Macrophage transcriptome changes in response to cardiac injuries are determined by the type of lesion rather than their ontogenetic nature

Abstract Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): CRC1425 - DFG Introduction Resident macrophages account for 5% of the cells in the healthy heart. In response to cardiac injury, monocytes infiltrate and differentiate into recruited macrophages complementing the original resident macrophage population, and together orchestrate cardiac remodeling. We aim to identify the extent to which macrophage origin, tissue location and type of cardiac injury determine macrophage phenotypes in ischemic and non-ischemic cardiac injuries over time. Results and methods We use a reporter mouse line CX3CR1Yfp CreER/+:R26tdT/+ to visualize and quantify fluxes of resident and recruited macrophages and their respective localizations within the heart following ischemia and reperfusion (I/R) injury and pressure overload after transversal aortic constriction (TAC), respectively. Interestingly, macrophage numbers peaked during the first week post-surgery in both the local/acute and the global/continuous injury models. Initially, recruited macrophages outnumbered the resident macrophage pool within the infarct area but ultimately reached a near 1:1 equilibrium at 4 weeks post I/R injury. In the remote myocardium the 1:1 equilibrium established already during the first week. In TAC-injured hearts, the 1:1 ratio of recruited and resident hearts also established within the first week post-surgery and persisted up to 8 weeks of follow-up even as cardiac function deteriorated. In the infarct, recruited macrophages showed increased cytokine synthesis, defensive response, and cell adhesion pathways during the first week after injury. Many inflammatory pathways remained activated up to day 28 post MI despite simultaneous activation of the anti-inflammatory response. Formerly resident macrophages and their progeny also mounted an immune response throughout the healing process. Unexpectedly, by day 28 post MI when a stable scar had formed, resident and recruited macrophage population transcriptomes converted onto a common, pro-inflammatory steady state phenotype, that differed from their originally divergent gene expression profiles in the healthy heart by over one thousand genes. In the remote zone, however, resident macrophages maintained cellular homeostasis function, unlike recruited macrophages that retained an inflammatory program. Following TAC surgery, transcriptional profiles remained differentially regulated at all time points tested, where recruited subset induced immune response and resident macrophages upregulated tissue remodeling genes. Conclusion Ischemic and non-ischemic cardiac injuries induce a transient peak of monocyte recruitment and macrophage differentiation in and around cardiac lesions which lead to a permanent integration of recruited monocyte-derived macrophages into the pool of tissue resident macrophages – albeit at different paces. The type of injury and macrophage localization within the changing tissue microenvironment determine partial or complete override of ontogenic cell programs.