Segmental flexibility and avidity of IgM in the interaction of polyvalent antigens

We prepared IgG and IgM with identical combining sites to a hapten, (4-hydroxy-3-nitrophenyl)acetic acid (NP), and used surface plasmon resonance to evaluate the association constants (K-a) in interactions of these antibodies (Abs) with antigens (Ags) which differed in the size of carriers and NP valence as well as in the stoichiometry of Ag to Ab in the immune complexes. It was found that IgM was unable to form an Ag(1)Ab(1) complex with the highly haptenated Ag, NP18.6-bovine serum albumin (BSA), such that one NP18.6-BSA molecule was held by multiple contacts with Fab arms from five subunits, although IgM was capable of forming an Ag(4)Ab(1), complex in which each subunit was bound to one NP18.6-BSA molecule. IgM was superior to IgG in interactions with large Ags of low hapten density. The K-a values of IgM to these Ags were estimated to be similar to1 x 10(9) M-1, about 20-fold higher than those of IgG. Reduction of inter-subunit and inter-chain disulfide bonds resulted in a decrease in Ka values to large Ags but no change in those to small Ags. (C) 2003 Elsevier Ltd. All rights reserved.