Single Cell RNA Sequencing of Synovial Tissue in Adolescents Undergoing ACL Reconstruction

Background: Loss of motion and arthrofibrosis after anterior cruciate ligament reconstruction (ACLR) can be a devastating complication for athletes. The cellular and molecular pathogenesis of arthrofibrosis is poorly understood, limiting prevention and treatment options. Synovial inflammation may contribute to post-ACLR arthrofibrosis. Hypothesis/Purpose: We hypothesized that higher synovial immune cell infiltration and inflammatory/catabolic gene expression patterns at the time of ACLR would correlate with poorer motion-related outcomes. Study Design: Case Series Methods: Patients aged 10-18 undergoing primary ACLR were enrolled in a prospective pilot study, and synovial tissue biopsies were obtained during ACLR. Flow cytometry and single cell RNA-sequencing explored synovial cell types/frequencies and gene expression. Principle component analysis followed by clustering grouped patients into distinct immunophenotypes based on their synovial cell composition. Clinical follow-up with knee range of motion (ROM), need for lysis of adhesions, and patient reported outcome measures were collected and compared between immunophenotypes. Results: Enrolled patients (n = 17) underwent ACLR at a median of 37 days post-injury. Analysis revealed three distinct immunophenotypes. Type 1 comprised patients with the longest time between injury and surgery and the lowest hematopoietic and T cell infiltration. Types 2 and 3 had similar times between injury and surgery, and Type 2 had intermediate while Type 3 had the highest hematopoietic and T cell percentages. Type 3 was associated with worse ROM at 2- and 6-weeks post-op; T cell prevalence and ROM were inversely correlated at those time points. The only patient requiring lysis of adhesions for arthrofibrosis was in Type 3. Conclusion: Synovial immune infiltration after ACL injury shows variability between patients that clusters into three immunophenotypes correlating with early ROM and the risk of arthrofibrosis. T cell recruitment and infiltration was the strongest factor correlated with ROM outcomes and presents an exciting venue for future research on post-ACLR arthrofibrosis. Synovial tissue biopsies were obtained during ACL reconstruction from six patients, enzymatically digested to obtain heterogenous single-cell suspensions, and analyzed by scRNA-seq. The six scRNA-seq samples as well as eleven other patient samples were categorized into three Immunotypes based on flow cytometry quantification of the immune cells present in the synovium tissue. The scRNA-seq output was analyzed for a more comprehensive characterization of the cell types present as well as the relative expression of select inflammatory markers and immunomodulatory genes.