3 Plant-Made Antigens for Developing Vaccines
The production of plant-based pharmaceuticals is strongly motivated especially
in the field of vaccines, which are the most effective tools for the prevention
and treatment of infections and, in some cases, cancer. Unfortunately, the progress
toward commercialization of vaccines takes the biggest effort and time compared to
other biopharmaceuticals, and this is also the case of plant-based vaccine candidates.
The possibility of manufacturing recombinant biopharmaceutical proteins in
plants initially gave the prospect of a possible vaccine delivery by edible plant
material (“edible vaccines”) (Haq et al. 1995). Nevertheless, the idea of vaccination
through ingestion of raw plant material (e.g. fruits or tubers) turned out to be not
feasible due to different issues (uppermost, uniformity in dosage). The concept of
oral antigen delivery in processed plant material is now considered more feasible
(Streatfield 2006). A limited number of clinical trials involving oral delivery of
antigens have been performed. In all cases no major safety concerns were detected,
and formulations turned out to be well tolerated by vaccinees.
The first trials were conducted with the non-toxic B subunit of heat-labile
enterotoxin (LTB) of enterotoxigenic E. coli contained either in raw potato or
maize administered orally to healthy volunteers for safety and immunogenicity
testing. No adverse effects of vaccination were noticed. LTB-specific IgA-secreting
cells in peripheral blood and increased levels of LTB-specific serum IgG and
IgA were detected after vaccination (Tacket et al. 1998; Tacket 2007).
Norovirus capsid protein VP1 was produced in potato tubers. Vaccination with
approximately 500 μg of recombinant VP1 was done. Despite the high infectivity of
the virus resulting in pre-existing serum immunity in a portion of volunteers,
vaccinees developed VP1-specific serum IgG titres (Tacket et al. 2000).
Also a pathogen like the hepatitis B virus (HBV), able to evolve chronic infection
and cancer both in adults and in infants, was targeted by PMF. HBV surface antigen
(HBsAg) transgenic lettuce leaves containing 0.1–0.5 μg of HBsAg/100 g of fresh
tissue were given to three adult volunteers showing transient protective levels of
specific IgG after vaccination, but no HBsAg-specific serum IgA (Kapusta et al.
1999). When HBsAg was produced in transgenic potato (Thanavala et al. 2005), a
dose-dependent, elevated serum HBsAg antibody titre induction was observed after
administration of 850 Æ 210 μg of antigen/dose, in a clinical trial on volunteers over
the 70-day follow-up period after the first immunization.
Fragments encoding a chimeric protein of G protein and N protein of the rabies
virus fused with that of alfalfa mosaic virus coat protein were introduced into a
TMV-derived plant expression vector. The vaccine transiently expressed in spinach
was orally administered (three doses of spinach; 20 g corresponding to 84 μg of
chimeric rabies peptide each) in a clinical trial revealing the induction of elevated
rabies-specific IgG in a quote of both in individuals previously vaccinated with
a commercial injection-type vaccine and in volunteers with no history of rabies
vaccination (Yusibov et al. 2002).
Overall, these studies have indicated that an immune response can be mounted in
individuals fed plant material expressing a disease antigen.
76
S. Massa et al.
The production of plant-based pharmaceuticals is strongly motivated especially
in the field of vaccines, which are the most effective tools for the prevention
and treatment of infections and, in some cases, cancer. Unfortunately, the progress
toward commercialization of vaccines takes the biggest effort and time compared to
other biopharmaceuticals, and this is also the case of plant-based vaccine candidates.
The possibility of manufacturing recombinant biopharmaceutical proteins in
plants initially gave the prospect of a possible vaccine delivery by edible plant
material (“edible vaccines”) (Haq et al. 1995). Nevertheless, the idea of vaccination
through ingestion of raw plant material (e.g. fruits or tubers) turned out to be not
feasible due to different issues (uppermost, uniformity in dosage). The concept of
oral antigen delivery in processed plant material is now considered more feasible
(Streatfield 2006). A limited number of clinical trials involving oral delivery of
antigens have been performed. In all cases no major safety concerns were detected,
and formulations turned out to be well tolerated by vaccinees.
The first trials were conducted with the non-toxic B subunit of heat-labile
enterotoxin (LTB) of enterotoxigenic E. coli contained either in raw potato or
maize administered orally to healthy volunteers for safety and immunogenicity
testing. No adverse effects of vaccination were noticed. LTB-specific IgA-secreting
cells in peripheral blood and increased levels of LTB-specific serum IgG and
IgA were detected after vaccination (Tacket et al. 1998; Tacket 2007).
Norovirus capsid protein VP1 was produced in potato tubers. Vaccination with
approximately 500 μg of recombinant VP1 was done. Despite the high infectivity of
the virus resulting in pre-existing serum immunity in a portion of volunteers,
vaccinees developed VP1-specific serum IgG titres (Tacket et al. 2000).
Also a pathogen like the hepatitis B virus (HBV), able to evolve chronic infection
and cancer both in adults and in infants, was targeted by PMF. HBV surface antigen
(HBsAg) transgenic lettuce leaves containing 0.1–0.5 μg of HBsAg/100 g of fresh
tissue were given to three adult volunteers showing transient protective levels of
specific IgG after vaccination, but no HBsAg-specific serum IgA (Kapusta et al.
1999). When HBsAg was produced in transgenic potato (Thanavala et al. 2005), a
dose-dependent, elevated serum HBsAg antibody titre induction was observed after
administration of 850 Æ 210 μg of antigen/dose, in a clinical trial on volunteers over
the 70-day follow-up period after the first immunization.
Fragments encoding a chimeric protein of G protein and N protein of the rabies
virus fused with that of alfalfa mosaic virus coat protein were introduced into a
TMV-derived plant expression vector. The vaccine transiently expressed in spinach
was orally administered (three doses of spinach; 20 g corresponding to 84 μg of
chimeric rabies peptide each) in a clinical trial revealing the induction of elevated
rabies-specific IgG in a quote of both in individuals previously vaccinated with
a commercial injection-type vaccine and in volunteers with no history of rabies
vaccination (Yusibov et al. 2002).
Overall, these studies have indicated that an immune response can be mounted in
individuals fed plant material expressing a disease antigen.
76
S. Massa et al.