TLR4-Deficiency Prevents Inflammatory Polarization in Cardiac Mesenchymal Stromal Cells and Increases their Reparative Properties in Cell-Therapy after MI

Background:
Little is known about the potentially unfavorable effects of mesenchymal stromal cell (MSC) activation on the heart. MSCs can respond to tissue injury by anti or pro-inflammatory activation. We sought to study the potential negative interaction between left ventricular dysfunction (LVD) and MSC activation.

Methods and Results:
We isolated MSCs from cardiac (c) and subcutaneous (sc) fat of mice with LVD, 28 days after extensive myocardial infarction (MI), or sham operation. We found that LVD switched cMSCs toward an inflammatory phenotype, with increased secretion of inflammatory cytokines, as well as chemokines. While cell-therapy of cMSCs and scMSCs from LVD and sham hearts did not improve LV remodeling and function, transplantation of cMSCs from LVD worsened LV dilatation, 28 days after MI. The inflammatory polarization of cMSCs by LVD was mediated by Toll-like receptor 4 (TLR4), as we found less secretion of inflammatory cytokines and chemokines, and higher secretion of anti-inflammatory cytokines from activated cMSCs of TLR4-deficient mice, compared with control cMSCs. Significantly, TLR4-deficiency preserved the expression of the anti-phagocytic receptor; CD47 (“don’t eat me" signal) on cMSCs after inflammatory stimulation in vitro and increased their survival after transplantation into the infarcted heart. Compared with WT cMSCs and saline, TLR4 ^-/- cMSCs survived and maintained their reparative properties, reduced infarct size, increased scar thickness and attenuated LV dilatation, 7 days after MI.

Conclusions:
The environment of the failing and infarcted myocardium drives resident and transplanted MSCs toward a pro-inflammatory phenotype, and restricts their survival and reparative properties in a mechanism mediated by TLR4. Thus, TLR4 could be a therapeutic target to improve the outcome of cell-therapy for infarct repair.