The type 1 receptor (CD120a) is the high-affinity receptor for soluble tumor necrosis factor

Abstract

Tumor necrosis factor (TNF) can induce a variety of cellular responses at low picomolar concentrations. This is in apparent conf lict with the published dissociation constants for TNF binding to TNF receptors in the order of 100-500 pM. To elucidate the mechanisms underlying the outstanding cellular sensitivity to TNF, we determined the binding characteristics of TNF to both human TNF receptors at 37°C. Calculation of the dissociation constant (K d ) from the association and dissociation rate constants determined at 37°C revealed a remarkable high affinity for TNF binding to the 60-kDa TNF type 1 receptor (TNF-R1; K d ‫؍‬ 1.9 ؋ 10 ؊11 M) and a significantly lower affinity for the 80-kDa TNF type 2 receptor (TNF-R2; K d ‫؍‬ 4.2 ؋ 10 ؊10 M). The high affinity determined for TNF-R1 is mainly caused by the marked stability of ligand-receptor complexes in contrast to the transient interaction of soluble TNF with TNF-R2. These data can readily explain the predominant role of TNF-R1 in induction of cellular responses by soluble TNF and suggest the stability of the TNF-TNF receptor complexes as a rationale for their differential signaling capability. In accordance with this reasoning, the lower signaling capability of homotrimeric lymphotoxin, compared with TNF, correlates with a lower stability of the lymphotoxin-TNF-R1 complex at 37°C.Tumor necrosis factor (TNF) is a pleiotropic cytokine that is a major mediator of immunological and pathophysiological reactions. As with many other cytokines, the concentration of TNF found in body fluids in pathophysiological situations is usually very low (1, 2). Various cellular responses of cultured cells can be initiated by low picomolar or even femtomolar concentrations of TNF (3-6). Because the binding of TNF to its cell surface receptors is a prerequisite for TNF responses, high-affinity binding sites for TNF with affinity constants in the same range should be assumed.Two distinct membrane receptors for TNF (TNF-Rs) with apparent molecular weights of 55-60 kDa (TNF-R1) and 70-80 kDa (TNF-R2) have been identified and molecularly cloned (for review, see ref. 7). Both TNF-Rs have been objects of intense physiological and biochemical investigations. Likewise, the ligand binding properties of the TNF-Rs have been extensively studied (8-11) although a detailed analysis became feasible only after the molecular cloning of the two individual receptor molecules. Equilibrium binding studies with 125 Ilabeled TNF ( 125 I-TNF) at 0°C defined high-affinity binding of TNF to both TNF-Rs with K d values of approximately 300-600 pM for TNF-R1 and 70-200 pM for TNF-R2 (12-18).Although most cell lines and primary tissues coexpress both receptor types, cellular responses to the soluble 17-kDa form of TNF seems to be dominated by the interaction with TNF-R1 (19-21). On the other hand, we have shown recently in different cellular systems that TNF-R2 can be strongly stimulated by the 26-kDa transmembrane form of TNF (transmembrane TNF) rather than by the soluble form, suggesting that transmemb...

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