The Human ACL Sheath Exhibits Distinct Transcriptomic Signatures and Functional Properties

The anterior cruciate ligament (ACL) has poor intrinsic healing capacity due to exposure to the intra-articular synovial environment of the knee. Therefore, tendon auto- or allograft reconstruction is the mainstay of treatment for a torn ACL. Following reconstruction, there is a high rate of post-traumatic osteoarthritis, likely due to alteration of joint biomechanics, which supports the clinical need to advance primary ACL repair strategies. The ACL is enclosed by a vascularized, synovial-like sheath (ACL-s), which supplies nutrients to the central core (ACL-c). The ACL-s may act as a selective barrier to solute diffusion, protecting the ACL-c from damaging synovial enzymes and/or hyaluronic acid, while allowing nutrient diffusion. Like the synovium, the ACL-s is rich with collagens and resident macrophages, thus may be susceptible to change of function following injury and inflammation. We investigated the healthy and inflammed ACL-s structure, diffusivity, and cellular profile in comparison to the ACL-c and synovium to guide the development and optimization of primary repair technologies. Diffusion and histologic assessments revealed the ACL-s functions as a semi-permeable barrier through lower solute diffusivity and unique collagen organization. RNAseq demonstrated differential expression of extracellular matrix, inflammatory, and immune-related genes across ACL-s, ACL-c, and synovium, with the ACL-s uniquely showing upregulated expression of CX3CR1 and CLDN5 suggestive of an immunologic barrier. This study establishes the potential of the ACL-s to serve an immunologic and functional barrier that protects the underlying ACL-c from the inflammatory milieu following injury, and therefore, should be recapitulated when developing primary ACL repair technologies.