Social transfer of pathogenic fungus promotes active immunisation in ant colonies

Due to the omnipresent risk of epidemics, insect societies have evolved sophisticated disease defences at the individual and colony level. An intriguing yet little understood phenomenon is that social contact to pathogen-exposed individuals reduces susceptibility of previously naive nestmates to this pathogen. We tested whether such social immunisation in Lasius ants against the entomopathogenic fungus Metarhizium anisopliae is based on active upregulation of the immune system of nestmates following contact to an infectious individual or passive protection via transfer of immune effectors among group members--that is, active versus passive immunisation. We found no evidence for involvement of passive immunisation via transfer of antimicrobials among colony members. Instead, intensive allogrooming behaviour between naive and pathogen-exposed ants before fungal conidia firmly attached to their cuticle suggested passage of the pathogen from the exposed individuals to their nestmates. By tracing fluorescence-labelled conidia we indeed detected frequent pathogen transfer to the nestmates, where they caused low-level infections as revealed by growth of small numbers of fungal colony forming units from their dissected body content. These infections rarely led to death, but instead promoted an enhanced ability to inhibit fungal growth and an active upregulation of immune genes involved in antifungal defences (defensin and prophenoloxidase, PPO). Contrarily, there was no upregulation of the gene cathepsin L, which is associated with antibacterial and antiviral defences, and we found no increased antibacterial activity of nestmates of fungus-exposed ants. This indicates that social immunisation after fungal exposure is specific, similar to recent findings for individual-level immune priming in invertebrates. Epidemiological modeling further suggests that active social immunisation is adaptive, as it leads to faster elimination of the disease and lower death rates than passive immunisation. Interestingly, humans have also utilised the protective effect of low-level infections to fight smallpox by intentional transfer of low pathogen doses ("variolation" or "inoculation").     

Trained immunity: a memory for innate host defense

Immune responses in vertebrates are classically divided into innate and adaptive, with only the latter being able to build up immunological memory. However, although lacking adaptive immune responses, plants and invertebrates are protected against reinfection with pathogens, and invertebrates even display transplant rejection. In mammals, past "forgotten" studies demonstrate cross-protection between infections independently of T and B cells, and more recently memory properties for NK cells and macrophages, prototypical cells of innate immunity, have been described. We now posit that mammalian innate immunity also exhibits an immunological memory of past insults, for which we propose the term "trained immunity." Understanding trained immunity will revolutionize our view of host defense and immunological memory, and could lead to defining a new class of vaccines and immunotherapies.     

Within and transgenerational immune priming in an insect to a DNA virus

Invertebrates mount a sophisticated immune response with the potential to exhibit a form of immune memory through ‘priming’. Increased immune protection following early exposure to bacteria has been found both later in life (within generation priming) and in the next generation (transgeneration priming) in a number of invertebrates. However, it is unclear how general immune priming is and whether immune priming occurs in response to different parasites, including viruses. Here, using Plodia interpuctella (Lepidoptera) and its natural DNA virus, Plodia interpunctella granulosis virus, we find evidence for both within generation and transgeneration immune priming. Individuals previously exposed to low doses of virus, as well as the offspring of exposed individuals, are subsequently less susceptible to viral challenge. Relatively little is known about the mechanisms that underpin viral immunity but it is probable that the viral immune response is somewhat different to that of bacteria. We show that immune priming may, however, be a characteristic of both responses, mediated through different mechanisms, suggesting that immune memory may be a general phenomenon of insect immunity. This is important because immune priming may influence both host–parasite population and evolutionary dynamics.     

Maternal transfer of strain-specific immunity in an invertebrate

The most celebrated component of the vertebrate immune system is the acquired response in which memory cells established during primary infection enhance the proliferation of antibodies during secondary infection. Additionally, the strength of vertebrate acquired immune responses varies dramatically depending on the infecting pathogen species or on the pathogen genotype within species. Because invertebrates lack the T-cell receptors and Major Histocompatibility Complex (MHC) molecules that mediate vertebrate adaptive immune responses, they are thought to lack adaptive immunity and be relatively unspecific in their interactions with pathogens. With only innate immunity, invertebrate hosts are believed to be nai;ve at each new encounter with pathogens. Nevertheless, some forms of facultative immunity appear to be important in insects; some individuals have enhanced immunity due to population density, and some social insects benefit when their nest-mates have been exposed to a pathogen or pathogen mimic (; see for a predation example.) Here we provide evidence for acquired strain-specific immunity in the crustacean Daphnia magna infected with the pathogenic bacteria Pasteuria ramosa. Specifically, the fitness of hosts was enhanced when challenged with a bacterial strain their mother had experienced relative to cases when mother and offspring were challenged with different strains.     

Innate immunity against viruses

Viruses are obligate parasites which are able to infect cells of all living organisms. Multiple antiviral defense mechanisms have appeared early in evolution of the immune system. Higher vertebrates have the most complex antiviral immunity which is based on both innate and adoptive immune responses. However, majority of living organisms, including plants and invertebrates, rely exclusively on innate immune mechanisms for protection against viral infections. There are some striking similarities in several components of the innate immune recognition between mammals, plants and insects, rendering these signaling cascades as highly conserved in the evolution of the immune system. This review summarizes recent advances in the field of innate immune recognition of viruses, with particular interest on pattern-recognition receptors.     

Molecular evolution of the NF-κB signaling system

The mechanisms of innate immunity in vertebrates show certain overall resemblances to immune mechanisms of insects. Two hypotheses have been proposed to explain these resemblances. (1) According to the evolutionary continuity hypothesis, innate immune mechanisms evolved in the common ancestor of vertebrates and insects and have been conserved since that time. (2) In the independent-evolution hypothesis, the mechanisms of innate immunity in vertebrates evolved independently from invertebrate immune mechanisms. Phylogenetic analysis of five gene families (Pelle, Rel, IkappaB, Toll, and TRAF) whose members are involved in NF-kappaB signaling in vertebrates and insects were used to decide between these hypotheses. The phylogenies of the Rel and TRAF families strongly supported independent evolution of immune functions in vertebrates and invertebrates, and, except for a possible case in the Pelle family, orthologous molecules having immune functions in both vertebrates and invertebrates were not found. The results suggest that NF-kappaB represents an ancient, generalized signaling system that has been co-opted for immune system roles independently in vertebrate and insect lineages.     

Innate mechanisms of viral recognition

Viruses are obligate parasites which can infect cells of all living organisms. Multiple antiviral defense mechanisms appeared early in the evolution of the immune system. Higher vertebrates possess the most complex antiviral immunity based on both innate and adoptive immune responses. However, a majority of living organisms, including plants and invertebrates, rely exclusively on innate immune mechanisms for protection against viral infections. There are some striking similarities in several components of innate immune recognition in mammals, plants, and insects suggesting that these signaling cascades are highly conserved in the evolution of the immune system. This review summarizes recent advances in the field of innate immune recognition of viruses, with a focus on pattern-recognition receptors.     

A Shift from Cellular to Humoral Responses Contributes to Innate Immune Memory in the Vector Snail Biomphalaria glabrata

Discoveries made over the past ten years have provided evidence that invertebrate antiparasitic responses may be primed in a sustainable manner, leading to the failure of a secondary encounter with the same pathogen. This phenomenon called “immune priming” or "innate immune memory" was mainly phenomenological. The demonstration of this process remains to be obtained and the underlying mechanisms remain to be discovered and exhaustively tested with rigorous functional and molecular methods, to eliminate all alternative explanations. In order to achieve this ambitious aim, the present study focuses on the Lophotrochozoan snail, Biomphalaria glabrata, in which innate immune memory was recently reported. We provide herein the first evidence that a shift from a cellular immune response (encapsulation) to a humoral immune response (biomphalysin) occurs during the development of innate memory. The molecular characterisation of this process in Biomphalaria/Schistosoma system was undertaken to reconcile mechanisms with phenomena, opening the way to a better comprehension of innate immune memory in invertebrates. This prompted us to revisit the artificial dichotomy between innate and memory immunity in invertebrate systems.     

Analogies in the evolution of individual and social immunity

We compare anti-parasite defences at the level of multicellular organisms and insect societies, and find that selection by parasites at these two organisational levels is often very similar and has created a number of parallel evolutionary solutions in the host's immune response. The defence mechanisms of both individuals and insect colonies start with border defences to prevent parasite intake and are followed by soma defences that prevent the establishment and spread of the parasite between the body's cells or the social insect workers. Lastly, germ line defences are employed to inhibit infection of the reproductive tissue of organisms or the reproductive individuals in colonies. We further find sophisticated self/non-self-recognition systems operating at both levels, which appear to be vital in maintaining the integrity of the body or colony as a reproductive entity. We then expand on the regulation of immune responses and end with a contemplation of how evolution may shape the different immune components, both within and between levels. The aim of this review is to highlight common evolutionary principles acting in disease defence at the level of both individual organisms and societies, thereby linking the fields of physiological and ecological immunology.     

The evolution of immune mechanisms

From early on in evolution, organisms have had to protect themselves from pathogens. Mechanisms for discriminating "self" from "non-self" evolved to accomplish this task, launching a long history of host-pathogen co-evolution. Evolution of mechanisms of immune defense has resulted in a variety of strategies. Even unicellular organisms have rich arsenals of mechanisms for protection, such as restriction endonucleases, antimicrobial peptides, and RNA interference.In multicellular organisms, specialized immune cells have evolved, capable of recognition, phagocytosis, and killing of foreign cells as well as removing their own cells changed by damage, senescence, infection, or cancer. Additional humoral factors, such as the complement cascade, have developed that co-operate with cellular immunity in fighting infection and maintaining homeostasis. Defensive mechanisms based on germline-encoded receptors constitute a system known as innate immunity. In jaw vertebrates, this system is supplemented with a second system, adaptive immunity, which in contrast to innate immunity is based on diversification of immune receptors and on immunological memory in each individual.Usually, each newly evolved defense mechanism did not replace the previous one, but supplemented it, resulting in a layered structure of the immune system. The immune system is not one system but rather a sophisticated network of various defensive mechanisms operating on different levels, ranging from mechanisms common for every cell in the body to specialized immune cells and responses at the level of the whole organism. Adaptive changes in pathogens have shaped the evolution of the immune system at all levels.     

Life is a huge compromise: Is the complexity of the vertebrate immune-neuroendocrine system an advantage or the price to pay?

Recent advances in comparative immunology have established that invertebrates produce hypervariable molecules probably related to immunity, suggesting the possibility of raising a specific immune response. “Priming” and “tailoring” are terms now often associated with the invertebrate innate immunity. Comparative immunologists contributed to eliminate the idea of a static immune system in invertebrates, making necessary to re-consider the evolutive meaning of immunological memory of vertebrates. If the anticipatory immune system represents a maximally efficient immune system, why can it be observed only in vertebrates, especially in consideration that molecular hypervariability exists also in invertebrates? Using well-established theories concerning the evolution of the vertebrate immunity as theoretical basis we analyze from an Eco-immunology-based perspective why a memory-based immune system may have represented an evolutive advantage for jawed vertebrates. We hypothesize that for cold-blooded vertebrates memory represents a complimentary component that flanks the robust and fundamental innate immunity. Conversely, immunological memory has become indispensable and fully exploited in warm-blooded vertebrates, due to their stable inner environment and high metabolic rate, respectively.     

The expanding realm of heterologous immunity: friend or foe?

Antecedent or current infections can alter the immunopathologic outcome of a subsequent unrelated infection. Immunomodulation by co-infecting pathogens has been referred to as 'heterologous immunity' and has been postulated to play a role in host susceptibility to disease, tolerance to organ transplant, and autoimmune disease. The effect of various infections on heterologous immune responses has been well studied in the context of shared epitopes and cross-reactive T cells. It has been shown that prior infections can modulate protective immunity and immunopathology by forming a pool of memory T cells that can cross-react with antigens from heterologous organisms or through the generation of a network of regulatory cells and cytokines. While it is not feasible to alter a host's history of prior infection, understanding heterologous immune responses in the context of simultaneous unrelated infections could have important therapeutic implications. Here, we outline key evidence from animal and human studies demonstrating the effect of heterologous immunity on the outcome of disease. We briefly review the role of T cells, but expand our discussion to explore other immune mechanisms that may modulate the response to concurrent active infections. In particular, we underscore the role of the innate immune system, polarized responses and regulatory mechanisms on heterologous immune responses.     

The evolution of resistance through costly acquired immunity

We examine the evolutionary dynamics of resistance to parasites through acquired immunity. Resistance can be achieved through the innate mechanisms of avoidance of infection and reduced pathogenicity once infected, through recovery from infection and through remaining immune to infection: acquired immunity. We assume that each of these mechanisms is costly to the host and find that the evolutionary dynamics of innate immunity in hosts that also have acquired immunity are quantitatively the same as in hosts that possess only innate immunity. However, compared with resistance through avoidance or recovery, there is less likely to be polymorphism in the length of acquired immunity within populations. Long-lived organisms that can recover at intermediate rates faced with fast-transmitting pathogens that cause intermediate pathogenicity (mortality of infected individuals) are most likely to evolve long-lived acquired immunity. Our work emphasizes that because whether or not acquired immunity is beneficial depends on the characteristics of the disease, organisms may be selected to only develop acquired immunity to some of the diseases that they encounter.     

Immune defense and host life history

Recent interest has focused on immune response in an evolutionary context, with particular attention to disease resistance as a life-history trait, subject to trade-offs against other traits such as reproductive effort. Immune defense has several characteristics that complicate this approach, however; for example, because of the risk of autoimmunity, optimal immune defense is not necessarily maximum immune defense. Two important types of cost associated with immunity in the context of life history are resource costs, those related to the allocation of essential but limited resources, such as energy or nutrients, and option costs, those paid not in the currency of resources but in functional or structural components of the organism. Resource and option costs are likely to apply to different aspects of resistance. Recent investigations into possible trade-offs between reproductive effort, particularly sexual displays, and immunity have suggested interesting functional links between the two. Although all organisms balance the costs of immune defense against the requirements of reproduction, this balance works out differently for males than it does for females, creating sex differences in immune response that in turn are related to ecological factors such as the mating system. We conclude that immune response is indeed costly and that future work would do well to include invertebrates, which have sometimes been neglected in studies of the ecology of immune defense.     

Reproduction and population density affect humoral immunity in bank voles under field experimental conditions

Density dependence is a common feature in the dynamics of animal populations. Availability of food resources critical to immunity is likely to be one of the mechanisms mediating the effect of population density on individual fitness. The ability to mount an immune response to an antigen is also affected by levels of immunosuppressive hormones associated with reproduction or mediating the response to ecological and social stress. We assessed variation in condition and intensity of humoral immune response to a T-cell-dependent antigen in bank voles (Clethrionomys glareolus) by experimentally altering population density before immunisation. Consistent with our prediction, males had lower humoral immunocompetence in the breeding than in the non-breeding season. Contrary to our expectation, males did not show enhanced immunocompetence and females showed depressed humoral immune response under experimentally lowered population density. Variation of immune response in relation to population density depended on sex, with females but not males showing lower immune response under experimentally reduced density. We conclude that humoral immunity of bank voles was affected by reproduction and social environment rather than by population density. Received: 2 November 1999 / Accepted: 22 March 2000     

A trade-off switch of two immunological memories in Caenorhabditis elegans reinfected by bacterial pathogens

Recent studies have suggested that innate immune responses exhibit characteristics associated with memory linked to modulations in both vertebrates and invertebrates. However, the diverse evolutionary paths taken, particularly within the invertebrate taxa, should lead to similarly diverse innate immunity memory processes. Our understanding of innate immune memory in invertebrates primarily comes from studies of the fruit fly Drosophila melanogaster, the generality of which is unclear. Caenorhabditis elegans typically inhabits soil harboring a variety of fatal microbial pathogens; for this invertebrate, the innate immune system and aversive behavior are the major defensive strategies against microbial infection. However, their characteristics of immunological memory remains infantile. Here we discovered an immunological memory that promoted avoidance and suppressed innate immunity during reinfection with bacteria, which we revealed to be specific to the previously exposed pathogens. During this trade-off switch of avoidance and innate immunity, the chemosensory neurons AWB and ADF modulated production of serotonin and dopamine, which in turn decreased expression of the innate immunity-associated genes and led to enhanced avoidance via the downstream insulin-like pathway. Therefore, our current study profiles the immune memories during C. elegans reinfected by pathogenic bacteria and further reveals that the chemosensory neurons, the neurotransmitter(s), and their associated molecular signaling pathways are responsible for a trade-off switch between the two immunological memories.     

Non-immunological defense in an evolutionary framework

After parasite infection, invertebrates activate immune system-based defenses such as encapsulation and the signaling pathways of the innate immune system. However, hosts are often able to defend against parasites without using these mechanisms. The non-immunological defenses, such as behaviors that prevent or combat infection, symbiont-mediated defense, and fecundity compensation, are often ignored but can be important in host-parasite dynamics. We review recent studies showing that heritable variation in these traits exists among individuals, and that they are costly to activate and maintain. We also discuss findings from genome annotation and expression studies to show how immune system-based and non-immunological defenses interact. Placing these studies into an evolutionary framework emphasizes their importance for future studies of host-parasite coevolution.     

The evolution and genetics of innate immunity

The immune system provides protection from a wide range of pathogens. One component of immunity, the phylogenetically ancient innate immune response, fights infections from the moment of first contact and is the fundamental defensive weapon of multicellular organisms. The Toll family of receptors has a crucial role in immune defence. Studies in fruitflies and in mammals reveal that the defensive strategies of invertebrates and vertebrates are highly conserved at the molecular level, which raises the exciting prospects of an increased understanding of innate immunity.