Finding the right combination

Diversity may be the spice of life, but it’s also the key to an effective immune system, as B lymphocytes rely on extensive recombination to shuffle their antibody-coding genes to produce molecules that can recognize a diverse array of potential threats.

Antibodies with established targets can also undergo further alterations to modulate the immune response that they trigger upon antigen binding. Known as ‘class switch recombination’ (CSR), this process relies on activation-induced cytidine deaminase (AID), an enzyme that induces major rearrangements in antibody-coding loci.

Unregulated, AID can generate cancer-causing genomic rearrangements, and a team led by Tasuku Honjo and Hitoshi Nagaoka at the University of Kyoto, with Sidonia Fagarasan’s group at the RIKEN Research Center for Allergy and Immunology in Yokohama, recently set about exploring the mechanisms that help constrain expression of the Aicda gene.

“AID is tightly regulated in activated B cells and speculated to be a B cell-specific factor—however, the Aicda promoter is not lymphocyte specific,” says Thinh Huy Tran, lead author of the team’s recent article in Nature Immunology1. Comparison of the mouse and human versions of this promoter revealed four discrete segments that had been closely conserved throughout evolution. To assess their contributions to gene specificity, the researchers generated artificial promoters consisting of various subsets of these conserved regions, which they used to regulate a bioluminescence-producing ‘reporter’ gene in cultured lymphocytes.

They found that two of these four segments directly contribute to specificity. ‘Region 2’ contains binding sites for transcription factors known to guide B lymphocyte development, but also contains sequences that strongly inhibit Aicda expression. The other promoter segment, ‘region 4’, appears to participate in the strong induction of this gene in response to signaling factors that trigger CSR in vivo.

“Our results demonstrate for the first time that two separate regions contribute together to regulating Aicda expression, in which silencers are derepressed by B lineage-specific and stimulation-responsive enhancers,” says Tran. “The negative factors that restrict Aicda expression might contribute to retaining genomic stability, while region 4 is essential for Aicda response in B cells to environmental stimulation … and is critical to generate antibody diversification.”

The investigators are now examining the individual importance of these various putative Aicda regulators, but also intend to further explore the bigger picture of the effects of AID dysregulation. “We plan to investigate the correlation between Aicda expression levels with mutation frequency in non-immune genes … and the role of AID in tumor development,” says Tran.

The corresponding author for this highlight is based at the Laboratory for Mucosal Immunity, Research Center for Allergy and Immunology

Journal information
1. Tran, T.H., Nakata, M., Suzuki, K., Begum, N.A., Shinkura, R., Fagarasan, S., Honjo, T. & Nagaoka, H. B cell–specific and stimulation-responsive enhancers derepress Aicda by overcoming the effects of silencers. Nature Immunology 11 148-154 (2010).