... And using bacteria to stymie viruses.

When a virus infects a cell, the immune system calls in killer T cells to prevent the virus from spreading throughout the body. But killer T cells can attack only those cells that the virus has already infected--a response known as cell-mediated immunity. Now, Los Angeles researchers are using a ubiquitous bacterium to produce viral peptides inside cells in order to provoke a response from killer T cells.

The bacterium, Listeria monocytogenes (LM), is found everywhere, especially in soil and soil-grown food products. When LM infects, it invades cells of the body and lives inside them--much as a virus does. The immune system mounts an attack, including the cell-mediated response, against the bacterium.

Because LM has no ill effects on most people, Jeff F. Miller and his team from the University of California, Los Angeles, wondered if it could serve as a vehicle for protection against other infections. They inserted peptides from a number of viruses into the single chromosome of LM to see if infection with altered bacteria would generate a protective response.

Miller's colleague Hao Shen inserted the genetic sequence for an

eight-amino-acid peptide from the lymphocytic meningitis virus (LCMV) into the chromosome of LM. Mice that he infected with the altered LM bacteria didn't get LCMV when exposed to the virus.

And colleague Eric R. Jensen used the technique to arrest the cottontail rabbit papilloma virus in rabbits--a model for viruses that induce human tumors, such as cervical cancers. Jensen, however, needed to include an entire viral protein in the LM chromosome, not just a small peptide. The rabbits first became infected with the papilloma virus and developed wartlike growths. But after 5 weeks, the papillomas in the rabbits that had been infected with the altered LM had shrunk or completely disappeared. The papillomas on the control animals continued to grow.

The researchers are a long way from using LM to vaccinate humans against viral disease. While protection from the viruses continued for a long time, LM has some serious risks. People with compromised immune systems can become quite ill with LM. "Clearly, an attenuated form of the bacteria would need to be developed before it could be used in humans," says Jensen.