Evolution Of Immune System Research Articles

Toll-Like Receptors in Adaptive Immunity.

The immune (innate and adaptive) system has evolved to protect the host from any danger present in the surrounding outer environment (microbes and associated MAMPs or PAMPs, xenobiotics, and allergens) and dangers originated within the host called danger or damage-associated molecular patterns (DAMPs) and recognizing and clearing the cells dying due to apoptosis. It also helps to lower the tissue damage during trauma and initiates the healing process. The pattern recognition receptors (PRRs) play a crucial role in recognizing different PAMPs or MAMPs and DAMPs to initiate the pro-inflammatory immune response to clear them. Toll-like receptors (TLRs) are first recognized PRRs and their discovery proved milestone in the field of immunology as it filled the gap between the first recognition of the pathogen by the immune system and the initiation of the appropriate immune response required to clear the infection by innate immune cells (macrophages, neutrophils, dendritic cells or DCs, and mast cells). However, in addition to their expression by innate immune cells and controlling their function, TLRs are also expressed by adaptive immune cells. We have identified 10 TLRs (TLR1-TLR10) in humans and 12 TLRs (TLR1-TLR13) in laboratory mice till date as TLR10 in mice is present only as a defective pseudogene. The present chapter starts with the introduction of innate immunity, timing of TLR evolution, and the evolution of adaptive immune system and its receptors (T cell receptors or TCRs and B cell receptors or BCRs). The next section describes the role of TLRs in the innate immune function and signaling involved in the generation of inflammation. The subsequent sections describe the expression and function of different TLRs in murine and human adaptive immune cells (B cells and different types of T cells, including CD4+T cells, CD8+T cells, CD4+CD25+Tregs, and CD8+CD25+Tregs, etc.). The modulation of TLRs expressed on T and B cells has a great potential to develop different vaccine candidates, adjuvants, immunotherapies to target various microbial infections, including current COVID-19 pandemic, cancers, and autoimmune and autoinflammatory diseases.

The Immune System and Responses to Cancer: Coordinated Evolution

This review explores the incessant evolutionary interaction and co-development between immune system evolution and somatic evolution, to put it into context with the short, over 60-year, detailed human study of this extraordinary protective system. Over millions of years, the evolutionary development of the immune system in most species has been continuously shaped by environmental interactions between microbes, and aberrant somatic cells, including malignant cells. Not only has evolution occurred in somatic cells to adapt to environmental pressures for survival purposes, but the immune system and its function has been successively shaped by those same evolving somatic cells and microorganisms through continuous adaptive symbiotic processes of progressive simultaneous immunological and somatic change to provide what we observe today. Indeed, the immune system as an environmental influence has also shaped somatic and microbial evolution. Although the immune system is tuned to primarily controlling microbiological challenges for combatting infection, it can also remove damaged and aberrant cells, including cancer cells to induce long-term cures. Our knowledge of how this occurs is just emerging. Here we consider the connections between immunity, infection and cancer, by searching back in time hundreds of millions of years to when multi-cellular organisms first began. We are gradually appreciating that the immune system has evolved into a truly brilliant and efficient protective mechanism, the importance of which we are just beginning to now comprehend. Understanding these aspects will likely lead to more effective cancer and other therapies.

S6K1/S6 axis-regulated lymphocyte activation is important for adaptive immune response of Nile tilapia

Ribosomal protein S6 kinase beta-1 (S6K1) is a serine/threonine kinase downstream of the mechanistic target of rapamycin (mTOR) pathway, and plays crucial roles in immune regulation. Although remarkable progress has been achieved with a mouse model, how S6K1 regulates adaptive immunity is largely unknown in early vertebrates. In this study, we identified an S6K1 from Nile tilapia Oreochromis niloticus (OnS6K1), and further investigated its potential regulatory role on the adaptive immunity of this fish species. Both sequence and structure of OnS6K1 were highly conserved with its homologs from other vertebrates and invertebrates. OnS6K1 was widely expressed in immune tissues, and with a relative higher expression level in the liver, spleen and head kidney. At the adaptive immune stage of Nile tilapia that infected with Aeromonas hydrophila, mRNA expression of OnS6K1 and its downstream effector S6 was significantly up-regulated in spleen lymphocytes. Meanwhile, their phosphorylation level was also enhanced during this process, suggesting that S6K1/S6 axis participated in the primary response of anti-bacterial adaptive immunity in Nile tilapia. Furthermore, after spleen lymphocytes were activated by the T cell-specific mitogen PHA or lymphocytes agonist PMA in vitro, mRNA and phosphorylation levels of S6K1 were elevated, and phosphorylation of S6 was also enhanced. Once S6K1 activity was blocked by a specific inhibitor, both mRNA and phosphorylation levels of S6 were severely impaired. More importantly, blockade of S6K1/S6 axis reduced the expression of T cell activation marker IFN-γ and CD122 in PHA-activated spleen lymphocytes, indicating the essential role of S6K1/S6 axis in regulating T cell activation of Nile tilapia. Together, our study suggests that S6K1 and its effector S6 regulate lymphocyte activation of Nile tilapia, and in turn promote lymphocyte-mediated adaptive immunity. This study enriched the mechanism of adaptive immune response in teleost and provided useful clues to understand the evolution of adaptive immune system.

Meningeal γδ T cells regulate anxiety-like behavior via IL-17a signaling in neurons.

Interleukin-17a (IL-17a) has been highly conserved during evolution of the vertebrate immune system and widely studied in contexts of infection and autoimmunity. Studies suggest that IL-17a promotes behavioral changes in experimental models of autism and aggregation behavior in worms. Here, through a cellular and molecular characterization of meningeal γδ17 T cells, we defined the nearest central nervous system associated source of IL-17a under homeostasis. Meningeal γδ T cells express high levels of the chemokine receptor CXCR6 and seed meninges shortly after birth. Physiological release of IL-17a by these cells was correlated with anxiety-like behavior in mice and was partially dependent on T cell receptor engagement and commensal-derived signals. IL-17a receptor was expressed in cortical glutamatergic neurons under steady state and its genetic deletion decreased anxiety-like behavior in mice. Our findings suggest that IL-17a production by meningeal γδ17 T cells represents an evolutionary bridge between this conserved anti-pathogen molecule and survival behavioral traits in vertebrates.

Toward the Discovery of Host-Defense Peptides in Plants.

Defense peptides protect multicellular eukaryotes from infections. In biomedical sciences, a dominant conceptual framework refers to defense peptides as host-defense peptides (HDPs), which are bifunctional peptides with both direct antimicrobial and immunomodulatory activities. No HDP has been reported in plants so far, and the very concept of HDP has not been captured yet by the plant science community. Plant science thus lacks the conceptual framework that would coordinate research efforts aimed at discovering plant HDPs. In this perspective article, I used bibliometric and literature survey approaches to raise awareness about the HDP concept among plant scientists, and to encourage research efforts aimed at discovering plant HDPs. Such discovery would enrich our comprehension of the function and evolution of the plant immune system, and provide us with novel molecular tools to develop innovative strategies to control crop diseases.

Evolution of animal immunity in the light of beneficial symbioses.

Immune system processes serve as the backbone of animal defences against pathogens and thus have evolved under strong selection and coevolutionary dynamics. Most microorganisms that animals encounter, however, are not harmful, and many are actually beneficial. Selection should act on hosts to maintain these associations while preventing exploitation of within-host resources. Here, we consider how several key aspects of beneficial symbiotic associations may shape host immune system evolution. When host immunity is used to regulate symbiont populations, there should be selection to evolve and maintain targeted immune responses that recognize symbionts and suppress but not eliminate symbiont populations. Associating with protective symbionts could relax selection on the maintenance of redundant host-derived immune responses. Alternatively, symbionts could facilitate the evolution of host immune responses if symbiont-conferred protection allows for persistence of host populations that can then adapt. The trajectory of immune system evolution will likely differ based on the type of immunity involved, the symbiont transmission mode and the costs and benefits of immune system function. Overall, the expected influence of beneficial symbiosis on immunity evolution depends on how the host immune system interacts with symbionts, with some interactions leading to constraints while others possibly relax selection on immune system maintenance. This article is part of the theme issue 'The role of the microbiome in host evolution'.

Superorganism Immunity: A Major Transition in Immune System Evolution

Social insect colonies can express adaptive, organism-like design. In some cases, colonies so resemble a unique, cohesive and integrated “individual” that they are termed superorganisms. The major evolutionary transitions framework explains, via inclusive fitness theory, how new levels of biological individuality, including genes into genomes within cells, cells into multicellular organisms and organisms into superorganisms can emerge. Importantly, it highlights how at each major transition similar challenges arose and why seemingly convergent solutions evolved. One challenge faced at each transition is exploitation, caused internally by social cheaters, and externally by parasites and pathogens. To overcome the problem of exploitation transitions in biological individuality required novel immune systems to maintain the integrity of newly emerged individuals. Multicellular organisms evolved an immune system while social insect colonies evolved a social immune system. In this review, we take a major transitions perspective of immunity to highlight the interdependency between the evolution of immune systems and the emergence of biological individuality. We build on the notion that superorganisms have evolved an immune system to promote the fitness of the colony. We draw parallels between the evolution of the metazoan immune system and the social immune system, and their expression as cognitive networks. Moreover, we discuss how research on other group-living species, such as family-based cooperative breeders, can inform our understanding of how social immune systems evolve. We conclude that “superorganism immunity” is an adaptive suite of organismal traits that evolves to maximise the fitness of advanced social insect colonies, fulfilling the same function as the immune system of Metazoa.

Immune priming against bacteria in spiders and scorpions?

Empirical evidence of immune priming in arthropods keeps growing, both at the within- and trans-generational level. The evidence comes mostly from work on insects and it remains unclear for some other arthropods whether exposure to a non-lethal dose of a pathogen provides protection during a second exposure with a lethal dose. A poorly investigated group are arachnids, with regard to the benefits of immune priming measured as improved survival. Here, we investigated immune priming in two arachnids: the wolf spider Lycosa cerrofloresiana and the scorpion Centruroides granosus. We injected a third of the individuals with lipopolysaccharides of Escherichia coli (LPS, an immune elicitor), another third were injected with the control solution (PBS) and the other third were kept naive. Four days after the first inoculations, we challenged half of the individuals of each group with an injection of a high dose of E. coli and the other half was treated with the control solution. For scorpions, individuals that were initially injected with PBS or LPS did not differ in their survival rates against the bacterial challenge. Individuals injected with LPS showed higher survival than that of naive individuals as evidence of immune priming. Individuals injected with PBS tended to show higher survival rates than naive individuals, but the difference was not significant—perhaps suggesting a general immune upregulation caused by the wounding done by the needle. For spiders, we did not observe evidence of priming, the bacterial challenge reduced the survival of naive, PBS and LPS individuals at similar rates. Moreover; for scorpions, we performed antibacterial assays of hemolymph samples from the three priming treatments (LPS, PBS and naive) and found that the three treatments reduced bacterial growth but without differences among treatments. As non-model organisms, with some unique differences in their immunological mechanisms as compared to the most studied arthropods (insects), arachnids provide an unexplored field to elucidate the evolution of immune systems.

The Potential Role of Endogenous Viral Elements in the Evolution of Bats as Reservoirs for Zoonotic Viruses.

Despite a small genome size, bats have comparable diversity of retroviral and non-retroviral endogenous sequences to other mammals. These include Class I and Class II retroviral sequences, foamy viruses, and deltaretroviruses, as well as filovirus, bornavirus, and parvovirus endogenous viral elements. Some of these endogenous viruses are sufficiently preserved in bat genomes to be expressed, with potential effects for host biology. It is clear that the bat immune system differs when compared with other mammals, yet the role that virus-derived endogenous elements may have played in the evolution of bat immunity is poorly understood. In this review, we discuss some of the bat-specific immune mechanisms that may have resulted in a virus-tolerant phenotype and link these to the long-standing virus-host coevolution that may have allowed a large diversity of endogenous retroviruses and other endogenous viral elements to colonize bat genomes. We also consider the possible effects of endogenization in the evolution of the bat immune system.

Extreme genomic volatility characterizes the evolution of the immunoglobulin heavy chain locus in cyprinodontiform fishes.

The evolution of the adaptive immune system has provided vertebrates with a uniquely sophisticated immune toolkit, enabling them to mount precise immune responses against a staggeringly diverse range of antigens. Like other vertebrates, teleost fishes possess a complex and functional adaptive immune system; however, our knowledge of the complex antigen-receptor genes underlying its functionality has been restricted to a small number of experimental and agricultural species, preventing systematic investigation into how these crucial gene loci evolve. Here, we analyse the genomic structure of the immunoglobulin heavy chain (IGH) gene loci in the cyprinodontiforms, a diverse and important group of teleosts present in many different habitats across the world. We reconstruct the complete IGH loci of the turquoise killifish (Nothobranchius furzeri) and the southern platyfish (Xiphophorus maculatus) and analyse their in vivo gene expression, revealing the presence of species-specific splice isoforms of transmembrane IGHM. We further characterize the IGH constant regions of 10 additional cyprinodontiform species, including guppy, Amazon molly, mummichog and mangrove killifish. Phylogenetic analysis of these constant regions suggests multiple independent rounds of duplication and deletion of the teleost-specific antibody class IGHZ in the cyprinodontiform lineage, demonstrating the extreme volatility of IGH evolution. Focusing on the cyprinodontiforms as a model taxon for comparative evolutionary immunology, this work provides novel genomic resources for studying adaptive immunity and sheds light on the evolutionary history of the adaptive immune system.

Fear of Covid-19: Insights from Evolutionary Behavioral Science.

Fear of infectious disease is substantially different from that evoked by other medical conditions. Such a difference depends on psychological and behavioral adaptations shaped by natural selection throughout the evolutionary history of Homo sapiens. Selective pressures have favored the evolution of a behavioral immune system that is separate from, and complementary to, the physiological immune system. The two systems interact in a complex way. The psychological mechanisms (i.e. disgust and fear) involved in the behavioral immune system impact also on aspects that pertain to social psychology (i.e. xenophobia, conformism, and authoritarianism). Acknowledging the existence of psychological and behavioral adaptations to avoid infection has important implications for public health programs, including the necessity of fighting stigma and the dubious utility of trauma debriefing for healthcare workers facing the COVID-19 emergency.

The evolutionarily conserved MAPK/Erk signaling promotes ancestral T-cell immunity in fish via c-Myc–mediated glycolysis

The mitogen-activated protein kinase (MAPK) cascade is an ancient and evolutionarily conserved signaling pathway involved in numerous physiological processes. Despite great advances in understanding MAPK-mediated regulation of adaptive immune responses in mammals, its contribution to T-cell immunity in early vertebrates remains unclear. Herein, we used Nile tilapia (Oreochromis niloticus) to investigate the regulatory roles of MAPK/extracellular signal-regulated kinase (Erk) signaling in ancestral T-cell immunity of jawed fish. We found that Nile tilapia possesses an evolutionarily conserved MAPK/Erk axis that is activated through a classical three-tier kinase cascade, involving sequential phosphorylation of RAF proto-oncogene serine/threonine-protein kinase (Raf), MAPK/Erk kinase 1/2 (Mek1/2), and Erk1/2. In Nile tilapia, MAPK/Erk signaling participates in adaptive immune responses during bacterial infection. Upon T-cell activation, the MAPK/Erk axis is robustly activated, and MAPK/Erk blockade by specific inhibitors severely impairs T-cell activation. Furthermore, signals from MAPK/Erk were indispensable for primordial T cells to proliferate and exert their effector functions. Mechanistically, activation of the MAPK/Erk axis promoted glycolysis via induction of the transcriptional regulator proto-oncogene c-Myc (c-Myc), to ensure the proper activation and proliferation of fish T cells. Our results reveal the regulatory mechanisms of MAPK/Erk signaling in T-cell immunity in fish and highlight a close link between immune signals and metabolic programs. We propose that regulation of T-cell immunity by MAPK/Erk is a basic and sophisticated strategy that evolved before the emergence of the tetrapod lineage. These findings shed light on the evolution of the adaptive immune system.

Identification, recombinant expression and immunological study of Lja-SHP2 in Lampetra japonica.

The protein tyrosine phosphatase SHP2 of higher vertebrates, encoded by ptpn11 gene, catalyzes the dephosphorylation of tyrosine phosphorylation site, and plays regulatory roles in various signaling pathways by cooperating with other protein tyrosine kinase. Previous studies have shown that SHP2 plays an important role in the activation and signal transduction of T and B cells in higher vertebrates. To study the role of a SHP2 homologous molecule of lampreys (Lja-SHP2) in immune response, we cloned and expressed the open reading frame sequence of Lja-SHP2 gene in prokaryotic expression vector pET-32a. The recombinant protein was successfully expressed in E. coli and the rabbit-derived polyclonal antibody was prepared. Lampetra japonica were immunized with mixed bacteria, and the mRNA and protein of Lja-SHP2 in immune-related cells and tissues were detected by real-time quantitative PCR and Western blotting after immunization. The Lja-SHP2 mRNA and protein were not significantly affected in leukocytes and supraneural myeloid bodies, but up-regulated significantly in gill tissues (P<0.05) after challenged by mixed bacteria, which indicated that Lja-SHP2 mainly participates in the immune response of gill tissues after mixed bacteria stimulation. To further investigate whether Lja-SHP2 level was affected in three lymphocyte subsets, the B-cell mitogen lipopolysaccharide (LPS) and T-cell mitogen phytohaemagglutinin (PHA) were employed to boost the immune response in L. japonica. LPS immune stimulation increased Lja-SHP2 in leucocytes significantly compared with the control group, and but had a marginal effect on Lja-SHP2 expression in gills and supraneural myeloid bodies. PHA immune stimulation could up-regulate Lja-SHP2 level in leukocytes, gill tissues and supraneural myeloid bodies. The change of Lja-SHP2 was especially dramatical in leukocytes, which was about 2.5 times higher than that in the control group, suggesting that Lja-SHP2 is involved in the lamprey immune response mediated by PHA. Consistent with the previous finding that PHA could induce the activation of VLRA+ lymphocytes, our results showed that Lja-SHP2 might be included in the immune response of VLRA+ lymphocytes mediated by PHA in gills. This research will benefit exploring the functions of Lja-SHP2 in the immune response of lamprey and will provide clues for understanding the phylogenesis of SHP2 molecular family, and its roles in the early occurrence and evolution of adaptive immune system in higher vertebrates.

Ca2+-Calcineurin Axis-Controlled NFAT Nuclear Translocation Is Crucial for Optimal T Cell Immunity in an Early Vertebrate.

Calcium ion (Ca2+) is a widespread and primitive second messenger that regulates physiological cell functions in almost all life beings. Ca2+ influx-induced NFAT activation is essential for T cell function and adaptive immunity. However, whether and how Ca2+ signaling modulates T cell immunity in early vertebrates, especially in nontetrapods, remains largely unknown. To address these questions, a Nile tilapia (Oreochromis niloticus) model was employed to investigate the regulation of ancestral T cell immunity by Ca2+-NFAT signaling in jawed fish. In Nile tilapia, an evolutionarily conserved Ca2+-NFAT signaling pathway is involved in the primary adaptive immune response during Streptococcus agalactiae infection. Meanwhile, T cell signals trigger several events along the Ca2+-NFAT axis in this early vertebrate, including Ca2+ influx, calcineurin activation, and NFAT nuclear import. More critically, suppression of Ca2+-NFAT signaling by the calcineurin inhibitor cyclosporine A impairs primordial T cell activation, clonal expansion, and infection clearance. Mechanistically, Nile tilapia NFAT interacts with several other transcription factors for potent gene expression, and T cells in this nontetrapod employ Cabin1 and DYRK1A to regulate NFAT nuclear import and export, respectively. To the best of our knowledge, this study is the first to demonstrate the regulatory mechanism of Ca2+-NFAT signaling on T cell immunity in a nontetrapod species. We suggest that modulation of T cell immunity by Ca2+-NFAT signaling is a primitive strategy that already existed prior to the divergence of bony fish from the tetrapod lineage. The findings of this study provide valuable perspectives for understanding the evolution of adaptive immune system.

Raptor/mTORC1 Acts as a Modulatory Center to Regulate Anti-bacterial Immune Response in Rockfish.

The mammalian target of rapamycin (mTOR) is an evolutionarily highly conserved atypical serine/threonine protein kinase, which regulates cell growth, proliferation, apoptosis, autophagy, and metabolism. As a regulatory protein, Raptor is awfully important for the stability and function of mTOR complex 1 (mTORC1). However, the studies about how Raptor/mTORC1 participates in and regulates immune response in lower vertebrates are still limited. In this study, we investigated the regulation of immune response by the Raptor/mTORC1 signaling pathway in rockfish Sebastes schlegelii. Sebastes schlegelii Raptor (Ss-Raptor) is a highly conserved protein during the evolution, in both primary and tertiary structure. Ss-Raptor mRNA was widely distributed in various tissues of rockfish and has a relative higher expression in spleen and blood. After infected by Micrococcus luteus or Listonella anguillarum, mRNA expression of Ss-Raptor rapidly increased within 48 h. Once Raptor/mTORC1 signaling was blocked by rapamycin, expression of the pro-inflammatory cytokines IL-1β and IL-8 was severely impaired, suggesting potential regulatory role of Raptor/mTORC1 signaling in the innate immune response of rockfish. In addition, Raptor/mTORC1 pathway participated in lymphocyte activation of rockfish through promoting 4EBP1 and S6 phosphorylation. Inhibition of Raptor/mTORC1 signaling crippled the lymphocyte expansion during primary adaptive immune response, manifesting by the decrease of lymphoid organ weight and lymphocyte numbers. More importantly, inhibition of Raptor/mTORC1 signaling impaired the lymphocyte mediated cytotoxic response, and made the fish more vulnerable to the bacterial infection. Together, our results suggested that Raptor and its tightly regulated mTORC1 signaling acts as modulatory center to regulate both innate and lymphocyte-mediated adaptive immune response during bacterial infection. This research has shed new light on regulatory mechanism of teleost immune response, and provide helpful evidences to understand the evolution of immune system.

Game of neutrophils: modeling the balance between apoptosis and necrosis

BackgroundNeutrophils are one of the key players in the human innate immune system (HIIS). In the event of an insult where the body is exposed to inflammation triggering moieties (ITMs), neutrophils are mobilized towards the site of insult and antagonize the inflammation. If the inflammation is cleared, neutrophils go into a programmed death called apoptosis. However, if the insult is intense or persistent, neutrophils take on a violent death pathway called necrosis, which involves the rupture of their cytoplasmic content into the surrounding tissue that causes local tissue damage, thus further aggravating inflammation. This seemingly paradoxical phenomenon fuels the inflammatory process by triggering the recruitment of additional neutrophils to the site of inflammation, aimed to contribute to the complete neutralization of severe inflammation. This delicate balance between the cost and benefit of the neutrophils’ choice of death pathway has been optimized during the evolution of the innate immune system. The goal of our work is to understand how the tradeoff between the cost and benefit of the different death pathways of neutrophils, in response to various levels of insults, has been optimized over evolutionary time by using the concepts of evolutionary game theory.ResultsWe show that by using evolutionary game theory, we are able to formulate a game that predicts the percentage of necrosis and apoptosis when exposed to various levels of insults.ConclusionBy adopting an evolutionary perspective, we identify the driving mechanisms leading to the delicate balance between apoptosis and necrosis in neutrophils’ cell death in response to different insults. Using our simple model, we verify that indeed, the global cost of remaining ITMs is the driving mechanism that reproduces the percentage of necrosis and apoptosis observed in data and neutrophils need sufficient information of the overall inflammation to be able to pick a death pathway that presumably increases the survival of the organism.

Natural variation in the contribution of microbial density to inducible immune dynamics.

Immune responses evolve to balance the benefits of microbial killing against the costs of autoimmunity and energetic resource use. Models that explore the evolution of optimal immune responses generally include a term for constitutive immunity, or the level of immunological investment prior to microbial exposure, and for inducible immunity, or investment in immune function after microbial challenge. However, studies rarely consider the functional form of inducible immune responses with respect to microbial density, despite the theoretical dependence of immune system evolution on microbe- versus immune-mediated damage to the host. In this study, we analyse antimicrobial peptide (AMP) gene expression from seven wild-caught flour beetle populations (Tribolium spp.) during acute infection with the virulent bacteria Bacillus thuringiensis (Bt) and Photorhabdus luminescens (P.lum) to demonstrate that inducible immune responses mediated by the humoral IMD pathway exhibit natural variation in both microbe density-dependent and independent temporal dynamics. Beetle populations that exhibited greater AMP expression sensitivity to Bt density were also more likely to die from infection, while populations that exhibited higher microbe density-independent AMP expression were more likely to survive P.luminescens infection. Reduction in pathway signalling efficiency through RNAi-mediated knockdown of the imd gene reduced the magnitude of both microbe-independent and dependent responses and reduced host resistance to Bt growth, but had no net effect on host survival. This study provides a framework for understanding natural variation in the flexibility of investment in inducible immune responses and should inform theory on the contribution of nonequilibrium host-microbe dynamics to immune system evolution.

Distinct evolution of toll-like receptor signaling pathway genes in cetaceans.

The relatively rapid spread and diversity of marine pathogens posed an initial and ongoing challenge for cetaceans (whales, dolphins, and porpoises), descendants of terrestrial mammals that transitioned from land to sea approximately 56 million years ago. Toll-like receptors (TLRs) play important roles in regulating immunity against pathogen infections by detecting specific molecular patterns and activating a wide range of downstream signaling pathways. The ever-increasing catalogue of mammalian genomes offers unprecedented opportunities to reveal genetic changes associated with evolutionary and ecological processes. This study aimed to explore the molecular evolution of TLR signaling pathway genes in cetaceans. Genes involved in the TLR signaling pathway were retrieved by BLAST searches using human coding sequences as queries. We tested each gene for positive selection along the cetacean branches using PAML and Hyphy. Physicochemical property changes of amino acids at all positively selected residues were assessed by TreeSAAP and visualized with WebLogo. Bovine and dolphin TLR4 was assessed using human embryonic kidney cell line HEK293, which lacks TLR4 and its co-receptor MD-2. We demonstrate that eight TLR signaling pathway genes are under positive selection in cetaceans. These include key genes in the response to Gram-negative bacteria: TLR4, CD14, and LY96 (MD-2). Moreover, 41 out of 65 positively selected sites were inferred to harbor substitution that dramatically changes the physicochemical properties of amino acids, with most of them situated in or adjacent to functional regions. We also found strong evidence that positive selection occurred in the lineage of the Yangtze finless porpoise, likely reflecting relatively recent adaptions to a freshwater milieu. Species-specific differences in TLR4 response were observed between cetacean and terrestrial species. Cetacean TLR4 was significantly less responsive to lipopolysaccharides from a terrestrial E. coli strain, possibly a reflection of the arms race of host-pathogen co-evolution faced by cetaceans in an aquatic environment. This study provides further impetus for studies on the evolution and function of the cetacean immune system.

Ancestral T Cells in Fish Require mTORC1-Coupled Immune Signals and Metabolic Programming for Proper Activation and Function.

T cells suddenly appeared in jawed fish ∼450 million years ago. Biological studies of fish T cells may provide helpful evidence to understand evolution of adaptive immune systems. To this end, using a Nile tilapia (Oreochromis niloticus) model, we revealed the regulatory mechanism of adaptive immunity mediated by ancestral T cells in jawed fish. Nile tilapia T cells as well as a tightly regulated mammalian/mechanistic target of rapamycin complex 1 (mTORC1) pathway participate in the cellular adaptive immune response during Streptococcus agalactiae infection. Blockade of mTORC1 signaling by rapamycin impairs T cell activation and Ag-induced proliferation in this early vertebrate. More critically, we show that signals from mTORC1 are indispensable for primordial effector T cells to eliminate infection by promoting the expression of proinflammatory cytokines, cytotoxic-related molecules, and proapoptotic genes. Mechanistically, teleost mTORC1 directs effector T cell function by coordinating multiple metabolic programs, including glycolysis, glutaminolysis, and lipogenesis through activating key transcription factors c-Myc, HIF-1α, and sterol regulatory element-binding proteins, and thus links immune signals to metabolic reprogramming in jawed fish. To our knowledge, these results represent the first description of the regulatory mechanism for T cell-mediated adaptive immunity in a fish species. From an evolutionary viewpoint, our study suggests that primordial T cells are armed with sophisticated regulatory strategies like those in modern T cells prior to the divergence of bony fish from the tetrapod lineage. Therefore, our findings fill in an important gap regarding evolution of the adaptive immune system.

Still Enigmatic: Innate Immunity in the Ctenophore Mnemiopsis leidyi.

Innate immunity is an ancient physiological response critical for protecting metazoans from invading pathogens. It is the primary pathogen defense mechanism among invertebrates. While innate immunity has been studied extensively in diverse invertebrate taxa, including mollusks, crustaceans, and cnidarians, this system has not been well characterized in ctenophores. The ctenophores comprise an exclusively marine, non-bilaterian lineage that diverged early during metazoan diversification. The phylogenetic position of ctenophore lineage suggests that characterization of the ctenophore innate immune system will reveal important features associated with the early evolution of the metazoan innate immune system. Here, we review current understanding of the ctenophore immune repertoire and identify innate immunity genes recovered from three ctenophore species. We also isolate and characterize Mnemiopsis leidyi cells that display macrophage-like behavior when challenged with bacteria. Our results indicate that ctenophores possess cells capable of phagocytosing microbes and that two distantly related ctenophores, M. leidyi and Hormiphora californiensis, possess many candidate innate immunity proteins.