Identification of novel, immune-mediating extracellular vesicles in human lymphatic effluent draining primary cutaneous melanoma

Epithelial tumors including melanoma often first metastasize to regional, sentinel lymph nodes (SLN). Thus, the presence of SLN metastases is a critical prognostic factor of survival. Prior to metastasis, accumulating evidence suggests the SLN is immunologically compromised; however, the process by which pre-metastatic niche formation occurs remains unknown. In this prospective study, freshly dissected, afferent lymphatic fluid was obtained during SLN biopsy in three patients with primary cutaneous melanoma. Lymphatic extracellular vesicles (L-EV) were visualized by transmission electron microscopy and proteomic cargo profiled by mass spectrometry. Flow cytometry assessed L-EV effects on autologous dendritic cell maturation in vitro. Immunogold electron microscopy and immunohistochemistry visualized expression of EV cargo within the primary tumor and SLN. Lymphatic extracellular vesicles from each afferent lymphatic channel demonstrated inhibition of autologous dendritic cell maturation. Proteomic profiling identified 81 peptides shared among the L-EV preparations including a signature of 18 immune-modulating proteins including previously established inhibitor of dendritic cell maturation, S100A9. Immunohistochemistry and immunogold electron microscopy confirmed S100A9 tracking along the lymphatic path, from keratinocytes in the primary tumor to sub-capsular macrophages in the SLN. Our findings suggest L-EV cargo may serve as early mediators of tumor-induced immune subversion in regional lymph nodes, by preceding malignant cells and trafficking within the lymphatic vasculature to harbor the first pre-metastatic niche.