Rationale: Hypertension is a major risk factor for cerebral small vessel disease, the most prevalent cause of vascular cognitive impairment. As we have shown, hypertension induced by a prolonged Angiotensin II infusion is associated with increased permeability of the blood-brain barrier (BBB), chronic activation of microglia and myelin loss. In this study we therefore aim to determine the contribution of microglia to hypertension-induced cognitive impairment in an experimental hypertension model by a pharmacological depletion approach. Methods: For this study, adult Cx3Cr1 gfp/wt x Thy1 yfp/0 reporter mice were infused for 12 weeks with Angiotensin II or saline and subgroups were treated with PLX5622, a highly selective CSF1R tyrosine kinase inhibitor. Systolic blood pressure (SBP) was measured via tail-cuff. Short- and long-term spatial memory was assessed during an Object Location task and a Morris Water Maze task (MWM). Microglia depletion efficacy was assessed by flow cytometry and immunohistochemistry. BBB leakages, microglia phenotype and myelin integrity were assessed by immunohistochemistry. Results: SBP, heart weight and carotid pulsatility were increased by Ang II and were not affected by PLX5622. Short-term memory was significantly impaired in Ang II hypertensive mice, and partly prevented in Ang II mice treated with PLX5622. Histological and flow cytometry analysis revealed almost complete ablation of microglia and a 60% depletion of brain resident perivascular macrophages upon CSF1R inhibition. Number and size of BBB leakages were increased in Ang II hypertensive mice, but not altered by PLX5622 treatment. Microglia acquired a pro-inflammatory phenotype at the site of BBB leakages in both Saline and Ang II mice and were successfully depleted by PLX5622. There was however no significant change in myelin integrity at the site of leakages. Conclusion: Our results show that depletion of microglia and PVMs, by CSF1R inhibition prevents short-term memory impairment in Ang II induced hypertensive mice. We suggest this beneficial effect is mediated by the major decrease of pro-inflammatory microglia within BBB leakages. This novel finding supports the critical role of brain immune cells in the pathogenesis of hypertension-related cognitive impairment. An adequate modulation of microglia /PVM density and phenotype may constitute a relevant approach to prevent and/or limit the progression of vascular cognitive impairment.

Chronic Graft-versus-Host Disease is a life-threatening inflammatory condition that affects many patients after allogeneic hematopoietic stem cell transplantation. Although we have made substantial progress in understanding disease pathogenesis and the role of specific immune cell subsets, treatment options are still limited. To date, we lack a global understanding of the interplay between the different cellular players involved, in the affected tissues and at different stages of disease development and progression. In this review we summarize our current knowledge on pathogenic and protective mechanisms elicited by the major involved immune subsets, being T cells, B cells, NK cells and antigen presenting cells, as well as the microbiome, with a special focus on intercellular communication of these cell types via extracellular vesicles as up-and-coming fields in chronic Graft-versus-Host Disease research. Lastly, we discuss the importance of understanding systemic and local aberrant cell communication during disease for defining better biomarkers and therapeutic targets, eventually enabling the design of personalized treatment schemes.

Rationale: Hypertension is a major risk factor for cerebral small vessel disease, the most prevalent cause of vascular cognitive impairment. As we have shown, hypertension induced by a prolonged Ang II infusion is associated with increased permeability of the blood-brain barrier (BBB) and chronic activation of microglia. In this study we therefore aim to determine the contribution of microglia to hypertension-induced cognitive impairment in an experimental hypertension model by a pharmacological depletion approach.Methods: For this study, adult Cx3Cr1 gfp/wt xThy1 yfp/0 reporter mice were infused for 12 weeks with Angiotensin II or saline and subgroups were treated with PLX5622, a highly selective CSF-1R inhibitor. Systolic blood pressure (SBP) was measured via tail-cuff. Short-and long-term spatial memory were assessed during an Object Location task and a Morris Water Maze task (MWM). At the end of the study, microglia depletion efficacy and BBB leakages were assessed using flow cytometry and immunohistochemistry.Results: SBP, heart weight and carotid pulsatility were increased by Ang II and were not affected by PLX5622. Short-term memory was significantly impaired in Ang II hypertensive mice, but not in Ang II mice treated with PLX5622. Histological and flow cytometry analyses revealed almost complete ablation of microglia upon CSF1R inhibition, while brain resident perivascular macrophages, were reduced by 60%. Number and size of BBB leakages were increased in Ang II hypertensive mice, but not altered by PLX5622 treatment. Conclusion:Our results show that depletion of microglia, and less so PVMs, by CSF1R inhibition prevents short-term memory impairment in Ang II induced hypertensive mice. This novel finding supports the critical role of brain immune cells, and most in particular microglia, in the pathogenesis of hypertension-related cognitive impairment. VCI vascular cognitive impairment;WMH white matter hyperintensities.

Haematopoietic stem cell transplantation (HSCT) is the treatment of choice for malignant haematological diseases. Despite continuous improvements in pre- and post-transplantation procedures, the applicability of allo-HSCT is limited by life-threatening complications such as graft-versus-host disease (GvHD), engraftment failure, and opportunistic infections. Extracorporeal photopheresis (ECP) is used to treat steroid resistant GvHD with significant success. However, the molecular mechanisms driving its immunomodulatory action, whilst preserving immune function, require further understanding. As ECP is safe to administer with few significant adverse effects, it has the potential for earlier use in the post-HSCT treatment of GvHD. Thus, further understanding the immunomodulatory mechanisms of ECP action may justify more timely use in clinical practice, as well as identify biomarkers for using ECP as first line or pre-emptive GvHD therapy. This review aims to discuss technical aspects and response to ECP, review ECP as an immunomodulatory treatment modality for chronic GvHD including the effect on regulatory T cells and circulating vs. tissue-resident immune cells and consider the importance of emerging biomarkers for ECP response.