Only after the epithelial cells begin differentiating does the virus emerge as distinct, infective viral particles, called virions. Thus, said Ahlquist, infectious HPV virus particles could only be obtained by laboriously differentiating cultured cells into artificial skin. This process takes weeks, and produces small amounts of virions, which permitted only limited study of the virus, said Ahlquist.
"The available culture techniques limited the ability to study critical early stages of infection," said Ahlquist. "We couldn't pursue a number of experimental approaches because of the small amounts of virus available," he said. "Also, we were restricted in genetically manipulating the virus because the available approaches required a fully functional viral genome to make infectious virus."
Two discoveries made it possible to develop a mass-production technique for the virus, said Ahlquist. Researchers had long known that the HPV capsid proteins, which were necessary for virion formation, could self-assemble. Also, said Ahlquist, John Schiller and his colleagues at the National Institutes of Health recently found that by introducing the genes for these self-assembling capsid proteins into cells along with smaller pieces of target DNA, the target genes could be packaged into viral like particles.
Based on these findings, Ahlquist and his colleagues developed a technique by which they could introduce the full HPV genome along with the genes for HPV capsid proteins into human cells and manipulate them to produce active, infectious viral particles. The resulting technique enables the scientists to produce over a thousand times more infectious virus per culture dish and takes only two days, he said. "With this new system of mass-producing packaged, replicating virions, we also can create mutations at will in the virus genome and recover these mutant genomes in infectious virus particle
Source:Howard Hughes Medical Institute