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Immunostimulatory RNA Blocks Suppression by Regulatory T Cells


Anz, David; Koelzer, Viktor H; Moder, Stefan; Thaler, Raffael; Schwerd, Tobias; Lahl, Katharina; Sparwasser, Tim; Besch, Robert; Poeck, Hendrik; Hornung, Veit; Hartmann, Gunther; Rothenfusser, Simon; Bourquin, Carole; Endres, Stefan (2010). Immunostimulatory RNA Blocks Suppression by Regulatory T Cells. Journal of Immunology, 184(2):939-946.

Abstract

The role of immune suppression by regulatory T (Treg) cells in the maintenance of immune homeostasis is well established. However, little is known about how Treg cell function is inhibited on viral infection to allow the development of a protective immune response. As viral RNA is a crucial mediator for activation of antiviral immunity, we examined the effects of immunostimulatory RNA and infection with RNA viruses on Treg cell function. We show that synthetic RNA oligonucleotides potently inhibit Treg cell-induced suppression in a sequence-dependent manner. This effect is entirely dependent on TLR7 activation of APCs and subsequent IL-6 production. In addition, stimulation with the RNA viruses encephalomyocarditis virus and Sendai virus that specifically activate the RNA-sensing helicases melanoma differentiation-associated gene 5 (MDA-5) and retinoic acid-inducible gene I (RIG-I) also blocks Treg cell function. Interestingly, this effect is seen even in the absence of APCs. Consistent with this, both Treg and T effector cells express RIG-I and MDA-5. Using MDA-5–deficient mice, we demonstrate that the loss of Treg cell function on infection with encephalomyocarditis virus is strictly dependent on MDA-5 expression by Treg cells. Thus, we show in this study for the first time that activation of a RIG-I–like helicase on Treg cells blocks their suppressive function.
Regulatory T (Treg) cells play a central role in the suppression of immune reactions and in the prevention of autoimmune responses harmful to the host (1, 2). In particular, Treg cells suppress the activation of naive T effector (Teff) cells by inhibiting TCR-triggered proliferation and differentiation of these cells (3). In the early phase of microbial infection, the suppressive effect of Treg cells must, however, be overcome to enable the generation of an efficient immune response against invading pathogens (4–6). In recent years, it has been shown that several bacterial components can block the suppressive function of Treg cells. Two distinct mechanisms have so far been described: first, bacterial molecules, such as LPS or CpG DNA, can activate APCs to produce inflammatory cytokines that render Teff cells resistant to suppression by Treg cells (7). Alternatively, Treg cells themselves can be directly stimulated: targeting human Treg cells with CpG DNA or with the bacterial lipoprotein Pam3Cys-SK4 abrogates their suppressive function even in the absence of APCs (8–10). The direct action of bacterial components on Treg cells can, however, also have the opposite effect: activation of Treg cells by flagellin enhances their suppressive function (11).

Whereas the effect of bacterial ligands on Treg cell suppression has been described, little is known about the regulation of Treg cell function upon viral infection. Although Treg cells are capable of suppressing antiviral immunity (12), most viral infections can be cleared through the induction of an efficient host immune response, demonstrating that Treg cell suppression can be overcome. Viruses are recognized by the innate immune system through host pattern-recognition receptors (13, 14), and several receptors recognizing molecular patterns present within viral RNA have recently been described. Single-stranded RNA from influenza virus initiates the secretion of type I IFN through activation of TLR7 within the endosome (15). In our laboratory, we found that synthetic short RNA oligonucleotides potently activate TLR7 in a sequence-dependent manner (16). Furthermore, we showed that modification of the RNA by phosphorothioate (PTO) linkage of the backbone enhances TLR7 stimulation (17). Viral RNA can also be recognized by cytosolic RNA sensors, the retinoic acid-inducible gene I (RIG-I)–like helicases (18). This family of structurally related proteins includes RIG-I, melanoma differentiation-associated gene 5 (MDA-5) and LGP2. Many RNA viruses, such as the measles virus, the rabies virus, the vesicular stomatitis virus, the Sendai virus, or the encephalomyocarditis virus (EMCV) induce type I IFN through these receptors (19–22). We have recently identified virally encoded 5′-triphosphate RNA as the natural ligand for the RIG-I receptor (23), whereas MDA-5 recognizes the synthetic long double-stranded RNA polyinosinic-polycytidylic acid [poly (I:C)] on cytosolic delivery (22).
Although the detection of viral RNA by pattern recognition receptors leads to strong immune activation, little is known about the effect of RNA ligands on Treg cell-induced immune suppression. We show in this study that RNA oligonucleotides block suppression by Treg cells in a sequence-specific and TLR7-dependent manner through inflammatory cytokines produced by APC. Infection with viruses known to stimulate the cytosolic RNA-sensing receptors RIG-I and MDA-5 blocked Treg cell function in a direct, APC-independent manner. Indeed, we show that Treg cells express RIG-I-like helicases, and activation of the MDA-5 receptor leads to an arrest of their suppressive function. Thus, suppression can be overcome both indirectly by RNA oligonucleotides activating TLR7 on APC and through direct activation of RIG-I–like helicases expressed by Treg cells.

Abstract

The role of immune suppression by regulatory T (Treg) cells in the maintenance of immune homeostasis is well established. However, little is known about how Treg cell function is inhibited on viral infection to allow the development of a protective immune response. As viral RNA is a crucial mediator for activation of antiviral immunity, we examined the effects of immunostimulatory RNA and infection with RNA viruses on Treg cell function. We show that synthetic RNA oligonucleotides potently inhibit Treg cell-induced suppression in a sequence-dependent manner. This effect is entirely dependent on TLR7 activation of APCs and subsequent IL-6 production. In addition, stimulation with the RNA viruses encephalomyocarditis virus and Sendai virus that specifically activate the RNA-sensing helicases melanoma differentiation-associated gene 5 (MDA-5) and retinoic acid-inducible gene I (RIG-I) also blocks Treg cell function. Interestingly, this effect is seen even in the absence of APCs. Consistent with this, both Treg and T effector cells express RIG-I and MDA-5. Using MDA-5–deficient mice, we demonstrate that the loss of Treg cell function on infection with encephalomyocarditis virus is strictly dependent on MDA-5 expression by Treg cells. Thus, we show in this study for the first time that activation of a RIG-I–like helicase on Treg cells blocks their suppressive function.
Regulatory T (Treg) cells play a central role in the suppression of immune reactions and in the prevention of autoimmune responses harmful to the host (1, 2). In particular, Treg cells suppress the activation of naive T effector (Teff) cells by inhibiting TCR-triggered proliferation and differentiation of these cells (3). In the early phase of microbial infection, the suppressive effect of Treg cells must, however, be overcome to enable the generation of an efficient immune response against invading pathogens (4–6). In recent years, it has been shown that several bacterial components can block the suppressive function of Treg cells. Two distinct mechanisms have so far been described: first, bacterial molecules, such as LPS or CpG DNA, can activate APCs to produce inflammatory cytokines that render Teff cells resistant to suppression by Treg cells (7). Alternatively, Treg cells themselves can be directly stimulated: targeting human Treg cells with CpG DNA or with the bacterial lipoprotein Pam3Cys-SK4 abrogates their suppressive function even in the absence of APCs (8–10). The direct action of bacterial components on Treg cells can, however, also have the opposite effect: activation of Treg cells by flagellin enhances their suppressive function (11).

Whereas the effect of bacterial ligands on Treg cell suppression has been described, little is known about the regulation of Treg cell function upon viral infection. Although Treg cells are capable of suppressing antiviral immunity (12), most viral infections can be cleared through the induction of an efficient host immune response, demonstrating that Treg cell suppression can be overcome. Viruses are recognized by the innate immune system through host pattern-recognition receptors (13, 14), and several receptors recognizing molecular patterns present within viral RNA have recently been described. Single-stranded RNA from influenza virus initiates the secretion of type I IFN through activation of TLR7 within the endosome (15). In our laboratory, we found that synthetic short RNA oligonucleotides potently activate TLR7 in a sequence-dependent manner (16). Furthermore, we showed that modification of the RNA by phosphorothioate (PTO) linkage of the backbone enhances TLR7 stimulation (17). Viral RNA can also be recognized by cytosolic RNA sensors, the retinoic acid-inducible gene I (RIG-I)–like helicases (18). This family of structurally related proteins includes RIG-I, melanoma differentiation-associated gene 5 (MDA-5) and LGP2. Many RNA viruses, such as the measles virus, the rabies virus, the vesicular stomatitis virus, the Sendai virus, or the encephalomyocarditis virus (EMCV) induce type I IFN through these receptors (19–22). We have recently identified virally encoded 5′-triphosphate RNA as the natural ligand for the RIG-I receptor (23), whereas MDA-5 recognizes the synthetic long double-stranded RNA polyinosinic-polycytidylic acid [poly (I:C)] on cytosolic delivery (22).
Although the detection of viral RNA by pattern recognition receptors leads to strong immune activation, little is known about the effect of RNA ligands on Treg cell-induced immune suppression. We show in this study that RNA oligonucleotides block suppression by Treg cells in a sequence-specific and TLR7-dependent manner through inflammatory cytokines produced by APC. Infection with viruses known to stimulate the cytosolic RNA-sensing receptors RIG-I and MDA-5 blocked Treg cell function in a direct, APC-independent manner. Indeed, we show that Treg cells express RIG-I-like helicases, and activation of the MDA-5 receptor leads to an arrest of their suppressive function. Thus, suppression can be overcome both indirectly by RNA oligonucleotides activating TLR7 on APC and through direct activation of RIG-I–like helicases expressed by Treg cells.

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Item Type:Journal Article, refereed, original work
Communities & Collections:04 Faculty of Medicine > University Hospital Zurich > Institute of Pathology and Molecular Pathology
Dewey Decimal Classification:610 Medicine & health
Scopus Subject Areas:Health Sciences > Immunology and Allergy
Life Sciences > Immunology
Language:English
Date:2010
Deposited On:18 Sep 2019 14:09
Last Modified:31 Jul 2020 03:40
Publisher:American Association of Immunologists
ISSN:0022-1767
OA Status:Hybrid
Free access at:Publisher DOI. An embargo period may apply.
Publisher DOI:https://doi.org/10.4049/jimmunol.0901245
PubMed ID:19966212

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