Integrin inactivation slows down neutrophils congesting the pre-metastatic lung in a model of breast cancer

Neutrophils are thought to be critical to the process whereby breast cancers establish an immunosuppressive and tumour cell nurturing ‘pre-metastatic’ niche before overt metastasis can be detected. However, the spatial localization of neutrophils and their interaction with other cell types in the lung pre-metastatic niche is not well described. We used a spontaneously metastatic mammary cancer model combined with a multiplexed three- and four- dimensional imaging approach to investigate the behaviour of neutrophils in the pre-metastatic niche. Volume fixed tissue three-dimensional imaging showed that approximately 40% of CD8 ⁺ T cells are adjacent to neutrophils at this stage. In live tissue, we found neutrophils with impaired intravascular motility congested the capillaries of pre-metastatic lungs potentially obstructing CD8 ⁺ T cell access. Slowed neutrophil transit was dependent on conformational inactivation of β ₂ -integrin and could be recapitulated by treating non-tumour bearing mice with G-CSF, a potent systemic mediator of granulopoiesis. Finally, we observed differential accumulation of intravenously injected micro-beads in the lung, suggestive of transient circulatory dead spaces which were also dependent on β ₂ -integrin inactivation. Overall, our study proposes that integrin-mediated neutrophil congestion of the alveolar capillaries could contribute to the generation of the pulmonary pre-metastatic niche.