Pre-metastatic cancer exosomes induce immune surveillance by patrolling monocytes

Background: Cancer exosomes are often involved in the suppression of innate immune responses. Monocytes and macrophages are essential in the metastatic microenvironments, in tumour-promoting or tumour-suppressive capacities. Non-classical or patrolling Ly6C low monocytes (PMo) were identified for the ability to remove damaged cells and rely on nuclear receptor Nr4a1 for survival. Recently, Nr4a1-positive PMo were implicated in scavenging metastatic tumour cells in the lungs. However, the events that control PMo at the metastatic niche remain unknown. Methods: We isolated and tested exosomes from spontaneously occurring and artificially generated metastatic/ non-metastatic melanoma cells and tested them in vivo for altering metastatic capacity of human and mouse cells. The effect on bone marrow myeloid cells was examined by FACS and dependence on specific cell types was determined using clodronate liposomes and neutralizing antibodies. The effects on macrophages were examined in functional and biochemical assays. The relevance of the findings was assessed by a functional and biomarker analysis of patient exosomes. Results: Exosomes from non-metastatic melanoma cells (ExoNM) are taken up by myeloid cells in the bone marrow and cause an expansion of Ly6C low monocytes, which display elevated levels of integrin-ß2, CX3CR1, and Nr4a1, which define patrolling monocytes. Pigment epithelium-derived factor (PEDF) is known for its anti-angiogenic, anticancer effects. In melanoma, PEDF suppresses ameboid invasion and metastasis. PEDF is also implicated in control of macrophage polarization via unknown mechanisms. Here, we demonstrate that PEDF is present at high levels on the surface of exosomes from nonmetastatic melanoma cells and its presence is critical for the activation of an innate immune response and elimination of melanoma metastasis. The resultant events induce Nr4a1 in myeloid cells, cause PMo expansion, recruitment, and expansion of TRAIL-positive macrophages, which kill and engulf tumour cells. Together, PMo and NK cells eliminate metastasis as is shown by depletion experiments. Summary/Conclusion: Our results suggest that non-metastatic tumours generate triggers of innate immune response(s) such as PEDF, which are delivered to the cells of the immune system by exosomes and maintain tumour clearance.