Aims: To elucidate the role of reactive oxygen species (ROS) in arthritis and to identify targets of arthritis treatment in conditions with different levels of oxidant stress.
Results: Through establishing an arthritis model by injecting arthritogenic serum into wild-type and NADPH oxidase 2 (NOX2)-deficient mice, we found that arthritis had a neutrophilic infiltrate and was more severe in Ncf1(-/-) mice, a mouse strain lacking the expression of the NCF1/p47(phox) component of NOX2. The levels of interleukin-1β (IL-1β) and IL-6 in inflamed joints were higher in Ncf1(-/-) than in controls. Antagonists of tumor necrosis factor-α (TNFα) and IL-1β were equally effective in suppressing arthritis in wild-type mice, while IL-1β blockade was more effective than TNFα blockade in Ncf1(-/-) mice. A treatment of caspase inhibitor and the combination treatment of a caspase inhibitor and a cathepsin inhibitor, but not a cathepsin inhibitor alone, suppressed arthritic severity in the wild-type mice, while a treatment of cathepsin inhibitor and the combination treatment of a caspase inhibitor and a cathepsin inhibitor, but not a caspase inhibitor alone, were effective in treating Ncf1(-/-) mice. Consistently, cathepsin B was found to proteolytically process pro-IL-1β to its active form and this activity was suppressed by ROS.
Innovation: This novel mechanism of a redox-mediated immune regulation of arthritis through leukocyte-produced ROS is important for devising an optimal treatment for patients with different levels of tissue ROS.
Conclusion: Our results suggest that ROS act as a negative feedback to constrain IL-1β-mediated inflammation, accounting for the more severe arthritis in the absence of NOX2.