Immunogenetics

Genes of the major class I and II histocompatibility complex have been extensively studied in mammals. Studies of these antigens in reptiles are very scarce. Here we describe the characteristics of these genes in the suborder Serpentes. We identified the presence of a much larger number of molecules of class I and beta chains of class II than found in mammals. Snakes only have one gene for the class II alpha chain. In these species, class I genes can be classified into two types. Approximately half of the genes lack 10 amino acids in the 1 domain, producing a structural alteration in the interaction region with the T lymphocyte receptor. In the genome of Thamnophis elegans, two haplotypes of an individual were studied revealing a different number and location of class I genes between these haplotypes. The results indicate that in these species, the diversity in the MHC is generated by the presence or absence of genes, independent of the presence of alleles.

In teleost fishes there are three immunoglobulin isotypes named immunoglobulin M (IgM), D (IgD) and T (IgT). IgT has been the last to be described and is considered a teleosts-fish specific isotype. From the recent availability of genome sequences of fishes, an in-depth analysis of Actinopterygii immunoglobulin heavy chain genes was undertaken. With the aid of a bioinformatics pipeline, a machine learning software, CHfinder, was developed that identifies the coding exons of the CH domains of fish immunoglobulins. Using this pipeline, a high number of such sequences were obtained from teleosts and holostean fishes. IgT was found in teleost and holostean fishes that had not been previously described. A phylogenetic analysis reveals that IgT CH1 exons are similar to the IgM CH1. This analysis also demonstrates that the other three domains (CH2, CH3 and CH4) were not generated by recent duplication processes of IgM in Actinopterygii, indicating it is an immunoglobulin with an earlier origin.

Over millennia, the selective breeding of dogs (Canis lupus familiaris) has generated remarkable genetic diversity among breeds, highlighting the need for comprehensive genomic and immunogenetic studies. This research provides detailed immunoglobulin kappa light chain locus (IGK) analysis across multiple dog breeds. It aims to uncover breed-specific genetic variations and their implications for immunology and veterinary medicine. The primary objectives were to do the biocuration of the IGK locus in nine canine genome assemblies, investigate structural variations, polymorphisms, and gene diversity, and to enrich the IMGT(R) database with comprehensive IGK data from diverse breeds, creating a more inclusive genetic resource. Our extensive annotation of breeds, including the Bernese Mountain Dog, Boxer, Cairn Terrier, Labrador Retriever, Great Dane, Basenji, and German Shepherd, identified 40 genes and 97 alleles, revealing both conserved genes and unique variants across these breeds, with in silico validation through Sanger sequencing. Notably, we analyzed discrepancies in the first reference assembly from the Boxer breed (Canfam3.1), highlighting potential errors in assembly, challenges in gene and allele nomenclature, and a low-density region within the canine IGK locus. This study not only refines the understanding of IGK locus diversity but also contributes to the IMGT(R) databases, advancing future research on immunogenetic variability, somatic mutations, and immune response dynamics in canine health and disease.

Squamata exhibit a loss of genes for the gamma/delta T-lymphocyte receptor chains and a significant decrease in the number of V genes at the TRBV locus. Through genome analysis, I have discovered a new locus that contains V, J, C, and TM genes that have a similar structure to the classical TCR chains. This gene is viable, as demonstrated by the presence of messenger RNAs in the transcriptomes. Analyses using the AlphaFold2 program indicate that the deduced protein chain is associated with the alpha chain of the TCR. I have named this new chain "epsilon," and it forms a new TCR alpha/epsilon. Evolutionarily, the epsilon chain arose from a duplication of the beta chain gene at the time of the divergence of amphibians and reptiles and has since been specifically maintained in Squamata.

Immunoglobulin Y (IgY) represents the major serum antibody in reptiles and birds, serving as the evolutionary precursor to mammalian IgG and IgE. While IgY diversification has been documented in several reptilian lineages, the structural basis underlying subclass divergence remains poorly understood. Here, we present a comprehensive phylogenetic and structural analysis of IgY sequences from 20 snake species, revealing two distinct evolutionary lineages (A and B) that arose through gene duplication. Structural modeling of the constant regions from Arizona elegans identified a fundamental difference in the light chain-heavy chain (CL-CH1) disulfide bond architecture between lineages. Lineage B utilizes CYS16 in the CH1 domain (alignment position 13) for the inter-chain disulfide bond with the light chain CYS98, whereas Lineage A employs CYS136 (alignment position 99), representing N-terminal versus C-terminal positioning within the CH1 domain. Analysis of 50 diagnostic amino acid positions between lineages revealed that changes are distributed across all constant domains (CH1-CH4), with 13 positions showing radical substitutions affecting charge or polarity. Sliding window dN/dS analysis demonstrated purifying selection ({omega} < 1) across both lineages, consistent with functional constraint following duplication. These findings provide structural evidence for subfunctionalization of snake IgY genes and suggest that alternative disulfide bond configurations may confer distinct biophysical or functional properties to each antibody subclass. This work advances our understanding of immunoglobulin evolution in reptiles and highlights the structural plasticity of antibody architecture.

Turbot (Scophthalmus maximus) is an important aquaculture species, but the genomic organization and expressed diversity of its antibody repertoire remain incompletely characterized. In this study, we annotated the immunoglobulin heavy chain (IGH) locus using the haplotype resolved fScoMax1.1 genome assembly, and we used this as a reference to profile the expressed turbot IgM, IgD and IgT repertoires in skin and spleen. The primary IGH locus was located on chromosome 19, spanned approximately 72 kb, and contained 25 IGHV genes, including 24 functional genes and one pseudogene, together with three IGHD, seven IGHJ and three IGHC genes corresponding to IgT, IgM and IgD. Comparison with the alternate fScoMax1.1 haplotype and a second turbot genome assembly showed conserved IGHD, IGHJ and IGHC content, whereas IGHV gene number differed among assemblies. High throughput 5RACE repertoire sequencing revealed isotype and tissue associated differences in expressed IGH diversity. IgM represented the dominant productive repertoire in both skin and spleen and showed the highest clonotypic diversity, particularly in spleen. IgD displayed an intermediate profile, whereas IgT was more enriched in skin and exhibited the strongest clonal restriction. IGHV subgroup usage was dominated by IGHV3 in IgM and IgD, whereas IgT showed a distinct profile characterized by preferential use of IGHV4, especially in skin. Gene level analysis further showed broad IGHV-IGHJ pairing in IgM and IgD, with preferential use IGHJ3 segment, while IgT sequences paired exclusively with IGHJT. Clonotype sharing between skin and spleen was isotype dependent, being strongest for IgT, intermediate for IgM, and negligible for IgD, suggesting that clonal expansion did not necessarily predict inter tissue trafficking. Together, these results provide a curated genomic and expressed repertoire framework for turbot IGH genes and reveal isotype specific organization of antibody diversity, with IgT displaying a particular repertoire pattern.

Oncorhynchus masou formosanus (Formosa land-locked salmon) is a critically endangered salmonid fish endemic to Taiwan. To begin to understand how its drastic change in lifestyle from anadromous to exclusively river-dwelling is reflected in its immune genes, we characterized the genes encoding six cytokines (IL-2A, IL-2B, IL-4A, IL-4B1, IL-4B2, and IL-17A/F2a) important for T cell responses as no genomic data is available for this fish. Interestingly, all genes appeared homozygous indicative of a genetic bottleneck. The IL2 and IL17A/F2a genes and their products are highly similar to their characterized homologs in Oncorhynchus mykiss (rainbow trout) and other salmonid fish. Two notable differences were observed in IL4 family important for type 2 immune responses. First, O. m. formosanus carries not only one but two genes encoding IL-4B1 proteins and expansions of these genes are present in other salmonid fish. Second, the OmfoIL4A gene carries a 228 bp deletion that results in a premature stop codon and hence a non-functional IL-4A cytokine. This suggests a reduced ability for T cell responses against parasitic infections in this species.

Leukocyte Immunoglobulin (Ig)-like Receptors (LILR) LILRB1 and LILRB2 play a pivotal role in maintaining self-tolerance and modulating the immune response through interaction with classical and non-classical Human Leukocyte Antigen (HLA) molecules. Although both diversity and natural selection patterns over HLA genes have been extensively evaluated, little information is available concerning the genetic diversity and selection signatures on the LIRB1/2 regions. Therefore, we identified the LILRB1/2 genetic diversity using next-generation sequencing in a population sample comprising 528 healthy control individuals from Sao Paulo State, Brazil. We identified 58 LILRB1 Single Nucleotide Variants (SNVs), which gave rise to 13 haplotypes with at least 1% of frequency. For LILRB2, we identified 41 SNVs arranged into 11 haplotypes with frequencies above 1%. We found evidence of either positive or purifying selection on LILRB1/2 coding regions. Some residues in both proteins showed to be under the effect of positive selection, suggesting that amino acid replacements in these proteins resulted in beneficial functional changes. Finally, we have shown that allelic variation (six and five amino acid exchanges in LILRB1 and LILRB2, respectively) affects the structure and/or stability of both molecules. Nonetheless, LILRB2 has shown higher average stability, with no D1/D2 residue affecting protein structure. Taken together, our findings demonstrate that LILRB1 and LILRB2 are highly polymorphic and provide strong evidence supporting the directional selection regime hypothesis.

Cetaceans correspond to mammals that have returned to the marine environment. Adaptive changes are very significant with the conversion of the limbs into flippers. It is studied the changes that have occurred in immunoglobulins, MHC class I and II and T cell receptors genes. Constant regions of immunoglobulins are similar to those of the rest of mammals. An exception is the IgD gene, which is composed of three CH domains but CH1 similar to CH1 of immunoglobulin M. In the IGHV locus, it exist a decrease in the number of VH genes with the absence of genes within Clan I. The number of V{lambda} genes is greater than that of V{kappa}. In the genes for T lymphocyte receptors, it exists a decrease in the number of V genes with loss of significant clades and subclades. In V{beta} and V{gamma}, there is also the loss of clades. These declines of V, V{beta} and V{gamma} are not present Artiodactyla, and they are specific to Cetaceans. In MHC present tree evolutive lines of class I genes. These species have DQ, DR, DO and DM genes, but they are no present DP genes.

The emergence of jaws in early vertebrates introduced a novel feeding apparatus and potent oral defenses but also increased the risk of physical injury and pathogen exposure. Innate immunity and inflammation constitute the bodys first line of defense against invading microbes and tissue damage, aiming to eliminate threats and restore internal homeostasis. Interferon regulatory factor 5 (IRF5) plays a critical role in orchestrating innate immunity and inflammation by regulating the transcription of genes that encode type I interferons (IFNs) and pro-inflammatory cytokines. Despite this, the evolution of IRF5 has remained poorly understood. We have identified the IRF5 and IRF6 genes in cartilaginous fish, including sharks. As cartilaginous fish represent one of the oldest surviving jawed vertebrate lineages, the presence of these genes suggests the genes have ancient origins potentially dating back hundreds of millions of years to early jawed vertebrates. Furthermore, our analysis shows that IRF5 has conserved nuclear export sequences and phosphorylation sites for activation throughout evolution from cartilaginous fish to humans, indicating these regulatory elements evolved early and have been maintained across jawed vertebrates. Additionally, the shift in subcellular localization of IRF5 from nucleus to cytosol, and of other interferon related IRFs, aligns with functional enhancements of IRFs in innate immunity and the emergence of IFNs across jawed vertebrates. This analysis implies that the evolution of jaws may have driven the emergence of new IRF members, the expansion of their functions, and the development of a unique inflammation and innate immune system.

Histocompatibility is the ability to discriminate between self and non-self tissues, and has been described in species throughout the metazoa. Despite its universal presence, histocompatibility genes utilized by different phyla are unique-those found in sponges, cnidarians, ascidians and vertebrates are not orthologous. Thus, the origins of these sophisticated recognition systems, and any potential functional commonalities between them are not understood. A well-studied histocompatibility system exists in the botryllid ascidians, members of the chordate subphylum, Tunicata, and provides an opportunity to do so. Histocompatibility in the botryllids occurs at the tips of an extracorporeal vasculature that come into contact when two individuals grow into proximity. If compatible, the vessels will fuse, forming a parabiosis between the two individuals. If incompatible, the two vessels will reject-an inflammatory reaction that results in melanin scar formation at the point of contact, blocking anastomosis. Compatibility is determined by a single, highly polymorphic locus called the fuhc with the following rules: individuals that share one or both fuhc alleles will fuse, while those who share neither will reject. The fuhc locus encodes multiple proteins with roles in allorecognition, including one called uncle fester, which is necessary and sufficient to initiate the rejection response. Here we report the existence of genotype-specific expression levels of uncle fester, differing by up to 8-fold at the mRNA-level, and that these expression levels are constant and maintained for the lifetime of an individual. We also found that these differences had functional consequences: the expression level of uncle fester correlated with the speed and severity of the rejection response. These findings support previous conclusions that uncle fester levels modulate the rejection response, and may be responsible for controlling the variation observed in the timing and intensity of the reaction. The maintenance of genotype specific expression of uncle fester is also evidence of an education process reminiscent of that which occurs in mammalian Natural Killer (NK) cells. In turn, this suggests that while histocompatibility receptors and ligands evolve via convergent evolution, they may utilize conserved intracellular machinery to interpret binding events at the cell surface.

Immunoglobulin M (IgM) is the most ancient and conserved antibody class in jawed vertebrates and is typically encoded by a single gene. In contrast, geckos and related lizards (infraorder Gekkota) possess multiple IgM genes within the immunoglobulin heavy chain locus. Here, we analyze 52 IgM constant-region sequences from 13 Gekkota species to clarify the evolutionary origin and functional consequences of this expansion. Phylogenetic reconstruction showed that IgM1 (the canonical form) is nearly monophyletic (86.7% clade purity), whereas internal-locus IgM2-6 variants display complex, lineage-specific duplication patterns. We identified 53 diagnostic amino acid positions distinguishing IgM1 from other variants, concentrated in CH1 (19 positions) and CH2 (25 positions). These differences are accompanied by a pronounced physicochemical shift in CH2: IgM1 carries a net positive charge (+2.01) while other IgMs are negatively charged (-2.13), a {Delta} of +4.14 charge units. Conservation analyses indicate stronger constraint on IgM1 in CH1/CH2, while internal-locus IgMs are more conserved in CH4, consistent with maintained polymerization function. Three-dimensional structural comparison of IgM1 and IgM4 supports functional divergence in assembly: IgM4 adopts an "open mouth" CH1-CH2 conformation with increased heavy-light chain contacts and a more electrostatically enriched interface, suggesting compensatory stabilization mechanisms. Together, these results support specialization of internal-locus IgMs through combined sequence and structural divergence.

The major histocompatibility complexs (MHC) role in the vertebrate adaptive immune response and its exceptional polymorphism make it a key target for studying adaptive gene evolution. However, previous studies on carnivore MHC have mostly focused on populations which experienced a severe bottleneck or are of general conservation concern. Hence, sample sizes are often small and generalizations about MHC diversity are unreliable. Furthermore, studies often focus on one MHC class and do not cover the whole peptide binding groove of the MHC molecule. Here, we characterize MHC class I (MHC-I) exon 2 and 3, encoding both the 1- and 2-domain of the MHC-I molecule, as well as MHC-II DRB exon 2 for a large sample (N = 282-485) of a wild mammal of least conservation concern, the banded mongoose. We found that MHC-I generally showed higher allelic diversity and polymorphism compared to MHC-II, which is in line with findings in humans that show higher diversifying selection acting on MHC-I. However, MHC-I exon 3 showed the lowest diversity, possibly due to its different role in generating the peptide binding groove of the class I molecule compared to exon 2. Moreover, we found selection to act more strongly on MHC-I exon 2 (domain 1) than exon 3 (domain 2). Despite frequent inbreeding, phylogenetic comparative analysis showed banded mongooses to have MHC diversity levels comparable with other carnivores of least concern. Phylogenetic analysis indicated a longer evolutionary trajectory for MHC-II compared to MHC-I as well as species-specific gene duplication of nonclassical sequences of MHC-I clustering with classical sequences. Trans-species polymorphism was detected for nonclassical MHC-I sequences suggesting homology or convergent evolution for these genes. Our study is the first to characterize both MHC classes of a social, wild carnivore using a high throughput sequencing approach with a large sample size and thereby provides the basis for further investigation of MHC structure and function within the banded mongoose and other carnivores.

The dog serves as a key translational model in cancer immunotherapy. Understanding the T cell receptor (TCR) repertoire is needed for various cancer immunotherapies. Compared to humans where >300 million TCRs have been identified, <100 canine TCRs are reported. To address this deficiency, we assembled >200,000 complete TCR complementarity-determining region 3 (CDR3) sequences from RNA-seq data published for [~]2,000 canine samples of blood, lymph node, and other tissues, of which 613 are tumors. We collected 1,324 human RNA-seq samples to compare the similarities and differences in T-cell repertoires between humans and dogs. Notably, our analysis revealed distinct variable gene usage patterns between blood samples and solid tissues in both canine and human samples for TRA and TRB loci. Moreover, our investigation led to the discovery of novel V gene and allele candidates in the canine genome. Our findings also revealed that the canine CDR3 resembled human CDR3 in terms of length and motifs. Additionally, our study unveiled shared traits in cancer TCRs between dogs and humans, including longer lengths and higher hydrophobicity of private CDR3s. Our results indicated the diversity of canine to be more comparable to that of humans than mice. Our study provides an initial landscape of the canine TCR repertoire, highlighting both its similarities and differences with the human counterpart, thus laying the groundwork for future research in comparative immunology and vaccine development.

Bats are known to be natural reservoirs of many viruses, yet their unique immune system enables them to coexist with viruses without frequently exhibiting disease symptoms. The current understanding of the bat adaptive immune system is limited, as there is no database or tool capable of processing T-cell receptor (TCR) sequences for bats, and the diverse nature of the Chiroptera. We established a reference library of TCR-{beta} germline genes by annotating three Chiroptera: The Greater Horseshoe Bat (Rhinolophus ferrumequinum, R. ferrumequinum), The Pale spear-nosed Bat (Phyllostomus discolor, P. discolor), and the Common Pipistrelle (Pipistrellus pipistrellus, P. pipistrellus). The distinct variations in the distribution of TRBV genes among the three types of bats could have a direct impact on the diversity of the TCR repertoire, as evidenced by the presence of conserved amino acids that indicate the T-cell recognition of antigens in bats is MHC-restricted. Furthermore, we conducted an analysis of the TCR-{beta} repertoire in the Intermediate Horseshoe Bat (Rhinolophus affinis, R. affinis) using high-throughput sequencing (HTS). The bats TCR-{beta} repertoire is formed through rearrangement of the V-D-J-C genes, with D-J/V-D deletions and insertion resulting a high diversity.

Understanding how the evolutionary relationship between immune cells and the blood-brain is important to devise therapeutic strategies that can regulate their critical function. In vertebrates, immune cells follow either a paracellular or transcellular pathway to infiltrate the BBB. In drosophila glial cells form the BBB that regulates the access of immune-like cells to the drosophila brain. However, it is still not known which route immune-like cells follow to infiltrate the drosophila brain. In vertebrates, paracellular migration is dependent on PECAM1, while transcellular migration is dependent on the expression of CAV1. Interestingly drosophila genome lacks both genes. Tre1 superfamily (Tre1, Moody, and Dmel_CG4313) play a diverse role in regulating transepithelial migration in drosophila. However, its evolutionary history and origin are not yet known. We performed phylogenetic analysis, together with HH search, positive selection, and ancestral reconstruction to investigate the Tre1 family Interestingly we found that Tre1 exists in mollusks, insects, ambulacria, and sclaidphora. Moody is shown to be a more ancient protein and it existed since cnidaria emergence and has a homolog (GPCR84) in mammals. The third family member (Dmel_CG4313) only exists in insects. The origin of the family seems to be related to the rhodopsin-like family and in particular family . We found that opsin is the nearest receptor to have a common ancestor with the Tre1 superfamily that seems to have diverged in sponges. We investigated the positive selection of the Tre1 family using PAML. Tre1 seems to have evolved under negative selection, whereas Moody has evolved during positive selection. The sites that we found under positive selection are Likely to play a role in the speciation of function in the case of Moody. We have identified an SH3, in Tre1 and, moody and Dmel_CG4313. Sh3 is known to play a fundamental role in regulating actin movement in a Rho-dependent manner. We suggest that Tre1 could be playing an important role in paracellular diapedesis in drosophila.

HLA, the coding genes of human major histocompatibility (MHC) proteins, play a crucial role in the human adaptive immune system by presenting antigenic peptides to T cell receptors on T cells. HLA-A, HLA-B and HLA-C, these 3 Class I HLA genes are one of the most polymorphic loci in the human genome. For decades, HLA typing has been performed prior to tissue and stem cell transplantation. However, beyond the role in tissue matching, HLA has also been implicated in a wide array of autoimmune diseases and HLA genotypes and expression levels are closely associated with cancer patients prognosis as recent studies have revealed. Recently methods have been developed to perform HLA typing and HLA expression quantification together by using RNA-seq techniques. However, these bulk RNA-seq experiments are measuring an averaged signal of cell populations. Single-cell RNA-seq (scRNA-seq) has regained its popularity due to its power to reliably resolve single RNA transcriptomes at large scales. In our present study, we did HLA typing using three independent scRNA-seq datasets. Interestingly, we found that single cells from the same donor could be classified into different groups where each group has a distinct expressed HLA genotype (e.g., HLA-A, heterozygous or homozygous); in other words, HLA class I genes show abundant allele specific expression in single cells. This phenomenon has been repeatedly observed in a total of 14 donors from 3 independent datasets (one is breast epithelium, another two are multiple myeloma). Our systematic analysis of HLA class I gene expression using multiple scRNA-seq datasets has uncovered a putative mechanism, where by fine tuning HLA class I expressions both at the quantity and allele levels, our immune system is able to handle various internal challenges through single cells equipped with extraordinary diverse HLA expression patterns.

Virtual crossmatch (VXM) compares a transplant candidates unacceptable antigens to the HLA typing of the donor before an organ offer is accepted and, in selected cases, supplant a prospective physical crossmatch. However, deceased donor typing can be ambiguous, leading to uncertainty in compatibility prediction. We have developed a web application that utilizes ambiguous HLA molecular typing data to assist in VXM assessments. The application compares a candidates listed unacceptable antigens to computed probabilities of all possible two-field donor HLA alleles and UNOS antigens. The VIrtual CrossmaTch for mOleculaR HLA typing (VICTOR) tool can be accessed at http://www.transplanttoolbox.org/victor. We reanalyzed historical VXM cases where a transplant centers manual interpretation of molecular typing results influenced offer evaluation. We found that VICTORs automated interpretation of ambiguous donor molecular typing data would influence VXM decisions. Standardized interpretation of molecular typing data, if applied to the match run, could also change which offers are made. HLA typing ambiguity has been an underappreciated source of immunological risk in organ transplantation. The VICTOR tool can serve as a testbed for development of allocation policies with the aim of decreasing offers refused due to HLA incompatibility.

TAPBP is a key chaperone of the peptide-loading complex that facilitates peptide loading onto major histocompatibility complex class I (MHC I) molecules. This study characterized TAPBP alleles in Korean Native Chickens (KNCs), identified novel variants, and evaluated haplotypic associations with BF2. Thirty-six samples representing six KNC lines were genotyped using LEI0258 and the MHC-B SNP panel, and individuals homozygous at both markers were classified into 16 groups. The same samples were subjected to Sanger sequencing of TAPBP exons 3-8. Sequences were assembled and aligned against MHC-B reference haplotypes and the Red Junglefowl reference. Additional comparisons with "tapasin allele" datasets enabled the identification of novel variants. Six novel nucleotide variants were detected across exons 3-6, including one nonsynonymous substitution in exon 4 (D251H). This residue corresponds to position Q265 in human TAPBP and lies adjacent to residues involved in MHC I interaction, suggesting potential functional relevance. Furthermore, TAPBP exhibited high haplotype diversity (Hd = 0.93) and moderate nucleotide diversity ({pi} = 0.00892), with exon 5 showing the highest diversity ({pi} = 0.01). B9 was the most frequent haplotype at the nucleotide level, whereas B6/B24 predominated at the amino acid level. Comparison with BF2 data revealed haplotype-dependent pairing patterns: BF2-B9 consistently matched TAPBP-B9, whereas BF2-B6 was associated with distinct TAPBP nucleotide variants, indicating allelic diversification within a shared haplotypic background. Homozygosity at LEI0258 and the SNP panel corresponded with TAPBP homozygosity, supporting marker-based prediction. These findings highlight potential BF2-TAPBP associations and provide a foundation for understanding variation in MHC I peptide loading.

T-cell receptor (TCR) diversity is generated by VDJ recombination. The classical course of TCR beta (TRB) chain production starts with D and J segment recombination and finishes with subsequent recombination between the resulting DJ junction and V segment. In this study, we performed deep sequencing of poorly explored incomplete TRBD1 to TRBD2 rearrangements in T-cell genomic DNA. We reconstructed full repertoires of human incomplete TRB DD rearrangements and validated its authenticity by detecting excision circles with RSS (recombination signal sequence) junctions for the first time. The identified rearrangements generated in compliance with the classical 12/23 rule are common for humans, rats, and mice and contain typical VDJ recombination footprints. Detected bimodal distribution of DD junctions indicates two active recombination sites producing long and short DD rearrangements. Unlike long DD rearrangements, the short ones have unusual origin resulting from non-canonical intrachromosomal RSSs junctions formation. Identified DD rearrangements lead to deleting J1 and C1 segments and creating diverse hybrid D segments, which recombine further with J2 and V segments. Resulting functional TRB VDDJ rearrangements are present in the memory T-cells subset proving its participation in antigen recognition.