Endothelial Rap1B mediates T-cell exclusion to promote tumor growth -a novel mechanism underlying vascular immunosuppression

Overcoming vascular immunosuppression: lack of endothelial cell (EC) responsiveness to inflammatory stimuli in the proangiogenic environment of tumors, is essential for successful cancer immunotherapy. The mechanisms through which Vascular Endothelial Growth Factor (VEGF) modulates tumor EC response to exclude T cells are not well understood. The goal was to determine the role of EC Rap1B, a small GTPase that positively regulates VEGF- angiogenesis during development, in tumor growth in vivo. Using mouse models of Rap1B deficiency, Rap1B+/- and EC-specific Rap1B KO (Rap1Bi{Delta}EC) we demonstrate that EC Rap1B restricts tumor growth and angiogenesis. More importantly, EC-specific Rap1B deletion leads to an altered tumor microenvironment with increased recruitment of leukocytes and increased activity of tumor CD8+ T cells. We find that tumor growth, albeit not angiogenesis, is restored in Rap1Bi{Delta}EC mice by depleting CD8+ T cells. Mechanistically, global transcriptome analysis indicated upregulation of the tumor cytokine, TNF-, -induced signaling and NF{kappa}B transcriptional activity in Rap1B-deficient ECs. Functionally, EC Rap1B deletion led to upregulation of NF{kappa}B activity and enhanced Cell Adhesion Molecules (CAMs) expression in TNF- stimulated ECs. Importantly, CAM expression was upregulated also in tumor ECs from Rap1Bi{Delta}EC mice, vs. controls. Significantly, deletion of Rap1B abrogated VEGF immunosuppressive downregulation of CAM expression, demonstrating that Rap1B is essential for VEGF-suppressive signaling. Thus, our studies identify a novel endothelial-endogenous mechanism underlying VEGF-dependent desensitization of EC to pro-inflammatory stimuli. Significantly, they identify EC Rap1 as a potential novel vascular target in cancer immunotherapy.