Interferon gamma


Interferon gamma is a dimerized soluble cytokine that is the only member of the type II class of interferons. The existence of this interferon, which early in its history was known as immune interferon, was described by E. F. Wheelock as a product of human leukocytes stimulated with phytohemagglutinin, and by others as a product of antigen-stimulated lymphocytes. It was also shown to be produced in human lymphocytes. or tuberculin-sensitized mouse peritoneal lymphocytes challenged with Mantoux test ; the resulting supernatants were shown to inhibit growth of vesicular stomatitis virus. Those reports also contained the basic observation underlying the now widely employed interferon gamma release assay used to test for tuberculosis. In humans, the IFNG protein is encoded by the IFNG gene.
Through cell signaling, interferon gamma plays a role in regulating the immune response of its target cell. A key signaling pathway that is activated by type II IFN is the JAK-STAT signaling pathway. IFNG plays an important role in both innate and adaptive immunity. Type II IFN is primarily secreted by CD4+ T helper 1 cells, natural killer cells, and CD8+ cytotoxic T cells. The expression of type II IFN is upregulated and downregulated by cytokines. By activating signaling pathways in cells such as macrophages, B cells, and CD8+ cytotoxic T cells, it is able to promote inflammation, antiviral or antibacterial activity, and cell proliferation and differentiation. Type II IFN is serologically different from interferon type 1, binds to different receptors, and is encoded by a separate chromosomal locus. Type II IFN has played a role in the development of cancer immunotherapy treatments due to its ability to prevent tumor growth.

Function

IFNG, or type II interferon, is a cytokine that is critical for innate and adaptive immunity against viral, some bacterial and protozoan infections. IFNG is an important activator of macrophages and inducer of major histocompatibility complex class II molecule expression. Aberrant IFNG expression is associated with a number of autoinflammatory and autoimmune diseases. The importance of IFNG in the immune system stems in part from its ability to inhibit viral replication directly, and most importantly from its immunostimulatory and immunomodulatory effects. IFNG is produced predominantly by natural killer cells and natural killer T cells as part of the innate immune response, and by CD4 Th1 and CD8 cytotoxic T lymphocyte effector T cells once antigen-specific immunity develops as part of the adaptive immune response. IFNG is also produced by non-cytotoxic innate lymphoid cells, a family of immune cells first discovered in the early 2010s.
The primary cells that secrete type II IFN are CD4+ T helper 1 cells, natural killer cells, and CD8+ cytotoxic T cells. It can also be secreted by antigen presenting cells such as dendritic cells, macrophages, and B cells to a lesser degree. Type II IFN expression is upregulated by the production of interleukin cytokines, such as IL-12, IL-15, IL-18, as well as type I interferons. Meanwhile, IL-4, IL-10, transforming growth factor-beta and glucocorticoids are known to downregulate type II IFN expression.
Type II IFN is a cytokine, meaning it functions by signaling to other cells in the immune system and influencing their immune response. There are many immune cells type II IFN acts on. Some of its main functions are to induce IgG isotype switching in B cells; upregulate major histocompatibility complex class II expression on APCs; induce CD8+ cytotoxic T cell differentiation, activation, and proliferation; and activate macrophages. In macrophages, type II IFN stimulates IL-12 expression. IL-12 in turn promotes the secretion of IFNG by NK cells and Th1 cells, and it signals naive T helper cells to differentiate into Th1 cells.

Structure

The IFNG monomer consists of a core of six α-helices and an extended unfolded sequence in the C-terminal region. This is shown in the structural models below. The α-helices in the core of the structure are numbered 1 to 6.
Image:IFN2.jpeg|350px|none|thumb|Figure 1. Line and cartoon representation of an IFN-γ monomer.
The biologically active dimer is formed by anti-parallel inter-locking of the two monomers as shown below. In the cartoon model, one monomer is shown in red, the other in blue.
Image:IFN3.jpeg|350px|thumb|none|Figure 2. Line and cartoon representation of an IFN-γ dimer.

Receptor binding

Cellular responses to IFNG are activated through its interaction with a heterodimeric receptor consisting of Interferon gamma receptor 1 and Interferon gamma receptor 2. IFN-γ binding to the receptor activates the JAK-STAT pathway. Activation of the JAK-STAT pathway induces upregulation of interferon-stimulated genes, including MHC II. IFNG also binds to the glycosaminoglycan heparan sulfate at the cell surface. However, in contrast to many other heparan sulfate binding proteins, where binding promotes biological activity, the binding of IFNG to HS inhibits its biological activity.
The structural models shown in figures 1-3 for IFNG are all shortened at their C-termini by 17 amino acids. Full length IFNG is 143 amino acids long, the models are 126 amino acids long. Affinity for heparan sulfate resides solely within the deleted sequence of 17 amino acids. Within this sequence of 17 amino acids lie two clusters of basic amino acids termed D1 and D2, respectively. Heparan sulfate interacts with both of these clusters. In the absence of heparan sulfate the presence of the D1 sequence increases the rate at which IFNG-receptor complexes form. Interactions between the D1 cluster of amino acids and the receptor may be the first step in complex formation. By binding to D1 HS may compete with the receptor and prevent active receptor complexes from forming.
The biological significance of heparan sulfates interaction with IFNG is unclear; however, binding of the D1 cluster to HS may protect it from proteolytic cleavage.

Signaling

IFNG binds to the type II cell-surface receptor, also known as the IFN gamma receptor which is part of the class II cytokine receptor family. The IFNGR is composed of two subunits: the IFNGR1 and IFNGR2. IFNGR1 is associated with JAK1 and IFNGR2 is associated with JAK2. Upon IFNG binding the receptor, IFNGR1 and IFNGR2 undergo conformational changes that result in the autophosphorylation and activation of JAK1 and JAK2. This leads to a signaling cascade and eventual transcription of target genes. The expression of 236 different genes has been linked to type II IFN-mediated signaling. The proteins expressed by type II IFN-mediated signaling are primarily involved in promoting inflammatory immune responses and regulating other cell-mediated immune responses, such as apoptosis, intracellular IgG trafficking, cytokine signaling and production, hematopoiesis, and cell proliferation and differentiation.

JAK-STAT pathway

One key pathway triggered by IFNG binding IFNGRs is the Janus Kinase and Signal Transducer and Activator of Transcription pathway, more commonly referred to as the JAK-STAT pathway. In the JAK-STAT pathway, activated JAK1 and JAK2 proteins regulate the phosphorylation of tyrosine in STAT1 transcription factors. The tyrosines are phosphorylated at a very specific location, allowing activated STAT1 proteins to interact with each other come together to form STAT1-STAT1 homodimers. The STAT1-STAT1 homodimers can then enter the cell nucleus. They then initiate transcription by binding to gamma interferon activation site elements, which are located in the promoter region of Interferon-stimulated genes that express for antiviral effector proteins, as well as positive and negative regulators of type II IFN signaling pathways.
The JAK proteins also lead to the activation of phosphatidylinositol 3-kinase. PI3K leads to the activation of protein kinase C delta type which phosphorylates the amino acid serine in STAT1 transcription factors. The phosphorylation of the serine in STAT1-STAT1 homodimers are essential for the full transcription process to occur.

Other signaling pathways

Other signaling pathways that are triggered by IFNG are the mTOR signaling pathway, the MAPK signaling pathway, and the PI3K/AKT signaling pathway.

Biological activity

IFNG is secreted by T helper cells, cytotoxic T cells, macrophages, mucosal epithelial cells and NK cells. IFNG is both an important autocrine signal for professional APCs in early innate immune response, and an important paracrine signal in adaptive immune response. The expression of IFNG is induced by the cytokines IL-12, IL-15, IL-18, and type I IFN. IFNG is the only Type II interferon and it is serologically distinct from Type I interferons; it is acid-labile, while the type I variants are acid-stable.
IFNG has antiviral, immunoregulatory, and anti-tumor properties. It alters transcription in up to 30 genes producing a variety of physiological and cellular responses. Among the effects are:
  • Promotes NK cell activity
  • Increases antigen presentation and lysosome activity of macrophages.
  • Activates inducible nitric oxide synthase
  • Induces the production of IgG2a and IgG3 from activated plasma B cells
  • Causes normal cells to increase expression of class I MHC molecules as well as class II MHC on antigen-presenting cells—to be specific, through induction of antigen processing genes, including subunits of the immunoproteasome, as well as TAP and ERAAP in addition possibly to the direct upregulation of MHC heavy chains and B2-microglobulin itself
  • Promotes adhesion and binding required for leukocyte migration
  • Induces the expression of intrinsic defense factors—for example, with respect to retroviruses, relevant genes include TRIM5alpha, APOBEC, and Tetherin, representing directly antiviral effects
  • Primes alveolar macrophages against secondary bacterial infections.
IFNG is the primary cytokine that defines Th1 cells: Th1 cells secrete IFNG, which in turn causes more undifferentiated CD4+ cells to differentiate into Th1 cells, representing a positive feedback loop—while suppressing Th2 cell differentiation.
NK cells and CD8+ cytotoxic T cells also produce IFNG. IFNG suppresses osteoclast formation by rapidly degrading the RANK adaptor protein TRAF6 in the RANK-RANKL signaling pathway, which otherwise stimulates the production of NF-κB.