Myocardial infarction promotes neutrophil-like monocyte expansion in the bone marrow via interleukin-6 signaling

BACKGROUND: Myocardial infarction (MI) triggers a systemic inflammatory response, mainly sustained by emergency hematopoiesis in the bone marrow (BM). While it is well established that hematopoietic progenitors and precursors react to MI by increasing leukocyte production, it remains elusive whether immune cell phenotypes also change during the inflammatory response.

METHODS&RESULTS: We performed single-cell RNA sequencing (scRNA-seq) on BM leukocytes from C57BL/6 mice at steady state and at 12-, 24-, 48-, and 72-hours post-MI (permanent ligation of the left ascending coronary artery). This analysis identified an expansion/the occurrence of a distinct subpopulation of Ly6C^high monocytes in response to infarction. In particular, this population expressed genes typically associated with neutrophils, including S100a9, S100a8, Lcn2, and Camp. Thus, this subpopulation was classified as “neutrophil-like monocytes” (Neu-Mo). We validated Neu-Mo dynamics over time in the BM, blood, and heart of infarcted mice using flow cytometry and identified this population as S100A9^highMHCII^negLy6C^high monocytes. Our analysis indicates that Neu-Mo represent a small monocyte subpopulation (6.98±2.92% of circulating monocytes). Following MI, Neu-Mo numbers start to rise first in the BM, then in the blood and ultimately in the heart at 48 hours post-infarction (15.36±9.03 cells/mg). These Neu-Mo kinetics were further confirmed using S100a9^EGFP/+ transgenic mice, which allow tracking of Neu-Mo post-MI.

We next investigated potential blood-borne factors driving Neu-Mo production. In-vitro experiments identified interleukin 6 (IL-6) as a primary inducer of Neu-Mo production. Wild-type mice injected with IL-6 confirmed Neu-Mo expansion in the BM (15,093±14,359 cells/femur vs. 98,025±57,775 cells/femur). Conversely, experiments in Il6^-/- mice demonstrated a significant reduction in Neu-Mo accumulation in the ischemic area 48 hours after MI (21.66±10.29 cells/mg vs. 4.139±2.382 cells/mg). To explore whether MI induces phenotypic shifts in monocyte precursors, we performed RNA-seq on sorted BM monocyte precursors at steady state and 12 hours post-MI. Monocyte precursors upregulate neutrophil-related genes such as Lcn2 and S100a9 in response to MI, indicating that Neu-Mo expansion is a direct consequence of phenotypic changes in upstream monocyte precursors.

CONCLUSIONS: Our findings identify a previously uncharacterized monocyte subpopulation that significantly contributes to the inflammatory response post-MI, highlighting the active role of the BM in modulating leukocyte phenotypes. Rather than representing a simple acceleration of baseline hematopoiesis, emergency hematopoiesis following MI involves a more specialized and branched hematopoietic process. This process facilitates the expansion of distinct subpopulations, such as Neu-Mo cells, which do not arise during typical hematopoiesis. Additionally, we demonstrate the critical role of IL-6 in regulating the heart-BM axis to promote Neu-Mo production. Ongoing research aims to elucidate the specific functions of Neu-Mo in the context of MI.