Chronic low-dose cadmium exposure accelerates the onset and metastasis of invasive mammary carcinoma and promotes immunosuppression in the tumor microenvironment in BALB–neuT mice

Cadmium (Cd), a heavy metal known to act as an endocrine disruptor, has been implicated in breast cancer (BC) via mechanisms that involve both estrogen receptor (ER)-dependent and -independent pathways. We used BALB-neuT transgenic mice, a model of aggressive ErbB2-driven mammary carcinogenesis, to evaluate the effects of chronic exposure to an environmentally relevant, low-dose of Cd on tumor onset, multiplicity, progression, immune microenvironment, and metastasis. Cd exposure significantly accelerated tumor onset and increased tumor multiplicity and weight, reducing both tumor-free and overall survival. Cd-exposed mice displayed elevated serum estrogen levels, and increased expression of progesterone receptor and tumor progression markers, including CD31, Ki67, and phosphorylated Akt, in tumor tissues. Despite the increased recruitment of CD4⁺ and CD8⁺ T cells to the peritumoral stroma, Cd exposure fostered an immunosuppressive microenvironment with elevated Tregs and PD-1⁺ exhausted T cells, both locally and systemically. Notably, lung metastases were threefold more frequent in Cd-exposed mice. Our results demonstrate that low-dose Cd exposure promotes the development and progression of ErbB2-driven breast tumors through hormonal, proliferative, and immune-modulating mechanisms. These findings reveal that exposure to environmentally relevant doses of Cd not only accelerates ErbB2-driven breast cancer but also reshapes the immune landscape toward dysfunction and immunosuppression, likely compromising both anti-tumor immunity and response to immunotherapies. These data advocate for tighter regulation of Cd exposure in populations at risk for breast cancer.