Le rejet aigu humoral (RAH) est une complication post-transplantation qui peut conduire à la dysfonction puis la perte du greffon. Le RAH est caractérisé, au niveau histologique, par la présence de lésions endothéliales et d’un infiltrat de macrophages intravasculaires. Cette étude avait pour objectif d’identifier les mécanismes et voies de signalisation impliqués dans l’altération des cellules endothéliales (CE) au cours du RAH et de nouvelles molécules régulatrices. Nous montrons que l’induction du ligand de la voie Notch Dll4 dans les CE et les macrophages est caractéristique du RAH dans les greffons cardiaques. L’expression endothéliale de Dll4et la sécrétion de l’IL-6 par les CE induit la polarisation des macrophages vers un profil pro-inflammatoire de type M1. Nous identifions aussi Dll4 comme un régulateur négatif et pro-apoptotique de la différentiation vers un phénotype suppressif de typeM2. Nous montrons que les cellules microvasculaires sont les cibles cellulaires privilégiées des anticorps non HLA préformés associés à certains RAH en transplantation rénale. Enfin, nous avons mis en évidence l’activité d’une famille de coumarines issues de végétaux sur l’expression endothéliale de molécules de l’inflammation et de l’immunité. Pour conclure,notre étude montre l’importance de la voie Notch dansl’inflammation liée au RAH et identifie DLL4 et IL-6comme de nouveaux médiateurs de l’inflammation et d’une interaction spatiale et fonctionnelle des CE et des macrophages. Ces travaux proposent donc DLL4 etl’IL-6 comme des cibles moléculaires et les coumarines comme nouvelles molécules bioactives pour le contrôle de l’inflammation et du RAH en transplantation. / Acute humoral rejection (RAH) is a post-transplantcomplication that can lead to dysfunction and graftloss. RAH is characterized histologically by thepresence of endothelial lesions and of an infiltrate ofintravascular macrophages. This study aimed toidentify the mechanisms, the signaling pathwaysinvolved in the alteration of endothelial cells (EC) upon RAH as well as new regulatory molecules fortherapeutic approaches. Here, we show that theinduction of the Notch ligand DLL4 in both the EC andmacrophages is a feature of RAH in cardiac allografts.The expression of Dll4 and secretion of IL-6 inducedpolarization of macrophages into proinflammatory M1-type. We further identify Dll4 as a negative and pro-apoptoticregulator of macrophage differentiation towards a suppressive phenotype M2-type. We foundthat glomerular microvascular cells are the cell targetsof preformed non HLA antibodies causing RAH inkidney transplantation. Finally, we have demonstratedthe inhibitory activity of a series of coumarins issued from plants on the endothelial expression of a panel of inflammation and immunity molecules. In conclusion,our study shows the importance of the Notch pathwayin inflammation-related RAH and identifies DLL4 and IL-6 as new mediators of inflammation and spatial andfunctional interaction of the EC and macrophages. Thiswork therefore propose DLL4 and IL-6 as moleculartargets and coumarins as new bioactive molecules forthe control of inflammation and RAH.

Multipotent haematopoietic stem cells (HSCs) supply the organism with mature blood cells of all lineages throughout adult life. These cells first originate in the dorsal aorta (DA) of the vertebrate embryo, and a multitude of signalling pathways regulate their specification in the embryo. The emergence of HSCs is dependent on appropriate arterial specification and vessel maturation, processes which are heavily dependent on Notch signalling. This arterial involvement of Notch obscures its later roles in HSC specification. The Notch ligand dll4 is crucially involved in arterial development in the mammalian embryo, while zebrafish embryos deficient for dll4 activity only exhibit minor arterial defects at the time of HSC emergence. Here, the zebrafish model has been exploited to reveal the first specific evidence for a role of dll4 in HSC specification. Dll4 is required for the expression of runx1, a transcription factor (TF) required for HSC specification, prior to any observed effects on vascular development. HSCs and all their derivatives are depleted in dll4 morphants. To disentangle the genetic requlatory cascade downstream of dll4 and upstream of runx1, RNA-seq was employed to discover downstream effectors of this signalling. Expression and functional screening of best candidate genes revealed seven genes with novel roles in HSC development. Foxc1b is a dll4 target predominantly mirroring the dll4 phenotype, and is thus likely to be the downstream effector of dll4, upstream of runx1. Interestingly, foxc1b also has a later dll4-independent role in HSC development, remarkably similar to that of cmyb. Taken together I show here for the first time a requirement of dll4 upstream of runx1 in HSC specification, mediated by foxc1b, followed by a later dll4-independent phase in HSC development.