Abstract
The naturally isolated avian coronavirus infectious bronchitis virus (IBV) generally cannot replicate in chicken kidney (CK) cells. To explore the molecular mechanism of IBV adapting to CK cells, a series of recombinant viruses were constructed by chimerizing the S genes of CK cell-adapted strain H120 and non-adapted strain IBYZ. The results showed that the S2 subunit determines the difference in cell tropism of the two strains. After comparing the amino acid sequences of S protein of CK cell-adapted strain YZ120, with its parental strain IBYZ, three amino acid substitutions, A138V, L581F, and V617I, were identified. Using YZ120 as the backbone, one or more of the above-mentioned substitutions were eliminated to verify the correlation between these sites and CK cell tropism. The results showed that the CK cell tropism of the YZ120 strain depends on the V617I substitution, the change of L581F promoted the adaptation in CK cells, and the change at 138 position was not directly related to the CK cell tropism. Further validation experiments also showed that V617I had a decisive role in the adaptation of IBV to CK cells, but other areas of the virus genome also affected the replication efficiency of the virus in CK cells.
Highlights
Avian coronavirus infectious bronchitis virus (IBV) is a positive-sense RNA enveloped virus, which belongs to the order Nidovirales, family Coronaviridae, genus Gammacoronavirus (Cavanagh, 2007)
The results showed that the S gene replacement with the corresponding sequence of chicken kidney (CK) cell non-adapted strain leads to the loss of replication in CK cells, while replacement with one of the adapted strains would provide the infection ability, demonstrating the S protein to be a determinant of CK cell tropism of IBVs (Jiang et al, 2017)
After evaluation by Immunofluorescence Assay (IFA) and proliferation curve tests of IBVinfected CK cells, we demonstrated the key role of V617I in the CK cell tropism of the YZ120 strain, which was confirmed using other recombinants, in which the amino acid encoded at S codon 617 had been changed from that in the genomic backbone of rH120, rH120-S/YZ, and rIBYZ strains
Summary
Avian coronavirus IBV is a positive-sense RNA enveloped virus, which belongs to the order Nidovirales, family Coronaviridae, genus Gammacoronavirus (Cavanagh, 2007). A few amino acid alterations in the RBD of the S1 subunit can change the host species tropism of the CoV (Krempl et al, 1997; Li et al, 2005; Qu et al, 2005). Four amino acid substitutions in the S2 subunit of MHV-A59 can extend the host range in the non-permissive mammalian cell types of this virus, which indicates that the S2 subunit plays an essential role in the viral cell and tissue tropism (Baric et al, 1999; McRoy and Baric, 2008). PEDV (porcine epidemic diarrhea virus) acquires the ability of replication in non-enteric tissues by a single-amino acid substitution at the S2’ cleavage site (Li, 2015)
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