Our October 12, 2017 post, Douglas Lowy, John Schiller, and the Vaccine Against Cervical Cancer, has reached a gratifying number of people. Since some readers might welcome a bit more background vis-à-vis the remarkable human papillomavirus (HPV) life cycle, or details concerning the use of virus-like particles (VLPs) in the experimental stages of the vaccine’s development, or how the vaccine might actually work, here are a few additional points.
The post noted that the replication cycle of HPV is regulated by the differentiation states of the cells making up the layers of an intact, stratified epithelium or mucosae. “Since the outer layer of the skin is comprised of dead cells, cutaneous HPV infection requires a break or puncture of the skin for the virus to access cells of the underlying germinal stratum of the epithelium. In the actively dividing basal cells, the viral genome replicates more frequently than the cellular genome, thus amplifying the viral genome copy number. However, because the viral genes that encode the capsid proteins are not expressed in these cells, progeny virus particles, which might induce an immune response, are not yet produced. As the basal cells differentiate and move up in the epithelium, the viral genomes replicate only once per cell cycle, on average, to maintain the viral genome copy number. Then, as the infected cells go through their final stages of differentiation in the outer layers of the epithelium, the virus life cycle switches to its productive phase. Capsid proteins are produced, and thousands of virus particles are generated from the each of the infected, terminally differentiated cells.” [How cellular differentiation regulates HPV gene expression and replication is detailed in the textbook, Virology: Molecular Biology and Pathogenesis.]
The post noted that by coupling its replication cycle to the differentiated state of the host cell within the stratified epithelium, HPV can produce progeny virus particles only in the terminally differentiated cells that comprise the outermost live cells of the epithelium. In this way, HPV productive infection does not activate an antiviral immune response. [The host’s immune response eventually does clear many HPV infections. Also, the incidence of HPV-associated lesions is higher in immunosuppressed patients.]
Here then is an additional key point. After the amplification stage, the viral genomes replicate in the basal cells, but only in conjunction with cellular DNA replication. In that way, the viral genome copy number is maintained in the basal cells. Moreover, and importantly, when the basal cells divide, one daughter cell remains behind as a basal cell, while the other daughter cell migrates up into the epithelium. Thus, one daughter cell will differentiate and thereby enable the virus to complete its replication cycle—at a level in the epithelium or mucosae beyond the reach of immune attack—while the other daughter cell remains behind in the basal layer, where it sustains the persistent infection.
Another consequence of this remarkable replication cycle is as follows. Since there are no blood or lymphatic vessels in the stratum of the epithelium or mucosae where the productive replication is occurring, the infection tends to remain localized, thereby giving rise to warts or tumors.
Since HPVs are difficult to study and propagate, one might ask how Lowy and Schiller were able to assess the antibody titers that were induced by inoculation with the HPV VLPs. The answer is that they used a pseudovirion-based immune assay. Pseudovirions are essentially VLPs that contain a plasmid that carries a reporter gene.
One last point. I believe it is generally the case that vaccines due not prevent virus infections per se. Rather, they enable the host to bring an infection under control more quickly, before symptoms might arise. Considering that cervical carcinomas may develop after years of virus persistence, despite a continuing immune response against the virus the whole time, how then might the vaccine protect against the cancer? Here is a thought. Bearing in mind that the human immune response naturally clears many HPV infections over time, perhaps the vaccine protects the host by enhancing immune surveillance to clear the infection before the emergence, or malignant progression of HPV-induced lesions. Or, perhaps the vaccine actually prevents infection.