Immunization of pigs with replication-incompetent adenovirus-vectored African swine fever virus multi-antigens induced humoral immune responses but no protection following contact challenge

Abstract

IntroductionAfrican swine fever virus (ASFV) is a pathogen of great economic importance given that continues to threaten the pork industry worldwide, but there is no safe vaccine or treatment available. Development of a vaccine is feasible as immunization of pigs with some live attenuated ASFV vaccine candidates can confer protection, but safety concerns and virus scalability are challenges that must to be addressed. Identification of protective ASFV antigens is needed to inform the development of efficacious subunit vaccines.MethodsIn this study, replication-incompetent adenovirus-vectored multicistronic ASFV antigen expression constructs that covered nearly 100% of the ASFV proteome were generated and validated using ASFV convalescent serum. Swine were immunized with a cocktail of the expression constructs, designated Ad5-ASFV, alone or formulated with either Montanide ISA-201™ (ASFV-ISA-201) or BioMize® adjuvant (ASFV-BioMize).ResultsThese constructs primed strong B cell responses as judged by anti-pp62-specific IgG responses. Notably, the Ad5-ASFV and the Ad5-ASFV ISA-201, but not the Ad5-ASFV BioMize®, immunogens primed significantly (p < 0.0001) higher anti-pp62-specific IgG responses compared with Ad5-Luciferase formulated with Montanide ISA-201™ adjuvant (Luc-ISA-201). The anti-pp62-specific IgG responses underwent significant (p < 0.0001) recall in all the vaccinees after boosting and the induced antibodies strongly recognized ASFV (Georgia 2007/1)-infected primary swine cells. However, following challenge by contact spreaders, only one pig nearly immunized with the Ad5-ASFV cocktail survived. The survivor had no typical clinical symptoms, but had viral loads and lesions consistent with chronic ASF.DiscussionBesides the limited sample size used, the outcome suggests that in vivo antigen expression, but not the antigen content, might be the limitation of this immunization approach as the replication-incompetent adenovirus does not amplify in vivo to effectively prime and expand protective immunity or directly mimic the gene transcription mechanisms of attenuated ASFV. Addressing the in vivo antigen delivery limitations may yield promising outcomes.

An Oil-Based Adjuvant Improves Immune Responses Induced by Canine Adenovirus-Vectored Vaccine in Mice

“…While previous studies have confirmed the efficacy and safety of ISA 201 in potentiating immune responses with recombinant antigens [ 12 , 13 , 20 , 21 ], our study provides evidence of its potential benefits for live recombinant viral-vectored vaccines. In contrast, Zajac et al reported a marginal increase in the IgG response to the pp62 antigen when an experimental African swine fever virus vaccine, based on the human adenovirus, was combined with ISA 201 [ 22 ]. Nonetheless, the interpretation of this finding is hindered by several factors, including the use of a cocktail of 42 adenoviral vectors expressing multiple ASFV antigens and the potential for interference with the host’s immune system.…”

Advancement in the development of gene/protein-based vaccines against African swine fever virus

“…Appropriate immune adjuvants can nonspecifically enhance or modify the body's adaptive immune response to the corresponding antigen, thereby boosting the immunogenicity of the vaccine. The Ad5-ASFV adenovirus vector vaccine prepared with BioMize (VaxLiant, NE, USA) and ISA-201™ (Seppic, NJ, USA) adjuvants was used to immunize piglets with a cocktail therapy, and the results showed that the pp62-specific antibody were significantly higher in the ISA-201™ group compared to the BioMize adjuvant group ( Zajac et al, 2023 ). Chen et al used the new Porcine reproductive and respiratory syndrome virus (PRRSV)-specific IgM monoclonal antibody (Mab)-PR5nf1 as a vaccine adjuvant to study the efficacy of in enhancing the protection ( Chen et al, 2022a ).…”

Saccharomyces cerevisiae oral immunization in mice using multi-antigen of the African swine fever virus elicits a robust immune response

“…Live attenuated ASFV vaccines require robust safety testing, before being considered safe for widespread use ( 16 – 19 ). Subunit vaccines have limited efficacy because of the lack of effective delivery systems and knowledge about protective antigens ( 20 , 21 ). A previous study demonstrated that a mixture of eight viral antigens protected pigs against a lethal dose of ASFV ( 22 ).…”