The Relationship Between TIGIT⁺ Regulatory T cells and Autoimmune Disease

Abstract

The role of regulatory T cells (Treg cell) in controlling autoimmune disease is an area of intense study. As such, the characterization and understanding the function of Treg markers has the potential to provide a considerable impact in developing treatments and understanding the pathogenesis of autoimmune diseases. One such inhibitory Treg cell marker that has been recently discovered is T cell immunoglobulin and ITIM domain (TIGIT). In this review, we discuss what is known about the expression and function of TIGIT on Treg cells, and we discuss the relationship between TIGIT expressing Treg cells and different autoimmune diseases such as atopic dermatitis, autoimmune thyroiditis, type 1 diabetes, autoimmune uveitis, aplastic anemia, multiple sclerosis, systemic lupus erythematosus, arthritis, and colitis.

1. Introduction

1.1. Regulatory T cells in autoimmune disease

The “re-discovery” of Tregs in 1995 was marked by the observation that CD25⁺CD4⁺ T cells suppress autoimmune disease when transferred to athymic BALB/c nude mice [1]. The FoxP3 gene was linked to Scurfy mice that suffer from fatal autoimmune disease [2], and loss of function mutations in the human FOXP3 gene were connected with immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome (IPEX), inflammatory bowel disease, and severe allergy [3–5]. Subsequent study of regulatory T cells (Tregs) has continued to expand to identify additional surface and intracellular markers. Tregs function through the production of soluble factors such as TGF-β, IL-10, IL-35, adenosine, and fibrinogen like 2 (FGL2) [6–9], and contact dependent factors such as CTLA-4, PD-1/PD-L1, LAG3, and TIM3 [7–10]. Single cell analysis is allowing for identification and characterization of multiple Treg subsets found in diverse tissue, and tumor environments [11]. The recently discovered T cell immunoglobulin and ITIM domain (TIGIT) is an inhibitory receptor expressed on Treg cells that have the potential to control autoimmune disease. In this review, we discuss what is known about TIGIT, its role in Treg function and expression, and what has been demonstrated regarding TIGIT⁺ Tregs with autoimmune diseases such as atopic dermatitis, autoimmune thyroiditis, type 1 diabetes, autoimmune uveitis, aplastic anemia, multiple sclerosis, systemic lupus erythematosus, arthritis, and colitis.

1.2. TIGIT background

TIGIT is also known as WUCAM, VSTM3, or VSIG9 and is part of the poliovirus receptor (PVR) or nectin family of proteins that consists of TIGIT, CD226 (DNAM-1), CD96, CD112R, PVR (CD155), CD112 (PVRL2/nectin-2), and CD113 (PVRL3/nectin-3) [12–16]. TIGIT binds with high affinity to PVR and with lower affinity to PVRL2 and PVRL3, but PVRL3 has not been demonstrated to bind to human TIGIT [12]. Nectins and nectin-like proteins are expressed on the cell surface and homophilic and heterophilic trans-interactions mediate cell adhesion with other cells, cell polarization, tissue organization, and signal transduction [17,18]. The structure of TIGIT includes an extracellular immunoglobulin variable-set (IgV) domain, type 1 transmembrane domain, intracellular domain with a canonical immunoreceptor tyrosine-based inhibitory motif (ITIM) and an immunoglobulin tyrosine tail (ITT) motif [12]. TIGIT expression has been observed on CD4⁺ T cells, CD8⁺ T cells, follicular T cells (Tfh cells), and NK cells [12,19–22].

TIGIT was first reported by Grogan, et al. in 2009 as an inhibitory receptor that suppresses T cell activation by inducing regulatory dendritic cells [12]. Grogan, et al., screened for proteins exclusively expressed by T cells [23] that had an ITIM [24,25], and further demonstrated TIGIT expression on activated human T cells, high affinity binding to PVR whose interaction promotes regulatory dendritic cells in an IL-10 mediated manner [12]. This report just ten years ago initiated the study of TIGIT⁺ Treg cells in cancer and autoimmunity.

1.3. Regulation of TIGIT expression

TIGIT expression on naïve T cells is undetectable, but is upregulated following activation. The transcription factors that control TIGIT expression include BLIMP1 and low expression of Bach2 in Tfh cells [26]. The transcription factor, Eomes, is expressed on CD8⁺ T cells from patients with newly diagnosed acute myeloid leukemia, and Eomes positively regulates TIGIT expression [27]. As of the writing of this review, the transcriptional program that controls TIGIT expression is not well understood, and additional work needs to be done to further define how TIGIT expression is regulated in Treg cells.

1.4. TIGIT Signaling

TIGIT ligation results in recruitment of Grb2 and SHIP1 in NK cells through the tyrosine region [28]. Presumably the ITIM or ITT region in T cells also recruits SHIP1 and/or SHP2 as with PD-1. As such, the human T-cell leukemia virus type 1 (HTLV-1) inhibits SHP-2 to allow for T-cell proliferation despite PD-1 and TIGIT ligation [29]. While Treg studies of downstream TIGIT signaling are lacking, it has been demonstrated that upregulation of TIGIT in Treg cells results in the demethylation of FOXP3 [30].

Another interesting observation that demonstrates a role for the microbiome in TIGIT signaling was made with Fusobacterium nucleatum [31]. F. nucleatum is a common oral anaerobic Gram-negative found in human tumors, adenocarcinoma tumors in particular [32,33]. Using a library of F. nucleatum mutants, the Fap2 protein expressed by F. nucleatum was identified as a ligand for TIGIT and suppresses T cell activity to promote tumor survival [31].

1.5. Mechanism of action

Three mechanisms have been identified through which TIGIT accomplishes immunoregulation (Fig 1). First, TIGIT can act as a ligand for PVR, and with less affinity for PVRL2 and PVRL3, to suppress the denritic cell or antigen presenting cell that expresses PVR. Ligation of PVR with TIGIT results in phosphorylation of PVR, Erk, and p38 promoting a regulatory dendritic cell that secretes IL-10 [12]. Second, TIGIT can act as a receptor on T cells that when ligated with PVR results in a block to T cell priming [12], possibly through downregulation of the TCR [34], and triggers the production of the immunosuppressive molecule, FGL2 [35]. In NK cells, TIGIT phosphorylation recruits Grb2 and β-arrestin which recruits SHIP1 and SHP2 to inhibit PI3K and NF-κB [28,36], while this has not yet been shown in T cells the NK cell work suggests that TIGIT signaling in T cells may work through SHIP1 and SHP2. Finally, TIGIT binds CD226 with higher affinity than PVR binding with CD226 [12,37], to prevent CD226 mediated activation. CD226 is an Ig superfamily member that is an activating receptor when bound by PVR that is disrupted through TIGIT binding to CD226. Therefore, it is through one or more mechanisms that TIGIT may suppress T cell activation to control an autoimmune response that makes TIGIT expressing Treg cells an attractive therapeutic for controlling autoimmune disease.

Figure 1.

Mechanisms of TIGIT mediated suppression. TIGIT can block CD226-PVR mediated activation of T cells because it binds to CD226 with higher affinity than PVR (A). Ligation of TIGIT with PVR on naïve T cells blocks T cell priming, and triggers the production of the immunosuppressive molecule, FGL2 (B). Ligation of PVR with TIGIT promotes a regulatory dendritic cell that secretes IL-10 (C). Artwork by Ryan E. Lee.

2. Autoimmune diseases connected with TIGIT⁺ Tregs

2.1. Atopic dermatitis

Clinical analysis of 17 patients with atopic dermatitis (AD) and 14 healthy individuals found the number of TIGIT expressing CD4⁺ T cells was increased in AD patients, but the four most severe AD cases showed a significant reduction in the number of TIGIT⁺ CD4⁺ T cells [38]. The increased number of TIGIT⁺ CD4⁺ T cells from AD patients may be compensatory because they had a significant reduction in proliferation capacity with anti-CD3 and anti-CD28 stimulation compared with healthy individuals [38]. This study suggests that a reduction in TIGIT⁺ T cells may contribute to more severe disease.

2.2. Autoimmune thyroiditis

A role for TIGIT⁺ Treg cells with autoimmune thyroiditis has been reported in humans and mice. It has been demonstrated in mice that OX40L-JAG1 treatment expanded CTLA4⁺ and TIGIT⁺ Tregs that alleviated experimental autoimmune thyroiditis, and humanized NSG mice also showed an expansion of the Tregs in the liver [39]. Analysis of thirty autoimmune thyroiditis (AT) patients compared with ten healthy controls revealed a correlation between expression of Fc Receptor Like 3 (FCRL3), an inhibitory receptor, and TIGIT with different subtypes of AT [40]. These mouse and clinical studies suggest that TIGIT⁺ Tregs function to control autoimmune thyroiditis.

2.3. Diabetes

There have been several observations linking TIGIT expressing Tregs with diabetes in different diabetic mouse models. TIGIT⁺ Tregs have been identified in the islets of NOD mice [41], and another group found that high-affinity TCR Tregs from the pancreas expressed TIGIT [41]. The S1PR1 agonist, CYM-5442, prevented type 1 diabetes and upregulated Tigit in T cells in the mouse Rip-LCMV T1D model [42]. A sub-immunogenic vaccination with strong insulin mimetopes was effective in preventing the onset of diabetes and induced TIGIT expression in Tregs in a humanized NSG mouse [43].

Importantly, TIGIT⁺ Tregs have also been shown to be expressed by T cells from human diabetes patients. In a large transcriptional profiling and functional analysis of T cell subsets from type 1 diabetic patients, TIGIT⁺CD226⁻ Tregs from type 1 diabetic patients are stable and suppressive, compared to TIGIT⁻CD226⁺ T cells that were a mixed population of T_CM, T_EM, and Treg with a decreased suppressive capacity, effector cytokine and IL-10 production [44]. Another study examining microRNA in CD4⁺ T cells from 23 pre-diabetic patients compared with 29 healthy controls revealed that expression of miR-26a correlates with FoxP3 and TIGIT expression, and the mechanism is through negative regulation of the histone methyltransferase EZH2 whose inhibition correlates with decreased Tregs [44]. These mouse and clinical studies suggest that TIGIT⁺ Tregs are capable of suppressing type 1 diabetes.

2.4. Aplastic anemia

Using a combination of mouse studies and clinical samples, it has been demonstrated that TIGIT⁺ Tregs improved the red blood cell count in an aplastic anemia model [45]. This group further showed that the long, non-coding RNA, MEG3 functions to absorb miR-23a that reduces TIGIT expression, resulting in upregulation of TIGIT and reduced expansion of Th1 and Th17 cells [46]. These studies suggest that TIGIT⁺ Tregs suppress aplastic anemia by suppressing Th1 and Th17 cells, and identify the lncRNA, MEG3, as a positive regulator of TIGIT.

2.5. Autoimmune uveitis

A study examining TIGIT⁺ Tregs from 50 autoimmune uveitis patients compared with ten control subjects revealed a positive correlation in the number of TIGIT⁺ Tregs with patients in remission [47]. This clinical study suggests that TIGIT⁺ Tregs contribute to remission of autoimmne uveitis. As of the time of writing of this report, not other studies involving TIGIT and uveitis have been published. Therefore, additional study of TIGIT expressing Tregs in the pathogenesis of autoimmune uveitis is necessary.

While the above discussed autoimmune diseases show a positive correlation with TIGIT expressing Tregs and autoimmune disease, there are reports of a correlation with a reduction in TIGIT expressing Tregs with autoimmune diseases with TIGIT⁺ Tregs, as discussed below.

2.6. Multiple Sclerosis

In mice, antigen specific immunotherapy in experimental autoimmune encephalomyelitis induces TIGIT expressing T cells [48]. In contrast, it has been reported that TIGIT expressing CD4 cells is lower in multiple sclerosis (MS) patients with a sample size of 57 MS patients and 19 healthy controls [49]. However, the TIGIT signaling pathway was found to be active in another report that included nine MS patients and seven healthy controls with the observation that TIGIT stimulation of PBMCs from MS patients reduces Th1 differentiation [50]. These studies suggest TIGIT-expressing Tregs are involved in suppression of MS. In mice, TIGIT⁺ Tregs are associated with suppression of EAE. In MS patients with active disease the low number of TIGIT⁺ Tregs suggests their absence may contribute to disease.

2.7. Colitis

In a dextran sulfate sodium mouse model of colitis, estrogen receptor β activation was effective in reducing disease and increased the number of infiltrating TIGIT⁺ Tregs [51]. However, CyTOF analysis of PBMCs from 33 ulcerative colitis patients compared with 14 healthy donors revealed an increase in TIGIT expression on NK and γδ T cells, but was decreased in CD4⁺ T cells [52]. In these studies, the discrepancy between mouse and patient observations could be due to differences in the state of disease, but suggests a defect in TIGIT expression on CD4⁺ T cells could contribute to disease.

2.8. Lupus

It has been demonstrated that recombinant TIGIT-Ig is an effective prevention and treatment for murine lupus [53]. However, a study that included 49 systemic lupus erythematosus (SLE) patients and 22 healthy controls found that patients with renal manifestations had a decreased frequency of TIGIT⁺ T cells [54]. In contrast, another study that included 50 SLE patients and 27 healthy controls found that TIGIT-expressing CD4 T cells are significantly elevated in SLE patients with high anti-dsDNA antibodies, high anti-Sm antibodies, and high levels of urine microalbumin compared with controls [55]. Another larger study including 94 SLE patients and 64 healthy controls also found the number of TIGIT⁺ CD4 T cells to be elevated in SLE patients and TIGIT⁺ CD4 T cells have less activation potential compared with TIGIT⁻ CD4 T cells [56]. While the first study discussed above suggests that a decrease in TIGIT⁺ T cells is related to disease, the conflicting observations in the second two studies that find an increase in TIGIT⁺ T cells could be due to differences in sample sizes and could be compensatory to keep the disease controlled.

2.9. Arthritis

Mouse models show TIGIT overexpression reduces the severity of rheumatoid arthritis (RA) [57]. However, there is a contradiction in the literature regarding TIGIT expression on T cells as a correlation with disease activity. Two groups analyzed PBMCs from over 70 RA patients that were being treated with various therapies and one found a correlation with TIGIT expression and rheumatoid arthritis [58], but the second group found no correlation with TIGIT expression [59]. Both studies also compared expression with the severity of disease and found opposite correlations. These observations suggest there may be differences between geographic regions, ethnicities, and treatments. Since both studies analyzed PBMCs and not the lymphocytes at the site of inflammation, another group analyzed the CD4 T cells from synovial fluid of RA patients and found a positive correlation with TIGIT expression and disease [60].

2.10. Disease summary

The above observations suggest that in some autoimmune diseases and disease states the TIGIT⁺ T cell population may be increased in an attempt to keep the disease under control, and in some cases when this population is absent or reduced, this may indicate the disease has progressed into a more severe disease state. While this review is focused on TIGIT-expressing Treg cell mediated expression it should be noted that because TIGIT is expressed on NK cells, Tfh cells, and CD8 T cells it likely that some of the suppressive mechanisms utilized by Treg cells to suppress through TIGIT are shared. Unfortunately, an extensive comparative analysis of the suppressive mechanisms of the various TIGIT-expressing cells has not been done. However, it is likely the extrinsic mechanisms of suppression such as induction of immunosuppressive dendritic cells would be shared between all cell types, but Treg cell specific mechanisms such as FGL2 production may not be shared.

3. Conclusions

A role for TIGIT⁺ Tregs in some autoimmune diseases is clear, but the heterogeneity of autoimmune diseases, disease status, and geographic variations of patient composition likely contributes to inconsistent observations regarding TIGIT⁺ Tregs as a definitive marker for the status of autoimmune diseases. Regardless of utilizing TIGIT⁺ Tregs as a marker for autoimmune disease, it is clear that this Treg cell represents a population of suppressive cells that may be used for the treatment of many different autoimmune diseases. As such, further investigation of TIGIT expression and function on Tregs is necessary.