Characterization of an IL-1 receptor from Asterias forbesi coelomocytes

The tremendous importance of cytokines to immune defensive systems suggests that they have been conserved through evolution. The existence of interleukin (IL)-1-like molecules in several invertebrate groups substantiates this hypothesis. To characterize further the relationship of invertebrate IL-1-like molecules, we have used competitive binding assays to show that invertebrate coelomocytes of the starfish Asterias forbesi possess an IL-1-specific binding protein. Competitive binding experiments used radiolabeled human IL-1alpha. IL-1 bound specifically to the coelomocytes by a single high-affinity binding site (K(d) = 8.72 x 10(-10)/M). There are approximately 6000 binding sites per cell. The specificity of the receptor was confirmed by demonstrating that, among a group of cytokines and lymphokines tested, only vertebrate IL-1- or echinoderm IL-1-like molecules and the vertebrate IL-1 receptor antagonist inhibit IL-1 binding. Treatment of coelomocytes (labeled with IL-1alpha) with bivalent water-soluble crosslinkers identified a membrane protein of approximately 70 kDa to which IL-1 is specifically crosslinked.