Effects of Type I Interferon Signaling and JAK Kinase Inhibition in a Mouse ACL Reconstruction Model

Improving anterior cruciate ligament reconstruction (ACL-R) healing remains a clinical challenge. Macrophages play a crucial role in musculoskeletal tissue injury repair. However, the mechanism by which macrophages modulate the healing after ACL-R remains incompletely characterized. Macrophages with an interferon gene expression signature gradually accumulate at the bone-to-tendon interface during healing. This study investigated the effects of interferon signaling on ACL-R healing using type I interferon receptor 1 (IFNAR1)-deficient mice and an FDA-approved JAK inhibitor baricitinib. Sixteen 12-week-old wildtype (WT) and Ifnar1^-/- mice underwent ACL-R surgery. Additionally, twenty-two 12-week-old WT mice underwent ACL-R and were treated with either baricitinib (10 mg/kg q24h p.o.) or vehicle control. The primary outcome measure was new bone formation in the bone tunnel, measured by μCT at 2 weeks post-operatively. Secondary outcomes included knee histology, radiographs, and gait analysis. Results showed that new bone formation occurred in control conditions, and blocking interferon response via IFNAR1 deletion or baricitinib had minimal impact on this process. Histological analysis confirmed no significant differences in bone-to-tendon healing. Additionally, IFNAR1 deficiency or baricitinib treatment did not significantly affect post-traumatic osteoarthritis (PTOA) development or gait. These findings suggest that interferon signaling does not play a major role in mediating the effects of macrophages on the bone-to-tendon healing processes after ACL-R. Modulating interferon responses is unlikely to have a significant impact on healing outcomes in individuals undergoing ACL-R surgery. This study provides insights into macrophage-related healing mechanisms in ACL-R and suggests that alternative pathways may be more relevant therapeutic targets.