Tumor Endothelium FasL Establishes a Selective Immune Barrier Promoting Tolerance in Tumors

We describe a novel mechanism regulating the tumor endothelial barrier and T cell homing to tumors. Selective expression of the death mediator Fas ligand (FasL/CD95L) was detected in the vasculature of many human and mouse solid tumors but not in normal vasculature, and in these tumors it was associated with scarce CD8(+) infiltration and predominance of FoxP3(+) T regulatory (Treg) cells. Tumor-derived vascular endothelial growth factor A (VEGF-A), interleukin 10 (IL-10) and prostaglandin E(2) (PGE(2)) cooperatively induced FasL expression on endothelial cells, which acquired the ability to kill effector CD8(+) T cells, but not Treg cells, due to higher levels of cFLIP expression in Tregs. In the mouse, genetic or pharmacologic suppression of FasL produced a significant increase in the influx of tumor-rejecting CD8(+) over FoxP3(+) T cells. Pharmacologic inhibition of VEGF and PGE(2) attenuated tumor endothelial FasL expression, produced a significant increase in the influx of tumor-rejecting CD8(+) over FoxP3(+) T cells, which was FasL-dependent, and led to CD8-dependent tumor growth suppression. Thus, tumor paracrine mechanisms establish a tumor endothelial death barrier, which plays a critical role in establishing immune tolerance and determining the fate of tumors.