SPECIFIC BINDING OF INTERLEUKIN-1 (IL-1)BETA AND IL-1 RECEPTOR ANTAGONIST (IL-1RA) TO HUMAN SERUM - HIGH-AFFINITY BINDING OF IL-1RA TO SOLUBLE IL-1 RECEPTOR TYPE-I

Molecules that bind recombinant interleukin 1 (rIL-1)β and rIL-1 receptor antagonist (rIL-1ra) with high affinity were detected in sera of healthy individuals. rIL-1β bound with dissociation constants in the nanomolar range, and the serum binding capacity was 40-50 ng/ml. rIL-1ra bound with 30 times higher affinity, and the serum binding capacity was 0.7-1 ng/ml. Rabbit antibodies against the recombinant-derived extracellular part of human IL-1 receptor type 1 (rsIL-1RI) selectively inhibited the binding of 125I-rIL-1ra to the serum factor(s). Almost 70% of the high-affinity IL-1ra-binding capacity was recovered after immunosorption with these antibodies. Binding of 125I-rIL-1ra to rsIL-1RI was blocked by rIL-1α and by rIL-1β. In contrast, the purified rIL-1ra-binding factor (IL-1raBF) failed to bind rIL-1α and rIL-1β. Gel filtration chromatography indicated a 1:1 binding of rIL-1β and rIL-1ra to their respective serum factors. The apparent molecular size of both serum factors was 70-80 kDa. Using SDS-PAGE and autoradiography, IL-1raBF had a molecular size of 60 kDa. We conclude that IL-1raBF, a serum factor which selectively and with high affinity binds IL-1ra (Kd = 70 pM), is related to or identical with a soluble form of IL-1RI. If upregulated during disease, IL-1raBF may constitute yet another level of natural regulation of IL-1 bioactivities. © 1993 Academie Press Limited.