B-cell Integration Cluster Research Articles

Regulation of B- and T-cell differentiation by a single microRNA

miRs (microRNAs) post-transcriptionally regulate gene expression mainly by repressing translation or by inducing mRNA degradation. Dicer, an enzyme responsible for miR biogenesis, is required for T-cell function, suggesting regulatory roles for miRs in lymphocytes. However, specific roles for individual miRs are only just beginning to emerge. miR-155 is encoded within an exon of the non-coding RNA known as bic (B-cell integration cluster) and high levels of bic expression are induced upon antigen receptor stimulation of B- and T-cells, as well as TLR (Toll-like receptor) stimulation of macrophages and dendritic cells. High levels of bic/miR-155 are found in B-cell lymphomas and solid tumours, indicating that this locus may also be linked to cancer. Indeed, transgenic mice overexpressing miR-155 develop B-cell malignancies. To define the in vivo role of bic/miR-155 (bic), we have studied bic-deficient mice. These mice are immunodeficient and fail to generate high levels of class-switched antibody upon immunization with thymus-dependent and thymus-independent antigens. This defect is intrinsic to B-cells and manifested at the level of differentiation of switched plasmablasts into mature antibody secreting plasma cells. In addition, bic-deficient T-cells show skewed differentiation into the Th2 lineage under a variety of in vitro culture conditions. Microarray analysis of bic-deficient B- and T-cells under different conditions has revealed a wide spectrum of targets regulated by an miR-155 and suggested mechanisms for the regulation of lymphocyte differentiation by a single miR.

B-cell Receptor Activation Induces BIC/miR-155 Expression through a Conserved AP-1 Element

microRNA-155 is an oncogenic microRNA that has been shown to be critical for B-cell maturation and immunoglobulin production in response to antigen. In line with its function in B-cell activation, miR-155, and its primary transcript, B-cell integration cluster (BIC), is induced by B-cell receptor (BCR) cross-linking. Using pharmacological inhibitors in the human B-cell line, Ramos, we show that activation of BIC and miR-155 expression by BCR signaling occurs through the extracellular signaling-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK) pathways but not the p38 pathway. BCR activation results in the induction of c-Fos, FosB, and JunB, and expression of these are suppressed by ERK and JNK inhibitors. Reporter analysis established a key role for a conserved AP-1 site approximately 40 bp upstream from the site of initiation but not an upstream NF-kappaB site or a putative c-Ets located at the site of initiation. Lastly, chromatin immunoprecipitation analysis demonstrated the recruitment of FosB and JunB to the miR-155 promoter following BCR activation. These results identify key determinants of BCR-mediated signaling that lead to the induction of BIC/miR-155.