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Global amphibian declines are compounded by deadly disease outbreaks caused by the chytrid fungus,Batrachochytrium dendrobatidis(Bd). Much has been learned about the roles of amphibian skin-produced antimicrobial components and microbiomes in controllingBd, yet almost nothing is known about the roles of skin-resident immune cells in anti-Bddefenses. Mammalian mast cells reside within and serve as key immune sentinels in barrier tissues like skin. Accordingly, we investigated the roles ofXenopus laevisfrog mast cells duringBdinfections. Our findings indicate that enrichment ofX. laevisskin mast cells confers anti-Bdprotection and ameliorates the inflammation-associated skin damage caused byBdinfection. This includes a significant reduction in infiltration ofBd-infected skin by neutrophils, promoting mucin content within cutaneous mucus glands, and preventingBd-mediated changes to skin microbiomes. Mammalian mast cells are known for their production of the pleiotropic interleukin-4 (IL4) cytokine and our findings suggest that theX. laevisIL4 plays a key role in manifesting the effects seen following cutaneous mast cell enrichment. Together, this work underscores the importance of amphibian skin-resident immune cells in anti-Bddefenses and illuminates a novel avenue for investigating amphibian host–chytrid pathogen interactions. 

Global amphibian declines are largely driven by deadly disease outbreaks caused by the chytrid fungus, Batrachochytrium dendrobatidis (Bd). In the time since these disease outbreaks were first discovered, much has been learned about the roles of amphibian skin-produced antimicrobial components and skin microbiomes in controlling Bd. Yet almost nothing is known about the roles of skin-resident immune cells in anti-Bd defenses. Notably, mammalian mast cells reside within and serve as key immune sentinels in barrier tissues like the skin. Thus, they are critical to immune recognition of pathogens and to orchestrating the ensuing immune responses. Accordingly, we investigated the roles of Xenopus laevis frog mast cells during Bd infections. Our findings indicate that enrichment of X. laevis skin mast cells confers significant anti-Bd protection and ameliorates the inflammation-associated skin damage caused by Bd infection. Moreover, enriching X. laevis mast cells promotes greater mucin content within cutaneous mucus glands and protects frogs from Bd-mediated changes to their skin microbiomes. Together, this work underlines the importance of amphibian skin-resident immune cells in anti-Bd defenses and introduces a novel approach for investigating amphibian host-chytrid pathogen interactions.