Discussion

The saliva of haematophagous arthropods is a cocktail of substances meant to facilitate bloodfeeding, by counteracting the host’s healing processes. These salivary components can also elicit significant immune responses, and while most research has focused on the salivary proteins, the sugars that modify them remain overlooked. Glycans influence a protein’s biological role, and so can be partially responsible for the saliva’s modulatory effects before, during and after pathogen transmission; furthermore, they can induce severe allergic reactions in some people. In some cases, salivary glycosylation pathways could influence viral glycosylation before transmission to the next host. In this work we studied the salivary glycome Amblyomma cajennense, Anopheles gambiae, Aedes aegypti, Glossina morsitans, Lutzomyia longipalpis and Rhodnius prolixus. To characterise the sugar structures in saliva we used a glycomics approach that included analyses by HPLC in combination with highly sensitive LC-MS/MS. Our work shows that the salivary glycoproteins of all these vectors are mostly composed of mannose-type sugars, with differences mainly in abundance of the various structures, as well as the presence of some hybrid sugars. Overlay work using recombinant fractions of human mannose receptors showed that salivary glycoproteins are positively recognised, which hints at in vivo interactions with macrophages and dendritic cells. These interactions may be responsible for the saliva-specific immune responses that affect the process of pathogen infection; additionally, they can have a role in the clearance (half-life) of the salivary glycoproteins themselves. Finally, the similarities of the sugars found indicate suggests the presence of conserved pathways of salivary protein glycosylation.