The DENV-2 PL046 strain (Genbank accession: AJ968413.1) was isolated from a Taiwanese DF patient. The DENV-2 prototype New Guinea C (NGC) strain was kindly provided by D. J. Gubler of the Centers for Disease Control and Prevention, USA. These viruses were propagated in the mosquito cell line C6/ 36 (ATCC: CRL-1660) grown in RPMI 1640 medium containing 5% fetal bovine serum (FBS) [24] . The J774A.1 mouse macrophage cell line (ATCC: TIB-67), A549 human lung epithelial carcinoma cell line (ATCC: CCL-185), and African green monkey kidney epithelial cell line Vero (ATCC: CCL-81) were cultured in DMEM medium supplemented with 10% FBS (Invitrogen). The TLR3 ligand polyinosine-polycytidylic acid (polyI:C) and TLR9 ligand CpG oligodeoxynucleotides 1826 (CpG ODN 1826; hereafter CpG) were from InvivoGen. The TLR4 ligand LPS (Sigma-Aldrich), anti-ERK antibody, anti-phospho-ERK antibody (Cell Signaling, catalog# 9102 and 9101S) and anti-NF-kB p65 antibody (sc-372, Santa Cruz Biotechnology) were used.