Sex-specific immune component contribution to Escherichia coli killing ability in Trachemys scripta elegans

Organisms are resource-limited and allocation toward physiological processes may shift with contexts, like reproductive investment and stress. The immune system comprises a myriad of components such as leukocytes and immune proteins. The costs of innate immune components are likely not uniform due to differences in roles and composition, and it may be beneficial to prioritize specific components through within-immune system tradeoffs. We investigated the differential contribution of innate immune components in wild adult red-eared slider turtles (Trachemys scripta elegans) during the beginning of the nesting season in Arkansas, USA. We collected blood samples within 3, 120, and 240 min of capture following the standardized stressor of capture. Enzyme-linked immunosorbent assays verified that corticosterone was elevated following capture. To assess immune component contribution, we systematically removed immune components from the isolated serum and buffy layer (serum+BL) to assess the contributions of immune cells, complement, and non-specific, heat-labile proteins (including antimicrobial peptides and natural, non-specific antibodies). After manipulating the serum+BL, we conducted functional microbial killing assays using Escherichia coli, a Gram-negative bacterium. Our findings suggest that complement and heat-labile proteins are upregulated during the acute stress response and the duration of upregulation differs by sex during the nesting season. Females were unable to maintain an increased protein-based immune response for as long as males following a stressor, indicating that females may be more energetically limited during the nesting season. This work provides evidence of tradeoffs within the innate immune system and enhances understanding of immunity in reptiles.