Isolation and Culture of Amphibian (Xenopus laevis) Sub-Capsular Liver and Bone Marrow Cells

Amphibian mast cells serve as barriers to chytrid fungus infections

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 controlling Bd, yet almost nothing is known about the roles of skin-resident immune cells in anti-Bd defenses. Mammalian mast cells reside within and serve as key immune sentinels in barrier tissues like skin. 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 anti-Bd protection and ameliorates the inflammation-associated skin damage caused by Bd infection. This includes a significant reduction in infiltration of Bd-infected skin by neutrophils, promoting mucin content within cutaneous mucus glands, and preventing Bd-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 X. laevis 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-Bd defenses and illuminates a novel avenue for investigating amphibian host-chytrid pathogen interactions.

Myelopoiesis of the Amphibian Xenopus laevis Is Segregated to the Bone Marrow, Away From Their Hematopoietic Peripheral Liver

Across vertebrates, hematopoiesis takes place within designated tissues, wherein committed myeloid progenitors further differentiate toward cells with megakaryocyte/erythroid potential (MEP) or those with granulocyte/macrophage potential (GMP). While the liver periphery (LP) of the Xenopus laevis amphibian functions as a principal site of hematopoiesis and contains MEPs, cells with GMP potential are instead segregated to the bone marrow (BM) of this animal. Presently, using gene expression and western blot analyses of blood cell lineage-specific transcription factors, we confirmed that while the X. laevis LP hosts hematopoietic stem cells and MEPs, their BM contains GMPs. In support of our hypothesis that cells bearing GMP potential originate from the frog LP and migrate through blood circulation to the BM in response to chemical cues; we demonstrated that medium conditioned by the X. laevis BM chemoattracts LP and peripheral blood cells. Compared to LP and by examining a comprehensive panel of chemokine genes, we showed that the X. laevis BM possessed greater expression of a single chemokine, CXCL12, the recombinant form of which was chemotactic to LP and peripheral blood cells and appeared to be a major chemotactic component within BM-conditioned medium. In confirmation of the hepatic origin of the cells that give rise to these frogs' GMPs, we also demonstrated that the X. laevis BM supported the growth of their LP-derived cells.