Splicing factor SRSF1 controls autoimmune-related molecular pathways in regulatory T cells distinct from FoxP3

Regulatory T cells (Tregs) are vital for maintaining immune self-tolerance, and their impaired function leads to autoimmune disease. Mutations in FoxP3, the master transcriptional regulator of Tregs, leads to immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome in humans and the early lethal "scurfy" phenotype with multi-organ autoimmune disease in mice. We recently identified serine/arginine-rich splicing factor 1 (SRSF1) as an indispensable regulator of Treg homeostasis and function. Intriguingly, Treg-conditional SRSF1-deficient mice exhibit early lethal systemic autoimmunity with multi-organ inflammation reminiscent of the scurfy mice. Importantly, SRSF1 is decreased in T cells from patients with the autoimmune disease systemic lupus erythematosus (SLE), and low SRSF1 levels inversely correlate with disease severity. Given that the Treg-specific deficiency of SRSF1 causes similarly profound autoimmune disease outcomes in mice as the deficiency/mutation in FoxP3, we aimed to evaluate the genes and molecular pathways controlled by these two indispensable regulatory proteins. We performed comparative bioinformatic analyses of transcriptomic profiles of Tregs from Srsf1-knockout mice and two Foxp3 mutant mice--the FoxP3-deficient ΔFoxp3 and the Foxp3 M370I mutant mice. We identified 132 differentially expressed genes (DEGs) unique to Srsf1-ko Tregs, 503 DEGs unique to Foxp3 M370I Tregs, and 1367 DEGs unique to ΔFoxp3 Tregs. Gene set enrichment and pathway analysis of DEGs unique to Srsf1-ko Tregs indicate that SRSF1 controls cytokine and immune response pathways. Conversely, FoxP3 controls pathways involved in DNA replication and cell cycle. Besides the distinct gene signatures, we identified only 30 shared genes between all three Treg mutants, mostly contributing to cytokine and immune defense pathways. Prominent genes included the chemokines CXCR6 and CCL1 and the checkpoint inhibitors FASLG and PDCD1. Thus, we demonstrate that SRSF1 and FoxP3 control common and distinct molecular pathways implicated in autoimmunity. Our analyses suggest that SRSF1 controls crucial immune functions in Tregs contributing to immune tolerance, and perturbations in its levels lead to systemic autoimmunity via mechanisms that are largely distinct from FoxP3.

Splicing factor SRSF1 is essential for CD8 T cell function and host antigen-specific viral immunity

Cytotoxic CD8 T cells are crucial for the host antigen-specific immune response to viral pathogens. Here we report the identification of an essential role for the serine/arginine-rich splicing factor (SRSF) 1 in CD8 T cell homeostasis and function. Specifically, SRSF1 is necessary for the maintenance of normal CD8 T lymphocyte numbers in the lymphoid compartment, and for the proliferative capacity and cytotoxic function of CD8 T cells. Furthermore, SRSF1 is required for antigen-specific IFN-γ cytokine responses in a viral infection challenge in mice. Transcriptomics analyses of Srsf1-deficient T cells reveal that SRSF1 controls proliferation, MAP kinase signaling and IFN signaling pathways. Mechanistically, SRSF1 controls the expression and activity of the Mnk2/p38-MAPK axis at the molecular level. Our findings reveal previously unrecognized roles for SRSF1 in the physiology and function of cytotoxic CD8 T lymphocytes and a potential molecular mechanism in viral immunopathogenesis.

Splicing factor SRSF1 controls distinct molecular programs in regulatory and effector T cells implicated in systemic autoimmune disease

Systemic autoimmune diseases are characterized by hyperactive effector T cells (Teffs), aberrant cytokines and chemokines, and dysfunctional regulatory T cells (Tregs). We previously uncovered new roles for serine/arginine-rich splicing factor 1 (SRSF1) in the control of genes involved in T cell signaling and cytokine production in human T cells. SRSF1 levels are decreased in T cells from patients with systemic lupus erythematosus (SLE), and low levels correlate with severe disease. Moreover, T cell-conditional Srsf1-deficient mice recapitulate the autoimmune phenotype, exhibiting CD4 T cell hyperactivity, dysfunctional Tregs, systemic autoimmunity, and tissue inflammation. However, the role of SRSF1 in controlling molecular programs in Teffs and Tregs and how these pathways are implicated in autoimmunity is not known. Here, by comparative bioinformatics analysis, we demonstrate that SRSF1 controls largely distinct gene programs in Tregs and Teffs in vivo. SRSF1 regulates 189 differentially expressed genes (DEGs) unique to Tregs, 582 DEGs unique to Teffs, and 29 DEGs shared between both. Shared genes included IL-17A, IL-17F, CSF1, CXCL10, and CXCR4, and were highly enriched for inflammatory response and cytokine-cytokine receptor interaction pathways. SRSF1 controls distinct pathways in Tregs, which include chemokine signaling and immune cell differentiation, compared with pathways in Teffs, which include cytokine production, T cell homeostasis, and activation. We identified putative mRNA binding targets of SRSF1 which include CSF1, CXCL10, and IL-17F. Finally, comparisons with transcriptomics profiles from lupus-prone MRL/lpr mice reveal that SRSF1 controls genes and pathways implicated in autoimmune disease. The target genes of SRSF1 and putative binding targets we discovered, have known roles in systemic autoimmunity. Our findings suggest that SRSF1 controls distinct molecular pathways in Tregs and Teffs and aberrant SRSF1 levels may contribute to their dysfunction and immunopathogenesis of systemic autoimmune disease.

Complement activation and increased expression of Syk, mucin-1 and CaMK4 in kidneys of patients with COVID-19

Acute and chronic kidney failure is common in hospitalized patients with COVID-19, yet the mechanism of injury and predisposing factors remain poorly understood. We investigated the role of complement activation by determining the levels of deposited complement components (C1q, C3, FH, C5b-9) and immunoglobulin along with the expression levels of the injury-associated molecules spleen tyrosine kinase (Syk), mucin-1 (MUC1) and calcium/calmodulin-dependent protein kinase IV (CaMK4) in the kidney tissues of people who succumbed to COVID-19. We report increased deposition of C1q, C3, C5b-9, total immunoglobulin, and high expression levels of Syk, MUC1 and CaMK4 in the kidneys of COVID-19 patients. Our study provides strong rationale for the expansion of trials involving the use of inhibitors of these molecules, in particular C1q, C3, Syk, MUC1 and CaMK4 to treat patients with COVID-19.

Splicing factor SRSF1 is indispensable for regulatory T cell homeostasis and function

The ability of regulatory T (Treg) cells to control the immune response and limit the development of autoimmune diseases is determined by distinct molecular processes, which are not fully understood. We show here that serine/arginine-rich splicing factor 1 (SRSF1), which is decreased in T cells from patients with systemic lupus erythematosus, is necessary for the homeostasis and proper function of Treg cells, because its conditional absence in these cells leads to profound autoimmunity and organ inflammation by elevating the glycolytic metabolism and mTORC1 activity and the production of proinflammatory cytokines. Our data reveal a molecular mechanism that controls Treg cell plasticity and offer insights into the pathogenesis of autoimmune disease.

Estrogen-induced hsa-miR-10b-5p is elevated in T cells from patients with systemic lupus erythematosus and downregulates splicing factor SRSF1

Objective

Autoimmune diseases affect women disproportionately more than men. Estrogen is implicated in immune cell dysfunction, yet its precise molecular roles are not fully known. We recently identified new roles for serine/arginine‐rich splicing factor 1 (SRSF1) in T cell function and autoimmunity. SRSF1 levels are decreased in T cells from patients with systemic lupus erythematosus (SLE) and are associated with active disease and comorbidity. However, the molecular mechanisms that control SRSF1 expression are unknown. Srsf1 messenger RNA (mRNA) has a long 3′‐untranslated region (3′‐UTR), suggesting posttranscriptional control. This study was undertaken to investigate the role of estrogen and posttranscriptional mechanisms of SRSF1 regulation in T cells and SLE.

Methods

In silico bioinformatics analysis of Srsf1–3′‐UTR revealed multiple microRNA (miRNA; miR)–binding sites. Additional screening and literature searches narrowed down hsa‐miR‐10b‐5p for further study. Peripheral blood T cells from healthy individuals and SLE patients were evaluated for mRNA and miRNA expression by quantitative reverse transcription–polymerase chain reaction, and SRSF1 protein levels were assessed by immunoblotting. T cells were cultured with β‐estradiol, and transient transfections were used to overexpress miRNAs. Luciferase assays were used to measure 3′‐UTR activity.

Results

We demonstrated that estrogen increased hsa‐miR‐10b‐5p expression in human T cells, and hsa‐miR‐10b‐5p down‐regulated SRSF1 protein expression. Mechanistically, hsa‐mir‐10b‐5p regulated SRSF1 posttranscriptionally via control of its 3′‐UTR activity. Importantly, hsa‐miR‐10b‐5p expression levels were elevated in T cells from healthy women compared to healthy men and also elevated in T cells from SLE patients.

Conclusion

We identified a previously unrecognized molecular link between estrogen and gene regulation in immune cells, with potential relevance to systemic autoimmune disease.

Activation of classical and alternative complement pathways in the pathogenesis of lung injury in COVID-19

Lung inflammation and damage is prominent in people infected with SARS-Cov-2 and a major determinant of morbidity and mortality. We report the deposition of complement components in the lungs of people who succumbed to COVID-19 consistent with the activation of the classical and the alternative pathways. Our study provides strong rationale for the expansion of trials involving the use of complement inhibitors to treat patients with COVID-19.

Splicing factor SRSF1 limits IFN-γ production via RhoH and ameliorates experimental nephritis

OBJECTIVE

CD4 T helper 1 (Th1) cells producing IFN-γ contribute to inflammatory responses in the pathogenesis of SLE and lupus nephritis. Moreover, elevated serum type II IFN levels precede the appearance of type I IFNs and autoantibodies in patient years before clinical diagnosis. However, the molecules and mechanisms that control this inflammatory response in SLE remain unclear. Serine/arginine-rich splicing factor 1 (SRSF1) is decreased in T cells from SLE patients, and restrains T cell hyperactivity and systemic autoimmunity. Our objective here was to evaluate the role of SRSF1 in IFN-γ production, Th1 differentiation and experimental nephritis.

METHODS

T cell-conditional Srsf1-knockout mice were used to study nephrotoxic serum-induced nephritis and evaluate IFN-γ production and Th1 differentiation by flow cytometry. RNA sequencing was used to assess transcriptomics profiles. RhoH was silenced by siRNA transfections in human T cells by electroporation. RhoH and SRSF1 protein levels were assessed by immunoblots.

RESULTS

Deletion of Srsf1 in T cells led to increased Th1 differentiation and exacerbated nephrotoxic serum nephritis. The expression levels of RhoH are decreased in Srsf1-deficient T cells, and silencing RhoH in human T cells leads to increased production of IFN-γ. Furthermore, RhoH expression was decreased and directly correlated with SRSF1 in T cells from SLE patients.

CONCLUSION

Our study uncovers a previously unrecognized role of SRSF1 in restraining IFN-γ production and Th1 differentiation through the control of RhoH. Reduced expression of SRSF1 may contribute to pathogenesis of autoimmune-related nephritis through these molecular mechanisms.

Splicing factor SRSF1 controls T cell homeostasis and its decreased levels are linked to lymphopenia in systemic lupus erythematosus

OBJECTIVE

Lymphopenia is a frequent clinical manifestation and risk factor for infections in SLE, but the underlying mechanisms are not fully understood. We previously identified novel roles for the RNA-binding protein serine arginine-rich splicing factor 1 (SRSF1) in the control of genes involved in signalling and cytokine production in human T cells. SRSF1 is decreased in T cells from patients with SLE and associates with severe disease. Because SRSF1 controls the expression of apoptosis-related genes, we hypothesized that SRSF1 controls T cell homeostasis and, when reduced, leads to lymphopenia.

METHODS

We evaluated SRSF1 expression in T cells from SLE patients by immunoblots and analysed its correlation with clinical parameters. T cell conditional Srsf1 knockout mice were used to evaluate lymphoid cells and apoptosis by flow cytometry. Quantitative PCR and immunoblots were used to assess Bcl-xL mRNA and protein expression. SRSF1 overexpression was performed by transient transfections by electroporation.

RESULTS

We found that low SRSF1 levels correlated with lymphopenia in SLE patients. Selective deletion of Srsf1 in T cells in mice led to T cell lymphopenia, with increased apoptosis and decreased expression of the anti-apoptotic Bcl-xL. Lower SRSF1 expression correlated with low Bcl-xL levels in T cells and lower Bcl-xL levels associated with lymphopenia in SLE patients. Importantly, overexpression of SRSF1 rescued survival of T cells from patients with SLE.

CONCLUSION

Our studies uncovered a previously unrecognized role for SRSF1 in the control of T cell homeostasis and its reduced expression as a molecular defect that contributes to lymphopenia in systemic autoimmunity.

Aging, Immunity, and COVID-19: How Age Influences the Host Immune Response to Coronavirus Infections?

The novel coronavirus severe acute respiratory syndrome coronavirus 2 causing the Coronavirus disease (COVID-19) pandemic has ravaged the world with over 72 million total cases and over 1.6 million deaths worldwide as of early December 2020. An overwhelming preponderance of cases and deaths is observed within the elderly population, and especially in those with pre-existing conditions and comorbidities. Aging causes numerous biological changes in the immune system, which are linked to age-related illnesses and susceptibility to infectious diseases. Age-related changes influence the host immune response and therefore not only weaken the ability to fight respiratory infections but also to mount effective responses to vaccines. Immunosenescence and inflamm-aging are considered key features of the aging immune system wherein accumulation of senescent immune cells contribute to its decline and simultaneously increased inflammatory phenotypes cause immune dysfunction. Age-related quantitative and qualitative changes in the immune system affect cells and soluble mediators of both the innate and adaptive immune responses within lymphoid and non-lymphoid peripheral tissues. These changes determine not only the susceptibility to infections, but also disease progression and clinical outcomes thereafter. Furthermore, the response to therapeutics and the immune response to vaccines are influenced by age-related changes within the immune system. Therefore, better understanding of the pathophysiology of aging and the immune response will not only help understand age-related diseases but also guide targeted management strategies for deadly infectious diseases like COVID-19.

COVID-19 and Systemic Lupus Erythematosus: Focus on Immune Response and Therapeutics

The SARS-CoV-2 novel coronavirus has caused the COVID-19 pandemic with over 35 million cases and over a million deaths worldwide as of early October 2020. The populations most affected are the elderly and especially those with underlying comorbidities. In terms of race and ethnicity, black and hispanic populations are affected at disproportionately higher rates. Individuals with underlying conditions that cause an immune-compromised state are considered vulnerable to this infection. The immune response is an important determinant in viral infections including coronaviruses, not only in the antiviral defense but also in the disease progression, severity, and clinical outcomes of COVID-19. Systemic lupus erythematosus is a chronic autoimmune disease which also disproportionately afflicts black and hispanic populations. In lupus patients, an aberrant immune response is characterized by the presence of circulating autoantibodies, lymphopenia, aberrant T cells, and proinflammatory cytokines along with defective regulatory mechanisms, leading to immune-mediated damage to tissues. Lupus patients are often treated with immune-suppressants and therefore are immune-compromised and more susceptible to infections and may be vulnerable to coronavirus infection. While the anti-viral immune response is important to protect from coronavirus infection, an uncontrolled proinflammatory cytokine response can lead to cytokine storm which causes damage to the lungs and other organs, causing significant morbidity and mortality. Better understanding of the underlying immune response and therapeutic strategies in lupus and COVID-19 is important to guide management of this deadly infectious disease in the context of lupus and vice-versa.

What's Sex Got to Do With COVID-19? Gender-Based Differences in the Host Immune Response to Coronaviruses

The novel severe acute respiratory syndrome coronavirus 2, the cause of the coronavirus disease 2019 (COVID-19) pandemic, has ravaged the world, with over 22 million total cases and over 770,000 deaths worldwide as of August 18, 2020. While the elderly are most severely affected, implicating an age bias, a striking factor in the demographics of this deadly disease is the gender bias, with higher numbers of cases, greater disease severity, and higher death rates among men than women across the lifespan. While pre-existing comorbidities and social, behavioral, and lifestyle factors contribute to this bias, biological factors underlying the host immune response may be crucial contributors. Women mount stronger immune responses to infections and vaccinations and outlive men. Sex-based biological factors underlying the immune response are therefore important determinants of susceptibility to infections, disease outcomes, and mortality. Despite this, gender is a profoundly understudied and often overlooked variable in research related to the immune response and infectious diseases, and it is largely ignored in drug and vaccine clinical trials. Understanding these factors will not only help better understand the pathogenesis of COVID-19, but it will also guide the design of effective therapies and vaccine strategies for gender-based personalized medicine. This review focuses on sex-based differences in genes, sex hormones, and the microbiome underlying the host immune response and their relevance to infections with a focus on coronaviruses.

Splicing factor SRSF1 controls T cell hyperactivity and systemic autoimmunity

Systemic lupus erythematosus (SLE) is a devastating autoimmune disease in which hyperactive T cells play a critical role. Understanding molecular mechanisms underlying the T cell hyperactivity will lead to identification of specific therapeutic targets. Serine/arginine-rich splicing factor 1 (SRSF1) is an essential RNA-binding protein that controls posttranscriptional gene expression. We have demonstrated that SRSF1 levels are aberrantly decreased in T cells from patients with SLE and that they correlate with severe disease, yet the role of SRSF1 in T cell physiology and autoimmune disease is largely unknown. Here we show that T cell-restricted Srsf1-deficient mice develop systemic autoimmunity and lupus-nephritis. Mice exhibit increased frequencies of activated/effector T cells producing proinflammatory cytokines, and an elevated T cell activation gene signature. Mechanistically, we noted increased activity of the mechanistic target of rapamycin (mTOR) pathway and reduced expression of its repressor PTEN. The mTOR complex 1 (mTORC1) inhibitor rapamycin suppressed proinflammatory cytokine production by T cells and alleviated autoimmunity in Srsf1-deficient mice. Of direct clinical relevance, PTEN levels correlated with SRSF1 in T cells from patients with SLE, and SRSF1 overexpression rescued PTEN and suppressed mTORC1 activation and proinflammatory cytokine production. Our studies reveal the role of a previously unrecognized molecule, SRSF1, in restraining T cell activation, averting the development of autoimmune disease, and acting as a potential therapeutic target for lupus.

Systemic lupus erythematosus favors the generation of IL-17 producing double negative T cells

Mature double negative (DN) T cells are a population of αβ T cells that lack CD4 and CD8 coreceptors and contribute to systemic lupus erythematosus (SLE). The splenic marginal zone macrophages (MZMs) are important for establishing immune tolerance, and loss of their number or function contributes to the progression of SLE. Here we show that loss of MZMs impairs the tolerogenic clearance of apoptotic cells and alters the serum cytokine profile, which in turn provokes the generation of DN T cells from self-reactive CD8⁺ T cells. Increased Ki67 expression, narrowed TCR V-beta repertoire usage and diluted T-cell receptor excision circles confirm that DN T cells from lupus-prone mice and patients with SLE undergo clonal proliferation and expansion in a self-antigen dependent manner, which supports the shared mechanisms for their generation. Collectively, our results provide a link between the loss of MZMs and the expansion of DN T cells, and indicate possible strategies to prevent the development of SLE.

Splenic marginal zone macrophages can establish immune tolerance and limit the development of systemic lupus erythematosus (SLE). Here the authors show that these cells do this by clearing apoptotic cells, and defects in these cells result in the generation of self-reactive double negative T cells that are known to contribute to SLE pathogenesis.

PP2A enables IL-2 signaling by preserving IL-2Rβ chain expression during Treg development

Tregs require IL-2 signaling for signal transducer and activator of transcription 5 (STAT5)-mediated induction of Foxp3. While phosphatase 2A (PP2A) is a negative regulator of IL-2 production in effector T cells and Tregs do not produce IL-2, it is not known whether PP2A controls IL-2 signaling in Tregs. To address the role of PP2A in IL-2 signaling in Tregs we studied mice engineered to lack PP2A in all Foxp3-expressing cells. We report that PP2A is required to enable Foxp3 expression and to maintain sufficient numbers of Tregs in the thymus. We show for the first time that PP2A prevents the selective loss of surface IL-2Rβ and preserves IL-2R signaling potency in Tregs. The loss of IL-2Rβ in thymus- and spleen-derived Tregs that lack PP2A is due to increased sheddase activity. Pan-sheddase or selective A disintegrin and metalloproteinase 10 (ADAM10) inhibition, like forced expression of IL-2Rβ in PP2A-deficient Tregs restored IL-2Rβ expression and signaling. Thus, PP2A restrains the sheddase activity of ADAM10 in Treg cells to prevent the cleavage of IL-2Rβ from the cell surface to enable competent IL-2R signaling which is essential for Tregs development and homeostasis.

Sex Hormones in Acquired Immunity and Autoimmune Disease

Women have stronger immune responses to infections and vaccination than men. Paradoxically, the stronger immune response comes at a steep price, which is the high incidence of autoimmune diseases in women. The reasons why women have stronger immunity and higher incidence of autoimmunity are not clear. Besides gender, sex hormones contribute to the development and activity of the immune system, accounting for differences in gender-related immune responses. Both innate and adaptive immune systems bear receptors for sex hormones and respond to hormonal cues. This review focuses on the role of sex hormones particularly estrogen, in the adaptive immune response, in health, and autoimmune disease with an emphasis on systemic lupus erythematosus.

Decreased Expression of Serine/Arginine-Rich Splicing Factor 1 in T Cells From Patients With Active Systemic Lupus Erythematosus Accounts for Reduced Expression of RasGRP1 and DNA Methyltransferase 1

OBJECTIVE

T cells from systemic lupus erythematosus (SLE) patients have reduced protein levels of RasGRP1, a guanine nucleotide exchange factor for Ras, and increased transcript of alternatively spliced (AS) forms lacking exon 11. Serine/arginine-rich splicing factor 1 (SRSF1) binds pre-messenger RNA (pre-mRNA) to regulate AS forms of several genes, including CD3ζ in SLE T cells. This study was undertaken to assess whether SRSF1 controls the expression of RasGRP1 in T cells from patients with SLE.

METHODS

We studied T cells from 45 SLE patients and 18 healthy subjects. Expression levels of SRSF1, wild-type (WT) RasGRP1, and DNA methyltransferase 1 (DNMT1) were assessed by quantitative polymerase chain reaction. Direct binding of SRSF1 to exon 11 of RasGRP1 mRNA was evaluated with an oligonucleotide-protein pulldown assay. Healthy T cells and SLE T cells were treated with SRSF1-specific small interfering RNA or SRSF1 expression vector, respectively, and then evaluated for mRNA/protein expression.

RESULTS

SRSF1 expression levels were significantly lower in T cells from SLE patients compared to those from healthy subjects, and correlated inversely with disease activity and positively with levels of RasGRP1-WT and DNMT1. SRSF1 bound directly to exon 11 of RasGRP1 mRNA. Silencing of SRSF1 in human T cells led to increased ratios of RasGRP1-AS to RasGRP1-WT and decreased levels of RasGRP1 protein, whereas overexpression of SRSF1 in SLE T cells caused recovery of RasGRP1, which in turn induced DNMT1/interleukin-2 expression.

CONCLUSION

SRSF1 controls the alternative splicing of RasGRP1 and subsequent protein expression. Our findings extend evidence that alternative splicing plays a central role in the aberrant T cell function in patients with SLE by controlling the expression of multiple genes.

Precision DNA demethylation ameliorates disease in lupus-prone mice

Defective DNA methylation in T cells leads to a series of T cell abnormalities in lupus; however, the full effect of T cell lineage-specific DNA methylation on disease expression has not been explored. Here, we show that 5-azacytidine, a DNA methyltransferase inhibitor, targeted to either CD4 or CD8 T cells in mice with established disease using a nanolipogel delivery system dramatically ameliorates lupus-related pathology through distinct mechanisms. In vivo targeted delivery of 5-azacytidine into CD4 T cells favors the expansion and function of Foxp3+ Tregs, whereas targeted delivery to CD8 T cells enhances the cytotoxicity and restrains the expansion of pathogenic TCR-αβ+CD4-CD8- double-negative T cells. Our results signify the importance of cell-specific inhibition of DNA methylation in the treatment of established lupus.

Aberrant T Cell Signaling and Subsets in Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a chronic multi-organ debilitating autoimmune disease, which mainly afflicts women in the reproductive years. A complex interaction of genetics, environmental factors and hormones result in the breakdown of immune tolerance to "self" leading to damage and destruction of multiple organs, such as the skin, joints, kidneys, heart and brain. Both innate and adaptive immune systems are critically involved in the misguided immune response against self-antigens. Dendritic cells, neutrophils, and innate lymphoid cells are important in initiating antigen presentation and propagating inflammation at lymphoid and peripheral tissue sites. Autoantibodies produced by B lymphocytes and immune complex deposition in vital organs contribute to tissue damage. T lymphocytes are increasingly being recognized as key contributors to disease pathogenesis. CD4 T follicular helper cells enable autoantibody production, inflammatory Th17 subsets promote inflammation, while defects in regulatory T cells lead to unchecked immune responses. A better understanding of the molecular defects including signaling events and gene regulation underlying the dysfunctional T cells in SLE is necessary to pave the path for better management, therapy, and perhaps prevention of this complex disease. In this review, we focus on the aberrations in T cell signaling in SLE and highlight therapeutic advances in this field.

Downregulation of CD3ζ in NK Cells from Systemic Lupus Erythematosus Patients Confers a Proinflammatory Phenotype

Cytotoxic function and cytokine profile of NK cells are compromised in patients with systemic lupus erythematosus (SLE). CD3ζ, an important molecule for NK cell activation, is downregulated in SLE T cells and contributes to their altered function. However, little is known about the role of CD3ζ in SLE NK cells. We studied CD3ζ levels and its contribution to cytotoxic, degranulation, and cytokine production capacity of NK cells from patients with SLE. Furthermore, we studied the human NK cell line, NKL, in which manipulation of CD3ζ levels was achieved using small interfering RNA and NK cells from Rag2 mice deficient in CD3ζ. We found reduced CD3ζ expression in NK cells from SLE patients independent of disease activity. Downregulation of CD3ζ expression in NK cells is mediated, at least in part, by Caspase 3, the activity of which is higher in NK cells from patients with SLE compared with NK cells from healthy donors. CD3ζ levels correlated inversely with natural cytotoxicity and the percentage of cells capable of producing the proinflammatory cytokines IFN-γ and TNF. In contrast, CD3ζ levels showed a direct correlation with levels of Ab-dependent cellular cytotoxicity. Experiments performed in CD3ζ-silenced NKL and CD3ζ-deficient NK cells from Rag2 mice confirmed the dependence of NK cell function on CD3ζ levels. Our results demonstrate a differential role for CD3ζ in natural cytotoxicity and Ab-dependent cellular cytotoxicity. We conclude that downregulated CD3ζ confers a proinflammatory phenotype to SLE NK cells and contributes to their altered function in patients with SLE.

Pathogenesis of Human Systemic Lupus Erythematosus: A Cellular Perspective

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease affecting multiple organs. A complex interaction of genetics, environment, and hormones leads to immune dysregulation and breakdown of tolerance to self-antigens, resulting in autoantibody production, inflammation, and destruction of end-organs. Emerging evidence on the role of these factors has increased our knowledge of this complex disease, guiding therapeutic strategies and identifying putative biomarkers. Recent findings include the characterization of genetic/epigenetic factors linked to SLE, as well as cellular effectors. Novel observations have provided an improved understanding of the contribution of tissue-specific factors and associated damage, T and B lymphocytes, as well as innate immune cell subsets and their corresponding abnormalities. The intricate web of involved factors and pathways dictates the adoption of tailored therapeutic approaches to conquer this disease.

Serine Arginine-Rich Splicing Factor 1 (SRSF1) Contributes to the Transcriptional Activation of CD3ζ in Human T Cells

T lymphocytes from many patients with systemic lupus erythematosus (SLE) express decreased levels of the T cell receptor (TCR)-associated CD3 zeta (ζ) signaling chain, a feature directly linked to their abnormal phenotype and function. Reduced mRNA expression partly due to defective alternative splicing, contributes to the reduced expression of CD3ζ chain. We previously identified by oligonucleotide pulldown and mass spectrometry approaches, the serine arginine-rich splicing factor 1 (SRSF1) binding to the 3’ untranslated region (UTR) of CD3ζ mRNA. We showed that SRSF1 regulates alternative splicing of the 3’UTR of CD3ζ to promote expression of the normal full length 3`UTR over an unstable splice variant in human T cells. In this study we show that SRSF1 regulates transcriptional activation of CD3ζ. Specifically, overexpression and silencing of SRSF1 respectively increases and decreases CD3ζ total mRNA and protein expression in Jurkat and primary T cells. Using promoter-luciferase assays, we show that SRSF1 enhances transcriptional activity of the CD3ζ promoter in a dose dependent manner. Chromatin immunoprecipitation assays show that SRSF1 is recruited to the CD3ζ promoter. These results indicate that SRSF1 contributes to transcriptional activation of CD3ζ. Thus our study identifies a novel mechanism whereby SRSF1 regulates CD3ζ expression in human T cells and may contribute to the T cell defect in SLE.

T cell signaling abnormalities contribute to aberrant immune cell function and autoimmunity

Systemic lupus erythematosus (SLE) is a prototype systemic autoimmune disease that results from a break in immune tolerance to self-antigens, leading to multi-organ destruction. Autoantibody deposition and inflammatory cell infiltration in target organs such as kidneys and brain lead to complications of this disease. Dysregulation of cellular and humoral immune response elements, along with organ-defined molecular aberrations, form the basis of SLE pathogenesis. Aberrant T lymphocyte activation due to signaling abnormalities, linked to defective gene transcription and altered cytokine production, are important contributors to SLE pathophysiology. A better understanding of signaling and gene regulation defects in SLE T cells will lead to the identification of specific novel molecular targets and predictive biomarkers for therapy.

Ubiquitination regulates expression of the serine/arginine-rich splicing factor 1 (SRSF1) in normal and systemic lupus erythematosus (SLE) T cells

T cells from patients with systemic lupus erythematosus (SLE) exhibit reduced expression of the critical T cell receptor (TCR)-associated CD3ζ signaling chain and are poor producers of the vital cytokine IL-2. By oligonucleotide pulldown and mass spectrometry discovery approaches, we identified the splicing regulator serine/arginine-rich splicing factor (SRSF) 1 or splicing factor 2/alternative splicing factor (SF2/ASF) to be important in the expression of CD3ζ chain. Importantly, increases in the expression of SRSF1 rescued IL-2 production in T cells from patients with SLE. In this study, we investigated the regulation of SRSF1 expression in resting and activated human T cells. We found that T cell stimulation induced a rapid and significant increase in mRNA expression of SRSF1; however, protein expression levels did not correlate with this increase. Co-engagement of CD28 induced a similar mRNA induction and reduction in protein levels. Proteasomal but not lysosomal degradation was involved in this down-regulation as evidenced by blocking with specific inhibitors MG132 and bafilomycin, respectively. Immunoprecipitation studies showed increased ubiquitination of SRSF1 in activated T cells. Interestingly, T cells from patients with SLE showed increased ubiquitination of SRSF1 when compared with those from healthy individuals. Our results demonstrate a novel mechanism of regulation of the splicing factor SRSF1 in human T cells and a potential molecular mechanism that controls its expression in SLE.

Estrogen upregulates cyclic AMP response element modulator α expression and downregulates interleukin-2 production by human T lymphocytes

Systemic lupus erythematosus (SLE) is an autoimmune disease with a complex multifactorial pathogenesis. T lymphocytes play a critical role in disease pathogenesis and display abnormal gene expression and poor interleukin (IL)-2 production. We previously showed that the expression of the transcriptional repressor cyclic AMP response element modulator α (CREMα) is increased in SLE T cells and contributes to reduced IL-2 production. Although estrogen is implicated in the onset and exacerbation of SLE, the precise nature of molecular events regulated by estrogen in immune cell function is not well understood. Here, we asked whether estrogen regulates the expression of CREMα in human T lymphocytes. We show that exposure of human T cells to 17-β-estradiol leads to a dose-dependent increase in CREMα mRNA expression, and this increase appears to be mediated through the estrogen receptors α and β. We show that the increased expression of CREMα is due to increased transcriptional activity of the CREM promoter and is mediated by increased expression and binding of the Sp1 transcriptional activator. We further show that estrogen treatment leads to a dose-dependent decrease in IL-2 mRNA and cytokine production by T cells. Finally, the effect of β-estradiol on CREMα is observed more frequently in T cells from women than from men. We conclude that estrogen can modulate the expression of CREMα and lead to IL-2 suppression in human T lymphocytes, thus revealing a molecular link between hormones and the immune system in SLE.

Methods and protocols to study T cell signaling abnormalities in human systemic lupus erythematosus

Abnormal expression of key signaling molecules and defective functions of T lymphocytes play a significant role in the pathogenesis of systemic lupus erythematosus (SLE). T cell receptor (TCR/CD3)-mediated stimulation of SLE T cells show increased protein tyrosine phosphorylation of cellular proteins with faster kinetics, heightened calcium flux response, and decreased IL-2 production. The molecular mechanisms of T cell signaling abnormalities in SLE T cells are complex. Current research has been directed towards investigating various factors that contribute to abnormal tyrosine phosphorylation, intracellular calcium response, and cytokine production. Central to this dysfunction is the aberrant expression and function of the TCR/CD3ζ chain. Latest developments suggest multiple explanations are involved, including altered receptor structure, supramolecular assembly, modulation of membrane clustering, aberrant cellular distribution, and pre-compartmentalization with lipid-rafts. The methods and protocols described here pertaining to T cell signaling abnormalities in SLE T cells are optimized in many ways and are derived by the combined task and continuous efforts of many researchers in the lab over a long period of time. These simplified protocols can be readily applied to study T cell signaling abnormalities in SLE to identify the genetic, molecular, and biochemical factors contributing to aberrant immune cell function and unravel the pathophysiology of SLE.

Abnormalities of T cell signaling in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease resulting from a loss of tolerance to multiple self antigens, and characterized by autoantibody production and inflammatory cell infiltration in target organs, such as the kidneys and brain. T cells are critical players in SLE pathophysiology as they regulate B cell responses and also infiltrate target tissues, leading to tissue damage. Abnormal signaling events link to defective gene transcription and altered cytokine production, contributing to the aberrant phenotype of T cells in SLE. Study of signaling and gene transcription abnormalities in SLE T cells has led to the identification of novel targets for therapy.

Alternative splicing factor/splicing factor 2 regulates the expression of the zeta subunit of the human T cell receptor-associated CD3 complex

T cells from patients with systemic lupus erythematosus express decreased levels of the T cell receptor-associated CD3 zeta chain, a feature directly linked to their aberrant function. The decrease in CD3zeta protein expression is in part due to decreased levels of functional wild type isoform of the 3'-untranslated region (UTR) of CD3zeta mRNA with concomitant increased levels of an unstable alternatively spliced isoform. In order to identify factors involved in the post-transcriptional regulation of CD3zeta, we performed mass spectrometric analysis of Jurkat T cell nuclear proteins "pulled down" by a CD3zeta 3'-UTR oligonucleotide, which identified the splicing protein alternative splicing factor/splicing factor 2 (ASF/SF2). We show for the first time that ASF/SF2 binds specifically to the 3'-UTR of CD3zeta and regulates expression of CD3zeta protein by limiting the production of the alternatively spliced isoform. During activation of human T cells, an increase in the wild type CD3zeta mRNA is associated with increased expression of ASF/SF2. Finally, we show a significant correlation between ASF/SF2 and CD3zeta protein levels in T cells from systemic lupus erythematosus patients. Thus, our results identify ASF/SF2 as a novel factor in the regulation of alternative splicing of the 3'-UTR of CD3zeta and protein expression in human T cells.

Alternative Splicing Factor/Splicing Factor 2 (ASF/SF2) regulates the expression of human T cell receptor CD3ζ chain (48.8)

T cells in patients with systemic lupus erythematosus (SLE) fail to control the aberrant autoimmune response and also infiltrate target organs. SLE T cells express low levels of the T cell receptor (TCR) CD3ζ chain, the main intracellular signal transducer upon antigen encounter. This deficiency is partially due to altered splicing of the CD3ζ mRNA. SLE T cells express a short alternatively spliced (AS) isoform of CD3ζ which is missing 562 nucleotides within its 3` untranslated region (UTR). This transcript is unstable and leads to reduced CD3ζ protein expression. We hypothesized that proteins binding to the splice-deleted region of the 3`UTR of CD3ζ play a role in regulating post transcriptional processing. Using an RNA oligonucleotide pulldown assay and mass spectrometry, we identified the serine/arginine-rich (SR) protein alternative splicing factor/ splicing factor 2 (ASF/SF2) as a putative protein binding to the CD3ζ 3`UTR. Specific knockdown of ASF/SF2 in Jurkat T cells led to a significant reduction in CD3ζ protein expression. Blocking of ASF/SF2 in human T cells with di-erythro-C6-ceramide enhanced the alternative splicing of CD3ζ, while overexpression of ASF/SF2 repressed this splicing. ASF/SF2 did not alter the RNA stability of CD3ζ. These data reveal a novel factor in the regulation of the post transcriptional processing of CD3ζ that may lead to altered CD3ζ chain expression in SLE.

Ischemia-mediated aggregation of the actin cytoskeleton is one of the major initial events resulting in ischemia-reperfusion injury

Ischemia-reperfusion (IR) injury represents a major clinical challenge, which contributes to morbidity and mortality during surgery. The critical role of natural immunoglobulin M (IgM) and complement in tissue injury has been demonstrated. However, cellular mechanisms that result in the deposition of natural IgM and the activation of complement are still unclear. In this report, using a murine intestinal IR injury model, we demonstrated that the beta-actin protein in the small intestine was cleaved and actin filaments in the columnar epithelial cells were aggregated after a transient disruption during 30 min of ischemia. Ischemia also led to deposition of natural IgM and complement 3 (C3). A low dose of cytochalasin D, a depolymerization reagent of the actin cytoskeleton, attenuated this deposition and also attenuated intestinal tissue injury in a dose-dependent manner. In contrast, high doses of cytochalasin D failed to worsen the injury. These data indicate that ischemia-mediated aggregation of the actin cytoskeleton, rather than its disruption, results directly in the deposition of natural IgM and C3. We conclude that ischemia-mediated aggregation of the actin cytoskeleton leads to the deposition of natural IgM and the activation of complement, as well as tissue injury.

The RNA-stabilizing protein HuR regulates the expression of zeta chain of the human T cell receptor-associated CD3 complex

T cell dysfunction is crucial to the pathogenesis of systemic lupus erythematosus (SLE); however, the molecular mechanisms involved in the deficient expression of the T cell receptor-associated CD3zeta chain in SLE are not clear. SLE T cells express abnormally increased levels of an alternatively spliced isoform of CD3zeta that lacks a 562-bp region in its 3'-untranslated region (UTR). We showed previously that two adenosine/uridine-rich elements (ARE) in this splice-deleted region of CD3zeta transcript are critical for the mRNA stability and protein expression of CD3zeta. In this study we show for the first time that the mRNA-stabilizing protein HuR binds to these two ARE bearing regions of CD3zeta 3'-UTR. Knockdown of HuR resulted in decreased expression of the CD3zeta chain, whereas overexpression led to the increase of CD3zeta chain levels. Additionally, overexpression of HuR in human T cells resulted in increased mRNA stability of CD3zeta. Our results identify the 3'-UTR of CD3zeta as a novel target for the mRNA-stabilizing protein HuR. Thus, the absence of two critical AREs in the alternatively spliced CD3zeta 3'-UTR found in SLE T cells may result in decreased HuR binding, representing a possible molecular mechanism contributing to the reduced stability and expression of CD3zeta in SLE.

Divergent Generation of Heterogeneous Memory CD4 T Cells

Mechanisms for the generation of memory CD4 T cells and their delineation into diverse subsets remain largely unknown. In this study, we demonstrate in two Ag systems, divergent generation of heterogeneous memory CD4 T cells from activated precursors in distinct differentiation stages. Specifically, we show that influenza hemagglutinin- and OVA-specific CD4 T cells activated for 1, 2, and 3 days, respectively, exhibit gradations of differentiation by cell surface phenotype, IFN-gamma production, and proliferation, yet all serve as direct precursors for functional memory CD4 T cells when transferred in vivo into Ag-free mouse hosts. Using a conversion assay to track the immediate fate of activated precursors in vivo, we show that day 1- to 3-activated cells all rapidly convert from an activated phenotype (CD25(high)IL-7R(low)CD44(high)) to a resting memory phenotype (IL-7R(high)CD25(low)CD44(high)) 1 day after antigenic withdrawal. Paradoxically, stable memory subset delineation from undifferentiated (day 1- to 2-activated) precursors was predominantly an effector memory (CD62L(low)) profile, with an increased proportion of central memory (CD62L(high)) T cells arising from more differentiated (day 3-activated) precursors. Our findings support a divergent model for generation of memory CD4 T cells directly from activated precursors in multiple differentiation states, with subset heterogeneity maximized by increased activation and differentiation during priming.

Committed to memory: lineage choices for activated T cells

The mechanisms for the generation of memory T cells and their delineation into heterogeneous subsets remain unknown. The linear model for memory T-cell generation from differentiated effector cells has been favored, although there is evidence that memory T cells can emerge directly from naive T cells undergoing homeostatic expansion and from activated T cells lacking effector functions. Here, we discuss the evidence from diverse studies of memory generation that support a new 'intersecting pathway' model for memory T-cell generation in which antigen-driven effector differentiation and homeostasis-driven memory differentiation follow distinct but analogous pathways. Antigen withdrawal during effector differentiation enables intersection with the memory pathway through a pre-memory intermediate, and memory heterogeneity is influenced by homeostasis, migration and persistence in vivo.