Given the number of rotator cuff (RC) repairs performed annually and the high rate of structural failure, there remains a significant clinical need for new approaches to augment the repair by enhancing the rate and quality of the tendon healing processes. Tissue-engineering approaches that combine the use of scaffolds and bioactive molecules represent promising new solutions for RC repair. In this study, we investigated the effect of the incorporation of two innate immune pattern recognition receptor agonists (PRRAs) into surgically implanted hydrogels on healing in vitro using ovine RC tendon tissues and in vivo in a translational rat model of RC injury. To address the impact of these innate immune agonists on shoulder healing, we assessed gait function, surgical site histopathology, and quantification of local immune cell infiltrates. We also treated tendon tissues in vitro to assess the impact on tendon transcriptomic responses. We hypothesized that early stimulation of innate immune responses at the site of tendon injury would improve functional and structural tendon healing. We found that of the three PRRAs evaluated, only polyinosine-polycytidylic acid [Poly(I:C)] improved functional gait quality in the postinjury period. However, PRRA injection exerted minimal effects on tendon histology or the density of immune infiltrates. In vitro transcriptomic analysis of tendon blocks treated with PRAAs provided evidence of activation of interferon pathways by Poly(I:C)-treated tissues, suggesting a role of these innate immune cytokines in the pain reduction response. Thus, we conclude that incorporation of certain PRRAs in hydrogels may improve functional recovery after shoulder tendon repair surgery, but also recognize that the timing and release kinetics of agonists delivered in gels at the surgery site can be further optimized. Impact Statement The immunological cascade of healing rotator cuff tissue is a large determinant of whether the tissue will heal or scar. Immunomodulation through biologics has shown mixed success in clinical applications for rotator cuff repair, perpetuating high retear rates. As such, there is a need to investigate novel, immunologically instructive therapies. Herein, we demonstrate that incorporating Toll-like receptor 3 agonist, polyinosine-polycytidylic acid, into a methylcellulose/hyaluronic acid blend hydrogel can induce functional, but interestingly, not tissue, level changes in a rat model of rotator cuff damage. Indicating initial efficacy for a novel potential immunotherapy for rotator cuff injury.
Keywords: immunomodulation; pattern recognition receptor agonists; rotator cuff; tendon healing.