MicroRNA-146a has been characterized as a negative regulator of the NF?B and interferon signaling pathways in myeloid and/or T cells, but its function in B cells is not well defined. Because microRNAs often target different pathways in immune cells, this proposal serves to fill this gap in knowledge in B cells to better understand its dysregulation in human autoimmune diseases, specifically systemic lupus erythematosus. We use miR-146a knockout mice (KO) to examine the effects of miR-146a on B cell development. We found that KO mice show a T cell-independent defect in marginal zone B (MZ) cell development, although all other immature, transitional B cells, follicular cells, and B1 cells were increased. This MZ developmental defect is likely cell intrinsic, as bone marrow progenitors were not different between KO and WT, and there were no differences in splenic immune cell egress. We then used high-throughput sequencing to compare the various spleen B cell subsets stage-specific transcriptomes, and determined the MZ cell differentiation was impaired due to decreases in the Notch2 signaling pathway. These findings show an important role of miR-146a regulating normal peripheral B cell development. We then examine the role of miR-146a on B cell function. We utilize in vitro isolated B cell assays to better characterize KO B cell characteristics compared to WT when stimulated with various T-independent (TI) or T-dependent (TD) antigens. We show that KO cells have higher proliferative capacity and activation markers in vitro than WT, in particular to TI antigen stimulation. We then perform immunization experiments with TI and TD antigens. Similar to our in vitro studies, KOs show higher primary immunization responses to TI antigen immunization, although not memory responses. We then use molecular genetics to define mechanisms of miR-146a target repression in B cells, utilizing RNA-Seq and genetic complementation studies currently underway. The combination of cellular and molecular studies will allow for better characterization of miR-146a in B cells. Finally, we end with a human application using a large, longitudinal systemic lupus cohort and discuss the importance of exploring microRNAs in human autoimmune research as promising biomarkers in an era of precision medicine.