Regulatory mechanism for expression of two distinct ligands for the cell adhesion molecule, selectin, on lymphocytes.

T淋巴细胞在免疫反应中起主要作用。我们研究了人类辅助性T细胞亚群部位特异性募集的机制，特别关注了选择素，这是一种细胞黏附分子家族，与特定的碳水化合物对抗受体相互作用。L-选择素是选择素家族的一员，参与了幼稚辅助T淋巴细胞通过高内皮微静脉(HEV)归巢到外周淋巴结的过程。我们通过产生特异性抗体和通过转染α1，3岩藻糖基转移酶VII和GlcNAcβ：6-Ο-磺基转移酶的基因重组了具有功能性的L选择素配体，确定了HEV内皮细胞上的L选择素的糖基配体为唾液酸基6-磺基乐^x。在Peyer氏斑和阑尾的HEV上也表达了相同的决定子，在那里它介导了含有α_4β_7整合素的肠归巢辅助记忆T淋巴细胞的黏附。还发现一种不同的静息辅助记忆T淋巴细胞亚群表达唾液酸化…α_4、β_7-整合素不表达，提示这些细胞是皮肤归巢的辅助性记忆T淋巴细胞，通过与真皮内皮细胞上的E-选择素和P-选择素相互作用而定位于皮肤。值得注意的是，唾液酸基6-磺基Le^X优先参与非炎症条件下不同亚群的辅助性T淋巴细胞的归巢过程。相比之下，常规唾液酸基Le^X在HEV或静息外周T淋巴细胞上几乎不表达，但在TPA或ConA-刺激下显著诱导淋巴细胞表达。伴随而来的是对岩藻糖基转移酶VII的显著转录诱导，该酶是白细胞合成唾液酸Le^X的限速酶。外周T淋巴细胞的刺激抑制了唾液酸基6-磺酸Le^X的表达，表明T淋巴细胞的活化伴随着选择素配体的优势分子种类从唾液酸基6-磺酸Le^X向常规唾液酸基Le^X的转变。我们认为唾液酸基6-磺酸基Le^X主要介导静息T淋巴细胞的常规归巢，而常规唾液酸基Le^X优先参与激活的T淋巴细胞向炎性病变的募集。唾液酸基6-磺基Le^X对人淋巴细胞的选择素结合活性受其唾液酸部分的翻译后修饰所调节，从而导致“环唾液酸”的形成，从而防止淋巴细胞在常规归巢过程中在血管床上过度聚集。较少

T-lymphocytes play principal roles in immune response. We studied the mechanism for site-specific recruitment of human helper T cell subsets with special attention to selectin, a family of cell adhesion molecules, which interact with specific carbohydrate counter-receptors. L-selectin, a member of the selectin family, is involved in the homing of naive helper T-lymphocytes into peripheral lymph nodes through high endothelial venules (HEV). We identified the carbohydrate ligand for L-selectin on HEV endothelial cells to be sialyl 6-sulfo Le^x, by generating specific antibodies, and by the reconstitution of functional L-selectin ligand by transfection of cDNAs for α1, 3 fucosyltransferase VII and GlcNAcβ : 6-Ο-sulfotransferase. The same determinant was expressed on HEVs of Peyer's patches and appendices, where it mediatesadhesion of the gut-homing helper memory T-lymphocytes bearing α_4β_7-integrin. A distinct subset of resting helper memory T-lymphocytes was also found to express sialyl … More 6-sulfo Le^X. The subset strongly co-expressed PSGL-1 and CCR4, but not α_4β_7-integrin, indicating these cells are skin-homing helper memory T-lymphocytes, which home to the skin by interacting with E- and P-selectins on dermal endothelial cells. It is notable that the sialyl 6-sulfo Le^X is preferentially involved in routine homing process of various subsets of helper T-lymphocytes under non-inflammatory conditions.In contrast, conventional sialyl Le^X was virtually not expressed on HEVs or on resting peripheral T-lymphocytes, but was prominently induced on lymphocytes upon TPA- or Con A-stimulation. This was accompanied by a remarkable transcriptional induction of fucosyltransferase VII, the rate-limiting enzyme for sialyl Le^X synthesis in leukocytes. Stimulation of peripheral T-lymphocytes abrogated the sialyl 6-sulfo Le^X expression, indicating that T-lymphocyte activation is accompanied by a switching of dominant molecular species of selectin ligands from sialyl 6-sulfo Le^X to conventional sialyl Le^X. We propose that sialyl 6-sulfo Le^X primarily mediates routine homing of resting T-lymphocytes, while conventional sialyl Le^X is preferentially involved in the recruitment of activated T-lymphocytes to inflammatory lesions. Selectin-binding activity of sialyl 6-sulfo Le^X on human lymphocytes was found to be regulated by a post-translational modification of its sialic acid moiety leading to the formation of "cyclicsialic acid", which would prevent excessive accumulation of lymphocytes at vascular beds in the routine homing process. Less