The OKT4 monoclonal antibody specifically recognizes CD4 which is also known as T4 and Leu-3. CD4 is expressed on most thymocytes, T-helper/inducer cells, regulatory T cells (Tregs), and NKT cells. It is also variably expressed on monocytes, macrophages, and some dendritic cells. CD4 is a ~55 kDa type I transmembrane glycoprotein that is encoded by CD4 which belongs to the immunoglobulin superfamily (IgSF). CD4 binds to a nonpolymorphic region of MHC class II and serves as a coreceptor, along with the T cell receptor for antigen (TCR), for the MHC Class II-restricted recognition of antigens on antigen-presenting cells. CD4 also functions as a receptor for human immunodeficiency viruses (HIV). CD4 has four immunoglobulin-like extracellular domains in its extracellular region and a CXCP binding motif for p56Lck in its cytoplasmic domain which regulates the antigen-bound TCR-induced signaling cascade.
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.