RAGE Deficiency Impairs Bacterial Clearance in Murine Staphylococcal Sepsis, but Has No Significant Impact on Staphylococcal 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.

Staphylococcus aureus lipoproteins in infectious diseases

Infections with the Gram-positive bacterial pathogen Staphylococcus aureus remain a major challenge for the healthcare system and demand new treatment options. The increasing antibiotic resistance of S. aureus poses additional challenges, consequently inflicting a huge strain in the society due to enormous healthcare costs. S. aureus expresses multiple molecules, including bacterial lipoproteins (Lpps), which play a role not only in immune response but also in disease pathogenesis. S. aureus Lpps, the predominant ligands of TLR2, are important for bacterial survival as they maintain the metabolic activity of the bacteria. Moreover, Lpps possess many diverse properties that are of vital importance for the bacteria. They also contribute to host cell invasion but so far their role in different staphylococcal infections has not been fully defined. In this review, we summarize the current knowledge about S. aureus Lpps and their distinct roles in various infectious disease animal models, such as septic arthritis, sepsis, and skin and soft tissue infections. The molecular and cellular response of the host to S. aureus Lpp exposure is also a primary focus.

Commensal Bacteria Augment Staphylococcus aureus septic Arthritis in a Dose-Dependent Manner

Background

Septic arthritis is considered one of the most dangerous joints diseases and is mainly caused by the Gram-positive bacterium Staphylococcus aureus (S. aureus). Human skin commensals are known to augment S. aureus infections. The aim of this study was to investigate if human commensals could augment S. aureus-induced septic arthritis.

Method

NMRI mice were inoculated with S. aureus alone or with a mixture of S. aureus together with either of the human commensal Staphylococcus epidermidis (S. epidermidis) or Streptococcus mitis (S. mitis). The clinical, radiological and histopathological changes due to septic arthritis were observed. Furthermore, the serum levels of chemokines and cytokines were assessed.

Results

Mice inoculated with a mixture of S. aureus and S. epidermidis or S. mitis developed more severe and frequent clinical arthritis compared to mice inoculated with S. aureus alone. This finding was verified pathologically and radiologically. Furthermore, the ability of mice to clear invading bacteria in the joints but not in kidneys was hampered by the bacterial mixture compared to S. aureus alone. Serum levels of monocyte chemoattractant protein 1 were elevated at the early phase of disease in the mice infected with bacterial mixture compared with ones infected with S. aureus alone. Finally, the augmentation effect in septic arthritis development by S. epidermidis was bacterial dose-dependent.

Conclusion

The commensal bacteria dose-dependently augment S. aureus-induced septic arthritis in a mouse model of septic arthritis.

The Expression of von Willebrand Factor-Binding Protein Determines Joint-Invading Capacity of Staphylococcus aureus, a Core Mechanism of Septic Arthritis

Septic arthritis, one of the most dangerous joint diseases, is predominantly caused by Staphylococcus aureus In contrast, coagulase-negative staphylococci are rarely found in septic arthritis. We hypothesize that coagulases released by S. aureus, including coagulase (Coa) and von Willebrand factor-binding protein (vWbp), play potent roles in the induction of septic arthritis. Four isogenic S. aureus strains differing in expression of coagulases (wild-type [WT] Newman, Δcoa, Δvwb, and Δcoa Δvwb) were used to induce septic arthritis in both wild-type and von Willebrand factor (vWF)-deficient mice. Septic arthritis severity was greatly reduced when wild-type mice were infected with the Δcoa Δvwb and Δvwb variants compared to WT or Δcoa strains, suggesting that vWbp rather than Coa is a major virulence factor in S. aureus septic arthritis. vWF-deficient mice were more susceptible to bone damage in septic arthritis, especially when the Δvwb strain was used. Importantly, no difference in arthritis severity between the Δvwb and WT strains was observed in vWF-deficient mice. Collectively, we conclude that vWbp production by S. aureus enhances staphylococcal septic arthritis.IMPORTANCE Septic arthritis remains one of the most dangerous joint diseases with a rapidly progressive disease character. Despite advances in the use of antibiotics, permanent reductions in joint function due to joint deformation and deleterious contractures occur in up to 50% of patients with septic arthritis. So far, it is still largely unknown how S. aureus initiates and establishes joint infection. Here, we demonstrate that von Willebrand factor-binding protein expressed by S. aureus facilitates the initiation of septic arthritis. Such effect might be mediated through its interaction with a host factor (von Willebrand factor). Our finding contributes significantly to the full understanding of septic arthritis etiology and will pave the way for new therapeutic modalities for this devastating disease.

The role of Staphylococcus aureus lipoproteins in hematogenous septic arthritis

Permanent joint dysfunction is a devastating complication in patients with septic arthritis. Staphylococcus aureus (S. aureus) lipoproteins (Lpp), the predominant ligands for TLR2, are known to be arthritogenic and induce bone destruction when introduced directly into the joint. Here, we aim to investigate the importance of S. aureus Lpp and TLR2 in a hematogenous septic arthritis model, which is the most common route of infection in humans. C57BL/6 wild-type and TLR2 deficient mice were intravenously inoculated with S. aureus Newman parental strain or its lipoprotein-deficient Δlgt mutant strain. The clinical course of septic arthritis, radiological changes, and serum levels of cytokines and chemokines, were assessed. Newman strain induced more severe and frequent clinical septic polyarthritis compared to its Δlgt mutant in TLR2 deficient mice, but not in wild-type controls. Bone destruction, however, did not differ between groups. Lpp expression was associated with higher mortality, weight loss as well as impaired bacterial clearance in mouse kidneys independent of TLR2. Furthermore, Lpp expression induced increased systemic pro-inflammatory cytokine and neutrophil chemokine release. Staphylococcal Lpp are potent virulence factors in S. aureus systemic infection independent of host TLR2 signalling. However, they have a limited impact on bone erosion in hematogenous staphylococcal septic arthritis.

Lack of Receptor for Advanced Glycation End Products Leads to Less Severe Staphylococcal Skin Infection but More Skin Abscesses and Prolonged Wound Healing

Background

Lack of receptor for advanced glycation end products (RAGE) ameliorates several infections including Staphylococcus aureus pneumonia. We sought to investigate the role of RAGE in staphylococcal skin infection in mice.

Methods

Wild-type (WT) and RAGE deficient (RAGE-/-) mice were subcutaneously inoculated with S. aureus SH1000 strain in abscess-forming dose or necrotic dose. Clinical signs of dermatitis, along with histopathological changes, were compared between the groups.

Results

The skin lesion size was smaller in RAGE-/- mice. Infected RAGE-/- mice expressed lower proinflammatory cytokines in local skins compared to control mice. Low dose of bacteria caused more abscess formation in RAGE-/- mice compared to skin necrosis that was more often observed in WT mice. As a result of more abscess formation, the wound healing was prolonged in RAGE-/- mice. Importantly, RAGE-/- mice had lower bacterial loads in the skin than controls, which is correlated with higher local levels of myeloperoxidase before skin infection. In vitro, enhanced phagocytic capacity of neutrophils and macrophages obtained from RAGE-/- mice compared to control mice was observed.

Conclusions

RAGE deficiency up-regulates phagocytic capacity of phagocytes, resulting in lower bacterial burden in local skin and milder skin lesions in mice with staphylococcal skin infection.

Baicalin modulates apoptosis via RAGE, MAPK, and AP-1 in vascular endothelial cells during Haemophilus parasuis invasion

Glässer’s disease, caused by Haemophilus parasuis, is a chronic disease related to an inflammatory immune response. Baicalin exerts important biological functions. In this study, we explored the protective efficacy of treatment with baicalin and the potential mechanism of activation of the MAPK signaling pathway in porcine aortic vascular endothelial cells (PAVECs) induced by H. parasuis. H. parasuis stimulated expression of receptor for advanced glycation end products, induced a significant increase in the level of protein kinase-α and protein kinase-δ phosphorylation, and significantly up-regulated ERK, c-Jun N-terminal kinase, and p38 phosphorylation in PAVECs. H. parasuis also up-regulated the levels of apoptotic genes (Bax, C-myc, and Fasl) and the expression levels of c-Jun and c-Fos, and induced S-phase arrest in PAVECs. However, treatment with baicalin inhibited expression of RAGE, suppressed H. parasuis-induced protein kinase-α and protein kinase-δ phosphorylation, reduced ERK, c-Jun N-terminal kinase, and p38 phosphorylation, down-regulated apoptotic genes (Bax, C-myc, and Fasl), attenuated phospho-c-Jun production from the extracellular to the nuclei, and reversed S-phase arrest in PAVECs. In conclusion, baicalin treatment inhibited the MAPK signaling pathway, thereby achieving its anti-inflammatory responses, which provides a new strategy to control H. parasuis infection.

Relevance of Receptor for Advanced Glycation end Products (RAGE) in Murine Antibody-Mediated Autoimmune Diseases

It is incompletely understood how self-antigens become targets of humoral immunity in antibody-mediated autoimmune diseases. In this context, alarmins are discussed as an important level of regulation. Alarmins are recognized by various receptors, such as receptor for advanced glycation end products (RAGE). As RAGE is upregulated under inflammatory conditions, strongly binds nucleic acids and mediates pro-inflammatory responses upon alarmin recognition, our aim was to examine its contribution to immune complex-mediated autoimmune diseases. This question was addressed employing RAGE−/− animals in murine models of pristane-induced lupus, collagen-induced, and serum-transfer arthritis. Autoantibodies were assessed by enzyme-linked immunosorbent assay, renal disease by quantification of proteinuria and histology, arthritis by scoring joint inflammation. The associated immune status was determined by flow cytometry. In both disease entities, we detected tendentiously decreased autoantibody levels in RAGE−/− mice, however no differences in clinical outcome. In accordance with autoantibody levels, a subgroup of the RAGE−/− animals showed a decrease in plasma cells, and germinal center B cells and an increase in follicular B cells. Based on our results, we suggest that RAGE deficiency alone does not significantly affect antibody-mediated autoimmunity. RAGE may rather exert its effects along with other receptors linking environmental factors to auto-reactive immune responses.