Tumor Necrosis Factor (TNF) was originally discovered as a serum protein with necrotizing effects on certain transplantable mouse tumors in vivo and cytotoxic effects against some transformed cells in vitro. The TNF family consists of two proteins designated TNF-a, also called cachectin, and TNF-b, also called lymphotoxin, which are pleiotropic cytokines that can mediate a wide variety of biological effects. TNF-b is produced by activated lymphocytes, whereas TNF-a is mostly produced by activated macrophages. Soluble TNF-b is a T-cell derived glycoprotein of 25 kD encoded by a gene within the MHC. The molecule consists of a 17.5 to 18 kD polypeptide core and 7kD of N-linked carbohydrate. TNF-b is 28 % structurally homologous with the macrophage produced nonglycoprotein TNF-a. The genes for TNF-a and TNF-b are closely linked, and the proteins share biological activities. TNF has been shown to interact with a cell through specific highaffinity receptors with a few hundred up to more than 20.000 copies per cell. In a variety of cell lines, two different TNF-receptor proteins have been identified and the cDNAs cloned. Recently, an ELISA has been established by Adolf and Apfler which provides a simple, rapid, and highly sensitive method for the determination of soluble TNF-R (60 kDa) levels in body fluids or cell culture supernatants. TNF-b is induced in an antigen-specific MHC restricted fashion from class I and class II restricted T cells. Viral infection is also associated with TNF-b production by lymphoid cells. TNF-b has several effects on target cells including killing, growth stimulation, induction of adhesion molecule (ICAM-1) expression, and induction of differentiation. The mechanisms of TNF-b effects involve receptor binding and internalization and several sequelae including changes in prostaglandins and chromosome integrity. TNF-b participates in tumor immunity, and it has been reported to inhibit carcinogenesis as well as growth of some tumors in vivo. Recent studies have demonstrated that both TNF-a and TNF-b are capable of activating neutrophils in vitro. The exposure of neutrophils to TNF-a or TNF-b causes the production of superoxide radicals, induces phagocytic response and enhances antibody dependent cell cytotoxicity. The release of IL-1 from Human endothelial cells is also induced by TNF-a and TNF-b. All the in vitro studies suggest that TNF-b may play an important role in immunoregulation. In fibroblasts TNF-b induces the synthesis of colony-stimulating factors, IL-1, collagenase and prostaglandin-E2. Monocytes are stimulated for terminal differentiation. On B-cells TNF-b acts as mitogen. As TNF-b exerts proliferative capacity on fibroblasts it may participate in the process of wound healing.
- References to TNF-beta (Lymphotoxin-alpha, TNF superfamily member 1)