Association of the Host Immune Response with Protection Using a Live Attenuated African Swine Fever Virus Model

Abstract

African swine fever (ASF) is a lethal hemorrhagic disease of swine caused by a double-stranded DNA virus, ASF virus (ASFV). There is no vaccine to prevent the disease and current control measures are limited to culling and restricting animal movement. Swine infected with attenuated strains are protected against challenge with a homologous virulent virus, but there is limited knowledge of the host immune mechanisms generating that protection. Swine infected with Pretoriuskop/96/4 (Pret4) virus develop a fatal severe disease, while a derivative strain lacking virulence-associated gene 9GL (Pret4Δ9GL virus) is completely attenuated. Swine infected with Pret4Δ9GL virus and challenged with the virulent parental virus at 7, 10, 14, 21, and 28 days post infection (dpi) showed a progressive acquisition of protection (from 40% at 7 dpi to 80% at 21 and 28 dpi). This animal model was used to associate the presence of host immune response (ASFV-specific antibody and interferon (IFN)-γ responses, or specific cytokine profiles) and protection against challenge. With the exception of ASFV-specific antibodies in survivors challenged at 21 and 28 dpi, no association between the parameters assessed and protection could be established. These results, encompassing data from 65 immunized swine, underscore the complexity of the system under study, suggesting that protection relies on the concurrence of different host immune mechanisms.

Deletion of CD2-Like (CD2v) and C-Type Lectin-Like (EP153R) Genes from African Swine Fever Virus Georgia-∆9GL Abrogates Its Effectiveness as an Experimental Vaccine

“…While animals inoculated with ASFV-G-Δ9GL presented, at the time of challenge, an average antibody titer of log 10 4.00 (SD ± 0.00) antibody titers in those inoculated with ASFV-G-Δ9GL/ΔCD2v or ASFV-G-Δ9GL/ΔCD2v/ΔEP153R were much lower (log 10 2, SD ± 0.82 and log 10 1.5, SD ± 0.58, respectively). As we previously reported, in our experience, the induction of virus-specific antibodies was directly related to the presence of the replication of all the attenuated viruses tested [ 3 , 4 , 6 , 7 , 9 , 10 , 21 ].…”

“…The host immune mechanisms responsible for protection against ASF are still under discussion. In our experience, the only immune mechanism induced by attenuated virus strains that is clearly associated with protection against challenge is the presence of virus-specific circulating antibodies [ 9 ]. In all cases, including this report, there is a strong association between the presence of ASFV-specific circulating antibodies and protection against challenge [ 3 , 4 , 5 , 9 ].…”

“…Anti-ASFV antibodies in the sera of infected animals were quantified using an in-house developed ELISA as previously described [ 9 ]. Briefly, ASFV was amplified in Vero cells [ 16 ], infected cells were then treated with Tween 80 (G-Biosciences, Saint Louis, MO, USA) and sodium deoxycholate (Sigma, Saint Louis, MO, USA) to a final concentration of 1% ( v/v ) and cell antigens were stored at ≤−70 °C.…”

“…The deletion of 9GL, a highly conserved gene across ASFV strains, produced different attenuation profiles depending on the strain considered. While Malawi [ 8 ] and Pret4 [ 9 ] isolates were completely attenuated following the deletion of 9GL, the Georgia2010 isolate (ASFV-G-Δ9GL) remained virulent when inoculated at high doses [ 5 ]. Attempts to increase the attenuation of ASFV-G-Δ9GL by including the same deletion producing attenuation in the ASFV-G-ΔMGF virus resulted in a significant reduction in a protective effect of the resulting virus, ASFV-G-Δ9GL/ΔMGF [ 7 ].…”

Safety and immunogenicity of mammalian cell derived and Modified Vaccinia Ankara vectored African swine fever subunit antigens in swine

“…Interestingly, antigen-specific IFN-␥ + T-cell responses to B646L were also observed following an adenoviral-vector homologous prime/boost immunization strategy (Lokhandwala et al, 2016). However it should be noted that the correlation between IFN-␥ + T-cell responses and host protection against ASFV remains unclear, at least in the context of using live, attenuated ASFV models (Carlson et al, 2016). The current study results also correlate with the recent study using ASFV interserotypic CD2 v (EP402R)/C-type lectin (EP153R) recombinant chimeric viruses and vaccination/challenge experiments in swine, which demonstrated partial protection as defined by delayed timeto death (3 days), onset of fever, and onset of viraemia.…”