Host and viral determinants of flavivirus emergence and pathogenesis

Abstract

Arthropod-borne viruses (arboviruses) are emerging and re-emerging at an increasing rate. While increased globalization, urbanization, and a changing climate all contribute to this increase, molecular mechanisms at the host and virus level are also at play. In this dissertation research, I investigate aspects of both host and viral determinants of pathogenesis and emergence of present and future arboviral threats. In Chapter 2, I characterize the pathogenesis and transmission capacity of an under-characterized flavivirus, Spondweni virus (SPONV). I show that SPONV causes fetal harm comparable to Zika virus (ZIKV) in a mouse model of flavivirus pathogenesis during pregnancy and that SPONV can be transmitted by Aedes aegypti mosquitoes. In Chapter 3, I further characterize SPONV pathogenesis with the development of a non-human primate model of SPONV infection in macaques. Here, I find that while cynomolgus macaques (Macaca fascicularis) are resistant to SPONV infection, rhesus macaques (Macaca mulatta) are a viable model species. By serial re-challenge of rhesus macaques with the heterologous virus, I show that SPONV immunity does not protect against ZIKV infection, but ZIKV immunity is protective against SPONV infection. This one-way cross-protection has implications for understanding host susceptibility to emerging flaviviruses in the complex context of heterologous immune landscapes throughout human populations. In Chapter 4, I characterize viral genetic determinants of increased Zika virus (ZIKV) virulence and transmission potential. ZIKV is unique among arboviruses, in that in addition to host cycling between vertebrate and invertebrate hosts, it can also transmit directly via sexual transmission. We previously found that direct mammalian transmission chains of ZIKV result in viruses that are more virulent in mice, and this increase in virulence coincides with the acquisition of single amino acid substitutions in the viral genome. Here, I find that mouse-passaged viruses have increased transmission capacity in Aedes aegypti. Using a reverse genetics system to engineer the amino acid changes of these viruses into a ZIKV infectious clone, I define the contributions of these genetic changes to the enhanced virulence and transmission phenotype using in-vivo mouse and mosquito models. Here, I find that an NS4A substitution contributes to enhanced virulence and immune modulation in mice, however, additional factors seem to be required to fully recapitulate the increased lethality and transmission phenotype. Together, this dissertation characterizes the emergence potential of a novel flavivirus, SPONV, and the evolutionary trajectory and future risk of a current arboviral threat, ZIKV. More broadly, the studies described here highlight the role of basic virology in pandemic preparedness. Such studies characterizing flavivirus evolution, emergence, and pathogenesis strengthen the field’s capacity to predict and respond to arboviral threats.