CD Antigens

Key Biological Properties: CD55 is a 70 kDa membrane protein that attaches to the cell membrane via a glycophosphatidylinositol (GPI) anchor . It contains four complement control protein (CCP) repeats with a single N-linked glycan positioned between CCP1 and CCP2 .

Expression Patterns and Tissue Distribution: CD55 is broadly distributed among hematopoietic and non-hematopoietic cells. It is highly expressed in most epithelial cells, trophoblasts, and alveolar cells, with distinct positivity in connective tissue . It is also a determinant for the Cromer blood group system .

Primary Biological Functions: CD55 regulates the complement system by recognizing C4b and C3b fragments created during the activation of C4 and C3 . It interferes with the conversion of C2 to C2b and factor B to Bb, thereby preventing the formation of C4b2a and C3bBb, the amplification convertases of the complement cascade . This inhibition prevents complement damage to host cells .

Role in Immune Responses and Pathogen Recognition: CD55 plays a critical role in immune responses by preventing the formation of the membrane attack complex, thereby protecting cells from complement-mediated lysis . It is also used as a receptor by some coxsackieviruses and other enteroviruses .

Mechanisms with Other Molecules and Cells: CD55 interacts with cell-associated C4b and C3b polypeptides, interfering with their ability to catalyze the conversion of C2 and factor B to enzymatically active C2a and Bb . This interaction prevents the formation of C4b2a and C3bBb, the amplification convertases of the complement cascade .

Binding Partners and Downstream Signaling Cascades: CD55 modulates Toll-like receptor 9 (TLR9) signaling and supports the survival of marginal zone B cells . It also traffics to the nucleus in certain cancer cells, leading to cisplatin resistance and stemness by inducing PRC2 and H3K27 trimethylation on chromatin .

Regulatory Mechanisms Controlling Expression and Activity: CD55 expression is regulated by transcription factors such as TFCP2 and NF-κB, as well as miRNAs like miR-27a-3p . Post-translational modifications, such as glycosylation, also play a role in its function and localization .

Transcriptional Regulation and Post-Translational Modifications: The promoter activity of CD55 is influenced by TFCP2 and NF-κB binding sites . Glycosylation of CD55 is necessary for its nuclear entry and function in chemoresistant cancer cells .

Biomedical Research: CD55 is used in various scientific applications, including flow cytometry, Western blot, immunohistochemistry, immunocytochemistry, and ELISA .

Diagnostic Tools and Therapeutic Strategies: CD55 antibodies are used to detect its presence in human, mouse, non-human primate, and rat samples . Targeting CD55 has potential therapeutic applications in treating chemoresistant cancers and preventing complement-mediated damage in autoimmune diseases .

Role Throughout the Life Cycle: CD55 plays a role in various stages of life, from development to aging and disease. It is expressed by neurons in response to chronic autoimmune central nervous system inflammation . In cancer, CD55 contributes to chemoresistance and the maintenance of cancer stem cells, which are implicated in tumor recurrence and metastasis .