Immunogenicity of recombinant F4 (K88) fimbrial adhesin FaeG expressed in tobacco chloroplast

Plant-based oral vaccines against zoonotic and non-zoonotic diseases

The shared diseases between animals and humans are known as zoonotic diseases and spread infectious diseases among humans. Zoonotic diseases are not only a major burden to livestock industry but also threaten humans accounting for >60% cases of human illness. About 75% of emerging infectious diseases in humans have been reported to originate from zoonotic pathogens. Because antibiotics are frequently used to protect livestock from bacterial diseases, the development of antibiotic-resistant strains of epidemic and zoonotic pathogens is now a major concern. Live attenuated and killed vaccines are the only option to control these infectious diseases and this approach has been used since 1890. However, major problems with this approach include high cost and injectable vaccines is impractical for >20 billion poultry animals or fish in aquaculture. Plants offer an attractive and affordable platform for vaccines against animal diseases because of their low cost, and they are free of attenuated pathogens and cold chain requirement. Therefore, several plant-based vaccines against human and animals diseases have been developed recently that undergo clinical and regulatory approval. Plant-based vaccines serve as ideal booster vaccines that could eliminate multiple boosters of attenuated bacteria or viruses, but requirement of injectable priming with adjuvant is a current limitation. So, new approaches like oral vaccines are needed to overcome this challenge. In this review, we discuss the progress made in plant-based vaccines against zoonotic or other animal diseases and future challenges in advancing this field.

Cloning and Expression of Genes Encoding F107-C and K88-1NT Fimbrial Proteins of Enterotoxigenic Escherichia coli from Piglets

We cloned two genes coding F107-C and K88-1NT fimbrial subunits from strains E. coli C and 1NT isolated from Thua Thien Hue province, Vietnam. The mature peptide of faeG gene from strain E. coli 1NT (called faeG-1NT) is 100 % similarity with faeG gene, while the CDS of fedA gene from strain C (called fedA-C) has a similarity of 97 % with the fedA gene. Expression of the faeG-1NT and fedA-C genes in E. coli BL21 Star™ (DE3) produced proteins of ~31 and 22 kDa, respectively. The effect of IPTG concentration on the K88-1NT and F107-C fimbriae production was investigated. The results showed that 0.5 mM IPTG is suitable for higher expression of K88-1NT subunit, while 0.75 mM IPTG strongly stimulated expression of F107-C subunit. The optimal induction time for expression was also examined. Generally, highest expression of K88-1NT subunit occurred after 6 h of induction, while that of F107-C subunit is after 14 h.

Subcutaneous or oral immunization of mice with Lactococcus lactis expressing F4 fimbrial adhesin FaeG

Enterotoxigenic Escherichia coli (ETEC) is one of the most common causes of diarrhea in neonatal and postweaning piglets. Fimbrial adhesion of ETEC has been considered an important colonization factor with antigenicity. To safely and effectively deliver the F4 (K88) fimbrial adhesin FaeG to the immune system, we have previously constructed the secretory expression vector pNZ8112-faeG, and FaeG was produced in cytoplasmic form in Lactococcus lactis. In this work, BALB/c mice were immunized with recombinant L. lactis to further determine the immunogenicity of recombinant FaeG (rFaeG) via the subcutaneous or oral route. Subcutaneous immunization in mice with recombinant L. lactis induced a significant increase in the F4-specific serum IgG titer and the number of antibody-secreting cells (ASCs) in the spleen. Oral immunization of mice with recombinant L. lactis induced mucosal and systemic F4-specific immune responses and increased the number of ASCs in the spleen, mesenteric lymph nodes and Peyer's patches. High-dose (2.8 × 10(11) CFU) recombinant strains and adjuvant cholera toxin B subunit enhanced specific mucosal immune responses. The results suggest the feasibility of delivering rFaeG expressed in L. lactis to the immune system in order to induce an F4-specific immune response.

Production of a subunit vaccine candidate against porcine post-weaning diarrhea in high-biomass transplastomic tobacco

Post-weaning diarrhea (PWD) in piglets is a major problem in piggeries worldwide and results in severe economic losses. Infection with Enterotoxigenic Escherichia coli (ETEC) is the key culprit for the PWD disease. F4 fimbriae of ETEC are highly stable proteinaceous polymers, mainly composed of the major structural subunit FaeG, with a capacity to evoke mucosal immune responses, thus demonstrating a potential to act as an oral vaccine against ETEC-induced porcine PWD. In this study we used a transplastomic approach in tobacco to produce a recombinant variant of the FaeG protein, rFaeG(ntd/dsc), engineered for expression as a stable monomer by N-terminal deletion and donor strand-complementation (ntd/dsc). The generated transplastomic tobacco plants accumulated up to 2.0 g rFaeG(ntd/dsc) per 1 kg fresh leaf tissue (more than 1% of dry leaf tissue) and showed normal phenotype indistinguishable from wild type untransformed plants. We determined that chloroplast-produced rFaeG(ntd/dsc) protein retained the key properties of an oral vaccine, i.e. binding to porcine intestinal F4 receptors (F4R), and inhibition of the F4-possessing (F4+) ETEC attachment to F4R. Additionally, the plant biomass matrix was shown to delay degradation of the chloroplast-produced rFaeG(ntd/dsc) in gastrointestinal conditions, demonstrating a potential to function as a shelter-vehicle for vaccine delivery. These results suggest that transplastomic plants expressing the rFaeG(ntd/dsc) protein could be used for production and, possibly, delivery of an oral vaccine against porcine F4+ ETEC infections. Our findings therefore present a feasible approach for developing an oral vaccination strategy against porcine PWD.