Positive preclinical results on the safety and efficacy of an RNA vaccine (ZVAX) against the mosquito-borne Zika virus that severely compromises brain development in children of infected mothers, were published in the journal Microbiology Spectrum on September 28, 2022 “Replication-Deficient Zika Vector-Based Vaccine Provides Maternal and Fetal Protection in Mouse Model.” The investigators tested the vaccine in pregnant mice and report the vaccine prevents systemic Zika infection in both mothers and developing fetuses.
“The ongoing COVID-19 pandemic has shown us the power of a strong pandemic preparedness plan and clear communication about prevention methods—all culminating in the rapid rollout of safe and reliable vaccines,” said senior author of the study, Vaithilingaraja Arumugaswami, DVM, PhD, an associate professor of molecular and medical pharmacology at the University of California, Los Angeles (UCLA). “Our research is a crucial first step in developing an effective vaccination program that could curb the spread of Zika virus and prevent large-scale spread from occurring.”
Engineering the vaccine
The experimental vaccine is composed of RNA that encodes nonstructural proteins found within the pathogen that trigger an immune response against the virus.
Arumugaswami said, “Engineering the vaccine involved deleting the part of the Zika genome that codes for the viral shell. This modification both stimulates an immunogenic reaction and prevents the virus from replicating and spreading from cell to cell.”
Eliminating structural proteins that mutate rapidly to escape the immune system also ensures that the vaccine trains the recipient’s immune system to recognize viral elements that are less likely to alter. The researchers packaged the replication deficient Zika vaccine particles in human producer cells and verified antigen expression in vitro.
“We deleted not just the gene responsible for encoding the capsid, but also those encoding the viral envelope and membrane. This vaccine is replication-deficient—it cannot spread among cells,” said co-author of the study, Nikhil Chakravarty, a master’s student at the UCLA Fielding School of Public Health.
Chakravarty clarified, “The deletion itself does not lead to stimulation of immune response but it makes this vaccine safer by rendering it replication deficient. The nonstructural proteins encoded by the RNA packaged in the vaccine stimulate more of a T-cell immune response that can specifically recognize Zika-infected cells and prevent viral replication and the spread of infection.”
The team showed increased effector T cell numbers in vaccinated versus unvaccinated mouse models. Using mass cytometry, the researchers showed high levels of splenic CD81 positive T cells and effector memory T cell responses and low levels of proinflammatory cell responses in vaccinated animals, suggesting that endogenous expression of the nonstructural viral proteins by the vaccine induced cellular immunity. There were no changes in antibody mediated humoral immunity in the vaccinated mice.
“We saw complete protective immunity against Zika virus in both pregnant and nonpregnant animals, speaking to the strength and utility of our vaccine candidate,” said Chakravarty. “This supports the deployment of this vaccine in pregnant mothers—the population, perhaps, most at need—upon further clinical evaluation. This would help mitigate some of the socioeconomic fallout from a potential Zika outbreak, as well as prevent neurological and developmental deficits in Zika-exposed children.”
The investigators administered the RNA vaccine using a prime-boost regimen where an initial dose was followed up by a booster dose. To estimate the durability of the vaccine, the researchers monitored the mice for a month-and-a-half, which is equivalent to approximately seven years in humans.
Chakravarty said, “Since the vaccine is geared toward stimulating T-cell response, we anticipate it will induce longer-lasting immunity than if it were just stimulating antibody immune response.”
The global Zika outbreak in 2016, led to efforts in developing effective therapies and vaccines against the virus. However, no vaccines or treatments have been approved for Zika virus yet.
“Other Zika vaccine candidates mainly focused on using structural proteins as immunogens, which preferably stimulates antibody response. Our candidate is unique in that it targets nonstructural proteins, which are more conserved across viral variants, and stimulate T-cell-mediated immunity,” said Chakravarty.
Epidemiological studies have shown that the Zika virus spreads approximately every seven years. Moreover, the habitats of Zika-spreading mosquitoes are increasing due to climate change, increasing the likelihood of human exposure to the virus.
“Given that RNA viruses—the category to which both Zika and the SARS family of viruses belong—are highly prone to evolving and mutating rapidly, there will likely be more outbreaks in the near future,” said Arumugaswami.
“It’s only a matter of time before we start seeing the virus spread again,” said Kouki Morizono, MD, PhD, an associate professor of medicine at UCLA and co-senior author of this study.
Before the vaccine candidate can be tested in humans, the researchers will be test it non-human primate models.