Firefly Light Illuminates Course of Herpes Infection in Mice

St. Louis, November 4, 2002 ? Researchers are using a herpes virus that produces a firefly enzyme to illuminate the virus’s course of infection in mice and to help monitor the infection’s response to therapy. The work is published by scientists at Washington University School of Medicine in St. Louis in the December issue of the Journal of Virology.

“This study demonstrates the feasibility of monitoring microbial infections in living animals in real time,” says study leader David A. Leib, Ph.D., associate professor of ophthalmology and visual sciences and of molecular microbiology. “The technique enables us to follow an infection over time as it progresses and resolves, and we can do this repeatedly using the same animal.”

This technology may solve several problems in studying herpes infections and the genes that control them. To investigate the progress of an infection over a course of days, for example, researchers normally must sacrifice infected mice each day and analyze their tissues to determine the level of virus present. The process is further complicated by the fact that individual mice respond differently to infection.

“One must make a leap of faith that a mouse sacrificed on day three of an infection is responding to the virus in the same way as a mouse sacrificed on day two of the infection,” says Leib.

This new technology, an imaging method known as in vivo bioluminescence, enables investigators to track changes in the viral population in the same animal day after day. The device is located in the Molecular Imaging Center at the University’s Mallinckrodt Institute of Radiology.

“This technology can be used to explore questions about this virus that are possible only by studying entire living animals over time,” says Gary D. Luker, M.D., an assistant professor of radiology with the Molecular Imaging Center and first author of the paper.

“This is an excellent example of the unique information and new collaborations that are generated when we examine fundamental biological processes with molecular imaging tools,” says David Piwnica-Worms, M.D., Ph.D., professor of radiology and of molecular biology and pharmacology and director of the Molecular Imaging Center.

The investigators first added a gene for luciferase, an enzyme produced by fireflies, to a strain of herpes simplex type 1 virus. After determining that the modified virus behaves in cells like the normal virus, they injected the modified virus into several locations in mice, including the brain and abdominal cavity.

Daily for nine days, the mice were injected with luciferin, a compound also produced by fireflies that emits light when exposed to luciferase. They then were anesthetized, placed in a light-free box and photographed using a charged-coupled device, or CCD camera. The camera captures light emitted through the tissues of the mouse by the actively replicating virus. The image produced by the camera shows the location and amount of virus in a mouse as areas of color, ranging from blue (low levels) to red (high levels), superimposed on a photograph of the anesthetized animal. Light produced by the luciferase-luciferin reaction is known as bioluminescence because it is generated by biological chemicals.

This imaging method enabled the investigators to monitor the infection as it spread and receded over nine days. In a second experiment, mice infected with the modified virus were treated with the antiviral drug valacyclovir. The investigators found that decreases in bioluminescence correlated with the decline in the amount of virus present.

The method works in part because bioluminescence produced by fireflies contains a significant amount of red light, which penetrates tissues more effectively than other wavelengths of light. This effect can be seen by shining a flashlight through a finger; it is red light that penetrates the finger.