African swine fever virus vaccine development

Abstract

African swine fever (ASF) is a highly lethal hemorrhagic fever of domestic and wild pigs caused by African swine fever virus (ASFV), the only known DNA arbovirus and the sole member of the Asfarviridae family. Unfortunately, despite ongoing virus spread in several regions in the world, there is no safe and efficacious vaccine. In this study, replication-competent adenovirus-vectored multicistronic expression cassettes encoding nearly the entire ASFV proteome were developed. The virus constructs replicated in primary swine alveolar epithelial cells and expressed authentic ASFV antigens. Cocktails of the recombinant virus constructs, designated RC-Ad5-ASFV, formulated with or without Quil-A adjuvant, were well tolerated by commercial piglets following homologous prime-boost immunization and no shedding of the recombinant virus was detected. The RC-Ad5-ASFV cocktail rapidly elicited significant (p < 0.0001) antigen-specific IgG antibody responses that were significantly (p < 0.05) recalled upon boost. Notably, sera from the RC-Ad5-ASFV-vaccinees, but not from RC-Ad5-GFP negative controls, strongly recognized primary swine cells infected with ASFV (Georgia 2007/1), but not uninfected cells. Interestingly, five out of six pigs immunized with non-adjuvanted RC-Ad5-ASFV cocktail (G3) survived (p<0.0001) following challenge by contact with naïve pigs shedding the highly virulent ASFV (Georgia 2007/1) genotype-II strain. In contrast, all the pigs immunized with the RC-Ad5-ASFV cocktail formulated with Quil-A adjuvant (G1) or a cocktail lacking structural antigens formulated with the Quil-A adjuvant (G2), succumbed to the challenge. Notably, the survivors cleared the challenge virus, had no-to-mild clinical scores, gained ~2 lbs./day post-challenge, and they were healthy at study termination on day 39 after initiation of contact challenge. In addition, the survivors had mild gross and histological lesions, but all the non-survivors, include the RC-Ad5-GFP negative controls as well as the shedders, exhibited severe acute ASF characterized by lymphoid depletion, vasculopathy, and coagulopathy. Even though all the sera from the RC-Ad5-ASFV vaccinees recognized ASFV, they failed to neutralize in vitro infection of primary swine macrophages. Interestingly, ASFV-specific GrB+CD8α+ T-cell responses were detected in PBMCs of the survivors on days 26 and 33 post-challenge, and in terminal splenocytes on day 39, suggesting a potential role for cellular immunity in clearance of infected cells. There was no correlation between SLA-I and -II haplotypes and survival. The promising outcome from this study supports empirical identification of ASFV antigens that elicit GrB+CD8α+ T-cell responses to inform rational development and in vivo validation of a second-generation live-vectored prototype subunit vaccine.