Ligand-induced formation of p55 and p75 THF receptor heterocomplexes: a mechanism underlying THF receptor cooperativity

The p55 and p75 TNF receptors are thought to mediate their effects on cells through distinct signaling pathways. However, only one mechanism (ligand passing) has been defined thus far that can explain cooperativity between the two receptor classes. We have used a model of TNF-induced p75 shedding in mice to examine this issue in a physiologic setting. MuTNF (7.5 μg/mouse) elicited maximal shedding of 14 ng/ml of soluble p75 3 hours post-injection. In contrast, ligands which engage only p55 (HuTNF, p55-agonist mAb 55R593, or MuTNF plus p75-antagonist mAb TR75-54) induced p75 shedding at only 30% of that induced by MuTNF. In addition, p55-deficient mice did not shed p75 in response to MuTNF. Thus (1) p55 engagement is obligatorily required for p75 shedding while p75 engagement is not; and (2) dual ligation of p55 and p75 results in enhanced p75 shedding. Since no amount of p55 stimulation was able to generate the same level of shed p75 as occurs when both receptors were engaged, these results cannot be explained entirely by a model of ligand passing. Using TNF receptor-specific mAbs whose binding properties are not affected by the presence of ligand, we observed that TNF induces the formation of heterocomplexes consisting of both p55 and p75. Whereas p55 or p75 TNF receptor precipitates from untreated or HuTNFtreated cells contained only the relevant TNF receptor class, p55 or p75 precipitates from MuTNF-treated cells contained both receptor types. The ligand-induced formation of TNF receptor heterocomplexes may define a novel mechanism of TNF receptor cooperation that results in enhanced p75 shedding induced by MuTNF observed in vivo.