Memory B cell commitment, maintenance and terminal differentiation

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Memory B cells acquire several intrinsic properties that differ from na・e B cells, suggesting that memory B cells may have a unique gene expression that differs quantitatively and/or qualitatively from other stages of B cells. Therefore, to clarify the mechanism responsible for memory B cell commitment, survival and terminal differentiation upon antigen reexposure, we identified changes in gene expression that occur in the transition from a naive B cell to either GC B cells, memory B cells or plasma cells upon antigen stimulation. We have cloned several genes which are highly expressed in memory B cells, including E52 and 4010. E52 was turned out to be a human homologue of the survival of motor neuron gene (SMN)1, which is a causative gene for spinal muscular atrophy (SMA). Over expression of SMN in B cell lymphoma cell lines prolonged cell survival in proapoptotic culture conditions, raising the idea that the gene could be responsible for memory B cell survival. Therefore, we characte … More rized the role of SMN in cell survival by biochemical analysis and concluded that SMN prolonged cell survival upon oxidant stress and enhanced the activity of the mitochondrial respiratory chain complex I.The 4010 gene encodes an adopter molecule and its overexpression in splenic B cells results in an augmentation of IgG1 response upon stimulation with anti-Igs and anti-CD40 mAbs in vitro. Thus, 4010 could be involved in the signaling cascade responsible for either memory B cell terminal differentiation or antibody secretion. To examine this possibility we are now establishing conditional knock out mice.To know the regulatory network responsible for memory B cell commitment, we utilized Affymetrix GeneChip analysis and characterize the changes in gene expression in the transition from naive B cells to GC B cells, memory B cells or plasma cells upon antigen stimulation. Q-PCR confirmation revealed that memory B cell population expressed a group of transcripts selectively enriched in this population, with similar time-dependent changes in their expression patterns. To utilize such genetic markers for the memory B cell lineage, we analyzed the early events in antigen-specific B cell response and suggested that activated B cells progress to memory B cells, at least, from day 5 to day 6 after immunization, accompanied by class-switch recombination and efficient proliferation. Less