A Thermostable Needle-Free Vaccine for Cholera

Vibrio cholerae is the causal agent of cholera and is most commonly spread by unknowingly drinking contaminated water. Currently, there are several different needle-free vaccines available to prevent for cholera, made from live-attenuated or “whole killed” cholera cells. However, these vaccines generally all require cold storage.

Technology Networks spoke to Hiroshi Kiyono, DDS, PhD, from the Institute of Medical Science at the University of Tokyo who is currently also affiliated with Chiba University and University of California, San Diego, to find out more about MucoRice-CTB, a room-temperature stable oral vaccine being developed using genetically modified rice plants.

Kate Robinson (KR): How prevalent is cholera and how does it affect humans?

Hiroshi Kiyono (HK): Diarrheal disease is a global health issue, with an estimated 1.3–4.0 million cases of cholera and 20,000–140,000 cholera-related deaths worldwide each year.

Following the consumption of contaminated water, Vibrio cholerae expands and produces cholera toxin (CT) in the human intestinal tract. CT is composed of an A subunit (CTA) associated with toxicity and a B subunit (CTB), which is a non-toxic protein and is essential for binding to epithelial cells for the induction of severe diarrhea.

KR: How does the MucoRice-CTB vaccine differ from other available vaccines for cholera?

HK: Existing cholera vaccines are a formulation of whole bacteria (e.g., killed) and generally require refrigerated storage (or cold chain).

Conversely, MucoRice-CTB is a subunit formulation produced by a rice protein production system. Further, our data showed that MucoRice-CTB can be stored for a prolonged period (approximately three years) without any refrigeration while maintaining immunogenicity for the induction of the toxin-specific antibody responses with neutralizing activity. Thus, MucoRice-CTB is a cold-chain free vaccine.

Since oral cholera vaccines formulated from killed V. cholerae are already available in some countries, in future clinical studies, it might be necessary to include a head-to-head comparison between the currently available bacteria-based vaccines and MucoRice-CTB. Alternatively, we may need to consider the prime and boost strategy using these different oral vaccines (e.g., bacteria-based vaccine followed by MucoRice-CTB) for the induction of maximum protective immunity against CT-induced diarrhea.

KR: What are the benefits associated with “needle-free” vaccines? Are there any drawbacks?

HK: After massive vaccination efforts, disposable needles and syringes constitute a large volume of medical waste which might influence the environment. Considering sustainable development goals, we should seek to develop needle- and syringe-free vaccines.

Since the intestinal tract is equipped with digestive enzymes, we must come up with an effective and stable vaccine delivery system which can transport vaccine antigens to the gut mucosal immune system successfully. Further, it may require co-administration of a safe and effective adjuvant which can support induction of antigen-specific immune responses against orally administered vaccines.

KR: How are rice plants utilized in vaccine production and how is the final product generated?

HK: We have developed the transgenic rice plant that expresses and accumulates CTB in its seeds, mainly in the protein bodies (PBs). The gene encoding CTB was transduced into rice seeds using an Agrobacterium-mediated method. Each transgenic seed contains approximately 20–30 µg of recombinant protein (or CTB).

The grounded powder form of MucoRice-CTB is a bulk drug preparation, which can be dissolved in water, formulated into a tablet or incorporated into a capsule for swallowing.

KR: Can you elaborate on the design of the Phase I study and talk us through the key findings?

HK: A double-blind, randomized, placebo-controlled, dose-escalation, Phase 1 study was conducted at IMSUT Hospital, the Institute of Medical Science, the University of Tokyo (IMSUT) in Japan. Eligible participants were healthy adult men with no known allergy to rice. Participants were excluded if they had any of the following: history of cholera or travelers’ diarrhea, poorly controlled constipation, abnormal results on hepatic, renal or hematological screening tests, use of any over-the counter drugs within seven days before first administration, inability to use a medically acceptable means of contraception or other reasons by medical judgment of the investigator.

Three dose cohorts of participants were randomly assigned by block to receive oral MucoRice-CTB (1 g, 3 g, or 6 g) or placebo (1 g, 3 g, or 6 g), once every two weeks for eight weeks (for a total of four doses). Serum and fecal samples were obtained for the analyses of CTB-specific antibodies and microbiome.

Participants who received the oral MucoRice-CTB showed increased CTB-specific serum IgG and IgA antibodies without inducing serious adverse events, indicating that MucoRice-CTB could be a safe and potent oral vaccine to induce the toxin-specific immune responses for the prevention of diarrheal disease. The oral MucoRice-CTB vaccine induced neutralizing antibodies against diarrheal toxins in a gut microbiota-dependent manner.

KR: What impact can an individual’s gut microbiome have on vaccine effectiveness?

HK: The current study showed an interesting finding – the condition of microflora may influence the efficacy of the induction of antigen-specific immune responses against orally administered MucoRice-CTB. However, we need to further elucidate how the gut microbiome influences the antigen-specific immune responses against oral vaccines. This avenue of research might lead to the development of new types of immunomodulator which can augment the efficacy of oral vaccines.

Hiroshi Kiyono was speaking with Kate Robinson, Editorial Assistant for Technology Networks