A Guide to T Cell Markers

Extensive research on the mammalian immune system as well as the growing success of immunotherapies have deepened our understanding of the complexities of T lymphocytes. We now know that T lymphocytes, or T cells, comprise many subsets and can be categorized by certain properties. This article will review phenotypic markers that have been used to identify various T cell types. In particular, we will focus on proteins that define function, such as helper, cytotoxic, or regulatory, as well as markers for resolving levels of T cell differentiation into varying memory and effector states. 

The function and identity of a given T cell can be broadly determined by its cell surface proteins, beginning in early development. Originating from the bone marrow, T lymphocyte precursors migrate to the thymus where they mature and undergo further selection based on receptor activity. These developing thymocytes undergo gene rearrangement within the beta locus of the T cell receptor (TCR). Those that successfully express a complete alpha-beta TCR complex move on to become double-positive thymocytes that express both CD4 and CD8. Undergoing positive selection, cells with an affinity for MHC class II will develop into the CD4+ subset of T cells, while an affinity for MHC class I leads to CD8+ T cells.

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Finally, T cells that are too strongly reactive to antigens from normal tissues of the body are removed in a process called negative selection. The majority of new naïve, unactivated cells that successfully leave the thymus can be broadly sorted into two groups, the CD4+ helper T cells and CD8+ cytotoxic T cells, each having their own set of subpopulations. 

Helper T Cells

CD4+ T cells, also known as T helper (Th) cells, mainly support other immune cells via signaling. Upon activation, Th cells can be further segregated into subtypes with distinct roles that are largely determined by their secreted cytokines. Th1 secretes pro-inflammatory cytokines, including IFN gamma, TNF alpha, and IL-2. These cells direct cell-mediated immunity to intracellular pathogens (such as bacteria and viruses) and promote inflammation. Th2 cells are known for the defense against extracellular parasites and in promoting differentiation of plasma B cells and dendritic cells. This subset secretes IL-4, IL-5, IL-9, and IL-10. Th17 secretes a number of cytokines, including IL-21, IL-22, and IL-25. Most notably, it secretes IL-17, which promotes defense against extracellular pathogens, including bacteria, fungi, and eukaryotes. Th22 produces IL-22, believed to help keep gut bacterial populations in check and help protect against chronic inflammation. Th22 cells are also implicated in epidermal immunity and remodeling. Th9, a more recently discovered subpopulation, has been described to secrete IL-9, promote mucus production, activate mast cells, and play a role in autoimmune and inflammatory diseases. An additional subset, Tfh (follicular helper T cells), promotes antigen-specific development of B cells and secretes IL-21. In addition to cytokines, the T helper cell subsets can also be characterized by distinct surface protein markers and transcription factors.

Highlighted helper T cell marker antibodies: CD4 antibodies, CXCR3 antibodies (Th1), CCR4 antibodies (Th2), CCR6 antibodies (Th17), CCR10 antibodies (Th22)

Image: A simplified summary of T cell subtypes, including the spectrum of differentiated T cells following activation. Subtypes are displayed for CD4+ (green box), CD8+ (red box), and uncommon T cell types (grey box).

Cytotoxic T Cells

Mature CD8+ T cells are known as cytotoxic T cells (Tc) for their role in recognizing infected, damaged, or cancerous somatic cells and triggering death pathways via cytotoxic proteins. Their activation is mediated by the receptors TCR, CD8, and CD28, generating mature cells that can be further grouped into distinct Tc subsets. As with T helper cells, each subset can be defined by surface proteins, transcription factors, and produced cytokines. The subsets and relevant markers include: Tc1 (TNF alpha, INF gamma, IL-2 CXCR3, TBX21); Tc2 (IL-4, IL-5, CCR4, GATA3); Tc9 (IL-9, IL-10, IRF4); and Tc17 (CCR6, KLRB1, IL-17, IRF4, RORC).

Highlighted cytotoxic T cell marker antibodies: CD8 antibodies, TNF alpha antibodies, TBX21 antibodies

Regulatory T Cells

A distinct population of T cells functions in regulating or suppressing immune responses. These regulatory T cells, or Tregs, secrete anti-inflammatory cytokines, express inhibitory proteins, and induce apoptosis of effector T cells by cytokine deprivation. First thought to be a subtype limited to CD4-expressing T cells, the Treg classification is now believed to be applicable to CD8+ cells as well. The defining markers are FOXP3 and IL2RA (CD25). Other notably expressed proteins include STAT5A, CTLA4, and the cytokines IL-10 and TGF beta.

Highlighted regulatory T cell marker antibodies: FOXP3 antibodies, CD25 antibodies

Naïve, Memory and Effector T cells

CD4+ and CD8+ T cells exist in a naïve state until their activation, upon which they expand and differentiate into a population of effector and memory T cells. Effectors are the more numerous, yet short-lived cells that enact the T cell response. Meanwhile, a small population of cells are retained as longer-lasting memory cells in order to quickly build secondary antigen-specific immune responses. Naïve T cells can be readily determined by the selective expression of the CD45 (PTPRC) protein isoform, CD45RA. An alternative isoform, CD45RO, indicates an activated T cell population and is expressed on memory T cells. 

The activation of naïve cells leads to different memory/effector subsets with gradual changes in their phenotype that can be further categorized. The differentiation spectrum ranges from most naïve, including recent thymic emigrants (RTEs), stem cell memory T cells (Tscm), central memory T cells (Tcm), effector memory T cells (Tem), and effector T cells (Teff), which are the most terminally differentiated. Phenotypic markers have been described for these subtypes, such as PECAM1, CD103 (naïve, RTEs); CCR7, CD127, CD62L (Tscm, Tcm); IL2RA (Tcm), HLA-DR, CCR5, TBX21, GZMA (Tem, Teff). A distinct subset known as tissue-resident memory cells (Trm) are described as non-circulating and can be identified by the markers CD69, ITGAE (CD103) and CTLA4 (CD152). It is important to note that markers can be detected on more than one cell type, but at varying expression levels.

Highlighted naïve, effector and memory T cell marker antibodies: CD45RA antibodies, CD45RO antibodies, CCR7 antibodies, CD38 antibodies

Other T cell types

Less common T cell types exist apart from the ones mentioned above. The natural killer T cell (NKT) comprises about 1% of T cells in the peripheral blood and are defined by their selective recognition of the antigen-presenting CD1d molecule (as opposed to MHC molecules). NKT cell markers include NCAM1 (CD56) and CD16. 

The majority of circulating T cells express the αβ-T cell receptor, made up of the alpha and beta TCR chains. A rare population, known as gamma delta T cells, have been found to express a distinct TCR isoform comprising of gamma and delta type chains. 

Table of T Cell Markers

The table lists common defining proteins for characterizing various subtypes of T cells. These markers are centered on cell surface receptors, transcription factors, and produced cytokines. Accompanying each entry are product catalogs of relevant antibodies and ELISA kits. These tools for immunodetection are routinely used in cell characterization studies via flow cytometry and immunostaining. The listings are sourced by an assortment of reagent suppliers, providing an extensive pool of products from which to compare and choose. 

Note: *Some proteins are protein isoforms or multi-subunit protein complexes composed of several distinct genes. Information on Protein Type, Localization, and Size (kDa) obtained from UniProt.org.

References

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