Research

The model organism and the role of tick saliva in modulating host homeostasis

Ticks of the Ixodes ricinus species (Photo) are significant vectors of both veterinary and medically important pathogens in Europe. The tick saliva plays a crucial role in their lifecycle and the transmission of diseases. At the interface between the tick and the vertebrate host, the tick saliva components modulate the host's haemostasis and immunity, allowing the tick to obtain a blood meal and promoting the survival and transmission of tick-borne pathogens. Our group has summarized the known interactions between tick salivary molecules and vertebrate host haemostasis (https://pubmed.ncbi.nlm.nih.gov/22564820/).

Questing adult ticks of the Ixodes ricinus species. A female adult tick surrounded by five male adult ticks – Photo courtesy: Mr. Jan Erhart

Moreover, tick saliva also suppresses the host's immune response, enabling the tick to remain attached to the host for an extended period, thereby facilitating the transmission of pathogens. In another publication (https://pubmed.ncbi.nlm.nih.gov/26189360/) of our group, we summarize the known interactions between tick saliva (or tick salivary gland extracts-SGE) and different immune cell populations participating in the vertebrate host immune response.

Systems Biology approaches to understand the complexity of tick saliva composition

To explore the composition of tick saliva, our group has utilized transcriptomics and proteomics. Transcriptomic analysis, continually improved with advancements in next-generation sequencing (NGS) platforms, has been a significant achievement in this field, enabling a better understanding of the molecular mechanisms underlying tick hematophagy, pathogen transmission, and tick-host-pathogen interactions. Quantitative proteomics has provided deeper insights into the temporal dynamics of expression of tick salivary components, allowing identification of many tick salivary constituents. Our use of Systems Biology techniques has been invaluable in advancing our knowledge of tick saliva composition and its role in tick-host interactions. For more details, please refer to our related review publications (Links: https://pubmed.ncbi.nlm.nih.gov/33135186/ and https://pubmed.ncbi.nlm.nih.gov/26520005/).

A significant number of novel antigens are now under investigation as antigens for developing either new anti-tick vaccines or tests for tick exposure diagnosis. However, more research is required to develop new and more efficient antigen-based vaccines and diagnostic tests, including assessing the efficiency of various epitopes against different tick species to confirm their cross-reactivity and high immunogenicity. For more details, please refer to our related review publication (Link: https://pubmed.ncbi.nlm.nih.gov/36902400/).

Now we know that Ixodes ricinus saliva contains a complex mixture of bioactive molecules that target a wide spectrum of host defence mechanisms, allowing ticks to feed on the vertebrate host for several days. Our work hypothesis is that tick salivary proteins cluster in multigenic protein families, and individual family members display redundancy and pluripotency in their action to ameliorate or evade host homeostatic responses against tick feeding. Members of different protein families can target the same cellular or molecular pathway of the host physiological response to tick feeding (redundancy of tick regulators that target the same host homeostatic mechanism). The same tick salivary molecule may also target more than one host homeostatic mechanism (pluripotency in the action of a given salivary molecule). For more details, please see our related review publication (Link: https://pubmed.ncbi.nlm.nih.gov/26830726/).

Tick salivary molecules at the tick-host interface

Protease inhibitors are highly represented in tick saliva, and they are unique because of their pharmacological properties, specificity, selectivity, and affinity to their target host proteases. Our group explores the structure and function of tick protease inhibitors, with an emphasis on the superfamilies abundant in tick saliva. We have demonstrated the pluripotent and redundant pharmacological action of tick salivary protease inhibitors to the vertebrate host (Figure 1), and our current efforts focus on the development of possible practical applications. We describe tick salivary protease inhibitors that are showing potential as candidates for drug development by testing them in in vivo animal models of disease with the ultimate goal of progressing some of them to preclinical and clinical trials. For more details, please see our related recent review publications (Links: https://pubmed.ncbi.nlm.nih.gov/36675071/ and https://pubmed.ncbi.nlm.nih.gov/35711658/  and https://pubmed.ncbi.nlm.nih.gov/33072132/).

Figure 1: The pluripotent action of tick salivary protease inhibitors at the tick-host interface. Figure courtesy: Dr. Amine Jmel Please click on the image to enlarge it

Tick saliva contains not only proteins and metabolites but also nucleic acids, including RNA silencing signals, which are crucial for cross-species communication. Recent transcriptomic and proteomic projects have identified several tick genes and non-coding RNAs (ncRNAs) in tick saliva that are involved in the vector-host interaction. These tick ncRNAs, including long non-coding RNAs (lncRNAs) and small non-coding RNAs (sncRNAs), act as crucial players in subverting the host defence mechanisms. The combinatorial effects of tick miRNAs on host target genes could sustain the robust regulation of host genes and pathways that are imperative for the tick-host interaction. These miRNAs are believed to target host KEGG pathways such as "gap junction" and "inflammatory mediator regulation of TRP channels," that have a significant role in the host homeostatic response (Figure 2A).

The staggering number of ncRNA sequences discovered by high-throughput projects suggests that tick ncRNAs may modulate host gene expression by binding to regulatory host miRNAs, which are involved in host immune cell responses. Tick ncRNAs could regulate protein-coding genes through mRNA cleavage, direct translational repression, and mRNA destabilization. The hypothesis that lncRNAs may act as "sponge" molecules to compete with host mRNAs for host miRNA binding is quite intriguing and may affect host homeostatic responses to tick feeding (Figure 2B). However, these in-silico predictions need to be validated using systems-based approaches to characterize tick ncRNAs and to comprehend their involvement in tick-host interactions. Several academic and commercial research groups are now exploring ncRNA-based therapies and investigating the potential of miRNA therapeutics, as miRNAs are the most studied ncRNAs to date. A few vertebrate miRNAs have entered preclinical and clinical testing, paving the way for their potential use in humans. As tick ncRNAs are likely to be less immunogenic by their intrinsic ability to hijack and bypass host immunity, we believe that this emerging field might lead to ncRNA therapeutics useful for the treatment of various diseases.
For more details, please see our related recent review publications (Links: https://pubmed.ncbi.nlm.nih.gov/31320293/ and https://pubmed.ncbi.nlm.nih.gov/33154170/ and https://pubmed.ncbi.nlm.nih.gov/33466803/).

Figure 2: Possible strategies used by tick ncRNAs in host cells. (A) miR-8-3p, bantam-3p, mir-317-3p, and miR-279a-3p from Ixodes ricinus have been predicted to target gap junctions and inflammatory mediator regulation of TRP channels, which play a role in host homeostatic responses. (B) Host miRNAs regulate protein coding genes through mRNA cleavage and/or direct translational repression and/or mRNA destabilization. Ticks secrete exosomes in their saliva containing lncRNAs, which may compete with host mRNAs for host miRNA binding by acting as “sponge” molecules that inhibit host miRNA interactions with host mRNAs, thus affecting host homeostatic responses to tick feeding. (Figure from https://pubmed.ncbi.nlm.nih.gov/33135186/)

Epidemiological significance of ticks

Establishing surveillance/monitoring mechanisms and increasing our preparedness for a scenario in which ticks may serve as disease vectors is crucial to mitigate the potential emergence of serious public health issues in Southern Europe. Ticks are known to transmit a variety of pathogens, including viruses, bacteria, and parasites, that can cause severe illnesses in humans and animals. With the effects of climate change and human activities, tick populations are expanding and becoming more prevalent in areas previously unexposed, leading to an increased risk of disease transmission. This trend is particularly alarming in Europe, where ticks are already established as significant vectors of disease. By implementing surveillance and monitoring mechanisms, we can identify the presence and distribution of tick-borne diseases and take appropriate measures to prevent their spread. Increasing our preparedness through measures such as tick-exposure diagnostics and anti-tick vaccine development, education, and effective surveillance and treatment protocols can help mitigate the impact of tick-borne diseases on public health and the economy. In summary, proactive measures to monitor, prevent, and respond to tick-borne diseases are essential for protecting public health and minimizing the risk of future epidemics.