Nature Reviews Immunology | AOP, published online 20 March 2015; doi:10.1038/nri3806

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Regulation of antiviral T cell responses by type I interferons Josh Crouse1, Ulrich Kalinke2 and Annette Oxenius1

Abstract | Type I interferons (IFNs) are pro-inflammatory cytokines that are rapidly induced in different cell types during viral infections. The consequences of type I IFN signalling include direct antiviral activity, innate immune cell activation and regulation of adaptive immune responses. In this Review, we discuss recent conceptual advances in our understanding of indirect and direct regulation of T cell immunity by type I IFNs, which can either promote or inhibit T cell activation, proliferation, differentiation and survival. This regulation depends, to a large extent, on the timing of type I IFN exposure relative to T cell receptor signalling. Type I IFNs also provide activated T cells with resistance to natural killer cell-mediated elimination.

Plasmacytoid dendritic cells (pDCs). A subset of DCs that are described as plasmacytoid because their microscopic appearance resembles that of plasmablasts. On a per-cell basis, pDCs are important producers of type I interferons in response to viral infections or Toll-like receptor stimulation.

Institute of Microbiology, Swiss Federal Institute of Technology (ETH) Zürich, 8093 Zürich, Switzerland. 2 Institute for Experimental Infection Research, TWINCORE, Centre for Experimental and Clinical Infection Research, a joint venture between the Hannover Medical School and the Helmholtz Centre for Infection Research, 30625 Hannover, Germany. Correspondence to A.O. e-mail: [email protected] doi:10.1038/nri3806 Published online 20 March 2015 1

In 1957, Isaacs and Lindenmann discovered that in cell culture, heat-inactivated influenza virus induces a soluble factor that inhibits the propagation of live influenza virus; they named this factor interferon (IFN)1. IFNs belong to a diverse family of cytokines that can have direct antiviral effects. Additionally, IFNs may modulate cell physiology — through effects on cell proliferation, survival and differentiation — and thus affect immune cell function. IFNs are assigned to one of three families: the type I, type II or type III IFN family. The type I IFN family is composed of multiple genes, including 13 functional IFNA genes in humans (and 14 in mice) and several other subtypes, including IFNε, IFNκ, IFNω and IFNδ (reviewed in REF. 2). The type II IFN family comprises one IFNγ2, and the type III IFN family comprises IFNλ1, IFNλ2 and IFNλ3 (also known as IL‑29, IL‑28A and IL‑28B, respectively), and the recently identified IFNλ4 (REF. 3). Each IFN family binds to its own common receptor; interestingly, type I and type II IFN receptors seem to be broadly expressed by most tissues and cell types, whereas the type III IFN receptor has a more restricted expression4. Canonical type I IFN receptor (IFNAR) signalling depends on the expression of IFNAR sub­ units, Janus kinase 1 (JAK1), tyrosine kinase 2 (TYK2), signal transducer and activator of transcription 1 (STAT1), STAT2 and IFN‑regulatory factor 9 (IRF9). Type I IFN responses can be induced by both viral and bacterial pathogens, which are sensed by several pattern recognition receptors; these include Toll-like receptors (TLRs), RIG-I-like receptors (RLRs), NODlike receptors (NLRs) and a growing family of intracellular DNA receptors, several of which promote

signalling through stimulator of IFN genes (STING)5 (FIG. 1). STING is also a direct sensor of prokaryotic cyclic dinucleotides, as well as of cyclic GMP–AMP (cGAMP), which is synthesized following DNA binding to the cytosolic receptor cGAMP synthase (cGAS)6. Notably, productive infection is only a prerequisite for some, but not all, signalling pathways that trigger type I IFN production. Depending on the type of stimulus, type I IFN production can be induced in a broad range of cell types. Whereas IFNβ can be expressed by almost any virusinfected cell, IFNα production is more limited to specific cell types, such as antigen-presenting cells (APCs), with plasmacytoid dendritic cells (pDCs) being a major source of type I IFNs7,8. In the context of viral infections, the in vivo relevance of pDCs, and their function as producers of type I IFNs, varies depending on the pathogen and the route of infection9. In mice that are depleted of pDCs, viral infections may still induce protective type I IFN responses, indicating that many cell types are able to provide physiologically relevant levels of type I IFNs10. In this Review, we discuss how and where type I IFNs directly and/or indirectly influence antiviral T cell immunity, and relate this to functional consequences on the cellular level and to physiological consequences on the organism level (FIG. 2). Indirect effects of type I IFNs on T cell immunity include: regulating the amount of newly produced antigen, inducing APC maturation, and promoting natural killer (NK) cell activation. In addition, direct signalling of type I IFNs in T cells can positively or negatively influence antiviral T cell responses, depending on the timing and the context of concomitant stimulation of other signalling pathways.

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REVIEWS Viral glycolipids TLR4

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Figure 1 | Induction and kinetics of type I interferon production following viral infection.  Viral constituents, mostly Reviews Immunology nucleic acids, are sensed by various pattern recognition receptors, including Toll-like receptorsNature (TLRs), RIG-I like |receptors (RLRs), NOD-like receptors (NLRs) and a growing family of intracellular DNA receptors (left panel). The activation of these receptors leads to the rapid production of type I interferons (IFNs) during viral infections, which precedes the induction of antiviral adaptive immunity (right panel). Type I IFNs are induced early in response to viral infections, with levels peaking in the first few days post-infection. The kinetics of type I IFN induction coincide with the time of early T cell activation such that type I IFNs can serve to regulate the T cell responses. cGAS, cGAMP synthase; dsRNA, double-stranded RNA; IFNAR, type I IFN receptor; MDA5, melanoma differentiation-associated protein 5; RIG-I, retinoic acid-inducible gene I; ssRNA, single-stranded RNA; STING, stimulator of IFN genes.

Indirect regulation of T cells by type I IFNs Type I IFN regulation of APCs. APCs are the most important cellular link between innate and adaptive immunity; therefore, changes in this cellular compartment will have substantial effects on the T cell response. APCs can take up, process and present antigen, migrate from the site of antigen encounter to sites of T cell activation, and express co-stimulatory molecules and cytokines. All of these activities are required for T cell priming and effector cell differentiation. Type I IFN signalling on APCs can affect each of these processes, thus having an indirect effect on the generation of T cell responses. Here, we discuss the various mechanisms by which type I IFNs can affect the activities of APCs. Type I IFN signalling affects APC development, starting with the differentiation from precursor cells. Type I IFNs enhance granulocyte–macrophage colonystimulating factor (GM-CSF)-mediated differentiation of monocytes into mature DCs11,12, but might also impair DC development through STAT2 activation13. In vitro exposure of differentiated DCs to type I IFNs induces their phenotypic maturation, indicated by enhanced expression of MHC class I, MHC class II and costimulatory molecules (such as CD40, CD80, CD83 and CD86), as well as increased expression of CC-chemokine receptor 5 (CCR5) and CCR7 (REFS  14,15), and the adhesion molecule lymphocyte function-associated antigen 1 (LFA1), which facilitates DC migration into draining lymph nodes15. Furthermore, IFNα induces

DC production of CXC-chemokine ligand 9 (CXCL9) and CXCL10, which can function as chemo­attractants for T cells16. Together, these type I IFN-mediated effects increase the ability of DCs to prime T cells in secondary lymphoid organs11,17–21. Consistent with this, mice that received an injection of IFNAR-blocking antibodies, or mice with DCs that lack IFNAR expression, showed suppressed DC maturation in response to polyinosinic–polycytidylic acid (poly(I:C)), which resulted in a reduced capacity to stimulate T cells22. Type I IFNs also enhance antigen presentation by sustaining MHC class II synthesis and antigen processing 23. Type I IFNs were also shown to enhance cross-­ presentation in various DC subsets, including CD8+ DCs in mice and pDCs in humans24,25. Infection with lymphocytic choriomeningitis virus (LCMV) — a potent inducer of type I IFNs — promoted the crosspriming of CD8+ T cells specific for an unrelated antigen in an IFNAR-dependent manner. Co-injection of IFNα with ovalbumin (OVA) protein also boosted the cross-­priming of OVA-specific CD8+ T cells26 via delayed endosomal acidification, and rerouting of antigens towards MHC class I processing pathways27,28. All of the type I IFN subtypes signal through the same receptor complex with minor differences in their receptor binding affinity29. Interestingly, different type I IFN subtypes were found to lead to distinct differentiation profiles of DCs in vitro. Specifically, transcription profiles of DCs that had differentiated from monocytes

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REVIEWS NK cell activation (↑ Cytotoxicity)

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Figure 2 | Type I interferon-mediated regulation of antiviral T cell immunity.  Type I interferons (IFNs) may Nature Reviews | Immunology regulate T cell immunity in a direct or indirect manner, in either a positive or negative way. Indirectly supporting T cell activation, type I IFNs promote dendritic cell maturation, migration and antigen presentation, and restrict viral replication, thereby modulating the amount of antigen that is presented to T cells. Notably, specific cell types, such as metallophilic macrophages in the spleen, enforce viral replication by interfering with type I IFN signalling and thereby function as viral sanctuaries that ensure the provision of sufficient amounts of antigen for T cell priming. Type I IFNs directly affect T cell activation, proliferation and survival by acting as signal 3 cytokines during T cell priming, and by protecting clonally expanding T cells against natural killer (NK) cell-mediated attack in viral infections that induce a type I IFN-dominated cytokine milieu. Type I IFN-mediated activation of NK cells enhances their cytotoxicity, which can contribute to early control of viral infection, but can also be directed against activated T cells and hence curtail the population of antiviral T cells. This is particularly pronounced when primed T cells fail to sense type I IFNs and to ‘shield’ themselves against NK cell recognition. APC, antigen-presenting cell; CCR7, CC-chemokine receptor 7; ISG, IFN-stimulated gene; IFNAR, type I IFN receptor; OAS, 2ʹ,5ʹ-oligoadenylate synthetase; NCR1, natural cytotoxicity triggering receptor 1; PKR, protein kinase R; TCR, T cell receptor; USP18, Ubl carboxy-terminal hydrolase 18.

in the presence of IFNβ and GM-CSF differed from DCs that had differentiated in the presence of IFNα1, IFNα2, IFNα8, IFNα21 or IFNω. Nevertheless, the migratory capacity and the overall T cell stimulatory capacity was equivalent, irrespective of the specific differentiation conditions, which highlights the apparent redundancy of type I IFN subtypes in this process30. These findings underscore the importance of type I IFNs in guiding the differentiation and maturation of DCs, which are able to migrate into secondary lymphoid organs to effectively prime T cell responses.

Chronic IFN exposure and immunoregulatory DCs. Although acute type I IFN signalling enhances the ability of DCs to stimulate T cell responses, chronic exposure can be detrimental to the control of infections with pathogens such as LCMV and Mycobacterium tubercu­ losis 31–34. During chronic LCMV infection, DCs from persistently infected mice exhibit an immunoregulatory phenotype that is characterized by interleukin‑10 (IL‑10) production and programmed cell death  1 ligand 1 (PDL1) expression. This phenotype is caused by persistent exposure to type I IFNs, as indicated by

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REVIEWS their increased expression of the type I IFN-stimulated genes 2′,5′-oligoadenylate synthetase (Oas) and myxovirus resistance 1 (Mx1)35. Inhibiting type I IFN signalling by the injection of IFNAR-blocking antibodies led to a reduction in the ratio of immunoregulatory to stimulatory DCs and enhanced virus-specific CD4+ T cell immunity, and thereby led to improved infection control33,34. These findings highlight the importance of the duration of IFN signalling for APC function, as prolonged exposure can lead to the generation of immunosuppressive DCs that attenuate T cell-mediated immunity and viral control.

Signal 3 cytokine During T cell activation, T cell receptor signals (signal 1) and co-stimulatory signals (signal 2) initiate proliferation of naive T cells; however, they also require a specific cytokine signal (known as signal 3) to differentiate into effector and memory T cells. The most prominent signal 3 cytokines for CD8+ T cell activation are interleukin‑12 and type I interferons.

Metallophilic macrophages A population of macrophages found in the spleen that is located around the white pulp adjacent to the marginal sinus. These cells express constitutive levels of Ubl carboxy-terminal hydrolase 18 (USP18), a negative regulator of type I interferon signalling, and are thus more vulnerable to infection with type I interferonsensitive viruses than other myeloid cells.

Type I IFN-induced cytokine production. Type I IFN signalling in APCs induces the production and secretion of cytokines that affect T cell-mediated immunity. One such cytokine is IL‑15 (REFS 36–38), which supports the proliferation and survival of memory T cells and NK cells37–39. In vitro incubation of macrophages with IFNα or IFNβ rapidly induces Il15 mRNA transcription37, and in vivo IFNα/β treatment enhances IL‑15 expression by splenic DCs36 in a STAT1‑dependent manner 38. This, in turn, promotes expression of the α-chain of the IL‑15 receptor (IL‑15Rα) by splenic conventional DCs36, which is required for the trans-presentation of IL‑15 to T cells40. In addition, type I IFNs promote IFNγ secretion by both DCs and NK cells21,41. This is important for the differentiation of T helper 1 (TH1) cells42, and for the induction of IL‑7 expression, which has an important role in thymic T cell development and the homeostasis of naive and memory T cells43. Another important cytokine that is regulated by type I IFNs is IL‑12; however, unlike IL‑7 and IL‑15, the production of IL‑12 is inhibited by type I IFN signalling 44,45. IL‑12 functions as a signal 3 cytokine during T cell activation and is therefore important for the differentiation and survival of activated T cells. However, type I IFN-mediated inhibition of IL‑12 does not hinder the T cell response because type I IFNs themselves also act as signal 3 cytokines (see below). There is currently an incomplete understanding of why such reciprocal inhibition between type I IFNs and IL‑12 exists, and whether it has physiological significance for viral control, but the reason for this could be a host adaptation that provides signal 3 cytokines for T cell differentiation during infections with viruses that actively inhibit type I IFN production or signalling. Type I IFNs in NK cell-mediated T cell regulation. Type I IFNs are potent activators of NK cell activity 38,46–48; they promote NK cell-mediated cytotoxicity and IFNγ secretion in a STAT1‑dependent manner; these are crucial effector functions for the early control of viral infections such as cytomegalovirus, hepatitis B virus and HIV41,48,49. IFNγ both activates innate antiviral responses and supports the differentiation of TH1 cells50,51. Aside from having direct antiviral functions, NK cells also contribute to the regulation of T cell responses (reviewed in REFS 52,53). Activated NK cells can either directly kill effector T cells, or can indirectly regulate T cell function by NK cell-mediated modulation of APC numbers

and/or function, as well as by cytokine secretion. Recent reports have demonstrated the ability of activated NK cells to directly recognize and eliminate activated T cells in a perforin-dependent manner54–61. Importantly, type I IFNs activate the cytotoxic activity of NK cells, while simultaneously acting directly on T cells to protect them against NK cell-mediated attack (discussed below). Thus, type I IFNs not only activate NK cells to exert direct antiviral effector functions, but also govern NK cell-mediated regulation of antiviral T cell responses. Viral load. One key aspect of type I IFN biology is its ability to act as an innate antiviral cytokine, which leads to the establishment of an antiviral state that is characterized by the expression of many proteins that are involved in the suppression of viral replication and spread, including proteins involved in RNA degradation, translational inhibition and cellular apoptosis62. Together, these effects contribute to an early reduction in viral load. As the overall viral load affects the extent of T cell activation and differentiation, direct antiviral type I IFN activity can also indirectly influence antiviral T cell responses. The beneficial effects of type I IFNs in mediating the early control of viral replication are essential; however, such control could compromise subsequent T cell priming by limiting amounts of viral antigens. Therefore, hosts have evolved mechanisms to counteract the potent antiviral effects of type I IFNs in a cell-specific manner, thus providing a source of sufficient viral antigens for adaptive immune cell activation. For example, the expression of the type I IFN signalling inhibitor Ubl carboxy-terminal hydrolase 18 (USP18) by metallophili­c macrophages in the spleen enforces viral replication in these cells, despite the presence of high levels of type I IFNs, thereby allowing sufficient viral antigen accumulation for the effective activation of virus-specific adaptive immunity 63. At the other extreme, excessive and prolonged viral replication, and hence excessive and prolonged exposure to antigens, can be detrimental to T cell function. During chronic infections with viruses such as LCMV or HIV, there is a strong correlation between viral load and T cell function, whereby high viral loads drive T cell exhaustion as a result of chronic antigenic stimulation64–67. Notably, this takes place despite the presence of continued type I IFN signalling 33,34. It is very likely that when high infection doses are used to establish chronic infections, type I IFNs do not have the capacity to mediate innate control of the infection and the type I IFNmediated induction of immunoregulatory DCs prevails in this setting 35.

Direct type I IFN signalling in T cells The phenotypic and functional consequences of direct IFNAR signalling on T cells are diverse and even seem to contradict each other. For example, although type I IFNs act as potent signal 3 cytokines during T cell activation — by promoting proliferation, survival and effector cell differentiation — they can also induce antiproliferative and pro-apoptotic programmes in T cells. It is presumed that the most decisive parameter is the relative timing of IFNAR and T cell receptor (TCR)

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REVIEWS signalling. In general, when TCR stimulation co­incides with, or shortly precedes, IFNAR signalling, the role of type I IFNs as signal 3 cytokines predominates. Conversely, if IFNAR signalling precedes TCR engagement, the anti-­proliferative and pro-apoptotic capacity of type I IFNs prevails (FIG. 3a). The molecular basis of these opposing effects is provided by different intracellular signalling pathways that are induced after IFNAR engagement in T cells in the presence or absence of concomitant or prior TCR signalling (reviewed in REF. 68). Type I IFNs have the unique ability to activate all seven known STATs, and induce the formation of many different STAT homodimer or heterodimer complexes in response to

a

IFNAR engagement (reviewed in REFS 69,70). Signalling through STAT1 is known to be pro-inflammatory, antiproliferative and pro-apoptotic, as opposed to signalling through STAT3, STAT4 and STAT5, which induces gene transcription that promotes cell survival, proliferation and differentiation71,72. Thus, the intracellular competition between STAT1, and STAT3, STAT4 and STAT5 determines the transcriptional and functional consequences of type I IFN signalling (FIG. 3a). TCR-activated T cells are unable to phosphorylate STAT1 (REF. 73), and so the type I IFN signal is propagated by other STATs, thereby explaining the pro-survival and pro-proliferative action of type I IFNs71,74–76.

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Figure 3 | Consequences of direct type I interferon signalling in T cells.  a | The timing of type I interferon (IFN) signalling relative to T cell receptor (TCR) triggering determines its pro-proliferative, anti-apoptotic or anti-proliferative, Nature Reviews | Immunology pro-apoptotic effect on T cells. Concomitant or slightly delayed type I IFN signalling relative to TCR engagement (that is, ‘in-sequence signalling’) promotes survival, effector cell differentiation and proliferation in a signal transducer and activator of transcription 4 (STAT4)-dependent manner, with STAT4 expression being upregulated relative to STAT1 in TCR-activated T cells. Pre-exposure of T cells to type I IFNs in relation to TCR engagement (that is, ‘out-of-sequence signalling’) induces a STAT1‑dependent anti-proliferative and pro-apoptotic programme. (Numbers in circles indicate the relative order of receptor triggering). b | In-sequence type I IFN signalling promotes: the survival of activated T cells through upregulation of the high-affinity interleukin‑2 receptor α-chain (CD25) and IL‑15 receptor induction (left); effector cell differentiation through regulation of the transcription factors T-bet, B lymphocyte-induced maturation protein 1 (BLIMP1) and eomesodermin (EOMES), and through increased sensing of IL‑2 (middle); and protection from natural killer (NK) cell-mediated killing by suppressing surface expression of ligands for the NK cell-activating receptor natural cytotoxicity triggering receptor 1 (NCR1), or upregulation of ligands (such as MHC class I) for NK cell-inhibitory receptors (right). IFNAR, type I IFN receptor.

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REVIEWS Owing to the importance of the timing of type I IFN signalling on T cells relative to TCR signalling for the downstream transcriptional and functional consequences, the following five sections address the impact of IFNAR signalling concomitant to T cell activation. Thereafter, type I IFN effects on T cells when provided ‘out of normal timing’ are discussed. Finally, a summary of the impact of sustained type I IFN signalling on T cells, as occurs during certain chronic viral infections, is provided.

Superantigen A microbial protein that activates all of the T cells expressing a particular set of T cell receptor (TCR) Vβ chains by crosslinking the TCR to a particular MHC molecule, regardless of the peptide presented.

Effects of in-sequence signalling on T cell proliferation and survival. Proper activation of T cells — resulting in clonal expansion, survival, acquisition of effector functions and memory formation — is only achieved when the responding T cells integrate three signals: TCR engagement by peptide–MHC complexes (signal 1), the ligation of co-stimulatory receptors (signal 2) and specific cytokine signals (signal 3). For CD8+ T cell responses, the best-studied signal 3 cytokines are IL‑12 and type I IFNs. In vitro stimulation of naive mouse CD8+ T cells with only signals 1 and 2 (that is, antigen and co-stimulation) induces limited proliferation and survival77, whereas the addition of type I IFNs or IL‑12 promotes survival of the clonally expanding T cells in a STAT4‑dependent manner74,78 (FIG. 3b). Also, in naive human CD8+ T cells, IFNα enhances proliferative capacity when the cells are stimulated with CD3and CD28‑specific monoclonal anti­b odies or with superantigen79,80, and increased IL‑2‑dependent cell division correlates with IFNα-induced expression of IL2RA (which encodes CD25; also known as IL‑2Rα), MYC and PIM1 (REF. 81). Enhanced survival of activated T cells is ascribed to the type I IFN-supported, STAT1‑dependent reversion of T cell blasts into a resting G0 or G1 state, together with upregulation of expression of the antiapoptotic molecule BCL-XL (also known as BCL‑2L1)82, although other mechanisms might also be involved83. Direct evidence for a role of type I IFNs in supporting the survival of antiviral T cells in vivo came from adoptive transfer experiments in which wild-type and IFNAR-deficient, LCMV-specific CD8 + and CD4 + T cells were transferred into wild-type hosts followed by LCMV infection. Despite comparable early division activity, IFNAR-deficient T cells failed to expand to numbers comparable to their wild-type counterparts56,60,84–88. Importantly, IFNAR-deficient T cells are not inherently unable to expand and differentiate into effector cells in the context of other cytokine signals (such as IL‑12), as the survival of IFNAR-deficient CD8+ T cells was much less affected following vaccinia virus or Listeria mono­ cytogenes infection60,85,86,88,89. Thus, the pro-survival activity of type I IFNs seems to be most pronounced during in vivo infections that induce strong type I IFN responses, and less pronounced in infectious settings that are more IL‑12 biased44,86,88,90. Similar observations were also made for in vivo recall responses of memory CD8+ T cells60,91. Consistent with these studies of viral infections, type I IFNs also enhance CD8+ T cell responses following protein or DC vaccination. Immunization with a protein antigen and concomitant IFNα administration prolonged the proliferation and enhanced the expansion of

CD8+ T cells92–94. Despite inducing upregulation of expression of CD25 — the high affinity receptor for IL‑2 — and IL‑15 receptor by responding T  cells, IL‑2 and IL‑15 signals, but not direct type I IFN signals, were dispensable for the enhanced accumulation of T cells92. By contrast, brief exposure to type I IFNs or IL‑12 following DC immunization did not yield an advantage to proliferation or survival at early time points but rather induced sustained and high expression of CD25, which promoted IL‑2‑triggered expression of genes that are associated with cell cycle progression95. Effects of in-sequence signalling on T cell differentiation. Properly timed IFNAR signalling in CD8+ T cells acts as signal 3 to instruct the acquisition of effector functions, such as cytotoxicity and cytokine secretion, and is involved in memory cell formation77,78,93,96 (FIG. 3b). The development of cytolytic effector function and IFNγ secretion is induced by type I IFNs in a STAT4‑dependent manner 41,74, and is antagonized by STAT1 (REF.  97). During TCR-induced activation, CD8+ T cells upregulate STAT4 expression while maintaining low STAT1 levels, thus explaining the predominant STAT4‑mediated pro-proliferative and effector cell-inducing functions of type I IFNs during CD8+ T cell priming 98. Elegant in vitro studies have analysed the transcriptional response of mouse CD8+ T cells that were activated by signals 1 and 2 together with either type I IFNs or IL‑12. Both cytokines induced a sustained transcriptional programme that was compatible with effector cell differentiation74,99. The maintenance of this gene expression programme involved chromatin remodelling through histone acetylation99. Similarly, in human CD8+ T cells, IFNα was shown to act as a third signal during the priming of naive CD8+ T  cells, as determined by transcriptional, protein expressio­n and functional analyses79. In vivo investigation of CD8+ T cell differentiation in the absence of IFNAR signalling in CD8+ T cells showed that type I IFN signalling in responding CD8+ T cells was required for the differentiation of short-lived effector cells in inflammatory environments that are dominated by type I IFNs60,89, but was not required in IL‑12‑dominated settings88,89. Despite the massively curtailed T cell expansion and the deficit of short-lived effector cell differentiation of IFNAR-deficient CD8+ T cells during LCMV infection, they were still capable of differentiating into memory precursor effector cells and consequently formed functional memory responses, albeit at much lower levels than their wild-type counterparts60,89,91,100. Type I IFN signalling was shown to directly upregulate expression of T-bet, a transcription factor that is required for effector cell differentiation89, and to induce sustained upregulation of CD25 expression95,101. In combination with studies demonstrating a pivotal role for strong IL‑2 signalling in effector cell differentiation102,103, the direct regulation of CD25 by type I IFNs supports its role in promoting short-lived effector cell formation. Whereas the effector functions of CD8 + T cells primed in vitro with signals 1 and 2 in the presence of either IL‑12 or type I IFNs seemed very comparable, the in vivo behaviour of these cells differed with respect

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REVIEWS to their ability to exert tumour control. IL‑12‑primed CD8+ T cells excelled in mediating tumour control in an adoptive immunotherapy model, whereas type I IFNprimed CD8+ T cells were more prone to exhaustion, as indicated by expression of programmed cell death protein 1 (PD1) at the tumour site104. The reasons for this differential expression of PD1 by IL‑12‑primed and type I IFN-primed cells during antige­n re-encounter currently remain unexplained. Effects of in-sequence signalling on CD4+ T cell polarization. One important aspect of CD4+ T cell activation is their polarization into distinct cytokine-producing effector and memory cell subsets, which is, to a large extent, enabled by the presence of various instructing innate cytokines during the priming period (reviewed in REF. 105). Both STAT4 activation and T-bet induction are essential for TH1 cell development. IL‑12, which signals through STAT4, is the classical TH1 cell-polarizing cytokine. Notably, type I IFNs can also signal via STAT4, which implies that they might also regulate TH1 cell differentiation. In human CD4+ T cells, type I IFNs can, to some extent, directly promote TH1 cell polarization106; however, this capacity is much reduced compared with that of IL‑12, owing to transient STAT4 activation by type I IFNs, as opposed to sustained STAT4 activation by IL‑12 (REFS 107,108). In mouse CD4+ T cells, there is little evidence for a direct role of type I IFNs in TH1 cell differentiation109,110. Instead, type I IFNs seem to promote TH1 cell differentiation in vivo by increasing the IL‑2 responsiveness of T cells (which is also seen in humans81), which leads to increased STAT5 signalling. STAT3, however, counter­ acts this indirect type I IFN-driven TH1 cell differentiation and instead promotes T follicular helper (TFH) cell differentiation111,112 by reducing CD25 expression (and hence IL‑2-induced signalling), and by directly competing with STAT5 for binding to the Bcl6 locus (which encodes B  cell lymphoma 6 (BCL‑6)) 113,114. Although type I IFNs induce the expression of BCL‑6, CXC-chemokine receptor 5 (CXCR5) and PD1 in CD4+ T cells, which are all key features of TFH cells115, they fail to induce the expression of IL‑21, which is an important effector cytokine of these cells116. Finally, type I IFNs also affect the activity of regulatory T (TReg) cells. Selective IFNAR deficiency in TReg cells resulted in impaired virus control after LCMV infection, and this was associated with compromised antiviral T cell function. Mechanistically, type I IFN signalling was shown to interfere with ICOS ligand (ICOSL)dependent and CD28‑dependent TReg cell activation and proliferation117. Protection against NK cell-mediated killing. As mentioned above, direct IFNAR signalling in LCMVspecific T cells is required for their sustained expansion and effector cell differentiation84,85,87. Recent in vivo studies have shown that direct IFNAR signalling on T cells during LCMV is not directly anti-proliferative, but instead protects the activated T cells from NK cellmediated attack 56,60  (FIG.  3b) . As the cytotoxicity of

NK cells is potently induced by type I IFNs, concomitantly activated and expanding T cells seem to require ‘camouflage’ against NK cell recognition. Type I IFNs provide such protection by downregulating T cell ligands for the activating NK cell receptor natural cytotoxicity triggering receptor 1 (NCR1)60, or by upregulating ligands (such as MHC class I) for inhibitory NK cell receptors56. These recent findings offer some potential explanations for earlier observations that actively cycling thymocytes were NK cell targets unless they were exposed to type I IFNs that were induced by poly(I:C) treatment or LCMV infection118. Effects of in-sequence signalling on T cell migration and adhesion. Type I IFNs are also involved in the regulation of T cell migration and cell adhesion. T cells need to be retained in secondary lymphoid tissues for several days to facilitate their full activation and effector cell differentiation, which is, in part, coordinated by type I IFN-induced CD69 upregulation119 and concomitant sphingosine‑1‑phosphate receptor 1 (S1PR1) downregulation120,121. In human T cells, IFNα2 was shown to promote increased chemotaxis, integrin clustering and integrin-mediated adhesion in a process that was dependent on mitogen-activated protein kinase (MAPK) and phosphoinositide 3‑kinase signalling 122. Anti-proliferative and pro-apoptotic effects of ‘out-ofsequence’ signalling in T cells. In the following section, we summarize the anti-proliferative effects of IFNAR signalling in T cells. Treatment of resting human primary CD4+ T cells with IFNα for 20 hours in vitro, or pre-exposure of mouse T cells to type I IFNs in vivo, delayed cell cycle entry following TCR engagement 123,124. By contrast, IFNα treatment of activated T cells did not impair their proliferation, and activated T cells exhibited much weaker induction of IFN-responsive genes compared to their resting counterparts123. The antiproliferative and pro-apoptotic effects of type I IFNs are mediated by STAT1 (REFS 71,76,125) (FIG. 3a). Release from this negative regulation occurs in CD8+ T cells when they are activated by ligation of the TCR or the common γ-chain receptor, which both increase the abundance of other STATs relative to STAT1 (REFS 72,76). The growth-­inhibitory signals that are conveyed by type I IFNs require the expression of the TCR signalling molecules CD45, LCK and ZAP70 in association with the IFNAR signalling complex 126. In addition to the PD1 (REF. 127) and TNF-related apoptosis-inducing ligand (TRAIL; also known as TNFSF10)128 pathways, the proapoptotic effect of type I IFNs was shown to involve apoptosis induced by FAS (also known as CD95) and activation-induced cell death via the FAS–FAS ligand (FASL; also known as CD95L) pathway 129,130. Long-term exposure of primary CD4+ T cells to IFNα reduces cell surface CD3 and CD28 expression, which leads to reduced signal strength following TCR triggering and diminished IL‑2 production. Even when supplemented with exogenous IL‑2, IFNα pre-treatment conferred reduced proliferation, which was associated with reduced CD25 expression131.

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REVIEWS Following TCR-mediated activation, the expression of co-inhibitory receptors, such as cytotoxic T lymphocyte antigen 4 (CTLA4) and PD1, downregulates subsequent T cell activation and effector functions. In addition to being dependent on TCR activation, the extent and duration of PD1 expression in T cells is regulated by IFNAR signalling, and therefore in the context of continued T cell activation (for example, during chronic infections or cancer), IFNAR signalling leads to weakened T cell responses127. Out-of-sequence signalling: immunosuppression and effects on ‘bystander’ cells. Type I IFNs are also known to exert substantial effects on bystander T cells, including the induction of apoptosis of CD44hi memory T cells, which express slightly higher IFNAR levels than naive T cells132. This effect leads to attrition of bystander memory CD8+ T cells during the early phases of viral infection, which may favour the establishment of new antiviral T cell responses133. Interestingly, this type I IFN-induced caspase‑3‑dependent and caspase‑8‑ dependent apoptosis was not only observed in the absence of nominal antigen, but also affected naive or memory T cells in the presence of nominal antigen134. At later stages of infection, when antigen-specific T cells started to markedly expand in number, bystander T cells were substantially depleted by apoptosis135. By contrast, in vivo pre-exposure of naive CD8+ T cells to type I IFNs sensitized these cells for rapid effector functions, such as IFNγ production and degranulation following stimulation with high-affinity antigen. This sensitization was dependent on the presence of self MHC class I molecules and, in the case of increased granzyme B expression, was mediated by type I IFN-induced induction of IL‑15 (REF. 136). This indirect effect of type I IFNs on bystander memory CD8+ T cells is consistent with previous studies showing that type I IFN injection induces TCR‑independent CD8+ T cell proliferation in vivo, which is mediated via IL‑15 (REF. 37). The timing of type I IFN signalling relative to TCR triggering also determines whether the effects of type I IFNs are pro-­proliferative versus anti-proliferative or apop­ totic in vivo. When third-party CD8+ T cells were activated by antigen concomitant with a hetero­logous virus infection, the infection served as adjuvant for the thirdparty response. Conversely, when antigen stimulation was delayed 3–9 days into a viral infection (and T cells had been exposed to the inflammatory milieu induced by the viral infection), proliferation of third-party cells was substantially inhibited, and this inhibitory effect was largely dependent on out-of-sequence type I IFN signalling in the responding T cells137. It is still not entirely known how the exposure of T cells to type I IFNs prior to TCR activation inhibits T cell proliferation and effector cell differentiation. Aside from the documented role of STAT1, the active suppression of subsequent IFNAR signalling during TCR triggering might be involved in this process138; for example, suppressor of cytokine signalling 1 (SOCS1) inhibits type I IFN signalling through its inhibition of the IFNAR1‑associated kinase TYK2, leading to inhibition of STAT signalling and reduced cell surface expression of IFNAR1 (REF. 139).

Type I IFNs during chronic viral infections. Chronic type I IFN exposure may occur during persistent viral infections; the consequences of sustained (chronic infection) versus time-limited (acute infection) type I IFN exposure on T cell-mediated immunity are summarized in FIG 4. Sustained type I IFN exposure may lead to out-of-sequence IFNAR signalling in T cells (discussed above) or to IFNAR signalling in T cells in the absence of TCR triggering. In untreated human HIV‑1 infection or primate simian immunodeficiency virus (SIV) infection, sustained type I IFN activity correlates with a state of general immune activation (reviewed in REFS 140,141). T cells from HIV-infected individuals show increased IFNα-induced gene expression, which correlates with enhanced FAS-induced apoptosis and expression of the pro-apoptotic protein BCL‑2 homologous antagonist/killer (BAK) 130. In SIV infection, the timing of type I IFN responses determines whether their effects are beneficial or detrimental; IFNAR blockade during acute stages of infection has been shown to increase SIV reservoir size and promote disease progression, highlighting the direct antiviral effects of type I IFNs during the early phases of infection. Consistent with this, exogenous administration of IFN‑α2a during early phases of the infection promoted virus control, whereas sustained administration resulted in type I IFN desensitization and accelerated disease progression142. In line with these observations, chronic LCMV infection in mice is also associated with a state of immune activation, the expression of negative immune regulators, a sustained IFN signature and compromised lymphoid tissue architecture. Two recent studies showed that IFNAR blockade before or during established persistent LCMV infection accelerated virus control, which was associated with reduced immune activation and immune suppression, with restoration of lymphoid tissue architecture and a reduction in expression of negative immune regulators33,34. Furthermore, a blockade of type I IFN signalling prevented the lethal CD8+ T cellmediated immunopathology that was induced in NZB mice following chronic LCMV infection143. During acute or chronic LCMV infection, type I IFNs are mainly produced in a manner that depends on melanoma differentiation-associated protein 5 (MDA5; also known as IFIH1) by cells other than pDCs. In the absence of MDA5 and T cell help, acute LCMV infection cannot be controlled, which results in T cell exhaustion and viral persistence. Treatment with exo­ genous type I IFNs during the early stages of infection rescued the antiviral T cell response, which resulted in improved virus control144. Consistent with these observations, negative regulation of type I IFN activity by OAS-like 1 (OASL1) promoted the persistence of LCMV infection145. During persistent viral infection, de novo priming of virus-specific T cells can occur and, in some instances, is believed to be important for the maintenance of the overall population of virus-specific T cells146. Priming of naive CD4+ T cells during established chronic LCMV infection strongly biases their differentiation into

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REVIEWS Priming phase

Role of type I IFNs (acute and chronic infection) • Increased maturation, migration and antigen presentation by DCs • Signal 3 cytokine • Protection from NK cell-mediated attack • Control of viral replication

APC

MHC class I TCR Type I IFNs

CD8

Type I IFNs

T cell

Acute infection

Chronic infection

Memory T cells Exhausted T cells

Role of type I IFNs • Sustained IFN signature • General immune activation • Immunoregulatory DCs • Compromised secondary lymphoid organ architecture • Induction of TFH cells • Impaired effector-to-memory cell differentiation of heterologous T cell responses • ‘Out-of-sequence’ signalling results in T cell apoptosis

Memory phase

Chronic phase

Figure 4 | Role of type I interferons in T cell immunity in acute and chronic infections.  During early phases of viral Nature Reviews | Immunology infections, type I interferon (IFN) signalling supports T cell priming indirectly by inducing maturation, migration and antigen presentation in dendritic cells (DCs), and by restricting viral replication. In a direct manner, type I IFNs function as signal 3 cytokines, supporting T cell differentiation, proliferation and survival, and providing protection against natural killer (NK) cell-mediated elimination (top panel). Type I IFN responses are time-limited during acute infections as, after viral clearance, memory CD8+ T cells form (lower left panel and dashed arrow). By contrast, type I IFN responses may be maintained during chronic infections, which can have multiple downstream effects (lower right panel). In combination with sustained T cell receptor (TCR) signalling and other immune-regulatory mechanisms (such as expression of co-inhibitory receptors, immune-suppressive cytokines and increased numbers of regulatory T cells), this leads to dysfunctional CD8+ T cell responses, also termed T cell exhaustion. APC, antigen-presenting cell; TFH, T follicular helper.

TFH cells at the expense of TH1 cell development, with chronic type I IFN production being responsible for the failure of TH1 cell differentiation147. Also, the differentiation pathways of non-virus-specific T cells are influenced in a setting of chronic viral infection. Specifically, the transition from effector to memory CD8+ T cells seems to be impaired by a chronic bystander infection that promotes a sustained inflammatory signature148.

Conclusions Initially discovered for their essential role as innate antiviral cytokines, our improved understanding of how type I IFNs contribute to the regulation of T cell immunity will prove useful when exploiting their actions in clinical settings. Recent findings show the ability of type I IFNs to protect antiviral T cells from NK cell-mediated elimination. Identification of the involved signalling pathways may open the possibility of

harnessing this type I IFN effect to target NK cell killing activity towards unwanted T cells, such as autoreactive T cells. Although the beneficial role of type I IFNs during acute infections is well documented — both with respect to direct control of viral replication and induction of effector T cell responses — the consequences of chronic exposure to type I IFNs are just beginning to be understood. It is already clear that chronic type I IFN exposure is associated with T cell dysfunction and impaired T cell immunity to third-party immunogens. These findings highlight the important functional and physiological implications of the timing and duration of type I IFN signalling. Further insights into the under­ lying molecular pathways will be instrumental, not only to advance our understanding of the physiology and pathophysiology of these cytokines, but also to develop strategies that exploit this information to either enhance or dampen T cell-mediated immunity.

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REVIEWS Isaacs, A. & Lindenmann, J. Virus interference. I. The interferon. Proc. R. Soc. Lond. B Biol. Sci. 147, 258–267 (1957). 2. Pestka, S., Krause, C. D. & Walter, M. R. Interferons, interferon-like cytokines, and their receptors. Immunol. Rev. 202, 8–32 (2004). 3. Prokunina-Olsson, L. et al. A variant upstream of IFNL3 (IL28B) creating a new interferon gene IFNL4 is associated with impaired clearance of hepatitis C virus. Nature Genet. 45, 164–171 (2013). 4. Megjugorac, N. J., Gallagher, G. E. & Gallagher, G. Modulation of human plasmacytoid DC function by IFN‑λ1 (IL‑29). J. Leukoc. Biol. 86, 1359–1363 (2009). 5. Cavlar, T., Ablasser, A. & Hornung, V. Induction of type I IFNs by intracellular DNA-sensing pathways. Immunol. Cell Biol. 90, 474–482 (2012). 6. Hornung, V., Hartmann, R., Ablasser, A. & Hopfner, K. P. OAS proteins and cGAS: unifying concepts in sensing and responding to cytosolic nucleic acids. Nature Rev. Immunol. 14, 521–528 (2014). 7. Barchet, W. et al. Virus-induced interferon-α production by a dendritic cell subset in the absence of feedback signaling in vivo. J. Exp. Med. 195, 507–516 (2002). 8. Reizis, B., Bunin, A., Ghosh, H. S., Lewis, K. L. & Sisirak, V. Plasmacytoid dendritic cells: recent progress and open questions. Annu. Rev. Immunol. 29, 163–183 (2011). 9. Swiecki, M., Wang, Y., Gilfillan, S. & Colonna, M. Plasmacytoid dendritic cells contribute to systemic but not local antiviral responses to HSV infections. PLoS Pathog. 9, e1003728 (2013). 10. Swiecki, M., Gilfillan, S., Vermi, W., Wang, Y. & Colonna, M. Plasmacytoid dendritic cell ablation impacts early interferon responses and antiviral NK and CD8+ T cell accrual. Immunity 33, 955–966 (2010). 11. Paquette, R. L. et al. Interferon-α and granulocytemacrophage colony-stimulating factor differentiate peripheral blood monocytes into potent antigenpresenting cells. J. Leukoc. Biol. 64, 358–367 (1998). 12. Santini, S. M. et al. Type I interferon as a powerful adjuvant for monocyte-derived dendritic cell development and activity in vitro and in Hu-PBL-SCID mice. J. Exp. Med. 191, 1777–1788 (2000). 13. Hahm, B., Trifilo, M. J., Zuniga, E. I. & Oldstone, M. B. Viruses evade the immune system through type I interferon-mediated STAT2‑dependent, but STAT1‑independent, signaling. Immunity 22, 247–257 (2005). 14. Parlato, S. et al. Expression of CCR‑7, MIP‑3β, and TH-1 chemokines in type I IFN-induced monocytederived dendritic cells: importance for the rapid acquisition of potent migratory and functional activities. Blood 98, 3022–3029 (2001). 15. Rouzaut, A. et al. Dendritic cells adhere to and transmigrate across lymphatic endothelium in response to IFN-α. Eur. J. Immunol. 40, 3054–3063 (2010). 16. Padovan, E., Spagnoli, G. C., Ferrantini, M. & Heberer, M. IFN‑α2a induces IP‑10/CXCL10 and MIG/CXCL9 production in monocyte-derived dendritic cells and enhances their capacity to attract and stimulate CD8+ effector T cells. J. Leukoc. Biol. 71, 669–676 (2002). 17. Radvanyi, L. G., Banerjee, A., Weir, M. & Messner, H. Low levels of interferon-α induce CD86 (B7.2) expression and accelerates dendritic cell maturation from human peripheral blood mononuclear cells. Scand. J. Immunol. 50, 499–509 (1999). 18. Luft, T. et al. Type I IFNs enhance the terminal differentiation of dendritic cells. J. Immunol. 161, 1947–1953 (1998). 19. Blanco, P., Palucka, A. K., Gill, M., Pascual, V. & Banchereau, J. Induction of dendritic cell differentiation by IFN-α in systemic lupus erythematosus. Science 294, 1540–1543 (2001). 20. Gallucci, S., Lolkema, M. & Matzinger, P. Natural adjuvants: endogenous activators of dendritic cells. Nature Med. 5, 1249–1255 (1999). 21. Montoya, M. et al. Type I interferons produced by dendritic cells promote their phenotypic and functional activation. Blood 99, 3263–3271 (2002). 22. Longhi, M. P. et al. Dendritic cells require a systemic type I interferon response to mature and induce CD4+ TH1 immunity with poly IC as adjuvant. J. Exp. Med. 206, 1589–1602 (2009). 1.

23. Simmons, D. P. et al. Type I IFN drives a distinctive dendritic cell maturation phenotype that allows continued class II MHC synthesis and antigen processing. J. Immunol. 188, 3116–3126 (2012). 24. Joffre, O. P., Segura, E., Savina, A. & Amigorena, S. Cross-presentation by dendritic cells. Nature Rev. Immunol. 12, 557–569 (2012). 25. Nierkens, S., Tel, J., Janssen, E. & Adema, G. J. Antigen cross-presentation by dendritic cell subsets: one general or all sergeants? Trends Immunol. 34, 361–370 (2013). 26. Le Bon, A. et al. Cross-priming of CD8+ T cells stimulated by virus-induced type I interferon. Nature Immunol. 4, 1009–1015 (2003). 27. Spadaro, F. et al. IFN-α enhances cross-presentation in human dendritic cells by modulating antigen survival, endocytic routing, and processing. Blood 119, 1407–1417 (2012). 28. Savina, A. et al. The small GTPase Rac2 controls phagosomal alkalinization and antigen crosspresentation selectively in CD8+ dendritic cells. Immunity 30, 544–555 (2009). 29. Thomas, C. et al. Structural linkage between ligand discrimination and receptor activation by type I interferons. Cell 146, 621–632 (2011). 30. Garcin, G. et al. Differential activity of type I interferon subtypes for dendritic cell differentiation. PLoS ONE 8, e58465 (2013). 31. Antonelli, L. R. et al. Intranasal Poly-IC treatment exacerbates tuberculosis in mice through the pulmonary recruitment of a pathogen-permissive monocyte/macrophage population. J. Clin. Invest. 120, 1674–1682 (2010). 32. McNab, F. W. et al. TPL‑2–ERK1/2 signaling promotes host resistance against intracellular bacterial infection by negative regulation of type I IFN production. J. Immunol. 191, 1732–1743 (2013). 33. Teijaro, J. R. et al. Persistent LCMV infection is controlled by blockade of type I interferon signaling. Science 340, 207–211 (2013). 34. Wilson, E. B. et al. Blockade of chronic type I interferon signaling to control persistent LCMV infection. Science 340, 202–207 (2013). Chronic viral infections are often associated with sustained type I IFN activity; references 33 and 34 show that interfering with this activity during established chronic infection can improve antiviral T cell immunity, leading to enhanced control of the infection. 35. Wilson, E. B. et al. Emergence of distinct multiarmed immunoregulatory antigen-presenting cells during persistent viral infection. Cell Host Microbe 11, 481–491 (2012). 36. Mattei, F., Schiavoni, G., Belardelli, F. & Tough, D. F. IL‑15 is expressed by dendritic cells in response to type I IFN, double-stranded RNA, or lipopolysaccharide and promotes dendritic cell activation. J. Immunol. 167, 1179–1187 (2001). 37. Zhang, X., Sun, S., Hwang, I., Tough, D. F. & Sprent, J. Potent and selective stimulation of memoryphenotype CD8+ T cells in vivo by IL‑15. Immunity 8, 591–599 (1998). 38. Nguyen, K. B. et al. Coordinated and distinct roles for IFN-α β, IL‑12, and IL‑15 regulation of NK cell responses to viral infection. J. Immunol. 169, 4279–4287 (2002). 39. Lucas, M., Schachterle, W., Oberle, K., Aichele, P. & Diefenbach, A. Dendritic cells prime natural killer cells by trans-presenting interleukin 15. Immunity 26, 503–517 (2007). 40. Dubois, S., Mariner, J., Waldmann, T. A. & Tagaya, Y. IL‑15Rα recycles and presents IL‑15 in trans to neighboring cells. Immunity 17, 537–547 (2002). 41. Nguyen, K. B. et al. Critical role for STAT4 activation by type 1 interferons in the interferon-γ response to viral infection. Science 297, 2063–2066 (2002). 42. Swain, S. L., McKinstry, K. K. & Strutt, T. M. Expanding roles for CD4+ T cells in immunity to viruses. Nature Rev. Immunol. 12, 136–148 (2012). 43. Surh, C. D. & Sprent, J. Homeostasis of naive and memory T cells. Immunity 29, 848–862 (2008). 44. Cousens, L. P., Orange, J. S., Su, H. C. & Biron, C. A. Interferon-α/β inhibition of interleukin 12 and interferon-γ production in vitro and endogenously during viral infection. Proc. Natl Acad. Sci. USA 94, 634–639 (1997). 45. Byrnes, A. A. et al. Type I interferons and IL‑12: convergence and cross-regulation among mediators of cellular immunity. Eur. J. Immunol. 31, 2026–2034 (2001).

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46. Biron, C. A., Sonnenfeld, G. & Welsh, R. M. Interferon induces natural killer cell blastogenesis in vivo. J. Leukoc. Biol. 35, 31–37 (1984). 47. Gidlund, M., Orn, A., Wigzell, H., Senik, A. & Gresser, I. Enhanced NK cell activity in mice injected with interferon and interferon inducers. Nature 273, 759–761 (1978). 48. Lee, C. K. et al. Distinct requirements for IFNs and STAT1 in NK cell function. J. Immunol. 165, 3571–3577 (2000). 49. Jost, S. & Altfeld, M. Control of human viral infections by natural killer cells. Annu. Rev. Immunol. 31, 163–194 (2013). 50. Scott, P. IFN-γ modulates the early development of TH1 and TH2 responses in a murine model of cutaneous leishmaniasis. J. Immunol. 147, 3149–3155 (1991). 51. Lighvani, A. A. et al. T-bet is rapidly induced by interferon-γ in lymphoid and myeloid cells. Proc. Natl Acad. Sci. USA 98, 15137–15142 (2001). 52. Crome, S. Q., Lang, P. A., Lang, K. S. & Ohashi, P. S. Natural killer cells regulate diverse T cell responses. Trends Immunol. 34, 342–349 (2013). 53. Crouse, J., Xu, H. C., Lang, P. A. & Oxenius, A. NK cells regulating T cell responses: mechanisms and outcome. Trends Immunol. 36, 49–58 (2014). 54. Lu, L. et al. Regulation of activated CD4+ T cells by NK cells via the Qa‑1–NKG2A inhibitory pathway. Immunity 26, 593–604 (2007). 55. Waggoner, S. N., Taniguchi, R. T., Mathew, P. A., Kumar, V. & Welsh, R. M. Absence of mouse 2B4 promotes NK cell-mediated killing of activated CD8+ T cells, leading to prolonged viral persistence and altered pathogenesis. J. Clin. Invest. 120, 1925–1938 (2010). 56. Xu, H. C. et al. Type I interferon protects antiviral CD8+ T cells from NK cell cytotoxicity. Immunity 40, 949–960 (2014). 57. Rabinovich, B. A. et al. Activated, but not resting, T cells can be recognized and killed by syngeneic NK cells. J. Immunol. 170, 3572–3576 (2003). 58. Cerboni, C. et al. Antigen-activated human T lymphocytes express cell-surface NKG2D ligands via an ATM/ATR-dependent mechanism and become susceptible to autologous NK- cell lysis. Blood 110, 606–615 (2007). 59. Soderquest, K. et al. Cutting edge: CD8+ T cell priming in the absence of NK cells leads to enhanced memory responses. J. Immunol. 186, 3304–3308 (2011). 60. Crouse, J. et al. Type I interferons protect T cells against NK cell attack mediated by the activating receptor NCR1. Immunity 40, 961–973 (2014). References 56 and 60 show that direct type I IFN signalling in activated virus-specific T cells protects these cells against NK cell-mediated cytotoxicity. 61. Peppa, D. et al. Up-regulation of a death receptor renders antiviral T cells susceptible to NK cellmediated deletion. J. Exp. Med. 210, 99–114 (2013). This study shows that intrahepatic human NK cells induce apoptosis of hepatitis B virus-specific CD8+ T cells in a TRAIL-dependent manner. 62. Sadler, A. J. & Williams, B. R. Interferon-inducible antiviral effectors. Nature Rev. Immunol. 8, 559–568 (2008). 63. Honke, N. et al. Enforced viral replication activates adaptive immunity and is essential for the control of a cytopathic virus. Nature Immunol. 13, 51–57 (2012). This study shows that expression of USP18 — which inhibits type I IFN signalling — in metallophilic macrophages permits viral replication in a cell-specific manner, thereby enabling locally restricted viral replication that provides sufficient amounts of antigen for the priming of adaptive immune responses. 64. Wherry, E. J., Blattman, J. N., Murali-Krishna, K., van der Most, R. & Ahmed, R. Viral persistence alters CD8 T-cell immunodominance and tissue distribution and results in distinct stages of functional impairment. J. Virol. 77, 4911–4927 (2003). 65. Richter, K., Brocker, T. & Oxenius, A. Antigen amount dictates CD8+ T-cell exhaustion during chronic viral infection irrespective of the type of antigen presenting cell. Eur. J. Immunol. 42, 2290–2304 (2012). 66. Mueller, S. N. & Ahmed, R. High antigen levels are the cause of T cell exhaustion during chronic viral infection. Proc. Natl Acad. Sci. USA 106, 8623–8628 (2009). 67. Streeck, H. et al. Antigen load and viral sequence diversification determine the functional profile of HIV‑1‑specific CD8+ T cells. PLoS Med. 5, e100 (2008).

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REVIEWS 68. van Boxel-Dezaire, A. H., Rani, M. R. & Stark, G. R. Complex modulation of cell type-specific signaling in response to type I interferons. Immunity 25, 361–372 (2006). 69. Brierley, M. M. & Fish, E. N. Stats: multifaceted regulators of transcription. J. Interferon Cytokine Res. 25, 733–744 (2005). 70. Platanias, L. C. Mechanisms of type-I- and type-IIinterferon-mediated signalling. Nature Rev. Immunol. 5, 375–386 (2005). 71. Tanabe, Y. et al. Cutting edge: role of STAT1, STAT3, and STAT5 in IFN-α β responses in T lymphocytes. J. Immunol. 174, 609–613 (2005). 72. Gimeno, R., Lee, C. K., Schindler, C. & Levy, D. E. Stat1 and Stat2 but not Stat3 arbitrate contradictory growth signals elicited by α/β interferon in T lymphocytes. Mol. Cell. Biol. 25, 5456–5465 (2005). 73. Van De Wiele, C. J. et al. Loss of interferon-induced Stat1 phosphorylation in activated T cells. J. Interferon Cytokine Res. 24, 169–178 (2004). 74. Curtsinger, J. M., Valenzuela, J. O., Agarwal, P., Lins, D. & Mescher, M. F. Type I IFNs provide a third signal to CD8 T cells to stimulate clonal expansion and differentiation. J. Immunol. 174, 4465–4469 (2005). This study shows that type I IFNs act as signal 3 cytokines during CD8+ T cell activation, and promote survival and effector cell differentiation in a STAT4‑dependent manner. 75. Garcia-Sastre, A. & Biron, C. A. Type 1 interferons and the virus–host relationship: a lesson in detente. Science 312, 879–882 (2006). 76. Gil, M. P., Salomon, R., Louten, J. & Biron, C. A. Modulation of STAT1 protein levels: a mechanism shaping CD8 T-cell responses in vivo. Blood 107, 987–993 (2006). This study shows that type I IFNs mediate their anti-proliferative effect in T cells through STAT1, and that activated T cells escape this inhibition by lowering the expression of STAT1 relative to other STAT molecules. 77. Curtsinger, J. M., Lins, D. C. & Mescher, M. F. Signal 3 determines tolerance versus full activation of naive CD8 T cells: dissociating proliferation and development of effector function. J. Exp. Med. 197, 1141–1151 (2003). 78. Curtsinger, J. M., Johnson, C. M. & Mescher, M. F. CD8 T cell clonal expansion and development of effector function require prolonged exposure to antigen, costimulation, and signal 3 cytokine. J. Immunol. 171, 5165–5171 (2003). 79. Hervas-Stubbs, S. et al. Effects of IFN-α as a signal‑3 cytokine on human naive and antigen-experienced CD8+ T cells. Eur. J. Immunol. 40, 3389–3402 (2010). 80. Cha, L., de Jong, E., French, M. A. & Fernandez, S. IFN‑α exerts opposing effects on activation-induced & IL‑7‑induced proliferation of T cells that may impair homeostatic maintenance of CD4+ T cell numbers in reated HIV infection. J. Immunol. 193, 2178–2186 (2014). 81. Matikainen, S. et al. Interferon-α activates multiple STAT proteins and upregulates proliferationassociated IL‑2Rα, c-myc, and pim‑1 genes in human T cells. Blood 93, 1980–1991 (1999). 82. Pilling, D. et al. Interferon-β mediates stromal cell rescue of T cells from apoptosis. Eur. J. Immunol. 29, 1041–1050 (1999). 83. Marrack, P., Kappler, J. & Mitchell, T. Type I interferons keep activated T cells alive. J. Exp. Med. 189, 521–530 (1999). 84. Kolumam, G. A., Thomas, S., Thompson, L. J., Sprent, J. & Murali-Krishna, K. Type I interferons act directly on CD8 T cells to allow clonal expansion and memory formation in response to viral infection. J. Exp. Med. 202, 637–650 (2005). This study showed for the first time that the absence of type I IFN signalling on CD8+ T cells results in massively curtailed antiviral T cell immunity. 85. Aichele, P. et al. CD8 T cells specific for lymphocytic choriomeningitis virus require type I IFN receptor for clonal expansion. J. Immunol. 176, 4525–4529 (2006). 86. Thompson, L. J., Kolumam, G. A., Thomas, S. & Murali-Krishna, K. Innate inflammatory signals induced by various pathogens differentially dictate the IFN-I dependence of CD8 T cells for clonal expansion and memory formation. J. Immunol. 177, 1746–1754 (2006).

87. Havenar-Daughton, C., Kolumam, G. A. & Murali-Krishna, K. Cutting edge: the direct action of type I IFN on CD4 T cells is critical for sustaining clonal expansion in response to a viral but not a bacterial infection. J. Immunol. 176, 3315–3319 (2006). 88. Keppler, S. J., Rosenits, K., Koegl, T., Vucikuja, S. & Aichele, P. Signal 3 cytokines as modulators of primary immune responses during infections: the interplay of type I IFN and IL‑12 in CD8 T cell responses. PLoS ONE 7, e40865 (2012). 89. Wiesel, M. et al. Type-I IFN drives the differentiation of short-lived effector CD8+ T cells in vivo. Eur. J. Immunol. 42, 320–329 (2012). This study shows that direct type I IFN signalling in CD8+ T cells instructs the differentiation of short-lived effector T cells in vivo. 90. Frenz, T. et al. Concomitant type I IFN receptortriggering of T cells and of DC is required to promote maximal modified vaccinia virus Ankara-induced T-cell expansion. Eur. J. Immunol. 40, 2769–2777 (2010). 91. Keppler, S. J. & Aichele, P. Signal 3 requirement for memory CD8+ T-cell activation is determined by the infectious pathogen. Eur. J. Immunol. 41, 3176–3186 (2011). 92. Le Bon, A. et al. Direct stimulation of T cells by type I IFN enhances the CD8+ T cell response during crosspriming. J. Immunol. 176, 4682–4689 (2006). 93. Van Uden, J. H., Tran, C. H., Carson, D. A. & Raz, E. Type I interferon is required to mount an adaptive response to immunostimulatory DNA. Eur. J. Immunol. 31, 3281–3290 (2001). 94. Pham, N. L., Badovinac, V. P. & Harty, J. T. Differential role of ‘Signal 3’ inflammatory cytokines in regulating CD8 T cell expansion and differentiation in vivo. Front. Immunol. 2, 4 (2011). 95. Starbeck-Miller, G. R., Xue, H. H. & Harty, J. T. IL‑12 and type I interferon prolong the division of activated CD8 T cells by maintaining high-affinity IL‑2 signaling in vivo. J. Exp. Med. 211, 105–120 (2014). 96. Xiao, Z., Casey, K. A., Jameson, S. C., Curtsinger, J. M. & Mescher, M. F. Programming for CD8 T cell memory development requires IL‑12 or type I IFN. J. Immunol. 182, 2786–2794 (2009). 97. Nguyen, K. B. et al. Interferon-α/β-mediated inhibition and promotion of interferon-γ: STAT1 resolves a paradox. Nature Immunol. 1, 70–76 (2000). 98. Gil, M. P. et al. Regulating type 1 IFN effects in CD8 T cells during viral infections: changing STAT4 and STAT1 expression for function. Blood 120, 3718–3728 (2012). 99. Agarwal, P. et al. Gene regulation and chromatin remodeling by IL‑12 and type I IFN in programming for CD8 T cell effector function and memory. J. Immunol. 183, 1695–1704 (2009). 100. Keppler, S. J., Theil, K., Vucikuja, S. & Aichele, P. Effector T-cell differentiation during viral and bacterial infections: role of direct IL‑12 signals for cell fate decision of CD8+ T cells. Eur. J. Immunol. 39, 1774–1783 (2009). 101. Wiesel, M., Kratky, W. & Oxenius, A. Type I IFN substitutes for T cell help during viral infections. J. Immunol. 186, 754–763 (2011). 102. Kalia, V. et al. Prolonged interleukin‑2Rα expression on virus-specific CD8+ T cells favors terminal-effector differentiation in vivo. Immunity 32, 91–103 (2010). 103. Pipkin, M. E. et al. Interleukin‑2 and inflammation induce distinct transcriptional programs that promote the differentiation of effector cytolytic T cells. Immunity 32, 79–90 (2010). 104. Gerner, M. Y., Heltemes-Harris, L. M., Fife, B. T. & Mescher, M. F. Cutting edge: IL‑12 and type I IFN differentially program CD8 T cells for programmed death 1 re-expression levels and tumor control. J. Immunol. 191, 1011–1015 (2013). 105. Yamane, H. & Paul, W. E. Cytokines of the γc family control CD4+ T cell differentiation and function. Nature Immunol. 13, 1037–1044 (2012). 106. Brinkmann, V., Geiger, T., Alkan, S. & Heusser, C. H. Interferon-α increases the frequency of interferon γ-producing human CD4+ T cells. J. Exp. Med. 178, 1655–1663 (1993). 107. Athie-Morales, V., Smits, H. H., Cantrell, D. A. & Hilkens, C. M. Sustained IL‑12 signaling is required for TH1 development. J. Immunol. 172, 61–69 (2004). 108. Ramos, H. J., Davis, A. M., George, T. C. & Farrar, J. D. IFN-α is not sufficient to drive TH1 development due to lack of stable T-bet expression. J. Immunol. 179, 3792–3803 (2007).

NATURE REVIEWS | IMMUNOLOGY

109. Persky, M. E., Murphy, K. M. & Farrar, J. D. IL‑12, but not IFN-α, promotes STAT4 activation and TH1 development in murine CD4+ T cells expressing a chimeric murine/human Stat2 gene. J. Immunol. 174, 294–301 (2005). 110. Rogge, L. et al. The role of Stat4 in species-specific regulation of TH cell development by type I IFNs. J. Immunol. 161, 6567–6574 (1998). 111. Nurieva, R. I. et al. Generation of T follicular helper cells is mediated by interleukin‑21 but independent of T helper 1, 2, or 17 cell lineages. Immunity 29, 138–149 (2008). 112. Ma, C. S. et al. Functional STAT3 deficiency compromises the generation of human T follicular helper cells. Blood 119, 3997–4008 (2012). 113. Ray, J. P. et al. Transcription factor STAT3 and type I interferons are corepressive insulators for differentiation of follicular helper and T helper 1 cells. Immunity 40, 367–377 (2014). 114. Oestreich, K. J., Mohn, S. E. & Weinmann, A. S. Molecular mechanisms that control the expression and activity of Bcl‑6 in TH1 cells to regulate flexibility with a TFH-like gene profile. Nature Immunol. 13, 405–411 (2012). 115. Crotty, S. Follicular helper CD4 T cells (TFH). Annu. Rev. Immunol. 29, 621–663 (2011). 116. Nakayamada, S. et al. Type I IFN induces binding of STAT1 to Bcl6: divergent roles of STAT family transcription factors in the T follicular helper cell genetic program. J. Immunol. 192, 2156–2166 (2014). 117. Srivastava, S., Koch, M. A., Pepper, M. & Campbell, D. J. Type I interferons directly inhibit regulatory T cells to allow optimal antiviral T cell responses during acute LCMV infection. J. Exp. Med. 211, 961–974 (2014). 118. Hansson, M., Kiessling, R., Andersson, B. & Welsh, R. M. Effect of interferon and interferon inducers on the NK sensitivity of normal mouse thymocytes. J. Immunol. 125, 2225–2231 (1980). 119. Kamphuis, E., Junt, T., Waibler, Z., Forster, R. & Kalinke, U. Type I interferons directly regulate lymphocyte recirculation and cause transient blood lymphopenia. Blood 108, 3253–3261 (2006). 120. Shiow, L. R. et al. CD69 acts downstream of interferon-α/β to inhibit S1P1 and lymphocyte egress from lymphoid organs. Nature 440, 540–544 (2006). 121. Schulz, O. et al. Hypertrophy of infected Peyer’s patches arises from global, interferon-receptor, and CD69‑independent shutdown of lymphocyte egress. Mucosal Immunol. 7, 892–904 (2014). 122. Avraamides, G. et al. IFN‑α2 induces leukocyte integrin redistribution, increased adhesion, and migration. J. Interferon Cytokine Res. 27, 291–303 (2007). 123. Dondi, E., Rogge, L., Lutfalla, G., Uze, G. & Pellegrini, S. Down-modulation of responses to type I IFN upon T cell activation. J. Immunol. 170, 749–756 (2003). 124. Sun, S., Zhang, X., Tough, D. F. & Sprent, J. Type I interferon-mediated stimulation of T cells by CpG DNA. J. Exp. Med. 188, 2335–2342 (1998). 125. Bromberg, J. F., Horvath, C. M., Wen, Z., Schreiber, R. D. & Darnell, J. E. Jr. Transcriptionally active Stat1 is required for the antiproliferative effects of both interferon α and interferon γ. Proc. Natl Acad. Sci. USA 93, 7673–7678 (1996). 126. Petricoin, E. F. et al. Antiproliferative action of interferon-α requires components of T-cell-receptor signalling. Nature 390, 629–632 (1997). 127. Terawaki, S. et al. IFN-α directly promotes programmed cell death‑1 transcription and limits the duration of T cell-mediated immunity. J. Immunol. 186, 2772–2779 (2011). 128. Herbeuval, J. P. et al. Regulation of TNF-related apoptosis-inducing ligand on primary CD4+ T cells by HIV‑1: role of type I IFN-producing plasmacytoid dendritic cells. Proc. Natl Acad. Sci. USA 102, 13974–13979 (2005). 129. Kaser, A., Nagata, S. & Tilg, H. Interferon-α augments activation-induced T cell death by upregulation of Fas (CD95/APO‑1) and Fas ligand expression. Cytokine 11, 736–743 (1999). 130. Fraietta, J. A. et al. Type I interferon upregulates Bak and contributes to T cell loss during human immunodeficiency virus (HIV) infection. PLoS Pathog. 9, e1003658 (2013). 131. Zella, D. et al. IFN-α 2b reduces IL‑2 production and IL‑2 receptor function in primary CD4+ T cells. J. Immunol. 164, 2296–2302 (2000).

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REVIEWS 132. Bahl, K., Huebner, A., Davis, R. J. & Welsh, R. M. Analysis of apoptosis of memory T cells and dendritic cells during the early stages of viral infection or exposure to toll-like receptor agonists. J. Virol. 84, 4866–4877 (2010). 133. McNally, J. M. et al. Attrition of bystander CD8 T cells during virus-induced T-cell and interferon responses. J. Virol. 75, 5965–5976 (2001). 134. Bahl, K. et al. IFN-induced attrition of CD8 T cells in the presence or absence of cognate antigen during the early stages of viral infections. J. Immunol. 176, 4284–4295 (2006). 135. Jiang, J., Lau, L. L. & Shen, H. Selective depletion of nonspecific T cells during the early stage of immune responses to infection. J. Immunol. 171, 4352–4358 (2003). 136. Marshall, H. D., Prince, A. L., Berg, L. J. & Welsh, R. M. IFN-α β and self-MHC divert CD8 T cells into a distinct differentiation pathway characterized by rapid acquisition of effector functions. J. Immunol. 185, 1419–1428 (2010). 137. Marshall, H. D., Urban, S. L. & Welsh, R. M. Virus-induced transient immune suppression and the inhibition of T cell proliferation by type I interferon. J. Virol. 85, 5929–5939 (2011). 138. Urban, S. L. & Welsh, R. M. Out-of-sequence signal 3 as a mechanism for virus-induced immune suppression of CD8 T cell responses. PLoS Pathog. 10, e1004357 (2014). This study shows that the timing of type I IFN signalling in relation to TCR stimulation is crucial for its effect on T cell activation, with type I IFN

pre-treatment of T cells resulting in desensitization for subsequent type I IFN signalling events during TCR stimulation. 139. Piganis, R. A. et al. Suppressor of cytokine signaling (SOCS) 1 inhibits type I interferon (IFN) signaling via the interferon α receptor (IFNAR1)-associated tyrosine kinase Tyk2. J. Biol. Chem. 286, 33811–33818 (2011). 140. Manches, O. & Bhardwaj, N. Resolution of immune activation defines nonpathogenic SIV infection. J. Clin. Invest. 119, 3512–3515 (2009). 141. Hosmalin, A. & Lebon, P. Type I interferon production in HIV-infected patients. J. Leukoc. Biol. 80, 984–993 (2006). 142. Sandler, N. G. et al. Type I interferon responses in rhesus macaques prevent SIV infection and slow disease progression. Nature 511, 601–605 (2014). This study shows that the timing and duration of type I IFN activity during SIV infection governs its physiological outcome; although type I IFNs contribute to restricting viral replication during acute infection, and thereby restrict disease progression, sustained type I IFN signalling during chronic infection leads to desensitization and accelerated disease progression. 143. Baccala, R. et al. Type I interferon is a therapeutic target for virus-induced lethal vascular damage. Proc. Natl Acad. Sci. USA 111, 8925–8930 (2014). 144. Wang, Y. et al. Timing and magnitude of type I interferon responses by distinct sensors impact CD8 T cell exhaustion and chronic viral infection. Cell Host Microbe 11, 631–642 (2012).

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145. Lee, M. S., Park, C. H., Jeong, Y. H., Kim, Y. J. & Ha, S. J. Negative regulation of type I IFN expression by OASL1 permits chronic viral infection and CD8+ T-cell exhaustion. PLoS Pathog. 9, e1003478 (2013). 146. Vezys, V. et al. Continuous recruitment of naive T cells contributes to heterogeneity of antiviral CD8 T cells during persistent infection. J. Exp. Med. 203, 2263–2269 (2006). 147. Osokine, I. et al. Type I interferon suppresses de novo virus-specific CD4 TH1 immunity during an established persistent viral infection. Proc. Natl Acad. Sci. USA 111, 7409–7414 (2014). 148. Stelekati, E. et al. Bystander chronic infection negatively impacts development of CD8+ T cell memory. Immunity 40, 801–813 (2014).

Acknowledgements

The authors are grateful to the members of the Oxenius group for helpful discussions and critical reading of the manuscript. This work was supported by the Swiss Federal Institute of Technology (ETH) and the Swiss National Science Foundation (grant numbers 310030-113947 and 310030_146140 to A.O.) by the Niedersachsen-Research Network on Neuroinfectiology (N-RENNT) of the Ministry of Science and Culture of Lower Saxony, Germany (to U.K.), and by the SFB 854 (TP B15 to U.K.) of the German Research Council.

Competing interests statement

The authors declare no competing interests.

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