Zika virus evolution in the presence of dengue virus-elicited cross-reactive immunity

Abstract

Abstract Zika virus (ZIKV), belongs to Flavivirus, causes neurological disorders including Guillain Barré syndrome and fetal microcephaly, and is responsible for widespread epidemics in countries where other flaviviruses, such as dengue (DENV), and yellow fever viruses, are endemic. However, how the cross-reactive, pre-existing immunity impacts the evolution of flaviviruses is poorly understood. To mimic the natural transmission cycle of ZIKV between vertebrate (human) hosts with pre-existing flavivirus immunity and invertebrate hosts (mosquito), ZIKV was subjected to 10 alternating cycles of passaging between DENV-immune mice and mosquito cells. We identified two mutations in the passaged ZIKV strains- one in the NS2B gene (I39V), both after passage in DENV-naïve and DENV-immune mice, and a mutation in the E gene (T470M), only after passage in DENV-immune mice. These mutations increase ZIKV virulence and pathogenesis in non-pregnant and pregnant Ifnar1−/− mice and abrogate cross-protection mediated by the pre-existing DENV immunity. In addition, ZIKV strain with the I39V mutation was more virulent than the parental ZIKV strain in human fetal neural progenitor cells (NPCs), and ZIKV variants in position 39 of the NS2B protein were observed in recent clinical isolates of ZIKV. Our data thus identify a mutational hotspot in the ZIKV NS2B protein and a mutation in the ZIKV E protein that arose as a consequence of the pre-existing immune pressure. Our results uncover the evolutionary strategies by which ZIKV can become pathogenic in both DENV-naïve and DENV-immune populations. Mechanistic studies of evolution in the presence and absence of cross-reactive immunity will benefit the development of safe and efficient flavivirus vaccines.