Coagulation is required for spontaneous and experimental metastases. Tissue factor (TF) is a key molecule in the coagulation cascade. TF expression by tumour cells correlates with metastasis in clinical and experimental settings. We show that clot formation by TF enhances tumour cell survival after arrest, during experimental lung metastasis, by recruiting macrophages (CD11b+, CD68+, F4/80+ and CX3CR1+, but CD11c-). Genetic (expression of TFPI) or pharmacological (hirudin) inhibition of coagulation abrogated macrophage recruitment and tumour cell survival. Furthermore, impairment of macrophage function (Mac1 KO and CD11b-DTR mice) decreased tumour cell survival without altering clot formation, demonstrating that the recruitment of functional macrophages was essential for tumour cell survival. Moreover, a similar population of macrophages was also recruited to the lung during the formation of a premetastatic niche. Anticoagulation inhibited their accumulation and prevented the enhanced metastasis associated with the formation of the niche.
Leukocyte homing during inflammation is mediated by endothelial activation. We examined the role of endothelial activation in myeloid cell recruitment during early pulmonary metastasis. VCAM-1 and VAP-1 were upregulated after tumour cell arrest and associated with tumour cell-clot formation and myeloid cell recruitment. VCAM-1 expression was independent of functional monocytes/macrophages, but decreased if coagulation was inhibited. Inhibition of VCAM-1 (blocking antibody), VAP-1 (small molecule inhibitor) or both reduced myeloid cell recruitment and subsequent tumour cell survival without altering clot formation.
Thus, TF-expressing tumour cells form platelet clots on their surface soon after arrest in the pulmonary vasculature. Endothelial activation is induced after tumour cell arrest, and enhanced by tumour cell-clot formation, resulting in the recruitment and retention of monocytes/macrophages that are essential for tumour cell survival. Interference with clot formation, monocytes/macrophages function or endothelial activation leads to reduced metastasis. These results suggest a means for therapeutic intervention to combat early metastasis, for example in relapse after treatment.