Plant Vaccines: An Immunological Perspective.

The advent of technologies to express heterologous proteins in planta has led to the proposition that plants may be engineered to be safe, inexpensive vehicles for the production of vaccines and possibly even vectors for their delivery. The immunogenicity of a variety of antigens of relevance to vaccination expressed in different plants has been assessed. The purpose of this article is to examine the utility of plant-expression systems in vaccine development from an immunological perspective

Transient Expression of Hemagglutinin Antigen from Low Pathogenic Avian Influenza A (H7N7) in Nicotiana benthamiana

The influenza A virus is of global concern for the poultry industry, especially the H5 and H7 subtypes as they have the potential to become highly pathogenic for poultry. In this study, the hemagglutinin (HA) of a low pathogenic avian influenza virus of the H7N7 subtype isolated from a Swedish mallard Anas platyrhynchos was sequenced, characterized and transiently expressed in Nicotiana benthamiana. Recently, plant expression systems have gained interest as an alternative for the production of vaccine antigens. To examine the possibility of expressing the HA protein in N. benthamiana, a cDNA fragment encoding the HA gene was synthesized de novo, modified with a Kozak sequence, a PR1a signal peptide, a C-terminal hexahistidine (6×His) tag, and an endoplasmic retention signal (SEKDEL). The construct was cloned into a Cowpea mosaic virus (CPMV)-based vector (pEAQ-HT) and the resulting pEAQ-HT-HA plasmid, along with a vector (pJL3:p19) containing the viral gene-silencing suppressor p19 from Tomato bushy stunt virus, was agro-infiltrated into N. benthamiana. The highest gene expression of recombinant plant-produced, uncleaved HA (rHA0), as measured by quantitative real-time PCR was detected at 6 days post infiltration (dpi). Guided by the gene expression profile, rHA0 protein was extracted at 6 dpi and subsequently purified utilizing the 6×His tag and immobilized metal ion adsorption chromatography. The yield was 0.2 g purified protein per kg fresh weight of leaves. Further molecular characterizations showed that the purified rHA0 protein was N-glycosylated and its identity confirmed by liquid chromatography-tandem mass spectrometry. In addition, the purified rHA0 exhibited hemagglutination and hemagglutination inhibition activity indicating that the rHA0 shares structural and functional properties with native HA protein of H7 influenza virus. Our results indicate that rHA0 maintained its native antigenicity and specificity, providing a good source of vaccine antigen to induce immune response in poultry species

Novel Vaccines to Human Rabies

Rabies, the most fatal of all infectious diseases, remains a major public health problem in developing countries, claiming the lives of an estimated 55,000 people each year. Most fatal rabies cases, with more than half of them in children, result from dog bites and occur among low-income families in Southeast Asia and Africa. Safe and efficacious vaccines are available to prevent rabies. However, they have to be given repeatedly, three times for pre-exposure vaccination and four to five times for post-exposure prophylaxis (PEP). In cases of severe exposure, a regimen of vaccine combined with a rabies immunoglobulin (RIG) preparation is required. The high incidence of fatal rabies is linked to a lack of knowledge on the appropriate treatment of bite wounds, lack of access to costly PEP, and failure to follow up with repeat immunizations. New, more immunogenic but less costly rabies virus vaccines are needed to reduce the toll of rabies on human lives. A preventative vaccine used for the immunization of children, especially those in high incidence countries, would be expected to lower fatality rates. Such a vaccine would have to be inexpensive, safe, and provide sustained protection, preferably after a single dose. Novel regimens are also needed for PEP to reduce the need for the already scarce and costly RIG and to reduce the number of vaccine doses to one or two. In this review, the pipeline of new rabies vaccines that are in pre-clinical testing is provided and an opinion on those that might be best suited as potential replacements for the currently used vaccines is offered

Plants as alternative systems for production of vaccines

Subunit vaccine production is typically associated with bacterial, yeast, insect or mammalian cell culture systems. Plants, however, are emerging as an alternative platform for producing vaccine antigens, and offer some advantages over other recombinant systems. In particular, plant virus-based transient expression systems are suitable for rapid engineering, ease of scale-up and cost-effective production of target antigens. In addition, this system provides an ideal approach for producing large quantities of vaccine antigens in a short period of time, which is particularly important when faced with natural outbreaks or accidental or intended release of bio-threat agents such as Bacillus anthrax and Yersinia pestis. This commentary reviews the production and evaluation of antigens made in plants in an attempt to develop vaccines against B. anthracis and Y. pestis

Plant-produced recombinant transmission blocking vaccine candidates to combat malaria

Malaria is one of the world’s most devastating infectious diseases, resulting in over half a million deaths annually. No licensed vaccine is currently available, although the pre-erythrocytic vaccine RTS,S is in advanced stage clinical trials and significantly reduces cases of malaria. However, RTS,S alone will be insufficient to eradicate malaria and, consequently, a variety of other vaccines targeting erythrocytic and sexual stages of the parasite’s life cycle are under development. The latter comprise transmission blocking vaccines (TBVs) that elicit antibodies against surface proteins of the parasite’s sexual stages and so prevent sexual development and transmission. Therefore, the World Health Organization has proposed TBVs as an effective means to reduce malaria transmission and serve as a key element in potentially eradicating the disease. Several TBV candidates are under development, including subunit vaccines based on the gametocyte and gamete antigens P4 8/45 and P230 and the zygote and ookinete antigens P25 and P28. These antigens have proven challenging to produce in established recombinant systems, potentially in part due to the large number of disulfide bonds. However, recently, plant-based platforms have been recruited for the production of these challenging antigens. Recombinant plants have several advantages for the production of subunit vaccines and therapeutic proteins, including cost-effectiveness, scalability, lack of harbored mammalian pathogens, and possession of the machinery for eukaryotic posttranslational protein modifications. Here, we review ongoing efforts to produce TBV candidates in plant-based expression systems