Background
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MHC class I polypeptide-related sequence B, also known as MICB, is a heavily glycosylated protein serving as a ligandfor the type II receptor NKG2D. MICB is widely expressed with the exception of the central nervous system where it isabsent. MICB is expressed in many, but not all, epithelial tumors of lung, breast, kidney, ovary, prostate and colon. Inhepatocellular carcinomas, it is expressed in tumor cells but not in surrounding non-cancerous tissue. MICB shares85?% amino acid identity with MICA, a closely related protein, both of which contain three extracellular immunoglobulin-like domains, but without capacity to bind peptide or interact with beta-2-microglobulin. Acting as a stress-induced self-antigen, binding of MICB to the NKG2D receptor activates the cytolytic response of natural killer (NK) cells, CD8+αβ Tcells, and γδ T cells on which the receptor is expressed. MICA, a ligand of the activating immunoreceptor NKG2D, isreleased by tumor cells in a soluble form and can be detected in sera of tumor patients at significant levels. MICA/B areminimally expressed on normal cells, but are frequently expressed on epithelial tumors and can be induced by bacterialand viral infections. MICA/B recognition thus is involved in tumor surveillance, viral infections, and autoimmunediseases. Unlike classical MHC class I molecules, MICB does not form a heterodimer with beta-2-microglobulin. It binds as amonomer to a KLRK1 / NKG2D homodimer. KLRK1 forms a complex with HCST / DAP10 in which KLRK1 binds MICBwhile HCST acts as an adapter molecule which enables signal transduction. Receptor-ligand interaction inducesclustering of both proteins in ordered structures called immune synapses and also leads to their intercellular transfer. This is associated with a reduction in the cytotoxicity of KLRK1-expressing cells. MICB binds to human cytomegalovirusglycoprotein UL16 which causes sequestration of MICB in the endoplasmic reticulum and increases resistance toKLRK1-mediated cytotoxicity.
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