Osteal Tissue Macrophages Are Intercalated throughout Human and Mouse Bone Lining Tissues and Regulate Osteoblast Function In Vitro and In Vivo

Abstract

Resident macrophages are an integral component of many tissues and are important in homeostasis and repair. This study examines the contribution of resident tissue macrophages to bone physiology. Using immunohistochemistry, we showed that a discrete population of resident macrophages, OsteoMacs, was intercalated throughout murine and human osteal tissues. OsteoMacs were distributed among other bone lining cells within both endosteum and periosteum. Furthermore, OsteoMacs were coisolated with osteoblasts in murine bone explant and calvarial preparations. OsteoMacs made up 15.9% of calvarial preparations and persisted throughout standard osteoblast differentiation cultures. Contrary to previous studies, we showed that it was OsteoMacs and not osteoblasts within these preparations that responded to pathophysiological concentrations of LPS by secreting TNF. Removal of OsteoMacs from calvarial cultures significantly decreased osteocalcin mRNA induction and osteoblast mineralization in vitro. In a Transwell coculture system of enriched osteoblasts and macrophages, we demonstrated that macrophages were required for efficient osteoblast mineralization in response to the physiological remodeling stimulus, elevated extracellular calcium. Notably, OsteoMacs were closely associated with areas of bone modeling in situ, forming a distinctive canopy structure covering >75% of mature osteoblasts on diaphyseal endosteal surfaces in young growing mice. Depletion of OsteoMacs in vivo using the macrophage-Fas-induced apoptosis (MAFIA) mouse caused complete loss of osteoblast bone-forming surface at this modeling site. Overall, we have demonstrated that OsteoMacs are an integral component of bone tissues and play a novel role in bone homeostasis through regulating osteoblast function. These observations implicate OsteoMacs, in addition to osteoclasts and osteoblasts, as principal participants in bone dynamics.

Porous magnesium-based scaffolds for tissue engineering

“…Activated macrophages secrete BMP2 and transforming growth factor b (TGF-b). It has been shown that the discharging macrophages result in the inhibition of in vivo bone formation [65][66][67][68]. Thereby, magnesium scaffolds coated with -TCP showed desired osteogenesis over osteoclastogenesis, due to the upregulation of osteoinductive molecules secreted by macrophages in contact with -TCP coating [63].…”

Hematopoietic stem cell mobilizing agents G-CSF, cyclophosphamide or AMD3100 have distinct mechanisms of action on bone marrow HSC niches and bone formation

“…Immunohistochemistry was used to examine expression of F4/80 (rat anti-mouse F4/80, clone CI:A3-1, AbD Serotec, Oxford, UK) and osteocalcin (rabbit anti-mouse osteocalcin, Alexis Biochemicals, San Diego, CA, USA) as previously described. 13,23 Flow cytometry…”

“…11,13,14 Once these endosteal niches are shutdown, HSC mobilization is initiated and possibly further amplified by protease-dependant mechanisms 13 involving neutrophil proteases, 15 CD26, 16 activation of the complement cascade 17 --19 and the thrombolytic pathway. 20 It has recently been reported that specific populations of BM macrophages are necessary to maintain the function of HSC niches in the BM and for the mobilizing effect of G-CSF 13,21,22 in particular osteoblast-supportive endosteal phagocytic macrophages called osteomacs 13,23 and CD11b þ F4/80 þ Ly-6G þ phagocytic macrophages expressing CD169, which are in close association with MSC. 13,21 Whether these two macrophage populations are distinct according to niche location or functionally overlap remains unknown.…”

Unraveling macrophage contributions to bone repair

“…They were present in resting osteal tissues and were increased at sites undergoing active bone anabolism. [12][13][14] Osteomacs have pivotal roles in bone and BM homeostasis through the support of osteoblast maintenance and functional activity. 13,15 In mice, osteomacs expressed numerous myeloid lineage markers including but not limited to F4/80, CD115, Mac-3 and CD68 ( Figure 2 and refs 12,13 and unpublished data).…”