Key inflammatory pathways underlying vascular remodeling in pulmonary hypertension

Independent of the underlying cause, pulmonary hypertension (PH) remains a devastating condition that is characterized by limited survival. Cumulating evidence indicates that in addition to a dysbalance of mediators regulating vascular tone and growth factors promoting vascular remodeling, failure to resolve inflammation and altered immune processes play a pivotal role in the development and progression of PH. Here, we highlight the role of key inflammatory pathways in the pathobiology of vascular remodeling and PH, and discuss potential therapeutic interventions that may halt disease progression or even reverse pulmonary vascular remodeling. Perivascular inflammation is present in all forms of PH, and inflammatory pathways involve numerous mediators and cell types including macrophages, neutrophils, T cells, dendritic cells, and mast cells. Dysfunctional bone morphogenic protein receptor 2 (BMPR2) signaling and dysregulated immunity enable the accumulation of macrophages and other inflammatory cells in obliterative vascular lesions. Regulatory T cells (Tregs) were shown to be of particular relevance in the control of inflammatory responses. Key cytokines/chemokines include interleukin-6, functioning via classic or trans-signaling, macrophage migratory inhibitory factor (MIF), but also other mediators such as neutrophil-derived myeloperoxidase. The expanding knowledge on this topic has resulted in multiple opportunities for sophisticated therapeutic interventions.

P4146The six-transmembrane protein Stamp2 protects from hypoxia-induced pulmonary vascular remodeling and pulmonary hypertension via actions in mononuclear cells and CXCL12

Background

The six-transmembrane protein of prostate (Stamp2) is a potent anti-inflammatory player in adipocytes and also in macrophages. Stamp2's actions in these cells protects from diet-induced diabetes and from atherosclerosis mice. As chronic inflammation is a hallmark of pulmonary arterial hypertension (PH), we sought to investigate the role of Stamp2 in PH.

Methods and results

Morphometric analyses of small pulmonary arteries after 3 weeks of chronic hypoxia (10% O2) showed aggravated pulmonary vascular remodeling in Stamp2−/− mice as compared to WT, demonstrated by a significantly reduced number of non-muscularized vessels and higher extent of fully-muscularized vessels. Consequently, right ventricular systolic pressure (RVSP, Millar catheter via right jugular vein) was significantly higher in Stamp2−/− mice (33.4±0.7 mmHg vs. 30.3±1.4, p<0.05). As endothelial (EC) and smooth muscle cells (PASMC) are critical for remodeling processes in PH, the role of Stamp2 in these cells was explored. However, siRNA-mediated knock-down of Stamp2 in human microvascular EC had no effect on apoptotic susceptibility (CellDeath Detection ELISA), or release of IL-6 (qPCR). Furthermore, Stamp2-deficiency in isolated primary PASMC had no effect on proliferation (BrdU incorporation) and chemotaxis (modified Boyden chamber). As Stamp2 deficiency promotes higher expression of inflammatory cytokines (IL6, IL1b, MCP1, TNFa, CXCL12, qPCR) and increased numbers of CD68-positive cells in the lung, actions of Stamp2 in macrophages are potentially driving vascular remodeling in PH. To test this hypothesis, PASMC proliferation and chemotaxis were assessed in response to treatment with supernatants from primary thioglycolate-elicited peritoneal Stamp2−/− or WT-macrophages. These experiments revealed that supernatants from Stamp2−/− macrophages induced PASMC proliferation and chemotaxis significantly stronger, thus providing a link between inflammatory actions in Stamp2 deficiency and vascular remodeling. To gain further insights, a cytokine array was performed with supernatants from Stamp2−/− and WT-macrophages, revealing CXCL12 as the most relevant candidate. Experiments with neutralizing antibodies confirmed the role of CXCL12 in driving Stamp2's actions on vascular remodelling processes in PASMC.

Importantly, Stamp2 expression (qPCR, western blot analyses) was significantly lower in the lung of humans with idiopathic PAH (IPAH), as well as in experimental PH in rats (monocrotalin, sugen/hypoxia) and in mice (hypoxia).

Conclusions

Stamp2 deficiency aggravates hypoxia-induced pulmonary vascular remodeling and pulmonary hypertension in mice. On the cellular level, actions of Stamp2 in macrophages drive vascular remodelling processes in smooth muscle cells via secreted factors such as CXCL12. The finding of decreased expression of Stamp2 in human and various experimental forms of PH points towards a general protective role of Stamp2.

P4142PI3-kinase delta protects against atherosclerosis progression by governing regulatory T-cell biology

Introduction and purpose

Atherosclerosis is a chronic inflammatory disease of arteries and represents the main underlying cause of death worldwide. Macrophages are major drivers of atherosclerosis by ingestion of lipoproteins, foam cell formation, and secretion of pro-inflammatory mediators. Although macrophages outnumber other leukocytes in atherosclerotic plaques, T and B lymphocytes can shape the course of disease by promoting or mitigating inflammatory responses. Leukocytes highly express the phosphoinositide 3-kinase isoform delta (PI3Kd), exerting a key role in the regulation of immune responses including the activation, proliferation, differentiation, and effector function of lymphocytes. Since macrophages and lymphocytes are all major effectors of atherosclerosis, we aimed to understand the role of PI3Kd in these leukocytes during atherogenesis.

Methods and results

To investigate the role of haematopoietic PI3Kd in atherosclerosis, bone marrow from PI3Kd−/− or PI3Kd+/+ mice was transplanted into LDLR−/− mice. After 6 weeks of feeding on an atherogenic diet, PI3Kd−/− recipient LDLR−/− mice displayed significantly impaired CD4+ and CD8+ T-cell numbers, CD4+ T-cell activation, CD4+ effector T cells, and proatherogenic CD4+ T helper (Th) 1 responses in para-aortic lymph nodes and spleen compared with PI3Kd+/+ transplanted controls. Surprisingly, the net effect of PI3Kd deficiency was a substantial increase of aortic inflammation and atherosclerosis in LDLR−/− mice. Moreover, haematopoietic PI3Kd deficiency augmented macrophage accumulation in atherosclerotic plaques of LDLR−/− mice, whereas major macrophage functions including foam cell formation, efferocytosis, and cytokine secretion were unaffected by PI3Kd inactivation in these phagocytes. However, haematopoietic PI3Kd deficiency led to depletion of atheroprotective B-1 cells and reduction of proatherogenic B-2 cells in LDLR−/− mice. Moreover, haematopoietic PI3Kd deficiency caused a significant reduction of regulatory CD4+ T cells (Tregs) in plaques, para-aortic lymph nodes, and spleen of LDLR−/− mice. Furthermore, PI3Kd−/− Tregs exhibited reduced secretion of anti-inflammatory cytokines IL-10 and TGF-b as well as impaired suppression of CD4+ T-cell proliferation. Consequently, adoptive transfer of PI3Kd+/+ Tregs fully constrains the atherosclerotic burden in PI3Kd−/− transplanted LDLR−/− mice without affecting B cell numbers.

Conclusions

We demonstrate that PI3Kd plays a crucial role in B lymphocytes, Th1 cells, and Tregs during atherogenesis. Lack of PI3Kd signalling in atheroprotective Treg responses outplays its impact on proatherogenic Th1 responses, thus leading to aggravated atherosclerosis. Hence, PI3Kd is a key regulator of Treg biology and thereby protects against atherosclerosis progression.

Acknowledgement/Funding

Center for Molecular Medicine Cologne (CMMC) and the Marga and Walter Boll-Stiftung

P1937Deficiency of PI 3-kinase isoform p110alpha in smooth muscle cells impairs vascular integrity and promotes aortic aneurysm formation

Background

The pathobiology of aortic aneurysms is characterized by vascular inflammation, extracellular matrix degeneration, and particularly by loss and dedifferentiation of vascular smooth muscle cells (SMCs). In SMCs, the PI 3-kinase isoform p110α mediates receptor tyrosine kinase dependent proliferation, chemotaxis, and survival. Smooth muscle specific p110α deficient mice (SM-p110α−/− mice) display reduced medial wall thickness, substantially reduced neointima formation and media hypertrophy after balloon injury of the carotid artery.

Objective

We hypothesized that loss of p110α signaling impairs vascular integrity and promotes development and progression of abdominal aortic aneurysms (AAA). We aimed to elucidate the impact of p110α deficiency on vascular integrity, SMC phenotypic modulation, vascular inflammation, and AAA formation.

Methods and results

Ultra-structural characterization of aortic wall morphology in abdominal aortas from SM-p110α−/− mice by transmission electron microscopy (TEM) revealed disarranged structure of tunica media as indicated by disorganized elastic fibers, detached SMCs, and elastic fiber breaks. Western blots showed reduced elastin and fibrillin expression in SMCs from p110α−/− mice. Media thickness was significantly reduced in abdominal aortas from SM-p110α−/− mice compared to wild type (WT) controls (29.0±3.1 vs. 42.5±4.1 μm). Lack of p110α decreased expression of differentiation markers SM-α-actin and SM-MHC. p110α deficiency significantly diminished responsiveness of aortic rings to vasodilator acetylcholine. These data indicate loss of differentiation and impaired contractility of p110α−/− SMCs. We subjected SM-p110α−/− mice and WT littermate controls to the porcine pancreatic elastase (PPE) model of AAA. PPE was infused into the infrarenal aorta, respectively, to induce AAA formation. Ultrasonic examination of abdominal aortas demonstrated an enlarged aortic diameter in PPE challenged mice. AAA formation was significantly (p<0.01) enhanced in SM-p110α−/− (0.46±0.12 mm, n=8) compared to SM-p110α+/+ mice (0.18±0.03 mm, n=4). These data indicate a protective function of p110α in AAA formation. Immunocytochemistry of the aortic medial compartment from PPE-perfused SM-p110α−/− mice revealed significantly increased MOMA-2+ monocyte/macrophage content indicating augmented aortic inflammation during AAA formation compared to WT controls. Furthermore, SMCs from SM-p110α−/− mice expressed reduced amounts of anti-inflammatory angiopoietin1 compared to p110α+/+ SMCs. Moreover, frequent apoptotic/necrotic SMCs were found in the aortic media of SM-p110α−/− mice by TEM, potentially contributing to vascular inflammation in a critical fashion.

Conclusion

These data indicate that p110α signaling critically contributes to vascular integrity via maintaining SMC plasticity, elastic fiber homeostasis, and anti-inflammatory processes. Consequently, lack of proper p110α signaling promotes progression of AAA formation.

[Pathobiology, pathology and genetics of pulmonary hypertension: Recommendations of the Cologne Consensus Conference 2016]

The 2015 European Guidelines on Diagnosis and Treatment of Pulmonary Hypertension (PH) are also valid for Germany. While the guidelines contain detailed recommendations regarding clinical aspects of pulmonary arterial hypertension (PAH) and other forms of PH, they contain only a relatively short paragraph on novel findings on the pathobiology, pathology, and genetics. However, these are of great importance for our understanding of this complex disease both from a clinical and scientific point of view, and they are essential for the development of novel treatment strategies. To this end, a number of current data are relevant, prompting a detailed commentary to the guidelines, and the consideration of new scientific data. In June 2016, a Consensus Conference organized by the PH working groups of the German Society of Cardiology (DGK), the German Society of Respiratory Medicine (DGP) and the German Society of Pediatric Cardiology (DGPK) was held in Cologne, Germany. This conference aimed to solve practical and controversial issues surrounding the implementation of the European Guidelines in Germany. To this end, a number of working groups was initiated, one of which was specifically dedicated to the pathobiology, pathology and genetics of PH. This article summarizes the results and recommendations of this working group.