Systemic immune alterations in a murine experimental model of osteoarthritis

Osteoarthritis (OA) is accompanied by an inflamed synovium containing macrophages, dendritic cells, T and B lymphocytes. Macrophages predominate and drive cytokine-mediated cartilage catabolism, while T cells and B cells, though fewer, may shape chronic adaptive responses. However, systemic immune contributions, particularly within peripheral lymphoid organs such as the spleen, remain poorly characterized. Our study aims to profile systemic immune changes in experimental OA induced by injection of mono-iodoacetate (MIA) in mouse paw. At day 56 post-OA induction, analysis of splenocytes showed that macrophages and conventional dendritic cells (cDC1 and cDC2) displayed a significant downregulation of MHCII expression, suggesting a negative feedback mechanism that limits chronic T cell activation. OA is also associated with an increase in total DCs including mainly MHCII negative tolerogenic DCs. Notably, while the proportion of CD11b ^- tolerogenic DCs was reduced, CD11b ^low tolerogenic DCs markedly expanded in OA animals. Expression level of the CD11b integrin was upregulated on macrophages and cDC2 in MIA-induced OA mice potentially facilitating their adhesion and migration toward inflamed joint. OA mice showed a significant reduction in total splenic leukocytes, primarily due to a loss of B cells, while total T cell numbers remained stable. However, T cell composition shifted: CD4 ⁺ T cells including activated and regulatory subsets decreased, whereas activated CD8 ⁺ T cells increased. This indicates a systemic imbalance favoring cytotoxic over regulatory immune activity, possibly linked to chronic immune stress or redistribution of lymphocytes to inflamed joint. In conclusion, our data reveals that chronic OA induces a coordinated remodeling of systemic innate and adaptive immunity. These systemic immune dysregulations could reveal new biomarkers or therapeutic targets.