Live attenuated African swine fever viruses as ideal tools to dissect the mechanisms involved in viral pathogenesis and immune protection

Abstract

African swine fever virus (ASFV) is the causal agent of African swine fever, a hemorrhagic and often lethal porcine disease causing enormous economical losses in affected countries. Endemic for decades in most of the sub-Saharan countries and Sardinia, the risk of ASFV-endemicity in Europe has increased since its last introduction into Europe in 2007. Live attenuated viruses have been demonstrated to induce very efficient protective immune responses, albeit most of the time protection was circumscribed to homologous ASFV challenges. However, their use in the field is still far from a reality, mainly due to safety concerns. In this study we compared the course of the in vivo infection caused by two homologous ASFV strains: the virulent E75 and the cell cultured adapted strain E75CV1, obtained from adapting E75 to grow in the CV1 cell-line. Interestingly, the kinetics of both viruses not only differed on the clinical signs that they caused and in the virus loads found, but also in the immunological pathways activated throughout the infections. Furthermore, E75CV1 confirmed its protective potential against the homologous E75 virus challenge and allowed the demonstration of poor cross-protection against BA71, thus defining it as heterologous. The in vitro specificity of the CD8+ T-cells present at the time of lethal challenge showed a clear activation against the homologous virus (E75) but not against BA71. These findings will be of utility for a better understanding of ASFV pathogenesis and for the rational designing of safe and efficient vaccines against this virus.

Different routes and doses influence protection in pigs immunised with the naturally attenuated African swine fever virus isolate OURT88/3

“…Regarding the role played by cytokines in the mechanisms of protection, conversely to what other studies suggested (Lacasta et al., 2015), IL-10 did not seem to contribute to controlling ASFV replication or avoiding harmful inflammation. Instead, and given the ability of IL-10 to impair or counteract the innate and adaptive immune response (Moore et al., 1993, Moore et al., 2001), the high concentrations detected after challenge in most of the immunised non-protected pigs might be linked to a failure of protective immune responses that might compromise the pigs' survival.…”

“…Other studies where pigs were immunised with OURT88/3 (Oura et al., 2005) suggested that although anti-ASFV antibodies were not enough to protect pigs, their role should not be completely ruled out. As for cellular immune responses, subsets of NK cells and CD8 + T-cells have been indicated to be relevant for protection induced by some live attenuated vaccine candidates (Scholl et al., 1989, Leitão et al., 2001, Oura et al., 2005, Lacasta et al., 2015) and DNA vaccines (Argilaguet et al., 2012, Lacasta et al., 2014). More specifically, in protected pigs immunised by oronasal route with ASFV NH/P68 isolate, cytotoxic activity was mediated by CD8 + cells which lysed macrophages infected with homologous isolates under restriction of class I swine leukocyte antigen (SLA) (Martins et al., 1993).…”

“…Genetically modified attenuated viruses, for which genes for virulence factors have been deleted, may provide safe vaccines against ASF (Lewis et al., 2000, O'Donnell et al., 2015, Reis et al., 2016). Despite some safety problems, live attenuated vaccines have demonstrated a high effectiveness against experimental infections with homologous (Leitão et al., 2001, Boinas et al., 2004, Oura et al., 2005, King et al., 2011, Lacasta et al., 2015), and occasionally heterologous (King et al., 2011, Mur et al., 2013), virulent isolates of ASFV. Like other viruses, ASFV has developed strategies to manipulate essential cytokines for the regulation of the immune response and the outcome of the infection (Dixon et al., 2004, Correia et al., 2013).…”

African swine fever virus does not express viral microRNAs in experimentally infected pigs

“…Those animals inoculated with E75 showed low Ct values at 7 dpi coinciding with the peak of virus replication and ASF clinical signs, while in contrast, inoculation of the same dose of the homologous attenuated E75CV1 strain showed no amplification. The animals infected with E75CV1 and re-inoculated with the BA71 virulent strain showed amplification levels similar to those found after E75 infection, correlating with the lack of cross-protection observed for these two strains [15]. As expected, the negative control animal did not show any specific PCR amplification (Table 2).…”

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

“…Experimental vaccines based on the use of an inactivated virus or virus subunits have failed to induce solid protective immunity [3,4,5,6]. Homologous protective immunity does develop in swine surviving infections with moderately virulent or attenuated ASFV isolates [7,8,9,10]. Swine immunized with live attenuated ASFVs containing genetically engineered deletions of specific virulence-associated genes are similarly protected when challenged with homologous parental viruses.…”