Immunization of Pigs by DNA Prime and Recombinant Vaccinia Virus Boost To Identify and Rank African Swine Fever Virus Immunogenic and Protective Proteins

Abstract

African swine fever virus (ASFV) causes an acute hemorrhagic fever in domestic pigs, with high socioeconomic impact. No vaccine is available, limiting options for control. Although live attenuated ASFV can induce up to 100% protection against lethal challenge, little is known of the antigens which induce this protective response. To identify additional ASFV immunogenic and potentially protective antigens, we cloned 47 viral genes in individual plasmids for gene vaccination and in recombinant vaccinia viruses. These antigens were selected to include proteins with different functions and timing of expression. Pools of up to 22 antigens were delivered by DNA prime and recombinant vaccinia virus boost to groups of pigs. Responses of immune lymphocytes from pigs to individual recombinant proteins and to ASFV were measured by interferon gamma enzyme-linked immunosorbent spot (ELISpot) assays to identify a subset of the antigens that consistently induced the highest responses. All 47 antigens were then delivered to pigs by DNA prime and recombinant vaccinia virus boost, and pigs were challenged with a lethal dose of ASFV isolate Georgia 2007/1. Although pigs developed clinical and pathological signs consistent with acute ASFV, viral genome levels were significantly reduced in blood and several lymph tissues in those pigs immunized with vectors expressing ASFV antigens compared with the levels in control pigs.IMPORTANCE The lack of a vaccine limits the options to control African swine fever. Advances have been made in the development of genetically modified live attenuated ASFV that can induce protection against challenge. However, there may be safety issues relating to the use of these in the field. There is little information about ASFV antigens that can induce a protective immune response against challenge. We carried out a large screen of 30% of ASFV antigens by delivering individual genes in different pools to pigs by DNA immunization prime and recombinant vaccinia virus boost. The responses in immunized pigs to these individual antigens were compared to identify the most immunogenic. Lethal challenge of pigs immunized with a pool of antigens resulted in reduced levels of virus in blood and lymph tissues compared to those in pigs immunized with control vectors. Novel immunogenic ASFV proteins have been identified for further testing as vaccine candidates.

Current status and evolving approaches to African swine fever vaccine development

“…In this experiment, about 47 viral genes were cloned into individual plasmids as a DNA vaccine and also into recombinant vaccinia virus as a boosting/booster vaccine. Though the vaccinated pigs developed clinical and pathological signs post‐challenge, the viral loads were significantly reduced in blood and several lymphoid tissues as compared to the control group (Jankovich et al, ). This massive and systematic screening of antigens would be a roadmap (summarized in Figure ) to discover immunogenic and optimum complementary antigens for the development of novel ASFV subunit vaccine modalities.…”

“…Approaches to subunit ASF vaccine development: Hypothetical or target genes of the virus can be amplified by PCR and cloned into vectors for immunization. Then, groups of pigs can be immunized with pool of ORFs (Jankovich et al, ; McGuire et al, ). The cloned antigens from each pool can be further evaluated based on the results of the immune response…”

“…Broadly, more than fifty immunogenic ASFV proteins have been identified, and recently 44 virus‐packaged polypeptides have been reported. Nevertheless, the immunogenicity of the newly identified proteins remains unknown (Alejo et al, ; Jankovich et al, ). To develop ASF subunit vaccine platform, the pertinent ASFV protective antigens and the existing virus strain diversity need to be explored (Rock, ).…”

“…Hence, through a prime-boost vaccination, the immunogenicity of multiple ASFV antigens has been screened. Immunizations of pigs with three antigens B646L (p72), E183L (p54) and O61R (p12) (Jankovich et al, 2018). This massive and systematic screening of antigens would be a roadmap (summarized in Figure 1) to discover immunogenic and optimum complementary antigens for the development of novel ASFV subunit vaccine modalities.…”

“…Broadly, more than fifty immunogenic ASFV proteins have been identified, and recently 44 virus-packaged polypeptides have been reported. Nevertheless, the immunogenicity of the newly identified proteins remains unknown (Alejo et al, 2018;Jankovich et al, 2018).…”