Two years after graduating from medical school, Peyton Rous joined the Rockefeller Institute where he was appointed to lead a cancer research lab (6). Shortly after his assignment began, his lab received a Plymouth Barred Rock hen from an inbred flock that presented with a large sarcoma in its right breast (5, 6). A piece of the sarcoma was removed from this hen and re-implanted into its peritoneal cavity and left breast along with two other birds of its flock (6). The original hen passed 25 days later due to growth from the transplant in its peritoneal cavity, and one of the two inoculated hens developed tumors that were similar in morphology to the original tumor (5, 6). Transplantation of tumor fragments into birds from the same original flock was weakly successful with a transplantation success rate of 25%, yet the continued passage of these tumors in this inbred flock resulted in highly malignant masses that grew quickly (6). Removal of live cells through grinding and filtration resulted in a supernatant that when injected into relatives of the original hen resulted in slow-growing tumors at the injection site (6). By 1911, Rous published this work on transplantable tumors caused by a filterable agent that we now know as Rous Sarcoma Virus (RSV) (1, 5, 6, 19).
Rous wasn’t the only scientist at the time to isolate an infectious virus that caused cancer in chickens. In 1914, Fujinami and Inamoto detected another avian sarcoma virus which they named after Fujinami (1, 19). In 1908, Danish veterinarians Vilhelm Ellermann and Oluf Bang published their work on identifying a viral cause behind chicken leukosis, beating Rous to the punch in identifying a viral origin of chicken cancer (1, 5). This virus – Avian Leukosis Virus (ALV) – is grouped with RSV and other avian sarcoma viruses into the avian virus genus known as Avian Sarcoma/Leukosis Viruses (ASLV) (1, 5).
Though exciting in hindsight, at the time this work was dismissed: Leukemia as cancer originating from bone marrow was still a subject up for debate, cancer was thought to not be contagious in humans, and chickens were thought to be too unrelated to humans to be reliable models (1, 5). In time, RSV proved to be more useful than what anyone at the time was anticipating. This virus helped revolutionize oncology through detection of proto-oncogenes, added new tools to the molecular biologist’s toolkit, and changed the way scientist thought of the Central Dogma. Due to the latter, this virus would indirectly help with the identification of human cancer- and immunodeficiency-causing retroviruses (1, 7).
These were not the first reported instances of cancer transmissibility (5, 19). In 1842, nuns in Verona were reported to have a lower incidence of cervical cancer compared to married women (5). Similarly, Bovine leukosis and Jaagsiekte lung carcinoma were also known to be transmissible in the 19th century yet it would take until the 1980s to understand the retroviral origins of these illnesses (5, 19). Post- Rous, additional cancer-causing retroviruses were discovered. It was initially thought that breast cancer in mice was genetic since lines with high or low incidence produced pups of high or low incidence, respectively (19). However, in 1936 John Bittner establish that mammary carcinoma in mice was transmissible by a filterable agent found in milk by fostering pups from low incidence lines with mothers of high incidence background (1, 19). This virus, now known as Mouse Mammary Tumor Virus (MMTV), was finally identified in 1949 by Dmochowski using an electron microscope, which was the common – and often insensitive and cumbersome – technique at the time (19). This was followed by Ludwik Gross who identified a strain of Murine Leukemia Virus in 1951 within the AKR mice developed by Jacob Furth in 1933 (1, 19). This began a cascade of cancer-causing retroviral discovery in the following decades in cats, mice, birds, and primates (1, 5).
When the genome of RSV was shown to be RNA in 1961, oncogenic retroviruses came to be known as RNA tumor viruses (19). Interestingly, cells transformed with this virus maintained their transformed state for many rounds of replication even in the absence of replicating virus (19). This led Howard Temin to develop a widely panned hypothesis that retroviruses can produce a DNA copy of themselves within infected cells that were subsequently inserted into the host cell genome, thereby rewriting the Central Dogma (1, 7, 19). He called this the DNA provirus hypothesis, inspired by integrated prophages from bacteriophages (19). On one occasion, David Baltimore heard of this hypothesis and based upon his own work with vaccinia virus in 1967 believed him (1, 7). The following three years would push Baltimore into looking at replication in reovirus and vesicular stomatitis virus and Temin to do the same in RSV, to discover RNA dependent DNA polymerases independently but around the same time. Both Temin and Baltimore – technically – presented this finding at the June 1970 Transcription meeting at Cold Spring Harbor, where Baltimore presented his lab’s work while Temin sent a publication (7). The discovery of this enzyme, renamed as reverse transcriptase, would become important to the identification of other retroviruses.
The identification of reverse transcriptase in virions provided a simple and sensitive way to detect and quantify viruses, especially compared to electron microscopy (9). The mechanism by which retroviruses integrate into the host genome also provided a possible mechanism that would explain how RNA viruses could transform cells into cancer cells (9). This encouraged investigators to look for human retroviruses causing cancer – “human tumor viruses” – as a part of the Special Virus Cancer Program (9). Reports of retrovirus detection in human tumors started to appear in the early 1970s (8). This was later shown to be due to laboratory contamination from other animal retroviruses – either from passage through mice or from cross-contamination – or from mitochondria (8). After repeated events of contamination from one source or another, many investigators started to give up the search, calling investigations to identify a human cancer-causing retrovirus as a quest for “human rumor viruses” (9).
There was one group that did not give up. In 1972, Robert Gallo’s group detected an RNA sensitive DNA polymerase in stimulated human lymphocytes (8). To identify what this was, Gallo developed a series of sensitive experiments to isolate any potential viruses found in leukemia and lymphoma cells, part of which was improving culture conditions for T-cells to allow for their continued passage (8, 9). His group finally detected reverse transcriptase in 1979 from an established T-cell line and published on the isolation of the virus now known as Human T-Cell Lymphotropic Virus (HTLV) (7, 8). Interestingly enough, when Gallo attempted to publish his work on the isolation of HTLV his paper was rejected on the grounds that they were continuing to support a hypothesis that to that point was considered untrue (7, 8, 9, 11). Gallo’s work was followed by two additional papers from Yorio Hinuma on the isolation of a similar virus from primary cultured patient cells and the presence of antibodies against HTLV-1 in Japanese cancer patients, a virus that he named Adult T-cell Leukemia Virus (ATLV) due to the cellular origins that he found it in (8,9). A few years after their papers came out, it was realized that the viruses they individually isolated were isolates of the same virus and so they agreed on the name HTLV.
The tools used to culture T cells and to isolate HTLV was important for another virus isolation. By 1981, the first reported cases of a new infection appeared in California and New York City (15-17). French researchers in the Montagnier team received samples from a young man who presented with lymphadenopathy and was able to isolate a virus (7, 8, 10, 11). Electronic microscopy, DNA inhibitors, and antibodies specific to HTLV suggested that this virus – isolated in 1983 – was a new virus (8). Gallo would follow within the next year with 4 studies that provided more conclusive evidence that this virus – then known as LAV by Montagnier and HTLV-III by Gallo – was different from HTLV (7, 8, 10, 11). Subsequent years saw the development of new detection methods by Gallo for the identification of patients who carried the virus, and a lawsuit between the French and United States government over the identification of this new virus (8). By 1986, it was proposed that this virus would be renamed to Human Immunodeficiency Virus (HIV) (18).