To investigate whether Treg cells can suppress osteoclast differentiation, and to define a new potential link between the immune system and the skeleton.

Regulatory CD4+,CD25+,Foxp3+ T cells were isolated and purified from the spleen and cocultured with CD11b+ osteoclast precursor cells isolated from bone marrow. Osteoclastogenesis and bone erosion were assessed by tartrate‐resistant acid phosphatase staining and pit resorption assay, respectively. In addition, Transwell experiments and cytokine‐blocking experiments were performed to define the mechanisms of interaction between Treg cells and osteoclasts.

CD4+,CD25+,Foxp3+ T cells, but not CD4+,CD25− T cells, dose dependently inhibited macrophage colony‐stimulating factor– and RANKL‐dependent osteoclast formation. Pit formation was inhibited by up to 80% when Treg cells were added. The blockade of osteoclast formation was not based on the alteration of RANKL/osteoprotegerin balance but was essentially dependent on direct cell–cell contact via CTLA‐4. Treg cell–mediated expression of transforming growth factor β, interleukin‐4 (IL‐4), and IL‐10 contributed but was not essential to the inhibitory effect on osteoclastogenesis.

These data show that CD4+,CD25+,Foxp3+ Treg cells suppress osteoclast formation, provide a new link between the immune system and bone, and extend our knowledge on regulation of bone homeostasis by the immune system.