Potential of avian and mammalian species A rotaviruses to reassort as explored by plasmid only-based reverse genetics

Highlights

• Cloning of all 11 genome segments of avian rotavirus A strain 02V0002G3 into plasmids

• Transfection of the plasmids into cells did not lead to generation of viable virus

• Generation of viable reassortants of avian VP3 or VP4 in a simian rotavirus backbone

• The reassortants showed slightly slower growth than the parent simian rotavirus

• Results in line with rare naturally occurring avian/mammalian rotavirus reassortants

Abstract

Species A rotavirus (RVA) is an important gastrointestinal pathogen that is widely distributed in humans, mammalian animals and birds. The RVA genome consists of eleven double-stranded RNA segments, enabling the generation of novel strains with new pathogenic or antigenic features by genetic reassortment. While reassortants between human and mammalian animal RVAs have been repeatedly described, data on the reassortment potential of avian RVA strains are rare. To investigate genome segment exchanges between avian and mammalian RVA strains, a plasmid-based reverse genetics strategy originally developed for the simian RVA strain SA11 was used here. All eleven genome segments of the chicken RVA strain 02V0002G3 were cloned into similar plasmids as in the SA11 system. However, in contrast to SA11, no infectious virus could be generated by transfection of the eleven 02V0002G3 plasmids into cell culture under the same conditions. In another series of experiments, each of the genome segments of 02V0002G3 was transfected together with the remaining ten genome segments of SA11. Viable mono-reassortants were only retrieved for the avian VP3 and VP4 genes. The reassortant viruses were structurally indistinguishable from their parental viruses, but grew to slightly lower titers in cell culture. The results indicate that the VP3 and VP4 genes, but not the other genes of avian RVA, can functionally substitute their mammalian homologs and create viable reassortants. Further research should focus on the reasons behind the reassortment incompatibility and on the optimization of the system for the generation of viable avian RVA rescued entirely from cloned avian RVA genome segments.

Introduction

Rotaviruses are enteric viruses that can cause severe gastroenteritis in young children. Despite ongoing vaccination programs it is estimated that rotavirus infections resulted in 128,500 deaths in 2016 worldwide (GBD 2016 Diarrhoeal Disease Collaborators, 2018). Besides human infections, rotaviruses are also highly prevalent in several animal species including birds (Otto et al., 2012, 2015), and outbreaks can lead to significant economic losses in livestock and poultry industry (Castells et al., 2018; Dhama et al., 2015; Vlasova et al., 2017). Avian rotavirus infection has been linked to symptoms of diarrhea and reduced feed intake resulting in growth retardation and increased mortality in chicken and turkey, even though co-infections with other pathogens seem to play a role for disease development (Dhama et al., 2015). In addition, rotavirus was recently demonstrated to be the single causative agent of young pigeon disease syndrome, an illness with high mortality rates (McCowan et al., 2018; Rubbenstroth et al., 2019). So far, no protective vaccines against rotaviruses are commercially available for poultry and domestic birds.

Rotaviruses belong to the genus Rotavirus of the family Reoviridae and comprise the species A-I, as well as the tentative species J-L (Banyai et al., 2017; Johne et al., 2019; Matthijnssens et al., 2012). Rotaviruses of species A (RVA), D, F and G have been detected in several species of birds (Dhama et al., 2015; Otto et al., 2012). RVA are non-enveloped viruses that carry a segmented double-stranded RNA (dsRNA) genome that encodes six structural viral proteins (VPs) and five or six non-structural viral proteins (NSPs). The VPs form the triple-layered icosahedral capsid of rotaviruses. Therein, the inner core is formed by VP2 that encloses the eleven viral dsRNA segments associated with the viral RNA-dependent RNA polymerase VP1 and the viral capping enzyme VP3. VP3 has guanylyl- and methyltransferase activities (Chen et al., 1999; Pizarro et al., 1991), and also functions as interferon (IFN) antagonist by cleaving 2′-5′-oligoadenylates that act as second messengers in the antiviral IFN response (Zhang et al., 2013). VP6 forms the middle layer and anchors the outer layer consisting of the glycoprotein VP7 and the receptor-binding VP4 that is activated by proteases during the course of infection (Desselberger, 2014). Proteolytic cleavage of trimeric VP4 into VP5* and VP8* occurs before attachment of the virion to the host cell receptor. The NSPs are expressed after infection of the host cell and regulate many aspects of rotavirus infection and replication, as well as suppress the intrinsic host cell immune defenses. In addition, NSP4 can also act as a secreted enterotoxin in the intestinal tract of infected individuals (Desselberger, 2014). For full RVA classification, the segments encoding VP7-VP4-VP6-VP1-VP2-VP3-NSP1-NSP2-NSP3-NSP4-NSP5/6 are assigned to specific genotypes designated as Gx-P[x]-Ix-Rx-Cx-Mx-Ax-Nx-Tx-Ex-Hx, respectively (Matthijnssens et al., 2011, 2008). The high genetic variability of RVA is reflected by a large number of currently known genotypes, e.g. 36 G-Types and 51 P-Types (Rotavirus Classification Working Group, 2020).

Co-infections with different RVA strains can lead to genetic reassortment, which can result in RVA strains with novel antigenic and/or pathogenic features (Doro et al., 2015; Martella et al., 2010). By reassortment, the diversity of RVA strains can increase and the risk of newly emerging human pathogenic RVA strains may arise. Reassortants containing genome segments from human and mammalian animal RVA strains have been frequently described, underlining the zoonotic nature of mammalian animal RVAs (Martella et al., 2010; Doro et al., 2015). In contrast, only little is known about zoonotic transmission and reassortment between avian and mammalian RVA strains. Zoonotic infections of avian RVA in mammalian animals have only been rarely detected (Brussow et al., 1992; Mitake et al., 2015), and reassortment events of avian RVAs have mainly been confined to that with other avian RVA strains (Schumann et al., 2009; Trojnar et al., 2013). Phylogenetically, avian and mammalian RVA cluster separately, indicating a rather long independent evolution (Trojnar et al., 2009). However, the 5′- and 3′-noncoding regions of the genomic segments of both virus groups are highly conserved, suggesting that genetic reassortment might theoretically be possible (Trojnar et al., 2009). One avian RVA strain derived from a pheasant contained a VP4 gene with high sequence identity to mammalian RVA, which might confirm the possibility of reassortment under field conditions (Trojnar et al., 2013).

Under laboratory conditions, only a few viable avian/mammalian reassortant viruses have been created in cell culture. Co-infection of MA-104 cells resulted in a turkey RVA strain carrying the VP4-encoding segment from a simian RVA (Kool et al., 1992). Using a helper virus-dependent reverse genetics system (RGS), the VP4-encoding segment of the simian RVA strain SA11 could also be replaced by its chicken RVA counterpart (Johne et al., 2016). Recently, the development of a helper virus-free RGS for strain SA11 was a major breakthrough for the study of rotavirus biology (Kanai et al., 2017; Komoto et al., 2018). Using this system, we were able to readily reproduce the rescue of the viable SA11/chicken VP4 reassortant (Falkenhagen et al., 2019). However, other genome segments have not been tested and no RGS for avian RVA is available.

Therefore, one aim of this study was to establish a helper virus-free RGS for an avian RVA to enable further investigations on specific characteristics of avian RVAs and their zoonotic potential. In addition, the reassortment capacity of each genome segment of the avian RVA in a mammalian RVA background was assessed using the generated avian RVA plasmids together with the recently established plasmid-based RGS for the simian SA11 strain.