LIGAND-INDUCED FORMATION OF P55 AND P75 TUMOR-NECROSIS-FACTOR RECEPTOR HETEROCOMPLEXES ON INTACT-CELLS

The p55 and p75 tumor necrosis factor receptors are known to mediate t heir effects on cells through distinct signaling pathways. Under certa in circumstances, the two classes of TNF receptors cooperate with each another to produce enhanced cellular responses. The only molecular me chanism proposed thus far to explain this effect is the process of ''l igand passing,'' whereby TNF is concentrated at cell surfaces by bindi ng to p75 and then following dissociation from this receptor class bin ds with high efficiency to p55. Using the in vivo model of TNF-induced TNF receptor shedding we have uncovered a novel ligand-dependent inte raction of the two TNF receptors that occurs upon exposure of cells to TNF. Using TNF receptor-specific monoclonal antibodies that bind TNF receptors in the presence or absence of ligand, we report that TNF ind uces the formation of heterocomplexes consisting of both p55 and p75 T NF receptors. Whereas immunoprecipitates from untreated or human TNF-t reated cells formed with either p55 or p75 TNF receptor-specific monoc lonal antibodies contained only the relevant TNF receptor class, anti- p55 or anti-p75 precipitated both receptor types from murine TNF-treat ed cells. Ligand-induced complex formation was transient, occurred at physiologically relevant concentrations of TNF, and occurred with rece ptors lacking intracellular domains or that contained irrelevant trans membrane domains. Formation of TNF receptor heterocomplexes may theref ore 1) define a novel molecular mechanism of ligand passing and/or 2) contribute to cooperative TNF receptor signaling via the juxtaposition of the intracellular domains of the two receptor classes and the sign aling proteins that. they recruit.