Freund's complete adjuvant induces arthritis in mice lacking a functional interferon‐γ receptor by triggering tumor necrosis factor α–driven osteoclastogenesis

Objective To investigate the hypothesis that Freund's complete adjuvant (CFA) plays an essential role in the induction of collagen‐induced arthritis in mice, by testing whether CFA by itself is able to induce arthritis in interferon‐γ receptor–knockout (IFNγR‐KO) mice. Methods IFNγR‐KO and wild‐type mice were sensitized with a single intradermal injection of CFA containing heat‐killed Mycobacterium butyricum . Flow cytometric analysis and in vitro osteoclastogenesis assays were performed on blood, spleen, and bone marrow cells. Tumor necrosis factor (TNF) levels were measured in the serum, and levels of RANKL, osteoprotegerin (OPG), and TNFα in the synovium were determined by quantitative reverse transcriptase–polymerase chain reaction. Effects of treatment with the TNFα antagonist etanercept were assessed. Results Symptoms of arthritis appeared in IFNγR‐KO mice but not in wild‐type mice, and reached an incidence of 55%. The onset coincided with an expansion of CD11b+ splenocytes that spontaneously produced TNFα and with increased osteoclastogenesis in spleen and blood cells. Expansion of CD11b+ splenocytes and osteoclast precursor cells was more pronounced in arthritic than in nonarthritic mice. There was a >100‐fold increase in the RANKL:OPG ratio in the synovia of CFA‐sensitized mice compared with those of naive animals. Treatment with etanercept prevented the development of arthritis and mitigated the increased expansion of myeloid cells as well as the increase in osteoclast precursor numbers in the spleen and blood. Conclusion These results indicate that sensitization of mice with CFA creates a condition in which dysregulation of a single cytokine leads to arthritis by triggering TNFα‐driven osteoclastogenesis.