Arboviruses are transmitted between vertebrate hosts by arthropod vectors, such as
mosquitoes or ticks. In vertebrates arboviruses cause cytopathic effects and disease, however,
arbovirus infection of arthropods usually results in persistence. Control of arboviral infection
is mediated by the arthropod’s immune system. Pathways such as RNAi, JAK/STAT, Toll
and IMD have previously been implicated in controlling arbovirus infections. In contrast, the
antiviral role of other pathways in mosquitoes, such as melanisation, is unknown. Using high
through output 454 sequencing the transcriptome of U4.4 cells infected with the model
arbovirus Semliki Forest virus (SFV)(Togaviridae, Alphavirus) was generated. This
experiment revealed intriguing patterns of differential transcript abundance that suggest a
broad impact of SFV infection in U4.4 cells, such as in metabolism, cell structure and nucleic
acid processing. SFV infection induces differential expression of genes in pathways such as
apoptosis, stress response and cell cycle. Most interestingly, this study indicated that
melanisation might have an antiviral role in mosquitoes. In arthropods, melanisation is a
process involved in wound healing and antimicrobial defences. Phenoloxidase (PO), a key
enzyme involved in melanisation, is cytotoxic and therefore kept in its inactive form,
prophenoloxidase (PPO), until activation is triggered. The PPO activation process is tightly
regulated by serine protease inhibitors (serpins) which inhibit the proteolytic activation
reaction. In this thesis I demonstrate that the supernatant of cultured Aedes albopictus-derived
U4.4 cells contains a functional proPO-activating system, which is activated by infection with
bacteria and virions of SFV. Activation of this pathway reduces the spread and infectivity of
SFV in vitro and in vivo. In order to further characterise the PO cascade and its antiviral role
the serpins in Ae. albopictus were also investigated. Using the transcriptome sequencing and
bioinformatics we identified and classified 11 serpins. We silenced each of the serpins and
monitored PPO levels and antiviral activity showing that homologues to drosophila’s serpin-
27a plays a role in melanisation against SFV in vitro. Collectively, these results characterise
the mosquito PO cascade as a novel immune defence against arbovirus infection in
mosquitoes
