The 509F6 monoclonal antibody specifically binds to Fc receptor-like 5 (FCLR5), a type I transmembrane glycoprotein that is also known as CD307e, FCRH5 (Fc receptor homolog 5) and IRTA2 (immunoglobulin superfamily receptor translocation associated 2). FCRL5 (CD307e) is a member of the Immunoglobulin (Ig) gene superfamily with 9 extracellular Ig domains, an intracellular immunoreceptor tyrosine-based activating motif (ITAM)-like sequence and two intracellular immunoreceptor tyrosine-based inhibitory motif (ITIM)-like sequences. Like other Fc receptor-like proteins, FCRL5 (CD307e) is homologous to FcγR1 (CD64) and binds to immunoglobulins, specifically the IgG isotypes, and it may play a role in the regulatory interactions of immune complexes with B lymphocytes. FCRL5 (CD307e) is differentially expressed in the B cell lineage, with little expression on peripheral blood cells and upregulated expression on plasma cells and some leukemias and lymphomas. mAb 509F6 binds to an epitope within the first 3 Ig domains of FCRL5 and blocks FCRL5's interaction with immune complexes.
The antibody was conjugated to BD Horizon™ BUV661 which is part of the BD Horizon Brilliant™ Ultraviolet family of dyes. This dye is a tandem fluorochrome of BD Horizon BUV395 with an Ex Max of 348-nm and an acceptor dye with an Em Max at 661-nm. BD Horizon Brilliant BUV661 can be excited by the ultraviolet laser (355 nm) and detected with a 670/25 filter and a 630 nm LP. Due to cross laser excitation of this dye, there may be significant spillover into channels detecting APC-like emissions (eg, 670/25-nm filter).
Due to spectral differences between labeled cells and beads, using BD™ CompBeads can result in incorrect spillover values when used with BD Horizon BUV661 reagents. Therefore, the use of BD CompBeads or BD CompBeads Plus to determine spillover values for these reagents is not recommended. Different BUV661 reagents (eg, CD4 vs. CD45) can have slightly different fluorescence spillover therefore, it may also be necessary to use clone-specific compensation controls when using these reagents.