Single-cell RNA-Seq reveals changes in immune landscape in post-traumatic osteoarthritis

Abstract

Osteoarthritis (OA) is the most common joint disease, affecting over 300 million people world-wide. Accumulating evidence attests to the important roles of the immune system in OA pathogenesis. Understanding the role of various immune cells in joint degeneration or joint repair after injury is vital for improving therapeutic strategies for treating OA. Post-traumatic osteoarthritis (PTOA) develops in ~50% of individuals who have experienced an articular trauma like an anterior cruciate ligament (ACL) rupture. Here, using the high resolution of single-cell RNA sequencing, we delineated the temporal dynamics of immune cell accumulation in the mouse knee joint after ACL rupture. Our study identified multiple immune cell types in the joint including neutrophils, monocytes, macrophages, B cells, T cells, NK cells and dendritic cells. Monocytes and macrophage populations showed the most dramatic changes after injury. Further characterization of monocytes and macrophages reveled 9 major subtypes with unique transcriptomics signatures, including a tissue resident Lyve1hiFolr2hi macrophage population and Trem2hiFcrls+ recruited macrophages, both showing enrichment for phagocytic genes and growth factors such as Igf1, Pdgfa and Pdgfc. We also identified several genes induced or repressed after ACL injury in a cell type-specific manner. This study provides new insight into PTOA-associated changes in the immune microenvironment and highlights macrophage subtypes that may play a role in joint repair after injury.

Transcriptomic changes in porcine articular cartilage one year following disruption of the anterior cruciate ligament

“…IGF-1 is known to have chondroprotective effects, such as stimulating neocartilage formation, promoting cell survival, and enhancing the integrity of ECM [ 44 , 45 ]. A recent posttraumatic murine model has shown that subpopulations of resident knee joint macrophages constitute a significant source of this IGF1 expression, and chondrocytes (expectedly) have high expression of IGF1R in the 3–7 days following compressive ACL rupture [ 46 ]. In the current porcine study, expression of IGF1 was elevated at 7 days, and it remained elevated at 4 and 52 weeks following ACL transection—suggesting that cartilage may work to maintain a chondroprotective effort even a year after injury.…”

A two-stage digestion of whole murine knee joints for single-cell RNA sequencing

“…Among these studies in different tissues, immune cells were between 5 and 45% of all cells. A recent study using digested mouse knees found that 78.4%–70.4% of cells were Cd-45 positive using flow cytometry, with neutrophils composing the majority of those cells [ 34 ]. Cd-45 positive high immune cell component of our whole joint samples may be explained by the presence of bone marrow, which is a primary hematopoietic organ.…”

The impact of omics research on our understanding of osteoarthritis and future treatments

“…This manuscript presents one example where integration of mouse data from this study and human data from the pain study above [24 ▪ ] will be powerful. Sebastian et al [26] used a similar posttraumatic model to characterize immune cells in the injured mouse knee joint. While they identified a large number of different immune cells in the knee, macrophages and monocytes underwent the most changes post injury.…”