SOLUBLE TUMOR-NECROSIS-FACTOR (TNF) RECEPTORS ARE EFFECTIVE THERAPEUTIC AGENTS IN LETHAL ENDOTOXEMIA AND FUNCTION SIMULTANEOUSLY AS BOTH TNF CARRIERS AND TNF ANTAGONISTS

Two forms (monomeric or dimeric) of the extracellular, ligand-binding portion of the human p80 cell-surface receptor for TNF were used to an tagonize TNF activity in vitro and in vivo. The dimeric sTNFR:Fc molec ule was a more potent inhibitor of TNF than the monomeric sTNFR (50 to 1000X), as assessed in vitro by inhibition of TNF binding or bioactiv ity and in vivo by protection of mice from an otherwise lethal injecti on of LPS. Surprisingly, the dimeric sTNFR:Fc construct demonstrated a beneficial effect even when administered 3 h after a lethal LPS injec tion (i.e., after serum TNF levels had peaked and receded). To study t he mechanism by which the soluble TNFR functions in vivo, serum TNF le vels were examined in mice given LPS in the presence or absence of sol uble receptor. Administration of a mortality-reducing dose of sTNFR:Fc ablated the rise in serum TNF bioactivity that normally occurs in res ponse to LPS. However, TNF bioactivity was revealed in these ''TNF-neg ative'' serum samples when the L929 bioassay was modified by inclusion of a mAb that blocks the binding of murine TNF to the human soluble T NFReceptor. These results indicate that the absence of direct cytolyti c activity in the L929 assay was caused by neutralization of TNF, rath er than to an absence of TNF in the serum. Moreover, administration of either monomeric sTNFR or low doses of dimeric sTNFR:Fc actually resu lted in increased serum TNF levels compared to mice given LPS but no s oluble receptor. However, these ''agonistic'' doses of soluble recepto r did not lead to increased mortality when an LD60 dose of LPS was giv en. Thus, dimeric sTNFR are effective inhibitors of TNF and under some circumstances function simultaneously as both TNF ''carriers'' and an tagonists of TNF biologic activity.