Immunotoxic Effects of Persistent Organic Pollutants on the Northern Leopard Frog, Lithobates (Rana) pipiens

Lithobates (Rana) pipiens is a native amphibian species of north central North America and has experienced population declines, making it of interest to investigate whether organic contaminant exposure compromises immune function in this frog species. L. pipiens is exposed to a variety of contaminants in the wild, including the two that are the focus of this dissertation, polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs). Tadpoles, orally exposed to environmentally relevant levels of PBDEs, were assessed for innate and adaptive immune responses following metamorphosis. PBDE exposure significantly decreased secondary antibody response to the antigen, keyhole limpet hemocyanin (KLH). Innate immune measures were not significantly altered by PBDE exposure. When tadpoles were exposed to environmentally relevant, dietary levels of PCB-126, there were no effects of PCB-126 on survival, growth and development of tadpoles, but there were alterations in immune responses. PCB-exposed frogs, challenged with KLH, had significantly decreased levels of KLH-specific IgY antibodies compared to the control frogs. The amount of secreted antimicrobial skin peptides (AMPs) was also significantly decreased in PCB-treated animals, but the composition of AMPs was not affected by PCB exposure. We ran two additional exposure trials to test whether life stage affected immune response following PCB-126 exposure. When tadpoles were exposed to PCB-126 and also challenged with a primary injection of KLH, the level of KLH-specific IgY antibodies produced following metamorphosis was significantly suppressed compared to control frogs. However, suppression was not apparent when animals were exposed to PCB-126 only after metamorphosis. To assess innate immune response, we measured complement hemolytic activity in the juvenile frogs. Larval PCB-126 exposure decreased hemolytic activity of frogs compared to unexposed frogs, but this suppression was no longer apparent when frogs were exposed only after metamorphosis. This is the first study to demonstrate differences in immunotoxicity between anuran life stages, and that a carry-over effect of larval immune disruption due to PCB-126 exposure persisted through metamorphosis and suppressed post-metamorphic frog immunity. The collection of research in this dissertation provides evidence in juvenile L. pipiens for immunomodulation following exposure to both PBDEs and PCB-126.