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Douglas Lowy, John Schiller, and the Vaccine Against Cervical Cancer

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.

Douglas Lowy and John Schiller

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 lower acceptance 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).”

References:

  1. A Revolutionary Vaccine, New York Times, September 6, 2017.

 

 

 

 

 

 

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Harald zur Hausen, Papillomaviruses, and Cervical Cancer

Harald zur Hausen (1936- ) was awarded a share of the 2008 Nobel Prize in Physiology or Medicine for discovering that papillomaviruses cause cervical cancer. He received the award jointly with Luc Montagnier and Françoise Barré-Sinoussi, who were given their portion for discovering HIV (1). Before getting on with zur Hausen’s story per se, we begin with bit of earlier history.

zur hausen Harald zur Hausen in 2008

Genital warts are benign epithelial tumors that have been known and associated with sexual promiscuity since the time of the ancient Greeks. In 1907 these lesions were unequivocally proven to be an infectious disease by Italian researcher, G. Ciuffo, who showed that they can be transmitted by filtered extracts of wart tissue; a finding which also implied that the etiologic agent is a virus. Ciuffo inoculated himself to advance his case.

Ciuffo’s finding is relevant to our story since members of the papillomavirus family of DNA viruses are the cause of warts. What’s more, and importantly, some papillomaviruses  also cause malignant cervical carcinomas.

In 1933 Richard Shope, at the Rockefeller Institute, became the first researcher to isolate a papillomavirus, the cottontail rabbit papillomavirus. Shope went on to show that this virus is the cause of skin papillomas in its rabbit host. This finding by Shope was the first to demonstrate that a DNA virus can be tumorigenic.

Years earlier, in 1911, Peyton Rous discovered that an RNA virus—the Rous sarcoma virus (the prototype retrovirus)—causes solid tumors in chickens. Peyton Rous was Richard Shope’s friend and colleague at the Rockefeller Institute. In 1934 Shope asked Rous to characterize the warts that the rabbit papillomavirus induces in jackrabbits. Rous found those warts to be benign tumors that could progress to malignant carcinomas.

Despite the earlier findings of Ciuffo, Shope, and others, the notion that genital warts in humans is a sexually transmitted malady was slow to gain acceptance. Oddly, perhaps, recognition of that truth was prompted by a 1954 report that American servicemen, who had been serving in Korea, were transmitting genital warts to their wives upon returning to the U.S (T. J. Barrett, et al., J. Am. Med. Assoc. 154:333, 1954). [Sexually transmitted diseases were a long-standing problem in the military. Servicemen were most often infected by sex workers who frequented the vicinity of military quarters.]

The key discoveries of this tale are Harald zur Hausen’s 1983 and 1984 findings that two human papillomavirus subtypes, HPV-16 and HPV-18, together account for about 70% of all cervical cancers. Considering that more than 120 distinct HPV subtypes have been identified, the high degree of association of cervical carcinoma with only two of these subtypes provided compelling evidence for the viral etiology of this malignancy. Later studies showed that HPV-31, HPV-33, HPV-45, HPV-52, and HPV-58 are responsible for another 20% of cervical cancers. Indeed, an HPV infection is present in virtually all cervical carcinomas. See Aside 1.

[Aside 1: Cervical cancer was once the leading cause of cancer-related deaths in women in the United States. However, the number of cervical cancer deaths in the industrialized world decreased dramatically over the last 40 years, largely because of the Pap test, which can detect pre-cancer cervical lesions in their early stages. The CDC website reports 12,109 cervical cancer cases and 4,092 deaths from cervical cancer in the U.S. in 2011 (the most recent year for which data are available). Worldwide, cervical cancer was responsible for 275,000 deaths in 2008. About 88% of these deaths were in developing countries (J. Ferlay et al., Int. J. Cancer, 127:2893, 2010).]

Harald zur Hausen was a child in Germany during the Second World War, growing up in Gelsenkirchen-Buer, which was then a center for German coal production and oil refining and, consequently, a major target for allied bombing. [The city also contained a women’s sub-camp of the Buchenwald concentration camp. The Nazis used its prisoners for slave labor.] All members of zur Hausen’s family survived the war. However, zur Hausen’s primary education contained significant gaps because schools were closed during the allied bombing (2).

Despite the gaps in zur Hausen’s early education, he was keenly interested in biology and dreamed of becoming a scientist. Yet at the University of Bonn he opted to study medicine, rather than biology. After zur Hausen received his medical degree, he worked as a medical microbiologist at the University of Düsseldorf, where he enjoyed the opportunity that the University gave him to carry out research on virus-induced chromosomal aberrations.

Although zur Hausen was fascinated by his research, he was soon aware of the deficiencies in his scientific background. So, in 1966 he looked to enhance his proficiency as a scientist by securing a postdoctoral position in the laboratories of Gertrude and Werner Henle at the Children’s Hospital of Philadelphia.

The Henles were a German-born husband and wife research team, known for their work on flu vaccines. More apropos to our story, they are also known for demonstrating the link between the recently discovered Epstein-Barr virus (EBV; a herpesvirus) and infectious mononucleosis, as well as for showing that EBV is the etiologic agent of Burkitt’s lymphoma; a cancer found in parts of Africa. EBV was, in fact, the first virus associated with a cancer in humans. [Gertrude Henle’s mother was murdered by the Nazis in 1943.]

Although zur Hausen took part in the Henles’ experiments involving EBV, he did so grudgingly because he was intimidated by his inexperience in molecular biology. In his own words: “I urged Werner Henle to permit me to work with a different agent, namely adenovirus type 12, hoping that this relatively well established system would permit me to become acquainted with molecular methods. He reluctantly agreed. I started to work eagerly on the induction of specific chromosomal aberrations in adenovirus type 12-infected human cells…and, to please my mentor, I demonstrated electron microscopically the presence of EBV particles directly in… Burkitt’s lymphoma cells (2).”

In 1969 zur Hausen returned to Germany to take an appointment as an independent scientist at the University of Wurzburg. His research was now focused entirely on EBV. Specifically, he wanted to challenge the prevailing view that Burkitt’s lymphoma tumors are persistently infected with EBV (i.e., that the tumors continuously produce low levels of the virus).

I presume that zur Hausen was interested in this issue because it was reasonable to believe that EBV gene expression is necessary to maintain the neoplastic state of the Burkitt’s tumor cells. Persistent infection would be one means by which viral genes could be carried by the cells. But zur Hausen believed that EBV DNA might be maintained in Burkitt’s lymphoma cells, even if they did not produce EBV particles.

Werner Henle in Philadelphia (and also George Klein in Stockholm) sent zur Hausen a large number of Burkitt’s lymphoma cell lines and tumor biopsies to aid in his study. One of those cell lines, the Raji line of Burkitt’s lymphoma cells, did not produce EBV particles. Nevertheless, zur Hausen isolated sufficient EBV DNA from the Raji cells to prove that multiple copies of EBV DNA were maintained in them. This was the first time that tumor virus DNA was shown to be present in malignant human cells that were not producing virus. See Aside 2.

[Aside 2: In 1968 Renato Dulbecco and co-workers were the first to discover viral DNA integrated by covalent bonds into cellular DNA (J. Sambrook et al., Proc. Natl. Acad. Sci. U S A. 60:1288, 1968). They were studying cells transformed by the polyomavirus, SV40. Integration explained how SV40 genes could be stably maintained and expressed in transformed cells, in the absence of productive infection. Integration is now recognized as a key event in cell transformation by members of several virus families, including the polyomaviruses, papillomaviruses, and the oncogenic retroviruses.

The situation in the case of EBV, a herpesvirus, is different, as herpesviruses are able to enter into a latent state in host cells. In the latent state the viral genome is maintained as an episome, and only a subset of the viral genes (i.e., those concerned with latency) are expressed. The episomal viral genome is replicated by the cellular DNA replication machinery during the cell cycle S phase, and a viral gene product, EBNA-1, ensures that viral genomes are equally partitioned between the daughter cells. In 1978 George Klein and co-workers were the first to demonstrate episomal EBV DNA in a cell line derived from a Burkitt’s lymphoma biopsy (S. Koliais et al., J. Natl. Cancer. Inst. 60:991, 1978).]

In 1972, while zur Hausen’s attention was focused on EBV and Burkitt’s lymphoma, his research direction took a providential turn that would lead to his most important discoveries. It happened as follows.

Recent seroepidemiological evidence was suggesting a link between herpes simplex virus type 2 (HSV-2), a well known genital infection, and cervical cancer. Since HSV-2, like EBV, is a herpesvirus, and since zur Hausen had already demonstrated that EBV DNA is present in Burkitt’s lymphoma tumor cells, zur Hausen believed he was well positioned to search for HSV-2 DNA in cervical cancer biopsies. However, in this instance, all his repeated attempts failed.

Harald zur Hausen then came across anecdotal reports of genital warts converting to squamous cell carcinomas. Importantly, those genital warts were known to contain typical papillomavirus particles. Taking these two points into account, zur Hausen considered the possibility that papillomaviruses, rather than herpesviruses, might be the cause of cervical carcinomas. Indeed, his initial thought was that the genital wart papillomavirus might also be the etiologic agent for cervical carcinomas.

Thus, Harald zur Hausen began his foray into papillomavirus research. His first experiments found papillomavirus particles in benign plantar (cutaneous) warts. His next experiments demonstrated that there are multiple papillomavirus subtypes. [In brief, zur Hausen used in vitro-transcribed plantar papillomavirus RNA as a hybridization probe against the DNA from various plantar and genital warts. Only some of the plantar wart DNAs, and none of the genital wart DNAs, reacted with his planter wart RNA probe. Restriction endonuclease patterns of a variety of human papillomavirus isolates confirmed that the HPVs comprise a heterogeneous virus family.]

Harald zur Hausen’s next experiments sought to detect papillomavirus DNA in cervical carcinoma biopsies. However, his initial trials in this crucial undertaking were unsuccessful.  He was using DNA from HPV-6 (isolated from a genital wart) as a hybridization probe in those failed attempts. But zur Hausen and co-workers had at hand a number of additional HPV subtypes, from which they prepared other DNA probes. DNA from HPV-11 (from a laryngeal papilloma) indeed detected papillomavirus DNA in cervical carcinomas.

In 1983, two of Zur Hausen’s former students, Mathias Dürst and Michael Boshart, using HPV-11 DNA as a probe, isolated a new HPV subtype, designated HPV-16, from a cervical carcinoma biopsy. In the following year, they isolated HPV-18 from another cervical carcinoma sample. Harald zur Hausen’s group soon determined that HPV-16 is present in about 50% of cervical cancer biopsies, while HPV-18 is present in slightly more than 20%. [The famous HeLa line of cervical cancer cells contains HPV-18 DNA.]

Additional key discoveries took place during the next several years, including the finding that papillomavirus DNA is integrated into the cellular DNA of cervical carcinoma cells. This finding clarified how papillomavirus genes persist in the cancers, and also revealed that the cancers are clonal (see Aside 2, above). Moreover, while the integrated viral genomes often contain deletions, zur Hausen’s group found that two viral genes, E6 and E7, are present and transcribed in all cervical cancer cells. This finding implied that E6 and E7 play a role in initiating and maintaining the oncogenic state. [In 1990 Peter Howley and co-workers demonstrated that the interaction of the E6 gene product with the cellular tumor suppressor protein p53 results in the degradation of p53. In 1992 Ed Harlow and coworkers showed that the E7 gene product blocks the activity of the cellular tumor suppressor protein pRb. Reference 3 details the mechanisms of papillomavirus carcinogenesis.]

The above findings led to widespread acceptance that cervical carcinoma is caused by papillomaviruses. Yet acceptance was not immediate. The prevailing belief, that herpesviruses cause cervical carcinoma, was well-entrenched and slow to fade away. It was based on the observation that many women afflicted with cervical carcinoma also had a history of genital herpes. But, individuals infected with one sexually transmitted pathogen are often infected with others as well. Apropos that, genital warts were long thought to be associated with syphilis, and later with gonorrhea. In any case, in 1995 the World Health Organization officially accepted that HPV-16 and HPV-18 are oncogenic in humans.

Harald zur Hausen was awarded one half of the 2008 Nobel Prize for Medicine or Physiology for proving that cervical cancer is caused by human papillomaviruses. By the time of his award, his findings had led to key insights into the mechanism of HPV-mediated carcinogenesis and, importantly, to the development of a vaccine to prevent cervical cancer. See Aside 3.

[Aside 3: The first generation of Gardasil, made by Merck & Co., helped to prevent cervical cancer by immunizing against HPV types 16 and 18, which are responsible for an estimated 70% of cervical cancers. Moreover it also immunized against HPV types 6 and 11, which are responsible for an estimated 90% of genital warts cases. Apropos genital warts, there are 500,000 to one million new cases of genital warts (also known as condylomata acuminate) diagnosed each year in the United States alone.

The original vaccine was approved by the USFDA on June 8, 2006. An updated version of Gardasil, Gardasil 9, protects against the HPV strains covered by the first generation of the vaccine, as well as five additional HPV strains (HPV-31, HPV-33, HPV-45, HPV-52, and HPV-58), which are responsible for another 20% of cervical cancers. Gardasil 9 was approved by the USFDA in December 2014.]

Harald zur Hausen reviewed the overall contribution of viruses to human cancer in his 2008 Nobel lecture (4). Some of his key points are as follows. HPVs were discussed above with respect to cervical carcinoma. HPVs also are associated with squamous cell carcinomas of the vagina, anus, vulva, and oropharynx. What’s more, 40% of the 26,300 cases of penile cancer reported worldwide in 2002 could be attributed to HPV infection.

Epstein-Barr virus too was discussed above. This member of the herpesvirus virus family causes nasopharyngeal carcinoma, as well as Burkitt’s lymphoma. Another herpesvirus, human herpesvirus 8, causes Kaposi’s sarcoma; the most frequent cancer affecting AIDS patients. Hepatitis B virus (HBV, a hepadnavirus), as well as hepatitis C virus (HCV, a flavivirus), causes hepatocellular carcinoma. The human T-lymphotropic virus 1 (HTLV-1), a retrovirus, induces adult T-cell leukemia. And the recently discovered Merkel cell polyomavirus (MCPyV) is responsible for Merkel cell carcinoma.

Harald zur Hausen estimated that viruses directly cause about 20% of all human cancers, and a similar percentage of all deaths due to cancer! And while 20% might seem to be a remarkably high figure for the extent of viral involvement in human cancer, zur Hausen suggests that it is actually a minimal estimate. That is so because it is difficult to determine that a particular virus is actually the cause of a cancer. Consequently, it is likely that other examples of viral involvement in human cancer will be discovered.

Harald zur Hausen gave two principal reasons for why it is difficult to establish that an infectious agent is the cause of a cancer in humans. First: “… no human cancer arises as the acute consequence of infection. The latency periods between primary infection and cancer development are frequently in the range of 15 to 40 years…” Second: “Most of the infections linked to human cancers are common in human populations; they are ubiquitous… Yet only a small proportion of infected individuals develops the respective cancer type.”

Viruses also contribute to the human cancer burden in an indirect way. For instance, HIV types 1 and 2 play an indirect role in cancer via their immunosuppressive effect, which is the reason for the extraordinarily high prevalence and aggressiveness of Kaposi’s sarcoma in AIDS patients.

Bacterial infections also contribute to the cancer burden. For example, Helicobacter pylori infections may lead to stomach cancer. What’s more, the parasites Schistosoma, Opisthorchis, and Clonorchis have been linked to rectum and bladder cancers in parts of Northern Africa and Southeast Asia, where they are prevalent.

Obviously, but important enough to state anyway, knowing that a particular cancer is caused by a particular infectious agent opens the possibility of developing a rational strategy to prevent that cancer. Gardasil is an exmple. A vaccine against HBV is also available, and one against HCV is under development.

References:

1. Who discovered HIV, Posted on the blog January 23, 2014.

2. MLA style: “Harald zur Hausen – Biographical”. Nobelprize.org. Nobel Media AB 2014. Web. 27 May 2015. <http://www.nobelprize.org/nobel_prizes/medicine/laureates/2008/hausen-bio.html&gt;

3. Norkin, Leonard C. (2010) Virology: Molecular Biology and Pathogenesis. ASM Press, Washington, D.C. See Chapters 15 and 16.

4. Zur Hausen, Harold, The search for infectious causes of human cancers: where and why. Nobel Lecture, December 7, 2008.