Objective: Annexin I is an endogenous antiinflammatory mediator, expressed in rheumatoid arthritis (RA) synovium, the contribution of which to autoregulation of the synovial inflammatory response has not been examined in models of RA. We investigated the antiinflammatory role of annexin I in rat adjuvant arthritis.

Methods: Rats with adjuvant-induced arthritis (AIA) were treated with a specific anti-annexin I monoclonal antibody (mAb), isotype control IgG, and/or dexamethasone. Clinical outcomes and synovial synthesis of tumor necrosis factor alpha (TNFalpha), prostaglandin E2 (PGE2), and nitric oxide were examined, and annexin I expression was assessed by flow cytometry and reverse transcription-polymerase chain reaction.

Results: Anti-annexin I mAb reversed the effects of dexamethasone on the clinical features of AIA and exacerbated AIA in the absence of exogenous glucocorticoid. Clinical exacerbation of AIA by anti-annexin I mAb was accompanied by significantly increased synovial TNFalpha and PGE2, suggesting that annexin I tonically inhibits the production of these mediators. Anti-annexin I mAb treatment was associated with significantly reduced leukocyte intracellular annexin I, despite increased annexin I messenger RNA expression, consistent with a depletion effect of extracellular mAb via the cell surface.

Conclusion: Annexin I is a key endogenous inhibitory mediator of arthritis via mechanisms that include inhibition of cytokine and effector molecule production. Moreover, a synthesis-independent depletion of intracellular annexin I by extracellular antibody supports the hypothesis that externalization of annexin I is involved in its mode of action.