**1. Introduction**

Ebola fever or Ebola virus disease (EVD) is an acute disease resulting in high rates of mortality. It is caused by RNA-containing viruses of *Filoviridae* family, genus *Ebolavirus.* Viruses of genus *Ebolavirus* belong to five species with different fatality rates and serologic properties: Zaire ebolavirus, Sudan ebolavirus, Bundibugyo ebolavirus, Tai Forrest ebolavirus, and Reston ebolavirus. The first outbreaks of EVD were registered in 1976 initially in Zaire (currently the Democratic Republic of the Congo) in the Ebola river area (Zaire species, genus Ebola) and almost concurrently in Sudan (Sudan species, genus Ebola). After that, sporadic outbreaks were registered over a period of 40 years in Central Africa countries, affecting from one to several dozens or even hundreds of people. All those outbreaks were successfully and timely controlled. The Ebola fever outbreak in Western Africa in 2014–2015 was found to be significantly extensive. To eliminate it, efforts of several countries across the world were required [1].

The main problems the doctors met with controlling Ebola fever included the absence of a vaccine and prophylactic drugs against this disease. Despite the high fatality rate, an epidemic danger of this agent was always believed to be insignificant. Expensive development of vaccines and therapeutic drugs against rare although lethal disease in each case seemed to be unprofitable and attracted interest only due to a potential bioterrorism threat. The 2014–2015 Ebola fever outbreak claimed more than 11 thousand lives, which enforced studies on countermeasures against this infection. Currently, active studies on development of control measures against the virus are being carried out including small interfering RNA, low-molecular compounds, and antibodies [2,3], drugs based on monoclonal antibodies [3], and, certainly, vaccines. There are a number of approaches to designing vaccines against Ebola virus including DNA vaccines, subunit vaccines, as well as vaccines based on virus-like particles and viral vectors such as adenoviruses HAdV-5, HAdV-26, ChAdV-3, vesicular stomatitis virus (VSV), human cytomegalovirus, and modified vaccinia virus Ankara (MVA) [4–6]. Their protective efficacy was evaluated in non-human primate models. Furthermore, to date, several vaccines to control the virus in humans were described, i.e., rVSV-ZEBOV [7], Ad5-ZEBOV [8], GamEvac-Combi [9], and others.

The majority of developed experimental vaccines were constructed based on genetically modified viruses encoding full-length viral antigens that induce responses of both antibodies and cytotoxic T-lymphocytes (CTL) [10]. However, it should be noted that data on the protective effect of neutralizing antibodies against filoviruses obtained in studies on NHP are contradictory. It was shown that some antibodies protect animals against further infection but fail to neutralize the virus, while others neutralize the virus but fail to protect animals [11]. Consequently, the relative significance of neutralizing antibodies compared with those that can provide protection using other mechanisms (e.g., antibody-dependent cell cytotoxicity or Fc-dependent mechanisms) is still unclear. Besides this, the question deserves to be asked about the role of non-neutralizing antibodies during protection against Ebola virus, considering the well-known effect of antibody-dependent enhancement of infection [12].

A number of Ebola virus vaccine candidates develop base on glycoprotein (GP). However, antibodies induced by such GP vaccines are typically autologous and limited to the other members of the same species. In contrast, T-cell vaccines designed on the basis of conservative regions of the filovirus proteins can protect against different members of the filovirus family. It was shown that simian adenovirus- and poxvirus MVA-vectored vaccines encoding cross-filovirus immunogen provided broad immunogenicity and a solid protection of the BALB/c and C57BL/6J mice against high, lethal challenges with Ebola and Marburg viruses, two distant members of the family [5].

Another promising trend in virus T-cell vaccine design is DNA vaccine. Compared with virus-vectored vaccines, DNA vaccines demonstrate a number of advantages [13]. They are inexpensive, non-infective, and simply produced in large quantities; they can be reused since previously existing immunity is of no importance for DNA-vectors compared to viral vectors. In addition, DNA-vaccination provides the most natural way of antigen presentation by both MHC class I and class II molecules, focusing the immune response only on antigen of interest, providing long-term persistence of immunogenicity, polarizing T-helper cells toward type 1 or type 2, and inducing protective humoral and cellular immune responses. The other important feature of DNA vaccines is the ability to put several antigens or several epitopes from different antigens in the plasmid, resulting in immunization against all of the agents; and a mixture of DNA plasmids can be used to form a broad spectrum of vaccines. At last, in vivo expression of gene of interest ensures the protein resembles the normal eukaryotic structure more closely, with accompanying post-translational modifications. The only disadvantage of DNA vaccine is their relatively low immunogenicity; thus, they require administration of several doses to achieve the desirable immunity [14]. However, this disadvantage can be evaded at present due to the strategy of intramuscular electroporation making it possible to significantly enhance DNA-vaccination efficiency [15–17]. In addition, to increase the immunogenicity of DNA vaccines it is possible to use other methods for delivery, for example, using VLPs [18] that include DNA plasmid, as well as by translating DNA vaccines into RNA vaccine format [19].

One of the promising trends in virus vaccine design is DNA vaccine encoding artificial polyepitope immunogens. These vaccines comprise a combination of conservative cytotoxic T-lymphocytes (CTL)- and T-helper (Th)-epitopes selected from different viral proteins and combined in one molecule [20–22]. Progress in identifying T-cell epitopes, as well as understanding the mechanisms of processing and presentation of antigens by Major Histocompatibility Complex (MHC) class I and II pathways are instrumental for rational designing of artificial polyepitope vaccines inducing responses of cytotoxic (CD8+) and helper (CD4+) T-lymphocytes [20,22,23].

This study aims to design artificial polyepitope T-cell immunogens—candidate DNA vaccines against Ebola virus using computer-aided molecular design, and to study their capacity to induce a specific immune response in a laboratory animals model.
