Regulated Expression of miR-155 is Required for iNKT Cell Development

An update on the roles of transcription factor Ets1 in autoimmune diseases

Transcription factors are crucial to regulate gene expression in immune cells and in other cell types. In lymphocytes, there are a large number of different transcription factors that are known to contribute to cell differentiation and the balance between quiescence and activation. One such transcription factor is E26 oncogene homolog 1 (Ets1). Ets1 expression is high in quiescent B and T lymphocytes and its levels are decreased upon activation. The human ETS1 gene has been identified as a susceptibility locus for many autoimmune and inflammatory diseases. In accord with this, gene knockout of Ets1 in mice leads to development of a lupus-like autoimmune disease, with enhanced activation and differentiation of both B cells and T cells. Prior reviews have summarized functional roles for Ets1 based on studies of Ets1 knockout mice. In recent years, numerous additional studies have been published that further validate ETS1 as a susceptibility locus for human diseases where immune dysregulation plays a causative role. In this update, new information that further links Ets1 to human autoimmune diseases is organized and collated to serve as a resource. This update also describes recent studies that seek to understand molecularly how Ets1 regulates immune cell activation, either using human cells and tissues or mouse models. This resource is expected to be useful to investigators seeking to understand how Ets1 may regulate the human immune response, particularly in terms of its roles in autoimmunity and inflammation. This article is categorized under: Immune System Diseases > Genetics/Genomics/Epigenetics Immune System Diseases > Molecular and Cellular Physiology.

B Cell Activation Results in IKK-Dependent, but Not c-Rel- or RelA-Dependent, Decreases in Transcription of the B Cell Tolerance-Inducing Gene Ets1

Ets1 is a key transcription factor in B cells that is required to prevent premature differentiation into Ab-secreting cells. Previously, we showed that BCR and TLR signaling downregulate Ets1 levels and that the kinases PI3K, Btk, IKK, and JNK are required for this process. PI3K is important in activating Btk by generating the membrane lipid phosphatidylinositol (3,4,5)-trisphosphate, to which Btk binds via its PH domain. Btk in turn is important in activating the IKK kinase pathway, which it does by activating phospholipase Cγ2→protein kinase Cβ signaling. In this study, we have further investigated the pathways regulating Ets1 in mouse B cells. Although IKK is well known for its role in activating the canonical NF-κB pathway, IKK-mediated downregulation of Ets1 does not require either RelA or c-Rel. We also examined the potential roles of two other IKK targets that are not part of the NF-κB signaling pathway, Foxo3a and mTORC2, in regulating Ets1. We find that loss of Foxo3a or inhibition of mTORC2 does not block BCR-induced Ets1 downregulation. Therefore, these two pathways are not key IKK targets, implicating other as yet undefined IKK targets to play a role in this process.

TGF‑β1 affects the renal cancer miRNome and regulates tumor cells proliferation

TGF-β1 is a pleiotropic cytokine that can either promote or inhibit cancer development and progression. It was previously found that TGF-β1 can regulate the expression of several microRNAs (miR or miRNA) involved in the progression of renal cell carcinoma (RCC). Therefore, the present study aimed to analyze the effects of TGF-β1 on the global RCC miRNome. It was found that TGF-β1 can regulate a complex network consisting of miRNAs and mRNAs involved in RCC transformation. In particular, TGF-β1 was revealed to regulate the proliferation of RCC cells while concomitantly modifying the expression of oncogenic regulators, including avian erythroblastosis virus E26 (V-Ets) oncogene homolog-1 (ETS1). In addition, TGF-β1 was demonstrated to regulate the expression of a number of miRNAs including miR-30c-5p, miR-155-5p, miR-181a-5p and miR-181b-5p. By contrast, TGF-β1 reciprocally modified the expression of genes encoding TGF-β1 receptors and SMADs, indicating a novel regulatory feedback mechanism mediated through the miRNAs. These data suggested that ETS1 served different roles in different subtypes of RCC tumors, specifically by functioning as an oncogene in clear cell RCC while as a tumor suppressor in papillary RCC.

miR-21 sustains CD28 signalling and low-affinity T-cell responses at the expense of self-tolerance

Objective

miR-21 is highly expressed in iNKT and activated T cells, but its T-cell autonomous functions are poorly defined. We sought to investigate the role of miR-21 in the development and functions of T and iNKT cells, representing adaptive and innate-like populations, respectively.

Methods

We studied mice with a conditional deletion of miR-21 in all mature T lymphocytes.

Results

Thymic and peripheral T and iNKT compartments were normal in miR-21 KO mice. Upon activation in vitro, miR-21 depletion reduced T-cell survival, T_H17 polarisation and, remarkably, T- and iNKT cell ability to respond to low-affinity antigens, without altering their response to high-affinity ones. Mechanistically, miR-21 sustained CD28-dependent costimulation pathways required to lower the T-cell activation threshold, inhibiting its repressors in a positive feedback circuit, in turn increasing T-cell sensitivity to antigenic stimulation and survival. Upon immunisation with the low-affinity self-epitope MOG_35-55, miR-21 KO mice were indeed less susceptible than WT animals to the induction of experimental autoimmune encephalomyelitis, whereas they mounted normal T-cell responses against high-affinity viral epitopes generated upon lymphocytic choriomeningitis virus infection.

Conclusion

The induction of T-cell responses to weak antigens (signal 1) depends on CD28 costimulation (signal 2). miR-21 sustains CD28 costimulation, decreasing the T-cell activation threshold and increasing their sensitivity to antigenic stimulation and survival, broadening the immune surveillance range. This occurs at the cost of unleashing autoimmunity, resulting from the recognition of weak self-antigens by autoreactive immune responses. Thus, miR-21 fine-tunes T-cell response and self-/non-self-discrimination.

microRNA dynamic expression regulates invariant NKT cells

Invariant natural killer T cells (iNKT) are a prevalent population of innate-like T cells in mice, but quite rare in humans that are critical for regulation of the innate and adaptive immune responses during antimicrobial immunity, tumor rejection, and inflammatory diseases. Multiple transcription factors and signaling molecules that contribute to iNKT cell selection and functional differentiation have been identified. However, the full molecular network responsible for regulating and maintaining iNKT populations remains unclear. MicroRNAs (miRNAs) are an abundant class of evolutionarily conserved, small, non-coding RNAs that regulate gene expression post-transcriptionally. Previous reports uncovered the important roles of miRNAs in iNKT cell development and function using Dicer mutant mice. In this review, we discuss the emerging roles of individual miRNAs in iNKT cells reported by our group and other groups, including miR-150, miR-155, miR-181, let-7, miR-17 ~ 92 cluster, and miR-183-96-182 cluster. It is likely that iNKT cell development, differentiation, homeostasis, and functions are orchestrated through a multilayered network comprising interactions among master transcription factors, signaling molecules, and dynamically expressed miRNAs. We provide a comprehensive view of the molecular mechanisms underlying iNKT cell differentiation and function controlled by dynamically expressed miRNAs.

MicroRNA-155 Regulates MAIT1 and MAIT17 Cell Differentiation

Mucosal-associated invariant T (MAIT) cells are innate-like T cells that develop in the thymus through three maturation stages to acquire effector function and differentiate into MAIT1 (T-bet⁺) and MAIT17 (RORγt⁺) subsets. Upon activation, MAIT cells release IFN-γ and IL-17, which modulate a broad spectrum of diseases. Recent studies indicate defective MAIT cell development in microRNA deficient mice, however, few individual miRNAs have been identified to regulate MAIT cells. MicroRNA-155 (miR-155) is a key regulator of numerous cellular processes that affect some immune cell development, but its role in MAIT cell development remains unclear. To address whether miR-155 is required for MAIT cell development, we performed gain-of-function and loss-of-function studies. We first generated a CD4Cre.miR-155 knock-in mouse model, in which miR-155 is over-expressed in the T cell lineage. We found that overexpression of miR-155 significantly reduced numbers and frequencies of MAIT cells in all immune organs and lungs and blocked thymic MAIT cell maturation through downregulating PLZF expression. Strikingly, upregulated miR-155 promoted MAIT1 differentiation and blocked MAIT17 differentiation, and timely inducible expression of miR-155 functionally inhibited peripheral MAIT cells secreting IL-17. miR-155 overexpression also increased CD4^–CD8⁺ subset and decreased CD4^–CD8^– subset of MAIT cells. We further analyzed MAIT cells in conventional miR-155 knockout mice and found that lack of miR-155 also promoted MAIT1 differentiation and blocked MAIT17 differentiation but without alteration of their overall frequency, maturation and function. Overall, our results indicate that adequate miR-155 expression is required for normal MAIT1 and MAIT17 cell development and function.

MicroRNA-155 Controls iNKT Cell Development and Lineage Differentiation by Coordinating Multiple Regulating Pathways

The development of invariant natural killer T (iNKT) cells requires a well-attuned set of transcription factors, but how these factors are regulated and coordinated remains poorly understood. MicroRNA-155 (miR-155) is a key regulator of numerous cellular processes that affects cell development and homeostasis. Here, we found that miR-155 was highly expressed in early iNKT cells upon thymic selection, and then its expression is gradually downregulated during iNKT cell development. However, the mice with miR-155 germline deletion had normal iNKT cell development. To address if downregulated miR-155 is required for iNKT cell development, we made a CD4Cre.miR-155 knock-in (KI) mouse model with miR-155 conditional overexpression in the T cell lineage. Upregulated miR-155 led to interruption of iNKT cell development, diminished iNKT17 and iNKT1 cells, augmented iNKT2 cells, and these defects were cell intrinsic. Furthermore, defective iNKT cells in miR-155KI mice resulted in the secondary innate-like CD8 T cell development. Mechanistically, miR-155 modulated multiple targets and signaling pathways to fine tune iNKT cell development. MiR-155 modulated Jarid2, a critical component of a histone modification complex, and Tab2, the upstream activation kinase complex component of NF-κB, which function additively in iNKT development and in promoting balanced iNKT1/iNKT2 differentiation. In addition, miR-155 also targeted Rictor, a signature component of mTORC2 that controls iNKT17 differentiation. Taken together, our results indicate that miR-155 serves as a key epigenetic regulator, coordinating multiple signaling pathways and transcriptional programs to precisely regulate iNKT cell development and functional lineage, as well as secondary innate CD8 T cell development.

Role of miRNAs in Normal and Myasthenia Gravis Thymus

The thymus, a primary lymphoid organ, provides a complex environment essential for the generation of the T-cell repertoire. Thymic alterations occur during life either in the context of thymic involution upon aging or the pathophysiological context of Myasthenia Gravis (MG). These changes involve complicated regulatory networks, in which microRNAs (miRNAs) are key players. Here, we analyzed the role of miRNAs in thymocyte maturation and differentiation sustained by thymic epithelial cells. We compared data from the literature regarding the role of mouse thymic miRNAs and original data obtained from a human thymic miRnome study. We identified a set of highly expressed miRNAs defined as ThymiRs and investigated miRNA expression in infants as compared to adults to determine those associated with human thymic involution. Thymic changes are also frequently observed in MG, an autoimmune disease which results in the production of anti-acetylcholine receptor (AChR) antibodies that lead to muscle weaknesses. Alterations such as thymoma in late-onset MG patients and hyperplasia with ectopic germinal centers (GCs) in early-onset (EOMG) patients are found. Thymic miRNA expression has been studied in AChR-MG patients both in thymoma-associated MG (TAMG) and EOMG, and their function through their mRNA targets investigated. Most of the dysregulated thymic miRNAs in EOMG are associated with GC development, such as miR-7, miR-24, miR-139, miR-143, miR-145, miR-146, miR-150, miR-452, miR-548 or thymic inflammation, such as miR-125b, miR-146, or miR-29. Understanding these pathways may provide therapeutic targets or biomarkers of disease manifestations.

Increased expression of microRNA-155-5p by alveolar type II cells contributes to development of lethal ARDS in H1N1 influenza A virus-infected mice

Alveolar type II (ATII) cells are essential to lung function and a primary site of influenza A virus (IAV) replication. Effects of IAV infection on ATII cell microRNA (miR) expression have not been comprehensively investigated. Infection of C57BL/6 mice with 10,000 or 100 pfu/mouse of IAV A/WSN/33 (H1N1) significantly altered expression of 73 out of 1908 mature murine miRs in ATII cells at 2 days post-infection (d.p.i.) and 253 miRs at 6 d.p.i. miR-155-5p (miR-155) showed the greatest increase in expression within ATII cells at both timepoints and the magnitude of this increase correlated with inoculum size and pulmonary edema severity. Influenza-induced lung injury was attenuated in C57BL/6-congenic miR-155-knockout mice without affecting viral replication. Attenuation of lung injury was dependent on deletion of miR-155 from stromal cells and was recapitulated in ATII cell-specific miR-155-knockout mice. These data suggest that ATII cell miR-155 is a potential therapeutic target for IAV-induced ARDS.

miR-183-96-182 Cluster Is Involved in Invariant NKT Cell Development, Maturation, and Effector Function

The development, differentiation and function of invariant NKT (iNKT) cells require a well-defined set of transcription factors, but how these factors are integrated to each other and the detailed signaling networks remain poorly understood. Using a Dicer-deletion mouse model, our previous studies have demonstrated the critical involvement of microRNAs (miRNAs) in iNKT cell development and function, but the role played by individual miRNAs in iNKT cell development and function is still not clear. In this study, we show the dynamic changes of miRNA 183 cluster (miR-183C) expression during iNKT cell development. Mice with miR-183C deletion showed a defective iNKT cell development, sublineage differentiation, and cytokine secretion function. miRNA target identification assays indicate the involvement of multiple target molecules. Our study not only confirmed the role of miR-183C in iNKT cell development and function but also demonstrated that miR-183C achieved the regulation of iNKT cells through integrated targeting of multiple signaling molecules and pathways.

Development of Unconventional T Cells Controlled by MicroRNA

Post-transcriptional gene regulation through microRNA (miRNA) has emerged as a major control mechanism of multiple biological processes, including development and function of T cells. T cells are vital components of the immune system, with conventional T cells playing a central role in adaptive immunity and unconventional T cells having additional functions reminiscent of both innate and adaptive immunity, such as involvement in stress responses and tissue homeostasis. Unconventional T cells encompass cells expressing semi-invariant T cell receptors (TCRs), such as invariant Natural Killer T (iNKT) and Mucosal-Associated Invariant T (MAIT) cells. Additionally, some T cells with diverse TCR repertoires, including γδT cells, intraepithelial lymphocytes (IEL) and regulatory T (Treg) cells, share some functional and/or developmental features with their semi-invariant unconventional counterparts. Unconventional T cells are particularly sensitive to disruption of miRNA function, both globally and on the individual miRNA level. Here, we review the role of miRNA in the development and function of unconventional T cells from an iNKT-centric point of view. The function of single miRNAs can provide important insights into shared and individual pathways for the formation of different unconventional T cell subsets.

Functional Role of MicroRNAs in Thymocyte Development

MicroRNAs (miRNAs) are a class of endogenous noncoding single-stranded RNAs widely distributed in eukaryotes, which can modulate target gene expression at posttranscriptional level and participate in cell proliferation, differentiation, and apoptosis. Related studies have shown that mi-RNAs are instrumental to many aspects of immunity, including various levels of T-cell immunity. In addition, multiple miRNAs have been ascribed key roles in T-cell development, differentiation, and function. In this review, we highlight the current literature regarding the functional role of miRNAs at various stages of thymocyte development. A better understanding of the relationship between miRNAs and thymocyte development is helpful for the exploration of the exact roles of miRNAs in the development and function of the immune system, as well as related clinical diseases.

MicroRNA Functions in Thymic Biology: Thymic Development and Involution

During the entire processes of thymus organogenesis, maturation, and involution, gene regulation occurs post-transcriptionally via recently discovered microRNA (miRNA) transcripts. Numerous reports indicate that miRNAs may be involved in the construction of a normal thymic microenvironment, which constitutes a critical component to support T lymphocyte development. MiRNAs are also expressed in thymic stromal cells including thymic epithelial cells (TECs) during maturation and senescence. This review focuses on the function of miRNAs in thymic development and involution. A better understanding of these processes will provide new insights into the regulatory network of TECs and further comprehension of how genes control TECs to maintain the thymic microenvironment during thymus development and aging, thus supporting a normal cellular immune system.

Regulation of invariant NKT cell development and function by a 0.14 Mbp locus on chromosome 1: a possible role for Fcgr3

Invariant NKT (iNKT) cells are tissue-resident innate-like T cells critical to the host immune response. We previously identified a 6.6 Mbp region on chromosome 1 as a major regulator of iNKT cell number and function in C57BL/6 and 129X1/SvJ mice. Here, we fine-mapped this locus by assessing the iNKT cell response to alpha-galactosylceramide (αGalCer) in a series of B6.129 congenic lines. This analysis revealed the presence of at least two genetic elements that regulate iNKT cell cytokine production in response to αGalCer. While one of these genetic elements mapped to the B6.129c6 interval containing Slam genes, the dominant regulator in this region mapped to the 0.14 Mbp B6.129c3 interval. In addition, we found that numbers of thymic iNKT cells and DP thymocytes were significantly lower in B6.129c3 mice, indicating that this interval also regulates iNKT cell development. Candidate gene analysis revealed a 5-fold increase in Fcgr3 expression in B6.129c3 iNKT cells, and we observed increased expression of FcγR3 protein on B6.129c3 iNKT cells, NK cells, and neutrophils. These data identify the B6.129c3 interval as a novel locus regulating the response of iNKT cells to glycosphingolipid, revealing a link between this phenotype and a polymorphism that regulates Fcgr3 expression.

Control of T cell effector functions by miRNAs

The differentiation of effector T cells is a tightly regulated process that relies on the selective expression of lineage-defining master regulators that orchestrate unique transcriptional programs, including the production of distinct sets of effector cytokines. miRNAs are post-transcriptional regulators that are now viewed as critical players in these gene expression networks and help defining cell identity and function. This review summarises the role of individual miRNAs in the regulation of the differentiation of effector T cell subsets, including CD4⁺ T helper cells, cytotoxic CD8⁺ T cells and innate-like NKT cells. Moreover, we refer to miRNAs that have been identified to affect simultaneously two or more effector T cell populations, impacting on the balance between effector T cells in vivo, thus constituting potential biomarkers or targets for therapies aiming at boosting immunity or controlling autoimmunity.

Transcriptional and epigenetic regulation of innate-like T lymphocyte development

Invariant Natural Killer T (iNKT) cells are a heterogeneous innate T cell population that recognizes lipid antigens. Despite the monospecific nature of their T cell receptor, iNKT cells differentiate into stable sublineages during thymic development, before foreign antigen encounter. How iNKT cell subsets acquire and maintain their functional programs is a central question in innate lymphocyte biology. Global transcriptional and epigenetic profiling of iNKT subsets has provided insights into the internal wiring of these subsets that defines their identity. Comparison of the iNKT transcriptional programs with those of other adaptive and innate lymphocyte lineages revealed common core regulatory circuits that may dictate effector functions. In this review, we summarize recent advances on the molecular mechanisms involved in iNKT cell development.

Invariant natural killer T-cell development and function with loss of microRNA-155

Invariant natural killer T (iNKT) cells are adaptive T cells with innate-like characteristics including rapid cytokine production and a proliferative response to stimulation. Development of these cells in the thymus is dependent on expression of the microRNA (miRNA) processing enzyme Dicer, indicating that iNKT cells probably have distinct miRNA requirements for gene regulation during development. The miRNA miR-155 has previously been shown to have numerous roles in T cells, including regulation of proliferation and differentiation, and positive modulation of interferon-γ expression. We examined the role of miR-155 in the development and function of iNKT cells. Using germline-deficient miR-155 mice, we showed that loss of miR-155 resulted in unchanged iNKT cell frequency and cell number. Although miR-155 was up-regulated in iNKT cells upon activation with α-galactosylceramide, loss of miR-155 did not affect cytokine production or proliferation by iNKT cells. Hence, cytokine production occurs in iNKT cells independently of miR-155 expression.

miR-17∼92 family clusters control iNKT cell ontogenesis via modulation of TGF-β signaling

Invariant natural killer T cells (iNKT) cells are T lymphocytes displaying innate effector functions, acquired through a distinct thymic developmental program regulated by microRNAs (miRNAs). Deleting miRNAs by Dicer ablation (Dicer KO) in thymocytes selectively impairs iNKT cell survival and functional differentiation. To unravel this miRNA-dependent program, we systemically identified transcripts that were differentially expressed between WT and Dicer KO iNKT cells at different differentiation stages and predicted to be targeted by the iNKT cell-specific miRNAs. TGF-β receptor II (TGF-βRII), critically implicated in iNKT cell differentiation, was found up-regulated in iNKT Dicer KO cells together with enhanced TGF-β signaling. miRNA members of the miR-17∼92 family clusters were predicted to target Tgfbr2 mRNA upon iNKT cell development. iNKT cells lacking all three miR-17∼92 family clusters (miR-17∼92, miR-106a∼363, miR-106b∼25) phenocopied both increased TGF-βRII expression and signaling, and defective effector differentiation, displayed by iNKT Dicer KO cells. Consistently, genetic ablation of TGF-β signaling in the absence of miRNAs rescued iNKT cell differentiation. These results elucidate the global impact of miRNAs on the iNKT cell developmental program and uncover the targeting of a lineage-specific cytokine signaling by miRNAs as a mechanism regulating innate-like T-cell development and effector differentiation.

DNA damage response (DDR) and senescence: shuttled inflamma-miRNAs on the stage of inflamm-aging

A major issue in aging research is how cellular phenomena affect aging at the systemic level. Emerging evidence suggests that DNA damage response (DDR) signaling is a key mechanism linking DNA damage accumulation, cell senescence, and organism aging. DDR activation in senescent cells promotes acquisition of a proinflammatory secretory phenotype (SASP), which in turn elicits DDR and SASP activation in neighboring cells, thereby creating a proinflammatory environment extending at the local and eventually the systemic level. DDR activation is triggered by genomic lesions as well as emerging bacterial and viral metagenomes. Therefore, the buildup of cells with an activated DDR probably fuels inflamm-aging and predisposes to the development of the major age-related diseases (ARDs). Micro (mi)-RNAs - non-coding RNAs involved in gene expression modulation - are released locally and systemically by a variety of shuttles (exosomes, lipoproteins, proteins) that likely affect the efficiency of their biological effects. Here we suggest that some miRNAs, previously found to be associated with inflammation and senescence - miR-146, miR-155, and miR-21 - play a central role in the interplay among DDR, cell senescence and inflamm-aging. The identification of the functions of shuttled senescence-associated miRNAs is expected to shed light on the aging process and on how to delay ARD development.

MicroRNA-133b Regulation of Th-POK Expression and Dendritic Cell Signals Affect NKT17 Cell Differentiation in the Thymus

NKT17 cells represent a functional subset of Vα14 invariant NKT (iNKT) cells with important effector functions in infections and autoimmune diseases. The mechanisms that drive NKT17 cell differentiation in the thymus are still largely unknown. The percentage of NKT17 cells has a high variability between murine strains due to differential thymic differentiation. For example, the NOD strain carries a high percentage and absolute numbers of NKT17 cells compared with other strains. In this study, we used the NOD mouse model to analyze what regulates NKT17 cell frequency in the thymus and peripheral lymphoid organs. In accordance with previous studies showing that the zinc finger transcription factor Th-POK is a key negative regulator of thymic NKT17 cell differentiation in the thymus, our data indicate that excessive NKT17 cell frequency in NOD mice correlates with defective Th-POK expression by thymic Vα14iNKT cells. Moreover, we found that Th-POK expression is under epigenetic regulation mediated by microRNA-133b whose expression is reduced in Vα14iNKT cells of NOD mice. We also demonstrated in a conditional knockout model of dendritic cell (DC) depletion (CD11cCreXDTA.B6 and CD11cCreRosa26DTA.NOD mice) that DCs play a crucial role in regulating Vα14iNKT cell maturation and their acquisition of an NKT17 cytokine secretion phenotype in the thymus. Overall, our data show that mechanisms regulating NKT17 cell differentiation are unique and completely different from those of Vα14iNKT cells. Specifically, we found that epigenetic regulation through microRNA-133b-regulated Th-POK expression and signals provided by DCs are fundamental for thymic NKT17 cell differentiation.