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™ BV650 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 650-nm. BD Horizon BV650 can be excited by the violet laser and detected in a filter used to detect APC-like dyes (eg, 660/20-nm filter). Due to the excitation and emission characteristics of the acceptor dye, there will be spillover into the APC and Alexa Fluor® 700 detectors. However, the spillover can be corrected through compensation as with any other dye combination.