CD Antigens

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CD Antigens

Peter J Delves, University College London, London, UK

Published online: January 2016

DOI: 10.1002/9780470015902.a0000924.pub2

Full Article on Wiley Online Library

Abstract
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Abstract

CD (clusters of differentiation) antigens are molecules originally defined as being present on the cell surface of leucocytes and recognised by specific antibody molecules, but now including some intracellular molecules and molecules present on cells other than leucocytes. Such molecules are assigned a CD number following a consensus reached at regularly held international workshops. CD antigens include a large number of cell surface receptors, signalling molecules and adhesion molecules. Their identification is helpful in defining different populations and subpopulations of cells, particularly those of the immune system, in health and disease. Analysis of the surface phenotype of cells can be carried out on cell suspensions or using tissue sections. Methods are available for the enrichment or depletion of cells bearing a particular CD antigen, for example, from a mixed population of peripheral blood cells. CD antigens can be targeted by monoclonal antibodies for the treatment of tumours and autoimmune disease and to help limit graft rejection following transplantation.

Key Concepts

CD antigens are molecules that are mostly, but not exclusively, present on cell surfaces.
They are defined using panels of monoclonal antibodies.
CD designations are assigned at international workshops held every few years.
There are over 300 different CD antigens characterised.
They are mostly detected for the identification of populations and subpopulations of immune system cells.
In some cases, they also have utility in the identification of tumours.
Antibodies targeting CD antigens are used clinically in the treatment of tumours and autoimmune diseases and to limit the rejection of transplanted organs.

Keywords: leucocyte; cell surface; flow cytometry; antigen; monoclonal antibody

	Figure 1. Binding of five monoclonal antibodies (mAb1–mAb5) to CD antigens on the surface of a cell. If one of the antibodies is labelled, for example, with a fluorescent dye (fluorochrome), the ability of the other antibodies to block binding of the labelled mAb can be assessed. In the example given mAb1 and mAb4 cross‐block each other suggesting that they recognise the same antigen, whereas there is no inhibition of binding of the other three mAbs.
	Figure 2. Cell surface expression of CD4 and CD8 changes as early T cells develop in the thymus. Cells that are destined to become T cells initially develop in the bone marrow and then travel to the thymus. After entering the thymus, these cells lack cell surface expression of both the CD4 and CD8 molecules. They are therefore described as CD4⁻CD8⁻ cells. As they mature in the thymus, they go through a developmental stage during which they express both of these molecules, that is, become CD4⁺CD8⁺ T cells. Upon gaining full maturity, they lose expression of one or other of these CD antigens to become either CD4⁺CD8⁻ cells (usually referred to as simply CD4⁺ T cells or just CD4 T cells) or CD4⁻CD8⁺ cells (CD8⁺ T cells, CD8 T cells) before leaving the thymus. Note that there will be many thousands of CD4 and/or CD8 molecules on the surface of a single cell, not just the one molecule shown diagrammatically, and that there will also be many other molecules on the cell surface, including other CD antigens such as CD3 and CD5 (cf. Table).
	Figure 3. Diagrammatic representation of the typical course of HIV infection, characterised by a rapid decline in the number of circulating CD4⁺ T cells (CD4 count). A few weeks after initial infection with HIV the CD4⁺, T‐cell count recovers somewhat but to a lower level than before infection. No symptoms are apparent but a slow depletion of CD4⁺ T cells is occuring. Eventually, CD4⁺ T‐cell depletion is so pronounced (< 400 CD4⁺ T cells mm⁻³ peripheral blood) that immune defences are compromised and resistance to a variety of fungal, bacterial, viral and parasitic infections begins to wane, leading ultimately to progression to the acquired immunodeficiency syndrome (AIDS) once the CD4 count falls below 200 CD4⁺ T cells mm⁻³. Seroconversion refers to the time at which anti‐HIV antibodies (Abs) become detectable in peripheral blood. HIV‐specific cytotoxic T lymphocytes (CTLs), most of which are CD8⁺, also develop early on. Reproduced with permission from Delves PJ, Martin SJ, Burton DR and Roitt IM. Roitt's Essential Immunology. 12th Edition. 2011. © Published Wiley‐Blackwell.
	Figure 4. Six‐parameter flow cytometry optical system for multicolour immunofluorescence analysis. Cell fluorescence excited by the blue laser is divided into green (fluorescein) and orange (phycoerythrin) signals, while fluorescence excited by the orange laser is reflected by a mirror and divided into near red (Texas red) and far red (allophycocyanin) signals. Blue light scattered at small forward angles and at 90° is also measured in this system, providing information on cell size and internal granularity respectively. PMT, photomultiplier tube. Reproduced with permission from Delves PJ, Martin SJ, Burton DR and Roitt IM. Roitt's Essential Immunology. 12th Edition. 2011. © Published Wiley‐Blackwell.
	Figure 5. Cells that have bound labelled monoclonal antibodies (mAb) directed towards cell surface antigens can be isolated from a mixed population of cells using (a) a fluorescence‐activated cell sorter (FACS) if the antibodies have been tagged with fluorescent dyes (fluorochromes) or (b) a magnet if the antibodies have been conjugated to magnetic beads.

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CD Antigens

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Key Concepts

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