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Journal of Virology, November 2003, p. 11849-11854, Vol. 77, No. 21
0022-538X/03/$08.00+0 DOI: 10.1128/JVI.77.21.11849-11854.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Department of Medicine, Division of Rheumatic and Autoimmune Diseases, University of Minnesota, Minneapolis, Minnesota
Received 5 May 2003/ Accepted 29 July 2003
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In 1979, Ray et al. described a model of chronic inflammatory myopathy (CIM) caused by infection of newborn mice with coxsackievirus B1 isolated from a patient with pleurodynia (19). This virus was later referred to as the Tucson strain (CVB1T) (22). In this model, the acute viral infection resolves within 2 weeks and is followed by chronic postviral myopathy that peaks at 1 month and is manifest as inflammation and weakness of proximal hind limb skeletal muscle. Weakness is apparent clinically as a change in hind limb mobility and gait. Chronic histopathology consists of perimysial and endomysial infiltration of muscle by mononuclear cells, accompanied by ongoing muscle necrosis and regeneration (22). CIM development is driven by a T-cell-dependent immunopathic response, as nude or neonatally thymectomized mice do not develop chronic weakness or inflammation (30, 31), and the H-2 haplotype influences disease severity (25).
The importance of the infecting strain of virus in the pathogenesis of CIM is illustrated by studies of CVB1T variants (24). Infection with myopathic variants causes CIM, whereas amyopathic variants are equally infectious but attenuated for the later development of CIM. Thus, acute cytopathic infection of muscle is not sufficient by itself to induce CIM. The objective of the present study was to identify the viral genetic determinants that cause CIM. We used a reverse genetics approach, producing infectious cDNA clones of myopathic and amyopathic variants combined with construction of viral chimeras and whole-genome sequencing.
Induction of CIM by cloned virus. Infectious cDNA clones were produced from myopathic and amyopathic CVB1T (24). SY8 is a descendant of the original Tucson strain (22). Virus MP1 was derived directly from SY8 and has a myopathic phenotype equivalent to that of its SY8 parent (24). AMP2 is an amyopathic variant produced by passage of MP1 in BGMK cells (24). To produce viral clones, viral RNA was purified with TriPure (Roche, Indianapolis, Ind.) and full-length cDNA clones were constructed by long reverse transcriptase PCR(RT-PCR) (2, 26). First-strand viral cDNA was synthesized with 200 U of reverse transcriptase (SuperScript II; Invitrogen, Carlsbad, Calif.) and 5 pmol of primer 3DTSP6-CP3 (5'-CGTGTCAAGCTTATTTAGGTGACACTATAGAT14-3') or primer 3DT-CP4 (5'-CGAGAGCGTGTCAAGCTTACGT24) according to the manufacturer's recommendations. Primer components included a HindIII site (italicized), an SP6 promoter (underlined), and an oligo(dT) region. Viral cDNA was amplified in a 100-µl hot-start PCR containing 25 pmol of each primer and 5 U of rTth DNA polymerase XL (PE Applied Biosystems, Foster City, Calif.). The downstream primer was 3DTSP6-CP3 or 3DT-CP4. The upstream primer was 5UTR-CP2 (5'-CGAGGTTCTAGATAATACGACTCACTATAGTTAAAACAGCCTGTGGGTTG-3'), which contains a 5' XbaI site (italicized) followed by a T7 promoter (underlined) and the first 20 bases of the CVB1N sequence (bold) (7). The amplification profile consisted of a single cycle of 94°C for 1 min, 50°C for 1 min, and 72°C for 6 min followed by 30 cycles of 94°C for 30 s, 55°C for 1 min, and 72°C for 6 min, with a 6-s extension per cycle. The 7.4-kb amplicon was digested with XbaI and HindIII, gel purified, and ligated into pUC18 with T4 ligase. Plasmid DNA was transformed into Escherichia coli SURE2 cells (Stratagene, La Jolla, Calif.), and all clones were grown at a reduced temperature of 30°C to maintain the stability of the viral insert. Clones containing a 7.4-kb insert were transfected into BGMK cells with Lipofectin (Invitrogen) according to the manufacturer's protocol. When cultures exhibited 100% cytopathic effect, virus was extracted by three freeze-thaw cycles, centrifugation at 2,500 x g, and storage of the supernatant at -80°C.
To test for induction of CIM, specific-pathogen-free ICR (CD-1) mice (Harlan, Indianapolis, Ind.) were injected intraperitoneally with 50 PFU of virus in 50 µl of phosphate-buffered saline within 48 h after birth. Negative controls were injected with phosphate-buffered saline. CIM was evaluated at 1 month postinfection. For histopathology, hamstring muscle was flash-frozen in isopentane cooled in liquid nitrogen and sectioned completely (sections were 8 µm thick), with every 10th section being mounted, stained with hematoxylin and eosin, and graded as shown in Fig. 1. The level of inflammation was fairly consistent throughout the entire piece of muscle. The incidences and levels of severity of weakness and inflammation were comparable between mice infected with cloned myopathic virus MP1.23 or MP1.24 and mice infected with the respective parent virus MP1 or SY8 (Table 1). Similarly, the amyopathic cloned viruses AMP2.17 and AMP2.22 caused markedly reduced levels of weakness and inflammation that were comparable to those of their parent virus, AMP2. Pairwise comparisons indicated that all myopathic parental viruses and clones differed from AMP2 and its clones in mean weakness grade (P
0.0004) and mean inflammation grade (P
0.003). Weakness and inflammation in amyopathic virus-infected mice did not differ from those in the uninfected control (P > 0.14). These results are consistent with the original characterization of the viruses MP1 and AMP2, which revealed only minor differences in virulence (24), and indicate that the myopathic phenotype of the parental virus is preserved in the cloned viruses.
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FIG. 1. Inflammatory histopathology grades 0 to 3 in hamstring muscle at 1 month postinfection. Representative examples of the different grades of inflammation in hamstring muscle are shown (original magnification, x150). (A) Grade 0, uninfected control; (B) grade 1, MP1.24-infected mouse showing small numbers of interstitial inflammatory cells; (C) grade 2, MP1.24-infected mouse with more widespread infiltration and occasional medium sized infiltrates; (D) grade 3, MP1.24-infected mouse with widespread inflammation and many medium- to large-sized infiltrates (arrow). This sample also shows fatty replacement (arrowhead). At the experimental end point, mice were euthanized by cervical dislocation under ketamine plus xylazine anesthesia according to the Public Health Service policy on Humane Care and Use of Laboratory Animals and the research protocol approved by the Institutional Animal Care and Use Committee at the University of Minnesota.
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TABLE 1. Induction of CIM by cloned viruses and chimeras of CVB1T
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Viral clones and chimeras were also evaluated for their ability to cause mortality, replicate in skeletal muscle, and induce antiviral immunoglobulin G (IgG) antibodies (Table 2). Mice infected with myopathic virus showed a slightly lower rate of survival than that of mice infected with amyopathic virus. Mice infected with MP1 or its clone MP1.23 showed the lowest survival rates, which differed from those of mice infected with AMP2 and its clones, all the viral chimeras, and the uninfected control (P
0.038). Survival of mice infected with MP1.24 differed only from that of mice infected with AMP2.17, AMP2, or MP-2492 or the uninfected control mice (P
0.036), while the survival of mice infected with SY8 or the MP1.24 sham control did not differ from the survival of mice infected with any other viruses or chimeras (P
0.098). Although myopathic viruses achieved slightly higher titers in muscle during acute infection, none of the virus titers differed significantly from each other. The chimeras and parental viruses also showed comparable levels of replication as determined with one-step growth curves of viruses in BGMK cells (data not shown). Similar results were obtained in our original studies of the parental CVB1T variants, where a more detailed analysis of growth kinetics indicated that the parental AMP2 virus is not defective for replication in vivo or in vitro (24).
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TABLE 2. Comparison of levels of virus virulence and replication and the host antiviral antibody responses in mice infected with CVB1T clones and chimeras
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0.0001 to P = 0.042). This finding indicates that differences in the immunogenicities of viral determinants are linked to the pathogenic phenotype of the virus, resulting in either a qualitative or quantitative difference in the immune response to infection. This difference is also apparent in the production of autoantibodies to muscle and nuclear antigens, which are found in mice infected with myopathic, but not amyopathic, virus (23). Candidate determinants identified by sequencing. Two myopathic (MP1.23 and MP1.24) and two amyopathic (AMP2.17 and AMP2.22) cDNA clones were completely sequenced. Automated sequencing was performed by PCR using Big Dye Terminator cycle sequencing protocols (PE Applied Biosystems). Sequence was first obtained by gene walking for pMP1.24 (Amplicon Express, Pullman, Wash.). These same primers were then used to sequence clones MP1.23, AMP2.17, and AMP2.22 on an ABI 377 sequencer through the Advanced Genetic Analysis Center at the University of Minnesota. Both strands were sequenced for all clones, with coverage at two to three times per base. Contig assembly and alignments were performed with SeqMan and MegAlign, respectively (DNAStar, Madison, Wis.), with manual editing (16). The nucleotide sequences of MP1.23, MP1.24, and AMP2.17 indicated that the CVB1T genome was 7391 nt in length. The AMP2.22 clone contained three rather than two terminal guanine residues and thus was 7392 nt in length.
CVB1N is the only other type B1 coxsackievirus that has been completely sequenced (7). Sequence information obtained from MP1.24 was compared to that from CVB1N (GenBank accession number M16560) (7), CVB2 Ohio (AF081485), CVB3 (M88483), CVB4 Benschoten (X05690), CVB5 Faulkner (AF114383), and CVB6 Schmitt (AF114384). In the 5' UTR, pMP1.24 showed the highest percentage of identity with CVB6 (85.3%), followed by CVB5 and CVB3 (84.5%), CVB1 (83.9%), CVB2 (83.3%), and CVB4 (80.6%). The base composition for MP1.24 showed a higher adenine content (28.3% A, 24.6% G, 23.5% C, and 23.7% U), similar to that of CVB1N. Comparative alignments were used to predict the translational start site at nt 743 and cleavage sites within the polyprotein. Seven AUG codons were located upstream of the translational start site. Two of these, at nt 381 and 393, did not occur in CVB1N, but all seven were conserved in CVB2, CVB3, CVB5, and CVB6. The predicted polyprotein of CVB1T consists of 2,182 residues and aligns with no deletions or additions in the coding region relative to that of CVB1N. The percent identity of the capsid region for these two CVB1 strains was 94.6%. In contrast, the capsid region of CVB1T showed at most 82.9% identity with CVB types 2 through 6. The amino acids flanking all predicted cleavage sites of CVB1T were identical to those in CVB1N, CVB3, CVB4, and CVB5. Thus, CVB1T appears to be a typical member of the coxsackievirus group with a high degree of similarity to CVB1N.
Comparison of the four CVB1T clones that were completely sequenced revealed one mutation in the 5' UTR and four coding changes that differed consistently between myopathic and amyopathic clones (Table 3). Three of the coding changes occurred in capsid protein VP1, with the Thr-to-Ala-276 mutation located only three residues upstream of the VP1/2A cleavage site. The fourth coding mutation occurred in protease 3C. Sequencing of additional virus clones and of amplicons produced by RT-PCR of parental virus RNA indicated that these five primary nucleotide differences were strictly associated with the pathogenic phenotype of the virus. Six additional conserved differences were located within the coding region but were translationally silent, and the possibility remains that one or more of these changes may be involved through effects on RNA secondary structure (5).
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TABLE 3. Nucleotide sequence and coding differences between myopathic and amyopathic viruses
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Nucleotide sequence accession numbers. Sequences for all four clones starting with nt 21 to the 3' end are available in GenBank (accession numbers AY186745 for pMP1.24, AY186746 for pMP1.23, AY186747 for pAMP2.17, and AY186748 for pAMP2.22).
We thank Stacey Orvik and Wei Li for excellent technical assistance. We also thank Maribeth Sandager and the College of Biological Sciences Imaging Center at the University of Minnesota for assistance with digital imaging.
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