Last updated 18 April 2022
To develop vaccines that could be mass-produced, researchers first had to grow the viruses or bacteria with which to develop those vaccines – in large quantities and with great consistency. Compared with bacteria, which can be grown in a laboratory environment when placed in a suitable growth medium, viruses cannot reproduce on their own and require living cells to infect. After a virus infects a cell, it uses the cell’s own components to produce more copies of itself.
So, while material for early bacterial vaccines could be grown in a lab without laboratory animals, researchers trying to develop material for viral vaccines faced an additional challenge. With techniques for growing viruses outside of live hosts not yet available, they were limited to obtaining materials from infected animal hosts.
During the early efforts to develop a vaccine against polio, researchers discovered that the virus could cause disease not only in humans, but also in monkeys. This led to early field trials in the 1930s of vaccine candidates developed using material taken from polio-infected monkeys, such as monkey spinal cords. These candidates proved dangerous, sometimes causing paralysis in the limb where the vaccine was administered. Vaccines derived using nervous system tissue have a higher side effect profile than those developed using other methods (the myelin in the vaccine material can stimulate an adverse neurological reaction). The trials ceased, and researchers moved on to find another way to grow the virus for vaccine development.
The Promise of Cell Culture in Vaccine Development
Hopes of growing poliovirus in the lab without the use of live animals drove many researchers in the 1930s and 1940s. Cell cultures involve growing cells in a culture dish, often with a supportive growth medium like collagen. They offer a level of control unavailable using live animals, and can also support large-scale virus production. (For more about cell cultures and cell lines, as well as cell lines made using human cells, see our article “.”) Early efforts to grow poliovirus in culture, however, repeatedly ended in failure.
In 1936, Albert Sabin and Peter Olitsky at the Rockefeller Institute successfully grew poliovirus in a culture of brain tissue from a human embryo. The virus grew quickly, which was promising, but Sabin and Olitsky were concerned about using this as starting material for a vaccine, fearing nervous system damage for vaccine recipients. They tried to grow poliovirus in cultures using tissue taken from other sources, but were unsuccessful.
Breakthrough in Boston
Thirteen years after Sabin and Olitsky’s success with growing poliovirus in brain tissue, researchers at the lab of John Enders at the Children’s Hospital in Boston successfully grew the virus in a culture of skin and muscle tissue from a human embryo—in a fortunate happenstance. At the time, the researchers were focused on trying to isolate and grow varicella, the chickenpox virus. They had already succeeded in growing mumps and influenza viruses, and had moved on to varicella, which they knew grew in human cells. After preparing flasks with human embryonic tissue, they inoculated four flasks with throat washings from chickenpox patients. Another four flasks were inoculated with a strain of poliovirus as a control group. The chickenpox virus did not grow in this case, but to the researchers’ great surprise, poliovirus did.
They went on to grow two other strains of poliovirus, and in many types of human embryonic tissue, without using nervous system tissue. They were able to grow the virus rapidly and to very high concentrations using the “roller tube” apparatus created by researcher George Otto Gey in the 1930s. (Gey also established perhaps the most famous human cell line, the HeLa, or Henrietta Lacks line.) While many tissue cultures at the time were done in flasks, Gey realized that the environment in the flask did not adequately simulate the environment inside a living body, where tissues are exposed to periods of nutrients being supplied, as well as waste removal. Instead of a flask, he placed tissue on the sides of test tubes, and then placed the tubes horizontally into holes in a wooden cylinder. The cylinder slowly turned like a wheel, rotating the tubes so that the tissue would alternate, coming into contact with air and a nutrient fluid added to the tube.
The researchers in Enders’s lab used the same technique, growing poliovirus much more rapidly than in static flasks. For demonstrating that poliovirus could be reliably grown without using nervous tissue, Enders and his colleagues Thomas Weller and Frederick Robbins were awarded the Nobel Prize in Physiology or Medicine in 1954.
Their discovery proved to be the breakthrough needed to develop a polio vaccine. In 1951, Jonas Salk and his colleagues at the University of Pittsburgh found that poliovirus could also be propagated on a large scale in monkey kidney cells.
Over time, most vaccine development efforts shifted to cell strains—cultures made up of only a single type of cell. These strains can be derived from tissue cultures, which contain multiple types of cells. While viruses can be grown in tissue cultures, cell strains allow continuous observation and control that may not be possible in cultures containing multiple types of cells. This same transition was made in the development of polio vaccines. A monkey kidney cell strain is used to grow poliovirus for the inactivated polio vaccine made today.
Current Vaccines Developed Using Animal Cell Strains
Today, many different animal cell strains are available for use in scientific research and development. Several vaccines currently available in the United States were developed using the Vero cell line, started from African green monkey kidney cells:
Rotavirus vaccines [Rotarix/GlaxoSmithKline, RotaTeq/Merck]
Polio [IPOL/Sanofi Pasteur]
Smallpox [ACAM2000/Sanofi Pasteur – Used only for selected military personnel]
Japanese encephalitis [Ixiaro/Intercell – Used only for those traveling to areas with known outbreaks of disease]
Future U.S. vaccines may use other animal cell strains, including the Madin Darby Canine Kidney (MDCK) line, started in 1958 with kidney cells from a cocker spaniel. (Some European vaccines are already made using MDCK.)
Sources
- John Franklin Enders, PhD -- (Accessed June 2021)
- GlaxoSmithKline. (2011). . (PDF: 425 KB) Accessed 01/10/2018.
- Intercell Biomedical. (2010). . (PDF: 224 KB). Accessed 01/10/2018.
- Merck & Co., Inc. (2011). . (PDF: 261 KB). Accessed 01/10/2018.
- Sanofi Pasteur. (2013). . (PDF: 140 KB). Accessed 01/10/2018.
- Sanofi Pasteur. (2009). . (PDF: 285 KB). Accessed 01/10/2018.