Lymphatic Vessels, Inflammation, and Immunity in Skin Cancer

Abstract

Skin is a highly ordered immune organ that coordinates rapid responses to external insult while maintaining self-tolerance. In healthy tissue, lymphatic vessels drain fluid and coordinate local immune responses; however, environmental factors induce lymphatic vessel dysfunction, leading to lymph stasis and perturbed regional immunity. These same environmental factors drive the formation of local malignancies, which are also influenced by local inflammation. Herein, we discuss clinical and experimental evidence supporting the tenet that lymphatic vessels participate in regulation of cutaneous inflammation and immunity, and are important contributors to malignancy and potential biomarkers and targets for immunotherapy.

Significance: The tumor microenvironment and tumor-associated inflammation are now appreciated not only for their role in cancer progression but also for their response to therapy. The lymphatic vasculature is a less-appreciated component of this microenvironment that coordinates local inflammation and immunity and thereby critically shapes local responses. A mechanistic understanding of the complexities of lymphatic vessel function in the unique context of skin provides a model to understand how regional immune dysfunction drives cutaneous malignancies, and as such lymphatic vessels represent a biomarker of cutaneous immunity that may provide insight into cancer prognosis and effective therapy.

Figure 1. Structure and function of initial and collecting lymphatic vessels

(A) The lymphatic vessels of skin are composed of two plexuses one superficial extending into the dermal papillae near the subpapillary arterial network, which drains vertically into the deep lymphatic plexus below the second arterial network. (B) Initial lymphatic capillaries are blind-ended vessels with discontinuous basement membrane and no associated smooth muscle cells (SMC). At resting state, the lymphatic endothelial cells that comprise the initial capillaries are characterized by unique overlapping, button-like junctions that allow for passive flow and leukocyte trafficking through interendothelial gaps in an integrin-independent manner. Local inflammation, results in vascular leakiness driving increased interstitial fluid pressures (IFP) and enhanced flows. At least in mouse respiratory tract, inflammation is associated with a remodeling of the interendothelial junctions of initial capillaries into tight, zipper-like junctions. Lymphatic capillaries are anchored directly to the extracellular matrix (ECM) through anchoring filaments, such that under high levels of IFP, stretching of ECM results in distension of initial capillaries and enhanced fluid flows and cellular trafficking both by intercellular and transcellular mechanisms. (C) Collecting vessels are larger vessels that have both a continuous basement membrane and SMC coverage. Collecting vessels are notably defined by the presence of a system of valves, which separates the vessel into functional units or lymphangions. SMCs mediate contraction of individual lymphangions that drives the opening of downstream valves while closely valves immediate upstream. This system of local contraction and relaxation drives unidirectional fluid flows from peripheral tissues to draining lymph nodes.

Figure 2. Lymphatic vessels, inflammation and immunity

(A) Homeostatic lymphatic capillaries support immune surveillance through steady-state homing of resident immune cells, including dendritic cells (DC) and some subsets of memory T cells. (B) Local inflammation and damage activate a series of danger signaling as well as increased interstitial fluid pressures (IFP) that activate initial lymphatic capillaries resulting in remodeling (either proliferative or non-proliferative), upregulation of adhesion molecules and enhanced expression of the homing chemokine, CCL21. Altered adhesions and CCL21 coordinate to facilitate entry of activated CCR7⁺ DCs into afferent lymphatic vessels and migration towards draining lymph nodes where they interact with and activate naïve T cells. The decoy receptor, D6, ensures proper presentation of homeostatic chemokines by LECs by scavenging inflammatory chemookines to specifically facilitate mature over immature DC migration. Changes in lymphatic flows that result from altered signaling in both initial capillaries and collecting vessels may influence accumulation of inflammatory cytokines that help to perpetuate local inflammation leading to infiltration and accumulation of leukocytes in tissue, which further drive lymphatic remodeling. (C) Though important for immune induction, evidence also indicates that lymphatic capillaries importantly regulate resolution of local inflammation and immunity through leukocyte egress and chemokine sequestration. Both macrophages and some T cells exit peripheral tissue through draining lymphatic capillaries using CCL21 and shingosine kinase (SPHK) conversion of sphingosine into Sphingosine-1-P (S1P) as signals for their exit, all produced by initial lymphatic vessels. Cellular exit is required for resolution of disease. (D) Novel immunomodulatory roles of lymphatic endothelial cells (LEC) have been described, largely in the context of lymphoid organs. LECs inhibit both antigen-dependent and independent T cell activation through production of nitric oxide (NO) and non-specific inhibition of DC/T cell interactions. Inflamed LECs inhibit maturation of DCs through ICAM-1 and receive peptide-loaded MHCII complexes from mature DCs. Additionally, LECs promiscuously present endogenous and scavenge exogenous antigen for cross-presentation on MHCI molecules and direct deletion of antigen-specific CD8 T cells.

Figure 3. Deregulation of lymphatic vessel function, inflammation and skin carcinogenesis by environmental factors

Environmental factors that predispose to skin cancer (ultraviolet radiation, infection and surgery or physical trauma) simultaneously impact lymphatic vessel dysfunction. Lymphatic remodeling as a result of ultraviolet exposure, infection or surgery, may result in altered fluid flows and local inflammation that generate a local microenvironment more permissive to the oncogenic effects of the agents.

Figure 4. Proposed model for feedback between lymphatic vessels, inflammation and skin carcinogenesis: implications for immunotherapy

(A) Tumor-promoting inflammation induces the initiation and progression of skin cancer as well as remodeling of local lymphatic vessels, which in turn may be further tumor-promoting by facilitating the resolution response characterized by immune suppressive leukocyte infiltrates and local tolerance. Lymphatic remodeling results in enhanced fluid flows to draining lymph nodes facilitating metastatic progression of developing skin cancers. Furthermore, in addition to the pro-tumor, suppressive inflammation, lymphatic vessels may directly inhibit anti-tumor immunity preventing local control of the growing tumor, though whether this would occur in tumor microenvironments or their draining lymph nodes remains unknown. (B) Immunotherapy endeavors to switch the balance in this network toward anti-tumor immunity through methods of both direct and indirect activation of adaptive immune responses against tumors. Enhanced anti-tumor immunity will control primary growth but may also influence local remodeling of lymphatic vessels through an interferon-γ–dependent mechanism. Mechanisms of resistance to this approach have already been described where infiltrating leukocytes impair local T cell infiltration and function. Given the novel immunomodulatory roles of lymphatic vessels it remains to be seen as to whether their status may be additionally predictive of response or alternatively a targetable mechanism of resistance.