Tendon healing in interleukin-4 and interleukin-6 knockout mice

Cytokines have been shown to play an important role in tendon and ligament healing by regulating cellular differentiation and activity. The majority of studies that have investigated the role of cytokines in tendon and ligament healing have added them to injured tissue and assessed their effect. Because the efficacy of exogenously applying cytokines is dependent upon many factors such as the correct dosage, timing, and frequency, conflicting results are often reported. To avoid these factors, this study used transgenic mice with knockouts of interleukin-4 (IL4 -/-) and interleukin-6 (IL6 -/-) to investigate their role in tendon healing. Because of the reported roles of both of these cytokines in inflammation and fibroplasia, it was hypothesized that the order of organizational, geometric, and mechanical properties would be (greatest to least) injured IL6 -/-, injured control, and injured IL4 -/- mice. In addition, it was hypothesized that specific cytokines would be upregulated in each knockout group, but not compensate for the lack of IL-4 or IL-6. Mechanical and organizational properties of injured tendons from IL6 -/- mice were inferior to that of control and IL4 -/- mice despite the upregulation of the pro-inflammatory cytokine TNF-alpha. Temporal levels of IL-10 and IL-13 in the IL4 -/- mice resulted in comparable and even superior properties when compared to CTL mice. This study shows that IL-6 could not be compensated for and plays an important role in tendon healing. This study also supports the use of this animal model to further investigate tendon healing.