Genetic Mapping of Monocyte Fate Decisions Following Myocardial Infarction

Inflammation contributes to the pathogenesis of cardiac disease and represents a viable therapeutic target for heart failure. Cardiac injury elicits recruitment of neutrophils, monocytes, and T-cells. Monocytes and their progeny represent are highly abundant, display incredible functional diversity, and are key determinants of myocardial inflammation. Much remains to be learned regarding mechanisms and signaling events that instruct monocyte fate decisions. We devised a genetic lineage tracing strategy using Ccr2crERT2Rosa2LSL-tdTomato mice in combination with single cell RNA-sequencing to map the fate and differentiation trajectories of monocytes that infiltrate the heart after reperfused myocardial infarction (MI). We observe that monocyte recruitment is restricted to the first 5 days following MI. Infiltrating monocytes give rise to transcriptionally distinct and spatially restricted macrophage and dendritic cell-like subsets, dynamically shift over time, and chronically persist within the myocardium. Pseudotime analysis predicted two differentiation trajectories of monocyte-derived macrophages that are initially partitioned into the border and infarct zones, respectively. Among these trajectories, we show that macrophages expressing a type I IFN responsive signature are an intermediate population localized within the border zone and promote myocardial protection. Collectively, these data uncover new complexities of monocyte differentiation in the infarcted heart and suggest that modulating monocyte fate decisions may have clinical implications. Overall design: We performed 90 minutes of closed chest ischemia of the left anterior descending coronary artery followed by reperfusion. For Ccr2:CreERT2,Rosa26:tdTomato mice, a single IP dose of 60mg/kg tamoxifen was given 1 day prior to injury. Hearts were collected at indicated timepoints and cells flow sorted for monocytes and macrophages for single cell RNA sequencing. For Cx3cr1:CreERT2,Rosa26:tdTomato mice, tamoxifen was injected IP daily for 5 days beginning on the day of instrumentation followed by 9 days of chase. Hearts were collected 2 days after injury and flow sorted for monocytes and macrophages and separated into tomato positive and negative libraries for single cell RNA sequencing. n=3 mice pooled for each library.