A comparison of amphibian (Xenopus laevis) peritoneal- and bone marrow-derived mast cells

Mast cells are tissue-resident granulocytes that are often viewed through a negative lens due to their roles in allergies and hypersensitivity. While central to these conditions, mast cells have broader roles in vertebrate immune systems that are often overlooked. Whereas most mast cell research to date has been conducted in mammalian models, mast-like cells are found across vertebrates, presumably with conserved and divergent functions in disparate immune systems. Our past studies defined a role of amphibian (Xenopus laevis) mast cells in skin antifungal defenses but stopped short of functionally characterizing these immune effectors. Presently, to further elucidate the biology of amphibian mast cells and to delineate evolutionarily conserved and divergent mast cell characteristics, we developed in vitro X. laevis mast cell cultures from peritoneal-derived and bone marrow-derived precursors. The phenotypes, transcriptomes and degranulation capacities were examined and compared between these frog mast cell cultures and to those of frog neutrophils, a distinct granulocyte lineage. Our results indicate that X. laevis mast cells are distinct from frog neutrophils and share many canonical features that are associated with mammalian mast cells. Akin to their mammalian counterparts, frog mast cells are capable of degranulation and release potent immune proteins and lipid mediators. Notably, although many characteristics appear to be shared between frog and mammalian mast cells, our findings also indicate unique aspects to these frog leukocytes. Future studies of nonmammalian mast cells will grant unique perspectives of the evolutionary pressures that shape mast cell defenses and associated pathologies.