Functional and biochemical inflammatory responses to a 75 ng intra-articular recombinant equine interleukin-1β (reIL-1β) synovitis model.

Recently, a synovitis model using 75 ng intra-articular recombinant equine interleukin-1 (reIL 1β), showed promise for inducing a mild, self-limiting inflammatory response without overtly altering upper-body movement asymmetry in n = 3 horses. This study aimed to evaluate the effect of this 75 ng reIL-1β model on gait biomechanics and synovial fluid (SF) inflammatory, pro-resolution, and cartilage turnover biomarkers in a larger equine sample. SF samples were collected from the left intercarpal joint of n = 11 horses at baseline and at 6- and 12-hours following 75 ng reIL-1β injection and were analyzed for prostaglandin E₂ (PGE₂), nitric oxide (NO), glycosaminoglycan (GAG), chondroitin sulfate 846, and resolvin D1. Prior to each arthrocentesis, horses underwent subjective (AAEP) and objective (inertial measurement unit (IMU)) gait analysis during in-hand trot trials. Upper-body movement asymmetry parameters (MinDiff and MaxDiff) were calculated from poll and pelvis IMU data. Linear mixed models and Spearman correlation coefficient compared biomarker concentration and asymmetry parameters between timepoints. At 6-hours, PGE₂ and NO concentrations and pelvis MinDiff increased significantly compared to baseline (p < 0.05). At 12-hours, GAG concentration significantly decreased and poll and pelvis MinDiff significantly increased compared to baseline (p < 0.05). Importantly, changes in poll and pelvis movement asymmetry (<7 and ±1.2 mm, respectively) fell within thresholds associated with between-session variability, and changes in intra-articular PGE₂ and NO concentrations were comparable to those observed following mild exercise. These findings provide further evidence to support the use of this ethically acceptable 75 ng reIL-1β model for inducing a low-grade, transient inflammatory response, without overtly altering movement.