CD3 (Cluster of differentiation 3), originally known as T3, is one of the first groups of human T lymphocyte surface antigens recognized by monoclonal antibodies. It has been reported that antibodies against CD3 could either stimulate T cells to divide or inhibit the progress of effector functions according to the conditions used. It plays an important role in T cell and the CD3 T-cell co-receptor helps to activate the cytotoxic T-Cell and T helper cells. The CD3 molecule is composed of a protein complex and consists of four distinct chains. The complex of mammals contains a CD3γ chain, a CD3δ chain, and two CD3ε chains. All these chains associate with T-cell receptor and the ζ-chain to generate an activation signal in T lymphocytes.
ICOS (CD278), a member of the CD28 superfamily of costimulatory molecules, is a type I transmembrane glycoprotein expressed predominantly on activated T lymphocytes. Structurally, it comprises an extracellular immunoglobulin (Ig)-like variable (V) domain and a constant (C) domain forming a characteristic Ig-fold tertiary structure, followed by a transmembrane helix and a cytoplasmic tail containing conserved tyrosine-based signaling motifs. This configuration facilitates ligand binding through its extracellular V domain and initiates intracellular signaling via phosphoinositide 3-kinase (PI3K) recruitment upon engagement with ICOS ligand (ICOS-L) on antigen-presenting cells. Functionally, CD278 mediates critical costimulatory signals for T-cell activation and differentiation, particularly in follicular helper T (Tfh) cells and regulatory T (Treg) cells. Its interaction with ICOS-L potentiates IL-4, IL-10, and IL-21 production, driving germinal center formation and antibody class-switching.
CEA (Carcinoembryonic antigen) is a type of glycoprotein attached to the cell membrane via glycosylphosphatidylinositol (GPI) anchor and stands for a set of highly related substance implicated in cell adhesion. It belongs to the immunoglobulin superfamily (IgSF) and there are two immunoglobulin domains including an N-terminal domain of 108 amino acids homologous to IgV-like and six domains homologous to the IgC2-like. The CAE gene family in humans is clustered on chromosome 19q and comprises 29 genes, 18 of which are commonly found. CEA is generally expressed in gastrointestinal tissue during embryonic development, but produced in very small amounts after birth. The serum level of it is not raised unless certain cancers tested. The CEA has become one of the most widely used tumor markers in clinical diagnosis and its main application is in gastrointestinal cancers, particularly in colorectal malignancy.