Investigating the Human Immune Response to Ixodes scapularis Tick Bites

National Institute Of Allergy And Infectious Diseases

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Abstract

Tick-borne diseases (TBDs) are a serious public health problem in the United States, with the number of reported cases more than doubling during 2004 to 2016. The deer tick (Ixodes scapularis) is the vector of at least 7 pathogens that cause human diseases: Lyme disease (Borrelia burgdorferi and Borrelia mayonii), anaplasmosis (Anaplasma phagocytophilum), babesiosis (Babesia microti), Borrelia miyamotoi disease (Borrelia miyamotoi), ehrlichiosis (Ehrlichia muris eauclairensis), and tick-borne encephalitis (deer tick virus/Powassan virus). Lyme disease accounts for more than 80% of the reported cases of tickborne diseases, with estimates of over 476,000 cases of Lyme disease been diagnosed and treated per year in the US. Ticks are obligate blood-feeding arthropods and require a blood meal at every active life stage. To acquire its blood meal, an ixodid tick must remain attached to the skin of a vertebrate host and complete its multi-day feeding process. Tick saliva is composed of a large variety of proteins that are differentially expressed throughout the process of tick feeding. Some of these proteins have anti-clotting, anti-platelet, vasodilatory, and immunomodulatory activities that allow successful feeding and pathogen transmission. The skin response to tick salivary proteins has been implicated in tick rejection as certain animals (e.g., rabbits, guinea pigs) that are repeatedly infested develop an immune response against tick proteins. Resistance to tick bites is manifested by reduced numbers and body weights of engorged ticks or tick death in subsequent infestations in these animal models and consequently protection against Borrelia burgdorferi transmission. Most people will not feel a tick bite, as tick bites do not tend to cause itching or immediate skin irritation. But people who have had many tick bites can become sensitized to tick bites over time, and this can protect against Lyme disease. Little is known about the nature of the protective immune response. We have combined our expertise to study the response to tick bites in a well standardized clinical setting to identify critical aspects of the human innate and adaptive immune responses in skin and blood following exposure to uninfected Ixodes scapularis ticks; and monitor the acquisition of a tick-associated skin immunity, including itch. This research could lead to the identification of tick salivary proteins that are targets of host immunity and might serve as targets for an anti-tick vaccine. The Lyme Disease Studies Unit at NIAID has been involved for many years in studies to examine the use of larval I. scapularis ticks in xenodiagnosis of B. burgdorferi infection and have extensive experience with placement of larval I. scapularis ticks in humans. The Vector Molecular Biology Section at the Laboratory of Malaria and Vector Research, and the Tick-Pathogen Transmission Unit, at the Laboratory of Bacteriology at Rocky Mountain Laboratories (RML), have extensive experience in working with the tick Ixodes scapularis, the isolation of Ixodes salivary components, molecular aspects of tick salivary proteins and the understanding of guinea pig immune responses to tick exposure, tick rejection and the role tick salivary proteins on tick rejection. Major accomplishments of the past year are the continuation of accrual of participants into the Major accomplishments of the past year are the continuation of accrual of participants into the study, which is now 65% completed. As planned in the clinical study, we continue to collect blood and skin biopsies from the participants at different time points after tick attachment. RNA from biopsies were extracted and RNA Illumina next generation sequencing is being performed. Biopsies have also been processed and preserved in paraffin for histopathology and guidance of the spatial analysis. Ticks fed on participants are being collect for RNA and protein extraction, which will be used for transcriptomic and proteomic analysis. We developed a basophil activation assay using flow cytometry to measure the activation of basophil to tick saliva in the blood of individuals exposed to tick bites. For feasibility, we tested one set of biopsies from normal skin, and skin bitten by ticks at time 24h and 48h by single cell spatial gene analysis using CosMx technology by Nanostring. This system gives single-cell resolution spatial transcriptomics and proteomics that will further our understanding of the biological processes involved in the host response.