Mode of Interaction Between Tumor Necrosis Factor α and a Monoclonal Antibody Expressing a Recurrent Idiotype

Abstract

Tumor Necrosis Factor α (TNFα) 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 TNFα (huTNFα). One of these mAb (mAb78, mouse IgGlk) was studied in detail. mAb78 expresses a recurrent idiotype typical of the BALB/c anti-huTNFα antibody response. HuTNFα bound to mAb78 with an affinity constant (K_obs) of 3.2 x lO¹⁰M^-1. The number of huTNFα-binding sites per mAb78 molecule was ≈0.7. At concentrations higher than the K_obs mAb78 neutralized huTNFα at a ≈ 1.3 : 1 molar ratio. mAb78 precipitated huTNFα in a double immunodiffusion assay in agar. Gel-filtration experiments of mAb78-huTNFα 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 huTNFα molecules are the smallest units formed upon interaction of the reagents. In view of this model we discuss how huTNFα and mAb78 can undergo a precipitin reaction.