SummaryUsing a multistep polymerase chain reaction method, we have produced a construct in which a cDNA sequence encoding the extraceUular domain of the human 55-kD tumor necrosis factor (TNF) receptor is attached to a sequence encoding the Fc portion and hinge region of a mouse IgG1 heavy chain through an oligomer encoding a thrombin-sensitive peptide linker. This construct was placed downstream from a cytomegalovirus promoter sequence, and expressed in Chinese hamster ovary cells. A secreted protein, capable of binding TNF and inactivating it, was produced by the transfected cells. Molecular characterization revealed that this soluble version of the TNF receptor was dimeric. Moreover, the protein could be quantitatively cleaved by treatment with thrombin. However, the monovalent extracellular domain prepared in this way has a greatly reduced TNF inhibitory activity compared with that of the bivalent inhibitor. Perhaps because of its high affinity for TNF, the chimeric protein is far more effective as a TNF inhibitor than are neutralizing monodonal antibodies. This molecule may prove very useful as a reagent for the antagonism and assay of TNF and lymphotoxin from diverse species in health and disease, and as a means of deciphering the exact mechanism through which TNF interacts with the 55-kD receptor.T he recent cloning of the 55-kD (1-3) and 75-kD (3-5) TNF receptors has opened the way for further studies of TNF effects and signal transduction. Moreover, it appears that truncated receptor molecules, lacking the transmembrane or cytoplasmic domains, are capable of interacting with TNF, and therefore have been isolated from urine (6, 7) and serum (2) as TNF inhibitors. We considered that derivatives of such molecules might prove useful as antagonists of TNF action in vivo, as high affinity ligands to be applied as the basis of a more sensitive assay for TNF, and as reagents to be used in defining the molecular interaction between TNF and its receptor.Unfortunately, truncated forms of the TNF receptor are highly unstable in vivo, and therefore are poor substitutes for antibodies as a means of antagonizing TNF action in living animals. The naturally occurring TNF receptor fragments are univalent and therefore have an avidity that is effectively far lower than that of a bivalent ligand. The production of large quantities of a truncated receptor by recombinant means has, in our hands, been problematic, since the protein is produced in an inactive and insoluble form in bacteria (Peppel, K., and B. Beutler, unpublished observations). This difficulty may arise from the highly cysteine-rich structure of the receptor-binding domain (1, 2). When produced in roammalian cells through recombinant techniques, the soluble receptor fragment is active, but produced at low levels, and therefore difficult to purify (Beutler, B., and T. Brown, unpublished observations).To circumvent these problems, we have engineered a chimeric protein in which the extracellular domain of the TNF receptor, which normally engages the TNF molecu...