Project Summary MicroRNAs (miRNAs) are critical post-transcriptional regulators of gene expression in immunity and autoimmunity, although many miRNAs remain poorly characterized. One such miRNA, miRNA-21 (miR-21), is upregulated in systemic lupus erythematosus (SLE) patients and promotes various diseases in mouse models. These include experimental autoimmune encephalomyelitis (EAE), asthma, cancer, and our findings on its role in toll-like receptor 7 (TLR7)-driven SLE in mice. We found that TLR7 overexpressing SLE-prone B6.Sle1bYaa mice deficient in miR-21 (Sle1bYaamiR-21-/-) had reduced germinal center (GC) B cell, T follicular helper (Tfh) cell and plasma cell (PC) responses. These responses in Sle1bYaamiR-21-/- mice were strongly associated with reduced autoantibody, and splenic and bone marrow autoantibody-producing antibody forming cell (AFC) responses. These data substantiate a role for miR-21 in the loss of B cell tolerance and autoimmunity, which necessitates identification of mechanisms (focus of the grant) by which miR-21 promotes autoantibody production and SLE. This will fill gaps in knowledge and may help discover an urgently needed targeted therapy for SLE, a debilitating disease that affects millions of Americans and people world-wide. TLR signaling, especially TLR7, promotes SLE in humans and murine models, and an attractive therapeutic target. TLR7 signaling activates the NF-kB inflammatory signaling pathway. TLR7 upregulates miR-21 expression in B cells which can target the negative regulator of NF-kB signaling, Peli1, although the role of miR-21 in regulating NF- kB signaling in SLE-prone B cells by targeting Peli1 is unknown. Preliminary data showed that B cells from Sle1bYaamiR-21-/- mice have increased Peli1 expression compared to control Sle1bYaa B cells. In addition, previous studies implicated TLR signaling in metabolic reprogramming, primarily through the activation of the PI3K/Akt/mTOR pathway in various immune conditions, although the role of this pathway in TLR7-mediated metabolic reprograming in B cells in SLE is not clear. miR-21 was also shown to regulate the PI3K/Akt/mTOR pathway in various models; however, its role in regulating B cell metabolism in TLR7-driven SLE is not known. Interestingly, our RNA-seq analysis of B cells from a TLR7-induced model showed negatively enriched genes in the PI3K/Akt/mTOR pathway in the absence of miR-21. Additional preliminary data showed increased extracellular acidification (ECAR) and oxygen consumption (OCR) rates in Sle1bYaa B cells overexpressing TLR7 compared to Sle1b B cells, indicating a more energetic phenotype in response to TLR7 signaling. Conversely, Sle1bYaamiR-21-/- B cells exhibited reduced ECAR and OCR compared to Sle1bYaa B cells. We hypothesize that miR-21 promotes TLR7-driven systemic autoimmune responses and SLE disease by regulating TLR7-NF-kB signaling and metabolic reprogramming in B cells. Proposed hypothesis and specific aims will help delineate the mechan...