Morphological and functional characterization of honey bee, Apis mellifera, hemocyte cell communities

Abstract

-Among invertebrates, cellular innate immunity is critical for wound healing and defense against parasites and pathogens. While the study of cellular immunity has received much attention in model insects, the study of hemocytes, including immune cells, in honey bees has received little attention. Much of our understanding of honey bee hemocytes is derived from a limited set of methodologies, predominately utilizing bright-field microscopy, which makes broad conclusions about honey bee cellular immunity difficult to infer. We build upon existing methodologies using differential cell staining, in vitro phagocytosis assays, and an analysis of respiratory burst activity as measured through reactive oxygen species (ROS) production. Further, we characterize the morphological diversity and functional capacity of honey bee hemocytes in both adult workers and young queen bees as well as the ontogeny of the hemocyte population across larval and adult stages of the worker caste. Our findings suggest that granulocytes are the major phagocytic cells in honey bees and that circulating larval granulocytes undergo mitotic cell division. Additionally, we demonstrate that ROS production in larval hemocytes can be stimulated with the protein kinase C activator, phorbol 12-myristate 13-acetate. This indicates the presence of a functional protein kinase C-dependent phagocyte oxidase system, though further experimentation is needed to confirm phagocyte oxidase as the source of ROS. Overall, this work expands our knowledge of honey bee hemocytes and provides additional methodological tools for studying immune mechanisms in insects. cellular immunity / plasmatocytes / granulocytes / phagocytosis / ROS production

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