T Cells Mount Powerful Defense Against Zika Virus

Zika virus vaccines have long faced a difficult immunological problem: the antibodies they generate may protect against infection, but they can also interact with related mosquito-borne viruses in ways that increase disease severity. A new study from researchers at the La Jolla Institute for Immunology suggests that CD8+ T cells can provide an alternative layer of protection. In experiments with mice, an experimental vaccine designed to reduce the production of problematic cross-reactive antibodies protected animals from Zika virus infection primarily through T-cell activity. The protection, however, faded within several weeks, revealing both the promise and the limitations of relying on cellular immunity alone.

Zika virus first became a global public-health concern in 2016, when transmission across the Americas was associated with a sharp increase in congenital abnormalities. Infection during pregnancy can cause microcephaly, in which the brain and skull fail to develop normally, as well as miscarriage, eye and ear abnormalities, joint problems and other features now grouped under congenital Zika syndrome. The World Health Organization declared the outbreak and its associated birth defects a public health emergency of international concern in February 2016, a designation that remained in place until November of that year. Although reported cases declined substantially after 2017, Zika transmission has been documented in at least 97 countries and territories, and researchers warn that population susceptibility can rebuild as immunity wanes.

Zika is transmitted primarily by Aedes mosquitoes, several species of which have expanded into new regions. Climate change may further extend the areas in which these vectors can survive, while insecticide resistance could make outbreaks more difficult to control. Despite the continuing risk, there is no approved vaccine or specific antiviral treatment for Zika virus. The virus belongs to the orthoflavivirus group, which also includes dengue virus and Japanese encephalitis virus. This viral family presents a special challenge because closely related viruses share structural features, particularly in the envelope proteins that form the outer surface of the viral particle and are readily recognized by antibodies.

Most conventional vaccines are designed to induce neutralizing antibodies. These proteins attach to viral structures and prevent the pathogen from entering susceptible cells. B cells, the immune cells that produce antibodies, can also generate long-lived populations that rapidly respond when the same pathogen is encountered again. For Zika virus, however, an antibody response must be carefully controlled. Antibodies directed against Zika may recognize dengue virus because the two viruses possess similar envelope proteins. If antibodies bind to a virus without fully neutralizing it, they can sometimes help the virus enter immune cells through antibody receptors, a process known as antibody-dependent enhancement, or ADE. Rather than blocking infection, these antibody-virus complexes can increase the number of cells exposed to the virus and potentially intensify disease.

Sujan Shresta, a professor at the La Jolla Institute for Immunology, has spent years studying how T cells might provide protection against Zika, dengue and other orthoflaviviruses without creating the same risks associated with cross-reactive antibodies. T cells recognize short fragments of viral proteins displayed on the surface of infected cells. CD8+ T cells, often called cytotoxic T cells, can then destroy those infected cells, limiting the production and spread of new virus. Unlike antibodies, which act most effectively before a virus enters a cell, CD8+ T cells target infection after viral replication has begun. This makes them an important component of immune defense, particularly when antibody-mediated protection is incomplete or undesirable.

For the new study, Shresta and colleagues Annie Elong Ngono and Kantinan Chuensirikulchai compared two experimental Zika vaccines in mice genetically susceptible to infection. Both vaccines were based on the virus’s envelope proteins, which are commonly used in vaccine development because they contain important targets for neutralizing antibodies. One vaccine retained the envelope proteins in their natural form. The second carried mutations in a small structural region known as the fusion loop. This region helps the virus merge its membrane with the membrane of a host cell, a necessary step for infection, but it is also a major source of antibodies that cross-react with related viruses and may contribute to ADE.

The researchers expected that modifying the fusion loop might reduce the production of dangerous antibodies, but they also wanted to determine whether the change would alter T-cell responses. The two vaccines produced a strikingly different pattern of protection. Mice receiving the unmodified vaccine were protected by a combination of antibodies and T cells. When CD8+ T cells from these vaccinated animals were transferred into unvaccinated mice, the recipient animals showed reduced levels of Zika virus, demonstrating that the cells could suppress infection independently. This result indicated that cellular immunity was not merely a secondary effect of the vaccine’s antibody response; it was capable of making a measurable contribution to protection on its own.

The fusion-loop mutant vaccine produced an even more unexpected outcome. In laboratory tests, the antibodies generated by the mutant and unmodified vaccines shared many characteristics, but the mutant vaccine’s antibodies failed to protect unvaccinated animals when transferred experimentally. Removing CD8+ T cells from vaccinated mice eliminated the protection that the mutant vaccine had provided. Together, these findings showed that the vaccine was working predominantly, if not entirely, through CD8+ T cells. The cells recognized and destroyed infected targets, preventing the virus from reaching the high levels observed in unvaccinated animals. The result suggests that altering a viral antigen can redirect the immune system toward a protective T-cell response, even when antibody measurements alone might suggest that two vaccines are performing similarly.

That protection was not durable. Twelve weeks after the final vaccination, mice that had received the fusion-loop mutant vaccine were no better protected than unvaccinated animals, whereas mice immunized with the unmodified vaccine remained protected. The finding highlights a central challenge in vaccine design: avoiding harmful antibody responses may come at the cost of weakening or shortening other forms of immunity. T cells can form long-lived memory populations, but their persistence and effectiveness depend on the signals provided during vaccination, the tissues in which they are generated and the quality of the immune memory that follows. Shresta’s team is now investigating how to produce a more durable population of Zika-specific CD8+ T cells while retaining the safety advantages of limiting ADE-associated antibodies.

The implications extend beyond Zika virus. Shresta’s earlier work has shown that T cells can cross-react with more than one related orthoflavivirus, raising the possibility of a broad vaccine capable of protecting against Zika, dengue and other members of the group. Such a vaccine would be especially valuable in regions where multiple viruses circulate and where a person’s previous exposure to one virus can influence the response to another. The new findings support an approach that combines carefully selected neutralizing antibodies with virus-specific and cross-reactive T cells. They also emphasize that antibody levels should not be the only measure used to evaluate an orthoflavivirus vaccine. In situations where antibodies may be insufficient or could contribute to unwanted immune effects, the strength, specificity and durability of T-cell immunity may be equally important. The current work was performed in animals and does not establish protection in humans, but it provides a framework for developing and testing next-generation vaccines against Zika and related viral threats.

Web References: https://www.lji.org/labs/shresta-lab/ ; https://www.who.int/emergencies/situations/zika-virus-outbreak ; https://www.who.int/publications/m/item/zika-epidemiology-update-may-2026

References: Nature Microbiology; DOI: 10.1038/s41564-026-02465-6; Article publication date: 26-Aug-2026