Human CD200R1 Fluorescein-conjugated Antibody

CD200 R1, also known as OX-2 receptor, is a 90 kDa transmembrane protein in the immunoglobulin superfamily (1‑3). The standard human CD200 R1 cDNA encodes a 325 amino acid (aa) precursor that includes a 28 aa signal sequence, a 215 aa extracellular domain (ECD), a 21 aa transmembrane segment, and a 61 aa cytoplasmic domain. The ECD is composed of one Ig-like V-type domain and one Ig-like C2-type domain (4). Within the ECD, human CD200 R1 shares 56% aa sequence identity with mouse and rat CD200 R1. Alternate splicing of the human CD200 R1 mRNA generates four isoforms, two of which are truncated in the Ig-C2 domain and are likely secreted. The protein expressed here contains a mature region that is identical to that of the standard form. There is an N-terminal extension of 25 aa that, in the standard form, is part of the signal sequence. In human, a separate CD200 R12 gene encodes a protein that shares 81% ECD aa identity with CD200 R11. In mouse, at least four genes for CD200 R1-like molecules have been described (4‑6). CD200 R1 expression is restricted primarily to mast cells, basophils, macrophages, and dendritic cells (7‑9), while its ligand, CD200, is widely distributed (10). Disruption of this receptor-ligand system by knockout of the CD200 gene in mice leads to increased macrophage number and activation and predisposition to autoimmune disorders (11). Association of CD200 with CD200 R1 takes place between their respective N-terminal Ig-like domains (12). The capacity of CD200 R1-like molecules to interact with CD200 is controversial (5, 13). CD200 R1 propagates inhibitory signals despite its lacking a cytoplasmic ITIM (immunoreceptor tyrosine-based inhibitory motif) (8, 9, 14, 15) CD200 R1-like molecules, in contrast, are potentially activating receptors by means of their association with DAP12 (4, 6).