Myocardial infarction (MI) remains a major cause of death in the western world. After MI, infarct-related hematopoiesis (IRH) in the bone marrow (BM) is activated in response to the excessive demand for inflammatory monocytes. IRH also triggers myeloid progenitor cells to seed the spleen for extramedullary hematopoiesis as splenic Ly6c^high monocytes are among the first cell populations to respond to injury. In this study, we aimed to investigate if extramedullary hematopoiesis after MI induces persistent long-term changes to the splenic monocyte compartment. 

MI was induced by permanent coronary ligation in C57BL/6 mice. Using flow cytometry, leukocyte subpopulations from various compartments were characterized in controls and on day 3 and day 28 after MI. Ly6C^high monocytes from bone marrow or spleen were subjected to transcriptomic profiling using bulk-RNA sequencing. Furthermore, scratch assays from bone marrow and splenic monocyte-derived macrophages were performed to screen for functional changes. In response to IRH splenic Ly6C^high monocytes increased in absolute numbers on day 3 after MI while other compartments resided in equilibrium. 

Intriguingly, by RNA sequencing splenic monocytes displayed a unique transcriptomic phenotype that was distinct from bone marrow monocytes already at baseline and persisted over time. On day 28 after MI, pathways from upregulated genes involved WNT signaling and pro-inflammatory pathways, which were not observed in BM monocytes. Furthermore, scratch assays revealed that macrophages derived from splenic monocytes on day 28 after MI were functionally activated with accelerated growth in comparison to macrophages derived from splenic monocytes on day 3 after MI, controls or the BM at any time after MI.

Our study shows that MI induces persistent changes in the transcriptional profile of splenic Ly6c^high monocytes and increases activity of monocyte derived macrophages in vitro, which may have relevant consequences for subsequent cardiovascular injury and can be a possible target for new therapies.