Background/Purpose: Regulatory T cells (Tregs) are pivotal in enforcing immune tolerance to prevent and protect from autoimmunity.  Defects in Tregs lead to unchecked immune cell activation and promote autoimmune disease. Numbers and/or function of Tregs are frequently impaired in patients with autoimmune diseases. Therefore, understanding mechanisms and molecules that control Treg homeostasis, stability and function is necessary to better target Tregs in autoimmune disease. Serine/arginine-rich splicing factor 1 (SRSF1) is the prototype member of the highly conserved serine arginine (SR) family of RNA-binding proteins. We previously found that SRSF1 expression levels are decreased in T cells from systemic lupus erythematosus (SLE) patients, and associate with active disease. In addition, using Srsf1 T cell-conditional knockout (Srsf1-T cell cko) mice, we have shown that deletion of Srsf1 in T cells leads to systemic autoimmunity and lupus-nephritis in vivo through effector T cell hyperactivity. However, the role of Srsf1 in Tregs is unknown.

Methods: Treg specific Srsf1 conditional knockout (Srsf1-Treg cko) mice were generated by crossing B6.Srsf1-flox mice with B6.Foxp3.YFP.Cre
transgenic mice to delete SRSF1 in FoxP3+ Tregs. Peripheral lymphoid organs were analyzed for immune cell phenotype and function by flow cytometry. Serum autoantibodies were measured by ELISA. Tissues were formalin fixed and processed for histopathology. Apoptosis was assessed by Annexin V/7AAD staining and flow cytometry and expression of Bcl-x genes by RT-qPCR. To assess Treg function, Srsf1-deficient Tregs from Srsf1-T cell ko mice were utilized. In vitro co-culture assays were performed with CFSE-labeled conventional T (Tconv) cells followed by measurement of Tconv cell proliferation by flow cytometry. In vivo Treg function was assessed by adoptive transfer of Tregs into B6 mice, followed by induction of colitis by oral dextran sodium sulfate.

Results: Srsf1-Treg cko mice develop early fatal systemic autoimmune disease and succumb to disease at 3-4 weeks of age. Mice develop systemic autoimmunity, exhibit increased inflammatory cytokine producing T cells, and inflammatory infiltration in vital organs including the lungs and liver.  Tregs are reduced in the peripheral lymphoid tissues and display increased levels of apoptosis. Induced (i) Tregs display skewed ratios of the anti-apoptotic Bcl-xL to pro-apoptotic Bcl-xs isoforms of the Bcl-x apoptosis-related gene with a shift towards the pro-apoptotic isoform. Srsf1-deficient Tregs from the Srsf1-T cell cko mice exhibit defects in suppressive function assessed by both in vitro and in vivo suppressive function assays. In addition, these Tregs produce proinflammatory cytokines including IFN-γ, IL-17 and IL-4. RNA-seq data analysis of Srsf1-deficient Tregs reveals that SRSF1 controls the expression of genes involved in homeostasis, inflammatory cytokines and chemokines.

Conclusion: SRSF1 is a novel regulator of Treg homeostasis and function, and its deficiency in Tregs leads to fatal systemic inflammation and autoimmunity. Therefore, deficiency of SRSF1 in Tregs may represent a molecular defect that contributes to the pathogenesis of systemic autoimmune disease.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/serine-arginine-rich-splicing-factor-1-srsf1-is-indispensable-for-homeostasis-and-function-of-regulatory-t-cells/