Mode of interaction between tumor necrosis factor alpha and a monoclonal antibody expressing a recurrent idiotype.

Tumor Necrosis Factor alpha (TNF alpha) is an inflammatory cytokine which exists mainly as a 51kD complex built up of 3 identical, noncovalently-linked polypeptide subunits. We have raised monoclonal antibodies (mAb) against human TNF alpha (huTNF alpha). One of these mAb (mAb78, mouse IgG1k) was studied in detail. mAb78 expresses a recurrent idiotype typical of the BALB/c anti-huTNF alpha antibody response. HuTNF alpha bound to mAb78 with an affinity constant (Kobs) of 3.2 x 10(10)M-1. The number of huTNF alpha-binding sites per mAb78 molecule was approximately 0.7. At concentrations higher than the Kobs mAb78 neutralized huTNF alpha at a approximately 1.3:1 molar ratio. mAb78 precipitated huTNF alpha in a double immunodiffusion assay in agar. Gel-filtration experiments of mAb78-huTNF alpha mixtures, that had been set up in large antigen excess, detected complexes of 570 kD as the smallest ones formed under these conditions. We propose that these results are accommodated best by a model according to which cyclic complexes built up of 3 mAb78 and 2 huTNF alpha molecules are the smallest units formed upon interaction of the reagents. In view of this model we discuss how huTNF alpha and mAb78 can undergo a precipitin reaction.