Quantification of macrophage and neutrophil response to early metastatic seeds in the lungs. (A) Schema for assessment of the innate immune response to an initial metastatic seed. Tumor clusters from the mammary tumors of K14-actin-GFP;MMTV-PyMT;ROSA^mT/mG mice were injected into the tail veins of immunocompetent mice, and the lung microenvironment was assessed for macrophages and neutrophils after 6 h. (B) Representative slide scanned images of early metastatic seeds staining for F4/80 (macrophages, white) and neutrophil-elastase (neutrophils, white) around K14⁺ (green) metastatic seeds (magenta). Scale bar, 20 µm. (C) Schema for the control experiment in which PBS was injected into immunocompetent host mice, and the lung microenvironment was assessed for macrophages and neutrophils after 6 h. (D) Representative slide scanned images of staining for NK1.1 (NK cells, white), F4/80 (macrophages, white), and neutrophil-elastase (neutrophils, white) around tumor clusters. Scale bar, 20 µm.