Persistent infiltration and pro-inflammatory differentiation of monocytes cause unresolved inflammation in brain arteriovenous malformation

Abstract

An abnormally high number of macrophages are present in human brain arteriovenous malformations (bAVM) with or without evidence of prior hemorrhage, causing unresolved inflammation that may enhance abnormal vascular remodeling and exacerbate the bAVM phenotype. The reasons for macrophage accumulation at the bAVM sites are not known. We tested the hypothesis that persistent infiltration and pro-inflammatory differentiation of monocytes in angiogenic tissues increase the macrophage burden in bAVM using two mouse models and human monocytes. Mouse bAVM was induced through deletion of AVM causative genes, Endoglin (Eng) globally or Alk1 focally, plus brain focal angiogenic stimulation. An endothelial cell and vascular smooth muscle cell co-culture system was used to analyze monocyte differentiation in the angiogenic niche. After angiogenic stimulation, the Eng-deleted mice had fewer CD68+ cells at 2 weeks (P=0.02), similar numbers at 4 weeks (P=0.97), and more at 8 weeks (P=0.01) in the brain angiogenic region compared with wild-type (WT) mice. Alk1-deficient mice also had a trend towards more macrophages/microglia 8 weeks (P=0.064) after angiogenic stimulation and more RFP+ bone marrow-derived macrophages than WT mice (P=0.01). More CD34+ cells isolated from peripheral blood of patients with ENG or ALK1 gene mutation differentiated into macrophages than those from healthy controls (P<0.001). These data indicate that persistent infiltration and pro-inflammatory differentiation of monocytes might contribute to macrophage accumulation in bAVM. Blocking macrophage homing to bAVM lesions should be tested as a strategy to reduce the severity of bAVM.

Pentraxin 3 level is elevated in hereditary hemorrhagic telangiectasia and reflects the severity of disease‐associated epistaxis

“…Previous studies on animal models have shown that proangiogenic or inflammatory stimuli are required for the development of the vascular lesions observed in HHT . More recently, increased macrophage recruitment, rather than angiogenic stimulation alone, was linked to brain AVM progression . These studies, taken together with our present finding of elevated levels of markers of vascular inflammation and in particular PTX3 in HHT patients, point toward persistent inflammation as an perhaps underestimated trait of the disease.…”

The role of endoglin in post-ischemic revascularization

“…This phenomenon can be explained by the reduced homing of monocytes in the injured tissue in Eng-deficient mice, as previously reported in other models [20,27,239]. However, Eng +/− animals showed more tissue macrophages at 8 weeks the stimulus, when arteriovenous malformations were already formed, resulting in persistent inflammation, which can be explained by an impaired clearance of Eng-deficient macrophages in the injured tissue (angiogenic region) [239,240].…”

“…Moreover, after distal middle cerebral artery occlusion, Eng +/− mice showed larger infarct/atrophic volumes and fewer infiltrating macrophages than wild type animals, suggesting that endoglin deficiency impairs brain injury recovery by impairing macrophage homing, delaying inflammation resolution, and reducing angiogenesis [239]. Furthermore, in an experimental model of brain arteriovenous malformations in mice induced by local injection of the VEGF gene, it has been reported that, compared with control mice, Eng-deleted mice show fewer macrophages homed to the angiogenic area 2 weeks after angiogenic stimulation [240]. This phenomenon can be explained by the reduced homing of monocytes in the injured tissue in Eng-deficient mice, as previously reported in other models [20,27,239].…”

Consensus guidelines for the use and interpretation of angiogenesis assays

“…These assays have been instrumental in the study of vascular biology in growth and development [6–8] but also play a key role in the design, development, and evaluation of drugs that positively or negatively modulate vessel function for the treatment of many diseases [9–11]. Some examples of where the use of such bioassays has been imperative are: (1) the development of angiostatic drugs for the treatment of cancer, ocular diseases, and other pathologic conditions where angiogenesis is implicated and also angiogenic treatment strategies in ischemic cardiovascular disease [12, 13], (2) screening of natural anti-angiogenic compounds [14], (3) the efforts to design combination therapies including angiogenesis inhibitors [15–20], (4) the unraveling of mechanisms regulating lymphangiogenesis [21, 22], (5) the interrelationship of angiogenesis and immunity [23–25], (6) the development of imaging as diagnostic strategy [26], (7) the study of drug resistance mechanisms [27–29], (8) development of compounds and strategies for the revascularization of ischemic injuries [30, 31], and (9) to improve the vascular fitness in aging vessels [32, 33]. The current paper describes a large collection of assays and techniques for the evaluation of angiogenesis and aims at explaining their respective advantages and limitations.…”