CD30, also known as TNFRSF8, is a cell membrane protein of the tumor necrosis factor receptor family, which regulates proliferation/apoptosis and antibody responses. CD30 is expressed by activated, but not by resting, T and B cells. Aberrant expression of CD30 by mastocytosis mast cells and interaction with its ligand CD30L (CD153) appears to play an important role in the pathogenesis and clinical presentation of systemic mastocytosis. CD30 has been considered as a specific diagnostic biomarker of anaplastic large cell lymphoma (ALCL) and classical Hodgkin lymphoma (cHL). CD30 is also a biomarker used for targeted therapy by an antibody–drug conjugate. Soluble CD30 retains the ability to bind CD30 Ligand and functions as an inhibitor of normal CD30 signaling.
Platelet endothelial cell adhesion molecule (PECAM-1, CD31) is a type I transmembrane glycoprotein adhesion molecule in the immunoglobulin superfamily. PECAM-1 is concentrated at cell junctions and is required for transendothelial migration (TEM). The extracellular domain (ECD) of PECAM-1 has ten potential N-linked glycosylation sites and six C2-type Ig-like domains, the first of which is critical for adhesion and extravasation. The cytoplasmic domain contains immunoregulatory tyrosine-based inhibitory and switch motifs (ITIM, ITSM) that mediate both inhibition and activation via phosphotyrosine-mediated engagement of SH2-containing signaling molecules. Expression is restricted to cells involved in circulation, especially endothelial cells, platelets, monocytes, neutrophils and lymphocyte subsets. PECAM-1 participates with other adhesion molecules in some functions, but is the critical molecule for TEM. Homotypic PECAM-1 adhesion in trans, combined with cycling of PECAM-1 to and from surface-connected endothelial cell vesicles, leads leukocytes across endothelial tight junctions.
Low affinity immunoglobulin gamma Fc region receptor II (CD32B) is a single-pass type I membrane protein and contains 2 Ig-like C2-type (immunoglobulin-like) domains. The inhibitory CD32B is expressed on B cells and myeloid dendritic cells. Ligation of CD32B on B cells downregulates antibody production and may, in some circumstances, promote apoptosis. Co-ligation of CD32B on dendritic cells inhibits maturation and blocks cell activation. CD32B may also be a target formonoclonal antibody therapy for malignancies.
Receptors for Fc portion of IgG (Fcγ Rs) are members of the Ig superfamily, and are divided into three classes designated Fcγ RI (CD64), Fcγ RII (CD32), and Fcγ RIII (CD16). CD32 protein is a low affinity receptor for IgG that binds only IgG immune complexes and is expressed on a diverse range of cells such as monocytes, macrophages, neutrophils, eosinophils, platelets, and B cells. Human CD32 class is encoded by three closely related genes, and designated Fcγ RII A, B, and C which share 94-99% amino acid identity in their extracellular domains but differ substantially in their transmembrane and cytoplasmic domains. CD32 is involved in a number of immune responses including antibody-dependent cell-mediated cytotoxicity, clearance of immune complexes, release of inflammatory mediators, and regulation of antibody production.
Cluster of Differentiation 34 (CD34) is a member of a family of single-pass transmembrane sialomucin proteins, and may function as a cell-cell adhesion factor. CD34 protein is selectively expressed on hematopoietic progenitor cells and the small vessel endothelium of a variety of tissues. It has been widely used as a stem and progenitor cell marker, and clinical CD34+ stem cell transplantation (CD34+SCT) has been performed for tumor purging. CD34 monoclonal antibodies are widely used to identify and isolate hemopoietic progenitors and to classify acute and chronic leukemias.
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