Volume 5, Issue 38 July/August 2008
A Fairer Fight Against Pathogens
Daniel Portnoy is also associate faculty director of the Center for Emerging and Neglected Diseases. Photo credit: courtesy Daniel Portnoy
To learn how the body fights infections, Daniel Portnoy studies at the tiny feet of the masters.
"Pathogens are the best cell biologists. They've spent many millions of years figuring out our cells," says Portnoy, a Berkeley professor of molecular and cell biology and public health.
Among the most challenging to fight are the intracellular pathogens, which include the microbes that cause AIDS, tuberculosis and malaria. These organisms can spread from cell to cell without being detected by antibodies, rendering traditional vaccines largely ineffective against them.
Portnoy is working to tip the scales of this fight in our favor. His studies of the food-borne bacterium Listeria monocytogenes are revealing how intracellular pathogens interact with the immune system. His findings are leading to new insights into the infection process and the development of vaccines against diseases such as cancer.
Once taken up by a cell, listeria uses the cell's own cytoskeleton (stippled areas) to form cellular projections. These projections are readily swallowed by neighboring cells, allowing the pathogen to invade neighboring cells.
A human cell that encounters listeria will encase it in a cellular bubble and swallow the package whole. Once inside the cell membrane, the microbes will break out of their transport vehicle and enter the cytosol, or inner sanctum of the cell.
Portnoy and his colleagues found that listeria hijacks the cell's own cytoskeleton-the internal scaffolding that allows cells to move and divide-so that it can spread from cell to cell. It deploys one component of this system, a protein called actin, to form tiny bacteria-laden projections on the surface of its host cell. Neighboring cells envelop these projections and wind up infected themselves.
Portnoy isolated strains of listeria with mutations that halted this process at each stage of the cell infection process. He then infected mice with the mutants and observed what happened.
He found that mutants unable to hijack actin weren't infectious because they were marooned in their original host cells. Even so, they triggered a strong immunological response among the T-cells of mice. Part of the body's cell-mediated immune system, T-cells can kill infected body cells without involving antibodies. The T-cells Portnoy found were primed to attack proteins or antigens that listeria release outside of the cell.
The finding helps explain why only live listeria trigger an immune response among mice and humans. "If a bacterium isn't alive or growing, it doesn't make or secrete the antigens. So you may make some immunity with dead microbes, but it targets the wrong antigens," Portnoy says.
Listeria bacteria (green) hijacking molecules of cellular actin (red). Image credit: Daniel Portnoy
Vaccines against HIV and the bacterium that causes tuberculosis have proven notoriously difficult to develop. Understanding why live microbes are better at triggering immunity will help break this impasse.
Though unable to spread and cause disease, Portnoy's mutants could still train the immune system to attack infected body cells. They could not only activate T-cells but also cause the release of chemicals that attract immune cells to the site of infection.
Portnoy's mutant listeria strains have also been conscripted in the battle against cancer. Just as the mutants can teach T-cells to attack cells producing listeria proteins, they can sensitize the immune system to kill cancer cells manufacturing proteins characteristic of cancer. Because they are targeted directly at cancer cells, these treatments promise to cause fewer side effects than traditional chemotherapies. Anza Therapeutics is using the approach to sensitize the immune system against a protein expressed in pancreatic cancer cells; the product is already in clinical trials among humans. Portnoy has a consulting relationship with and a financial interest in Anza.
After working on listeria for so long, Portnoy says, "It's extremely gratifying to see work that I've done for 20 years being translated to help humans."