Dengue virus serotype 2 (DENV-2) strains 16681 and New Guinea C were used in this study. DENV was grown in C6/36 insect cells for 6 days as described elsewhere [51] . Briefly, C6/36 cells were infected at a multiplicity of infection (MOI) of 0.01, and 6 days after infection, cell supernatants were collected, clarified, and stored at 80uC. The titers of DENV stocks were determined by limiting-dilution plaque assay on BHK cells [52] . Semliki Forest virus (SFV) expressing GFP and DENV-NS2B3 were generated as described previously [36] and titrated in BHK cells by immunofluorescence [53] .