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 BUV737 which is part of the BD Horizon Brilliant™ Ultraviolet family of dyes. This dye is a tandem fluorochrome with an Ex Max near 350 nm and an Em Max near 737 nm. BD Horizon Brilliant BUV737 can be excited by the ultraviolet laser (355 nm) and detected with a 740/35 nm filter. Due to the excitation of the acceptor dye by the red laser line, there may be significant spillover into red laser detectors with filters in the 700-720 nm range.