Antiviral Immunity and Virus-Mediated Antagonism in Disease Vector Mosquitoes

Abstract

More than 100 pathogens, spanning multiple virus families, broadly termed 'arthropod-borne viruses (arboviruses)' have been associated with human and/or animal diseases. These viruses persist in nature through transmission cycles that involve alternating replication in susceptible vertebrate and invertebrate hosts. Collectively, these viruses are among the greatest burdens to global health, due to their widespread prevalence, and the severe morbidity and mortality they cause in human and animal hosts. Specific examples of mosquito-borne pathogens include Zika virus (ZIKV), West Nile virus (WNV), dengue virus serotypes 1-4 (DENV 1-4), Japanese encephalitis virus (JEV), yellow fever virus (YFV), chikungunya virus (CHIKV), and Rift Valley fever virus (RVFV). Interactions between arboviruses and the immune pathways of vertebrate hosts have been extensively reviewed. In this review we focus on the antiviral immune pathways present in mosquitoes. We also discuss mechanisms by which mosquito-borne viruses may antagonize antiviral pathways in disease vectors. Finally, we elaborate on the possibility that mosquito-borne viruses may be engaged in an evolutionary arms race with their invertebrate vector hosts, and the possible implications of this for understanding the transmission of mosquito-borne viruses.

Keywords: RNA interference; RNA silencing; RNAi; VSR; arbovirus; suppressor.

Figure 1. Viruses have evolved diverse proteins (VSRs) that interfere with various steps of the RNA silencing pathway

Viral replication triggers Dcr-2 processing of dsRNA intermediates into siRNA duplexes. Virus-derived siRNAs are then loaded into the RISC, a guide strand acting as a specificity determinant for the targeting and degradation of cognate viral mRNAs. VSRs bind long dsRNAs (YFV C, FHV B2, DCV 1A, H1N1 NS1 or TCV P38) preventing processing by Dicer, and/or the smaller siRNA duplexes (FHV B2, H1N1 NS1, TCV P38 or CymRSV) preventing loading into the RISC. VSRs may in some cases also interact directly with Ago2 (CrPV 1A or TCV P38) to inhibit slicing activity.

Figure 2. Overview of molecular pathways with antiviral or potential antiviral roles in mosquitoes

A) The RNAi pathway has emerged as the major antiviral response in mosquitoes; although, most of the mechanistic details of the pathway are derived from work in D. melanogaster or other model organisms. Recognition of viral dsRNA triggers Dicer-2 to process dsRNA into viral siRNAs which are then loaded into the RISC complex to target cognate viral RNAs for sequence specific degradation. B) Virus-derived piRNAs exhibit hallmarks of a ping-pong-mediated mechanism for biogenesis, but these products have not yet been ascribed any specific role in antiviral immunity. C) There is some evidence to suggest that the evolutionarily conserved Imd, Toll and Jak-STAT pathways contribute to the antiviral response in mosquitoes, possibly in a virus or tissue specific role. A connection between the siRNA and Jak-STAT pathways mediated by Vago, a secreted protein, has been postulated. D) The mosquito has recently been postulated to have adaptive antiviral responses mediated by reverse transcribed viral DNA (vDNA), which has been proposed to enter the piRNA pathway. E) In flies, a mechanism for the production of a non-canonical class of viral siRNAs released from exosome-like vesicles in circulating hemocytes has been described. As yet, it remains unclear if similar mechanisms exist in mosquito vectors.

Figure 3. Model for the co-evolutionary relationship between mosquito-borne viral pathogens and their disease vectors

If viral mechanisms antagonizing antiviral immunity are too weak, viral replication will decrease to unsustainable levels leading to non-productive infections. However, viral mechanisms antagonizing antiviral immunity too robustly will result in levels of virus replication consistent with disease and possibly death, which would also be disruptive to the transmission cycles maintaining these viruses in nature. Thus, levels of viral replication conducive for transmission to a susceptible vertebrate host must be defined by upper and lower limits, which may serve as a constraint on the evolution of pathogens with this type of biology.