highendothelial

High endothelium refers to the specialized endothelial cells that line high endothelial venules (HEVs), a subset of postcapillary venules found predominantly in secondary lymphoid organs such as lymph nodes, tonsils, and Peyer's patches. The endothelial cells of HEVs have a distinctive tall or cuboidal appearance, giving rise to the term high endothelium. This morphology supports the unique role of HEVs in guiding lymphocytes from the bloodstream into lymphoid tissue.

Molecularly, HEVs express a set of adhesion molecules and chemokines that promote lymphocyte recruitment. Peripheral node addressin (PNAd) is commonly expressed on HEVs in peripheral lymph nodes and mediates L-selectin–dependent rolling of naive T and B cells. In gut-associated lymphoid tissue, HEVs may express MAdCAM-1, enabling homing of gut-tropic lymphocytes. HEVs also produce chemokines such as CCL21, which helps orient lymphocytes for diapedesis through the endothelium. The combination of selectins, chemokines, and other adhesion molecules creates a specialized portal for lymphocyte entry into lymphoid organs.

Functionally, HEVs serve as critical gateways for immune surveillance, allowing naive and central memory lymphocytes to exit the blood and participate in antigen sampling within lymphoid tissues. Their activity can be modulated during immune responses, with changes in caliber, adhesion molecule expression, and chemokine production to accommodate increased lymphocyte traffic. In chronic inflammation, HEV remodeling may occur, and in oncology, tumor-associated HEVs have been observed to facilitate intratumoral lymphocyte infiltration, potentially influencing anti-tumor immunity.

In summary, high endothelium describes the distinctive endothelial lining of HEVs that enables regulated lymphocyte trafficking and plays a central role in adaptive immunity.