Because most of cervical cancer cases are present in the third world, there is an urgent need for alternative low-cost vaccines, like DNA-based vaccines (see above) or plant-expressed proteins.
Plant-based therapeutic vaccination could be ideal for an application in the developing word
The most successful HPV prophylactic vaccine candidates to date are based on L1 major capsid protein virus-like particles (VLPs) produced by recombinant baculovirus and yeast. These VLPs are almost indistinguishable from native virions in morphology, and induce effectively identical immune responses. Prophylactic VLP based vaccines are now available e.g. from GlaxoSmithKline (Gardasil®). However, these are very expensive vaccines. The idea of using plants as production vehicles to produce a host of different proteins has come a long way. Production of vaccines in plants results in a cheap, effective alternative to conventional methods [Fischer, R. et al., Curr Opin Plant Biol 7 (2004) 152-158 and Stephen, J.et al., Int J for Parasit 33 (2003) 479-493]. Indeed, plant expressed proteins were already used in the past with success in immunization experiments [Wang, L. et al., Int J Parasitol 38 (2008) 103-10].
Experience of vaccine expression in plants
One problem encountered so far is the low yield of proteins in plants, which in turn increases cost of production. To increase the yield of plant produced proteins has been the focus of the research of our cooperation partner at the University of Cape Town, South Africa (Prof. Rybicki and Dr. Hitzeroth, Department of Molecular and Cell Biology), for the last few years [personal communication, Dr. Hitzeroth and Kohl, T. et al., Clin Vaccine Immunol 13 (2006) 845-853 and Maclean, J. et al., J Gen Virol 88 (2007) 1460-1469 and Varsani, A., et al., Virus Res 120 (2006) 91-96]. Today, our cooperation partners are able to produce proteins in plants in large amounts and a protocol for a fast and cheap purification of recombinant proteins has been established.
In the past we have cloned the artificial genes of HPV-16 and HPV-18 (see above) into pTRaC- ZERA® plant expression vectors (ERA Biotech, Spain) that targets the recombinant protein to protein bodies within the plant cell. ZERA® is an engineered peptide assembly domain derived from the y-zein storage protein of maize and directs the formation of membrane-bound structures called protein bodies (StorProTM). Recombinant vectors were transformed into competent Agrobacterium tumefaciens and N. benthamiana plants were infiltrated with the recombinant Agrobacterium and recombinant proteins were isolated from homogenised leaf material by density-gradients.
In immunization experiments in BL/6 mice we found high immunogenicity of plant produced HPV-proteins similar to our DNA-based therapeutic HPV-16 vaccine (Öhlschläger et al., in preparation).
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