|
High endothelial venules (HEV) are specialized post-capillary venous swellings characterized by plump endothelial cells as opposed to the usual thinner endothelial cells found in regular venules. HEVs enable lymphocytes circulating in the blood to directly enter a lymph node (by crossing through the HEV).〔 Table 14-1〕 In humans, HEVs are found in all secondary lymphoid organs (with the exception of spleen, where blood exits through open arterioles and enters the red pulp), including hundreds of lymph nodes dispersed in the body, tonsils and adenoids in the pharynx, Peyer's patches (PIs) in the small intestine, appendix, and small aggregates of lymphoid tissue in the stomach and large intestine. In contrast to the endothelial cells from other vessels, the high endothelial cells of HEVs have a distinctive appearance, consisting of a cuboidal morphology and with various receptors to interact with leukocytes (express specialized ligands for lymphocytes and are able to support high levels of lymphocyte extravasation).〔 HEVs enable naïve lymphocytes to move in and out of the lymph nodes from the circulatory system. HEV cells express addressins, which are specific adhesion molecules that attach to the L-selectins on lymphocytes and anchor them to the HEV wall in preparation for crossing the endothelium. The endothelial cells of HEVs have a 'plump' appearance different from the flat morphology of endothelial cells that line other vessels, and are therefore called high endothelial cells by reference to their thickness.〔 Another characteristic of HEVs, revealed by light-microscopic examination, is the presence of a large number of lymphocytes within their walls. This illustrates the function of HEVs in lymphocyte recruitment, and explains why these vessels were implicated in lymphocyte traffic from the time of their initial description. ==The need for HEV== In order for an adaptive immune response to occur T cells need to be activated. T cells become activated by recognising foreign antigens bound to antigen presenting cells (APC), in particular, dendritic cells. In order for naïve T cells to bind to their specific antigen, they need to experience physical contact with those cells. Since antigen levels are usually low, contact in blood circulation would be unlikely. Therefore, T cells need a region where they can go to sample foreign antigens that have entered the body. When an APC, such as a dendritic cell, binds a foreign antigen it becomes activated and moves into the lymph nodes (sites for antigen sampling by T cells) via afferent lymphatic vessels. Naïve T cells in the circulation regularly move through the lymph nodes via HEV in order to scan the APC for foreign antigens. When they encounter such an antigen, the cell becomes activated, resulting in the immune system mounting a response against the causative agent of the infection. Depletion of CD11c+ dendritic cells in mouse significantly altered the phenotype of HEV. The normal phenotype of HEV is possibly maintained by DC-secreted lymphotoxin (TNF-beta).〔Dendritic cells control lymphocyte entry to lymph nodes through high endothelial venules. Nature. 2011 Nov 13;479(7374):542-6. 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「High endothelial venules」の詳細全文を読む スポンサード リンク
|