Innate and adaptive immune defenses against a fungus linked to global amphibian declines in the South African Clawed Frog, <i>Xenopus laevis</i> (43.24)

Abstract Many amphibian species have suffered population declines due to an emerging infectious disease, chytridiomycosis, caused by the chytrid fungus, Batrachochytrium dendrobatidis (Bd). Little is known about immune defenses against this skin pathogen. Here we show that antimicrobial peptides in the skin mucus of the South African clawed frog, Xenopus laevis, inhibited Bd growth in vitro, and skin peptides secreted under natural conditions were within the range of concentrations required to inhibit Bd growth. Peptide secretion was induced by norepinephrine stimulation in a dose-dependent fashion. Maximal stimulation resulted in peptide depletion and increased susceptibility to infection. Frogs required approximately 28-50 days for peptide renewal. Following experimental exposure, infection intensity in the skin was greatest from 20-30 days and declined significantly by 45 days. Thus, immune defenses effectively cleared the infection by seven weeks. Sublethal X-irradiation of frogs resulted in decreased leukocyte numbers in the spleen and greater susceptibility to infection. Immunization against Bd resulted in elevated IgM and IgY antibody responses in vivo that persisted for greater than four weeks. Mucus secretions from X. laevis previously exposed to Bd five months earlier contained IgM, IgY, and IgX antibodies. These data strongly suggest that both innate and adaptive immune defenses are critical in the resistance of X. laevis to lethal Bd infections.