Trained immunity of synovial macrophages is associated with exacerbated joint inflammation and damage after Staphylococcus aureus infection.

Objectives: Investigate whether and which synoviocytes would acquire trained immunity characteristics that could exacerbate joint inflammation following a secondary Staphylococcus aureus infection.

Methods: Lipopolysaccharide (LPS) and S. aureus were separately or double injected (21 days of interval) into the tibiofemoral joint cavity of male C57BL/6 mice. At different time points after these stimulations, mechanical nociception was analyzed followed by the analysis of signs of inflammation and damage in the affected joints. The trained immunity markers, including the glycolytic and mTOR pathway, were analyzed in whole tissue or isolated synoviocytes. A group of mice was treated with Rapamycin, an mTOR inhibitor before LPS or S. aureus stimulation.

Results: The double LPS - S. aureus hit promoted intense joint inflammation and damage compared to single joint stimulation, including markers in synoviocyte activation, production of proinflammatory cytokines, persistent nociception, and bone damage, despite not reducing the bacterial clearance. The double LPS - S. aureus hit joints increased the synovial macrophage population expressing CX3CR1 alongside triggering established epigenetic modifications associated with trained immunity events in these cells, such as the upregulation of the mTOR signaling pathway (p-mTOR and HIF1α) and the trimethylation of histone H3. Mice treated with Rapamycin presented reduced CX3CR1⁺ macrophage activation, joint inflammation, and bone damage.

Conclusions: There is a trained immunity phenotype in CX3CR1⁺ synovial macrophages that contributes to the exacerbation of joint inflammation and damage during septic arthritis caused by S. aureus.