It has been about 30 years since the first plant engineering technology was established. Although the concept of plant-based pharmaceuticals or vaccines motivates us to develop practicable commercial products using plant engineering, there are some difficulties in reaching the final goal: to manufacture an approved product. At present, the only plant-made vaccine approved by the United States Department of Agriculture is a Newcastle disease vaccine for poultry that is produced in suspension-cultured tobacco cells. The progress toward commercialization of plant-based vaccines takes much effort and time, but several candidate vaccines for use in humans and animals are in clinical trials. This review discusses plant engineering technologies and regulations relevant to the development of plant-based vaccines and provides an overview of human and animal vaccines currently under clinical trials.

To establish a safer and more effective vaccine against pneumococcal respiratory infections, current knowledge regarding the antigens common among pneumococcal strains and improvements to the system for delivering these antigens across the mucosal barrier must be integrated. We developed a pneumococcal vaccine that combines the advantages of pneumococcal surface protein A (PspA) with a nontoxic intranasal vaccine delivery system based on a nanometer-sized hydrogel (nanogel) consisting of a cationic cholesteryl group-bearing pullulan (cCHP). The efficacy of the nanogel-based PspA nasal vaccine (cCHP-PspA) was tested in murine pneumococcal airway infection models. Intranasal vaccination with cCHP-PspA provided protective immunity against lethal challenge withStreptococcus pneumoniaeXen10, reduced colonization and invasion by bacteria in the upper and lower respiratory tracts, and induced systemic and nasal mucosal Th17 responses, high levels of PspA-specific serum immunoglobulin G (IgG), and nasal and bronchial IgA antibody responses. Moreover, there was no sign of PspA delivery by nanogel to either the olfactory bulbs or the central nervous system after intranasal administration. These results demonstrate the effectiveness and safety of the nanogel-based PspA nasal vaccine system as a universal mucosal vaccine against pneumococcal respiratory infection.

We previously established a nanosized nasal vaccine delivery system by using a cationic cholesteryl group-bearing pullulan nanogel (cCHP nanogel), which is a universal protein-based antigen-delivery vehicle for adjuvant-free nasal vaccination. In the present study, we examined the central nervous system safety and efficacy of nasal vaccination with our developed cCHP nanogel containing pneumococcal surface protein A (PspA-nanogel) against pneumococcal infection in nonhuman primates. When [¹⁸F]-labeled PspA-nanogel was nasally administered to a rhesus macaque (Macaca mulatta), longer-term retention of PspA was noted in the nasal cavity when compared with administration of PspA alone. Of importance, no deposition of [¹⁸F]-PspA was seen in the olfactory bulbs or brain. Nasal PspA-nanogel vaccination effectively induced PspA-specific serum IgG with protective activity and mucosal secretory IgA (SIgA) Ab responses in cynomolgus macaques (Macaca fascicularis). Nasal PspA-nanogel-induced immune responses were mediated through T-helper (Th) 2 and Th17 cytokine responses concomitantly with marked increases in the levels of miR-181a and miR-326 in the serum and respiratory tract tissues, respectively, of the macaques. These results demonstrate that nasal PspA-nanogel vaccination is a safe and effective strategy for the development of a nasal vaccine for the prevention of pneumonia in humans.

Mucosal vaccines are considered the most suitable type of vaccines to combat emerging and re-emerging infectious diseases because of their ability to induce both mucosal and systemic immunity. Considerable advances have been made toward the development of mucosal vaccines against influenza virus and rotavirus. Many additional mucosal vaccines are in development, including vaccines against cholera, typhoid, traveler's diarrhea and respiratory infections. In addition to oral and nasal vaccines, transcutaneous (or skin patch) and sublingual immunizations are now part of a new generation of mucosal vaccines. Furthermore, a rice-based oral vaccine (MucoRice) has been receiving global attention as a new form of cold chain-free vaccine, because it is stable at room temperature for a prolonged period. This review describes recent developments in mucosal vaccines with promising preclinical and clinical results.

Rotavirus-induced diarrhea is a life-threatening disease in immunocompromised individuals and in children in developing countries. We have developed a system for prophylaxis and therapy against rotavirus disease using transgenic rice expressing the neutralizing variable domain of a rotavirus-specific llama heavy-chain antibody fragment (MucoRice-ARP1). MucoRice-ARP1 was produced at high levels in rice seeds using an overexpression system and RNAi technology to suppress the production of major rice endogenous storage proteins. Orally administered MucoRice-ARP1 markedly decreased the viral load in immunocompetent and immunodeficient mice. The antibody retained in vitro neutralizing activity after long-term storage (>1 yr) and boiling and conferred protection in mice even after heat treatment at 94°C for 30 minutes. High-yield, water-soluble, and purification-free MucoRice-ARP1 thus forms the basis for orally administered prophylaxis and therapy against rotavirus infections.