**1.1 Epstein-Barr virus**

Epstein-Barr virus (EBV) or infectious mononucleosis virus, together with other herpesvirus infections, is a prototype of persistent viral infections characterized by latency. In the mid-1980s, the problem of chronic EBV infection or "chronic mononucleosis" attracted the attention of researchers. It has been shown that EBV causes the development of chronic or recurrent infectious mononucleosis-like symptoms in immunocompetent individuals, which persist for a long time and are characterized by constant fatigue, headaches, myalgias, lymphadenopathy, subfebrile fever (37.1–37.3°С), hepatosplenomegaly. Additionally, gastrointestinal diseases may develop hematological, neurological, and skin lesions [1].

EBV infection is accompanied by high production of IgG antibodies to viral capsid (VCA) and early antigens (EA) when compared with the control group, as well as very low production or absence of antibodies to Epstein-Barr virus nuclear antigen (EBNA) [2]. That is why it was suggested that the cause of the syndrome is a chronic EBV infection [3]. However, some patients may not have abnormally high antibody titers associated with EBV [4]. According to data published by Kanegane et al., the severity of the disease directly correlates with the level of EBV DNA copies number in serum or plasma [5]. In later studies, it was shown that an increased level of EBV DNA in the blood is a more specific criterion for chronic EBV than the levels of antibodies to EBV [6]. In 1983, Hellman et al. first proposed an abbreviation for this syndrome "Chronic Active EBV infection" (CAEBV) [7]. As a result of the analysis of the literature, including work on chronic EBV as well as work on herpesvirus infections, immunodeficiencies, and three types were identified based on clinical, pathological, and virological data characteristic of this syndrome [8, 9]:

1.Chronic fatigue syndrome of unknown etiology (CFS), which is characterized by profound, debilitating fatigue and a combination of symptoms leading to a significant decline in occupational, personal, social, and educational status;

2.CAEBV;

3. Severe CAEBV (SCAEBV).

EBV is known to spread through contact with saliva and penetrate the epithelium that lines the nasopharynx. Waldeyer's ring, which includes the adenoids, tonsils, and the lymphoid system surrounding the nasopharyngeal region, forms a continuous structure called lymphoepithelium [10]. The virus infects epithelial cells, replicates, then is released, followed by infection of resting naive B cells in nearby areas by activating latent proteins (six EBV nuclear antigens (EBNAs: EBNA1, EBNA2, EBNA3A, EBNA3B, EBNA3C, EBNA-LP), two latent membrane proteins (LMPs: LMP1, LMP2) and viral anti-apoptotic protein BHRF1) encoded by the growth program. As a result, the cell becomes a proliferating lymphoblast (lymphoblastic burst). This program leads to the expansion of EBV-infected B cells in the lymphoid tissue of the oropharynx and the appearance of infected B cells in the blood. In some infected B cells, EBV inhibits its growth transformation program, allowing the cells to enter the memory B cell pool, while the virus persists as a truly latent infection devoid of viral gene expression. Therefore, the target of the incoming virus is a resting naive B cell, which becomes infected as it passes through the epithelium. Naive B cells are continuously recirculated throughout the body, extravasating from the peripheral circulation to secondary lymphoid tissue through specialized structures called high endothelial vesicles (HEVs) located in the lymphoepithelium. Naive B cells migrate to the mantle zone of the follicles below

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

the epithelium and remain there for several days, and then return to the circulation. Infection of new naive B cells occurs in the intraepithelial layer, and not in the mantle zone [11], that is, when an infected B cell enters the follicle, it is already a blast and cannot migrate to the mantle zone. EBV infects cells through the interaction of viral glycoproteins gp350/220 with CD21 and gp42/gH/gL with class II HLA in a B cell. Thus, memory B cells are a place of long-term viral persistence, where the virus can remain throughout the patient's life, because immunological memory is formed, and the virus ceases to be pathogenic for the host since the genes that induce cell proliferation and contribute to the development of neoplastic disease are disabled. However, EBV can infect T and NK (natural killer) cells of the tonsils [12] and peripheral blood [13]. Expression of CD21 on T cells [14] and NK cells can be induced by trogocytosis and the formation of an immunological synapse that occurs when EBV contacts B cells, which leads to a possible EBV infection [15].

It has been shown that the level of infected B cells in the population ranges from 5 to 3000 in every 107 memory B cells, both in peripheral blood (on average 110/ 107) and in Waldeyer's ring (average value 175/107), then there is a virus evenly distributed throughout the ring. Thus, the level of infected cells is similar between peripheral blood and Waldeyer's ring, but 20 times lower than in other lymphoid tissue (spleen and mesenteric lymph node) [16]. The total body load in humans amounts to 104–107 (on average 0.5/106) infected memory B-cells, representing a small, stable, and most critically "safe" pool of infected cells, which guarantees long-term persistence. Only about 1% of these cells are found in the peripheral blood. One latently infected memory B cell in the amygdala can differentiate into a plasma cell and secrete a virus that infects epithelial cells. The virus constantly seeps into the oral cavity, where it mixes with saliva for about 2 minutes before swallowing. Thus, the oral cavity is a reservoir for EBV flow and not a static reservoir. About 250 cells begin to replicate in the Waldeyer ring at any given time. The oral cavity and peripheral blood are important anatomical sites for the localization and persistence of EBV infection. These two compartments are connected by oropharyngeal lymphoid tissue such as the lingual, palatine, and pharyngeal tonsils. EBV-infected B cells can re-enter the tonsils, where memory lymphocytes express characteristic sets of adhesion receptors, through which they are able to return to target organs, where they first encountered antigens. Thus, EBV-infected cells can release viral particles through lytic replication, reinfect cells in lymphoepithelial tissue, and subsequently release viruses into the oral cavity [17].

The release of viruses into plasma from different anatomical sites indicates that different viral strains can persist in different tissue compartments. Therefore, EBV can be detected in the tissues of various anatomical structures of the human body.

Despite the fact that EBV is an oncogenic virus, the vast majority of EBVinfected people do not suffer from any long-term consequences. This is due to the antiviral immune response that develops during primary infection with EBV, and further supports subsequent lifelong control to ensure the mutual coexistence of the virus and its host. Early control of EBV infection is associated with the expansion of innate immune cells (primarily NK cells) and CD8+ and CD4+ T cells, specific for a wide range of EBV proteins expressed during the lytic and latent stages of viral infection. Patients with persistent EBV infection develop a specific CD8+ T-cell response to antigens of the lytic and latent cycles, the former being more frequent. An individual lytic epitope-specific response can account for up to 2% of the total population of CD8+ T cells. The response to immediate-early antigens dominates the response to early antigens, and the response to late expressed antigens rarely develops [18]. CD8+ T cells play a major role in the formation of responses to proteins EBNA3A, 3B, and 3C. A less specific immune response develops against EBNA1, EBNA2, and LMP2. Individuals expressing HLA-B\*3801 have been shown

to have strong responses to the EBNA2 epitope, and carriers of HLA-A\*0203 have strong responses to the epitope from EBNA-LP. In persistently infected individuals the EBV-specific T cell pool contains resting antigen-expressed T cells that are not active and do not proliferate. Lymphoid markers CCR7 and CD62L, specific for the latent antigen, are expressed on T cells. The phenotype, functional profile, and clonotypic composition of TCRs specific for CD8+ T cells remain stable for many years [19]. The EBV-specific CD4+ T cell response in healthy carriers is much weaker and may be 10 times less pronounced than the CD8+ T cell response to the same antigen. EBV-specific CD4+ memory T cells share the same phenotype regardless of whether they are specific for latent or lytic antigens. CD4+ T cells do not express perforin and granzyme, and upon ex vivo stimulation, the cytokine polyfunctionality of cells increases, and TNF-alpha production predominates [20]. NK-T cells are a conserved population of congenital T cells expressing the semiinvariant Va24-Ja18/Vβ11 T cell receptor. Only one study evaluated the frequency of NK T cells in the blood during EBV infection, and it was shown that the number of NK T cells was increased in the first month of infection. A change in cellular phenotype and function was noted with an increase in the content of CD56 (bright) with a high ability to destroy EBV-infected cells. NK-T cells play an important role in the control of primary EBV infection by eliminating infected B cells and increasing the antigen-specific response of T cells through the release of immunomodulatory cytokines [21]. It has been shown that patients with primary immunodeficiency are predisposed to the development of EBV-associated disease. The presence of NK T cells reduces the EBV transformation of B cells in vitro. With EBV infection of blood lymphocytes, the previous depletion of NK T cells leads to both an increase in the number of B cells infected with EBV and an increase in the total viral load in culture. It has been suggested that NK T cells play a role in the early immune recognition of newly EBV-infected B cells [22].
