The Ticking Clock to Fight Ticks

Art Courtesy of Diya Naik.

Tick spray might not be the first thing you think of when preparing to go backpacking. However, if gone undetected, these small arachnids can feed and feed, increasing the likelihood of transmission of dangerous bacteria. Named for the city of Lyme, Connecticut, where it was first detected, Lyme disease is most commonly spread by Ixodes ticks and is the most common tick-borne illness in the Northeast region of the United States. If left untreated, the disease can cause debilitating effects on the heart, joints, muscles, and nervous system. According to the CDC, reports of Lyme disease have doubled in the United States since 2000, reflecting a growing need for prevention.

In 1993, the Yale Scientific Magazine reported on a novel vaccine being tested against Lyme disease (Vol. 67 No. 2). The writer, Emily Ho, estimated that the vaccine would be “available for the public sometime in 1995.” However, as of today, there are no Lyme disease vaccines available for humans. What went wrong?

Revisiting the Past

The story began in 1988, when Erol Fikrig—then a postdoctoral researcher—met with Richard Flavell, who was the newly appointed chair of immunobiology at the Yale School of Medicine. “We decided, why don’t we make a vaccine against Lyme disease?” said Fikrig, now the Waldemar von Zedtwitz Professor of Medicine at Yale. So, they started looking for a target.

Lyme disease is caused by Borrelia burgdorferi, which is a pathogenic spirochete (a type of slender, spiral-shaped bacteria). It was known that a key part of protection against Lyme disease in animals is mediated by humoral immunity, a type of immune response that can be passive or active. In active immunity, B cells (a type of white blood cell) produce antibodies that specifically recognize and destroy an encountered foreign antigen, conferring future protection against that antigen. In passive immunity, blood serum containing these antibodies can be transferred to other animals to confer protection against the target antigen.

Previously, it was shown that immunization of hamsters with serum-containing antibodies against B. burgdorferi prevented infection when the hamsters were challenged with the bacterial strain. However, scientists did not know what specific antigens trigger the production of protective antibodies against Lyme disease. Fikrig and Flavell would have to solve that mystery.

In their 1990 paper published in Science, Fikrig and Flavell—along with collaborators Stephen Barthold and Fred Kantor—hypothesized that a possible candidate for the antigen was outer surface protein A (OspA), which is a lipoprotein (a particle composed of fats and proteins), on B. burgdorferi. To test this hypothesis, the gene for OspA in a B. burgdorferi strain was cloned into the bacteria E. coli, such that the E. coli would express OspA. Mice then received either the OspA-transformed bacteria or a control. Four weeks later, an antibody response was detected in mice that received the OspA-transformed bacteria, suggesting that humoral immunity had developed in response to OspA.

The team found that the mice that received the OspA-transformed bacteria were successfully protected from infection from B. burgdorferi. While the control mice showed evidence of arthritis, inflamed heart tissue, and infection in blood and spleen cultures, mice that received the OspA-transformed bacteria showed no sign of infection—all signs pointed towards successful immunity. Lastly, the researchers found that the serum of the protected mice conferred similar prevention against infection when injected into new mice, demonstrating effective passive immunization. 

Studies like this one paved the way for a human vaccine against Lyme disease. Indeed, in the YSM article published thirty years ago, which covered a similar study highlighting the potential of the OspA vaccine, the writer was optimistic about the vaccine’s release in 1995. 

What happened, then, over these three decades?

LYMErix and The Revival of The OspA Vaccine

According to Fikrig, biopharmaceutical company GlaxoSmithKline signed an agreement with Yale to develop the vaccine. Fikrig and Flavell advised them as they developed a three-dose vaccine for humans, under the name LYMErix. The last phase of clinical trials for the vaccine enrolled over ten thousand patients in endemic areas, and demonstrated that LYMErix was safe and effective in reducing the occurrence of Lyme disease in humans. Following FDA approval, LYMErix was released to the public in 1998. 

Unfortunately, LYMErix’s success was short-lived. The vaccine was discontinued in 2002, even though it remains FDA-approved. 

LYMErix’s brief time on the market saw low demand and a potential onslaught of lawsuits alleging the vaccine caused arthritis. “These rumors were circulating that the vaccine was making people sick. There was absolutely no evidence for that, and there still is absolutely no evidence for that,” said Flavell, who is now a Sterling Professor of Immunobiology at Yale. However, the controversy, combined with the lack of interest, was enough for GlaxoSmithKline to remove LYMErix from the general public. There has not been another Lyme disease vaccine for humans since then.

However, that may soon change. Major pharmaceutical companies such as Pfizer and Valneva are now interested in developing a new Lyme vaccine. The principle behind how these new vaccines would function is similar to findings from past research. “You need a high titer [concentration] of OspA antibodies to work in both [the old and the new vaccines]… they’re fundamentally the same in that regard,” Fikrig said. The new vaccines are now being tested in clinical trials, offering the possibility that a human vaccine for Lyme disease may soon return to the market. 

Beyond Lyme Disease: A Vaccine Against Ticks

While pharmaceutical companies try to revive OspA vaccine efforts, Fikrig and Flavell have moved on. They are searching for success with a different approach—one that might be able to prevent all tick-borne illnesses. Besides Lyme disease, Ixodes ticks are also carriers of illnesses such as babesiosis, Powassan virus, and anaplasmosis, which can cause debilitating health effects in humans. Fikrig and colleagues utilized an anti-tick approach to a vaccine in their recent paper published in Science Translational Medicine in 2021. 

The idea of using an mRNA vaccine for tick immunity was formulated in 2019 in collaboration with Drew Weissman, who is now a co-winner of the 2023 Nobel Prize in Physiology or Medicine for his work on mRNA vaccine technology (which was foundational for the development of the Pfizer-BioNTech COVID-19 vaccine). The mechanism of these vaccines relies on the delivery of mRNA, which directs the production of a small, harmless piece of a target antigen in host cells so that the immune system will learn to mount a response against them if encountered again. Rather than targeting specific antigens associated with Lyme disease, the researchers focused on the source—proteins found in the tick’s saliva, which is secreted into humans at the site of the bite.

After identifying nineteen salivary proteins with high immunogenicity (ability to trigger an immune response), the researchers created a cocktail of mRNA encoding those proteins. They placed this cocktail, called 19ISP, in lipid nanoparticles—which protect the mRNA from premature degradation—for delivery. Two weeks after injecting it into guinea pigs, antibodies against ten of the nineteen proteins were detected in the serum of the immunized guinea pigs, while none were observed in the control group, suggesting that exposure to 19ISP triggered a humoral response. 

Following this, the guinea pigs then underwent tests to examine whether 19ISP vaccination was effective in generating tick immunity and resistance against Lyme disease. “We showed that if you give [19ISP] to a guinea pig and then put ticks on it, the ticks feed very poorly, you get redness where the ticks feed, and the [ticks] detach and die quickly. And then we showed that if you put ticks that have the disease-causing agent in them, the guinea pigs will not get Lyme disease,” Fikrig said. 

In comparison, control guinea pigs had low rates of tick detachment, did not show redness, and were susceptible to B. burgdorferi. These results suggest that 19ISP may be a viable solution in both the early detection of tick attachment and the facilitation of tick detachment. “I think we have a vaccine that can increase tick recognition and prevent Borrelia transmission,” Fikrig said. “And hopefully, at some point, that may be available to the public. We don’t know yet, but it may be useful in preventing more than just Lyme disease.”

Future Directions

The “anti-tick” approach is crucial because Ixodes represent just one genus of ticks that carry illness. Other ticks pose their own threats. Saliva from Amblyomma ticks, for example, is thought to cause red meat allergy in humans, according to Fikrig. The tick’s saliva proteins may contain the sugar molecule alpha-gal, which is found in most mammals but not in humans. When the tick’s saliva is transferred to humans, alpha-gal is flagged as a foreign antigen, triggering a severe immune response when alpha-gal is encountered again, such as in red meat and milk. “I think that our anti-tick vaccine can be the first ever vaccine against an allergic condition,” Fikrig said. In the past, it was demonstrated that immunity to Ixodes ticks can protect against other disease-spreading tick species (e.g. Amblyomma and Dermacentor); however, their specific 19ISP cocktail has yet to be tested for this cross-protection.

In addition to solving these mysteries, Fikrig and Flavell are working with a worldwide network of collaborators to explore whether the anti-insect approach can be applied to other infectious diseases. Flavell highlighted that this approach may be applicable to other creatures that spread disease. “We’ve brought together a consortium of people to develop this concept, and if we’re very lucky, we could hopefully make a dent in diseases like malaria, which is a huge problem,” Flavell said. Despite previous roadblocks in the history of Lyme disease vaccines, the researchers have continued to forge ahead to meet greater success than before—and pioneered an approach that could revolutionize vaccines for allergies and broader infectious disease prevention.