Feb. 1 (HealthDay News) -- In another finding that points to an estrogen-breast cancer connection, U.S. researchers say the female hormone appears to shield breast cancer cells from attack by the body's immune cells.

It was already known that estrogen enhances the growth and migration of breast cancer cells. This study, by a team from the University of Illinois at Urbana-Champaign, found that estrogen also stimulates the expression of an inhibitor -- protease inhibitor 9 (PI-9) -- that hinders the ability of immune cells to kill tumor cells.

This is the first time that estrogen's role in shielding breast cancer cells from immune cells has been identified, the researchers said. The finding may help in the development of new breast cancer therapies.

The study was published online in the journal Oncogene.

"It wasn't known that estrogen could do this in breast cancer cells. The amounts of estrogen required to do this are quite small," principal investigator David J. Shapiro, a professor of biochemistry in the university's School of Molecular and Cellular Biology, said in a prepared statement.

PI-9 has also been implicated in other kinds of cancers. For example, high levels of PI-9 in some lymphomas are associated with poor patient prognoses.