Structure and dynamics of pathway activation by the Toll immunoreceptor from the viral mosquito vector Aedes aegypti

A. aegypti has evolved to become an efficient vector for arboviruses such as Dengue but the mechanisms of host-pathogen immune tolerance are unknown. Toll receptors and Spaetzle (Spz) ligands have undergone duplication raising the possibility of neofunctionalization and mutualism to develop between arboviruses and mosquitoes. Here we present cryo-EM structures and biophysical characterisation of low affinity Toll5A-Spz1C complexes that display transient but specific interactions. Binding of the first ligand alters receptor-receptor interactions and promotes asymmetric contacts in the vicinity of the Z-loop in Toll5A. This conformation then restricts binding of a second ligand, while temporarily bridging the C-termini that promote signalling. Increased receptor concentrations promote inactivating head-to-head receptor assemblies. Furthermore, the transcriptional signature of Spz1C differs from other Spz cytokines in the control of genes involved in innate immunity, lipid metabolism and tissue regeneration. Given the remarkable DENV-induced expression patterns of these proteins, our data rationalises how Spz1C upregulation might promote antimicrobial defence in the midgut, and Toll5A upregulation, viral tolerance in the salivary glands.