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.