Reverse Genetics System for Porcine Enteric Calicivirus, a Prototype Sapovirus in the Caliciviridae

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Abstract
A porcine enteric calicivirus (PEC), strain Cowden in the genus Sapovirus of the Caliciviridae family, can be propagated in a porcine kidney continuous cell line (LLC-PK) in the presence of bile acids in the cell culture medium. A full-length cDNA copy of the Cowden PEC genome was cloned into a plasmid vector directly downstream from the T7 RNA polymerase promoter, and capped RNA transcripts derived from this clone were infectious when transfected into LLC-PK cells. The recovery of PEC after transfection of RNA transcripts was dependent on the presence of bile acids, consistent with our recent identification of a bile acid-mediated signaling pathway required for PEC replication (Chang et al., Proc. Natl. Acad. Sci. USA 101:8733-8788, 2004). Recovery of virus was verified by detection of PEC antigen in transfected cells by immunofluorescence and enzyme-linked immunosorbent assays, direct observation of recovered viral particles by electron microscopy, and partial sequence analysis of their genomes (first 1,070 nucleotides) to differentiate them from tissue culture-adapted parental virus. The recovered virus retained its ability to infect piglets when administered by the oral route and showed an attenuated phenotype similar to that of the tissue culture-adapted parental virus. This reverse genetics system for PEC provides a new tool to study the molecular basis of replication and pathogenesis for caliciviruses associated with diarrheal disease.