2024 Theses Doctoral

The Inflammatory Response in Tendon Fibroblasts is Multi-Factorial and Alters Their Responses to Mechanical Stimulation

Sup, McKenzie

Tendon pathologies, including both chronic injuries and acute tendon tears, are some of the most common musculoskeletal injuries. Chronic tendon injury, or tendinopathy, occurs both in athletes and in the general population, and can interfere with quality of life and ability to work. Overuse of the tendon during exercise plays a role in up to 50% of injuries in athletes, and affects multiple parts of the body including the supraspinatus tendon in the shoulder, and the Achilles tendon in the ankle. Acute tears of tendons and ligaments, on the other hand, add substantially to the socioeconomic burden of tendon disease as a whole.

These injuries also affect both upper and lower extremities, including the shoulder, ankle, hand, and wrist. Historically, inflammatory processes have been thought to be of little importance in tendon pathology, due to the largely avascular nature of the healthy tissue. However, more recent literature has identified the presence of inflammation in both acute and chronic tendon injury. Because the literature on inflammation in tendon is in its relatively nascent stages, there remain gaps in knowledge that hinder progress in the development of therapeutics to improve healing. A more complete characterization of the inflammatory response in tendon is needed, by defining the relative roles of different molecular pathways, and determining how these pathways interact with tendon mechanobiology.

To investigate these questions, an in vitro model was developed, wherein the complexity of the in vivo healing environment was simulated by applying M1 macrophage conditioned media (M1-CM) to tendon fibroblasts (TFs). This was motivated by the well-established role of macrophages in driving tendon inflammatory responses. Characterization of the M1-CM and its effect on TFs revealed a robust inflammatory response, including upregulation of over 500 genes and increased secretion of several cytokines in TFs.

Next, multiple immune-related pathways were manipulated in TFs in order to identify those necessary for inflammatory responses. Both the NF-kB pathway and the JAK/STAT signaling pathway were inhibited, to determine their respective roles in propagating inflammation. It was determined that both JAK/STAT and NF-kB were necessary for the response to M1-CM, and each pathway was responsible for different downstream responses to inflammation in TFs. Finally, the role of mechanical loading in tendon responses to inflammation was assessed, as mechanical stimulation is crucial for proper tendon function in homeostasis and in healing. We found that the TF response to loading was altered by the presence of an inflammatory stimulus, with more genes being downregulated by loading under inflammatory conditions. Analysis of the genes that responded differently to loading with inflammation present suggested changes in pathways involving extracellular matrix organization and G protein signaling.

In summary, this work served to more completely characterize the tendon inflammatory response. The results of these studies indicate that inflammation in tendon is mediated by both NF-kB and JAK/STAT signaling. Additionally, the application of loading may serve to reduce ECM degradation processes, and calm the inflammatory response in tendon, without suppressing it entirely.