| The four serotypes of dengue virus (DENV1-4) are the causative agents of the mosquito-borne illness dengue fever, which affects 50-100 million people and results in 240,000-500,000 cases of the life-threatening dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS) each year. An improved animal model for DENV infection would greatly facilitate characterization of the immune response and the mechanism of DENV pathogenesis in vivo. Two models were initially chosen in which to characterize viral and immunological parameters, the A/J and AG129 (129/Pas mice lacking IFN-alpha/beta and/or -gamma receptors) strains. The AG129 mice turned out to be the most appropriate model, and we therefore defined the cellular tropism of DENV in these mice, thereby establishing an alternative endpoint for studies of immune response and pathogenesis. During the first week post-infection via a subcutaneous route, DENV was present in lymph nodes, spleen, bone marrow, and circulating white blood cells. F4/80 +/CD11b+ macrophages and CD11c+ dendritic cells were demonstrated to be targets for DENV2 infection in the spleen by flow cytometry directed to structural and nonstructural DENV proteins and by magnetic bead separation followed by strand-specific RT-PCR. Importantly, this demonstrates that the initial cellular tropism of DENV in mice is similar to that reported in humans.; We then used viral tropism as an endpoint to characterize secondary DENV infection in mice, because previous infection can be either protective or pathogenic. AG129 mice were infected sequentially with DENV1 followed by DENV2 or with DENV2 followed by DENV4 at intervals of 4, 15, 33, and 52 weeks. In order to separate the role of antibodies and immune cells in protection observed during sequential infections, mice were passively transferred with either homologous or heterologous antibodies, or with homologous or heterologous DENV-immune spleen cells, followed by viral challenge. Both sequential heterologous infections and passive transfer of heterologous DENV-immune serum were found to be protective, as evidenced by reduced titers of the heterologous challenge virus. In contrast, adoptive transfer of DENV-immune cells only partially protected mice from challenge with homologous virus.; We have therefore defined both the cellular tropism and mechanism of heterologous protection in AG129 mice infected with DENV. Further studies will correlate in vivo protection studies with the in vitro assays used to evaluate natural and vaccine-induced immunity, thereby providing much needed information regarding the nature of anti-DENV antibodies necessary for understanding protection in natural infections and developing a safe and effective tetravalent dengue vaccine. |
