Neutrophils activated by membrane attack complexes increase the permeability of melanoma blood vessels

The microenvironment of malignant melanomas defines the properties of tumor blood vessels and regulates infiltration and vascular dissemination of immune and cancer cells, respectively. Previous research in other cancer entities suggested the complement system as an essential part of the tumor microenvironment. Here, we confirm activation of the complement system in samples of melanoma patients and murine melanomas. We identified the tumor endothelium as the starting point of the complement cascade. Generation of complement-derived C5a promoted the recruitment of neutrophils. Upon contact with the vascular endothelium, neutrophils were further activated by complement membrane attack complexes (MACs). MAC-activated neutrophils release neutrophil extracellular traps (NETs). Close to the blood vessel wall, NETs opened the endothelial barrier as indicated by an enhanced vascular leakage. This facilitated the entrance of melanoma cells into the circulation and their systemic spread. Depletion of neutrophils or lack of MAC formation in complement component 6 (C6)-deficient animals protected the vascular endothelium and prevented vascular intravasation of melanoma cells. Our data suggest that inhibition of MAC-mediated neutrophil activation is a potent strategy to abolish hematogenous dissemination in melanoma.

Automated summary

The microenvironment of malignant melanomas plays a crucial role in the infiltration and vascular dissemination of immune and cancer cells. In this study, we found that the complement system is activated in melanoma patients and murine melanomas. Complement-derived C5a recruits neutrophils, and activated neutrophils release neutrophil extracellular traps (NETs), which open the endothelial barrier and facilitate the entrance of melanoma cells into circulation. Inhibition of neutrophil activation could prevent vascular intravasation of melanoma cells.