In vivo quantification of lymph viscosity and pressure in lymphatic vessels and draining lymph nodes of arthritic joints in mice

Previously, it was found that the popliteal lymph node (PLN) enlarges during the pre‐arthritic ‘expanding’ phase, and then ‘collapses’ with adjacent knee flare and is associated with the loss of the intrinsic lymphatic pulse. However, the mechanisms responsible are unknown and we therefore developed in vivo methods to measure lymph viscosity, lymphatic pumping pressure (LPP) in the lymphatic vessels afferent to the PLN, and lymph node pressure (LNP). Multiphoton fluorescence recovery after photobleaching (MP‐FRAP) was used to calculate lymph viscosity and speed; no difference was found among mice with wild‐type (WT), expanding or collapsed PLN in lymph viscosity, but lymph speed was found to be decreased in mice with collapsed PLN compared to WT and expanding PLN mice. LPP was measured indirectly by slowly releasing a pressurized cuff occluding ICG fluorescent dye; we found that mice with expanding PLN exhibit a higher LPP compared to WT and mice with collapsed PLN show an extremely low LPP. Direct measurement of LNP demonstrated a decrease in expanding PLN versus WT pressure, which dramatically increased in collapsed PLN. The decrease in lymphatic flow and loss of LPP during PLN collapse are consistent with decreased drainage from the joint during arthritic flare, and validate these biomarkers of rheumatoid arthritis progression and possibly other chronic inflammatory conditions