Acceleration of Bone Repair in NOD/SCID Mice by Human Monoosteophils, Novel LL-37-Activated Monocytes

BackgroundAn incomplete understanding of bone forming cells during wound healing and ectopic calcification has led to a search for circulating cells that may fulfill this function. Previously, we showed that monoosteophils, a novel lineage of calcifying/bone-forming cells generated by treatment of monocytes with the natural peptide LL-37, are candidates. In this study, we have analyzed their gene expression profile and bone repair function.Methods and FindingsHuman monoosteophils can be distinguished from monocytes, macrophages and osteoclasts by their unique up-regulation of integrin α3 and down-regulation of CD14 and CD16. Monoosteophils express high mRNA and protein levels of SPP1 (osteopontin), GPNMB (osteoactivin), CHI3L1 (cartilage glycoprotein-39), CHIT1 (Chitinase 1), MMP-7, CCL22 and MAPK13 (p38MAPKδ). Monocytes from wild type, but not MAPK13 KO mice are also capable of monoosteophil differentiation, suggesting that MAPK13 regulates this process. When human monoosteophils were implanted in a freshly drilled hole in mid-diaphyseal femurs of NOD/SCID mice, significant bone repair required only 14 days compared to at least 24 days in control treated injuries.ConclusionHuman derived monoosteophils, characterized as CD45+α3+α3β+CD34−CD14−BAP (bone alkaline phosphatase)− cells, can function in an animal model of bone injury.

研究背景
由于对伤口愈合及异位钙化过程中成骨细胞的认知仍不全面，学界亟需探寻可发挥成骨功能的循环细胞。此前本团队已证实，单核骨细胞（monoosteophils）——一种通过天然肽LL-37处理单核细胞所生成的新型钙化/成骨细胞谱系——正是此类候选细胞。本研究对其基因表达谱及骨修复功能展开了分析。

方法与结果
单核骨细胞可通过其独特的整合素α3（integrin α3）上调、CD14与CD16下调的表型特征，与单核细胞、巨噬细胞及破骨细胞相区分。该细胞高表达SPP1（骨桥蛋白，osteopontin）、GPNMB（骨激活素，osteoactivin）、CHI3L1（软骨糖蛋白-39，cartilage glycoprotein-39）、CHIT1（几丁质酶1，Chitinase 1）、MMP-7（基质金属蛋白酶7）、CCL22（CC家族趋化因子配体22）及MAPK13（p38MAPKδ）的mRNA与蛋白水平。野生型小鼠（而非MAPK13基因敲除（KO）小鼠）的单核细胞同样可分化为单核骨细胞，这表明MAPK13调控该分化过程。将人源单核骨细胞植入NOD/SCID小鼠股骨干中段新鲜钻孔损伤处后，仅需14天即可实现显著骨修复，而对照组损伤的修复时长至少需要24天。

结论
经鉴定为CD45+α3+α3β+CD34−CD14−BAP（骨碱性磷酸酶，bone alkaline phosphatase）−的人源单核骨细胞，可在骨损伤动物模型中发挥修复功能。