CTLA4‐Ig directly inhibits osteoclastogenesis by interfering with intracellular calcium oscillations in bone marrow macrophages

: CTLA4-Ig (cytotoxic T-lymphocyte antigen 4-immunoglobulin; Abatacept) is a biologic drug for rheumatoid arthritis. CTLA4 binds to the CD80/86 complex of antigen-presenting cells and blocks the activation of T cells. Although previous reports showed that CTLA4-Ig directly inhibited osteoclast differentiation, the whole inhibitory mechanism of CTLA4-Ig for osteoclast differentiation is unclear. Bone marrow macrophages (BMMs) from WT mice were cultured with M-CSF and RANKL with or without the recombinant mouse chimera CTLA4-Ig. Intracellular calcium oscillations of BMMs with RANKL were detected by staining with calcium indicator fura-2 immediately after administration of CTLA4-Ig or after one day of treatment. Calcium oscillations were analyzed using Fc receptor gamma- (FcRγ-) deficient BMMs. CTLA4-Ig inhibited osteoclast differentiation and reduced the expression of the nuclear factor of activated T cells NFATc1 in BMMs in vitro. Calcium oscillations in BMMs were suppressed by CTLA4-Ig both immediately after administration and after one day of treatment. CTLA4-Ig did not affect osteoclastogenesis and did not cause remarkable changes in calcium oscillations in FcRγ-deficient BMMs. Finally, to analyze the effect of CTLA4-Ig in vivo, we used an LPS-induced osteolysis model. CTLA4-Ig suppressed LPS-induced bone resorption in WT mice, not in FcRγ-deficient mice. In conclusion, CTLA4-Ig inhibits intracellular calcium oscillations depending on FcRγ and downregulates NFATc1 expression in BMMs. © 2019 American Society for Bone and Mineral Research.