CD63 is a widely studied protein marker in biology and medicine, found both on the surface and inside many cell types, including immune cells and platelets. This molecule serves as a reliable indicator of cellular activity, allowing scientists to monitor events like activation and the release of tiny communication packages. Understanding CD63 is central to identifying certain cell states and is increasingly relevant to advanced medical diagnostics.
What Is CD63? A Structural Overview
CD63 belongs to the tetraspanin superfamily of proteins, also known as TSPAN30, characterized by a distinct structural motif. This protein is a multi-pass transmembrane molecule that spans the cell membrane four times. This structure creates four distinct domains: a short N-terminal and C-terminal tail within the cell, and two extracellular loops exposed outside the cell.
The most prominent feature is the large second extracellular loop, which is highly accessible and allows CD63 to interact with other molecules. Tetraspanin proteins cluster together with other membrane proteins, such as integrins, to form dynamic microdomains known as “tetraspanin webs.” These webs organize the cell surface and mediate signal transduction events involved in cell motility and adhesion. Within a resting cell, CD63 is primarily located in internal storage compartments, specifically late endosomes, lysosomes, and multivesicular bodies.
Role in Cellular Activation and Degranulation
The primary function of CD63 is to act as a dynamic marker for cellular activation, involving the rapid movement of the protein from the cell’s interior to its surface. In immune cells like mast cells and basophils, CD63 is stored in the membranes of secretory granules containing inflammatory mediators like histamine. When the cell is triggered, these internal granules fuse with the outer cell membrane, a process called degranulation.
This fusion immediately exposes the stored CD63 to the cell exterior, indicating that the cell has recently activated and released its contents. Similarly, in platelets—small blood components involved in clotting—CD63 is located within alpha-granules. Upon activation, these granules release their contents, and CD63 is translocated to the platelet surface, where its expression is measured to quantify the degree of activation. Surface CD63 is frequently used in flow cytometry to detect this activity in both clinical and experimental settings.
CD63 as a Key Marker for Extracellular Vesicles
CD63 is a recognized marker for identifying a specific type of extracellular vesicle (EV) known as an exosome. EVs are tiny, membrane-bound particles released by almost all cell types, serving as a means of cell-to-cell communication. Exosomes, typically ranging from 30 to 150 nanometers in diameter, originate from internal compartments called multivesicular bodies (MVBs).
The enrichment of CD63 on the exosomal membrane is a direct consequence of its location within these MVBs. As the MVB matures, inward budding forms small intraluminal vesicles, which are the precursors to exosomes. Since CD63 is concentrated in the MVB membrane, it is naturally incorporated into these forming vesicles. When the MVB fuses with the outer cell membrane and releases its contents, the exosomes are secreted into the extracellular space with CD63 decorating their surface. This consistent presence makes CD63 a standard marker for identifying and isolating exosomes from complex biological fluids.
Diagnostic Potential in Health and Disease
The measurement of CD63 has significant practical applications in clinical diagnostics, particularly through the analysis of extracellular vesicles. Since CD63 is enriched on exosomes, measuring the amount of CD63-positive vesicles in bodily fluids provides a quantifiable assessment of cellular activity and disease state. This is often performed as part of a liquid biopsy, a non-invasive technique that samples biological fluids like blood or urine to detect disease biomarkers.
In inflammatory and allergic conditions, the level of CD63 on basophils is directly measured to diagnose and monitor allergic reactions, as it indicates recent mast cell and basophil degranulation. CD63-positive exosomes are studied in cancer research, as tumor cells often release high amounts of these vesicles. These tumor-derived exosomes carry cancer-specific cargo, and elevated levels of CD63-positive EVs have been observed in patients with cancers like pancreatic ductal adenocarcinoma and melanoma. Detecting and quantifying these vesicles, often using techniques like Enzyme-Linked Immunosorbent Assay (ELISA) or flow cytometry, allows researchers to assess tumor burden and predict disease progression.