Plants as biotechnological tools to transiently express candidate antigens of different human papillomaviruses

Over the last years plants have been proposed as biotechnological tools to produce proteins with pharmaceutical use, including antigens and monoclonal antibodies. Plants offer the advantage of relative ease of production and high scalability at reduced costs, combined to good product safety. Different strategies have been attempted aimed at maximizing protein yield, optimizing gene expression and translation and overcoming RNA-mediated gene silencing. We investigated the potential of the dycot species Nicotiana benthamiana to transiently express proteins of Human papillomavirus, as candidate prophylactic or therapeutic vaccines. Plant expression strategies included classical binary vectors associated with silencing suppressors, replicating plant viral vectors based on Tobacco mosaic virus, as well as non-replicating viral-based vectors. The expression vectors were delivered to plants through the simple agroinfiltration procedure. As HPV antigens suitable as prophylactic vaccines, we expressed the L1 major capsid protein of two HPVs, the high-risk mucosal HPV-16 causing cervical cancers and the cutaneous HPV-8, ethiologically related to non-melanoma skin cancer in immunocompetent people and immunosuppressed organ transplant recipients. Starting from human codon optimised synthetic genes, HPV-16 L1 protein was expressed either as a 'native' polypeptide or as a chimeric protein carrying heterologous epitopes. Electron microscopy showed that HPV-8 L1 and the native and chimeric proteins of HPV-16 assembled into virion-like particles, a prerequisite for their biological activity in immunogenicity studies. As candidate therapeutic vaccine, we optimised the expression in planta of the non-structural E7 protein of HPV-8 and tested its biological activity in mice. The plant-made antigen induced a specific cytotoxic response and delayed the onset of papillomatous lesions in a transgenic mouse model.