SELECTin

Selectins are a family of cell adhesion molecules that mediate the initial step of leukocyte trafficking from the bloodstream to tissues. They are calcium-dependent lectins with a characteristic structure that includes a C-type lectin domain, an epidermal growth factor–like domain, a single transmembrane region, and a small cytoplasmic tail. The family comprises L-selectin (CD62L), E-selectin (CD196), and P-selectin (CD62P).

L-selectin is constitutively expressed on most leukocytes and supports the early capture and rolling of naive lymphocytes on high endothelial venules in secondary lymphoid organs. E-selectin is induced on activated vascular endothelium by inflammatory cytokines such as IL-1 and TNF and promotes leukocyte rolling at sites of inflammation. P-selectin is stored in Weibel-Palade bodies of endothelial cells and alpha granules of platelets and rapidly appears on the cell surface after activation, mediating tethering and rolling through interactions with leukocyte ligands.

Ligands and glycosylation: Interactions involve sialylated Lewis X–containing epitopes on leukocytes. PSGL-1 is a principal ligand for P-selectin, while other ligands for L- and E-selectin include mucin-type glycoproteins such as CD34, GlyCAM-1, and ESL-1, often requiring specific glycosylation to present the active epitopes.

Physiological and clinical relevance: Selectins mediate leukocyte recruitment during inflammation and immune surveillance and can influence tumor cell adhesion and metastasis in certain cancers. Therapeutic strategies targeting selectin–ligand interactions, including antibodies and small molecules, have been explored in inflammatory diseases and oncology, though clinical approval remains limited.