Single Cell Sequencing of Mouse Heart Immune Infiltrate in Pressure Overload-Driven Heart Failure Reveals Extent of Immune Activation.

Background: Inflammation is a key component of cardiac disease, with macrophages and T lymphocytes mediating essential roles in the progression to heart failure (HF). Nonetheless, little insight exists on other immune subsets involved in the cardiotoxic response. Methods: Here we used single-cell RNA sequencing to map the cardiac immune composition in the standard murine non-ischemic, pressure-overload HF model. By focusing our analysis on CD45+ cells, we obtained a higher resolution identification of the immune cell subsets in the heart, at early and late stages of disease and in controls. We then integrated our findings using multi-parameter flow cytometry, immunohistochemistry and tissue clarification immunofluorescence, in mouse and human. Results: We found that most major immune cell subpopulations, including macrophages, B cells, T cells and regulatory T cells (Treg), dendritic cells, NK cells, neutrophils and mast cells are present in both healthy and diseased hearts. Most cell subsets are found within the myocardium, whilst mast cells are found also in the epicardium. Upon induction of pressure overload, immune activation occurs across the entire range of immune cell types. Activation led to upregulation of key subset-specific molecules, such as Osm in pro-inflammatory macrophages and PD-1 in Treg, that may help explain clinical findings such as the refractivity of HF patients to anti-TNF therapy and cardiac toxicity during anti-PD-1 cancer immunotherapy, respectively. Conclusions: Despite the absence of infectious agents or an autoimmune trigger, induction of disease leads to immune activation that involves far more cell types than previously thought, including neutrophils, B cells, NK cells and mast cells. This opens up the field of cardio-immunology to further investigation using toolkits that have already been developed to study the immune subsets above. The subset-specific molecules that mediate their activation may thus become useful targets for diagnostics or therapy of HF.