Repertoire shift in the humoral response to phosphocholine-keyhole limpet hemocyanin: V_H somatic mutation in germinal center B cells impairs T15 Ig function

Phosphocholine (PC) is a naturally occurring Ag common to many pathogenic microorganisms. Early in the primary response to PC conjugated to keyhole limpet hemocyanin (KLH), T15 Id⁺ Abs constitute >90% of the serum Ig in BALB/c mice. During the late primary and memory response to PC-protein, a shift in the repertoire occurs and T15 Id⁺ Abs lose dominance. In this study, we use immunohistochemistry and single germinal center microdissection to locate T15 Id⁺ cells in the spleen in a primary response to PC-KLH. We demonstrate T15 Id⁺ B cells and V_H1-DFL16.1-JH1 and Vκ22-Jκ5 rearrangements in germinal centers early in the immune response; thus loss of T15 dominance is not due to lack of T15 cells within germinal centers. One-hundred thirty one V_H1 and 57 Vκ22 rearrangements were cloned and sequenced. Thirty four percent of the V_H1 clones and 37% of the Vκ22 clones contained somatic mutations indicating participation in the germinal center response. Six variant T15 H clones were expressed with wild-type T15 L chain in vitro. Two of these Abs were defective in secretion providing the first evidence that mutation occurring in vivo can disrupt Ig assembly and secretion. Of the four secretion-competent Abs, two failed to display binding to PC-protein, while the other two displayed altered carrier recognition. These results indicate that somatic mutation of T15 in vivo can result in the loss of binding and secretion, potentially leading to B cell wastage. The failure of T15 to gain affinity enhancing mutations in the face of these detrimental changes may contribute to repertoire shift.