Molecular basis for the lack of mimicry of Brucella polysaccharide antigens by Ab2 gamma antibodies

The crystal structure of the complex of an anti-Id Fab with an Fab specific for a Brucella polysaccharide antigen has previously been reported (Evans et al., 1994, J. Mol. Biol. 241, 691-705). To complement this study, the bi nding characteristics and immunological properties of this Ab2 and two othe rs raised with a second anti-Brucella antibody were investigated, including quantitative kinetic measurements by surface plasmon resonance. The affini ties of the Fabs from the Ab2s for the Ab1s were three orders of magnitude greater than those estimated for the antigen, but the Ab2s failed to induce antigen-binding Ab3s, that is, they were of the Ab2 gamma type. The avidit ies of the Ab1s for antigen were however within one order of magnitude of t heir avidities for Ab2. Tests of 16 other anti-Brucella polysaccharide anti bodies showed that the two idiotopes were not present in them, and in confi rmation of the lack of a dominant idiotope, N-terminal sequencing of their H and L chains showed a wide variety of V genes were employed in the immune response to the Brucella polysaccharides. The failure of the Ab2 to induce antigen-reactive Ab3 thus appears to be due to neither intrinsic affinity nor idiotope frequency, but arises instead from structural reasons, for exa mple, the incomplete penetration of the Ab2 into the binding-site cleft of the Abl. The surface topography of polysaccharide antigens and their bindin g-sites thus appears to be especially difficult for Ab2s to mimic and will restrict their routine use as surrogates for T-cell independent polysacchar ide antigens. Crown Copyright (C) 1999 Published by Elsevier Science Ltd. A ll rights reserved.