Abstract
Ticks are obligate hematophagous parasites and are important vectors of a wide variety of pathogens. These pathogens include spirochetes in the genus Borrelia that cause Lyme disease, rickettsial pathogens, and tick-borne encephalitis virus, among others. Due to their prolonged feeding period of up to two weeks, hard ticks must counteract vertebrate host defense reactions in order to survive and reproduce. To overcome host defense mechanisms, ticks have evolved a large number of pharmacologically active molecules that are secreted in their saliva, which inhibits or modulates host immune defenses and wound healing responses upon injection into the bite site. These bioactive molecules in tick saliva can create a privileged environment in the host’s skin that tick-borne pathogens take advantage of. In fact, evidence is accumulating that tick-transmitted pathogens manipulate tick saliva composition to enhance their own survival, transmission, and evasion of host defenses. We review what is known about specific and functionally characterized tick saliva molecules in the context of tick infection with the genus Borrelia, the intracellular pathogen Anaplasma phagocytophilum, and tick-borne encephalitis virus. Additionally, we review studies analyzing sialome-level responses to pathogen challenge.
Highlights
Ticks are capable of transmitting a wide variety of pathogens including viruses, bacteria, protozoans, fungi, and nematodes of medical and veterinary importance [1]
A. phagocytophilum, B. burgdorferi, and tick-borne encephalitis virus (TBEV), all manipulate gene expression in tick salivary glands, in particular those coding for anticlotting factors, immune inhibitors, and proteins involved in cement protein production
To support the uptake of large volumes of blood over a long duration of time, ticks have evolved a large repertoire of salivary molecules to counteract host defense mechanisms
Summary
Ticks are capable of transmitting a wide variety of pathogens including viruses, bacteria, protozoans, fungi, and nematodes of medical and veterinary importance [1]. Other immune cells residing in the skin such as Langerhans and dermal dendritic cells (DC) migrate to the lymph nodes and activate inflammatory and immune responses upon encountering antigens [14,15] To counteract these challenges, ticks have evolved and acquired several effectors that diminish these immune and hemostatic responses. Tick salivary glands release a wide number of immunomodulatory and anti-hemostatic molecules These components maintain blood flow and reduce itching, inflammation, and immune rejection at the skin interface with the attached tick, allowing blood feeding to succeed. Macrophages [22] and can alter dendritic cell activation and function [23,24] Through these bioactive components, tick saliva creates an immune-privileged local environment at the bite site that facilitates pathogen transmission. B. burgdorferi, Anaplasma phagocytophilum, and tick-borne encephalitis virus (TBEV)
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