Single cell level characterization of injury-induced immune changes in MRL/MpJ knee joints

Post traumatic osteoarthritis (PTOA) is a painful joint disease characterized by the degradation of bone, cartilage, and connective tissues. PTOA is initiated by trauma to joint-stabilizing tissues, such as the anterior cruciate ligament, medial meniscus or by intra-articular fractures. In humans, ~50% of joint injuries progress to chronic PTOA, while the rest spontaneously resolve. To understand molecular programs contributing to PTOA development, we examined injury-induced fluctuations in immune cell populations and transcriptional shifts by single-cell sequencing of synovial joints in PTOA -susceptible C57BL6/J (B6) and -resistant MRL/MpJ (MRL) mice. A potent myeloid driven anti-inflammatory response was discovered in MRL injured joints that significantly contrasted the pro-inflammatory signaling seen in B6 joints. We found Trem2+ macrophages classically described as M2 (CD206+Trem2+) to be enriched in the uninjured joints of MRL mice, and these populations were consistently elevated in this strain, post injury. To evaluate whether Trem2 is essential to macrophage-mediated homeostasis in the joint, we examined Trem2 deficient mice and determined that these mice develop spontaneous OA. Overall, these data suggest that the PTOA resistant MRL mouse strain displays enhanced capacity of clearing debris and apoptotic cells induced by inflammation after injury due to an increase in activated macrophages within the synovial tissue and joint space. Furthermore, we showed that Trem2 is essential for preserving cartilage homeostasis in B6, and we postulate that the elevated levels of Trem2+ joint-resident macrophages in MRL are in part responsible for the PTOA-resistant phenotype in this strain.