updated 6/9/2011 12:43:48 PM ET 2011-06-09T16:43:48

During pregnancy a decline in the number of natural killer cells, which are a type of immune system cell, ripens conditions in the body for existing tumors to spread to new locations, a new study shows.

The finding may explain why researchers have long-observed that pregnant women with cancer face an increased risk of their tumors metastasizing, said study researcher Ivan Stamenkovic, director of the Department of Experimental Pathology at the University of Lausanne in Switzerland.

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Further, the findings may help researchers better understand what happens when cancer metastasizes, and could reveal a new approach to keeping metastasis at bay, the researchers said.

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"Our work provides evidence that innate immunity plays a major role in protecting tissues against metastatic tumor growth," Stamenkovic said.

How it works
Several years ago, scientists observed that tumors send out signals that change the chemistry of other sites in the body in ways that make it easier for cells that break free of a tumor to begin growing in those other sites, Stamenkovic said.

But normally, such growths are kept in check by immune system cells called natural killer cells. They move through the body and kill cancerous cells that they encounter.

Stamenkovic and his colleagues found pregnant mice had fewer natural killers than other mice, and the natural killer cells they had were less effective at doing their job.

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Further, the scientists found that what's keeping the natural killers from doing their job are other cells called myeloid-derived suppressor cells. Therefore, a treatment that blocks the activity of these cells could boost the immune system and make the body more resistant to metastasis, Stamenkovic said.

While it is unlikely that such a treatment could alone prevent metastasis, it could be part of a multipronged approach to lessening the spread of established tumors.

Pregnancy and immunity
Normally, metastasis is an inefficient process — less than 0.1 percent of cells that break away from a primary tumor and enter the bloodstream will sprout new growths elsewhere in the body, according to the study.

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The findings may point scientists toward the genes at work in metastases that are successful, the study said. The researchers compared changes in gene expression in the pregnant mice to changes in gene expression in human lung cancer patients. They found that the same genes that had decreased expression in pregnant mice also had decreased expression in the lung cancer patients that had the poorest prognosis. This suggests that these genes are at work in the immune suppression that happens during both pregnancy and tumor growth.

It's unclear why these changes happen during pregnancy, Stamenkovic said. It's possible that their activity is part of the normal immunity-lowering that happens during pregnancy. Or it could be that any condition that causes a certain amount of stress of the body — pregnancy or otherwise — causes these changes.

The study was published today (June 6) in the Journal of Clinical Investigation.

Pass it on: A decline in natural killer cells during pregnancy may make it easier for an existing tumor to spread to other sites in the body.

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