**1.2 Interferon-γ**

Interferons (IFNs) are important biological regulatory proteins called cytokines and mediators of cellular homeostatic reactions that are produced in response to viral infection inhibiting the replication of a wide range of DNA and RNA viruses, thereby creating negative feedback. Inhibition of the viral replication cycle is carried out with the help of the synthesis of viral polypeptides [23]. When IFNs are administered in vivo, the level of viremia decreases, that is, interferons can be used as antiviral drugs, and the antiviral effect is mediated both by the immune system itself and by intracellular antiviral mechanisms. All types of IFNs inhibit more than one step in the viral life cycle: viral entry and decay, viral mRNA transcription, viral protein synthesis, viral genome replication, and progeny assembly and isolation of virions. According to the amino acid sequence, IFNs are divided into three types: I, II, and III.

IFN-γ is the only representative of type II IFN. It is structurally unrelated to type I IFNs, binds to another receptor and is encoded by a separate chromosomal locus. Type II IFN or immune IFN—IFN-γ is a highly pleiotropic cytokine, secreted not in response to viral infection, but indirectly by mitogen-activated T cells and NK cells, which are the primary producers of IFN-γ during the innate and adaptive phases of the immune response to viral infection. Other cells such as B cells, NK T cells, and professional antigen-presenting cells (APCs) have now been shown to secrete IFNγ. The production of IFN-γ by monocytes/macrophages and dendritic cells acting locally is important in cell activation [24].

IFN-γ plays an important role in the activation of macrophages for the production of tumor necrosis factor-α (TNF-α), increases macrophage phagocytosis and microbicidal activity, the formation of active nitrogen and oxygen intermediates,

### *Recombinant Human Interferon-Gamma: Prospects for the Treatment of Chronic Epstein-Barr… DOI: http://dx.doi.org/10.5772/intechopen.101325*

including superoxide radicals - nitric oxide and hydrogen peroxide, which are powerful cytotoxic effectors, stimulates the Th1-T cell response and has a strong inflammatory activity. IFN-γ is the main product of Th1 cells and further shifts the immune response towards the Th1 phenotype. IFN-γ achieves this by stimulating characteristic Th1-effector mechanisms: innate cell-mediated immunity (through the activation of NK cell effector functions), specific cytotoxic immunity (through the interaction of T cells with APC), and macrophage activation. IFN-γ increases the content of lymphocytes and leads to their long-term persistence in the tissue, induces activation of the complement cascade and acute phase response, plays a role in the switch of IgG class production, and has a direct antiviral effect [24]. Normally, in the early stages of the host's immune response, IFN-γ production by NK cells, CD4+ T (Th1) cells, and CD8+ T cells is aimed at improving antigen recognition in APCs such as macrophages and dendritic cells. IFN-γ is one of the key cytokines that differentiate naive CD4 cells into Th1 effector T cells, which produce the main mediators of cellular immunity against viral and intracellular bacterial infections [25]. Together, IFN-γ and IL-12 generate a very strong Th1 response. Th1 cell-mediated immunity and Th2 cell-mediated humoral immunity are modulated by IFN-γ, which affects the differentiation of naive T cells into Th1 or Th2 cells.

When activated, almost all CD8+ T cells, NK cells, and Th1 lymphocytes produce IFN-γ, which stimulates cytokine activity and increases the expansion of low avid NK cells. Of all the interferons/cytokines of the Th1 response, IFN-γ is most strongly correlated with the Th1 response [26]. The effects induced by IFN-γ lead to increased immune surveillance. In addition, IFN-γ blocks the production of IL-4, an inducer of Th2 cell differentiation and proliferation. The synergistic effects of IL-21, IL-18, and IL-15 increase IFN-γ production. The most potent regulator of IFN-γ production is IL-15 compared to IL-21 in human NK and T cells. The cytokines IL-15 and IL-18 are produced by macrophages, while IL-21 is mainly produced by activated T cells. IFN-γ increases the expression of the HLA (major histocompatibility complex) class I and II antigen by increasing the expression of subunits, increasing the expression and activity of proteasomes, resulting in increased sensitivity of the host to an infectious pathogen and an increased ability to identify and respond to this pathogen [26]. Thus, IFN-γ has many important immunostimulatory and immunomodulatory effects.

With the development of inflammation, a high level of IFN-γ leads to the activation of both canonical and non-canonical pathways. In the canonical signaling pathway, IFN-γ dimerizes and binds to two IFN-γ receptors, which are composed of two different ligand-binding chains: high-affinity IFNGR-1 (α) with high expression and two signal-transforming low-affinity IFNGR-2 (β),—with related signaling mechanisms. The IFNGR1 and IFNGR2 chains belong to the class II cytokine receptor family. The IFNGR2 chain limits sensitivity to IFN-γ and the IFNGR1 chain is usually in excess. But the expression level of IFNGR2 can be tightly regulated depending on the state of cell differentiation or activation. Receptors are expressed on the surface of almost all cell types. The expression level is determined by the cell type and its activation status. Initially, IFN-γ binds to IFNGR1, and the formed IFNγ\*IFNGR1 complex facilitates its binding to IFNGR2, then downstream signaling pathway events are initiated [27]. IFN-γ gene transcription is induced through several mechanisms. The most studied response to IFN-γ, mediated by STAT-1 containing transcription factor GAF (gamma-activated factor), which is activated by tyrosine kinases Jak1 and Jak2 and binds to the gamma-activating sequence GAS (Gamma Activating Sequence), which is present in the promoter regions of many genes. As a result of gene activation, the formation of cellular immune response to a viral infection begins [28]. The JAK/STAT pathway is the main signaling pathway initiated by IFN-γ stimulation. Further, IFN-γ, together with one of its receptor

subunits IFNGR1 and pSTAT1, is translocated into the cytoplasmic domain in combination with endocytosis and induces gene expression by binding to GAS elements in the promoter region of inducible IFN genes [29]. When viruses inhibit the functions of STAT1 molecules, IFN-γ can independently induce a noncanonical signaling pathway [30]. That is, IFN-γ is capable of inducing gene expression in STAT1/ bone marrow macrophages, suggesting that IFN-γ acts independently of STAT-1 or in an alternative non-canonical manner. Typically, activation of noncanonical pathways occurs later, after STAT1 activation. However, there is evidence that noncanonical pathways can be activated in the absence or presence of STAT1 in a dependent manner [31]. The IFN-γ and IFN-α/β signaling pathways intersect at several levels, partially overlap, which allows cross-interaction of certain functions within the cell. This crossover mechanism is relevant because in vivo cells are not stimulated in isolation by a single cytokine, but rather a cytokine cocktail that induces gene expression through the integration of multiple signaling pathways.

When infected with a virus, IFN-γ can induce apoptosis by regulating Fas ligands to remove virus-infected cells, enhancing the expression of type I IFNs, proinflammatory cytokines, and chemokines by endothelial, epithelial cells and fibroblasts to attract macrophages, neutrophils, and T cells to the sites of infection [32]. IFN-γ can also initiate the expression of dsRNA-specific adenosine deaminase (ADAR), which inhibits viral replication by editing or disrupting the translation of viral proteins [33].

Virus infection of a cell begins with the attachment of the virus to the surface of the host cell through a receptor and/or through cell membrane molecules such as glycans. Viruses can release their genomes directly into the cell after fusion of its membrane with the plasma membrane, while other viruses enter cells through cellular endocytosis, which allows the virus to release the core virion containing the viral genome directly into the cytoplasm [34]. The isolated genome, either naked or associated with viral proteins, moves to certain regions of the cytoplasm or nucleus for its replication [35]. IFN-γ can inhibit the entry of the virus from the endosome into the cytoplasm.

Virus replication is the primary goal of the virus life cycle [36]. Suppression of any stage of the life cycle can lead to suppression of viral genome replication during viral infection. IFN-γ is a potent antiviral cytokine that interferes with various stages of the viral life cycle in stimulated cells using the following mechanisms [35]:


*Recombinant Human Interferon-Gamma: Prospects for the Treatment of Chronic Epstein-Barr… DOI: http://dx.doi.org/10.5772/intechopen.101325*
