1999/12/01 by G. Tau, Gregory Tau, P. Rothman +1
Medicine · Immunology and Microbiology · #Cytokine Signaling Pathways and Interactions #interferon and immune responses #Immune Cell Function and Interaction
paper · pdf · doi:10.1034/j.1398-9995.1999.00099.x
Interferon-gamma (IFN-γ) is a cytokine that plays an important role in inducing and modulating an array of immune responses. Cellular responses to IFN-γ are mediated by its heterodimeric cell-surface receptor (IFN-γR), which activates downstream signal transduction cascades, ultimately leading to the regulation of gene expression. In order to study the role of IFN-γ in a number of immune responses and pathways, researchers have generated mice with altered patterns of IFN-γR gene expression. These studies, together with analyses of naturally occurring mutations of the IFN-γR in man, have been instrumental in elucidating the diverse functions of IFN-γ, and are the subject of this review. Originally identified 30 years ago as an agent with antiviral activity, IFN-γ has since been characterized as a homodimeric glycoprotein with pleiotropic immunologic functions ( 1–3). IFN-γ is primarily secreted by activated T cells and natural killer (NK) cells, and can promote macrophage activation, mediate antiviral and antibacterial immunity, enhance antigen presentation, orchestrate activation of the innate immune system, coordinate lymphocyte–endothelium interaction, regulate Th1/Th2 balance, and control cellular proliferation and apoptosis ( 1, 2). It was not until 20 years after the identification of IFN-γ that its cell-surface receptor was discovered ( 4–9). The α chain of the IFN-γR, also known as IFN-γR1 or CD119, was the first component of the receptor to be identified and cloned ( 10–14). Although it binds IFN-γ with relatively high affinity, IFN-γR1 alone is unable to mediate the biologic responses to this cytokine ( 12, 15–17). Subsequent complementation studies led to the identification and cloning of an accessory factor (AF-1), also known as the β receptor chain or IFN-γR2, as the protein required, in addition to IFN-γR1, to endow a cell with the ability to respond to IFN-γ ( 15, 18–21). Specific residues within the cytoplasmic domains of both the α and β chains of the IFN-γR are critical for transducing the IFN-γ signal from the cell surface to the nucleus through the activation of intracellular signaling pathways ( 22–25). Mutations in either component of the IFN-γ receptor that impair or alter the ability of cells to respond to this ligand have global consequences for IFN-γ-mediated immunity, and therefore serve as an important tool for analyzing the pleiotropic effects of this cytokine ( 26, 27). The receptor complex that mediates the full biologic function of IFN-γ consists of at least two species-matched chains: IFN-γR1, a 90-kDa glycoprotein (which is 472 amino acids (aa) long in man, and 451aa in mice) encoded on human chromosome 6 and mouse chromosome 10, and IFN-γR2, a 60–67-kDa glycoprotein (which is 316aa in man and 314aa in mice) encoded on human chromosome 21 and mouse chromosome 16 ( 4, 18, 20, 28–35). IFN-γR1 is the major ligand-binding subunit, binding IFN-γ with a Ka of 109–1010 M−1 and a receptor-to-ligand ratio of 2:1, as inferred from the crystal structure of the occupied receptor and other studies ( 4, 34, 36, 37). IFN-γR2 increases the affinity of IFN-γR1 for its ligand, presumably by enhancing the stability of the complex, but plays only a minor role in direct ligand binding ( 38). The β chain is, however, obligatory for transducing the IFN-γ signal ( 21, 38, 39). Both chains of the IFN-γ receptor are members of the class II family of cytokine receptors that includes tissue factor, the IL-10 ligand-binding component, and both chains of the IFN-α receptor (IFN-αR) ( 40, 41). Like other family members, the IFN-γR α and β chains lack intrinsic kinase activity. Signaling through the IFN-γR is mediated through JAK1 and JAK2, members of the Janus family of protein tyrosine kinases, which are constitutively associated with specific membrane-proximal residues on the cytoplasmic domains of IFN-γR ( 25, 34, 39, 42–44). JAK1 binds the 266LPKS269 motif (also known as the box 1 motif) on IFN-γR1, while JAK2 binds the 263PPSIPLQIEEYL274 motif (or the box 1, box 2 motif) on IFN-γR2 ( 23, 25, 44–46). Ligand binding leads to receptor oligomerization, with two IFN-γR1 chains bound to one IFN-γ homodimer, and the subsequent recruitment of two IFN-γR2 chains to the complex ( 25, 37, 38, 45–48). IFN-γ-mediated aggregation of its receptor components brings the inactive JAKs associated with the cytoplasmic tails of the α and β chains into close proximity with one another ( Fig. 1). Once clustered, the JAKs are reciprocally activated through sequential auto- and transphosphorylation events ( 42, 49). Activated JAKs phosphorylate a specific tyrosine residue near the C-terminus of the IFN-γR1 (Y440 in man) ( 24, 42, 45, 50) ( Fig. 1). This phosphorylated tyrosine residue pair (one on each IFN-γR1 chain) is embedded within a recognition sequence (440YDKPH444) to which STAT1 (a member of the Signal Transducers and Activators of Transcription family of latent cytoplasmic proteins) binds through its SH2 (src homology 2) domain ( 24, 45, 51, 52). The docking of STAT1 molecules at their target sequences on the IFN-γR complex is followed by their phosphorylation on tyrosine residue Y701 by the receptor-associated JAKs ( 53–55) ( Fig. 1). Once phosphorylated, two STAT1 proteins homodimerize via reciprocal SH2-phosphotyrosine interactions, forming a protein complex first identified as GAF (gamma-activated factor) ( 51, 56). The STAT1 homodimer then translocates to the nucleus, where it binds a nine-nucleotide consensus sequence, TTNCNNNAA, known as a GAS (gamma-activated site) element ( 57–59). This binding site has been identified in the regulatory regions of over 200 genes; therefore, recognition of this element by STAT1 homodimers can modulate the expression of a vast array of genes, thereby mediating the biologic functions of IFN-γ ( 2). IFN-γ signaling cascade. A) IFN-γR is composed of α and β chains. JAK1 is constitutively associated with IFN-γR1 while JAK2 is constitutively associated with IFN-γR2. B) IFN-γ binding to its receptor leads to aggregation of receptor components. Subsequently, JAKs are activated through auto- and transphosphorylation events. Activated JAKs then phosphorylate tyrosine residue near C-terminus of IFN-γR1. C) STAT1 molecules dock at phosphorylated receptor, and are then phosphorylated by activated JAKs. D) Phosphorylated STAT1 proteins homodimerize via reciprocal SH2-phosphotyrosine interactions, and translocate to nucleus, where they regulate gene transcription. Mechanistically, it has been suggested that after ligand binding, IFN-γ signaling is initiated by JAK2 autophosphorylation, followed by phosphorylation of JAK1 ( 34, 60). Activated JAK1 is then thought to phosphorylate IFN-γR1, providing a docking site for STAT1. After binding to its receptor site, STAT1 is to be activated through phosphorylation by with JAK1 and JAK2 have that while the JAK2 gene the kinase of JAK1 is not for its role in transducing the IFN-γ signal ( 49). 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by cells in this and then antigen in the of class and the ( antigen through the class II is by IFN-γ, antigen through the class can be by either IFN-γ or IFN-α ( cells, which the of the IFN-γ that is thought to in of their function ( It was therefore of to function in IFN-γR1 mice in order to the of IFN-γ to the function of from IFN-γR1 mice with target cells with this that are of and functions ( cells in IFN-γR1 mice of as to that they are to cell to activation (which be to either an antigen or an intrinsic ( number of studies that T cells are important of the of responses to ( Although tissue in to and was in IFN-γR1 mice and mice ( of the of this of immunity, be by in either the cell or antigen or activation or of this ( in mice not be by an intrinsic Although also in ( of and to in the ability of and IFN-γR1 mice to ( These for the intrinsic function to be T cells can and functions their to an IFN-γ Both and the mice to be in elucidating the specific of and II and IFN-γ, in ( In studies have that signaling via their cell-surface receptors in order to cells the effects of ( 26, In studies, on the other that while are for the of IFN-γ is that the antiviral of II are as they of ( mice to be to as and while to as and ( mice to they in to as to control mice an with but of as to mice ( IFN-γR1 mice and an of cells to control These studies that the immune a number of antiviral responses. of be on IFN-γ, not be for a or and of with intracellular are innate and ( mice or of to of mice with is characterized by a in mice ( mice and with intracellular as of and in mice ( IFN-γ is an important for which are in a for and intracellular Activated also a and both of which are important for their function ( 2). it is that an macrophage is the of the of IFN-γR1 mice to with intracellular ( of is the of an immune In responses are associated with responses to intracellular as responses are associated with from as ( The innate immune is also important in The role of IFN-γ in and in the of and immune responses was with IFN-γR1 with major on macrophage and function ( of IFN-γR1 mice with major or is associated with that in control mice in the of of ( mice a in to to the IFN-γR1 ( These studies that the IFN-γ is critical for intracellular and a in macrophage function as a to the of with that the in in mice not be to a ( with a of this mice are to control but the other IFN-γR1 mice are unable to control a and ultimately to the T cells from mice are unable to to in IFN-γR1 mice of and ( These that the specific to in IFN-γR1 mice be to of macrophage of IFN-γR1 mice with of the immune in other studies ( to the of with a number of of mice from to of The immune in mice to of was to the in that in the of IFN-γ the of responses is In mice with or immune responses that alter the of the and at to as to are to be mediated and by cells and IFN-γ ( This however, is not since the IFN-γ and is IFN-γ has been to have effects on on the and or in of its or ( the IFN-γ of the IFN-γR1 mice have been in a number of and ( In the of in mice this Th1/Th2 IFN-γR1 mice are to and or of as to mice in as and the and ( of IFN-γR1 (which the of in mice ( These studies the of the IFN-γ in the of is in mouse as by with II protein ( This has been as a tool for the and of in The and regulation of in mice has been to on on IFN-γ ( of leads to of that it is with cellular and the with and IFN-γR1 mice this mice to ( in other studies of IFN-γR mice a in IFN-γ have functions ( IFN-γ also to the of in the ( mice are to IFN-γR1 are This that IFN-γ can not only promote but also serve in an in In have been to IFN-γ to its ability to in a which has a on activation and proliferation ( of from mice can in the of or T cells a component in this ( number of studies of mice that are to in order to the of IFN-γ in the of and immune complex on the and the IFN-γR1 expression to mice ( IFN-γR1 mice in and therefore, immune and of as to control activation, interaction, or an intrinsic in the of IFN-γ 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mice with expression of this receptor generated with the ( In T cells to the effects of IFN-γ as to T the not in T cell the of or the of In of T cells the by have been the role IFN-γ in this This that the in of cells can of an IFN-γ signal in the of IFN-γ has been to in immune responses ( 1, 2). the effects of this cytokine on the as to the have not been The was to the role of IFN-γ signaling by cells in immune responses in a The IFN-γR1 was into the cell cells to IFN-γ ( cells to in a and a the cell cells in their to subsequent and not into a by the cell of is that cells are not in but are their In a two cell and to be to IFN-γ, a IFN-γR1 and their ability to in mouse ( cell IFN-γ antigen through both the class and class II pathways, thereby the of cells and enhancing their and by immune and ( It is also that IFN-γ the of that are through the class by inducing expression of and in the of IFN-γ cells be unable to that immunity, thereby by study the of the β chain of the IFN-γR for mediating the biologic functions of IFN-γ, generated mice a in the IFN-γR2 its expression ( 27). These mice in a with a of in IFN-γ however, was at of the cascade. from mice unable to JAK2, and STAT1 at or to in to IFN-γ on T cells from of IFN-γR2 in their ability to the the of either IFN-γ or but ( Fig. 2). the was cells with in IFN-γR2 mice have in immunity, as responses to with protein The ability of IFN-γR2 cells to class to as was also the of class to as and cells are in the of both and IFN-γ, was not in IFN-γR1 macrophage function be to the immune in IFN-γR2 β mice are to with a that primarily and their function for a responses in These studies not only the obligatory role of IFN-γR2 in transducing the IFN-γ signal and its biologic but also that IFN-γ is critical for the of cells and cells are to IFN-γ of their ability to regulate to this cytokine their impair their and the in IFN-γR2 mice in to which or T not to have a in this system, this function of In mice in either α or β chains of the IFN-γR in their immune while in T cells the IFN-γR1 to IFN-γ as as cells, T cells from mice The studies be to in the of T cells with IFN-γR2 T cells with This is with studies that have that the of the can T ( It is also that the the the in of T cells from two of mice is that the IFN-γR1 not the IFN-γ signal in T cells it have cells are to IFN-γ, cells are unable to IFN-γ signaling in to this cytokine ( These studies of and IFN-γ with only the to the effects of this cytokine ( of the components of the IFN-γ in cells cells, cells not IFN-γR2 ( ( Fig. 2). an IFN-γR2 into cells their ( These that T to IFN-γ is by expression of IFN-γR2. of the immune functions of mice that IFN-γ cell and cells be to respond to this cytokine their other their their cells IFN-γR2 and to that of IFN-γR2 and therefore the of to IFN-γ, is a in the of and in be important for cell this mice in which expression of IFN-γR2 is by the human control cells and constitutively this receptor ( The β chain the of cells, as cells cloned from mice to IFN-γ, cells Like IFN-γR2 mice and and in however, IFN-γR2 mice to have specific immunologic IFN-γR2 mice in their ability to cells in ( Fig. 2). 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