Quantitative proteomics analysis reveals core and variable tick salivary proteins at the tick-vertebrate host interface

Tick exposure biomarkers: A One Health approach to new tick surveillance tools

The spread of tick-borne disease (TBD) is escalating globally, driven by climate change and socio-economic shifts, underlining the urgency to improve surveillance, diagnostics, and control strategies. Ticks can transmit a range of pathogens increasing the risk of transmission of human and veterinary diseases such as Lyme disease, tick-borne encephalitis, theileriosis, anaplasmosis, or Crimean-Congo hemorrhagic fever. Surveillance methods play a crucial role in monitoring the spread of tick-borne pathogens (TBP). However, there are shortcomings in the current surveillance methods regarding risks related to ticks. Human-tick encounters offer a novel metric for disease risk assessment, integrating human behavior into traditional surveillance models. However, to more reliably measure tick exposure, a molecular marker is needed. The identification of antibodies against arthropod salivary proteins as biomarkers for vector exposure represents a promising avenue for enhancing existing diagnostic and surveillance metrics. Here we explore how the use of tick saliva biomarkers targeting recombinant proteins and synthetic peptides could significantly improve the assessment of TBD transmission risk and the effectiveness of vector control measures. With focused efforts on creating a biomarker against tick exposure suitable for humans and domestic animals alike, tick surveillance, diagnosis and control would be more achievable and aid in reducing the mounting threat of TBP through a One Health lens.

The immune factors involved in the rapid clearance of bacteria from the midgut of the tick Ixodes ricinus

Ticks are obligate hematophagous arthropods that transmit a wide range of pathogens to humans as well as wild and domestic animals. They also harbor a non-pathogenic microbiota, although our previous study has shown that the diverse bacterial microbiome in the midgut of Ixodes ricinus is quantitatively poor and lacks a core. In artificial infections by capillary feeding of ticks with two model bacteria (Gram-positive Micrococcus luteus and Gram-negative Pantoea sp.), rapid clearance of these microbes from the midgut was observed, indicating the presence of active immune mechanisms in this organ. In the current study, RNA-seq analysis was performed on the midgut of I. ricinus females inoculated with either M. luteus or Pantoea sp. or with sterile water as a control. While no immune-related transcripts were upregulated by microbial inoculation compared to that of the sterile control, capillary feeding itself triggered dramatic transcriptional changes in the tick midgut. Manual curation of the transcriptome from the midgut of unfed I. ricinus females, complemented by the proteomic analysis, revealed the presence of several constitutively expressed putative antimicrobial peptides (AMPs) that are independent of microbial stimulation and are referred to here as 'guard' AMPs. These included two types of midgut-specific defensins, two different domesticated amidase effector 2 (Dae2), microplusin/ricinusin-related molecules, two lysozymes, and two gamma interferon-inducible lysosomal thiol reductases (GILTs). The in vitro antimicrobial activity assays of two synthetic mature defensins, defensin 1 and defensin 8, confirmed their specificity against Gram-positive bacteria showing exceptional potency to inhibit the growth of M. luteus at nanomolar concentrations. The antimicrobial activity of midgut defensins is likely part of a multicomponent system responsible for the rapid clearance of bacteria in the tick midgut. Further studies are needed to evaluate the role of other identified 'guard' AMPs in controlling microorganisms entering the tick midgut.

An Unusual Two-Domain Thyropin from Tick Saliva: NMR Solution Structure and Highly Selective Inhibition of Cysteine Cathepsins Modulated by Glycosaminoglycans

The structure and biochemical properties of protease inhibitors from the thyropin family are poorly understood in parasites and pathogens. Here, we introduce a novel family member, Ir-thyropin (IrThy), which is secreted in the saliva of Ixodes ricinus ticks, vectors of Lyme borreliosis and tick-borne encephalitis. The IrThy molecule consists of two consecutive thyroglobulin type-1 (Tg1) domains with an unusual disulfide pattern. Recombinant IrThy was found to inhibit human host-derived cathepsin proteases with a high specificity for cathepsins V, K, and L among a wide range of screened cathepsins exhibiting diverse endo- and exopeptidase activities. Both Tg1 domains displayed inhibitory activities, but with distinct specificity profiles. We determined the spatial structure of one of the Tg1 domains by solution NMR spectroscopy and described its reactive center to elucidate the unique inhibitory specificity. Furthermore, we found that the inhibitory potency of IrThy was modulated in a complex manner by various glycosaminoglycans from host tissues. IrThy was additionally regulated by pH and proteolytic degradation. This study provides a comprehensive structure-function characterization of IrThy-the first investigated thyropin of parasite origin-and suggests its potential role in host-parasite interactions at the tick bite site.

Cellular stress is triggered by tick-borne encephalitis virus and limits the virus replication in PMJ2-R mouse macrophage cell line

Viral infection may represent a stress condition to the host cell. Cells react to it by triggering the defence programme to restore homeostasis and these events may in turn impact the viral replication. The knowledge about tick-borne encephalitis virus (TBEV) infection-associated stress is limited. Here we investigated the interplay between TBEV infection and stress pathways in PMJ2-R mouse macrophage cell line, as macrophages are the target cells in early phases of TBEV infection. First, to determine how stress influences TBEV replication, the effect of stress inducers H2O2 and tunicamycin (TM) was tested. Viral multiplication was decreased in the presence of both stress inducers suggesting that the stress and cellular stress responses restrict the virus replication. Second, we investigated the induction of oxidative stress and endoplasmic reticulum (ER) stress upon TBEV infection. The level of oxidative stress was interrogated by measuring the reactive oxygen species (ROS). ROS were intermittently increased in infected cells at 12 hpi and at 72 hpi. As mitochondrial dysfunction may result in increased ROS level, we evaluated the mitochondrial homeostasis by measuring the mitochondrial membrane potential (MMP) and found that TBEV infection induced the hyperpolarization of MMP. Moreover, a transient increase of gene expression of stress-induced antioxidative enzymes, like p62, Gclm and Hmox1, was detected. Next, we evaluated the ER stress upon TBEV infection by analysing unfolded protein responses (UPR). We found that infection induced gene expression of two general sensors BiP and CHOP and activated the IRE1 pathway of UPR. Finally, since the natural transmission route of TBEV from its tick vector to the host is mediated via tick saliva, the impact of tick saliva from Ixodes ricinus on stress pathways in TBEV-infected cells was tested. We observed only marginal potentiation of UPR pathway. In conclusion, we found that TBEV infection of PMJ2-R cells elicits the changes in redox balance and triggers cellular stress defences, including antioxidant responses and the IRE1 pathway of UPR. Importantly, our results revealed the negative effect of stress-evoked events on TBEV replication and only marginal impact of tick saliva on stress cellular pathways.

Insight Into the Dynamics of the Ixodes ricinus Nymphal Midgut Proteome

Ticks are ectoparasites that feed on blood and have an impressive ability to consume and process enormous amounts of host blood, allowing extremely long periods of starvation between blood meals. The central role in the parasitic lifestyle of ticks is played by the midgut. This organ efficiently stores and digests ingested blood and serves as the primary interface for the transmission of tick-borne pathogens. In this study, we used a label-free quantitative approach to perform a novel dynamic proteomic analysis of the midgut of Ixodesricinus nymphs, covering their development from unfed to pre-molt stages. We identified 1534 I. ricinus-specific proteins with a relatively low proportion of host proteins. This proteome dataset, which was carefully examined by manual scrutiny, allowed precise annotation of proteins important for blood meal processing and their dynamic changes during nymphal ontogeny. We focused on midgut molecules related to lipid hydrolysis, storage, and transport, opening a yet unexplored avenue for studying lipid metabolism in ticks. Further dynamic profiling of the tick's multi-enzyme digestive network, protease inhibitors, enzymes involved in redox homeostasis and detoxification, antimicrobial peptides, and proteins responsible for midgut colonization by Borrelia spirochetes promises to uncover new targets for targeting tick nymphs, the most critical life stage for transmission the pathogens that cause tick-borne diseases.

The diurnal salivary glands transcriptome of Dermacentor nuttalli from the first four days of blood feeding

The ixodid tick Dermacentor nuttalli is distributed from southern Siberia to North China and is a vector of many pathogens. This species can have severe impacts on animal husbandry and human health. To date, the control of D. nuttalli is limited to the use of acaricides such as organophosphorus, synthetic pyrethroids and amidine pesticides. There are no environmentally friendly or reliable prevention and control measures, and little is known regarding key antigens involved in blood feeding. Salivary glands are major tissues involved in the blood feeding and pathogen transmission of ticks. Therefore, this study focused on salivary glands tissue to identify the dominant antigens of D. nuttalli involved in tick feeding. For this, high-throughput RNA sequencing (RNA-seq) was used for analysis. The transcriptome of female D. nuttalli ticks was assembled and characterized, and differentially expressed genes (DEGs) were identified in the salivary glands of ticks that had not fed (0 h) and of ticks after 24, 48, 72 and 96 h of feeding. There were 22,802,784, 22,275,013, 26,629,453, 24,982,389, and 22,596,230 high-quality clean reads obtained from salivary glands tissues at the five different blood feeding time points. The total number of annotated unigenes was 100,347. The differences in gene expression between different time points were compared, and functional enrichment was performed. Quantitative reverse transcription PCR (RT‒qPCR) was used to validate the RNA-seq results, the results of which showed that the differences in expressed transcripts presented similar trends. Among the identified DEGs, the most numerous were those with catalytic and binding activities and those involved in diverse metabolic pathways and cellular processes. The expression patterns of homologous and family-member proteins throughout the blood feeding period exhibited significant differences, strongly suggesting that the transcriptome composition is highly dynamic and likely subjected to important variation throughout the life cycle. Studies of gene sequences in D. nuttalli will greatly increase the information on tick protective antigens, which could potentially function as effective vaccine candidates or drug targets for the development of environmentally friendly acaricides.