Endothelial cells (EC) line the lymphatic and vascular vessels of the body, forming a barrier between the lumen and surrounding tissue. ECs actively coordinate the exchange of migrating cells, fluids and molecules thus regulating tissue homeostasis. Correctly working endothelium is also important during the embryonic development, as progenitor cells have tight time windows to reach and colonize target tissues. Failures to do so lead to perturbed cell composition and lasting adverse effects in the adulthood. In the blood endothelium, plasmalemma vesicle-associated protein (PLVAP) forms diaphragms on specific plasma membrane structures (transendothelial channels, fenestrae and caveolae) and has been showed to regulate the EC permeability in blood vessels. However, the role of PLVAP in the lymphatic endothelium is not characterized, nor its role in the immune system during development.

The aim of this thesis was to use in vivo and in vitro -methods to investigate endothelial cells in the lymph node and liver. The results of this thesis show that PLVAP is unexpectedly expressed in distinct lymphatic ECs of the lymph node and there, PLVAP regulates the antigen and lymphocyte entry to the lymph node. During development PLVAP expression is crucial for migration of macrophage progenitors and the emerging of a F4/80Hi tissue-resident macrophage population in several organs. In mammary gland, the F4/80Hi macrophages form a major population of the steady state macrophage pool and that their unique functions cannot be performed by other macrophage types.