Elevated Galectin-9 Suppresses Th1 Effector Function and Induces Apoptosis of Activated CD4⁺ T Cells in Osteoarthritis

T cell immunoglobulin and mucin domain 3 (Tim-3) is a critical regulatory molecule found on activated Th1 cells, exhausted CD8⁺ T cells, and resting monocytes/macrophages. Galectin-9 (Gal-9) is an identified ligand for Tim-3. Interaction between Tim-3 and Gal-9 is thought to inhibit Th1 responses. The regulation and function of Tim-3 and Gal-9 in osteoarthritis (OA) have not been intensively investigated. We found that in peripheral blood, CD4⁺ T cells, but not CD8⁺ T cells or CD14⁺ monocytes, from OA patients presented significantly elevated Tim-3 and Gal-9 expression compared to those from healthy controls (HC). The CD4⁺ T cells from OA did not present altered Th1, Th2, and Th17 composition in the peripheral blood, but secreted less Th1 cytokine interleukin 2 (IL-2) and interferon gamma (IFN-γ) after activation. Further investigation demonstrated that Gal-9 induced high levels of apoptosis in activated CD4⁺ T cells from OA patients. Inhibition of Gal-9 resulted in significantly higher IL-2 and IFN-γ expression that was directly correlated with the number of non-apoptotic cells. In the synovial fluid, both secreted Gal-9 and surface Gal-9 levels were significantly higher in less-severe grade 2 OA patients than in more-severe grade 4 OA patients. Surface Tim-3 was also higher in synovial fluid CD8⁺ T cells and CD14⁺ monocytes from grade 2 OA patients and lower in grade 4 OA patients. Together, these results suggested that Tim-3 and Gal-9 could downregulate T cell inflammation in OA, and could be utilized as a novel therapeutic strategy.