Objective. To investigate the anti-arthritic effect of recombinant human interleukin-1 receptor antagonist protein (IRAP) in two experimental models of arthritis.

Methods. Recombinant IRAP was administered daily to mice with type II collagen–induced arthritis (CIA) or with antigen-induced arthritis (AIA) provoked by methylated bovine serum albumin (mBSA). Disease incidence and severity were assessed by a clinical index and histologic features. Serum antibody to type II collagen, spleen cell proliferation to mBSA, and anti-IRAP antibodies were measured as indices of immune function.

Results. IRAP reduced the incidence and delayed the onset of CIA and suppressed the antibody response to type II collagen. In contrast, IRAP did not affect the pathogenesis of AIA and had no effect on either humoral or cellular immune responses to mBSA in arthritic mice.

Conclusion. These observations suggest that interleukin-1 may play a prominent role in the development of some, but not all, forms of arthritis.