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Lymphocyte Populations in Jawless Vertebrates: Insights Into the Origin and Evolution of Adaptive Immunity
Chapter 3
Lymphocyte Populations in Jawless Vertebrates: Insights Into the Origin and Evolution of Adaptive Immunity Yoichi Sutoh*, Masanori Kasahara** *Emory Vaccine Center and Department of Pathology and Laboratory Medicine, Emory University, Atlanta, GA, United States; **Department of Pathology, Hokkaido University Graduate School of Medicine, Sapporo, Japan
1 INTRODUCTION The human body is thought to have tens of millions of lymphocyte clones, each expressing antigen receptors with distinct specificities. When infected with pathogens, lymphocyte clones expressing specific receptors undergo proliferation and differentiate into effector lymphocytes. Most of these effector lymphocytes die by apoptosis shortly after the elimination of pathogens. Some lymphocytes, however, survive for a long time to build an immunological memory. These lymphocytes—known as memory lymphocytes—enable the host to mount a more prompt and vigorous immune response upon reexposure to the same pathogen. Although the adaptive immune system (AIS) is a complex biological system, its backbone is formed by highly diverse antigen receptors clonally expressed on lymphocytes. The origin and evolution of the AIS and lymphocytes has attracted the interest of immunologists for a long time. Animals that have received particular attention in this regard are the jawless vertebrates represented by lampreys and hagfish. Studies conducted in the 1960s and 1970s showed that both lampreys and hagfish are capable of producing specific agglutinins against particulate antigens and rejecting skin allografts with immunological memory,1–8 suggesting that the origin of adaptive immunity can be traced back to the emergence of jawless vertebrates. Consistent with this, jawless vertebrates have blood cells morphologically indistinguishable from mammalian lymphocytes. In lampreys, naive lymphocyte-like cells measure about 10 mm in diameter and have a round shape, with thin cytosol and a large nucleus.9,10 In electron microscopy, thin The Evolution of the Immune System. http://dx.doi.org/10.1016/B978-0-12-801975-7.00003-7 Copyright © 2016 Elsevier Inc. All rights reserved.
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heterochromatin is found on the inside rim of the nuclear membrane.9 After stimulation with particulate antigens such as sheep blood-cells and anthrax spore-coats, the lymphocyte-like cells undergo proliferation, differentiating into plasma cell-like cells with enlarged cytosol that contain well-developed rough endoplasmic reticulum.11 Transcriptome analysis revealed that lamprey and hagfish lymphocyte-like cells express many genes whose mammalian counterparts are expressed by lymphocytes, such as the genes coding for a homolog of Spi, B-cell receptorassociated protein (BCAP), GATA2/3, CXC chemokine receptor (CXCR) 4, and CD98.12–14 However, the transcripts coding for major histocompatibility complex (MHC) molecules, T-cell receptors (TCRs), B-cell receptors (BCRs), or the RAG (recombination activating gene) enzymes were never identified from jawless vertebrates. This was puzzling because, at first glance, it contradicted the earlier studies that clearly demonstrated the ability of jawless vertebrates to reject skin allografts and to produce agglutinins specific for particulate antigens. Ultimately, this puzzle was resolved by the discovery that, instead of TCRs and BCRs, jawless vertebrates use a unique antigen receptor now known as variable lymphocyte receptors (VLR).15–20
2 OVERVIEW OF VLRs 2.1 Structure of VLR Proteins and Gene Assembly VLR was discovered in lampreys as a gene coding for proteins with highly diverse sequences, through the analysis of a cDNA library, enriched for transcripts upregulated in antigen-stimulated lymphocyte-like cells.21 Structurally, VLRs are a member of the leucine-rich repeat (LRR) family of proteins composed of a signal peptide (SP), an N-terminal LRR cassette (LRRNT), an 18-residue N-terminal LRR cassette (LRR1), a variable number of 24-residue LRR cassettes (LRRV), a 24-residue end LRRV cassette (LRRVe), a 13-residue truncated LRR cassette [also called the connecting peptide (CP)], a C-terminal LRR cassette (LRRCT), and an invariant domain containing a stalk region (Fig. 3.1A). The sequences of the regions ranging from the 3′-half of LRRNT (3′-LRRNT) to the 5′-half of LRRCT (5′-LRRCT) are highly diverse. By contrast, the sequences of the remaining regions—the SP, the 5′-half of LRRNT, the 3′-half of LRRCT, and the stalk region—are invariant. Remarkably, the VLR gene has an incomplete structure incapable of encoding any protein in the genome of nonlymphoid cells, such as erythrocytes; this germline gene encodes only the invariant region of VLR proteins and lacks the sequences coding for the diversity region (Fig. 3.1B). Cassettes such as 3′-LRRNT, LRR1, LRRV, LRRVe, CP, and 5′-LRRCT, constituting the diversity region, are located in multiple copies (sometimes in several hundred copies) in the vicinity of the germline VLR gene. During lymphocyte development, the invariant intervening sequence of the
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FIGURE 3.1 Domain organization, gene assembly, and tertiary structure of VLRs. (A) Domain organization of mature VLR protein. The diversity region is composed of the following LRR cassettes: the N-terminal LRR cap (LRRNT), LRR1, a variable number of LRRVs, LRRVe, a connecting peptide (CP), and the C-terminal LRR cap (LRRCT). SP, signal peptide. (B) Assembly of VLR genes. The germline VLR gene has an invariant intervening sequence instead of highly diverse LRR cassettes and is unable to encode functional proteins (top). During lymphocyte development, LRR cassettes, which are scattered around the germline VLR, replace the intervening sequence in a stepwise manner from either the 5′- or 3′-end (middle and bottom). This process, which resembles gene conversion, is thought to be mediated by cytidine deaminases of the AID-APOBEC family. (C) Tertiary structure of VLRs. VLR proteins have a horseshoe-shaped structure characteristic of LRR family proteins. The majority of variable amino acid residues are located on the b-sheet, facing the concave surface (shared by all three receptors), and the hypervariable loop in LRRCT (present only in VLRA and VLRB receptors; indicated by red arrows). Both ends of the b-sheet are capped by LRRNT and LRRCT (indicated by blue and red, respectively). Unlike VLRA or VLRB, lamprey VLRC has an invariable loop in LRRNT (indicated by a blue arrow). Like TCRs and BCRs, all three VLRs lack a signaling domain in the cytoplasmic region. Therefore, it is assumed that VLRs associate with signal transducers yet to be identified.
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germline VLR gene is replaced by a gene conversion-like mechanism in a stepwise manner, beginning either from its 5′- or 3′-end, by adding flanking cassettes, and eventually forming a completely assembled VLR gene.22–25 Short stretches of nucleotide homology (10–30 bps) are found between donor and acceptor sequences.24,26,27 Therefore, the sequence located at the ends of the most newly copied cassette presumably determines which flanking LRR cassettes should be copied into the germline VLR gene in the next step. The sequences of LRR cassettes are highly variable and the number of copied LRRV cassettes is also variable. This enables assembled VLR genes to acquire sequence diversity comparable to that of TCRs and BCRs.22,24 The assembly of VLR genes is mediated by cytidine deaminases (CDA) of the activation-induced cytidine deaminase (AID)-apolipoprotein B mRNA editing enzyme (APOBEC) family.24 In lampreys, two CDAs named CDA1 and CDA2 have been identified.24
2.2 Both Lampreys and Hagfish Have Three VLR Genes Initially, only one VLR gene was identified in lampreys.21 Subsequent studies revealed that lampreys have two more VLR genes.24,28 The three VLR genes are now known as VLRA, VLRB, and VLRC. Hagfish also have three VLR genes named VLRA, VLRB, and VLRC, thought to be orthologous to lamprey VLRA, VLRB, and VLRC genes,29,30 indicating that these three genes already existed in a common ancestor of lampreys and hagfish. Functional studies in lampreys demonstrated that the products of the three VLR genes are expressed on three distinct populations of lymphocyte-like cells31,32 (Fig. 3.2). VLRA and VLRC, which are more closely related to each other in sequence than they are to VLRB, are membrane-bound receptors with no secretory form. VLRA+ cells and VLRC+ cells are T-cell-like and develop in the “thymoid,” an organ assumed to be the equivalent of the gnathostome thymus.33 On the other hand, VLRB is a glycosylphosphatidylinositolanchored protein that also occurs in a secretory form. Secreted VLRB molecules form pentamers or tetramers of dimers and have 8–10 antigen binding sites.34 This type of subunit organization resembles the subunit arrangement of gnathostome IgM antibodies and accounts for the strong agglutinating activities of VLRB antibodies. Interestingly, VLRB+ cells are B-cell-like, develop in hematopoietic organs, undergo clonal proliferation in response to antigen stimulation, and differentiate into plasma cell-like cells secreting VLRB antibodies.11,22,31 Furthermore, similar to the allelic exclusion in TCR and BCR genes, each lymphocyte expresses only a single functional VLR allele, indicating that VLR assembly occurs in a monoallelic manner.21,28,31 Collectively, these observations provided convincing evidence that lymphocyte-like cells in jawless vertebrates resemble gnathostome lymphocytes, not only morphologically, but also functionally, and hence should be regarded as authentic lymphocytes.
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FIGURE 3.2 Features of three populations of lamprey lymphocytes. Lampreys have two lymphocyte lineages whose gene expression profiles resemble gnathostome T cells and B cells, respectively. VLRA and VLRC are membrane-bound receptors without any secretory form. VLRA+ cells and VLRC+ cells express molecules related to T-cell development and/or function. They also secrete IL-17, a pro-inflammatory cytokine released from immune cells, including Th17 cells in jawed vertebrates. VLRC+ cells specifically express genes such as SOX13, ITGAL, and TLR3, which are characteristically expressed in gnathostome gd T cells. VLRB receptors occur in both membrane-bound and secretory forms. VLRB+ cells express molecules typically expressed in gnathostome B cells.
2.3 Crystal Structure of VLR Proteins and Antigen Recognition Crystallographic analysis revealed that VLR proteins adopt a horseshoe-like solenoid structure characteristic of the LRR protein family, capped by LRRNT and LRRCT cassettes at N- and C-terminal ends, respectively (Fig. 3.1C).35 The inner concave surface of the solenoid structure is formed from multiple b-strands (derived from LRRNT, LRR1, LRRV, LRRVe, and CP), which assemble into a continuous b-sheet. In VLRA and VLRB receptors, this b-sheet and a protruding loop formed in the 5′-LRRCT cap contain the majority of variable residues. Structural analysis of VLRB receptors in complex with antigens such as Type O blood antigen (H-trisaccharide) and hen egg-white lysozyme (HEL) demonstrated that VLRB binds antigens via the b-sheet and protruding loop in 5′-LRRCT.36 Interestingly, the protrusion in 5′-LRRCT penetrated deeply into the catalytic cleft of HEL in the VLRB-HEL complex.37 Immunoglobulins made up of VH and VL chains normally recognize epitopes exposed on the surface of molecules. The ability of VLRs to bind to residues hidden in the cleft is reminiscent of camel and shark VH antibodies that preferentially target clefts.38,39
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In one study, lampreys immunized with HEL produced not only specific VLRBs, but also specific VLRAs with binding affinity comparable to that of IgG.40 The crystal structure of lamprey VLRA in a complex with HEL revealed that VLRA interacts with HEL through its concave surface and LRRCT.41 Unlike VLRA or VLRB receptors, VLRC lacks a protrusion in its LRRCT region28,30,35 (Fig. 3.1C), and its LRRCT is semiinvariant. Also, unlike VLRA or VLRB receptors, the LRRNT of lamprey VLRC has a loop that protrudes into the concave surface.42,43 This loop shows little sequence variation. These structural features of VLRC suggest that VLRC might recognize antigens in a unique manner. It is possible that VLRC interacts with putative antigen-presenting molecules through its invariant or semiinvariant LRRCT region and LRRNT loop.
2.4 VLRA and VLRC Genes are Tightly Linked in the Lamprey Genome VLRA and VLRC loci are located close to each other in the lamprey genome.26 Similar to the gnathostome TCRA/TCRD locus that shares some V segments, lamprey VLRA and VLRC genes often use the same LRR cassettes for their assembly.44 Such sharing of LRR cassettes is also observed in assembled hagfish VLRA and VLRC genes, suggesting that VLRA and VLRC genes are also tightly linked in the hagfish genome. In contrast, neither lamprey nor hagfish VLRB receptors share identical LRR cassettes with the VLRA or VLRC receptors of respective species, suggesting that the germline VLRB gene is not situated close to the VLRA/C locus. Consistent with this, previous fluorescence in situ hybridization analysis showed that hagfish VLRB and VLRC (then known as VLRA) genes are physically well-separated, although they are on the same chromosome.45 It appears that VLR genes increased their copy number by tandem duplication; subsequently, chromosomal inversion or intrachromosomal translocation presumably separated the VLRB gene from the VLRA/C genes, facilitating the functional specialization of the receptor genes.
3 THREE POPULATIONS OF AGNATHAN LYMPHOCYTES 3.1 VLRA+ Cells and VLRC+ Cells Resemble Gnathostome T Cells, Whereas VLRB+ Cells Resemble Gnathostome B Cells VLRA+ cells and VLRC+ cells resemble gnathostome T cells in that they undergo proliferation in response to a T-cell mitogen such as phytohemagglutinin (PHA). They also resemble T cells in terms of gene expression profiles (Fig. 3.2). For instance, they express transcription factors involved in T-cell development, such as GATA2/3, B-cell CLL/lymphoma 11B (BCL11b), and T-cell factor 1 (TCF1), signaling molecules such as LAT (linker for activation of T cells), cell surface molecules such as NOTCH1, CD45, and aryl hydrocarbon receptor (AHR), cytokines such as IL-17, and cytokine receptors such
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as chemokine receptor 9/7 (CCR9/7), which regulate the migration of T-cell progenitors to the thymus.31,32 VLRC+ cells generally have a gene expression profile similar to that of VLRA+ cells, but differ from VLRA+ cells in that they express genes characteristically expressed in gnathostome gd T cells, such as sex-determining region Y-box 13 (SOX13) coding for a fate-determining factor for the gd T cell lineage, and Toll-like receptor 3 (TLR3).32 VLRC+ cells also express an integrin family of adhesion molecules involved in the epithelial localization of gd T cells, such as integrins aL (ITGAL), a4 (ITGA4), and b1 (ITGB1). On the other hand, VLRB+ cells resemble gnathostome B cells, in that they differentiate into plasma cells and secrete specific VLRB molecules as antibodies, when challenged with particulate antigens.11 Also, the gene expression profile of VLRB+ cells is similar to that of gnathostome B cells; they express transcription factors such as B-lymphocyte-induced maturation protein 1 (BLIMP-1), B-cell CLL/lymphoma 6 (BCL6), and paired box protein 5 (PAX5), signal-transducing molecules such as spleen tyrosine kinase (Syk) and B-cell adaptor protein (BCAP), and Toll-like receptors such as TLR2a-c, TLR7, and TLR10. Interestingly, T-cell-like VLRA+ cells express the IL-8 receptor and IL17, whereas B-cell-like VLRB+ cells express IL-8 and the IL-17 receptor. This observation suggests that VLRA+ and VLRB+ cells may have a crosstalk via cytokines and possibly interact in a manner analogous to T–B cell collaboration32 (Fig. 3.3). In rodents, gd T cells are a minor population of T lymphocytes in blood and peripheral lymphoid tissues; however, they occupy the majority of lymphocytes in epithelial layers of tissues such as skin, intestine, tongue, and the reproductive tract. These intraepithelial gd T cells express TCRs with limited variability. Most notably, gd T cells residing in the epidermis, known as dendritic epidermal T cells (DETC), express an invariant Vg5Vd1 (alternate nomenclature Vg3Vd1) TCR, lacking junctional diversity.47,48 Interestingly, lamprey VLRC+ cells are located predominantly in epithelial tissues and express restricted antigen receptor diversity, showing intriguing similarity to mouse DETCs and intraepithelial lymphocytes (Fig. 3.2).49 When stimulated with poly I:C, a synthetic viral double-strand RNA (dsRNA) analog, VLRC+ cells respond via their TLR3 and upregulate IL-16 expression. IL-16 is a chemoattractant recruiting CD4+ leukocytes, such as T cells, monocytes, and eosinophils to the site of infection.50 Therefore, similar to DETCs, epithelial VLRC+ cells may perform a sentinel function in the epithelium.
3.2 Antigen Recognition by Agnathan T-Like Cells is Still Full of Mystery When lampreys were immunized with anthrax spores, they generated VLRB+ cells capable of binding to the spores, but VLRA+ cells with such ability were
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FIGURE 3.3 The presumed role of three populations of agnathan lymphocytes in eliminating pathogens. Invading pathogens are recognized by VLRs with specific binding specificities. VLRC+ cells also express TLR3 that recognizes viral double-stranded RNAs. Activated VLRA+ cells and VLRC+ cells secrete pro-inflammatory cytokines, such as IL-17, MIF, and IL-16, which presumably induce the migration of various immune cells to the site of infection. IL-17 released from VLRA+ cells may activate VLRB+ cells that express the IL-17 receptor and induce the secretion of VLRB antibodies into the serum. Secreted VLRB antibodies form an antigen–antibody complex that activates the complement pathway via binding to C1q-like protein and mannose-binding lectin-associated serine protease (MASP). VLRB+ cells might use IL-8 for cross-talk with VLRA+ cells and VLRC+ cells that express the IL-8 receptor. It is still uncertain whether jawless vertebrates have antigen-presenting molecules with functions equivalent to MHC molecules. Also unknown is whether VLRA and VLRC require antigen-presenting molecules for antigen recognition. This figure was modified from Sutoh and Kasahara.46
not detectable,29 suggesting that VLRA receptors do not bind native bacterial surface epitopes and might recognize processed antigens in vivo similar to gnathostome ab T cells. However, screening of a yeast display library resulted in the identification of lamprey VLRA molecules that directly bound to HEL.40 Although this observation demonstrates that VLRA molecules are capable of binding unprocessed antigens, it is in contrast to the fact that ab T cells, the presumed counterpart of agnathan VLRA+ cells, recognize only processed antigens bound to MHC class I or class II molecules. At present, it remains unknown whether VLRA receptors always recognize antigens without any requirement for antigen processing, or direct antigen recognition occurs only in some or exceptional cases. Also unknown is whether direct recognition occurs in vivo.
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Currently, no information is available as to how VLRC+ cells recognize antigens. If VLRC+ cells are gd T-cell-like, as discussed previously, they might recognize antigens directly in a manner similar to gd T cells. This is, however, in apparent conflict with the speculation that lamprey VLRC receptors might interact with putative antigen-presenting molecules through their invariant loop in LRRNT and semiinvariant LRRCT. Much remains to be learned about antigen recognition by agnathan T-cell-like lymphocytes.
4 LYMPHOCYTE DEVELOPMENT IN JAWLESS VERTEBRATES 4.1 Development of VLRB+ Cells B cells develop in hematopoietic organs such as the bone marrow in mammals, the bursa of Fabricius in birds, the pronephros in bony fishes, and the spleen and spiral valves in cartilaginous fishes.51–53 In adult lampreys, VLRB+ cells are abundant in kidneys and blood. On the other hand, in lamprey larvae, VLRB+ cells are abundant in the typhlosole, an invaginated spiral valve spanning the length of the intestine. Recent flow cytometric analysis showed that 26 and 41% of lymphocytes are VLRB+ cells in typhlosoles and kidneys, respectively.29 In both organs, cell proliferation was activated within 28 days after immunization by anthrax spore-coats. Moreover, in situ hybridization analysis detected CDA2 expression in VLRB+ cells, in typhlosoles and kidneys supporting the idea that VLRB gene assembly occurs in these organs.33 Although rigorous analysis with molecular markers remains to be conducted, available evidence indicates strongly that VLRB+ cells develop in hematopoietic organs such as typhlosoles and kidneys, consistent with the idea that VLRB+ cells represent B-lineage cells (Fig. 3.4).
4.2 Development of VLRA+ Cells and VLRC+ Cells In jawed vertebrates, T-lineage progenitors generated in hematopoietic tissues migrate to the thymus. After successful V(D)J recombination of TCR genes, T cells are positively and negatively selected in the thymic cortex and medulla, respectively. As a result of thymic selection, only T cells expressing self-tolerant, MHC-restricted TCRs are allowed to survive and exit to the periphery. The thymus was histologically and/or functionally identified in all jawed vertebrates ranging from cartilaginous fish to mammals, consistent with the fact that they have MHC molecules and TCRs. In contrast, jawless vertebrates lack anatomical structures with discrete cortical and medullary regions resembling the thymus. Therefore, it was generally assumed until recently that the thymus emerged in a common ancestor of jawed vertebrates, concomitant with the emergence of TCR, BCR, MHC, and RAG molecules.54,55 However, the discovery that VLRA+ cells are T-cell-like,31 and that jawless vertebrates have a genetic network involved in thymopoiesis,54 initiated a renewed search for a thymus equivalent in lampreys, which led to the identification of the “thymoid”33 (Fig. 3.4). “Thymoids” are located at the tips of the gill filaments in the gill basket, thus occurring, not
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FIGURE 3.4 Lymphocyte development in jawless vertebrates. VLRB+ cells are thought to develop in hematopoietic tissues such as kidneys and typhlosoles. Some lymphocytes in these tissues express a cytidine deaminase named CDA2, assumed to mediate VLRB gene assembly (top). On the other hand, VLRA+ cells and VLRC+ cells are thought to develop in an organ named “thymoid,” located at the gill tip (bottom). Some lymphocytes in the “thymoid” express a cytidine deaminase named CDA1, assumed to mediate the assembly of VLRA and VLRC genes. Assembled VLRA and VLRC sequences incapable of encoding functional proteins are frequently observed in the “thymoid.” Such nonfunctional gene assembly is rarely seen in circulating lymphocytes and other tissues, suggesting that the assembly of VLRA and VLRC genes occurs in the “thymoid” and that some sort of quality control mechanism is in operation in the “thymoid.” It remains to be examined whether VLRA+ cells and VLRC+ cells are derived from the precursor cells that migrate from the hematopoietic tissue to the “thymoid” in a manner similar to gnathostome T cells. Orange and green arrows indicate unsuccessful and successful VLR gene assembly, respectively.
as a single organ, but as a constellation of specialized lymphoid tissues with no obvious corticomedullary differentiation. In “thymoids,” VLRA+ or VLRC+ lymphocytes expressing CDA1 occur in close proximity to pharyngeal epithelial cells expressing FOXN1, a marker of the thymopoietic microenvironment in jawed vertebrates. Also, sequencing of VLRA and VLRC genes isolated from gill-tip tissues contained nonfunctionally assembled sequences over fourfold more frequently than in peripheral blood cells. Furthermore, VLRA+ cells and VLRC+ cells in peripheral blood, but not in “thymoids” underwent proliferation in response to PHA. These results suggest that the “thymoid” is a primary, rather than a secondary, lymphoid organ.33 The thymus and the gill are both derived from the pharyngeal arch, and the thymus is located in the vicinity of the gill in cartilaginous and bony fishes. Therefore, the thymus and the “thymoid” appear to be homologous rather than analogous organs.
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4.3 Cell Fate Determination of VLRA+ Cells and VLRC+ Cells VLRB+ cells never express the VLRA gene nor the VLRC gene. Conversely, neither VLRA+ nor VLRC+ cells express the VLRB gene, consistent with the observation that the assembly of VLRA/C and VLRB genes occur in different primary lymphoid organs. In contrast, VLRA+ cells sometimes express incompletely assembled, nonfunctional VLRC transcripts. Likewise, a fraction of VLRC+ cells expresses incompletely assembled, nonfunctional VLRA transcripts, indicating that the assembly of VLRA and VLRC genes occurs concomitantly or sequentially. When analyzed by genomic PCR, the assembly of the VLRA gene was hardly found in VLRC+ cells. In contrast, the assembly of the VLRC gene was observed in the majority of VLRA+ cells, although 79% of assembly was nonproductive. The higher frequency of nonfunctional VLRC assembly in VLRA+ cells than nonfunctional VLRA assembly in VLRC+ cells suggests that the assembly of the VLRC gene precedes that of the VLRA gene, and that the germline VLRA gene undergoes assembly when both copies of the germline VLRC gene fail to undergo successful assembly.
4.4 Evidence for Selection in “Thymoids” Accumulating evidence suggests that selection, somewhat similar to thymic selection, may be operating on VLRA+ cells and VLRC+ cells. On average, VLRB receptors have about 1.5 LRRV cassettes (excluding LRRVe), with the copy number of LRRV cassettes showing the binomial distribution.56 In contrast, the average number of LRRV cassettes is about 3 in VLRA/VLRC receptors, and the copy number distribution of LRRV cassettes deviates from the binomial distribution, mainly because transcripts with two or fewer LRRV cassettes occur infrequently, thus suggesting that the VLRA/VLRC receptors with two or fewer LRRV cassettes may be selected against. Indeed, nonfunctional VLRC transcripts in VLRA+ cells shows greater variation in the copy number of LRRV cassettes than functional VLRC transcripts in VLRC+ cells.42 Furthermore, the copy number of LRRV cassettes shows greater variation in VLRC transcripts isolated from the “thymoid” than in those isolated from the peripheral blood.42 These observations suggest that VLRA/VLRC receptors expressed on the surface of lymphocytes are selected in the “thymoid”. The observation that caspase-3 positive cells are frequently found in the “thymoid”33 is also consistent with this suggestion. Interestingly, the second LRRV module in VLRA/C, but not VLRB transcripts, has distinctive sequence signatures. Therefore, the presumed selection in “thymoids” may influence not only the copy number of LRRV modules, but also the sequences in the second LRRV module.56
4.5 Evolution of Lymphocytes in Vertebrates Jawless vertebrates have two major lineages of lymphocytes: one resembling gnathostome T cells and the other resembling gnathostome B cells.31 Therefore, it is
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very likely that a common ancestor of jawed and jawless vertebrates had two lymphocyte lineages: one specialized for cellular immunity, and the other endowed with phagocytic activity but also oriented toward humoral immunity. It appears that jawless and jawed vertebrates then coopted different antigen receptors within the context of such lymphocyte lineages (Fig. 3.5). Because there is no evidence that urochordates (ascidians and other tunicates, described in chapter: Origin and Functions of Tunicate Hemocytes) or cephalochordates (amphioxus or lancelets) have lymphocytes, an ancestral lymphocyte appears to have emerged in the vertebrate lineage after it diverged from the invertebrate lineages of chordates. (For further discussion about the evolution of vertebrate lymphocytes, see chapter: The Evolution of Lymphocytes in Ectothermic Gnathostomata.) Like jawed vertebrates, lampreys have two lineages of T-cell-like cells, with VLRA+ cells and VLRC+ cells resembling ab and gd T cells, respectively, in terms of gene expression profiles and tissue distribution.32 Therefore, not only the separation of lymphocyte lineages into T-cell-like and B-cell-like cells, but also the separation of T-cell-like cells into ab T-like and gd T-like cells appears to have taken place in a common ancestor of vertebrates. Searches for immunoglobulin superfamily proteins in jawless vertebrates identified potential evolutionary precursors of TCR/BCR, such as the lamprey “TCR-like gene” coding for an immunoreceptor tyrosine-based inhibition motifbearing membrane protein, with one V-type and one C2-type immunoglobulinlike domain,57 and “agnathan paired receptors resembling antigen receptors” (APAR) of hagfish, having a single extracellular V-type immunoglobulin-like domain with a canonical J segment.58 The occurrence of these receptors in jawless vertebrates indicates that a common ancestor of jawed and jawless vertebrates had V-type immunoglobulin-like domains that could evolve into those of TCR/BCR.59 On the other hand, VLR is assumed to have emerged from a glycoprotein Iba (GPIba)-like protein, a component of the platelet glycoproteinreceptor complex conserved in all vertebrates.24 Therefore, it is likely that a common ancestor of jawed and jawless vertebrates possessed building blocks for both VLR-like and TCR/BCR-like receptors. Hence, it is reasonable to assume that a common ancestor of vertebrates had the potential for developing both VLR-based and TCR/BCR-based adaptive immunity. Then, what kind of receptors did a common ancestor of vertebrates use for antigen recognition? A key observation in addressing this question is that both jawed and jawless vertebrates have cytidine deaminases of the AID/APOPEC family. In jawless vertebrates, two cytidine deaminases named CDA1 and CDA2 generate diversity of VLRs by a gene-conversion-like mechanism.24 In jawed vertebrates, AID is involved in gene conversion, class switch recombination, and somatic hypermutation,60 thus having functions overlapping with those of CDA1 and CDA2. Furthermore, in some animals, such as rabbits, sheep, and chickens, AID plays a major role in the diversification of antibody repertoire.61 In contrast, the RAG enzymes involved in the V(D)J recombination of TCR/BCR genes are of transposon origin62–64 and are present only in jawed vertebrates. These observations
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FIGURE 3.5 Origin and evolution of lymphocytes in vertebrates. Available evidence indicates that three lineages of lymphocytes (two T-cell-like and one B-cell-like lineage) emerged in a common ancestor of jawed and jawless vertebrates. This common ancestor most likely possessed cytidine deaminases (CDA) and VLR-like antigen receptors because CDA and GPIba are present in both jawed and jawless vertebrate lineages. This ancestor also appears to have possessed potential precursors of TCRs/BCRs that became the targets of RAG insertion. Because the RAG enzymes are present only in the jawed vertebrate lineage, this insertion appears to have occurred only in a common ancestor of jawed vertebrates. Therefore, it is likely that a common ancestor of jawed and jawless vertebrates had an AIS based on VLRs and CDAs, and that this AIS was superseded by a more efficient TCR/ BCR-based AIS in the jawed vertebrate lineage after the acquisition of RAG enzymes.
suggest that a common ancestor of jawed and jawless vertebrates probably used VLR-like receptors and employed the members of the AID/APOPEC family to generate the diversity of their antigen receptors. Presumably, VLRs were superseded by TCRs and BCRs in the jawed-vertebrate lineage because the acquisition of RAG transposons enabled the development of a more efficient and powerful antigen-recognition system.16,65 The RAG transposon is thought to have been inserted into an ancestor of TCR/BCR genes. Therefore, the functional specialization of a gnathostome antigen-receptor into TCRs and BCRs, and then of the former into abTCRs and gdTCRs, must have occurred under the strong functional constraint of the three lineages of specialized lymphocytes.
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5 CONCLUSIONS Despite the fact that jawed and jawless vertebrates use structurally unrelated receptors for antigen recognition, their AISs have much in common. Particularly striking is the conservation of lymphocyte lineages. Extant invertebrates, either protostomes or deuterostomes, lack lymphocytes. Therefore, it appears that T- and B-lymphocyte lineages specialized for cellular and humoral arms of adaptive immunity emerged in a common ancestor of jawed and jawless vertebrates. Jawless vertebrates have neither MHC class I nor MHC class II molecules.19,66,67 An important issue that remains unanswered is whether jawless vertebrates have antigen-presenting molecules with functions equivalent to gnathostome MHC molecules. The observation that VLRC+ cells (and presumably VLRA+ cells as well) undergo selection in the “thymoid”42 suggests that jawless vertebrates have antigen-presenting molecules with functions equivalent to those of MHC molecules. The basic design of the AIS is well-conserved between jawed and jawless vertebrates, most likely because the lymphocyte lineages had been established in their common ancestor.16,20 Given the overall similarity of the gnathostome and agnathan AISs, it seems likely that jawless vertebrates have antigen-presenting molecules that function as substitutes of gnathostome MHC molecules.
ACKNOWLEDGMENTS This work has been supported by Grants-in-Aid for Scientific Research from The Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. We thank Dr Yukiko Miyatake for her kind help with the preparation of figures.
REFERENCES 1. Finstad J, Good RA. The evolution of the immune response III. Immunologic responses in the lamprey. J Exp Med 1964;120:1151–68. 2. Fujii T, Nakagawa H, Murakawa S. Immunity in lamprey II. Antigen-binding responses to sheep erythrocytes and hapten in the ammocoete. Dev Comp Immunol 1979;3:609–20. 3. Linthicum DS, Hildemann WH. Immunologic responses of Pacific hagfish III. Serum antibodies to cellular antigens. J Immunol 1970;105:912–8. 4. Litman GW, Finstad FJ, Howell J, Pollara BW, Good RA. The evolution of the immune response III. Structural studies of the lamprey immunoglobulin. J Immunol 1970;105:1278–85. 5. Marchalonis JJ, Edelman GM. Phylogenetic origins of antibody structure III. Antibodies in the primary immune response of the sea lamprey, petromyzon marinus. J Exp Med 1968;127:891–914. 6. Pollara B, Litman GW, Finstad J, Howell J, Good RA. The evolution of the immune response VII. Antibody to human “O” cells and properties of the immunoglobulin in lamprey. J Immunol 1970;105:738–45. 7. Acton RT, Weinheimer PF, Hildemann WH, Evans EE. Induced bactericidal response in the hagfish. J Bacteriol 1969;99:626–8. 8. Perey DY, Finstad J, Pollara B, Good RA. Evolution of the immune response VI. First and second set skin homograft rejections in primitive fishes. Lab Invest 1968;19:591–7.
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9. Fujii T. Electron microscopy of the leucocytes of the typhlosole in ammocoetes, with special attention to the antibody-producing cells. J Morphol 1982;173:87–100. 10. Mayer WE, Uinuk-Ool T, Tichy H, Gartland LA, Klein J, Cooper MD. Isolation and characterization of lymphocyte-like cells from a lamprey. Proc Natl Acad Sci USA 2002;99:14350–5. 11. Alder MN, Herrin BR, Sadlonova A, Stockard CR, Grizzle WE, Gartland LA, et al. Antibody responses of variable lymphocyte receptors in the lamprey. Nat Immunol 2008;9:319–27. 12. Shintani S, Terzic J, Sato A, Saraga-Babic M, O’hUigin C, Tichy H, et al. Do lampreys have lymphocytes? The spi evidence. Proc Natl Acad Sci USA 2000;97:7417–22. 13. Suzuki T, Shin IT, Kohara Y, Kasahara M. Transcriptome analysis of hagfish leukocytes: a framework for understanding the immune system of jawless fishes. Dev Comp Immunol 2004;28:993–1003. 14. Uinuk-Ool T, Mayer WE, Sato A, Dongak R, Cooper MD, Klein J. Lamprey lymphocyte-like cells express homologs of genes involved in immunologically relevant activities of mammalian lymphocytes. Proc Natl Acad Sci USA 2002;99:14356–61. 15. Boehm T, McCurley N, Sutoh Y, Schorpp M, Kasahara M, Cooper MD. VLR-based adaptive immunity. Annu Rev Immunol 2012;30:203–20. 16. Kasahara M, Sutoh Y. Two forms of adaptive immunity in vertebrates: similarities and differences. Adv Immunol 2014;122:59–90. 17. Pancer Z, Cooper MD. The evolution of adaptive immunity. Annu Rev Immunol 2006;24:497–518. 18. Cooper MD, Alder MN. The evolution of adaptive immune systems. Cell 2006;124:815–22. 19. Flajnik MF, Kasahara M. Origin and evolution of the adaptive immune system: genetic events and selective pressures. Nat Rev Genet 2010;11:47–59. 20. Boehm T. Design principles of adaptive immune systems. Nat Rev Immunol 2011;11:307–17. 21. Pancer Z, Amemiya CT, Ehrhardt GR, Ceitlin J, Gartland GL, Cooper MD. Somatic diversification of variable lymphocyte receptors in the agnathan sea lamprey. Nature 2004;430:174–80. 22. Alder MN, Rogozin IB, Iyer LM, Glazko GV, Cooper MD, Pancer Z. Diversity and function of adaptive immune receptors in a jawless vertebrate. Science 2005;310:1970–3. 23. Nagawa F, Kishishita N, Shimizu K, Hirose S, Miyoshi M, Nezu J, et al. Antigen-receptor genes of the agnathan lamprey are assembled by a process involving copy choice. Nat Immunol 2007;8:206–13. 24. Rogozin IB, Iyer LM, Liang L, Glazko GV, Liston VG, Pavlov YI, et al. Evolution and diversification of lamprey antigen receptors: evidence for involvement of an AID-APOBEC family cytosine deaminase. Nat Immunol 2007;8:647–56. 25. Kishishita N, Nagawa F. Evolution of adaptive immunity: implications of a third lymphocyte lineage in lampreys. Bioessays 2014;36:244–50. 26. Das S, Hirano M, Aghaallaei N, Bajoghli B, Boehm T, Cooper MD. Organization of lamprey variable lymphocyte receptor C locus and repertoire development. Proc Natl Acad Sci USA 2013;110:6043–8. 27. Das S, Li J, Hirano M, Sutoh Y, Herrin BR, Cooper MD. Evolution of two prototypic T cell lineages. Cell Immunol 2015;296:87–94. 28. Kasamatsu J, Sutoh Y, Fugo K, Otsuka N, Iwabuchi K, Kasahara M. Identification of a third variable lymphocyte receptor in the lamprey. Proc Natl Acad Sci USA 2010;107:14304–8. 29. Pancer Z, Saha NR, Kasamatsu J, Suzuki T, Amemiya CT, Kasahara M, et al. Variable lymphocyte receptors in hagfish. Proc Natl Acad Sci USA 2005;102:9224–9. 30. Li J, Das S, Herrin BR, Hirano M, Cooper MD. Definition of a third VLR gene in hagfish. Proc Natl Acad Sci USA 2013;110:15013–8. 31. Guo P, Hirano M, Herrin BR, Li J, Yu C, Sadlonova A, et al. Dual nature of the adaptive immune system in lampreys. Nature 2009;459:796–802.
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32. Hirano M, Guo P, McCurley N, Schorpp M, Das S, Boehm T, et al. Evolutionary implications of a third lymphocyte lineage in lampreys. Nature 2013;501:435–8. 33. Bajoghli B, Guo P, Aghaallaei N, Hirano M, Strohmeier C, McCurley N, et al. A thymus candidate in lampreys. Nature 2011;470:90–4. 34. Herrin BR, Alder MN, Roux KH, Sina C, Ehrhardt GR, Boydston JA, et al. Structure and specificity of lamprey monoclonal antibodies. Proc Natl Acad Sci USA 2008;105:2040–5. 35. Kim HM, Oh SC, Lim KJ, Kasamatsu J, Heo JY, Park BS, et al. Structural diversity of the hagfish variable lymphocyte receptors. J Biol Chem 2007;282:6726–32. 36. Han BW, Herrin BR, Cooper MD, Wilson IA. Antigen recognition by variable lymphocyte receptors. Science 2008;321:1834–7. 37. Velikovsky CA, Deng L, Tasumi S, Iyer LM, Kerzic MC, Aravind L, et al. Structure of a lamprey variable lymphocyte receptor in complex with a protein antigen. Nat Struct Mol Biol 2009;16:725–30. 38. Stanfield RL, Dooley H, Flajnik MF, Wilson IA. Crystal structure of a shark single-domain antibody V region in complex with lysozyme. Science 2004;305:1770–3. 39. De Genst E, Silence K, Decanniere K, Conrath K, Loris R, Kinne J, et al. Molecular basis for the preferential cleft recognition by dromedary heavy-chain antibodies. Proc Natl Acad Sci USA 2006;103:4586–91. 40. Tasumi S, Velikovsky CA, Xu G, Gai SA, Wittrup KD, Flajnik MF, et al. High-affinity lamprey VLRA and VLRB monoclonal antibodies. Proc Natl Acad Sci USA 2009;106:12891–6. 41. Deng L, Velikovsky CA, Xu G, Iyer LM, Tasumi S, Kerzic MC, et al. A structural basis for antigen recognition by the T cell-like lymphocytes of sea lamprey. Proc Natl Acad Sci USA 2010;107:13408–13. 42. Holland SJ, Gao M, Hirano M, Iyer LM, Luo M, Schorpp M, et al. Selection of the lamprey VLRC antigen receptor repertoire. Proc Natl Acad Sci USA 2014;111:14834–9. 43. Kanda R, Sutoh Y, Kasamatsu J, Maenaka K, Kasahara M, Ose T. Crystal structure of the lamprey variable lymphocyte receptor C reveals an unusual feature in its N-terminal capping module. PLoS ONE 2014;9:e85875. 44. Das S, Li J, Holland SJ, Iyer LM, Hirano M, Schorpp M, et al. Genomic donor cassette sharing during VLRA and VLRC assembly in jawless vertebrates. Proc Natl Acad Sci USA 2014;111:14828–33. 45. Kasamatsu J, Suzuki T, Ishijima J, Matsuda Y, Kasahara M. Two variable lymphocyte receptor genes of the inshore hagfish are located far apart on the same chromosome. Immunogenetics 2007;59:329–31. 46. Sutoh Y, Kasahara M. The immune system of agnathans (jawless vertebrates). In: Ratcliffe MJH, editor. Encyclopedia of Immunobiology. Cambridge (Massachusetts, USA): Academic Press, Elsevier; 2016. 47. Girardi M. Immunosurveillance and immunoregulation by gd T cells. J Invest Dermatol 2006;126:25–31. 48. Hayday AC. gd cells: a right time and a right place for a conserved third way of protection. Annu Rev Immunol 2000;18:975–1026. 49. Vantourout P, Hayday A, Six-of-the-best. Unique contributions of gd T cells to immunology. Nat Rev Immunol 2013;13:88–100. 50. Cruikshank WW, Kornfeld H, Center DM. Interleukin-16. J Leukoc Biol 2000;67:757–66. 51. Cooper MD, Peterson RD, Good RA. Delineation of the thymic and bursal lymphoid systems in the chicken. Nature 1965;205:143–6. 52. Hansen JD, Zapata AG. Lymphocyte development in fish and amphibians. Immunol Rev 1998;166:199–220.
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53. Glick G, Chang TS, Jaap RG. The bursa of Fabricius and antibody production. Poultry Sci 1956;35:224–34. 54. Bajoghli B, Aghaallaei N, Hess I, Rode I, Netuschil N, Tay BH, et al. Evolution of genetic networks underlying the emergence of thymopoiesis in vertebrates. Cell 2009;138:186–97. 55. Amemiya CT, Saha NR, Zapata A. Evolution and development of immunological structures in the lamprey. Curr Opin Immunol 2007;19:535–41. 56. Sutoh Y, Kasahara M. Copy number and sequence variation of leucine-rich repeat modules suggests distinct functional constraints operating on variable lymphocyte receptors expressed by agnathan T cell-like and B cell-like lymphocytes. Immunogenetics 2014;66:403–9. 57. Pancer Z, Mayer WE, Klein J, Cooper MD, Prototypic T. Cell receptor and CD4-like coreceptor are expressed by lymphocytes in the agnathan sea lamprey. Proc Natl Acad Sci USA 2004;101:13273–8. 58. Suzuki T, Shin IT, Fujiyama A, Kohara Y, Kasahara M. Hagfish leukocytes express a paired receptor family with a variable domain resembling those of antigen receptors. J Immunol 2005;174:2885–91. 59. Kasahara M, Kasamatsu J, Sutoh Y. Two types of antigen receptor systems in vertebrates. Zoolog Sci 2008;25:969–75. 60. Honjo T, Muramatsu M, Fagarasan S. AID: how does it aid antibody diversity? Immunity 2004;20:659–68. 61. Flajnik MF. Comparative analyses of immunoglobulin genes: surprises and portents. Nat Rev Immunol 2002;2:688–98. 62. Schatz DG, Swanson PC. V(D)J recombination: mechanisms of initiation. Annu Rev Genet 2011;45:167–202. 63. Kapitonov VV, Jurka J. RAG1 core and V(D)J recombination signal sequences were derived from transib transposons. PLoS Biol 2005;3:e181. 64. Kim MS, Lapkouski M, Yang W, Gellert M. Crystal structure of the V(D)J recombinase RAG1RAG2. Nature 2015;518:507–11. 65. Kato L, Stanlie A, Begum NA, Kobayashi M, Aida M, Honjo T. An evolutionary view of the mechanism for immune and genome diversity. J Immunol 2012;188:3559–66. 66. Flajnik MF, Kasahara M. Comparative genomics of the MHC: glimpses into the evolution of the adaptive immune system. Immunity 2001;15:351–62. 67. Smith JJ, Kuraku S, Holt C, Sauka-Spengler T, Jiang N, Campbell MS, et al. Sequencing of the sea lamprey (Petromyzon marinus) genome provides insights into vertebrate evolution. Nat Genet 2013;45:415–21.
Lymphocyte Populations in Jawless Vertebrates: Insights Into the Origin and Evolution of Adaptive Immunity Yoichi Sutoh*, Masanori Kasahara** *Emory Vaccine Center and Department of Pathology and Laboratory Medicine, Emory University, Atlanta, GA, United States; **Department of Pathology, Hokkaido University Graduate School of Medicine, Sapporo, Japan
1 INTRODUCTION The human body is thought to have tens of millions of lymphocyte clones, each expressing antigen receptors with distinct specificities. When infected with pathogens, lymphocyte clones expressing specific receptors undergo proliferation and differentiate into effector lymphocytes. Most of these effector lymphocytes die by apoptosis shortly after the elimination of pathogens. Some lymphocytes, however, survive for a long time to build an immunological memory. These lymphocytes—known as memory lymphocytes—enable the host to mount a more prompt and vigorous immune response upon reexposure to the same pathogen. Although the adaptive immune system (AIS) is a complex biological system, its backbone is formed by highly diverse antigen receptors clonally expressed on lymphocytes. The origin and evolution of the AIS and lymphocytes has attracted the interest of immunologists for a long time. Animals that have received particular attention in this regard are the jawless vertebrates represented by lampreys and hagfish. Studies conducted in the 1960s and 1970s showed that both lampreys and hagfish are capable of producing specific agglutinins against particulate antigens and rejecting skin allografts with immunological memory,1–8 suggesting that the origin of adaptive immunity can be traced back to the emergence of jawless vertebrates. Consistent with this, jawless vertebrates have blood cells morphologically indistinguishable from mammalian lymphocytes. In lampreys, naive lymphocyte-like cells measure about 10 mm in diameter and have a round shape, with thin cytosol and a large nucleus.9,10 In electron microscopy, thin The Evolution of the Immune System. http://dx.doi.org/10.1016/B978-0-12-801975-7.00003-7 Copyright © 2016 Elsevier Inc. All rights reserved.
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heterochromatin is found on the inside rim of the nuclear membrane.9 After stimulation with particulate antigens such as sheep blood-cells and anthrax spore-coats, the lymphocyte-like cells undergo proliferation, differentiating into plasma cell-like cells with enlarged cytosol that contain well-developed rough endoplasmic reticulum.11 Transcriptome analysis revealed that lamprey and hagfish lymphocyte-like cells express many genes whose mammalian counterparts are expressed by lymphocytes, such as the genes coding for a homolog of Spi, B-cell receptorassociated protein (BCAP), GATA2/3, CXC chemokine receptor (CXCR) 4, and CD98.12–14 However, the transcripts coding for major histocompatibility complex (MHC) molecules, T-cell receptors (TCRs), B-cell receptors (BCRs), or the RAG (recombination activating gene) enzymes were never identified from jawless vertebrates. This was puzzling because, at first glance, it contradicted the earlier studies that clearly demonstrated the ability of jawless vertebrates to reject skin allografts and to produce agglutinins specific for particulate antigens. Ultimately, this puzzle was resolved by the discovery that, instead of TCRs and BCRs, jawless vertebrates use a unique antigen receptor now known as variable lymphocyte receptors (VLR).15–20
2 OVERVIEW OF VLRs 2.1 Structure of VLR Proteins and Gene Assembly VLR was discovered in lampreys as a gene coding for proteins with highly diverse sequences, through the analysis of a cDNA library, enriched for transcripts upregulated in antigen-stimulated lymphocyte-like cells.21 Structurally, VLRs are a member of the leucine-rich repeat (LRR) family of proteins composed of a signal peptide (SP), an N-terminal LRR cassette (LRRNT), an 18-residue N-terminal LRR cassette (LRR1), a variable number of 24-residue LRR cassettes (LRRV), a 24-residue end LRRV cassette (LRRVe), a 13-residue truncated LRR cassette [also called the connecting peptide (CP)], a C-terminal LRR cassette (LRRCT), and an invariant domain containing a stalk region (Fig. 3.1A). The sequences of the regions ranging from the 3′-half of LRRNT (3′-LRRNT) to the 5′-half of LRRCT (5′-LRRCT) are highly diverse. By contrast, the sequences of the remaining regions—the SP, the 5′-half of LRRNT, the 3′-half of LRRCT, and the stalk region—are invariant. Remarkably, the VLR gene has an incomplete structure incapable of encoding any protein in the genome of nonlymphoid cells, such as erythrocytes; this germline gene encodes only the invariant region of VLR proteins and lacks the sequences coding for the diversity region (Fig. 3.1B). Cassettes such as 3′-LRRNT, LRR1, LRRV, LRRVe, CP, and 5′-LRRCT, constituting the diversity region, are located in multiple copies (sometimes in several hundred copies) in the vicinity of the germline VLR gene. During lymphocyte development, the invariant intervening sequence of the
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FIGURE 3.1 Domain organization, gene assembly, and tertiary structure of VLRs. (A) Domain organization of mature VLR protein. The diversity region is composed of the following LRR cassettes: the N-terminal LRR cap (LRRNT), LRR1, a variable number of LRRVs, LRRVe, a connecting peptide (CP), and the C-terminal LRR cap (LRRCT). SP, signal peptide. (B) Assembly of VLR genes. The germline VLR gene has an invariant intervening sequence instead of highly diverse LRR cassettes and is unable to encode functional proteins (top). During lymphocyte development, LRR cassettes, which are scattered around the germline VLR, replace the intervening sequence in a stepwise manner from either the 5′- or 3′-end (middle and bottom). This process, which resembles gene conversion, is thought to be mediated by cytidine deaminases of the AID-APOBEC family. (C) Tertiary structure of VLRs. VLR proteins have a horseshoe-shaped structure characteristic of LRR family proteins. The majority of variable amino acid residues are located on the b-sheet, facing the concave surface (shared by all three receptors), and the hypervariable loop in LRRCT (present only in VLRA and VLRB receptors; indicated by red arrows). Both ends of the b-sheet are capped by LRRNT and LRRCT (indicated by blue and red, respectively). Unlike VLRA or VLRB, lamprey VLRC has an invariable loop in LRRNT (indicated by a blue arrow). Like TCRs and BCRs, all three VLRs lack a signaling domain in the cytoplasmic region. Therefore, it is assumed that VLRs associate with signal transducers yet to be identified.
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germline VLR gene is replaced by a gene conversion-like mechanism in a stepwise manner, beginning either from its 5′- or 3′-end, by adding flanking cassettes, and eventually forming a completely assembled VLR gene.22–25 Short stretches of nucleotide homology (10–30 bps) are found between donor and acceptor sequences.24,26,27 Therefore, the sequence located at the ends of the most newly copied cassette presumably determines which flanking LRR cassettes should be copied into the germline VLR gene in the next step. The sequences of LRR cassettes are highly variable and the number of copied LRRV cassettes is also variable. This enables assembled VLR genes to acquire sequence diversity comparable to that of TCRs and BCRs.22,24 The assembly of VLR genes is mediated by cytidine deaminases (CDA) of the activation-induced cytidine deaminase (AID)-apolipoprotein B mRNA editing enzyme (APOBEC) family.24 In lampreys, two CDAs named CDA1 and CDA2 have been identified.24
2.2 Both Lampreys and Hagfish Have Three VLR Genes Initially, only one VLR gene was identified in lampreys.21 Subsequent studies revealed that lampreys have two more VLR genes.24,28 The three VLR genes are now known as VLRA, VLRB, and VLRC. Hagfish also have three VLR genes named VLRA, VLRB, and VLRC, thought to be orthologous to lamprey VLRA, VLRB, and VLRC genes,29,30 indicating that these three genes already existed in a common ancestor of lampreys and hagfish. Functional studies in lampreys demonstrated that the products of the three VLR genes are expressed on three distinct populations of lymphocyte-like cells31,32 (Fig. 3.2). VLRA and VLRC, which are more closely related to each other in sequence than they are to VLRB, are membrane-bound receptors with no secretory form. VLRA+ cells and VLRC+ cells are T-cell-like and develop in the “thymoid,” an organ assumed to be the equivalent of the gnathostome thymus.33 On the other hand, VLRB is a glycosylphosphatidylinositolanchored protein that also occurs in a secretory form. Secreted VLRB molecules form pentamers or tetramers of dimers and have 8–10 antigen binding sites.34 This type of subunit organization resembles the subunit arrangement of gnathostome IgM antibodies and accounts for the strong agglutinating activities of VLRB antibodies. Interestingly, VLRB+ cells are B-cell-like, develop in hematopoietic organs, undergo clonal proliferation in response to antigen stimulation, and differentiate into plasma cell-like cells secreting VLRB antibodies.11,22,31 Furthermore, similar to the allelic exclusion in TCR and BCR genes, each lymphocyte expresses only a single functional VLR allele, indicating that VLR assembly occurs in a monoallelic manner.21,28,31 Collectively, these observations provided convincing evidence that lymphocyte-like cells in jawless vertebrates resemble gnathostome lymphocytes, not only morphologically, but also functionally, and hence should be regarded as authentic lymphocytes.
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FIGURE 3.2 Features of three populations of lamprey lymphocytes. Lampreys have two lymphocyte lineages whose gene expression profiles resemble gnathostome T cells and B cells, respectively. VLRA and VLRC are membrane-bound receptors without any secretory form. VLRA+ cells and VLRC+ cells express molecules related to T-cell development and/or function. They also secrete IL-17, a pro-inflammatory cytokine released from immune cells, including Th17 cells in jawed vertebrates. VLRC+ cells specifically express genes such as SOX13, ITGAL, and TLR3, which are characteristically expressed in gnathostome gd T cells. VLRB receptors occur in both membrane-bound and secretory forms. VLRB+ cells express molecules typically expressed in gnathostome B cells.
2.3 Crystal Structure of VLR Proteins and Antigen Recognition Crystallographic analysis revealed that VLR proteins adopt a horseshoe-like solenoid structure characteristic of the LRR protein family, capped by LRRNT and LRRCT cassettes at N- and C-terminal ends, respectively (Fig. 3.1C).35 The inner concave surface of the solenoid structure is formed from multiple b-strands (derived from LRRNT, LRR1, LRRV, LRRVe, and CP), which assemble into a continuous b-sheet. In VLRA and VLRB receptors, this b-sheet and a protruding loop formed in the 5′-LRRCT cap contain the majority of variable residues. Structural analysis of VLRB receptors in complex with antigens such as Type O blood antigen (H-trisaccharide) and hen egg-white lysozyme (HEL) demonstrated that VLRB binds antigens via the b-sheet and protruding loop in 5′-LRRCT.36 Interestingly, the protrusion in 5′-LRRCT penetrated deeply into the catalytic cleft of HEL in the VLRB-HEL complex.37 Immunoglobulins made up of VH and VL chains normally recognize epitopes exposed on the surface of molecules. The ability of VLRs to bind to residues hidden in the cleft is reminiscent of camel and shark VH antibodies that preferentially target clefts.38,39
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In one study, lampreys immunized with HEL produced not only specific VLRBs, but also specific VLRAs with binding affinity comparable to that of IgG.40 The crystal structure of lamprey VLRA in a complex with HEL revealed that VLRA interacts with HEL through its concave surface and LRRCT.41 Unlike VLRA or VLRB receptors, VLRC lacks a protrusion in its LRRCT region28,30,35 (Fig. 3.1C), and its LRRCT is semiinvariant. Also, unlike VLRA or VLRB receptors, the LRRNT of lamprey VLRC has a loop that protrudes into the concave surface.42,43 This loop shows little sequence variation. These structural features of VLRC suggest that VLRC might recognize antigens in a unique manner. It is possible that VLRC interacts with putative antigen-presenting molecules through its invariant or semiinvariant LRRCT region and LRRNT loop.
2.4 VLRA and VLRC Genes are Tightly Linked in the Lamprey Genome VLRA and VLRC loci are located close to each other in the lamprey genome.26 Similar to the gnathostome TCRA/TCRD locus that shares some V segments, lamprey VLRA and VLRC genes often use the same LRR cassettes for their assembly.44 Such sharing of LRR cassettes is also observed in assembled hagfish VLRA and VLRC genes, suggesting that VLRA and VLRC genes are also tightly linked in the hagfish genome. In contrast, neither lamprey nor hagfish VLRB receptors share identical LRR cassettes with the VLRA or VLRC receptors of respective species, suggesting that the germline VLRB gene is not situated close to the VLRA/C locus. Consistent with this, previous fluorescence in situ hybridization analysis showed that hagfish VLRB and VLRC (then known as VLRA) genes are physically well-separated, although they are on the same chromosome.45 It appears that VLR genes increased their copy number by tandem duplication; subsequently, chromosomal inversion or intrachromosomal translocation presumably separated the VLRB gene from the VLRA/C genes, facilitating the functional specialization of the receptor genes.
3 THREE POPULATIONS OF AGNATHAN LYMPHOCYTES 3.1 VLRA+ Cells and VLRC+ Cells Resemble Gnathostome T Cells, Whereas VLRB+ Cells Resemble Gnathostome B Cells VLRA+ cells and VLRC+ cells resemble gnathostome T cells in that they undergo proliferation in response to a T-cell mitogen such as phytohemagglutinin (PHA). They also resemble T cells in terms of gene expression profiles (Fig. 3.2). For instance, they express transcription factors involved in T-cell development, such as GATA2/3, B-cell CLL/lymphoma 11B (BCL11b), and T-cell factor 1 (TCF1), signaling molecules such as LAT (linker for activation of T cells), cell surface molecules such as NOTCH1, CD45, and aryl hydrocarbon receptor (AHR), cytokines such as IL-17, and cytokine receptors such
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as chemokine receptor 9/7 (CCR9/7), which regulate the migration of T-cell progenitors to the thymus.31,32 VLRC+ cells generally have a gene expression profile similar to that of VLRA+ cells, but differ from VLRA+ cells in that they express genes characteristically expressed in gnathostome gd T cells, such as sex-determining region Y-box 13 (SOX13) coding for a fate-determining factor for the gd T cell lineage, and Toll-like receptor 3 (TLR3).32 VLRC+ cells also express an integrin family of adhesion molecules involved in the epithelial localization of gd T cells, such as integrins aL (ITGAL), a4 (ITGA4), and b1 (ITGB1). On the other hand, VLRB+ cells resemble gnathostome B cells, in that they differentiate into plasma cells and secrete specific VLRB molecules as antibodies, when challenged with particulate antigens.11 Also, the gene expression profile of VLRB+ cells is similar to that of gnathostome B cells; they express transcription factors such as B-lymphocyte-induced maturation protein 1 (BLIMP-1), B-cell CLL/lymphoma 6 (BCL6), and paired box protein 5 (PAX5), signal-transducing molecules such as spleen tyrosine kinase (Syk) and B-cell adaptor protein (BCAP), and Toll-like receptors such as TLR2a-c, TLR7, and TLR10. Interestingly, T-cell-like VLRA+ cells express the IL-8 receptor and IL17, whereas B-cell-like VLRB+ cells express IL-8 and the IL-17 receptor. This observation suggests that VLRA+ and VLRB+ cells may have a crosstalk via cytokines and possibly interact in a manner analogous to T–B cell collaboration32 (Fig. 3.3). In rodents, gd T cells are a minor population of T lymphocytes in blood and peripheral lymphoid tissues; however, they occupy the majority of lymphocytes in epithelial layers of tissues such as skin, intestine, tongue, and the reproductive tract. These intraepithelial gd T cells express TCRs with limited variability. Most notably, gd T cells residing in the epidermis, known as dendritic epidermal T cells (DETC), express an invariant Vg5Vd1 (alternate nomenclature Vg3Vd1) TCR, lacking junctional diversity.47,48 Interestingly, lamprey VLRC+ cells are located predominantly in epithelial tissues and express restricted antigen receptor diversity, showing intriguing similarity to mouse DETCs and intraepithelial lymphocytes (Fig. 3.2).49 When stimulated with poly I:C, a synthetic viral double-strand RNA (dsRNA) analog, VLRC+ cells respond via their TLR3 and upregulate IL-16 expression. IL-16 is a chemoattractant recruiting CD4+ leukocytes, such as T cells, monocytes, and eosinophils to the site of infection.50 Therefore, similar to DETCs, epithelial VLRC+ cells may perform a sentinel function in the epithelium.
3.2 Antigen Recognition by Agnathan T-Like Cells is Still Full of Mystery When lampreys were immunized with anthrax spores, they generated VLRB+ cells capable of binding to the spores, but VLRA+ cells with such ability were
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FIGURE 3.3 The presumed role of three populations of agnathan lymphocytes in eliminating pathogens. Invading pathogens are recognized by VLRs with specific binding specificities. VLRC+ cells also express TLR3 that recognizes viral double-stranded RNAs. Activated VLRA+ cells and VLRC+ cells secrete pro-inflammatory cytokines, such as IL-17, MIF, and IL-16, which presumably induce the migration of various immune cells to the site of infection. IL-17 released from VLRA+ cells may activate VLRB+ cells that express the IL-17 receptor and induce the secretion of VLRB antibodies into the serum. Secreted VLRB antibodies form an antigen–antibody complex that activates the complement pathway via binding to C1q-like protein and mannose-binding lectin-associated serine protease (MASP). VLRB+ cells might use IL-8 for cross-talk with VLRA+ cells and VLRC+ cells that express the IL-8 receptor. It is still uncertain whether jawless vertebrates have antigen-presenting molecules with functions equivalent to MHC molecules. Also unknown is whether VLRA and VLRC require antigen-presenting molecules for antigen recognition. This figure was modified from Sutoh and Kasahara.46
not detectable,29 suggesting that VLRA receptors do not bind native bacterial surface epitopes and might recognize processed antigens in vivo similar to gnathostome ab T cells. However, screening of a yeast display library resulted in the identification of lamprey VLRA molecules that directly bound to HEL.40 Although this observation demonstrates that VLRA molecules are capable of binding unprocessed antigens, it is in contrast to the fact that ab T cells, the presumed counterpart of agnathan VLRA+ cells, recognize only processed antigens bound to MHC class I or class II molecules. At present, it remains unknown whether VLRA receptors always recognize antigens without any requirement for antigen processing, or direct antigen recognition occurs only in some or exceptional cases. Also unknown is whether direct recognition occurs in vivo.
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Currently, no information is available as to how VLRC+ cells recognize antigens. If VLRC+ cells are gd T-cell-like, as discussed previously, they might recognize antigens directly in a manner similar to gd T cells. This is, however, in apparent conflict with the speculation that lamprey VLRC receptors might interact with putative antigen-presenting molecules through their invariant loop in LRRNT and semiinvariant LRRCT. Much remains to be learned about antigen recognition by agnathan T-cell-like lymphocytes.
4 LYMPHOCYTE DEVELOPMENT IN JAWLESS VERTEBRATES 4.1 Development of VLRB+ Cells B cells develop in hematopoietic organs such as the bone marrow in mammals, the bursa of Fabricius in birds, the pronephros in bony fishes, and the spleen and spiral valves in cartilaginous fishes.51–53 In adult lampreys, VLRB+ cells are abundant in kidneys and blood. On the other hand, in lamprey larvae, VLRB+ cells are abundant in the typhlosole, an invaginated spiral valve spanning the length of the intestine. Recent flow cytometric analysis showed that 26 and 41% of lymphocytes are VLRB+ cells in typhlosoles and kidneys, respectively.29 In both organs, cell proliferation was activated within 28 days after immunization by anthrax spore-coats. Moreover, in situ hybridization analysis detected CDA2 expression in VLRB+ cells, in typhlosoles and kidneys supporting the idea that VLRB gene assembly occurs in these organs.33 Although rigorous analysis with molecular markers remains to be conducted, available evidence indicates strongly that VLRB+ cells develop in hematopoietic organs such as typhlosoles and kidneys, consistent with the idea that VLRB+ cells represent B-lineage cells (Fig. 3.4).
4.2 Development of VLRA+ Cells and VLRC+ Cells In jawed vertebrates, T-lineage progenitors generated in hematopoietic tissues migrate to the thymus. After successful V(D)J recombination of TCR genes, T cells are positively and negatively selected in the thymic cortex and medulla, respectively. As a result of thymic selection, only T cells expressing self-tolerant, MHC-restricted TCRs are allowed to survive and exit to the periphery. The thymus was histologically and/or functionally identified in all jawed vertebrates ranging from cartilaginous fish to mammals, consistent with the fact that they have MHC molecules and TCRs. In contrast, jawless vertebrates lack anatomical structures with discrete cortical and medullary regions resembling the thymus. Therefore, it was generally assumed until recently that the thymus emerged in a common ancestor of jawed vertebrates, concomitant with the emergence of TCR, BCR, MHC, and RAG molecules.54,55 However, the discovery that VLRA+ cells are T-cell-like,31 and that jawless vertebrates have a genetic network involved in thymopoiesis,54 initiated a renewed search for a thymus equivalent in lampreys, which led to the identification of the “thymoid”33 (Fig. 3.4). “Thymoids” are located at the tips of the gill filaments in the gill basket, thus occurring, not
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FIGURE 3.4 Lymphocyte development in jawless vertebrates. VLRB+ cells are thought to develop in hematopoietic tissues such as kidneys and typhlosoles. Some lymphocytes in these tissues express a cytidine deaminase named CDA2, assumed to mediate VLRB gene assembly (top). On the other hand, VLRA+ cells and VLRC+ cells are thought to develop in an organ named “thymoid,” located at the gill tip (bottom). Some lymphocytes in the “thymoid” express a cytidine deaminase named CDA1, assumed to mediate the assembly of VLRA and VLRC genes. Assembled VLRA and VLRC sequences incapable of encoding functional proteins are frequently observed in the “thymoid.” Such nonfunctional gene assembly is rarely seen in circulating lymphocytes and other tissues, suggesting that the assembly of VLRA and VLRC genes occurs in the “thymoid” and that some sort of quality control mechanism is in operation in the “thymoid.” It remains to be examined whether VLRA+ cells and VLRC+ cells are derived from the precursor cells that migrate from the hematopoietic tissue to the “thymoid” in a manner similar to gnathostome T cells. Orange and green arrows indicate unsuccessful and successful VLR gene assembly, respectively.
as a single organ, but as a constellation of specialized lymphoid tissues with no obvious corticomedullary differentiation. In “thymoids,” VLRA+ or VLRC+ lymphocytes expressing CDA1 occur in close proximity to pharyngeal epithelial cells expressing FOXN1, a marker of the thymopoietic microenvironment in jawed vertebrates. Also, sequencing of VLRA and VLRC genes isolated from gill-tip tissues contained nonfunctionally assembled sequences over fourfold more frequently than in peripheral blood cells. Furthermore, VLRA+ cells and VLRC+ cells in peripheral blood, but not in “thymoids” underwent proliferation in response to PHA. These results suggest that the “thymoid” is a primary, rather than a secondary, lymphoid organ.33 The thymus and the gill are both derived from the pharyngeal arch, and the thymus is located in the vicinity of the gill in cartilaginous and bony fishes. Therefore, the thymus and the “thymoid” appear to be homologous rather than analogous organs.
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4.3 Cell Fate Determination of VLRA+ Cells and VLRC+ Cells VLRB+ cells never express the VLRA gene nor the VLRC gene. Conversely, neither VLRA+ nor VLRC+ cells express the VLRB gene, consistent with the observation that the assembly of VLRA/C and VLRB genes occur in different primary lymphoid organs. In contrast, VLRA+ cells sometimes express incompletely assembled, nonfunctional VLRC transcripts. Likewise, a fraction of VLRC+ cells expresses incompletely assembled, nonfunctional VLRA transcripts, indicating that the assembly of VLRA and VLRC genes occurs concomitantly or sequentially. When analyzed by genomic PCR, the assembly of the VLRA gene was hardly found in VLRC+ cells. In contrast, the assembly of the VLRC gene was observed in the majority of VLRA+ cells, although 79% of assembly was nonproductive. The higher frequency of nonfunctional VLRC assembly in VLRA+ cells than nonfunctional VLRA assembly in VLRC+ cells suggests that the assembly of the VLRC gene precedes that of the VLRA gene, and that the germline VLRA gene undergoes assembly when both copies of the germline VLRC gene fail to undergo successful assembly.
4.4 Evidence for Selection in “Thymoids” Accumulating evidence suggests that selection, somewhat similar to thymic selection, may be operating on VLRA+ cells and VLRC+ cells. On average, VLRB receptors have about 1.5 LRRV cassettes (excluding LRRVe), with the copy number of LRRV cassettes showing the binomial distribution.56 In contrast, the average number of LRRV cassettes is about 3 in VLRA/VLRC receptors, and the copy number distribution of LRRV cassettes deviates from the binomial distribution, mainly because transcripts with two or fewer LRRV cassettes occur infrequently, thus suggesting that the VLRA/VLRC receptors with two or fewer LRRV cassettes may be selected against. Indeed, nonfunctional VLRC transcripts in VLRA+ cells shows greater variation in the copy number of LRRV cassettes than functional VLRC transcripts in VLRC+ cells.42 Furthermore, the copy number of LRRV cassettes shows greater variation in VLRC transcripts isolated from the “thymoid” than in those isolated from the peripheral blood.42 These observations suggest that VLRA/VLRC receptors expressed on the surface of lymphocytes are selected in the “thymoid”. The observation that caspase-3 positive cells are frequently found in the “thymoid”33 is also consistent with this suggestion. Interestingly, the second LRRV module in VLRA/C, but not VLRB transcripts, has distinctive sequence signatures. Therefore, the presumed selection in “thymoids” may influence not only the copy number of LRRV modules, but also the sequences in the second LRRV module.56
4.5 Evolution of Lymphocytes in Vertebrates Jawless vertebrates have two major lineages of lymphocytes: one resembling gnathostome T cells and the other resembling gnathostome B cells.31 Therefore, it is
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very likely that a common ancestor of jawed and jawless vertebrates had two lymphocyte lineages: one specialized for cellular immunity, and the other endowed with phagocytic activity but also oriented toward humoral immunity. It appears that jawless and jawed vertebrates then coopted different antigen receptors within the context of such lymphocyte lineages (Fig. 3.5). Because there is no evidence that urochordates (ascidians and other tunicates, described in chapter: Origin and Functions of Tunicate Hemocytes) or cephalochordates (amphioxus or lancelets) have lymphocytes, an ancestral lymphocyte appears to have emerged in the vertebrate lineage after it diverged from the invertebrate lineages of chordates. (For further discussion about the evolution of vertebrate lymphocytes, see chapter: The Evolution of Lymphocytes in Ectothermic Gnathostomata.) Like jawed vertebrates, lampreys have two lineages of T-cell-like cells, with VLRA+ cells and VLRC+ cells resembling ab and gd T cells, respectively, in terms of gene expression profiles and tissue distribution.32 Therefore, not only the separation of lymphocyte lineages into T-cell-like and B-cell-like cells, but also the separation of T-cell-like cells into ab T-like and gd T-like cells appears to have taken place in a common ancestor of vertebrates. Searches for immunoglobulin superfamily proteins in jawless vertebrates identified potential evolutionary precursors of TCR/BCR, such as the lamprey “TCR-like gene” coding for an immunoreceptor tyrosine-based inhibition motifbearing membrane protein, with one V-type and one C2-type immunoglobulinlike domain,57 and “agnathan paired receptors resembling antigen receptors” (APAR) of hagfish, having a single extracellular V-type immunoglobulin-like domain with a canonical J segment.58 The occurrence of these receptors in jawless vertebrates indicates that a common ancestor of jawed and jawless vertebrates had V-type immunoglobulin-like domains that could evolve into those of TCR/BCR.59 On the other hand, VLR is assumed to have emerged from a glycoprotein Iba (GPIba)-like protein, a component of the platelet glycoproteinreceptor complex conserved in all vertebrates.24 Therefore, it is likely that a common ancestor of jawed and jawless vertebrates possessed building blocks for both VLR-like and TCR/BCR-like receptors. Hence, it is reasonable to assume that a common ancestor of vertebrates had the potential for developing both VLR-based and TCR/BCR-based adaptive immunity. Then, what kind of receptors did a common ancestor of vertebrates use for antigen recognition? A key observation in addressing this question is that both jawed and jawless vertebrates have cytidine deaminases of the AID/APOPEC family. In jawless vertebrates, two cytidine deaminases named CDA1 and CDA2 generate diversity of VLRs by a gene-conversion-like mechanism.24 In jawed vertebrates, AID is involved in gene conversion, class switch recombination, and somatic hypermutation,60 thus having functions overlapping with those of CDA1 and CDA2. Furthermore, in some animals, such as rabbits, sheep, and chickens, AID plays a major role in the diversification of antibody repertoire.61 In contrast, the RAG enzymes involved in the V(D)J recombination of TCR/BCR genes are of transposon origin62–64 and are present only in jawed vertebrates. These observations
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FIGURE 3.5 Origin and evolution of lymphocytes in vertebrates. Available evidence indicates that three lineages of lymphocytes (two T-cell-like and one B-cell-like lineage) emerged in a common ancestor of jawed and jawless vertebrates. This common ancestor most likely possessed cytidine deaminases (CDA) and VLR-like antigen receptors because CDA and GPIba are present in both jawed and jawless vertebrate lineages. This ancestor also appears to have possessed potential precursors of TCRs/BCRs that became the targets of RAG insertion. Because the RAG enzymes are present only in the jawed vertebrate lineage, this insertion appears to have occurred only in a common ancestor of jawed vertebrates. Therefore, it is likely that a common ancestor of jawed and jawless vertebrates had an AIS based on VLRs and CDAs, and that this AIS was superseded by a more efficient TCR/ BCR-based AIS in the jawed vertebrate lineage after the acquisition of RAG enzymes.
suggest that a common ancestor of jawed and jawless vertebrates probably used VLR-like receptors and employed the members of the AID/APOPEC family to generate the diversity of their antigen receptors. Presumably, VLRs were superseded by TCRs and BCRs in the jawed-vertebrate lineage because the acquisition of RAG transposons enabled the development of a more efficient and powerful antigen-recognition system.16,65 The RAG transposon is thought to have been inserted into an ancestor of TCR/BCR genes. Therefore, the functional specialization of a gnathostome antigen-receptor into TCRs and BCRs, and then of the former into abTCRs and gdTCRs, must have occurred under the strong functional constraint of the three lineages of specialized lymphocytes.
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5 CONCLUSIONS Despite the fact that jawed and jawless vertebrates use structurally unrelated receptors for antigen recognition, their AISs have much in common. Particularly striking is the conservation of lymphocyte lineages. Extant invertebrates, either protostomes or deuterostomes, lack lymphocytes. Therefore, it appears that T- and B-lymphocyte lineages specialized for cellular and humoral arms of adaptive immunity emerged in a common ancestor of jawed and jawless vertebrates. Jawless vertebrates have neither MHC class I nor MHC class II molecules.19,66,67 An important issue that remains unanswered is whether jawless vertebrates have antigen-presenting molecules with functions equivalent to gnathostome MHC molecules. The observation that VLRC+ cells (and presumably VLRA+ cells as well) undergo selection in the “thymoid”42 suggests that jawless vertebrates have antigen-presenting molecules with functions equivalent to those of MHC molecules. The basic design of the AIS is well-conserved between jawed and jawless vertebrates, most likely because the lymphocyte lineages had been established in their common ancestor.16,20 Given the overall similarity of the gnathostome and agnathan AISs, it seems likely that jawless vertebrates have antigen-presenting molecules that function as substitutes of gnathostome MHC molecules.
ACKNOWLEDGMENTS This work has been supported by Grants-in-Aid for Scientific Research from The Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. We thank Dr Yukiko Miyatake for her kind help with the preparation of figures.
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