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The expression of miR-29b is shown to be repressed in tonsil B cells relative to peripheral naïve B cells.Human AICDA contains a miR-29b target site in its 3′ UTR that is not conserved in rodents.Knockdown of miR-29b in naïve B cells augments AID expression, conversely miR29b overexpression in total B cells reduces AID expression.miR-29b directly targets the miR-29 binding site within the AICDA 3′ UTR.miR-29 overexpression in stimulated tonsil cells dampens CSR to IgE.Class-switch recombination (CSR) is an essential B cell process that alters the isotype of antibody produced by the B cell, tailoring the immune response to the nature of the invading pathogen. CSR requires the activity of the mutagenic enzyme AID (encoded by AICDA) to generate chromosomal lesions within the immunoglobulin genes that initiate the class switching recombination event. These AID-mediated mutations also participate in somatic-hypermutation of the immunoglobulin variable region, driving affinity maturation. As such, AID poses a significant oncogenic threat if it functions outside of the immunoglobulin locus. We found that expression of the microRNA, miR-29b, was repressed in B cells isolated from tonsil tissue, relative to circulating naïve B cells. Further investigation revealed that miR-29b was able to directly initiate the degradation of AID mRNA. Enforced overexpression of miR-29b in human B cells precipitated a reduction in overall AID protein and a corresponding diminution in CSR to IgE. Given miR-29b's ability to potently target AID, a mutagenic molecule that can initiate chromosomal translocations and “off-target” mutations, we propose that miR-29b acts to silence premature AID expression in naïve B cells, thus reducing the likelihood of inappropriate and potentially dangerous deamination activity.