Lymphatic dynamics visualized by native transient hypoxia imaging in areas of tissue damage, edema and in sentinel lymph nodes.

Imaging lymphatic compartments and their function has always been challenging, yet this capability is key to understanding the dynamics of immune response and lymph dysfunction in disease states. This study reports the first ever visualization of murine lymphatic pumping and function imaged from the inherent transient hypoxia that occurs within the lymph ducts and nodes. The lymphatic system appears as one of the few naturally hypoxic areas in vivo. Hypoxia in lymphatics is detected via delayed fluorescence (DF) of endogenous protoporphyrin IX (PpIX), enabling real-time imaging. Lymph nodes and their function were localized by hypoxia transcutaneous imaging and in surgically exposed nodes, followed by correlation of localization to indocyanine green (ICG) local injection. The lymphatic pumping frequency was altered through progressive damage from mild, moderate, and severe wound injuries, and hypoxia appeared readily in the sentinel lymph nodes near tumor regions. Cyclical pumping was observed at sites of edema and in nodes near tumors. Control data from uninjured anesthetized mice showed little lymphatic contrast, whereas awake mice exhibited hypoxia localized to lymph nodes. Unlike contrast injection-based regional lymph node imaging by ICG or MRI, DF hypoxia imaging appears to provide a natural whole-body contrast mechanism, highlighting its potential for visualizing lymphatic function and associated hypoxia dynamics. The value for localization of sentinel lymph nodes and for allowing for visualization of damaged lymph has very practical potential applications.