AB0097 NOVEL EX VIVOMODEL OF SEPTIC ARTHRITIS DEVELOPED TO IDENTIFY BIOMARKERS RELEASED UPON ARTICULAR INFECTION
Authors: Kathryn McCall, Caroline Atherton, Neal L. Millar, Carl Goodyear, Tom Evans, Iain Mcinnes
Journal: Annals of the Rheumatic Diseases (2019)

<h3>Background:</h3> Septic arthritis (SA) is most commonly caused by infectious agents such as <i>Staphylococcus aureus</i>. Due to the morbidity and mortality associated with the disease, joint infections are treated as a medical emergency and therefore early diagnosis is essential. Unfortunately, no reliable biomarkers are available to assist diagnosis, beyond serum CRP and synovial fluid culture. <h3>Objectives:</h3> To generate a novel <i>ex vivo</i> model of septic arthritis comprised of cartilage matrix, neutrophils and SA-associated <i>Staph. aureus</i>, to identify biomarkers released upon an inflammatory host response to infection. <h3>Methods:</h3> Cartilage explants biopsied from human femoral heads were compared to chondrocyte disks differentiated from mesenchymal stem cells (MSC). MSC differentiation into chondrocytes under defined media and growth factor conditions was confirmed by IHC staining for collagen II on the disks. Explants and disks were infected for 48h with 10<sup>6</sup> cfu <i>Staph. aureus</i> (SA patient derived). Structural damage was measured by glycosaminoglycan (GAG) release. Neutrophils were purified from health donor blood samples using the StemCell whole blood isolation kit, and 10<sup>6</sup> cell were added in the final 4h of bacterial infection. Disks were stained with eosin to visualise microscopic damage, CMFDA for viability and PI to identify cell death. <h3>Results:</h3> Co-culture of MSC-derived chondrocyte disks with neutrophils or <i>Staph. aureus</i> alone resulted in microscopically identifiable structural damage to the disk matrix. The addition of neutrophils in the final 4h of the <i>Staph. aureus</i> co-cultures resulted in enhanced structural damage. Assessment of cellular viability revealed that both neutrophils and <i>Staph. aureus</i> alone induced cell death throughout the disk whilst the combination of both resulted in total abrogation of viable chondrocytes. Subsequent evaluation of GAG release, however, showed no clear differentiation between the conditions and as such this model was judged not to be sufficient for biomarker development. In comparison, femoral head cartilage explants studies demonstrated that <i>Staph. aureus</i> infection in this explant model resulted in significant GAG release after 24h and 48h, which was consistent between donors. <h3>Conclusion:</h3> Cartilage explants provide a superior and more physiologically relevant cartilage matrix model for the <i>ex vivo</i> analysis of septic arthritis. <i>Staph. aureus</i> infection resulted in reproducible damage to cartilage matrices that was not emulated in an MSC-derived chondrocyte disk system. These finding suggest that the femoral head cartilage explants are the optimal matrix model to identify biomarkers for joint infections. <h3>Disclosure of Interests:</h3> Kathryn McCall: None declared, Caroline Atherton: None declared, Neal L Millar: None declared, Carl Goodyear Grant/research support from: AstraZeneca, BMS, Celgene, Janssen, MedAnnex, Pfizer and UCB, Speakers bureau: Abbvie, Tom Evans: None declared, Iain McInnes Grant/research support from: AstraZeneca, Celgene, Compugen, Novartis, Roche, UCB Pharma, Consultant for: AbbVie, Celgene, Galvani, Lilly, Novartis, Pfizer, UCB Pharma