Quality control in self/nonself discrimination

Honeybees' novel complementary sex-determining system: function and origin

Complementary sex determination regulates female and male development in honeybees (Apis mellifera) via heterozygous versus homo-/hemizygous genotypes of the csd (complementary sex determiner) gene involving numerous naturally occurring alleles. This lineage-specific function offers a rare opportunity to understand an undescribed regulatory mechanism and the molecular evolutionary path leading to this mechanism. We reviewed recent advances in understanding how Csd recognizes different versus identical protein variants, how these variants regulate downstream pathways and sexual differentiation, and how this mechanism has evolved and been shaped by evolutionary forces. Finally, we highlighted the shared regulatory principles of sex determination despite the diversity of primary signals and demonstrated that lineage-specific mutations are very informative for characterizing newly evolved functions.

A journey to your self: The vague definition of immune self and its practical implications

The identification of immunogenic peptides has become essential in an increasing number of fields in immunology, ranging from tumor immunotherapy to vaccine development. The nature of the adaptive immune response is shaped by the similarity between foreign and self-protein sequences, a concept extensively applied in numerous studies. Can we precisely define the degree of similarity to self? Furthermore, do we accurately define immune self? In the current work, we aim to unravel the conceptual and mechanistic vagueness hindering the assessment of self-similarity. Accordingly, we demonstrate the remarkably low consistency among commonly employed measures and highlight potential avenues for future research.

The Narrow Footprint of Ancient Balancing Selection Revealed by Heterokaryon Incompatibility Genes in Aspergillus fumigatus

In fungi, fusion between individuals leads to localized cell death, a phenomenon termed heterokaryon incompatibility. Generally, the genes responsible for this incompatibility are observed to be under balancing selection resulting from negative frequency-dependent selection. Here, we assess this phenomenon in Aspergillus fumigatus, a human pathogenic fungus with a very low level of linkage disequilibrium as well as an extremely high crossover rate. Using complementation of auxotrophic mutations as an assay for hyphal compatibility, we screened sexual progeny for compatibility to identify genes involved in this process, called het genes. In total, 5/148 (3.4%) offspring were compatible with a parent and 166/2,142 (7.7%) sibling pairs were compatible, consistent with several segregating incompatibility loci. Genetic mapping identified five loci, four of which could be fine mapped to individual genes, of which we tested three through heterologous expression, confirming their causal relationship. Consistent with long-term balancing selection, trans-species polymorphisms were apparent across several sister species, as well as equal allele frequencies within A. fumigatus. Surprisingly, a sliding window genome-wide population-level analysis of an independent dataset did not show increased Tajima's D near these loci, in contrast to what is often found surrounding loci under balancing selection. Using available de novo assemblies, we show that these balanced polymorphisms are restricted to several hundred base pairs flanking the coding sequence. In addition to identifying the first het genes in an Aspergillus species, this work highlights the interaction of long-term balancing selection with rapid linkage disequilibrium decay.

Genotype-specific expression of uncle fester suggests a role in allorecognition education in a basal chordate

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

Recognition of polymorphic Csd proteins determines sex in the honeybee

Sex in honeybees, Apis mellifera, is genetically determined by heterozygous versus homo/hemizygous genotypes involving numerous alleles at the single complementary sex determination locus. The molecular mechanism of sex determination is however unknown because there are more than 4950 known possible allele combinations, but only two sexes in the species. We show how protein variants expressed from complementary sex determiner (csd) gene determine sex. In females, the amino acid differences between Csd variants at the potential-specifying domain (PSD) direct the selection of a conserved coiled-coil domain for binding and protein complexation. This recognition mechanism activates Csd proteins and, thus, the female pathway. In males, the absence of polymorphisms establishes other binding elements at PSD for binding and complexation of identical Csd proteins. This second recognition mechanism inactivates Csd proteins and commits male development via default pathway. Our results demonstrate that the recognition of different versus identical variants of a single protein is a mechanism to determine sex.

Comparative study of structures and functional motifs in lectins from the commercially important photosynthetic microorganisms

Photosynthetic microorganisms, specifically cyanobacteria and microalgae, can synthesize a vast array of biologically active molecules, such as lectins, that have great potential for various biotechnological and biomedical applications. However, since the structures of these proteins are not well established, likely due to the presence of intrinsically disordered regions, our ability to better understand their functionality is hampered. We embarked on a study of the carbohydrate recognition domain (CRD), intrinsically disordered regions (IDRs), amino acidic composition, as well as and functional motifs in lectins from cyanobacteria of the genus Arthrospira and microalgae Chlorella and Dunaliella genus using a combination of bioinformatics techniques. This search revealed the presence of five distinctive CRD types differently distributed between the genera. Most CRDs displayed a group-specific distribution, except to C. sorokiniana possessing distinctive CRD probably due to its specific lifestyle. We also found that all CRDs contain short IDRs. Bacterial lectin of Arthrospira prokarionte showed lower intrinsic disorder and proline content when compared to the lectins from the eukaryotic microalgae (Chlorella and Dunaliella). Among the important functions predicted in all lectins were several specific motifs, which directly interacts with proteins involved in the cell-cycle control and which may be used for pharmaceutical purposes. Since the aforementioned properties of each type of lectin were investigated in silico, they need experimental confirmation. The results of our study provide an overview of the distribution of CRD, IDRs, and functional motifs within lectin from the commercially important microalgae.

Plant Communication

Communication occurs when a sender emits a cue perceived by a receiver that changes the receiver's behavior. Plants perceive information regarding light, water, other nutrients, touch, herbivores, pathogens, mycorrhizae, and nitrogen-fixing bacteria. Plants also emit cues perceived by other plants, beneficial microbes, herbivores, enemies of herbivores, pollinators, and seed dispersers. Individuals responding to light cues experienced increased fitness. Evidence for benefits of responding to cues involving herbivores and pathogens is more limited. The benefits of emitting cues are also less clear, particularly for plant–plant communication. Reliance on multiple or dosage-dependent cues can reduce inappropriate responses, and plants often remember past cues. Plants have multiple needs and prioritize conflicting cues such that the risk of abiotic stress is treated as greater than that of shading, which is in turn treated as greater than that of consumption. Plants can distinguish self from nonself and kin from strangers. They can identify the species of competitor or consumer and respond appropriately. Cues involving mutualists often contain highly specific information.

Reproduction Multitasking: The Male Gametophyte

The gametophyte represents the sexual phase in the alternation of generations in plants; the other, nonsexual phase is the sporophyte. Here, we review the evolutionary origins of the male gametophyte among land plants and, in particular, its ontogenesis in flowering plants. The highly reduced male gametophyte of angiosperm plants is a two- or three-celled pollen grain. Its task is the production of two male gametes and their transport to the female gametophyte, the embryo sac, where double fertilization takes place. We describe two phases of pollen ontogenesis-a developmental phase leading to the differentiation of the male germline and the formation of a mature pollen grain and a functional phase representing the pollen tube growth, beginning with the landing of the pollen grain on the stigma and ending with double fertilization. We highlight recent advances in the complex regulatory mechanisms involved, including posttranscriptional regulation and transcript storage, intracellular metabolic signaling, pollen cell wall structure and synthesis, protein secretion, and phased cell-cell communication within the reproductive tissues.

Search for MHC/TCR-Like Systems in Living Organisms

Highly polymorphic loci evolved many times over the history of species. These polymorphic loci are involved in three types of functions: kind recognition, self-incompatibility, and the jawed vertebrate adaptive immune system (AIS). In the first part of this perspective, we reanalyzed and described some cases of polymorphic loci reported in the literature. There is a convergent evolution within each functional category and between functional categories, suggesting that the emergence of these self/non-self recognition loci has occurred multiple times throughout the evolutionary history. Most of the highly polymorphic loci are coding for proteins that have a homophilic interaction or heterophilic interaction between linked loci, leading to self or non-self-recognition. The highly polymorphic MHCs, which are involved in the AIS have a different functional mechanism, as they interact through presented self or non-self-peptides with T cell receptors, whose diversity is generated by somatic recombination. Here we propose a mechanism called “the capacity of recognition competition mechanism” that might contribute to the evolution of MHC polymorphism. We propose that the published cases corresponding to these three biological categories represent a small part of what can be found throughout the tree of life, and that similar mechanisms will be found many times, including the one where polymorphic loci interact with somatically generated loci.

Bringing Together the Power of T Cell Receptor Mimic and Bispecific Antibodies for Cancer Immunotherapy: Still a Long Way to Go

Antibody-based cancer immunotherapy has revolutionized oncology. The first successful therapeutic antibodies relied on eliciting immune-mediated cytotoxicity (rituximab) or modulation of intracellular signaling (trastuzumab). Further attempts to enhance the antitumor effects led to the development of immunoconjugates with radioactive or cytotoxic compounds (tositumomab, brentuximab vedotin). Another line of research led to the bispecific antibodies that can enhance the formation of immunological synapse between cancer and cytotoxic T cells (blinatumomab). Despite the constant advances in design and production, the application of monoclonal antibodies in cancer treatment remains limited by the presence of specific cell surface markers. A rational approach to target intracellular cancer antigens was proposed almost two decades ago by the development of anti-peptide human leukocyte antigen (HLA) complex (T cell receptor-like/mimic) antibodies. They could recognize specifically cancer neoantigens expressed in the context of specific HLA molecules theoretically providing unprecedented specificity. Furthermore, they can be developed in a semigeneric format, that is, to target common neoantigens expressed in the context of common HLA molecules. It is rationale to expect that the development of such antibodies in the format of bispecific antibody constructs can bring together the power of specific antibody-based recognition and that of T cell-mediated lysis. There are already some preliminary reports that suggest such constructs would be an achievable goal. In this brief review, we discuss some of the successful preclinical developments in the field and the major challenges that are yet to be addressed.

Evolution of thymopoietic microenvironments

In vertebrates, the development of lymphocytes from undifferentiated haematopoietic precursors takes place in so-called primary lymphoid organs, such as the thymus. Therein, lymphocytes undergo a complex differentiation and selection process that culminates in the generation of a pool of mature T cells that collectively express a self-tolerant repertoire of somatically diversified antigen receptors. Throughout this entire process, the microenvironment of the thymus in large parts dictates the sequence and outcome of the lymphopoietic activity. In vertebrates, direct genetic evidence in some species and circumstantial evidence in others suggest that the formation of a functional thymic microenvironment is controlled by members of the Foxn1/4 family of transcription factors. In teleost fishes, both Foxn1 and Foxn4 contribute to thymopoietic activity, whereas Foxn1 is both necessary and sufficient in the mammalian thymus. The evolutionary history of Foxn1/4 genes suggests that an ancient Foxn4 gene lineage gave rise to the Foxn1 genes in early vertebrates, raising the question of the thymopoietic capacity of the ancestor common to all vertebrates. Recent attempts to reconstruct the early events in the evolution of thymopoietic tissues by replacement of the mouse Foxn1 gene by Foxn1-like genes isolated from various chordate species suggest a plausible scenario. It appears that the primordial thymus was a bi-potent lymphoid organ, supporting both B cell and T cell development; however, during the course of vertebrate, evolution B cell development was gradually diminished converting the thymus into a site specialized in T cell development.

HLA genotyping meets response to immune checkpoint inhibitors prediction: A story just started

The implementation of the immune checkpoint blockade as a therapeutic option in contemporary oncology is one of the significant immunological achievements in the last century. Constantly accumulating evidence suggests that the response to immune checkpoint inhibitors (ICIs) is not universal. Therefore, it is critical to identify determinants for response, resistance and adverse effects of immune checkpoint therapy that could be developed as prognostic and predictive markers. Recent large scale analyses of cancer genome data revealed the key role of HLA class I and class II molecules in cancer immunoediting, and it appears that HLA diversity can predict response to ICIs. In the present review, we summarize the emerging data on the role of HLA germline variations as a marker for response to ICIs.

The gut microbiome defines social group membership in honey bee colonies

In the honey bee, genetically related colony members innately develop colony-specific cuticular hydrocarbon profiles, which serve as pheromonal nestmate recognition cues. Yet, despite high intracolony relatedness, the innate development of colony-specific chemical signatures by individual colony members is largely determined by the colony environment, rather than solely relying on genetic variants shared by nestmates. Therefore, it is puzzling how a nongenic factor could drive the innate development of a quantitative trait that is shared by members of the same colony. Here, we provide one solution to this conundrum by showing that nestmate recognition cues in honey bees are defined, at least in part, by shared characteristics of the gut microbiome across individual colony members. These results illustrate the importance of host-microbiome interactions as a source of variation in animal behavioral traits.

Conflict, Competition, and Cooperation Regulate Social Interactions in Filamentous Fungi

Social cooperation impacts the development and survival of species. In higher taxa, kin recognition occurs via visual, chemical, or tactile cues that dictate cooperative versus competitive interactions. In microbes, the outcome of cooperative versus competitive interactions is conferred by identity at allorecognition loci, so-called kind recognition. In syncytial filamentous fungi, the acquisition of multicellularity is associated with somatic cell fusion within and between colonies. However, such intraspecific cooperation entails risks, as fusion can transmit deleterious genotypes or infectious components that reduce fitness, or give rise to cheaters that can exploit communal goods without contributing to their production. Allorecognition mechanisms in syncytial fungi regulate somatic cell fusion by operating precontact during chemotropic interactions, during cell adherence, and postfusion by triggering programmed cell death reactions. Alleles at fungal allorecognition loci are highly polymorphic, fall into distinct haplogroups, and show evolutionary signatures of balancing selection, similar to allorecognition loci across the tree of life.

Origins of the RAG Transposome and the MHC

How innate immunity gave rise to adaptive immunity in vertebrates remains unknown. We propose an evolutionary scenario beginning with pathogen-associated molecular pattern(s) (PAMPs) being presented by molecule(s) on one cell to specific receptor(s) on other cells, much like MHC molecules and T cell receptors (TCRs). In this model, mutations in MHC-like molecule(s) that bound new PAMP(s) would not be recognized by original TCR-like molecule(s), and new MHC-like gene(s) would be lost by neutral drift. Integrating recombination activating gene (RAG) transposon(s) in a TCR-like gene would result in greater recognition diversity, with new MHC-like variants recognized and selected, along with a new RAG/TCR-like system. MHC genes would be selected to present many peptides, through multigene families, allelic polymorphism, and peptide-binding promiscuity.

A stigmatic gene confers interspecies incompatibility in the Brassicaceae

Pre-zygotic interspecies incompatibility in angiosperms is a male-female relationship that inhibits the formation of hybrids between two species. Here, we report on the identification of STIGMATIC PRIVACY 1 (SPRI1), an interspecies barrier gene in Arabidopsis thaliana. We show that the rejection activity of this stigma-specific plasma membrane protein is effective against distantly related Brassicaceae pollen tubes and is independent of self-incompatibility. Point-mutation experiments and functional tests of synthesized hypothetical ancestral forms of SPRI1 suggest evolutionary decay of SPRI1-controlled interspecies incompatibility in self-compatible A. thaliana. Hetero-pollination experiments indicate that SPRI1 ensures intraspecific fertilization in the pistil when pollen from other species are present. Our study supports the idea that SPRI1 functions as a barrier mechanism that permits entrance of pollen with an intrinsic signal from self species.

UVB-induced depletion of donor-derived dendritic cells prevents allograft rejection of immune-privileged hair follicles in humanized mice

Dendritic cells (DCs) are key targets for immunity and tolerance induction; they present donor antigens to recipient T cells by donor- and recipient-derived pathways. Donor-derived DCs, which are critical during the acute posttransplant period, can be depleted in graft tissue by forced migration via ultraviolet B light (UVB) irradiation. Here, we investigated the tolerogenic potential of donor-derived DC depletion through in vivo and ex vivo UVB preirradiation (UV) combined with the injection of anti-CD154 antibody (Ab) into recipients in an MHC-mismatched hair follicle (HF) allograft model in humanized mice. Surprisingly, human HF allografts achieved long-term survival with newly growing pigmented hair shafts in both Ab-treated groups (Ab-only and UV plus Ab) and in the UV-only group, whereas the control mice rejected all HF allografts with no hair regrowth. Perifollicular human CD3⁺ T cell and MHC class II⁺ cell infiltration was significantly diminished in the presence of UV and/or Ab treatment. HF allografts in the UV-only group showed stable maintenance of the immune privilege in the HF epithelium without evidence of antigen-specific T cell tolerance, which is likely promoted by normal HFs in vivo. This immunomodulatory strategy targeting the donor tissue exhibited novel biological relevance for clinical allogeneic transplantation without generalized immunosuppression.

The cuticular hydrocarbon profiles of honey bee workers develop via a socially-modulated innate process

Large social insect colonies exhibit a remarkable ability for recognizing group members via colony-specific cuticular pheromonal signatures. Previous work suggested that in some ant species, colony-specific pheromonal profiles are generated through a mechanism involving the transfer and homogenization of cuticular hydrocarbons (CHCs) across members of the colony. However, how colony-specific chemical profiles are generated in other social insect clades remains mostly unknown. Here we show that in the honey bee (Apis mellifera), the colony-specific CHC profile completes its maturation in foragers via a sequence of stereotypic age-dependent quantitative and qualitative chemical transitions, which are driven by environmentally-sensitive intrinsic biosynthetic pathways. Therefore, the CHC profiles of individual honey bees are not likely produced through homogenization and transfer mechanisms, but instead mature in association with age-dependent division of labor. Furthermore, non-nestmate rejection behaviors seem to be contextually restricted to behavioral interactions between entering foragers and guards at the hive entrance.

Honey bees are social insects that live in large groups called colonies, within structures known as hives. The young adult bees stay within the hive to build nests and care for the young, while the older bees leave the hive to forage for food. Honey bees store food and other valuable resources in their hives, so they are often targeted by predators, parasites and ‘robber’ bees from other colonies. Therefore, it is important for bees to determine whether individuals trying to enter the nest are group members or intruders.

While it is known that social insects use blends of waxy chemicals called cuticular hydrocarbons to identify group members at the entrance to the colony, it is not clear how members of the same colony acquire a similar blend of cuticular hydrocarbons. Some previous work suggested that in some ant species (which are also social insects), colony members exchange cuticular hydrocarbons with each other so that all members of the colony are covered with a similar blend of chemicals. However, it was not known whether honey bees also share cuticular hydrocarbons between colony members in order to identify members of a hive.

Vernier et al. used chemical, molecular and behavioral approaches to study the cuticular hydrocarbons found on honey bees. The results show that, rather than exchanging chemicals with other members of their colony, individual bees make their own blends of cuticular hydrocarbons. As a bee ages it makes different blends of cuticular hydrocarbons, and by the time it starts to leave the hive to forage it makes a blend that is specific to the colony it belongs to. The production of this final blend is influenced by the environment within the hive.

Thus, the findings of Vernier et al. indicate that honey bees guarding the entrance to a hive can only identify non-colony-member forager bees as intruders, rather than any non-colony-member bee that happens upon the hive entrance. Honey bees play an essential role in pollinating many crop plants so understanding how these insects maintain their social groups may help to improve agriculture in the future. Furthermore, this work may aid our understanding of how other social insects interact in a variety of biological situations.

Dynamics of Cas10 Govern Discrimination between Self and Non-self in Type III CRISPR-Cas Immunity

Adaptive immune systems must accurately distinguish between self and non-self in order to defend against invading pathogens while avoiding autoimmunity. Type III CRISPR-Cas systems employ guide RNA to recognize complementary RNA targets, which triggers the degradation of both the invader's transcripts and their template DNA. These systems can broadly eliminate foreign targets with multiple mutations but circumvent damage to the host genome. To explore the molecular basis for these features, we use single-molecule fluorescence microscopy to study the interaction between a type III-A ribonucleoprotein complex and various RNA substrates. We find that Cas10-the DNase effector of the complex-displays rapid conformational fluctuations on foreign RNA targets, but is locked in a static configuration on self RNA. Target mutations differentially modulate Cas10 dynamics and tune the CRISPR interference activity in vivo. These findings highlight the central role of the internal dynamics of CRISPR-Cas complexes in self versus non-self discrimination and target specificity.

Danger signals activate a putative innate immune system during regeneration in a filamentous fungus

The ability to respond to injury is a biological process shared by organisms of different kingdoms that can even result in complete regeneration of a part or structure that was lost. Due to their immobility, multicellular fungi are prey to various predators and are therefore constantly exposed to mechanical damage. Nevertheless, our current knowledge of how fungi respond to injury is scarce. Here we show that activation of injury responses and hyphal regeneration in the filamentous fungus Trichoderma atroviride relies on the detection of two danger or alarm signals. As an early response to injury, we detected a transient increase in cytosolic free calcium ([Ca2+]c) that was promoted by extracellular ATP, and which is likely regulated by a mechanism of calcium-induced calcium-release. In addition, we demonstrate that the mitogen activated protein kinase Tmk1 plays a key role in hyphal regeneration. Calcium- and Tmk1-mediated signaling cascades activated major transcriptional changes early following injury, including induction of a set of regeneration associated genes related to cell signaling, stress responses, transcription regulation, ribosome biogenesis/translation, replication and DNA repair. Interestingly, we uncovered the activation of a putative fungal innate immune response, including the involvement of HET domain genes, known to participate in programmed cell death. Our work shows that fungi and animals share danger-signals, signaling cascades, and the activation of the expression of genes related to immunity after injury, which are likely the result of convergent evolution.

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.

Electrostatic potentials of the S-locus F-box proteins contribute to the pollen S specificity in self-incompatibility in Petunia hybrida

Self-incompatibility (SI) is a self/non-self discrimination system found widely in angiosperms and, in many species, is controlled by a single polymorphic S-locus. In the Solanaceae, Rosaceae and Plantaginaceae, the S-locus encodes a single S-RNase and a cluster of S-locus F-box (SLF) proteins to control the pistil and pollen expression of SI, respectively. Previous studies have shown that their cytosolic interactions determine their recognition specificity, but the physical force between their interactions remains unclear. In this study, we show that the electrostatic potentials of SLF contribute to the pollen S specificity through a physical mechanism of 'like charges repel and unlike charges attract' between SLFs and S-RNases in Petunia hybrida. Strikingly, the alteration of a single C-terminal amino acid of SLF reversed its surface electrostatic potentials and subsequently the pollen S specificity. Collectively, our results reveal that the electrostatic potentials act as a major physical force between cytosolic SLFs and S-RNases, providing a mechanistic insight into the self/non-self discrimination between cytosolic proteins in angiosperms.

Dichotomy between T Cell and B Cell Tolerance to Neonatal Retroviral Infection Permits T Cell Therapy

Elucidation of the immune requirements for control or elimination of retroviral infection remains an important aim. We studied the induction of adaptive immunity to neonatal infection with a murine retrovirus, under conditions leading to immunological tolerance. We found that the absence of either maternal or offspring adaptive immunity permitted efficient vertical transmission of the retrovirus. Maternal immunodeficiency allowed the retrovirus to induce central Th cell tolerance in the infected offspring. In turn, this compromised the offspring’s ability to mount a protective Th cell–dependent B cell response. However, in contrast to T cells, offspring B cells were not centrally tolerized and retained their ability to respond to the infection when provided with T cell help. Thus, escape of retrovirus-specific B cells from deletional tolerance offers the opportunity to induce protective retroviral immunity by restoration of retrovirus-specific T cell help, suggesting similar T cell immunotherapies for persistent viral infections.

Synthetic multivalency for biological applications

Current directions and emerging possibilities under investigation for the integration of synthetic and semi-synthetic multivalent architectures with biology are discussed. Attention is focussed around multivalent interactions, their fundamental role in biology, and current and potential approaches in emulating them in terms of structure and functionality using synthetic architectures.

The natural defense system and the normative self model

Infectious agents are not the only agressors, and the immune system is not the sole defender of the organism. In an enlarged perspective, the ‘normative self model’ postulates that a ‘natural defense system’ protects man and other complex organisms against the environmental and internal hazards of life, including infections and cancers. It involves multiple error detection and correction mechanisms that confer robustness to the body at all levels of its organization. According to the model, the self relies on a set of physiological norms, and NONself (meaning : Non Obedient to the Norms of the self) is anything ‘off-norms’. The natural defense system comprises a set of ‘civil defenses’ (to which all cells in organs and tissues contribute), and a ‘professional army ‘, made of a smaller set of mobile cells. Mobile and non mobile cells differ in their tuning abilities. Tuning extends the recognition capabilities of NONself by the mobile cells, which increase their defensive function. To prevent them to drift, which would compromise self/NONself discrimination, the more plastic mobile cells need to periodically refer to the more stable non mobile cells to keep within physiological standards.

Characterization of Greenbeard Genes Involved in Long-Distance Kind Discrimination in a Microbial Eukaryote

Microorganisms are capable of communication and cooperation to perform social activities. Cooperation can be enforced using kind discrimination mechanisms in which individuals preferentially help or punish others, depending on genetic relatedness only at certain loci. In the filamentous fungus Neurospora crassa, genetically identical asexual spores (germlings) communicate and fuse in a highly regulated process, which is associated with fitness benefits during colony establishment. Recognition and chemotropic interactions between isogenic germlings requires oscillation of the mitogen-activated protein kinase (MAPK) signal transduction protein complex (NRC-1, MEK-2, MAK-2, and the scaffold protein HAM-5) to specialized cell fusion structures termed conidial anastomosis tubes. Using a population of 110 wild N. crassa isolates, we investigated germling fusion between genetically unrelated individuals and discovered that chemotropic interactions are regulated by kind discrimination. Distinct communication groups were identified, in which germlings within one communication group interacted at high frequency, while germlings from different communication groups avoided each other. Bulk segregant analysis followed by whole genome resequencing identified three linked genes (doc-1, doc-2, and doc-3), which were associated with communication group phenotype. Alleles at doc-1, doc-2, and doc-3 fell into five haplotypes that showed transspecies polymorphism. Swapping doc-1 and doc-2 alleles from different communication group strains was necessary and sufficient to confer communication group affiliation. During chemotropic interactions, DOC-1 oscillated with MAK-2 to the tips of conidial anastomosis tubes, while DOC-2 was statically localized to the plasma membrane. Our data indicate that doc-1, doc-2, and doc-3 function as "greenbeard" genes, involved in mediating long-distance kind recognition that involves actively searching for one's own type, resulting in cooperation between non-genealogical relatives. Our findings serve as a basis for investigations into the mechanisms associated with attraction, fusion, and kind recognition in other eukaryotic species.

Allorecognition, via TgrB1 and TgrC1, mediates the transition from unicellularity to multicellularity in the social amoeba Dictyostelium discoideum

The social amoeba Dictyostelium discoideum integrates into a multicellular organism when individual starving cells aggregate and form a mound. The cells then integrate into defined tissues and develop into a fruiting body that consists of a stalk and spores. Aggregation is initially orchestrated by waves of extracellular cyclic adenosine monophosphate (cAMP), and previous theory suggested that cAMP and other field-wide diffusible signals mediate tissue integration and terminal differentiation as well. Cooperation between cells depends on an allorecognition system comprising the polymorphic adhesion proteins TgrB1 and TgrC1. Binding between compatible TgrB1 and TgrC1 variants ensures that non-matching cells segregate into distinct aggregates prior to terminal development. Here, we have embedded a small number of cells with incompatible allotypes within fields of developing cells with compatible allotypes. We found that compatibility of the allotype encoded by the tgrB1 and tgrC1 genes is required for tissue integration, as manifested in cell polarization, coordinated movement and differentiation into prestalk and prespore cells. Our results show that the molecules that mediate allorecognition in D. discoideum also control the integration of individual cells into a unified developing organism, and this acts as a gating step for multicellularity.

Identification of Allorecognition Loci in Neurospora crassa by Genomics and Evolutionary Approaches

Understanding the genetic and molecular bases of the ability to distinguish self from nonself (allorecognition) and mechanisms underlying evolution of allorecognition systems is an important endeavor for understanding cases where it becomes dysfunctional, such as in autoimmune disorders. In filamentous fungi, allorecognition can result in vegetative or heterokaryon incompatibility, which is a type of programmed cell death that occurs following fusion of genetically different cells. Allorecognition is genetically controlled by het loci, with coexpression of any combination of incompatible alleles triggering vegetative incompatibility. Herein, we identified, characterized, and inferred the evolutionary history of candidate het loci in the filamentous fungus Neurospora crassa. As characterized het loci encode proteins carrying an HET domain, we annotated HET domain genes in 25 isolates from a natural population along with the N. crassa reference genome using resequencing data. Because allorecognition systems can be affected by frequency-dependent selection favoring rare alleles (i.e., balancing selection), we mined resequencing data for HET domain loci whose alleles displayed elevated levels of variability, excess of intermediate frequency alleles, and deep gene genealogies. From these analyses, 34 HET domain loci were identified as likely to be under balancing selection. Using transformation, incompatibility assays and genetic analyses, we determined that one of these candidates functioned as a het locus (het-e). The het-e locus has three divergent allelic groups that showed signatures of positive selection, intra- and intergroup recombination, and trans-species polymorphism. Our findings represent a compelling case of balancing selection functioning on multiple alleles across multiple loci potentially involved in allorecognition.

Histocompatibility as adaptive response to discriminatory within-organism conflict: a historical model

Multicellular tissue compatibility, or histocompatibility, restricts fusion to close kin. Histocompatibility depends on hypervariable cue genes, which often have more than 100 alleles in a population. To explain the evolution of histocompatibility, I here take a historical approach. I focus on the specific example of marine invertebrate histocompatibility. I use simple game-theoretical models to show that histocompatibility can evolve through five steps. These steps include the evolution of indiscriminate fusion, the evolution of discriminatory within-organism conflict, the evolution of minor histocompatibility, the evolution of major histocompatibility, and the evolution of major histocompatibility cue polymorphism. Allowing for gradual evolution reveals discriminatory within-organism conflict as a selective pressure for histocompatibility and associated cue polymorphism. Existing data from marine invertebrates and other organisms are consistent with this hypothesis.

Sociogenomics of self vs. non-self cooperation during development of Dictyostelium discoideum

Background

Dictyostelium discoideum, a microbial model for social evolution, is known to distinguish self from non-self and show genotype-dependent behavior during chimeric development. Aside from a small number of cell-cell recognition genes, however, little is known about the genetic basis of self/non-self recognition in this species. Based on the key hypothesis that there should be differential expression of genes if D. discoideum cells were interacting with non-clone mates, we performed transcriptomic profiling study in this species during clonal vs. chimeric development. The transcriptomic profiles of D. discoideum cells in clones vs. different chimeras were compared at five different developmental stages using a customized microarray. Effects of chimerism on global transcriptional patterns associated with social interactions were observed.

Results

We find 1,759 genes significantly different between chimera and clone, 1,144 genes associated significant strain differences, and 6,586 genes developmentally regulated over time. Principal component analysis showed a small amount of the transcriptional variance to chimerism-related factors (Chimerism: 0.18%, Chimerism × Timepoint: 0.03%). There are 162 genes specifically regulated under chimeric development, with continuous small differences between chimera vs. clone over development. Almost 60% of chimera-associated differential genes were differentially expressed at the 4 h aggregate stage, which corresponds to the initial transition of D. discoideum from solitary life to a multicellular phase.

Conclusions

A relatively small proportion of over-all variation in gene expression is explained by differences between chimeric and clonal development. The relatively small modifications in gene expression associated with chimerism is compatible with the high level of cooperation observed among different strains of D. discoideum; cells of distinct genetic backgrounds will co-aggregate indiscriminately and co-develop into fruiting bodies. Chimeric development may involve re-programming of the transcriptome through small modifications of the developmental genetic network, which may also indicate that response to social interaction involves many genes with individually small transcriptional effect.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-616) contains supplementary material, which is available to authorized users.

Molecular recognition by a polymorphic cell surface receptor governs cooperative behaviors in bacteria

Cell-cell recognition is a fundamental process that allows cells to coordinate multicellular behaviors. Some microbes, such as myxobacteria, build multicellular fruiting bodies from free-living cells. However, how bacterial cells recognize each other by contact is poorly understood. Here we show that myxobacteria engage in recognition through interactions between TraA cell surface receptors, which leads to the fusion and exchange of outer membrane (OM) components. OM exchange is shown to be selective among 17 environmental isolates, as exchange partners parsed into five major recognition groups. TraA is the determinant of molecular specificity because: (i) exchange partners correlated with sequence conservation within its polymorphic PA14-like domain and (ii) traA allele replacements predictably changed partner specificity. Swapping traA alleles also reprogrammed social interactions among strains, including the regulation of motility and conferred immunity from inter-strain killing. We suggest that TraA helps guide the transition of single cells into a coherent bacterial community, by a proposed mechanism that is analogous to mitochondrial fusion and fission cycling that mixes contents to establish a homogenous population. In evolutionary terms, traA functions as a rare greenbeard gene that recognizes others that bear the same allele to confer beneficial treatment.

Recent advances in the pathogenesis of autoimmune hair loss disease alopecia areata

Alopecia areata is considered to be a cell-mediated autoimmune disease, in which autoreactive cytotoxic T cells recognize melanocyte-associated proteins such as tyrosinase. This review discusses recent advances in the understanding of the pathogenesis of alopecia areata, focusing on immunobiology and hormonal aspects of hair follicles (HFs). The HF is a unique “miniorgan” with its own immune and hormonal microenvironment. The immunosuppressive milieu of the anagen hair bulb modulated by immunosuppressive factors is known as “hair follicle immune privilege.” The collapse of the hair follicle immune privilege leads to autoimmune reactions against hair follicle autoantigens. Alopecia areata is sometimes triggered by viral infections such as influenza that causes excess production of interferons (IFN). IFN-γ is one of the key factors that lead to the collapse of immune privilege. This paper reviews the interactions between the endocrine and immune systems and hair follicles in the pathogenesis of alopecia areata.

Evolution of vertebrate immunity

All multicellular organisms protect themselves against pathogens using sophisticated immune defenses. Functionally interconnected humoral and cellular facilities maintain immune homeostasis in the absence of overt infection and regulate the initiation and termination of immune responses directed against pathogens. Immune responses of invertebrates, such as flies, are innate and usually stereotyped; those of vertebrates, encompassing species as diverse as jawless fish and humans, are additionally adaptive, enabling more rapid and efficient immune reactivity upon repeated encounters with a pathogen. Many of the attributes historically defining innate and adaptive immunity are in fact common to both, blurring their functional distinction and emphasizing shared ancestry and co-evolution. These findings provide indications of the evolutionary forces underlying the origin of somatic diversification of antigen receptors and contribute to our understanding of the complex phenotypes of human immune disorders. Moreover, informed by phylogenetic considerations and inspired by improved knowledge of functional networks, new avenues emerge for innovative therapeutic strategies.

Social discrimination by quantitative assessment of immunogenetic similarity

Genes of the major histocompatibility complex (MHC) that underlie the adaptive immune system may allow vertebrates to recognize their kin. True kin-recognition genes should produce signalling products to which organisms can respond. Allelic variation in the peptide-binding region (PBR) of MHC molecules determines the pool of peptides that can be presented to trigger an immune response. To examine whether these MHC peptides also might underlie assessments of genetic similarity, we tested whether Xenopus laevis tadpoles socially discriminate between pairs of siblings with which they differed in PBR amino acid sequences. We found that tadpoles (four sibships, n = 854) associated preferentially with siblings with which they were more similar in PBR amino acid sequence. Moreover, the strength of their preference for a conspecific was directly proportional to the sequence similarity between them. Discrimination was graded, and correlated more closely with functional sequence differences encoded by MHC class I and class II alleles than with numbers of shared haplotypes. Our results thus suggest that haplotype analyses may fail to reveal fine-scale behavioural responses to divergence in functionally expressed sequences. We conclude that MHC-PBR gene products mediate quantitative social assessment of immunogenetic similarity that may facilitate kin recognition in vertebrates.

Detect thy neighbor: identity recognition at the root level in plants

Some plant species increase root allocation at the expense of reproduction in the presence of non-self and non-kin neighbors, indicating the capacity of neighbor-identity recognition at the root level. Yet in spite of the potential consequences of root identity recognition for the relationship between plant interactions and community structure and functioning, this phenomenon still remains poorly understood. We first critically assess the evidence for the existence of self/non-self and kin recognition at the root level in plants. While root identity recognition most likely exists to some degree, there remain valid points of criticism regarding experiments that have documented this, particularly concerning the effects of pot volume in self/non-self recognition experiments and the roles of size inequality and asymmetric competition in kin recognition studies. Subsequently we review and propose some plausible physiological mechanisms that may underlie these responses. Finally we briefly discuss the relation between under- and aboveground interactions and the potential consequences of root identity recognition for agriculture, and conclude with raising several questions for future studies.

Formyl peptide receptors from immune and vomeronasal system exhibit distinct agonist properties

The formyl peptide receptor (Fpr) family is well known for its contribution to immune defense against pathogens in human and rodent leukocytes. Recently, several structurally related members of these receptors were discovered in sensory neurons of the mouse vomeronasal organ (VNO), key detectors of pheromones and related semiochemicals. Although the biological role of vomeronasal Fprs is not yet clear, the known contribution of other Fprs to host immune defense suggested that they could contribute to vomeronasal pathogen sensing. Precise knowledge about the agonist properties of mouse Fprs is required to determine their function. We expressed all seven mouse and three human Fprs using an in vitro system and tested their activation with 32 selected compounds by conducting high throughput calcium measurements. We found an intriguing functional conservation between human and mouse immune Fprs that is most likely a consequence of closely similar biological constraints. By contrast, our data suggest a neofunctionalization of the vomeronasal Fprs. We show that the vomeronasal receptor mFpr-rs1 can be activated robustly by W-peptide and structural derivatives but not by other typical ligands of immune Fprs. mFpr-rs1 exhibits a stereo-selective preference for peptides containing d-amino acids. The same peptide motifs are contained in pathogenic microorganisms. Thus, the ligand profile of mFpr-rs1 is consistent with a role in vomeronasal pathogen sensing.

MHC I-associated peptides preferentially derive from transcripts bearing miRNA response elements

MHC I-associated peptides (MIPs) play an essential role in normal homeostasis and diverse pathologic conditions. MIPs derive mainly from defective ribosomal products (DRiPs), a subset of nascent proteins that fail to achieve a proper conformation and the physical nature of which remains elusive. In the present study, we used high-throughput proteomic and transcriptomic methods to unravel the structure and biogenesis of MIPs presented by HLA-A and HLA-B molecules on human EBV-infected B lymphocytes from 4 patients. We found that although HLA-different subjects present distinctive MIPs derived from different proteins, these MIPs originate from proteins that are functionally interconnected and implicated in similar biologic pathways. Secondly, the MIP repertoire of human B cells showed no bias toward conserved versus polymorphic genomic sequences, were derived preferentially from abundant transcripts, and conveyed to the cell surface a cell-type-specific signature. Finally, we discovered that MIPs derive preferentially from transcripts bearing miRNA response elements. Furthermore, whereas MIPs of HLA-disparate subjects are coded by different sets of transcripts, these transcripts are regulated by mostly similar miRNAs. Our data support an emerging model in which the generation of MIPs by a transcript depends on its abundance and DRiP rate, which is regulated to a large extent by miRNAs.

A role for microchimerism in obesity and evolution?

Cells exchanged between individuals, such as those passing the placenta from the mother to the child and vice versa, may survive in the fetal or maternal circulation and tissues for decades and result in microchimerism. Microchimeric cells may play a role in tissue repair, but they have also been implicated as inducers of chronic inflammation, leading to autoimmunity or even cancer. Here we propose that microchimerism may play a more fundamental role in health and evolution by setting a limit to genomic variability within populations. This means that microchimerism allows immune recognition of genomic differences between donor and host which may, depending on the level of variability, cause chronic inflammation. Since chronic inflammation has been experimentally linked to metabolic syndrome, we propose that genomic variability could affect the individual's weight. Thus, metabolic syndrome, which is a growing health problem, may not only result from our lifestyle, but in part be caused by global migration and the increasingly diverse origin of the present human population. Moreover, since in nature weight gain is associated with an increased risk of predation, we discuss the possibility that immunological incompatibility normally promotes the continuous development of new species.

Self/non-self discrimination in angiosperm self-incompatibility

Self-incompatibility (SI) in angiosperms prevents inbreeding and promotes outcrossing to generate genetic diversity. In many angiosperms, self/non-self recognition in SI is accomplished by male-specificity and female-specificity determinants (S-determinants), encoded at the S-locus. Recent studies using genetic, molecular biological and biochemical approaches have revealed that angiosperms utilize diverse self/non-self discrimination systems, which can be classified into two fundamentally different systems, self-recognition and non-self recognition systems. The self-recognition system, adopted by Brassicaceae and Papaveraceae, depends on a specific interaction between male and female S-determinants derived from the same S-haplotype. The non-self recognition system, found in Solanaceae, depends on non-self (different S-haplotype)-specific interaction between male and female S-determinants, and the male S-determinant genes are duplicated to recognize diverse non-self female S-determinants.

Roles for receptors, pheromones, G proteins, and mating type genes during sexual reproduction in Neurospora crassa

Here we characterize the relationship between the PRE-2 pheromone receptor and its ligand, CCG-4, and the general requirements for receptors, pheromones, G proteins, and mating type genes during fusion of opposite mating-type cells and sexual sporulation in the multicellular fungus Neurospora crassa. PRE-2 is highly expressed in mat a cells and is localized in male and female reproductive structures. Δpre-2 mat a females do not respond chemotropically to mat A males (conidia) or form mature fruiting bodies (perithecia) or meiotic progeny (ascospores). Strains with swapped identity due to heterologous expression of pre-2 or ccg-4 behave normally in crosses with opposite mating-type strains. Coexpression of pre-2 and ccg-4 in the mat A background leads to self-attraction and development of barren perithecia without ascospores. Further perithecial development is achieved by inactivation of Sad-1, a gene required for meiotic gene silencing. Findings from studies involving forced heterokaryons of opposite mating-type strains show that presence of one receptor and its compatible pheromone is necessary and sufficient for perithecial development and ascospore production. Taken together, the results demonstrate that although receptors and pheromones control sexual identity, the mating-type genes (mat A and mat a) must be in two different nuclei to allow meiosis and sexual sporulation to occur.

The Papaver self-incompatibility pollen S-determinant, PrpS, functions in Arabidopsis thaliana

Many angiosperms use specific interactions between pollen and pistil proteins as "self" recognition and/or rejection mechanisms to prevent self-fertilization. Self-incompatibility (SI) is encoded by a multiallelic S locus, comprising pollen and pistil S-determinants. In Papaver rhoeas, cognate pistil and pollen S-determinants, PrpS, a pollen-expressed transmembrane protein, and PrsS, a pistil-expressed secreted protein, interact to trigger a Ca(2+)-dependent signaling network, resulting in inhibition of pollen tube growth, cytoskeletal alterations, and programmed cell death (PCD) in incompatible pollen. We introduced the PrpS gene into Arabidopsis thaliana, a self-compatible model plant. Exposing transgenic A. thaliana pollen to recombinant Papaver PrsS protein triggered remarkably similar responses to those observed in incompatible Papaver pollen: S-specific inhibition and hallmark features of Papaver SI. Our findings demonstrate that Papaver PrpS is functional in a species with no SI system that diverged ~140 million years ago. This suggests that the Papaver SI system uses cellular targets that are, perhaps, common to all eudicots and that endogenous signaling components can be recruited to elicit a response that most likely never operated in this species. This will be of interest to biologists interested in the evolution of signaling networks in higher plants.

Origin and plasticity of MHC I-associated self peptides

Endogenous peptides presented by MHC I molecules represent the essence of self for CD8 T lymphocytes. These MHC I peptides (MIPs) regulate all key events that occur during the lifetime of CD8 T cells. CD8 T cells are selected on self-MIPs, sustained by self-MIPs, and activated in the presence of self-MIPs. Recently, large-scale mass spectrometry studies have revealed that the self-MIP repertoire is more complex and plastic than previously anticipated. The composition of the self-MIP repertoire varies from one cell type to another and can be perturbed by cell-intrinsic and -extrinsic factors including dysregulation of cellular metabolism and infection. The complexity and plasticity of the self-MIP repertoire represent a major challenge for the maintenance of self tolerance and can have pervasive effects on the global functioning of the immune system.

The MHC I immunopeptidome conveys to the cell surface an integrative view of cellular regulation

Self/non-self discrimination is a fundamental requirement of life. Endogenous peptides presented by major histocompatibility complex class I (MHC I) molecules represent the essence of self for CD8 T lymphocytes. These MHC I peptides (MIPs) are collectively referred to as the immunopeptidome. From a systems-level perspective, very little is known about the origin, composition and plasticity of the immunopeptidome. Here, we show that the immunopeptidome, and therefore the nature of the immune self, is plastic and moulded by cellular metabolic activity. By using a quantitative high-throughput mass spectrometry-based approach, we found that altering cellular metabolism via the inhibition of the mammalian target of rapamycin results in dynamic changes in the cell surface MIPs landscape. Moreover, we provide systems-level evidence that the immunopeptidome projects at the cell surface a representation of biochemical networks and metabolic events regulated at multiple levels inside the cell. Our findings open up new perspectives in systems immunology and predictive biology. Indeed, predicting variations in the immunopeptidome in response to cell-intrinsic and -extrinsic factors could be relevant to the rational design of immunotherapeutic interventions.

Self-recognition in social amoebae is mediated by allelic pairs of tiger genes

Free-living cells of the social amoebae Dictyostelium discoideum can aggregate and develop into multicellular fruiting bodies in which many die altruistically as they become stalk cells that support the surviving spores. Dictyostelium cells exhibit kin discrimination--a potential defense against cheaters, which sporulate without contributing to the stalk. Kin discrimination depends on strain relatedness, and the polymorphic genes tgrB1 and tgrC1 are potential components of that mechanism. Here, we demonstrate a direct role for these genes in kin discrimination. We show that a matching pair of tgrB1 and tgrC1 alleles is necessary and sufficient for attractive self-recognition, which is mediated by differential cell-cell adhesion. We propose that TgrB1 and TgrC1 proteins mediate this adhesion through direct binding. This system is a genetically tractable ancient model of eukaryotic self-recognition.

Allorecognition in a basal chordate consists of independent activating and inhibitory pathways

Histocompatibility in the basal chordate Botryllus schlosseri is controlled by the polymorphisms of a single gene: the fuhc. A polymorphic candidate receptor (fester) appeared to play roles in both initiating the reaction and discriminating between fuhc alleles. Here we report the characterization of a related protein, uncle fester. uncle fester is not polymorphic, and although coexpressed with fester, has different functional properties. Loss-of-function studies demonstrate that uncle fester was required for incompatible reactions but has no role in interactions between compatible individuals. Furthermore, stimulation with monoclonal antibodies could initiate a rejection phenotype on a single colony, and in both assays the severity of the rejection could be manipulated. These findings suggest that allorecognition in Botryllus consists of independent pathways that control compatible and incompatible outcomes that are integrated within the interacting cells, and may provide insight into basal processes conserved in allorecognition responses throughout the metazoa.

Design principles of adaptive immune systems

Both jawless vertebrates, such as lampreys and hagfish, and jawed vertebrates (encompassing species as diverse as sharks and humans) have an adaptive immune system that is based on somatically diversified and clonally expressed antigen receptors. Although the molecular nature of the antigen receptors and the mechanisms of their assembly are different, recent findings suggest that the general design principles underlying the two adaptive immune systems are surprisingly similar. The identification of such commonalities promises to further our understanding of the mammalian immune system and to inspire the development of new strategies for medical interventions targeting the consequences of faulty immune functions.