Fibroblasts, which are traditionally recognized as a quiescent cell responsible for extracellular matrix production, are more and more appreciated as an active key player of the immune system. This review describes how fibroblasts and immune cells reciprocally influence the pathogenesis of fibrosis. An overview is given how fibroblasts are triggered by components of the innate and adaptive immunity on the one hand and how fibroblasts modulate immune cell behaviour via conditioning the cellular and cytokine microenvironment on the other hand. Finally, latest insights into the role of cardiac fibroblasts in the orchestration of inflammatory cell infiltration in the heart, and their impact on heart failure, are outlined.

AimsCoxsackievirus B3 (CVB3)-induced myocarditis, initially considered a sole immune-mediated disease, also results from a direct CVB3-mediated injury of the cardiomyocytes. Mesenchymal stem cells (MSCs) have, besides immunomodulatory, also anti-apoptotic features. In view of clinical translation, we first analysed whether MSCs can be infected by CVB3. Next, we explored whether and how MSCs could reduce the direct CVB3-mediated cardiomyocyte injury and viral progeny release, in vitro, in the absence of immune cells. Finally, we investigated whether MSC application could improve murine acute CVB3-induced myocarditis.Methods and resultsPhase contrast pictures and MTS viability assay demonstrated that MSCs did not suffer from CVB3 infection 4–12–24–48 h after CVB3 infection. Coxsackievirus B3 RNA copy number decreased in this time frame, suggesting that no CVB3 replication took place. Co-culture of MSCs with CVB3-infected HL-1 cardiomyocytes resulted in a reduction of CVB3-induced HL-1 apoptosis, oxidative stress, intracellular viral particle production, and viral progeny release in a nitric oxide (NO)-dependent manner. Moreover, MSCs required priming via interferon-γ (IFN-γ) to exert their protective effects. In vivo, MSC application improved the contractility and relaxation parameters in CVB3-induced myocarditis, which was paralleled with a reduction in cardiac apoptosis, cardiomyocyte damage, left ventricular tumour necrosis factor-α mRNA expression, and cardiac mononuclear cell activation. Mesenchymal stem cells reduced the CVB3-induced CD4− and CD8− T cell activation in an NO-dependent way and required IFN-γ priming.ConclusionWe conclude that MSCs improve murine acute CVB3-induced myocarditis via their anti-apoptotic and immunomodulatory properties, which occur in an NO-dependent manner and require priming via IFN-γ.

Material

Systemic application of mesenchymal stromal cells (MSCs) in inflammatory cardiomyopathy exerts cardiobeneficial effects. The mode of action is unclear since a sufficient and long-acting cardiac homing of MSCs is unlikely. We therefore investigated the regulation of the immune response in coxsackievirus B3 (CVB3)-induced acute myocarditis after intravenous application of MSCs. Wildtype mice were infected with CVB3 and treated with either PBS, human MSCs or human cardiac fibroblasts intravenously 1 day after infection. Seven days after infection, MSCs could be detected in the spleen, heart, pancreas, liver, lung and kidney, whereby the highest presence was observed in the lung. MSCs increased significantly the myocardial expression of HGF and decreased the expression of the proinflammatory cytokines TNFα, IL1β and IL6 as well as the severity of myocarditis and ameliorated the left ventricular dysfunction measured by conductance catheter. MSCs upregulated the production of IFNγ in CD4+ and CD8+ cells, the number of IL10-producing regulatory T cells and the apoptosis rate of T cells in the spleen. An increased number of CD4+CD25+FoxP3 could be found in the spleen as well as in the circulation. In contrast, application of human cardiac fibroblasts had no effect on the severity of myocarditis and the systemic immune response observed after MSCs-administration. In conclusion, modulation of the immune response in extracardiac organs is associated with cardiobeneficial effects in experimental inflammatory cardiomyopathy after systemic application of MSCs.

NOD2 is an important mediator in the viral uptake and inflammatory response during the pathogenesis of CVB3 myocarditis.

BackgroundUnder conventional heart failure therapy, inflammatory cardiomyopathy typically has a progressive course, indicating a need for alternative therapeutic strategies to improve long-term outcomes. We recently isolated and identified novel cardiac-derived cells from human cardiac biopsies: cardiac-derived adherent proliferating cells (CAPs). They have similarities with mesenchymal stromal cells, which are known for their anti-apoptotic and immunomodulatory properties. We explored whether CAPs application could be a novel strategy to improve acute Coxsackievirus B3 (CVB3)-induced myocarditis.Methodology/Principal FindingsTo evaluate the safety of our approach, we first analyzed the expression of the coxsackie- and adenovirus receptor (CAR) and the co-receptor CD55 on CAPs, which are both required for effective CVB3 infectivity. We could demonstrate that CAPs only minimally express both receptors, which translates to minimal CVB3 copy numbers, and without viral particle release after CVB3 infection. Co-culture of CAPs with CVB3-infected HL-1 cardiomyocytes resulted in a reduction of CVB3-induced HL-1 apoptosis and viral progeny release. In addition, CAPs reduced CD4 and CD8 T cell proliferation. All CAPs-mediated protective effects were nitric oxide- and interleukin-10-dependent and required interferon-γ. In an acute murine model of CVB3-induced myocarditis, application of CAPs led to a decrease of cardiac apoptosis, cardiac CVB3 viral load and improved left ventricular contractility parameters. This was associated with a decline in cardiac mononuclear cell activity, an increase in T regulatory cells and T cell apoptosis, and an increase in left ventricular interleukin-10 and interferon-γ mRNA expression.ConclusionsWe conclude that CAPs are a unique type of cardiac-derived cells and promising tools to improve acute CVB3-induced myocarditis.

Mesenchymal stromal cell (MSC) application in Coxsackievirus B3 (CVB3)‐induced myocarditis reduces myocardial inflammation and fibrosis, exerts prominent extra‐cardiac immunomodulation, and improves heart function. Although the abovementioned findings demonstrate the benefit of MSC application, the mechanism of the MSC immunomodulatory effects leading to a final cardioprotective outcome in viral myocarditis remains poorly understood. Monocytes are known to be a trigger of myocardial tissue inflammation. The present study aims at investigating the direct effect of MSC on the mobilization and trafficking of monocytes to the heart in CVB3‐induced myocarditis. One day post CVB3 infection, C57BL/6 mice were intravenously injected with 1 x 106 MSC and sacrificed 6 days later for molecular biology and flow cytometry analysis. MSC application reduced the severity of myocarditis, and heart and blood pro‐inflammatory Ly6Chigh and Ly6Cmiddle monocytes, while those were retained in the spleen. Anti‐inflammatory Ly6Clow monocytes increased in the blood, heart, and spleen of MSC‐treated CVB3 mice. CVB3 infection induced splenic myelopoiesis, while MSC application slightly diminished the spleen myelopoietic activity in CVB3 mice. Left ventricular (LV) mRNA expression of the chemokines monocyte chemotactic protein‐1 (MCP)−1, MCP‐3, CCL5, the adhesion molecules intercellular adhesion molecule‐1, vascular cell adhesion molecule‐1, the pro‐inflammatory cytokines interleukin‐6, interleukin‐12, tumor necrosis factor‐α, the pro‐fibrotic transforming growth factorβ1, and circulating MCP‐1 and MCP‐3 levels decreased in CVB3 MSC mice, while LV stromal cell‐derived factor‐1α RNA expression and systemic levels of fractalkine were increased in CVB3 MSC mice. MSC application in CVB3‐induced myocarditis modulates monocytes trafficking to the heart and could be a promising strategy for the resolution of cardiac inflammation and prevention of the disease progression. Stem Cells Translational Medicine 2017;6:1249–1261

Inflammation in myocarditis induces cardiac injury and triggers disease progression to heart failure. NLRP3 inflammasome activation is a newly identified amplifying step in the pathogenesis of myocarditis. We previously have demonstrated that mesenchymal stromal cells (MSC) are cardioprotective in Coxsackievirus B3 (CVB3)-induced myocarditis. In this study, MSC markedly inhibited left ventricular (LV) NOD2, NLRP3, ASC, caspase-1, IL-1β, and IL-18 mRNA expression in CVB3-infected mice. ASC protein expression, essential for NLRP3 inflammasome assembly, increased upon CVB3 infection and was abrogated in MSC-treated mice. Concomitantly, CVB3 infection in vitro induced NOD2 expression, NLRP3 inflammasome activation and IL-1β secretion in HL-1 cells, which was abolished after MSC supplementation. The inhibitory effect of MSC on NLRP3 inflammasome activity in HL-1 cells was partly mediated via secretion of the anti-oxidative protein stanniocalcin-1. Furthermore, MSC application in CVB3-infected mice reduced the percentage of NOD2-, ASC-, p10- and/or IL-1β-positive splenic macrophages, natural killer cells, and dendritic cells. The suppressive effect of MSC on inflammasome activation was associated with normalized expression of prominent regulators of myocardial contractility and fibrosis to levels comparable to control mice. In conclusion, MSC treatment in myocarditis could be a promising strategy limiting the adverse consequences of cardiac and systemic NLRP3 inflammasome activation.