The Impact of Myeloid Lineage Cells on Skeletal Homeostasis and Repair

Abstract

Bone is in a constant state of remodeling, a process which was once attributed solely to osteoblasts and osteoclasts. Decades of research has identified many other populations of cells in the bone that participate and mediate skeletal homeostasis. Recently, osteal macrophages emerged as vital participants in skeletal remodeling and osseous repair. The exact mechanistic roles of these tissue-resident macrophages are currently unknown. Macrophages are highly plastic in response to their micro-environment and are typically classified as being pro- or anti-inflammatory (pro-resolving) in nature. Given that inflammatory states result in decreased bone mass, proinflammatory macrophages may be negative regulators of bone turnover. Pro-resolving macrophages have been shown to release anabolic factors and present a potential target for therapeutic intervention in inflammation-induced bone loss and fracture healing. To better understand the role of macrophages in bone, an in vitro approach was used to study mechanisms of bone marrow macrophages. Additionally, macrophage functions in vivo were assessed in normal bone turnover and in oral osseous wound healing.

The process of apoptotic cell clearance, termed efferocytosis, is mediated by pro-resolving macrophages and may contribute to steady-state bone turnover as well as fracture healing. This process of efferocytosis by bone marrow macrophages was investigated in vitro. Interleukin-10, a pro-resolving cytokine, enhanced bone marrow macrophage efferocytosis of apoptotic bone marrow stromal cells (apBMSCs) in a phospho-STAT3 dependent manner. Additionally, macrophages engulfing apBMSCs secreted a unique profile of cytokines compared to macrophages engulfing apoptotic neutrophils. Macrophages displayed increased secretion of monocyte chemotactic protein 1/chemokine (C-C motif) ligand 2 (MCP-1/CCL2) and transforming growth factor beta 1 (TGF-β1) in response to apBMSCs. This secreted protein profile suggests efferocytosis of apoptotic bone cells signals to recruit new osteoblast progenitors to repopulate apoptotic cells and initiate a new round of bone formation.

Milk fat globule-EGF factor 8 (MFG-E8) is a secreted glycoprotein that facilitates the process of efferocytosis by acting as a bridge between apoptotic cells and phagocytes. The role of MFG-E8 in bone turnover was investigated. Mice deficient in MFG-E8 displayed decreased bone with age which correlated with an increased inflammatory phenotype and accumulations of apoptotic cells in the spleen. Increased inflammation leads to increased osteoclast differentiation and activation, resulting in decreased bone mass. MFG-E8 deficient mice displayed increased osteoclast numbers per bone surface. Intermittent daily parathyroid hormone (iPTH) administration is a known anabolic bone agent yet its use in patients is currently limited to cases of severe osteoporosis. MFG-E8 deficient mice responded to iPTH administration with a greater anabolic response than WT control mice. This suggests iPTH treatment may be beneficial in patients with inflammation-induced bone loss.

Ineffective oral wound healing is detrimental to patients’ oral health related quality of life. Delineating the cellular mechanisms involved in optimal healing will elicit better approaches to treating patients with compromised healing. To study the role of phagocytic myeloid cells on oral wound healing, phagocytes were depleted with clodronate-loaded liposomes at the time of tooth extraction in mice. Depletion of macrophages and osteoclasts via clodronate treatment increased extraction socket bone fill 14 days after surgery, suggesting a crucial balance of macrophages and osteoclasts is necessary to promote effective healing.

A better understanding of the exact mechanisms by which macrophages mediate bone homeostasis and healing will lead to an expansion of pharmacologic targets for the treatment of osteoporosis and inflammation-induced bone loss.