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Amphibian metamorphosis represents a dramatic example of post-embryonic development. In the anuran Xenopus laevis frog, this process involves extensive changes to larval tissues, structures, and physiology to produce its adult form. As a long-standing model to study tissue remodeling, both amphibian metamorphosis and mammalian development are under the control of thyroid hormone. Successful remodeling though, also requires precise temporospatial regulation of immune activation. Yet there is much to learn about the immune components linked to metamorphosis. In turn, granulocytes are a class of innate immune cells recently touted for their participation in processes beyond classical immune defenses, including in pathological and non-pathological tissue remodeling. In this manuscript, we explore the roles of granulocytes in perhaps the most conspicuous anuran metamorphic event: tadpole tail reabsorption. We characterize granulocyte infiltration into the tail as metamorphosis progresses. Although some granulocyte subpopulations exist in both Xenopus and mammals, our previous work has identified additional Xenopus-specific populations. Thus, here we further explored subpopulation dynamics through distinct stages of natural metamorphosis, their likely roles during this process, and their relationship with thyroid hormone. As endocrine disruptors continue to threaten species across the animal kingdom, the work described here offers much-needed insight into immune contributions to endocrine-linked development. 

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.