Divergent IL18-STAT1 Immune Responses Underlie Differential Susceptibility to Aeromonas hydrophila in Geoclemys hamiltonii and Trachemys scripta: A Comparative Transcriptomic Perspective

Background/Objectives: The IUCN endangered spotted pond turtle (Geoclemys hamiltonii) demonstrates markedly reduced resistance to Aeromonas hydrophila-induced infections compared to the red-eared slider (Trachemys scripta). This study investigates the immunological basis for this disparity by analyzing infection outcomes and splenic transcriptomes of both species post-pathogen challenge. Methods: In a preliminary experiment, 32 turtles (16 G. hamiltonii and 16 T. scripta) were exposed to A. hydrophila. Results: G. hamiltonii developed skin ulcer syndrome at a significantly higher incidence (81.25%) than T. scripta (12.5%) (p < 0.05). Comparative transcriptomic analysis identified 19 differentially expressed immune-related genes, with qPCR validation across five tissues (heart, liver, spleen, intestine, blood) revealing pronounced interspecies differences in IL18, STAT1, IFIH1, and CD28 expression. Notably, IL18 and its downstream effector STAT1 were robustly upregulated in T. scripta but were considerably lower in G. hamiltonii, correlating with delayed IFN-γ pathway activation and impaired epidermal barrier repair. Concurrently, CD28 upregulation in T. scripta facilitated rapid T-cell-mediated pathogen clearance, whereas its delayed induction in G. hamiltonii hindered adaptive immunity. These findings implicate dysregulated innate (IL18/STAT1) and adaptive (CD28) immune pathways as key determinants of G. hamiltonii’s susceptibility to bacterial infection. Conclusions: Despite the critical conservation status of G. hamiltonii, the immunological basis underlying its heightened susceptibility to bacterial infections remains largely unexplored; this study addresses this gap by comparing the splenic transcriptomes of G. hamiltonii and T. scripta following A. hydrophila challenge, identifying the dysregulated IL18-STAT1 Immune Axis and CD28-mediated adaptive immunity as key determinants, thereby providing actionable immune targets for conservation breeding and susceptibility screening in this endangered species.