Evidences that syngeneic alpha-type antiidiotypic antibodies may noncompetitively inhibit idiotype oligomeric antigen interactions by affecting idiotype avidity

The effects of syngeneic anti-Id antibodies on the multivalent interaction between human TNF-alpha, a homotrimeric Ag, and an anti-TNF mAb (mAb(1)78) have been studied. Eight anti-mAb(1)78 Ig secreting hybridoma, able to inhibit TNF binding in a competitive or non-competitive mode, have been generated. Two representative clones (mAb(2)1G3 and mAb(2)9F1) were selected for studying the inhibition mechanism of TNF-mAb(1)78 interaction. Idiotype-paratope topography studies indicated that mAb(2)1G3 (IgG2a) and mAb(2)9F1 (IgG1) bind two sterically distinct idiotopes on mAb(1)78 (IgG1) V regions. In particular, mAb(2)1G3 was found to bind an idiotope located within (or spatially close to) the Ag combining site suggesting that competitive inhibition of TNF binding to mAb(1)78 by mAb(2)1G3 occurs through paratope blockade. On the other hand, mAb(2)9F1 recognizes an idiotope located outside the paratope, being able to bind mAb(1)78 even in the presence of saturating amounts of TNF. mAb(1)78-TNF molar ratio in complexes, at stoichiometric equivalence, was unchanged in the presence of a large excess of mAb(2)9F1, suggesting that the functional bivalency of mAb(1)78 was not impaired by this anti-Id antibody. However, bivalent mAb(2)9F1 was able to partially inhibit the binding of bivalent mAb(1)78 to oligomeric TNF in liquid-phase as well as in solid-phase assays, whereas no inhibition was observed with monovalent mAb(2)9F1-F(ab) or mAb(1)78-F(ab). This suggests that inhibition is based on a decrease of the avidity of bivalent mAb(1)78 and not on allosteric effects on antigen binding sites. The effect of mAb(2)9F1 on mAb(1)78 arm flexibility and paratope orientation is discussed. In conclusion, the results indicate that anti-Id antibodies may inhibit Ag antibody multivalent interactions by paratope blockade or by affecting the antibody avidity.