CCL20/CCR6 Signaling Regulates Bone Mass Accrual in Mice

CCL20 is a member of the macrophage inflammatory protein family and is reported to signal monogamously through the receptor CCR6. Although studies have identified the genomic locations of both Ccl20 and Ccr6 as regions important for bone quality, the role of CCL20/CCR6 signaling in regulating bone mass is unknown. By micro–computed tomography (μCT) and histomorphometric analysis, we show that global loss of Ccr6 in mice significantly decreases trabecular bone mass coincident with reduced osteoblast numbers. Notably, CCL20 and CCR6 were co-expressed in osteoblast progenitors and levels increased during osteoblast differentiation, indicating the potential of CCL20/CCR6 signaling to influence osteoblasts through both autocrine and paracrine actions. With respect to autocrine effects, CCR6 was found to act as a functional G protein–coupled receptor in osteoblasts and although its loss did not appear to affect the number or proliferation rate of osteoblast progenitors, differentiation was significantly inhibited as evidenced by delays in osteoblast marker gene expression, alkaline phosphatase activity, and mineralization. In addition, CCL20 promoted osteoblast survival concordant with activation of the PI3K-AKT pathway. Beyond these potential autocrine effects, osteoblast-derived CCL20 stimulated the recruitment of macrophages and T cells, known facilitators of osteoblast differentiation and survival. Finally, we generated mice harboring a global deletion of Ccl20 and found that Ccl20^-/- mice exhibit a reduction in bone mass similar to that observed in Ccr6^-/- mice, confirming that this phenomenon is regulated by CCL20 rather than alternate CCR6 ligands. Collectively, these data indicate that CCL20/CCR6 signaling may play an important role in regulating bone mass accrual, potentially by modulating osteoblast maturation, survival, and the recruitment of osteoblast-supporting cells.