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The AID enzyme induces class switch recombination in fibroblasts


The switch of the immunoglobulin isotype from IgM to IgG, IgE or IgA is mediated by class switch recombination (CSR). CSR changes the immunoglobulin heavy chain constant region (CH) gene from Cμ to one of the other CH genes1,2. Somatic hypermutation introduces massive numbers of point mutations in the immunoglobulin variable (V) region gene, giving rise to immunoglobulin with higher affinity3. Activation-induced cytidine deaminase (AID), a putative RNA-editing cytidine deaminase, is expressed strictly in activated B cells and is indispensable in both CSR and somatic hypermutation4,5. But the exact function of AID is unknown. Here we show that ectopic expression of AID induces CSR in an artificial switch construct in fibroblasts at a level comparable to that in stimulated B cells. Sequences around recombination junctions in the artificial substrate have features similar to endogenous CSR junctions. Furthermore, AID-induced CSR in fibroblasts is dependent on transcription of the target S region, as shown in endogenous CSR in B cells. The results show that AID is the only B-cell-specific factor required for initiation of the CSR reaction in the activated locus.

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Figure 1: Induction of CSR in an artificial substrate in CH12F3-2 cells by overexpression of AID without stimulation.
Figure 2: Induction of CSR in the artificial substrate SCI(μ,α) in NIH3T3 cells by expression of AID.
Figure 3: AID-induced CSR in NIH3T3 cells mimics physiological CSR.
Figure 4: Time course and dose dependency of AID-induced phenotypes in NIH3T3-SCI(μ,α) clone number 37.


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We are grateful to H. Yagita and Targeted Genetics Co. for providing the HM40-3 monoclonal antibody and the tgCMV/HyTK plasmid, respectively. We thank Y. Sakakibara, K. Ikuta, H. Nagaoka, S. Fagarasan and T. Okazaki for their critical reading of the manuscript; T. Muto, T. Eto and T. Doi for discussions; Y. Tabuchi, T. Toyoshima and K. Yurimoto for their technical assistance; and T. Nishikawa for secretarial help. This work was supported by a Center of Excellence Grant from the Ministry of Education, Culture, Sports, Science and Technology of Japan. I.O. is a research fellow of the Japan Society for the Promotion of Science.

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Correspondence to Tasuku Honjo.

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Okazaki, Im., Kinoshita, K., Muramatsu, M. et al. The AID enzyme induces class switch recombination in fibroblasts. Nature 416, 340–345 (2002).

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