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The lymphatics transport material from peripheral tissues to lymph nodes, where immune responses are formed, before being transported into systemic circulation. With key roles in transport and fluid homeostasis, lymphatic dysregulation is linked to diseases, including lymphedema. Fluid within the interstitium passes into initial lymphatic vessels where a valve system prevents fluid backflow. Additionally, lymphatic endothelial cells produce key chemokines, such as CCL21, that direct the migration of dendritic cells and lymphocytes. As a result, lymphatics are an attractive delivery route for transporting immune modulatory treatments to lymph nodes where immunotherapies are potentiated in addition to being an alternative method of reaching systemic circulation. In this review, we discuss the physiology of lymphatic vessels and mechanisms used in the transport of materials from peripheral tissues to lymph nodes. We then summarize nanomaterial-based strategies to take advantage of lymphatic transport functions for delivering therapeutics to lymph nodes or systemic circulation. We also describe opportunities for targeting lymphatic endothelial cells to modulate transport and immune functions.

Lymphatic vessels have received considerable attention in recent years as delivery route for immune modulatory therapies to the lymph nodes. Lymph node targeting of immunotherapies and vaccines has been shown to significantly enhance their therapeutic efficacy. Lymphatics transport functions materials from peripheral tissues to the lymph nodes, including small 10 – 250 nm therapeutic nanoparticles. While size required to enter lymphatic vessels, surface chemistry is more poorly studied. Here, we probed the effects of surface poly(ethylene glycol) (PEG) density on nanoparticle transport across lymphatic endothelial cells (LECs). We differentially PEGylated model carboxylate-modified polystyrene nanoparticles to form either a brush or dense brush PEG conformation on the nanoparticle surfaces. Using an established in-vitro lymphatic transport model, we found that the addition of any PEG improved the transport of nanoparticles through lymphatic endothelial cells (2.5 - 2.6 ± 0.9% transport efficiency at 24 hours) compared to the unmodified PS-COOH nanoparticles (0.05 ± 0.05% transport efficiency at 24 hours). Additionally, we found that transcellular transport is maximized (4.2 ± 0.7% transport efficiency at 24 hours) when the PEG is in a dense brush conformation on nanoparticle surfaces, corresponding with a high grafting density (Rf/D = 4.9). These results suggest that PEG conformation has a crucial role in determining translocation of nanoparticles across LECs and into lymphatic vessels. Thus, we identified PEG density as a major design criteria for maximizing lymphatic targeting of therapeutic nanoparticle formulations that can be widely applied to enhance immunotherapeutic and vaccine outcomes in future studies.

Lymphatic vessels have received significant attention as drug delivery targets, as they shuttle materials from peripheral tissues to the lymph nodes, where adaptive immunity is formed. Delivery of immune modulatory materials to the lymph nodes via lymphatic vessels has been shown to enhance their efficacy and also improve bioavailability of drugs when delivered to intestinal lymphatic vessels. In this study we generated a three-compartment model of a lymphatic vessel with a set of kinematic differential equations to describe the transport of nanoparticles from surrounding tissues into lymphatic vessels. We used previously published data and collected additional experimental parameters, including transport efficiency of nanoparticles over time, and also examined how nanoparticle formulation affected the cellular transport mechanisms using small molecule inhibitors. This experimental data was incorporated into a system of kinematic differential equations and non-linear, least squares curve fitting algorithms were employed to extrapolate transport coefficients within our model. The subsequent computational framework produced some of the first parameters to describe transport kinetics across lymphatic endothelial cells and allows for the quantitative analysis of the driving mechanisms of transport into lymphatic vessels. Our model indicates that transcellular mechanisms, such as micro- and macropinocytosis, drive transport into lymphatics. This information is crucial to further design strategies that will modulate lymphatic transport for drug delivery, particularly in diseases like lymphedema, where normal lymphatic functions are impaired.

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