Consequences of avidity in lymphocyte receptor-multivalent antigen binding in affinity maturation

A mathematical model proposed by Crothers & Metzger (Immunochemistry 9, 341-357, 1972), describing the influence of multiple site interactions on the effective antigen binding property of antibodies was used to assess the possible role of lymphocyte receptor affinity in antibody affinity maturation. The model proposed that observed association constants (KobsS) were primarily dependent on the number of allowed active site-antigen interactions (n), the density and orientation of antigenic sites and the intrinsic association constant (K1) of an univalent antibody active site interacting with an univalent antigen. Because of the inherent uncertainty involved in the estimation of lymphocyte membrane structure and molecular parameters necessary to utilize other mathematical models, the Crothers and Metzger model was applied to the case of surface membrane bound immunoglobulin (smIg) on B-lymphocytes interacting with polyvalent antigens as a simple and direct concept. Consideration of red blood cells, dinitrophenyl16 guinea-pig albumin, and fluorescyl220 keyhole limpet hemocyanin as model antigens revealed that Kobs was many orders of magnitude higher than K1 when only a few interactions were allowed. A dissociation rate of several hours for release of polyvalent antigen from cooperating smIg was calculated for receptors with a K1 as low as 10(3) M-1. Assuming prolonged antigen contact as necessary to activate lymphocytes, cells bearing low-affinity receptors are functionally indistinguishable from those carrying higher-affinity smIg in their ability to retain antigen. The presumed role for antigen-driven selection of lymphocytes bearing increasingly higher-affinity smIg, which results in antibody affinity maturation, must be reexamined if the K1 of smIg does not confer a selective advantage to lymphocytes for retaining antigen.