biology

TBX21

TBX21 is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand TBX21 rather than just read about it. In short: T-box transcription factor TBX21, also called T-bet (T-box expressed in T cells), is a protein that in humans is encoded by the TBX21 gene. Though being for long thought of only as a master regulator of type 1 immune response, T-bet has recently been shown to be implicated in development of various immune cell subsets and maintenance of mucosal homeostasis.

Key takeaways

TBX21 belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect TBX21 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of TBX21 from memory before moving on to harder problems.

Reference excerpt

T-box transcription factor TBX21, also called T-bet (T-box expressed in T cells), is a protein that in humans is encoded by the TBX21 gene. Though being for long thought of only as a master regulator of type 1 immune response, T-bet has recently been shown to be implicated in development of various immune cell subsets and maintenance of mucosal homeostasis.

Function This gene is a member of a phylogenetically conserved family of genes that share a common DNA-binding domain, the T-box. T-box genes encode transcription factors involved in the regulation of developmental processes. This gene is the human ortholog of mouse Tbx21/Tbet gene. Studies in mouse show that Tbx21 protein is a Th1 cell-specific transcription factor that controls the expression of the hallmark Th1 cytokine, interferon-gamma (IFNg). Expression of the human ortholog also correlates with IFNg expression in Th1 and natural killer cells, suggesting a role for this gene in initiating Th1 lineage development from naive Th precursor cells. The function of T-bet is best known in T helper cells (Th cells). In naïve Th cells the gene is not constitutively expressed, but can be induced via 2 independent signalling pathways, IFNg-STAT1 and IL-12-STAT4 pathways. Both need to cooperate to reach stable Th1 phenotype. Th1 phenotype is also stabilised by repression of regulators of other Th cell phenotypes (Th2 and Th17). In a typical scenario it is thought that IFNg and T cell receptor (TCR) signalling initiates the expression of Tbet, and once TCR signalling stops, signalling via IL-12 receptor can come to play as it was blocked by repression of expression of one of its receptor subunits (IL12Rb2) by TCR signalling. IL-2 signalling enhances the expression of IL-12R. The 2-step expression of T-bet can be viewed as a safety mechanism of sort, which ensures, that cells commit to the Th1 phenotype only when desired. T-bet controls transcription of many genes, for example proinflammatory cytokines like lymphotoxin-a, tumour necrosis factor and ifng, which is a hallmark cytokine of type one immunity. Certain chemokines are also regulated by T-bet, namely xcl1, ccl3, ccl4 and chemokine receptors cxcr3, ccr5. The expression of T-bet controlled genes is facilitated by 2 distinct mechanisms: chromatin remodelation via enzyme recruitment and direct binding to enhancer sequences promoting transcription or 3D gene structure supporting transcription. T-bet also recruits other transcription factors like HLX, RUNX1, RUNX3 which aid it in setting Th1 transcription profile. Apart from promoting type 1 immune response (Th1), T-bet also suppresses the other types of immune response. Type 2 immune response (Th2) phenotype is repressed by sequestering of its master regulator, GATA3 away from its target genes. Gata3 expression is further silenced by promotion of silencing epigenetic changes in its region. In addition to that the Th2 specific cytokines are also silenced by binding of T-bet and RUNX3 to il4 silencer region. Type 17 immune response (Th17) phenotype is suppressed by RUNX1 recruitment, which disallows it to mediate Th17 specific genes, like rorc, a Th17 master regulator. Rorc is also silenced by epigenetic changes promoted by T-bet and STAT4. T-bet also performs function in cytotoxic T cells and B cells. In cytotoxic T cells it promotes IFNg, granzyme B expression and in cooperation with another transcription factor EOMES their maturation. The role of T-bet in B cells seems to be to direct the cell towards type 1 immune response expression profile, which involves secretion of antibodies IGg1 and IGg3 and is usually elevated during viral infections. These populations of B cells differ from standard ones by their lack of receptors CD21 and CD27, also given that these cells have undergone antibody class switch, they are regarded as memory B cells. These cells have been shown to secrete IFNg and in vitro to polarise naïve T helper cells towards Th1 phenotype. Populations of T-bet positive B cells were also identified in various autoimmune diseases like systemic lupus erythematosus, Crohn's disease, multiple sclerosis and rheumatoid arthritis.

Role in mucosal homeostasis It has been identified that T-bet contributes to the maintenance of mucosal homeostasis and mucosal immune response. Mice lacking adapative immune cells and T-bet (RAG -/-, T-bet -/-) developed disease similar to human ulcerative colitis (hence the name TRUC), which was later attributed to the outgrowth Gram-negative bacteria, namely Helicobacter typhlonius. The dysbiosis appears to be a consequence of multiple factors, firstly the innate lymphoid cells 1 (ILC1) population and a subset of ILC3s are missing, because the expression of T-bet is needed for their maturation. Secondly, T-bet ablation causes increased levels of TNF, as its expression is not repressed in dendritic cells and immune system is more biased away from Th1.

Role in disease

Atherosclerosis Atherosclerosis is an autoimmune disease caused by inflammation and associated infiltration of immune cells in fatty deposits in arteries called atherosclerosis plaques. Th1 cells are responsible for production of proinflammatory cytokines contributing to the progression of the disease by promoting expression of adhesive (e.g., ICAM1) and homing molecules (mainly CCR5) needed for cellular migration. Experimental vaccination of patients with peptides derived from apolipoprotein B, part of low-density lipoprotein, which is deposited on arterial walls, has shown increased T regulatory cells (TREGs) and cytotoxic T cells. The vaccination has showed smaller Th1 differentiation, though the mechanism behind it remains unresolved. Currently it is hypothesised that the decrease of Th1 differentiation is caused by the destruction of dendritic cells presenting auto antigens by cytotoxic T cells and increased differentiation of TREGs suppressing immune response. Taken together T-bet might serve as a potential target in treatment of atherosclerosis.

Asthma The transcription factor encoded by TBX21 is T-bet, which regulates the development of naive T lymphocytes. Asthma is a disease of chronic inflammation, and it is known that transgenic mice born without TBX21 spontaneously develop abnormal lung function consistent with asthma. It is thought that TBX21, therefore, may play a role in the development of asthma in humans as well.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with TBX21

Start with the simplest possible case. Write down what TBX21 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to TBX21 before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about TBX21 ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of TBX21

In research: TBX21 appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses TBX21 in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: TBX21 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Genes on human chromosome 17, Transcription factors, so understanding it makes those chapters shorter.
In everyday life: Look for TBX21 outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study TBX21 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what TBX21 means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain TBX21 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is TBX21 in simple terms?

Why does TBX21 matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study TBX21?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on TBX21.