Multiple roles for CCR2 during fracture healing

Abstract

SUMMARYBone injury induces an inflammatory response that involves neutrophils, macrophages and other inflammatory cells. The recruitment of inflammatory cells to sites of injury occurs in response to specific signaling pathways. The CC chemokine receptor type 2 (CCR2) is crucial for recruiting macrophages, as well as regulating osteoclast function. In this study, we examined fracture healing in Ccr2-/-mice. We first demonstrated that the expression of Ccr2 transcripts and the filtration of macrophages into fracture calluses were most robust during the early phases of fracture healing. We then determined that the number of macrophages at the fracture site was significantly lower in Ccr2-/-mice compared with wild-type controls at 3 days after injury. As a result, impaired vascularization, decreased formation of callus, and delayed maturation of cartilage were observed at 7 days after injury in mutant mice. At day 14, Ccr2-/-mice had less bone in their calluses. At day 21, Ccr2-/-mice had larger calluses and more bone compared with wild-type mice, suggesting a delayed remodeling. In addition, we examined the effect of Ccr2 mutation on osteoclasts. We found that a lack of Ccr2 did not affect the number of osteoclasts within fracture calluses at 21 days after injury. However, Ccr2-/-osteoclasts exhibited a decreased ability to resorb bone compared with wild-type cells, which could contribute to the delayed remodeling of fracture calluses observed in Ccr2-/-mice. Collectively, these results indicate that a deficiency of Ccr2 reduces the infiltration of macrophages and impairs the function of osteoclasts, leading to delayed fracture healing.

Unraveling macrophage contributions to bone repair

“…32 In a mouse model of fracture, interleukin-1 and TNF were localized predominately to macrophages and inflammatory cells in the marrow and periosteum adjacent to fracture sites. 31 In mouse models of fracture in which recruitment of inflammatory macrophages 28 or inflammatory cytokine signaling [33][34][35] was compromised through germline genetic alterations, there were prolonged negative impacts on fracture healing. These data support the theory that either reduction of inflammatory macrophages (without specific assessment of resident macrophages) or diminution of their inflammatory cytokine output from the time of injury compromises fracture healing.…”

“…Altered endochondral callus formation has been reported as a result of manipulation of macrophages and or macrophage-expressed molecules, but the experimental designs did not specifically target these molecules during the anabolic phase. 28,29,34,45 Therefore, altered inflammatory progression/resolution cannot be ruled out as a contributing mechanism to the observed compromised fracture repair. Carefully timed manipulation of macrophages and/or their products is required to elucidate the pro-anabolic contributions of macrophages to bone repair via endochondral ossification.…”

The roles of immune cells in bone healing; what we know, do not know and future perspectives

“…Macrophages participate in the induction of angiogenesis and a substantial reduction in macrophages is associated with impaired vascularisation and delayed formation of callus as revealed in CCR2 À/À mice model [73]. Bone-lining macrophages participate in the intramembranous bone healing as shown in mouse model of tibial fracture [74].…”

Interactions Between Multipotential Stromal Cells (MSCs) and Immune Cells During Bone Healing

“…Macrophages are recruited into the fracture site from the periosteum or peripheral blood (31,32). Macrophage-colony stimulating factor (M-CSF), which is known to target mainly macrophages (33), is expressed in the fractured bone, peaking during inflammatory phase then again when remodelling phase starts (34) indicating an involvement of the macrophages during all phases of the repair.…”

“…Macrophages are known to be involved in the intramembranous ossification during the repair phase (31,32). Macrophages also participate in the bone repair via the induction of angiogenesis.…”