Oral vaccines are currently part of the strategy to control the acute diarrheal disease caused by Vibrio cholera, especially in areas with poor sanitation. Yet, even at less than US $2 per dose, these vaccines can be costly for widespread use by poorer countries, and there remain 1.3–4 million estimated cases of cholera worldwide per year, and about 21,000 to 143,000 deaths from the disease. Now, the results of the first human trial of an edible cholera vaccine made from engineered rice show it increased antibody concentrations against a diarrheal toxin without inducing severe adverse events in the study participants, according to a report published June 25 in The Lancet Microbe.
The idea of using plants as biological vaccine factories is decades old. It’s a beautiful concept, says mucosal immunologist Hiroshi Kiyono of the University of Tokyo, but such vaccines haven’t materialized partly because most plants cannot be preserved for a long time period, which raises the cost of distribution and storage. Kiyono, who led the current work, says he and his colleagues thought that cereals such as rice could potentially overcome that obstacle, as seeds can be stored for a long time without refrigeration.
The idea of using rice to make a heat-stable vaccine is really creative, says Vanessa Harris, an infectious disease specialist at the Amsterdam University Medical Center who did not participate in the study. Furthermore, “it has real significance for low-[to]-middle-income countries,” she says, as its characteristics make it “easy to administer, store, and distribute.”
Kiyono and his colleagues chose to target the cholera toxin, which is responsible for the severe watery diarrhea associated with a cholera infection. They first reported the development of this rice-based vaccine in 2007, showing that genetically engineered seeds successfully expressed the cholera toxin B subunit (CTB) and induced an antigen-specific immune response in vitro. Further studies in mice and macaques confirmed that oral immunization with the vaccine, dubbed MucoRice-CTB, induced high levels of CTB-specific antibodies in the serum of these animal models.
The recent human trial involved 60 healthy Japanese men who were randomized to receive four doses of either oral MucoRice-CTB or a placebo over a period of eight weeks. For the oral administration, the engineered rice seeds were ground into fine powder and sealed in aluminum pouches whose contents were suspended in a salt solution at the time of vaccination so participants could drink it. Of the participants who completed the trial, 10 received vaccine doses of 1 g each, nine received doses of 3 g, and eight received 6 g doses. Three of the original 30 participants receiving the treatment did not complete the study.
The serum CTB-specific antibody concentrations of participants were assessed before and throughout the treatment, up through 16 weeks after the first dose. Individuals in all three dose cohorts showed, on average, a trend of increased CTB-specific immunoglobulin G (IgG) and A (IgA) antibody concentrations over time, but only those who received the highest dose had statistically significantly higher antibody concentrations over the corresponding placebo group.
It is not clear whether CTB-specific antibodies are sufficient to prevent cholera-induced diarrhea, especially for long periods of time. The basis for immunity against cholera is primarily the production of antibodies directed against another antigen of V. cholera, and secondarily, antibody production against the cholera toxin, explains John Clemens, a senior scientific advisor at the International Vaccine Institute in Seoul who led the team that developed one of the currently available oral cholera vaccines. The antibodies induced here “can play a role, but it’s very unlikely that by themselves they will be sufficient to confer meaningful protection against cholera,” he says. The authors acknowledge this potential limitation.
The current work is mainly a “proof of concept and safety,” says Kiyono. The sample size of the study was not only small but also quite homogeneous, in that all participants were male and Japanese. Kiyono says he and his and colleagues plan to perform larger, more diverse trials going forward—they are currently analyzing the data for a Phase 1b study on MucoRice-CTB conducted in the United States.
Clemens adds that vaccines such as MucoRice-CTB that use genetically engineered organisms have additional challenges ahead. “One of the motivations for plant-based vaccines was their inexpensive production,” he says, but the regulatory hurdles associated with growing these engineered plants might affect their price. This is not necessarily a deal breaker, and it may be overcome in the future, he says, and regardless, the work by Kiyono and colleagues “represents a triumph of bioengineering and also a triumph for perseverance.”
A role for the microbiome?
Although participants who received the vaccine generally saw increased antigen-specific antibody responses afterward, the vaccine didn’t show the same immunogenicity in all subjects. In its analysis, the study team classified participants as responders or nonresponders based on whether the maximum CTB-specific antibody concentration measured over the 16 weeks was at least four times the level before vaccination. According to this classification, there were 16 responders and 11 nonresponders. Kiyono and his colleagues hypothesized that this variability could be due to gut bacterial composition, and tested their ideas by performing metagenomic analyses of fecal samples obtained before vaccination of 20 of the participants (12 responders and 8 nonresponders).
Their results suggest that responders had a more diverse microbiome, suggesting they may host higher numbers of beneficial bacteria, explains Kiyono, “which may create a better gut environment for responding to vaccine antigen.” Additionally, in contrast with nonresponders, responders had a higher abundance of bacterial DNA corresponding to enterobacteria such as Escherichia coli and Shigella species. The team speculates that exposure to this bacterial group and its enterotoxins, which are closely related to CTB, may influence the immune response to the candidate vaccine.
Given the limited number of samples, the authors caution against drawing conclusions about the role of the microbiome in response to the vaccine just yet. Harris agrees, adding, “but they found correlations, and that’s a nice starting point.” Her team has also observed that the presence of enteric bacteria correlated with rotavirus vaccine response. So these new observations align with previous findings, she says.