The role of accessory proteins in the replication of feline infectious peritonitis virus in peripheral blood monocytes

Abstract

The ability to productively infect monocytes/macrophages is the most important difference between the low virulent feline enteric coronavirus (FECV) and the lethal feline infectious peritonitis virus (FIPV). In vitro, the replication of FECV in peripheral blood monocytes always drops after 12h post inoculation, while FIPV sustains its replication in the monocytes from 45% of the cats. The accessory proteins of feline coronaviruses have been speculated to play a prominent role in virulence as deletions were found to be associated with attenuated viruses. Still, no functions have been ascribed to them. In order to investigate if the accessory proteins of FIPV are important for sustaining its replication in monocytes, replication kinetics were determined for FIPV 79-1146 and its deletion mutants, lacking either accessory protein open reading frame 3abc (FIPV-Δ3), 7ab (FIPV-Δ7) or both (FIPV-Δ3Δ7). Results showed that the deletion mutants FIPV-Δ7 and FIPV-Δ3Δ7 could not maintain their replication, which was in sharp contrast to wt-FIPV. FIPV-Δ3 could still sustain its replication, but the percentage of infected monocytes was always lower compared to wt-FIPV. In conclusion, this study showed that ORF7 is crucial for FIPV replication in monocytes/macrophages, giving an explanation for its importance in vivo, its role in the development of FIP and its conservation in field strains. The effect of an ORF3 deletion was less pronounced, indicating only a supportive role of ORF3 encoded proteins during the infection of the in vivo target cell by FIPVs.

Mutations in the 3c and 7b genes of feline coronavirus in spontaneously affected FIP cats

“…The latter exhibited more effective replication in macrophages than do FIPVs with intact ORF 3abc. In contrast, another study detected a crucial role of ORF 7ab in FIPV replication in monocytes/macrophages and not in ORF 3abc (Dedeurwaerder et al, 2013). Additionally, the ORF 7a unit combined with ORF 3-encoded proteins seems to be an effective antagonist of IFN-alpha-induced anti-viral response (Dedeurwaerder et al, 2014).…”

“…This led to the assumption that deletions in ORF 3c may result in an increase of virulence even if FIPV strains with intact ORF 3c could be found as well. On the other hand, deletions in ORF 7b were considered to play a role in virus attenuation (Dedeurwaerder et al, 2013;Herrewegh et al, 1995b;Vennema et al, 1998). The relevance of all these findings is still controversial (Balint et al, 2012;Brown et al, 2009;Chang et al, 2010;Dedeurwaerder et al, 2013Dedeurwaerder et al, , 2014Kennedy et al, 2001Kennedy et al, , 2006Kiss et al, 2000;Lin et al, 2009b;Pedersen et al, 2009Pedersen et al, , 2012Porter et al, 2014;Tekes et al, 2012;Vennema et al, 1998).…”

Accessory proteins of SARS-CoV and other coronaviruses

“…It has been demonstrated that the ORF7 proteins are essential for efficient replication in vitro and for virulence in vivo (Haijema et al, 2004). The ORF3 proteins were found to have only supportive roles during infection of the target cell by FIPV (Dedeurwaerder et al, 2013). It has also been shown that the FIPV p7a protein is a type-I IFN antagonist but needs the presence of ORF3 proteins to fully exert its antagonistic function (Dedeurwaerder et al, 2014).…”

An update on feline infectious peritonitis: Virology and immunopathogenesis

“…This finding altered the conclusions of many previous studies using WSU-79-1683 and WSU-79-1146 as prototypic FECVs and FIPVs, respectively. Dedeurwaerder et al (2013a) tried to link ORF 7b with macrophage tropism and hence with the evolution of FIPVs by measuring the replication kinetics of FIPV WSU-79-1146 and several specific deletion mutants in monocyte cultures. The mutants lacked either the ORF 3abc accessory genes (FIPV-Δ3), ORF 7ab (FIPV-Δ7) or both ORF 3abc and ORF 7b (FIPV-Δ3Δ7).…”