Sex-specific innate immunity and ageing in long-lived fresh water turtles (Kinosternon flavescens: Kinosternidae)

BACKGROUND

The progressive deregulation of the immune system with age, termed immunosenescence, has been well studied in mammalian systems, but studies of immune function in long-lived, wild, non-mammalian populations are scarce. In this study we leverage a 38-year mark-recapture study to quantify the relationships among age, sex, survival, reproductive output and the innate immune system in a long-lived reptile, yellow mud turtles (Kinosternon flavescens; Testudines; Kinosternidae).

METHODS

We estimated rates of survival and age-specific mortality by sex based on mark-recapture data for 1530 adult females and 860 adult males over 38 years of captures. We analyzed bactericidal competence (BC), and two immune responses to foreign red blood cells - natural antibody-mediated haemagglutination (NAbs), and complement-mediated haemolysis ability (Lys) - in 200 adults (102 females; 98 males) that ranged from 7 to 58 years of age captured in May 2018 during their emergence from brumation, and for which reproductive output and long-term mark-recapture data were available.

RESULTS

We found that females are smaller and live longer than males in this population, but the rate of accelerating mortality across adulthood is the same for both sexes. In contrast, males exhibited higher innate immunity than females for all three immune variables we measured. All immune responses also varied inversely with age, indicating immunosenescence. For females that reproduced in the preceding reproductive season, egg mass (and therefore total clutch mass) increased with age,. In addition to immunosenescence of bactericidal competence, females that produced smaller clutches also had lower bactericidal competence.

CONCLUSIONS

Contrary to the general vertebrate pattern of lower immune responses in males than females (possibly reflecting the suppressive effects of androgens), we found higher levels of all three immune variables in males. In addition, contrary to previous work that found no evidence of immunosenescence in painted turtles or red-eared slider turtles, we found a decrease in bactericidal competence, lysis ability, and natural antibodies with age in yellow mud turtles.

Immunity in Sea Turtles: Review of a Host-Pathogen Arms Race Millions of Years in the Running

The immune system of sea turtles is not completely understood. Sea turtles (as reptiles) bridge a unique evolutionary gap, being ectothermic vertebrates like fish and amphibians and amniotes like birds and mammals. Turtles are ectotherms; thus, their immune system is influenced by environmental conditions like temperature and season. We aim to review the turtle immune system and note what studies have investigated sea turtles and the effect of the environment on the immune response. Turtles rely heavily on the nonspecific innate response rather than the specific adaptive response. Turtles' innate immune effectors include antimicrobial peptides, complement, and nonspecific leukocytes. The antiviral defense is understudied in terms of the diversity of pathogen receptors and interferon function. Turtles also mount adaptive responses to pathogens. Lymphoid structures responsible for lymphocyte activation and maturation are either missing in reptiles or function is affected by season. Turtles are a marker of health for their marine environment, and their immune system is commonly dysregulated because of disease or contaminants. Fibropapillomatosis (FP) is a tumorous disease that afflicts sea turtles and is thought to be caused by a virus and an environmental factor. We aim, by exploring the current understanding of the immune system in turtles, to aid the investigation of environmental factors that contribute to the pathogenesis of this disease and provide options for immunotherapy.

Adaptive Immunity in Reptiles: Conventional Components but Unconventional Strategies

Recent studies have established that the innate immune system of reptiles is broad and robust, but the question remains: What role does the reptilian adaptive immune system play? Conventionally, adaptive immunity is described as involving T and B lymphocytes that display variable receptors, is highly specific, improves over the course of the response, and produces a memory response. While reptiles do have B and T lymphocytes that utilize variable receptors, their adaptive response is relatively non-specific, generates a prolonged antibody response, and does not produce a typical memory response. This alternative adaptive strategy may allow reptiles to produce a broad adaptive response that complements a strong innate system. Further studies into reptile adaptive immunity cannot only clarify outstanding questions on the reptilian immune system but can shed light on a number of important immunological concepts, including the evolution of the immune system and adaptive immune responses that take place outside of germinal centers.

Herpesviruses in Reptiles

Since the 1970s, several species of herpesviruses have been identified and associated with significant diseases in reptiles. Earlier discoveries placed these viruses into different taxonomic groups on the basis of morphological and biological characteristics, while advancements in molecular methods have led to more recent descriptions of novel reptilian herpesviruses, as well as providing insight into the phylogenetic relationship of these viruses. Herpesvirus infections in reptiles are often characterised by non-pathognomonic signs including stomatitis, encephalitis, conjunctivitis, hepatitis and proliferative lesions. With the exception of fibropapillomatosis in marine turtles, the absence of specific clinical signs has fostered misdiagnosis and underreporting of the actual disease burden in reptilian populations and hampered potential investigations that could lead to the effective control of these diseases. In addition, complex life histories, sampling bias and poor monitoring systems have limited the assessment of the impact of herpesvirus infections in wild populations and captive collections. Here we review the current published knowledge of the taxonomy, pathogenesis, pathology and epidemiology of reptilian herpesviruses.

Serological Survey on the Occurrence of Anti-Leptospira spp. Antibodies in Red-Eared Terrapins (Trachemys scripta elegans) Living in a Natural Park of Northern Italy

Simple Summary

Although it has been supposed that reptiles, including chelonians, are involved in the epidemiology of Leptospira, data about leptospirosis in turtles are very scant. In the present survey, Trachemys scripta elegans terrapins living in a natural park were tested to verify the presence of antibodies against Leptospira spp., with 6% being positive. Infected turtles could be a source of infection for other animals and humans, shedding leptospirae into the environment. However, further studies are necessary to verify if Leptospira may cause disease in turtles or if these animals may serve as reservoirs.

Abstract

Turtles are suspected to be involved in the epidemiology of Leptospira; however, data about the dissemination of this zoonotic pathogen among chelonians are scant. In the present study, the serum samples collected from 49 Trachemys scripta elegans living in a natural park of northern Italy were tested by a microagglutination test to measure detectable antibodies against different Leptospira serovars. Three (6.12%) turtles had agglutinins to the serovar Tarassovi, suggesting that they were exposed to the spirochaetes. Currently, it is not clear if Leptospira can cause disease in chelonians or if these animals can serve as reservoirs of leptospirae. Considering that chelonians often share the same environment with other animals and humans, and considering the One Health perspective, investigations to better understand the role of chelonians as a source of Leptospira infection are necessary.

The reptilian perspective on vertebrate immunity: 10 years of progress

Ten years ago, ‘Understanding the vertebrate immune system: insights from the reptilian perspective’ was published. At the time, our understanding of the reptilian immune system lagged behind that of birds, mammals, fish and amphibians. Since then, great progress has been made in elucidating the mechanisms of reptilian immunity. Here, I review recent discoveries associated with the recognition of pathogens, effector mechanisms and memory responses in reptiles. Moreover, I put forward key questions to drive the next 10 years of research, including how reptiles are able to balance robust innate mechanisms with avoiding self-damage, how B cells and antibodies are used in immune defense and whether innate mechanisms can display the hallmarks of memory. Finally, I briefly discuss the links between our mechanistic understanding of the reptilian immune system and the field of eco-immunology. Overall, the field of reptile immunology is poised to contribute greatly to our understanding of vertebrate immunity in the next 10 years.

Plasma complement activation mechanisms differ in ornate (Terrapene ornata ornata) and eastern box turtles (Terrapene carolina carolina)

Eastern (Terrapene carolina carolina) and ornate (Terrapene ornata ornata) box turtles have robust plasma antibacterial activity, however, the mechanism behind this activity is unknown. We used sheep red blood cell (SRBC) hemolysis assays, mannan-affinity chromatography, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) to explore the mechanisms of complement activity in box turtles. Plasma from both species demonstrated volume, time, and temperature-dependent SRBC hemolysis, with significantly greater hemolytic activity in ornate box turtle plasma. Hemolytic activity was highly attenuated following treatment with heat, EDTA, and salicylaldoxime in both species, but was unchanged after treatment with methylamine and ammonium hydroxide. Two abundant mannan-binding proteins (presumed C-type lectins) were identified in eastern box turtle plasma using SDS-PAGE and MALDI-TOF, but ornate box turtles did not express either protein. Eastern box turtles appear to rely on the lectin pathway of complement activation while ornate box turtles utilize the alternative pathway. This study provides further evidence that mechanisms underlying immune function are not always conserved between closely related species. This finding may have important implications for explaining species differences in susceptibility to emerging threats such as disease, toxicants, and climate change.

Immunosenescence and its influence on reproduction in a long-lived vertebrate

Immunosenescence is a well-known phenomenon in mammal systems, but its relevance in other long-lived vertebrates is less understood. Further, the influence of age and reproductive effort on immune function in long-lived species can be challenging to assess, as long-term data are scarce and it is often difficult to sample the oldest age classes. We used the painted turtle (Chrysemys picta) to test hypotheses of immunosenescence and a trade-off between reproductive output and immune function in a population of a long-lived vertebrate that has been monitored for over 30 years. These long-term data were utilized to employ a unique approach of aging turtles with mark-recapture data and population-specific growth modeling to obtain more accurate estimates of age. We analyzed natural antibodies, lysis ability and bactericidal competence in 126 individuals from 1 to 33 years of age captured during May and June 2011. Older turtles exhibited greater natural antibody levels than young individuals. Young females with large clutches exhibited greater lysis ability, while older females with large clutches had decreased lysis ability, suggesting a trade-off between reproductive output and immune function conditional upon age. However, bactericidal competence increased later in the nesting season for older females. Our study rejects the hypothesis of immunosenescence in a long-lived turtle, despite evidence of actuarial and reproductive senescence in this population. Additionally, we detected mixed evidence for a trade-off between reproduction and immune health.

Intestinal B cells in the red-eared slider turtle, Trachemys scripta: Anatomical distribution and implications for ecological interactions with pathogenic microbes

Disease is a significant threat in the global decline of reptile species. Many aquatic reptiles live in habitats with high levels of opportunistic microbial pathogens, yet little is known about their immune system. Gut-associated lymphoid tissue is vital for protection against ingested pathogens and maintenance of normal gut microbiota. In mammals, gut mucosal immunity is well-characterized and mucosal surfaces are coated in protective antibodies. However, reptiles lack lymph nodes and Peyer's patches, which are the major sites of mammalian B cell responses. The presence or distribution of mucosal B cells in reptiles is unknown. In this study, we first set out to determine if B cells could be detected in intestinal tissues of red-eared slider turtles, Trachemys scripta. Using whole-mount immunochemistry and a primary antibody to turtle antibody light chains, we identified widely distributed B cell aggregates within the small intestine of hatchling turtles. These aggregates appeared similar to isolated lymphoid follicles (ILFs) in mammals and the frequency was much higher in distal intestinal sections than in proximal sections. To determine if these structures were inducible in the presence of microbes, we introduced an enteric Salmonella species through oral gavage. Analysis of intestinal tissues revealed that hatchlings exposed to Salmonella exhibited significantly more of these aggregates when compared with those that did not receive bacteria. These studies provide the first evidence for B cell-containing ILF-like structures in reptiles and provide novel information about gut immunity in nonmammalian vertebrates that could have important implications for ecological interactions with pathogens.

Relationships between parasitic infection and natural antibodies, age, and sex in a long-lived vertebrate

Reptilian immune systems are believed to rely more heavily on the innate response than the adaptive response. Past research on reptilian immune systems has indicated that natural antibodies (NAbs) play an important role in fighting antigens as the first responder and initiating the innate immune response. However, there is a gap in research on NAbs in reptiles as there is little data on how they may influence parasite prevalence in reptiles. Furthermore, no studies have examined NAbs at mucosal sites, which represent an important point of entry for parasites. We investigated NAbs in male and female red-eared slider turtles (Trachemys scripta) in order to begin to fill this gap in our understanding. Plasma and mucosal samples were collected from T. scripta to determine total immunoglobulin levels in relation to intestinal parasite load, which was determined from fecal samples. We also determined antibody levels to various antigens to examine if NAbs are present in mucosal samples and how levels compare to plasma NAbs. In female turtles, parasite intensity decreased with increasing host age and mucosal antibody levels. We also found that the levels of antibodies that bind to a novel antigen and an antigen they are likely to have encountered were significantly related in both plasma and mucosal samples, indicating a NAb response. This study contributes to our understanding of how reptiles may rely on a broad innate response that allows them to resist potential pathogens while potentially avoiding some negative consequences of immunosenescence.