All cell lines were maintained at 37uC and 5% CO 2 in Dulbecco's modified Eagle's medium supplemented with 10% fetal bovine serum. Vero (African green monkey kidney epithelial) cells (ATCC CCL-81) stably expressing human signaling lymphocytic activation molecule (CD150w/SLAM), called in this study Vero-SLAM cells [74] , and baby hamster kidney (BHK-21) cells stably expressing T7 polymerase (BSR-T7/5 (BHK-T7) cells [75] ) were incubated at every third passage in the presence of G-418 (Geneticin) at a concentration of 100 mg/ml. Lipofectamine 2000 (Invitrogen) was used for cell transfections. Peripheral blood mononuclear cells (PBMCs) were prepared through overlay of whole blood samples from mixed, healthy human donors (Emory University Institutional Review Board approval IRB00045690, Phlebotomy of Healthy Adults for Research in Infectious Diseases and Immunology) on Ficoll Hypaque solution, followed by centrifugation at 2406g for 30 minutes at room temperature and removal of the interphase material. Red blood cells were lysed with RBC lysis solution (Sigma), followed by repeated washing of extracted PBMCs with phosphate buffered saline and transfer to tissue culture plates pre-coated with poly-L-lysine (Sigma). Other primary human cell lines were obtained from PromoCell, Germany. Virus strains used in this study were MeV isolate MVi/Alaska.USA/16.00, genotype H2 (MeV-Alaska) [76] , HPIV3, MuV strain South Africa, RSV strain Long, laboratory adapted influenza A strains WSN (H1N1) and PR8/34 (H1N1), swine-origin influenza virus isolates S-OIV Texas and Mexico, vaccinia virus and sindbis virus. To prepare virus stocks, cells permissive for the virus to be amplified (Vero-Slam, Vero, HepG2 (ATCC HB-8065), and Madin-Darby canine kidney (MDCK)) were infected and incubated at 37uC. Cell-associated paramyxovirus and vaccinia virus particles were harvested by scraping cells in OPTIMEM (Invitrogen), followed by release of virus through two consecutive freeze-thaw cycles. Influenza virus and sindbis virus particles were harvested from cell culture supernatants. Titers of MeV and MuV were determined through 50% tissue culture infective dose (TCID 50 ) titration according to the Spearman-Karber method [77] as described [78] , titer of all other viruses were determined by plaque assay on permissive cells.