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.

Introduction: Interleukin (IL)‐7 is a potent regulator of lymphocyte development, which has significant effects on bone. Bone marrow cell cultures from IL‐7 deficient (IL‐7KO) mice produced significantly more TRACP⁺ osteoclasts (OCs) than did cells from wildtype (WT) mice. A previous study found that treatment of mice with a neutralizing antibody to IL‐7 blocked ovariectomy (OVX)‐induced bone loss. We examined if differences exist between the bones of WT and IL‐7KO mice and if OVX altered bone mass in IL‐7KO mice.

Materials and Methods: Studies were in 2‐month‐old sham‐operated (SHAM) and OVX female mice that were killed 4 weeks after surgery. IL‐7KO mice and WT controls were in a C57BL/6 background. Both vertebrae (L₁) and femora were evaluated by DXA, μCT, and histomorphometry. IL‐7KO mice were confirmed as IL‐7 deficient by their almost total lack of mature B cells in their bone marrow.

Results: There was significantly less trabecular bone volume in the vertebrae of IL‐7KO mice than in WT mice. In addition, IL‐7KO mice had significantly decreased (p < 0.05) trabecular number (13%) and increased trabecular spacing (15%). OVX decreased vertebral trabecular bone volume (TBV) by 21% (p < 0.05) in WT mice and by 22% (p < 0.05) in IL‐7KO mice compared with SHAM. IL‐7KO SHAM mice also had significantly less (30%) TBV (TA/TTA) in their femurs, as measured histomorphometrically, than did WT SHAM mice. Femurs from IL‐7KO SHAM mice had significantly increased percent OC surface (23%) compared with WT SHAM. As in the vertebrae, OVX significantly decreased femoral TBV in both WT and IL‐7KO mice by similar amounts (47% and 48%, respectively, p < 0.05 for both) compared with SHAM. However, OVX decreased cortical bone mass in WT but not in IL‐7KO bones. We also examined bone marrow cells from WT and IL‐7KO mice. Bone marrow cells from IL‐7KO animals showed a significant increase in the number of TRACP⁺ osteoclast‐like cells (OCLs), which formed in cultures that were stimulated with macrophage‐colony stimulating factor (M‐CSF) and RANKL (both at 30 ng/ml). However, there was no significant difference in the number of OCLs that formed in B lymphocyte–depleted (B220⁻) bone marrow cell cultures from WT and IL‐7KO mice.

Conclusions: IL‐7 deficiency in mice caused increased OC number in bone and decreased bone mass. OVX‐induced bone loss in IL‐7–deficient mice was selective and occurred in trabecular but not cortical bone.