High-affinity binding sites for human 26-kDa protein (interleukin 6, B cell stimulatory factor-2, human hybridoma plasmacytoma growth factor, interferon-beta 2), different from those of type I interferon (alpha, beta), on lymphoblastoid cells.

The human 26-kDa glycoprotein (26K) is a cytokine produced by lymphoid as well as nonlymphoid cells. So far it is active as (a) a potent hybridoma and plasmacytoma growth factor on mouse cells, (b) a B cell differentiating factor on human cells, and (c) (for some authors) an interferon (IFN). Internally labeled recombinant human 26K, obtained by translation of mRNA in Xenopus oocytes, was used to investigate the presence of specific receptors for this new cytokine and to analyze its binding to responsive cells. The results indicate that (a) the 26K-responsive human lymphoblastoid CESS cells express about 1500 high-affinity (Kd = 30 pM) binding sites for this cytokine, (b) this binding is not competed for by interleukin (IL)1, IL2, tumor necrosis factor (TNF), IFN-alpha 2, IFN-beta or IFN-gamma, and (c) these 26K-binding sites are different from the classical type I (alpha-beta) IFN receptors by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.