Lymphatic Control of the Tumor Immune Microenvironment

Tumor engagement or activation of surrounding lymphatic vessels is well-known to correlate with tumor progression and metastasis in melanoma and many other cancers. We and others have identified several mechanisms by which the lymphatic growth factor VEGF-C and lymphangiogenesis can promote metastasis, including (i) increasing immune suppressive cell types and factors in the tumor microenvironment both directly and indirectly, (ii) inhibiting maturation of antigen-presenting cells and T cell activation, and (iii) driving changes in the stromal microenvironment that promote both cancer invasion and immune suppression. However, lymphatic activation also enhances communication with cells in the draining lymph node by antigen and cell transport, which may trigger the initiation of adaptive immune responses against the tumor. Under normal conditions, the potential anti-tumor effects are rendered 'dormant' by the pro-tumor immune suppression, and the tumor progresses. However, we are now observing that lymphangiogenic tumors are exceptionally responsive to immunotherapy, implying that the anti-tumor aspects can be unleashed when the overall balance of pro- and anti-tumor immune aspects is tipped enough towards the latter (e.g., upon tumor cell killing). On the mechanistic side, we are finding that 'lymphangiogenic potentiation' depends on tumor cell infiltration of both CD103⁺ dendritic cells and naïve T cells, driving local T cell education post-immunotherapy and antigen spreading. On the translational side, we are developing novel strategies to exploit lymphangiogenesis for cancer immunotherapy. Understanding the yin and yang of lymphatic activation in the tumor microenvironment and how it affects immunity may lead to exciting new translational strategies for cancer immunotherapy.