Identification and partial characterization of a Salp15 homolog from Ixodes ricinus

Host Immune Responses to Salivary Components - A Critical Facet of Tick-Host Interactions

Tick sialome is comprised of a rich cocktail of bioactive molecules that function as a tool to disarm host immunity, assist blood-feeding, and play a vibrant role in pathogen transmission. The adaptation of the tick's blood-feeding behavior has lead to the evolution of bioactive molecules in its saliva to assist them to overwhelm hosts' defense mechanisms. During a blood meal, a tick secretes different salivary molecules including vasodilators, platelet aggregation inhibitors, anticoagulants, anti-inflammatory proteins, and inhibitors of complement activation; the salivary repertoire changes to meet various needs such as tick attachment, feeding, and modulation or impairment of the local dynamic and vigorous host responses. For instance, the tick's salivary immunomodulatory and cement proteins facilitate the tick's attachment to the host to enhance prolonged blood-feeding and to modulate the host's innate and adaptive immune responses. Recent advances implemented in the field of "omics" have substantially assisted our understanding of host immune modulation and immune inhibition against the molecular dynamics of tick salivary molecules in a crosstalk between the tick-host interface. A deep understanding of the tick salivary molecules, their substantial roles in multifactorial immunological cascades, variations in secretion, and host immune responses against these molecules is necessary to control these parasites. In this article, we reviewed updated knowledge about the molecular mechanisms underlying host responses to diverse elements in tick saliva throughout tick invasion, as well as host defense strategies. In conclusion, understanding the mechanisms involved in the complex interactions between the tick salivary components and host responses is essential to decipher the host defense mechanisms against the tick evasion strategies at tick-host interface which is promising in the development of effective anti-tick vaccines and drug therapeutics.

Changing the Recipe: Pathogen Directed Changes in Tick Saliva Components

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.

What do we still need to know about Ixodes ricinus?

In spite of many decades of intensive research on Ixodes ricinus, the castor bean tick of Europe, several important aspects of its basic biology remain elusive, such as the factors determining seasonal development, tick abundance and host specificity, and the importance of water management. Additionally, there are more recent questions about the geographical diversity of tick genotypes and phenotypes, the role of migratory birds in the ecoepidemiology of I. ricinus, the importance of protective immune responses against I. ricinus, particularly in the context of vaccination, and the role of the microbiome in pathogen transmission. Without more detailed knowledge of these issues, it is difficult to assess the likely effects of changes in climate and biodiversity on tick distribution and activity, to predict potential risks arising from new and established tick populations and I. ricinus-borne pathogens, and to improve prevention and control measures. This review aims to discuss the most important outstanding questions against the backdrop of the current state of knowledge of this important tick species.

Strong interactions between Salp15 homologues from the tick I. ricinus and distinct types of the outer surface OspC protein from Borrelia

Ticks belonging to the genus Ixodes are parasites feeding on vertebrate blood and vectors for many pathogenic microbes, including Borrelia burgdorferi sensu lato spirochetes, the causative agent of Lyme borreliosis. The tick saliva contains a mixture of bioactive molecules showing a wide range of properties for efficient engorgement. One of the most extensively studied components of tick saliva is a 15-kDa salivary gland protein (Salp15) from Ixodes scapularis. This multifunctional protein suppresses the immune response of hosts through pleiotropic action on a few crucial defense pathways. Salp15 and its homologue from I. ricinus Iric1 have been also shown to bind to Borrelia burgdorferi sensu stricto outer surface protein C (OspC) permitting the spirochetes to evade antibody-mediated killing in the human host. Further studies revealed that Salp15 and Iric1 protected B. burgdorferi s. s. and B. garinii expressing OspC against the complement system. OspC is the most variable protein on the outer surface of Borrelia, which in addition to Salp15 can also bind other ligands, such as plasminogen, fibrinogen, fibronectin or complement factor 4. So far several OspC variants produced by B. burgdorferi s. l. spirochetes were shown to be capable of binding Salp15 or its homologue, but the protection against borreliacidal antibodies has only been proven in the case of B. burgdorferi s. s. The question of Salp15 contribution to Borrelia survival during the infection has been comprehensively studied during the last decades. In contrast, the organization of the OspC-Salp15 complex has been poorly explored. This report describes the binding between three Salp15 homologues from the tick Ixodes ricinus (Iric1, Iric2 and Iric3) and OspC from four B. burgdorferi sensu lato strains in terms of the binding parameters, analyzed with two independent biophysical methods - Microscale thermophoresis (MST) and Biolayer interferometry (BLI). The results of both experiments show a binding constant at the nanomolar level, which indicates very strong interactions. While the Iric1-OspC binding has been reported before, we show in this study that also Iric2 and Iric3 are capable of OspC binding with high affinity. This observation suggests that these two Salp15 homologues might be used by B. burgdorferi s. l. in a way analogous to Iric1. A comparison of the results from the two methods let us propose that N-terminal immobilization of OspC significantly increases the affinity between the two proteins. Finally, our results indicate that the Iric binding site is located in close proximity of the OspC epitopes recognized by human antibodies, which may have important biological and medical implications.

Salp15, a Multifunctional Protein From Tick Saliva With Potential Pharmaceutical Effects

Ixodes ticks are the main vectors for a number of zoonotic diseases, including Lyme disease. Ticks secrete saliva directly into a mammalian host while feeding on the host's blood. This action serves to modulate host immunity and coagulation, thus allowing ticks to attach and feed upon their host. One of the most extensively studied components of tick saliva is Salp15. Research has shown that this protein binds specifically to CD4 molecules on the surface of T lymphocytes, interferes with TCR-mediated signaling transduction, inhibits CD4+ T cell activation and proliferation, and impedes the secretion of interleukin 2 (IL-2). Salp15 also binds specifically to dendritic cell dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN) to up-regulate the expression of CD73 in regulatory T cells. Collectively, these findings render this salivary protein a potential candidate for a range of therapeutic applications. Here, we discuss our current understanding of Salp15 and the mechanisms that might be used to treat disease.

The Essential Role of Tick Salivary Glands and Saliva in Tick Feeding and Pathogen Transmission

As long-term pool feeders, ticks have developed myriad strategies to remain discreetly but solidly attached to their hosts for the duration of their blood meal. The critical biological material that dampens host defenses and facilitates the flow of blood-thus assuring adequate feeding-is tick saliva. Saliva exhibits cytolytic, vasodilator, anticoagulant, anti-inflammatory, and immunosuppressive activity. This essential fluid is secreted by the salivary glands, which also mediate several other biological functions, including secretion of cement and hygroscopic components, as well as the watery component of blood as regards hard ticks. When salivary glands are invaded by tick-borne pathogens, pathogens may be transmitted via saliva, which is injected alternately with blood uptake during the tick bite. Both salivary glands and saliva thus play a key role in transmission of pathogenic microorganisms to vertebrate hosts. During their long co-evolution with ticks and vertebrate hosts, microorganisms have indeed developed various strategies to exploit tick salivary molecules to ensure both acquisition by ticks and transmission, local infection and systemic dissemination within the vertebrate host.

Characterization of the early local immune response to Ixodes ricinus tick bites in human skin

Little is known about the immunomodulation by tick saliva during a natural tick bite in human skin, the site of the tick-host interaction. We examined the expression of chemokines, cytokines and leucocyte markers on the mRNA levels and histopathologic changes in human skin biopsies of tick bites (n=37) compared to unaffected skin (n=9). Early tick-bite skin lesions (<24 hours of tick attachment) were characterized by a predominance of macrophages and dendritic cells, elevated mRNA levels of macrophage chemoattractants (CCL2, CCL3, CCL4) and neutrophil chemoattractants (CXCL1, CXCL8), of the pro-inflammatory cytokine, IL-1β, and the anti-inflammatory cytokine, IL-5. In contrast, the numbers of lymphocytes and mRNA levels of lymphocyte cell markers (CD4, CD8, CD19), lymphocyte chemoattractants (CXCL9, CXCL10, CXCL11, CXCL13, CCL1, CCL22), dendritic cell chemoattractants (CCL20), and other pro- (IL-6, IL-12p40, IFN-γ, TNF-α) and anti-inflammatory cytokines (IL-4, IL-10, TGF-β) did not differ from normal skin. With longer tick attachment (>24 hours), the numbers of innate immune cells and mediators (not significantly) declined, whereas the numbers of lymphocytes (not significantly) increased. Natural tick bites by Ixodes ricinus ticks initially elicit a strong local innate immune response in human skin. Beyond 24 hours of tick attachment, this response usually becomes less, perhaps because of immunomodulation by tick saliva.

An investigation of binding ability of Ixodes persulcatus Schulze Salp15 with Lyme disease spirochetes

Salp15, a 15-kDa tick salivary gland protein, has several suppressive modes of activity against host immunity and plays a critical role in the transmission of Lyme disease spirochetes in Ixodes scapularis and Ixodes ricinus, major vectors of Lyme disease in North America and Western Europe. Salp15 adheres to Borrelia burgdorferi and specifically interacts with its outer surface protein C (OspC), protecting the spirochete from antibody-mediated cytotoxicity and facilitating infection in the mice. Recently, we identified two Salp15 homologues, IperSalp15-1 and IperSalp15-2, in Ixodes persulcatus, a vector for Lyme disease in Japan. Here we describe the function of IperSalp15 in the transmission of Lyme borreliosis. To investigate the function of IperSalp15, recombinant IperSalp15-1 and IperSalp15-2 were prepared in bacterial and insect cells. Both were identified in the sera of tick-immunized hamsters, indicating that these are secretory proteins in exposed host animals. Solid-phase overlay and indirect fluorescence assays showed that IperSalp15 binds to OspC from B. burgdorferi, Borrelia garinii, and Borrelia afzelii. Importantly, this binding likely protected the spirochete from antibody-mediated cytotoxicity in vitro. In addition, IperSalp15 tended to facilitate infection in mice. Thus, further characterization of tick molecules, including IperSalp15, could lead to the development of new strategies to prevent the transmission of tick-borne diseases.