Spleen Tyrosine Kinase Exacerbates Anti‐Citrullinated Protein/Peptide Antibody‐Mediated Osteoclast Bone Resorption via Promotion of Vav3 Phosphorylation

Rheumatoid arthritis (RA) is a chronic systemic autoimmune disease characterized by abnormal bone resorption. Anti-citrullinated protein/peptide antibodies (ACPAs), detected in most RA patients, can stimulate osteoclasts differentiation by targeting osteoclast precursors, thereby enhancing bone resorption. However, the underlying mechanism of ACPAs-induced osteoclast activation on bone resorption in RA remains unknown. In this study, ACPA-positive (ACPA⁺) IgG promoted the maturation of osteoclasts. Phalloidin and wheat germ agglutinin (WGA) staining demonstrated that the ACPA⁺ IgG group exhibited significantly higher mean fluorescence intensity, cell volume, and wheat agglutinin coloring in osteoclasts compared to the ACPA-negative (ACPA^-) IgG group. Additionally, ACPA⁺ IgG stimulation significantly upregulated the p-SYK/SYK ratio in osteoclasts. SYK knockdown had no effect on osteoblast differentiation, but significantly decreased the area of bone lacunae, and attenuated osteoclasts bone resorption. Furthermore, SYK knockdown significantly decreased Vav3 phosphorylation, and colocalization of SYK and Vav3 was observed in osteoclasts. Notably, SYK and Vav3 enrichment at the leading edge of the osteoclasts was abrogated in the SYK-shRNA group. The number of actin rings was also significantly lower in the SYK-shRNA group compared to the SYK-shRNA-NC group. In conclusion, ACPA⁺ IgG induction not only promoted osteoclastogenesis but also increased SYK phosphorylation and bone resorption. SYK exacerbated osteoclast bone resorption by promoting Vav3 phosphorylation. These findings provide valuable insights for identifying novel therapeutic targets in RA.