In 1911 Peyton Rous, at the Rockefeller Institute, discovered the Rous sarcoma virus; the first virus known to cause solid tumors (1). Although Rous’ eponymous virus also would be known as the prototype retrovirus, his discovery generated only scant interest at the time, and would not be recognized by the Nobel Committed until 65 years later! [Nobel prizes are not awarded posthumously. Fortunately, Rous had longevity on his side. He died 4 years after receiving the prize, at age 87.]
In 1976 Harold Varmus and J. Michael Bishop, then at the University of California San Francisco, discovered that the Rous sarcoma virus oncogene, v-src, as well as the oncogenes of several other tumorgenic retroviruses, actually were derived from cellular genes that normally play an important role in controlling cell division and differentiation (2). Moreover, Varmus and Bishop showed that these cellular “proto-oncogenes” can be altered by mutation, to become “oncogenes” that contribute to cancer. [Varmus and Bishop received the 1989 Nobel Prize in Physiology or Medicine for their discovery of proto-oncogenes.]
But what is the actual activity of the protein coded for by the normal cellular c-src, and by v-src as well? The story of that discovery is rather delightful and begins as follows.
In 1978, Raymond Erikson and coworker Marc Collette, then at the University of Colorado Medical Center, were the first researchers to isolate the Src protein. They accomplished this by first preparing lysates from avian and mammalian cells, which had been transformed in culture into tumor cells by Rous sarcoma virus. Next, they precipitated those lysates with antisera from rabbits that bore Rous sarcoma virus-induced tumors. The premise of their strategy was that antibodies from the tumor-bearing rabbits would recognize and precipitate proteins that were specific to cells transformed by the virus .
With the Src protein now in hand, Ericson and Collette next sought its function. They initially asked whether Src might have protein kinase activity (i.e., an activity that adds a phosphate group to a protein.). This was a reasonable possibility because protein phosphorylation was already known to play a role in regulating various cellular processes, including cell growth and differentiation.
Ericson and Collette tested their premise by incubating their Src immunoprecipitates with [γ-32P] ATP (i.e. 32P-labelled adenosine triphosphate). In agreement with their proposal, they found that the antibody molecules in the Src immunoprecipitates had been phosphorylated. [Note that Src’s protein kinase activity was simultaneously and independently discovered by Varmus and Bishop.]
Ericson and Collette also carried out control experiments that were particularly revealing. When the same rabbit antisera was used to immunoprecipitate extracts from normal cells, or extracts from cells infected with a transformation-defective mutant of Rous sarcoma virus, no signs of protein kinase activity were seen in those immunoprecipitates. What’s more, the protein kinase activity was found to be temperature sensitive in immunoprecipitates from cells infected with a mutant Rous sarcoma virus that was temperature-sensitive for transformation.
These control experiments confirmed that the protein kinase activity in the immunoprecipitates was coded for by the virus. What’s more, they confirmed that the kinase activity of the retroviral Src protein plays an essential role in transformation. Furthermore, when taken with the earlier findings of Varmus and Bishop, they implied that the kinase activity of the cellular Src protein plays a key role in the control of normal cell proliferation.
While Erickson and coworkers were carrying out the above experiments in Denver, Walter Eckhart and TonyHunter, at the Salk Institute, were looking into the basis for the transforming activity of the mouse polyomavirus middle T (MT) protein. [Unlike Rous sarcoma virus, which is a retrovirus, the mouse polyomavirus is a member of the Polyomavirus family of small DNA tumor viruses. SV40 is the prototype Polyomavirus.]
Since Erickson’s group was finding that Src expresses protein kinase activity, Eckhart and Hunter asked whether the polyomavirus MT protein might likewise be a protein kinase. Thus, as Erickson and Collette had done in the case of Src, Eckhart and Hunter examined immunoprecipitates of MT to see if they too might express a protein kinase activity, and found that indeed they did.
Interestingly, it was not known at the time of these experiments that MT actually does not express any intrinsic enzymatic activity of its own. Instead, MT interacts with the cellular Src protein to activate its protein kinase activity. See Aside 1.
[Aside 1: For aficionados, MT is a membrane-associated protein that interacts with several cellular proteins. Importantly, the phosphorylation events carried out by MT-activated Src cause a variety of signal adaptor molecules [e.g., Shc, Grb2, and Sos] and other signal mediators [e.g., PI3K and PLCγ] to bind to the complex, thereby triggering a variety of mitogenic signaling pathways. These facts were not yet known when Eckhart and Hunter were doing their experiments.]
At the time of these experiments, serine and threonine were the only amino acids known to be phosphorylated by protein kinases. In fact, Erikson and Collette, as well as Varmus and Bishop, believed that threonine was the amino acid phosphorylated by the Src kinase (see below). Consequently, Hunter asked whether the polyomavirus MT protein likewise would phosphorylate threonine. [Recall that MT actually does not express any intrinsic enzymatic activity of its own.]
Hunter’s experimental procedure was relatively straightforward and reminiscent of Erikson’s and Collette’s. It involved incubating immunoprecipitates of MT with [γ-32P]ATP, hydrolyzing the immunoglobulin, and then separating the amino acids in the hydrolysate by electrophoresis. But, to Hunter’s surprise, the position of the labeled amino acid in his electropherogram did not correspond to that of either threonine or serine.
Hunter was well aware that tyrosine is the only other amino acid with a free hydroxyl group that might be a target for the MT kinase activity. And, while there was no precedent for a tyrosine-specific protein kinase, Hunter proceeded to ask whether the polyomavirus MT protein indeed might phosphorylate tyrosine.
Hunter began by synthesizing a phosphotyrosine molecule that could be used as a standard marker against which to compare the labeled amino acid in a repeat of his earlier experiment. And, to his pleasure, Hunter found that the amino acid that was phosphorylated by the MT kinase activity ran precisely with the phosphotyrosine standard marker in his new electropherograms.
But why had other researchers not detected tyrosine phosphorylation earlier? It was partly because phosphotyrosine accounts for only about 0.03% of phosphorylated amino acids in normal cells. The remaining 99.97% are phosphoserine and phosphothreonine. But, again, that is not the entire explanation. The rest is truly precious.
In Hunter’s own words, he was “too lazy to make up fresh buffer” before doing his experiments. Had the buffer been fresh, its pH would have been the usual 1.9; a pH that, unbeknownst to all at the time, does not separate phosphotyrosine from phosphothreonine during the electrophoresis procedure. The pH of the old buffer that Hunter used in his experiment had inadvertently dropped to 1.7; a pH at which phosphotyrosine is resolved from phosphothreonine. That fact enabled Hunter to discriminate phosphotyrosine from phosphothreonine for the first time. Thus, Hunter attributes his hugely important discovery to his laziness.
The finding that tyrosine is the amino acid phosophorylated by the polyomavirus MT protein kinase activity led Hunter and his Salk Institute-colleague Bart Sefton to ask whether Src too might phosphorylate tyrosine, rather than serine or threonine (4). Indeed, they found that the retroviral Src protein, as well the normal cellular Src protein, function as tyrosine-specific protein kinases. [Recall that it became clear only later that MT actually has no intrinsic enzyme activity of its own and that it acts through Src.] Moreover, the levels of phosphotyrosine were 10-fold higher in cells infected with wild-type Rous sarcoma virus than in control cells, consistent with the premise that Src’s protein tyrosine kinase activity accounts for the altered growth potential of those cells.
Subsequently, Stanley Cohen, at Vanderbilt University, discovered that the epidermal growth factor (EGF) receptor contains an intrinsic protein-tyrosine kinase activity, further underscoring the importance of protein-tyrosine kinases in the normal control of cell proliferation. [Cohen shared the 1986 Nobel Prize in Physiology or Medicine with Rita Levi-Montalcini for their discoveries of growth factors, including EGF.] Subsequent studies identified additional receptor protein-tyrosine kinases, such as the fetal growth factor (FGF) receptor, and non-receptor protein-tyrosine kinases, such as Abl, each of which activates a mitogenic intracellular signaling pathway.
Tony Hunter and coworkers went on to demonstrate that protein-tyrosine kinases play key roles in additional crucial cellular processes, including cellular adhesion, vesicle trafficking, cell communication, the control of gene expression, protein degradation, and immune responses. Moreover, discoveries regarding the role of protein-tyrosine kinases in cell transformation and cancer gave rise to a promising new rational approach to cancer therapy; i.e., the targeting of protein-tyrosine kinases. For example, the drug Gleevec, which inhibits activation of the Abl and platelet-derived growth factor (PDGF) tyrosine kinases, was approved by the U.S. Food and Drug Administration for the treatment of chronic myelogenous leukemia and several types of gastrointestinal tumors.
Harold Varmus and J. Michael Bishop changed cancer research in a fundamental way in the 1970s, when they discovered proto-oncogenes at the University of California at San Francisco (UCSF). Proto-oncogenes are cellular genes that normally play an important role in controlling cell division and differentiation. However, Varmus and Bishop found that proto-oncogenes can be altered by mutation, to become oncogenes that contribute to cancer. When Varmus and Bishop first began their collaboration in 1970, cancer research was, for the most part, focused on epidemiology (e.g., studies linking smoking to lung cancer) and empirical approaches to therapy (e.g., radiation and chemotherapy).
The discovery of proto-oncogenes is a pertinent topic for our Virology blog because it depended crucially on Varmus and Bishop’s earlier finding that retroviral oncogenes are mutated versions of cellular genes that retroviruses “captured” from their host cells. Varmus and Bishop hypothesized and then demonstrated that since retroviral oncogenes are versions of genes that actually are part of a normal cell’s genetic makeup, mutations in those genes, or their inappropriate expression, can lead to cancer. The v-src gene of Rous sarcoma virus was the first retroviral oncogene that Varmus and Bishop showed is derived from a cellular genome (1).
Varmus and Bishop continued searching for proto-oncogenes in the 1980s. Varmus also began investigating HIV (also a retrovirus and the cause of AIDS). In 1989 Varmus and Bishop were awarded the Nobel Prize in Physiology or Medicine for their discovery of proto-oncogenes.
In the early 1990s Varmus stepped out from his role as a research scientist to take up the cause of public funding for biomedical research. In 1993 President Bill Clinton acknowledged Varmus’ efforts in that regard, as well as his stature as a scientist, by appointing him to serve as Director of the National Institutes of Health (NIH). Thus, Varmus became the first Nobel laureate to head the NIH.
In 2000 Varmus left the NIH to accept the presidency of the Memorial Sloan-Kettering Cancer Center in New York. In 2010 Varmus returned to the NIH, this time appointed by President Barak Obama to serve as director of the National Cancer Institute (NCI). In 2015 Varmus was back again in New York where he is the Lewis Thomas University Professor of Medicine at Weill-Cornell Medical College.
Varmus was featured in two earlier blog postings. The first of these described how he mediated the dispute between Robert Gallo and Luc Montagnier over the right to name the AIDS virus (2). The second posting covered some of the political and social dilemmas Varmus faced during his days leading the NIH (3).
Here, we relate first how Varmus opted for a career in biomedical science and, second, how his collaboration with Bishop came about. This is an interesting tale because Varmus’ remarkable career as a science researcher, administrator, and spokesperson happened despite his initial intention to become a teacher of English literature. Indeed, his career in science did not begin until after he earned an M.A. degree in English from Harvard University, and then spent four years in medical school preparing for a career in clinical medicine.
We begin our story in 1950 as Varmus recounts how, as a ten-year-old, he witnessed his physician father receive a call that conveyed shocking news: “one of my mother’s favorite cousins, a robust man in the middle of his life, had just been diagnosed with leukemia. Of course, I did not know very much about leukemia, but I did know immediately from my parents’ expressions–and within a few weeks, from our cousin’s death—that his disease was a veritable tidal wave.” [All quotations are from Varmus’ book The Art and Politics of Science (4), in which he reflects back on his entire career.]
His cousin’s leukemia actually resulted from a mutation in one of the genes that Varmus would discover more than two decades later. And Varmus notes just how far the science in general had progressed during that 25-year interim: “…when my father heard about our cousin’s leukemia, biologists were not even sure that genes were made of DNA, had no idea how genetic information could be encoded in genes, and, of course, had no way of knowing that cancers are driven by mutations.”
Varmus was urged by his father to prepare for a career in medicine. Nonetheless, when Varmus enrolled as a freshman at Amherst College he strongly favored studying the humanities. Thus, he “toyed with the idea of majoring in philosophy (ultimately too abstract), physics (ultimately too hard), and English literature (ultimately selected).”
Throughout his undergraduate days, Varmus envisioned preparing for an academic career teaching literature. Still, he dutifully fulfilled premed requirements to keep open the possibility of obliging his father’s wishes that he become a medical doctor. Yet he never considered majoring in biology. “I couldn’t understand how some of my close friends (among them, some now distinguished scientists) could spend long afternoons and evenings incarcerated in a laboratory, when they could be reading books in a soft library chair or reciting poetry on Amherst’s green hills.”
Varmus began having doubts about his career choice when his Amherst College classmate Art Landy (later a well-known molecular biologist at Brown University) won an Amherst biology prize that allowed him to attend a 1961 international biochemistry meeting in Moscow. Importantly, Landy invited Varmus to accompany him to the Moscow meeting, where Varmus learned that Marshal Nirenberg had deciphered the genetic code. “Even though I did not understand its meaning or its importance at the time, I was not oblivious to the excitement around me…Scientists seemed likely to discover new, deep, and useful things about the world, and other scientists would be excited by these discoveries and eager to build on them. Would this be true of literary critics and teachers?”
Notwithstanding these misgivings, Varmus continued on his path to a career in English literature after graduating from Amherst College in 1961, earning an M.A. in English from Harvard in 1962 (his focus was on Anglo-Saxon poetry). But his uncertainties about his future only grew stronger. “Despite outward signs that I had chosen a life of studying and teaching literature, soon after starting my graduate work at Harvard I began to suffer some further internal doubts about abandoning medicine. The graduate curriculum in English literature was not especially onerous, but it felt like a prolongation of college. Most of my courses were heavily populated with Harvard and Radcliff undergraduates.” Varmus leaves the impression that he looked upon much of his course work at Harvard as a tiresome chore.
Varmus was also aware of the enthusiasm of former Amherst College classmates who were then studying at Harvard Medical School. “Occasionally, on Saturday mornings, I traveled across the Charles River to join some Amherst classmates at Harvard Medical School, while they sat in the Ether Dome at Massachusetts General Hospital, entranced by diagnostic dilemmas discussed at the weekly pathology conference. These stories struck me as far more interesting than those I was reading, and my medical school friends expressed general excitement about their work. They also seemed to have formed a community of scholars, with shared interests in the human body and its diseases and common expectations that they would soon be able to do something about those diseases…These Saturday excursions probably account for an influential dream I had one night about my continuing indecision. In that dream, my future literature students were relieved when I didn’t turn up to teach a class, but my future patients were disappointed when I didn’t appear. It seemed I wanted to be wanted.”
So, Varmus finally came to grips with his qualms about a career teaching English literature, hastily preparing an application to Harvard Medical School and biking across the frozen Charles River to deliver it just in time to meet the deadline. But it was to no avail, since the dean of admissions thought Varmus was “too inconstant and immature” for medical school.
Varmus next sent off an application to Columbia University’s College of Physicians and Surgeons (P&S). His interviewer at Columbia was an esteemed physician named David Seegal, who also happened to be rather literate. Seegal asked Varmus if he might translate the Anglo Saxon phrase Ich ne wat. “This was easy; it simply means ‘I don’t know.’” Seegal used his question as a lead-in to discuss why a physician might admit fallibility to a patient. In any case: “By the fall of 1962, I was happily enrolled at P&S, helped for the first, but not the last, time by someone’s exaggerated appreciation of my competence in two cultures.”
Now ensconced at P&S, Varmus thought he might become a psychiatrist; an ambition stoked by an interest in Freud and by his winning of an essay prize at P&S in psychiatry. But, he found his “first hour alone in a room with a psychotic patient to be more difficult and less interesting than an hour reading Freud.” So, Varmus’ interests in medical school turned from the “elusive mind” to the physical brain and then, more generally, to diseases that might be explained by known physiology and biochemistry.
When graduation from medical school was impending, Varmus had to consider his career options more deliberately than he had in the past. A key factor was the Vietnam War, which was in progress, and which he and many others of that era vehemently opposed. “I was determined not to serve in it. Medical graduates were subject to the draft; however, we did have the more palatable option of two years training at one of the agencies of the Public Health Service. For most of my classmates with academic ambitions similar to my own, the NIH was the favored choice. As the largest biomedical research campus in the world, it offered unequaled opportunities to learn virtually any form of biomedical research…”
Varmus confesses that he had a “woeful lack of laboratory credentials.” Nonetheless, he entered the competition for one of the coveted research slots at the NIH. But, because of his lack of research experience, he was not encouraged by most of the NIH laboratory chiefs who interviewed him. However, one of them, endocrinologist Jack Robbins, suggested to Varmus that he speak to Ira Pastan; a young endocrinologist who, at that time, was interested in the production of thyroid hormones.
As Varmus relates, “The recommendation proved to be wise and fateful. My schooling in literature turned out to be more important than my interest in endocrinology, Ira’s field, because Ira’s wife Linda, a poet, had often complained that Ira’s colleagues seldom talked about books…When matches were announced I was told I would become Ira’s first clinical associate, having been passed over by the more senior investigators. This outcome could not have been more fortunate.”
But, before Varmus could take up his position at the NIH, he received a “shocking phone call” from Pastan, to the effect that he (Pastan) was giving up his work on thyroid hormones because he and colleague Bob Perlman “had made a shocking discovery about gene regulation in bacteria.” Pastan and Perlman found that cyclic AMP is a major regulator of bacterial gene activity, and that it plays a similar role in animal cells—findings which led Pastan to pioneer the field of receptor biology in animal cells.
The discovery by Pastan and Perlman had important consequences for Varmus. First, it immediately forced him to give up his plan to train in endocrinology. Instead, Pastan assigned Varmus to find out whether cyclic AMP augments bacterial gene expression by increasing transcription of mRNA. Second, as explained below, Pastan’s new research direction led to Varmus’ introduction to and fascination with virology.
So, Varmus was now a budding molecular biologist. But, since he had no prior research experience, his first days in the Pastan lab were a near disaster, leading Pastan to half jokingly ask, “Now remind me why I took you into the lab.”
In any case, Varmus worked closely with Pastan to develop a molecular hybridization assay to measure transcription of E. coli lac mRNA. [Their specific the goal was determine whether the mechanism by which cyclic AMP reverses catabolite repression of the E. coli lac operon is by enhancing transcription of lac mRNA.] And, they used an E. coli phage, which had incorporated the lac operon into its genome, as their source of isolated lac operon DNA. Thus, Varmus was introduced to virology. [Aficionados, note, “These experiments with the lac operon proved to be analogous in several ways to experiments that revealed the first proto-oncogene a few years later.”]
The satisfaction that Varmus derived from his research in Pastan’s lab caused him to reconsider his aspirations for a career in clinical medicine, and instead to think about a future in biomedical research. He thought he might next try his hand at cancer research, motivated in part, by his mother’s breast cancer, first diagnosed in 1968, to which she succumbed two years later. But there were other factors at work as well. In particular, Varmus’ use of the E. coli phage in Pastan’s lab led to his fascination with virology. And his interest in virology was relevant to his new plans because the DNA and RNA tumor viruses held immense potential for cancer research. 1970s technology could not identify which one of the tens of thousands of cellular genes might have mutated to result in a cancer. However, that technology was potentially able to identify which of the handful of a tumor virus’ genes might underlie its ability to transform a normal cell into a tumor cell.
That line of thought led Varmus to apply for a research position in Renato Dulbecco’s lab at the Salk Institute. [Dulbecco would win a share of the 1975 Nobel Prize in Physiology or Medicine for his pioneering studies of the DNA tumor viruses (5).] However, reminiscent of Varmus’ unsuccessful application to Harvard Medical School, he was “rebuffed by not one but two letters from his (Dulbecco’s) secretary.”
While the rejection from Dulbecco was a disappointment, it would be another of the seemingly providential happenings in Varmus’ career. In the summer of 1969 he chanced to visit Harry Rubin, an eminent Rous sarcoma virus researcher at U Cal Berkeley. Rubin, who had earlier introduced Howard Temin to virology (another auspicious happening; see reference 6), told Varmus about a new group at UCSF that had begun to study retroviruses. Importantly, the goal of the UCSF group was to discover cancer-causing genes. Thus, Varmus stopped over at UCSF, where he met members of the group, including a smart young virologist named Mike Bishop. Varmus reports, “we recognized from the first moments that we were destined to work together.”
Varmus came to Bishop’s lab in 1970 as a postdoctoral fellow. However, their relationship quickly evolved to one of equals, and they made all of their major discoveries in the 1970s and 1980s as a team, and they rose together through the UCSF academic ranks. Bishop relates that their bond formed not just by a shared fascination with cancer viruses but “by our mutual love of words and language.” Varmus, for his part, notes that “after many years of ambivalence and indecision…I appeared to be headed in a clear direction, even if not towards medicine or literature.”
1. Stehelin D, Varmus HE, Bishop JM, Vogt PK., 1976. DNA related to the transforming gene(s) of avian sarcoma viruses is present in normal avian DNA. Nature 260:170-173.
2. How the Human Immunodeficiency Deficiency Virus (HIV) Got Its Name, posted on the blog February 4, 2014.
3. The Politics of Science: Vignettes Featuring Nobel Laureate Harold Varmus during his Tenure as Director of the NIH, posted on the blog June 2, 2014.
4. Varmus, H. 2009. The Art and Politics of Science, (W. W. Norton & Company).
5. Renato Dulbecco and the Beginnings of Quantitative Animal Virology, posted on the blog December 3, 2013.
6. Howard Temin: “In from the Cold”, posted on the blog December 14, 2013.
There is a cautionary note on the info sheet accompanying the influenza vaccine, which advises individuals who are allergic to eggs to speak with their doctors before receiving the vaccine. As most readers know, the reason for the warning is that the usual flu vaccine is grown in embryonated chicken eggs.
[Aside 1: The current trivalent influenza vaccine is prepared by inoculating separate batches of fertile chicken eggs; each with one of the three influenza strains (representing an H1N1, an H3N2, and a B strain) recommended by the WHO for the upcoming winter flu season. The monovalent viral yields are then combined to make the trivalent vaccine.]
But, why chicken eggs, and how did this state of affairs come to be? The backdrop to this tale is that until the third decade of the twentieth century, virologists were still searching for fruitful means to cultivate viruses outside of a live laboratory animal. This was so despite the fact that, as early as 1907, researchers had been developing procedures for maintaining viable tissues in culture. And, soon afterwards, virologists began to adapt tissue cultures as substrates for propagating viruses.
Yet as late as 1930, there were still only two antiviral vaccines—the smallpox vaccine developed by Edward Jenner in 1798 (1) and the rabies vaccine developed by Louis Pasteur in 1885. Bearing in mind that Jenner’s vaccine preceded the germ-theory of disease by a half century, and that Pasteur’s vaccine came 15 years before the actual discovery of viruses (as microbial agents that are distinct from bacteria), the development of these first two viral vaccines was fortunate indeed (2).
The principal factor holding up the development of new viral vaccines was that viruses, unlike bacteria, could not be propagated in pure culture. Instead, for reasons not yet understood, viruses could replicate only within a suitable host. And, notwithstanding early attempts to propagate viruses in tissue culture (reviewed below), developments had not yet reached a stage where that approach was fruitful enough to generate a vaccine. How then were Jenner and Pasteur able to produce their vaccines? See Aside 2 for the answers.
[Aside 2: Jenner, without any awareness of the existence of infectious microbes, obtained his initial inoculate by using a lance to pierce a cowpox postule on the wrist of a young milkmaid, Sarah Nelmes. Jenner then propagated the vaccine, while also transmitting immunity, by direct person-to-person transfer. (The rationale underlying Jenner’s vaccine, and his story, is told in detail in reference 1.)
Jenner’s live cowpox vaccine protected against smallpox because cowpox, which produces a relatively benign infection in humans, is immunologically cross-reactive with smallpox. Thus, inoculating humans with cowpox induces immunity that is active against cowpox and against smallpox as well. Jenner’s discovery of the smallpox vaccine, while not entirely fortuitous, was still providential, since immunity per se, as well as microbes, were unknown in Jenner’s day.
Following a successful worldwide vaccination program, smallpox was officially declared to be eradicated in 1977. The smallpox vaccine currently stockpiled in the United States contains live vaccinia; a virus that is immunologically related to cowpox and smallpox. Like cowpox, vaccinia causes a mild infection in humans.
The existing smallpox vaccine was grown in the skin of calves. It is now more than 40 years old and has not been used for years, but it is still believed to be effective.
Pasteur (probably the greatest and most famous microbiologist) was a pioneer of the germ theory of disease. Yet he developed his rabies vaccine more than a decade before the discovery of viruses. He did so by applying the same principle that he used earlier to produce a vaccine against cholera. That is, he “attenuated” the rabies agent. He began with virus that was contained in an extract from a rabid dog. Pasteur attenuated the virus for humans by successively passing extracts in the spinal cords of live rabbits, and then aging the last extracts in the series. Modern rabies vaccines are generally killed virus vaccines, prepared by chemically inactivating tissue culture lysates.]
In the years following the pioneering 19th century contributions of Pasteur, Koch, and Lister, and with the widespread acceptance of the germ theory of disease, microbiologists (that is, bacteriologists) appreciated the importance of working with “pure cultures” that could be grown in a sterilized medium. Yet this was proving to be impossible in the case of viruses. Moreover, as late as the 1930s, it was not understood why that should be so
At the very least, virologists would have liked to be able to cultivate viruses outside of a living animal host. The possibility of achieving that goal began to emerge when Ross G. Harrison, working at Johns Hopkins in 1907, became the first researcher to maintain bits of viable tissue outside of an animal. Harrison maintained frog neuroblasts in hanging drops of lymph medium. What’s more, under those conditions, the neuroblasts gave rise to outgrowths of nerve fibers.
In 1913, Edna Steinhardt became the first researcher to cultivate (or at least maintain) a virus (cowpox) in a tissue culture. Steinhardt did this by infecting hanging-drop cultures with corneal extracts from the eyes of cowpox-infected rabbits and guinea pigs. However, there was no methodology at the time for Steinhardt to determine whether the virus might have replicated in her tissue cultures.
In 1912, Alexis Carrel, working at the Rockefeller Institute, began a two-decade-long experiment that significantly increased interest in tissue culture. Carrel maintained tissue fragments from an embryonic chicken heart in a closed flask, which he regularly supplied with fresh nutrients. Later, he claimed that he maintained the viability of the culture for more than 20 years; well beyond the normal lifespan of a chicken. See Aside 3.
[Aside 3: Carrel’s experimental results could never be reproduced. In fact, in the 1960s, Leonard Hayflick and Paul Moorhead made the important discovery that differentiated cells can undergo only a limited number of divisions in culture before undergoing senescence and dying. It is not known how Carrel obtained his anomalous results. But, Carrel was an honored, if controversial scientist, having been awarded the 1912 Nobel Prize in Physiology or Medicine for pioneering vascular suturing techniques. In the 1930s Carrel developed an intriguing and close friendship with Charles Lindbergh, which began when Lindbergh sought out Carrel to see if Carrel might help Lindbergh’s sister, whose heart was damaged by rheumatic fever. Carrel could not help Lindbergh’s sister, but Lindbergh helped Carrel build the first perfusion pump, which laid the groundwork for open heart surgery and organ transplants. Carrel and Lindbergh also co-authored a book, The Culture of Organs. In the 1930s, Carrel, promoted enforced eugenics. During the Second World War, Carrel, who was French by birth, helped the Vichy French government put eugenics policies into practice. Moreover, he praised the eugenics policies of the Third Reich, leading to inconclusive investigations into whether he collaborated with the Nazis. Carrel died in November, 1944.]
In 1925 Frederic Parker and Robert Nye, at the Boston City Hospital, provided the first conclusive evidence for viral growth in a tissue culture. The virus was a strain of herpes simplex, which Parker and Nye received in the form of an extract from Ernest Goodpasture; soon to be the major character in our story. Parker and Nye established their first culture from the brain of a rabbit that was inoculated intracerebrally with an extract from an infected rabbit brain. The animal was sacrificed when in a convulsive state, and its brain was then removed aseptically. Small pieces of normal rabbit testes were added to pieces of brain in the cultures, to provide another potential host cell for the virus. Virus multiplication was demonstrated by inoculating diluents of subculture extracts into laboratory animals. A 1:50,000 diluent was able to transmit the infection.
At this point in our chronology, the pathologist Ernest Goodpasture, and the husband-wife team of Alice and Eugene Woodruff, enters our story. Goodpasture’s principal interest was then, as always, in pathology. He became interested in viruses while he was serving as a Navy doctor during World War I. But his focus was on the pathology of the 1918 influenza pandemic, which he studied in the first sailors stricken by the infection (3). He was later interested in herpetic encephalitis, and in how rabies virus made its way to the central nervous system, but always from the perspective of a pathologist.
In 1927, Eugene Woodruff was a newly graduated physician who joined Goodpasture in the Pathology Department at Vanderbilt University for training as a pathologist. Eugene’s wife, Alice, a Ph.D., came to the Vanderbilt Pathology Department a year later, as a research fellow in Goodpasture’s laboratory.
Goodpasture set Eugene Woodruff to work on fowlpox; a relative of smallpox, which, unlike cowpox, can not infect humans. Goodpasture was interested in the cellular pathology of fowlpox infection; specifically, in the nature of the inclusion bodies seen in fowlpox-infected cells. Using a micropipette, Woodruff was able to pick single inclusion bodies from infected chicken cells, and to then determine that inclusion bodies are intracellular crystalline arrays of the virus.
More apropos to our story, in the late 1920s, virologists still could not generate large amounts of virus that were free of bacteria and contaminating tissue elements. For that reason, Goodpasture believed that future important advancements in virology would require the development of methods to grow large amounts of virus in pure culture; an impossible goal. In any case, Goodpasture delegated Alice Woodruff to develop a method for growing fowlpox outside of a live chicken.
Goodpasture had already adapted Carrel’s tissue culture methods, which he used to maintain chick kidney tissue in culture. So, Alice’s first experiments were attempts to get fowlpox to propagate in cultures of chick kidney tissue. However, the virus stubbornly declined to grow in the tissue cultures. Goodpasture then suggested to Alice that she try to grow the virus in embryonated chicken eggs. But why did Goodpasture make that suggestion?
The answer isn’t clear. But, back in 1910, Peyton Rous and colleague James Murphy, at the Rockefeller Institute, fruitfully made use of fertile chick eggs to cultivate a virus, as described in Aside 4. However, Rous’ accomplishments, which eventually would be recognized as huge, were largely ignored for the next 50 or so years. (The reasons are discussed in reference 4.) Goodpasture may well have been unaware of Rous’ earlier work when he suggested to Alice that she try to cultivate fowlpox in chicken eggs. If so, then his suggestion to Alice may have been an original idea on his part, perhaps inspired by his thinking of the chick embryo as a sterile substrate that is enclosed in a naturally sterile container. On the other hand, he and Alice did note the earlier work of Rous and Murphy in the 1931 report of their own work. (In that paper, they state: “The production of experimental infection in the chorio-allantoic membrane has, however, been done only in the one instance where Rous and Murphy grew the virus of the Rous sarcoma.”). In any case, the chick embryo method for growing viruses had lain dormant for twenty years.
[Aside 4: Rous and Murphy cut a small window into the shells of six-to-sixteen-day-old embryonated chicken eggs, and then placed a bit of a filtered, cell-free extract from a chicken sarcoma into each. By one week’s time there was a tumor mass growing in each of the inoculated embryos. These studies led to Rous’ 1911 report of a filterable, infectious agent, eventually named the Rous sarcoma virus, which causes sarcomas in chickens. The Rous sarcoma virus was the first virus known to cause solid tumors and, moreover, it was the prototype of a virus family that eventually would be known as the retroviruses (4).]
Alice Woodruff’s procedure for infecting the chicken eggs began with her making a small window in the egg shell, at the site of the air sac. (An egg cup served as the operating table, and the window was cut with a dentist’s drill.) She then inoculated the viral extract into the outermost layer of the chorio-allantoic membrane, which encloses the embryo and provides an air channel into its body. Alice then closed the window with a piece of glass, held in place with Vaseline.
Alice tried to maintain sterility at all stages of her procedure. Yet despite the elegance of her techniques, she had nothing to show for these efforts except dead embryos that were overgrown with mold or bacteria. She then turned to her husband, Eugene, who was working in a separate laboratory, down the hall from her lab.
Alice and Eugene, working together, developed procedures to sterilely remove fowlpox lesions from the heads of chicks. In brief, the chick heads were shaved and then bathed in alcohol. Then, the lesions were excised with sterile instruments. Next, the excised lesions were tested for bacterial or fungal contamination by incubating fragments in nutrient broth. If a lesion was sterile by that test, it was deemed fit to be inoculated into the eggs.
Eugene further contributed to the effort by applying a technique that he developed earlier; picking out individual inclusion bodies from fowlpox-infected cells. When he discovered that the inclusion bodies could be disrupted into individual virus particles by incubating them in trypsin, he was able to provide Alice with virtually pure virus that she could inoculate the eggs with.
As Greer Williams relates in Virus Hunters (5): “Then, one morning when she peeked into the window of an egg that had been incubating for about a week after she had infected it with the virus, she saw something different. This chick embryo was still alive…She removed the embryo from the shell and examined it. It had a swollen claw. ‘Could this be due to fowlpox infection?’…She went to Goodpasture and put the same question to him…”
In Alice’s own words, “I can’t forget the thrill of that moment when Dr. Goodpasture came into my lab, and we stood by the hood where the incubator was installed and I showed him this swollen claw from the inoculated embryo (5).”
The swollen claw indeed resulted from the fowlpox infection. This was shown by the fact that when bits of the swollen tissue were transferred to other embryos, they in turn induced more swollen tissue. Moreover, these swollen tissues contained fowlpox inclusion bodies. Additionally, when transferred to adult chickens, those bits of swollen tissue produced typical fowlpox lesions.
During the next year, Goodpasture, Alice Woodruff, and Gerritt Budding (a lab assistant, who dropped out of medical school to participate in the chick embryo work) reported that cowpox and herpes simplex viruses could also be grown in the embryonated chicken eggs.
Later studies by Goodpasture and Buddingh showed that each embryonated chicken egg could produce enough vaccinia to produce more than 1,000 doses of smallpox vaccine. They also showed, in a case-study involving 1,074 individuals, that the chick-grown smallpox vaccine works as well in humans as the vaccine produced by inoculating the skin of calves. Regardless, the chick vaccine never caught on to replace the long-established, but cruder calf-grown vaccine (see Aside 2).
Goodpasture placed Alice’s name ahead of his own on their report describing the propagation of fowlpox in chicken eggs. Alice says that Goodpasture was “over-generous” in that regard. Howevever, much of the day-to-day lab work resulted from her initiatives. Eugene’s name also came before Goodpasture’s on the report describing the inclusion body study.
Shortly after completing these studies, Alice left research to raise a family. Eugene’s name also disappeared from the virus literature. But in his case that was because his interests turned to tuberculosis.
In 1932, soon after the above breakthroughs in Godpasture’s laboratory, Max Theiler and Eugen Haagen developed their yellow fever vaccine (6), which initially was generated in embryo tissue from mice and chickens. But, starting in 1937, production of the yellow fever vaccine was switched to the embryonated egg method, in part, to “cure” the live yellow fever vaccine of its neurotropic tendencies.
Recall our introductory comments regarding the warning that individuals allergic to eggs should get medical advice before receiving the standard flu vaccine. In 1941, Thomas Francis, at the University of Michigan, used embryonated chicken eggs to produce the first influenza vaccine (see Asides 5 and 6). Remarkably, even today, in the era of recombinant DNA and proteomics, this seemingly quaint procedure is still the preferred means for producing the standard trivalent flu vaccine (see Aside 1).
[Aside 5: Thomas Francis produced his 1941influenza vaccine in response to urging by U.S. Armed Forces Epidemiological Board. With the Second World War underway in Europe and Asia, and with the 1918 influenza pandemic in mind, there was fear that if an influenza epidemic were to emerge during the upcoming winter, it might impede the military training that might be necessary. An epidemic did not materialize that winter, but the vaccine was ready, and we were at war.]
[Aside 6: Thomas Francis was one of the great pioneers of medical virology. The same year that he developed his flu vaccine, Jonas Salk (recently graduated from NYU medical school) came to his laboratory for postgraduate studies. Francis taught Salk his methodology for vaccine development, which ultimately enabled Salk to develop his polio vaccine (7).]
Next, Hillary Koprowski developed a safer, less painful and more effective rabies vaccine that is grown in duck eggs, and that is still widely used. Why duck eggs? The reason is that duck eggs require four weeks to hatch, instead of the three weeks required by chicken eggs. So, duck eggs give the slow-growing rabies virus more time to replicate.
By any measure, the procedures for growing viruses in embryonated chicken eggs, developed by Ernest Goodpasture and Alice Woodruff, were a major step forward in vaccine development. Sir Macfarlane Burnet (a Nobel laureate for his work on immunological tolerance) commented 25 years later, “Nearly all the later practical advances in the control of viral diseases of man and animals sprang from this single discovery.”
Addendum 1: Several major advances in cell and tissue culture (the other means for growing viruses outside of an animal) happened after Woodruff and Goopasture reported the development of their embryonated egg method in 1931. For the sake of completeness, several of these are noted.
In 1933, George Gey, at Johns Hopkins, developed the roller tube technique, in which the tissue is placed in a bottle that is laid on its side and continuously rotated around its cylindrical axis. In that way, the media continually circulates around the tissue. Compared to the older process of growing tissues in suspension, the roller culture method allowed the prolonged maintenance of the tissues in an active state and, consequently, the growth of large amounts of virus. The roller tube technique also works very well for cell cultures that attach to the sides of the bottle. [Incidentally, Gey is probably best known for having established the HeLa line of human carcinoma cells from cancer patient, Henrietta Lacks. HeLa cells comprise the first known human immortal cell line and they have served as one of the most important tools for medical research. (See The Immortal Life of Henrietta Lacks, by Rebecca Skloot, 2010.)]
In 1948, John Enders, and colleagues Thomas Weller and Frederick Robbins, used Gey’s methods, to demonstrate for the first time that poliovirus could be grown in non-nervous tissue. This was significant because the potential hazard of injecting humans with nervous tissue was holding up the development of a polio vaccine.
Next, Renato Dulbecco and Marguerite Vogt, working at Caltech, developed procedures to grow large amounts poliovirus in cell culture, adding to the feasibility of an eventual polio vaccine (8). Additionally, Dulbecco and Vogt developed a plaque assay procedure to measure the titer of animal viruses grown in cell culture (7).
Addendum 2: The following excerpt tells of the chance encounter that led Howard Temin to become a virologist (4). Temin was the Nobel laureate who first proposed the retroviral strategy of replication, and who co-discovered reverse transcriptase.
“Howard Temin began working on Rous sarcoma virus in the 1950s, while a graduate student in Renato Dulbecco’s laboratory at Caltech (see reference 7 for more on Dulbecco). However, he worked under the direct supervision of Harry Rubin, an early star in the field, who was, at the time, a postdoctoral fellow in the Dulbecco lab. Nothing was known as yet about the replication of the RNA tumor viruses, as the retroviruses were then known. Moreover, little more was known about the molecular basis of cancer in the 1950s than was known in 1911, when Rous first isolated his virus; a state of affairs that would be much alleviated by future studies of the oncogenic retroviruses.
Rubin was a veterinarian by training, perhaps accounting for his somewhat unique appreciation of an oncogenic virus of chickens, well after even Rous himself had lost interest. And, Rubin was responsible for introducing other young investigators to the RNA tumor virus field, both at Caltech and later at UC Berkely.
Rubin’s mentorship of Temin began somewhat fortuitously, as follows. When they first met, Temin was actually doing his graduate research in another laboratory at Caltech, looking into the embryology of the innkeeper worm, Urechis caupo. But he was also serving as a laboratory assistant in the Caltech general biology course. In that capacity, he was dispatched to Dulbecco’s laboratory to obtain some fertilized chicken eggs for use in the general biology lab. Harry Rubin supplied the chicken eggs. But the chance visit from Temin gave Rubin the opportunity to tell Temin about the chicken sarcoma viruses that were being studied in the Dulbecco laboratory.
Rubin had just recently found that he could induce the neoplastic transformation of a normal chicken cell with a single Rous sarcoma virus particle. He then demonstrated that the transformed cell produced hundreds more transformed daughter cells in a week’s time. During their chance conversation, Rubin suggested to Temin that he (Temin) might make use of that observation to develop a quantitative tissue culture assay for Rous sarcoma virus. Sufficiently intrigued by Rubin’s proposition, Temin switched from embryology to virology and proceeded to develop a focus-forming cell culture assay for Rous sarcoma virus; an assay analogous in principle to a plaque assay. But instead of forming plaques of dead cells, the non-cytocidal Rous sarcoma virus induces the growth of visible foci of morphologically transformed neoplastic cells.”
[Addendum 3: Today, viruses are usually cultivated in readily available continuous cell lines. That said, when I first entered the field in 1970, as a postdoctoral studying the murine polyomavirus, my first task of the week was to prepare the baby-mouse-kidney and mouse-embryo primary cell cultures, which at that time served as the cellular host for that virus. This rather unpleasant chore was a reason I eventually turned to SV40, since I could grow that virus in continuous lines of monkey kidney cells.
1. Edward Jenner and the Smallpox Vaccine, posted on the blog September 16, 2014.
2. Leonard C. Norkin, Virology: Molecular Biology and Pathogenesis, ASM Press, 2010. Chapter 1 tells how viruses were discovered and how their distinctive nature was brought to light.
3. Opening Pandora’s Box: Resurrecting the 1918 Influenza Pandemic Virus andTransmissible H5N1 Bird Flu, posted on the blog April 15, 2014.
4. Howard Temin: “In from the Cold,” posted on the blog December 14, 2013.
5. Greer Williams, Virus Hunters, Alfred A. Knopf, 1960.
6. The Struggle Against Yellow Fever: Featuring Walter Reed and Max Theiler, posted on the blog May 12, 2014.
7. Renato Dulbecco and the Beginnings of Quantitative Animal Virology, posted on the blog December 3, 2013.
8. Jonas Salk and Albert Sabin: One of the Great Rivalries of Medical Science, posted on the blog March 27, 2014.