Immune cell research guide: how to choose T cell markers

Over the years, scientists have conducted extensive research on the mammalian immune system, which has deepened our understanding of the complexity of T lymphocytes
.

We now know that T lymphocytes (T cells) contain multiple subsets, which can be classified according to certain characteristics

The function and identity of specific T cells are determined by their cell surface proteins
.

T cell precursors originate in the bone marrow and then migrate to the thymus, where they mature and are further selected based on receptor activity

Those cells that successfully express the complete αβ TCR complex will continue to develop into double-positive thymocytes, expressing CD4 and CD8
.

After positive selection, cells with affinity for MHC class II molecules will develop into CD4+ T cells, and affinity for MHC class I molecules will lead to the formation of CD8+ T cells

Finally, through the process of negative selection, those T cells that are too reactive to the antigen of normal tissues are removed
.

Most unactivated cells that successfully leave the thymus can be roughly divided into two types: CD4+ helper T cells and CD8+ cytotoxic T cells, each of which has its own subgroups

Helper T cells

CD4+ T cells, also called helper T cells (Th), have almost no cytolytic or phagocytic activity, and mainly support other immune cells through signal transduction
.

After activation, Th cells can be further divided into subtypes with different roles, which largely depends on the cytokines they secrete

Th1 cells secrete pro-inflammatory cytokines, including IFNγ, TNFα and IL-2
.

These cells mainly direct cell-mediated immunity to pathogens (such as bacteria and viruses) in the cell and promote inflammation

Th17 cells secrete a variety of cytokines, including IL-21, IL-22 and IL-25
.

It is worth noting that it also secretes IL-17A, which promotes defense against extracellular pathogens, including bacteria, fungi, and eukaryotes

Th9 is a newly defined subset of Th cells that can secrete IL-9, promote mucus production, activate mast cells, and play a role in autoimmune diseases and inflammatory diseases
.

Another subgroup Tfh (follicular helper T cells) promotes the development of B cells and secretes IL-21

(Picture from Thermo Fisher Scientific)

Antibodies for detecting helper T cell markers include: CD4 antibody, CXCR3 antibody (Th1), CCR4 antibody (Th2), CCR6 antibody (Th17) and CCR10 antibody (Th22)

Cytotoxic T cells

Mature CD8+ T cells are called cytotoxic T cells (Tc) because they can recognize infected, damaged or cancerous somatic cells and trigger the death pathway through cytotoxic proteins
.
The activation of these cells is mediated by TCR, CD8 and CD28 receptors, and the resulting mature cells can be further divided into different Tc subgroups
.

Like various helper T cell subsets, cytotoxic T cells also express specific transcription factors and produce different cytokines and effector molecules
.
For example, Tc1 cells are produced in the presence of IL-2 and IL-12, and secrete cytokines INFγ and TNFα
.
Tc2 cells are induced in the presence of IL-4 and secrete IL-5 and IL-13
.

Phenotype	Cytokines required for differentiation	Transcription factor	Effector	Features
Tc1	IL-2 , IL-12	T-bet , BLIMP1 , Id2 , IRF-4	IFNγ , TNFα , granzyme, perforin	Fight against intracellular pathogens and tumor immunity
Tc2	IL-4	GATA-3	IL-5 , IL-13 , IL-4 , granzyme, perforin	Exacerbate Th2 cell-mediated allergy, which is related to arthritis
Tc9	TGFβ , IL-4	IRF-4	IL-9 , IL-10	Intensify Th2 cell-mediated allergy and anti-tumor response
Tc17	TGFβ , IL-6 , IL-21	ROR-γT , RORa , IRF-4	IL-17 , IL-21	Intensify autoimmunity, virus infection immunity, anti-tumor response

Antibodies for detecting cytotoxic T cell markers include: CD8 antibody, TNFα antibody, T-bet antibody

Regulatory T cells

The function of another group of T cells is to regulate or suppress the immune response
.
These regulatory T cells (Treg) secrete anti-inflammatory cytokines, express inhibitory proteins, and induce the apoptosis of effector T cells
.
Regulatory T cells can be roughly divided into thymus-derived tTreg, pTreg induced in vivo, and iTreg induced in vitro
.
Commonly used markers are FOXP3 and IL2RA (CD25)
.
Other prominently expressed proteins include STAT5A, CTLA4, and the cytokines IL-10 and TGFβ
.

Antibodies for detecting regulatory T cell markers include: FOXP3 antibody, CD25 antibody

Naïve, memory and effector T cells

CD4+ and CD8+ T cells remain in the naïve state before they are activated, after which they will expand and differentiate into effector and memory T cells
.
Effector T cells are cells with a large number but a short life span, which will produce a T cell response
.
At the same time, another part of the cells will become more durable memory cells in order to quickly establish a second antigen-specific immune response
.
The initial T cells and memory T cells can be easily determined by the expression of CD45RA and CD45RO
.

The activation of initial cells can lead to the appearance of different memory/effect subgroups, whose phenotypes gradually change and can be further classified
.
The differentiation lineage includes newly emigrating thymus cells (RTE), stem cell memory T cells (Tscm), central memory T cells (Tcm), effector memory T cells (Tem) and effector T cells (Teff)
.

The phenotypic markers of these subtypes have been determined, such as PECAM1, CD103 (initial and RTE); CCR7, CD127, CD62L (Tscm and Tcm); IL2RA (Tcm); HLA-DR, CCR5, TBX21, GZMA (Tem and Teff) )
.
Tissue-resident memory T cells (Trm) are a unique subpopulation that can be identified by the markers CD69, ITGAE (CD103) and CTLA4 (CD152)
.
It should be noted that multiple markers can be detected on more than one cell type, but their expression levels are different
.

(Picture from Thermo Fisher Scientific)

Antibodies for detecting initial, effector and memory T cell markers include: CD45RA antibody, CD45RO antibody, CCR7 antibody, CD38 antibody

Other types of T cells

In addition to those mentioned above, there are some less common T cell types
.
Natural killer T cells (NKT) account for approximately 1% of T cells in peripheral blood and can be defined by their selective recognition of antigen-presenting CD1d molecules
.
Markers of NKT cells include NCAM1 (CD56) and CD16
.

Most circulating T cells express αβ T cell receptors, which are composed of TCR α and β chains
.
Another rare subpopulation is called γδ T cells, and they express a unique TCR isoform composed of γ and δ chains
.

Further reading:

Immune cell research guide: how to choose macrophage markers