The Roles of Granulocytes in Amphibian Physiology and Immune Defenses
Open AccessInflammation is thought to have evolved as a means of reestablishing homeostasis, and thus is closely tied to many physiological and pathological processes. From the perspective of metazoan phylogenetics, granulocyte-lineage cells are relatively ancient and ubiquitous inflammatory mediators. Named for their conspicuous granules, these innate immune cells have historically been studied for their antimicrobial roles, primarily in higher vertebrates. Only recently though, has remarkable diversity of granulocyte form and function across and within species been illuminated. Discoveries of granulocyte subtypes and polarization states have ushered significant interest in and appreciation for these cell types, which are now known to participate in biological processes as varied as reproduction, metabolism, and development. Many features of the classical granulocyte subtypes (neutrophils, eosinophils, basophils, and mast cells) are conserved across distantly related vertebrate species. But each vertebrate class - each species - is subject to disparate selective pressures that presumably shaped the many differences which also exist within this cell lineage. Amphibians, facing distinct biotic and abiotic factors, certainly have granulocyte populations to meet the challenges imposed by those pressures. Yet almost the entirety of existing amphibian granulocyte literature is limited to superficial descriptions of cells morphologically resembling mammalian granulocytes. The current information gap, although daunting, is a source of grand scientific adventures. In this dissertation, I chronicle my own scientific adventures. I present the work of my collaborators and myself, designed to expand our knowledge of amphibian granulocyte biology. Toward this end, we studied this cell lineage in the African clawed frog Xenopus laevis through a series of comprehensive approaches. This anuran has served as a lucrative research tool, particularly for comparative immunologists, for over a century, offering biological insight where newer model organisms are inadequate. In the first part of this thesis, we explore the fundamental immune roles of several X. laevis granulocyte subtypes during host responses to key emerging amphibian pathogens. The experiments described in the second part of this thesis explore X. laevis granulocytes beyond canonical immunity. We instead shift our attention to their participation in non-pathological tissue remodeling, focusing on tail regression during metamorphosis.Altogether, this work unveils a great deal about amphibian granulocyte biology, but also represents just a modest start to fully understanding the variations of the cells that so elegantly meet species-specific needs. Indeed, “every living thing is a masterpiece, written by nature and edited by evolution.”
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