Engineering Lymphangiogenesis

Melanoma causes the vast majority of skin cancer deaths, and there is currently no conventional treatment available that is able to cure metastatic melanoma patients. However, a novel treatment modality has recently emerged: cancer immunotherapy. Cancer immunotherapy aims at raising potent, tumor-specific T cell responses that are able to recognize and eliminate cancerous cells. Despite the fact that for the first time in history some metastatic melanoma patients can be cured, it is currently unclear why a large fraction of patients does not respond to immunotherapy. Thus, the overarching goal of this thesis was to gain a better understanding of the molecular mechanisms dictating the outcome to immunotherapy. Pre-existing inflammation within the tumor microenvironment is associated with good clinical prognosis following immunotherapy. Because our lab has previously shown that tumor associated lymphatic vessels (LVs) can modulate tumor inflammation, we hypothesized that LVs might be involved in modulating the outcome of cancer immunotherapy. To explore this hypothesis, we characterized two mouse models that recapitulate primary human lymphangiogenic melanoma. Using these models, we found that tumor-associated lymphatics secrete CCL21 to actively attract naïve T cells into the tumor microenvironment. Interestingly, we found that antigen-specific immunotherapy induced activation of these naïve T cell infiltrates, which not only resulted in eradication of the primary melanoma tumors, but also conferred the mice with long-term protection against tumor re-challenge. We thus demonstrate that LVs increase the quantity and quality of tumor inflammation, thereby establishing a microenvironment that is able to potentiate antigen-specific immunotherapy. We then show that controlled release of VEGFC from injectable hydrogels leads to lymphangiogenesis in the mouse dermis. Similarly to what we found in lymphangiogenic tumors, engineered lymphangiogenic skin sites displayed increased expression of CCL21, and increased infiltrations of CD4+ and CD8+ T cells. Delivery of the model antigen ovalbumin into lymphangiogenic sites led to enhanced release of the effector cytokine interferon-¿ upon antigen-restimulation, suggesting that engineered lymphangiogenic sites could be exploited for therapeutic immunomodulation. Finally, to better understand mechanisms underlying successful cancer immunotherapy, we established a tool for the observation of anti-tumor immune responses in the context of the native tumor microenvironment. By combining existing methods in a novel way, we developed an intravital microscopy method based on immunofluorescence that allows simultaneous, high resolution and dynamic visualization of the tumor microenvironment including immune cells and extracellular matrix proteins. In conclusion, this thesis demonstrates that lymphatic endothelial cells (LECs) orchestrate immune cell recruitment into peripheral tissues by secretion of CCL21 chemokine, both in steady-state and disease. Our findings add more evidence to the hypothesis that LECs might play an underappreciated role in driving the formation of peripheral sites of immunomodulation. Our findings have translational potential, as immune cell infiltrates attracted by local lymphangiogenesis can be exploited for therapeutic immune regulation, such as to improving the efficacy of cancer immunotherapy.