Lymphatic-derived 25-hydroxycholesterol promotes immunity in melanoma by inhibiting PPAR-? in tumor associated macrophages and monocytes
In melanoma, lymphangiogenesis correlates with metastasis and poor prognosis and promotes immunosuppression. However, it also potentiates immunotherapy by supporting trafficking of immune cells. Lymphatic endothelial cells (LECs) are highly plastic cells that shape their phenotype according to distinct microenvironments, which likely define their functional properties, such as immunomodulation. Here, we show in a lymphangiogenic murine melanoma model that LECs upregulate the enzyme Ch25h, which catalyzes the formation of 25-hydroxycholesterol (25-HC) from cholesterol and plays important roles in regulating lipid metabolism, gene expression, and immune activation. LECs represent the main source of extracellular 25-HC in tumors, leading to the inhibition of PPAR-? in intra-tumoral macrophages and monocytes. This prevents their immunosuppressive phenotype and instead promotes their conversion into proinflammatory myeloid cells that support effector T cell functions. We identify here in mechanistic detail a novel LEC function that supports anti-tumor immunity, which can be therapeutically exploited in combination with immunotherapy. Overall design: mRNA profiles of lymphatic endothelial cells derived from tumors,  tumor draining lymph nodes and non draining lymph nodes in mice transplanted with lymphangiogenic mouse melanoma cell line