Human pericardial macrophages suppress cardiac fibrosis through cystatin C signaling after myocardial infarction.

The pericardium plays an important homeostatic role for the neighboring heart, providing both lubrication and structural support. In vivo models have further identified a protective role for the pericardium in modulating cardiac remodeling following myocardial infarction, possibly through the actions of tissue-resident pericardial macrophages. Using patient-derived pericardial samples, we establish that human pericardial immune cells directly inhibit cardiac fibroblast fibrotic activity, and this action is dampened following myocardial infarction. Using single-cell RNA sequencing of patient pericardial fluid cells, we identify two pericardial macrophage subsets that are uniquely altered in response to myocardial infarction, which contributes to a shift in their effector molecule expression profiles. We confirm that fibronectin-expressing human pericardial macrophages are the primary driver of the pericardial antifibrotic actions through the release of cystatin C. Finally, we establish cystatin C as a myeloid cell-derived cardioprotective effector molecule in an in vivo model of myocardial infarction. Collectively, we uncover a molecular mechanism of the local immune environment that regulates cardiac remodeling after myocardial infarction.