Abstract
To prolong residence on their hosts, ticks secrete many salivary factors that target host defense molecules. In particular, the tick Rhipicephalus sanguineus has been shown to produce three salivary glycoproteins named "evasins," which bind to host chemokines, thereby inhibiting the recruitment of leukocytes to the location of the tick bite. Using sequence similarity searches, we have identified 257 new putative evasin sequences encoded by the genomes or salivary or visceral transcriptomes of numerous hard ticks, spanning the genera Rhipicephalus, Amblyomma, and Ixodes of the Ixodidae family. Nine representative sequences were successfully expressed in Escherichia coli, and eight of the nine candidates exhibited high-affinity binding to human chemokines. Sequence alignments enabled classification of the evasins into two subfamilies: C8 evasins share a conserved set of eight Cys residues (four disulfide bonds), whereas C6 evasins have only three of these disulfide bonds. Most of the identified sequences contain predicted secretion leader sequences, N-linked glycosylation sites, and a putative site of tyrosine sulfation. We conclude that chemokine-binding evasin proteins are widely expressed among tick species of the Ixodidae family, are likely to play important roles in subverting host defenses, and constitute a valuable pool of anti-inflammatory proteins for potential future therapeutic applications.
Highlights
To prolong residence on their hosts, ticks secrete many salivary factors that target host defense molecules
To identify additional evasin candidates, we used the aligned sequences of the initial hits to generate a hidden Markov model (HMM)[4] with which we interrogated four databases: the UniProtKB protein sequence database; the genomic, transcriptomic, and peptide data in VectorBase; data from the NCBI Transcriptome Shotgun Sequence Assembly Database; and salivary gland and viscera transcriptomes from the cattle tick Rhipicephalus microplus and the Australian paralysis tick Ixodes holocyclus (Fig. 1, Phase 2)
In combination with the initial search results, these searches identified a total of 428 sequences, which were curated for conserved features to yield a final database of 257 putative evasins with 9 – 46% sequence identity to R. sanguineus evasin-1 and -4
Summary
To prolong residence on their hosts, ticks secrete many salivary factors that target host defense molecules. The tick Rhipicephalus sanguineus has been shown to produce three salivary glycoproteins named “evasins,” which bind to host chemokines, thereby inhibiting the recruitment of leukocytes to the location of the tick bite. We have identified 257 new putative evasin sequences encoded by the genomes or salivary or visceral transcriptomes of numerous hard ticks, spanning the genera Rhipicephalus, Amblyomma, and Ixodes of the Ixodidae family. Leukocyte recruitment is mediated by chemokine proteins, which are secreted into the vasculature at the site of the injury. There, they bind to chemokine receptors on circulating leukocytes and thereby stimulate leukocyte trafficking to the damaged tissue (5, 6). Humans and other mammals produce an array of chemokines and chemokine receptors, which collectively orchestrate the migration of different types of leukocytes in response to different inflammatory stimuli (7, 8)
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