Objective: To measure lymphocyte velocity, lymphatic contraction, and shear stress in phasically contracting lymphatics in situ. Methods: A high‐speed video system was used to capture multiple contraction cycles in rat mesenteric lymphatic preparations. The images were analyzed to determine fluid velocity, volume flow rate, wall shear stress, and retrograde flow. Results: Lymphocyte density and flux varied from 326 to 35,500 cells/μ L and 206 to 2030 cells/min, respectively. Lymphatics contracted phasically, with a mean diameter 91 ± 9.0 μ m and amplitudes of 39%. Lymph velocity varied with the phasic contractions in both direction and magnitude with an average of 0.87 ± 0.18 and peaks of 2.2–9.0 mm/s. The velocity was ∼ 180° out of phase with the contraction cycle. The average lymph flow was 13.95 ± 5.27 μ L/h with transient periods of flow reversal. This resulted in an average shear of 0.64 ± 0.14 with peaks of 4–12 dynes/cm². Conclusions: High‐speed lymphocyte tracking provided the spatial and temporal resolution to measure lymphocyte flux throughout the phasic contraction. Poiseuille flow was a reasonable model for estimating wall shear stress through most of the phasic contraction cycle of the intervalvular lymphatic segments. Shear rate was low but had large variations in magnitude compared to that seen in blood vessels.

Rationale: Lymphatic transport of peripheral interstitial fluid and dendritic cells (DCs) is important for both adaptive immunity and maintenance of tolerance to self-antigens. Lymphatic drainage can change rapidly and dramatically on tissue injury or inflammation, and therefore increased fluid flow may serve as an important early cue for inflammation; however, the effects of transmural flow on lymphatic function are unknown. Objective: Here we tested the hypothesis that lymph drainage regulates the fluid and cell transport functions of lymphatic endothelium. Methods and Results: Using in vitro and in vivo models, we demonstrated that lymphatic endothelium is sensitive to low levels of transmural flow. Basal-to-luminal flow (0.1 and 1 μm/sec) increased lymphatic permeability, dextran transport, and aquaporin-2 expression, as well as DC transmigration into lymphatics. The latter was associated with increased lymphatic expression of the DC homing chemokine CCL21 and the adhesion molecules intercellular adhesion molecule-1 and E-selectin. In addition, transmural flow induced delocalization and downregulation of vascular endothelial cadherin and PECAM-1 (platelet/endothelial cell adhesion molecule-1). Flow-enhanced DC transmigration could be reversed by blocking CCR7, intercellular adhesion molecule-1, or E-selectin. In an experimental model of lymphedema, where lymphatic drainage is greatly reduced or absent, lymphatic endothelial expression of CCL21 was nearly absent. Conclusions: These findings introduce transmural flow as an important regulator of lymphatic endothelial function and suggest that flow might serve as an early inflammatory signal for lymphatics, causing them to regulate transport functions to facilitate the delivery of soluble antigens and DCs to lymph nodes.

It is well documented that cells secrete exosomes, which can transfer biomolecules that impact recipient cells’ functionality in a variety of physiologic and disease processes. The role of lymphatic drainage and transport of exosomes is as yet unknown, although the lymphatics play critical roles in immunity and exosomes are in the ideal size-range for lymphatic transport. Through in vivo near-infrared (NIR) imaging we have shown that exosomes are rapidly transported within minutes from the periphery to the lymph node by lymphatics. Using an in vitro model of lymphatic uptake, we have shown that lymphatic endothelial cells actively enhanced lymphatic uptake and transport of exosomes to the luminal side of the vessel. Furthermore, we have demonstrated a differential distribution of exosomes in the draining lymph nodes that is dependent on the lymphatic flow. Lastly, through endpoint analysis of cellular distribution of exosomes in the node, we identified macrophages and B-cells as key players in exosome uptake. Together these results suggest that exosome transfer by lymphatic flow from the periphery to the lymph node could provide a mechanism for rapid exchange of infection-specific information that precedes the arrival of migrating cells, thus priming the node for a more effective immune response.

Lymphedema is a common debilitating condition with very limited treatment options, and leukotriene B₄may be a key pathogenic molecule and therapeutic target.