8536d_ch05_105-136 8/22/02 2: 46 PM Page 105 mac46 mac46: 1256_deh: 8536d: Goldsby et al./Immuno chapter 5 rganization and Expression of Vx JK Jx Immunoglobulin G enes RNA splicing NE OF THE MOST REMARKABLE FEATURES OF the vertebrate immune system is its ability to respond to an apparently limitless array of for eign antigens As immunoglobulin(Ig)sequence data accu- Genetic Model Compatible with Ig Structure mulated, virtually every antibody molecule studied was found to contain a unique amino acid sequence in its vari- n Multigene Organization of Ig Genes able region but only one of a limited number of invariant se Variable-Region Gene Rearrangements quences in its constant region. The genetic basis for this combination of constancy and tremendous variation in a Mechanism of Variable-Region DNA single protein molecule lies in the organization of the Rearrangements munoglobulin genes a Generation of Antibody Diversity In germ-line DNA, multiple gene segments encode por tions of a single immunoglobulin heavy or light chain. These a Class Switching among Constant-Region Genes gene segments are carried in the germ cells but cannot be a Expression of Ig Genes transcribed and translated into complete chains until they are rearranged into functional genes During B-cell matura a Synthesis, Assembly, and Secretion of tion in the bone marrow, certain of these gene segments are Immunoglobulins randomly shuffled by a dynamic genetic system capable of a Regulation of Ig.-Ge ene trans generating more than 10combinations.Subsequent processes increase the diversity of the repertoire of antibody a Antibody Genes and Antibody Engineering binding sites to a very large number that exceeds 10 by at least two or three orders of magnitude. The processes of B- cell development are carefully regulated the maturation of a progenitor B cell progresses through an ordered sequence of DNA. While we think of genomic DNA as a stable genetic Ig-gene rearrangements, coupled with modifications to the blueprint, the lymphocyte cell lineage does not retain an in- gene that contribute to the diversity of the final product. By tact copy of this blueprint. Genomic rearrangement is an es- the end of this process, a mature, immunocompetent B cell sential feature of lymphocyte differentiation, and no other will contain coding sequences for one functional heavy- vertebrate cell type t describes the detailed organization of has been shown to undergo this process hain variable-region and one light-chain variable-region his chapter fir The individual B cell is thus antigenically committed to a the immunoglobulin genes, the process of lg-gene rearrange- specific epitope. After antigenic stimulation of a mature B ment, and the mechanisms by which the dynamic im cell in peripheral lymphoid organs, further rearrangement munoglobulin genetic system generates more than 10 of constant-region gene segments can generate changes in different antigenic specificities. Then it describes the mecha the isotype expressed, which produce changes in the biolog- nism of class switching, the role of differential RNA process ical effector functions of the immunoglobulin molecule ing in the expression of immunoglobulin genes, and the without changing its specificity. Thus, mature B cells contain regulation of Ig-gene transcription. The chapter concludes chromosomal dNa that is no longer identical to germ-line plication of our knowledge of the molecularchapter 5 DNA. While we think of genomic DNA as a stable genetic blueprint, the lymphocyte cell lineage does not retain an in￾tact copy of this blueprint. Genomic rearrangement is an es￾sential feature of lymphocyte differentiation, and no other vertebrate cell type has been shown to undergo this process. This chapter first describes the detailed organization of the immunoglobulin genes, the process of Ig-gene rearrange￾ment, and the mechanisms by which the dynamic im￾munoglobulin genetic system generates more than 108 different antigenic specificities. Then it describes the mecha￾nism of class switching, the role of differential RNA process￾ing in the expression of immunoglobulin genes, and the regulation of Ig-gene transcription. The chapter concludes with the application of our knowledge of the molecular Vκ Jκ Jκ 3′ Cκ VJ Cκ Polyadenylation RNA splicing (A)n 5′ L L ■ Genetic Model Compatible with Ig Structure ■ Multigene Organization of Ig Genes ■ Variable-Region Gene Rearrangements ■ Mechanism of Variable-Region DNA Rearrangements ■ Generation of Antibody Diversity ■ Class Switching among Constant-Region Genes ■ Expression of Ig Genes ■ Synthesis, Assembly, and Secretion of Immunoglobulins ■ Regulation of Ig-Gene Transcription ■ Antibody Genes and Antibody Engineering Organization and Expression of Immunoglobulin Genes O       the vertebrate immune system is its ability to respond to an apparently limitless array of for￾eign antigens. As immunoglobulin (Ig) sequence data accu￾mulated, virtually every antibody molecule studied was found to contain a unique amino acid sequence in its vari￾able region but only one of a limited number of invariant se￾quences in its constant region. The genetic basis for this combination of constancy and tremendous variation in a single protein molecule lies in the organization of the im￾munoglobulin genes. In germ-line DNA, multiple gene segments encode por￾tions of a single immunoglobulin heavy or light chain. These gene segments are carried in the germ cells but cannot be transcribed and translated into complete chains until they are rearranged into functional genes. During B-cell matura￾tion in the bone marrow, certain of these gene segments are randomly shuffled by a dynamic genetic system capable of generating more than 106 combinations. Subsequent processes increase the diversity of the repertoire of antibody binding sites to a very large number that exceeds 106 by at least two or three orders of magnitude. The processes of B￾cell development are carefully regulated: the maturation of a progenitor B cell progresses through an ordered sequence of Ig-gene rearrangements, coupled with modifications to the gene that contribute to the diversity of the final product. By the end of this process, a mature, immunocompetent B cell will contain coding sequences for one functional heavy￾chain variable-region and one light-chain variable-region. The individual B cell is thus antigenically committed to a specific epitope. After antigenic stimulation of a mature B cell in peripheral lymphoid organs, further rearrangement of constant-region gene segments can generate changes in the isotype expressed, which produce changes in the biolog￾ical effector functions of the immunoglobulin molecule without changing its specificity. Thus, mature B cells contain chromosomal DNA that is no longer identical to germ-line Kappa Light-Chain Gene Rearrangement 8536d_ch05_105-136 8/22/02 2:46 PM Page 105 mac46 mac46:1256_des:8536d:Goldsby et al. / Immunology 5e: