HLA-DQ

Key takeaways

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

Reference excerpt

HLA-DQ (DQ) is a cell surface receptor protein found on antigen-presenting cells. It is an αβ heterodimer of type MHC class II. The α and β chains are encoded by two loci, HLA-DQA1 and HLA-DQB1, that are adjacent to each other on chromosome band 6p21.3. Both α-chain and β-chain vary greatly. A person often produces two α-chain and two β-chain variants and thus 4 isoforms of DQ. The DQ loci are in close genetic linkage to HLA-DR, and less closely linked to HLA-DP, HLA-A, HLA-B and HLA-C. Different isoforms of DQ can bind to and present different antigens to T-cells. In this process T-cells are stimulated to grow and can signal B-cells to produce antibodies. DQ functions in recognizing and presenting foreign antigens (proteins derived from potential pathogens). But DQ is also involved in recognizing common self-antigens and presenting those antigens to the immune system in order to develop tolerance from a very young age. When tolerance to self proteins is lost, DQ may become involved in autoimmune disease. Two autoimmune diseases in which HLA-DQ is involved are coeliac disease and type 1 diabetes. DQ mediates autoimmunity by skewing the T-cell receptor (TCR) repertoire during thymic selection. Carriers of risk serotypes such as DQ8 have a higher proportion of circulating T-cell receptors that may bind insulin, the primary autoantigen in type 1 diabetes. DQ is one of several antigens involved in rejection of organ transplants. As a variable cell surface receptor on immune cells, these D antigens, originally HL-A4 antigens, are involved in graft-versus-host disease when lymphoid tissues are transplanted between people. Serological studies of DQ recognized that antibodies to DQ bind primarily to the β-chain. The currently used serotypes are HLA-DQ2, -DQ3, -DQ4, -DQ5, -DQ6, -DQ7, -DQ8, -DQ9. HLA-DQ1 is a weak reaction to the α-chain and was replaced by DQ5 and DQ6 serology. Serotyping is capable of identifying most aspects of DQ isoform structure and function, however sequence specific PCR is now the preferred method of determining HLA-DQA1 and HLA-DQB1 alleles, as serotyping cannot resolve, often, the critical contribution of the DQ α-chain. This can be compensated for by examining DR serotypes as well as DQ serotypes.

Structure, Functions, Genetics

Function The name 'HLA DQ' originally describes a transplantation antigen of MHC class II category of the major histocompatibility complex of humans; however, this status is an artifact of the early era of organ transplantation. HLA DQ functions as a cell surface receptor for foreign or self antigens. The immune system surveys antigens for foreign pathogens when presented by MHC receptors(like HLA DQ). The MHC Class II antigens are found on antigen presenting cells (APC) (macrophages, dendritic cells, and B-lymphocytes). Normally, these APC 'present' class II receptor/antigens to a great many T-cells, each with unique T-cell receptor (TCR) variants. A few TCR variants that recognize these DQ/antigen complexes are on CD4 positive (CD4+) T-cells. These T-cells, called T-helper cells, can promote the amplification of B-cells which, in turn recognize a different portion of the same antigen. Alternatively, macrophages and other megalocytes consume cells by apoptotic signaling and present self-antigens. Self antigens, in the right context, form a regulatory T-cell population that protects self tissues from immune attack or autoimmunity.

Genetics

HLA-DQ (DQ) is encoded on the HLA region of chromosome 6q21.3, in what was classically known as the "D" antigen region. This region encoded the subunits for DP,-Q and -R which are the major MHC class II antigens in humans. Each of these proteins have slightly different functions and are regulated in slightly different ways. DQ is made up of two different subunits to form an αβ-heterodimer. Each subunit is encoded by its own "gene" (a coding locus). The DQ α subunit is encoded by the HLA-DQA1 gene and the DQ β subunit is encoded by the HLA-DQB1 gene. Both loci are variable in the human population (see regional evolution).

Detecting DQ isoforms In the human population DQ is highly variable, the β subunit more so than the alpha chain. The variants are encoded by the HLA DQ genes and are the result of single nucleotide polymorphisms (SNP). Some SNP result in no change in amino-acid sequence. Others result in changes in regions that are removed when the proteins is processed to the cell surface, still others result in change in the non-functional regions of the protein, and some changes result in a change of function of the DQ isoform that is produced. The isoforms generally change in the peptides they bind and present to T-cells. Much of the isoform variation in DQ is within these 'functional' regions. Serotyping. Antibodies raised against DQ tend to recognize these functional regions, in most cases the β-subunit. As a result, these antibodies can discriminate different classes of DQ based on the recognition similar DQβ proteins known as serotypes. An example of a serotype is DQ2.

Recognize HLA-DQB1*02 gene products which include gene products of the following alleles: HLA-DQB1*02:01 HLA-DQB1*02:02 HLA-DQB1*02:03 Sometimes DQ2 antibodies recognize other gene products, such as DQB1*03:03, resulting in serotyping errors. Because of this mistyping serotyping is not as reliable as gene sequencing or SSP-PCR. While the DQ2 isoforms are recognized by the same antibodies, and all DQB1*02 are functionally similar, they can bind different α subunit and these αβ isoform variants can bind different sets of peptides. This difference in binding is an important feature that helps to understand autoimmune disease. The first identified DQ were DQw1 to DQw3. DQw1 (DQ1) recognized the alpha chain of DQA1*01 alleles. This group was later split by beta chain recognition to DQ5 and DQ6. DQ3 is known as broad antigen serotypes, because they recognize a broad group of antigens. However, because of this broad antigen recognition their specificity and usefulness is somewhat less than desirable. For most modern typing the DQ2, DQ4 - DQ9 set is used.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with HLA-DQ

Start with the simplest possible case. Write down what HLA-DQ 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 HLA-DQ 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 HLA-DQ 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 HLA-DQ

In research: HLA-DQ 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 HLA-DQ 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: HLA-DQ is common in secondary-school and first-year university syllabi. It links to neighbouring topics Genes on human chromosome 6, HLA-DQ haplotypes, Human MHC mediated diseases, so understanding it makes those chapters shorter.
In everyday life: Look for HLA-DQ 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 HLA-DQ in 20 minutes

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

Frequently asked questions

What is HLA-DQ in simple terms?

HLA-DQ (DQ) is a cell surface receptor protein found on antigen-presenting cells. It is an αβ heterodimer of type MHC class II.

Why does HLA-DQ 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 HLA-DQ?

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 HLA-DQ.