Membrane-Associated IL-1 Contributes to Chronic Synovitis and Cartilage Destruction in Human IL-1α Transgenic Mice

IL-1 molecules are encoded by two distinct genes, IL-1α and IL-1β. Both isoforms possess essentially identical activities and potencies, whereas IL-1α, in contrast to IL-1β, is known to act as a membrane-associated IL-1 (MA-IL-1) and plays an important role in a variety of inflammatory situations. The transgenic (Tg) mouse line (Tg1706), which was generated in our laboratory, overexpresses human IL-1α (hIL-1α) and exhibits a severe arthritic phenotype characterized by autonomous synovial proliferation with subsequent cartilage destruction. Because the transgene encoded Lys⁶⁴ to Ala²⁷¹ of the hIL-1α amino acid sequence, Tg mice may overproduce MA-IL-1 as well as soluble IL-1α. The present study investigated whether MA-IL-1 contributes to synovial proliferation and cartilage destruction in the development of arthritis. Flow cytometric analysis revealed that both macrophage-like and fibroblast-like synoviocytes constitutively produce MA-IL-1. D10 cell proliferation assay revealed MA-IL-1 bioactivity of paraformaldehyde-fixed synoviocytes and the further induction of endogenous mouse MA-IL-1 via autocrine mechanisms. MA-IL-1 expressed on synoviocytes triggered synoviocyte self-proliferation through cell-to-cell (i.e., juxtacrine) interactions and also promoted proteoglycan release from the cartilage matrix in chondrocyte monolayer culture. Interestingly, the severity of arthritis was significantly correlated with MA-IL-1 activity rather than with soluble IL-1α activity or concentration of serum hIL-1α. Moreover, when the Tg1706 line was compared with the Tg101 line, which selectively overexpresses the 17-kDa mature hIL-1α, the severity of arthritis was significantly higher in the Tg1706 line than in the Tg101 line. These results suggest that MA-IL-1 contributes to synoviocyte self-proliferation and subsequent cartilage destruction in inflammatory joint disease such as rheumatoid arthritis.