In the 18th century, physicians noticed nuns had some of the highest rates of breast cancer. It was one of the earliest clues that led scientists to suspect that child-bearing and breastfeeding could protect against the disease.
Modern data has confirmed the centuries-old observation but the biological reasons behind it have remained unclear. Explanations have often focused on pregnancy-related hormonal changes, but research published Tuesday in Nature has found breastfeeding provides long-lasting immune protection.
Prof Sherene Loi, a clinician scientist at the Peter MacCallum Cancer Centre in Melbourne, said the most sophisticated level of the immune system, called the adaptive immune system, involves T-cells which react against particular viruses or bacteria – and also against cancer. This response is “one of our very modern therapeutic weapons against cancer”.
Loi, the senior author of the new study, said her team had noticed some breast cancers had large amounts of these specialised immune cells, while others had very few.
Patients who had more cells had better outcomes, particularly for one of the most aggressive types: triple-negative breast cancer. These immune cells were also found in healthy breast tissue.
The study sought to investigate why these T-cells were already there, whether they were related to pregnancy and breastfeeding, and if they protected against breast cancer formation and growth.
The researchers studied noncancerous breast tissue of more than 260 women from diverse populations who had breast reductions or surgery to reduce their risk of breast cancer.
They found those who had children had more of these specialised cells, called CD8⁺ T-cells. These cells remained in the breasts for more than 30 years after pregnancy.
Using mice models, the researchers then implanted cancerous cells into the equivalent of their breast tissue, and found they grew less in the mice that had had pups and breastfed, than in virgin mice.
When they depleted the T-cells from the mice with pups, they lost the protection. This suggested the T-cells were directly responsible for the effect, Loi said.
The researchers then investigated whether women who breastfed lived longer with triple-negative breast cancer. They looked at studies involving more than 1,000 breast cancer patients diagnosed after having children, and had a record of their breastfeeding history.
They found that women who had breastfed did better from their triple-negative breast cancer than those who had not. Their tumours also had more immune cells, suggesting there was ongoing immune activation and regulation from the body against their breast cancer.
“The key take-home messages [are] that pregnancy and breastfeeding will leave behind long-lived protective immune cells in the breast and the body, and these cells help to reduce risk and improve defence against breast cancer, particularly triple-negative breast cancer, but potentially other cancers as well as disease,” Loi said.
Loi said the study provided an explanation why breastfeeding is protective, so that it can be used in the future to try to recreate this effect in women who don’t have children or are unable to breastfeed.
Understanding the biological mechanism could help develop vaccines and new strategies to mimic this protection, she said.
She emphasised even if women do breastfeed, it’s not a 100% guarantee that they won’t get breast cancer. “The effects are really quite small for every individual, but population wide the effects are large.”
Associate Prof Wendy Ingman from the University of Adelaide’s Medical School said the longer the duration of breastfeeding, the greater the benefits. She said, for every year of breastfeeding, there is a 4% lifetime reduction in the mother’s breast cancer risk.
“This study shows that having babies and breastfeeding causes long-lasting changes in immune cells that could help protect the breast from cancer,” she said. “I’m hopeful that this type of research will lead to new approaches to reduce women’s breast cancer risk.”
