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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. 

The amphibian declines are compounded by emerging pathogens that often preferentially target distinct amphibian developmental stages. While amphibian immune responses remain relatively unexplored, macrophage (Mφ)-lineage cells are believed to be important to both amphibian host defenses and to their pathogen infection strategies. As such, a greater understanding of tadpole and adult amphibian Mφ functionality is warranted. Mφ biology is interdependent of interleukin-34 (IL-34) and colony-stimulating factor-1 (CSF-1) cytokines and we previously showed that CSF-1- and IL-34-derived Mφs of the Xenopus laevis frog are morphologically, transcriptionally, and functionally distinct. Presently, we directly compared the cytology and transcriptomes of X. laevis tadpole and frog CSF-1- and IL-34-Mφs. Our results indicate that tadpole and frog CSF-1-Mφs possess greater non-specific esterase activity, typically associated with Mφ-lineage cells. By contrast, both tadpole and frog IL-34-Mφs have greater specific esterase activity, which is typically attributed to granulocyte-lineage cells. Our comparisons of tadpole CSF-1-Mφ transcriptomes with those of tadpole IL-34-Mφs indicate that the two tadpole populations possess significantly different transcriptional profiles of immune and non-immune genes. The frog CSF-1-Mφ gene expression profiles are likewise significantly disparate from those of frog IL-34-Mφs. Compared to their respective tadpole Mφ subtypes, frog CSF-1- and IL-34-Mφs exhibited greater expression of genes associated with antigen presentation. Conversely, compared to their frog Mφ counterparts, tadpole CSF-1- and IL-34-Mφs possessed greater levels of select Fc-like receptor genes. Presumably, these cytological and transcriptional differences manifest in distinct biological roles for these respective tadpole and frog Mφ subtypes.