Alternative Methods of Vaccine Delivery: An Overview of Edible and Intradermal Vaccines

Criscuolo, E.; Caputo, V.; Diotti, R. A.; Sautto, G. A.; Kirchenbaum, G. A.; Clementi, N.

Owing to the fact that plants are edible, the notion that they could serve as a delivery vehicle, as well as biofactories, led to their use for oral vaccination in the early 1990s [23] . In recent years, additional studies have sought to overcome the limitations of conventional vaccines through development of edible formulations [24, 25] . Since the inception of the idea, it has been evident that using plants to produce vaccines would have several advantages. First, plant vaccines would likely have a low production cost and could be easily scaled-up, as has been demonstrated by the biopharmaceutical industry. Molecular farming gained visibility thanks to the success of ZMapp, the experimental drug against the Ebola virus that was produced in Nicotiana plants [26] . However, unlike biomolecule production, edible vaccine formulations do not need processing or purification steps before administration, which serves to further lower productionassociated costs. Indeed, another advantage of this strategy is that plant cells would provide antigen protection due to their rigid cell wall. This is also known as the bioencapsulation effect and could increase bioavailability of antigenic molecules to the GALTs through preserving structural integrity of vaccine components through the stomach to elicit both a mucosal and a systemic immune response. Additional strategies for antigen protection can be achieved through targeting biomolecule expression inside chloroplasts or other storage organelles [27] or in the protein bodies of seeds [28, 29] . This technology also offers advantages in terms of storage and cold chain-free delivery due to the high stability of the expressed antigens bioencapsulated within the plant and seed tissues. Moreover, plant materials can be stored at elevated temperatures for longer periods and grown where needed, making this strategy even more attractive for developing countries [30] . Finally, plant-based oral vaccines are characterized by improved safety relative to traditional recombinant vaccine platforms, especially since contamination from mammalian-specific pathogens can be eliminated [30] . Indeed, some studies using lyophilized leaves have shown their advantages over fresh materials such as long-term stability, higher antigen content, and lower microbial contamination. As an example, freeze-dried CTB-EX4-expressing (CTB: cholera toxin B subunit; EX4: exendin-4) leaves were shown to be stable for up to 10 months at room temperature, and lettuces expressing soluble antigen (PA; protective antigen from Bacillus anthracis) were successfully stored for up to 15 months at room temperature without showing antigen degradation [31] . The antigen content in lyophilized leaf materials was also 24-fold higher than fresh leaves. An additional benefit of lyophilization was its ability to remove microbial contamination. While lyophilized lettuce had no detectable microbes, fresh leaves contained up to approximately 6000 cfu/g microbes when plated on various growing media [31] .