Inflammation in the central nervous system (CNS) and disruption of its immune privilege are major contributors to the pathogenesis of multiple sclerosis (MS) and of its rodent counterpart, experimental autoimmune encephalomyelitis (EAE). We have previously identified developmental endothelial locus-1 (Del-1) as an endogenous anti-inflammatory factor, which inhibits integrin-dependent leukocyte adhesion. Here we show that Del-1 contributes to the immune privilege status of the CNS. Intriguingly, Del-1 expression decreased in chronic active MS lesions and in the inflamed CNS in the course of EAE. Del-1-deficiency was associated with increased EAE severity, accompanied by increased demyelination and axonal loss. As compared to control mice, Del-1−/− mice displayed enhanced disruption of the blood brain barrier and increased infiltration of neutrophil granulocytes in the spinal cord in the course of EAE, accompanied by elevated levels of inflammatory cytokines, including IL-17. The augmented levels of IL-17 in Del-1-deficiency derived predominantly from infiltrated CD8+ T cells. Increased EAE severity and neutrophil infiltration due to Del-1-deficiency was reversed in mice lacking both Del-1 and IL-17-receptor, indicating a crucial role for the IL-17/neutrophil inflammatory axis in EAE pathogenesis in Del-1−/− mice. Strikingly, systemic administration of Del-1-Fc ameliorated clinical relapse in relapsing-remitting EAE. Therefore, Del-1 is an endogenous homeostatic factor in the CNS protecting from neuroinflammation and demyelination. Our findings provide mechanistic underpinnings for the previous implication of Del-1 as a candidate MS susceptibility gene and suggest that Del-1-centered therapeutic approaches may be beneficial in neuroinflammatory and demyelinating disorders.

No abstract

Bone marrow endothelium plays an important role in the homing of hematopoietic stem and progenitor cells upon transplantation, but surprisingly little is known on how the bone marrow endothelial cells regulate local permeability and hematopoietic stem and progenitor cells transmigration. We show that temporal loss of vascular endothelial-cadherin function promotes vascular permeability in BM, even upon low-dose irradiation. Loss of vascular endothelial-cadherin function also enhances homing of transplanted hematopoietic stem and progenitor cells to the bone marrow of irradiated mice although engraftment is not increased. Intriguingly, stabilizing junctional vascular endothelial-cadherin in vivo reduced bone marrow permeability, but did not prevent hematopoietic stem and progenitor cells migration into the bone marrow, suggesting that hematopoietic stem and progenitor cells use the transcellular migration route to enter the bone marrow. Indeed, using an in vitro migration assay, we show that human hematopoietic stem and progenitor cells predominantly cross bone marrow endothelium in a transcellular manner in homeostasis by inducing podosome-like structures. Taken together, vascular endothelial-cadherin is crucial for BM vascular homeostasis but dispensable for the homing of hematopoietic stem and progenitor cells. These findings are important in the development of potential therapeutic targets to improve hematopoietic stem and progenitor cell homing strategies.

In proliferative retinopathies, like proliferative diabetic retinopathy and retinopathy of prematurity (ROP), the hypoxia response is sustained by the failure of the retina to revascularise its ischaemic areas. Non-resolving retina ischaemia/hypoxia results in upregulation of pro-angiogenic factors and pathologic neovascularisation with ectopic, fragile neovessels. Promoting revascularisation of the retinal avascular area could interfere with this vicious cycle and lead to vessel normalisation. Here, we examined the function of endothelial junctional adhesion molecule-C (JAM-C) in the context of ROP. Endothelial-specific JAM-C-deficient (EC-JAM-C KO) mice and littermate JAM-C-proficient (EC-JAM-C WT) mice were subjected to the ROP model. An increase in total retinal vascularisation was found at p17 owing to endothelial JAM-C deficiency, which was the result of enhanced revascularisation and vessel normalisation, thereby leading to significantly reduced avascular area in EC-JAM-C KO mice. In contrast, pathologic neovessel formation was not affected by endothelial JAM-C deficiency. Consistent with improved vessel normalisation, tip cell formation at the interface between vascular and avascular area was higher in EC-JAM-C KO mice, as compared to their littermate controls. Consistently, JAM-C inactivation in endothelial cells resulted in increased spreading on fibronectin and enhanced sprouting in vitro in a manner dependent on β1-integrin and on the activation of the small GTPase RAP1. Together, endothelial deletion of JAM-C promoted endothelial cell sprouting, and consequently vessel normalisation and revascularisation of the hypoxic retina without altering pathologic neovascularisation. Thus, targeting endothelial JAM-C may provide a novel therapeutic strategy for promoting revascularisation and vessel normalisation in the treatment of proliferative retinopathies.

Actin-binding protein cortactin promotes pathogenesis of experimental autoimmune encephalomyelitis by supporting leukocyte infiltration into the central nervous system

Leukocyte entry into the CNS is a crucial step in the development of multiple sclerosis and its animal model experimental autoimmune encephalomyelitis (EAE). Adhesion molecules mediating the docking of leukocytes to the endothelium of the blood-brain barrier (BBB) represent valuable targets for interference with the disease. However, little is known about the adhesion and signaling mechanisms in endothelial cells that mediate the diapedesis through the BBB. Here, we show that conditional Tie-2-Cre driven gene inactivation of CD99L2 inhibits leukocyte entry into the CNS during active MOG -induced EAE and alleviates severity of the disease. No detrimental effect on the immune response was observed. The number of perivascular cuffs around vessels of the CNS was reduced, as was the number of inflammatory foci, sites of demyelination and expression levels of pro-inflammatory cytokines. Three-dimensional analysis of vibratome sections of the CNS revealed an accumulation of leukocytes between endothelial cells and the underlying basement membrane, whereas leukocyte docking to the luminal surface of the endothelium of the BBB was unaffected. Collectively, these results suggest that CD99L2 participates in the development of EAE by supporting diapedesis of leukocytes through the endothelial basement membrane of blood vessels of the BBB in the CNS.

No abstract

All leukocytes express chemokine receptor CXCR4 on their surface and efficiently migrate in response to its sole ligand CXCL12. Here we report for the first time that CXCR4 is also prominently expressed in erythroblasts, the bone marrow erythroid precursor cells but, instead of chemotaxis, CXCR4 signaling promoted erythroblast enucleation, a unique mammalian process of their asymmetrical division into an anuclear reticulocyte and a nucleus-containing pyrenocyte. Mechanistically, CXCL12 ligation of erythroblast CXCR4 triggered its endocytosis and translocation into multiple intracellular compartments. These included, surprisingly, the nuclear envelope and nucleus where CXCL12 and CXCR4 colocalized with either Gαi or phosphorylated β-arrestin1, respectively, suggesting an ongoing differential CXCR4 signaling around and within the erythroblast nucleus. Supporting the notion of intracellular signaling, stimulation of erythroblasts with CXCL12 caused a persistent phosphorylation of intracellular and intranuclear CXCR4 and a delayed activation of multiple protein kinases. Furthermore, CXCL12 induced erythroblast elongation, a condensation and ex-centric positioning of their nuclei and triggered rapid perinuclear Ca2+ transients, which led directly to erythroblast enucleation. The CXCR4-mediated sequential erythroblast responses taking place during ultimate steps of their development and culminating in enucleation pinpoint a new physiological role of CXCR4 and the bone marrow-derived CXCL12 in regulating erythrocyte production.