A soluble protein that binds specifically to interleukin-1 (IL-1)β was released from a B cell line (Raji). The covalently cross-linked binding protein/ [125I]IL-1β migrated at 60 kDa by SDS-PAGE. The IL-1 receptor (IL-1R) on Raji cells had the same ligand specificity. Stimulation of Raji with dexamethasone increased surface expression of the IL-1R and the rate of release of soluble binding protein. A serine protease inhibitor prevented release of the binding protein and increased IL-1R expression on the cells. These results suggest that the soluble IL-1β binding protein is a proteolytically cleaved form of the novel B cell IL-1R.
