In the midst of a global health emergency, many look to researchers and health care providers to develop and administer treatments or vaccines to curtail the spread of infectious diseases. For some diseases, development of such resources prior to an outbreak is hindered due to lack of prior risk, public interest, and/or funding. The onset of an outbreak, however, can spark a sudden increase in efforts to address previously understudied pathogens, as was the case with Zika virus (ZIKV).
ZIKV is a mosquito-borne flavivirus that was identified in 1947, but until recently, it was thought to be associated with only mild illness. In recent years, it has become apparent that ZIKV infection during pregnancy is associated with microcephaly in fetuses, and ZIKV has also been linked to the neurological disorder Guillian-Barre syndrome in adults. The 2015-2016 ZIKV outbreak led to a burst in research to understand its pathogenesis and to develop effective vaccines.
A recent Nature publication co-authored by MVP student Mike Hogan from Dr. Drew Weissman’s lab demonstrates the effectiveness of a nucleoside-modified mRNA vaccine in protecting against ZIKV infection. mRNA-based vaccines are non-infectious gene vectors that produce high levels of protein and could be inexpensive and easy to manufacture, which makes mRNA an appealing vaccine platform. Unlike DNA vaccines, mRNA does not need to enter the nucleus to produce protein and poses fewer risks because it cannot integrate into the cellular genome. However, mRNA is unstable and the presence of foreign RNA can activate detrimental immune signaling. To circumvent these issues, Mike and colleagues took advantage of a vaccine platform recently developed in the Weissman lab. “One exciting thing about this project was that the timing was perfect,” says Mike. “My lab had just made some breakthroughs in developing an extremely potent vaccine platform using mRNA. So, when the Zika outbreak emerged and everyone wanted a vaccine, we realized that we were in the perfect position to use mRNA to make a Zika vaccine.”
Their vaccine platform has three important elements: nucleoside modification, HPLC purification of the mRNA, and delivery by lipid nanoparticles. Incorporation of 1-methylpseudouridine, a modified nucleoside, into their mRNA construct prevents detection by intracellular immune sensors. Delivery by lipid nanoparticles increases stability of the mRNA, and all three elements enhance translation.
Mike and colleagues designed their vaccine to express the ZIKV pre-membrane (prM) and envelope (E) glycoproteins, which are sufficient to form subviral particles that can be secreted from cells. Mike and colleagues delivered their mRNA construct or a control mRNA to mice by intradermal injection of the lipid nanoparticles. They observed cytokine expression from CD4+ T cells upon stimulation with the ZIKV E-glycoprotein two weeks after immunization, demonstrating antigen-specific responses. Immunized mice also produced E-protein-specific neutralizing antibodies within two weeks of immunization at levels higher than had been observed with other ZIKV vaccines. Immunized and control mice were infected with ZIKV two or twenty weeks after immunization to measure short- and long-term protection, respectively. While almost all control mice had viral RNA in their blood within three days of infection, all immunized mice were protected during both challenges, demonstrating short- and long-term protection from a single immunization. The authors also demonstrated that their vaccine provides protection against ZIKV infection in rhesus macaques. Immunized macaques produced neutralizing antibodies within two weeks after immunization, and antibodies were detectable up to twelve weeks post-immunization. Immunized and control macaques were challenged with ZIKV five weeks after immunization, and four out of five immunized macaques were protected.
Zika virus protection by a single low-dose nucleoside-modified mRNA vaccination. A single dose of a modified mRNA vaccine delivered via lipid nanoparticles was shown by Pardi*, Hogan* et al. to protect mice and rhesus macaques from experimental infection of Zika virus (ZIKV). The mRNA is uniquely designed because it encodes the signal peptide from MHC class II and two viral glycoproteins from a particular strain of ZIKV. This allows the translated products to be expressed and secreted by host cells. The group also observed that neutralizing ZIKV antibodies were detectable in rhesus macaques up to twelve weeks after vaccination, suggesting that a single dose provides long-lasting protection. Ultimately, human clinical trials will determine the safety and efficacy of this vaccine for preventing ZIKV infection and disease.
Currently, Mike and colleagues are examining whether their vaccine can protect against fetal ZIKV transmission in a susceptible mouse model. They are also investigating whether the antibodies produced in response to their vaccine enhance infection by the closely related Dengue virus due to antibody-dependent enhancement. The ultimate question, of course, is whether the vaccine would protect humans from ZIKV infection. A similar ZIKV mRNA vaccine was developed by Moderna Therapeutics and is moving into human clinical trials, demonstrating the potential of an mRNA vaccine platform for ZIKV protection.The 2015-2016 ZIKV outbreak turned a relatively unheard of virus into a household name and sparked a flurry of research. Not surprisingly, Mike describes his ZIKV vaccine project as much more fast-paced than his other projects. “It was very interesting to follow how the basic and vaccine research developed for Zika in real time,” says Mike. “There was a big movement for labs to post their data online in real time, or to publish results in pre-peer-review journals before they later came out in peer-reviewed ones.”
Mike is co-mentored by Drs. Drew Weissman and Jim Hoxie and will defend his thesis on July 10, 2017.
To learn more about ongoing ZIKV research at Penn, check out our previous article.
Pardi, N.*, Hogan M.J.* et al. Weissman, D. Zika virus protection by a single low-dose nucleoside-modified mRNA vaccination. Nature 2017; 543: 248-251.