Tumor-associated endothelial cells (ECs) line the blood vessels that promote the growth and support the dissemination and survival of cancer cells. The tumor vasculature is also a gatekeeper that controls the passage of immune cells both into and out of the tumor microenvironment. We recently used single cell RNA sequencing (sc-RNAseq) to characterize EC heterogeneity in a mammary tumor model; from these studies, we turned our attention to DNA methyltransferase1 (DNMT1) which has well-defined roles in stem/progenitor cell self-renewal via it's ability to re-establish patterns of methylation in dividing cells, but no known role in regulating EC function in tumors. Using mice with conditional deletion of DNMT1 in ECs (DNMT1iECKO mice), we show inhibition of tumor growth and metastatic seeding and reduced vessel complexity/branching. We propose these effects are due to a loss of methylation-dependent EC specification required for neovascularization and are due to de-repression of Th1 chemokines (e.g. Cxcl9/Cxcl10, and Cxcl11) and cell adhesion molecules (e.g. Vcam1, Icam1/2, and E-selectin) in ECs that recruit and retain cytotoxic T- lymphocytes to impair tumor growth. In aim 1 we will use DNMT1iECKO mice and vascular-tropic nanoparticles to determine how targeting DNMT1 regulates EC morphogenesis, perfusion, and permeability during cancer cell survival. In aim 2 we will use metastasis models to assess how vascular DNMT1 shapes the tumor immune microenvironment via its ability to regulate cell adhesion molecules (CAMs) and CTL-mobilizing chemokines in ECs. In aim 3 we will examine mechanisms of immune suppression by a FGF2/DNMT1 axis that triggers methylation-induced silencing of CAMs and chemokines in tumor-associated ECs. To complete our goals, we have assembled a team of investigators with expertise in DNA methylation (S. Bhatnager), tumor immune micro environments (V. Engelhard), and the development of microfluidics devices to study EC-to-T-cell interactions (R. Kamm). Together, our study characterizes a completely unexplored area; namely, identifying how methylation-dependent pathways regulate the complex functional diversity, specification, and immunosuppressive features of tumor-associated ECs.