A plasmid-based reverse genetics system for animal double-stranded RNA viruses

Abstract

Mammalian orthoreoviruses (reoviruses) are highly tractable experimental models for studies of double-stranded (ds) RNA virus replication and pathogenesis. Reoviruses infect respiratory and intestinal epithelium and disseminate systemically in newborn animals. Until now, a strategy to rescue infectious virus from cloned cDNA has not been available for any member of the Reoviridae family of dsRNA viruses. We report the generation of viable reovirus following plasmid transfection of murine L929 (L) cells using a strategy free of helper virus and independent of selection. We used the reovirus reverse genetics system to introduce mutations into viral capsid proteins sigma1 and sigma3 and to rescue a virus that expresses a green fluorescent protein (GFP) transgene, thus demonstrating the tractability of this technology. The plasmid-based reverse genetics approach described here can be exploited for studies of reovirus replication and pathogenesis and used to develop reovirus as a vaccine vector.

Figure 1

Experimental Strategy to Generate Reovirus from Cloned cDNA (A) Prototype reovirus gene segment cDNA in plasmid. Cloned cDNAs representing each of the ten full-length reovirus RNA gene segments are flanked by the bacteriophage T7 RNA polymerase promoter (T7P) and the antigenomic hepatitis delta virus (HDV) ribozyme (Rib). (B) Schematic of approach. The ten reovirus cDNA constructs are transfected into murine L cells expressing T7 RNA polymerase from recombinant vaccinia virus strain rDIs-T7pol, which is replication defective. Nascent transcripts correspond to viral mRNAs containing the native 5′ end. Self cleavage by the HDV ribozyme generates the native viral 3′ end. Following 5 days of incubation, transfected cells are lysed by freeze-thaw, and viable viruses rescued from cloned cDNAs are isolated by plaque assay using L cells. (C) Kinetics of virus production following plasmid transfection of L cells. Cells were transfected with plasmid DNA according to the protocol in (B) and lysed at the intervals shown. Viral titers in cell lysates were determined by plaque assay. (D) Infectious center assay following plasmid transfection of L cells. Cells were transfected with plasmid DNA, trypsinized at the intervals shown posttransfection, washed, counted, diluted, and applied directly to monolayers of untreated L cells. The number of the infectious centers was determined by plaque assay.

Figure 2

Rescue of rsT3D and rsT3D/T1LS1 (A) Electropherotypes of T1L, T3D, rsT3D, and rsT3D/T1LS1. Viral dsRNA was extracted from purified virions and electrophoresed in an SDS-polyacrylamide gel, followed by ethidium bromide staining to visualize viral gene segments. Size classes of gene segments (L, M, S) are indicated. (B) Recombinant viruses contain a novel mutation in the L1 gene. The single nucleotide difference in L1 unique to rsT3D and rsT3D/T1LS1 is shown in the alignment as an asterisk. The G→A substitution at position 2205 is a signature change engineered into the cloned T3D L1 cDNA used for marker rescue. (C) Sequence analysis of L1 gene RT-PCR products from rescued reoviruses. A fragment of the L1 gene was amplified by one-step RT-PCR performed using viral dsRNA extracted from purified virions of T3D, rsT3D, and rsT3D/T1LS1. Products were subjected to direct sequence analysis and compared to the L1 sequence of T3D. Shown are sequence chromatograms demonstrating G→A substitution at position 2205 of the rsT3D and rsT3D/T1LS1 L1 genes.

Figure 3

Infectivity of Native and Recombinant Reoviruses (A) Immunofluorescence of cells infected with T3D and rsT3D. L cells were mock infected or infected with either T3D or rsT3D, stained at 18 hr postinfection with anti-μNS antiserum to visualize reovirus inclusions (green) and TO-PRO3 to visualize nuclei (blue), and imaged by confocal laser scanning microscopy. Representative digital fluorescence (top panel) and DIC images (bottom panel) for mock-, T3D-, and rsT3D-infected cells are shown. Scale bar, 10 μM. (B) One-step growth curve for T1L, T3D, rsT3D, and rsT3D/T1LS1 in L cells (left) and MEL cells (right). Cells were infected with virus, incubated for the intervals shown, and lysed by freeze-thaw. Viral titers in cell lysates were determined by plaque assay. The results are presented as the mean viral titers for triplicate experiments. Error bars indicate SD.

Figure 4

The σ1 Protein of rsT3D-σ1T249I Is Resistant to Trypsin (A) Model of σ1 generated by adding five β spiral repeats to the N terminus of the crystallized fragment of σ1 (Chappell et al., 2002). The three monomers in the crystallized fragment are shown in red, yellow, and blue; the model is shown in gray. The inset shows an enlarged view of the β spiral region in σ1 that influences susceptibility of the molecule to cleavage by intestinal proteases (Chappell et al., 1998). Side chains of Arg245 and Thr249 are depicted in ball-and-stick representation. (B) Electrophoretic analysis of viral structural proteins of rsT3D and rsT3D-σ1T249I during treatment with trypsin to generate ISVPs. Purified ³⁵S-methionine-labeled virions were treated with trypsin for the indicated intervals and loaded into wells of a 4%–20% polyacrylamide gradient gel. After electrophoresis, the gel was prepared for fluorography and exposed to film. Samples of untreated virions appear in the lanes labeled “V.” Viral proteins are labeled. Positions of molecular weight standards (in kDa) are indicated. The experiments shown are representative of two performed for each virus. (C) Infectivity of rsT3D and rsT3D-σ1T249I during treatment with trypsin to generate ISVPs. Purified virions were treated with trypsin at 37°C for the intervals shown. Titers of virus in the treatment mixtures were determined by plaque assay. The results are presented as the mean viral titers for triplicate experiments. Error bars indicate SD.

Figure 5

rsT3D-σ1T249I Infects the Murine Intestine and Disseminates to the CNS Titers of rsT3D and rsT3D-σ1T249I after either PO (A) or IC (B) inoculation. Mice were inoculated with virus and euthanized at the indicated times postinoculation. Viral titers in organ homogenates were determined by plaque assay. The limit of detection was 100 PFU/ml of organ homogenate. Each data point represents the average viral titer from 3 to 12 mice. Error bars indicate SD.

Figure 6

A Single Mutation in Outer-Capsid Protein σ3 Accelerates Proteolytic Disassembly of Reovirus (A) Crystal structure of T3D σ3 (Olland et al., 2001), in which cathepsin L cleavage sites are depicted in blue between amino acids 243 and 244 and between 250 and 251 (Ebert et al., 2002). Surrounding residues, from amino acids 241 to 253, are shown in yellow. The C-terminal residues of σ3, from amino acids 340–365, are colored red. Tyr354, which is altered in several PI (Wetzel et al., 1997), D-EA (Ebert et al., 2001), and ACA-D viruses (Clark et al., 2006), is shown in green. The virion-distal end of σ3 is at the top of the figure, and the virion-proximal end and N terminus are at the bottom. The inset shows an enlarged view of the boxed region of σ3 using the same color scheme. Side chains of amino acids 243, 244, 250, 251, and 354 are depicted in ball-and-stick representation. (B) Chymotrypsin treatment of rsT3D and rsT3D-σ3Y354H. Purified virions were treated with chymotrypsin for the indicated intervals and loaded into wells of 10% polyacrylamide gels. After electrophoresis, the gels were stained with Coomassie blue. Viral proteins are labeled. The experiments shown are representative of two performed for each virus. (C) Growth of rsT3D and rsT3D-σ3Y354H in L cells treated with E64. L cells were preincubated in medium with or without E64 at the concentrations shown. The medium was removed, cells were adsorbed with virus for 1 hr, and fresh medium with or without E64 was added. After 24 hr incubation, viral titers in cell lysates were determined by plaque assay. The results are presented as the mean viral yields, calculated by dividing titer at 24 hr by titer at 0 hr for each concentration of E64, for triplicate experiments. Error bars indicate SD.

Figure 7

Expression of GFP by rsT3D/S4-GFP (A) Schematic of pT7-S4GFP. The GFP ORF is flanked by S4 gene nucleotides 1–149 and 769–1196. (B) RT-PCR analysis of rsT3D and rsT3D/S4-GFP. Viral dsRNA was extracted from purified virions and subjected to RT-PCR using primers specific for T3D S4 and GFP sequences. Numbers delineate the S4 RNA nucleotide position corresponding to the 5′ end of S4-specific primers. (C) Viral protein expression in cells infected with rsT3D/S4-GFP. L cells were infected with rsT3D or rsT3D/S4-GFP at an MOI of 1 PFU per cell and incubated for 24 hr. Cell lysates were analyzed by immunoblotting using antibodies specific for μNS, σNS, and σ3 proteins. (D) Image analysis of cells infected with rsT3D/S4-GFP. L cells were infected with rsT3D/S4-GFP, stained with antibodies specific for μNS (blue) and σ3 (red) proteins at 24 hr postinfection, and imaged by confocal laser scanning microscopy. Scale bar, 10 μM.