Plasma Fibronectin Drives Macrophage Elongation via Integrin β3–Tie2 Axis in Blood Clots

<b>Background/Objectives</b><b>:</b> Plasma fibronectin (pFN) supports lung metastasis by promoting tumor cell invasion and survival in the context of blood clotting. Here, we set out to test if myeloid cells reiterate the clot-invasive mechanisms that have been established for tumor cells. <b>Methods:</b> We analyzed lung tissue sections from transgenic pFN-deficient mice for the co-localization of intravenously injected B16F1 tumor cells and the surrounding fibrin with myeloid cells, granulocytes, and macrophages. We also tested the role of pFN for macrophage differentiation and invasion in a three-dimensional fibrin matrix. <b>Results:</b> B16F1 melanoma cells, entrapped in the lungs of pFN-competent C57BL/6-<i>Fn(fl/fl)</i><i>Mx-Cre^-</i> mice, were surrounded by a fibrin matrix, CD11b-positive myeloid cells, and Gr-1-positive granulocytes within 1 h of intravenous injection, while homing F4/80-positive macrophages to lung-born tumor cells occurred within 16 h. Compared to pFN-competent C57BL/6-<i>Fn(fl/fl)</i><i>Mx-Cre^-</i> mice, the co-localization of CD11b⁺, Gr-1⁺, and F4/80⁺ cells with B16F1 cells was significantly reduced in the lungs of pFN-deficient C57BL/6-<i>Fn(fl/fl)</i><i>Mx-Cre^-</i> mice. Mechanistically, we found that fibrin-fibronectin complexes promoted macrophage adhesion, differentiation, and invasion in clotted plasma. The pro-invasive function of fibrin-fibronectin depended on the upregulation of integrin β3 and Tie2 expression in macrophages and was reversed after knocking-down integrin β3 and Tie2 with siRNA. <b>Conclusions:</b> Our results suggest that blood clotting plays an important role in the recruitment of macrophages to circulating tumor cells and that the underlying mechanism of macrophage recruitment involves fibrin-fibronectin complexes, integrin β3, and Tie2.