Overexpression of the T-cell receptor V beta 3 in transgenic mice increases mortality during infection by enterotoxin A-producing Staphylococcus aureus

Impacts of enterotoxin gene cluster-encoded superantigens on local and systemic experimental Staphylococcus aureus infections

Staphylococcus aureus is both a component of the normal skin flora and an important pathogen. It expresses a range of recognized and putative virulence factors, such as enterotoxins with superantigenic properties. Several superantigen genes, i.e., seg, sei, selm, seln, and selo, are encoded by the enterotoxin gene cluster (egc), which is found in the majority of S. aureus isolates. Carriage of egc is associated with fitness of S. aureus in the gut microbiota, but it is not known if it contributes to pathogenicity. We constructed egc+ (functional for the seg, selm, and selo genes) and isogenic egc- S. aureus mutants, and investigated their virulence profiles in murine infection models. No effect of egc was seen in a local skin and soft tissue infection model, but in an invasive infection model, increased weight loss was observed after infection with the egc+ as compared to the egc- mutant. Mortality and arthritis were not affected by egc status. Our data suggest that egc has limited effects on the virulence of S. aureus. It may primarily function as a colonization factor increasing commensal fitness, although it might have some aggravating effects on the infection when the bacteria reach the blood.

The staphylococcal enterotoxin (SE) family: SEB and siblings

Staphylococcus aureus plays an important role in numerous human cases of food poisoning, soft tissue, and bone infections, as well as potentially lethal toxic shock. This common bacterium synthesizes various virulence factors that include staphylococcal enterotoxins (SEs). These protein toxins bind directly to major histocompatibility complex class II on antigen-presenting cells and specific Vβ regions of T-cell receptors, resulting in potentially life-threatening stimulation of the immune system. Picomolar concentrations of SEs ultimately elicit proinflammatory cytokines that can induce fever, hypotension, multi-organ failure, and lethal shock. Various in vitro and in vivo models have provided important tools for studying the biological effects of, as well as potential vaccines/therapeutics against, the SEs. This review succinctly presents known physical and biological properties of the SEs, including various intervention strategies. In particular, SEB will often be portrayed as per biodefense concerns dating back to the 1960s.

A polymorphism in a staphylococcal enterotoxin receptor gene (T cell receptor BV3 recombination signal sequence) is not associated with unexplained sudden unexpected death in infancy in an Australian cohort

Polymorphisms in genes that influence the expression of toxin receptors could contribute to Sudden Infant Death Syndrome (SIDS) and unexplained Sudden Unexpected Death in Infancy (uSUDI) for which there is evidence of toxin involvement. We aimed to determine whether TCRBV3S1 allele 2 could be involved in a staphylococcal toxic shock hypothesis for uSUDI. Observed frequencies of the TCRBV3S1*2 allele and genotype in 48 Australian uSUDI cases and 96 live comparison infants did not differ. In future the role of other toxin receptor gene polymorphisms deserves investigation.

Bacterial cell wall-expressed protein A triggers supraclonal B-cell responses upon in vivo infection with Staphylococcus aureus

We have previously shown that staphylococcal protein A (SpA) anchored to the cell wall of Staphylococcus aureus acts as a virulence factor in septic arthritis. Apart from the ability of SpA to interact with Fcgamma, it also binds to Fab-regions with immunoglobulin heavy chains encoded by the V(H) clan III gene family. The objective of the present study was to investigate whether in vivo expression of SpA by staphylococci induces V(H)III-dependent supraclonal B-cell responses, and whether such responses may affect the ability of the host to produce anti-staphylococcal antibodies. Upon primary infection of mice, a SpA-expressing staphylococcal strain gave rise to significantly higher serum levels of V(H)III-encoded antibodies specific for SpA devoid of Fcgamma-binding ability (MSpA) than an isogeneic spa deletion mutant strain. The V(H)III-dependence of MSpA-specific antibody responses was affected by the size of the staphylococcal inoculum, and differed for IgM and IgG isotypes. Mice that had recovered from a prior mild infection from a SpA-expressing strain were protected against infection-induced weight loss upon reinfection. Although no lasting MSpA-specific IgG was induced by previous mild infection, these protected mice possessed IgG specific for clumping factor A, a conventional staphylococcal protein antigen. Our findings demonstrate that the expression of a B-cell superantigen during staphylococcal infection causes supraclonal changes to the immune system. Notably, while superantigen-triggered B-cell responses do not favor the development of SpA-specific memory B-cells, such responses do not interfere with the development of antibodies specific for a staphylococcal protein antigen associated with protective immunity.

Polarization of cytokine responses in B- and T-lymphocytes during Staphylococcus aureus infection

The destructive course of Staphylococcus aureus arthritis is due to certain leukocytes and their products, mainly cytokines. The cellular source of cytokines mediating this inflammatory process has not been previously assessed on a protein level. Using a mouse model of hematogenously induced S. aureus infection the intracellular production of IFN-gamma, IL-4 and IL-10 in splenic B and T cells have been determined. This has enabled us to define distinct Th1 vs Th2 and Be1 vs Be2 populations of lymphocytes participating in S. aureus infection. Spleen cells were obtained before, and at different time intervals during the first week of infection and re-stimulated in vitro with staphylococcal peptidoglycan, formalin killed staphylococci and TSST-1. The different antigens used for re-stimulation gave rise to different cytokine profiles in analysed T cells (identified as CD4+) and B cells (identified as CD19+). TSST-1 acted as the most potent re-stimulator and we found that 40% of the CD4+ cells responded with IFN-gamma production, and unexpectedly almost 20% of the CD19+ cells. As to IL-4 and IL-10 production, the percentage of B cells expressing these cytokines was higher than the percentage T cells and the peak of their appearance appeared later than that of IFN-gamma. This finding indicates that Be1 cells are an important source of IFN-gamma early during the infection and that the production of the Th2 cytokines in B cells downregulates its production of IFN-gamma. In conclusion this study shows that both B and T cells contribute to the cytokine production during S. aureus infection in a complex pattern.

Anti-septicaemic effect of polysaccharide from Panax ginseng by macrophage activation

The aim of the present research was conducted to elucidate anti-septicaemic effect of a polysaccharide (PS) isolated from Panax ginseng C.A. Meyer (Araliaceae) by nitric oxide production from stimulated macrophage. In vitro assays for the activity measurement of PS, NO production test with Greiss reagent, phagocytic activity test using zymosan and cytokines production test using ELISA kit were also conducted. In vivo anti-septicaemic activity was assessed by using C57BL/6J mice. This was done with Staphylococcus aureus infection test. PS used at 0.025 mg/kg concentration showed a potent anti-septicaemic activity (80%, survival). However, it did not directly inhibit S. aureus in a minimum inhibitory concentration (MIC) test, conducted in vitro (data not shown). Nitric oxide production via macrophage activation showed the highest value of 5.5 nmol/ml at 1 microg/ml PS. In in vitro phagocytic activity test, PS at 10 microg/ml concentration showed a potent phagocytic activity for zymosan with 167% of the control. Production of TNF-alpha by macrophage activation at 10 microg/ml of PS was 96% lysis of L929. Also production of IL-1 and IL-6 by stimulation of macrophage with 100 microg/ml PS dose increased to 235 pg/ml and 0.47 ng/ml, respectively. The low mortality of PS treated (0.025 mg/kg) infected mice was concurrent with decreased bacterial content in the blood. Nitric oxide production in S. aureus infected mice whose macrophage was stimulated by PS (0.025 mg/kg) increased approximately 4 times than the untreated S. aureus infected group at 24 and 48 h incubation. In the PS treated (0.025 mg/kg) group, the intracellular concentration of S. aureus in macrophages decreased approximately by 50%, compared with the untreated group. Combine treatment with PS (0.025 mg/kg body weight) and vancomycin (10 mg/kg B.W.) resulted in 100% survival of the animals, whereas only 67% or 50% of the animals survived, respectively, when treated with PS or vancomycin alone. These results suggest that PS from Panax ginseng possess a potent anti-septicaemic activity by stimulating macrophage and a potentiality as an immunomodulator against sepsis occurred by Staphylococcus aureus.

Enhanced Th1 response to Staphylococcus aureus infection in human lactoferrin-transgenic mice

Lactoferrin (Lf) is an iron-binding protein of external secretions and neutrophil secondary granules with antimicrobial and immunomodulatory activities. To further define these properties of Lf, we have investigated the response to Staphylococcus aureus infection in transgenic mice carrying a functional human Lf gene. The transgenic mice cleared bacteria significantly better than congenic littermates, associated with a trend to reduced incidence of arthritis, septicemia, and mortality. We identified two pathways by which S. aureus clearance was enhanced. First, human Lf directly inhibited the growth of S. aureus LS-1 in vitro. Second, S. aureus-infected transgenic mice exhibited enhanced Th1 immune polarization. Thus, spleen cells from infected transgenic mice produced higher levels of TNF-alpha and IFN-gamma and less IL-5 and IL-10 upon stimulation ex vivo with the exotoxin toxic shock syndrome toxin-1 compared with congenic controls. To confirm that these effects of Lf transgene expression could occur in the absence of live bacterial infection, we also showed that Lf-transgenic DBA/1 mice exhibited enhanced severity of collagen-induced arthritis, an established model of Th1-induced articular inflammation. Higher levels of stainable iron in the spleens of transgenic mice correlated with human Lf distribution, but all other parameters of iron metabolism did not differ between transgenic mice and wild-type littermates. These results demonstrate that human Lf can mediate both antimicrobial and immunomodulatory activities with downstream effects on the outcome of immune pathology in infectious and inflammatory disease.

Model systems: modeling human staphylococcal arthritis and sepsis in the mouse

The staphylococci have been recognized as serious pathogens for over a century and are the etiological agent of a variety of diseases ranging from mild cutaneous infections to often fatal forms of septic arthritis, endocarditis, toxic shock syndrome and sepsis. Despite intensive efforts to halt their spread, they remain the most common cause of community- and nosocomially acquired bacteremia. Murine models of Staphylocococus aureus-mediated arthritis and sepsis exist and are being used to gain a better understanding of the host-bacterium relationship as well to develop better methods of prevention and treatment.

Staphylococcus aureus-induced inflammation and bone destruction in experimental models of septic arthritis

Staphylococcus aureus is the most common cause of septic arthritis. This disease often leads to severe joint destruction and high mortality. An experimental model of S. aureus arthritis has been developed to study the course of inflammation and joint destruction, to elucidate the role of bacterial and host factors for joint pathology and mortality, and to develop therapeutical and preventive devices against septic arthritis and sepsis. Results show that the innate immune system is crucial in defending the host against staphylococcal infection while components of the specific immune system, T and B lymphocytes and their products, are detrimental to the host, mediating joint destruction and increasing mortality rates. Staphylococcal capsule polysaccharides, toxins, cell wall-attached adhesins and possibly also the chromosomal DNA are virulence determinants in S. aureus arthritis. Several vaccine candidates have recently been described which protects against staphylococcal infections, e.g. staphylococcal surface polysaccharides, enterotoxins devoid of their superantigenic properties and collagen adhesin. There are also new approaches suggested for treatment of ongoing infections, such as the combined use of antibiotics and corticosteroids.

Altered Immune Responses and Susceptibility to Leishmania major and Staphylococcus aureus Infection in IL-18-Deficient Mice

IL-18, formerly designated IFN-inducing factor, is a novel cytokine produced by activated macrophages. It synergizes with IL-12 in the induction of the development of Th1 cells and NK cells. To define the biological role of IL-18 in vivo, we have constructed a strain of mice lacking IL-18. Homozygous IL-18 knockout (−/−) mice are viable, fertile, and without evident histopathologic abnormalities. However, in contrast to the heterozygous (+/−) or wild-type (+/+) mice, which are highly resistant to the infection of the protozoan parasite Leishmania major, the IL-18−/− mice are uniformly susceptible. The infected IL-18−/− mice produced significantly lower levels of IFN-γ and larger amounts of IL-4 compared with similarly infected +/− and +/+ mice. In contrast, when infected with the extracellular Gram-positive bacteria Staphylococcus aureus, the IL-18−/− mice developed markedly less septicemia than similarly infected wild-type (+/+) mice. However, the mutant mice developed significantly more severe septic arthritis than the control wild-type mice. This was accompanied by a reduction in the levels of Ag-induced splenic T cell proliferation, decreased IFN-γ and TNF-α synthesis, but increased IL-4 production by the mutant mice compared with the wild-type mice. These results therefore provide direct evidence that IL-18 is not only essential for the host defense against intracellular infection, but it also plays a critical role in regulating the synthesis of inflammatory cytokines, and therefore could be an important target for therapeutic intervention.

V beta 11+ T-lymphocyte expansion by toxic shock syndrome toxin-1 differs in mice bearing H-2q versus H-2b haplotypes

We have recently demonstrated that toxic shock syndrome toxin-1 (TSST-1) expanded V beta 11+ T lymphocytes contribute to Staphylococcus aureus arthritis and sepsis-induced mortality. Interestingly, V beta 11+ T-cell mediated joint pathology varies in different mouse strains. In this study, we characterized the in vitro pattern of V beta 11+ T-cell expansion by TSST-1 in mice with various genetic backgrounds. Mice expressing major histocompatibility complex (MHC) class II I-E molecules did not expand V beta 11+ T cells upon stimulation with TSST-1. Using B10 congeneic I-E negative mouse strains, we found that the TSST-1-expanded V beta 11+ T cells in B10Q (H-2q) and B10M (H-2f) mice but not in B10B (H-2b) mice. Antigen-presenting cells (APC) from B10Q mice, L cells and lymphoma cell line transfected with a q gene did not restore the deficient V beta 11+ T-cell expansion by TSST-1 in purified T cells from B10B mice. In contrast, I-Ab APC were able to stimulate V beta 11+ T cells from H-2q mice. Furthermore, V beta 11+ T cells in H-2b mice did expand when exposed to staphylococcal enterotoxin A (SEA). These findings suggest that the T-cell repertoire, skewed by clonal deletion and inactivation of self-reactive T cells, accounts for the different magnitude of V beta 11+ T-cell expansion among the different mouse strains.

The dual role of interferon-gamma in experimental Staphylococcus aureus septicaemia versus arthritis

To evaluate the role of interferon-gamma (IFN-gamma) in Staphylococcus aureus infection, we investigated the effects of supplementation with and neutralization of IFN-gamma during septicaemia and arthritis in a murine model. In vivo administration of IFN-gamma both before and after bacterial inoculation significantly decreased mortality on one hand but enhanced the development of arthritis on the other. Treatment of mice with anti-IFN-gamma monoclonal antibodies (mAb) before and after bacterial inoculation did not significantly influence the survival rate but decreased the frequency and severity of arthritis. The beneficial effect of supplementation with IFN-gamma on septicaemia was correlated to the increased phagocytosis and bacterial clearance from liver and kidneys. The down-regulation of the development of arthritis by anti-IFN-gamma mAb was accompanied by the decreased serum tumour necrosis factor-alpha, interleukin-6 and interleukin-1 beta levels. These results demonstrate a significant role for IFN-gamma in simultaneous protection against septicaemia but promotion for the development of septic arthritis.

Septic Arthritis Following Staphylococcus aureus Infection in Mice Lacking Inducible Nitric Oxide Synthase

Nitric oxide (NO), produced in large amounts by inducible NO synthase (iNOS), has emerged recently as an important microbicidal and immunomodulatory mediator. We have investigated its role in bacterial septic arthritis caused by Staphylococcus aureus infection using iNOS-deficient mice. The incidence, rate of development, and severity of arthritis were greater in iNOS-deficient than in heterozygous or wild-type control mice. Similarly, the incidence and severity of septicemia and mortality were significantly higher in iNOS-deficient mice compared with controls. Increased TNF-α synthesis in vivo and in vitro and enhanced IFN-γ compared with IL-4 production in vitro in iNOS-mutant mice demonstrated exaggerated Th1 polarization of the host response. These data indicate that high output NO production is not a prerequisite for severe articular destruction and imply that NO is of importance in synovial defense against staphylococcal infection.

Arthritis and sepsis caused by Staphylococcus aureus: can the tissue injury be reduced by modulating the host's immune system?

In recent years, a number of virulence factors that are either expressed, secreted or sequestered by staphylococci have been shown to affect the outcome of the infective process. Secretion of superantigens and exposure of the immune system to sequestered bacterial DNA aggravates the inflammatory response of the infected host to a point that can be life threatening. This exaggerated inflammatory response is mediated by cytokines originating from activated T cells and macrophages. Recent studies show that downregulation of lymphocyte and macrophage responses (e.g. by treatment with corticosteroids) significantly alleviates the outcome of staphylococcal infections when combined with appropriate antibiotics.

Genetically engineered superantigens as tolerable antitumor agents

Superantigens (SAg) are a family of bacterial and viral proteins with strong immunostimulatory properties. SAg bound to major histocompatibility complex (MHC) class II molecules activate a high frequency of T cells and represent the most potent known activators of T cells to date. To explore the use of SAg for T cell-based tumor therapy we have created a tumor-reactive SAg by engineering a fusion protein composed of a tumor-reactive mAb (C215Fab) and the bacterial SAg staphylococcal enterotoxin A (SEA). A point mutation D227A was introduced at the major MHC class II binding site in SEA to reduce systemic toxicity. Treatment of tumor bearing mice with the Fab-SEA D227A fusion protein resulted in profound antitumor effects with a markedly reduced toxicity as compared with the wild-type Fab-SEA fusion protein. The reduced toxicity was probably due to a weak distribution of the SEA D227A fusion protein in tissues with a high MHC class II expression and low systemic cytokine levels as exhibited in mice and rabbits. The data presented demonstrate the efficacy of immunoconjugates containing a mutated SAg in directing a T cell attack against tumor cells with minimal systemic immune activation.