Immunobiological and molecular genetic properties of non-hemadsorbing African swine fever virus strains (review)

African swine fever virus (ASF, Asfivirus , Asfarviridae ) is the most serious problem for the swine industry worldwide. The proposed review presents the results of the study of non-hemadsorbing strains of the African swine fever virus (ASF, African swine fever virus). According to published data, most of the non-hemadsorbing strains of the ASF virus isolated in nature or obtained in laboratory conditions are weak or avirulent and have the property of forming immunological protection against homologous virulent hemadsorbing isolates or strains in subsequent infection of pigs (J.D. Vigário et al., 1970). On the African continent, avirulent non-hemadsorbent strains of ASF virus were usually isolated from persistently infected warthogs ( Phacochoerus spp.), bush pigs ( Potomochorus porcus ) and soft mites Ornithodoros moubata (A. Pini, 1976; G.R. Thomson et al., 1979). In Europe (Portugal, Spain) and Asia (China) - from persistently infected domestic pigs ( Sus scrofa domesticus ), wild boars ( Sus scrofa ) and from Ornithodoros erraticus ( marocanus ) ticks (F.S. Boinas et al., 2004; C. Gallardo et al., 2019; Sun E. et al., 2021). The review focuses on the use of non-hemadsorbing strains in order to study immunological mechanisms of protection against ASF. In experiments with the OURT88/3 strain, CD8+ T-cells were shown to have an important role in immunological protection against ASF. The cross-protection induced by the OURT88/3 strain against infection with virulent isolates of unrelated genotypes correlated with the ability of these isolates to specifically stimulate the production of IFNγ by lymphocytes of the immunized pigs (C.C. Abrams et al., 2013). Experiments with the non-hemadsorbing strain NH/P68 demonstrated that a high levels of specific antibodies to the ASF virus is characteristic to the chronic form of the disease, while low levels of antibodies were noted in asymptomatic pigs after intranasal and intramuscular immunization (A. Leitão et al., 2001; C. Gallardo et al., 2019). The low pathogenicity of non-hemadsorbing isolates is associated with the loss of virulence factors due to large deletions close to the left end of the genome or smaller deletions and substitutions in genes encoding virulence factors elsewhere in the genome (F.S. Boinas et al., 2004). The loss of the hemadsorbing properties of the ASF virus is associated with deletions and/or a shift in the reading frame in the EP402R gene (R.J. Rowlands et al., 2009; R. Portugal et al., 2015; K.A. Mima et al., 2015). In terms of possible practical application of non-hemadsorbing strains this paper presents results on reducing adverse clinical reactions in pigs inoculated with deletion mutants of strains OURT88/3 and NH/P68 (M.L. Nogal et al., 2001; C. Hurtado et al., 2004; A.G. Granja et al., 2009). Naturally attenuated non-hemadsorbent strains of the ASF virus are used in research focused on the creation of candidate live vaccines. In experiments conducted with their use, up to a 100 % protection against homologous virulent isolates and strains of ASF virus was obtained in domestic pigs (K. King et al., 2011; P.J. Sánchez-Cordón et al., 2017; C. Gallardo et al., 2018; C. Gallardo et al., 2019; P.J. Sanchez-Cordon et al., 2020) and wild boars (J.A. Barasona et al., 2019).