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™ BUV563 which is part of the BD Horizon Brilliant™ Ultraviolet family of dyes. This dye is a tandem fluorochrome of BD Horizon BUV395 which has an Ex Max of 348 nm and an acceptor dye. The tandem has an Em Max at 563 nm. BD Horizon BUV563 can be excited by the 355 nm ultraviolet laser. On instruments with a 561 nm Yellow-Green laser, the recommended bandpass filter is 585/15 nm with a 535 nm long pass to minimize laser light leakage. When BD Horizon BUV563 is used with an instrument that does not have a 561 nm laser, a 560/40 nm filter with a 535 nm long pass may be more optimal. Due to the excitation and emission characteristics of the acceptor dye, there may be spillover into the PE and PE-CF594 detectors. However, the spillover can be corrected through compensation as with any other dye combination.