Biological responses of ligament and tendon graft-derived fibroblasts to clinical levels of cyclic strain

Abstract

Anterior Cruciate Ligament (ACL) is one of the major ligaments in the knee. ACL injury is the partial or complete tear of the ACL usually as a consequence of a traumatic sports related injury. ACL injury commonly affects athletes in the late adolescence and are usually more common in females than males. Factors such as metabolic response, cellular crosstalk among tissues and sex differences are not fully understood. Therefore, an increased comprehension of the biology of ACL injury and healing is required. Management of ACL injuries is typically achieved by surgical stabilization of the knee. For this, arthroscopic ACL reconstruction with different tendon autografts is the gold standard technique. Additionally, ligament repair by suturing the torn ends of the ACL is an alternative procedure. Although current surgical treatment, is generally successful, ACL graft and repair healing biology needs to be further explored to decrease failure rates. Moreover, whether a specific activity level would in turn affect the healing process is still not known. Several in vitro and in vivo preclinical models have been developed to explore current ACL injury and healing biology. Among these, large animals offer the advantage to be similar to the human. The preferred model is the spontaneous onset of ACL injury, commonly found in dogs. In both species, extracellular matrix (ECM) remodeling driven by cellular responses to load can influence disease, as well as response to treatment. However, most in vitro models do not include the multiple cell types in the joint or the mechanical environment that modulates their responses. Consequently, we aimed to evaluate the metabolic responses of fibroblasts obtained from canine intraarticular tissues potentially involved in ACL injury as well as tendon grafts commonly used for ACL reconstruction subjected to varying levels of mechanical stress in a monolayer culture. We hypothesized that fibroblasts from these tissues would produce significantly different levels of inflammatory and remodeling biomarkers in response to a spectrum of cyclic strains in vitro. We were able to demonstrate that ACL surrounding tissues are an important source of inflammatory and degradative biomarkers, and that metabolic responses of tendon graft fibroblasts are strain and tissue dependent. Moreover, male canine tendon fibroblasts seem to increase degradative responses while females tend to increase inflammatory responses when stimulated with different strain magnitudes. Also, synoviocytes are an important source of inflammation and human intraarticular tissue fibroblasts and tendon graft-derived fibroblasts have variable responses to stress but is not differentially affected by sex. Consequently, further studies are required to elucidate complex regulation mechanisms in both canines and humans.