Immune Defenses againstBatrachochytrium dendrobatidis, a Fungus Linked to Global Amphibian Declines, in the South African Clawed Frog,Xenopus laevis

Abstract

Batrachochytrium dendrobatidisis a chytrid fungus that causes the lethal skin disease chytridiomycosis in amphibians. It is regarded as an emerging infectious disease affecting diverse amphibian populations in many parts of the world. Because there are few model amphibian species for immunological studies, little is known about immune defenses againstB. dendrobatidis. We show here that the South African clawed frog,Xenopus laevis, is a suitable model for investigating immunity to this pathogen. After an experimental exposure, a mild infection developed over 20 to 30 days and declined by 45 days postexposure. Either purified antimicrobial peptides or mixtures of peptides in the skin mucus inhibitedB. dendrobatidisgrowthin vitro. Skin peptide secretion was maximally induced by injection of norepinephrine, and this treatment resulted in sustained skin peptide depletion and increased susceptibility to infection. Sublethal X-irradiation of frogs decreased leukocyte numbers in the spleen and resulted in greater susceptibility to infection. Immunization againstB. dendrobatidisinduced elevated pathogen-specific IgM and IgY serum antibodies. Mucus secretions fromX. laevispreviously exposed toB. dendrobatidiscontained significant amounts of IgM, IgY, and IgX antibodies that bind toB. dendrobatidis. These data strongly suggest that both innate and adaptive immune defenses are involved in the resistance ofX. laevisto lethalB. dendrobatidisinfections.