Memory B cells are essential for generating rapid and robust secondary antibody responses. It has been thought that the unique cytoplasmic domain of IgG causes the prompt activation of antigen-experienced IgG memory B cells. To assess this model, we have generated a mouse containing IgG1 B cells that have never encountered antigen. We found that, upon challenge, antigen-experienced IgG1 memory B cells rapidly differentiated into plasma cells, whereas nonexperienced IgG1 B cells did not, suggesting the importance of the stimulation history. In addition, our results suggest that repression of the Bach2 transcription factor, which results from antigen experience, contributes to predisposition of IgG1 memory B cells to differentiate into plasma cells.

We found that two distinct antibody maturation pathways exist in the immune response of C57BL/6 mice to (4-hydroxy-3-nitrophenyl)acetyl and that the junctional amino acid introduced by a process far preceding somatic hypermutation determined the pathway of affinity maturation. Antibodies belonging to each pathway clearly separated into two separate branches of a phylogenic tree. We also constructed a three-dimensional fitness landscape for antibody evolution by introducing the association constants of the antibodies into the phylogenic tree as the third axis, allowing us to comprehend the significance of junctional diversity in the "evolvability" of antibodies. Thermodynamic analyses of the antigen-antibody interactions suggested that a high conformational versatility in the antigen-combining site allows for the enhanced evolvability of antibodies.

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Akatsu and colleagues show that CD72 specifically recognizes Sm/RNP, a lupus-related self-antigen and an endogenous TLR7 ligand, and inhibits B cell responses to Sm/RNP. In mice, CD72 prevents production of anti-Sm/RNP antibodies crucial for lupus development.

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