Abstract Metastasis is the main driver of breast cancer mortality and is more common in older patients (55-85 years), yet most experimental work still relies on young (∼8-week-old, equivalent to 20 human years) mice, which fail to capture the age-related dynamics of metastasis. Moreover, most studies focus exclusively on lung metastasis, even though breast cancer often spreads to the liver, which responds poorly to immune checkpoint inhibitors and other systemic therapies. Prior work shows that an aged lung microenvironment can promote breast cancer progression via stromal factors, but how aging reshapes immune surveillance, particularly natural killer (NK) cells, in lung and liver niches remains unclear. To address this, we developed syngeneic breast cancer metastasis models in young (8 weeks), middle-aged (12-16 months), and geriatric (22-26 months) C57BL/6 mice, generating primary tumors and lung or liver metastases. Strikingly, primary tumors progressed rapidly in geriatric mice and were associated with larger lung metastases, whereas young and middle-aged mice showed limited outgrowth. Tail-vein models recapitulated this whereby older mice had increased lung metastasis. Immune profiling of pre-metastatic lung revealed that NK cells were decreased in aged and geriatric mice relative to young mice. We then depleted NK cells in young mice to test their role in restraining outgrowth. NK-cell depletion significantly increased primary tumor size and lung metastatic burden, and surprisingly, in the tail-vein model, produced large liver metastases, a phenotype not seen in any age group with tail-vein injection. Pre-metastatic liver analysis confirmed reduced NK cells in middle-aged and geriatric mice. To assess their role, we used hydrodynamic tail-vein injection, which promotes colonization of the liver specifically. Interestingly, liver metastases peaked in middle-aged mice but were reduced in both young and geriatric mice, recapitulating human data where breast cancer incidence is low in young adults (45 years), peaks between 65-75 years and then declines in women ≥80 years. Immune profiling showed overall NK cells in lung and liver were largely unchanged across ages in tumor-bearing mice; however, NK cells in middle-aged mice livers alone exhibited a dysfunctional phenotype, with increased expression of inhibitory receptors. Importantly, immunosuppressive PMN-MDSCs and regulatory T cells were elevated in middle-aged livers but remained low in young and geriatric mice. Together, these findings define an age- and organ-specific regulation of immune cells that drives metastatic outgrowth in middle-aged and geriatric lungs and enhances liver metastasis in middle-aged mice but, surprisingly, limits liver metastasis in geriatric mice. This work underscores the need for age-appropriate models and highlights NK cells as a key target to prevent metastatic outgrowth in older breast cancer patients. Citation Format: Pulkit Datt, Jhon Pasamonte, Kelly Coutant, Anastasiia Burtseva, Mitchell Fane. Aging impairs NK-cell surveillance to promote metastatic outgrowth in breast cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 2071.
Datt et al. (Fri,) studied this question.