Mice with a DC-specific deletion of the transcriptional repressor B lymphocyte–induced maturation protein-1 (Blimp1) exhibit a lupus-like phenotype, secondary to enhanced DC production of IL-6. Here we explored further phenotypic changes in Blimp1-deficient DCs, the molecular mechanism underlying these changes, and their relevance to human disease. Blimp1-deficient DCs exhibited elevated expression of MHC II, and exposure to TLR agonists increased secretion of proinflammatory cytokines. This phenotype reflects enhanced expression of the microRNA let-7c, which is regulated by BLIMP1. Let-7c reciprocally inhibited Blimp1 and also blocked LPS-induced suppressor of cytokine signaling-1 (SOCS1) expression, contributing to the proinflammatory phenotype of Blimp1-deficient DCs. DCs from Blimp1 SLE-risk allele carriers exhibited analogous phenotypic changes, including decreased BLIMP1 expression, increased let-7c expression, and increased expression of proinflammatory cytokines. These results suggest that let-7c regulates DC phenotype and confirm the importance of BLIMP1 in maintaining tolerogenic DCs in both mice and humans.
(A) Diagram of a mouse genome sequence encompassing let-7c. Premature let-7c gene encompassing mature miRNAs (in upper case) is in bold, and a putative Blimp1 binding sequence is boxed. Arrows indicate primers for the ChIP assay. (B) EMSA was performed with nuclear extracts (NE) prepared from BM-DCs that were stimulated with LPS for 24 hours. In vitro binding of proteins and oligos was performed under various conditions, and the reaction mixtures were separated on a 4% acrylamide gel. Images were scanned and analyzed by Odyssey (LI-COR Biosciences). (C) ChIP assay was performed with LPS-stimulated BM-DCs generated from control or DCBlimp1ko mice. Total input and elutions after immunoprecipitation with either control IgG or anti–Blimp1 IgG were diluted, and PCR was performed with primers as depicted in (A). A representative image of 4 independent experiments is shown. Quantitative PCR was performed and the amount compared with total input was calculated using mean ± SEM of 4 independent experiments.