antibody detected during the "primary" response was completely susceptible to reduction with mercaptoethanol. Most of the antibody detected after a secondary antigenic stimulation at 10 weeks was also susceptible to mercaptoethanol. The antibody titers correlated well with mouse protective activity, and this activity of the serum was also shown to be associated with the IgM fraction.     

大剂量HBsAg对HBV转基因小鼠T细胞免疫调节的研究

目的：研究大剂量HBsAg对HBV转基因小鼠其T细胞免疫效果的影响。方法：用大剂量血源性HBsAg免疫HBV转基因小鼠,采用ELISA方法观察转基因小鼠所诱生的HBsAg特异性Th1类细胞因子的水平,ELISPOT方法检测不同免疫方案对小鼠HBsAg特异性分泌IFN-γT细胞数量的影响,同时检测对小鼠淋巴细胞增殖的影响。结果：HBsAg组免疫后脾细胞产生的Th1类细胞因子（IFN-γ、IL-2）、HBsAg特异性分泌IFN-γT细胞及T细胞增殖水平较对照组显著增加（P〈0.05）。结论：大剂量的HBs-Ag可以诱导乙肝转基因小鼠产生高水平Th1类细胞因子并打破免疫耐受。     

大剂量HBsAg 对HBV 转基因小鼠T 细胞免疫调节的研究

目的:研究大剂量HBsAg对HBV转基因小鼠其T细胞免疫效果的影响。方法:用大剂量血源性HBsAg免疫HBV转基因小鼠,采用ELISA方法观察转基因小鼠所诱生的HBsAg特异性Th1类细胞因子的水平,ELISPOT方法检测不同免疫方案对小鼠HBsAg特异性分泌IFN-γT细胞数量的影响,同时检测对小鼠淋巴细胞增殖的影响。结果:HBsAg组免疫后脾细胞产生的Th1类细胞因子(IFN-γ、IL-2)、HBsAg特异性分泌IFN-γT细胞及T细胞增殖水平较对照组显著增加(P<0.05)。结论:大剂量的HBs-Ag可以诱导乙肝转基因小鼠产生高水平Th1类细胞因子并打破免疫耐受。     

Action of heliomycin on protein and mucleic acid synthesis in Staphylococcus aureus]

Heliomycin inhibited synthesis of RNA in Staph. aureua which was clearly shown in the study of the antibiotic effect on RNA synthesis in the lag phase of the culture development: heliomycin markedly lowered the maximum RNA level in the biomass observed in the culture at the beginning of the exponential growth. On further growth of the culture heliomycin induced a significant retardness of the process of the natural decrease in the RNA biomass level resulting in increased content of RNA in the cells growing in the presence of heliomycin as compared to the control culture. Retarded natural decrease in the RNA biomass level in the presence of heliomycin was observed also on the antibiotic addition just at the beginning of the exponential growth, during the period of maximum RNA accumulation in the cells. Heliomycin had no effect on synthesis and biomass levels of DNA. Heliomycin inhibited the protein synthesis and was close to chloramphenicol by the level of inhibition of the summation protein synthesis in the biomass. However, comparison of the effect of the above antibiotics on synthesis of beta-galactosidase, an individual enzyme protein showed that heliomycin was much less active as an inhibitor of protein synthesis in comparison to chloramphenicol.     

Quantifying the Early Immune Response and Adaptive Immune Response Kinetics in Mice Infected with Influenza A Virus

Seasonal and pandemic influenza A virus (IAV) continues to be a public health threat. However, we lack a detailed and quantitative understanding of the immune response kinetics to IAV infection and which biological parameters most strongly influence infection outcomes. To address these issues, we use modeling approaches combined with experimental data to quantitatively investigate the innate and adaptive immune responses to primary IAV infection. Mathematical models were developed to describe the dynamic interactions between target (epithelial) cells, influenza virus, cytotoxic T lymphocytes (CTLs), and virus-specific IgG and IgM. IAV and immune kinetic parameters were estimated by fitting models to a large data set obtained from primary H3N2 IAV infection of 340 mice. Prior to a detectable virus-specific immune response (before day 5), the estimated half-life of infected epithelial cells is ∼1.2 days, and the half-life of free infectious IAV is ∼4 h. During the adaptive immune response (after day 5), the average half-life of infected epithelial cells is ∼0.5 days, and the average half-life of free infectious virus is ∼1.8 min. During the adaptive phase, model fitting confirms that CD8⁺ CTLs are crucial for limiting infected cells, while virus-specific IgM regulates free IAV levels. This may imply that CD4 T cells and class-switched IgG antibodies are more relevant for generating IAV-specific memory and preventing future infection via a more rapid secondary immune response. Also, simulation studies were performed to understand the relative contributions of biological parameters to IAV clearance. This study provides a basis to better understand and predict influenza virus immunity.Current strategies for preventing or decreasing the severity of influenza infection focus on increasing virus-neutralizing antibody titers through vaccination, as experience indicates that this is the best way to prevent morbidity and mortality. Influenza A virus (IAV) undergoes mutations of the genes encoding the hemagglutinin (HA) and neuraminidase (NA) proteins that the neutralizing antibodies are directed against. When the variation is low (antigenic drift), prior vaccination often confers substantial heterologous immunity against a new seasonal IAV strain. In contrast, major genetic changes (antigenic shift) can result in pandemic IAV strains, since for novel strains, the humoral immune response is a primary response, and heterologous immunity is lacking. The emergence of such pandemic strains and the fact that young children are more vulnerable to influenza diseases highlight the need to better understand which viral and immune parameters determine the outcome of infection with viruses novel to the individual.Conventional experimental methods to measure influenza virus immunity have been limited to animal models and studies of adult human peripheral blood leukocytes. The advantages of using animal models include the ability to intensively sample multiple tissues and to utilize genetic and other interventions, such as blocking or depleting antibodies, to dissect the contribution of individual arms of the immune system. However, it is easy to question the relevance of these experiments to humans because of the many important biological differences between human and murine immune systems (29). In both the animal and human systems, we are limited to measuring those parameters and variables for which assays are available, most of them being ex vivo. Parameters such as cell-to-cell spread of the virus in vivo, trafficking of immune cells to the lung, and the in vivo interactions in an intact immune system are much more difficult or impossible to measure with contemporary techniques, particularly in humans. Computational approaches have the potential to offset some of these limitations and provide additional insight into the kinetics of the IAV infection and the associated immune response.Animal models of influenza virus infection in which different arms of the immune system have been suppressed suggest that some components of the adaptive immune system are required for complete viral clearance, often termed a sterilizing immune response. For example, abrogation of the CD4 T-cell response by cytotoxic antibody therapy or through knockout of major histocompatibility complex (MHC) class II slightly delays viral clearance but has little overall effect on the ability to control the infection (21, 54, 55). Elimination of the CD8 T-cell response typically results in delayed viral clearance (12, 20, 47), although animals with intact CD4 T-cell and B-cell compartments are able to control the infection in the absence of CD8 T cells. Presumably, this occurs through antibody-mediated mechanisms (54). Most animals depleted of both CD8 T cells and B cells are not able to clear the virus, which results in death (14, 32, 53). CD4⁺ T cells certainly contribute to the control of IAV infection, although cytotoxic CD4 T cells are not frequently observed unless cultured in vitro (8, 22, 45). Thus, it is generally accepted that CD8 T cells and/or antibodies are sufficient for timely and complete IAV clearance. Studies that strictly separate the relative roles of CD8 T cells and virus-specific antibodies are less satisfying. Animals depleted of both CD4 and CD8 T cells generally do not control the infection, despite substantial production of anti-IAV IgM antibodies (4, 23, 33, 34). However, adoptive transfer of IAV-specific IgM or IgG antibodies is protective (40, 51), suggesting that the timing and magnitude of the antibody response, i.e., the affinity, avidity, and antibody isotype, are important protective factors.While murine gene knockout or lymphocyte depletion studies are highly informative, they also have a number of limitations. Most importantly, the near-complete ablation of one component of the adaptive immune system often causes profound and unpredictable effects on many other immune components. Although the reported experimental measurements are highly quantitative, they often focus only on a limited number of time points or measurements and do not capture the complexity of the altered, or intact, immune response. In contrast, high-frequency experimental sampling, coupled with mathematical modeling techniques and new statistical approaches, can give insights into the complex biology of IAV infection and test the assumptions inherent in the model. We have learned from other systems, particularly HIV (19, 35, 37, 38, 56), that quantitative analysis of the biology can reveal important factors that are not intuitively obvious. For example, our current estimates for the rates of HIV production and the life span of productively infected cells in vivo were obtained via mathematical modeling (35).Mathematical models have long been used to investigate viral dynamics and immune responses to viral infections, including influenza A virus (3, 5, 7, 15, 16, 31, 36, 48). We recently described a complex differential equation model to simulate and predict the adaptive immune response to IAV infection (24). This model involves 15 equations and 48 parameters, and because of its complexity, many of the parameter values that could not be directly measured were unidentifiable. Thus, it is difficult to estimate all model parameters by fitting experimental data directly to this complex model, although the model can be used to perform simulation predictions (25). This issue can, however, be addressed by reducing the model into smaller submodels with smaller but identifiable sets of parameters, which can be estimated from experimental data. In this paper, we describe such an approach which focuses on IAV infection and the immune response solely within the lung.In the present report, we have fitted a model of primary murine influenza virus infection to data. In naïve subjects, our data suggested that there is no adaptive immune response (e.g., IAV-specific CD8⁺ T cells or antibodies) detectable in the spleen, lymph nodes, or lung until approximately 5 days after infection; therefore, we have divided the analysis into the following two phases: the initial preadaptive (innate) phase and the later adaptive phase. We use direct experimental data from infection of mice with the H3N2 influenza virus A/X31 strain (2, 24) to obtain key kinetic parameters. The model fitting has revealed that the duration of the infection depends on a small set of immune components, and even large fluctuations in other arms of the immune system or IAV behavior have surprisingly little impact on the outcome of the infection.     

Effect of Diet and Strain Difference on the Virulence of Staphylococcus aureus for Mice

Years of intensive investigations in our laboratories on staphylococcal infections in mice have indicated a gradual decrease in the virulence of Staphylococcus aureus for this animal, particularly the strain we were using, the random-bred BT mouse. The present investigations, based on changes in diet and strain differences in mice, were undertaken in an attempt to improve susceptibility of our stock strain, on the one hand, and to determine whether susceptibility was influenced by genetic strain differences, on the other. In the first series of experiments, littermate mice of the BT strain were placed on different diets: one group received a commercial diet; the other, a basic diet previously established in our laboratories as adequate for growth requirements of the animals. An increase in the susceptibility of the animals to staphylococcal infection was noted in the mice receiving the basic diet. In the second series of investigations, three other strains of mice (the Swiss albino, the C3H/HeJ, and the C57B1) previously not used by our laboratories in staphylococcal experiments were studied for staphylococcal susceptibility. These experiments revealed that all of these strains of mice were highly susceptible to the infection, even though they were maintained on a commercial diet.     

Resistance of Staphylococcus aureus to Desiccasion,Heat and Ultraviolet Rays in Relation to Phage Pattern

Resistance of staphylococci to desiccation, dry heat, wet heat and ultraviolet irradiation was studied using 128 strains obtained from patients in our surgical clinics. Staphylococci of phage group I including type 81, particularly of type 80/81, showed significantly high death rates after desiccation, dry heating, and wet heating, and significantly low death rates after ultraviolet irradiation. In contrast, group-II strains responded inversely to these physical agents. The mean death rate of group-III strains approximated that of group I after desiccation, dry heating, and ultraviolet irradiation, but after wet heating it was lower than that of group II. The results indicate that transmissibility of staphylococci might be influenced to a great extent by physical factors in the environment outside the body.     

CpG壳聚糖纳米颗粒对小鼠乙型肝炎疫苗免疫应答的影响

为探索高效安全的分子免疫增强剂,本实验设计合成含11个CpG基序的寡聚核苷酸(CpG ODN),制备壳聚糖纳米颗粒包裹CpG ODN,研究新型CpG ODN壳聚糖纳米颗粒(CpG-CNP)对人乙型肝炎疫苗的免疫佐剂效应.在肌注乙型肝炎疫苗免疫6周龄昆明小鼠的同时,肌注接种裸CpG ODN 30 pmol/只(A1组)和CpG-CNP 5 pmol/只(A2组),口服CpG-CNP 30 pmol/只(B组),并设生理盐水空白疫苗对照组(C组).在接种后每周采血,Sandwich ELISA检测实验小鼠IL-2、IL-4、IL-6、IgG、IgA和IgM水平以及特异性抗体(HbsAb)含量和免疫细胞数量的变化.结果 发现各实验组小鼠的白细胞介素、血清免疫球蛋白、乙肝特异抗体和外周血免疫细胞均较对照组显著增多(P<0.05);CpG-CNP肌注组小鼠血液的免疫球蛋白、特异性乙肝表面抗体和白细胞介素含量、淋巴细胞和单核细胞数量较A1组和B组显著增加(P<0.05).CpG-CNP口服组与裸CpG肌肉注射组无显著差异(P＞0.05).这些表明CpG-CNP不仅能高效诱导小鼠对乙肝疫苗的体液免疫应答,同时也显著增强其细胞免疫应答;CNP包裹可提高CpG的免疫刺激效应,减少CpG剂量,提示新CpG-CNP肌注或口服可用于HBV的免疫预防.     

Improving the Safety of Staphylococcus aureus Polyvalent Phages by Their Production on a Staphylococcus xylosus Strain

Team1 (vB_SauM_Team1) is a polyvalent staphylococcal phage belonging to the Myoviridae family. Phage Team1 was propagated on a Staphylococcus aureus strain and a non-pathogenic Staphylococcus xylosus strain used in industrial meat fermentation. The two Team1 preparations were compared with respect to their microbiological and genomic properties. The burst sizes, latent periods, and host ranges of the two derivatives were identical as were their genome sequences. Phage Team1 has 140,903 bp of double stranded DNA encoding for 217 open reading frames and 4 tRNAs. Comparative genomic analysis revealed similarities to staphylococcal phages ISP (97%) and G1 (97%). The host range of Team1 was compared to the well-known polyvalent staphylococcal phages phi812 and K using a panel of 57 S. aureus strains collected from various sources. These bacterial strains were found to represent 18 sequence types (MLST) and 14 clonal complexes (eBURST). Altogether, the three phages propagated on S. xylosus lysed 52 out of 57 distinct strains of S. aureus. The identification of phage-insensitive strains underlines the importance of designing phage cocktails with broadly varying and overlapping host ranges. Taken altogether, our study suggests that some staphylococcal phages can be propagated on food-grade bacteria for biocontrol and safety purposes.     

Increased Susceptibility of Humanized NSG Mice to Panton-Valentine Leukocidin and Staphylococcus aureus Skin Infection

Staphylococcus aureus is a leading cause of skin and soft-tissue infections worldwide. Mice are the most commonly used animals for modeling human staphylococcal infections. However a supra-physiologic S. aureus inoculum is required to establish gross murine skin pathology. Moreover, many staphylococcal factors, including Panton-Valentine leukocidin (PVL) elaborated by community-associated methicillin-resistant S. aureus (CA-MRSA), exhibit selective human tropism and cannot be adequately studied in mice. To overcome these deficiencies, we investigated S. aureus infection in non-obese diabetic (NOD)/severe combined immune deficiency (SCID)/IL2rγ^null (NSG) mice engrafted with human CD34⁺ umbilical cord blood cells. These “humanized” NSG mice require one to two log lower inoculum to induce consistent skin lesions compared with control mice, and exhibit larger cutaneous lesions upon infection with PVL⁺ versus isogenic PVL^- S. aureus. Neutrophils appear important for PVL pathology as adoptive transfer of human neutrophils alone to NSG mice was sufficient to induce dermonecrosis following challenge with PVL⁺ S. aureus but not PVL^- S. aureus. PMX53, a human C5aR inhibitor, blocked PVL-induced cellular cytotoxicity in vitro and reduced the size difference of lesions induced by the PVL⁺ and PVL^- S. aureus, but PMX53 also reduced recruitment of neutrophils and exacerbated the infection. Overall, our findings establish humanized mice as an important translational tool for the study of S. aureus infection and provide strong evidence that PVL is a human virulence factor.