Ixodes ticks spend several days feeding on blood while attached to the victim's skin. B. burgdorferi is carried into the victim's skin in the Ixodes tick's saliva starting 3-4 days (on average) after attachment. Tick saliva contains a blend of biological substances that aid the tick in drinking blood from its victim. These substances include cement proteins to keep the tick's feeding apparatus tightly bound to the skin, anti-coagulants to keep the blood flowing into the tick, and anti-inflammatory factors that ward off the local inflammatory response. The activity of these substances also promote transmission of B. burgdorferi from the feeding tick to the victim. Hence a vaccine that targets a saliva component may protect humans from Lyme disease.
A Lyme vaccine that targets the tick has a few advantages over one that targets the spirochete. First, a tick-based Lyme disease vaccine is unlikely to interfere with laboratory diagnosis, which currently relies on detection of antibodies against the Lyme Borrelia spirochete. Second, an effective vaccine that targets the tick may also prevent transmission of other pathogens carried by the Ixodes tick by interfering with tick feeding or with the tick's countermeasures against the host inflammatory response at the feeding site.
In their recent work, the investigators focused their efforts on a salivary protein called tick histamine release factor (tHRF). Because tHRF levels in the salivary glands of feeding Ixodes ticks were higher when B. burgdorferi was present in the tick, they guessed that tHRF was doing something to help transmit B. burgdorferi from the tick to the victim. The authors turned out to be correct. Transmission of B. burgdorferi was impaired when they knocked down the tick's production of tHRF by RNAi.
The investigators went on to test the vaccine potential of tHRF in their mouse model. They passively immunized mice with antiserum raised against tHRF or actively immunized the rodents with recombinant tHRF. Actively immunized mice were also given booster injections with tHRF (the paper did not say how many). Control mice were not immunized. They then challenged the mice with ticks infected with B. burgdorferi. One or three weeks later, tissues were removed from the mice, and the bacterial load of B. burgdorferi in skin, heart, and joints was measured by quantitative PCR.
Their data showed that immunization with tHRF was somewhat effective. Depending on the experiment, B. burgdorferi DNA could not be detected in any of the three tissues in 20-33% of immunized mice, whereas the spirochete's DNA was detected in at least one tissue in all control mice. Even in immunized mice with detectable B. burgdorferi DNA, the levels were often lower than the average level found in the control mice. It would have been nice to know how much inflammation was present in the tissues of the immunized mice. Unfortunately, the histopathology of the tissues was not presented in the paper.
|Figure 4, panels E-G from Dai et al., 2010. Bacterial burden in skin (day 7 after challenge) and joint and heart (day 21) was determined by quantitative PCR with flaB primers. Horizonal lines represent the mean value ± SEM. * p < 0.05 and ** p < 0.01. Results were pooled from 3 independent experiments.|
tHRF is not the first Lyme vaccine candidate to target a protein found in tick saliva. An earlier report from Fikrig's group demonstrated that active and passive immunization with Salp15, another tick salivary protein, was also somewhat effective in protecting mice from colonization with B. burgdorferi. tHRF was superior to Salp15 in impairing feeding by ticks. Ticks feeding on Salp15-immunized mice were able to complete their blood meal. In contrast, most of the ticks had a hard time feeding on mice immunized with tHRF and could not complete their blood meal, as assessed by tick weights following detachment from the mice.
Although immunization with tHRF and Salp15 prevented colonization in only some mice, Fikrig's work shows for the first time that it may be possible to design a Lyme disease vaccine that targets the tick vector. Ultimately, the most effective vaccine may be a mixture that targets multiple components in both the tick and the spirochete.
Dai, J., Narasimhan, S., Zhang, L., Liu, L., Wang, P., & Fikrig, E. (2010). Tick histamine release factor is critical for Ixodes scapularis engorgement and transmission of the Lyme disease agent PLoS Pathogens, 6 (11) DOI: 10.1371/journal.ppat.1001205
Dai, J., Wang, P., Adusumilli, S., Booth, C.J., Narasimhan, S., Anguita, J., & Fikrig, E. (2009). Antibodies against a tick protein, Salp15, protect mice from the Lyme disease agent. Cell Host & Microbe, 6 (5), 482-492 DOI: 10.1016/j.chom.2009.10.006