Crosstalk Between the Bone and Immune Systems: Osteoclasts Function as Antigen-Presenting Cells and Activate CD4⁺ and CD8⁺ T Cells.

Abstract 918

Bone is a dynamic tissue that is constantly being remodeled by bone-resorbing osteoclasts (OCs) and bone-forming osteoblasts (OB). Bone destruction, observed in patients with autoimmune disease rheumatoid arthritis and malignancies such as multiple myeloma, is believed to be caused by hyperactivation of OCs. OCs differentiate from hematopoietic monocytic precursors under stimulation by the cytokines RANKL (receptor activator of nuclear factor κB ligand) and M-CSF (macrophage colony-stimulating factor), which are produced primarily by bone marrow stromal cells, OBs, and activated T cells. As OCs are derived from monocytes/macrophages lineage similar to dendritic cells (DCs), we hypothesized that OCs could serve as antigen-presenting cells (APCs) to activate T cells. In this study, OCs were generated from human monocytes with the stimulation of RANKL and M-CSF in vitro. DCs derived from monocytes with the stimulation of GM-CSF and IL-4 were used as positive controls. First, we examined the phenotype of OCs by flow cytometry, and OCs were detached by non-enzymatic cell dissociation solution. Results showed that OCs expressed MHC class I and II molecules and costimulatory molecules important for APCs such as CD80, CD86 and CD40. The expression of these molecules could be upregulated by LPS and IFN-γ. Second, we showed by PCR that OCs expressed IL-10, TGF-β, IL-6 and TNF-α, but not IL-12p35 and p40. Third, we examined the ability of OCs to present alloantigens and activate alloreactive T cells in a mixed lymphocyte reaction assay. OCs could present allogeneic antigens and activate both CD4⁺ and CD8⁺ alloreactive T cells to proliferate as detected by [³H]thymidine incorporation and CFSE dilution assay. The activation was restricted by MHC class I and II molecules. Finally, we recruited tetanus toxoid (TT)-immunized healthy donors to test whether OCs could uptake exogenous soluble antigens and present them to CD4⁺ T cells. OCs pulsed with TT could activate autologous specific CD4⁺ T cells, which was MHC II molecule restricted. These findings indicate that OCs could function as APCs and activate both CD4⁺ and CD8⁺ T cells. Thus, our study provides new insight to the effect of OCs on the immune system especially T cells. This is not only important for a better understanding of the crosstalk between the bone and immune system but also may help develop novel strategies for treating diseases such as rheumatoid arthritis and multiple myeloma, which affect both the bone and immune systems.