CD138

CD138, also known as syndecan-1, is a small cell-surface proteoglycan encoded by the SDC1 gene. It is a member of the syndecan family of heparan sulfate proteoglycans and functions as a type I transmembrane protein. The extracellular domain carries glycosaminoglycan chains, primarily heparan sulfate, which can bind a range of ligands including extracellular matrix components and growth factors. This enables CD138 to participate in cell–matrix adhesion and signal modulation.

CD138 is most prominently expressed on mature plasma cells and is widely used as a marker to identify plasma cells in bone marrow and other tissues. In normal tissues, expression on non-plasma cells is limited and context-dependent, including some epithelial and secretory cells. The ectodomain can be proteolytically shed, generating soluble CD138 that circulates in serum.

In clinical practice, CD138 is used in immunophenotyping by flow cytometry and in immunohistochemistry to detect plasma cells and diagnose plasma cell disorders such as multiple myeloma and plasmacytomas. Soluble CD138 levels in serum correlate with tumor burden and can have prognostic value, with higher levels associated with more advanced disease and poorer outcomes in multiple myeloma.

CD138 is a target of therapeutic development for plasma cell neoplasms. Several CD138-targeted therapies, including antibody-drug conjugates, have been investigated in clinical trials, reflecting ongoing interest in exploiting syndecan-1 as a therapeutic axis in multiple myeloma and related disorders.