This module will help you
T Cell
[antigen] Receptor (TCR)
TCR Somatic Recombination
T Cell [antigen] Receptor (TCR)
The absence of a secreted form of T Cell [antigen] Receptor (TCR), and the requirement for TCR recognition of both peptide and MHC, made its isolation and characterization much more difficult than that of BCR. As T cells develop in the thymus, they rearrange TCR gene segments to produce a unique TCR. The T cells are then screened for their ability to bind self peptide on self MHC, and only those that bind with the appropriate affinity leave the thymus for the periphery.
Antigen-specific receptors on T cells are similar but not identical to those on B cells. TCR is a member of the Ig superfamily, with Ig-like domains. Like Ig, each chain has a variable and a constant region; variable regions have CDR which define the antigen-binding specificity and framework residues. TCR is encoded in gene segments that undergo somatic recombination during T cell development to generate antigen-binding diversity. Each T cell bears a single specificity and a single allele of TCR. Clonotypic monoclonal antibodies can recognize TCR idiotypes.
TCR is a heterodimer composed usually of a and b chains or, on a minority of T cells, g and d chains. The two chains are disulfide-bonded just outside the T cell plasma membrane in a short extended stretch of amino acids resembling the Ig hinge region; like Ig, TCR have very short cytoplasmic tails. Both chains of the TCR are glycosylated at sites on their V and C regions. Each TCR has a single binding site for antigen, CDR 3 (the most variable), while CDR 1 and CDR 2 bind MHC. TCR generally only binds peptide antigens presented on MHC. Antigen-binding affinity is lower than that of Ig for native antigen, but MHC binding by the T cell membrane co-receptors CD4 (on helper T cells) or CD8 (on cytotoxic T cells) increases the binding avidity of the T cell for the antigen-MHC complex. CD4 and CD8 also signal the T cell to become activated.
TCR is not secreted from the T cell. TCR is expressed on the T cell membrane with a signal transduction complex, CD3, also called the invariant TCR chains because CD3 molecules on all T cells are formed from identical subunits. CD3 is composed of three dimers: gamma epsilon (ge), delta epsilon (de), and either two zetas (zz)or a zeta/eta (zh) heterodimer. The g and d chains of CD3 are not the same molecules found in the gd TCR.
CD4 is a monomeric protein with four Ig-like domains; the two most membrane distal domains are thought to bind Class II MHC b2 domain. CD8 is a disulfide-linked dimer; its a and b chains each have one Ig-like domain with a long extended region connecting it to the transmembrane region. CD8 binds to the a3 region of Class I MHC. The cytoplasmic tails of both CD4 and CD8 associate with a cytoplasmic tyrosine kinase, Lck, to initiate signal transduction. (see Receptor Signaling)
TCR gene organization is similar to that for Ig genes, with TCR genes located in clusters on two (human) or three (mouse) different chromosomes. Genes for the d chain segments are located completely within the region containing the a gene segments . Va and Vg regions of TCR proteins, like VL regions of Ig, are encoded by V and J segments. Humans have approximately 70 different Va, 60 different Ja, and a single Ca segment. A cluster of twelve Vg is followed by 3 Jg with Cg1 and 2 Jg with Cg2.
Vb and Vd chain regions, like VH, are encoded by V, D, and J gene segments. Humans have approximately 50 Vb and two clusters containing 1 Db, 6-7 Jb , and 1 Cb segment. Three each of Vd, Dd, and Jd segments and a single Cd segment have been counted; the Vd segments are interspersed with Va segments The constant region domain has separate gene segments for the constant, hinge, transmembrane, and cytoplasmic regions.
The gene segments for TCR are flanked by the same recombination signal sequences as are the Ig gene segments, and the same RAG-1 and RAG-2 encoded recombinase and TdT are required for somatic recombination. The joining regions for Va and Ja and for Vb, Db, and Jb occur in CDR3, while CDR1 and CDR2 sequences are encoded within Va and Vb. P and N nucleotides are added to the junctions between Vb, Db, and Jb and between Va and Ja. Generation of antigen-binding diversity for TCR, therefore, depends on the same combinatorial and junctional mechanisms used for Ig diversity. Somatic hypermutation does not seem to be an important diversity mechanism for TCR. TCR genes undergo somatic recombination in a defined sequence during T cell development in the thymus. (see T Cell Development)
Practice Quiz
Pick the one BEST answer for each question by clicking on the letter of the correct choice.
1. Which of the following is NOT True about TCR?
a. All TCRs on a particular T cell have identical idiotypes.
b. CDR3 of TCR has the most sequence variability from molecule to molecule.
c. TCR has binding sites for both antigen and self MHC.
d. TCR is a disulfide-bonded heterodimer.
e. The ab or gd isotype of TCR determines the biological function of its secreted form.
2. The antigen-binding region of TCR is formed by the folding of
a. Va and Vb chains.
b. Va, Vb, and CD3 chains.
c. Va and Vb2-microglobulin chains.
d. Vg and Va chains.
e. VL and VH chains.
3. Which of the following properties are NOT shared by TCR and BCR?
a. Antigen-binding avidity is increased by the presence of two antigen binding regions on each receptor.
b. Antigen-binding diversity is generated through gene rearrangement.
c. Folding of protein domains is maintained by intrachain disulfide bonds.
d. Membrane expression and lymphocyte activation by antigen require receptors to be associated with signal transduction molecules.
e. Receptor antigen-binding sites are formed from two polypeptide chains.
4. TCR most closely resembles
a. Class I MHC.
b. Class II MHC.
c. Fab region of immunoglobulin.
d. Fc region of immunoglobulin.
e. light chain of immunoglobulin.
5. Rearrangement of both TCR and BCR gene segments does NOT
a. generate diversity of antigen binding by recombination of a large pool of germline V, D, and J segments.
b. lead to CDR3 being the most hypervariable region in the receptor chains.
c. require RAG-1, RAG-2, and TdT expression.
d. result in allelic exclusion of membrane receptors.
e. result in isotype switching after antigen stimulation of the mature lymphocytes.
6. The amount of diversity in TCR generated within one individual by somatic recombination
a. is higher than BCR diversity.
b. is about the same as for BCR diversity.
c. is lower than BCR diversity.
d. is lower than Class I MHC diversity.
e. is lower than Class II MHC diversity.
7. T cells use all of the following for generating antigen-recognition diversity on the TCR, except
a. combinatorial association of chains.
b. combinatorial association of segments.
c. large germline pool of gene sequences.
d. N region addition of nucleotides.
e. somatic hypermutation.
8. CD8 is a co-receptor on T cells that binds
a. CD3.
b. endogenous antigen peptide.
c. the constant region of Class I MHC.
d. the constant region of TCR.
e. the variable region of Class I MHC.
9. All of the following are true for antigen receptors on both B cells and T cells EXCEPT
a. associated with signal transduction molecules in the membrane.
b. generated by somatic recombination during lymphocyte development.
c. members of the Ig gene superfamily.
d. MHC-restricted in their ability to bind antigen.
e. specific for a single antigen epitope.
10. Which of the following statements is FALSE?
a. TCR is allelically excluded on individual T cells.
b. CD4 and CD8 co-receptors are also signal transducing molecules for T cell activation.
c. The arrangement of a chain gene segments most closely resembles that of k chain.
d. The gene segments for the d chain are interspersed with those for the g chain.
e. The T cells that are most likely to react against allogeneic kidney cells are CD8+ cytotoxic T cells.
Problems
1. Like BCR, TCR have idiotypes. How could you prepare an anti-idiotype antibody to TCR that are specific for influenza virus hemagglutinin (HA), a protein on the envelope of influenza virus that binds RBC and causes them to agglutinate, restricted to HLA-B2.?
2. Blood leukocytes have been prepared from a person who has antibody k chain alleles k2 and k4, TCR a chain alleles a5 and a 7, HLA-A 1,3 and HLA-DP 8,10. You analyze the surface markers on the blood leukocytes by flow cytometry. Draw flow cytometry plots for staining with a) FITC anti-TCR a5 and PE anti-TCR a7, b) FITC anti-TCR a5 and PE anti-HLA-A1, and c) FITC anti-k2 and PE anti-DP10 by writing the names of the cells which will appear in each quadrant. Hint: Think about which cell types are in the blood and whether each cell type has TCR, BCR, Class I or Class II on its membrane. Cells positive for FITC fluorescence are in the top boxes, cells negative for FITC fluorescence in the bottom boxes. Cells positive for PE fluorescence are in the right-hand boxes, cells negative for PE fluorescence are in the left-hand boxes.
