Abstract
Regulatory T cells (Tregs) are believed to be dysfunctional in autoimmunity. Juvenile idiopathic arthritis (JIA) and juvenile dermatomyositis (JDM) result from a loss of normal immune regulation in specific tissues such as joints or muscle and skin, respectively. Here, we discuss recent findings in regard to Treg biology in oligo-/polyarticular JIA and JDM, as well as what we can learn about Treg-related disease mechanism, treatment and biomarkers in JIA/JDM from studies of other diseases. We explore the potential use of Treg immunoregulatory markers and gene signatures as biomarkers for disease course and/or treatment success. Further, we discuss how Tregs are affected by several treatment strategies already employed in the therapy of JIA and JDM and by alternative immunotherapies such as anti-cytokine or co-receptor targeting. Finally, we review recent successes in using Tregs as a treatment target with low-dose IL-2 or cellular immunotherapy. Thus, this mini review will highlight our current understanding and identify open questions in regard to Treg biology, and how recent findings may advance biomarkers and new therapies for JIA and JDM.
Introduction
CD4+FOXP3+ regulatory T cells (Tregs) are a subset of CD4+ T helper cells present in lymphoid and non-lymphoid tissues, and are crucial for mediating tolerance to self, preventing allergies and controlling immune reactions after infections (). They develop in the thymus or are induced in the periphery and exhibit contact-dependent and -independent mechanisms of action (). Inactivating mutations in FOXP3 lead to multi-organ autoimmune disease [immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome (IPEX)], highlighting the importance of Tregs (). Importantly, it is becoming clear that the local microenvironment affects the phenotype and function of tissue-localized Tregs, which also have additional roles in repair and regeneration ().
Treg–tissue interaction might be particular important in autoimmunity with tissue-specific presentation, such as juvenile idiopathic arthritis (JIA) and juvenile dermatomyositis (JDM). While JIA is the most common inflammatory rheumatic disease in children, JDM is rare. JIA is characterized by persistent arthritis and subtype-dependent symptoms [reviewed in ()]. Here, we focus on polyarticular and oligoarticular JIA, which present without involvement of systemic organs or skin. JDM is characterized by inflammation of muscles and skin, resulting in muscle weakness and rashes [reviewed in ()]. Interestingly, for both conditions researchers may take advantage of clinical sample collection from the site of inflammation: synovial fluid (SF) drained during therapeutic joint injection (JIA) and biopsies (mostly muscle, JDM). While some patients respond to therapy, others do not and studying the underlying differences may lead to better understanding and treatments.
Here, we discuss recent advances in the understanding of Treg biology in oligo-/ polyarticular JIA and JDM, and what we can learn about Treg-related disease mechanisms, treatments and biomarkers from other diseases.
Altered Tregs in JIA and JDM
The phenotype of CD4+FOXP3+ Tregs in JIA has been considerably characterized in the past () with the molecular roles of FOXP3 in JIA reviewed by Copland and Bending in this special collection (). It is now clear that the Treg TCR (T cell receptor) repertoire is highly restricted in JIA, both at the site of inflammation (–) and in circulation (, ). Interestingly, in blood only Tregs but not conventional CD4+ non-Treg cells (Tconv) are more clonal (, ). Some suggest that the TCR repertoires of Tregs from SF and peripheral blood (PB) significantly overlap (), while others only found a very small overlap (, ). These differences might be explained by different sequencing depth and analysis strategies and/or by different Treg subsets studied: total () or effector Tregs defined by HLA-DR () or CD161 expression (). Further, one study found that SF Tregs, but not Tconv, share specificity at an amino acid sequence level among different patients (), suggesting disease-associated Treg clones might foster JIA.
Besides a restricted TCR repertoire, Tregs from the JIA inflammatory sites show unstable FOXP3 and CD25 (), altered homing markers (), cytokine production (, ), deficiency in specific chemokine production (), and low responsiveness to IL-2 ()—indicating impaired Treg function in JIA. Nevertheless, many reports found that JIA SF and PB Tregs are fully demethylated (, ), thus committed to the Treg-lineage, and suppressive in vitro (, , , , ). Hence, JIA Tregs are likely functioning inappropriately or insufficiently in the context of the inflammatory microenvironment. Interestingly, adding SF to in vitro cultures can both increase/stabilize Treg FOXP3 expression (, ) and in situ induce effector T cells to be resistant to Treg-mediated suppression ex vivo (, ). Thus, more research is needed to decipher the effects of the inflammatory microenvironment on Treg function.
In comparison, we know little about the contribution of Tregs to JDM pathogenesis. Similar to JIA, the Treg repertoire is restricted with a lack of diversity (). FOXP3+ Tregs were found to be enriched in JDM muscle compared to muscle tissue from patients with Duchenne muscular dystrophy (). Since the latter is already enriched in Tregs compared to normal muscle (), this suggests a hyper-enrichment in JDM in response to autoimmune inflammation. PB Tregs of active JDM also appear less suppressive in vitro with decreased expression of CTLA4 (). Adult DM/ polymyositis muscle biopsies are also enriched with Tregs (). Interestingly, both Treg and effector T cell numbers decreased post immunosuppressive therapy in adult myositis, suggesting that Treg enrichment is a response to inflammation. However, juvenile and adult DM have different clinical presentation () and JDM PB express more Th17-type and FOXP3 transcripts (). JDM and other myopathies are characterized by a type 1 IFN signature (–) and interferons may be a potential therapeutic target (), but their effects on Tregs remain to be investigated.
Tregs are crucial in resolving muscle injury in animal studies () and Treg-deficient mice develop more severe myopathies in response to antigen, while adoptive Treg transfer prevents inflammation (, ). Thorough immune-profiling recently revealed pan-tissue and tissue-specific signatures and enhancers of murine Tregs (). The muscle Treg signature was highly enriched in cell cycle genes, showed a dynamic response to injury and was more similar to circulating Treg signatures than to other tissue Tregs (), indicating that muscle Tregs might acutely infiltrate muscle and are not necessarily long-term resident cells. While myopathy is a defining characteristic of JDM, skin inflammation and rash are other symptoms (). Skin-resident Tregs are crucial for immune homeostasis () and have been characterized in health and various disease settings (). However, studies on JDM-affected skin are lacking, and more work is needed to characterize JDM skin-resident Tregs.
Tregs as a Biomarker?
JIA and JDM can exhibit an unpredictable disease course. While mounting evidence indicates that an early aggressive treatment is best for severe disease (, , , ), the disease course is unpredictable at presentation. Additionally, due to potential short- and long-term side effects children should not be exposed to unnecessary medication. Unfortunately, once a patient appears to be in clinical remission (on or off medications), disease may flare without any notice or obvious trigger (Figure 1A). Indeed, among JIA patients who are in clinical remission, 30–50% experience flares (, ).
Figure 1
Hence, reliable biomarkers need to predict (i) the future disease course, (ii) treatment response, and (iii) the safety for medication withdrawal during clinical remission (Figure 1B).
Inflammation markers in the serum can indicate disease activity and potentially treatment response in JIA [reviewed in (
Only a few putative biomarkers probe the immunoregulatory balance in autoimmune arthritis and myopathies. The frequency of inflammation-associated Tregs (HLA-DR+) in PB was proposed as a biomarker for disease activity in arthritis (
In the recent past, gene signatures have been defined as multi-parameter biomarkers. Thus, far, efforts to define JIA immune-based gene biomarkers have focused on whole genome expression profiling (
In summary, some progress has been made, but more biomarkers are needed for biological disease activity, prognosis, treatment success, and risk of flares. Further, a consensus of criteria to describe active/inactive disease is needed to better estimate the currently widely variable incidence of clinically inactive JIA disease (
Tregs as Therapeutic Target/Tool?
Convincing evidence demonstrates that functioning Tregs are crucial to prevent autoimmunity and our understanding of how different immunotherapies affect Tregs has improved.
(Unforeseen) Treg Effects of Immuno-Therapy
High levels of TNF-α in the inflamed JIA joint (
Ustekinumab is another potentially attractive anti-cytokine therapy which targets the p40 subunit of IL-12 and IL-23, key cytokines driving Th1, Th17, and Th17.1, (ex-)Th17 cells with a Th1-like phenotype, function (
IL-6 also drives inflammatory environments, including skewing the Treg/Th17 balance toward Th17 (
Whether drugs such as ustekinumab and tocilizumab act on Tregs directly or through changing the microenvironment is unclear. Human Tregs can express the receptors for IL-6 (86), IL-12 (87), and IL-23 (88), but evidence for direct drug action on Tregs is lacking.
Alternatively, co-receptors can be targeted to manipulate the immunoregulatory balance. Initially established for cancer therapy (checkpoint blockade), mimicking checkpoints such as CTLA4 (CTLA4-Ig, abatacept, belatacept) is used as treatment for autoimmunity. Abatacept has been shown to be safe and effective in oligo- and polyarticular JIA (
Treg (-Targeted) Therapy
Adoptive transfer of Tregs has been shown to be safe and possibly effective at reducing inflammation, inducing transplant tolerance, preventing graft-vs.-host disease (GVHD) and treating autoimmunity [reviewed in (101)].
Important considerations for Treg-therapy currently under investigation are the source of therapeutic cells, antigen-specificity and possibly tailoring homing characteristics for improved activity. Isolating and expanding sufficient numbers of Tregs from patients awaiting transplantation, under immunosuppression or with autoimmune disease is feasible and can restore their function (101–103), although achieving clinically relevant Treg numbers from pediatric JIA and JDM patients might prove challenging. Third-party Tregs from umbilical cord blood have been found safe and possibly effective as GVHD prophylaxis in adults (104, 105) and pediatric thymus—routinely removed during pediatric cardiac surgery—might be a plentiful source for highly functional therapeutic Tregs (106, 107). Antigen-specific Tregs are more effective than polyclonal Tregs for therapy and with recent successes of chimeric antigen receptor (CAR) T effector therapies for cancer, there has been a surge to adapt this technology to generate CAR-Tregs [reviewed in (108)]. While generation of antigen-specific Tregs recognizing allogeneic HLA-molecules is relatively straightforward in transplantation, generation of CAR-Tregs for autoimmunity without known antigen (i.e., JIA) might be difficult. Still, CAR-Tregs reacting with antigen found at the site of inflammation (i.e., JIA joints or JDM muscle) could activate Tregs locally. Alternatively, Tregs could be conditioned in vitro to home to specific sites (107) or Tregs could be injected locally, as shown with intra-dermal injection of Tregs to inhibit murine allograft skin inflammation (109).
Since Treg cell therapies are challenging and expensive, targeting Treg expansion in vivo might be more feasible for conditions such as JIA and JDM. The most promising advances of non-cellular therapies targeting Tregs have been low-dose IL-2, IL-2 complexes, or IL-2 bio-similars (110–112). While high doses of IL-2 stimulate mainly effector cells, low-dose IL-2 [0.3–3 × 106 units/day (112)] skews the response toward Tregs. Low-dose IL-2 increases the frequency of activated, functional and fully demethylated CD25+FOXP3+ Tregs (113–115) and induces STAT5 phosphorylation in vivo (114, 115). Low-dose IL-2 therapy has been deemed safe and successful in the treatment of T1D (112, 114, 115), GVHD (116, 117), and SLE (113). Indeed, low-dose IL-2 therapy rescued Tregs with low levels of CD25 in SLE (113), indicating that it might also rescue JIA Tregs with low CD25 expression (
However, increasing Treg numbers alone might not be sufficient to overcome the highly inflammatory environment and effector cell resistance. Thus, to achieve sustained remission combination-therapy might be necessary to reduce the inflammatory milieu paralleled with boosting Tregs to maintain a renewed tolerance.
Concluding Remarks
Taken together, it is clear that Tregs present challenges and opportunities in JIA and JDM research and clinical management (Figure 2). Their phenotype and function are clearly altered in JIA and JDM, targeting them might improve disease outcome and Tregs could be used as biomarkers to gage the state and progress of disease.
Figure 2

Tregs provide challenges and opportunities in JIA/JDM pathogenesis, treatment and monitoring. (A) Tregs are altered in repertoire, phenotype and frequency in JIA/JDM, particularly at the site of inflammation. (B–D) Treatment options to restore the immunoregulatory balance include targeting pro-inflammatory cytokines (B), targeting Tregs to enhance their activity (C) or using Tregs as a cellular therapy (D). (E) Further, changes in Treg gene signatures could aid as biomarkers to measure or predict disease/treatment response.
The role of the microenvironment on Treg function and phenotype in JIA- and JDM-affected tissues remains to be explored further. Researchers should take advantage of biopsies taken for clinical diagnosis (JDM) and SF aspirated during therapeutic joint injections (JIA). Novel techniques, such as single cell sequencing, multidimensional mass/flow cytometry and microscopy, will aid using clinical samples to their full potential (125–127). Additionally, co-culture with SF or muscle-derived cells could highlight how the microenvironment affects Tregs. Since JDM in particular is a rare disease, collaborations between groups are crucial to increase sample size for fundamental research, biomarker-finding and -validation studies and controlled treatment trials. This could be achieved by consortiums similar to juvenile diabetes research foundation (JDRF) biomarker working group for T1D (114) and the immune tolerance network trials (128).
While there is progress toward unified measures of disease activity (
Success of various agents affecting the immunoregulatory balance in other diseases point to potential uses in JIA and JDM. Any (new) therapy will need to be considered in regards to both effector cells AND Tregs, since some therapies might have unexpected effects on Tregs. Thus, it is important to continuously build our understanding of how various agents affect the immunoregulatory balance.
In conclusion, important recent advances might lead to valid future contributions to the widened arsenal of treatment options available to restore the immunoregulatory balance in a heterogeneous disease spectrum.
Statements
Author contributions
AP conceived the review. RH and AP reviewed the literature and co-wrote the manuscript.
Funding
AP is funded by a Versus Arthritis career development fellowship (ARUK CDF21738).
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
regulatory T cells, juvenile idiopathic arthritis, juvenile dermatomyositis, biomarker, therapy
Citation
Hoeppli RE and Pesenacker AM (2019) Targeting Tregs in Juvenile Idiopathic Arthritis and Juvenile Dermatomyositis—Insights From Other Diseases. Front. Immunol. 10:46. doi: 10.3389/fimmu.2019.00046
Received
31 October 2018
Accepted
09 January 2019
Published
25 January 2019
Volume
10 - 2019
Edited by
David Bending, University of Birmingham, United Kingdom
Reviewed by
Jorg Van Loosdregt, University Medical Center Utrecht, Netherlands; Klaus Tenbrock, RWTH Aachen Universität, Germany
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© 2019 Hoeppli and Pesenacker.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Anne M. Pesenacker a.pesenacker@ucl.ac.uk
This article was submitted to Inflammation, a section of the journal Frontiers in Immunology
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