Resolution of inflammation in murine autoimmune arthritis is disrupted by cyclooxygenase-2 inhibition and restored by prostaglandin E2-mediated lipoxin A4 production.

Acute inflammation follows defined phases of induction, inflammation and resolution, and resolution occurs by an active process that requires cyclooxygenase-2 (COX-2) activity. This study aims to address whether this paradigm extends to recognized model of chronic inflammation. We demonstrated that murine collagen-induced arthritis follows a similar sequential course. Interestingly, COX-2 and its metabolite, the presumably proinflammatory PGE(2), are present in the joints during resolution, and blocking COX-2 activity and PGE(2) production within this period perpetuated, instead of attenuated, inflammation. Repletion with PGE(2) analogs restored homeostasis, and this function is mediated by the proresolving lipoxygenase metabolite, lipoxin A(4), a potent stop signal. Thus, the study provided in vivo evidence for a natural, endogenous link between the cyclooxygenase-lipoxygenase pathways and showed that PGE(2) serves as a feedback inhibitor essential for limiting chronic inflammation in autoimmune arthritis. These findings may explain the enigma regarding why COX-2 inhibitors are palliative rather than curative in humans, because blocking resolution may mitigate the benefit of preventing induction.