Review of NKG2D function and its related ligands: review article

The natural killer group 2D (NKG2D) is a transmembrane protein and a member of the CD94/NKG2 family of C-type lectin-like receptors. NKG2D is encoded by the KLRK1 gene, which is located in the NK-gene complex (NKC) placed on chromosomes 6 and 12 in mice and humans, respectively. NKG2D forms a homodimer structure and binds through ectodomains with its related ligands. Each of its monomers consists of two β-sheets, two α-helices, and four disulfide bands and also contains a β-strand that distinguishes it from other C-type lectin-like receptors. NKG2D ligands are homologs of major histocompatibility complex (MHC) class I molecules in mice and humans. MHC class I chain-related protein A (MICA) and B (MICB) and human cytomegalovirus UL16-binding proteins (ULBP1-6) are recognized by the human NKG2D. In Natural Killer (NK) cells, NKG2D-mediated cytotoxicity can be elicited via two different systems by signaling from immunoreceptor tyrosine-based activation motifs in DAP12 or via a Syk-independent pathway activated by DAP10. Therefore, NKG2D is an activating immunoreceptor which was first recognized on NK cells but subsequently found on γδT cells, CD8 αβT cells, and macrophages. NKG2D-ligand diversity may facilitate the detection of the presence of a broad range of viruses and may provide protection against rapidly evolving cancers. NKG2D ligand recognition induces and/or improves immune responses to cancer cells. NK cells recognize a wide range of stressed cells. The activation of NKG2D receptor can lead to the lysis of the target cell and the production of various cytokines and chemokines depending on the nature of the stimulation as a result of NK and myeloid-mediated innate immunity and as well as T γδ and CD8 mediated-adaptive immune system. However, inappropriate expression of NKG2D ligands could cause autoimmune diseases in healthy cells, including rheumatoid arthritis, colitis, celiac disease, multiple sclerosis, alopecia areata, type 1 diabetes, and chronic obstructive pulmonary disease. Therefore, a precise understanding of the structure and function of NKG2D receptor and its interaction with various ligands may lead to the development of strategies to treat autoimmune diseases. Hence, the purpose of this review is to examine the detailed studies on the function of NKG2D receptor and their related ligands.