Two Forms of Adaptive Immunity in Vertebrates

Cytokine Receptor Diversity in the Lamprey Predicts the Minimal Essential Cytokine Networks of Vertebrates

The vertebrate adaptive immune systems (Agnatha and Gnathostomata) use sets of T and B lymphocyte lineages that somatically generate highly diverse repertoires of Ag-specific receptors and Abs. In Gnathostomata, cytokine networks regulate the activation of lymphoid and myeloid cells, whereas little is known about these components in Agnathans. Most gnathostome cytokines are four-helix bundle cytokines with poorly conserved primary sequences. In contrast, sequence conservation across bilaterians has been observed for cognate cytokine receptor chains, allowing their structural classification into two classes, and for downstream JAK/STAT signaling mediators. With conserved numbers among Gnathostomata, human cytokine receptor chains (comprising 34 class I and 12 class II) are able to interact with 28 class I helical cytokines (including most ILs) and 16 class II cytokines (including all IFNs), respectively. Hypothesizing that the arsenal of cytokine receptors and transducers may reflect homologous cytokine networks, we analyzed the lamprey genome and transcriptome to identify genes and transcripts for 23 class I and five class II cytokine receptors alongside one JAK signal mediator and four STAT transcription factors. On the basis of deduction of their respective orthologs, we predict that these receptors may interact with 16 class I and 3 class II helical cytokines (including IL-4, IL-6, IL-7, IL-12, IL-10, IFN-γ, and thymic stromal lymphoprotein homologs). On the basis of their respective activities in mammals, this analysis suggests the existence of lamprey cytokine networks that may regulate myeloid and lymphoid cell differentiation, including potential Th1/Th2 polarization. The predicted networks thus appear remarkably homologous to those of Gnathostomata, albeit reduced to essential functions.

Recurrent expansions of B30.2-associated immune receptor families in fish

B30.2 domains, also known as PRY/SPRY, are key components of specific subsets of two large families of proteins involved in innate immunity: the tripartite motif proteins (TRIMs) and the Nod-like receptors (NLRs). TRIM proteins are important, often inducible factors of antiviral innate immunity, targeting multiple steps of viral cycles through a variety of mechanisms. NLRs prime and regulate systemic innate defenses, especially against bacteria, and control inflammation. Large TRIM and NLR subsets characterized by the presence of a B30.2 domain have been reported from a few fish species including zebrafish and seem to be strongly prone to gene duplication/expansion. Here, we performed a large-scale survey of these receptors across about 150 fish genomes, focusing on ray-finned fishes. We assessed the number and genomic distribution of domains and domain combinations associated with TRIMs, NLRs, and other genes containing B30.2 domains and looked for gene expansion patterns across fish groups. We then used a model to test the impact of taxonomy, genome size, and environmental variables on the copy numbers of these genes. Our findings reveal novel domain structures, clade-specific gains and losses. They also assist with the timing of the gene expansions, reveal patterns associated with the MHC, and lay the groundwork for further studies delving deeper into the forces that drive the copy number variation of immune genes on a species level.

The immune system of jawless vertebrates: insights into the prototype of the adaptive immune system

Jawless vertebrates diverged from an ancestor of jawed vertebrates approximately 550 million years ago. They mount adaptive immune responses to repetitive antigenic challenges, despite lacking major histocompatibility complex molecules, immunoglobulins, T cell receptors, and recombination-activating genes. Instead of B cell and T cell receptors, agnathan lymphocytes express unique antigen receptors named variable lymphocyte receptors (VLRs), which generate diversity through a gene conversion-like mechanism. Although gnathostome antigen receptors and VLRs are structurally unrelated, jawed and jawless vertebrates share essential features of lymphocyte-based adaptive immunity, including the expression of a single type of receptor on each lymphocyte, clonal expansion of antigen-stimulated lymphocytes, and the dichotomy of cellular and humoral immunity, indicating that the backbone of the adaptive immune system was established in a common ancestor of all vertebrates. Furthermore, recent evidence indicates that, unlike previously thought, agnathans have a unique classical pathway of complement activation where VLRB molecules act as antibodies instead of immunoglobulins. It seems likely that the last common ancestor of all vertebrates had an adaptive immune system resembling that of jawless vertebrates, suggesting that, as opposed to jawed vertebrates, agnathans have retained the prototype of vertebrate adaptive immunity.

Expression profile of kalliklectin, a soluble-type mannose receptor, during embryogenesis and early larval development in fugu (Takifugu rubripes)

Kalliklectin is a unique fish-specific lectin, whose sequence is similar to the heavy chain of mammalian plasma kallikrein and coagulation factor XI. In this study, we aimed to evaluate dynamic expression profiles of the lectin gene, during early developmental stages, in fugu, Takifugu rubripes. Reverse transcription-polymerase chain reaction (RT-PCR) showed that the kalliklectin gene was not expressed until 14 h post-fertilization (hpf), while the mRNA was detected after 30 hpf. In real-time quantitative PCR (qPCR), the gene was first expressed at 10.5 hpf; then, the expression level increased with a peak at 30 hpf and then gradually decreased. On the other hand, western blotting with specific antibody detected the lectin protein at all tested stages, including the unfertilized egg, which suggests that the lectin detected in the early stages was a maternal factor. Immunohistochemistry demonstrated that kalliklectin was localized at the basement membranes of the newly hatched larvae, while the lectin was widely detected in epidermal cells in larva at 5 dph. A 40-kDa lectin was partially purified from unfertilized eggs using mannose-affinity chromatography, and the lectin was determined as kalliklectin by liquid chromatography with quadrupole time-of-flight tandem mass spectrometry (LC/Q-TOF-MS) analysis, which indicated that the lectin is functional in the eggs. The egg lectin can bind to Gram-positive bacterial pathogens of fish, such as Lactococcus garvieae and Streptococcus iniae. We conclude that fugu kalliklectin might be an important immunocomponent, transferred from mother to offspring.

Transcriptome sequencing of hybrid bester sturgeon: Responses to poly (I:C) in the context of comparative immunogenomics

Sturgeons represent a substantial scientific interest due to their high economic value, endangered status and also as the most primitive group of ray-finned fishes. Rapid progress in knowledge of sturgeon immunity was achieved recently with use of RNA sequencing. We report transcriptome sequencing of gill, head kidney, and spleen of bester sturgeon (a hybrid of beluga Huso huso and sterlet Acipenser ruthenus) injected with synthetic double-stranded RNA (polyI:C). The composition of transcriptome and responses to treatment were examined in the context of comparative genomics with focus on immune genes. Sturgeon transcripts matched to 21.5 k different proteins (blastx). With reference to Atlantic salmon, the functional groups and pathways of the immune system were uniformly represented: at average 36.5 ± 0.8% genes were found. Immune genes comprise a significant fraction of transcriptome. Among twenty genes with highest transcription levels, five are specialized immune genes and two encode heme and iron binding proteins (serotransferrin and hemopexin) also known as acute phase proteins. Challenge induced multiple functional groups including apoptosis, cell cycle and a number of metabolic pathways. Treatment stimulated innate antiviral immunity, which is well conserved between sturgeon and salmon, the most responsive genes were mx, rsad2 (viperin), interferon induced protein 44 and protein with tetratricopeptide repeats 5, cd87 and receptor transporting protein 3. Results added to knowledge of immune phylogeny. Gain and loss of genes was assessed by comparison with genomes from different phylogenetic groups. Among differentially expressed genes, percentage of acquired and lost genes was much lower in comparison with genes present in all vertebrates. Innate antiviral immunity was subject to the greatest changes in evolution of jawed vertebrates. A significant fraction of genes (15%) was lost in mammals and only half of genes is annotated in public databases as involved in antiviral responses. Change of function may have an important role in evolution of immunity together with gain and loss of genes.

Zebrafish and Medaka: Two Teleost Models of T-Cell and Thymic Development

Over the past two decades, studies have demonstrated that several features of T-cell and thymic development are conserved from teleosts to mammals. In particular, works using zebrafish (Danio rerio) and medaka (Oryzias latipes) have shed light on the cellular and molecular mechanisms underlying these biological processes. In particular, the ease of noninvasive in vivo imaging of these species enables direct visualization of all events associated with these processes, which are, in mice, technically very demanding. In this review, we focus on defining the similarities and differences between zebrafish and medaka in T-cell development and thymus organogenesis; and highlight their advantages as two complementary model systems for T-cell immunobiology and modeling of human diseases.

Origin and evolution of the specialized forms of proteasomes involved in antigen presentation

Proteasomes are a multi-subunit protease complex that produces peptides bound by major histocompatibility complex (MHC) class I molecules. Phylogenetic studies indicate that two specialized forms of proteasomes, immunoproteasomes and thymoproteasomes, and the proteasome activator PA28αβ emerged in a common ancestor of jawed vertebrates which acquired adaptive immunity based on the MHC, T cell receptors, and B cell receptors ~ 500 million years ago. Comparative genomics studies now provide strong evidence that the genes coding for the immunoproteasome subunits emerged by genome-wide duplication. On the other hand, the gene encoding the thymoproteasome subunit β5t emerged by tandem duplication from the gene coding for the β5 subunit. Strikingly, birds lack immunoproteasomes, thymoproteasomes, and the proteasome activator PA28αβ, raising an interesting question of whether they have evolved any compensatory mechanisms.

The immunoproteasome and thymoproteasome: functions, evolution and human disease

The basic principle of adaptive immunity is to strictly discriminate between self and non-self, and a central challenge to overcome is the enormous variety of pathogens that might be encountered. In cell-mediated immunity, immunological discernment takes place at a molecular or cellular level. Central to both mechanisms of discernment is the generation of antigenic peptides associated with MHC class I molecules, which is achieved by a proteolytic complex called the proteasome. To adequately accomplish the discrimination between self and non-self that is essential for adaptive immunity and self-tolerance, two proteasome subtypes have evolved via gene duplication: the immunoproteasome and the thymoproteasome. In this Review, we describe various aspects of these immunity-dedicated proteasomes, from their discovery to recent findings.

Origin and Evolution of Dendritic Epidermal T Cells

Dendritic epidermal T cells (DETCs) expressing invariant Vγ5Vδ1 T-cell receptors (TCRs) play a crucial role in maintaining skin homeostasis in mice. When activated, they secrete cytokines, which recruit various immune cells to sites of infection and promote wound healing. Recently, a member of the butyrophilin family, Skint1, expressed specifically in the skin and thymus was identified as a gene required for DETC development in mice. Skint1 is a gene that arose by rodent-specific gene duplication. Consequently, a gene orthologs to mouse Skint1 exists only in rodents, indicating that Skint1-dependent DETCs are unique to rodents. However, dendritic-shaped epidermal γδ T cells with limited antigen receptor diversity appear to occur in other mammals. Even lampreys, a member of the most primitive class of vertebrates that even lacks TCRs, have γδ T-like lymphocytes that resemble DETCs. This indicates that species as divergent as mice and lampreys share the needs to have innate-like T cells at their body surface, and that the origin of DETC-like cells is as ancient as that of lymphocytes.

Beyond antibody engineering: directed evolution of alternative binding scaffolds and enzymes using yeast surface display

Pioneered exactly 20 years ago, yeast surface display (YSD) continues to take a major role in protein engineering among the high-throughput display methodologies that have been developed to date. The classical yeast display technology relies on tethering an engineered protein to the cell wall by genetic fusion to one subunit of a dimeric yeast-mating agglutination receptor complex. This method enables an efficient genotype–phenotype linkage while exploiting the benefits of a eukaryotic expression machinery. Over the past two decades, a plethora of protein engineering efforts encompassing conventional antibody Fab and scFv fragments have been reported. In this review, we will focus on the versatility of YSD beyond conventional antibody engineering and, instead, place the focus on alternative scaffold proteins and enzymes which have successfully been tailored for purpose with regard to improving binding, activity or specificity.

Development of chimeric antigen receptors targeting T-cell malignancies using two structurally different anti-CD5 antigen binding domains in NK and CRISPR-edited T cell lines

Relapsed T-cell malignancies have poor outcomes when treated with chemotherapy, but survival after allogeneic bone marrow transplantation (BMT) approaches 50%. A limitation to BMT is the difficulty of achieving remission prior to transplant. Chimeric antigen receptor (CAR) T-cell therapy has shown successes in B-cell malignancies. This approach is difficult to adapt for the treatment of T-cell disease due to lack of a T-lymphoblast specific antigen and the fratricide of CAR T cells that occurs with T-cell antigen targeting. To circumvent this problem two approaches were investigated. First, a natural killer (NK) cell line, which does not express CD5, was used for CAR expression. Second, CRISPR-Cas9 genome editing technology was used to knockout CD5 expression in CD5-positive Jurkat T cells and in primary T cells, allowing for the use of CD5-negative T cells for CAR expression. Two structurally distinct anti-CD5 sequences were also tested, i) a traditional immunoglobulin-based single chain variable fragment (scFv) and ii) a lamprey-derived variable lymphocyte receptor (VLR), which we previously showed can be used for CAR-based recognition. Our results show i) both CARs yield comparable T-cell activation and NK cell-based cytotoxicity when targeting CD5-positive cells, ii) CD5-edited CAR-modified Jurkat T cells have reduced self-activation compared to that of CD5-positive CAR-modified T cells, iii) CD5-edited CAR-modified Jurkat T cells have increased activation in the presence of CD5-positive target cells compared to that of CD5-positive CAR-modified T cells, and iv) although modest effects were seen, a mouse model using the CAR-expressing NK cell line showed the scFv-CAR was superior to the VLR-CAR in delaying disease progression.

Building conventions for unconventional lymphocytes

Falling between the classical characteristics of innate immune cells and adaptive T and B cells are a group of lymphocytes termed "unconventional." These cells express antigen-specific T or B cell receptors, but behave with innate characteristics. Well-known members of this group include the gamma-delta T cell and the Natural Killer T cell. Recent literature has greatly expanded scientific knowledge of unconventional lymphocytes, but key questions remain unresolved in the field, including why these cells have been maintained concurrently with conventional innate and adaptive immune cells. Here, we summarize current literature that suggests what their unique purposes may be, including specialized functions with the microbiota and in early development. From the consensus literature, we discuss where we see unconventional lymphocytes fit into the logical organization of the complete immune system.

An evolutionary perspective on the systems of adaptive immunity

We propose an evolutionary perspective to classify and characterize the diverse systems of adaptive immunity that have been discovered across all major domains of life. We put forward a new function-based classification according to the way information is acquired by the immune systems: Darwinian immunity (currently known from, but not necessarily limited to, vertebrates) relies on the Darwinian process of clonal selection to 'learn' by cumulative trial-and-error feedback; Lamarckian immunity uses templated targeting (guided adaptation) to internalize heritable information on potential threats; finally, shotgun immunity operates through somatic mechanisms of variable targeting without feedback. We argue that the origin of Darwinian (but not Lamarckian or shotgun) immunity represents a radical innovation in the evolution of individuality and complexity, and propose to add it to the list of major evolutionary transitions. While transitions to higher-level units entail the suppression of selection at lower levels, Darwinian immunity re-opens cell-level selection within the multicellular organism, under the control of mechanisms that direct, rather than suppress, cell-level evolution for the benefit of the individual. From a conceptual point of view, the origin of Darwinian immunity can be regarded as the most radical transition in the history of life, in which evolution by natural selection has literally re-invented itself. Furthermore, the combination of clonal selection and somatic receptor diversity enabled a transition from limited to practically unlimited capacity to store information about the antigenic environment. The origin of Darwinian immunity therefore comprises both a transition in individuality and the emergence of a new information system - the two hallmarks of major evolutionary transitions. Finally, we present an evolutionary scenario for the origin of Darwinian immunity in vertebrates. We propose a revival of the concept of the 'Big Bang' of vertebrate immunity, arguing that its origin involved a 'difficult' (i.e. low-probability) evolutionary transition that might have occurred only once, in a common ancestor of all vertebrates. In contrast to the original concept, we argue that the limiting innovation was not the generation of somatic diversity, but the regulatory circuitry needed for the safe operation of amplifiable immune responses with somatically acquired targeting. Regulatory complexity increased abruptly by genomic duplications at the root of the vertebrate lineage, creating a rare opportunity to establish such circuitry. We discuss the selection forces that might have acted at the origin of the transition, and in the subsequent stepwise evolution leading to the modern immune systems of extant vertebrates.

The Appearance and Diversification of Receptors for IgM During Vertebrate Evolution

Three different receptors that interact with the constant domains of IgM have been identified: the polymeric immunoglobulin (Ig) receptor (PIGR), the dual receptor for IgA/IgM (FcαµR) and the IgM receptor (FcµR). All of them are related in structure and located in the same chromosomal region in mammals. The functions of the PIGRs are to transport IgM and IgA into the intestinal lumen and to saliva and tears, whereas the FcαµRs enhance uptake of immune complexes and antibody coated bacteria and viruses by B220+ B cells and phagocytes, as well as dampening the Ig response to thymus-independent antigens. The FcµRs have broad-spectrum effects on B-cell development including effects on IgM homeostasis, B-cell survival, humoral immune responses and also in autoantibody formation. The PIGR is the first of these receptors to appear during vertebrate evolution and is found in bony fish and all tetrapods but not in cartilaginous fish. The FcµR is present in all extant mammalian lineages and also in the Chinese and American alligators, suggesting its appearance with early reptiles. Currently the FcαµR has only been found in mammals and is most likely the evolutionary youngest of the three receptors. In bony fish, the PIGR has either 2, 3, 4, 5 or 6 extracellular Ig-like domains, whereas in amphibians, reptiles and birds it has 4 domains, and 5 in all mammals. The increase in domain number from 4 to 5 in mammals has been proposed to enhance the interaction with IgA. Both the FcαµRs and the FcµRs contain only one Ig domain; the domain that confers Ig binding. In both of these receptors this domain shows the highest degree of sequence similarity to domain 1 of the PIGR. All Ig domains of these three receptors are V type domains, indicating they all have the same origin although they have diversified extensively in function during vertebrate evolution by changing expression patterns and cytoplasmic signaling motifs.

Development of a novel therapeutic vaccine carrier that sustains high antibody titers against several targets simultaneously

With the aim to improve the efficacy of therapeutic vaccines that target self-antigens, we have developed a novel fusion protein vaccine on the basis of the C-terminal multimerizing end of the variable lymphocyte receptor B (VLRB), the Ig equivalent in jawless fishes. Recombinant vaccines were produced in Escherichia coli by fusing the VLRB sequence to 4 different cancer-associated target molecules. The anti-self-immune response generated in mice that were vaccinated with VLRB vaccines was compared with the response in mice that received vaccines that contained bacterial thioredoxin (TRX), previously identified as an efficient carrier. The anti-self-Abs were analyzed with respect to titers, binding properties, and duration of response. VLRB-vaccinated mice displayed a 2- to 10-fold increase in anti-self-Ab titers and a substantial decrease in Abs against the foreign part of the fusion protein compared with the response in TRX-vaccinated mice (P < 0.01). VLRB-generated Ab response had duration similar to the corresponding TRX-generated Abs, but displayed a higher diversity in binding characteristics. Of importance, VLRB vaccines could sustain an immune response against several targets simultaneously. VLRB vaccines fulfill several key criteria for an efficient therapeutic vaccine that targets self-antigens as a result of its small size, its multimerizing capacity, and nonexposed foreign sequences in the fusion protein.-Saupe, F., Reichel, M., Huijbers, E. J. M., Femel, J., Markgren, P.-O., Andersson, C. E., Deindl, S., Danielson, U. H., Hellman, L. T., Olsson, A.-K. Development of a novel therapeutic vaccine carrier that sustains high antibody titers against several targets simultaneously.

Genetic engineering of chimeric antigen receptors using lamprey derived variable lymphocyte receptors

Chimeric antigen receptors (CARs) are used to redirect effector cell specificity to selected cell surface antigens. Using CARs, antitumor activity can be initiated in patients with no prior tumor specific immunity. Although CARs have shown promising clinical results, the technology remains limited by the availability of specific cognate cell target antigens. To increase the repertoire of targetable tumor cell antigens we utilized the immune system of the sea lamprey to generate directed variable lymphocyte receptors (VLRs). VLRs serve as membrane bound and soluble immune effectors analogous but not homologous to immunoglobulins. They have a fundamentally different structure than immunoglobulin (Ig)-based antibodies while still demonstrating high degrees of specificity and affinity. To test the functionality of VLRs as the antigen recognition domain of CARs, two VLR-CARs were created. One contained a VLR specific for a murine B cell leukemia and the other contained a VLR specific for the human T cell surface antigen, CD5. The CAR design consisted of the VLR sequence, myc-epitope tag, CD28 transmembrane domain, and intracellular CD3ζ signaling domain. We demonstrate proof of concept, including gene transfer, biosynthesis, cell surface localization, and effector cell activation for multiple VLR-CAR designs. Therefore, VLRs provide an alternative means of CAR-based cancer recognition.

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.

Comparative genomics of the NKG2D ligand gene family

NKG2D ligands (NKG2DLs) are a group of stress-inducible major histocompatibility complex (MHC) class I-like molecules that act as a danger signal alerting the immune system to the presence of abnormal cells. In mammals, two families of NKG2DL genes have been identified: the MIC gene family encoded in the MHC region and the ULBP gene family encoded outside the MHC region in most species. Some mammals have a third family of NKG2DL-like class I genes which we named MILL (MHC class I-like located near the leukocyte receptor complex). Despite the fact that MILL genes are more closely related to MIC genes than ULBP genes are to MIC genes, MILL molecules do not function as NKG2DLs, and their function remains unknown. With the progress of mammalian genome projects, information on the MIC, ULBP, and MILL gene families became available in many mammalian species. Here, we summarize such information and discuss the origin and evolution of the NKG2DL gene family from the viewpoint of host-pathogen coevolution.

The p53-Mdm2 interaction and the E3 ligase activity of Mdm2/Mdm4 are conserved from lampreys to humans

Here, Coffill et al. characterize Tp53, Tp63, and Tp73 in a jawless vertebrate, the Japanese lamprey, as well as the Mdm2 and Mdm4 genes using genome analysis. Functional analysis reveals conservation of p63 and p73 compared with p53, which shows substantial variability within the C-terminal and N-terminal domains, and that lamprey Mdm2 degrades human p53 with great efficiency; however, this interaction is not inhibited by currently available small molecule inhibitors of the human HDM2 protein.

The extant jawless vertebrates, represented by lampreys and hagfish, are the oldest group of vertebrates and provide an interesting genomic evolutionary pivot point between invertebrates and jawed vertebrates. Through genome analysis of one of these jawless vertebrates, the Japanese lamprey (Lethenteron japonicum), we identified all three members of the important p53 transcription factor family—Tp53, Tp63, and Tp73—as well as the Mdm2 and Mdm4 genes. These genes and their products are significant cellular regulators in human cancer, and further examination of their roles in this most distant vertebrate relative sheds light on their origin and coevolution. Their important role in response to DNA damage has been highlighted by the discovery of multiple copies of the Tp53 gene in elephants. Expression of lamprey p53, Mdm2, and Mdm4 proteins in mammalian cells reveals that the p53–Mdm2 interaction and the Mdm2/Mdm4 E3 ligase activity existed in the common ancestor of vertebrates and have been conserved for >500 million years of vertebrate evolution. Lamprey Mdm2 degrades human p53 with great efficiency, but this interaction is not blocked by currently available small molecule inhibitors of the human HDM2 protein, suggesting utility of lamprey Mdm2 in the study of the human p53 signaling pathway.

Along the Axis between Type 1 and Type 2 Immunity; Principles Conserved in Evolution from Fish to Mammals

A phenomenon already discovered more than 25 years ago is the possibility of naïve helper T cells to polarize into TH1 or TH2 populations. In a simplified model, these polarizations occur at opposite ends of an "immune 1-2 axis" (i1-i2 axis) of possible conditions. Additional polarizations of helper/regulatory T cells were discovered later, such as for example TH17 and Treg phenotypes; although these polarizations are not selected by the axis-end conditions, they are affected by i1-i2 axis factors, and may retain more potential for change than the relatively stable TH1 and TH2 phenotypes. I1-i2 axis conditions are also relevant for polarizations of other types of leukocytes, such as for example macrophages. Tissue milieus with "type 1 immunity" ("i1") are biased towards cell-mediated cytotoxicity, while the term "type 2 immunity" ("i2") is used for a variety of conditions which have in common that they inhibit type 1 immunity. The immune milieus of some tissues, like the gills in fish and the uterus in pregnant mammals, probably are skewed towards type 2 immunity. An i2-skewed milieu is also created by many tumors, which allows them to escape eradication by type 1 immunity. In this review we compare a number of i1-i2 axis factors between fish and mammals, and conclude that several principles of the i1-i2 axis system seem to be ancient and shared between all classes of jawed vertebrates. Furthermore, the present study is the first to identify a canonical TH2 cytokine locus in a bony fish, namely spotted gar, in the sense that it includes RAD50 and bona fide genes of both IL-4/13 and IL-3/ IL-5/GM-CSF families.

Food-Nonfood Discrimination in Ancestral Vertebrates: Gamete Cannibalism and the Origin of the Adaptive Immune System

Adaptive immunity is a complex system that appeared twice in vertebrates (in gnathostomes and in jawless fish) although it is not required for invertebrate defence. The adaptive immune system is tightly associated with self-non-self discrimination, and it is now clear that this interplay is not limited to the prevention of autoreactivity. Micro-organisms are usually considered for their pathogenicity or symbiotic ability, but, for most small metazoans, they mainly constitute food. Vertebrates are characterized by feeding by predation on larger preys, when compared to their ancestors who were filter feeders and ate micro-organisms. Predation gives a strong selective advantage, not only due to the availability of new food resources but also by the ability to eliminate competitors for environmental resources (intraguild predation (IGP)). Unlike size-structured IGP, intraspecific predation of juveniles, zygotes or gametes can be detrimental for species fitness in some circumstances. The ability of individuals to recognize highly polymorphic molecules on the surface of gametes present in the plankton and so distinguish self versus non-self gametes might have constituted a strong selective advantage in intraspecific competition. Here, I propose the theory that the capacity to rearrange receptors has been selected in ancestral vertebrates as a consequence of this strong need for discriminating between hetero-cannibalism versus filial cannibalism. This evolutionary origin sheds light on presently unexplained features of the immune system, including the existence of regulatory T cells and of non-pathogenic natural autoimmunity.

Identification, expression pattern and potential role of variable lymphocyte receptor Aj-VLRA from Apostichopus japonicus in response to bacterial challenge

The variable lymphocyte receptors (VLRs) are found in jawless vertebrates (agnathans), and specifically recognize bacteria and viruses via their leucine-rich repeats (LRRs). VLRs are believed to be adaptive immune response molecules. Echinoderms do not have adaptive immune systems; however, in the present study, a VLR cDNA named Aj-VLRA was cloned and characterized from sea cucumber, Apostichopus japonicus. The complete cDNA of Aj-VLRA was 3072 bp, including a 1995 bp open reading frame encoding 664 amino acids comprising LRR domains, a predicted transmembrane helix and an N-terminal signal peptide. As determined by quantitative real-time reverse transcription polymerase chain reaction, Aj-VLRA transcripts are ubiquitously expressed in the body wall, longitudinal muscles, intestine and respiratory tree of A. japonicus. The expression level of Aj-VLRA was upregulated after challenge with four common marine bacteria. In situ hybridization showed that the expression of Aj-VLRA was widely distributed in the four tissues, particularly in the cytoplasm of epidermal cells. Recombinantly expressed Aj-VLRA (including the LRR domains) could bind to bacteria including Micrococcus lysodeikticus (Gram+) and Vibrio anguillarum (Gram-). Collectively, the results suggested that Aj-VLRA is related to an innate immune response of A. japonicus.

Defining Mononuclear Phagocyte Subset Homology Across Several Distant Warm-Blooded Vertebrates Through Comparative Transcriptomics

Mononuclear phagocytes are organized in a complex system of ontogenetically and functionally distinct subsets, that has been best described in mouse and to some extent in human. Identification of homologous mononuclear phagocyte subsets in other vertebrate species of biomedical, economic, and environmental interest is needed to improve our knowledge in physiologic and physio-pathologic processes, and to design intervention strategies against a variety of diseases, including zoonotic infections. We developed a streamlined approach combining refined cell sorting and integrated comparative transcriptomics analyses which revealed conservation of the mononuclear phagocyte organization across human, mouse, sheep, pigs and, in some respect, chicken. This strategy should help democratizing the use of omics analyses for the identification and study of cell types across tissues and species. Moreover, we identified conserved gene signatures that enable robust identification and universal definition of these cell types. We identified new evolutionarily conserved gene candidates and gene interaction networks for the molecular regulation of the development or functions of these cell types, as well as conserved surface candidates for refined subset phenotyping throughout species. A phylogenetic analysis revealed that orthologous genes of the conserved signatures exist in teleost fishes and apparently not in Lamprey.

Variable Lymphocyte Receptors: A Current Overview

Jawless vertebrates represented by lampreys and hagfish mount antigen-specific immune responses using variable lymphocyte receptors. These receptors generate diversity comparable to that of T-cell and B-cell receptors by assembling multiple leucine-rich repeat modules with highly variable sequences. Although it is true that jawed and jawless vertebrates have structurally unrelated antigen receptors, their adaptive immune systems have much in common. Most notable is the conservation of lymphocyte lineages. It appears that specialized lymphocyte lineages emerged in a common vertebrate ancestor and that jawed and jawless vertebrates co-opted different antigen receptors within the context of such lymphocyte lineages.

Characterization of the Runx gene family in a jawless vertebrate, the Japanese lamprey (Lethenteron japonicum)

The cyclostomes (jawless vertebrates), comprising lampreys and hagfishes, are the sister group of jawed vertebrates (gnathostomes) and are hence an important group for the study of vertebrate evolution. In mammals, three Runx genes, Runx1, Runx2 and Runx3, encode transcription factors that are essential for cell proliferation and differentiation in major developmental pathways such as haematopoiesis, skeletogenesis and neurogenesis and are frequently associated with diseases. We describe here the characterization of Runx gene family members from a cyclostome, the Japanese lamprey (Lethenteron japonicum). The Japanese lamprey contains three Runx genes, RunxA, RunxB, and RunxC. However, phylogenetic and synteny analyses suggest that they are not one-to-one orthologs of gnathostome Runx1, Runx2 and Runx3. The major protein domains and motifs found in gnathostome Runx proteins are highly conserved in the lamprey Runx proteins. Although all gnathostome Runx genes each contain two alternative promoters, P1 (distal) and P2 (proximal), only lamprey RunxB possesses the alternative promoters; lamprey RunxA and RunxC contain only P2 and P1 promoter, respectively. Furthermore, the three lamprey Runx genes give rise to fewer alternative isoforms than the three gnathostome Runx genes. The promoters of the lamprey Runx genes lack the tandem Runx-binding motifs that are highly conserved among the P1 promoters of gnathostome Runx1, Runx2 and Runx3 genes; instead these promoters contain dispersed single Runx-binding motifs. The 3'UTR of lamprey RunxB contains binding sites for miR-27 and miR-130b/301ab, which are conserved in mammalian Runx1 and Runx3, respectively. Overall, the Runx genes in lamprey seem to have experienced a different evolutionary trajectory from that of gnathostome Runx genes which are highly conserved all the way from cartilaginous fishes to mammals.

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.

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

Unlike jawed vertebrates that use T cell and B cell receptors for antigen recognition, jawless vertebrates represented by lampreys and hagfish use variable lymphocyte receptors (VLR) as antigen receptors. VLRs generate high levels of diversity by assembling variable leucine-rich repeat (LRR) modules. Of the three VLRs thus far identified, VLRB is expressed on B cell-like lymphocytes and functions as antibodies, whereas VLRA and VLRC are expressed on T cell-like lymphocytes and function as membrane-bound receptors. In the present study, we show that the copy number of LRRV modules in lamprey and hagfish VLRB transcripts follows a binominal distribution with the success rates of 15.5 and 22.4 %, respectively. By contrast, the copy number distribution of LRRV modules in VLRA and VLRC transcripts deviates from the binominal distribution mainly because transcripts with two or less LRRV modules occur infrequently. Notably, the second LRRV module shows distinctive sequence signatures in VLRA and VLRC, but not in VLRB transcripts. These observations suggest that distinct functional constraints operate on VLRs expressed by agnathan T cell-like and B cell-like lymphocytes.

Evolution of integrin I domains

In humans, an ~200-residue "inserted" I domain, a von Willebrand factor A domain (vWFA), buds out from the β-propeller domain in 9 of 18 integrin α subunits. The vWFA domain is not unique to the α subunit as it is an integral part of all integrin β subunits and many other proteins. The βI domain has always been a component of integrins but the αI domain makes its appearance relatively late, in early chordates, since it is found in tunicates and later diverging species. The tunicate αI domains are distinct from the human collagen and leukocyte recognizing integrin α subunits, but fragments of integrins from agnathastomes suggest that the human-type αI domains arose in an ancestor of the very first vertebrate species. The rise of integrins with αI domains parallels the enormous changes in body plan and systemic development of the chordate line that began some 550 million or more years ago.