Local Shifts in Inflammatory and Resolving Lipid Mediators in Response to Tendon Overuse

Tendon inflammation has been implicated in both adaptive connective tissue remodeling and overuse-induced tendinopathy. Lipid mediators control the initiation and resolution of inflammation, but their roles within tendon are largely unknown. Here we profiled local shifts in intratendinous lipid mediators via liquid chromatography-tandem mass spectrometry in response to synergist ablation-induced plantaris tendon overuse. Sixty-four individual lipid mediators were detected in homogenates of habitually loaded plantaris tendons from healthy ambulatory rats. This included many bioactive metabolites of the cyclooxygenase (COX), lipoxygenase (LOX), and epoxygenase (CYP) pathways. Synergist ablation induced a robust inflammatory response at day 3 post-surgery characterized by epitenon infiltration of polymorphonuclear leukocytes (PMNs) and macrophages (M{Phi}), heightened expression of inflammation-related genes, and increased intratendinous concentrations of the pro-inflammatory eicosanoids thromboxane B2 (TXB2) and prostaglandin E2 (PGE2). By day 7, M{Phi} became the predominant myeloid cell type in tendon and there were further delayed increases in other COX metabolites including PGD2, PGF2 and PGI2. Specialized pro-resolving mediators (SPMs) including protectin D1 (PD1) and resolvin D6 (RvD6), as well as related pathway markers of D-resolvins (17-HDoHE), E-resolvins (18-HEPE) and lipoxins (15-HETE) were also increased locally in response to tendon overuse, as were many anti-inflammatory fatty acid epoxides of the CYP pathway (e.g. EpETrEs). Nevertheless, intratendinous prostaglandins remained markedly increased even following 28 days of tendon overuse together with a lingering M{Phi} presence. These data reveal a delayed and prolonged local inflammatory response to tendon overuse characterized by an overwhelming predominance of pro-inflammatory eicosanoids and a relative lack of pro-resolving lipid mediators.