Background/Purpose: Lymphopenia is a common clinical feature in patients with systemic lupus erythematosus (SLE), and associates with high disease activity and comorbidities including infections. However, the mechanisms of lymphopenia in SLE patients are not fully understood. SLE T cells exhibit increased susceptibility to apoptosis and display altered expression of apoptosis/survival related genes such as the Bcl-2 family genes, many of which have pro- and anti-apoptotic isoforms. Using discovery approaches we previously identified the serine arginine-rich splicing factor 1 (SRSF1) in human T cells, and showed that it regulates normal expression of the CD3zeta chain and is required for IL-2 production. We showed that SRSF1 expression levels are decreased in T cells from SLE patients, and this decrease associates with severe disease. Because it is reported that SRSF1 regulates alternative splicing of the apoptosis-related gene BcL-x to promote the anti-apoptotic long (L) isoform over the pro-apoptotic short (s) isoform, we hypothesized that SRSF1 is important for T lymphocyte homeostasis, and its deficiency leads to apoptosis and lymphopenia in mice and in SLE patients.

Methods: 42 SLE patients, and age-, race- and gender-matched healthy individuals were enrolled, and clinical and laboratory parameters recorded. Peripheral blood T cells were isolated by negative selection and SRSF1 protein levels assessed by western blot. SLE patients were divided into lymphopenic (<1000/μL) and non-lymphopenic groups, and correlations were assessed with relative SRSF1 expression levels, and compared to gender- and age-matched healthy controls. To generate T cell-specific Srsf1 conditional knockout (Srsf1-cko) mice, Srsf1-flox mice were crossed with d.Lck.Cre transgenic mice. Mice were euthanized at 10-20 weeks of age or aged to >1 year, and lymphoid tissues (spleen and lymph nodes) were analyzed. Immune cell phenotype was assessed by flow cytometry. Apoptosis was assessed in splenocytes ex vivo or after induction by anti-CD95 (Fas) crosslinking, by flow cytometry staining for 7AAD and Annexin V.

Results: SLE patients with lymphopenia presented significantly lower expression levels of SRSF1 (0.65 vs. 1.05, p=0.0074), whereas there was no correlation of SRSF1 with hemoglobin, platelet counts, or serum complement levels. In parallel, peripheral T cell lymphopenia was observed in Srsf1-cko mice. Lymphopenia was more evident in younger mice, and was more profound in the CD8 than CD4 T cells. Crosslinking with anti-CD95 (Fas) antibody led to increased apoptosis in T cells from Srsf1-cko mice. The expression levels of anti-apoptotic gene Bcl-xL were decreased in spleen cells from the Srsf1-cko mice both ex vivo and after anti-CD95 (Fas) crosslinking. These results indicate that a deficiency of SRSF1 induces apoptosis in T cells through decreased expression of Bcl-xL.

Conclusion: SRSF1 controls the expression of the anti-apoptotic gene Bcl-xL and is an important regulator of T lymphocyte homeostasis in vivo and its reduced expression levels associate with lymphopenia in SLE patients. Therefore, deficiency of SRSF1 may represent a molecular defect that contributes to the pathophysiology of systemic autoimmune disease.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/serine-arginine-rich-splicing-factor-1-srsf1-is-a-novel-regulator-of-t-lymphocyte-homeostasis-in-vivo-and-its-deficiency-associates-with-lymphopenia-in-sle-patients/