Immune-regulatory and anti-inflammatory functions of lymphatic endothelial cells

Abstract

Chronic inflammatory diseases are largely affecting the population and pose a major burden on quality of life for the patients. Furthermore, chronic inflammatory conditions are associated with high cost as well as comorbidities. These conditions include rheumatoid arthritis, atopic dermatitis, inflammatory bowel disease, and psoriasis. A variety of oral agents and biologicals are capable to effectively treat the symptoms. However, side effects and poor patient adherence greatly affect treatment efficacy. Furthermore, treatments only reduce symptoms which upon treatment discontinuation in many cases reappear. Effective and long-term protective therapies are still needed. In the first project, we aimed to investigate the long-term impact of targeted VEGF-C delivery in a contact hypersensitivity-induced chronic dermatitis model in mice with epidermal over-expression of VEGF-A, focusing on the longevity of lymphatic expansion and its protective abilities upon recurring inflammation. Activation of lymphatic vessels has been shown to alleviate immediate inflammation in several inflammatory disease models such as psoriasis and chronic colitis by enhancing local drainage. It is unclear, however, whether the benefits of therapeutically induced lymphangiogenesis are transient or long-lasting, preventing recurrent inflammation. Intravenous injection of a fully humanized VEGF-C fusion protein targeting the EDA domain of fibronectin (F8-VEGF-C) resulted in reduced inflammation upon initial contact hypersensitivity challenge. Importantly, targeted VEGF-C-mediated expansion of lymphatic vessels persisted for more than 170 days, long beyond primary inflammation had resolved. Furthermore, the treatment markedly decreased the severity of ear skin edema upon contact sensitizer re-challenge. At the same time, infiltration of leukocytes including CD4+ T cells, macrophages and dendritic cells was significantly reduced in the treated group. In sum, targeted delivery of VEGF-C and subsequent site-specific lymphangiogenesis leads to long-lasting lymphatic expansion and long-term protection against inflammation. These findings represent a promising approach for the effective treatment of chronic inflammatory skin diseases. Immunotherapies such as checkpoint inhibitors that activate the immune system to fight the tumor have recently gained high interest. However, their efficacy seems to be limited to a narrow set of patients. Further understanding of factors in the tumor microenvironment influencing immune responses might strengthen the understanding of tumor immune evasion. In the second project of the thesis, we sought to understand the specific role of lymphatic PD-L1 (CD274) in tumor immunity. In physiological conditions, LECs present self-antigens on MHC-I promoting self-tolerance by constitutive expression of PD-L1, leading to T cell inhibition. It has been demonstrated that tumor-associated LECs upregulate PD-L1 but the impact on the immune response has not yet been elucidated. We subjected mice lacking lymphatic PD-L1 to either B16F10 melanoma or MC38 colorectal carcinoma implantation. This resulted in expansion of tumor-specific CD8+ T cells in tumor-associated LNs of both tumor models and reduction of the primary tumor mass in MC38 colorectal carcinoma. Furthermore, in the B16F10 melanoma model, adoptive T cell therapy showed an increased efficacy. Interestingly, we were able to identify apoptosis in tumor-specific CD8+ central memory T cells as an underlying mechanism of lymphatic PD-L1 signaling. Overall, these findings demonstrate that LECs can restrain tumor-specific immunity via PD-L1, which may explain why some patients with cancer without PD-L1 expression in the tumor microenvironment still respond to PD-L1/PD-1-targeted immunotherapy. Collectively, we demonstrated that inflammation site-specific delivery of VEGF-C promotes persisting lymphatic expansion with protective capabilities for future inflammation. This represents a promising approach for chronic inflammatory diseases. In addition, we were able to demonstrate the importance of lymphatic PDL1 in adaptive immune response to tumors.