Inhibition of nitric oxide synthase (NOS) aggravates Staphylococcus aureus septicaemia and septic arthritis

Exploring the role of bacterial virulence factors and host elements in septic arthritis: insights from animal models for innovative therapies

Septic arthritis, characterized as one of the most aggressive joint diseases, is primarily attributed to Staphylococcus aureus (S. aureus) and often results from hematogenous dissemination. Even with prompt treatment, septic arthritis frequently inflicts irreversible joint damage, leading to sustained joint dysfunction in a significant proportion of patients. Despite the unsatisfactory outcomes, current therapeutic approaches for septic arthritis have remained stagnant for decades. In the clinical context, devising innovative strategies to mitigate joint damage necessitates a profound comprehension of the pivotal disease mechanisms. This entails unraveling how bacterial virulence factors interact with host elements to facilitate bacterial invasion into the joint and identifying the principal drivers of joint damage. Leveraging animal models of septic arthritis emerges as a potent tool to achieve these objectives. This review provides a comprehensive overview of the historical evolution and recent advancements in septic arthritis models. Additionally, we address practical considerations regarding experimental protocols. Furthermore, we delve into the utility of these animal models, such as their contribution to the discovery of novel bacterial virulence factors and host elements that play pivotal roles in the initiation and progression of septic arthritis. Finally, we summarize the latest developments in novel therapeutic strategies against septic arthritis, leveraging insights gained from these unique animal models.

Arginine-dependent immune responses

A growing body of evidence indicates that, over the course of evolution of the immune system, arginine has been selected as a node for the regulation of immune responses. An appropriate supply of arginine has long been associated with the improvement of immune responses. In addition to being a building block for protein synthesis, arginine serves as a substrate for distinct metabolic pathways that profoundly affect immune cell biology; especially macrophage, dendritic cell and T cell immunobiology. Arginine availability, synthesis, and catabolism are highly interrelated aspects of immune responses and their fine-tuning can dictate divergent pro-inflammatory or anti-inflammatory immune outcomes. Here, we review the organismal pathways of arginine metabolism in humans and rodents, as essential modulators of the availability of this semi-essential amino acid for immune cells. We subsequently review well-established and novel findings on the functional impact of arginine biosynthetic and catabolic pathways on the main immune cell lineages. Finally, as arginine has emerged as a molecule impacting on a plethora of immune functions, we integrate key notions on how the disruption or perversion of arginine metabolism is implicated in pathologies ranging from infectious diseases to autoimmunity and cancer.

Inhibition of Host Arginase Activity Against Staphylococcal Bloodstream Infection by Different Metabolites

Staphylococcus aureus is a notorious bacterial pathogen that often causes soft tissue and bloodstream infections and invariably garners resistance mechanisms against new antibiotics. Modulation of the host immune response by metabolites is a powerful tool against bacterial infections, but has not yet been used against S. aureus infections. In this study, we identified four metabolite biomarkers: L-proline, L-isoleucine, L-leucine, and L-valine (PILV), through a metabolomics study using animal models of S. aureus bloodstream infection. The exogenous administration of each metabolite or of PILV showed anti-infective effects, and a higher protection was achieved with PILV in comparison to individual metabolites. During the staphylococcal infection, the expression of most host arginase and nitric oxide synthase (NOS) isozymes was simultaneously induced in mouse liver, kidney, and blood samples. However, the induction of arginase isozymes was dramatically stronger than that of NOS isozymes. This elevated arginase activity was inhibited by the metabolite biomarkers thus killing S. aureus, and PILV exhibited the strongest inhibition of arginase activity and bacterial inhibition. The suppression of arginase activity also contributed to the metabolite-mediated phagocytic killing of S. aureus in mouse and human blood. Our findings demonstrate the metabolite-mediated arginase inhibition as a therapeutic intervention for S. aureus infection.

The role of macrophages in the innate immune response to Streptococcus pneumoniae and Staphylococcus aureus: mechanisms and contrasts

Macrophages are critical mediators of innate immune responses against bacteria. The Gram-positive bacteria Streptococcus pneumoniae and Staphylococcus aureus express a range of virulence factors, which challenge macrophages' immune competence. We review how macrophages respond to this challenge. Macrophages employ a range of strategies to phagocytose and kill each pathogen. When the macrophages capacity to clear bacteria is overwhelmed macrophages play important roles in orchestrating the inflammatory response through pattern recognition receptor-mediated responses. Macrophages also ensure the inflammatory response is tightly constrained, to avoid tissue damage, and play an important role in downregulating the inflammatory response once initial bacterial replication is controlled.

Role of Toll-like receptor 13 in innate immune recognition of group B streptococci

Murine Toll-like receptor 13 (TLR13), an endosomal receptor that is not present in humans, is activated by an unmethylated motif present in the large ribosomal subunit of bacterial RNA (23S rRNA). Little is known, however, of the impact of TLR13 on antibacterial host defenses. Here we examined the role of this receptor in the context of infection induced by the model pathogen group B streptococcus (GBS). To this end, we used bacterial strains masked from TLR13 recognition by virtue of constitutive expression of the ErmC methyltransferase, which results in dimethylation of the 23S rRNA motif at a critical adenine residue. We found that TLR13-mediated rRNA recognition was required for optimal induction of tumor necrosis factor alpha and nitrous oxide in dendritic cell and macrophage cultures stimulated with heat-killed bacteria or purified bacterial RNA. However, TLR13-dependent recognition was redundant when live bacteria were used as a stimulus. Moreover, masking bacterial rRNA from TLR13 recognition did not increase the ability of GBS to avoid host defenses and replicate in vivo. In contrast, increased susceptibility to infection was observed under conditions in which signaling by all endosomal TLRs was abolished, i.e., in mice with a loss-of-function mutation in the chaperone protein UNC93B1. Our data lend support to the conclusion that TLR13 participates in GBS recognition, although blockade of the function of this receptor can be compensated for by other endosomal TLRs. Lack of selective pressure by bacterial infections might explain the evolutionary loss of TLR13 in humans. However, further studies using different bacterial species are needed to prove this hypothesis.

Strategies to improve drug development for sepsis

Sepsis, a common and potentially fatal systemic illness, is triggered by microbial infection and often leads to impaired function of the lungs, kidneys or other vital organs. Since the early 1980s, a large number of therapeutic agents for the treatment of sepsis have been evaluated in randomized controlled clinical trials. With few exceptions, the results from these trials have been disappointing, and no specific therapeutic agent is currently approved for the treatment of sepsis. To improve upon this dismal record, investigators will need to identify more suitable therapeutic targets, improve their approaches for selecting candidate compounds for clinical development and adopt better designs for clinical trials.

Regulation of IDO activity by oxygen supply: inhibitory effects on antimicrobial and immunoregulatory functions

Tryptophan is an essential amino acid for human beings as well as for some microorganisms. In human cells the interferon-γ (IFN-γ) inducible enzyme indoleamine 2,3-dioxygenase (IDO) reduces local tryptophan levels and is therefore able to mediate broad-spectrum effector functions: IDO activity restricts the growth of various clinically relevant pathogens such as bacteria, parasites and viruses. On the other hand, it has been observed that IDO has immunoregulatory functions as it efficiently controls the activation and survival of T-cells. Although these important effects have been analysed in much detail, they have been observed in vitro using cells cultured in the presence of 20% O₂ (normoxia). Such high oxygen concentrations are not present in vivo especially within infected and inflamed tissues. We therefore analysed IDO-mediated effects under lower oxygen concentrations in vitro and observed that the function of IDO is substantially impaired in tumour cells as well as in native cells. Hypoxia led to reduced IDO expression and as a result to reduced production of kynurenine, the downstream product of tryptophan degradation. Consequently, effector functions of IDO were abrogated under hypoxic conditions: in different human cell lines such as tumour cells (glioblastoma, HeLa) but also in native cells (human foreskin fibroblasts; HFF) IDO lost the capacity to inhibit the growth of bacteria (Staphylococcus aureus), parasites (Toxoplasma gondii) or viruses (herpes simplex virus type 1). Additionally, IDO could no longer efficiently control the proliferation of T-cells that have been co-cultured with IDO expressing HFF cells in vitro. In conclusion, the potent antimicrobial as well as immunoregulatory functions of IDO were substantially impaired under hypoxic conditions that pathophysiologically occurs in vivo.

Oxidative modification of proteins: an emerging mechanism of cell signaling

There are a wide variety of reactive species which can affect cell function, including reactive oxygen, nitrogen, and lipid species. Some are formed endogenously through enzymatic or non-enzymatic pathways, and others are introduced through diet or environmental exposure. Many of these reactive species can interact with biomolecules and can result in oxidative post-translational modification of proteins. It is well documented that some oxidative modifications cause macromolecular damage and cell death. However, a growing body of evidence suggests that certain classes of reactive species initiate cell signaling by reacting with specific side chains of peptide residues without causing cell death. This process is generally termed "redox signaling," and its role in physiological and pathological processes is a subject of active investigation. This review will give an overview of oxidative protein modification as a mechanism of redox signaling, including types of reactive species and how they modify proteins, examples of modified proteins, and a discussion about the current concepts in this area.

Hypoxia-mediated impairment of the mitochondrial respiratory chain inhibits the bactericidal activity of macrophages

In infected tissues oxygen tensions are low. As innate immune cells have to operate under these conditions, we analyzed the ability of macrophages (Mφ) to kill Escherichia coli or Staphylococcus aureus in a hypoxic microenvironment. Oxygen restriction did not promote intracellular bacterial growth but did impair the bactericidal activity of the host cells against both pathogens. This correlated with a decreased production of reactive oxygen intermediates (ROI) and reactive nitrogen intermediates. Experiments with phagocyte NADPH oxidase (PHOX) and inducible NO synthase (NOS2) double-deficient Mφ revealed that in E. coli- or S. aureus-infected cells the reduced antibacterial activity during hypoxia was either entirely or partially independent of the diminished PHOX and NOS2 activity. Hypoxia impaired the mitochondrial activity of infected Mφ. Inhibition of the mitochondrial respiratory chain activity during normoxia (using rotenone or antimycin A) completely or partially mimicked the defective antibacterial activity observed in hypoxic E. coli- or S. aureus-infected wild-type Mφ, respectively. Accordingly, inhibition of the respiratory chain of S. aureus-infected, normoxic PHOX(-/-) NOS2(-/-) Mφ further raised the bacterial burden of the cells, which reached the level measured in hypoxic PHOX(-/-) NOS2(-/-) Mφ cultures. Our data demonstrate that the reduced killing of S. aureus or E. coli during hypoxia is not simply due to a lack of PHOX and NOS2 activity but partially or completely results from an impaired mitochondrial antibacterial effector function. Since pharmacological inhibition of the respiratory chain raised the generation of ROI but nevertheless phenocopied the effect of hypoxia, ROI can be excluded as the mechanism underlying the antimicrobial activity of mitochondria.

Phagosomal degradation increases TLR access to bacterial ligands and enhances macrophage sensitivity to bacteria

Signaling by innate immune receptors initiates and orchestrates the overall immune responses to infection. Macrophage receptors recognizing pathogens can be broadly grouped into surface receptors and receptors restricted to intracellular compartments, such as phagosomes and the cytoplasm. There is an expectation that ingestion and degradation of microorganisms by phagocytes contributes to activation of intracellular innate receptors, although direct demonstrations of this are rare, and many model ligands are studied in soluble form, outside of their microbial context. By comparing a wild-type strain of Staphylococcus aureus and a lysozyme-sensitive mutant, we have been able directly to address the role of degradation of live bacteria by mouse macrophages in determining the overall innate cellular inflammatory response. Our investigations revealed a biphasic response to S. aureus that consisted of an initial signal resulting from the engagement of surface TLR2, followed by a later, second wave on inflammatory gene induction. This second wave of inflammatory signaling was dependent on and correlated with the timing of bacterial degradation in phagosomes. We found that TLR2 signaling followed by TLR2/TLR9 signaling enhanced sensitivity to small numbers of bacteria. We further found that treating wild-type bacteria with the peptidoglycan synthesis-inhibiting antibiotic vancomycin made S. aureus more susceptible to degradation and resulted in increased inflammatory responses, similar to those observed for mutant degradation-sensitive bacteria.

Maternal/fetal mortality and fetal growth restriction: role of nitric oxide and virulence factors in intrauterine infection in rats

OBJECTIVE

The mechanism of infection-related deaths of pregnant rats and intrauterine growth restriction are not understood. We assessed whether nitric oxide (NO) has differential effects on infection with Escherichia coli Dr/Afa mutants that lack either AfaE or AfaD invasins.

STUDY DESIGN

Sprague-Dawley rats were infected intrauterinally with the clinical strain of E coli AfaE(+)D(+) or 1 of its isogenic mutants in the presence or absence of the NO synthesis inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). Maternal/fetal mortality rates, fetoplacental weight, and infection rates were evaluated.

RESULTS

Maternal and/or fetal death was associated with the presence of at least 1 virulence factor (AfaE(+)D(+)>AfaE(+)D(-)>AfaE(-)D(+)) and was increased by L-NAME treatment. The fetal and placental weights were lower than controls and were further reduced by L-NAME treatment.

CONCLUSION

These results demonstrate that NO enhanced AfaE- and AfaD-mediated virulence and plays an important role in Dr/Afa(+)E coli gestational tropism.

NADPH-oxidase but not inducible nitric oxide synthase contributes to resistance in a murine Staphylococcus aureus Newman pneumonia model

Staphylococcus aureus is a pathogen that often causes severe nosocomial infections including pneumonia. The present study was designed to examine innate phagocyte mediated immune mechanisms using a previously described murine S. aureus Newman pneumonia model. We found that BALB/c mice represent a more susceptible mouse strain compared to C57BL/6 mice after intranasal S. aureus Newman challenge. Depletion experiments revealed that neutrophils are a crucial determinant for resistance whereas depletion of alveolar macrophages protected mice to some degree from acute pulmonary S. aureus challenge. C57BL/6 mice lacking the subunit gp91phox of the NADPH-oxidase (gp91phox⁻/⁻ mice) proved to be highly susceptible against the pathogen. In contrast, C57BL/6 inducible nitric oxidase synthase deficient (iNOS⁻/⁻) mice did not differ in their clinical outcome after infection. Neither bone marrow macrophages from iNOS-/- nor from gp91phox⁻/⁻ mice were impaired in controlling intracellular persistence of S. aureus. Our data suggest that neutrophil and NADPH-oxidase mediated mechanisms are essential components in protecting the host against pulmonary S. aureus Newman challenge. On contrary, macrophages as well as NO mediated mechanisms do not seem to play a critical role for resistance in this model.

Angiotensin-converting enzyme overexpression in mouse myelomonocytic cells augments resistance to Listeria and methicillin-resistant Staphylococcus aureus

Gene targeting in ES cells was used to substitute control of angiotensin converting enzyme (ACE) expression from the endogenous promoter to the mouse c-fms promoter. The result is an animal model called ACE 10/10 in which ACE is overexpressed by monocytes, macrophages, and other myelomonocytic lineage cells. To study the immune response of these mice to bacterial infection, we challenged them with Listeria monocytogenes or methicillin-resistant Staphylococcus aureus (MRSA). ACE 10/10 mice have a significantly enhanced immune response to both bacteria in vivo and in vitro. For example, 5 days after Listeria infection, the spleen and liver of ACE 10/10 mice had 8.0- and 5.2-fold less bacteria than wild type mice (WT). In a model of MRSA skin infection, ACE 10/10 mice had 50-fold less bacteria than WT mice. Histologic examination showed a prominent infiltrate of ACE-positive mononuclear cells in the skin lesions from ACE 10/10. Increased bacterial resistance in ACE 10/10 is directly due to overexpression of ACE, as it is eliminated by an ACE inhibitor. Critical to increased immunity in ACE 10/10 is the overexpression of iNOS and reactive nitrogen intermediates, as inhibition of iNOS by the inhibitor 1400W eliminated all in vitro and in vivo differences in innate bacterial resistance between ACE 10/10 and WT mice. Increased resistance to MRSA was transferable by bone marrow transplantation. The overexpression of ACE and iNOS by myelomonocytic cells substantially boosts innate immunity and may represent a new means to address serious bacterial infections.

Infectious arthritis

In contrast to the outstanding achievements made in therapy for autoimmune arthritides, not least rheumatoid arthritis, the pace of progress in therapy for infectious arthritis remains slow. This has primarily to do with the complex task of, on the one hand, killing the invading microorganisms and, on the other, to down-regulate the exaggerated immune response which participates in the microbial clearance but at the same time contributes to the tissue destruction. The use of experimental models of microbial arthritides has clarified several bacterial virulence factors as well as many haematopoietic cell types and their products that are involved in the pathogenesis of joint infection. Recent studies have documented that T-cell-mediated responses are not only prominent but also decisive with respect to disease sequelae. This chapter also reviews the primarily protective non-antigen-specific immune responsiveness to microbial agents, including the impact of neutrophils, complement system, and nitric oxide. The knowledge gained regarding microbial virulence factors and the host immune responses has prompted researchers to develop new strategies on how to interact in vivo with the infectious process. Some of these approaches are commented upon in this review: e.g. vaccination procedures to prevent septic arthritis and sepsis, as well as therapeutic procedures to minimize joint damage during an ongoing infection.

The nitrosative stress response of Staphylococcus aureus is required for resistance to innate immunity

Staphylococcus aureus is a highly virulent human pathogen with an extensive array of strategies to subvert the innate immune response. An important aspect of innate immunity is the production of the nitrogen monoxide radical (Nitric Oxide, NO.). Here we describe an adaptive response to nitrosative stress that allows S. aureus to replicate at high concentrations of NO.. Microarray analysis revealed 84 staphylococcal genes with significantly altered expression following NO. exposure. Of these, 30 are involved with iron-homeostasis, potentially under the control of the Fur regulator. Another seven induced genes are involved in hypoxic/fermentative metabolism, including the flavohaemoprotein, Hmp. The SrrAB two-component system has been shown to regulate the expression of many of the NO.-induced metabolic genes. Indeed, inactivation of hmp, srrAB and fur resulted in heightened NO. sensitivity. Hmp was responsible for c. 90% of measurable staphylococcal NO. consumption and therefore critical for efficient NO. detoxification. While SrrAB was required for maximal hmp expression, srrAB mutants still exhibited significant NO. scavenging and NO.-dependent induction of hmp. Yet S. aureus lacking SrrAB were more sensitive to nitrosative stress than hmp mutants, indicating that the contribution of SrrAB to NO. resistance extends beyond the regulation of hmp expression. Both Hmp and SrrAB were required for full virulence in a murine sepsis model, however, only the attenuation of the hmp mutant was restored by the abrogation of host NO. production. Thus, the S. aureus Hmp protein has evolved to serve as an iNOS-dependent virulence determinant.

Inhibition of nitric oxide synthase exacerbates group B streptococcus sepsis and arthritis in mice

The role of nitric oxide in group B Streptococcus (GBS) infection was evaluated by inhibiting its production with aminoguanidine (AG). AG-treated mice displayed higher mortality rates and more frequent and severe arthritis than controls. Worsening of arthritis correlated with a higher number of GBS cells in the joints and local interleukin-1 beta production.

Cytokine milieu of atopic dermatitis, as compared to psoriasis, skin prevents induction of innate immune response genes

Atopic dermatitis (AD) and psoriasis are the two most common chronic skin diseases. However patients with AD, but not psoriasis, suffer from frequent skin infections. To understand the molecular basis for this phenomenon, skin biopsies from AD and psoriasis patients were analyzed using GeneChip microarrays. The expression of innate immune response genes, human beta defensin (HBD)-2, IL-8, and inducible NO synthetase (iNOS) was found to be decreased in AD, as compared with psoriasis, skin (HBD-2, p = 0.00021; IL-8, p = 0.044; iNOS, p = 0.016). Decreased expression of the novel antimicrobial peptide, HBD-3, was demonstrated at the mRNA level by real-time PCR (p = 0.0002) and at the protein level by immunohistochemistry (p = 0.0005). By real-time PCR, our data confirmed that AD, as compared with psoriasis, is associated with elevated skin production of Th2 cytokines and low levels of proinflammatory cytokines such as TNF-alpha, IFN-gamma, and IL-1beta. Because HBD-2, IL-8, and iNOS are known to be inhibited by Th2 cytokines, we examined the effects of IL-4 and IL-13 on HBD-3 expression in keratinocyte culture in vitro. We found that IL-13 and IL-4 inhibited TNF-alpha- and IFN-gamma-induced HBD-3 production. These studies indicate that decreased expression of a constellation of antimicrobial genes occurs as the result of local up-regulation of Th2 cytokines and the lack of elevated amounts of TNF-alpha and IFN-gamma under inflammatory conditions in AD skin. These observations could explain the increased susceptibility of AD skin to microorganisms, and suggest a new fundamental rule that may explain the mechanism for frequent infection in other Th2 cytokine-mediated diseases.

Current status of pathogenetic mechanisms in staphylococcal arthritis

Interactions between staphylococci and the joint tissues of the host lead typically to rapidly progressing and highly destructive processes. Staphylococci possess a vast arsenal of components and products that contribute to the pathogenesis of joint infection. Occasionally these compounds have overlapping activities and act either in concert or alone. Host responsiveness to staphylococcal infection displays an even more complex pattern. Most of the cells and molecules that participate in the innate immune system protect the host against bacteria. However, the staphylococci have developed systems that counteract endogenous protective mechanisms. Interestingly, certain cells and molecules of the acquired immune system potentiate the severity of infection by triggering exaggerated responses to the staphylococcal danger signals. This review deals with the intricate host-bacterium interactions that occur during experimental septic arthritis, and outlines potential preventive and treatment modalities.

Acute septic arthritis

Acute septic arthritis may develop as a result of hematogenous seeding, direct introduction, or extension from a contiguous focus of infection. The pathogenesis of acute septic arthritis is multifactorial and depends on the interaction of the host immune response and the adherence factors, toxins, and immunoavoidance strategies of the invading pathogen. Neisseria gonorrhoeae and Staphylococcus aureus are used in discussing the host-pathogen interaction in the pathogenesis of acute septic arthritis. While diagnosis rests on isolation of the bacterial species from synovial fluid samples, patient history, clinical presentation, laboratory findings, and imaging studies are also important. Acute nongonococcal septic arthritis is a medical emergency that can lead to significant morbidity and mortality. Therefore, prompt recognition, rapid and aggressive antimicrobial therapy, and surgical treatment are critical to ensuring a good prognosis. Even with prompt diagnosis and treatment, high mortality and morbidity rates still occur. In contrast, gonococcal arthritis is often successfully treated with antimicrobial therapy alone and demonstrates a very low rate of complications and an excellent prognosis for full return of normal joint function. In the case of prosthetic joint infections, the hardware must be eventually removed by a two-stage revision in order to cure the infection.

Involvement of NO in the failure of neutrophil migration in sepsis induced by Staphylococcus aureus

1. Sepsis induced by S. aureus was used to investigate whether neutrophil migration failure to infectious focus correlates with lethality in Gram-positive bacteria-induced sepsis in mice. 2. By contrast with the sub-lethal (SL-group), the lethal (L-group) intraperitoneal inoculum of S. aureus caused failure of neutrophil migration (92% reduction), high CFU in the exudate, bacteremia and impairment of in vitro neutrophil chemotactic activity. 3. Pre-treatments of L-group with adequate doses of aminoguanidine prevented the neutrophil migration failure and improved the survival of the animals (pre-treated: 43%; untreated: 0% survival). Thus, the impairment of neutrophil migration in the L-group appears to be mediated by nitric oxide (NO). 4. The injection of S. aureus SL-inoculum in iNOS deficient (-/-) or aminoguanidine-treated wild-type mice (pre- and post-treatment), which did not present neutrophil migration failure, paradoxically caused severe peritonitis and high mortality. This fact is explainable by the lack of NO dependent microbicidal activity in migrated neutrophils. 5. In conclusion, although the NO microbicidal mechanism is active in neutrophils, the failure of their migration to the infectious focus may be responsible for the severity and outcome of sepsis.

Low molecular weight heparin aggravates infectious arthritis triggered by Staphylococcus aureus

PURPOSE

Staphylococcus aureus is responsible for the majority of wound infections in clean surgical procedures that involve implantation of foreign material, grafts or prosthetic devices. The aim of the study was to assess the effect of low molecular weight heparin on the development and progression of S. aureus arthritis.

MATERIALS AND METHODS

The murine model of hematogenously acquired septic arthritis was used injecting intravenously toxic shock syndrome toxin-1 (TSST-1) producing S. aureus of LS-1 strain. Mice lacking prosthetic implants were treated with intraperitoneal injections of low molecular weight heparin, used routinely as anti-thrombotic prophylaxis following joint prosthetic surgery. Evaluation of arthritis was performed clinically and histopathologically. In addition, the effect of low molecular weight heparin on T cell dependent and independent inflammation was assessed.

RESULTS

Seven days after inoculation with bacteria 18 out of 19 low molecular weight heparin treated mice displayed clinical symptoms of arthritis as compared to 9 out of 23 control animals (p < 0.05), and the severity of arthritis, expressed as arthritic index, was 2.6+/-0.5 versus 1.6+/-0.5 (p = 0.05). The histopathological examination confirmed the clinical findings showing that both inflammation and joint destruction were more substantial in heparin treated animals.

CONCLUSION

Our findings indicate that the routine anti-coagulation treatment with heparin contributes to more severe course of joint infection.

Intracisternally localized bacterial DNA containing CpG motifs induces meningitis

Unmethylated CpG motifs are frequently found in bacterial DNA, and have recently been shown to exert immunostimulatory effects on leukocytes. Since bacterial infections in the CNS will lead to local release of prokaryotic DNA, we wanted to investigate whether such an event might trigger meningitis. To that end, we have intracisternally injected mice and rats with bacterial DNA and oligonucleotides containing CpG motifs. Histopathological signs of meningitis were evident within 12 h and lasted for at least 14 days, and were characterized by an influx of monocytic, Mac-3(+) cells and by a lack of T lymphocytes. To study the mechanisms whereby unmethylated CpG DNA gives rise to meningitis, we deleted the monocyte/macrophage population leading to abrogation of brain inflammation. Also, interaction with NF-kappaB using antisense technology led to down-regulation of proinflammatory cytokine production and frequency of meningitis. Furthermore, specific interactions with vascular selectin expression and inhibition of NO synthase led to a significant amelioration of meningitis, altogether indicating that this condition is dependent on macrophages and their products. In contrast, neutrophils, NK cells, T/B lymphocytes, IL-12, and complement system were not instrumental in meningitis triggered by bacterial DNA containing CpG motifs. This study proves that bacterial DNA containing unmethylated CpG motifs induces meningitis, and indicates that this condition is mediated in vivo by activated macrophages.

Staphylococcal resistance to antimicrobial peptides of mammalian and bacterial origin

Antimicrobial host defense peptides, such as defensins, protegrins, and platelet microbicidal proteins are deployed by mammalian skin, epithelia, phagocytes, and platelets in response to Staphylococcus aureus infection. In addition, staphylococcal products with similar structures and activities, called bacteriocins, inhibit competing microorganisms. Staphylococci have developed resistance mechanisms, which are either highly specific for certain host defense peptides or bacteriocins or which broadly protect against a range of cationic antimicrobial peptides. Experimental infection models can be used to study the molecular mechanisms of antimicrobial peptides, the peptide resistance strategies of S. aureus, and the therapeutic potential of peptides in staphylococcal diseases.

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.

Nitric oxide inhibits aggrecan degradation in explant cultures of equine articular cartilage

Arthroses are debilitating diseases of articular joints which result in erosion of the cartilage extracellular matrix. Nitric oxide (NO) is a major component of the inflammatory response, and has been implicated as a mediator of some of the effects of the proinflammatory cytokine, interleukin-1 (IL-1). In this study, we investigated the role of NO in the regulation of proteoglycan degradation in equine articular cartilage. NO fully mediated the suppressive effect of IL-1 on proteoglycan synthesis. However, NO was also antagonistic to proteoglycan degradation, irrespective of whether degradation was initiated by 10 ng/ml IL-1 or 1 micromol/l all-trans retinoic acid (RA) which (unlike IL-1) does not elevate NO production. This was confirmed using the NO donor 2,2'-(hydroxynitrosohydrazono) bis-ethanamine (DETA-NONOate) and the iNOS inhibitor L-N5-iminoethyl ornithine (dihydrochloride) (L-NIO). The G1 fragments of aggrecan were detected in the media and extracts of cartilage explant cultures treated with all-trans RA, DETA-NONOate and L-NIO. The presence of exogenous NO in culture resulted in a decrease in the appearance of the 'aggrecanase' cleavage epitope. Therefore, changes in the appearance of the G1 fragment expressing the 'aggrecanase' cleavage epitope in the media emulated the glycosaminoglycan loss from the tissue. These results lend further support to the hypothesis that NO has an anticatabolic role in equine cartilage proteoglycan degradation, and suggest that this may be mediated by the regulation of 'aggrecanase' activity. Therefore, any pharmacological intervention using NO as a target must take into account both its catabolic and anticatabolic roles in joint tissue turnover.

Staphylococcus aureus stimulates inducible nitric oxide synthase in articular cartilage

OBJECTIVE

To determine if Staphylococcus aureus stimulates the L-arginine-nitric oxide (NO) synthase pathway in articular cartilage.

METHODS

A heat-killed and sonicated (denatured) S. aureus preparation was added to cultures of bovine articular cartilage. NO production was measured as accumulated nitrite in the culture medium and by the NO synthase-dependent conversion of 3H-L-arginine to 3H-L-citrulline in cartilage homogenates. Inducible NO synthase (iNOS) messenger RNA (mRNA) expression was analyzed by Northern blot. Proteoglycan synthesis was measured by 35SO4 incorporation into glycosaminoglycan.

RESULTS

Nitrite accumulation and 3H-L-citrulline formation in cartilage were elevated by denatured S. aureus (compared with unstimulated control cartilage) and inhibited by the NO synthase inhibitor N(G)-monomethyl-L-arginine. Northern blot analysis revealed increased iNOS mRNA expression in bovine chondrocytes in response to denatured S. aureus stimulation. Denatured S. aureus suppressed the accumulation of 35SO4-labeled macromolecules representing newly synthesized proteoglycans in bovine articular cartilage. The suppressed proteoglycan synthesis was due to the presence of NO.

CONCLUSION

These findings support the hypothesis that a component of S. aureus can stimulate iNOS in articular cartilage, and that NO generated from this enzyme down-regulates cartilage matrix proteoglycan synthesis.

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.

Interaction with complement receptor 1 (CD35) leads to amelioration of sepsis-triggered mortality but aggravation of arthritis during Staphylococcus aureus infection

The aim of this study was to assess the importance of complement receptor 1 (CR1, CD35) in Staphylococcus aureus arthritis and sepsis. The murine model of haematogenously acquired septic arthritis was used, injecting toxic shock syndrome toxin 1 (TSST-1)-producing S. aureus LS-1 intravenously. CR1 was blocked using immunoglobulin G (IgG) rat antimouse CR1 monoclonal antibody (MoAb) (8C12). Evaluation of arthritis was performed clinically and histopathologically. In addition, the effect of blocking CR1 was assessed on the phagocytic activity of leucocytes and on T-cell dependent and independent inflammation. Seven days after inoculation with bacteria, 96% of CR1 MoAb-treated mice had clinical symptoms of arthritis compared with 58% of the control animals (P < 0.01). The severity of arthritis, expressed as mean arthritic index, was 2.9 +/- 0.5 and 1.4 +/- 0.5, respectively (P = 0.004). Fifteen days after bacterial inoculation, all CR1 MoAb-treated mice had severe arthritis (mean arthritic index 6.3 +/- 0.6), while only 77% of controls were affected (mean arthritic index 2.9 +/- 0.6; P = 0.002). The potential explanation of these findings is that treatment with CR1 MoAb significantly increases the polymorphonuclear cell-dependent inflammatory response as a result of enhanced vasodilatation in treated animals. We conclude that treatment with CR1 MoAb leads to amelioration of sepsis-induced mortality during S. aureus infection, possibly as a result of the increased phagocytic activity of peripheral phagocytes.

Gram-positive sepsis. Mechanisms and differences from gram-negative sepsis

This article has reviewed the mechanisms by which gram-positive bacteria lead to septic shock, with regard to bacterial structure and toxicology and the host responses elicited both in animal models and in the clinical setting. Gram-positive organisms are better suited to invade host tissues and elicit, in general, a brisker phagocytic response than gram-negative organisms. The lack of endotoxin in the outer cell wall is compensated for by the presence of exposed peptidoglycan and a range of other toxic secreted products. It appears that cell wall components of gram-positive bacteria may signal via the same receptor as gram-negative endotoxin, although the type of signal and coreceptor may differ. Both animal and clinical data suggest that, unlike endotoxin-mediated shock, gram-positive infection produces a modest TNF response only and does not respond well to anti-TNF therapies. This leads one to conclude that the mechanisms leading to shock in gram-positive infection may be multifactorial and perhaps more difficult to treat. A thorough review of gram-positive mechanisms of sepsis is hampered by a lack of basic research in this field. Understanding of gram-negative bacterial structure and the regulation of virulence genes is at an advanced stage, yet the molecular tools to analyse virulence factors in the gram-positive genome have only recently become available. There is a paucity of good animal models of gram-positive infection and a lack of microbiologic data from some of the major trials in sepsis that might have given greater insight into the mechanisms leading to shock in various infections.

The role of nitric oxide in articular cartilage damage

The production of large amounts of NO in vitro by cytokine-activated chondrocytes has been established. In vitro studies suggest that NO compromises chondrocyte survival. The role of NO in regulating matrix biosynthesis and degradation has received much attention. Most studies indicate that NO is at least partly responsible for IL-1-induced suppression of glycosaminoglycan and collagen synthesis. NO also may be involved as a mediator of IL-1-induced expression of MMP, mRNA, and protein and may contribute as an activator of the latent forms of the enzymes. Although the interaction of NO and prostaglandins is of considerable interest, current data are inconclusive with respect to the role of NO in the regulation of prostaglandin synthesis, although it seems clear that prostaglandin is not involved in NO synthesis. It is important to note that NO does have protective effects in cartilage and other tissues. Under certain conditions, NO may have anabolic and anticatabolic effects in cartilage. In other tissues, notably in skin and muscle, NO has been found to have a stimulatory role in extracellular matrix repair. In antimicrobial defense, in general, and in bacterial arthritis specifically, NO is an important protective molecule. Production of NO in arthritis-affected cartilage and synovium is a consistent feature of human and experimentally induced arthritis. The production of NO is associated with matrix degradation and chondrocyte apoptosis. The administration of NO synthase inhibitors in experimentally induced arthritis has resulted in reduction of synovial inflammation and destruction of cartilage and bone.

Modulation of inflammation and immunity by arginine supplements

Arginine holds a key position in the cellular functions and interactions that occur during inflammation and immune responses. The competition for arginine as a substrate between nitric oxide synthase and arginase appears to be at the core of the regulation of the inflammatory process. This review examines some of the recently defined effects of arginine on various inflammatory processes and immune cell functions.

Staphylococcus aureus-Induced Septic Arthritis and Septic Death Is Decreased in IL-4-Deficient Mice: Role of IL-4 as Promoter for Bacterial Growth

Lack of IL-4 has been shown to be protective in some experimental models of infectious diseases in mice such as cutaneous leishmaniasis. At the same time IL-4, together with other Th2 cytokines, including IL-10 and IL-13, is known as an anti-inflammatory cytokine with the potential to down-regulate proinflammatory cytokine production. To investigate the role of IL-4 in experimental Staphylococcus aureus-induced and T lymphocyte-mediated arthritis, IL-4-deficient C57BL/6 mice (IL-4−/−) and their congenic controls (IL-4+/+) were inoculated with a toxic shock syndrome toxin-1-producing S. aureus strain. In IL-4+/+ mice, arthritis peaked 14 days after bacterial inoculation, whereas, at that time, IL-4−/− mice displayed significantly less frequent (p &lt; 0.05) joint inflammation. Paralleling lower frequency of arthritis, IL-4-deficient mice showed a decreased bacterial burden in joints (p = 0.014) and kidneys (p = 0.029), as well as lower infection-triggered weight decrease and mortality. In vitro, IL-4 inhibited intracellular killing of S. aureus in infected macrophages, without affecting phagocytosis. This finding may explain the enhanced staphylococcal clearance observed in IL-4−/− mice in vivo. Our results suggest that IL-4 and IL-4-dependent Th2 responses promote septic arthritis and sepsis-related mortality by inhibition of bacterial clearance during S. aureus infection.