Functional capacities of high endothelial venules appear not to be controlled by recirculating lymphocytes

The influence of recirculating lymphocytes on the function and morphology of high endothelial venules (HEV) has been studied. Mice were depleted of lymphocytes by lethal (1200 cGy) total body irradiation; subsequently, the HEV in mesenteric and cervical lymph nodes were studied up to 7 days after irradiation for: 1) capacity to bind lymphocytes by using the in vitro HEV-binding assay, 2) for morphological aspects such as ultrastructure and endothelial height, 3) for presence of RNA (pyroninophylia) and MECA-325 expression. Although, commencing 3 days after irradiation, lymphocyte depletion was intense and no extravasation of lymphocytes was observed; HEV were capable of binding lymphocytes at normal levels. Also the ratio of B/T cell binding to HEV was comparable to normal. MECA-325 expression, pyroninophilia, and ultrastructure of high endothelial cells were not affected by lymphocyte depletion. However, the average height of endothelial cells, which is a measurement related to cell volume, declined during lymphocyte depletion, stabilizing at about 70% of normal levels from day 4. After intravenous injection of viable lymph node cells, endothelial cell height rapidly increased within a few hours in conjunction with lymphocyte extravasation and homing into the nodes. Restoration of endothelial cell height was not observed after infusion of thymocytes, lethally irradiated lymph node cells or supernatants rich in cytokines. We conclude that recirculating lymphocytes in blood and lymphoid tissues are not involved in controlling high endothelial cell activity including the specific function in lymphocyte extravasation. However, recirculating/extravasating lymphocytes contribute to the development of endothelial cell height. The significance of non-lymphoid (radioresistant) cells in the control of characteristic high endothelial function is suggested.