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28 Cards in this Set
- Front
- Back
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How does light chain recombination occur?
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One of the many V gene segments is recombined with one of the J gene segments (Coding Joint) and the DNA between (the Signal Joint) is deleted. The coding joint gets transcribed into a primary RNA transcript.
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When does the region between the J and C region spliced out?
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This region is spliced out at the RNA level leaving an mRNA with V,J, and C segements juxtaposed
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How does heavy chain rearrangement occur?
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There are 2 different deletions at the DNA level. D rearrangement occurs first with one D segment and one J segment chosen and spliced together the signal joint lacking and origin of replication is lost. Then rearragement between V and D segments occurs.
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What is the RSS?
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the recombination signal sequence (RSS) flanks rearranging gene segments. It is made up of a conserved 7bp region, a non-conserved region of 12 or 23 bp, and another conserved 9bp region. The RSS is on the 3' end of the V gene and 5' end of the J gene. Machinery cuts at the end of the nonomer.
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What is the 12-23 Rule?
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A 12bp spacer regions always bond with 23bp spacers in VJ recombination. V-lambda are 23bp, J-lambda are 12bp in light chain. In heavy chain V-lambda and J-lambda are 23bp, but are combined with D region with 12bp on each side.
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What occurs during the initiation stage of V(D)J recombination?
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The initiation stage is lymphocyte or B-cell specific. RAGs (recombination Activating Gene) initiates V(D)J recombination. RAG 1 and 2 bind and recognize RSS segments, cuts, and forms hairpin structure cutting out signal joint. Holds synaptic complex together
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What occurs during the next step in V(D)J recombination?
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DNA repair occurs in the next step with Ku binding to ends of DNA and recruiting DNA protein kinase, DNAses nibbling the ends, TdT adds nucleotides to the broken ends, artemis cuts the hairpin, DNA ligase4 covalently seals both the signal and coding joints.
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Which hypervariable region is the most important in the recognition of antigen for the light chain?
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The 3rd HV region (CDR3) occuring between V and J. Changes here change the antigen recognition properties of the Ig.
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In the heavy chain what regions are found in CDR3?
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In the heavy chain both the V-D and D-J junctions occur in the third HV region.
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How is additional diversity generated in antigen binding in relation to the structure of the heavy chain?
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The addition of bases at the V-D and D-J junctions via TdT (non-template dependent manner) and P-region addition occuring when asymmetric cutting of the hairpin occurs.
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What are the consequences of V(D)J recombination?
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Huge amount of diversity generated through random combinatorial mechanisms, junctional diversity at CDR3, deletions and additions make every junction unique even if the same V and J regions are brought together. Allows for editing to alter potentially self-reactive antibodies.
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What does the heavy chain constant region confer on the antibody?
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Antigen elimination properties and also where the the antibody itself resides
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What is the odering of the heavy chain loci?
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After the J region you have the Cm and Cd regions then a break after which there are 2Cg, 1 Ca, 2Cg, 1Ce regions.
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What makes the expression of Cm and Cd different from Ca, Ce, or Cd expression?
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No class switch recombination is needed because they are processed at the mRNA level.
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What is CSR?
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The main mechanism by which the heavy chain constant region is generated.
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How does CSR take place?
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It is an addition and loss of DNA. Cm and Cd are initally expressed. CSR loops out and deletes between different switch regions, repetitive elements located in front of every class region except delta.
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Does class switching only occur once?
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Depends. CSR can occur multiple times, but only if there are intervening segments that can be looped out.
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How is CSR different than V(D)J recombination in terms of specificity?
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CSR can be sloppier because it is taking place in intervening sequences that will later be removed in RNA processing. Unlike V(D)J recombination which takes place in exons.
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How does mRNA processing of mu and delta constant regions proceed?
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After VDJ regions have been joined. Splicing at poly adenylation sites found following the mu region and one following the delta region. Spkcing at pA1 gives IgM and pA2 gives IgD
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What does the mRNA processing of mu and delta constant regions mean for the antibodies produced by a single B-cell clone?
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With differential polyadenylation and splicing a single B-cell clone with a single V(D)J recombination could express both mu and delta simultaneously
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What determines if an antibody will be secreted or membrane bound?
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This is determined by which polyA site is spliced. If the first of the two polyA sites is spliced the antibody will be secreted if the second is spliced the antibody will become a membrane bound antibody
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Where are the I region promoters and what do they do?
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the I region promoters are upstream of all the switch regions and the constant regions. These promoters must be transcribed before rearrangements of heavy chain class can occur.
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How are the I region promoters activated?
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Different cytokines from T-cells activate different promoters allowing them to be transcribed and heavy class switching to occur. IL-4 will induce IgG1 and IgE and inhibit the rest
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Compare VDJ and CSR.
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Join exon (VDJ), Joins Introns (CSR)
RAGs required (VDJ), AID required (CSR) Repair enzymes, Repair enzymes Generates diversity (VDJ), Changes Isotype (CSR) Ag specificity (VDJ), Ag elimination (CSR) Random (VDJ), Regualted by T-cell signals (CSR) |
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AID
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Activation Induced Deaminase. Deaminates cysteins on DNA. Recognized by repair enzymes
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SCID
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Severe Combined Immune Deficiency (SCID). Patient has no B or T cells and are radio-sensitive due to a defective artemis protein
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Hyper IgM syndrome
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Patients can make IgM but can't switch.
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Omen's syndrome
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Profound immunodeficiency because no B or T cells due to defective RAGs
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