Inflammatory factors released by ACL reconstruction tendon grafts and their potential for macrophage polarization

Anterior cruciate ligament (ACL) rupture requires reconstruction (ACLR), often using tendon autografts, to restore joint stability. However, postoperative issues persist, especially in younger patients, with the primary cause still unclear. These issues are believed to result from a detrimental in vivo remodeling response, characterized by macrophage-involving inflammation that is driven by dead and dying graft resident cells. This research aimed to (1) analyze factors secreted by different tendon grafts, (2) investigate how these factors influence macrophage polarization, and (3) explore the influence of patients' age on these secreted factors. An ex vivo set-up was used to provide new insights into the role of tendon grafts in the inflammatory response observed after ACLR in a patient-specific way. It was found that the dying cells in fresh autografts released a mixture of pro- and anti-inflammatory cytokines and matrix metalloproteinases. This release was reduced in grafts where the cells were already dead (frozen) or in grafts from which the cells were removed (decellularized). The release of pro-inflammatory cytokines by fresh grafts may contribute to the inflammatory response after ACLR. However, secreted factors did not polarize (THP-1) macrophages in our in vitro setup. A key finding of this study was the observed patient-specific response, which was not age-related in the used cohort, which emphasizes the need for personalized rehabilitation. In the future, correlating these findings with clinical patients' outcomes can enhance understanding of patient-specific recovery and aid in finding a biomarker for personalized rehabilitation.