A novel mice model of acute flares in osteoarthritis elicited by intra‐articular injection of cultured mast cells

Purpose Mast cells are multifunctional in osteoarthritis (OA), and infiltration of activated mast cells likely contributes to disease severity and progression. However, the detailed mechanisms of action are unclear. The purpose of this study was to elucidate the role of mast cell infiltration in OA at histological level using a new mice model and to investigate pharmacological inhibitory effects of existing mast cell stabilizers in this model. Methods Mice were injected intra‐articularly with monosodium iodoacetate (MIA 0.5 mg) or PBS on day 0, and PBS, with or without mast cells (MC: 1 × 10 6 cells) on day 14. They were divided into four groups: OA flare (MIA + MC), OA (MIA + PBS), MC non‐OA (PBS + MC), and PBS non‐OA (PBS + PBS). In OA flare, the MC stabilizer drug (tranilast: 400 mg/kg/day) or PBS was administered intraperitoneally from days 15 to 21. Results Histologically, modified Mankin score of the OA flare was significantly higher than that of OA (7.0 [1.8] vs. 3.3 [1.3], P  < 0.05), and a larger number of mast cells was observed in OA flare than in OA (34.5 [6.3]/mm 2 vs. 27.2 [2.3]/mm 2 , P  < 0.05) on day 22. OA flare also showed acute exacerbation of pain and increased gene expression of pro‐inflammatory cytokines and aggrecanase compared with OA. Administration of tranilast to OA flare‐up provoked significant improvements in term of histological changes, pain, and gene expression at day 22. Conclusion Our novel model possibly mimics OA flare conditions, which may open a new strategy of disease‐modifying treatment for OA, focused on controlling the multiple functions of mast cells.