High- and low-affinity receptors for murine interleukin 6. Distinct distribution on B and T cells.

The distribution of the murine receptor for interleukin (IL) 6 was examined on a variety of cells. Binding sites for IL 6 were found on many cell lines including fibroblasts and bone marrow-derived macrophages. The highest density of binding sites was found on B cell hybridomas and plasmacytomas, irrespective of their IL 6 dependence for growth. Scatchard analysis carried out with these cells identified about 1000 high-affinity sites (Kd = 25 pM) and 10,000 low-affinity sites (Kd = 2.5 nM). The binding was specific for IL 6 and led to the internalization of the ligand. Receptors for IL 6 were also present on other B cell lines, but binding on normal resting or activated B cells was below the limit of detection (less than 3 molecules/cell). In contrast, receptors were found on mature thymocytes and on peripheral T cells. However, unlike plasmacytomas and hybridomas, T cells expressed only high-affinity binding sites. The difference in the relative numbers of high- and low-affinity receptors on different cells suggests that IL 6 interacts with several proteins, the expression of which varies from one cell type to another.