The receptors and their binding properties

The terms TNF and TNF receptors relate respectively to a number of ligands and receptor molecules that can interact in different ways (see Figure 1). Three structurally related ligands, TNF«, lymphotoxin a (LTa) (or TNF/3) and LTj3, and three receptors with structurally related extracellular domains are currently known to operate in this response system. Two of the receptors have received CD designations, though other names are also still in use. One of these receptors is designated CD120a (or the type I, the p55 or the p60 receptor) and the other CD120b (or the type II, the p75 or the p80 receptor). The third is termed the LT/3 receptor (LT/3-R), the type III TNF receptor or TNFR-RP. Both the ligands and the receptors belong to growing families of structurally related molecules, known as the TNF-ligand and the TNF/NGF receptor families.

TNFa and LTa have been shown to act as homo-trimers, and this is apparently true for all other members of the TNF-ligand family. Another common feature of most of these ligands, including TNFa and LT/3, is that they are functionally active not only as soluble molecules (like most of the known cytokines) but also as cell surface, type II transmembrane proteins, from which the soluble forms are derived pro-teolytically. LTa is the only known ligand of the TNF family which is released by cells as a secreted protein. It has the ability to bind to LTj3 molecules and thus, even though formed as a soluble molecule, it also occurs in a cell-bound form, comprised of het-erotrimers of LTa and LTj3 (see Figure 1). As discussed below, both the ligands and the TNF receptors occur in cell-bound as well as in soluble forms, the latter being derived from the cell surface forms following cleavage by cell bound metalloprotease(s).

The known interactions between the three ligands and the three receptors are illustrated in Figure 1. Both CD120a and CD120b can bind to the cell-bound as well as to the soluble forms of homotrimers of TNFa and LTa. CD120b, however, binds more effectively to the cell-bound form of TNFa and is preferentially activated by it. The LT/3-R binds only to heterotrimers of LTa and LT/3.

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