Zika virus is easy to underestimate. For many people, infection causes no symptoms at all, or only a fever, rash, and joint pain. But when the mosquito-borne virus spread explosively through the Americas in 2015 and 2016, it exposed a far darker side: infection during pregnancy could impact fetal brain development, causing microcephaly and other birth defects now grouped as congenital Zika syndrome.
Part of the challenge is that Zika does not circulate alone. The virus belongs to the same mosquito-borne family as dengue, yellow fever, West Nile, and Japanese encephalitis viruses, and it is especially similar to dengue, which spreads in many of the same regions.
That resemblance matters because antibodies raised against one virus can sometimes recognize another without fully neutralizing it. In some cases, those cross-reactive antibodies can make infection worse through antibody-dependent enhancement (ADE), raising concerns that a conventional Zika vaccine could complicate later dengue infection—or that prior dengue immunity could reshape responses to Zika. That concern has prompted some researchers to look beyond antibodies and toward another arm of immunity: virus-killing CD8+ T cells.
Now, researchers at La Jolla Institute for Immunology (LJI) have shown that an experimental Zika vaccine can protect mice by leaning heavily on those T cells rather than on neutralizing antibodies. The study, published in Nature Microbiology, is titled “A Zika Virus Vaccine with E Protein Fusion Loop Mutations Fails to Elicit Neutralizing Antibodies Against Mature Virions but Protects via CD8+ T Cells.”
Led by senior author Sujan Shresta, PhD, the team compared two experimental Zika vaccines in mice bred to be susceptible to the virus. Both vaccine candidates were built around Zika’s outer envelope proteins, which are common targets for antibody-based vaccines. But one vaccine carried mutations in a small region of the envelope protein called the fusion loop—a site known to generate many of the cross-reactive antibodies implicated in ADE.
The unmodified vaccine worked as expected, producing both antibodies and T cell responses. When the researchers transferred CD8+ T cells from vaccinated mice into unvaccinated animals, those cells alone reduced Zika levels, suggesting that T cells were contributing meaningful protection even when antibodies were also present.
Although the fusion-loop mutant vaccine’s antibodies looked similar to those generated by the unmodified vaccine in cell-based and test-tube assays, they did not protect animals when transferred to unvaccinated mice. Removing CD8+ T cells, however, eliminated the vaccine’s protection. “This vaccine wasn’t protecting via antibodies,” Shresta said. “It was protecting via T cells.”
The finding points to a possible way around one of the thorniest issues in Zika vaccine development: how to avoid antibody responses that might worsen later infection with a related virus. But the approach also revealed a limitation. Twelve weeks after the final dose, mice that received the fusion-loop mutant vaccine were no better protected than unvaccinated animals, while mice that received the unmodified vaccine remained protected.
In other words, the mutation reduced a potential antibody liability but also cost the vaccine its staying power. Shresta and colleagues are now investigating how to build a more durable pool of Zika-fighting T cells that could persist for years after vaccination.
“Our study highlights the importance of considering T cell-mediated immunity alongside neutralizing antibodies,” said first author Kantinan Chuensirikulchai, PhD. “This concept may inspire new vaccine strategies for other orthoflaviviruses, particularly in situations where antibody responses alone are insufficient or may contribute to unwanted immune effects.”
The work may also have implications beyond Zika. Because T cells can recognize features shared across related orthoflaviviruses, the researchers say the findings could help guide efforts toward broader vaccines that protect against Zika, dengue, and other mosquito-borne threats without relying solely on antibodies.
