Dendritic cells (DCs) are important for the initiation and regulation of immune responses. In this study, we demonstrate that DC homeostatic development in peripheral lymphoid organs is negatively regulated by the transcriptional repressor, Blimp-1, which is critical for regulation of plasma cell differentiation and T cell homeostasis and function. Deletion of Prdm1, the gene encoding Blimp-1, in mouse hematopoietic lineages resulted in an increase in the steady-state number of conventional DCs (cDCs). Specifically, Prdm1 deletion increased immediate CD8− cDC precursors in peripheral lymphoid organs, causing selective expansion of the CD8− cDC population. Upon stimulus-induced maturation, Blimp-1 was up-regulated in bone marrow-derived DCs via the p38 MAPK and NF-κB pathways. Notably, Blimp-1-deficient DCs matured poorly upon stimulation in vitro and in vivo. Blimp-1 binds to the proinflammatory cytokine/chemokine genes, Il-6 and Ccl2, and negatively regulates their expression. Collectively, our findings reveal two new roles for Blimp-1: negative regulation of a select subset of cDCs during homeostatic development, and enhancement of DC maturation.

Transcriptional repressor B lymphocyte-induced maturation protein-1 (Blimp-1) is a master regulator of plasma cell differentiation. Here we show that Blimp-1 is covalently modified by SUMO1 at lysine 816, a modification mediated by SUMO E3 ligase PIAS1. Mutation of Blimp-1 lysine 816 reduces transcriptional repression--correlating with a reduced interaction with a histone deacetylase, HDAC2--and impairs differentiation of antibody-secreting cells. Thus, the SUMO pathway critically regulates Blimp-1 function during plasma cell differentiation.

The transcriptional repressor B lymphocyte-induced maturation protein-1 (Blimp-1) has crucial roles in the control of plasma cell differentiation and in maintaining survival of plasma cells. However, how Blimp-1 ensures the survival of plasma cell malignancy, multiple myeloma (MM), has remained elusive. Here we identified Aiolos, an anti-apoptotic transcription factor of MM cells, as a Blimp-1-interacting protein by mass spectrometry. ChIP coupled with DNA microarray was used to profile the global binding of Aiolos and Blimp-1 to endogenous targets in MM cells, which revealed their co-binding to a large number of genes, including apoptosis-related genes. Accordingly, Blimp-1 and Aiolos regulate similar transcriptomes in MM cells. Analysis of the binding motifs for Blimp-1 and Aiolos uncovered a partial motif that was similar across sites for both proteins. Aiolos promotes the binding of Blimp-1 to target genes and thereby enhances Blimp-1-dependent transcriptional repression. Furthermore, treatment with an anti-MM agent, lenalidomide, caused ubiquitination and proteasomal degradation of Blimp-1, leading to the de-repression of a new Blimp-1 direct target, CULLIN 4A (CUL4A), and reduced Aiolos levels. Accordingly, lenalidomide-induced cell death was partially rescued by reintroduction of Blimp-1 or knockdown of CUL4A. Thus, we demonstrated the functional impacts and underlying mechanisms of the interaction between Aiolos and Blimp-1 in maintaining MM cell survival. We also showed that interruption of Blimp-1/Aiolos regulatory pathways contributes to lenalidomide-mediated anti-MM activity.

Ag-primed B cells that result from an immune response can form either memory B cells or Ab-secreting plasma cells; however, the molecular machinery that controls this cellular fate is poorly understood. In this study, we show that activated B cell factor-1 (ABF-1), which encodes a basic helix-loop-helix transcriptional repressor, participates in this regulation. ABF-1 was prevalently expressed in purified memory B cells and induced by T follicular helper cell–mediated signals. ABF-1 expression declined by the direct repression of B lymphocyte–induced maturation protein-1 during differentiation. Ectopic expression of ABF-1 reduced the formation of Ab-secreting cells in an in vitro differentiation system of human memory B cells. Accordingly, knockdown of ABF-1 potentiates the formation of Ab-secreting cells. A transgenic mouse that expresses inducible ABF-1 in a B cell–specific manner was generated to demonstrate that the formation of germinal center and memory B cells was augmented by induced ABF-1 in an immune response, whereas the Ag-specific plasma cell response was dampened. This effect was associated with the ability of ABF-1 to limit cell proliferation. Together, our results demonstrate that ABF-1 facilitates formation of memory B cells but prevents plasma cell differentiation.

Using genome-wide approaches, we studied the microRNA (miRNA) expression profile during human plasma cell (PC) differentiation induced by stimulation of human blood B cells with T follicular helper cell–dependent signals. Combining the profiles of differentially expressed genes in PC differentiation with gene ontology (GO) analysis revealed that a significant group of genes involved in the transcription factor (TF) activity was preferentially changed. We thus focused on studying the effects of differentially expressed miRNAs on several key TFs in PC differentiation. Cohorts of differentially expressed miRNAs cooperating as miRNA hubs were predicted and validated to modulate key TFs, including a down-regulated miRNA hub containing miR-101-3p, -125b-5p, and -223-3p contributing to induction of PRDM1 as well as an up-regulated miRNA hub containing miR-34a-5p, -148a-3p, and -183-5p suppressing BCL6, BACH2, and FOXP1. Induced expression of NF-κB and PRDM1 during PC differentiation controlled the expression of up- and down-regulated miRNA hubs, respectively. Co-expression of miR-101-3p, -125b-5p, and -223-3p in stimulated B cells showed synergistic effects on inhibition of PC formation, which can be rescued by re-introduction of PRDM1. Together, we catalogue the complex roadmap of miRNAs and their functional interplay in collaboratively directing PC differentiation.