Lipid Interactions Between Flaviviruses and Mosquito Vectors

Lipid Metabolism in Insect Vectors of Diseases

According to the World Health Organization vector-borne diseases account for more than 17% of all infectious diseases, causing more than 700,000 deaths annually. Vectors are organisms that are able to transmit infectious pathogens between humans, or from animals to humans. Many of these vectors are hematophagous insects, which ingest the pathogen from an infected host during a blood meal, and later transmit it into a new host. Malaria, dengue, African trypanosomiasis, yellow fever, leishmaniasis, Chagas disease, and many others are examples of diseases transmitted by insects.Both the diet and the infection with pathogens trigger changes in many metabolic pathways, including lipid metabolism, compared to other insects. Blood contains mostly proteins and is very poor in lipids and carbohydrates. Thus, hematophagous insects attempt to efficiently digest and absorb diet lipids and also rely on a large de novo lipid biosynthesis based on utilization of proteins and carbohydrates as carbon source. Blood meal triggers essential physiological processes as molting, excretion, and oogenesis; therefore, lipid metabolism and utilization of lipid storage should be finely synchronized and regulated regarding that, in order to provide the necessary energy source for these events. Also, pathogens have evolved mechanisms to hijack essential lipids from the insect host by interfering in the biosynthesis, catabolism, and transport of lipids, which pose challenges to reproduction, survival, fitness, and other insect traits.In this chapter, we have tried to collect and highlight the current knowledge and recent discoveries on the metabolism of lipids in insect vectors of diseases related to the hematophagous diet and pathogen infection.

Evidence of Differences in Cellular Regulation of Wolbachia-Mediated Viral Inhibition between Alphaviruses and Flaviviruses

The intracellular bacterium Wolbachia is increasingly being utilised in control programs to limit the spread of arboviruses by Aedes mosquitoes. Achieving a better understanding of how Wolbachia strains can reduce viral replication/spread could be important for the long-term success of such programs. Previous studies have indicated that for some strains of Wolbachia, perturbations in lipid metabolism and cholesterol storage are vital in Wolbachia-mediated antiviral activity against the flaviviruses dengue and Zika; however, it has not yet been examined whether arboviruses in the alphavirus group are affected in the same way. Here, using the reporters for the alphavirus Semliki Forest virus (SFV) in Aedes albopictus cells, we found that Wolbachia strains wMel, wAu and wAlbB blocked viral replication/translation early in infection and that storage of cholesterol in lipid droplets is not key to this inhibition. Another alphavirus, o'nyong nyong virus (ONNV), was tested in both Aedes albopictus cells and in vivo in stable, transinfected Aedes aegypti mosquito lines. The strains wMel, wAu and wAlbB show strong antiviral activity against ONNV both in vitro and in vivo. Again, 2-hydroxypropyl-β-cyclodextrin (2HPCD) was not able to rescue ONNV replication in cell lines, suggesting that the release of stored cholesterol caused by wMel is not able to rescue blockage of ONNV. Taken together, this study shows that alphaviruses appear to be inhibited early in replication/translation and that there may be differences in how alphaviruses are inhibited by Wolbachia in comparison to flaviviruses.

The influence of the larval microbiome on susceptibility to Zika virus is mosquito genotype-dependent

The microbiome of the mosquito Aedes aegypti is largely determined by the environment and influences mosquito susceptibility for arthropod-borne viruses (arboviruses). Larval interactions with different bacteria can have carry-over effects on adult Ae. aegypti replication of arboviruses, but little is known about the role that mosquito host genetics play in determining how larval-bacterial interactions shape Ae aegypti susceptibility to arboviruses. To address this question, we isolated single bacterial isolates and complex microbiomes from Ae. aegypti larvae from various field sites in Senegal. Either single bacterial isolates or complex microbiomes were added to two different genetic backgrounds of Ae. aegypti in a gnotobiotic larval system. Using 16S amplicon sequencing we showed that the bacterial community structure differs between the two genotypes of Ae. aegypti when given identical microbiomes, and the abundance of single bacterial taxa differed between Ae. aegypti genotypes. Using single bacterial isolates or the entire preserved complex microbiome, we tested the ability of specific larval microbiomes to drive differences in infection rates for Zika virus in different genetic backgrounds of Ae. aegypti. We observed that the proportion of Zika virus-infected adults was dependent on the interaction between the larval microbiome and Ae. aegypti host genetics. By using the larval microbiome as a component of the environment, these results demonstrate that interactions between the Ae. aegypti genotype and its environment can influence Zika virus infection. As Ae. aegypti expands and adapts to new environments under climate change, an understanding of how different genotypes interact with the same environment will be crucial for implementing arbovirus transmission control strategies.

Profiling lipidomic changes in dengue-resistant and dengue-susceptible strains of Colombian Aedes aegypti after dengue virus challenge

The mosquito Aedes aegypti is the primary vector for all four serotypes of dengue viruses (DENV1-4), which infect millions across the globe each year. Traditional insecticide programs have been transiently effective at minimizing cases; however, insecticide resistance and habitat expansion have caused cases of DENV to surge over the last decade. There is an urgent need to develop novel vector control measures, but these are contingent on a detailed understanding of host-parasite interactions. Here, we have utilized lipidomics to survey the profiles of naturally DENV-resistant (Cali-MIB) or susceptible (Cali-S) populations of Ae. aegypti, isolated from Cali, Colombia, when fed on blood meals containing DENV. Control insects were fed on a DENV-free blood meal. Midguts were dissected from Cali-MIB and Cali-S females at three time points post-infectious blood meal, 18, 24 and 36h, to identify changes in the lipidome at key times associated with the entry, replication and exit of DENV from midgut cells. We used principal component analysis to visualize broad patterns in lipidomic profiles between the treatment groups, and significance analysis of microarray to determine lipids that were altered in response to viral challenge. These data can be used to identify molecules or metabolic pathways particular to the susceptible or refractory phenotypes, and possibly lead to the generation of stable, DENV-resistant strains of Ae. aegypti.

The influence of the larval microbiome on susceptibility to Zika virus is mosquito genotype dependent

The microbiome of the mosquito Aedes aegypti is largely determined by the environment and influences mosquito susceptibility for arthropod-borne viruses (arboviruses). Larval interactions with different bacteria can influence adult Ae. aegypti replication of arboviruses, but little is known about the role that mosquito host genetics play in determining how larval-bacterial interactions shape Ae aegypti susceptibility to arboviruses. To address this question, we isolated single bacterial isolates and complex microbiomes from Ae. aegypti larvae from various field sites in Senegal. Either single bacterial isolates or complex microbiomes were added to two different genetic backgrounds of Ae. aegypti in a gnotobiotic larval system. Using 16S amplicon sequencing we show that similarities in bacterial community structures when given identical microbiomes between different genetic backgrounds of Ae. aegypti was dependent on the source microbiome, and the abundance of single bacterial taxa differed between Ae. aegypti genotypes. Using single bacterial isolates or the entire preserved complex microbiome, we tested the ability of specific microbiomes to drive differences in infection rates for Zika virus in different genetic backgrounds of Ae. aegypti . We observed that the proportion of Zika virus-infected adults was dependent on the interaction between the larval microbiome and Ae. aegypti host genetics. By using the larval microbiome as a component of the environment, these results demonstrate that interactions between the Ae. aegypti genotype and its environment can influence Zika virus infection. As Ae. aegypti expands and adapts to new environments under climate change, an understanding of how different genotypes interact with the same environment will be crucial for implementing arbovirus transmission control strategies.

Cell Type Variability in the Incorporation of Lipids in the Dengue Virus Virion

A lipid bilayer produced from the host membrane makes up around 20% of the weight of the dengue virus (DENV) virion and is crucial for virus entry. Despite its significance, the virion's lipid composition is still poorly understood. In tandem with lipid profiles of the cells utilised to generate the virions, this work determined a partial lipid profile of DENV virions derived from two cell lines (C6/36 and LLC-MK₂). The results showed distinctive profiles between the two cell types. In the mammalian LLC-MK₂ cells, 30.8% (73/237 identified lipid species; 31 upregulated, 42 downregulated) of lipid species were altered in response to infection, whilst in insect C6/36 cells only 12.0% (25/208; 19 upregulated, 6 downregulated) of lipid species showed alterations in response to infection. For virions from LLC-MK₂ cells, 14 lipids were detected specifically in virions with a further seven lipids being enriched (over mock controls). For virions from C6/36 cells, 43 lipids were detected that were not seen in mock preparations, with a further 16 being specifically enriched (over mock control). These results provide the first lipid description of DENV virions produced in mammalian and mosquito cells, as well as the lipid changes in the corresponding infected cells.

Japanese Encephalitis Vaccine Generates Cross-Reactive Memory T Cell Responses to Zika Virus in Humans

Objective. Zika virus (ZIKV) and Japanese encephalitis virus (JEV) are mosquito-borne flaviviruses with sequence homology. ZIKV circulates in some regions where JEV also circulates, or where JE vaccination is used. Cross-immunity between flaviviruses exists, but the precise mechanisms remain unclear. We previously demonstrated that T cell immunity induced by the live-attenuated Japanese encephalitis (JE) SA14-14-2 vaccine conferred protective immunity against ZIKV infection in mice, which could even bypass antibody-dependent enhancement. However, the role of T cell immune, especially memory T cell subsets, in cross-reactive immune responses between JE vaccine and ZIKV in humans has not been reported. Methods. We examined central and effector memory CD4+ and CD8+ T cell (TCM and TEM) responses (including degranulation, cytokines, and chemokines) in the presence of JEV and ZIKV, respectively, by using qualified peripheral blood mononuclear cell samples from 18 children who had recently received a two-dose course of JE vaccine SA14-14-2 as well as seven children without JE vaccination. Results. Cross-reactive CD8+ TCM in response to ZIKV was characterized by secretion of IFN-γ, whereas CD8+ TEM did not show significant upregulation of functional factors. In the presence of ZIKV, IFN-γ and TNF-α expression was upregulated by CD4+ TEM, and the expression signature of CD4+ TCM is more cytotoxic potential. Conclusions. We profiled the cross-reactive memory T cell responses to ZIKV in JE vaccine recipients. These data will provide evidence for the mechanism of cross-reactive memory T cell immune responses between JEV and ZIKV and a more refined view of bivalent vaccine design strategy.