The lymphatic vasculature is the second largest component of the circulatory system and consists of a hierarchical network of lymphatic vessels (LVs) that is important for the passive transport of fluids, immune cells, and macromolecules. Recent evidence highlights the important immunomodulatory role of lymphatic endothelial cells (LECs) lining LVs, which regulate immune cell traCicking, antigen cross-presentation, and peripheral tolerance. Although tumor-associated LVs are traditionally associated with poor prognosis and overall survival in many cancers, recent evidence in melanoma has uncovered a more immunostimulatory role for LECs. In this tumor-context, LVs promote immune cell infiltration and enhance therapeutic eCicacy of immune checkpoint blockade (ICB).

We have previously shown that a heterologous prime boost vaccination strategy of KISIMA/VSV-GP with matching tumor associated antigens (TAAs), combined with ICB, anti-PD-1 (αPD-1) therapy, demonstrated preclinical eCicacy in non-lymphangiogenic tumor models. However, these eCects were not durable in murine melanoma-bearing mice and the tumor intrinsic factors determining response to this treatment remain incompletely understood. Here we show that both the therapeutic eCicacy and dependency on the combination with αPD-1 is shaped by the lymphatic architecture of the tumor microenvironment. Using murine melanoma models, with two distinct lymphatic landscapes, we demonstrate that combining the chimeric peptide vaccine, KISIMA with recombinant VSV-GP-TAA viral vector in a heterologous prime-boost regimen, induces robust tumor control in lymphangiogenic B16ovaVC tumors without the need for ICB. Elevated lymphatic density was associated with an increased CD45+ immune infiltration and a sustained recruitment of naive like T lymphocytes, supporting intratumoral priming and diCerentiation into eCector or memory T cells. In addition, vaccination also induces a pro-inflammatory program in tumoral LECs. In contrast, in poorly lymphangiogenic B16ovaWT tumors, limited LVs restricted immune influx into the TME, vaccination promotes T cell exhaustion, rendering its eCicacy dependent on checkpoint blockade-mediated T cell reinvigoration. Collectively, these findings highlight the active role of tumor-associated LECs in shaping the eCicacy to combination therapies.