Sialome diversity of ticks revealed by RNAseq of single tick salivary glands

Abstract

Ticks salivate while feeding on their hosts. Saliva helps blood feeding through host anti-hemostatic and immunomodulatory components. Previous transcriptomic and proteomic studies revealed the complexity of tick saliva, comprising hundreds of polypeptides grouped in several multi-genic families such as lipocalins, Kunitz-domain containing peptides, metalloproteases, basic tail secreted proteins, and several other families uniquely found in ticks. These studies also revealed that the composition of saliva changes with time; expression of transcripts from the same family wax and wane as a function of feeding time. Here, we examined whether host immune factors could influence sialome switching by comparing sialomes of ticks fed naturally on a rabbit, to ticks artificially fed on defibrinated blood depleted of immune components. Previous studies were based on transcriptomes derived from pools of several individuals. To get an insight into the uniqueness of tick sialomes, we performed transcriptomic analyses of single salivary glands dissected from individual adult female I. ricinus ticks. Multivariate analysis identified 1,279 contigs differentially expressed as a function of time and/or feeding mode. Cluster analysis of these contigs revealed nine clusters of differentially expressed genes, four of which appeared consistently across several replicates, but five clusters were idiosyncratic, pointing to the uniqueness of sialomes in individual ticks. The disclosure of tick quantum sialomes reveals the unique salivary composition produced by individual ticks as they switch their sialomes throughout the blood meal, a possible mechanism of immune evasion.

Wonders of tick saliva

“…Examination of salivary glands from single female I. ricinus after 24 h, 48 h, and 72 h feeding on rabbits by RNA sequencing confirmed the individuality of tick sialomes: single ticks were found to express unique clusters of genes (Perner et al, 2018). The pooling of 'quantum sialomes' has obvious benefits for ticks.…”

“…Currently, ~70,000 protein sequences deposited on GenBank are assigned to tick salivary glands . Transcriptomic studies indicate ticks of a given species may secrete >500 different proteins and peptides in their saliva over the course of feeding (Giovanni et al, 2014;Esteves et al, 2017;Perner et al, 2018). These are grouped into several multigene families, including lipocalins, Kunitz-domain containing proteins/peptides, metalloproteases, and basic tail secreted proteins, and several families apparently unique to ticks.…”

“…Thus, complement becomes available when the tick bites. The importance of complement for ticks is illustrated by the number and diversity of complement inhibitors in tick saliva (Jore et al, 2016;Šimo et al, 2017;Perner et al, 2018). In the salivary glands of I. ricinus, anti-complement activity was detected at all days of feeding and in salivary glands of unfed adult females, suggesting complement inhibitors are prepared before feeding commences and presumably stored within granular acini (Lawrie et al, 1999).…”

“…Dynamic differential expression of saliva genes is substantially greater in adult female compared with male ixodid ticks, reflecting the different goals of feeding females (maximise bloodmeal size to maximise egg production) and males (mating and mate guarding) (De Castro et al, 2017) (section 2.6). Transcriptomic analysis at the level of individual salivary glands confirmed expression of saliva genes in defined clusters as feeding progresses for adult female I. ricinus (Perner et al, 2018). The term 'sialome switching' has been coined to describe the changes in saliva composition as engorgement progresses (a physiological feeding 'clock') although it could also refer to changes in feeding environment resulting from e.g.…”

Tick-Borne Encephalitis Virus Infection Alters the Sialome of Ixodes ricinus Ticks During the Earliest Stages of Feeding

“…All the sequences derived from 5 day feeding ticks inhabit clades that are not shared by those upregulated by TBEV. This is not surprising, since ticks switch their sialomes with time (Karim and Ribeiro, 2015;Perner et al, 2018), possibly as an immune evasion mechanism. Thus, the different clades may represent proteins with different functions, or with different antigenic character.…”