Cellular targets and receptors for interleukin‐6 II. Characterization of IL‐6 binding and receptors in peripheral blood cells and macrophages

. Within 15 min, approximately 2–5% of 125 I‐labelled interleukin‐6 (IL‐6) injected intravenously into rats was taken up by the spleen. As determined by light microscopic autoradiography, uptake was mainly (60%) accounted for by macrophages in the red pulp. 125 I‐IL‐6 binding in rat peritoneal macrophages was quantitatively similar to that in cultured human monocytes and T‐cells. By comparison, IL‐6 binding to polymorphonuclear granulocytes and freshly isolated monocytes was low. Stimulation with antigen, but not with mitogen (PWM), induced receptor presentation in B‐cells, whereas antigen and mitogen downregulated the binding in T‐cells. At 4 C, labelled IL‐6 bound to cells with a half‐time of about 1.5 h. Binding appeared reversible, but dissociation was slow and incomplete. The apparent K d for IL‐6 binding was about 30 pmol 1 ‐1 in most cell types, however, values of approximately 120 pmol 1 ‐1 were obtained in polymorphonuclear granulocytes. At 37 c C, 125 I‐IL‐6 was rapidly internalized by T‐cells and monocyte‐macrophages, and after a lag time, TCA‐soluble radioactivity was released from the cells following a sigmoidal curve. Polyacrylamide gel electro‐phoresis of radiolabelled IL‐6 cross‐linked to its binding sites in T‐cells, yielded receptor‐ligand complexes with molar masses of 70–80 and 120–140 kg mo ‐1 . This would agree with a dimeric conformation of the IL‐6 receptor.,