Menopausal timing and senescent–immune coupling in age-related lobular involution of the human breast: a longitudinal cohort study

Background Incomplete postmenopausal breast involution leaves persistent epithelial-rich lobules and elevated breast density in about 40% of women and is associated with higher breast cancer risk, but why remodelling stalls remains unclear. Methods We studied a longitudinal cohort of 81 women with paired benign breast biopsies (baseline age 45–55 years; follow-up 2–10 years), all with baseline NanoString transcriptomics and two-timepoint digital morphometry, and with multiplex immunofluorescence in spatial-imaging subsets (baseline n=14–16 depending on panel; follow-up n=14). A separate postmenopausal endpoint cohort (12 women: eight noninvoluted, four completely involuted), profiled by genome-wide expression array and multiplex immunofluorescence, defined the persistent-lobule phenotype. Findings Noninvoluted postmenopausal tissue retained a proliferation-competent, tumour-associated epithelial state and showed immune accumulation at lobular boundaries with reduced access to p16 + (senescence-associated) epithelial foci. The same SASP and innate immune programmes that predicted slower involution across the menopausal transition predicted faster involution after menopause. Follow-up boundary CD45→p16 engagement was directionally consistent with this reversal in Pre→Post and Post→Post women. Spatial imaging resolved this reversal into a perimenopausal stall architecture and a postmenopausal clearance-associated architecture marked by direct CD16 + innate-effector engagement of p16 + epithelium; macrophage targeting provided convergent support (two-sided exact permutation interaction p=0.0077). Interpretation Menopausal timing conditions whether senescent–immune programmes couple to productive clearance or to spatially uncoupled surveillance and persistent risk-associated tissue. Biomarker interpretation should therefore be anchored to menopausal timing. Funding Casey DeSantis Cancer Fund and US National Cancer Institute.