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.
The 2017 Lasker-DeBakey Prize for Clinical Research went to two virologists at the National Cancer Institute, Douglas Lowy, 75, and John Schiller, 64, for developing technologies that led to FDA-approved vaccines against human papillomavirus (HPV) strains that cause cervical carcinoma and other cancers. Lasker awards are considered the United States’ most prestigious biomedical research awards. They often precede a Nobel Prize in Physiology or Medicine. Thus, they are referred to as “America’s Nobels.” Eighty-seven Lasker awardees have gone on to win a Nobel.
Lowy and Schiller’s achievements were prompted by Harald zur Hausen’s 1983 discovery that two HPV subtypes, HPV-16 and HPV-18, together account for about 70% of all cervical cancers. Since more than 120 distinct HPV subtypes had been identified, the high degree of association of cervical carcinoma with only two of these subtypes provided compelling evidence for the viral etiology of cervical carcinoma. Later studies showed that HPV-31, HPV-33, HPV-45, HPV-52, and HPV-58 are responsible for another 20% of cervical cancers. Thus, an HPV infection can be detected in virtually all cervical carcinomas. Harald zur Hausen was awarded a share of the 2008 Nobel Prize in Physiology or Medicine for his discovery. [His story is told in Harald zur Hausen, Papillomaviruses, and Cervical Cancer, posted June 19, 2015.]
Lowy and Schiller did not begin their work on papillomaviruses with the intent to produce a vaccine. Instead, like many papillomavirus researchers at the time, they were investigating how papillomavirus oncogene products affected cell growth and replication (i.e., how they cause cancer). Toward that end, they were making use of bovine papilloma virus (BPV) in their studies, rather than HPV. BPV was easier to work with than HPV, because BPV, but not HPV, could be studied in standard cell cultures (see Aside 1).
[Aside 1: The replication cycle of HPV depends upon the differentiation states of the cells making up the layers of an intact, stratified epithelium. Details are as follow. 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. Thus, the HPV life cycle is regulated by the differentiated state of the host cell within the stratified epithelium. Because virus production is restricted to the outermost layers of the epithelium, the virus can evade the immune system, such that the infection can persist, and be passed on for years. However, in most instances, the host appears to eventually mount a successful immune response, which clears the infection.
The development of so-called organotypic raft cultures eventually made it possible to study HPV in cell culture. But one could produce only very limited amounts of the virus in that system.]
Working with BPV, Lowy and Schiller developed protocols they would later use when they turned their attention towards an HPV vaccine. One of these protocols was for an assay to measure the titer of neutralizing antibodies against BPV. Importantly, they also discovered that they could generate “virus-like particles” (VLPs), comprised only of the major BPV coat protein (L1). The BPV L1 proteins (which were generated by a baculovirus vector in insect cells) self-assembled into VLPs that were morphologically like actual BPV particles. What’s more, using their assay to measure the titer of neutralizing serum antibodies, they found that the VLPs induced neutralizing antibodies in rabbits that were effective against the actual virus. Importantly, since the VLPs did not contain viral genes, they could not cause cancer.
Again, using their assay for measuring the titer of neutralizing antibodies against BPV, Lowy and Schiller compared the immunogenicity of BVP VLPs, to that of individual BPV proteins. The VLPs indeed are more immunogenic than individual viral proteins, since they induced higher levels of neutralizing antibodies than were induced by individual L1 proteins (see Aside 2).
[Aside 2: The activation of antibody-producing B-cells is triggered by the cross-linking of their antigen-binding B-cell receptors, which is facilitated by the multimeric VLPs, but not by individual viral proteins.]
The innovations resulting from their work with BPV would enable Lowy and Schiller to overcome the formidable challenges they faced when working to develop the HPV vaccine. One obstacle was that HPV cannot replicate in standard cell cultures. Thus, it was difficult to study HPV, and importantly, it also was difficult to propagate it. Being able to propagate substantial amounts of the virus would be necessary to produce a vaccine.
Another obstacle to an HPV vaccine was the potentially unacceptable risk of inoculating people with a virus (either attenuated or killed) that contains known oncogenes. Lowy and Schiller overcame this impediment, and the one noted above, by implementing protocols they previously developed while researching BPV. Specifically, they generated HPV VLPs that were comprised only of the HPV L1 capsid protein, and which induced an immune response that produced protective antibodies. [They used the L1 protein of HPV-16; the most carcinogenic strain of HPV.] In addition, they developed cell lines, which contained high copy numbers of the plasmid that encoded the HPV L1 protein; a step which enabled them to scale-up production of the VLPs.
Together, these breakthroughs made a compelling case for the feasibility of an HPV vaccine. So, Lowy and Schiller prevailed upon several pharmaceutical companies to produce a vaccine in commercial amounts, and to see the vaccine through the clinical trials process. Most companies remained skeptical about the ultimate success of the vaccine. But two companies, Merck and GlaxoSmithKline (which later bought Merck), accepted the challenge. Thus, Merck developed Gardasil, while GlaxoSmithKline developed Cervarix. [The VLPs in Gardasil are produced in yeast, whereas the VLPs from Cervarix are produced in insect cells, via a recombinant baculovirus.]
Clinical trials showed that the Merck and the GlaxoSmithKline vaccines induce significant antibody titers against high-risk HPVs. The US FDA approved the respective HPV vaccines in 2006 and 2009.
The HPV vaccines have had a substantial effect on human health. Consider the following: Cervical cancer is the second most common cause of death from cancer among women worldwide. HPV infection is the cause of virtually all cases of cervical cancer. HPVs also cause 95% of anal cancers, 70% of oropharyngeal cancers (more common in men than in women), 65% of vaginal cancers, 50% of vulvar cancers, and 35% of penile cancers. Next, consider that, since Gardasil and Cervarix were introduced, HPV infection rates have dropped by 50 percent among teen-age girls in U.S., even though only a third of teens between 13 to 17 years-old have received the full course (3 shots) of the vaccine (see Aside 3).
[Aside 3: Current CDC recommendations are as follows: “All kids who are 11 or 12 years old should get two shots of HPV vaccine six to twelve months apart. Adolescents who receive their two shots less than five months apart will require a third dose of HPV vaccine…If your teen hasn’t gotten the vaccine yet, talk to their doctor or nurse about getting it for them as soon as possible. If your child is older than 14 years, three shots will need to be given over 6 months. Also, three doses are still recommended for people with certain immunocompromising conditions aged 9 through 26 years.”]
Although he HPV vaccines have significantly reduced the incidence of cervical cancer in the developed world, the rates of cervical cancer in the United States are needlessly high, in comparison to the rates in other industrialized nations. The HPV vaccines have a loweracceptance rate than other childhood vaccines in the United States, perhaps because many American parents, some of whom associate with the religious right, have reservations about vaccinating their children against a sexually transmitted disease. Other individuals, liberals as well as conservatives, may oppose vaccines in general because they distrust pharmaceutical companies, or because they resent government interference in their lives. In any case, the CDC found no evidence of any increase in sexual activity among teenage girls who received the vaccine. Nor did it report any major ill effects]. See Aside 4.
[Aside 4: Since HPVs alone account for about 5% of all human cancers worldwide, we might ask what percentage of human cancers have a viral etiology. Hepatitis C virus, a flavivirus, and hepatitis B virus, a hepadnavirus, cause hepatocellular carcinoma; Epstein-Barr virus (EBV), a herpesvirus, causes Burkitt’s lymphoma and nasopharyngeal carcinoma; human herpesvirus 8 (HHV-8), causes Kaposi’s sarcoma, the most frequent cancer seen in AIDS patients; the human T-lymphotropic retrovirus I (HTLV-I) induces adult T-cell leukemia; and Merkel cell polyomavirus (MCV) causes its eponymous cancer. Together, viruses may account for as many as 20% of all human cancers, and a similar percentage of all deaths due to cancer!
As shown by the HPV vaccine, and earlier by vaccines against hepatitis B, cancers that have a viral etiology might be prevented by vaccination. Apropos hepatitis B, in the late 1980s, Merck and GlaxoSmithKline developed the respective hepatitis B vaccines, Recombivax and Engerix. Like, the HPV vaccines, they are based on VLPs, and they have significantly reduced the incidence of HBV-associated hepatoma; once one of the most lethal cancers.
Bacterial and parasitic infections too may lead to cancer. For example, Heliobacter pylori infections may lead to stomach cancer, and Schistosoma, Opisthorchis, and Clonorchis have been linked to rectum and bladder cancers in areas of Northern Africa and Southeast Asia, where those pathogens are prevalent.]
Lowy and Schiller’s achievement stands out as a superb example of basic research translating into very considerable public health benefits. Moreover, it serves as a strong endorsement for government support of basic research. To these points, Schiller noted that companies would not likely have carried out the necessary basic research and development necessary to produce the HPV vaccine, considering the seemingly small likelihood of success, as suggested by earlier failed attempts to develop a vaccine.
At a September 6, 2017 press conference announcing the Lasker-DeBakey Clinical Medical Research Award, Lowry related that he first learned about vaccines in 1955, when he went with his mother, a physician, to a talk by Jonas Salk about his then new polio vaccine. “I learned far more about polio virus and the vaccine than was probably appropriate for a 12-year-old boy.” Many years afterwards, Lowy began his “extraordinarily effective” collaboration with Schiller, which has endured for more than 30 years.
Schiller said that a high point in his career was taking his daughter to get the vaccine he helped to create. “We first came up with the idea of the vaccine when she was born and it became available when she was 13 years old (1).”
A Revolutionary Vaccine, New York Times, September 6, 2017.