V(D)J recombination

V(D)J recombination is the unique mechanism of genetic recombination that occurs only in developing lymphocytes during the early stages of T and B cell maturation. It involves somatic recombination, and results in the highly diverse repertoire of antibodies/immunoglobulins (Igs) and T cell receptors (TCRs) found on B cells and T cells, respectively. The process is a defining feature of the adaptive immune system. V(D)J recombination is the unique mechanism of genetic recombination that occurs only in developing lymphocytes during the early stages of T and B cell maturation. It involves somatic recombination, and results in the highly diverse repertoire of antibodies/immunoglobulins (Igs) and T cell receptors (TCRs) found on B cells and T cells, respectively. The process is a defining feature of the adaptive immune system. V(D)J recombination occurs in the primary lymphoid organs (bone marrow for B cells and thymus for T cells) and in a nearly random fashion rearranges variable (V), joining (J), and in some cases, diversity (D) gene segments. The process ultimately results in novel amino acid sequences in the antigen-binding regions of Igs and TCRs that allow for the recognition of antigens from nearly all pathogens including bacteria, viruses, parasites, and worms as well as 'altered self cells' as seen in cancer. The recognition can also be allergic in nature (e.g. to pollen or other allergens) or may match host tissues and lead to autoimmunity. In 1987, Susumu Tonegawa was awarded the Nobel Prize in Physiology or Medicine 'for his discovery of the genetic principle for generation of antibody diversity'. Human antibody molecules (including B cell receptors) are composed of heavy and light chains, each of which contains both constant (C) and variable (V) regions, genetically encoded on three loci: Each heavy chain or light chain gene contains multiple copies of three different types of gene segments for the variable regions of the antibody proteins. For example, the human immunoglobulin heavy chain region contains 2 Constant (Cμ and Cδ) gene segments and 44 Variable (V) gene segments, plus 27 Diversity (D) gene segments and 6 Joining (J) gene segments. The light chains also possess 2 Constant (Cμ and Cδ) gene segments and numerous V and J gene segments, but do not have D gene segments. DNA rearrangement causes one copy of each type of gene segment to go in any given lymphocyte, generating an enormous antibody repertoire; roughly 3×1011 combinations are possible, although some are removed due to self reactivity. Most T-cell receptors are composed of an alpha chain and a beta chain. The T cell receptor genes are similar to immunoglobulin genes in that they too contain multiple V, D, and J gene segments in their beta chains (and V and J gene segments in their alpha chains) that are rearranged during the development of the lymphocyte to provide that cell with a unique antigen receptor. The T cell receptor in this sense is the topological equivalent to an antigen-binding fragment of the antibody, both being part of the immunoglobulin superfamily. Failure of the cell to create a successful product that does not self-react leads to apoptosis. Autoimmunity is prevented by eliminating lymphocytes that self-react in the thymus by testing them against an array of self antigens expressed through the function of Aire.The immunoglobulin lambda light chain locus contains protein-coding genes that can be lost with its rearrangement. This is based on a physiological mechanism and is not pathogenetic for leukemias or lymphomas. In the developing B cell, the first recombination event to occur is between one D and one J gene segment of the heavy chain locus. Any DNA between these two gene segments is deleted. This D-J recombination is followed by the joining of one V gene segment, from a region upstream of the newly formed DJ complex, forming a rearranged VDJ gene segment. All other gene segments between V and D segments are now deleted from the cell’s genome. Primary transcript (unspliced RNA) is generated containing the VDJ region of the heavy chain and both the constant mu and delta chains (Cμ and Cδ). (i.e. the primary transcript contains the segments: V-D-J-Cμ-Cδ). The primary RNA is processed to add a polyadenylated (poly-A) tail after the Cμ chain and to remove sequence between the VDJ segment and this constant gene segment. Translation of this mRNA leads to the production of the Ig M heavy chain protein.