The transcriptome of the salivary glands of the female western black-legged tick Ixodes pacificus (Acari: Ixodidae)
Introduction
Lyme disease is the most prevalent vector-borne disease in the US and is transmitted by the tick vectors Ixodes scapularis and I. pacificus in eastern and western North America, respectively (Barbour, 1998). Humans usually acquire Lyme disease when an infected nymphal-stage Ixodes sp. tick attaches and transmits the spirochete Borrelia burgdorferi (Burgdorfer et al., 1985). I. scapularis and I. pacificus transmit other zoonotic agents besides the Lyme disease spirochete, such as Anaplasma phagocytophilum (both species) or Babesia microti (I. scapularis only) (Barbour, 1998). Transmission is facilitated by tick saliva that operates not only as a carrier for Borrelia sp. but also contains a large repertoire of molecules that counteract the host response to injury (Ribeiro and Francischetti, 2003), allowing ticks to feed for days (Sonenshine, 1985). Accordingly, many biologic activities have been described in tick saliva, including molecules that impair platelet aggregation or neutrophil function (Ribeiro et al., 1985) in addition to coagulation inhibitors such as ixolaris and penthalaris that block Factor VIIa/tissue factor complex (Francischetti et al., 2002a, Francischetti et al., 2004a) and SALP 14, which targets Factor Xa (Narasimhan et al., 2002). Enzymes such as a kininase that degrades bradykinin (Ribeiro and Mather, 1998), an apyrase that destroys ADP (Ribeiro et al., 1985), and a metalloprotease with fibrin(ogen)olytic activity (Francischetti et al., 2003) also have been reported. Tick saliva is also rich in small molecules such as prostacyclin, a potent inhibitor of platelet activation and strong inducer of vasodilation (Ribeiro et al., 1988).
As for the immune system, an inhibitor of the alternative complement pathway exists in ixodid tick saliva (Valenzuela et al., 2000). Immunomodulators affecting NK cell function (Kopecky and Kuthejlova, 1998)—in addition to inhibitors of the proliferation of T lymphocytes and an IL-2 binding activity—also are present in this secretion (Ramachandra and Wikel, 1992; Gillespie et al., 2001). Finally, saliva is important in transmission of tick-borne pathogens, as it may enhance pathogen transmission (for a review, see Wikel, 1999).
The pace of discovery of tick salivary proteins has been greatly increased by novel molecular biology techniques and bioinformatics analysis (Ribeiro and Francischetti, 2003). Our goal here has been to further study the complexity of I. pacificus salivary glands. We report the full-length clone of 87 novel sequences and discuss their potential role in modulating host inflammatory and immune responses.
Section snippets
Reagents
All water used was of 18 MΩ quality and was produced using a MilliQ apparatus (Millipore, Bedford, MA, USA). Organic compounds were obtained from Sigma (St. Louis, MO, USA) or as stated otherwise.
Salivary gland cDNA library construction and sequencing
Ticks were collected in northern California by dragging low vegetation with a tick-drag. Salivary glands were excised and kept at −80 °C until use. The mRNA from two pairs of I. pacificus salivary glands was obtained using a Micro-Fast Track mRNA isolation kit (Invitrogen, San Diego, CA, USA) according
Results and discussion
I. scapularis and I. pacificus are the respective vectors for B. burgdorferi in the eastern and western US (Fig. 1). After attachment to the host, infected ticks transmit B. burgdoferi after 1–2 days of blood-feeding (Barbour, 1998) via saliva, a secretion that contains a cocktail of bioactive molecules (Ribeiro and Francischetti, 2003). Actually, the identification of the transcripts and proteins present in the salivary gland of ticks such as I. scapularis (Valenzuela et al., 2002), Boophilus
Acknowledgements
We thank Drs. Thomas E. Wellems, Robert W. Gwadz, and Thomas J. Kindt for encouragement and support. We are thankful to Brenda Rae Marshal for editorial assistance.
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