A cell population study of IgM affinity maturation

Abstract

Affinity changes of hapten-specific IgM antibodies were analysed by plaque inhibition assays and by inhibition with varying quantities of monovalent inhibitor. The resulting data were used for cell population studies, based on the affinity of plaque-forming antibody. Maturation of IgM plaque-forming cell populations was time- and antigen dose-dependent. Maturation was characterized by a consistent increase in the proportions of high affinity and medium affinity cells and a decrease in the proportion of low affinity cell populations. Thus progressive selection of high affinity cells occurred over the entire dose range of immunization. The hapten concentration at which 20% of cells was inhibited decreased with time at all administered antigen concentrations. The inhibitory hapten concentration at which 50 or 70% of cells was inhibited did not change after administration of very large immunizing doses. Thus recruitment of this type of plaque-forming cell depended on dose. In the secondary response, high affinity cells appeared initially but very transiently, and the subsequent rate of population changes was faster than that observed in the primary response. The short productive life of IgM plaque-forming populations may be responsible for the time restriction in IgM as compared to IgG maturation.