viruses Review Tick–Virus–Host Interactions at the Cutaneous Interface: The Nidus of Flavivirus Transmission Meghan E. Hermance 1 ID and Saravanan Thangamani 1,2,3,* 1 Department of Pathology, University of Texas Medical Branch (UTMB), 301 University Boulevard, Galveston, TX 77555-0609, USA; [email protected] 2 Institute for Human Infections and Immunity, University of Texas Medical Branch (UTMB), Galveston, TX 77555-0609, USA 3 Center for Tropical Diseases, University of Texas Medical Branch (UTMB), Galveston, TX 77555-0609, USA * Correspondence: [email protected]; Tel.: +1-409-747-2412 Received: 15 June 2018; Accepted: 6 July 2018; Published: 7 July 2018 Abstract: Tick-borne viral diseases continue to emerge in the United States, as clearly evident from the increase in Powassan encephalitis virus, Heartland virus, and Bourbon virus infections. Tick-borne flaviviruses (TBFVs) are transmitted to the mammalian host along with the infected tick saliva during blood-feeding. Successful tick feeding is facilitated by a complex repertoire of pharmacologically active salivary proteins/factors in tick saliva. These salivary factors create an immunologically privileged micro-environment in the host’s skin that influences virus transmission and pathogenesis. In this review, we will highlight tick determinants of TBFV transmission with a special emphasis on tick–virus–host interactions at the cutaneous interface. Keywords: tick; flavivirus; saliva; skin; cutaneous; interface; feeding 1. Introduction The interactions between tick-borne flaviviruses (TBFVs), tick vectors, and vertebrate hosts are essential for successful tick-borne disease transmission (Figure1). These three components interact with one another individually (tick–virus, host–virus, and tick–host) and shape the outcome of a tick-borne flaviviral infection; however, the tick feeding site is the one location where all three of these components interact together. This tripartite interaction facilitates the successful transmission and dissemination of a tick-borne flavivirus into the host. Skin serves as a physical barrier that provides the first line of defense against injury and infection. This complex organ possesses an array of cell populations, including immune sentinels and soluble mediators that contribute to the host’s local and systemic immune responses [1,2]. Skin is also the site where a tick initially attaches to a host and begins its lengthy feeding process. Pathogen transmission occurs during tick feeding, as skin is the first site where a pathogen gains access either to the host or to the tick vector. Therefore, the cutaneous interface is the only site in nature where TBFVs, tick vectors, and mammalian hosts contact each other simultaneously. The redundant host defense mechanisms of the skin pose a significant threat to successful tick feeding. However, tick saliva consists of a complex array of bioactive compounds that enable the tick to remain attached and undetected by the host, to successfully blood feed, and to evade the host’s immune response [2–4]. Mediators of the pain and itch responses are blocked by tick salivary factors, protecting the tick from discovery and subsequent removal by the host. Tick saliva also has antihaemostatic and anti-complement activities that enable the tick to overcome host vasoconstriction, platelet aggregation, blood coagulation, and inflammation. Viruses 2018, 10, 362; doi:10.3390/v10070362 www.mdpi.com/journal/viruses Viruses 20182018,, 1010,, 362x 22 of of 11 Figure 1. Interactions between tick-borne flaviviruses, tick vectors, and vertebrate hosts. The orange Figure 1. Interactions between tick-borne flaviviruses, tick vectors, and vertebrate hosts. The orange region with the question mark represents the cutaneous interface, which is the initial site where viruses region with the question mark represents the cutaneous interface, which is the initial site where gain access to a host or a vector. viruses gain access to a host or a vector. Since hard hard ticks ticks must must remain remain attached attached to tothe the host host for forextended extended periods periods of time of timecompared compared to other to otherblood-feeding blood-feeding arthropods, arthropods, they theyhave have evolved evolved salivary salivary countermeasures countermeasures directed directed against against host inflammationinflammation and immune defenses. Various Various components of tick saliva can modulate the cutaneous innate and and adaptive adaptive immune immune responses. responses. Tick Tick salivary salivary factors factors are are capable capable of ofaltering altering the the function function of ofneutrophils, neutrophils, natural natural killer killer cells, cells, dendritic dendritic cells, cells, macrophages, macrophages, basophils, basophils, B- and B- T-lymphocytes, and T-lymphocytes, and andsoluble soluble mediators mediators such such as complement, as complement, cytokines, cytokines, chemokines, chemokines, and and lectins lectins [2]. [2 ].As As a atick tick feeds, salivation isis not not a a continuous continuous process process [5], [5], and and many many salivary salivary proteins proteins are differentially are differentially expressed expressed during theduring course the ofcourse feeding of feeding [3,6]. Thus, [3,6]. the Thus, composition the composition of tick saliva of tick is intricatesaliva is andintricate dynamic, and dynamic, enabling itenabling to overcome it to overcome the many redundanciesthe many redundancies essential to essential the host to cutaneous the host defensescutaneous [7 ,defenses8]. In addition [7,8]. In to facilitatingaddition to successfulfacilitating blood successful feeding, blood these feeding, bioactive these tick bioactive salivary tick factors salivary are increasinglyfactors are increasingly recognized forrecognized playing afor role playing in tick-borne a role in pathogen tick-borne transmission pathogen transmission and establishment; and establishment; therefore, there therefore, is significant there scientificis significant interest scientific in the interest identification in the andidentification isolation of and the isolation salivary factorsof the salivary responsible factors for theseresponsible effects. for theseThe effects. focus of this review article will be on tick determinants of TBFV transmission in vivo. ThisThe perspective focus of willthis bereview emphasized article will by highlightingbe on tick determinants the role of theof TBFV cutaneous transmission interface in during vivo. This the earlyperspective timeline will of be flavivirus emphasized transmission by highlighting by tick feeding. the role of the cutaneous interface during the early timeline of flavivirus transmission by tick feeding. 2. Enhancement of Flavivirus Transmission by Tick Saliva 2. EnhancementSaliva-assisted of Flavivirus transmission Transmission (SAT), previously by Tick referred Saliva to as saliva-activated transmission, is the processSaliva-assisted by which bioactive transmission salivary (SAT), factors previously in tick saliva referred modulate to as saliva-activated the host environment, transmission, promoting is the transmissionprocess by which and bioactive establishment salivary of factors the tick-borne in tick saliva pathogen. modulate The the skin host feeding environment, site of promoting ticks is an ecologicallytransmission privileged and establishment niche that of can the be tick-borne exploitedby pathogen. pathogens. The During skin feeding SAT, tick-borne site of ticks pathogens is an exploitecologically the actions privileged of tick niche saliva that moleculescan be exploited at the feedingby pathogens. site of During the tick SAT, [9]. SATtick-borne was first pathogens used to describeexploit the the actions enhancement of tick saliva of Thogoto molecules virus at (THOV)the feeding transmission site of the bytickRhipicephalus [9]. SAT was appendiculatus first used to salivarydescribe glandthe enhancement extract (SGE) of [10 Thogoto]. In the virus seminal (THOV) work bytransmission Jones et al., by guinea Rhipicephalus pigs were appendiculatus infested with salivary gland extract (SGE) [10]. In the seminal work by Jones et al., guinea pigs were infested with Viruses 2018, 10, 362 3 of 11 uninfected R. appendiculatus and inoculated with a mixture of R. appendiculatus SGE and THOV, or with THOV alone. The number of ticks that acquired THOV from feeding on guinea pigs inoculated with virus plus SGE was approximately 10-fold greater than the number of ticks that became infected by feeding on guinea pigs inoculated with virus only, providing the first evidence that THOV transmission is enhanced by factors associated with the salivary glands of feeding ticks [10]. In addition to THOV, direct evidence of SAT has been demonstrated for several TBFVs [11,12]. When guinea pigs were infested with uninfected R. appendiculatus nymphs and inoculated with a mixture of tick-borne encephalitis virus (TBEV) plus SGE from partially fed uninfected female ticks or inoculated with TBEV alone, more guinea pigs developed a detectable viremia following inoculation with TBEV plus SGE compared to guinea pigs inoculated with virus in the absence of SGE [12]. Furthermore, the number of R. appendiculatus nymphs that became infected with TBEV was significantly higher in guinea pigs inoculated with TBEV plus SGE from partially fed ticks than the number of R. appendiculatus nymphs that became infected by feeding on guinea pigs inoculated with virus only or with virus plus SGE from unfed ticks [12]. More