Amphibian mast cells serve as barriers to chytrid fungus infections.

Global amphibian declines are compounded by deadly disease outbreaks caused by the chytrid fungus, (). Much has been learned about the roles of amphibian skin-produced antimicrobial components and microbiomes in controlling , yet almost nothing is known about the roles of skin-resident immune cells in anti- defenses. Mammalian mast cells reside within and serve as key immune sentinels in barrier tissues like skin. Accordingly, we investigated the roles of frog mast cells during infections. Our findings indicate that enrichment of skin mast cells confers anti- protection and ameliorates the inflammation-associated skin damage caused by infection. This includes a significant reduction in infiltration of -infected skin by neutrophils, promoting mucin content within cutaneous mucus glands, and preventing -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 the IL4 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- defenses and illuminates a novel avenue for investigating amphibian host-chytrid pathogen interactions.