Tumor-draining lymph nodes: At the crossroads of metastasis and immunity

Abstract

Early engagement of the lymphatic system by solid tumors in peripheral, nonlymphoid tissues is a clinical hallmark of cancer and often forecasts poor prognosis. The significance of lymph node metastasis for distant spread, however, has been questioned by large-scale lymph node dissection trials and the likely prevalence of direct hematogenous metastasis. Still, an emerging appreciation for the immunological role of the tumor-draining lymph node has renewed interest in its basic biology, role in metastatic progression, antitumor immunity, and patient outcomes. In this review, we discuss our current understanding of the early mechanisms through which tumors engage lymphatic transport and condition tumor-draining lymph nodes, the significance of these changes for both metastasis and immunity, and potential implications of the tumor-draining lymph node for immunotherapy.

Figure 1.. The lymph node undergoes structural changes as a function of tumor drainage.

A. Solid tumors are connected to lymph nodes (LN) through a network of lymphatic vessels that transport fluid, soluble factors, lipids, and cells. The sentinel LN is the first LN draining tumor-associated lymph and is assayed clinically to determine the metastatic potential of a nascent malignant lesion. This sentinel LN sits in a basin of tumor-draining LNs (TDLN) that are at risk of metastatic seeding and uniquely affected by a tumor when compared to distant, non-draining LNs (NDLN). B. Afferent lymph flows from the tumor to the TDLN and delivers tumor-derived material including antigens and tumor-derived extracellular vesicles, to the TDLN. TDLNs progressively expand and initiate three major stromal remodeling processes that affect TDLN structure and metastatic potential: (1) the TDLN undergoes extensive lymphangiogenesis, expanding lymphatic sinuses. LN lymphangiogenesis is initiated prior to tumor seeding and supports initial regional metastatic progression. (2) High endothelial venules (HEV) initially increase in density but ultimately undergo dilation and de-differentiation, which may impair lymphocyte recruitment. (3) Fibroblastic reticular cells (FRCs) proliferate in the TDLN, resulting in widened conduits, altered size-exclusion properties of the reticular conduits and antigen delivery into the LN paracortex.

Figure 2.. The subcapsular sinus intersects immunity and metastasis.

The subcapsular sinus (SCS) is the first site in the tumor-draining lymph node (TDLN) contacted by tumor-draining material carried in afferent lymph and exhibits profound molecular and structural alterations which impact both antigen presentation and metastatic progression. (A) Afferent lymph draining from the tumor carries soluble antigens (Ag), tumor-derived secreted factors (TDSF), tumor-derived extracellular vesicles (tdEVs), migratory dendritic cells (DCs), and metastatic tumor cells to the SCS. (B) SCS macrophages interdigitate the floor of the SCS and extend into the lumen serving both antigen presentation and barrier functions through the capture of particulate lymph-borne tumor-derived material (>70kDa), including tdEVs. While SCS macrophages may be protective against metastasis, they can be progressively lost during primary tumor development leading to inappropriate tdEV access to B cell follicles that drives pathogenic responses. (C) Lymphatic endothelial cells (LEC) form the boundaries of the SCS and create and maintain chemokine gradients (e.g. CCL21, CXCL12, CCL1) that direct DC migration towards the paracortex for antigen presentation. Small, soluble antigens (<70kDa) and proteins enter fibroblastic reticular cells lined collagen conduits that bridge the subcapsular sinus and paracortex. Tumor cells arrive in TDLN via afferent lymph and often coopt the same homing mechanisms used by DCs (e.g. CCL21/CCR7, CCL1/CCR8, and CXCL12/CXCR4) to facilitate their migration to and seeding within the SCS. Furthermore, expanded and activated LECs within the TDLN upregulate adhesion molecules, such as α4β1 integrin, that further support tumor cell adhesion and invasion. (D) Lymphatic sinuses are expanded prior to tumor cell arrival by both tumor- and B cell- derived lymphangiogenic growth factors (e.g. VEGF family members), and their growth restructures the carefully maintained guidance cues that direct antigen presentation and tumor cell seeding. These alterations culminate in the formation of a functional pre-metastatic niche that supports tumor cell adhesion, outgrowth, and invasion while also contributing to LN immune suppression.

Figure 3.. The role of the tumor-draining lymph node in PD-1/PD-L1 blockade.

The mechanism of action of immune checkpoint blockade (ICB) may differ by anatomic location and data supports an integral role for the tumor-draining lymph node (TDLN). (A) Within the tumor microenvironment, PD-L1 is expressed by tumor cells and the hematopoietic and non-hematopoietic tumor microenvironment. Initially, PD-L1 blockade was thought to directly improve intratumoral T cell function by relieving local immune suppression and reinvigorating chronically antigen-stimulated, exhausted tumor infiltrating lymphocytes. (B) When delivered systemically, however, reinvigorated CD8⁺PD-1⁺Ki67⁺ T cells can be detected in blood of responders over non-responders and de novo T cell responses are seen in blood, tumors, and normal adjacent tissue, indicate systemic expansion on therapy. (C) While initial experiments using FTY720 to block shingosine-1-phosphate-dependent T cell egress suggested that TDLN were not required for response to ICB, the TDLN is now emerging as a key player in response and a potential direct target. The TDLN exhibits higher expression of hematopoietic PD-L1 compared to non-draining LNs (NDLN) and are critical sites of antigen presentation and T cell priming. PD-L1/PD-1 antibodies administered via routes that enrich delivery to the TDLN boost CD8⁺ T cell proliferation and show equivalent efficacy even at substantially lower doses than delivered systemically. Targeting and activating the antigen-loaded TDLN, therefore, may represent an exciting strategy for immunotherapy moving forward.