Developing an Effective Lyme Disease Vaccine

As spring dawns and the hiking season begins, people may increasingly encounter ticks that latch onto ankles and dig into the skin. While most tick bites are harmless, some can spread bacterial infections like Lyme disease.

Usually caused by the bacterium Borrelia burgdorferi, Lyme disease affects an estimated 476,000 people in the US annually.¹ As such, researchers have been trying to design vaccines against the disease. In the last few decades, researchers have identified a promising vaccine antigen: complement regulator-acquiring surface protein 2 (CspZ) that helps bacteria evade the hosts’ immune response.

“We've known for years that CspZ would be an ideal vaccine target because it's produced in abundance during infection, but the challenge was that in its natural form, the protein doesn't trigger a strong immune response," said Yi-Pin Lin, an infectious disease biologist at Tufts University, in a press release.

Now, Lin and his team edited the gene encoding the protein and found mutants that elicited a strong immune response, killing disease-causing bacteria effectively and preventing disease-associated symptoms.² The study, published in Nature Communications, identified a vaccine antigen that could offer long-standing protection against Lyme disease while requiring fewer doses than other candidates.³

When bacteria infect hosts, CspZ binds to factor H, a host molecule responsible for detecting pathogens, preventing this protein from triggering an immune response. Previously, researchers, including Lin’s group, showed that CspZ carrying two altered amino acids (CspZ-YA) does not interact with factor H, resulting in the immune system recognizing the pathogen and eliciting a response.^4,5

In the present study, Lin and his team improved upon their results to generate a more stable protein that would be a more useful vaccine candidate. They elucidated the X-ray structure of CspZ-YA and identified two additional amino acid residues that could be modified to enhance its stability. They then cloned the genes carrying these mutations into the CspZ-YA background and purified both modified CspZ-YA proteins.

Mice immunized with either of the modified proteins produced high titers of antibody. The researchers compared sera from mice after two doses of immunization with either the modified or unmodified CspZ-YA and found that the former killed cultured B. burgdorferi more effectively.

Exposing immunized mice to infected ticks resulted in lower bacterial burden in the animals that had received one of the modified protein-containing vaccine. These mice showed less severe symptoms, such as inflammation at joints, of Lyme disease.

Lin and his team then investigated the molecular basis underlying the enhanced immune response of modified CspZ-YA. Using three-dimensional imaging, simulations, and algorithms to predict protein structures, the researchers discovered that the genetic tweaks led to increased interactions between the CspZ-YA amino acids in some parts of the protein adjacent to the binding site of factor H.

The researchers hypothesized that the increased molecular interactions would improve the modified protein’s heat tolerance, making it more stable at human body temperatures. Consistent with this, the unmodified protein denatured at a lower temperature compared to modified CspZ-YA.

“This allows the engineered CspZ protein to persist longer in the body to promote continuous production of protective antibodies, which significantly reduces how many vaccine booster shots are needed,” said Lin.

The researchers eventually hope to develop platforms to test the safety of their engineered CspZ protein-based vaccine strategy by conducting human clinical trials.

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