Scientists found that the natural peptide LL-37 can turn regular immune cells (monocytes) into a new type of bone‑forming cell called monoosteophils. When these cells were put into bone holes in special mice, the bones healed faster—about two weeks instead of three or more. The effect depends on a protein called MAPK13, and the cells show high levels of bone‑related genes.

An 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. Human 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. Human derived monoosteophils, characterized as CD45(+)α3(+)α3β(+)CD34(-)CD14(-)BAP (bone alkaline phosphatase)(-) cells, can function in an animal model of bone injury.