A viral cloaking device( PASADENA Calif.--Viruses achieve the...)

PASADENA, Calif.--Viruses achieve their definition of success when they can thrive without killing their host. Now, biologists Pamela Bjorkman and Zhiru Yang of the California Institute of Technology have uncovered how one such virus, prevalent in humans, evolved over time to hide from the immune system.

The human immune system and the viruses hosted by our bodies are in a continual dance for survival--viruses ever seek new ways to evade detection, and our immune system devises new methods to hunt them down. Human Cytomegalovirus (HCMV), says Bjorkman, Caltech's Delbrck Professor of Biology and a Howard Hughes Medical Institute (HHMI) Investigator, "is the definition of a successful virus--it thrives but it doesn't affect the host."

HCMV is carried by eight in 10 people. Although it generally harms only those who are immunocompromised, it has also been linked with brain tumors like the one for which Ted Kennedy recently had surgery. Understanding how HCMV survives may help in the development of a vaccine, as well as in the fight against other viruses with similar evasive tactics.

"We are interested in mechanisms taken by viruses to escape our immune system," says Caltech biology postdoc and HHMI associate Zhiru Yang. She and Bjorkman published their findings on HCMV survival mechanisms in the July 15 edition of Proceedings of the National Academy of Sciences. They describe the underpinnings of a viral cloaking device, partly made of stolen goods from healthy cells, that helps HCMV to move undetected through the body.

For 20 years, Bjorkman's lab has been dedicated to understanding class 1 major histocompatibility complex (MHC) proteins and the immune response, most recently related to AIDS research. MHC proteins carry peptides, small pieces that are chopped up from the cell's internal proteins, to the cell's surface. If a cell has been infected, MHC presents viral peptides to signal T cells to kill it. So some viruses
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