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THE RESULT OF SOMATIC GENE REARRANGEMENTS 1. Combination of gene segments results in a huge number of various variable regions of the heavy and light chains expressed by different B-cells SOMATIC GENE REARRANGEMENT 2. How B cells express one light chain species and one heavy chain species even though every B cell possesses a maternal and paternal locus of both genes. Since all other genes known at the time appeared to be expressed codominantly, how could B cells shut down the genes on one of their chromosomes? ALLELIC EXCLUSION Evidence for allelic exclusion ALLOTYPE- a polymorphism in the Heavy chain C region of Ig Allotypes can be identified by staining B cell surface Ig with antibodies B a B AND b b B B Y Suppression of H chain rearrangement by pre-B cell receptor prevents expression of two specificities of antibody per cell Y b Y B Y a a/b b/b Y Y a/a a Allelic exclusion is needed for efficient clonal selection Antibody S. typhi S. typhi All daughter cells must express the same Ig specificity otherwise the efficiency of the response would be compromised Suppression of H chain gene rearrangement helps to prevent the emergence of new daughter specificities during proliferation after clonal selection Allelic exclusion prevents unwanted responses One Ag receptor per cell IF there were two Ag receptors per cell Y YY S. aureus Y Y Y Y Anti S. aureus Antibodies B S. aureus Anti brain Abs Y Y Y Y Self antigen expressed by e.g. brain cells YY B Anti S. aureus Antibodies Suppression of H chain gene rearrangement ensures only one specificty of Ab expressed per cell. Prevents induction of unwanted responses by pathogens ALLELIC EXCLUSION Ig alleles are not differentially marked Blastocyst Close (methylated) chromatin structure Asynchronous -locus replication Pre-B cell Early replicating allele is chosen to undergo demethylation and chromatin changes Nucleosomes Modified histones The early replicating allele becomes accessible to rearrangement RESULT OF SOMATIC GENE REARRANGEMENT AND ALLELIC EXCLUSION 1. Somatic rearrangement of Ig gene segments in a highly controlled manner 2. Single B-cells become committed to the synthesis of one unique H-chain and one unique L-chain variable domain, which determine their specificities 3. In one individual a huge B-cell repertoire is generated consisting of B-cell clones with different H- and L-chain variable domains 4. This potential B-cell repertoire is able to recognize a wide array of various antigens INDEPENDENT ON ANTIGEN OCCURS IN THE BONE MARROW THE RESULT OF SOMATIC GENE REARRANGEMENTS 1. Combination of gene segments results in a huge number of various variable regions of the heavy and light chains expressed by different B-cells SOMATIC GENE REARRANGEMENT 2. Successful somatic rearrangement in one chromosome inhibits gene rearrangement in the other chromosome ALLELIC EXCLUSION 3. One B-cell produces only one type of heavy and one type of light chain COMMITMENT TO ONE TYPE OF ANTIGEN BINDING SITE 4. The B-cell pool consist of B-cells with differently rearranged immunoglobulin genes INDEPENDENT OF ANTIGEN OCCURS DURING B-CELL DEVELOPMENT IN THE BONE MARROW Allelic exclusion helps diagnose and monitor lymphoma: Due to clonal expansion of a single cell that contains a unique rearrangement the amount of cancer cells in blood or in bone marrow can be determined Can be used to monitor residual tumor cells upon treatment Allelic exclusion is needed to prevent holes in the repertoire One specificity of Ag receptor per cell IF there were two specificities of Ag receptor per cell Anti-brain Ig Anti-brain Ig AND anti-S. Aureus Ig B B Exclusion of anti-brain B cells i.e. self tolerance B Deletion OR B BUT anti S.Aureus B cells will be excluded leaving a “hole in the repertoire” Anergy B S. aureus