Abstract: BACKGROUND: During the repair process following tendon injury, an excessive inflammatory response can cause tendon cell apoptosis, thereby leading to a reduction in the biomechanical properties of the tendon. Meanwhile, a persistent inflammatory response can also trigger tissue fibrosis and adhesion. Studies have confirmed that interleukin-10 exerts an inflammatory regulatory role in connective tissue cells such as fibroblasts and can block inflammatory responses produced in various models.
OBJECTIVE: To explore the effect of interleukin-10 against inflammatory responses following acute tendon injury. 
METHODS: Forty-two Sprague-Dawley rats were randomly divided into a normal group (n=6), model group (n=12), control group (n=12), and intervention group (n=12). Except for the normal group, the other three groups underwent acute Achilles tendon injury modeling via intra-tendinous injection of type I collagenase solution (the model was successfully established after 3 days). On the day of modeling, the control and intervention groups were subjected to daily injections of PBS and interleukin-10 protein solution, respectively, at the 1 cm points on both sides of the intersection between the line connecting the hind limbs and the midline of the abdomen, once daily for 4 consecutive days. Ultrasound examination of the Achilles tendon was performed on day 3 after successful modeling. Tissue samples were collected on days 3 and 7 after successful modeling. Hematoxylin-eosin staining was used to observe pathological changes in the Achilles tendon, while immunohistochemical staining was used to detect the expression of phosphorylated nuclear factor κB and tumor necrosis factor α proteins in the Achilles tendon. RT-PCR was used to detect mRNA expression of nuclear factor κB, tumor necrosis factor α, and cyclooxygenase-2 in Achilles tendon tissue. Western blot analysis was performed to detect protein expression of phosphorylated nuclear factor κB, tumor necrosis factor α, and cyclooxygenase-2 in Achilles tendon tissue.
RESULTS AND CONCLUSION: (1) Ultrasound examination revealed that in the model group, the Achilles tendon tissue had a blurred boundary and increased thickness, whereas in the intervention group, the Achilles tendon tissue exhibited a clearer boundary and tissue thickness close to that of the normal group. (2) Hematoxylin-eosin staining demonstrated that on day 3 after successful modeling, in the model and control groups, the collagen fibers of the Achilles tendon arranged irregularly, with extensive infiltration of inflammatory cells, and the nuclei exhibited a round, concentrated distribution. In the intervention group, the disorganization of collagen fiber alignment was alleviated, with minimal infiltration of inflammatory cells, and a higher number of long spindle-shaped tendon cells. On day 7 after successful modeling, the model and control groups showed significantly improved collagen fiber alignment and reduced inflammatory cell infiltration in the Achilles tendon. Tendon cells exhibited a transition from round to spindle-shaped morphology. Most nuclei were neatly arranged and evenly distributed. In the intervention group, the collagen fibers of the Achilles tendon were arranged in parallel and orderly, tending toward normal. Infiltration of inflammatory cells was further reduced, and tendon cells predominantly exhibited a long spindle shape. (3) Immunohistochemical staining showed that on days 3 and 7 after successful modeling, the protein expression levels of phosphorylated nuclear factor κB and tumor necrosis factor-α) in the Achilles tendon of the model group were higher than those in the normal group and the intervention group (P < 0.05). RT-PCR detection showed that on days 3 and 7 after successful modeling, the mRNA expression levels of phosphorylated nuclear factor κB, tumor necrosis factor-α, and cyclooxygenase-2 in the Achilles tendon of the model group were higher than those in the normal group and the intervention group (P < 0.05). (4) Western blot analysis indicated that on days 3 and 7 after successful modeling, the protein and mRNA expression levels of phosphorylated nuclear factor κB, tumor necrosis factor-α, and cyclooxygenase-2 in the Achilles tendon of the model group were higher than those in the normal group (P < 0.05). On day 3 after successful modeling, the intervention group exhibited lower protein expression of phosphorylated nuclear transcription factor κB, tumor necrosis factor α, and cyclooxygenase-2 in the Achilles tendon compared with the model group (P < 0.05). To conclude, administration of interleukin-10 can effectively attenuate inflammatory responses during the repair of acute tendon injury.

Key words: interleukin-10, acute tendon injury, inflammation, nuclear factor κB, tumor necrosis factor α, cyclooxygenase-2, tissue construction