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Organization and Rearrangement of TCR Genes – II
Sqadia video is the demonstration of The T-Cell Receptor - II. TCR variable-region genes rearrange in a manner that is similar to antibody genes. The α chain, like the immunoglobulin L chain, is encoded by V, J, and C gene segments. The β chain, like the immunoglobulin H chain is encoded by V, D, J, and C gene segments. Rearrangement of the TCR α- and β-chain gene segments results in VJ joining for the α chain and VDJ joining for the β chain. After transcription of the rearranged TCR genes, RNA processing, and translation, the α and β chains are expressed as a disulfide-linked heterodimer on the membrane of the T cell. Each TCR constant region includes a connecting sequence, a transmembrane sequence, and a cytoplasmic sequence. The germ-line DNA encoding the TCR α and β chain constant regions is much simpler than the immunoglobulin heavy-chain germ-line DNA. TCR α-chain DNA has only a single C gene segment; the β-chain DNA has two C gene segments, but their protein products differ by only a few amino acids and have no known functional differences. The mechanisms by which TCR germ-line DNA is rearranged to form functional receptor genes appear to be similar to the mechanisms of Ig-gene rearrangements. All of the TCR-gene rearrangements follow the one-turn/two-turn joining rule observed for the Ig, so recombination can occur only between the two different types of RSSs. Like the pre-B cell, the pre-T cell expresses the recombination-activating genes (RAG-1 and RAG-2). RAG-1/2 introduces a nick on one DNA strand between the coding and signal sequences.
Organization and Rearrangement of TCR Genes - III
Studies with SCID mice, which lack functional T and B cells provide evidence for the similarity in the mechanisms of Ig-gene and TCR-gene rearrangements. SCID mice have a defect in a gene required for the repair of double-stranded DNA breaks. As a result of this defect, D and J gene segments are not joined during rearrangement of either Ig or TCR DNA. Presumably, the recombinase enzyme system is regulated in each cell lineage so that only rearrangement of the correct receptor DNA occurs. The δ genes are located within the α-gene complex and are deleted by α-chain rearrangements. The organization of the β-chain gene segments into two clusters means that, if a non-productive rearrangement occurs, the thymocyte can attempt a second rearrangement. This increases the likelihood of a productive rearrangement for the β chain. Studies with transgenic mice also indicate that allelic exclusion is less stringent for TCR α-chain genes than for β-chain genes. Mice that carry a productively rearranged αβ-TCR transgene do not rearrange and express the endogenous β-chain genes. One proposal suggests that when a T cell expresses two different αβ T-cell receptors only one is likely to be self-MHC restricted and therefore functional.
Organization and Rearrangement of TCR Genes - IV
The variable regions of T-cell receptors are, of course, encoded by rearranged VDJ and VJ sequences. In TCR genes, combinatorial joining of V gene segments appears to generate CDR1 and CDR2, whereas junctional flexibility and N-region nucleotide addition generate CDR3. The constant region of each TCR chain is encoded by a C gene segment that has multiple exons. TCR germ-line DNA contains far fewer V gene segments than Ig. Combinatorial joining of V-region gene segments generates a large number of random gene combinations. Antigen-binding specificity of a given TCR depends upon the variable region in both chains. Additional means to generate diversity in the TCR V genes also exist.
Organization and Rearrangement of TCR Genes - V
The location of one-turn (12-bp) and two-turn (23-bp) RSSs In TCR β- and δ-chain DNA differs from that in Ig heavy-chain DNA. Because of the arrangement of the RSSs in TCR germ-line DNA alternative joining of D gene segments can occur. While the one-turn/two-turn joining rule is observed. The joining of gene segments during TCR-gene rearrangement exhibits junctional flexibility. This flexibility can generate many non-productive rearrangements. It also increases diversity by encoding several alternative AAs at each junction. Addition of N-region nucleotides, catalyzed by a terminal deoxynucleotidyl transferase generates additional junctional diversity. Combined effects of P- and N-region nucleotide addition and joining flexibility can generate as many as 1013 possible amino acid sequences in the TCR junctional regions alone. The mechanism by which diversity is generated for the TCR must allow the receptor to recognize a very large number of processed antigens while restricting its MHC-recognition repertoire to a much smaller number. TCR DNA has far fewer V gene segments than Ig DNA. It has been postulated that the smaller number of V gene segments in TCR DNA have been selected to encode a limited number of CDR1 and CDR2 regions with affinity for regions of the α helices of MHC molecules.
T-Cell Receptor Complex: TCR-CD3
Membrane-bound immunoglobulin on B cells associates with another membrane protein. The Ig-α/Ig-β heterodimer, to form the B-cell antigen receptor. Similarly, the T-cell receptor associates with CD3, forming the TCR-CD3 membrane complex. In both cases, the accessory molecule participates in signal transduction after interaction of a B or T cell with antigen. T-cell receptor is associated with another membrane molecule. Experiments in which fluorescent antibody to the receptor was used cause aggregation of another membrane protein designated CD3. Loss of the genes encoding either CD3 or the TCR chains results in loss of the entire molecular complex from the membrane. CD3 is a complex of five invariant polypeptide chains that associate to form three dimers i.e a heterodimer of gamma and epsilon chains (γϵ), a heterodimer of delta and epsilon chains (δϵ), a homodimer of two zeta chains (ζζ), a heterodimer of zeta and eta chains (ζη). The γ, δ, and ϵ chains of CD3 are members of the Immunoglobulin superfamily. Each containing an immunoglobulin like extracellular domain, a transmembrane region, a cytoplasmic domain of more than 40 amino acids. The cytoplasmic tails of the CD3 chains contain a motif called the immunoreceptor tyrosine-based activation motif (ITAM). The ITAM sites have been shown to interact with tyrosine kinases and to play an important role in signal transduction.