When a blacklegged tick (Ixodes scapularis) sinks its mouthparts into a person’s skin, it releases a variety of salivary proteins. Some of these proteins suppress the innate immune response that would otherwise try to get rid of the tick and any invading microorganisms it might be carrying (1). The tick also lays down a cement that helps its mouthparts stay attached to its new host (2).
In most cases, these secretions not only help the tick feed longer but also facilitate a person’s infection by tick-borne pathogens, such as the bacteria that cause Lyme disease. But for some people who have encountered ticks before, the tick salivary proteins themselves trigger an acquired immune response. “Tick bites kind of hurt or itch” for people who have developed an immune response to them, said Thomas Hart, who is now a microbiologist at Johns Hopkins University.
Not only does this alert the person to the tick, but it can also force the tick to detach before getting a full bloodmeal. Since the bacteria that cause Lyme disease and many other tick-borne pathogens take hours or even days of feeding by a tick to infect a person, this response could be fast enough to prevent transmission.
That’s why Hart and his colleagues think tapping into this acquired immunity could be an effective way to reduce cases of tick-borne diseases. To that end, while Hart was a postdoctoral researcher at Yale School of Medicine, he and his colleagues created a tool that measures the host’s antibody response to a panel of tick salivary proteins (3). Using this tool, they identified multiple tick antigens that led to host resistance to tick bites.
“It is providing a really strong foundation to develop vaccines moving forward,” said Hart.
Thomas Hart studies Lyme disease and how the bacteria responsible interact with tick and human hosts.
Credit: Thomas Hart
To make this tool, Hart’s team first identified the proteins that ticks were most likely to secrete after biting a person and the portions of those proteins that would be most accessible to human antibodies. Then, the researchers created a yeast surface display library that included each of those antigens. They identified which antigens provoked an immune response by first combining antibodies taken from a person or an animal with the library of yeast cells. Then, they isolated the yeast cells that had attracted antibodies. They named their screening system I. scapularis rapid extracellular antigen monitoring (IscREAM).
Previously, Erol Fikrig’s team at Yale School of Medicine had developed their first anti-tick mRNA vaccine, which included sequences from 19 tick salivary proteins, so they used that vaccine to validate IscREAM (4). The scientists gave that vaccine to guinea pigs, isolated IgG antibodies from the animals, and then analyzed the antibodies with their new yeast-based tool. As expected, the antibodies primarily targeted salivary proteins included in the vaccine as well as closely related proteins.
Then, Hart and his colleagues, including Fikrig, developed a new mRNA vaccine that included sequences from 25 tick cement proteins. They gave the vaccine to guinea pigs that had never encountered ticks before and showed that the animals tended to develop redness within two days of being bitten by a tick. The ticks also tended to detach from vaccinated guinea pigs earlier than unvaccinated ones, providing evidence that the guinea pig's immune response affected the parasites. When the researchers analyzed the vaccinated guinea pigs’ immune responses with IscREAM, they confirmed that the animals’ IgG antibodies targeted the cement and closely related proteins, further validating their new screening platform.
Wen-Hsiang Chen, a chemical engineer at Baylor College of Medicine who was not involved in the study, considered it a notable achievement that the researchers have now induced tick resistance with two different vaccines. However, he thinks both formulas would ultimately face the same challenge. “One consists of 19 different antigens, the other consists of 25 mRNA antigens,” said Chen. Down the line, he said, that would necessitate 19 and 25 separate quality control checks for each respective vaccine.
Yi-Pin Lin, a microbiologist at Tufts University who also was not involved in the study, agreed that the high number of antigens could prove to be a burden. At the same time, he recognized that both ticks and people are diverse, meaning that reducing the number of antigens without careful consideration could render the vaccines less effective.
It is providing a really strong foundation to develop vaccines moving forward.
– Thomas Hart, Johns Hopkins University
Hart recognizes that developing a vaccine is a balancing act, and that’s why he thinks IscREAM will prove to be particularly valuable. To conclude their study, the researchers used their screening tool to identify the antigens that guinea pigs and humans who had previously encountered ticks responded to. They also screened mice, which never develop resistance, to see how their responses differed.
It turned out that antibodies from humans who had previously encountered ticks but did not have confirmed resistance recognized a diverse range of antigens. The researchers concluded that it would be important to zero in on the antigens most associated with tick resistance.
Hart and his colleagues saw that the antibodies collected from one human known to be resistant and from resistant guinea pigs tended to recognize a particular type of tick protein that mouse antibodies did not. These proteins were capable of sequestering histamines, the molecules that induce redness, itchiness, and swelling in response to allergies and pathogens. The researchers need to do more work to figure out how important targeting histamine-binding proteins might be for inducing tick resistance, but the results offer a promising avenue of future research as the scientists seek to refine their vaccine formulas.
For Hart, the question moving forward is: “How can we develop the optimal vaccine to lead to tick rejection as quickly as possible?”
References
- Marchal, C. et al. Antialarmin effect of tick saliva during the transmission of Lyme disease. Infect Immun 79, 774-785 (2011).
- Mulenga, A. et al. Identification and characterization of proteins that form the inner core Ixodes scapularis tick attachment cement layer. Sci Rep 12, 21300 (2022).
- Hart, T.M. et al. Tick feeding or vaccination with tick antigens elicits immunity to the Ixodes scapularis exoproteome in guinea pigs and humans. Sci Transl Med 17, eads9207 (2025).
- Sajid, A. et al. mRNA vaccination induces tick resistance and prevents transmission of the Lyme disease agent. Sci Transl Med 13, eabj9827 (2021).
