Viruses, cell lines, and mice.

Murine L2 fibroblasts were maintained in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum (FBS), HEPES (10 mM), and 1% penicillin-streptomycin. Plaque assays were performed on L2 cells as described previously (32). Murine 3T3/pLZ fibroblasts and control 3T3/neo (33) cells were grown in DMEM containing 10% FBS supplemented with G418 (350 μg/ml) and 1% penicillin-streptomycin. The recombinant coronaviruses inf-MHV-A59 (wild-type [WT] A59, referred to as A59 here) and inf-ns2-H126R (referred to as ns2^H126R here) were obtained from Stuart Siddell (University of Bristol, Bristol, United Kingdom) and have been described previously (2, 34). Newcastle disease virus expressing green fluorescent protein (NDV-GFP) (7, 29, 35) was obtained from Luis Martinez-Sobrido (University of Rochester School of Medicine). B6 mice were purchased from the National Cancer Institute (Frederick, MD). Rnasel^−/− mice (bred for 10 generations to obtain a B6 background) were described previously (14). Ifih1^−/− (36) and Ifnar1^−/− (37) mice, both with B6 background, were obtained from Michael S. Diamond (Washington University in St. Louis, St. Louis, MO). Both strains were further bred and maintained in the animal facility at the University of Pennsylvania, Philadelphia, PA.