What does CD antigens mean in immunology?
CD stands for "cluster of differentiation".
In the early 1980s, as many research groups were discovering more and more antibodies that could sometimes recognize the same antigen on human leukocytes, the scientific community decided to establish a standard for greater clarity.
The nomenclature, still in use today, was established at the 1st International Workshop and Conference on Human Leukocyte Differentiation Antigens (HLDA) in Paris (1). It was decided to assign a CD number to a group of monoclonal antibodies that recognize a membrane protein on immune cells. The usage then evolved to use the abbreviation CD for the transmembrane receptors themselves.
How many clusters of differentiation are there?
To our knowledge, as of today, the last known CD is CD371 or CLEC12A, a C-type lectin-like receptors (CTLR) localized on NK cells (2). This CD was added to the list after the last edition of HLDA conference in 2014.
But in reality, more than 400 names have been assigned since the HLDA nomenclature permits to use:
- A lowercase “w” preceding the number designation. It indicates that the CD designation is provisional, since there’s an insufficiently amount of validated information about the monoclonal or the CD antigen (CDw-).
- An uppercase letter following a CD number to designate a variant of the extracellular domain of the membrane protein. For example, the three exons of the primary transcripts of CD45 are alternatively spliced to generate up to eight different mature CD45 surface molecule (CD45RA, CD45RB, CD45RC, CD45RAB, CD45RAC, CD45RBC, CD45RO, CD45R).
- A lowercase letter following the CD number (e.g., CD1a, CD1b, CD1c, CD1d, or CD1e) to indicate several surface molecules that share a common chain, but with a different glycosylation profile.
What is the use of cluster of differentiation?
Based on the previous studies of Duque and Braylan in 1991, the works of Knowles and Willman in 1992, Richard Clatch finally set up a multiparametric method, called Immunophenotyping (Clatch, 2001). In fact, thanks to the labelling of cells with monoclonal antibodies and the use of a flow cytometer, we can detect a specific lineage with a particular differentiation state, on a per-cell basis.
Firstly, CD markers allow us to identify and isolate specific immune cells.
For instance, CD19 cell surface protein member is of great interest since CD19 is Pan-B cell marker (meaning this marker is ubiquitously expressed on all B cell lineages whatever their maturity stage) and exclusive to B lymphocyte cell type. B-lymphocytes can so be easily purify and isolate using CD19-coupled magnetic beads for instance.
Secondly, CD markers indicate a certain state of differentiation of the different immune cells, which allows the monitoring of the immunogenicity.
For example, B-cell development begins in the bone marrow from B220+ Pro B cells to immature B cells. Immature B cells then enter the transition phase where they become T1/T2/T3 B cells with a different phenotype (CD27-CD38+CD24+CD10+) and prevent autoantibody production. Selected non-self-reactive B cells first become naive B cells, then DN1/DN2 double-negative cells (IgD-CD27-) and finally differentiate into plasma cells (CD138+ antibody-secreting cells). All other B cell populations (memory, regulatory Breg, regulatory PCreg, marginal B1 cells, follicular B2 cells...) also show their own phenotype and other CD markers to further subdivide these various populations, and provide us with a concrete picture of the immune response.
Finally, a CD marker can present a diagnostic or therapeutic interest.
Lymphomas are cancers of white blood cells, either B cells or T cells. But since CD20 is an antigen that is found on the surface of B cells but not T cells, this marker helps distinguish diffuse large B-cell lymphoma (DLBCL) from anaplastic large cell T-lymphoma (ALCL). Upon the correct diagnostic, the appropriate treatment can then be administrated to treat DLBCL (anti-CD20 Rituximab monoclonal antibody for instance).
Is it difficult to generate a monoclonal antibody targeting cluster of differentiation?
CD molecules are membrane receptors important for immune cell function. They are found on the cell surface and can be entirely extracellular or transmembrane.
While it may seem easy to recognize an extracellular loop by injecting the dedicated peptide epitope, acting on a cell membrane glycoprotein is another matter.
The production of a functional antibody against a transmembrane receptor can be achieved by the unique combination of presenting the native conformation of the antigen to the immune system, excluding irrelevant plasma cell populations and isolating rare clones secreting the antibody of interest.
This was achieved by SYnabs by performing DNA immunization followed by syngeneic whole-cell immunization in proprietary LOU rat species.
We generated LO-CD2a (BTI-322), a functional rat human anti-CD2, which has been shown to have potential use as a T-cell depleting agent (3). Under license from Medimmune, the human version of LO-CD2a was named MEDI-507 and is now known as Siplizumab.
In parallel, SYnAbs has generated LO-TACT, a therapeutic rat anti-human CD25 (IL2Ra) depleting antibody, already chimerized for further clinical trials in human.
(1) The international workshops and conferences on human leukocyte differentiation antigens. Birth, current status and future. Boumsell L. Tissue Antigens. 1996 Oct; 48(4 Pt 1):238-41.
(2) Marshall AS, Willment JA, Pyz E, Dennehy KM, Reid DM, Dri P, Gordon S, Wong SY, Brown GD. Human MICL (CLEC12A) is differentially glycosylated and is down-regulated following cellular activation. Eur J Immunol. 2006 Aug;36(8):2159-69. doi: 10.1002/eji.200535628. PMID: 16838277.
(3) XU, Y., KOLBER-SIMONDS, D., HOPE, J.A., BAZIN, H., LATINNE, D., MONROY, R., WHITE-SCHARF, M.E. and SCHUURMAN, H.-J. (2004), The anti-CD2 monoclonal antibody BTI-322 generates unresponsiveness by activation-associated T cell depletion. Clinical & Experimental Immunology, 138: 476-483. https://doi.org/10.1111/j.1365-2249.2004.02650.x