Evolution of Alternative Adaptive Immune Systems in Vertebrates.

Adaptive immunity in jawless fishes is based on antigen recognition by three types of variable lymphocyte receptors (VLRs) composed of variable leucine-rich repeats, which are differentially expressed by two T-like lymphocyte lineages and one B-like lymphocyte lineage. The T-like cells express either VLRAs or VLRCs of yet undefined antigen specificity, whereas the VLRB antibodies secreted by B-like cells bind proteinaceous and carbohydrate antigens. The incomplete VLR germline genes are assembled into functional units by a gene conversion-like mechanism that employs flanking variable leucine-rich repeat sequences as templates in association with lineage-specific expression of cytidine deaminases. B-like cells develop in the hematopoietic typhlosole and kidneys, whereas T-like cells develop in the thymoid, a thymus-equivalent region at the gill fold tips. Thus, the dichotomy between T-like and B-like cells and the presence of dedicated lymphopoietic tissues emerge as ancestral vertebrate features, whereas the somatic diversification of structurally distinct antigen receptor genes evolved independently in jawless and jawed vertebrates.

Evolution of variable lymphocyte receptor B antibody loci in jawless vertebrates.

Three types of variable lymphocyte receptor (VLR) genes, VLRA, VLRB, and VLRC, encode antigen recognition receptors in the extant jawless vertebrates, lampreys and hagfish. The somatically diversified repertoires of these VLRs are generated by serial stepwise copying of leucine-rich repeat (LRR) sequences into an incomplete germline VLR gene. Lymphocytes that express VLRA or VLRC are T cell-like, while VLRB-expressing cells are B cell-like. Here, we analyze the composition of the VLRB locus in different jawless vertebrates to elucidate its configuration and evolutionary modification. The incomplete germline VLRB genes of two hagfish species contain short noncoding intervening sequences, whereas germline VLRB genes in six representative lamprey species have much longer intervening sequences that exhibit notable genomic variation. Genomic clusters of potential LRR cassette donors, fragments of which are copied to complete VLRB gene assembly, are identified in Japanese lamprey and sea lamprey. In the sea lamprey, 428 LRR cassettes are located in five clusters spread over a total of 1.7 Mbp of chromosomal DNA. Preferential usage of the different donor cassettes for VLRB assemblage is characterized in our analysis, which reveals evolutionary modifications of the lamprey VLRB genes, elucidates the organization of the complex VLRB locus, and provides a comprehensive catalog of donor VLRB cassettes in sea lamprey and Japanese lamprey.

Ancient BCMA-like Genes Herald B Cell Regulation in Lampreys.

The TNF superfamily ligands BAFF and APRIL interact with three receptors, BAFFR, BCMA, and TACI, to play discrete and crucial roles in regulating B cell selection and homeostasis in mammals. The interactions between these ligands and receptors are both specific and redundant: BAFFR binds BAFF, whereas BCMA and TACI bind to either BAFF or APRIL. In a previous phylogenetic inquiry, we identified and characterized a BAFF-like gene in lampreys, which, with hagfish, are the only extant jawless vertebrates, both of which have B-like and T-like lymphocytes. To gain insight into lymphocyte regulation in jawless vertebrates, in this study we identified two BCMA-like genes in lampreys, BCMAL1 and BCMAL2, which were found to be preferentially expressed by B-like lymphocytes. In vitro analyses indicated that the lamprey BAFF-like protein can bind to a BCMA-like receptor Ig fusion protein and to both BCMAL1- and BCMAL2-transfected cells. Discriminating regulatory roles for the two BCMA-like molecules are suggested by their differential expression before and after activation of the B-like lymphocytes in lampreys. Our composite results imply that BAFF-based mechanisms for B cell regulation evolved before the divergence of jawed and jawless vertebrates.

ADVERSE EFFECT OF MACULAR INTRARETINAL HEMORRHAGE ON THE PROGNOSIS OF SUBMACULAR HEMORRHAGE DUE TO RETINAL ARTERIAL MACROANEURYSM RUPTURE.

PURPOSE: To investigate the clinical course of submacular hemorrhage associated with ruptured retinal arterial macroaneurysm using swept-source optical coherence tomography. METHODS: This study included 23 eyes of 23 consecutive patients diagnosed with submacular hemorrhage associated with ruptured retinal arterial macroaneurysm. Cases underwent displacement of submacular hemorrhage (vitrectomy + subretinal injection of tissue plasminogen activator + air tamponade) and were followed up for 6 months after surgery. Localization of the preoperative hemorrhage and its effect on preoperative and postoperative best-corrected visual acuity, central retinal thickness, and continuity of the ellipsoid zone were measured. RESULTS: Macular intraretinal hemorrhage (IRH) was observed in 17 eyes (73.9%, IRH [+] group) and was not observed in 6 eyes (26.1%, IRH [-] group). The IRH (+) group showed worse postoperative best-corrected visual acuity values compared with the IRH (-) group (0.89 ± 0.47 in logarithm of the minimal angle of resolution units, Snellen equivalent 20/155 and 0.16 ± 0.23, 20/29, respectively; P < 0.01), smaller central retinal thickness values (97.7 ± 53.5 µm, 173.0 ± 32.3 µm, respectively; P < 0.01), and a higher rate of ellipsoid zone disruption (100%, 33.3%, respectively; P < 0.01). CONCLUSION: Patients with preoperative macular IRH showed lower postoperative visual acuity and worse macular contour after submacular hemorrhage displacement compared with patients without macular IRH.

EN FACE IMAGE-BASED ANALYSIS OF RETINAL TRACTION CAUSED BY EPIRETINAL MEMBRANE AND ITS RELATIONSHIP WITH VISUAL FUNCTIONS.

PURPOSE: To evaluate the relationship between retinal traction caused by epiretinal membrane and visual functions. METHODS: In this institutional study, en face swept-source optical coherence tomography images of 141 eyes of 130 patients with epiretinal membrane were analyzed to investigate maximum depth of retinal folds, which represents retinal traction strength and the distribution pattern of retinal folds. We investigated the relationships between the maximum depth and distribution pattern of retinal folds and visual functions as well as the effects of membrane peeling. RESULTS: Maximum retinal fold depth was significantly correlated with the metamorphopsia score (P < 0.001). Fifteen eyes showed retinal folds radially extending from the macular epiretinal membrane (radiating folds group), whereas 126 eyes showed a multidirectional pattern of retinal folds (multidirectional folds group). The radiating folds group showed a significantly lower metamorphopsia score (P = 0.014). Multiple regression analysis revealed that the metamorphopsia score was significantly related to maximum retinal fold depth (P = 0.003), distribution pattern (P = 0.015), and central retinal thickness (P < 0.001). One month after membrane peeling, parafoveal retinal folds resolved completely in all cases, and both visual acuity (P < 0.001) and average metamorphopsia score (P = 0.036) were significantly improved. CONCLUSION: Both the strength and the distribution pattern of retinal traction are significantly related to metamorphopsia in epiretinal membrane patients.

Evolutionary implications of a third lymphocyte lineage in lampreys.

Jawed vertebrates (gnathostomes) and jawless vertebrates (cyclostomes) have different adaptive immune systems. Gnathostomes use T- and B-cell antigen receptors belonging to the immunoglobulin superfamily. Cyclostomes, the lampreys and hagfish, instead use leucine-rich repeat proteins to construct variable lymphocyte receptors (VLRs), two types of which, VLRA and VLRB, are reciprocally expressed by lymphocytes resembling gnathostome T and B cells. Here we define another lineage of T-cell-like lymphocytes that express the recently identified VLRC receptors. Both VLRC(+) and VLRA(+) lymphocytes express orthologues of genes that gnathostome γδ and αß T cells use for their differentiation, undergo VLRC and VLRA assembly and repertoire diversification in the 'thymoid' gill region, and express their VLRs solely as cell-surface proteins. Our findings suggest that the genetic programmes for two primordial T-cell lineages and a prototypic B-cell lineage were already present in the last common vertebrate ancestor approximately 500 million years ago. We propose that functional specialization of distinct T-cell-like lineages was an ancient feature of a primordial immune system.

Characterization of Lamprey BAFF-like Gene: Evolutionary Implications.

BAFF (TNF superfamily [TNFSF] 13B/Blys) and APRIL (TNFSF13) are important regulatory factors for lymphocyte activation and survival in mammals. A BAFF/APRIL-like relative called BAFF- and APRIL-like molecule (BALM) has also been identified in cartilaginous and bony fishes, and we report in this study a BAFF-like gene in lampreys. Our phylogenetic analysis of these genes and a related TNFSF12 gene called TNF-like weak inducer of apoptosis (TWEAK) suggest that, whereas an ancestral homolog of BAFF and APRIL was already present in a common ancestor of jawed and jawless vertebrates, TWEAK evolved early on in the jawed vertebrate lineage. Like mammalian BAFF and APRIL, the lamprey BAFF-like gene is expressed in T-like, B-like, and innate immune cells. The predicted protein encoded by this BAFF-like gene in lampreys exhibits higher sequence similarity with mammalian BAFF than APRIL. Correspondingly, we find BAFF orthologs in all of the jawed vertebrate representatives that we examined, although APRIL and/or BALM orthologs are not identifiable in certain jawed vertebrates. For example, BALM is not identifiable in tetrapods, and APRIL is not identifiable in several bony fishes or in birds, the latter of which also lack a TWEAK-like gene. Our analysis further suggests that a hybrid molecule called TWE-PRIL, which is a product of an in-genomic fusion between APRIL and TWEAK genes evolved early in mammalian evolution.

Embedding of lamellar hole-associated epiretinal proliferation combined with internal limiting membrane inversion for the treatment of lamellar macular hole: a case report.

BACKGROUND: We recently reported that lamellar macular hole (LMH) with lamellar hole-associated epiretinal proliferation (LHEP) can be effectively treated by embedding the LHEP into the retinal cleavage to improve foveal contour and visual acuity. Here, we report a case of LMH with LHEP for which we performed embedding of the LHEP combined with internal limiting membrane (ILM) inversion. We then evaluated the effects of this surgery on macular morphology and visual functions. CASE PRESENTATION: A 62-year-old man presented with visual disturbance (20/29) and metamorphopsia in his right eye. B-scan optical coherence tomography (OCT) imaging revealed the presence of both partial-thickness defect of the macula with degenerative retinal cleavage and LHEP at the surface of the retina. En face OCT imaging showed the absence of retinal fold. We performed phacoemulsification with intraocular lens implantation, vitrectomy, embedding of LHEP into the retinal cleavage, and ILM inversion. Three months after the surgery, both foveal contour and visual acuity (20/20) were improved and metamorphopsia was reduced. CONCLUSION: Embedding of the LHEP combined with ILM inversion may be an effective treatment for LMH with LHEP.

Traumatic Globe Luxation with Complete Optic Nerve Transection Caused by Heavy Object Compression.

Traumatic eyeball luxation is a rare clinical condition with a dramatic presentation. Here, we describe a unique case of traumatic globe luxation and complete optic nerve transection caused by heavy object compression. A 45-year-old male automobile mechanic was injured when a truck slipped from its supports, crushing his head and face. On arrival, his right eyeball was obviously displaced anteriorly and he had no light perception. Computed tomography revealed complex frontal bone and facial fractures with underlying brain contusion in addition to complete transection of the right optic nerve. The patient was successfully treated using a multidisciplinary approach.

Impact of Polyp Regression on 2-year Outcomes of Intravitreal Aflibercept Injections: A Treat-and-Extend Regimen for Polypoidal Choroidal Vasculopathy.

We conducted intravitreal aflibercept injections (IVAs) for 37 Japanese patients (28 males, 9 females, mean age 73.4 years) with polypoidal choroidal vasculopathy (PCV), with a treat-and-extend regimen (TER). We evaluated the impact of polyp regression after a loading dose (2-mg IVA 1×/month for 3 months) on the patients' 2-year treatment outcomes. Thirty-seven eyes were treated with IVA by a TER for 2 years. We divided the patients into 2 groups based on their polyp status after the loading dose: polyp regression (PR+) (n=19) and no polyp regression (PR-) (n=18). We compared the groups' best-corrected visual acuity (BCVA), central retinal thickness (CRT), recurrence rate, total number of injections, and final treatment interval. Both the BCVA and CRT were significantly improved by the treatment in both groups, with no between-group difference in the amount of change (p=0.769). In the polyp regression (+) group, recurrence was significantly less common (p=0.03), the mean total number of injections was significantly lower (p=0.013), and the mean treatment interval was significantly longer (0.042). Regarding the 2-year outcomes for PCV, the eyes with post-loading-dose polyp regression demonstrated less frequent recurrence and required fewer numbers of injections compared to the eyes without polyp regression.

Results of a Treat-and-Eextend Regimen of Intravitreal Ranibizumab Injection for Macular Edema due to Branch Retinal Vein Occlusion.

To investigate the effectiveness of a treat-and-extend regimen (TAE) of intravitreal ranibizumab injections (IVR) for macular edema (ME) due to branch retinal vein occlusion (BRVO). We retrospectively examined 35 eyes of 35 patients with ME due to BRVO who underwent TAE for 1 year. Patients whose treatment interval extended to 12 weeks were switched to a pro re nata regimen (PRN; TAE to PRN group), while TAE was continued for patients whose treatment interval was less than 12 weeks (continued TAE group). Changes in best-corrected visual acuity (BCVA), central retinal thickness (CRT), and predictive factors for inclusion in the TAE to PRN group were analyzed. BCVA and CRT both improved significantly at 1 year compared with baseline (p<0.001). Sixteen eyes (45.7%) were included in the TAE to PRN group, while 19 eyes (54.3%) were included in the continued TAE group. BCVA in the TAE to PRN group was significantly better than that in the continued TAE group at 1 year (p=0.047). BCVA at baseline and macular BRVO were significant predictive factors for inclusion in the TAE to PRN group. TAE was effective for improving BCVA and CRT. The TAE to PRN group showed significantly better prognosis.

Characterization of Lamprey IL-17 Family Members and Their Receptors.

IL-17 is an ancient cytokine implicated in a variety of immune defense reactions. We identified five members of the sea lamprey IL-17 family (IL-17D.1, IL-17D.2, IL-17E, IL-17B, and IL-17C) and six IL-17R genes (IL-17RA.1, IL-17RA.2, IL-17RA.3, IL-17RF, IL-17RE/RC, and IL-17RD), determined their relationship with mammalian orthologs, and examined their expression patterns and potential interactions to explore their roles in innate and adaptive immunity. The most highly expressed IL-17 family member is IL-17D.1 (mammalian IL-17D like), which was found to be preferentially expressed by epithelial cells of skin, intestine, and gills and by the two types of lamprey T-like cells. IL-17D.1 binding to rIL-17RA.1 and to the surface of IL-17RA.1-expressing B-like cells and monocytes of lamprey larvae was demonstrated, and treatment of lamprey blood cells with rIL-17D.1 protein enhanced transcription of genes expressed by the B-like cells. These findings suggest a potential role for IL-17 in coordinating the interactions between T-like cells and other cells of the adaptive and innate immune systems in jawless vertebrates.

A thymus candidate in lampreys.

Immunologists and evolutionary biologists have been debating the nature of the immune system of jawless vertebrates--lampreys and hagfish--since the nineteenth century. In the past 50 years, these fish were shown to have antibody-like responses and the capacity to reject allografts but were found to lack the immunoglobulin-based adaptive immune system of jawed vertebrates. Recent work has shown that lampreys have lymphocytes that instead express somatically diversified antigen receptors that contain leucine-rich-repeats, termed variable lymphocyte receptors (VLRs), and that the type of VLR expressed is specific to the lymphocyte lineage: T-like lymphocytes express type A VLR (VLRA) genes, and B-like lymphocytes express VLRB genes. These clonally diverse anticipatory antigen receptors are assembled from incomplete genomic fragments by gene conversion, which is thought to be initiated by either of two genes encoding cytosine deaminase, cytosine deaminase 1 (CDA1) in T-like cells and CDA2 in B-like cells. It is unknown whether jawless fish, like jawed vertebrates, have dedicated primary lymphoid organs, such as the thymus, where the development and selection of lymphocytes takes place. Here we identify discrete thymus-like lympho-epithelial structures, termed thymoids, in the tips of the gill filaments and the neighbouring secondary lamellae (both within the gill basket) of lamprey larvae. Only in the thymoids was expression of the orthologue of the gene encoding forkhead box N1 (FOXN1), a marker of the thymopoietic microenvironment in jawed vertebrates, accompanied by expression of CDA1 and VLRA. This expression pattern was unaffected by immunization of lampreys or by stimulation with a T-cell mitogen. Non-functional VLRA gene assemblies were found frequently in the thymoids but not elsewhere, further implicating the thymoid as the site of development of T-like cells in lampreys. These findings suggest that the similarities underlying the dual nature of the adaptive immune systems in the two sister groups of vertebrates extend to primary lymphoid organs.

Evolution of vertebrate adaptive immunity: immune cells and tissues, and AID/APOBEC cytidine deaminases.

All surviving jawed vertebrate representatives achieve diversity in immunoglobulin-based B and T cell receptors for antigen recognition through recombinatorial rearrangement of V(D)J segments. However, the extant jawless vertebrates, lampreys and hagfish, instead generate three types of variable lymphocyte receptors (VLRs) through a template-mediated combinatorial assembly of different leucine-rich repeat (LRR) sequences. The clonally diverse VLRB receptors are expressed by B-like lymphocytes, while the VLRA and VLRC receptors are expressed by lymphocyte lineages that resemble αß and γδ T lymphocytes, respectively. These findings suggest that three basic types of lymphocytes, one B-like and two T-like, are an essential feature of vertebrate adaptive immunity. Around 500 million years ago, a common ancestor of jawed and jawless vertebrates evolved a genetic program for the development of prototypic lymphoid cells as a foundation for an adaptive immune system. This acquisition preceded the convergent evolution of alternative types of clonally diverse receptors for antigens in all vertebrates, as reviewed in this article.

Genomic donor cassette sharing during VLRA and VLRC assembly in jawless vertebrates.

Lampreys possess two T-like lymphocyte lineages that express either variable lymphocyte receptor (VLR) A or VLRC antigen receptors. VLRA(+) and VLRC(+) lymphocytes share many similarities with the two principal T-cell lineages of jawed vertebrates expressing the αß and γδ T-cell receptors (TCRs). During the assembly of VLR genes, several types of genomic cassettes are inserted, in step-wise fashion, into incomplete germ-line genes to generate the mature forms of antigen receptor genes. Unexpectedly, the structurally variable components of VLRA and VLRC receptors often possess partially identical sequences; this phenomenon of module sharing between these two VLR isotypes occurs in both lampreys and hagfishes. By contrast, VLRA and VLRC molecules typically do not share their building blocks with the structurally analogous VLRB receptors that are expressed by B-like lymphocytes. Our studies reveal that VLRA and VLRC germ-line genes are situated in close proximity to each other in the lamprey genome and indicate the interspersed arrangement of isotype-specific and shared genomic donor cassettes; these features may facilitate the shared cassette use. The genomic structure of the VLRA/VLRC locus in lampreys is reminiscent of the interspersed nature of the TCRA/TCRD locus in jawed vertebrates that also allows the sharing of some variable gene segments during the recombinatorial assembly of TCR genes.

Selection of the lamprey VLRC antigen receptor repertoire.

The alternative adaptive immune system of jawless vertebrates is based on different isotypes of variable lymphocyte receptors (VLRs) that are composed of leucine-rich repeats (LRRs) and expressed by distinct B- and T-like lymphocyte lineages. VLRB is expressed by B-like cells, whereas VLRA and VLRC are expressed by two T-like lineages that develop in the thymoid, a thymus-like structure in lamprey larvae. In each case, stepwise combinatorial insertions of different types of short donor LRR cassettes into incomplete germ-line genes are required to generate functional VLR gene assemblies. It is unknown, however, whether the diverse repertoires of VLRs that are expressed by peripheral blood lymphocytes are shaped by selection after their assembly. Here, we identify signatures of selection in the peripheral repertoire of VLRC antigen receptors that are clonally expressed by one of the T-like cell types in lampreys. Selection strongly favors VLRC molecules containing four internal variable leucine-rich repeat (LRRV) modules, although VLRC assemblies encoding five internal modules are initially equally frequent. In addition to the length selection, VLRC molecules in VLRC(+) peripheral lymphocytes exhibit a distinct pattern of high entropy sites in the N-terminal LRR1 module, which is inserted next to the germ-line-encoded LRRNT module. This is evident in comparisons to VLRC gene assemblies found in the thymoid and to VLRC gene assemblies found in some VLRA(+) cells. Our findings are the first indication to our knowledge that selection operates on a VLR repertoire and provide a framework to establish the mechanism by which this selection occurs during development of the VLRC(+) lymphocyte lineage.

Organization of lamprey variable lymphocyte receptor C locus and repertoire development.

Jawless vertebrates are pivotal representatives for studies of the evolution of adaptive immunity due to their unique position in chordate phylogeny. Lamprey and hagfish, the extant jawless vertebrates, have an alternative lymphocyte-based adaptive immune system that is based on somatically diversifying leucine-rich repeat (LRR)-based antigen receptors, termed variable lymphocyte receptors (VLRs). Lamprey T-like and B-like lymphocyte lineages have been shown to express VLRA and VLRB types of anticipatory receptors, respectively. An additional VLR type, termed VLRC, has recently been identified in arctic lamprey (Lethenteron camtschaticum), and our analysis indicates that VLRC sequences are well conserved in sea lamprey (Petromyzon marinus), L. camtschaticum, and European brook lamprey (Lampetra planeri). Genome sequences of P. marinus were analyzed to determine the organization of the VLRC-encoding locus. In addition to the incomplete germ-line VLRC gene, we have identified 182 flanking donor genomic sequences that could be used to complete the assembly of mature VLRC genes. Donor LRR cassettes were classifiable into five basic structural groups, the composition of which determines their order of use during VLRC assembly by virtue of sequence similarities to the incomplete germ-line gene and to one another. Bidirectional VLRC assembly was predicted by comparisons of mature VLRC genes with the sequences of donor LRR cassettes and verified by analysis of partially assembled intermediates. Biased and repetitive use of certain donor LRR cassettes was demonstrable in mature VLRCs. Our analysis provides insight into the unique molecular strategies used for VLRC gene assembly and repertoire diversification.

Definition of a third VLR gene in hagfish.

Jawless vertebrates (cyclostomes) have an alternative adaptive immune system in which lymphocytes somatically diversify their variable lymphocyte receptors (VLR) through recombinatorial use of leucine-rich repeat cassettes during VLR gene assembly. Three types of these anticipatory receptors in lampreys (VLRA, VLRB, and VLRC) are expressed by separate lymphocyte lineages. However, only two VLR genes (VLRA and VLRB) have been found in hagfish. Here we have identified a third hagfish VLR, which undergoes somatic assembly to generate sufficient diversity to encode a large repertoire of anticipatory receptors. Sequence analysis, structural comparison, and phylogenetic analysis indicate that the unique hagfish VLR is the counterpart of lamprey VLRA and the previously identified hagfish "VLRA" is the lamprey VLRC counterpart. The demonstration of three orthologous VLR genes in both lampreys and hagfish suggests that this anticipatory receptor system evolved in a common ancestor of the two cyclostome lineages around 480 Mya.

Genome-wide mutation avalanches induced in diploid yeast cells by a base analog or an APOBEC deaminase.

Genetic information should be accurately transmitted from cell to cell; conversely, the adaptation in evolution and disease is fueled by mutations. In the case of cancer development, multiple genetic changes happen in somatic diploid cells. Most classic studies of the molecular mechanisms of mutagenesis have been performed in haploids. We demonstrate that the parameters of the mutation process are different in diploid cell populations. The genomes of drug-resistant mutants induced in yeast diploids by base analog 6-hydroxylaminopurine (HAP) or AID/APOBEC cytosine deaminase PmCDA1 from lamprey carried a stunning load of thousands of unselected mutations. Haploid mutants contained almost an order of magnitude fewer mutations. To explain this, we propose that the distribution of induced mutation rates in the cell population is uneven. The mutants in diploids with coincidental mutations in the two copies of the reporter gene arise from a fraction of cells that are transiently hypersensitive to the mutagenic action of a given mutagen. The progeny of such cells were never recovered in haploids due to the lethality caused by the inactivation of single-copy essential genes in cells with too many induced mutations. In diploid cells, the progeny of hypersensitive cells survived, but their genomes were saturated by heterozygous mutations. The reason for the hypermutability of cells could be transient faults of the mutation prevention pathways, like sanitization of nucleotide pools for HAP or an elevated expression of the PmCDA1 gene or the temporary inability of the destruction of the deaminase. The hypothesis on spikes of mutability may explain the sudden acquisition of multiple mutational changes during evolution and carcinogenesis.

Evolution of two prototypic T cell lineages.

Jawless vertebrates, which occupy a unique position in chordate phylogeny, employ leucine-rich repeat (LRR)-based variable lymphocyte receptors (VLR) for antigen recognition. During the assembly of the VLR genes (VLRA, VLRB and VLRC), donor LRR-encoding sequences are copied in a step-wise manner into the incomplete germ-line genes. The assembled VLR genes are differentially expressed by discrete lymphocyte lineages: VLRA- and VLRC-producing cells are T-cell like, whereas VLRB-producing cells are B-cell like. VLRA(+) and VLRC(+) lymphocytes resemble the two principal T-cell lineages of jawed vertebrates that express the αß or γδ T-cell receptors (TCR). Reminiscent of the interspersed nature of the TCRα/TCRδ locus in jawed vertebrates, the close proximity of the VLRA and VLRC loci facilitates sharing of donor LRR sequences during VLRA and VLRC assembly. Here we discuss the insight these findings provide into vertebrate T- and B-cell evolution, and the alternative types of anticipatory receptors they use for adaptive immunity.