Patients with rheumatoid arthritis (RA) have defective CD4⁺CD25⁺ regulatory T (T_reg) cells and increased osteoclastogenesis. A similar situation has been described in collagen-induced arthritis (CIA). In this study, it was investigated whether a single transfer of polyclonally activated T_reg cells inhibits CIA and osteoclastogenesis.

Purified T_reg cells were expanded in vitro with anti-CD3 and anti-CD28 antibody-coated beads and injected into DBA/1 mice. Mice were immunised with collagen type II (CII) in complete Freund adjuvant (CFA) and scores of arthritis were recorded. In vitro osteoclastogenesis assays were performed on splenocytes by stimulation with macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor (NF)κB ligand (RANKL). Levels of anti-CII antibody and cytokines were determined in the supernatant using ELISA and Bio-Plex protein array system.

It was found that 10⁶ activated T_reg cells significantly counteracted the development of CIA, which was accompanied by decreased serum levels of TNFα and IL6, but not by inhibition of autoimmune antibody responses. The differentiation of osteoclasts in splenocyte cultures was significantly reduced in the presence of prestimulated T_reg cells. Expression of cytokines that are described to inhibit osteoclastogenesis, including granulocyte macrophage colony-stimulating factor (GM-CSF), interferon (IFN)γ, interleukin (IL)5 and IL10, were dramatically increased upon addition of T_reg cells. Furthermore, splenocytes from mice that had been treated with T_reg cells displayed an impaired capacity to develop into mature osteoclasts, suggesting that T_reg cells abrogated osteoclastogenesis in vivo.

Activated CD4⁺CD25⁺ T_reg cells improve clinical symptoms of CIA, regulate cytokine production and inhibit osteoclastogenesis in vitro and in vivo.