MOLECULAR-BASIS OF CROSS-REACTIVITY AND THE LIMITS OF ANTIBODY-ANTIGEN COMPLEMENTARITY

TWO major unanswered questions concerning the specificity of antibodie s are: how do structurally different antigens bind with high affinity to the same antibody, and what are the limits of the antibody combinin g site complementarity and flexibility that contribute to such crossre activity? We report here a comparative analysis of the X-ray structure s of five conformationally different steroids in complex with the Fab' fragment of an anti-progesterone antibody DB3 at 2.7 angstrom. This a ntibody is unable to complement completely the shape of the hydrophobi c antigen so that crossreactivity occurs with other ligands without ma jor structural rearrangements of the binding site. Antigen specificity can be explained through conserved interactions of DB3 with the stero id D-ring, whereas some of the crossreactivity is realized through dif ferent binding orientations of the steroid skeleton that place the A-r ing into alternative pockets on the antibody surface. The restricted g ene usage of the VGAM3.8 family in the generation of anti-progesterone monoclonal antibodies1,2 may be explained by the specific interaction Of V(H) hallmark residues with the steroid D-ring. This first detaile d structure of steroid interactions with a protein could be applied to the understanding of general mechanisms of steroid recognition as wel l as in the design of specific binding sites for small hydrophobic lig ands.