African swine fever (ASF) is a highly lethal transboundary disease that severely impacts the swine industry. Because of the complexity of the ASF virus (ASFV), vaccine development remains challenging, highlighting the need to robustly evaluate immune responses induced by vaccine candidates and their efficacy against different viral strains. This study evaluates the utility of the hemadsorption inhibition assay (iHAD) as an in vitro method to evaluate ASFV specific immune responses, and compares its results with in vivo protection outcomes.
Serum samples were selected from wild boars involved in two separate in vivo immunization trials against the genotype II isolate Arm07. In the first trial, animals were immunized with the naturally attenuated and non-hemadsorbing strain Lv17/WB/Rie1 (Rie1), followed by a challenge with the heterologous strain Ken06.bus. In the second trial, animals were immunized with the gene-deleted vaccine candidate Lv17/WB/Rie1ΔAB (dAB). In both trials, vaccinees exhibited high protection levels against Arm07, and selected sera demonstrated high ASFV-specific antibody titers. The inhibitory activity of these sera was tested by iHAD against homologous isolates (Arm07, Rie1, dAB) and heterologous isolates (Ken06.bus, Esp70, Ken07).
90% of the animals were protected in vivo against Arm07, while 30% of the sera exhibited in vitro iHAD. For the heterologous strain Ken06.bus, 30% of the animals showed in vivo protection, whereas 80% of the sera demonstrated in vitro inhibitory activity. Sera from animals previously immunized with genotype II vaccine strains inhibited hemadsorption by the virulent strains Arm07, Ken06.bus, and the moderately virulent strain Esp70.
The iHAD correlates with the inhibitory capacity of sera from vaccinated animals but cannot be considered a definitive marker of vaccine efficacy for Rie1 and dAB strains. The in vitro results suggest the existence of complete or partial cross-protection among phylogenetically distinct ASFV strains, including Arm07, Ken06.bus, and Esp70.
