Inflammatory Arthritis in Caspase 1 Gene-Deficient Mice Contribution of Proteinase 3 to Caspase 1-Independent Production of Bioactive Interleukin-1β

Objective. Caspase 1, a known cysteine protease, is a critical component of the inflammasome. Both caspase I and neutrophil serine proteases such as proteinase 3 (PR3) can process pro-interleukin-1 beta (proIL-1 beta), a crucial cytokine linked to the pathogenesis of rheumatoid arthritis. This study was undertaken to establish the relative importance of caspase I and serine proteases in mouse models of acute and chronic inflammatory arthritis. Methods. Acute and chronic arthritis were induced in caspase 1(-/-) mice, and the lack of caspase 1 was investigated for its effects on joint swelling, cartilage metabolism, and histopathologic features. In addition, caspase 1 activity was inhibited in mice lacking active cysteine proteases, and the effects of dual blockade of caspase I and serine proteases on arthritis severity and histopathologic features were evaluated. Results. Surprisingly, caspase 1(-/-) mice, in a model of acute (neutrophil-dominated) arthritis, developed joint swelling to an extent similar to that in wild-type control mice. Joint fluid concentrations of bioactive IL-1 beta were comparable in caspase 1(-/-) mice and controls. In contrast, induction of chronic arthritis (characterized by minimal numbers of neutrophils) in caspase 1(-/-) mice led to reduced joint inflammation and less cartilage damage, implying a caspase 1-dependent role in this process. In mice lacking neutrophil serine PR3, inhibition of caspase 1 activity resulted in decreased bioactive IL-1 beta concentrations in the synovial tissue and less suppression of chondrocyte anabolic function. In addition, dual blockade of both PR3 and caspase 1 led to protection against cartilage and bone destruction. Conclusion. Caspase 1 deficiency does not affect neutrophil-dominated joint inflammation, whereas in chronic arthritis, the lack of caspase 1 results in reduced joint inflammation and cartilage destruction. These findings suggest that inhibitors of caspase 1 are not able to interfere with the whole spectrum of IL-1 beta production, and therefore such inhibitors may be of therapeutic value only in inflammatory conditions in which limited numbers of neutrophils are present.