Structure and Molecular Characteristics

• CD81, also known as TAPA-1 (Target of Anti-proliferative Antibody 1), is part of the tetraspanin superfamily with a molecular weight of 26 kDa.
• Composed of four transmembrane domains forming two loops and internal N- and C-terminal tails.
• The large extracellular loop (LEL) includes conserved cysteine residues forming disulfide bonds, essential for stability and interaction with other molecules.
• The small extracellular loop (SEL) also plays a role in organizing membrane microdomains, aiding cell functions.

Expression Pattern and Distribution

• Widely expressed in various cell types and tissues, including immune cells (B cells, T cells), hepatocytes, neurons, endothelial cells, and epithelial tissues.
• CD81 levels can change dynamically based on cell activation, stress, or disease conditions.
• Present on exosomes and other extracellular particles, aiding in cell-to-cell communication.

Role in Immune System Function

• Forms a complex with CD19 and CD21, creating a co-receptor that lowers the threshold for B cell activation via the B cell receptor (BCR).
• Crucial for the functioning of antibody-producing cells and humoral immune responses.
• In T cells, CD81 influences T cell receptor signalling, activation, proliferation, and cytokine production.
• Impacts the formation of immune synapse, critical for interactions between immune cells.
• Regulates immune cell migration, antigen presentation, adhesion molecules, and chemokines.

Involvement in Disease Processes

• Acts as a co-receptor for hepatitis C virus, making it a target for anti-viral treatment.
• In cancer biology, CD81 can have oncogenic or tumor suppressor functions depending on the cancer type and cellular microenvironment.
• Involved in cardiovascular diseases by regulating platelet function and vascular cell behaviour.