Innate immune responses to TLR2 and TLR4 agonists differ between baboons, chimpanzees and humans

The buried gems of disease tolerance in animals: Evolutionary and interspecies comparative approaches: Interspecies comparative approaches are valuable tools for exploring potential new mechanisms of disease tolerance in animals: Interspecies comparative approaches are valuable tools for exploring potential new mechanisms of disease tolerance in animals

Host defense mechanisms are categorized into different strategies, namely, avoidance, resistance and tolerance. Resistance encompasses mechanisms that directly kill the pathogen while tolerance is mainly concerned with alleviating the harsh consequences of the infection regardless of the pathogen burden. Resistance is well-known strategy in immunology while tolerance is relatively new. Studies addressed tolerance mainly using mouse models revealing a wide range of interesting tolerance mechanisms. Herein, we aim to emphasize on the interspecies comparative approaches to explore potential new mechanisms of disease tolerance. We will discuss mechanisms of tolerance with focus on those that were revealed using comparative study designs of mammals followed by summarizing the reasons for adopting comparative approaches on disease tolerance studies. Disease tolerance is a relatively new concept in immunology, we believe combining comparative studies with model organism study designs will enhance our understanding to tolerance and unveil new mechanisms of tolerance.

Primate innate immune responses to bacterial and viral pathogens reveals an evolutionary trade-off between strength and specificity

Humans and our close evolutionary relatives respond differently to a large number of infections. Such differences are thought to result, at least in part, from interspecies differences in immune function. Here, we report on the whole-genome expression of blood leukocytes from four primate species responding to bacterial and viral stimulation. We show that apes mount a markedly stronger early transcriptional response to both viral and bacterial stimulation when compared to African and Asian monkeys. In addition, our findings suggest that apes activate a broader array of defense molecules that may be beneficial for early pathogen killing at the potential cost of increased energy expenditure and tissue damage. Our results provide insight into the evolution of immune responses in primates.

Despite their close genetic relatedness, apes and African and Asian monkeys (AAMs) differ in their susceptibility to severe bacterial and viral infections that are important causes of human disease. Such differences between humans and other primates are thought to be a result, at least in part, of interspecies differences in immune response to infection. However, because of the lack of comparative functional data across species, it remains unclear in what ways the immune systems of humans and other primates differ. Here, we report the whole-genome transcriptomic responses of ape species (human and chimpanzee) and AAMs (rhesus macaque and baboon) to bacterial and viral stimulation. We find stark differences in the responsiveness of these groups, with apes mounting a markedly stronger early transcriptional response to both viral and bacterial stimulation, altering the transcription of ∼40% more genes than AAMs. Additionally, we find that genes involved in the regulation of inflammatory and interferon responses show the most divergent early transcriptional responses across primates and that this divergence is attenuated over time. Finally, we find that relative to AAMs, apes engage a much less specific immune response to different classes of pathogens during the early hours of infection, up-regulating genes typical of anti-viral and anti-bacterial responses regardless of the nature of the stimulus. Overall, these findings suggest apes exhibit increased sensitivity to bacterial and viral immune stimulation, activating a broader array of defense molecules that may be beneficial for early pathogen killing at the potential cost of increased energy expenditure and tissue damage.

Sepsis and the evolution of human increased sensitivity to lipopolysaccharide

Among mammals, humans are exquisitely sensitive to lipopolysaccharide (LPS), an environmentally pervasive bacterial cell membrane component. Very small doses of LPS trigger powerful immune responses in humans and can even initiate symptoms of sepsis. Close evolutionary relatives such as African and Asian monkeys require doses that are an order of magnitude higher to do the same. Why humans have evolved such an energetically expensive antimicrobial strategy is a question that biological anthropologists are positioned to help address. Here we compare LPS sensitivity in primate/mammalian models and propose that human high sensitivity to LPS is adaptive, linked to multiple immune tactics against pathogens, and part of multi-faceted anti-microbial strategy that strongly overlaps with that of other mammals. We support a notion that LPS sensitivity in humans has been driven by microorganisms that constitutively live on us, and has been informed by human behavioral changes over our species' evolution (e.g., meat eating, agricultural practices, and smoking).

TLR4 Signaling by Heme and the Role of Heme-Binding Blood Proteins

Toll-like receptors (TLRs), also known as pattern recognition receptors, respond to exogenous pathogens and to intrinsic danger signals released from damaged cells and tissues. The tetrapyrrole heme has been suggested to be an agonist for TLR4, the receptor for the pro-inflammatory bacterial component lipopolysaccharide (LPS), synonymous with endotoxin. Heme is a double-edged sword with contradictory functions. On the one hand, it has vital cellular functions as the prosthetic group of hemoproteins including hemoglobin, myoglobin, and cytochromes. On the other hand, if released from destabilized hemoproteins, non-protein bound or “free” heme can have pro-oxidant and pro-inflammatory effects, the mechanisms of which are not fully understood. In this review, the complex interactions between heme and TLR4 are discussed with a particular focus on the role of heme-binding serum proteins in handling extracellular heme and its impact on TLR4 signaling. Moreover, the role of heme as a direct and indirect trigger of TLR4 activation and species-specific differences in the regulation of heme-dependent TLR4 signaling are highlighted.

New Approaches to Treat Sepsis: Animal Models «Do Not Work» (Review)

Like many other pathological infectious processes, sepsis is mainly studied in vivo using mice models. Over the past 30 years, such studies have led to significant achievements in understanding of the sepsis pathophysiology. However, unfortunately, none of them led to any «discoveries» in the treatment of patients. In this review, we question the relevance of the experimental models applied, list some aspects rarely taken into account and discuss ways to resolve the deadlock.

The text is a translation of the article: Cavail-lon J. M. New methods of treating sepsis: failure of animal models, Bull. Assoc. Anc. El. Inst. Pastor, 2017, 59,230, 58—60. Translation from French by «Akademperevod», Moscow, Russia.

Biochemical, Cellular, Physiological, and Pathological Consequences of Human Loss of N-Glycolylneuraminic Acid

About 2-3 million years ago, Alu-mediated deletion of a critical exon in the CMAH gene became fixed in the hominin lineage ancestral to humans, possibly through a stepwise process of selection by pathogen targeting of the CMAH product (the sialic acid Neu5Gc), followed by reproductive isolation through female anti-Neu5Gc antibodies. Loss of CMAH has occurred independently in some other lineages, but is functionally intact in Old World primates, including our closest relatives, the chimpanzee. Although the biophysical and biochemical ramifications of losing tens of millions of Neu5Gc hydroxy groups at most cell surfaces remains poorly understood, we do know that there are multiscale effects functionally relevant to both sides of the host-pathogen interface. Hominin CMAH loss might also contribute to understanding human evolution, at the time when our ancestors were starting to use stone tools, increasing their consumption of meat, and possibly hunting. Comparisons with chimpanzees within ethical and practical limitations have revealed some consequences of human CMAH loss, but more has been learned by using a mouse model with a human-like Cmah inactivation. For example, such mice can develop antibodies against Neu5Gc that could affect inflammatory processes like cancer progression in the face of Neu5Gc metabolic incorporation from red meats, display a hyper-reactive immune system, a human-like tendency for delayed wound healing, late-onset hearing loss, insulin resistance, susceptibility to muscular dystrophy pathologies, and increased sensitivity to multiple human-adapted pathogens involving sialic acids. Further studies in such mice could provide a model for other human-specific processes and pathologies involving sialic acid biology that have yet to be explored.

Loss of CMAH during Human Evolution Primed the Monocyte-Macrophage Lineage toward a More Inflammatory and Phagocytic State

Humans and chimpanzees are more sensitive to endotoxin than are mice or monkeys, but any underlying differences in inflammatory physiology have not been fully described or understood. We studied innate immune responses in Cmah^-/- mice, emulating human loss of the gene encoding production of Neu5Gc, a major cell surface sialic acid. CMP-N-acetylneuraminic acid hydroxylase (CMAH) loss occurred ∼2-3 million years ago, after the common ancestor of humans and chimpanzees, perhaps contributing to speciation of the genus HomoCmah^-/- mice manifested a decreased survival in endotoxemia following bacterial LPS injection. Macrophages from Cmah^-/- mice secreted more inflammatory cytokines with LPS stimulation and showed more phagocytic activity. Macrophages and whole blood from Cmah^-/- mice also killed bacteria more effectively. Metabolic reintroduction of Neu5Gc into Cmah^-/- macrophages suppressed these differences. Cmah^-/- mice also showed enhanced bacterial clearance during sublethal lung infection. Although monocytes and monocyte-derived macrophages from humans and chimpanzees exhibited marginal differences in LPS responses, human monocyte-derived macrophages killed Escherichia coli and ingested E. coli BioParticles better. Metabolic reintroduction of Neu5Gc into human macrophages suppressed these differences. Although multiple mechanisms are likely involved, one cause is altered expression of C/EBPβ, a transcription factor affecting macrophage function. Loss of Neu5Gc in Homo likely had complex effects on immunity, providing greater capabilities to clear sublethal bacterial challenges, possibly at the cost of endotoxic shock risk. This trade-off may have provided a selective advantage when Homo transitioned to butchery using stone tools. The findings may also explain why the Cmah^-/- state alters severity in mouse models of human disease.

Reservoir host immune responses to emerging zoonotic viruses

Zoonotic viruses, such as HIV, Ebola virus, coronaviruses, influenza A viruses, hantaviruses, or henipaviruses, can result in profound pathology in humans. In contrast, populations of the reservoir hosts of zoonotic pathogens often appear to tolerate these infections with little evidence of disease. Why are viruses more dangerous in one species than another? Immunological studies investigating quantitative and qualitative differences in the host-virus equilibrium in animal reservoirs will be key to answering this question, informing new approaches for treating and preventing zoonotic diseases. Integrating an understanding of host immune responses with epidemiological, ecological, and evolutionary insights into viral emergence will shed light on mechanisms that minimize fitness costs associated with viral infection, facilitate transmission to other hosts, and underlie the association of specific reservoir hosts with multiple emerging viruses. Reservoir host studies provide a rich opportunity for elucidating fundamental immunological processes and their underlying genetic basis, in the context of distinct physiological and metabolic constraints that contribute to host resistance and disease tolerance.

Immunological and technical considerations in application of alginate-based microencapsulation systems

Islets encapsulated in immunoprotective microcapsules are being proposed as an alternative for insulin therapy for treatment of type 1 diabetes. Many materials for producing microcapsules have been proposed but only alginate does currently qualify as ready for clinical application. However, many different alginate-based capsule systems do exist. A pitfall in the field is that these systems are applied without a targeted strategy with varying degrees of success as a consequence. In the current review, the different properties of alginate-based systems are reviewed in view of future application in humans. The use of allogeneic and xenogeneic islet sources are discussed with acknowledging the different degrees of immune protection the encapsulation system should supply. Also issues such as oxygen supply and the role of danger associated molecular patterns (DAMPS) in immune activation are being reviewed. A common property of the encapsulation systems is that alginates for medical application should have an extreme high degree of purity and lack pathogen-associated molecular patterns (PAMPs) to avoid activation of the recipient’s immune system. Up to now, non-inflammatory alginates are only produced on a lab-scale and are not yet commercially available. This is a major pitfall on the route to human application. Also the lack of predictive pre-clinical models is a burden. The principle differences between relevant innate and adaptive immune responses in humans and other species are reviewed. Especially, the extreme differences between the immune system of non-human primates and humans are cumbersome as non-human primates may not be predictive of the immune responses in humans, as opposed to the popular belief of regulatory agencies. Current insight is that although the technology is versatile major research efforts are required for identifying the mechanical, immunological, and physico-chemical requirements that alginate-based capsules should meet for successful human application.

A comparative review of toll-like receptor 4 expression and functionality in different animal species

Toll-like receptors (TLRs) belong to the pattern recognition receptor (PRR) family, a key component of the innate immune system. TLRs detect invading pathogens and initiate an immediate immune response to them, followed by a long-lasting adaptive immune response. Activation of TLRs leads to the synthesis of pro-inflammatory cytokines and chemokines and the expression of co-stimulatory molecules. TLR4 specifically recognizes bacterial lipopolysaccharide, along with several other components of pathogens and endogenous molecules produced during abnormal situations, such as tissue damage. Evolution across species can lead to substantial diversity in the TLR4’s affinity and specificity to its ligands, the TLR4 gene and cellular expression patterns and tissue distribution. Consequently, TLR4 functions vary across different species. In recent years, the use of synthetic TLR agonists as adjuvants has emerged as a realistic therapeutic goal, notably for the development of vaccines against poorly immunogenic targets. Given that an adjuvanted vaccine must be assessed in pre-clinical animal models before being tested in humans, the extent to which an animal model represents and predicts the human condition is of particular importance. This review focuses on the current knowledge on the critical points of divergence between human and the mammalian species commonly used in vaccine research and development (non-human primate, mouse, rat, rabbit, swine, and dog), in terms of molecular, cellular, and functional properties of TLR4.

Discordant biological and toxicological species responses to TLR3 activation

Toll-like receptors (TLRs) are highly conserved type 1 membrane proteins that initiate a multiplicity of transient gene transcriptions, resulting in innate and adaptive immune responses. These essential immune responses are triggered by common TLR pattern recognition receptors of microbial products expressed through the cytoplasmic carboxy-terminal Toll/IL-1 domain. Toll/IL-1 adapter protein cascades are induced by an activated Toll/IL-1 to induce transient transcription responses. All TLRs, with the exception of TLR3, use an MyD88 adapter to Toll/IL-1 to initiate a proinflammatory cascade. TLR3 uses the toll receptor 3/4 induction factor adapter to initiate a different cytosolic adapter cascade with double-stranded RNA agonists. This non-MyD88 pathway induces both NF-κB and type 1 interferon responses. By using a TLR3-restricted double-stranded RNA agonist, rintatolimod, we demonstrate significant unexpected differences in toxic responses between rats and primates. The mechanism of this differential response is consistent with a relative down-regulation of the NF-κB inflammatory cytokine induction pathway in the cynomolgus monkey and humans, but not observed systemically in rat. Our findings suggest evaluation of TLR3 agonists in drug therapy.

Thyroid autoantibodies are rare in nonhuman great apes and hypothyroidism cannot be attributed to thyroid autoimmunity

The great apes include, in addition to Homo, the genera Pongo (orangutans), Gorilla (gorillas), and Pan, the latter comprising two species, P. troglodytes (chimpanzees) and P. paniscus (bonobos). Adult-onset hypothyroidism was previously reported in 4 individual nonhuman great apes. However, there is scarce information on normal serum thyroid hormone levels and virtually no data for thyroid autoantibodies in these animals. Therefore, we examined thyroid hormone levels and TSH in all nonhuman great ape genera including adults, adolescents, and infants. Because hypothyroidism in humans is commonly the end result of thyroid autoimmunity, we also tested healthy and hypothyroid nonhuman great apes for antibodies to thyroglobulin (Tg), thyroid peroxidase (TPO), and the TSH receptor (TSHR). We established a thyroid hormone and TSH database in orangutans, gorillas, chimpanzees, and bonobos (447 individuals). The most striking differences are the greatly reduced free-T4 and free-T3 levels in orangutans and gorillas vs chimpanzees and bonobos, and conversely, elevated TSH levels in gorillas vs Pan species. Antibodies to Tg and TPO were detected in only 2.6% of adult animals vs approximately 10% in humans. No animals with Tg, TPO, or TSHR antibodies exhibited thyroid dysfunction. Conversely, hypothyroid nonhuman great apes lacked thyroid autoantibodies. Moreover, thyroid histology in necropsy tissues was similar in euthyroid and hypothyroid individuals, and lymphocytic infiltration was absent in 2 hypothyroid animals. In conclusion, free T4 and free T3 are lower in orangutans and gorillas vs chimpanzees and bonobos, the closest living human relatives. Moreover, thyroid autoantibodies are rare and hypothyroidism is unrelated to thyroid autoimmunity in nonhuman great apes.