Abstract
Ticks are ectoparasitic arthropods that necessarily feed on the blood of their vertebrate hosts. The success of blood acquisition depends on the pharmacological properties of tick saliva, which is injected into the host during tick feeding. Saliva is also used as a vehicle by several types of pathogens to be transmitted to the host, making ticks versatile vectors of several diseases for humans and other animals. When a tick feeds on an infected host, the pathogen reaches the gut of the tick and must migrate to its salivary glands via hemolymph to be successfully transmitted to a subsequent host during the next stage of feeding. In addition, some pathogens can colonize the ovaries of the tick and be transovarially transmitted to progeny. The tick immune system, as well as the immune system of other invertebrates, is more rudimentary than the immune system of vertebrates, presenting only innate immune responses. Although simpler, the large number of tick species evidences the efficiency of their immune system. The factors of their immune system act in each tick organ that interacts with pathogens; therefore, these factors are potential targets for the development of new strategies for the control of ticks and tick-borne diseases. The objective of this review is to present the prevailing knowledge on the tick immune system and to discuss the challenges of studying tick immunity, especially regarding the gaps and interconnections. To this end, we use a comparative approach of the tick immune system with the immune system of other invertebrates, focusing on various components of humoral and cellular immunity, such as signaling pathways, antimicrobial peptides, redox metabolism, complement-like molecules and regulated cell death. In addition, the role of tick microbiota in vector competence is also discussed.
Highlights
Ticks (Acari: Ixodida) are ectoparasitic arthropods that obligatorily feed on the blood of a diverse list of vertebrate hosts, including mammals, birds, reptiles, and even amphibians
The results showed that the 5.3-kDa antimicrobial peptides (AMPs) is an effector regulated by the JAK/STAT pathway, which is essential to restrict A. phagocytophilum proliferation in tick salivary glands and hemolymph but not in the gut, indicating that additional effectors under JAK/STAT pathway regulation are required in this organ [53]
Studies on the interactions between ticks and tick-borne pathogens (TBPs) have shown that both the immune deficiency (IMD) and JAK/STAT pathways are key for the control of bacterial infections (B. burgdorferi, A. marginale and A. phagocytophilum), while the Toll and RNA interference (RNAi) pathways might be involved in tick defense against viral infections
Summary
Ticks (Acari: Ixodida) are ectoparasitic arthropods that obligatorily feed on the blood of a diverse list of vertebrate hosts, including mammals, birds, reptiles, and even amphibians. In addition to ingesting blood, ticks secrete saliva into the host during feeding. Tick saliva, produced by their salivary glands, returns excess water and ions to the host, thereby concentrating the blood meal [5]. The antihemostatic and immunomodulatory properties of saliva can facilitate the infection of pathogens that use saliva as a vehicle to be transmitted to the host during tick blood feeding [6, 8]. The first organ that a pathogen acquired within the blood meal interacts with is the tick gut (Figure 1). The pathogen must reach the salivary glands In each of these organs, the pathogen must counteract tick immune factors to be successfully transmitted through saliva to the vertebrate host in a subsequent bloodfeeding [11]. The role of tick microbiota in vector competence is discussed
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