Osteoclast Activated FoxP3+ CD8+ T-Cells Suppress Bone Resorption in vitro

Background: Osteoclasts are the body's sole bone resorbing cells. Cytokines produced by pro-inflammatory effector T-cells (T-EFF) increase bone resorption by osteoclasts. Prolonged exposure to the T-EFF produced cytokines leads to bone erosion diseases such as osteoporosis and rheumatoid arthritis. The crosstalk between T-cells and osteoclasts has been termed osteoimmunology. We have previously shown that under non-inflammatory conditions, murine osteoclasts can recruit naive CD8 T-cells and activate these T-cells to induce CD25 and FoxP3 (Tc-REG). The activation of CD8 T-cells by osteoclasts also induced the cytokines IL-2, IL-6, IL-10 and IFN-gamma. Individually, these cytokines can activate or suppress osteoclast resorption. Principal Findings: To determine the net effect of Tc-REG on osteoclast activity we used a number of in vitro assays. We found that Tc-REG can potently and directly suppress bone resorption by osteoclasts. Tc-REG could suppress osteoclast differentiation and resorption by mature osteoclasts, but did not affect their survival. Additionally, we showed that Tc-REG suppress cytoskeletal reorganization in mature osteoclasts. Whereas induction of Tc-REG by osteoclasts is antigen-dependent, suppression of osteoclasts by Tc-REG does not require antigen or re-stimulation. We demonstrated that antibody blockade of IL-6, IL-10 or IFN-gamma relieved suppression. The suppression did not require direct contact between the Tc-REG and osteoclasts. Significance: We have determined that osteoclast-induced Tc-REG can suppress osteoclast activity, forming a negative feedback system. As the CD8 T-cells are activated in the absence of inflammatory signals, these observations suggest that this regulatory loop may play a role in regulating skeletal homeostasis. Our results provide the first documentation of suppression of osteoclast activity by CD8 regulatory T-cells and thus, extend the purview of osteoimmunology.