Page 179 - 2021_03-Haematologica-web

VWF-mediated thromboinflammation in stroke
stroke brain of VWF-deficient mice. Importantly, not only the overall number but also the density of neutrophils in the affected brain tissue was reduced. Furthermore, there was no direct link between the reduction in infarct size (2-fold) and the reduction of all subsets of leukocytes (2- to 5-fold). Together, these observations go against a potential nonspecific effect due to smaller infarct vol- umes.
Ischemic stroke induces an extensive inflammatory response, with various types of immune cells transmi- grating over the activated endothelium to invade the damaged brain.33 The key steps mediating the initial phase of leukocyte-mediated stroke brain damage are not yet fully resolved. During cerebral ischemia, the brain endothelium becomes rapidly activated, resulting in the upregulation of cell adhesion molecules.37 Together with transient disruption of the blood-brain barrier, this allows entry of leukocytes into the brain. Our results now sug- gest that VWF is an important molecule involved in the initial recruitment of inflammatory monocytes, T cells and neutrophils leading to reperfusion injury. The poten- tial role of monocytes in the acute phase of ischemic stroke is not clear27 but interactions between monocytes and VWF-platelet complexes have been shown to con- tribute to monocyte diapedesis in vitro.38 Both neutrophils and T cells have been found to cause cerebral ischemia/reperfusion injury by obstructing the microcir- culation, contributing to the no-reflow phenomenon.37,39,40
Interestingly, we observed a significant number of neu- trophils and T cells within the microvasculature of WT mice during the early phase after stroke. Whether these cells are in the process of extravasating or really plugging the microcirculation is hard to ascertain from our current study. Together with VWF/platelet aggregates, these intravascular immune cells can occlude brain capillaries and impair microcirculatory reperfusion. Our observa- tions are in line with those of other studies on neutrophil recruitment to the brain after stroke. Indeed, neutrophils have been found trapped in the cerebral microcirculation in both murine41,42 and baboon39 stroke models. Importantly, also in human stroke patients, intravascular neutrophil accumulation has been observed in post- mortem brain tissue.41,43 Similarly, regulatory T cells were found to increase cerebral thrombus formation, impair cerebral reperfusion and cause overall microvascular dys- function in the acute reperfusion phase in a murine stroke model.44 Accordingly, depletion of neutrophils or T cells in the acute phase of ischemic stroke has a protective effect.28,35,44-46 Our results now identify VWF as a key adhe- sion molecule that could be implicated in the recruitment of these immune cells, most likely via a platelet-depen- dent mechanism, potentially triggering microvascular obstructions in the reperfused ischemic stroke brain.
A key finding of this study is that we could pinpoint the pathogenic contribution of VWF to its A1 domain using a nanobody that specifically targets the platelet
AB
CD
Figure 4. Inhibition of the von Willebrand factor A1 domain protects mice from acute ischemic stroke. Transient focal cerebral ischemia was induced by occluding the right middle cerebral artery for 60 min, followed by 23 h of reperfusion. Immediately at the start of reperfusion, mice were intravenously treated with 10 mg/kg of either control (KB-VWF-004 bv) or inhibitory anti-VWF A1 nanobody (KB-VWF-006 bv). (A) Edema-corrected brain infarct volumes were quantified by planimetric analysis 24 h after stroke. (B) Representative TTC staining of three consecutive brain sections. (C) Motor function was examined using the grip test. (D) Neurological outcome 24 h after stroke was assessed using the Bederson test. Data are represented as scatter plots showing all data points and the median value, except for infarct size which is shown as mean ± standard deviation. *P<0.05; **P<0.01 (n=10-11).
haematologica | 2021; 106(3)
825