The term Cluster of Differentiation was coined as a means of classifying the mass of mAbs against leukocyte epitopes. The Cluster of Differentiation (CD) system for classifying antibodies and cell surface molecules, originally on leukocytes, has had a large impact in modern research and drug development, particularly in oncology. Antibodies have been seen as ‘magic bullets’ against a variety of diseases since the late 20th century, with antibody technology really taking off in 1975. Laboratories worldwide began producing monoclonal antibodies (mAbs), to target specific molecules.
Many labs however were preparing antibodies against similar targets and, with no simple way to cross reference these, classification and identification were becoming a problem. To combat this, the international HLDA (Human Leucocyte Differentiation Antigens) workshop was established in Paris in 1982 to ‘comprehend’ which mAbs were duplicates and compare them against each other. This and the following 10 workshops have provided independent validation of an antibody’s specificity and usability, backing the confident use of these life science reagents in research, diagnosis and therapy.
For the workshops, antibodies are submitted by researchers to be blind tested against a range of cell types and undergo IF assays using a fluorescent labelled secondary antibody and FACS analysis. This allows for staining patterns of each antibody against specific cell types to be compared against other submitted antibodies. Following this evaluation, antibodies are assigned a CD number. From this CD1 to CD350 molecules have been characterised within these workshops. Two of the most well-known CD markers are CD4 and CD8, used to identify helper and cytotoxic T cells respectively.
Anti-CD antibodies have a multitude of purposes, and are widely used in differentiating between specific diseases (such as distinguishing a primary lung squamous cell carcinoma (scc) from thymic carcinoma) and aiding in disease treatment. CD labelling originated solely for leukocytes, however, it is now relevant for all immune cells. A CD number is assigned when two mAbs specifically bind to the same surface molecule. Clusters of Differentiation are frequently used as cell markers, to help associate cells with various immune functions or characteristics. Therapeutic mAbs can be used in conjunction and aimed against multiple CD markers upon a target cell, particularly within cancer and immunotherapies. This increases specificity, limiting damage to normal cells. There are several renowned therapeutic mAbs targeting CD markers, such as the 2018 Nobel Prize winners for anti-CTLA4 (CD152) and anti-PD1 (CD279) immune checkpoint therapeutics, which prove effective against various cancers including lung, renal, lymphoma and melanoma.
Ximbio’s portfolio contains many examples of antibodies that target CD markers, including some of the earliest and best-known clones such as anti-CD3 (UCHT1) and anti-CD45RO (UCHL1) . Their inventor, Professor Peter Beverley, was a significant contributor to the organisation of the HLDA workshops. Some other interesting CD antibodies on Ximbio’s portfolio include:
Anti-PDL1 (29E.2A3)/CD274:
Anti-programmed death ligand 1 (PDL1) mAb binds to receptor PD-1 found on activated T cells, B cells, and myeloid cells, delivering a signal which inhibits TCR-mediated activation of IL-2 production and T cell proliferation. Upregulation of PDL1 is thought to assist immune evasion in cancers, with linked to increased tumour aggressiveness and risk of death. Antibodies such as PDL1 inhibitors are in development as immunotherapies, with positive results in clinical trials, providing a novel approach to cancer therapy. Find out more here.
Anti-CD19 (4G7):
This antibody binds CD19, a protein thought to play an active role in the growth of B-cell cancers predominantly via stabilising Myc oncoprotein concentrations. Anti-CD19 experimental therapeutics typically use the presence of CD19 to directly target B-cell cancers, however CD19 downstream signalling is becoming an attractive target. Anti-CD19 mAbs are a novel area of research in the pharmaceutical industry, for treatment of CD-19 positive lymphomas and autoimmune diseases. Take a look here.
Anti-CD68 (KP1):
Anti-CD68 (KP1) mAb recognises a fixation-resistant epitope in a wide variety of tissue macrophages and in granulocyte precursors. CD68 is a 110-kD transmembrane glycoprotein which is highly expressed by human monocytes and tissue macrophages. It is a member of the lysosomal/endosomal-associated membrane glycoprotein (LAMP) family and favours binding to lectins through its heavily glycosylated extracellular domain. This antibody proves useful for studying disorders of the monocyte/macrophage system, including both reactive and neoplastic states. Find out more here.