Bone fracture is a common event, with over 170 million fractures per year globally (1). An estimated 2% to 10% of these fractures go onto nonunion, which is defined by the FDA as a fracture that persists for more than nine months without signs of healing progress for three months, thus lacking the potential to heal without further intervention (2). Risk factors for nonunion are multifactorial and depend on the bone affected, but generally include the severity of the injury, impaired blood supply, age, body mass index, diabetes, osteoarthritis, autoimmune disease, HIV, cigarette smoking, excessive alcohol use, and medications such as opioids, nonsteroidal antiinflammatory drugs, antibiotics such as fluoroquinolones, and anticoagulants such as warfarin (3). Treatment for nonunion often requires surgical intervention with addition or replacement of internal fixation (4). Additional biologic stimulation with bone morphogenic protein (BMP) is approved for long bone nonunion, and systemic parathyroid hormone therapy (teriparatide) may also be used (2). Despite these therapies, nonunion is associated with reduced health status due to greater physical impairment and pain, results in increased unemployment rates as much as a year after the fracture, and more than doubles the cost of total care (5). Thus, new therapeutic approaches to promote fracture repair are still needed.

Fracture healing is a complex process, characterized by hematoma formation and an early inflammatory phase, a repair phase with cartilaginous callus and bony callus formation, and remodeling (6). In the last decade, the role of immune cells in orchestrating this complex process has been increasingly appreciated. Within minutes after a fracture occurs, platelets and innate immune cells, such as neutrophils and macrophages, populate the injury and produce proinflammatory cytokines and chemokines, while in later stages, neutrophils and macrophages remove cellular and tissue debris and acquire an antiinflammatory phenotype (6). Highlighting the roles of these innate cells, depletion studies of either macrophages or neutrophils in mouse models of fractures results in higher rates of nonunion (7, 8). In the later stages of the inflammatory phase, adaptive T cells arrive at the fracture site and regulate osteoblast-osteoclast equilibrium by secretion of cytokines, such as IFN-γ, TNF-α, and receptor activator of nuclear factor-κβ ligand (RANKL) (6). Regulatory T cells (Tregs), characterized by the expression of the transcription factor Foxp3, have also been observed at fracture sites, and decreased Tregs have been observed in patients with nonhealing fractures (9). Recently, punctual deletion of Tregs using the anti-CD25 antibody has been shown to compromise fracture healing with increased fracture gap and lower callus volume, but the mechanism by which Tregs promote healing has not been fully elucidated (10).