Larval Exposure to Polychlorinated Biphenyl-126 Led to a Long-Lasting Decrease in Immune Function in Postmetamorphic Juvenile Northern Leopard Frogs, Lithobates pipiens

Amphibian populations are decreasing worldwide, and pollution is a contributing factor. Polychlorinated biphenyls (PCBs) are a class of persistent organic pollutants known to exert immunotoxicity. To assess impacts of PCBs on frogs, we exposed Lithobates pipiens tadpoles to a diet of PCB-126 (0-5 ng PCB-126/g wet food) through metamorphic climax. Postmetamorphic frogs were immunized with keyhole limpet hemocyanin (KLH); then production of KLH-specific IgY, as well as total IgY and IgM, was measured (Trial I). A second larval study (0 and 7.3 ng PCB-126/g wet food) was performed to investigate whether PCB altered antigenic responses in prometamorphic tadpoles (Gosner Stage 36-39), and to measure the innate immune response of postmetamorphic frogs (Trial II). After larval PCB-126 exposure, both KLH-specific IgY levels and complement activity were reduced. Because postmetamorphic frogs carried a body burden of PCB-126 (2.4 ng/g or less), we wanted to determine whether the effect on immune response was due to larval exposure or to the resulting body burden as frogs. To test this, we reared tadpoles under control conditions (no PCB), and limited PCB exposure to postmetamorphosis only by injecting 2-week-old frogs with 10 ng PCB-126/g (Trial III). The resulting body burden (3.4 ng/g) was similar to that of frogs in Trial I, but we no longer detected suppression of KLH-specific IgY or hemolytic activity. These results suggest life-stage-specific immune responses; however, because we administered PCB-126 differently between trials, it is premature to conclude that these differences are intrinsically life stage dependent, and further study is warranted. Regardless, our study demonstrated a long-lasting effect of larval PCB-126 exposure that persisted through metamorphosis and suppressed frog immunity. Environ Toxicol Chem 2021;00:1-14. (c) 2021 SETAC

Automated summary

PCBs have a negative impact on the immune response of amphibians, especially when exposed during early stages of development. Frogs at different life stages show specific immune responses, although further research is needed to determine whether these effects are inherent to the life stage.