The tumor-draining lymph nodes (TDLNs) are primary sites for induction of tumor immunity. They are also common sites of metastasis, suggesting that tumor-induced mechanisms can subvert anti-tumor immune responses and promote metastatic seeding. The high endothelial venules (HEVs) together with CCL21-expressing fibroblastic reticular cells (FRCs) are essential for lymphocyte recruitment into the LNs. We established multicolor antibody panels for evaluation of HEVs and FRCs in TDLNs from breast cancer (BC) patients. Our data show that patients with invasive BC display extensive structural and molecular remodeling of the HEVs, including vessel dilation, thinning of the endothelium and discontinuous expression of the HEV-marker PNAd. Remodeling of the HEVs was associated with dysregulation of CCL21 in perivascular FRCs and with accumulation of CCL21-saturated lymphocytes, which we link to loss of CCL21-binding heparan sulfate in FRCs. These changes were rare or absent in LNs from patients with non-invasive BC and cancer-free organ donors and were observed independent of nodal metastasis. Thus, pre-metastatic dysregulation of core stromal and vascular functions within TDLNs reflect the primary tumor invasiveness in BC. This adds to the understanding of cancer-induced perturbation of the immune response and opens for prospects of vascular and stromal changes in TDLNs as potential biomarkers.

Lymph node (LN) lipomatosis is a common but rarely discussed phenomenon associated with aging that involves a gradual exchange of the LN parenchyma into adipose tissue. The mechanisms behind these changes and the effects on the LN are unknown. We show that LN lipomatosis starts in the medullary regions of the human LN and link the initiation of lipomatosis to transdifferentiation of LN fibroblasts into adipocytes. The latter is associated with a downregulation of lymphotoxin beta expression. We also show that isolated medullary and CD34⁺ fibroblasts, in contrast to the reticular cells of the T‐cell zone, display an inherently higher sensitivity for adipogenesis. Progression of lipomatosis leads to a gradual loss of the medullary lymphatic network, but at later stages, collecting‐like lymphatic vessels are found inside the adipose tissue. The stromal dysregulation includes a dramatic remodeling and dilation of the high endothelial venules associated with reduced density of naïve T‐cells. Abnormal clustering of plasma cells is also observed. Thus, LN lipomatosis causes widespread stromal dysfunction with consequences for the immune contexture of the human LN. Our data warrant an increased awareness of LN lipomatosis as a factor contributing to decreased immune functions in the elderly and in disease. © 2022 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Single-cell transcriptomics promises to revolutionize our understanding of the vasculature. Emerging computational methods applied to high dimensional single cell data allow integration of results between samples and species, and illuminate the diversity and underlying developmental and architectural organization of cell populations. Here, we illustrate these methods in analysis of mouse lymph node (LN) lymphatic endothelial cells (LEC) at single cell resolution. Clustering identifies five well-delineated subsets, including two medullary sinus subsets not recognized previously as distinct. Nearest neighbor alignments in trajectory space position the major subsets in a sequence that recapitulates known and suggests novel features of LN lymphatic organization, providing a transcriptional map of the lymphatic endothelial niches and of the transitions between them. Differences in gene expression reveal specialized programs for (1) subcapsular ceiling endothelial interactions with the capsule connective tissue and cells, (2) subcapsular floor regulation of lymph borne cell entry into the LN parenchyma and antigen presentation, and (3) medullary subset specialization for pathogen interactions and LN remodeling. LEC of the subcapsular sinus floor and medulla, which represent major sites of cell entry and exit from the LN parenchyma respectively, respond robustly to oxazolone inflammation challenge with enriched signaling pathways that converge on both innate and adaptive immune responses. Integration of mouse and human single-cell profiles reveals a conserved cross-species pattern of lymphatic vascular niches and gene expression, as well as specialized human subsets and genes unique to each species. The examples provided demonstrate the power of single-cell analysis in elucidating endothelial cell heterogeneity, vascular organization and endothelial cell responses. We discuss the findings from the perspective of LEC functions in relation to niche formations in the unique stromal and highly immunological environment of the LN.HighlightsComputational alignments (“trajectories”) predict LN LEC organizationin situ, revealing a continuum of phenotypes punctuated by specialized clustersMultiple intermediate phenotypes suggest LEC malleabilityGene profiles define niche-specific functional specializationMedullary sinus LECs are comprised of Ptx3-LECs and Marco-LECs Distinct mechanisms for pathogen interactions and matrix modeling Ptx3-LECs: paracortical and central medullary sinuses near hilus; enriched for genes driving lymphangiogenic responses and lymphocyte egress Marco-LECs: peri-follicular medullary sinuses; macrophage-associated genes, complement and coagulation cascade Niche-specific responses to inflammation IFN gene responses in SCS floor and medullary sinus LECs Suppression of LEC identity genes in responding subsets Conserved and unique LEC subsets and gene programs across species Core subsets common to mouse and human Greater diversity of subsets and intermediates in human LN LECs

Vascular remodeling is common in human cancer and has potential as future biomarkers for prediction of disease progression and tumor immunity status. It can also affect metastatic sites, including the tumor‐draining lymph nodes (TDLNs). Dilation of the high endothelial venules (HEVs) within TDLNs has been observed in several types of cancer. We recently demonstrated that it is a premetastatic effect that can be linked to tumor invasiveness in breast cancer. Manual visual assessment of changes in vascular morphology is a tedious and difficult task, limiting high‐throughput analysis. Here we present a fully automated approach for detection and classification of HEV dilation. By using 12,524 manually classified HEVs, we trained a deep‐learning model and created a graphical user interface for visualization of the results. The tool, named the HEV‐finder, selectively analyses HEV dilation in specific regions of the lymph nodes. We evaluated the HEV‐finder's ability to detect and classify HEV dilation in different types of breast cancer compared to manual annotations. Our results constitute a successful example of large‐scale, fully automated, and user‐independent, image‐based quantitative assessment of vascular remodeling in human pathology and lay the ground for future exploration of HEV dilation in TDLNs as a biomarker. © 2022 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Lymph node (LN) lipomatosis is a common, but rarely discussed phenomenon, associated with aging, involving a gradual exchange of the LN parenchyma into adipose tissue. The mechanisms behind these changes and the effects on the LN have been unknown. We show that LN lipomatosis starts in the medullary regions of the human LN and link the initiation of lipomatosis to transdifferentiation of LN medullary fibroblasts into adipocytes. The latter is associated with a downregulation of lymphotoxin beta expression. We also show that, medullary fibroblasts, in contrast to the reticular cells in the T-cell zone, display an inherent higher sensitivity for adipogenesis. Progression of lipomatosis leads to a gradual loss of the medullary lymphatic network, but at later stages, collecting-like lymphatic vessels, are found inside the adipose tissue. The stromal dysregulation includes a dramatic remodeling and dilation of the high endothelial venules associated with reduced density of naïve T-cells. Abnormal clustering of plasma cells is also observed. Thus, LN lipomatosis causes widespread stromal dysfunction with consequences for the immune contexture of the human LN. Our data warrant an increased awareness of LN lipomatosis as a factor contributing to decreased immune functions in the elderly and in disease. Graphical In lymph nodes (LNs) of young patients there is a normal lymph flow, normal and functioning high endothelial venules (HEVs) with a high density of surrounding naïve T-cells. With aging lymphotoxin beta (LTB) is downregulated in the medulla of the LN and the fibroblasts of the medulla, namely the medullary reticular cells (MedRCs), transdifferentiate into adipocytes inducing LN lipomatosis. LN lipomatosis leads to loss of lymphoid tissue, medullary sinuses and can be predicted to result in a shortcut of the lymph flow based on the presence of collecting-like vessels in the adipose tissue in late stage lipomatosis. Lipomatosis also induce extensive vascular remodeling with loss of medullary lymphatic vessels and dysfunctional, highly dilated HEVs with lower density of naïve T-cells and trapped plasma cells.

Epithelial-to-mesenchymal-transition (EMT) and upregulation of chemokine receptors are linked to lymphatic metastasis in various types of cancer including breast cancer. However, the underlying molecular mechanisms are not fully understood. Using a triple-negative mammary carcinoma model, conditioned for lymphatic metastasis through expression of the C-C chemokine receptor 7 (CCR7), we identified intrinsic tumor plasticity associated with EMT as a critical determinant for effective metastasis. Specifically, tumor cells upregulate Podoplanin (PDPN) as part of a spontaneous mesenchymal shift that promotes lymphatic dissemination and tumor growth. Intriguingly, the expression of CCR7, or PDPN alone, was not sufficient for these effects. CCR7- and PDPN-positive tumors displayed an immune-cold tumor profile, compared to control, characterized by reduced tumor-infiltrating T-cells. Consistent with this, we found that spontaneous upregulation of PDPN was linked to hypoxia and was associated with the downregulation of homeostatic interferon signaling, and increased expression of collagens. Analysis of single-cell data sets showed that PDPN expression was heterogeneous and correlated significantly with EMT markers, collagen expression, and hypoxia hallmark in human breast cancer cell lines and primary human triple negative (TN) breast cancer, mirroring findings in the mouse model. Further analysis of the human breast cancer METABRIC-microarray datasets supported an association between a high CCR7/PDPN mRNA expression score and aggressive breast cancer subtypes with an independent prognostic value in lymph node-positive tumors. Together, these findings highlight a critical role of tumor microenvironment-driven tumor plasticity and molecular synergy between CCR7 and PDPN-coupled EMT shift in promoting chemokine receptor-mediated tumor dissemination and progression. Summary: CCR7-driven lymphatic metastasis requires a mesenchymal shift with PDPN upregulation. This tumor plasticity, linked to hypoxia and immune evasion, suggests CCR7-PDPN synergy in aggressive breast cancer progression and poor prognosis.