Fracture healing is accelerated in the absence of the adaptive immune system

Abstract

Fracture healing is a unique biologic process starting with an initial inflammatory response. As in other regenerative processes, bone and the immune system interact closely during fracture healing. This project was aimed at further elucidating how the host immune system participates in fracture healing. A standard closed femoral fracture was created in wild-type (WT) and recombination activating gene 1 knockout (RAG1 À/À ) mice lacking the adaptive immune system. Healing was investigated using micro-computed tomography (mCT), biomechanical testing, and histologic and mRNA expression analyses. Biomechanical testing demonstrated a significantly higher torsional moment on days 14 and 21 in the RAG1 À/À mice compared to the WT group. mCT evaluation of RAG1 À/À specimens showed earlier mineralization and remodeling. Histologically, endochondral ossification and remodeling were accelerated in the RAG1 À/À compared with the WT mice. Histomorphometric analysis on day 7 showed a significantly higher fraction of bone and a significantly lower fraction of cartilage in the callus of the RAG1 À/À mice than in the WT mice. Endochondral ossification was accelerated in the RAG1 À/À mice. Lymphocytes were present during the physiologic repair process, with high numbers in the hematoma on day 3 and during formation of the hard callus on day 14 in the WT mice. Expression of inflammatory cytokines was reduced in the RAG1 À/À mice. In contrast, expression of anti-inflammatory interleukin 10 (IL-10) was strongly upregulated in RAG1 À/À mice, indicating protective effects. This study revealed an unexpected phenotype of enhanced fracture healing in RAG1 À/À mice, suggesting detrimental functions of lymphocytes on fracture healing. The shift from proinflammatory to anti-inflammatory cytokines suggests that immunomodulatory intervention strategies that maximise the regenerative and minimize the destructive effects of inflammation may lead to enhanced fracture repair. ß

Osteal macrophages promote in vivo intramembranous bone healing in a mouse tibial injury model

“…Taken together, our data demonstrate for the first time that osteomacs are an integral cellular component of osteal tissues during bone healing and implicate osteomacs as critical participants in intramembranous ossification during bone repair. The identification of this novel osteoimmunologic mechanism expands our understanding of the substantial contributions the immune system has on bone/callus outcomes during fracture repair (59,60) and provides a new research avenue for manipulating bone formation for therapeutic applications. Identification of osteomac-derived anabolic factors ultimately may lead to new approaches to improve fracture healing and reverse skeletal damage after pathologic bone loss.…”

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

“…Interestingly, the uncontrolled stimulation of B and T lymphocytes appear to be important in delayed bone healing. RAG -/-mice, which lack T and B cells, show an enhancement of callus mineralisation and bone remodelling and acceleration of the fracture healing (42). These effects were associated with a lower level of pro-inflammatory cytokines, TNF-, IFN-, IL-2 and IL-4 at local fracture tissues (42).…”

“…RAG -/-mice, which lack T and B cells, show an enhancement of callus mineralisation and bone remodelling and acceleration of the fracture healing (42). These effects were associated with a lower level of pro-inflammatory cytokines, TNF-, IFN-, IL-2 and IL-4 at local fracture tissues (42). Activated T lymphocytes are known to release RANKL, which enhance osteoclast differentiation from their progenitors and consequently induce the bone lysis (156).…”

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

“…Additionally, Cho et al demonstrated that resident macrophages (osteal) are significantly involved in parathyroid hormone-dependent bone healing [15]. Although there are no experimental models for NK cell depletion in factures, an important role of NK cells during bone repair has been implied when a high level of interferon-gamma (IFN-g) was detected in the diaphyseal regions of fractured femur in mice lacking T-and B-lymphocytes [16]. Conversely, as shown in immune-compromised animal model, bone marrow (BM) transplantation greatly enhanced the process of bone healing [17].…”