Immunons revisited: binding of multivalent antigens to B cells

The T-independent B cell response induced by highly multivalent hapten polymer preparations has been studied extensively. The in vitro measured dose-response curve tends to be roughly bell-shaped with the peak response occurring at very low ligand concentrations, between 0.1-1 ng/ml for a variety of different ligands. Furthermore, polymers with more than approximately 10 haptens tend to be stimulatory, whereas polymers with fewer than 10 haptens conjugated, tend to be inhibitory. These observations have been perplexing when viewed within the context of standard theories of receptor ligation by multivalent ligands. We present a new analysis of these previous experiments that reconciles the differences between theory and experiment. From this theory it is concluded that the peak in the observed dose response curve only weakly reflects properties of the ligand and the affinity of surface immunoglobulin for the hapten, but depends strongly on the density of antigen-specific B cells in the culture. The number of responding cells decreases at low ligand concentrations, because cells have to share limiting amounts of ligand and not because of the decreasing probability of receptors and ligands meeting each other. Our theory leads to the same conclusion as made by previous researchers, namely that a minimum number of receptor sites, of the order of 10, need to be bound to a single ligand in order to stimulate a B cell. While this conclusion was based on the lack of immunogenicity of antigens carrying less than a minimum number of haptens, the quantitative results of this study, derived from fitting experimental dose response curves obtained with highly multivalent antigens, provide evidence for the immunon hypothesis that is based upon the degree of receptor aggregation. Our theory also provides quantitative agreement with experimental observations on systems, in which both stimulatory and non-stimulatory polymers are mixed in the same system.