The blood cells, or hemocytes, in Drosophila participate in the immune response through the production of antimicrobial peptides, the phagocytosis of bacteria, and the encapsulation of larger foreign particles such as parasitic eggs; these immune reactions are mediated by phylogenetically conserved mechanisms. The encapsulation reaction is analogous to the formation of granuloma in vertebrates, and is mediated by large specialized cells, the lamellocytes. The origin of the lamellocytes has not been formally established, although it has been suggested that they are derived from the lymph gland, which is generally considered to be the main hematopoietic organ in the Drosophila larva. However, it was recently observed that a subepidermal population of sessile blood cells is released into the circulation in response to a parasitoid wasp infection. We set out to analyze this phenomenon systematically. As a result, we define the sessile hemocytes as a novel hematopoietic compartment, and the main source of lamellocytes. cellular immunity ͉ lamellocytes ͉ parasitoid wasp ͉ plasmatocytes ͉ niche

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Title: Cell lineage tracing reveals the plasticity of the hemocyte lineages and of the hematopoietic compartments in Drosophila melanogaster. Drosophila is a result of a precisely regulated succession of intracellular and intercellular events, though the nature and extent of these interactions are not known. We describe here a cell lineage tracing system set up to analyze the development of hemocyte lineages and functionally distinct hemocyte subsets. This system allowed us to distinguish two major embryonic hemocyte lineages, the crq and Dot lineages, in two, physically separated compartments, the embryonic macrophages and the embryonic lymph gland. We followed the fate and development of these lineages in the construction of the larval hematopoietic compartments and during the cell mediated immune response, the encapsulation reaction. Our results revealed the considerable plasticity and concerted action of the hematopoietic compartments and the hemocyte lineages in the development of the innate immune system and in the course of the cell mediated immune response in Drosophila. Journal: Molecular Immunology