# Obesity and breast density enhance immune exclusion in the primary tumor microenvironment and promote breast cancer metastasis

## TL;DR

Obesity and high breast density together create an immune exclusion pattern in breast tumors, increasing the risk of metastasis in mice and humans.

## Contribution

A novel murine model and human data showing that obesity and breast density synergistically alter the tumor immune microenvironment and promote metastasis.

## Key findings

- Mice with obesity and high breast density showed increased lung metastasis and immune exclusion in the tumor microenvironment.
- Human patients with both risk factors had a 45% metastasis incidence and similar immune cell localization patterns.
- The murine model mirrors human disease and can be used to study therapies targeting immune exclusion caused by these risk factors.

## Abstract

Recent epidemiological studies suggest that breast density and obesity together increase breast cancer risk. Although these risk factors have been explored individually, little is known about how they combine to alter the tumor immune microenvironment (TIME) and promote disease progression. To address this gap, we developed a murine model of both risk factors. Spatial analysis of the TIME revealed macrophages and T-cells predominantly localized in the stroma of both risk factor groups, indicating an immune exclusion phenotype. Mice with dual risk factors had significantly increased lung metastasis. To establish the human relevance of this model, we interrogated the TIME in biopsies from 158 patients with invasive ductal carcinoma and 10 years of follow-up data. We found that patients with both risk factors had the highest incidence of metastasis (45%). Furthermore, spatial immune profiling revealed exacerbated stromal localization of macrophages and T-cells in the dual risk factor group that progressed to metastasis. Overall, we uncovered an immune exclusion phenotype in metastatic breast cancer patients with obesity and breast density, and we present a relevant murine model that parallels human disease. The murine model will enable future investigation into therapies that intercept the mechanisms by which dual risk factors modulate the TIME.