Iron nanoparticle-labeled mesenchymal stromal cells persist and demonstrate anti-inflammatory mechanism of action in an osteoarthritis mouse model

Abstract

Background & Aim Osteoarthritis (OA) is characterized by cartilage degradation and chronic joint inflammation. Mesenchymal stromal cells (MSCs) have shown promising results in OA, but their mechanism of action is not fully understood. We hypothesize that MSCs polarize macrophages (MΦ) to more homeostatic sub-types. We tracked ferumoxytol (iron-nanoparticle)-loaded MSCs in mouse osteoarthritic joints, and investigated mechanisms of action of injected MSCs via immunohistochemical staining of homeostatic MΦ populations. Methods, Results & Conclusion 10-week-old C57/BL6 male mice were induced to undergo OA by destabilization of medical meniscus (DMM) or SHAM surgery. 3 weeks post-surgery, mice were injected intra-articularly with saline or ferumoxytol-labeled murine MSCs (50 × 103 MSCs/mouse) to yield the following groups (n=5/group): i) DMM+MSC with ferumoxytol; ii) DMM+MSC without ferumoxytol iii) DMM+Saline; iv) SHAM+MSC; and v) SHAM+Saline. MRIs of mice were taken at 1, 3 days and 1, 2, or 4 weeks after injection. At 4 weeks post-injection, mice were scored for i) OARSI to determine cartilage damage and synovitis and ii) immunohistochemical changes in CD206, F/480, Prussian Blue/Sca-1, and cleaved caspase-3 to detect homeostatic MΦ, total MΦ, ferumoxytol-labeled MSCs, and apoptotic cells respectively. Ferumoxytol-loaded MSCs persisted to time of euthanization (4 weeks) at greater levels in DMM- vs. SHAM-joints. Prussian Blue/Sca-1 co-staining confirmed the iron label resides in MSCs (at 1, 2, and 4 weeks), although some iron label was also detected in F4/80+ MΦ, likely due to phagocytosis of apoptotic MSCs. MRI analyses demonstrated a significant decrease in joint swelling at day 3 and 4 week groups in MSCs vs saline treated animals. However, OARSI synovitis scores did not capture any differences between MSCs vs saline treated animals. In depth examination by immunohistochemical staining showed for the first time that MSCs vs saline treated had increased proportions of CD206+ homeostatic MΦ (p We demonstrate that ferumoxytol-MSC can be imaged in OA mice by MRI for 4 weeks, and these mice have augmented MΦ infiltration (p