Tick Cell Lines in Research on Tick Control

Genetic manipulation of an Ixodes scapularis cell line

Although genetic manipulation is one of the hallmarks of model organisms, its applicability to non-model species has remained difficult due to our limited understanding of their fundamental biology. For instance, manipulation of a cell line originated from the black-legged tick Ixodes scapularis, an arthropod that serves as a vector for several human pathogens, has yet to be established. Here, we demonstrate the successful genetic modification of the commonly used tick ISE6 line through ectopic expression and clustered regularly interspaced palindromic repeats [(CRISPR)/CRISPR-associated protein 9 (Cas9)] genome editing. We performed ectopic expression using nucleofection and attained CRISPR-Cas9 editing via homology-dependent recombination. Targeting the E3 ubiquitin ligase x-linked inhibitor of apoptosis (xiap) and its substrate p47 led to an alteration in molecular signaling within the immune deficiency network and increased infection of the rickettsial agent Anaplasma phagocytophilum in I. scapularis ISE6 cells. Collectively, our findings complement techniques for the genetic engineering of I. scapularis ticks, which currently limit efficient and scalable molecular genetic screens in vivo.IMPORTANCEGenetic engineering in arachnids has lagged compared to insects, largely because of substantial differences in their biology. This study unveils the implementation of ectopic expression and CRISPR-Cas9 gene editing in a tick cell line. We introduced fluorescently tagged proteins in ISE6 cells and edited its genome via homology-dependent recombination. We ablated the expression of xiap and p47, two signaling molecules present in the immune deficiency (IMD) pathway of Ixodes scapularis. Impairment of the tick IMD pathway, an analogous network of the tumor necrosis factor receptor in mammals, led to enhanced infection of the rickettsial agent Anaplasma phagocytophilum. Altogether, our findings provide a critical technical resource to the scientific community to enable a deeper understanding of biological circuits in the black-legged tick I. scapularis.

New Cell Lines Derived from European Tick Species

Tick cell lines are important tools for research on ticks and the pathogens they transmit. Here, we report the establishment of ten new cell lines from European ticks of the genera Argas, Dermacentor, Hyalomma, Ixodes and Rhipicephalus originating from Germany and Spain. For each cell line, the method used to generate the primary culture, a morphological description of the cells and species confirmation by sequencing of the partial 16S rRNA gene are presented. Further molecular analysis of the two new Ixodes ricinus cell lines and three existing cell lines of the same species revealed genetic variation between cell lines derived from ticks collected in the same or nearby locations. Collectively, these new cell lines will support research into a wide range of viral, bacterial and protozoal tick-borne diseases prevalent in Europe.

Potential Mechanisms of Transmission of Tick-Borne Viruses at the Virus-Tick Interface

Ticks (Acari; Ixodidae) are the second most important vector for transmission of pathogens to humans, livestock, and wildlife. Ticks as vectors for viruses have been reported many times over the last 100 years. Tick-borne viruses (TBVs) belong to two orders (Bunyavirales and Mononegavirales) containing nine families (Bunyaviridae, Rhabdoviridae, Asfarviridae, Orthomyxovirida, Reoviridae, Flaviviridae, Phenuviridae, Nyamiviridae, and Nairoviridae). Among these TBVs, some are very pathogenic, causing huge mortality, and hence, deserve to be covered under the umbrella of one health. About 38 viral species are being transmitted by <10% of the tick species of the families Ixodidae and Argasidae. All TBVs are RNA viruses except for the African swine fever virus from the family Asfarviridae. Tick-borne viral diseases have also been classified as an emerging threat to public health and animals, especially in resource-poor communities of the developing world. Tick-host interaction plays an important role in the successful transmission of pathogens. The ticks' salivary glands are the main cellular machinery involved in the uptake, settlement, and multiplication of viruses, which are required for successful transmission into the final host. Furthermore, tick saliva also participates as an augmenting tool during the physiological process of transmission. Tick saliva is an important key element in the successful transmission of pathogens and contains different antimicrobial proteins, e.g., defensin, serine, proteases, and cement protein, which are key players in tick-virus interaction. While tick-virus interaction is a crucial factor in the propagation of tick-borne viral diseases, other factors (physiological, immunological, and gut flora) are also involved. Some immunological factors, e.g., toll-like receptors, scavenger receptors, Janus-kinase (JAK-STAT) pathway, and immunodeficiency (IMD) pathway are involved in tick-virus interaction by helping in virus assembly and acting to increase transmission. Ticks also harbor some endogenous viruses as internal microbial faunas, which also play a significant role in tick-virus interaction. Studies focusing on tick saliva and its role in pathogen transmission, tick feeding, and control of ticks using functional genomics all point toward solutions to this emerging threat. Information regarding tick-virus interaction is somewhat lacking; however, this information is necessary for a complete understanding of transmission TBVs and their persistence in nature. This review encompasses insight into the ecology and vectorial capacity of tick vectors, as well as our current understanding of the predisposing, enabling, precipitating, and reinforcing factors that influence TBV epidemics. The review explores the cellular, biochemical, and immunological tools which ensure and augment successful evading of the ticks' defense systems and transmission of the viruses to the final hosts at the virus-vector interface. The role of functional genomics, proteomics, and metabolomics in profiling tick-virus interaction is also discussed. This review is an initial attempt to comprehensively elaborate on the epidemiological determinants of TBVs with a focus on intra-vector physiological processes involved in the successful execution of the docking, uptake, settlement, replication, and transmission processes of arboviruses. This adds valuable data to the existing bank of knowledge for global stakeholders, policymakers, and the scientific community working to devise appropriate strategies to control ticks and TBVs.

Tick Cell Culture Analysis of Growth Dynamics and Cellular Tropism of Rickettsia buchneri, an Endosymbiont of the Blacklegged Tick, Ixodes scapularis

The blacklegged tick, Ixodes scapularis, a species of significant importance to human and animal health, harbors an endosymbiont Rickettsia buchneri sensu stricto. The symbiont is largely restricted to the ovaries, but all life stages can harbor various quantities or lack R. buchneri entirely. The endosymbiont is cultivable in cell lines isolated from embryos of Ixodes ticks. Rickettsia buchneri most readily grows and is maintained in the cell line IRE11 from the European tick, Ixodes ricinus. The line was characterized by light and electron microscopy and used to analyze the growth dynamics of wildtype and GFPuv-expressing R. buchneri. qPCR indicated that the genome copy doubling time in IRE11 was >7 days. Measurements of fluorescence using a plate reader indicated that the amount of green fluorescent protein doubled every 11 days. Two 23S rRNA probes were tested via RNA FISH on rickettsiae grown in vitro and adapted to evaluate the tissue tropism of R. buchneri in field-collected female I. scapularis. We observed strong positive signals of R. buchneri in the ovaries and surrounding the nucleus of the developing oocytes. Tissue tropism in I. scapularis and in vitro growth dynamics strengthen the contemporary understanding of R. buchneri as a transovarially transmitted, non-pathogenic endosymbiont.

Three-Dimensional Culture of Rhipicephalus (Boophilus) microplus BmVIII-SCC Cells on Multiple Synthetic Scaffold Systems and in Rotating Bioreactors

Tick cell culture facilitates research on the biology of ticks and their role as vectors of pathogens that affect humans, domestic animals, and wildlife. Because two-dimensional cell culture doesn't promote the development of multicellular tissue-like composites, we hypothesized that culturing tick cells in a three-dimensional (3-D) configuration would form spheroids or tissue-like organoids. In this study, the cell line BmVIII-SCC obtained from the cattle fever tick, Rhipicephalus (Boophilus) microplus (Canestrini, 1888), was cultured in different synthetic scaffold systems. Growth of the tick cells on macrogelatinous beads in rotating continuous culture system bioreactors enabled cellular attachment, organization, and development into spheroid-like aggregates, with evidence of tight cellular junctions between adjacent cells and secretion of an extracellular matrix. At least three cell morphologies were identified within the aggregates: fibroblast-like cells, small endothelial-like cells, and larger cells exhibiting multiple cytoplasmic endosomes and granular vesicles. These observations suggest that BmVIII-SCC cells adapted to 3-D culture retain pluripotency. Additional studies involving genomic analyses are needed to determine if BmVIII-SCC cells in 3-D culture mimic tick organs. Applications of 3-D culture to cattle fever tick research are discussed.

The antiviral immunity of ticks against transmitted viral pathogens

Ticks, being obligate hematophagous arthropods, are exposed to various blood-borne pathogens, including arboviruses. Consequently, their feeding behavior can readily transmit economically important viral pathogens to humans and animals. With this tightly knit vector and pathogen interaction, the replication and transmission of tick-borne viruses (TBVs) must be highly regulated by their respective tick vectors to avoid any adverse effect on the ticks' biological development and viability. Knowledge about the tick-virus interface, although gaining relevant advances in recent years, is advancing at a slower pace than the scientific developments related to mosquito-virus interactions. The unique and complicated feeding behavior of ticks, compared to that of other blood-feeding arthropods, also limits the studies that would further elaborate the antiviral immunity of ticks against TBVs. Hence, knowledge of molecular and cellular immune mechanisms at the tick-virus interface, will further elucidate the successful viral replication of TBVs in ticks and their effective transmission to human and animal hosts.