Tcr-seq

Key takeaways

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

Reference excerpt

TCR-Seq (T-cell Receptor Sequencing) is a method used to identify and track specific T cells and their clones. TCR-Seq utilizes the unique nature of a T-cell receptor (TCR) as a ready-made molecular barcode. This technology can apply to both single cell sequencing technologies and high throughput screens

Background

T-cell Receptor (TCR) T cells are a part of the adaptive immune system and play a critical role in protecting the body from foreign pathogens. T-cell receptors (TCRs) are a group of membrane proteins found on the surface of T cells which can bind to foreign antigens. TCRs interact with major histocompatibility complexes (MHC) on cell surfaces to recognize antigens. They are heterodimers made up of predominantly α and β chains (or more rarely δ and γ chains) and consist of a variable region and a constant region. Variable regions are produced through a process called VDJ recombination, which results in unique amino acid sequences for α, β, and γ chains. The result is that each TCR is unique and recognizes a specific antigen

Complementarity Determining Regions (CDRs)

Complementarity determining regions (CDRs) are a part of the TCR and play an essential role in TCR-MHC interactions. CDR1 and CDR2 are encoded by V genes, while CDR3 is made from the region between V and J genes or between D and J genes (termed "VDJ genes" when referred to together). CDR3 is the most variable of the CDRs, and is in direct contact with the antigen. As such, CDR3 is used as the “barcode region” to identify unique T cell populations, as it is highly unlikely for two T cells to have the same CDR3 sequence unless they came from the same parental T cell.

Clonality

VDJ recombination produces such a vast amount of unique TCRs that many receptors never encounter the antigen they are best suited for. When a foreign antigen is present in the body, the few T cells that recognize that antigen are positively selected for so that the body has an adequate number of T cells to mount an effective immune response. The selected T cells rapidly divide and differentiate into effector T-cells through a process called clonal expansion, which retains the TCR sequence (including the CDR3 sequence) that originally recognized the antigen TCR-Seq uses the unique nature of the TCR - in particular CDR3 - as a molecular barcode to track T cells through a variety of processes like differentiation and proliferation, which can be used for a wide variety of purposes.

Methods

Bulk vs Single-Cell Sequencing TCR sequencing can be performed in on pooled cell populations (“bulk sequencing”) or single cells (“single cell sequencing”). Bulk sequencing is useful to explore entire TCR repertoires - all the TCRs within an individual or a sample - and to generate comparisons between repertoires of different individuals. This method can sequence millions of cells in a single experiment. However, one major disadvantage is that bulk sequencing cannot determine which TCR chains pair together, only the frequency within the repertoire. The large amount of TCRs sampled also means that lower abundance TCRs may not be detected Single cell sequencing can determine TCR chain pairs, making them more useful for identifying specific TCRs. Some major disadvantages of this technique are its high costs, limited capacity of a few thousand cells, and the necessity of live cells which may be more challenging to obtain

Target Sequences Any TCR chain can be sequenced, although the α and β chain are more commonly chosen due to their abundance in the T cell population. In particular, the β chain is of interest due to its higher diversity and specificity compared to other chains. The presence of a D gene component in the β chain which is not present in the α chain allows more diverse combinations. As well, β chains are unique to each T cell, which can be used to identify distinct T cell populations within a sample To perform TCR-sequencing, polymerase chain reaction (PCR) amplification is performed on the CDR3 region as a measure of unique T cells within a population. The CDR3 region is chosen over CDR1 and CDR2 as it is directly responsible for antigen interactions and is generally unique to TCRs from the same lineage, which allows identification of distinct populations

Library Preparation

The goal of this step is to generate a library of transcripts to be sequenced. There are 3 major ways of generating a library for TCR sequencing.

Multiplex DNA Multiplex PCR can be employed on both genomic DNA (gDNA) or RNA which has been converted to double-stranded complementary DNA (cDNA). Primer pools with primer pairs targeting J and V alleles are used to amplify the CDR3 region of the TCR transcript. The transcript goes through two or more rounds of PCR to amplify the region of interest, then adaptors are ligated onto either end of the resulting transcript. This method is among the most used in the generation of libraries for TCR-seq as it can capture a great deal of the diversity of the TCR through the primer pool. However, as it is near-impossible to optimize PCR conditions for all the primers in the pool, multiplex DNA can result in amplification bias where some CDR3 regions with primers that bind poorly may not be amplified. This means the abundance of amplified segments may not correspond with the actual abundance within the cell

Target Enrichment In-Solution This method can use genomic gDNA or RNA converted to cDNA. The starting material is first processed to generate DNA or cDNA transcripts with indexed adaptors on the 5’ and 3’ ends. These transcripts are then incubated with RNA baits designed to bind to regions of interest, which is generally the CDR3 region. These baits, which are normally bound to magnetic beads, can be isolated using a magnet. This allows the isolation of transcripts of the CDR3 region which can then amplified using PCR. Target enrichment using RNA baits requires fewer PCR amplification steps, which may decrease amplification bias. However, the efficiency of the capture by magnets may affect the diversity of the amplified transcripts.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Tcr-seq

Start with the simplest possible case. Write down what Tcr-seq claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, 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 Tcr-seq 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 Tcr-seq 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 Tcr-seq

In research: Tcr-seq appears in chemistry 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 Tcr-seq 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: Tcr-seq is common in secondary-school and first-year university syllabi. It links to neighbouring topics DNA sequencing, Molecular biology techniques, so understanding it makes those chapters shorter.
In everyday life: Look for Tcr-seq 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.

How to study Tcr-seq in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Tcr-seq 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 Tcr-seq out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Tcr-seq in simple terms?

TCR-Seq (T-cell Receptor Sequencing) is a method used to identify and track specific T cells and their clones. TCR-Seq utilizes the unique nature of a T-cell receptor (TCR) as a ready-made molecular barcode.

Why does Tcr-seq matter?

Because it connects several chemistry 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 Tcr-seq?

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 Tcr-seq.