A human tumor necrosis factor (TNF) binding protein from serum of cancer patients was purified to homogeneity and partially sequenced. Synthetic DNA probes based on amino acid sequence information were used to isolate cDNA clones encoding a receptor for TNF. The TNF receptor (TNF-R) is a 415 amino acid polypeptide with a single membrane-spanning region. The extracellular cysteine-rich domain of the TNF-R is homologous to the nerve growth factor receptor and the B cell activation protein Bp50. Human embryonic kidney cells transfected with a TNF-R expression vector specifically bind both 125I-labeled and biotinylated TNF-alpha. Unlabeled TNF-alpha and TNF-beta were equally effective at displacing the binding of labeled TNF-alpha to TNF-R expressing cells. Northern analysis indicates a single species of mRNA for the TNF-R in a variety of cell types. Therefore, the soluble TNF binding protein found in human serum is probably proteolytically derived from the TNF-R.

Serum ultrafiltrates (SUF) from human patients with different types of cancer contain a blocking factor (BF) that inhibits the cytolytic activity ofhuman tumor necrosis factor a (TNF-a) in vitro. BF is a protein with a molecular mass of28 kDa on reducing sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS/PAGE). The active material was purified to homogeneity by a combination of affinity chromatography, PAGE, and high-pressure liquid chromatography. Amino add sequence analysis revealed that BF is derived from the membrane TNF receptor. Purified BF blocks the lytic activity ofrecombinant human and mouse TNF-a and recombinant human lymphotoxin on murine L929 cells in vitro. However, BF inhibits the lytic activity of TNF-a more effectively than it does that of lymphotoxin. The BF also inhibits the necrotizing activity ofrecombinant human TNF-a when cou jected into established cutaneous Meth A tumors in BALB/c mice. The BF may have an important role in (i) the regulation and control of TNF-a and lymphotoxin activity in cancer patients, (ii) interaction between the tumor and the host antitumor mechanisms, and (ii) use of systemically administered TNF-a in clinical trials with human cancer patients.

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