Lymphocyte phenotypes in wild-caught rats suggest potential mechanisms underlying increased immune sensitivity in post-industrial environments

Physiological strategies in wild rodents: immune defenses of commensal rats

The importance of issues associated with urban/commensal rats and mice (property damage, management costs, and health risks) press upon research on these animals. While the demography of commensal rodents is mostly studied, the need for understanding factors influencing their natural morbidity/mortality is also stressed. In this respect, more attention is expected to be paid to immunity, the physiological mechanism of defense against host survival threats (pathogens, parasites, diseases). Commensal rats and mice carry numerous pathogens that evoke diverse immune responses. The state of immunity in commensal house mice is studied in great detail, owing to the use of laboratory strains in biomedical research. Because commensal rats are, compared to mice, carriers of more zoonotic agents, rats' immunity is studied mainly in that context. Some of these zoonotic agents cause chronic, asymptomatic infections, which justified studies of immunological mechanisms of pathogen tolerance versus clearance regulation in rats. Occurrence of some infections in specific tissues/organs pressed upon analysis of local/regional immune responses and/or immunopathology. A survey of immunological activity/responses in commensal rats is given in this review, with mention of existing data in commensal mice. It should throw some light on the factors relevant to their morbidity and lifespan, supplementing the knowledge of commensal rodent ecology.

Adaptation to low parasite abundance affects immune investment and immunopathological responses of cavefish

Reduced parasitic infection rates in the developed world are suspected to underlie the rising prevalence of autoimmune disorders. However, the long-term evolutionary consequences of decreased parasite exposure on an immune system are not well understood. We used the Mexican tetra Astyanax mexicanus to understand how loss of parasite diversity influences the evolutionary trajectory of the vertebrate immune system, by comparing river with cave morphotypes. Here, we present field data affirming a strong reduction in parasite diversity in the cave ecosystem, and show that cavefish immune cells display a more sensitive pro-inflammatory response towards bacterial endotoxins. Surprisingly, other innate cellular immune responses, such as phagocytosis, are drastically decreased in cavefish. Using two independent single-cell approaches, we identified a shift in the overall immune cell composition in cavefish as the underlying cellular mechanism, indicating strong differences in the immune investment strategy. While surface fish invest evenly into the innate and adaptive immune systems, cavefish shifted immune investment to the adaptive immune system, and here, mainly towards specific T-cell populations that promote homeostasis. Additionally, inflammatory responses and immunopathological phenotypes in visceral adipose tissue are drastically reduced in cavefish. Our data indicate that long-term adaptation to low parasite diversity coincides with a more sensitive immune system in cavefish, which is accompanied by a reduction in the immune cells that play a role in mediating the pro-inflammatory response.

Early Immune Response to Acute Gastric Fluid Aspiration in a Rat Model of Lung Transplantation

OBJECTIVES

Chronic aspiration of gastric fluid contents can decrease long-term survival of pulmonary transplants due to development of obliterative bronchiolitis. However, little is known about the early immune response and the cascade of events involved in the development of obliterative bronchiolitis.

MATERIALS AND METHODS

We utilized a rat orthotopic pulmonary transplant model and a single aspiration of either gastric fluid or normal saline to investigate the histologic, cellular, and cytokine changes associated with an acute gastric fluid aspiration event compared with normal saline at 2 and 10 days after aspiration.

RESULTS

Our observations included a decrease in pulmonary compliance and increased airway inflammation and acute rejection of the transplanted lung, as well as increases in macrophages, granulocytes, and proinflammatory cytokines such as interleukin 1β, transforming growth factor β1 and β2, and tumor necrosis factor α in bronchoalveolar lavage fluid from the transplanted lung of gastric fluid-aspirated rats compared with normal saline-aspirated rats.

CONCLUSIONS

The acute inflammatory response observed in the present study is consistent with changes found in chronic models of aspiration-associated injury and suggests a potentially important role for mast cells in the development of obliterative bronchiolitis.

The Immunology of Wild Rodents: Current Status and Future Prospects

Wild animals' immune responses contribute to their evolutionary fitness. These responses are moulded by selection to be appropriate to the actual antigenic environment in which the animals live, but without imposing an excessive energetic demand which compromises other component of fitness. But, exactly what these responses are, and how they compare with those of laboratory animals, has been little studied. Here, we review the very small number of published studies of immune responses of wild rodents, finding general agreement that their humoral (antibody) responses are highly elevated when compared with those of laboratory animals, and that wild rodents' cellular immune system reveals extensive antigenic exposure. In contrast, proliferative and cytokine responses of ex vivo-stimulated immune cells of wild rodents are typically depressed compared with those of laboratory animals. Collectively, these responses are appropriate to wild animals' lives, because the elevated responses reflect the cumulative exposure to infection, while the depressed proliferative and cytokine responses are indicative of effective immune homeostasis that minimizes immunopathology. A more comprehensive understanding of the immune ecology of wild animals requires (i) understanding the antigenic load to which wild animals are exposed, and identification of any key antigens that mould the immune repertoire, (ii) identifying immunoregulatory processes of wild animals and the events that induce them, and (iii) understanding the actual resource state of wild animals, and the immunological consequences that flow from this. Together, by extending studies of wild rodents, particularly addressing these questions (while drawing on our immunological understanding of laboratory animals), we will be better able to understand how rodents' immune responses contribute to their fitness in the wild.

Immunization enhances the natural antibody repertoire

The role of immunization in the production of antibodies directed against immunogens is widely appreciated in laboratory animals and in humans. However, the role of immunization in the development of "natural antibodies" has not been investigated. Natural antibodies are those antibodies present without known history of infection or immunization, and react to a wide range of targets, including "cryptic" self-antigens that are exposed upon cell death. In this study, the ability of immunization to elicit the production of natural antibodies in laboratory rats was evaluated. Laboratory rats were immunized with a series of injections using peanut extracts (a common allergen), a high molecular weight protein conjugated to hapten (FITC-KLH), and a carbohydrate conjugated to hapten (DNP-Ficall). Significantly greater binding of antibodies from immunized animals compared to controls was observed to numerous autologous organ extracts (brain, kidney, liver, lung, prostate, and spleen) for both IgM and IgG, although the effect was more pronounced for IgM. These studies suggest that immunization may have at least one unforeseen benefit, enhancing networks of natural antibodies that may be important in such processes as wound repair and tumor surveillance. Such enhancement of natural antibody function may be particularly important in Western society, where decreased exposure to the environment may be associated with a weakened natural antibody repertoire.

Changing expression of vertebrate immunity genes in an anthropogenic environment: a controlled experiment

Background

The effect of anthropogenic environments on the function of the vertebrate immune system is a problem of general importance. For example, it relates to the increasing rates of immunologically-based disease in modern human populations and to the desirability of identifying optimal immune function in domesticated animals. Despite this importance, our present understanding is compromised by a deficit of experimental studies that make adequately matched comparisons between wild and captive vertebrates.

Results

We transferred post-larval fishes (three-spined sticklebacks), collected in the wild, to an anthropogenic (captive) environment. We then monitored, over 11 months, how the systemic expression of immunity genes changed in comparison to cohort-matched wild individuals in the originator population (total n = 299). We found that a range of innate (lyz, defbl2, il1r-like, tbk1) and adaptive (cd8a, igmh) immunity genes were up-regulated in captivity, accompanied by an increase in expression of the antioxidant enzyme, gpx4a. For some genes previously known to show seasonality in the wild, this appeared to be reduced in captive fishes. Captive fishes tended to express immunity genes, including igzh, foxp3b, lyz, defbl2, and il1r-like, more variably. Furthermore, although gene co-expression patterns (analyzed through gene-by-gene correlations and mutual information theory based networks) shared common structure in wild and captive fishes, there was also significant divergence. For one gene in particular, defbl2, high expression was associated with adverse health outcomes in captive fishes.

Conclusion

Taken together, these results demonstrate widespread regulatory changes in the immune system in captive populations, and that the expression of immunity genes is more constrained in the wild. An increase in constitutive systemic immune activity, such as we observed here, may alter the risk of immunopathology and contribute to variance in health in vertebrate populations exposed to anthropogenic environments.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0751-8) contains supplementary material, which is available to authorized users.

Got worms? Perinatal exposure to helminths prevents persistent immune sensitization and cognitive dysfunction induced by early-life infection

The incidence of autoimmune and inflammatory diseases has risen dramatically in post-industrial societies. "Biome depletion" - loss of commensal microbial and multicellular organisms such as helminths (intestinal worms) that profoundly modulate the immune system - may contribute to these increases. Hyperimmune-associated disorders also affect the brain, especially neurodevelopment, and increasing evidence links early-life infection to cognitive and neurodevelopmental disorders. We have demonstrated previously that rats infected with bacteria as newborns display life-long vulnerabilities to cognitive dysfunction, a vulnerability that is specifically linked to long-term hypersensitivity of microglial cell function, the resident immune cells of the brain. Here, we demonstrate that helminth colonization of pregnant dams attenuated the exaggerated brain cytokine response of their offspring to bacterial infection, and that combined with post-weaning colonization of offspring with helminths (consistent with their mothers treatment) completely prevented enduring microglial sensitization and cognitive dysfunction in adulthood. Importantly, helminths had no overt impact on adaptive immune cell subsets, whereas exaggerated innate inflammatory responses in splenic macrophages were prevented. Finally, helminths altered the effect of neonatal infection on the gut microbiome; neonatal infection with Escherichia coli caused a shift from genera within the Actinobacteria and Tenericutes phyla to genera in the Bacteroidetes phylum in rats not colonized with helminths, but helminths attenuated this effect. In sum, these data point toward an inter-relatedness of various components of the biome, and suggest potential mechanisms by which this helminth might exert therapeutic benefits in the treatment of neuroinflammatory and cognitive disorders.

The laboratory mouse and wild immunology

The laboratory mouse, Mus musculus domesticus, has been the workhorse of the very successful laboratory study of mammalian immunology. These studies--discovering how the mammalian immune system can work--have allowed the development of the field of wild immunology that is seeking to understand how the immune responses of wild animals contributes to animals' fitness. Remarkably, there have hardly been any studies of the immunology of wild M. musculus domesticus (or of rats, another common laboratory model), but the general finding is that these wild animals are more immunologically responsive, compared with their laboratory domesticated comparators. This difference probably reflects the comparatively greater previous exposure to antigens of these wild-caught animals. There are now excellent prospects for laboratory mouse immunology to make major advances in the field of wild immunology.

Increased biodiversity in the environment improves the humoral response of rats

Previous studies have compared the immune systems of wild and of laboratory rodents in an effort to determine how laboratory rodents differ from their naturally occurring relatives. This comparison serves as an indicator of what sorts of changes might exist between modern humans living in Western culture compared to our hunter-gatherer ancestors. However, immunological experiments on wild-caught animals are difficult and potentially confounded by increased levels of stress in the captive animals. In this study, the humoral immune responses of laboratory rats in a traditional laboratory environment and in an environment with enriched biodiversity were examined following immunization with a panel of antigens. Biodiversity enrichment included colonization of the laboratory animals with helminths and co-housing the laboratory animals with wild-caught rats. Increased biodiversity did not apparently affect the IgE response to peanut antigens following immunization with those antigens. However, animals housed in the enriched biodiversity setting demonstrated an increased mean humoral response to T-independent and T-dependent antigens and increased levels of “natural” antibodies directed at a xenogeneic protein and at an autologous tissue extract that were not used as immunogens.

Evolutionary biology and anthropology suggest biome reconstitution as a necessary approach toward dealing with immune disorders

Industrialized society currently faces a wide range of non-infectious, immune-related pandemics. These pandemics include a variety of autoimmune, inflammatory and allergic diseases that are often associated with common environmental triggers and with genetic predisposition, but that do not occur in developing societies. In this review, we briefly present the idea that these pandemics are due to a limited number of evolutionary mismatches, the most damaging being 'biome depletion'. This particular mismatch involves the loss of species from the ecosystem of the human body, the human biome, many of which have traditionally been classified as parasites, although some may actually be commensal or even mutualistic. This view, evolved from the 'hygiene hypothesis', encompasses a broad ecological and evolutionary perspective that considers host-symbiont relations as plastic, changing through ecological space and evolutionary time. Fortunately, this perspective provides a blueprint, termed 'biome reconstitution', for disease treatment and especially for disease prevention. Biome reconstitution includes the controlled and population-wide reintroduction (i.e. domestication) of selected species that have been all but eradicated from the human biome in industrialized society and holds great promise for the elimination of pandemics of allergic, inflammatory and autoimmune diseases.