Modulation of Osteoblastic Cell Efferocytosis by Bone Marrow Macrophages

Abstract

Apoptosis occurs at an extraordinary rate in the human body and the effective clearance of dead cells (efferocytosis) is necessary to maintain homeostasis and promote healing, yet the contribution and impact of this process in bone is unclear. Bone formation requires that bone marrow stromal cells (BMSCs) differentiate into osteoblasts which direct matrix formation and either become osteocytes, bone lining cells, or undergo apoptosis. A series of experiments were performed to identify the regulators and consequences of macrophage efferocytosis of apoptotic BMSCs (apBMSCs). Bone marrow derived macrophages treated with the anti-inflammatory cytokine interleukin-10 (IL-10) exhibited increased efferocytosis of apBMSCs compared to vehicle treated macrophages. Additionally, IL-10 increased anti-inflammatory M2-like macrophages (CD206+), and further enhanced efferocytosis within the CD206+ population. Stattic, an inhibitor of STAT3 phosphorylation, reduced the IL-10-mediated shift in M2 macrophage polarization and diminished IL-10-directed efferocytosis of apBMSCs by macrophages implicating the STAT3 signaling pathway. Cell culture supernatants and RNA from macrophages co-cultured with apoptotic bone cells showed increased secretion of monocyte chemotactic protein 1/chemokine (C-C motif) ligand 2 (MCP-1/CCL2) and transforming growth factor beta 1 (TGF-β1) and increased ccl2 gene expression. In conclusion, IL-10 increases M2 macrophage polarization and enhances macrophage-mediated engulfment of apBMSCs in a STAT3 phosphorylation-dependent manner. After engulfment of apoptotic bone cells, macrophages secrete TGF-β1 and MCP-1/CCL2, factors which fuel the remodeling process. A better understanding of the role of macrophage efferocytosis as it relates to normal and abnormal bone turnover will provide vital information for future therapeutic approaches to treat bone related diseases.

A Pronounced Inflammatory Activity Characterizes the Early Fracture Healing Phase in Immunologically Restricted Patients

“…Mesenchymal stem cells are essential for fracture healing [27]. They migrate to the fracture site and are important progenitors of osteoblasts and bone lining cells [28]. However, they are also assumed to contribute to the termination of the initial inflammation in fracture healing via their immunomodulatory properties such as the secretion of IL-10 [29].…”

Apoptosis-induced CXCL5 accelerates inflammation and growth of prostate tumor metastases in bone

“…Immunofluorescence and flow cytometric studies show that bone marrow-derived macrophages (MΦs) effectively efferocytose apoptotic cells (12,14). We hypothesized that macrophages discriminate between different types of apoptotic cells and orchestrate a distinctive response accordingly.…”

Macrophages and skeletal health

“…M2-like macrophages are present during the resolution phase of inflammation and are responsible for anti-inflammatory cytokine production and enhanced clearance of apoptotic cells, termed efferocytosis (Bystrom, et al, 2008; deCathelineau & Henson, 2003; Xu, et al, 2006). Exposure to anti-inflammatory cytokines IL-4, IL-12 and IL-10 leads to M2-like macrophage polarization (Mantovani, et al, 2004), and increased apoptotic cell clearance (Lingnau, et al, 2007; Michalski, et al, 2016; Ogden, et al, 2005). …”