Roles of non-structural protein 1 in dengue virus replication and immunomodulation <i>in vivo</i>

Dengue virus (DENV) is a vector-borne positive-sense RNA virus causing the dengue epidemic in tropical and subtropical regions. Non-structural viral protein 1 (NS1) is a key modulator of host-pathogen interactions. We investigated the in vivo effect of NS1 on DENV infection using an NS1-expressing recombinant vaccinia virus DIs strain (rDIs-DENV2C-NS1). NS1 expression and anti-NS1 antibody induction were observed. To assess its functional role, rDIs-DENV2C-NS1 was administered to AG129 mice lacking type I and II interferon receptors. Two weeks post-immunization, the mice were challenged with DENV2C (10⁵ PFU/mouse). Four days post-infection, the mice immunized with rDIs-DENV2C-NS1 exhibited higher serum viral loads than controls. Liver viral load was significantly increased 14 days after infection compared to that in the control group, and NS1 protein expression was confirmed in the sera. To delineate the effects of NS1, the mice were treated with recombinant NS1 before DENV2 infection. This treatment showed an increase in serum, liver, and spleen viral loads, indicating enhanced viral replication. Serum interleukin (IL)-3, IL-13, IL-18, and macrophage colony-stimulating factor levels were significantly upregulated in the NS1 protein-treated group in a dose-dependent manner. These findings provide in vivo evidences for the immunomodulatory and replication-promoting effects of NS1 during DENV infection.