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The function of the lymph node (LN) stroma is crucial for the induction of an adaptive immune response. The stroma includes lymphatic vessels, high endothelial venules (HEVs) and mesenchymal cells (MCs). In this thesis, I have been investigating the role of the LN stroma in cancer and aging. In Paper I, we demonstrate that LNs draining invasive breast cancer present remodeling of the HEVs and dysregulation of the perivascular MCs. Dilation of HEVs is correlating with inhibition of normal lymphocyte recruitment due to perivascular changes in the expression of molecular cues necessary for chemotaxis. As a follow up of Paper I, we created an automatic method for image analysis of HEVs by using artificial intelligence and deep learning. In paper II, we validate the HEV-finder that we suggest is a good automatic image analysis tool to study HEV remodeling in different subtypes of cancer and to perform survival studies in larger patient cohorts. In paper III, we present a mapping of the lymphatic endothelial cells (LECs) of the mouse and human LNs by using single-cell RNA sequencing and present a new LEC subset called PTX3-LECs. We demonstrate that both in mouse and human LNs there are four LEC subsets, forming the subcapsular and medullary sinuses where immune cells enter and exit the LN, respectively. Our mapping is a great tool for further investigation of different subsets and their specific genes in translational studies of homeostasis or disease. In paper IV, we are focusing on LN lipomatosis, which is an age-related phenomenon where the normal LN parenchyma is replaced by adipose tissue. Our data support that LN lipomatosis is developing in the medulla of the LN due to transdifferentiation of medullary MCs into adipocytes. We found that it is causing remodeling of the HEVs and loss of medullary lymphatic vessels, affecting both the entry and exit of lymphocytes. Based on our findings, we suggest that LN lipomatosis is a contributing factor in immunodeficiency in elderly. In paper V, we created a new transplantable breast cancer model in mouse to study the effect of LN metastasis by inducing the expression of the chemokine receptor CCR7 in the mammary carcinoma cell line EO771. The new model will allow us to study the effect of LN metastasis on the anti-tumor immunity and response to therapy. In summary, my thesis shed light on the importance of a functioning LN stroma in both cancer and aging and provides new tools for image analysis, translational studies of the LN stroma and models for LN metastasis.