OX2

OX2, also known as the OX-2 antigen, is a cell surface glycoprotein first identified by monoclonal antibody recognition. In humans the protein is encoded by the CD200 gene and is commonly referred to by the protein name CD200. It belongs to the immunoglobulin superfamily and is a single-pass transmembrane protein with extracellular Ig-like domains.

Expression and distribution: OX2/CD200 is highly expressed in the brain, particularly on neurons, and is also found in the placenta and on various other cell types including some endothelial and epithelial cells and subsets of hematopoietic cells. Its receptor, CD200R, is primarily expressed on cells of the myeloid lineage such as macrophages and dendritic cells, as well as on some T cells and natural killer cells.

Function: The interaction of CD200 with CD200R delivers inhibitory signals that dampen immune responses, contributing to immune tolerance and the regulation of inflammatory processes. In the central nervous system, CD200–CD200R signaling helps restrain microglial activation and neuroinflammation.

Clinical significance: Altered CD200 expression can influence disease outcomes. Many tumors express CD200 to evade immune surveillance by engaging CD200R and suppressing anti-tumor immunity. Conversely, agonists that activate the CD200–CD200R pathway are explored as potential therapies for autoimmune and inflammatory conditions, while blockade of this signaling is investigated to enhance anti-tumor responses. CD200 is studied as a biomarker in certain cancers and neurodegenerative disorders, though results are context-dependent.

History: The OX-2 antigen was described in the late 20th century through studies using the OX-2 monoclonal antibody, and the human homolog is CD200, now a focus of immunology and neuroimmunology research.

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