Viruses, Viral Stocks, VRPs, Plaque Assays MNV strains MNV1.CW3 or MNV1.CW1 were used in all virus infections [9, 28, 31] . Two mutations (that result in changes in the encoded amino acids) distinguish the genomes of MNV1.CW3 and MNV1.CW1 [28] . To generate a concentrated virus stock, RAW 264.7 cells (ATCC, Manassas, VA) were infected in VP-SFM media (Gibco, Carlsbad, CA) for 2 days at a multiplicity of infection (MOI) of 0.05. Supernatants were clarified by low-speed centrifugation for 20 min at 3,000 rpm. Virus was concentrated by centrifugation at 4uC for 3 h at 27,000 rpm (90,000 g) in a SW32 rotor. Viral pellets were resuspended in PBS and titered on RAW 264.7 cells as previously described [28] . Type I Lang reovirus was kindly provided by Dr. Terrence S. Dermody (Vanderbilt University, Nashville, TN). Plaque assays were performed as previously described [28] with the following modifications. Tissues were harvested into sterile, screw-top 2-ml tubes containing 500 ml of 1-mm zirconia/silica beads (BioSpec Products, Bartlesville, OK) and stored at 280uC. To obtain viral titers in these tissues 1 ml of complete DMEM was added to each sample on ice and homogenized using a MagNA Lyser (Roche Applied Science, Hague Road, IN) prior to plaque assay. The limit of detection was 20 plaque forming units (PFU)/ml. All VRPs were produced as previously described [60] . Briefly, ORFs 1, 2 and 3 from MNV1.CW3 and ORF2 from Lordsdale virus and Chiba virus were each cloned into VRP expression vectors. Following infection of BHK cells with VRPs for 24 h, culture supernatants were harvested and cells lysed. Proteins were separated by SDS-PAGE and analyzed by western blot with polyclonal rabbit anti-MNV serum [28] . VRP titers and efficient expression of recombinant protein were determined by immunofluorescence assay using mouse antisera generated from inoculation with respective antigens. Cell lysates from MNV ORF2, Chiba virus and Lordsdale virus VRP-infected cell cultures were further purified to obtain VLPs [56] .