A Distinct Macrophage Population Mediates Metastatic Breast Cancer Cell Extravasation, Establishment and Growth

TLDR

The stromal microenvironment, particularly macrophages, drives primary tumor malignancy, yet their role at metastatic sites remains largely unknown. In animal breast‑cancer metastasis models, a distinct host macrophage phenotype is recruited to extravasating pulmonary metastatic cells across species. Ablation of this macrophage population impairs metastatic seeding, growth, and extravasation, showing that these macrophages directly promote metastasis and represent a therapeutic target.

Abstract

Background The stromal microenvironment and particularly the macrophage component of primary tumors influence their malignant potential. However, at the metastatic site the role of these cells and their mechanism of actions for establishment and growth of metastases remain largely unknown. Methodology/Principal Findings Using animal models of breast cancer metastasis, we show that a population of host macrophages displaying a distinct phenotype is recruited to extravasating pulmonary metastatic cells regardless of species of origin. Ablation of this macrophage population through three independent means (genetic and chemical) showed that these macrophages are required for efficient metastatic seeding and growth. Importantly, even after metastatic growth is established, ablation of this macrophage population inhibited subsequent growth. Furthermore, imaging of intact lungs revealed that macrophages are required for efficient tumor cell extravasation. Conclusion/Significance These data indicate a direct enhancement of metastatic growth by macrophages through their effects on tumor cell extravasation, survival and subsequent growth and identifies these cells as a new therapeutic target for treatment of metastatic disease.