L-selectin

L-selectin, also known as CD62L, is a is_associated_with::cell adhesion molecule found on is_associated_with::lymphocytes and the preimplantation embryo. It belongs to the is_associated_with::selectin family of proteins, which recognize sialylated carbohydrate groups. It is cleaved by is_associated_with::ADAM17.

SELL is a cell surface component that is a member of a family of adhesion/homing receptors that play important roles in lymphocyte-endothelial cell interactions. The molecule is composed of multiple domains: one homologous to lectins, one to epidermal growth factor, and two to the consensus repeat units found in C3/C4-binding proteins.

Ligands

 * is_associated_with::GlyCAM-1, found in the is_associated_with::high endothelial venules of the is_associated_with::lymph nodes.
 * is_associated_with::CD34, found on endothelial cells.
 * is_associated_with::MadCAM-1, found on endothelial cells of is_associated_with::gut-associated lymphoid tissue.
 * is_associated_with::PSGL-1, binds with low affinity.

Function
L-selectin acts as a "homing receptor" for lymphocytes to enter secondary lymphoid tissues via high endothelial venules. is_associated_with::Ligands present on is_associated_with::endothelial cells will bind to lymphocytes expressing L-selectin, slowing lymphocyte trafficking through the blood, and facilitating entry into a secondary lymphoid organ at that point. The receptor is commonly found on the cell surfaces of T cells. Naive T-lymphocytes, which have not yet encountered their specific antigen, need to enter secondary lymph nodes to encounter their antigen. Central memory T-lymphocytes, which have encountered antigen, express L-selectin to localize in secondary lymphoid organs. Here they reside ready to proliferate upon re-encountering antigen. Effector memory T-lymphocytes do not express L-selectin, as they circulate in the periphery and have immediate effector functions upon encountering antigen.

High expression of L-selectin on human bone marrow progenitor cells is an early sign of cells becoming committed to lymphoid differentiation.

L-selectin is also present on the surface of human embryo trophoblasts prior to implantation into the uterus. Similar to its function in lymphocytes, L-selectin acts as a receptor to facilitate adhesion of the embryo to the site of invasion on the surface epithelium of the uterine endometrium. The embryo secretes human chorionic gonadotropin (hCG), which downregulates anti-adhesion factor, is_associated_with::MUC-1, located on the uterine epithelium at the site of invasion. Removal of MUC-1 exposes the oligosaccharide ligands of the uterine epithelium, thus allowing binding by the L-selectin receptor of the trophopblast cell, followed by embryo adhesion and invasion.