Dengue 1 (DENV1) strain Hawaii, Dengue 2 (DENV2) strains New Guinea C (NGC) and ST, Dengue 3 (DENV3) strain H87 and Dengue 4 (DENV4) strain H241 were passaged in Vero cells for all assays unless otherwise stated. Virus titers were determined by plaque assay on BHK cells. Immature DENV (imDENV) was produced as described previously [16] . Briefly, the media of infected Vero cells was replaced 2 dpi with M199 NH 4 Cl medium (M199 with 10% FBS, 1% sodium pyruvate, 1% non essential amino acids, 1% PS, 2.5 mM L-Glutathione and 20 mM NH 4 Cl). Culture supernatants were harvested after 5 days and imDENV was purified by PEG precipitation. Virus titer was determined as genome containing particles (GCP)/ml by quantifying real-time PCR (qPCR) described previously [17] .