Viral stocks and titration - The reference strains of DENV-1, DENV-2 New Guinea C (NGC), DENV-3, and DENV-4 were provided by the Centers for Disease Control (CDC, CO, USA). Viral stocks were obtained by inoculating a monolayer of C6/36 HT cells in a 75-cm2 tissue culture flask with the virus at a multiplicity of infection (MOI) of 0.05 diluted in 1 mL of L-15 medium supplemented with 2% FBS. After 3 h of adsorption, 10 mL of L15 medium supplemented with 2% FBS was added and the cells were cultured for five days at 34ºC without CO2. The supernatant was then removed from the cells and centrifuged for 5 min at 400 x g to pellet the cell debris. The supernatant was aliquoted and stored at -70ºC for future use. Clinical isolates of DENV-1 (strain Bga-07), DENV-2 (strain 109-05), and DENV-4 (strain Bga-06) were obtained from patients with dengue haemorrhagic fever in Antioquia, Colombia (kindly provided by Dr FJ Díaz, Grupo Inmunovirología, Facultad de Medicina, Universidad de Antioquia) and used to clone the DENV 3′ UTR (Castillo et al. 2016). Virus titration was performed by flow cytometry, as previously described (Castillo et al. 2016). Briefly, C6/36 HT cells were seeded in 12-well plates and cultured overnight at 34ºC without CO2. They were then infected with 10-fold serial dilutions of the virus, and at 24 h post-infection (hpi) they were harvested and resuspended in PBS. For flow cytometry analyses, the cells were fixed using a Fixation/Permeabilisation buffer (eBioscience, San Diego, CA, USA), centrifuged and washed twice with PBS, and stained with the monoclonal antibody 4G2 (kindly provided by Dr P Desprès, Institut Pasteur, Paris) in a final volume of 100 mL. As the secondary antibody, fluorescein isothiocyanate (FITC)-labelled goat anti-mouse IgG antibody (Invitrogen, Life Technologies, CA, USA) was used. The cells were analysed with a FACScan flow cytometer using the FACSdiva software. The percentage of infected cells in each sample and the total number of cells seeded per well were used to calculate the final virus titre.