Role of macrophages in pulmonary metastatic tumor cell seeding and growth

Abstract

Breast cancer is the most common malignancy among women. However it is metastasis the main cause of death of cancer patients. Tumor cells escape from the primary site into the lymphatic or blood system, travel to the secondary site, extravasate and become established. Each step is inefficient and of these, extravasation is a major rate-limiting step.;Work from our lab has shown that cancer cell derived-CCL2 is important for the recruitment of inflammatory monocytes to the lung, which facilitate cancer cell extravasation. Monocytes differentiate into metastasis-associated macrophages (MAM) and potentiate tumor cell survival and subsequent growth. Ablation of MAMs or their progenitors inhibits metastasis, suggesting that they may be good therapeutic targets. However, the mechanisms through which macrophages assist tumor cell extravasation and growth in the lungs are still unclear. We hypothesize that factors controlling macrophage polarization also modulate tumor cell extravasation in the lung. To test this, we evaluated whether cytokine signaling in macrophages affected trans-endothelial migration of tumor cells in vitro. Interleukin-4 (IL-4) enhanced macrophage-dependent tumor cell extravasation in a mechanism dependent on CXCR2 expression. We also found that mice that IL4Ra -/-mice develop fewer and smaller lung metastasis. IL-4 signaling in macrophages thus seems to be a key mediator in the process of seeding and growth of breast cancer metastatic foci in the lung.;To better understand the role of macrophages during extravasation of tumor cells in the lungs, we set our efforts towards high-resolution in vivo imaging. Using two-photon microscopy and a vacuum-stabilized window we observed that, upon arrival to the lungs, tumor cells were found exclusively in capillary vessels. Tumor cells initially displayed a high level of protrusive activity that decreased dramatically over time, with a concomitant increase in their positional stability. Tumor cells extravasated between 36-48 h after hematogenous spread, after which they establish stable physical interactions with macrophages without signs of tumor cell death. This technique opens new and invaluable opportunities to evaluate and quantify tumor cell behavior during tumor seeding and early growth, as well as to assess the interactions with different components of the tumor microenvironment such as macrophages.