Osteal macrophages (osteomacs) contribute to bone homeostasis and regeneration. To further distinguish their functions from osteoclasts, which share many markers and growth factor requirements, we developed a rapid, enzyme-free osteomac enrichment protocol that permitted characterization of minimally manipulated osteomacs by flow cytometry. Osteomacs differ from osteoclasts in expression of Siglec1 (CD169). This distinction was confirmed using the CD169-diphtheria toxin (DT) receptor (DTR) knock-in model. DT treatment of naïve CD169-DTR mice resulted in selective and striking loss of osteomacs, whilst osteoclasts and trabecular bone area were unaffected. Consistent with a previously-reported trophic interaction, osteomac loss was accompanied by a concomitant and proportionately striking reduction in osteoblasts. The impact of CD169+ macrophage depletion was assessed in two models of bone injury that heal via either intramembranous (tibial injury) or endochondral (internally-plated femoral fracture model) ossification. In both models, CD169+ macrophage, including osteomac depletion compromised bone repair. Importantly, DT treatment in CD169-DTR mice did not affect osteoclast frequency in either model. In the femoral fracture model, the magnitude of callus formation correlated with the number of F4/80+ macrophages that persisted within the callus. Overall these observations provide compelling support that CD169+ osteomacs, independent of osteoclasts, provide vital pro-anabolic support to osteoblasts during both bone homeostasis and repair.

中文翻译：

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CD169 +巨噬细胞对于成骨细胞的维持至关重要，并在骨修复过程中促进膜内和软骨内骨化。

骨巨噬细胞（osteomacs）有助于骨骼的稳态和再生。为了进一步区分破骨细胞的功能，破骨细胞具有许多标志物和生长因子的要求，我们开发了一种快速，无酶的富骨蛋白富集方案，该方案可通过流式细胞术表征最少操作的破骨细胞。成骨细胞与破骨细胞在Siglec1（CD169）的表达上有所不同。使用CD169-白喉毒素（DT）受体（DTR）敲入模型证实了这种区别。初次接受CD169-DTR小鼠的DT治疗导致选择性的和惊人的破骨细胞损失，而破骨细胞和小梁骨区域不受影响。与先前报道的营养相互作用一致，成骨细胞的丧失伴随着成骨细胞的减少并成比例地显着减少。在两种骨损伤模型中评估了CD169 +巨噬细胞耗竭的影响，这些模型可以通过膜内（胫骨损伤）或软骨内（骨覆盖的股骨骨折模型）骨化来治愈。在这两种模型中，CD169 +巨噬细胞（包括骨肉瘤耗竭）都会损害骨修复。重要的是，在两种模型中，CD169-DTR小鼠中的DT治疗均不影响破骨细胞频率。在股骨骨折模型中，愈伤组织形成的程度与持续存在于愈伤组织中的F4 / 80 +巨噬细胞的数量相关。总体而言，这些观察结果提供了令人信服的支持，即CD169 +破骨细胞独立于破骨细胞，在骨稳态和修复过程中为成骨细胞提供了重要的促合成代谢支持。