PFAS exposure linked to altered innate immune biomarkers in a sentinel apex predator, the bald eagle (Haliaeetus leucocephalus).

Per- and polyfluoroalkyl substances (PFAS) are persistent and ubiquitous environmental contaminants that can bioaccumulate in food webs, yet their effects on wildlife health-particularly in apex predators-remain poorly characterized. Bald eagles (Haliaeetus leucocephalus), which integrate contaminant exposure across aquatic and terrestrial prey, provide a valuable sentinel for assessing biological responses to PFAS in freshwater ecosystems. Here, we evaluated associations between PFAS levels in blood plasma of bald eagle nestlings sampled over multiple years across Wisconsin, USA, and biomarkers of innate immune function, including naturally circulating antibodies (IgY), complement lysis ability, and total white blood cell counts (WBC). To account for potential non-linear exposure-response relationships, we modeled associations between PFAS concentrations and immune biomarkers using a suite of candidate functional forms. Results indicate that a subset of individual PFAS exhibited significant associations with immune biomarkers after covariate adjustment, with predominantly negative relationships observed between several long-chain PFAS and complement-mediated lysis, alongside fewer but detectable associations with IgY. In contrast, complement activity was more frequently associated with PFAS exposure than humoral immunity, and exposure-response shapes varied across analytes, with log-linear and other non-linear forms often providing the best fit. In analyses of leukocyte profiles, heterophil proportions were positively associated with multiple PFAS structural groupings-including long-chain, short-chain, sulfonate, carboxylate, and legacy PFAS-while one individual analyte (PFHxA) showed a negative association, suggesting selective shifts in innate immune cell composition rather than broad changes in total leukocyte counts. Collectively, these findings suggest that environmentally relevant PFAS concentrations are associated with altered immune biomarkers in a sentinel apex predator, highlighting potential implications for wildlife health in contaminated freshwater ecosystems.