IL-7 is produced by stromal cells in bone marrow and is a major regulator of B and T lymphopoiesis. It is also a direct inhibitor of osteoclastogenesis in vitro. In this study we show that IL-7-deficient mice have increased OC and decreased trabecular bone volume compared with WT mice but mimic WT mice in the amount of trabecular but not cortical bone lost after ovariectomy.

We examined the direct effects of IL-7 on osteoclastogenesis in murine bone marrow cultures, using cells from wild-type and IL-7- and IL-7 receptor (IL-7R)-deficient mice. IL-7 inhibited osteoclast-like cells (OCL) formation in macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor kappaB ligand (RANKL)-stimulated (both at 30 ng/ml) murine bone marrow cultures. Significant inhibitory effects were seen at 1 ng/ml (57%) and 10 ng/ml (86%). IL-7 also inhibited (P < 0.05) OCL formation in bone marrow cultures that were stimulated with vitamin D(3) (10(-8) M, 60%), bovine PTH (bPTH) (100 ng/ml, 54%), or RANKL alone (30 ng/ml, 50%). IL-7 (10 ng/ml) increased expression of the B lymphocyte marker B220 from 40-86% of total nonadherent cells in cultures treated with M-CSF and RANKL. Bone marrow cells from IL-7-deficient [IL-7 knockout (KO)] mice showed a significant (P < 0.05) increase in tartrate-resistant acid phosphatase(+) OCL numbers in cultures that were stimulated with vitamin D(3) (136 +/- 13.3%), bPTH (196 +/- 18.8%), or M-CSF and RANKL (160 +/- 7.2%). In contrast, in vitro osteoclast formation in bone marrow from IL-7R-deficient (IL-7R KO) mice showed a significant decrease in tartrate-resistant acid phosphatase(+) OCL numbers in cultures that were stimulated with vitamin D(3), PTH, RANKL, or M-CSF and RANKL. These results demonstrate that there are differences in the mechanisms regulating OCL formation between IL-7 KO and IL-7R KO cells. It seems that IL-7 is a direct inhibitor of OCL formation in vitro, based on results of adding IL-7 to wild-type cultures and the responses of IL-7 KO cells. It is unknown why IL-7R KO cells behave differently from IL-7 KO cells in vitro. However, it is possible that additional cytokines interact with IL-7R and that loss of these signals contributes to the responses of IL-7R KO cells. Alternatively, IL-7 may interact with multiple receptors.

No abstract

Interleukin-7 is a critical cytokine for lymphoid development and a direct inhibitor of in vitro osteoclastogenesis in murine bone marrow cultures. To explore the role of IL-7 in bone, we generated transgenic mouse lines bearing the 2.3 Kb rat collagen 1A1 promoter driving the expression of human IL-7 specifically in osteoblasts. In addition we crossed these mice with IL-7 deficient mice to determine if the alterations in lymphopoiesis, bone mass and osteoclast formation observed in the IL-7 KO mice could be rescued by osteoblast-specific overexpression of IL-7. Here we show that mice overexpressing human IL-7 in the osteoblast lineage demonstrated increased trabecular bone volume in vivo by µCT and decreased osteoclast formation in vitro. Furthermore, targeted overexpression of IL-7 in osteoblasts rescued the osteopenic bone phenotype and B cell development of IL-7 KO mice but did not have an effect on T lymphopoiesis, which occurs in the periphery. The bone phenotypes in IL-7 KO mice and targeted IL-7 overexpressing mouse models were observed only in females. These results likely reflect both a direct inhibitory effects of IL-7 on osteoclastogenesis in vivo and gender specific differences in responses to IL-7.

We examined the effects that ovariectomy had on sclerostin mRNA and protein levels in the bones of 8-week-old mice that were either sham-operated (SHAM) or ovariectomized (OVX) and then sacrificed 3 or 6 wks. later. In this model bone loss occurred between 3 and 5 wks. Post-surgery. In calvaria OVX significantly decrease sclerostin mRNA levels at 6 wks. post-surgery (by 52%) but had no significant effect at 3 wks. In contrast, sclerostin mRNA levels were significantly lower in OVX femurs at 3 wks. post-surgery (by 53%) but equal to that of SHAM at 6 wks. The effects of OVX on sclerostin were not a global response of osteocytes since they were not mimicked by changes in the mRNA levels for 2 other relatively osteocyte-specific genes: DMP-1 and FGF-23. Sclerostin protein decreased by 83% and 60%, respectively at 3 and 6 wks. post-surgery in calvaria and by 38% in lumbar vertebrae at 6 wks. We also detected decreases in sclerostin by immunohistochemistry in cortical osteocytes of the humerus at 3 wks. post-surgery. However, there were no significant effects of OVX on sclerostin protein in femurs or on serum sclerostin at 3 and 6 wks. post-surgery. These results demonstrate that OVX has variable effects on sclerostin mRNA and protein in mice, which are dependent on the bones examined and the time after surgery. Given the discrepancy between the effects of OVX on serum sclerostin levels and sclerostin mRNA and protein levels in various bones, these results argue that, at least in mice, serum sclerostin levels may not accurately reflect changes in the local production of sclerostin in bones. Additional studies are needed to evaluate whether this is also the case in humans.