Biology of amphibian granulocytes - From evolutionary pressures to functional consequences

Granulocyte-lineage cells are important innate immune effectors across all vertebrates. Named for conspicuous secretory granules, granulocytes have historically been studied for their antimicrobial roles. Although versions of these cells are found in all vertebrate species examined to date, disparate environmental and physiological pressures acting on distinct vertebrate classes have shaped many of the facets dictating granulocyte biology. Immune pressures further determine granulopoietic constraints, ultimately governing granulocyte functions. For amphibians that inhabit pathogen-rich aquatic environments for some or all their lives, their unique granulocyte biologies satisfy many of their antimicrobial needs. Amphibians also occupy an intermediate position in the evolution of vertebrate immune systems, using combinations of primitive (e.g., subcapsular liver) and more recently evolved (e.g., bone marrow) tissue sites for hematopoiesis and specifically, granulopoiesis. The last decade of research has revealed vertebrate granulocytes in general, and amphibian granulocytes in particular, are more complex than originally assumed. With dynamic leukocyte phenotypes, granulocyte-lineage cells are being acknowledged for their multifaceted roles beyond immunity in other physiological processes. Here we provide an overview of granulopoiesis in amphibians, highlight key differences in these processes compared to higher vertebrates, and identify open questions.