Receptor-Ligand Biomarker Profile in Ankylosing Spondylitis: Associations Between AXL/MERTK Expression, Circulating...

(1) Background: Ankylosing spondylitis (AS) is a chronic inflammatory rheumatic disease associated with immune dysregulation. The TYRO3, AXL, and MER (TAM) signaling pathway, comprising AXL receptor tyrosine kinase (AXL), MER tyrosine kinase (MERTK), growth arrest-specific 6 (GAS6), and Protein S, is a key regulator of immune homeostasis. This study investigated these receptor-ligand components in patients with AS. (2) Methods: A total of 45 patients with AS and 44 healthy controls were enrolled. Serum GAS6 and Protein S levels were measured by ELISA, and AXL and MERTK expression levels were analyzed by RT-qPCR. Clinical and inflammatory parameters were also evaluated. (3) Results: ESR, CRP, and IL-6 levels were significantly higher, whereas Protein S levels were significantly lower in patients with AS than in controls. No significant differences were observed in GAS6, AXL, or MERTK expression levels, although lower expression trends were detected in patients. Logistic regression analysis identified CRP, IL-6, Protein S, and AXL expression as variables independently associated with AS. ROC analysis demonstrated significant discriminatory performance for AXL expression. (4) Conclusions: Alterations in the TAM signaling pathway, particularly reduced Protein S levels and altered AXL expression, may contribute to immune dysregulation and persistent inflammation in AS.