Interleukin-1 Receptor Antagonist Isoforms in Collagen-Induced Arthritis

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Interleukin-1 receptor antagonist (IL-1Ra) is a natural IL-1 inhibitor that possesses anti-inflammatory properties in vivo. IL-1Ra refers to three different proteins, one secreted (sIL-1Ra) and two intracellular (icIL-1RaI and icIL-1RaII). Levels of both sIL-1Ra and icIL-1RaI are increased in the rheumatoid synovium, although icIL-1RaI predominates, particularly in cultured rheumatoid synovial fibroblasts. Since collagen-induced arthritis (CIA) reproduces some of the pathologic characteristics of rheumatoid arthritis, we investigated the expression of IL-1Ra isoforms in the joints of mice with CIA. Total RNA and proteins were extracted from whole joints of immunized mice prior to the development or during the course of CIA, as well as from synovial tissues obtained from normal mice, immunized mice before the appearance of arthritis, and at different time points during the course of CIA (1 to 20 days). The presence of IL-1Ra mRNA and protein isoforms was examined by RT-PCR, RNase protection assay, and Western blot analysis. The results showed that sIL-1Ra mRNA was present at low levels in normal whole joints, as assessed by RT-PCR, and was further stimulated during arthritis, whereas icIL-1RaI mRNA was detected only in joints during CIA. By Western blot analysis, sIL-1Ra protein was detected in whole joint extracts of immunized mice prior to clinical signs of CIA, whereas icIL-1RaI was detected only in the joints of mice with CIA. icIL-1RaII protein followed the production of sIL-1Ra. icIL-1RaI was the predominant isoform in the isolated inflamed synovium, with levels increasing during the course of CIA. In conclusion, sIL-1Ra mRNA was present in low amounts in normal joints and its levels increased early during arthritis. In a similar fashion, sIL-1Ra protein was present in the joints at early stages of CIA. In contrast, icIL-1RaI mRNA and protein was produced in the joints of mice with CIA in a delayed manner and was the predominant isoform in the inflamed synovium.