1. Centre de Biologie du Développement, UMR 5547 and IFR 109, CNRS and Université Paul Sabatier, Toulouse III, 118 route de Narbonne, 31062 Toulouse cedex 09, France
2. Jagiellonian University, The Institute of Zoology, 6 Ingardena Street, 30-060 Cracow, Poland
3. UPR 9022 du CNRS, IBMC, 15 rue René Descartes, 67084 Strasbourg, France
4. Present address: Museum of Zoology, 29 boulevard de la Victoire, 67000 Strasbourg, France.

Correspondence to: Michèle Crozatier¹ Correspondence and requests for materials should be addressed to M.C. (Email: crozat@cict.fr).

Abstract

Drosophila haemocytes (blood cells) originate from a specialized haematopoietic organ—the lymph gland. Larval haematopoietic progenitors (prohaemocytes) give rise to three types of circulating haemocytes: plasmatocytes, crystal cells and lamellocytes. Lamellocytes, which are devoted to encapsulation of large foreign bodies, only differentiate in response to specific immune threats, such as parasitization by wasps. Here we show that a small cluster of signalling cells, termed the PSC (posterior signalling centre)¹, controls the balance between multipotent prohaemocytes and differentiating haemocytes, and is necessary for the massive differentiation of lamellocytes that follows parasitization. Communication between the PSC and haematopoietic progenitors strictly depends on the PSC-restricted expression of Collier, the Drosophila orthologue of mammalian early B-cell factor. PSC cells act, in a non-cell-autonomous manner, to maintain JAK/STAT signalling activity in prohaemocytes, preventing their premature differentiation. Serrate-mediated Notch signalling from the PSC is required to maintain normal levels of col transcription. The key role of the PSC in controlling blood cell homeostasis is reminiscent of interactions between haematopoietic progenitors and their micro-environment in vertebrates^{2, 3, 4}, thus further highlighting the interest of Drosophila as a model system for studying the evolution of haematopoiesis and cellular innate immunity.

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