L-Selectin was initially identified in 1983 as a lymph node-homing receptor. It is a type I transmembrane glycoprotein that is expressed on leukocytes and implicated in the recruitment of leukocytes to lymph nodes and inflamed tissues. L-Selectin is encoded by the SELL gene on human and mouse chromosome 1 and is also present in mice. T cells, B cells, and neutrophils are the cell types where the L-Selectin function is most understood. Later research revealed that there are additional L-Selectin ligands, including PSGL-1, which binds P- and E-Selectin as well as MAdCAM-1, which is found in the mesenteric lymph node of the gut mucosa.
Fig.1 Crystal structure of L-Selectin lectin/EGF domains. (PDB)
There are just a few small changes in the amino acid residues in the cytoplasmic tail and extracellular cleavage site between the structures of L-Selectin in humans and mice. Due to variations in glycosylation patterns, the molecular weight of L-Selectin ranges from 65 to 100 kDa depending on the type of cell. While L-Selectin attaches to sulphated sialyl-lewis X (sLex) epitopes on proteins including CD34, podocalyxin, and glycosylation-dependent cell adhesion molecule-1, E- and P-Selectins bind sialyl-lewis X (sLex) epitopes on scaffold proteins like PSGL-1 and CD44 (GlyCAM-1). Cells get tethered to the vasculature and begin to roll due to these interactions.
L-Selectin (CD62L) is made up of an intracellular domain (ICD) at the C-terminus, a transmembrane domain (TMD), which crosses the cell membrane, and an extracellular domain (ECD), which includes the C-type lectin domain at the N-terminus, the epidermal growth factor (EGF)-like domain, two short consensus repeat domains, and the cleavage site. L-Selectin is proteolytically shed by A disintegrin and metalloproteinase 17 (ADAM17).
Fig.2 The structure of transmembrane glycoprotein L-Selectin and enzyme ADAM17. (Reed & Ager, 2022)
Even in the absence of P- or E-Selectin, L-Selectin has the ability to regulate leukocyte homing. In order to protect the host from infection, leukocyte trafficking is a critical step that coordinates the immune response. It involves a number of homing molecules and receptors on leukocytes as well as blood arteries. Naive and central memory cells are traditionally guided to lymph nodes by L-Selectin through high endothelial venules (HEVs). According to recent research, L-Selectin is necessary for the homing of activated CD8+ T cells to lungs that are infected with influenza as well as for the lowering of viral load.
When naive T cells with high L-Selectin levels meet an antigen and become activated, they release L-selectin and transform into effector cells with downregulated L-Selectin expression. After the illness has been treated, these cells continue to grow and become memory cells. Although the process is yet unknown, memory cells are either low L-Selectin level or re-express L-Selectin. Acute inflammation, inflammatory bowel disease (IBD), and cancer are also connected to L-Selectin.
Modulating L-Selectin interactions has been done in a variety of ways. One strategy is to directly prevent L-Selectin and its ligands from interacting. Oligosaccharides as ligand analogs and soluble protein ligands are useful tools to block L-Selectin binding. Additionally, approaches that target processes upstream or downstream, including inhibiting the enzymes necessary for the crucial post-translational modifications of L-Selectin ligands, are potentially intriguing.
Fig.3 L-Selectin on T cells. (Ivetic, et al., 2019)
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