Different expression patterns and clinical significance of mAxl and sAxl in systemic lupus erythematosus

Axl is one of the TAM family members that downregulates activated immune responses to maintain immune homeostasis. We analyzed the expression and clinical relevance of Axl on the surface of CD14+ monocytes/macrophages (mAxl, membrane Axl) and in the plasma (sAxl, soluble Axl) from patients with systemic lupus erythematosus (SLE). Compared to healthy subjects, the concentrations of sAxl were significantly elevated in plasma from SLE patients, while the mAxl expression on CD14+ monocytes/macrophages from SLE patients was significantly downregulated. A series of severe disease clinical manifestations and laboratory features such as presence of autoantibodies, 24-hour proteinuria excretion or SLEDAI ≥10 were associated with decreased mAxl expression on monocytes/macrophages but elevated sAxl levels in plasma. The plasma level of Gas6, the main ligand of Axl, was slightly decreased in SLE patients, and was negatively correlated with anti-dsDNA antibodies and C-reactive protein. SLE patients with SLEDAI ≥10 showed significantly lower Gas6 levels. Our study suggests that abnormal mAxl and sAxl expression may be involved in the imbalance of immune regulation in SLE.