BioWorld International Correspondent
LONDON - Plants could one day provide a way of growing the ingredients needed for vaccines to protect against diseases such as the human immunodeficiency virus (HIV) or malaria. Researchers at the University of London have discovered how to make plants produce the antigens needed for vaccines in much larger quantities than ever before.
The study raises the prospect of producing vaccine components from genetically modified tobacco plants, for example, grown in greenhouses or in the open.
Patricia Obregon, senior research fellow in the department of cellular and molecular medicine at St. George’s Hospital Medical School, which is part of the University of London, said: "The approach we used may represent a generic strategy to increase the expression of recombinant proteins in plants. It could open the door to cheaper biopharmaceuticals. Plant-derived pharmaceuticals are of great interest because of their enormous potential for economy and scale of production."
The technology could, she added, lead to production of modern medicines that also will be accessible to poor populations in developing countries. "This is where these medicines are needed the most," she said.
The work is reported in the March 13, 2006, issue of Plant Biotechnology Journal in a paper titled "HIV-1 p24-immunoglobulin fusion molecule: A new strategy for plant-based protein production."
Obregon and her colleagues, working in the laboratory of Julian Ma, and with collaborators at the University of Warwick in Coventry, UK, found a way to boost the manufacture of vaccine components by plants - they fused the required antigen to part of an antibody molecule. That strategy also has the potential to enhance the effectiveness of the candidate vaccine, because the antibody molecule could target the vaccine to specific cells of the immune system.
Antigens required for vaccines can be manufactured after transferring the gene(s) that encode them into organisms such as Escherichia coli, yeast and baculovirus. The bacteria, yeast or viruses are then grown up in huge fermentation vats.
Recent work, however, has shown that plants, too, can produce recombinant pharmaceutical proteins. Because plants are higher eukaryotic organisms, they possess a system of intracellular membranes and a secretory pathway similar to that of mammalian cells. As a result, if the cells are provided with an appropriate package of genes, they can manufacture the antigen or protein required, correctly folded and finished.
The stumbling block for researchers to date has been the difficulty of stimulating the plant cells to produce enough of the desired protein. Researchers estimate that levels of the recombinant protein need to reach at least 1 percent of total soluble plant protein, if the transgenic protein is going to be worth extracting using current purification technologies.
Obregon and her colleagues now appear to have overcome that barrier. They linked the DNA sequence encoding the p24 antigen of HIV to that encoding part of a human immunoglobulin molecule. The DNA was then transferred into the chromosomal DNA of tobacco plants.
The researchers found that the plants produced the p24 antigen/immunoglobulin fusion protein at levels about 13 times higher than plants engineered to receive the DNA for p24 alone.
Obregon told BioWorld International: "We suggest that the plants produced higher levels of HIV p24 from the fusion protein due to the processes of protein folding and assembly, subcellular targeting and protein stability. The fusion of the immunoglobulin partner to the HIV p24 could lead to interaction with the appropriate molecular chaperones and determine the efficiency of protein folding and assembly."
The group chose to investigate whether the strategy would work with HIV p24 antigen because, Obregon said, "acquired immunodeficiency syndrome is one of the most important crises currently facing human development, and there is an urgent need to develop an efficient, low-cost HIV vaccine and therapeutic agents for this illness." P24 is the target of T-cell immune responses in both primary HIV infection and in those with chronic infection. Studies have shown that loss of the antibody response to p24 is associated with disease progression, so p24 is likely to be included in any multicomponent vaccine for HIV. Having purified the fusion protein produced by the transgenic plants, the researchers used it to immunize mice. The animals produced anti-p24 antibodies. Tests showed that their T cells would proliferate when brought into contact with p24.
Future studies will involve exploring the possibility that the addition of the immunoglobulin sequence confers stability to the fusion protein, as well as examining the immunological properties of the HIV p24-immunoglobulin fusion protein in more detail.