Intravenous application of CD271-selected mesenchymal stem cells during fracture healing

Circulating mesenchymal stem cells (MSCs) participate in fracture healing and can be used to enhance fracture healing. This study investigated how CD271-selected MSCs travel in circulation and when is the optimal time to apply MSCs intravenously during fracture healing.

Based on the expression of CD271, MSCs were isolated from human bone marrow and labeled with cypate, a near-infrared fluorochrome. A unilateral closed fracture was created at the femur in immunodeficient mice. The cypate-labeled MSCs were injected into the tail vein of the mice at days 1 and 3 after fracture and were tracked by near-infrared imaging. The mice were euthanized at 3 weeks after fracture. Immunohistochemistry was performed to detect human MSCs at the fracture sites. Migration of CD271-selected MSCs, under the influence of stem cell-derived factor-1, was assessed in vitro.

Intravenously injected at day 1, but not day 3, after fracture, CD271-selected MSCs accumulated at the fracture sites significantly and lasted for at least 7 days. All fractures, with or without MSC injections, healed in 3 weeks. Human cells were localized at the fracture sites in mice by immunohistochemistry. CD271-selected MSCs migrated toward the medium contained stem cell-derived factor-1 in vitro.

After intravenous injection, CD271-selected MSCs were recruited to the fracture sites. The stages of fracture healing influenced the homing of culture-expanded MSCs. In mice, an optimal window of intravenous injection of MSCs was around 24 hours after fracture.

Intravenous application of MSCs may serve as a practical route to deliver stem cells for the treatment of fracture nonunion and delayed union.