A new study reveals that breastfeeding leads to a long-term surge in specialized immune cells in breast tissue, potentially reducing cancer risk. Researchers found these CD8+ T cells persist for decades, acting as guards against malignant cells. The findings suggest breastfeeding could enhance natural protection against aggressive breast cancers.
Breastfeeding has been associated with a 4.3 percent reduction in breast cancer risk for every year of breastfeeding, with benefits especially for older mothers. However, the mechanisms were unclear until a recent study by Sherene Loi at the Peter MacCallum Cancer Centre in Victoria, Australia.
The team examined breast tissue from 260 women aged 20 to 70, from diverse ethnicities, who varied in parity and breastfeeding history but had no cancer diagnoses. They discovered that women who breastfed had elevated levels of CD8+ T cells in their breast tissue, persisting for decades post-childbirth—in some cases up to 50 years. "We found that women who have breastfed have more specialised immune cells, called CD8+ T cells, that live in the breast tissue for decades after childbirth," says Loi. "These cells act like local guards, ready to attack abnormal cells that might turn into cancer."
To explore causality, researchers studied mice. Those completing a full pregnancy, lactation, and weaning cycle showed a significant increase in these T cells in mammary tissue 28 days later, unlike non-lactating counterparts. When implanted with triple-negative breast cancer cells—an aggressive form—lactating mice exhibited slower tumor growth. Depleting the T cells accelerated tumor progression.
In clinical analysis of over 1,000 women diagnosed with triple-negative breast cancer post-full-term pregnancy, breastfeeders had tumors with higher CD8+ T cell density. "This suggests there was ongoing immune activation and regulation from the body against their breast cancer," says Loi. After adjusting for factors like age, these women showed longer survival, though breastfeeding duration's specific impact was inconclusive due to data variability.
The T cells likely accumulate during breastfeeding to combat infections like mastitis. Pregnancy's cancer link is complex, with risk reduction tied to younger age at pregnancy. "This has important implications for understanding why some women may be more naturally protected against aggressive forms of breast cancer, and also how we might tailor prevention or treatment strategies in the future," says Loi. She emphasizes breastfeeding is a personal choice, not always feasible, and does not guarantee cancer prevention.
Daniel Gray at the Walter and Eliza Hall Institute praises the multi-group analysis as a strength, noting it sets the stage for future research on T cell 'memory' from breastfeeding. The study appears in Nature (DOI: 10.1038/s41586-025-09713-5).