Tumor draining lymph nodes, immune response, and radiotherapy: Towards a revisal of therapeutic principles.

The tumor-draining lymph nodes (TDLNs) are the primary sites of the development of anti-tumor immunity. Primary tumor irradiation promotes 'radio-vaccination' by enhancing the release of tumor antigens and activating the interferon type-I pathway. Activated intratumoral dendritic cells (DCs) enter the lymphatics to reach the TDLNs. The adaptive anti-tumor immune responses are developed, as DCs will present tumor-related antigens to activate CD4+ and CD8+ T-cells. Strong experimental evidence suggests that post-irradiation tumor clearance is strongly dependent on the accumulation of such cytotoxic T-cells in the tumors. However, TDLNs are heavily irradiated during Radiotherapy to eradicate the clinical and subclinical metastatic disease. At the same time, irradiation depletes the critical immune cell population residing in TDLNs and primary tumors, blocking immune response and compromising the effectiveness of immuno-stimulatory interventions. Since TDLNs are essential for T-cell activation by inbound dendritic cells previously activated in the tumor environment, the practice of TDLN-irradiation demands re-evaluation. Interventions to preserve and handle the functional state of regional TDLNs or remote nodes, during or after Radiotherapy, may have great therapeutic importance. TDLNs represent the main playground for educating and expanding tumor-specific cytotoxic immune cells and controlling a delicate balance between immune surveillance and tumor spread. Their activation state may define the outcome of Radiotherapy and the manifestation of abscopal effects. In this critical review, we present the biological and clinical role of TDLNs and propose strategies to include in the design of immuno-radiotherapy trials aiming to eradicate cancer at a local and distant level.