It has long been known that childbirth and breastfeeding reduce the future risk of developing breast cancer (BC), attributed to the fact that differentiation and growth pathways of mammary epithelial cells change during pregnancy, lactation, and involution. Such changes are thought to reduce the likelihood that cells will acquire malignant traits over time.
Childbirth more strongly decreases the risk of hormone-dependent cancer forms, whereas breastfeeding is especially important for protection against triple-negative breast cancer (TNBC) – the most aggressive tumor subtype. However, the mechanisms underlying this association remain incompletely understood. Clarifying them may help develop new approaches to TNBC prevention and treatment, particularly for high-risk women – for example, carriers of hereditary mutations or patients with poor prognostic factors.
The immune system plays a key role in tumor protection. The presence of CD8+ T lymphocytes within the tumor is associated with better outcomes in TNBC. These cells participate in destroying cancer cells and enhance the efficacy of therapy, including immunotherapy. A subset of CD8+ T cells acquires memory-like properties (TRM), which provide long-term antitumor control and reduce recurrence risk. These cells are found not only in tumors but also in healthy breast tissue, indicating their potential role in long-term cancer protection.
After childbirth, the immune environment of the breast changes significantly. During involution – the regression of the mammary gland after lactation – apoptosis and active tissue remodeling occur. These processes are accompanied by shifts in immune cell composition: numbers of certain T-cell subsets, such as NKT cells involved in immune surveillance, increase. In addition, women after childbirth show elevated levels of circulating T cells that persist for months and are thought to contribute to autoimmune responses.
All these data indicate that pregnancy and lactation initiate long-lasting changes in the breast immune environment. Researchers at the Peter MacCallum Cancer Centre hypothesized that childbirth and post-lactational involution create a favorable microenvironment that promotes recruitment and long-term maintenance of CD8+ T cells, strengthening immune surveillance and reducing future BC risk.
Association Between Childbirth and T-Cell Numbers in Normal Human Breast Tissue
To understand how childbirth affects the breast immune environment, the researchers compared CD8+ T cells in healthy parous and nulliparous women.
Analysis of cells from normal breast tissue of 170 adult women with average BC risk showed that parous women had significantly higher proportions of CD3+, CD8+, and TRM-like CD8+ T cells, whereas other immune subpopulations did not differ substantially.
The results were confirmed in samples from high-risk women: parous women had more CD45+, CD3+, and CD8+ cells, particularly those expressing markers of tissue residency, CD69 and CD103 (TRM-like cells). These cells were located in the intraepithelial and periductal regions. CD69+ CD103+ CD8+ TRM-like cells persisted for more than 30 years after childbirth, indicating a long-lasting immune effect of pregnancy.
RNA analysis of TRM-like CD8+ cells from breast tissue of parous women without cancer, who breastfed for a total of 26 months, revealed 347 genes with altered expression compared with circulating CD8+ T cells, including ITGAE, ITGA1, and CXCR6 – markers forming a maternal TRM signature.
Thus, pregnancy and lactation promote accumulation and long-term maintenance of TRM-like CD8+ T cells, establishing a stable immune micro-landscape in the breast.
Lactation and Involution Increase CD8+ T-Cell Numbers in the Mammary Gland of Healthy Mice
Researchers examined mammary adipose tissue in mice after pregnancy, lactation, and involution. Three groups were compared: virgin mice, mice after a full lactation cycle, and mice undergoing early pup withdrawal.
Mice completing a full lactation cycle showed a significant increase in CD8+ T cells, including TRM-like subsets (CD69+ CD103+). The effect was strain-independent. Mice with early pup withdrawal – experiencing early involution – did not show these increases.
CD8+ T cells enriched in parous mice expressed tissue-resident markers (CD69, CD103, CD49a) and granzymes A, NK1.1, and CXCR3. Elevated expression of Tgfβ2, Tnf, and Cxcl16 indicated active TRM-cell formation. Fluorescence microscopy confirmed the accumulation of CD8+ TRM-like cells near the mammary epithelium after the full lactation cycle.
Thus, complete lactation and involution in mice lead to the formation of a stable CD8+ TRM pool, mirroring changes observed in normal human breast tissue.
Lactation and Involution Suppress Mammary Tumor Growth in Mice
Preclinical models show that early involution is associated with accelerated tumor growth due to myeloid-cell influx. To assess the impact of different involution stages, researchers compared TNBC tumor growth in mice after full lactation, during early involution, and in virgin controls.
In early involution, tumor growth did not differ from controls, whereas mice completing a full lactation cycle showed significantly slower tumor growth.
Compared with controls and early-involution mice, tumors in mice after full lactation contained more CD8+ T cells, including TRM-like and antigen-specific subsets, as well as more CD4+ Th1 cells and dendritic cells. In contrast, B cells, NK cells, and γδ T cells did not differ.
CD8+ T Cells Provide Tumor Protection After Childbirth
In mice lacking T, B, and NK cells, tumor growth was the same in parous and virgin animals, confirming that protection depends on T cells.
Transfer of naïve CD8+ OT-I cells (OVA-specific) before mating restored protection: mice completing lactation displayed slower tumor growth. Activated OT-I cells transferred before mating accumulated in the mammary gland and tumors, which did not occur in virgin mice or mice with early involution. Transfer of nonspecific T cells did not affect tumor growth.
Deletion of CD8α+ or CD8β+ T cells markedly reduced protection, whereas depletion of CD4+ T cells enhanced tumor control. Blocking CD8+ T-cell migration with FTY720 abolished the protective effect and reduced tumor-specific CD8+ T-cell numbers.
Thus, childbirth-associated tumor protection is mediated by CD8αβ+ T cells. Post-lactational involution and tumor challenge stimulate both mammary tissue-resident and circulating tumor-specific effector T cells, enhancing immune surveillance and tumor elimination.
Breastfeeding Enhances Antitumor Immunity in Human TNBC
Having established that lactation-associated tumor suppression in mice depends on CD8+ T cells, researchers examined whether a similar association exists in human TNBC, where high T-cell infiltration is known to improve outcomes.
Data from 934 Malaysian patients with early breast cancer showed that parous and breastfeeding women had higher tumor immune infiltration in basal-like breast cancer (TNBC) than nulliparous women. Parous breastfeeding women also showed enrichment of the maternal TRM-associated signature and increased CD8+ T-cell density in basal-like tumors compared with other BC subtypes.
In an independent Australian cohort of 270 women with BC-predisposing mutations who developed hormone-receptor-negative early breast cancer after at least one full-term pregnancy, breastfeeding was associated with longer survival after diagnosis and increased tumor-infiltrating lymphocyte numbers. Thus, breastfeeding, beyond childbirth, may provide additional immune protection.
Conclusion
The breast undergoes significant structural and immune changes during pregnancy and lactation. Parous women show increased numbers of CD8+ T cells, particularly TRM-like subsets, indicating the formation of a durable antitumor immune profile.
In mouse studies, completing the pregnancy–lactation cycle led to the accumulation of CD8+ TRM cells near the mammary epithelium and slowed tumor growth. CD8+ T-cell depletion or migration blockade eliminated this effect, confirming the essential protective role of both resident and circulating T cells.
Among women with TNBC, parous and breastfeeding patients displayed stronger CD8+ T-cell tumor infiltration and better survival. Breast remodeling associated with lactation provides durable protection against TNBC mediated by CD8+ T cells.
Reference
Parity and lactation induce T cell mediated breast cancer protection
