The 551 monoclonal antibody specifically recognizes Siglec-H, a type I transmembrane glycoprotein that is encoded by Siglech (Sialic acid binding Ig-like lectin H). Siglec-H contains two immunoglobulin domains in its extracellular region and a cytoplasmic domain that lacks tyrosine-based signaling motifs unlike other CD33-related Siglec-like molecules. Siglec-H is expressed on progenitors of plasmacytoid dendritic cells (pDCs) and depends on DAP12 for cell surface expression and intracellular signaling function. In addition to its expression on plasmacytoid dendritic cells (pDC), Siglec-H may also be expressed by splenic marginal zone macrophages, medullary macrophages in lymph nodes, and microglia. Siglec-H may regulate the production of type I interferons (IFN-I) by pDCs and other cell types. There is no clear human ortholog for mouse Siglec-H.
The antibody was conjugated to BD Horizon BV750 which is part of the BD Horizon Brilliant™ Violet family of dyes. This dye is a tandem fluorochrome of BD Horizon BV421 with an Ex Max of 405-nm and an acceptor dye with an Em Max at 750-nm. BD Horizon Brilliant BV750 can be excited by the violet laser (405 nm) and detected with a 750/30 nm filter with a 740 nm long pass. Due to spectral differences between labeled cells and beads, using BD™ CompBeads can result in incorrect spillover values when used with BD Horizon BV750 reagents. Therefore, the use of BD CompBeads or BD CompBeads Plus to determine spillover values for these reagents is not recommended.