P6598MicroRNA 21 and Hypoxia Inducible Factor 1 synchronize the impact of B lymphocytes on cardiac function after acute myocardial infarction

Background Myocardial infarction (MI) is a severe ischemic disease responsible for heart failure and sudden death. Mature B lymphocytes have been shown to exacerbate tissue injury and deterioration of cardiac function after MI. However, the cellular and molecular mechanisms governing B cell deleterious effects in the ischemic milieu remain to be defined. Purpose In this study, we speculate that endogenous activation of the miR21/HIFα-related pathways mediates the effect of B lymphocytes on post-ischemic cardiac remodeling. Methods Acute MI was induced by permanent ligation of the left anterior descending artery in mice. Cardiac function and remodeling was determined by echocardiography and immunohistochemistry. Inflammatory cell number and phenotype were defined by FACS analysis. To evaluate the role of HIFα isoforms in B cells, we generated mice with B cell lineage specific (Cd79aCre/+) conditional deletion of HIF1α (HIF1αflox/flox), HIF2α (HIF2αflox/flox), or both isoforms (HIF1α-HIF2αflox/flox). Results Acute MI increased miR21 levels in B cells. miR21 deficient mice showed reduced B cell numbers in the spleen, blood and subsequently in the injured cardiac tissue. Transplantation of bone marrow derived cells isolated from miR21-deficient mice (miR21−/−) improved cardiac function and remodeling when compared to administration of wild-type (WT) bone marrow cells. Similarly, in Rag1−/− immunodeficient mice with acute MI, re-supplementation with miR21−/− B lymphocytes restored cardiac repair and function when compared to injection of WT B cells. miR21 abrogated PTEN contents and subsequently enhanced HIF1α levels in cultured B cells. B cell deletion of HIF1α, but not that of HIF2α, reduced B cell accumulation and improved cardiac function after MI. Mice, which were equally deficient in HIF1α and HIF2α, also exhibited abrogation of adverse ventricular remodeling and showed recovery of cardiac function after MI. Toll like receptor agonist, CpG, fostered the release of the monocyte chemo-attractant protein, Ccl7, in cultured WT B cells but not in miR21- or HIF1α- deficient B cells. Ccl7 circulating levels were also reduced in miR21−/− and Cd79aCre/+/HIF1α flox/flox animals after acute MI. Ccl7 down-regulation hampered Ly6Chigh monocyte infiltration in the ischemic myocardium, leading to decreased infarct size and interstitial fibrosis, supporting cardiac repair. Conclusion This work reveals a novel function for miR21/HIF1α related pathways in B lymphocyte dependent effect on cardiac function and remodeling in the setting of acute MI.