Viruses are biological agents that hurt and can possibly kill living organisms. By taking over the cells of the host, viruses can replicate and produce viral progeny. In order for viruses to replicate, they must be able to survive the defense mechanisms for the host cell, or the innate immunity. Examples of innate immunity are the immune system, the skin, and mucus. In the experiment conducted, the defense mechanism studied was the immune system. Viruses must be able to evade the immune system in order to reach the host cell and successfully replicate, otherwise they would get destroyed by the immune system before reproducing in the host cells. In an immunocompetent host, interferons are produced and identify the pathogen and trigger the immune response. However, there are cells that are immunodeficient, meaning that they do not produce interferons. Viral selection occurs in immunocompetent hosts since the immune system is destroying the viruses that cannot evade it. This selection weeds out the viruses that cannot evade the immune system, resulting in only the reproduction of the virus population that do have the ability to evade the immune system. In immunodeficient cells, there is no selection occurring. The scientists began to experiment to find the connection between the virus populations that replicate and the viral progeny and see if the innate immunity, or the immune system, affects how a virus evolves.
The scientists cultured populations of the vesicular stomatitis virus in immunodeficient and immunocompetent hosts. HeLa cells (human carcinoma cells) were the immunodeficient hosts, and MDCK cells (canine kidney cells) were the immunocompetent hosts. The scientists hypothesized that the virus that adapts and replicates in the HeLa cells would only be able to adapt and survive in only immunodeficient cells and related hosts due to the fact that there was no immunity-selective pressure present. The researchers also hypothesized that the virus that adapted to the innate immunity of the MDCK cells would not only have a high fitness in the immunocompetent hosts, but also in immunodeficient hosts since there is no selection occurring. The researchers also tested a mix of the viral population, exposing some to HeLa cells and exposing a separate population to MDCK cells, predicting that they would also have high fitness, or being able to evade the immune system and replicate. To test their hypotheses, the scientists placed the viral progeny from each cell host into another set of immunodeficient and immunocompetent cells. LNCaP cells were the immunodeficient cells while PC-3 cells were immunocompetent.
After conducting research, the scientists discovered that the viral progeny from the virus populations cultured in HeLa cells could only survive in immunodeficient cells like its predecessors. However, the viral progeny from the MDCK population only had moderate fitness in immunocompetent cells. The viral progeny from the both cell types had different levels of fitness because each virion had different capabilities of evading the immune system since some of the population were exposed to immunity-selective pressure while the rest was not exposed. The scientists concluded that innate immune selection does affect the evolution of RNA viruses.