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Type I Interferon Receptors: Biochemistry and Biological Functions*

  • Nicole A. de Weerd
    Affiliations
    Centre for Functional Genomics and Human Disease, Monash Institute of Medical Research, Monash University, Clayton, Victoria 3168, Australia
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  • Shamith A. Samarajiwa
    Affiliations
    Centre for Functional Genomics and Human Disease, Monash Institute of Medical Research, Monash University, Clayton, Victoria 3168, Australia
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  • Paul J. Hertzog
    Correspondence
    To whom correspondence should be addressed. Tel.: 61-3-9594-7206
    Affiliations
    Centre for Functional Genomics and Human Disease, Monash Institute of Medical Research, Monash University, Clayton, Victoria 3168, Australia
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  • Author Footnotes
    * This minireview will be reprinted in the 2007 Minireview Compendium, which will be available in January, 2008. This work was supported by the NHMRC, Australia, and the Cooperative Research Centre for Chronic Inflammatory Diseases. This is the second article of four in the Interferon Minireview Series.
Open AccessPublished:May 14, 2007DOI:https://doi.org/10.1074/jbc.R700006200
      The type I interferon (IFN)
      The abbreviations used are: IFN, interferon; IFNAR, type I IFN receptor; IFNGR, type II IFNγ receptor; hu, human; hCR, helical cytokine receptor; ECD, extracellular domain; FBN, fibronectin; TF, tissue factor; SD, subdomain; PDB, Protein Data Bank; IL, interleukin; aa, amino acid(s); s (sIL, sIFN), soluble; JAK, Janus kinase; STAT, signal transducers and activators of transcription.
      2The abbreviations used are: IFN, interferon; IFNAR, type I IFN receptor; IFNGR, type II IFNγ receptor; hu, human; hCR, helical cytokine receptor; ECD, extracellular domain; FBN, fibronectin; TF, tissue factor; SD, subdomain; PDB, Protein Data Bank; IL, interleukin; aa, amino acid(s); s (sIL, sIFN), soluble; JAK, Janus kinase; STAT, signal transducers and activators of transcription.
      receptor (IFNAR) is comprised, as other cytokine receptors, of multiple components, in this case designated IFNAR1 and IFNAR2. However it is unique among cytokine receptors in the number of cognate ligands, including 13 IFNα subtypes, β, ω, ∊, κ, and others in some species. The type I IFN receptors are distinct from those required for the type II IFNγ (IFNGR1 and IFNGR2) and type III IFNs (IFNLR and IL10Rβ). Nevertheless, genes encoding a component of each type of IFN receptor, namely IFNAR1, IFNAR2, IFNGR2, and IL10Rβ, are located on human chromosome 21q22.1 in a cytokine receptor gene cluster, as typical of functionally related genes.
      Although IFNs were identified 50 years ago and the existence of IFN receptors 10 years later, it was in 1990 when the first type I IFN receptor, now designated IFNAR1, was cloned. This was achieved utilizing human gene libraries expressed in murine cells and rescue of the definitive, species specific antiviral activity of human IFNα8 (
      • Uze G.
      • Lutfalla G.
      • Gresser I.
      ). IFNAR2 cloning was achieved first by identifying a human IFN binding activity in urine, peptide sequencing, and then by gene library screening with derived oligonucleotides (
      • Novick D.
      • Cohen B.
      • Rubinstein M.
      ). It was subsequently discovered that the original cDNA encoded only one isoform of the IFNAR2 gene, which also encoded a long transmembrane isoform that transduced a signal, a truncated transmembrane isoform, and a soluble/secreted isoform (
      • Lutfalla G.
      • Holland S.J.
      • Cinato E.
      • Monneron D.
      • Reboul J.
      • Rogers N.C.
      • Smith J.M.
      • Stark G.R.
      • Gardiner K.
      • Mogensen K.E.
      ) (Fig. 1A).
      Figure thumbnail gr1
      FIGURE 1A, differential splicing of human and murine Ifnar2. i, murine Ifnar2 generates one transcript encoding a long Ifnar2c; two soluble specific transcripts are generated by either exon skipping (sIfnar2a′) or reading through into intron 8 (exon 7′)(sIfnar2a). ii, human IFNAR2 is alternatively spliced to generate transcripts encoding a long isoform (IFNAR2c) similar to the mouse, a short isoform (IFNAR2b), and soluble (sIFNAR2a) isoforms by exon skipping. Soluble IFNAR2a is generated by splicing at exon 7 into splice acceptor site (sa1) within exon 9 and uses poly(A) site 1 (*1). The long IFNAR2c uses exons 7 and 8 and sa2 (*2). Short IFNAR2b uses exons 7 and 8 and either poly(A) site *1 or *2. B, receptor complexes and IFN signaling. Step 1, conventional signaling occurs when IFN binds to IFNAR1 and tmIFNAR2c resulting in cross-phosphorylation of receptors and associated Janus kinases (Tyk2 and Jak1). This provides docking sites on the receptor complex for STAT proteins. STAT proteins are in turn phosphorylated and form homo- and heterodimeric complexes, which dissociate from the receptor and then translocate to the nucleus and bind to an ISRE or GAS element within the promoters of interferon-regulated genes, leading to their transcription. Step 2, IFNAR2b acts as a dominant negative modulator of IFN signaling by binding ligand but not transducing antiviral signals. Step 3, transsignaling by ligand-bound soluble IFNAR2a interacting with IFNAR1 can generate a biological response.
      Subsequently, the functions of the type I IFN receptors have been elucidated with respect to ligand interaction, mechanisms of signal transduction, and biological responses. The pioneering studies that discovered IFNARs and their mechanisms of actions in vitro have been largely validated in vivo using genetargeted mice. This body of work has highlighted the important roles of IFNARs in mediating type I IFN responses in hemopoiesis and innate and acquired immunity to infection and cancer. However, IFNs elicit many biological effects that can even be opposite in different cell types. For example, type I IFN inhibits proliferation and is proapoptotic for many cell types (
      • Platanias L.C.
      ), yet it prolongs the survival of memory T cells (
      • Tough D.F.
      • Sun S.
      • Zhang X.
      • Sprent J.
      ). Understanding the function of the IFNAR complex will elucidate how such a diversity of biological outcomes is generated.

      Genes and Gene Expression

      The IFNAR genes encode multiple isoforms that contribute to the potential complexity of the functional receptor (
      • Novick D.
      • Cohen B.
      • Rubinstein M.
      ,
      • Lutfalla G.
      • Holland S.J.
      • Cinato E.
      • Monneron D.
      • Reboul J.
      • Rogers N.C.
      • Smith J.M.
      • Stark G.R.
      • Gardiner K.
      • Mogensen K.E.
      ,
      • Owczarek C.M.
      • Hwang S.Y.
      • Holland K.A.
      • Gulluyan L.M.
      • Tavaria M.
      • Weaver B.
      • Reich N.C.
      • Kola I.
      • Hertzog P.J.
      ). Two splice variants of IFNAR1 have been identified in cell lines (
      • Abramovich C.
      • Ratovitski E.
      • Lundgren E.
      • Revel M.
      ,
      • Cook J.R.
      • Cleary C.M.
      • Mariano T.M.
      • Izotova L.
      • Pestka S.
      ). However, subsequent bioinformatic analyses of splice variants in expressed sequence tag (EST) data bases and rapid amplification of cDNA ends (
      • Lutfalla G.
      • Holland S.J.
      • Cinato E.
      • Monneron D.
      • Reboul J.
      • Rogers N.C.
      • Smith J.M.
      • Stark G.R.
      • Gardiner K.
      • Mogensen K.E.
      ′-RACE) analyses from normal cells identified only one isoform, suggesting that the former are either artifacts or aberrant transcripts found only in particular tumor cell lines.
      S. A. Samarajiwa and P. J. Hertzog, unpublished data.
      In contrast, four IFNAR2 transcripts encoding three isoforms are generated from the same gene by exon skipping, alternative splicing, and differential usage of polyadenylation sites (
      • Lutfalla G.
      • Holland S.J.
      • Cinato E.
      • Monneron D.
      • Reboul J.
      • Rogers N.C.
      • Smith J.M.
      • Stark G.R.
      • Gardiner K.
      • Mogensen K.E.
      ) (Fig. 1A) These transcripts encode a long trans-membrane IFNAR2c, a short transmembrane IFNAR2b chain, and a soluble sIFNAR2a chain. Transfection of human IFNAR1 and IFNAR2c, but not IFNAR2b, reconstituted the antiviral IFN response (
      • Cohen B.
      • Novick D.
      • Barak S.
      • Rubinstein M.
      ). This is consistent with data, at least in sarcomas, that IFNAR2b may act as a dominant negative regulator of IFN responses (
      • Gazziola C.
      • Cordani N.
      • Carta S.
      • De Lorenzo E.
      • Colombatti A.
      • Perris R.
      ) (Fig. 1B).
      The mouse has been the primary model for pathophysiological studies of IFNs due to the capability of generating knockout mice, which can demonstrate cause-and-effect associations in vivo. The mouse has a comparable type I IFN system to human with multiple ligands (α, β, ∊, etc.) and Ifnar1 and Ifnar2 genes (
      • Hardy M.P.
      • Owczarek C.M.
      • Trajanovska S.
      • Liu X.
      • Kola I.
      • Hertzog P.J.
      ,
      • Hardy M.P.
      • Sanij E.P.
      • Hertzog P.J.
      • Owczarek C.M.
      ). No IFNAR2b chain has been identified in mouse, but two transcripts capable of encoding soluble isoforms (sIfnar2a and sIfnar2a′) are generated by differential splicing. The more abundant 1.5-kb sIfnar2a transcript encodes the complete IFNAR2 extracellular domain and reads through the splice site on the exon 7–7′ boundary producing a transcript encoding 12 unique and mostly hydrophobic C-terminal residues (
      • Owczarek C.M.
      • Hwang S.Y.
      • Holland K.A.
      • Gulluyan L.M.
      • Tavaria M.
      • Weaver B.
      • Reich N.C.
      • Kola I.
      • Hertzog P.J.
      ). The sIfnar2a′ minor transcript is generated from a transcript missing 128 nucleotides after codon 236 of the Ifnar2 cDNA. This isoform originates by skipping the transmembrane-encoding exon 8, leading to a frameshift forming a stop codon that generates a transcript capable of producing a soluble receptor with 11 unique C-terminal amino acids (Fig. 1A).
      The transmembrane and soluble Ifnar2 transcripts are differentially regulated based on Northern blot analyses of expression in murine tissues. Ratios of tmIfnar2c:sIfnar2a range from >10:1 in some tissues to ∼1:1 in hemopoietic tissues (
      • Hardy M.P.
      • Owczarek C.M.
      • Trajanovska S.
      • Liu X.
      • Kola I.
      • Hertzog P.J.
      ). Analysis of the 5′ flanking region of Ifnar2 using promoter reporter constructs identified three regulatory regions that confer basal expression, inducible expression by IFNα + IFNγ, and a negative regulatory region (
      • Hardy M.P.
      • Owczarek C.M.
      • Trajanovska S.
      • Liu X.
      • Kola I.
      • Hertzog P.J.
      ,
      • Hardy M.P.
      • Sanij E.P.
      • Hertzog P.J.
      • Owczarek C.M.
      ,
      • Hardy M.P.
      • Hertzog P.J.
      • Owczarek C.M.
      ).
      Structure-Function Relationships—Studies of type I IFN receptors prior to their cloning have indicated that most cell types bind IFNs, with large variation in the number of binding sites (200–10,000/cell) and binding affinities. Scatchard analyses of binding usually identify two types of binding sites of low (μm) and high affinity (nm–pm) (
      • Langer J.A.
      • Pestka S.
      ). This pattern of binding is consistent with a multicomponent receptor containing a high affinity binding chain (often called the α or primary binding chain) and a β or signal-transducing chain that has low intrinsic ligand binding affinity and converts the affinity of interaction of ligand with α chain from moderate (nm) to high affinity (pm) (
      • Bazan J.F.
      ). Recently, elegant studies using recombinant IFNAR extracellular domains (ECDs) tethered to lipid membranes have clearly demonstrated that various type I IFNs bind to IFNAR2 with Kd values mostly in the nm range (from 0.1 to 1000 nm) and bind to IFNAR1 with Kd mostly in the μm range (from 0.05 to 10 μm) (
      • Jaks E.
      • Gavutis M.
      • Uze G.
      • Martal J.
      • Piehler J.
      ).
      IFNAR Structures—IFNAR1 and IFNAR2 belong to the class II helical cytokine receptor (hCR) family, which includes the receptor for type II IFN, tissue factor (TF), and IL10Rβ (
      • Bazan J.F.
      ). Members of the class II hCR family contain tandem ∼100 amino acid (aa) domains with a predicted topology analogous to the Ig constant domain (
      • Bazan J.F.
      ). The ECD of huIFNAR1, comprising 409 aa (403 in the murine form) contains four subdomains, referred to as SD1–SD4, each housing one fibronectin (FBN-III-like) domain (Fig. 2). SD1 contains conserved residues implicated in binding membrane glycosphingolipids (
      • Ghislain J.
      • Lingwood C.A.
      • Fish E.N.
      ). SD1–SD3 appears to house the ligand binding domain; SD4 is essential for ternary complex formation (
      • Lamken P.
      • Gavutis M.
      • Peters I.
      • Van der Heyden J.
      • Uze G.
      • Piehler J.
      ). The SD1-SD2 pair is structurally similar to the SD3-SD4 pair with characteristic disulfide-bonding cysteine pairs (
      • Bazan J.F.
      ) and 50% sequence homology (
      • Uze G.
      • Lutfalla G.
      • Mogensen K.E.
      ).
      Figure thumbnail gr2
      FIGURE 2The type I IFN receptor signaling complex. The three components, huIFNAR1ECD (left), huIFNα2(middle), and huIFNAR2ECD (right), were modeled using RasMol and are given at the approximate relative size. The N and C termini of each component and the four subdomains of IFNAR1ECD are indicated. The huIFNAR1ECD model (left, Spacefill format) was predicted from the crystal structure of human fibronectin (PDB reference code 1FNF). Amino acid residues of IFNAR1ECD that reportedly interact with membrane-bound glycosphingolipids are shown in yellow and those that are important for huIFNα2 binding in violet. The huIFNα2 structure (middle, ribbon format) was determined by NMR (PDB reference code 1ITF). Amino acid residues involved in IFNAR1 interactions are shown in green (stick format), and those involved in IFNAR2 interactions shown in violet (stick format). The huIFNAR2ECD structure (right, Spacefill format) was determined by NMR (PDB reference code 1N6U) and shows amino acid residues that are important for the binding of huIFNα2(red) or huIFNβ (green) and those residues that are important for binding both ligands (orange).
      NMR has been used to model huIFNAR2 structure and its interaction with huIFNα2 (
      • Chill J.H.
      • Quadt S.R.
      • Levy R.
      • Schreiber G.
      • Anglister J.
      ), which has similarities to and differences from the resolved crystal structures of two members of the class II hCR group, TF (
      • Harlos K.
      • Martin D.M.
      • O'Brien D.P.
      • Jones E.Y.
      • Stuart D.I.
      • Polikarpov I.
      • Miller A.
      • Tuddenham E.G.
      • Boys C.W.
      ) and IFNGR (
      • Walter M.R.
      • Windsor W.T.
      • Nagabhushan T.L.
      • Lundell D.J.
      • Lunn C.A.
      • Zauodny P.J.
      • Narula S.K.
      ). As predicted for class II hCR receptors (
      • Bazan J.F.
      ), all three proteins have two FBN-III domains with Ig-like folding topology and two conserved disulfide bonds (
      • Walter M.R.
      • Windsor W.T.
      • Nagabhushan T.L.
      • Lundell D.J.
      • Lunn C.A.
      • Zauodny P.J.
      • Narula S.K.
      ,
      • Runkel L.
      • deDios C.
      • Karpusas M.
      • Betzenhauser M.
      • Muldowney C.
      • Zafari M.
      • Benjamin C.D.
      • Miller S.
      • Hochman P.S.
      • Whitty A.
      ,
      • Cajean-Feroldi C.
      • Nosal F.
      • Nardeux P.C.
      • Gallet X.
      • Guymarho J.
      • Baychelier F.
      • Sempe P.
      • Tovey M.G.
      • Escary J.L.
      • Eid P.
      ). However, unlike TF and IFNGR, which show a conserved interdomain angle of ∼120° (
      • Harlos K.
      • Martin D.M.
      • O'Brien D.P.
      • Jones E.Y.
      • Stuart D.I.
      • Polikarpov I.
      • Miller A.
      • Tuddenham E.G.
      • Boys C.W.
      ,
      • Walter M.R.
      • Windsor W.T.
      • Nagabhushan T.L.
      • Lundell D.J.
      • Lunn C.A.
      • Zauodny P.J.
      • Narula S.K.
      ), huIFNAR2 has an interdomain angle approximating 90° (
      • Chill J.H.
      • Quadt S.R.
      • Levy R.
      • Schreiber G.
      • Anglister J.
      ) (Fig. 2). Based on the prediction that type I IFNs have an interaction surfaces for each receptor on opposing sides of the ligand (
      • Runkel L.
      • deDios C.
      • Karpusas M.
      • Betzenhauser M.
      • Muldowney C.
      • Zafari M.
      • Benjamin C.D.
      • Miller S.
      • Hochman P.S.
      • Whitty A.
      ) and on information regarding the locations of amino acids implicated in ligand binding on both IFNAR1 and IFNAR2, a three-dimensional model for the ligand-bound human type I IFN receptor complex has been proposed (
      • Cajean-Feroldi C.
      • Nosal F.
      • Nardeux P.C.
      • Gallet X.
      • Guymarho J.
      • Baychelier F.
      • Sempe P.
      • Tovey M.G.
      • Escary J.L.
      • Eid P.
      ). This model predicts that upon ligand binding, the N-terminal FBN-III domain of IFNAR1 forms a lid over the bound ligand (
      • Cajean-Feroldi C.
      • Nosal F.
      • Nardeux P.C.
      • Gallet X.
      • Guymarho J.
      • Baychelier F.
      • Sempe P.
      • Tovey M.G.
      • Escary J.L.
      • Eid P.
      ). However, this prediction fails to consider the residues of IFNAR1 implicated in binding membrane glycosphingolipids (
      • Ghislain J.
      • Lingwood C.A.
      • Fish E.N.
      ) so that the N terminus of IFNAR1 may fold toward the membrane, stabilizing the molecule before or after ligand binding.
      IFNAR-Ligand Interaction—The binding site of IFNα2 on IFNAR1 has been predicted from site-directed mutagenesis (
      • Cajean-Feroldi C.
      • Nosal F.
      • Nardeux P.C.
      • Gallet X.
      • Guymarho J.
      • Baychelier F.
      • Sempe P.
      • Tovey M.G.
      • Escary J.L.
      • Eid P.
      ,
      • Lamken P.
      • Lata S.
      • Gavutis M.
      • Piehler J.
      ) and epitope mapping with an anti-IFNAR1-neutralizing antibody (
      • Eid P.
      • Langer J.A.
      • Bailly G.
      • Lejealle R.
      • Guymarho J.
      • Tovey M.G.
      ) (Fig. 2), albeit the latter may be inaccurate because of steric hindrances. Residues involved in ligand binding are found on the three membrane-distal FBN-III SD domains. Residues 69VY70 have been identified as the key residues in IFNAR1 recognition by a neutralizing monoclonal antibody (
      • Eid P.
      • Langer J.A.
      • Bailly G.
      • Lejealle R.
      • Guymarho J.
      • Tovey M.G.
      ) and thus proposed to influence IFNα2 binding (
      • Cajean-Feroldi C.
      • Nosal F.
      • Nardeux P.C.
      • Gallet X.
      • Guymarho J.
      • Baychelier F.
      • Sempe P.
      • Tovey M.G.
      • Escary J.L.
      • Eid P.
      ). A number of other surface-exposed, aromatic residues within the surrounding region have also been demonstrated to aid IFNα2 binding (
      • Cajean-Feroldi C.
      • Nosal F.
      • Nardeux P.C.
      • Gallet X.
      • Guymarho J.
      • Baychelier F.
      • Sempe P.
      • Tovey M.G.
      • Escary J.L.
      • Eid P.
      ) (Fig. 2).
      Studies from our laboratory and others suggest that IFNAR1 is necessary for signaling and is possibly responsible for the differential recognition of the IFN ligands (
      • Jaks E.
      • Gavutis M.
      • Uze G.
      • Martal J.
      • Piehler J.
      ,
      • Lamken P.
      • Gavutis M.
      • Peters I.
      • Van der Heyden J.
      • Uze G.
      • Piehler J.
      ,
      • Lewerenz M.
      • Mogensen K.E.
      • Uze G.
      ,
      • Piehler J.
      • Schreiber G.
      ). Numerous studies have investigated the residues of IFNAR2 involved in ligand interactions with IFNα2 and IFNβ (
      • Chill J.H.
      • Quadt S.R.
      • Levy R.
      • Schreiber G.
      • Anglister J.
      ,
      • Lamken P.
      • Lata S.
      • Gavutis M.
      • Piehler J.
      ,
      • Lewerenz M.
      • Mogensen K.E.
      • Uze G.
      ,
      • Piehler J.
      • Schreiber G.
      ) (Fig. 2). Notably, the ligand binding site of huIFNAR2 is composed largely of aliphatic hydrophobic amino acids (
      • Chill J.H.
      • Quadt S.R.
      • Levy R.
      • Schreiber G.
      • Anglister J.
      ,
      • Harlos K.
      • Martin D.M.
      • O'Brien D.P.
      • Jones E.Y.
      • Stuart D.I.
      • Polikarpov I.
      • Miller A.
      • Tuddenham E.G.
      • Boys C.W.
      ,
      • Walter M.R.
      • Windsor W.T.
      • Nagabhushan T.L.
      • Lundell D.J.
      • Lunn C.A.
      • Zauodny P.J.
      • Narula S.K.
      ) (Fig. 2). Forming the core of the IFNAR2 ligand binding domain are three highly conserved residues Thr44, Met46, and Lys48, whereas residues, His76, Glu77, Tyr81, Trp100, Ile103, and Asp106 also facilitate IFNα2 binding (
      • Lewerenz M.
      • Mogensen K.E.
      • Uze G.
      ,
      • Piehler J.
      • Schreiber G.
      ). These residues are predicted by NMR to form an extensive and largely aliphatic hydrophobic patch on the surface of IFNAR2 (Fig. 2) (
      • Chill J.H.
      • Quadt S.R.
      • Levy R.
      • Schreiber G.
      • Anglister J.
      ). The interaction of IFNβ is predicted to be different from IFNα, involving IFNAR2 residues Ile45 and Trp100, with minor contributions from Thr44, Met46, Ser47, and Ile103 (Fig. 2) (
      • Piehler J.
      • Schreiber G.
      ). It appears that the sequence differences between type I IFNs result in different binding affinities with each IFNAR chain and consequent biological activities (
      • Jaks E.
      • Gavutis M.
      • Uze G.
      • Martal J.
      • Piehler J.
      ).
      Soluble IFNAR—Soluble cytokine receptors are present in body fluids and modulate cytokine activity during homeostasis and disease (
      • Fernandez-Botran R.
      ). Soluble IFNAR2 receptors are present in serum, urine, saliva, and the peritoneal fluid of both humans and mice (
      • Hardy M.P.
      • Owczarek C.M.
      • Trajanovska S.
      • Liu X.
      • Kola I.
      • Hertzog P.J.
      ,
      • Novick D.
      • Cohen B.
      • Rubinstein M.
      ). In vitro studies demonstrate that a soluble IFNAR2 can also be generated by cleavage of transmembrane IFNAR2 by intramembrane proteases in response to IFNs and other stimuli (
      • Saleh A.Z.
      • Fang A.T.
      • Arch A.E.
      • Neupane D.
      • El Fiky A.
      • Krolewski J.J.
      ), but there is no definitive in vivo evidence for this effect. Nevertheless, there is a precedent in the IL6 receptor system where the soluble IL6Rα is generated both by alternative splicing and by cleavage by ADAM 10 and 17 proteases (
      • Jones S.A.
      • Richards P.J.
      • Scheller J.
      • Rose-John S.
      ).
      Although a definitive function for the soluble IFNAR2 isoform has not yet been resolved, in vitro experiments have demonstrated that sIFNAR2a can act either as an agonist or antagonist (
      • Hardy M.P.
      • Owczarek C.M.
      • Trajanovska S.
      • Liu X.
      • Kola I.
      • Hertzog P.J.
      ). Soluble IFNAR2a can inhibit IFN signaling in normal cells, whereas in primary thymocytes from Ifnar2–/– mice, sIFNAR2a can bind IFNα or -β and generate an antiproliferative signal (Fig. 1B). In another study, ovine soluble IFNAR2 was able to mediate antiviral activity in vitro (
      • Han C.S.
      • Chen Y.
      • Ezashi T.
      • Roberts R.M.
      ). Transgenic mice over expressing the soluble receptor are more susceptible to LPS-induced, IFNβ-mediated septic shock, suggesting that high levels of the sIFNAR2a receptor can potentiate IFN signaling in vivo.
      S. A. Samarajiwa, M. Hardy, C. M. Owczarek, and P. J. Hertzog, unpublished data.
      Potentiation or agonist actions of soluble IFNAR2a may be mediated by the process of trans-signaling (see below), which is the major method of signal transduction by sIL6R and sIL15R (
      • Jones S.A.
      • Richards P.J.
      • Scheller J.
      • Rose-John S.
      ,
      • Rubinstein M.P.
      • Kovar M.
      • Purton J.F.
      • Cho J.H.
      • Boyman O.
      • Surh C.D.
      • Sprent J.
      ). Soluble IFNAR2a might also have a carrier function, as IFNβ bound sIFNAR2a increased the stability of IFNβ and enhanced the anti-tumor activity in a xenograft tumor model (
      • McKenna S.D.
      • Vergilis K.
      • Arulanandam A.R.
      • Weiser W.Y.
      • Nabioullin R.
      • Tepper M.A.
      ).

      Mechanisms of Signaling

      Signal Transduction Domains and Pathways—The type I IFN receptor, typical of class II hCR, lack intrinsic kinase activity and thus rely on associated Janus kinases (JAKs) to phosphorylate receptors and signal transducing molecules, such as STAT proteins, after ligand-induced receptor clustering. IFNAR1 is preassociated with Tyk2 (
      • Yan H.
      • Krishnan K.
      • Lim J.T.
      • Contillo L.G.
      • Krolewski J.J.
      ), which also stabilizes IFNAR1 cell surface expression levels (
      • Marijanovic Z.
      • Ragimbeau J.
      • Kumar K.G.
      • Fuchs S.Y.
      • Pellegrini S.
      ). The Tyk2 binding site on the huIFNAR1 cytoplasmic region has been localized to a region encompassing residues 479–511 (
      • Yan H.
      • Krishnan K.
      • Lim J.T.
      • Contillo L.G.
      • Krolewski J.J.
      ). HuIFNAR1 also bound STAT1 and STAT2 via phospho-Tyr466 and phospho-Tyr481 (
      • Li X.
      • Leung S.
      • Kerr I.M.
      • Stark G.R.
      ) when overexpressed in heterologous cells (
      • Platanias L.C.
      ). STAT3 reportedly undergoes a phosphotyrosine-dependent interaction with IFNAR1 (
      • Yang C.H.
      • Shi W.
      • Basu L.
      • Murti A.
      • Constantinescu S.N.
      • Blatt L.
      • Croze E.
      • Mullersman J.E.
      • Pfeffer L.M.
      ), consistent with STAT1 and STAT3 homo- and heterodimer formation after IFNα treatment (
      • Owczarek C.M.
      • Hwang S.Y.
      • Holland K.A.
      • Gulluyan L.M.
      • Tavaria M.
      • Weaver B.
      • Reich N.C.
      • Kola I.
      • Hertzog P.J.
      ).
      Using truncation mutants of the intracellular domain of huIFNAR2, the site of Jak1 binding was identified to a 47-aa region (
      • Domanski P.
      • Fish E.
      • Nadeau O.W.
      • Witte M.
      • Platanias L.C.
      • Yan H.
      • Krolewski J.
      • Pitha P.
      • Colamonici O.R.
      ). Jak1, STAT1, and STAT2 may also be preassociated with IFNAR2 (
      • Li X.
      • Leung S.
      • Kerr I.M.
      • Stark G.R.
      ). These data suggest that the intracellular domains and signal-transducing molecules such as STATs may form multimolecular signal transduction complexes in which each molecule has multiple interactions (
      • Nguyen V.P.
      • Saleh A.Z.
      • Arch A.E.
      • Yan H.
      • Piazza F.
      • Kim J.
      • Krolewski J.J.
      ).
      Further diversity of IFNAR signaling is achieved by the activation of other pathways including other STAT proteins and non-STAT proteins (
      • Platanias L.C.
      ). These “alternative” signaling pathways include CrkL, Rap1, MAP kinases, Vav, RAC1, PI 3-kinase, IRS1 and -2, PMRT1, and Sin1 (
      • Platanias L.C.
      ).
      Negative Regulation of Signal Transduction—The diversity of signals generated through IFNARs can protect the host against infection and cancer and mount controlled immune responses, whereas excessive or deregulated signaling can lead to toxicity, leucopenia, autoimmunity, and even death. Thus, IFNARs also interact with a number of negative regulatory molecules, including SOCS-1 (suppressor of cytokine signaling), UBP43, and SHP (
      • You M.
      • Yu D.H.
      • Feng G.S.
      ,
      • Malakhova O.A.
      • Kim K.I.
      • Luo J.K.
      • Zou W.
      • Kumar K.G.
      • Fuchs S.Y.
      • Shuai K.
      • Zhang D.E.
      ,
      • Fenner J.E.
      • Starr R.
      • Cornish A.L.
      • Zhang J.G.
      • Metcalf D.
      • Schreiber R.D.
      • Sheehan K.
      • Hilton D.J.
      • Alexander W.S.
      • Hertzog P.J.
      ), to limit the extent of signaling. The C terminus of IFNAR1 contains a highly conserved region spanning 14 amino acids, which mediates inhibition of type I IFN signaling (
      • Kumar K.G.
      • Krolewski J.J.
      • Fuchs S.Y.
      ). This may occur by binding a negative regulator such as SOCS-1 to inhibit JAK/STAT signaling (
      • Fenner J.E.
      • Starr R.
      • Cornish A.L.
      • Zhang J.G.
      • Metcalf D.
      • Schreiber R.D.
      • Sheehan K.
      • Hilton D.J.
      • Alexander W.S.
      • Hertzog P.J.
      ). Residues within this region are also essential for the recruitment of E3 ubiquitin ligases and ubiquitination and degradation of the receptor (
      • Kumar K.G.
      • Krolewski J.J.
      • Fuchs S.Y.
      ). Recently, a type I IFN-inducible cysteine protease, UBP43, was shown to interact directly with IFNAR2, blocking the interaction between Jak1 and the receptor (
      • Malakhova O.A.
      • Kim K.I.
      • Luo J.K.
      • Zou W.
      • Kumar K.G.
      • Fuchs S.Y.
      • Shuai K.
      • Zhang D.E.
      ).
      Trans-signaling—Soluble cytokine receptors such as sIL6R, sCNTFR, sIL11R, and sIL15R can mediate cytokine biological effects by a mechanism known as trans-signaling (Fig. 1B). Trans-signaling occurs when the soluble receptor bound ligand interacts with a complementary transmembrane receptor chain of the receptor complex (
      • Jones S.A.
      • Richards P.J.
      • Scheller J.
      • Rose-John S.
      ,
      • Rubinstein M.P.
      • Kovar M.
      • Purton J.F.
      • Cho J.H.
      • Boyman O.
      • Surh C.D.
      • Sprent J.
      ). In the case of IL6Rα and the signaling gp130 chain, this is the major mechanism of IL6 signaling because gp130 is expressed on most cells, but the transmembrane form of IL6Rα shows restricted expression (
      • Jones S.A.
      • Rose-John S.
      ). We have demonstrated in vitro that sIFNAR2a can bind IFNα or -β and transduce a signal through IFNAR1 (
      • Hardy M.P.
      • Owczarek C.M.
      • Trajanovska S.
      • Liu X.
      • Kola I.
      • Hertzog P.J.
      ). Furthermore, our experiments using mice overexpressing sIFNAR2a suggest that high levels of sIFNAR2a may act as trans-signaling molecules in vivo.4 Although more direct evidence of trans-signaling in the IFN system is required, it provides a compelling mechanism for transducing alternative signals.

      In Vivo Function

      Expression Patterns—All tissues and organs and most cell lines express transcripts for IFNAR1 and both soluble and transmembrane isoforms of IFNAR2 (
      • Hardy M.P.
      • Owczarek C.M.
      • Trajanovska S.
      • Liu X.
      • Kola I.
      • Hertzog P.J.
      ,
      • Hardy M.P.
      • Sanij E.P.
      • Hertzog P.J.
      • Owczarek C.M.
      ,
      • Hardy M.P.
      • Hertzog P.J.
      • Owczarek C.M.
      ) and thus bind and respond to IFNs. Microarray analysis of Ifnar expression in different tissues and cells indicate that there is some differential expression of IFNAR1 and IFNAR2, with IFNAR1 being more widespread and IFNAR2 more restricted. These studies need to be validated by other techniques, with reagents that enable the measurement of IFNAR protein levels, particularly at the cell surface, to allow a clearer picture of the composition of the IFNAR receptor on different cell types during homeostasis and disease.
      Knock-out Studies—Mice with null mutations in Ifnar1 have demonstrated that this component is essential for responses to multiple IFNα as well as IFNβ (
      • Hwang S.Y.
      • Hertzog P.J.
      • Holland K.A.
      • Sumarsono S.H.
      • Tymms M.J.
      • Hamilton J.A.
      • Whitty G.
      • Bertoncello I.
      • Kola I.
      ,
      • Muller U.
      • Steinhoff U.
      • Reis L.F.
      • Hemmi S.
      • Pavlovic J.
      • Zinkernagel R.M.
      • Aguet M.
      ). Extensive use of these mice has demonstrated the necessity of Ifnar1 for survival against most viral infections, myelopoiesis, and B and T cell-mediated immune responses and as a potent proinflammatory cytokine.
      We have also generated mice with a null mutation in the Ifnar2 gene that had a phenotype similar to Ifnar1 null mice in their susceptibility to viral and bacterial infections but were distinct in their abnormal thymic T cell development.
      P. J. Hertzog, B. Scott, C. M. Owczarek, M. Hardy, J. Gould, S. Noppest, and J. Fenner, unpublished data.
      The subtle differences in phenotype were notable in their homeostatic role in hemopoiesis and in the mediation of proinflammatory signals (
      • Fenner J.E.
      • Starr R.
      • Cornish A.L.
      • Zhang J.G.
      • Metcalf D.
      • Schreiber R.D.
      • Sheehan K.
      • Hilton D.J.
      • Alexander W.S.
      • Hertzog P.J.
      ). The mechanism and nature of these differential signals remain to be elucidated.

      Disease Associations

      Polymorphisms in promoters and genes encoding type I IFN receptors have been implicated in a number of diseases. Protection or susceptibility against cerebral malaria (
      • Aucan C.
      • Walley A.J.
      • Hennig B.J.
      • Fitness J.
      • Frodsham A.
      • Zhang L.
      • Kwiatkowski D.
      • Hill A.V.
      ), susceptibility to multiple sclerosis (
      • Leyva L.
      • Fernandez O.
      • Fedetz M.
      • Blanco E.
      • Fernandez V.E.
      • Oliver B.
      • Leon A.
      • Pinto-Medel M.J.
      • Mayorga C.
      • Guerrero M.
      • Luque G.
      • Alcina A.
      • Matesanz F.
      ), trypanosomaiasis (
      • Kierstein S.
      • Noyes H.
      • Naessens J.
      • Nakamura Y.
      • Pritchard C.
      • Gibson J.
      • Kemp S.
      • Brass A.
      ), HIV (
      • Diop G.
      • Hirtzig T.
      • Do H.
      • Coulonges C.
      • Vasilescu A.
      • Labib T.
      • Spadoni J.L.
      • Therwath A.
      • Lathrop M.
      • Matsuda F.
      • Zagury J.F.
      ), and hepatitis B and C virus (
      • Saito T.
      • Ji G.
      • Shinzawa H.
      • Okumoto K.
      • Hattori E.
      • Adachi T.
      • Takeda T.
      • Sugahara K.
      • Ito J.I.
      • Watanabe H.
      • Saito K.
      • Togashi H.
      • Ishii K.
      • Matsuura T.
      • Inageda K.
      • Muramatsu M.
      • Kawata S.
      ,
      • Frodsham A.J.
      • Zhang L.
      • Dumpis U.
      • Taib N.A.
      • Best S.
      • Durham A.
      • Hennig B.J.
      • Hellier S.
      • Knapp S.
      • Wright M.
      • Chiaramonte M.
      • Bell J.I.
      • Graves M.
      • Whittle H.C.
      • Thomas H.C.
      • Thursz M.R.
      • Hill A.V.
      ) are influenced by IFNAR polymorphisms.
      In Down syndrome, where HSA 21 containing the IFNAR1 and IFNAR2 genes is trisomic, cells are more sensitive to IFNα treatment; the aberrant immune response in this condition has been associated with aberrant IFN signaling (
      • Kola I.
      • Hertzog P.J.
      ). The levels of sIFNAR2a have not been assessed in this condition.
      Increased levels of sIFNAR2a have been reported in many chronic viral infections, cancers, and urological diseases (
      • Saito T.
      • Ji G.
      • Shinzawa H.
      • Okumoto K.
      • Hattori E.
      • Adachi T.
      • Takeda T.
      • Sugahara K.
      • Ito J.I.
      • Watanabe H.
      • Saito K.
      • Togashi H.
      • Ishii K.
      • Matsuura T.
      • Inageda K.
      • Muramatsu M.
      • Kawata S.
      ,
      • Ambrus Sr., J.L.
      • Dembinski W.
      • Ambrus Jr., J.L.
      • Sykes D.E.
      • Akhter S.
      • Kulaylat M.N.
      • Islam A.
      • Chadha K.C.
      ,
      • Mizukoshi E.
      • Kaneko S.
      • Kaji K.
      • Terasaki S.
      • Matsushita E.
      • Muraguchi M.
      • Ohmoto Y.
      • Kobayashi K.
      ). During chronic hepatitis C infection, total IFNAR2 transcript levels as well as sIFNAR2a serum levels, increase by more than 10-fold (
      • Mizukoshi E.
      • Kaneko S.
      • Kaji K.
      • Terasaki S.
      • Matsushita E.
      • Muraguchi M.
      • Ohmoto Y.
      • Kobayashi K.
      ). The increased serum levels of sIFNAR2a correlated with increases in serum 2–5 oligoadenylate synthetase activity (p < 0.001) suggestive of potentiated IFN activity (
      • Mizukoshi E.
      • Kaneko S.
      • Kaji K.
      • Terasaki S.
      • Matsushita E.
      • Muraguchi M.
      • Ohmoto Y.
      • Kobayashi K.
      ). Interestingly, other studies have not found a correlation between increased sIFNAR2a serum levels and lack of response to IFN therapy. Although the importance of soluble IFNAR2 in human disease will be clearer once it has been ascertained whether it participates mostly in agonist or antagonist activities, the above studies support the former.
      Not only do host cells produce IFNAR proteins during viral infections, but certain viruses have evolved a form of soluble type I IFN receptor as a means of evading the immune response. Poxvirus encode a soluble IFN receptor homologue that neutralizes all type I IFNs tested, which is essential for virulence (
      • Symons J.A.
      • Alcami A.
      • Smith G.L.
      ). This is an unusual protein in that it has low amino acid homology to IFNAR1 or IFNAR2 but has a tertiary structural similarity, based on modeling studies, and is a potent inhibitor of the antiviral activity of a broad range of type I IFNs without the species specificity of mammalian IFN-IFNAR interactions.

      Summary and Perspective

      The IFNAR complex is novel among cytokine receptors in mediating signaling by more than 15 different but related type I IFN ligands. This system has been instrumental in the discovery of the JAK/STAT signaling pathway, which is necessary for regulating genes involved in the characteristic antiviral response. However emerging data indicate that many more so-called alternative pathways are activated by IFNAR activation. This diversity of signals may explain how IFNs generate complex biological responses. There has been considerable advance in understanding the structure of the receptor signaling complex. However, more detailed structural studies and confirmation of the potential difference in receptor configurations, like that which would elicit trans-signaling, are necessary to elucidate how the different biological activities of type I IFNs can be regulated by the receptor.

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