Antigen-receptor genes of the agnathan lamprey are assembled by a process involving copy choice

Abstract

Jawless vertebrates have acquired immunity but do not have immunoglobulin-type antigen receptors. Variable lymphocyte receptors (VLRs) have been identified in lamprey that consist of multiple leucine-rich repeat (LRR) modules. An active VLR gene is generated by the assembly of a series of variable gene segments, including many that encode LRRs. Stepwise assembly of the gene segments seems to occur by replacement of the intervening DNA between the 5' and 3' constant-region genes. Here we report that lamprey (Lethenteron japonicum) assemble their VLR genes by a process involving 'copy choice'. Regions of short homology seemed to prime copying of donor LRR-encoding sequences into the recipient gene. Those LRR-encoding germline sequences were abundant and shared extensive sequence homologies. Such genomic organization permits initiation of copying anywhere in an LRR-encoding module for the generation of various hybrid LRRs. Thus, a vast repertoire of recombinant VLR genes could be generated not only by copying of various LRR segments in diverse combinations but also by the use of multiple sites in an LRR gene segment for priming.

Structure and specificity of lamprey monoclonal antibodies

“…The germline VLR genes are incomplete in that they have coding regions only for the invariant N-terminal and C-terminal sequences, which are separated by intervening sequences, lacking canonical splice sites (1,2). During development of lymphocyte lineage cells, components of the flanking LRR modular units are sequentially inserted into the incomplete VLR gene with a concomitant deletion of the intervening sequences (4,5). The stepwise assembly of a mature VLR gene is mediated via a gene conversion mechanism in which AID-APOBEC family members with lymphocyte-restricted expression may serve a catalytic role (3,5).…”

“…During development of lymphocyte lineage cells, components of the flanking LRR modular units are sequentially inserted into the incomplete VLR gene with a concomitant deletion of the intervening sequences (4,5). The stepwise assembly of a mature VLR gene is mediated via a gene conversion mechanism in which AID-APOBEC family members with lymphocyte-restricted expression may serve a catalytic role (3,5). This somatic gene rearrangement process has been estimated to produce a potential repertoire of Ͼ10 14 distinct VLR-B receptors (4).…”

Definition of a thirdVLRgene in hagfish

“…The germ-line VLR genes are incomplete in that they only contain coding sequences for the leader sequence, incomplete aminoand carboxyl-terminal LRR subunits (LRRNT and LRRCT) and the stalk region (4,7,8). However, each germ-line VLR gene is flanked by hundreds of different LRR-encoding sequences, which can be used as templates to add LRR sequences during the assembly of a mature VLR gene (4,(8)(9)(10)(11). This gene conversionlike process is postulated to involve the activation-induced cytidine deaminase (AID) orthologs, cytidine deaminases 1 and 2 (CDA1 and CDA2) (8,12).…”

High-affinity lamprey VLRA and VLRB monoclonal antibodies

“…Only small amino-and carboxyterminal portions of the diversity regions are contributed by the germline genes (Fig. S1); these serve as docking sites for the sequential incorporation of LRR cassettes via a gene conversionlike process (6,9).…”