Phenoloxidase Activity Acts as a Mosquito Innate Immune Response against Infection with Semliki Forest Virus

Michael R. Strand,Alain Kohl,Margo E. Chase-Topping,Rennos Fragkoudis,Esther Schnettler,Andres Merits,Markus H. Beck,John K. Fazakerley,Seema Rani,Mhairi C. Ferguson,Julio Rodriguez-Andres,Gerald Barry,Kenneth D. Vernick,Margus Varjak

doi:10.1371/journal.ppat.1002977

Abstract

Several components of the mosquito immune system including the RNA interference (RNAi), JAK/STAT, Toll and IMD pathways have previously been implicated in controlling arbovirus infections. In contrast, the role of the phenoloxidase (PO) cascade in mosquito antiviral immunity is unknown. Here we show that conditioned medium from the Aedes albopictus-derived U4.4 cell line contains a functional PO cascade, which is activated by the bacterium Escherichia coli and the arbovirus Semliki Forest virus (SFV) (Togaviridae; Alphavirus). Production of recombinant SFV expressing the PO cascade inhibitor Egf1.0 blocked PO activity in U4.4 cell- conditioned medium, which resulted in enhanced spread of SFV. Infection of adult female Aedes aegypti by feeding mosquitoes a bloodmeal containing Egf1.0-expressing SFV increased virus replication and mosquito mortality. Collectively, these results suggest the PO cascade of mosquitoes plays an important role in immune defence against arboviruses.

Highlights

The transmission of arboviruses by mosquitoes and other arthropod vectors has considerable adverse impacts on human and animal health
Semliki Forest virus, we show that this virus activates the PO cascade
By using recombinant Semliki Forest virus expressing an inhibitor of the PO cascade, we demonstrate that this pathway inhibits virus spread in cell culture

Summary

Introduction

The transmission of arboviruses by mosquitoes and other arthropod vectors has considerable adverse impacts on human and animal health. This group of pathogens consists primarily of viruses in the families Flaviviridae, Togaviridae Bunyaviridae, and Reoviridae [1,2,3,4]. Arboviruses replicate in both vertebrate and arthropod hosts. The genus Alphavirus (family Togaviridae) contains several mosquito-vectored arboviruses including models like Sindbis virus (SINV) and Semliki Forest virus (SFV) [5,6] and the re-emerging human pathogen chikungunya virus (CHIKV) [7]. Infection of mosquito cell cultures has been useful to study arbovirus replication, allowing increasingly detailed studies of arbovirus/vector interactions [13,14]

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