T time for ADAR: ADAR1 is required for T cell self‐tolerance

Abstract

ADAR1, an RNA‐editing enzyme, plays a key role in preventing self‐RNAs from triggering autoinflammatory responses. In this issue of EMBO reports, Nakahama and colleagues uncover a novel role for ADAR1 in T cells 1. The authors report that in T cells, ADAR1‐mediated suppression of type I interferon‐stimulated gene (ISG) expression is required for thymic T cell self‐tolerance and prevention of colitis. These findings establish a novel function of ADAR1 in T cells and suggest that autoreactive T cells may contribute to disease symptoms in autoinflammatory disorders.

ADAR (adenosine deaminase acting on RNA) is a critical enzyme responsible for adenosine (A)‐to‐inosine (I) editing of double‐stranded RNA (dsRNA). Massive A‐to‐I editing is common across metazoa, and understanding the biological significance of these edits is an important and emerging area of research. Since inosine can be read as guanosine by the translational machinery, ADAR editing can result in protein recoding. However, in mammals, the vast majority of A‐to‐I edits occur in non‐coding regions, especially within the SINE family of retrotransposons 2. Currently, the biological significance of these millions of A‐to‐I edits is incompletely understood.

ADAR1 plays a key role in preventing endogenous RNAs from inappropriately triggering an innate immune response, most notably the type I interferon (IFN) pathway. Type I IFN is produced when host cells sense viral nucleic acids. However, in ADAR1 knockout (KO) mice, which are embryonic lethal, type I IFN‐stimulated genes (ISGs) are elevated in the absence of viral infection 3, 4. ADAR1 also suppresses spontaneous ISG induction in humans. Mutations in ADAR1 have been associated with the type I interferonopathy Aicardi‐Goutières syndrome (AGS) 5, which shares features with the prevalent autoimmune disease systemic lupus erythematosus (SLE).

Then, how does ADAR1 suppress spontaneous ISG induction? To detect viral nucleic acids, the host cell relies on pattern recognition receptors (PRRs), which sense viral nucleic acids and relay a signal to produce type I IFN. Recent studies propose that ADAR1 suppresses aberrant ISG induction by preventing endogenous RNAs from activating MDA5, a PRR for dsRNA 3, 4, 6, 7. For example, ADAR1‐deficient mice exhibit an enhanced ISG signature in hematopoietic cells that are defective in hematopoiesis; these phenotypes can be corrected by concurrent ablation of MDA5 4.

Nakahama et al 1 now report a novel role for ADAR1 in the thymus, a critical lymphoid organ for T cell maturation and self‐tolerance. They demonstrate that ADAR1‐mediated suppression of MDA5 activation (and subsequent prevention of ISG induction) is required for proper thymic T cell maturation and protection from colitis.

Nakahama et al 1 made the striking observation that thymic CD4⁺CD8⁻ single‐positive (4SP) cells up‐regulate the expression of ADAR1 and multiple ISGs. These findings encouraged them to investigate the function of ADAR1 in CD4⁺ T cells. For this, they crossed ADAR1^flox/flox with CD4^cre mice to generate a mouse model in which ADAR1 is specifically ablated in CD4⁺ T cells. In CD4^creADAR1^flox/flox mice, although total thymocyte numbers were normal, T cell maturation—including positive and negative selection of thymic T cells—was impaired (Fig 1A). Moreover, CD4⁺Foxp3⁺ Treg cells, which are critical for maintenance of central immune tolerance, were also severely depleted in the thymus of CD4^creADAR1^flox/flox mice. These findings collectively demonstrate that ADAR1 is required for optimal thymic T cell maturation.

Figure 1. ADAR1 is required for thymic T cell maturation and intestinal homeostasis.

(A) Schematic of ADAR1's role in thymic T cells. In brief, ADAR1 is postulated to prevent endogenous double‐stranded RNAs (dsRNAs) from activating MDA5, a PRR for viral dsRNA. In ADAR1‐deficient thymocytes, MDA5 activation leads to excessive ISG expression and defective T cell receptor (TCR) signaling. As a result, ADAR1‐deficient thymocytes are impaired in positive and negative selection. Positive selection occurs when TCRs “moderately” bind to self‐peptide/MHC (major histocompatibility complex) complexes. Following positive selection, negative selection removes T cells with TCRs that “strongly” bind to self‐peptide/MHC complexes. Negative selection is an essential step to delete autoreactive T cells. APC, antigen presenting cell. (B) While wild‐type (WT) mice are healthy, mice with ADAR1‐deficient T cells spontaneously develop colitis. Colitis is likely caused by the impaired T cell maturation described in (A).

Next, the authors characterized ADAR1‐deficient thymic T cells to understand the mechanism(s) by which ADAR1 supports T cell maturation 1. Based on these studies, they propose that unedited endogenous dsRNAs trigger MDA5 activation, inducing excessive ISG expression, and impaired TCR signal transduction (Fig 1A). Ultimately, down‐regulated TCR signal transduction is thought to cause the defect in T cell maturation.

Strikingly, CD4^creADAR1^flox/flox mice develop spontaneous colitis (Fig 1B). The authors suggest that this could be due to impaired negative selection resulting in accumulation of autoreactive T cells. Remarkably, excessive ISG expression in T cells, impaired thymic T cell maturation, and spontaneous colitis could all be rescued by deleting MDA5 in the CD4^creADAR1^flox/flox mice. Altogether, these findings demonstrate that ADAR1 is required for thymic T cell tolerance, which is important for preventing autoimmune diseases such as colitis 1.

These findings reveal two novel functions of ADAR1. First, they extend the role of ADAR1 to maintaining T cell homeostasis, in particular proper T cell maturation. Second, they provide yet another example of distinct ADAR1 functions in different cell types. Interestingly, in several cell types, lack of ADAR1 leads to apoptosis and cell death 4, 6. However in T cells, Nakahama et al show that ADAR1 affects cell maturation, rather than directly regulating cell death.

The study also raises new and intriguing hypotheses regarding the role of ISGs in thymocyte maturation and the mechanism of ISG induction. First, these findings suggest that moderate (or low) ISG expression may play an active role in thymic T cell maturation. The authors clearly show that ISG expression is significantly elevated at the 4SP stage 1. In the absence of ADAR1, however, ISG expression becomes dysregulated leading to excessive ISG expression and defective T cell maturation. Second, Nakahama et al carefully raise the possibility that in ADAR1‐deficient T cells, MDA5 activation induces ISG expression in a type I IFN‐independent manner. This rests on their observation of up‐regulated ISG expression in ADAR1‐deficient thymocytes without detectable type I IFN expression 1. In line with this model, it is well known that certain ISGs can be directly induced by PRR activation 8, and other studies have revealed apparently IFN‐independent intrinsic expression of ISGs in certain cell types, in particular stem cells 9.

Revealing a requirement for ADAR1 in thymic T cell maturation has implications for human disease. Currently, it is not known how dysfunctional ADAR1 leads to AGS pathology, and the work of Nakahama et al now raises the exciting possibility that autoreactive T cells could be a contributing factor. Furthermore, these findings have broader implications for human disease beyond AGS. Enhanced ISG expression is one of the hallmarks of common autoinflammatory disorders such as SLE 10. Excessive ISG expression in T cells could impair T cell maturation/selection, causing or exacerbating a breakdown in thymic self‐tolerance in SLE.

A major hurdle for treating SLE and AGS patients is that the underlying molecular mechanisms of these diseases are poorly understood. For example, although patients display a significant ISG signature, the molecular trigger that initiates the type I IFN response is not known. The study by Nakahama and co‐workers now joins other studies suggesting that self‐nucleic acids may trigger autoimmune diseases 3, 4, 6, 7. Identifying the immunostimulatory self‐RNAs (presumably SINE RNAs or other endogenous RNA species) and investigating the molecular mechanisms by which ADAR1 prevents dsRNAs from activating PRRs could reveal insights into the underlying causes of various autoimmune diseases and clues on how to develop effective therapeutics.

Conflict of interest

The authors declare that they have no conflict of interest.

EMBO Reports (2018) 19: e47237