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It is well known that hormones influence and direct most facets of physiology; however, there is still contention regarding the directions of certain relationships, for example, between gonadal hormones and immunity. Among the many proposed relationships relating to gonadal–immune interactions, support for immunosuppressive effects of androgens remains prominent within physiological literature. Although ample study has been directed toward the immunosuppressive effects of androgens, considerable disagreement remains regarding their influence on immune function. In this study, we test the hypothesis that androgens inhibit immunocompetence in the American alligator (Alligator mississippiensis). Developing alligators were incubated at female‐producing temperatures with a subset of individuals being exposed to 17‐α‐methyltestosterone (MT) before sexual determination. 17‐α‐methyltestosterone is a potent androgen, not aromatizable by crocodilians, that has been found to exert masculinizing effects in exposed crocodilian populations in vivo and in vitro. Additionally, a subset of animals was exposed to a novel antigen to quantify innate and acquired immune function. We recovered no significant differences in leukocyte ratios or proportions between groups and found no significant differences in innate immune function as measured by hemolysis‐hemagglutination. However, we did find significant differences in acquired immune function, where masculinized individuals expressed greater antibody titers. Our findings reject the hypothesis that androgens suppress immune function; rather, androgens may be immunoenhancing to acquired humoral responses and neutral to innate humoral immunity in crocodilians. [ABSTRACT FROM AUTHOR]

Ecoimmunology affords us the ability to better understand immunological processes through consideration of external factors, such as the thermal microenvironment. This consideration is imperative when examining the immunological processes of ectothermic organisms like reptiles. Reptiles uniquely rely heavily on their innate immune function but remain poorly understood in immunological studies. In this study, we examined innate immunity in two zoo‐housed tortoise species, the Indian star tortoise (Geochelone elegans, Schoepff, 1795) and northern spider tortoise (Pyxis arachnoides brygooi, Vuillemin & Domergue, 1972). Bacterial killing assays (BKAs) were optimized and used to assess the monthly immunocompetence of these tortoises to three different bacteria: Escherichia coli, Salmonella enterica, and Staphylococcus aureus. We evaluated differences in blood biochemistry values (lactate and glucose) among months and species as well as fecal corticosterone (CORT) between species. Lastly, we examined the potential influences of individual thermal microenvironments on bactericidal ability. Both G. elegans and P. a. brygooi demonstrated immunocompetence against all bacterial challenges, but only bactericidal ability against E. coli varied over months. Optimal BKA serum dilutions, blood glucose levels, and fecal CORT concentrations differed between the two species. Finally, there was evidence that the thermal microenvironment influenced the tortoises' bactericidal ability against E. coli. Through use of nonmodel organisms, such as tortoises, we are given insight into the inner workings of innate immunity and a better understanding of the complexities of the vertebrate immune system. Research Highlights: •Innate immune ability and fecal corticosterone concentrations were species‐specific. •Bactericidal ability against Escherichia coli had a relationship with the thermal microenvironment. [ABSTRACT FROM AUTHOR]

The inflammatory response is a complex process that relies on interactions among multiple endocrine and immune modulators. Studies incorporating time‐related and integrative endocrine and immune responses to an immune challenge might shed light on the characterization of the phases of the inflammatory response in anurans. The present study investigated time‐related changes (1, 3, 6, and 18 h post‐challenge) in plasma corticosterone (CORT), melatonin (MEL) and testosterone (T) levels, phagocytosis percentage (PP), plasma bacterial killing ability (BKA), and neutrophil to lymphocyte ratio (NLR) following a lipopolysaccharide (LPS) immune challenge in Rhinella diptycha toads. Our results showed the response to LPS injection was characterized by increased CORT, PP, BKA, and NLR, with a concomitant decrease in plasma MEL and T. Increased CORT was more pronounced at 6 and 18 h, while increased NLR was observed only 18 h post‐LPS injection. Meanwhile, plasma MEL and T decreased independently of the time post‐LPS injection. Additionally, toads in better body condition showed higher BKA and PP in the LPS‐treated group, regardless of the time postinjection. Our results show that toads (R. diptycha) were sensitive to the LPS challenge, mounting an inflammatory response, which started quickly (after 1 h) and developed over time and was influenced by body condition. These results demonstrate a time‐related hormonal and immune variation as a consistent pattern of activation of the immune system, as well as of hypothalamic‐pituitary‐adrenal/interrenal and immune‐pineal axes following an immune challenge more deeply studied in mammals, suggesting the evolutionary conservation of the regulatory mechanisms for tetrapod vertebrates. Highlights: Increased CORT, and decreased MEL and T were observed following LPS treatment.LPS increased phagocytosis and BKA within 1 h and 6 h after, respectively.Increased immune redistribution (NLR) was observed 18 h after LPS treatment. [ABSTRACT FROM AUTHOR]

Environmental conditions experienced by developing animals have an impact on the development and maturity of the immune system. Specifically, the diet experienced during early development influences the maintenance and function of the immune system in young and adult animals. It is well known that exposure to low‐protein diets during early development are related to an attenuation of immunocompetence in adulthood. While this functional linkage has been widely studied in altricial models' mammals, it has been little explored how the nutritional history modulates the immune function in precocial animals. We evaluated the effect of dietary protein consumed during early development on the immune function and the oxidative costs in the precocial Caviomorph rodent Octodon degus, or degu. We evaluated components of the acute phase response (APR) and oxidative parameters before and after immune challenge. We found that after the immune challenge, the juveniles on the low‐protein dietary treatment exhibited an attenuation of body temperature but showed higher levels of lipid peroxidation than juvenile degus on the high‐protein diet. We did not find a significant effect of the interaction between diet and immune challenge on body mass, levels of inflammatory proteins, nor in the total antioxidant capacity. Our results suggest that some components of the immune function and the oxidative status in the degu can be modulated by diet during development. However, the modulation would depend on the immune variables analyzed, and the characteristics of the immune system of precocial rodents. Research Highlights: Diet experienced during early development influences the immune system and oxidative status in young animals.High‐protein diet is related to low lipid peroxidation after immune challengeLow‐protein diet is related to changes in body temperature after immune challenge. [ABSTRACT FROM AUTHOR]

Due to global changes, fish are increasingly exposed to immune challenges associated with disease outbreaks in aquatic ecosystems. Adjustments in physiology and behavior are generally critical to maintaining homeostasis after an immune challenge, but there is limited knowledge on the specific thresholds and dynamics of responses across levels of biological organization in fish. In this study, we tested how different concentrations of an antigens mixture (phytohemagglutinin and lipopolysaccharide) affected innate immunity with potential consequences on oxidative stress, energy reserves, body condition, and behavior across time, using the common gudgeon (Gobio sp.) as model species. The immune challenge induced a transitory increase in lytic enzyme activity (i.e., lysozyme) and local immune response (i.e., skin swelling) 2 days after the antigen injection. The available energy stored in muscle was also reduced 4 days after injection, without inducing oxidative stress at the cellular level. Overall, the immune challenge induced limited costs at the molecular and cellular levels but had strong effects at the whole organism level, especially on behavior. Indeed, fish swimming activity and sociability were affected in a dose‐ and time‐dependent manner. These results suggest that immune challenges have dose‐dependent effects across levels of biological organization and that behavior is a key response trait to cope with pathogen‐induced immune costs in the wild, although fitness consequences remain to be tested. Highlights: The costs induced by immune challenge vary in fish in a dose‐ and time‐dependent manner.Medium and high doses of antigens mostly affected immune and behavioral traits with limited effects on fish condition, likely due to energy reallocation. [ABSTRACT FROM AUTHOR]

The thymus is present in all gnathostome vertebrates and is an essential organ for the adaptive immune system via the generation of functional mature T‐cells. Over the life span of mammals, the thymus undergoes morphological and functional alterations, including an age‐related involution, which in humans starts in early life. Life history tradeoffs have been suggested as possible reasons for thymus involution. While in teleost fish, only a few studies have investigated alterations of thymus structure and function over different life stages, resulting in a fragmented database. Here, we investigated the thymus growth of zebrafish (Danio rerio) from early life, throughout puberty and reproductive stage, up to 1‐year‐old. We assessed thymus growth by histological and morphometric analyses and thymocyte numbers. Thymus function was assessed by measuring the transcripts of the thymocyte marker genes, ikaros, tcrα, and tcrδ. Additionally, we analyzed gonad maturity and tail homogenate vitellogenin concentrations to align thymus status with the status of the reproductive system. Our results showed that the zebrafish thymus, in contrast to the human thymus, grew strongly during early life and puberty but started to undergo involution when the fish reached the reproductive age. The involution was characterized by reduced thymus area and thymocyte number, altered histoarchitecture, and decreasing thymocyte marker gene transcript levels. Our findings suggest that age‐related changes of the zebrafish thymus do exist and could be partly explained in terms of resource tradeoffs, but also in terms of the ontogenetically late development of a functional adaptive immune system in teleosts. Research Highlights: Zebrafish thymus growth peaks at puberty and age‐related involution starts at reproduction, in contrast to mammals where thymus involution starts in early life, suggesting different ecological and evolutionary pressures on thymus development. [ABSTRACT FROM AUTHOR]

Climate change and emerging infectious diseases are often described as the main factors associated with the worldwide amphibian population decline. In this context, rising temperatures due to global warming might act as a chronic stressor for many amphibians, leading to immunosuppression. This study aimed to characterize the thermal sensitivity of the Bullfrog's (Lithobates catesbeianus) immune response and the effect of acclimation at different temperatures on it. Plasma bacterial killing ability (BKA) and phagocytosis activity of blood leukocytes were measured at different incubation temperatures (5–40°C) in individuals kept at 28°C and 34°C. First, all individuals were held under 28°C and sampled on the 16th day. Subsequently, one group was kept at 28°, and the other one was transferred to 34°C. Both groups were sampled at 83 and 106 days of maintenance. Plasma corticosterone (CORT) and testosterone (T) were assessed to evidence thermal stress and possible endocrine correlates of immune changes over time. The incubation temperature affected BKA both on animals kept at 28°C and 34°C, with maximum values at lower temperatures (5–20°C). Phagocytosis activity was constant over the range of assay temperatures. Immune and endocrine variables decreased over time in both thermal regimes, but frogs maintained at 34°C showed lower T and immunosuppression, evidencing stress response. Therefore, exposure to high temperatures might decrease immune function in bullfrogs due to chronic stress response and by exposition to temperatures of lower performance according to the thermal sensitivity curve, which might increase vulnerability to diseases in this anuran species. Research Highlights: Bullfrog's plasma bacterial killing ability showed thermal sensitivity, but phagocytosis did not.Bullfrog's immunity showed eurythermy and no thermal acclimation.Bullfrog's immune response decreased over time in individuals kept at 28 or 34°C. [ABSTRACT FROM AUTHOR]

Cane toads (Rhinella marina) were introduced worldwide and have become invasive in multiple locations, representing a major driver of biodiversity loss through competition (food, shelter, territory), predation, and the poisoning of native species. These toads have been used in Australia as a model for studies concerning invasion biology and ecoimmunology, as longer‐established (core) and invasion front (edge) populations show altered stress and immune response profiles. Although cane toads were also introduced into the United States in the 1950s, these patterns have yet to be evaluated for the populations spanning Florida. Toads introduced into Florida have dispersed primarily northward along a latitudinal gradient, where they encounter cooler temperatures that may further impact stress and immune differences between core and edge populations. In this study, we sampled cane toads from nine different locations spanning their invasion in Florida. Cane toads from southern populations showed higher plasma bacterial killing ability and natural antibody titers than the toads from the northern populations, indicating they have a better immune surveillance system. Also, southern toads were more responsive to a novel stressor (1 hr restraint), showing a higher increase in corticosterone levels. These results indicate that possible trade‐offs have occurred between immune and stress responses as these toads have become established in northern cooler areas in Florida. Research Highlights: Stress and immunity of invasive cane toads in Florida were affected from dispersal northSouthern (core) toads had higher reactivity to novel stressor than northern (edge) toadsCore populations had better immune surveillance system (BKA and Ab titers) [ABSTRACT FROM AUTHOR]

Understanding host immune function and ecoimmunology is increasingly important at a time when emerging infectious diseases (EIDs) threaten wildlife. One EID that has emerged and spread widely in recent years is chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), which is implicated unprecedented amphibian declines around the world. The impacts of Bd have been severe for many amphibian species, but some populations have exhibited signs of persistence, and even recovery, in some regions. Many mechanisms may underpin this pattern and amphibian immune responses are likely one key component. Although we have made great strides in understanding amphibian immunity, the complement system remains poorly understood. The complement system is a nonspecific, innate immune defense that is known to enhance other immune responses. Complement activation can occur by three different biochemical pathways and result in protective mechanisms, such as inflammation, opsonization, and pathogen lysis, thereby providing protection to the host. We currently lack an understanding of complement pathway activation for chytridiomycosis, but several studies have suggested that it may be a key part of an early and robust immune response that confers host resistance. Here, we review the available research on the complement system in general as well as amphibian complement responses to Bd infection. Additionally, we propose future research directions that will increase our understanding of the amphibian complement system and other immune responses to Bd. Finally, we suggest how a deeper understanding of amphibian immunity could enhance the conservation and management of amphibian species that are threatened by chytridiomycosis. Highlights: The complement system is an immune component present in vertebrates, but little is known about its role in amphibian chytridiomycosis.We review recent research, current gaps in knowledge, and we suggest future research to fill these gaps. [ABSTRACT FROM AUTHOR]

Exposure to stressors over prolonged periods can have fitness‐relevant consequences, including suppression of immune function. We tested for effects of presence of an invasive species threat on a broad panel of immune functions of a coexisting lizard. Eastern fence lizards (Sceloporus undulatus) have been exposed to invasive fire ants (Solenopsis invicta) for over 80 years. Fire ants sting and envenomate lizards, causing physiological stress, but we do not have a comprehensive understanding of the broad immune consequences of lizard exposure to fire ant presence. We conducted a suite of immune measures on fence lizards caught from areas with long histories of fire ant invasion and lizards from areas not yet invaded by fire ants. The effect of fire ant presence on immunity varied depending on the immune component measured: within fire ant invaded areas, some portions of immunity were suppressed (lymphocytic cell‐mediated immunity, complement), some were unaffected (phagocytic respiratory burst, natural antibodies), and some were enhanced (anti‐fire ant immunoglobulin M, basophils) compared to within uninvaded areas. Rather than fire ants being broadly immunosuppressing, as generally assumed, the immune response appears to be tailored to this specific stressor: the immune measures that were enhanced are important to the lizards' ability to handle envenomation, whereas those that were unaffected or suppressed are less critical to surviving fire ant encounters. Several immune measures were suppressed in reproductive females when actively producing follicles, which may make them more susceptible to immunosuppressive costs of stressors such as interactions with fire ants. Research Highlights: Immune function of fence lizards from areas with and without invasive venomous fire ants appear to be tailored to this stressor. Immune measures beneficial to surviving envenomation are enhanced and less critical measures are unchanged or reduced. [ABSTRACT FROM AUTHOR]

The immunocompetence of a community of free‐living animals can be affected by seasonality, sex, and parasite burden. However, each of these factors is often examined independently. Recent studies have also found that dehydration can enhance aspects of immunocompetence in drought‐adapted species. To explore how all of these factors interact, and their effect on the immune system in mesic‐adapted species, we collected blood samples from a community of free‐ranging snakes in coastal South Carolina, United States, across 2 years. We specifically examined (a) how sex and seasonality influence humoral and cellular immunocompetence and parasite burden, (b) the dynamics among hydration state, parasite burden, and immunocompetence, and (c) whether mesic‐adapted species also show enhanced innate immunity with dehydration. Consistent with previous work on drought‐adapted species, we found that dehydration enhances multiple aspects of humoral immunity in mesic species, and we are the first to report that dehydration also enhances aspects of cellular immunocompetence. Contrary to previous results in other squamates, sex and season did not impact immunocompetence or parasite prevalence. Our results also reveal complex interactions among parasite prevalence, immunocompetence, and hydration state demonstrating that hydration state and parasitism are two ubiquitous factors that should continue to be considered in future studies examining ecoimmunological variation. Research Highlights: Across a community of mesic‐adapted squamates, dehydration enhanced both innate and cellular immunity. These results also highlight the complex interactions among parasite prevalence, immunocompetence, and hydration state. [ABSTRACT FROM AUTHOR]

Invasive species have become a prolific environmental issue, second only to climate change, yet many of the phenomena that facilitate invasive success are not well understood (Phillip & Shine, Proc. Roy. Soc. B, 273, 1545‐1550). The combination of several generalist life‐history traits, certain physiological mechanisms, and environmental conditions is thought to play a significant role in invasion success. The ability to undergo fitness trade‐offs—to reallocate nutritional and energetic resources towards processes that increase reproduction, growth, and dispersal—is also thought to be an adaptive quality of many invasive species. Due to their inherent flexibility, phenotypically plastic traits in particular are often targeted for fitness reallocations. Immune function, for example, is determined by a highly plastic phenotype, which is crucial for combating a diverse array of pathogens. When active, immune function also demands extensive resources from the host. Laboratory and field studies suggest that certain aspects of the immune system are more costly than others, though, and that its components can be regulated independent of one another. In invasive species undergoing fitness trade‐offs, costly innate inflammatory responses are often downregulated, while antibody‐mediated responses may be enhanced. A combination of fixed physiological responses and environmentally induced trade‐offs are thought to regulate the immune system, though the relationship between these facets of regulation is still an area of active research. The field of ecoimmunology, then, has emerged in effort to understand the phenomena by which individual immune regulation can drive (and be driven by) species‐level ecology and evolution, and therefore be linked to invasive success (Downs et al., 2014. Integr. Compar. Biol., 54, 340–352). Research Highlights: Cross‐taxa studies suggest that selective regulation of immune system components ‐wherein more costly immune functions are down‐regulated in favor of up‐regulating more energetically‐efficient processes‐ may be contributing to invasive success. [ABSTRACT FROM AUTHOR]

As infectious diseases in ectothermic vertebrates increasingly threaten wild populations, understanding how host immune systems are affected by the environment is key to understanding the process of infection. In this study, we investigated how temperature change and simulated bacterial infection (via lipopolysaccharide [LPS] injection) interacted to regulate innate immunity, as measured by bactericidal ability (BA), phagocytosis rate, and heterophil:lymphocyte ratio (HLR) in common musk turtles (Sternotherus odoratus). We found that LPS stimulated an acute immune response, as measured by an increase in BA, phagocytosis rate, and HLR. When exposed to a 5 or 10°C temperature change for 48 hr, turtles rapidly acclimated to the new temperature by adjusting their immune output. This acclimation was compensatory as seen by elevated rates of immune output in colder animals and decreased rates of immune output in warmer animals. These results indicate that while temperature change may be a constraint on some animals, S. odoratus have the ability to rapidly adjust immunity to match environmental thermal demand. This rapid ability to adjust immunity may be related to the broad geographic distribution of musk turtles. Future research should focus on how immune acclimation in ectotherms varies both intraspecifically and interspecifically across regional scales and geographic distributions. Highlights: When body temperatures were changed for 48 hr, turtles adjusted immune output as measured by the bactericidal ability and phagocytosis to compensate for the changed temperature. Lipopolysaccharide triggered a temperature‐dependent acute response. Phagocytosis rate, as measured by proportion chemoluminescence change between pretreatment and posttreatment Sternotherus odoratus was negatively related to temperature in saline‐injected control turtles (p = 0.039, R2 = 0.217). There was not a significant relationship between phagocytosis rate change and temperature in lipopolysaccharide (LPS)‐injected animals (p = 0.619), although LPS‐injected animals had significantly higher phagocytosis rates than saline‐injected animals. [ABSTRACT FROM AUTHOR]