This article summarizes the study results on the generation of attenuated strains of African swine fever virus (ASFV) of seroimmunotypes I–VIII and the creation of live vaccines for temporary protection of pigs during a period of epizootics in the surveillance zone (a zone adjacent to the area of outbreak). These studies were initiated at the Federal Research Center for Virology and Microbiology (FRCVM, formerly VNIIVViM) at the time of introduction of the pathogen to the Iberian Peninsula in the middle of the 20th century. The developed experimental vaccines against ASFV seroimmunotypes I–V provided protection against virulent strains of homologous seroimmunotypes by day 14 after vaccination, lasting at least four months.

Understanding the immunological mechanisms of protection and the viral proteins involved in the induction of a protective immune response to the African swine fever virus (ASFV) is still limited. In the last years, the CD2v protein (gp110-140) of the ASFV has been proven to be a serotype-specific protein. Current work is devoted to the investigation of the possibility of creating protection against virulent ASFV strain Mozambique-78 (seroimmunotype III) in pigs previously vaccinated with vaccine strain FK-32/135 (seroimmunotype IV) and then immunized with the pUBB76A_CD2v plasmid, containing a chimeric nucleotide sequence from the CD2v protein gene (EP402R, nucleotides from 49 to 651) from the MK-200 strain (seroimmunotype III). Vaccination with the ASFV vaccine strain FK-32/135 protects pigs from the disease caused by the strain with homologous seroimmunotype—France-32 (seroimmunotype IV). Our attempt to create balanced protection against virulent strain Mozambique-78 (seroimmunotype III) by induction of both humoral factors of immunity (by vaccination with strain FK-32/135 of seroimmunotype IV) and serotype-specific cellular immunity (by immunization with the plasmid pUBB76A_CD2v of seroimmunotype III) was unsuccessful.

African swine fever (ASF) is an infectious disease of domestic and wild pigs of all breeds and ages, with the acute form of the disease being characterized by high fever, hemorrhages in the reticuloendothelial system and a high mortality rate. Registered safe and efficacious ASF vaccines are not available. The development of experimental ASF vaccines, particularly live attenuated, have considerably intensified in the last years. There is much variability in experimental approaches undertaken by laboratories attempting to develop first generation vaccines, rendering it difficult to interpret and make comparisons across trials. ASF virus (ASFV) genotyping does not fully correlate with available cross-protection data and may be of limited value in predicting cross-protective vaccine efficacy. Recently, ASFV strains were assigned to a respective nine groups by seroimmunotype (from I to IX): in vivo the grouping is based on results of cross protection of pigs survived after their infection with a virulent strain (bioassay), while in vitro this grouping is based on hemadsorption inhibition assay (HADIA) data. Here we demonstrate the antigenic and protective properties of two attenuated ASFV strains MK200 and FK-32/135. Pronounced differences in the HADIA and in immunological test in animals allow us to consider them and the corresponding reference virulent strains of the ASFV of Mozambique-78 (seroimmunotype III, genotype V) and France-32 (seroimmunotype IV, genotype I) as useful models for studying the mechanisms of protective immunity and evaluation of the candidate vaccines.

This review presents comparative results of simultaneously conducted studies on proteins responsible for the haemadsorption and serotype-specific properties of African swine fever virus (ASFV). An ASFV gene EP402R (or LMW8-DR) encoding protein CD2v homologous to murine, human or porcine T-cell adhesive receptor was found. The CD2v was shown to be directly involved into a haemadsorption process, and expressed in ASFV-infected cells as a glycoprotein with a molecular weight of approximately 105-110 kDa. In the presence of a glycosylation inhibitor, tunicamycin, its molecular weight is about 42 kDa. In ASFV-infected cells labeled with 3H-glucosamine or 14C-sodium acetate, a virus-specific major glycoprotein with a molecular weight of 110-140 kDa (gp 110-140) was identified using radioimmunoprecipitation assay. Using ASFV reference strains belonging to seroimmunotypes I-IV and the corresponding antisera active in haemadsorption inhibition assay (HADIA), we determined that gp 110-140 defines the serotype specificity. Genotyping on the basis of the genetic locus encoding the CD2v and a C-type lectin protein also showed a concurrence with the grouping of ASFV isolates and strains based on their seroimmunotypes. Immunization of pigs with the gp 110-140 within liposomes, or a recombinant haemagglutinin (CD2v) protected 67 to 100% of animals from death due to their subsequent infection with homologous virulent ASFV strains. Based on the physico-chemical and biological characteristics of the gp 110-140 and CD2v it is suggested that they are one and the same virus-specific glycoprotein crucial for induction of the immunological protection against ASF.Key words: ASFV; seroimmunotypes; serotype; glycoproteins; gp 110-140; CD2v; protectivity SEROTIPNO-SPECIFIČEN IN HEMADSORPCIJSKI PROTEIN VIRUSA AFRIŠKE PRAŠIČJE KUGE A.DPovzetek: Pregledni članek predstavlja primerjavo rezultatov sočasno izvedenih raziskav o beljakovinah, ki so pomembne za hemadsorpcijo in serotipno specifične lastnosti virusa afriške prašičje kuge (ASFV; iz angl. african swine fever virus). Pri virusu ASFV je bil odkrit gen EP402R (imenovan tudi LMW8-DR), ki kodira beljakovino CD2v, ki je homologna glodavskemu, človeškemu in prašičjemu T-celičnemu adhezivnemu receptorju. Pokazalo se je, da je CD2v neposredno vpletena v proces hemadsorpcije in je izražena v celicah, okuženih z ASFV kot glikoprotein z molekulsko maso okrog 105-110 kDa. V prisotnosti zaviralca glikozilacije tunicamicina je njegova molekulska masa približno 42 kDa. V celicah, okuženih z ASFV, označenih s 3H-glukozaminom ali 14C-natrijevim acetatom, je bil s testom radioimunoprecipitacije ugotovljen virusno specifični osrednji glikoprotein z molekulsko maso 110-140 kDa (gp 110-140). Z uporabo referenčnih sevov ASFV, ki pripadajo seroimunotipom I-IV, in ustreznim antiserumom, ki so bili aktivni pri preizkusu zaviranja hemadsorpcije (HADIA), smo ugotovili, da gp 110-140 določa specifičnost serotipa. Genotipizacija na osnovi genskega lokusa, ki kodira CD2v in C-tip lektinske beljakovine, je pokazala soizražanje s skupino izolatov in sevov ASFV na podlagi njihovih seroimunotipov. Imunizacija prašičev z gp 110-140 v liposomih ali z rekombinantnim hemaglutininom (CD2v) je zaščitila od 67 do 100 odstotkov živali pred smrtjo zaradi njihove naknadne okužbe z virulentnimi sevi ASFV. Na podlagi fizikalno-kemičnih in bioloških značilnosti beljakovin gp 110-140 in CD2v menimo, da gre za isti virusni glikoprotein, ki je ključnega pomena za vzpodbuditev imunološke zaščite pred ASF.Ključne besede: ASFV; seroimunotipi; serotip; glikoproteini; gp 110-140; CD2v; zaščita

Three recombinant plasmid constructs, expressing chimeric proteins containing human ubiquitin fused to an ectodomain of one of the potentially protective proteins (p30, p54 and CD2v) of the attenuated MK-200 strain of African swine fever virus (ASFV), were created as potential inductors of specific antiviral cellular immunity. Three-time immunization of pigs with the mixture of these plasmids led to the formation of virus-specific cytotoxic T-lymphocytes (CTL), but did not induce production of virus-specific antibodies. After challenge with the homologous parental virulent ASFV strain M-78 at a dose of 10 ³ HAD ₅₀ , all five animals (four immunized pigs and one naïve) fell between the 4th and 7th days post infection. The obtained results demonstrated that induction of CTL did not protect pigs against challenge with the virulent ASFV. Balanced activation of CTL and antibody-mediated cellular mechanisms should be investigated.