The antigen receptors of lymphocytes are remarkably diverse, and developing B cells and T cells use the same basic mechanism to achieve this diversity. In each cell, functional genes for the immunoglobulin and T-cell receptor chains are assembled by somatic recombination from sets of separate gene segments that together encode the V region. The substrates for the joining process are arrays of V, D, and J gene segments, which are similar in all the antigen-receptor gene loci. The lymphoid-specific proteins RAG-1 and RAG-2 direct the site-specific cleavage of DNA at RSSs flanking the V, D, and J segments to form double-strand breaks that initiate the recombination process in both T and B cells. These proteins function in concert with ubiquitous DNA-modifying enzymes acting in the double-strand break repair pathway, and with at least one other lymphoid-specific enzyme, TdT, to complete the gene rearrangements. As each type of gene segment is present in multiple, slightly different, versions, the random selection of one gene segment from each set is a source of substantial potential diversity. During assembly, the imprecise joining mechanisms at the coding junctions create a high degree of diversity concentrated in the CDR3 loops of the receptor, which lie at the center of the antigen-binding sites. The independent association of the two chains of immunoglobulins or T-cell receptors to form a complete antigen receptor multiplies the overall diversity available. An important difference between immunoglobulins and T-cell receptors is that immunoglobulins exist in both membrane-bound forms (B-cell receptors) and secreted forms (antibodies). The ability to express both a secreted and a membrane-bound form of the same molecule is due to alternative splicing of the heavy-chain mRNA to include exons that encode different forms of the carboxy terminus. Heavy-chain C regions of immunoglobulins contain three or four domains, whereas the T-cell receptor chains have only one. Other species have developed strategies to diversify receptors involved in immunity, and the agnathans use a system of VLRs that undergo somatic rearrangement that has some specific similarities to our own adaptive immune system. Adaptive immunity in jawed vertebrates—gnathostomes—appears to have arisen by the integration of a retrotransposon that encoded prototype RAG1/2 genes into a preexisting V-type immunoglobulin-like gene that subsequently diversified to generate Tand B-cell receptor genes.
Questions.
5.1 True or False: A developing T cell may by chance express both an αβ heterodimer and a γδ heterodimer if all the loci recombine successfully.
5.2 Multiple Choice: Which of the following factors involved in antigen-receptor recombination could be deleted without completing ablating antigen receptor formation?
A. Artemis
B. TdT C. RAG-2
D. Ku E. XRCC4 5.3 True or False: Both B and T cells can undergo somatic hypermutation of their antigen receptor in the context of an immune response in order to enhance antigen affinity.
5.4 Short Answer: What four processes contribute to the vast diversity of antibodies and B-cell receptors?
5.5 Matching: Match the protein(s) to its (their) function:
A. RAG-1 and RAG-2 i. Nontemplate addition of N-nucleotides
B. Artemis ii. Nuclease activity to open the DNA hairpin and generate P-nucleotides
C. TdT iii. Recognize(s) RSS and create(s) single-stranded break
D. DNA ligase IV and iv. Join(s) DNA ends XRCC4
E. DNA-PKcs v. Form(s) a complex with Ku to hold DNA together and phosphorylate Artemis 5.6 Short Answer: What is the 12/23 rule and how does it ensure proper V(D)J segment joining?
5.7 Matching: Match the clinical disorder to the gene defects: