CD14(+)CD16(−) monocytes are the main precursors of osteoclasts in rheumatoid arthritis via expressing Tyro3TK

BACKGROUND: Monocytes as precursors of osteoclasts in rheumatoid arthritis (RA) are well demonstrated, while monocyte subsets in osteoclast formation are still controversial. Tyro3 tyrosine kinase (Tyro3TK) is a member of the receptor tyrosine kinase family involved in immune homeostasis, the role of which in osteoclast differentiation was reported recently. This study aimed to compare the osteoclastic capacity of CD14(+)CD16(+) and CD14(+)CD16(−) monocytes in RA and determine the potential involvement of Tyro3TK in their osteoclastogenesis. METHODS: Osteoclasts were induced from CD14(+)CD16(+) and CD14(+)CD16(−) monocyte subsets isolated from healthy control (HC) and RA patients in vitro and evaluated by tartrate-resistant acid phosphatase (TRAP) staining. Then, the expression of Tyro3TK on CD14(+)CD16(+) and CD14(+)CD16(−) monocyte subsets in the peripheral blood of RA, osteoarthritis (OA) patients, and HC were evaluated by flow cytometry and qPCR, and their correlation with RA patient clinical and immunological features was analyzed. The role of Tyro3TK in CD14(+)CD16(−) monocyte-mediated osteoclastogenesis was further investigated by osteoclast differentiation assay with Tyro3TK blockade. RESULTS: The results revealed that CD14(+)CD16(−) monocytes were the primary source of osteoclasts. Compared with HC and OA patients, the expression of Tyro3TK on CD14(+)CD16(−) monocytes in RA patients was significantly upregulated and positively correlated with the disease manifestations, such as IgM level, tender joint count, and the disease activity score. Moreover, anti-Tyro3TK antibody could inhibit Gas6-mediated osteoclast differentiation from CD14(+)CD16(−) monocytes in a dose-dependent manner. CONCLUSIONS: These findings indicate that elevated Tyro3TK on CD14(+)CD16(−) monocytes serves as a critical signal for osteoclast differentiation in RA.