Principles of Virology. Jane Flint

Principles of Virology - Jane Flint


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cells which contact the basement membrane. In the alveoli the epithelium is made of a thin, single cell layer to facilitate air exchange. Air-liquid interface cultures may be produced from primary human bronchial cells or respiratory cell lines (Fig. 2.4).

       Evidence of Viral Reproduction in Cultured Cells

      Before quantitative methods for measuring viruses were developed, evidence of viral propagation was obtained by visual inspection of infected cells. Some viruses kill the cells in which they reproduce, and they may eventually detach from the cell culture plate. As more cells are infected, the changes become visible and are called cytopathic effects.

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      TERMINOLOGY

      In vitro and in vivo

      The terms “in vitro” and “in vivo” are common in the virology literature. In vitro means “in glass” and refers to experiments carried out in an artificial environment, such as a glass or plastic test tube. Unfortunately, the phrase “experiments performed in vitro” is used to designate not only work done in the cell-free environment of a test tube but also work done within cultured cells. The use of the phrase in vitro to describe living cultured cells leads to confusion and is inappropriate. In vivo means “in a living organism” but may be used to refer to either cells or animals. Those who work on plants avoid this confusion by using the term “in planta.”

      In this textbook, we use in vitro to designate experiments carried out in the absence of cells, e.g., in vitro translation. Work done in cells in culture is done ex vivo, while research done in animals is carried out in vivo.

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      The time required for the development of cytopathology varies considerably among animal viruses. For example, depending on the size of the inoculum, enteroviruses and herpes simplex virus can cause cytopathic effects in 1 to 2 days and destroy the cell monolayer in 3. In contrast, cytomegalovirus, rubella virus, and some adenoviruses may not produce such effects for several weeks.

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      Some viruses multiply in cells without causing obvious cytopathic effects. For example, many members of the families Arenaviridae, Paramyxoviridae, and Retroviridae do not cause obvious damage to cultured cells. Infection by such viruses must therefore be assessed using alternative methods, as described in “Assay of Viruses” below.

      In the early 1900s, when viruses were first isolated, freezers and cell cultures were not available, and it was necessary to maintain virus stocks by continuous passage from animal to animal. This practice not only was inconvenient but also, as we shall see, led to the selection of viral mutants (Volume II, Chapter 7). For example, monkey-to-monkey intracerebral passage of poliovirus selected a mutant that could no longer infect chimpanzees by the oral route, the natural means of infection.

      Although cell culture has supplanted animals for propagating most viruses, experimental infection of laboratory animals has always been, and will continue to be, obligatory for studying the processes by which viruses cause disease. The study in monkeys of poliomyelitis, the paralytic disease caused by poliovirus, led to an understanding of the basis of this disease and was instrumental in the development of a successful vaccine. Similarly, the development of vaccines against hepatitis B virus would not have been possible without experimental studies with chimpanzees. Understanding how the immune system or any complex organ reacts to a virus cannot be achieved without research on living animals. The development of viral vaccines, antiviral drugs, and diagnostic tests for veterinary medicine has also benefited from research on diseases in laboratory animals. Despite their utility, it must be appreciated


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