IDENTIFICATION OF AN EPITOPE OF TUMOR-NECROSIS-FACTOR (TNF) RECEPTOR-TYPE-1 (P55) RECOGNIZED BY A TNF-ALPHA-ANTAGONIST MONOCLONAL-ANTIBODY

The relationships between epitope topography and agonistic/antagonisti c effects of anti-TNF receptor type 1 (TNF-R1) antibodies on TNF-alpha cytotoxic activity have been studied. To this purpose various monoclo nal antibodies (mAbs) against the soluble form of TNF-R1 (sTNF-R1) hav e been generated and characterized. Epitope topography studies identif ied at least four distinct epitopes located outside (4E10) or within ( or close to) the TNF-alpha binding site of urinary sTNF-R1 (7H3, 4C1, 9B11). mAbs 7H3 and 4C1 were able to neutralize the inhibition of huma n TNF-alpha cytotoxicity on L-M cells by sTNF-R1, while 4E10 was unabl e. Moreover, 7H3 and 4C1 were able to antagonize the TNF-a cytotoxicit y on human U937 cells, while they were uneffective on mouse LM cells, suggesting that these antibodies recognize, in a species-specific mode , also the membrane form of the human receptor. No agonistic effects w ere observed when these antibodies were used in the absence of TNF-alp ha. Epitope topography studies carried out using overlapping decapepti des covering most of the sTNF-R1 sequence showed that residues 143-148 of the fourth cysteine-rich domain of the receptor (FFLREN) contain a ntigenic determinants recognized by the antagonist antibody 7H3. These results suggest that at least part of residues 143-148 of sTNF-R1 are surface exposed on the soluble as well as on the membrane forms of TN F-R1 and are accessible to TNF-alpha antagonists.