The strength of B cell interaction with antigen determines the degree of IgM polymerization

The induction of variable disulfide polymerization of IgM in the trout (Oncorhynchus mykiss) and its effect on its half-life were examined. An association between greater Ab affinity and increased disulfide polymerization was first indicated by the observation of this increased IgM disulfide polymerization during the process of affinity maturation. A direct association between Ab affinity and disulfide polymerization was then established by the fractionation of individual sera into high- and low-affinity subpopulations, which also resulted in the partitioning of high and low degrees of disulfide polymerization. The ability of high-affinity B cells to produce more highly polymerized Abs upon Ag induction was demonstrated by in vitro Ag-driven selection. Low Ag concentrations, which elicited only high-affinity Abs, also possessed the highest degree of polymerization, whereas higher concentrations of Ag elicited a broader array of Ab affinities, yielding a lower average affinity and degree of polymerization. Half-life studies revealed that the high-affinity, highly polymerized Abs possessed longer half-lives than the lower-affinity, lightly polymerized Abs. Finally, although the affinity for Ag is associated with elevated levels of polymerization, analysis of naive Ig revealed that the degree of polymerization alone, not affinity, appears sufficient to prolong Ig half-life.