Background: Chronic inflammation following myocardial infarction (MI) contributes to chronic ischemic heart failure (HF)and increased mortality. After MI, there are three phases of response: early inflammation, repair, and ventricular remodelling. Immune cells have diverse roles in these processes and immunosuppressive T regulatory cells have been shown to increase in the myocardium during the repair phase. However, the role of T-cells during ventricular remodelling/chronic phase is less understood. To better understand the immune cell kinetics and T-cell biology post-MI, we used single cell RNA sequencing (scRNA-seq) to delineate T-cell phenotypic changes post-MI.

Methods: To this end, we utilized cells from the non-cardiomyocyte fraction of cells in the mouse heart in an MI time course (day (d) 0, d1, d3, d7d, d14, d28 post-MI) using scRNA-seq (n=1 mouse/timepoint, n=10 780 cells total). For validation, we performed a second scRNA-seq post-MI time course and utilized single nucleus RNA sequencing (snRNA-seq) from human cardiac apex biopsies of healthy and chronic ischemic HF patients (n=3 donors, n=23 016 nuclei). 

Results: Predicted events post-MI related to neutrophil and monocyte infiltration were detected early in the first days of the time course and are consistent with the literature.  This was accompanied by a 1.5 -fold increase in T regulatory cell abundance at d1 relative to d0.  By d7, T regulatory cells proportions diminished and autoimmune Th17 cells increased through d28 (2.6-fold increase relative to d0, 1.6-fold increase relative to d7). Overall T-cell abundance is >2-fold in d28 hearts relative to d0.  T-cell subsets predominantly populated from d28 were phenotypically distinct in their upregulation of activation and homing markers (e.g. Cd69, Cxcr6, p<0.01).  Interestingly, markers for immunosuppression were nearly absent in T-cell subsets populated by d28, as seen in the diminution of Foxp1, Tcf7 and Lef1 genes relative to clusters populated by homeostasis (p<0.01). Expanded Th17 T cell populations could be confirmed in a second scRNA-seq cohort at d28 relative to homeostasis. Gene Ontology terms associated with upregulated genes were IL-17 signalling pathway and cellular response to tumor necrosis factor. Confirmation in a snRNA-seq validation study from human myocardial apex biopsies of healthy and chronic ischemic HF patients also show trending increases in T-cell abundance and T-cell activation (CD69).

Conclusions: T cell homeostatic imbalance resulting from diminished immunosuppressive T regulatory cells and expanded autoimmune Th17 cells during remodelling phase may induce tissue injury, dysfunction and aggravate heart failure. Disrupting this imbalance may serve as a useful target in chronic ischemic heart failure therapies.