Allogeneic Platelet-Rich Gel Supernatant Reprograms the Cytokine and Growth Factor Microenvironment in an Equine In Vitro Inflammatory Tendon System.

Tendinopathy involves a dysregulated inflammatory microenvironment in which cytokines, growth factors (GF) and extracellular matrix components interact dynamically. Platelet-rich plasma (PRP) is widely used as a regenerative therapy, but its mechanisms of action in inflamed tendon remain unclear. This study evaluated whether platelet-rich gel supernatant (PRGS) reprograms the inflammatory-anabolic mediator network in an equine in vitro tendon explant system stimulated with lipopolysaccharide (LPS). Tendon explants were cultured under six experimental conditions, including non-stimulated control, LPS-stimulated control, and LPS combined with 25% or 50% PRGS or platelet-poor gel supernatant (PPGS). Cytokines, GF, and hyaluronic acid (HA) were quantified at 1 h and 48 h and analyzed using linear mixed-effects models, mediator ratios, correlation networks, and principal component analysis. PRGS contained higher concentrations of PDGF-BB (2044 pg/mL, 95% CI 1382-2706) and IL-1ra (1196 pg/mL, 95% CI 424-1967) compared with PPGS. In LPS-stimulated explants, PRGS significantly increased IL-1ra and PDGF-BB, while IL-1β and HA exhibited significant time-dependent changes (F = 8.675 and F = 10.752, respectively). The PDGF-BB:HA ratio remained consistently higher in PRGS-treated groups (F = 46.100, p < 0.001). Multivariate analysis showed that the first two principal components explained 62% of the total variance and revealed coordinated shifts in mediator organization over time. These findings indicate that PRGS does not simply suppress inflammation but actively reprograms the tendon microenvironment toward a regulatory and reparative phenotype within this experimental system.