Breastfeeding has long been linked to lowering the risk of breast cancer. However, the precise mechanisms behind this effect remain elusive. Recent studies reveal that women who breastfeed possess a higher presence of specialized immune cells in their breasts that might inhibit malignant immune cells.

Previous findings indicate that the risk of breast cancer, which is the second most prevalent cancer globally, decreases by 4.3% for each year of breastfeeding. This preventive effect appears to be particularly advantageous for older mothers.

The exact reasons remain partially understood, but are believed to involve alterations in breast tissue and hormonal exposure. To investigate further, Shereen Roy and colleagues at the Peter McCallum Cancer Center in Victoria, Australia, examined breast tissue from 260 women from diverse ethnic backgrounds, aged 20 to 70. These women varied in their maternal status and breastfeeding experiences, with none having previously been diagnosed with breast cancer.

“We discovered that breastfeeding mothers have a greater quantity of specialized immune cells known as CD8+ T cells, which can persist in breast tissue for decades after childbirth,” says Roy. “These cells serve as local defenders, poised to combat abnormal cells that may lead to cancer.” In certain instances, these cells remained present for up to 50 years.

The researchers also investigated mice, some of which underwent a complete cycle of pregnancy, lactation, and breast recovery during the weaning of their pups. Their mammary tissue was analyzed 28 days later, by which point the mammary glands had reverted to their pre-pregnancy state. Other mice had their pups taken away shortly after birth, or they were not pregnant at all.

The study revealed that completing a full lactation cycle significantly increased the accumulation of specialized T cells in mammary tissue, a phenomenon not observed in the other mice. When triple-negative breast cancer cells, known for their aggressive nature, were transplanted into the mammary gland tissue, tumors developed much more slowly in mice that had experienced lactation. However, depleting these T cells led to rapid tumor growth.

The researchers also analyzed clinical data from over 1,000 women diagnosed with triple-negative breast cancer post at least one full-term pregnancy. They found that women who breastfed exhibited tumors with a higher density of CD8+ T cells. “This indicates that the body’s immune response against breast cancer is active and ongoing,” notes Roy.

After considering other risk factors linked to breast cancer mortality, such as age, the researchers noted that women who breastfed had substantially longer overall survival. However, the variability in the data made it challenging to determine whether the duration of breastfeeding impacted this outcome.

The research team believes that T cells accumulate during breastfeeding to fend off infections that can lead to mastitis. Additionally, the relationship between pregnancy and breast cancer is complex, with studies indicating the risk being mitigated primarily for pregnancies occurring at younger ages.

“These findings have significant implications for understanding why certain women possess a more inherent protection against aggressive breast cancer and how we might develop targeted prevention and treatment strategies in the future,” Roy explains. However, she emphasizes that the choice to breastfeed is personal, not feasible for everyone, and may not always prevent breast cancer development.

Daniel Gray, along with researchers from the Walter and Eliza Hall Medical Research Institute in Victoria, highlighted that one of the study’s strengths was the analysis of multiple groups of women. “This lays the groundwork for future research that may elucidate how CD8+ T cells retain ‘memory’ of breastfeeding,” he comments.