High Throughput Characterization of V(D)J Recombination Signal Sequences Redefines the Consensus Sequence

ABSTRACTIn the adaptive immune system, V(D)J recombination initiates the production of a diverse antigen receptor repertoire in developing B and T cells. Recombination activating proteins, RAG1 and RAG2 (RAG1/2), catalyze V(D)J recombination by cleaving adjacent to recombination signal sequences (RSSs) that flank antigen receptor gene segments. Previous studies defined the consensus RSS as containing conserved heptamer and nonamer sequences separated by a less conserved 12 or 23 base-pair spacer sequence. However, many RSSs deviate from the consensus sequence. Here, we developed a cell-based, massively parallel V(D)J recombination assay to evaluate RAG1/2 activity on thousands of RSSs. We focused our study on the RSS heptamer and adjoining spacer region, as this region undergoes extensive conformational changes during RAG-mediated DNA cleavage. While the consensus heptamer sequence (CACAGTG) was marginally preferred, RAG1/2 was highly active on a wide range of non-consensus sequences. RAG1/2 generally preferred select purine/pyrimidine motifs that may accommodate heptamer unwinding in the RAG1/2 active site. Our results suggest RAG1/2 specificity for RSS heptamers is primarily dictated by DNA structural features dependent on purine/pyrimidine pattern, and to a lesser extent, RAG:RSS base-specific interactions. Further investigation of RAG1/2 specificity using this new approach will help elucidate the genetic instructions guiding V(D)J recombination.Summary StatementPartially conserved recombination signal sequences (RSSs) govern antigen receptor gene assembly during V(D)J recombination. Here, a massively parallel analysis of randomized RSSs reveals key attributes that allow DNA sequence diversity in the RAG1/2 active site and that contribute to the differential utilization of RSSs in endogenous V(D)J recombination. Overall, these results will assist identification of RAG1/2 off-target sites, which can drive leukemia cell transformation, as well as characterization of bona fide RSSs used to generate antigen receptor diversity.

RSSs set the odds for exclusion

In this issue of JEM, Wu et al. (https://doi.org/10.1084/jem.20200412) provide new insights into allelic exclusion. They demonstrate that Vβ-to-DβJβ rearrangement occurs stochastically on two competing Tcrb alleles, with suboptimal Vβ recombination signal sequences limiting synchronous rearrangements and essential for allelic exclusion.

Inefficient V(D)J recombination underlies monogenic T cell receptor β expression

The assembly of T cell receptor (TCR) and immunoglobulin (Ig) genes by V(D)J recombination generates the antigen receptor (AgR) diversity that is vital for adaptive immunity. At most AgR loci, V(D)J recombination is regulated so that only one allele assembles a functional gene, ensuring that nearly every T and B cell expresses a single type, or specificity, of AgR. The genomic organizations of some AgR loci permit the assembly and expression of two distinct genes on each allele; however, this is prevented by undetermined mechanisms. We show that the poor qualities of recombination signal sequences (RSSs) flanking Vβ gene segments suppress the assembly and expression of two distinct TCRβ genes from a single allele. Our data demonstrate that an intrinsic genetic mechanism that stochastically limits Vβ recombination efficiency governs monogenic TCRβ expression, thereby restraining the expression of multiple AgRs on αβ T cells.

Poor quality Vβ recombination signal sequences stochastically enforce TCRβ allelic exclusion

The monoallelic expression of antigen receptor (AgR) genes, called allelic exclusion, is fundamental for highly specific immune responses to pathogens. This cardinal feature of adaptive immunity is achieved by the assembly of a functional AgR gene on one allele, with subsequent feedback inhibition of V(D)J recombination on the other allele. A range of epigenetic mechanisms have been implicated in sequential recombination of AgR alleles; however, we now demonstrate that a genetic mechanism controls this process for Tcrb. Replacement of V(D)J recombinase targets at two different mouse Vβ gene segments with a higher quality target elevates Vβ rearrangement frequency before feedback inhibition, dramatically increasing the frequency of T cells with TCRβ chains derived from both Tcrb alleles. Thus, TCRβ allelic exclusion is enforced genetically by the low quality of Vβ recombinase targets that stochastically restrict the production of two functional rearrangements before feedback inhibition silences one allele.

Sequence-dependent dynamics of synthetic and endogenous RSSs in V(D)J recombination

Developing lymphocytes of jawed vertebrates cleave and combine distinct gene segments to assemble antigen–receptor genes. This process called V(D)J recombination that involves the RAG recombinase binding and cutting recombination signal sequences (RSSs) composed of conserved heptamer and nonamer sequences flanking less well-conserved 12- or 23-bp spacers. Little quantitative information is known about the contributions of individual RSS positions over the course of the RAG–RSS interaction. We employ a single-molecule method known as tethered particle motion to track the formation, lifetime and cleavage of individual RAG–12RSS–23RSS paired complexes (PCs) for numerous synthetic and endogenous 12RSSs. We reveal that single-bp changes, including in the 12RSS spacer, can significantly and selectively alter PC formation or the probability of RAG-mediated cleavage in the PC. We find that some rarely used endogenous gene segments can be mapped directly to poor RAG binding on their adjacent 12RSSs. Finally, we find that while abrogating RSS nicking with Ca2+ leads to substantially shorter PC lifetimes, analysis of the complete lifetime distributions of any 12RSS even on this reduced system reveals that the process of exiting the PC involves unidentified molecular details whose involvement in RAG–RSS dynamics are crucial to quantitatively capture kinetics in V(D)J recombination.

Population matched (PM) germline allelic variants of immunoglobulin (IG) loci: New pmIG database to better understand IG repertoire and selection processes in disease and vaccination

At the population level, immunoglobulin (IG) loci harbor inter-individual allelic variants in the many different germline IG variable (V), Diversity (D) and Joining (J) genes of the IG heavy (IGH), IG kappa (IGK) and IG lambda (IGL) loci, which together form the genetic basis of the highly diverse antigen-specific B-cell receptors. These inter-individual allelic variants can be shared between or be specific to human populations. The current IG databases IMGT, VBASE2 and IgPdb hold information about germline alleles, most of which are partial sequences, obtained from a mixture of human (B-cell) samples, many with sequence errors and/or acquired (non-germline) IG variations, induced by somatic hypermutation (SHM) during antigen-specific B-cell responses. We systematically identified true germline alleles (without SHM) from 26 different human populations around the world, profiled by the “1000 Genomes data”. Our resource is uniquely enriched with complete IG allele sequences and their frequencies across human populations. We identified 409 IGHV, 179 IGKV, and 199 IGLV germline alleles supported by at least seven haplotypes (= minimum of four individuals), after removal of potential false-positives, based on using other genomic databases, i.e. ENSEMBL, TopMed, ExAC, ProjectMine. Remarkably, the positions of the identified variant nucleotides of the different alleles are not at random (as observed in case of SHM), but show striking patterns, restricted to limited nucleotide positions, the same as found in other IG data bases, suggesting over-time evolutionary selection processes. The identification of these specific patterns provides extra evidence that the identified variant nucleotides are not sequencing errors, but genuine allelic variants. The diversity of germline allelic variants in IGH and IGL loci is the highest in Africans, while the IGK locus is most diverse in Europeans. We also report on the presence of recombination signal sequences (RSS) in V pseudogenes, explaining their usage in V(D)J rearrangements. We propose that this new set of genuine germline IG sequences can serve as a new population-matched IG (pmIG) database for better understanding B-cell repertoire and B-cell receptor selection processes in disease and vaccination within and between different human populations. The database in format of fasta is available via GitHub (https://github.com/InduKhatri/pmIG).Contribution to the Field StatementWe present a catalogue of immunoglobulin (IG) germline-alleles of unprecedented completeness and accuracy from 26 different human populations belonging to five different large ethnicities (Source: 1000 Genomes). We identified the population distribution of several known germline alleles and identified multiple new alleles, especially in African populations, indicative of high allelic diversity of IG genes in Africa. Strikingly, the identified variant nucleotides of the different alleles are not at random, but show striking patterns, restricted to limited nucleotide positions, the same as found in other IG databases, suggesting over-time evolutionary selection processes. Furthermore, we identified recombination signal sequences in pseudogenes (previously not known). We provide an overview of IG germline alleles shared with and between known databases and also point to potential sources of non-germline variation and incompleteness of the existing IG databases. More importantly, we believe that this information can serve as a novel population-matched IG (pmIG) database, highly valuable for the research community in supporting the dissection and understanding of differences in effectiveness of antibody-based immune responses in infectious diseases, other (immune) diseases and vaccination within and between human populations. Such knowledge might be used in developing population-specific vaccination strategies e.g. for currently ongoing SARS-CoV2 pandemic.

Functional requirement of terminal inverted repeats for efficient ProtoRAG activity reveals the early evolution of V(D)J recombination

The discovery of ProtoRAG in amphioxus indicated that vertebrate RAG recombinases originated from an ancient transposon. However, the sequences of ProtoRAG terminal inverted repeats (TIRs) were obviously dissimilar to the consensus sequence of mouse 12/23RSS and recombination mediated by ProtoRAG or RAG made them incompatible with each other. Thus, it is difficult to determine whether or how 12/23RSS persisted in the vertebrate RAG system that evolved from the TIRs of ancient RAG transposons. Here, we found that the activity of ProtoRAG is highly dependent on its asymmetric 5′TIR and 3′TIR, which are composed of conserved TR1 and TR5 elements and a partially conserved TRsp element of 27/31 bp to separate them. Similar to the requirements for the recombination signal sequences (RSSs) of RAG recombinase, the first CAC in TR1, the three dinucleotides in TR5 and the specific length of the partially conserved TRsp are important for the efficient recombination activity of ProtoRAG. In addition, the homologous sequences flanking the signal sequences facilitate ProtoRAG- but not RAG-mediated recombination. In addition to the diverged TIRs, two differentiated functional domains in BbRAG1L were defined to coordinate with the divergence between TIRs and RSSs. One of these is the CTT* domain, which facilitates the specific TIR recognition of the BbRAGL complex, and the other is NBD*, which is responsible for DNA binding and the protein stabilization of the BbRAGL complex. Thus, our findings reveal that the functional requirement for ProtoRAG TIRs is similar to that for RSS in RAG-mediated recombination, which not only supports the common origin of ProtoRAG TIRs and RSSs from the asymmetric TIRs of ancient RAG transposons, but also reveals the development of RAG and RAG-like machineries during chordate evolution.