How is a virus attenuated?

By | January 15, 2015

How is a virus attenuated?

An attenuated virus is a form of virus that is genetically altered so that it is no longer pathogenic to humans. However, the immune system can produce defense against it. This defense is then sufficient to ward off the real, live virus if we ever become infected.

First techniques of attenuation involved growing the virus in cultured cells and selecting for the altered forms of the virus that no longer grew well in human cells. First, a virus would be isolated from a patient and grown in human cultured cells in a dish. Next, the cultured virus would be infected into other cells such as monkey cells. Over time, the virus mutates and new genetic strains of the virus are produced. For example, the Sabin polio vaccine strain differs from wild-type polio by only 10 of 7429 nucleotides. Eventually, what's left is a virus that no longer grows well in human cells. Hence, it is attenuated.

Attenuated viral vaccines are not without risk, unfortunately. In immunodeficient individuals, the virus could "back" mutate and become live again.

New approaches have since been developed using recombinant DNA technology that help overcome this setback. For example, the isolation and in vitro mutagenesis of specific viral genes can be accomplished now. This first requires the isolation of virulence genes within a specific virus. In other words, mutations can be so finely engineered so that these "back" mutations are practically impossible.


Further Reading:

Janeway's Immunobiology, Pgs 700-703.

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