Adaptive and Innate Immune Cell Responses in Tendons and Lymph Nodes After Tendon Injury and Repair

Tendon injuries are a common clinical condition with limited treatment options. The cellular components of the innate system such as neutrophils and macrophages, have been well studied in tendon injuries. However the adaptive immune system, comprised of specialized lymphocytes, plays an important role in orchestrating the healing of numerous tissues but less is known about these cells in tendon healing. To gain a greater understanding of the biological processes that regulate tendon healing, we sought to determine how the cellular components of the adaptive and innate immune system respond to a tendon injury using two-month old male mice. We determined that the lymphatic vasculature is present in the epitenon and superficial regions of Achilles tendons. We then created an acute Achilles tenotomy followed by repair, and collected tendons and lymph nodes one, two, and four weeks after injury. Using flow cytometry and histology, after tendon injury we observed a robust adaptive immune cell response that followed an initial innate immune cell response. There was an accumulation of monocytes, neutrophils, and macrophages one week after injury that declined thereafter. Dendritic cells and CD4+ T cells peaked two weeks after injury, while B cells and CD8+ T cells demonstrated a progressive increase over time. In parallel, immune cells of the draining popliteal lymph node demonstrated a similarly coordinated reaction to the injury. These results indicate that the cellular components of the adaptive immune cell population are recruited in response to tendon injury, and may play a role in regulating tendon healing.