Markedly Elevated Antibody Responses in Wild versus Captive Spotted Hyenas Show that Environmental and Ecological Factors Are Important Modulators of Immunity

Abstract

Evolutionary processes have shaped the vertebrate immune system over time, but proximal mechanisms control the onset, duration, and intensity of immune responses. Based on testing of the hygiene hypothesis, it is now well known that microbial exposure is important for proper development and regulation of the immune system. However, few studies have examined the differences between wild animals in their natural environments, in which they are typically exposed to a wide array of potential pathogens, and their conspecifics living in captivity. Wild spotted hyenas (Crocuta crocuta) are regularly exposed to myriad pathogens, but there is little evidence of disease-induced mortality in wild hyena populations, suggesting that immune defenses are robust in this species. Here we assessed differences in immune defenses between wild spotted hyenas that inhabit their natural savanna environment and captive hyenas that inhabit a captive environment where pathogen control programs are implemented. Importantly, the captive population of spotted hyenas was derived directly from the wild population and has been in captivity for less than four generations. Our results show that wild hyenas have significantly higher serum antibody concentrations, including total IgG and IgM, natural antibodies, and autoantibodies than do captive hyenas; there was no difference in the bacterial killing capacity of sera collected from captive and wild hyenas. The striking differences in serum antibody concentrations observed here suggest that complementing traditional immunology studies, with comparative studies of wild animals in their natural environment may help to uncover links between environment and immune function, and facilitate progress towards answering immunological questions associated with the hygiene hypothesis.

Fig 1. Immune defense component model modified from Schmid-Hempel and Ebert [28].

Patterns shown here are hypothesized based on existing literature and the locations within the plot were adjusted based on the results of this study. Bacterial killing capacity (BKC) is the least specific defense and also falls at the constitutive end of the x-axis. Specific IgG falls near the induced end of the x-axis, and is the most specific defense represented here. Anti-nuclear antibodies (ANAs) are specific to nuclear proteins and nucleic acids and can be induced following ligation of endosomal pattern recognition receptors. Natural IgG and IgM are non-specific and can be produced independent of CD4 T cell help. The dashed lines represent primary exposure (left) and secondary (right) exposure.

Fig 2. Relative concentrations of total IgG and IgM.

Total IgG (A) and total IgM (B) are significantly higher in wild hyenas (n = 14) than in captive hyenas (n = 15). Females are indicated by open circles and males by filled circles. ***p < 0.001.

Fig 3. Relative concentrations of anti-KLH natural antibodies.

Anti-KLH natural IgG (A) is significantly higher in wild hyenas (n = 14) than in captive hyenas (n = 11), but there is no difference in anti-KLH natural IgM (B) between wild (n = 14) captive (n = 11) hyenas. Females are indicated by open circles and males by filled circles. *** p < 0.001.

Fig 4. Relative concentrations of anti-nuclear antibodies.

Anti-nuclear IgG (A) and anti-nuclear IgM (B) are significantly higher in wild hyenas (n = 14 for IgG; n = 12 for IgM) than in captive hyenas (n = 15 for IgG and IgM). Females are indicated by open circles and males by filled circles. *** p < 0.001.

Fig 5. Bacterial killing capacity (BKC) of serum.

BKC is similar in captive (n = 15) and wild (n = 14) hyenas. Females are indicated by open circles and males by filled circles.