Clones encoding the type 1 (p80) and type 2 (p60) forms of the murine receptors for tumor necrosis factor (TNF) were isolated by cross-hybridization using probes derived from the cloned human TNF receptors. Each of the murine receptors shows strong sequence homology to the corresponding human receptor (approximately 65% amino acid identity) throughout the molecule but only modest homology, limited to ligand-binding domains, between themselves. The ligand-binding characteristics of the recombinant murine receptors mirror those of the human homologs: both receptor types bind TNF-alpha and -beta with multiple affinity classes, and the ligands cross-compete. Analysis of the murine transcripts encoding these receptors revealed the presence of RNAs for one or both forms of the receptors in all cells examined. It was also demonstrated that for both types of human TNF receptor, variably sized transcripts are observed in different cells. The murine cDNAs were further used to determine the chromosomal locations of the TNF receptor genes. They are not linked, in contrast to the ligands, and map to chromosomes 4 (type 1) and 6 (type 2).

A transcriptionally active open reading frame (T2) from Shope Fibroma Virus was recently shown to have striking sequence homology with members of a new superfamily of cell surface proteins, including a receptor for human tumor necrosis factor (1). Here we report that recombinant T2 protein expressed in COS cells is a soluble, secreted glycoprotein which specifically binds human TNFα and β, and inhibits binding of these cytokines to native TNF receptors on cells. T2 binding of TNF is not inhibited by nerve growth factor, although the nerve growth factor receptor is also a member of the same family, nor by nine other recombinant cytokines. Further, the repeating domain structure of T2 most closely resembles that of the type I TNF receptor (p75) and is significantly different from other family members, including the type II TNF receptor (p55). Since T2 possesses a leader sequence but lacks a transmembrane domain, these results confirm the original suggestion (1) that T2 represents a soluble form of the type I TNF receptor which is secreted from virally infected cells, and whose function is to immunosupress the host by abrogating the potentially destructive effects of TNF. This is the first such virally-encoded soluble cytokine receptor to be identified, and may represent a more general mechanism by which viruses subvert the host immune system.

We report the isolation from a placental library, of two cDNAs that can encode high affinity receptors for granulocyte colony-stimulating factor (G-CSF) when expressed in COS-7 cells. The cDNAs are predicted to encode integral membrane proteins of 759 and 812 amino acids in length. The predicted extracellular and membrane spanning sequences of the two clones are identical, as are the first 96 amino acids of their respective cytoplasmic regions. Different COOH termini of 34 or 87 residues are predicted for the two cDNAs, due apparently to alternate splicing. The receptor with the longer cytoplasmic domain is the closest human homologue of the murine G-CSF receptor recently described by Fukunaga et al. (Fukunaga, R., E. Ishizaka-Ikeda, Y. Seto, and S. Nagata. 1990. Cell. 61:341). A hybridization probe derived from the placental G-CSF receptor cDNA detects a approximately 3-kb transcript in RNAs isolated from placenta and a number of lymphoid and myeloid cells. The extracellular region of the G-CSF receptors is composed of four distinct types of structural domains, previously recognized in other cell surface proteins. In addition to the two domains of the HP receptor family-defining region (Patthy, L. 1990. Cell. 61:13) it incorporates one NH2-terminal Ig-like domain, and three additional repeats of fibronectin type III-like domains. The presence of both an NH2-terminal Ig-like domain and multiple membrane-proximal FN3-like domains suggests that the G-CSF receptor may be derived from an ancestral NCAM-like molecule and that the G-CSF receptor may function in some adhesion or recognition events at the cell surface in addition to the binding of G-CSF.

Tumor necrosis factor alpha and beta (TNF-alpha and TNF-beta) bind surface receptors on a variety of cell types to mediate a wide range of immunological responses, inflammatory reactions, and anti-tumor effects. A cDNA clone encoding an integral membrane protein of 461 amino acids was isolated from a human lung fibroblast library by direct expression screening with radiolabeled TNF-alpha. The encoded receptor was also able to bind TNF-beta. The predicted cysteine-rich extracellular domain has extensive sequence similarity with five proteins, including nerve growth factor receptor and a transcriptionally active open reading frame from Shope fibroma virus, and thus defines a family of receptors.