Soluble TAM receptor tyrosine kinases in rheumatoid arthritis: correlation with disease activity and bone destruction

The TAM receptor tyrosine kinases (TAM RTK) are a subfamily of receptor tyrosine kinases, the role of which in autoimmune diseases such as systemic lupus erythematosus has been well explored, while their functions in rheumatoid arthritis (RA) remain largely unknown. In this study, we investigated the role of soluble TAM receptor tyrosine kinases (sAxl/sMer/sTyro3) in patients with RA. A total of 306 RA patients, 100 osteoarthritis (OA) patients and 120 healthy controls (HCs) were enrolled into this study. The serum concentrations of sAxl/sMer/sTyro3 were measured by enzyme-linked immunosorbent assay (ELISA), then the associations between sAxl/sMer/sTyro3 levels and clinical features of RA patients were analysed. We also investigated whether sTyro3 could promote osteoclast differentiation in vitro in RA patients. The results showed that compared with healthy controls (HCs), sTyro3 levels in the serum of RA patients were elevated remarkably and sMer levels were decreased significantly, whereas there was no difference between HCs and RA patients on sAxl levels. The sTyro3 levels were correlated weakly but positively with white blood cells (WBC), immunoglobulin (Ig)M, rheumatoid factor (RF), swollen joint counts, tender joint counts, total sharp scores and joint erosion scores. Conversely, there were no significant correlations between sMer levels and the above indices. Moreover, RA patients with high disease activity also showed higher sTyro3 levels. In-vitro osteoclast differentiation assay showed further that tartrate-resistant acid phosphatase (TRAP)⁺ osteoclasts were increased significantly in the presence of sTyro3. Collectively, our study indicated that serum sTyro3 levels were elevated in RA patients and correlated positively with disease activity and bone destruction, which may serve as an important participant in RA pathogenesis.