CD Antigens

Key Biological Properties: CD5 is a 67 kDa glycoprotein composed of an extracellular domain with three SRCR domains, a transmembrane region, and a cytoplasmic tail. The extracellular domain is involved in ligand binding, while the cytoplasmic tail is essential for signal transduction.

Expression Patterns: CD5 is predominantly expressed on the surface of T cells, particularly thymocytes and mature T cells. It is also found on a subset of B cells known as B-1a cells.

Tissue Distribution: CD5 is widely distributed in lymphoid tissues, including the thymus, spleen, and lymph nodes. It is also present in peripheral blood and other non-lymphoid tissues.

Primary Biological Functions: CD5 functions as a co-receptor in T cell receptor (TCR) signaling, modulating T cell activation and differentiation. It also plays a role in B cell receptor (BCR) signaling in B-1a cells.

Role in Immune Responses: CD5 is involved in the regulation of immune responses by modulating TCR and BCR signaling thresholds. It helps maintain self-tolerance and prevents autoimmunity by negatively regulating T cell activation.

Pathogen Recognition: CD5 has been implicated in the recognition of pathogen-associated molecular patterns (PAMPs) and contributes to the immune response against infections.

Mechanisms with Other Molecules and Cells: CD5 interacts with various ligands, including CD72 and galectin-1, to modulate immune cell functions. It also forms complexes with other surface receptors, such as the TCR and BCR, to influence signaling pathways.

Binding Partners: CD5 binds to CD72, a C-type lectin expressed on B cells, and galectin-1, a β-galactoside-binding protein. These interactions play a role in immune regulation and cell-cell communication.

Downstream Signaling Cascades: Upon ligand binding, CD5 initiates downstream signaling cascades involving the activation of protein tyrosine kinases (PTKs) and the recruitment of adaptor proteins. This leads to the modulation of TCR and BCR signaling and the regulation of immune cell activation.

Regulatory Mechanisms Controlling Expression and Activity: CD5 expression is regulated at both the transcriptional and post-transcriptional levels. Transcription factors such as NF-κB and AP-1 play a role in the transcriptional regulation of CD5.

Transcriptional Regulation: The CD5 gene promoter contains binding sites for various transcription factors, including NF-κB, AP-1, and Ets family members. These factors regulate CD5 expression in response to different stimuli.

Post-Translational Modifications: CD5 undergoes post-translational modifications, including glycosylation and phosphorylation. These modifications influence its stability, localization, and signaling activity.

Biomedical Research: CD5 is used as a marker for T cells and B-1a cells in immunological studies. It is also employed in research on autoimmune diseases and immune regulation.

Diagnostic Tools: CD5 expression is used as a diagnostic marker in certain hematological malignancies, such as chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL).

Therapeutic Strategies: Targeting CD5 has potential therapeutic applications in autoimmune diseases and cancer. CD5-specific antibodies and inhibitors are being explored as therapeutic agents.

Development: CD5 plays a role in the development and maturation of T cells in the thymus. It is involved in the selection processes that ensure self-tolerance and the generation of a functional T cell repertoire.

Aging: CD5 expression and function may change with age, potentially impacting immune responses and susceptibility to infections and autoimmune diseases.

Disease: Altered CD5 expression and signaling have been associated with various diseases, including autoimmune disorders, infections, and cancers. Understanding the role of CD5 in these conditions may provide insights into disease mechanisms and therapeutic targets.