Ly-6Chigh monocytes depend on Nr4a1 to balance both inflammatory and reparative phases in the infarcted myocardium.

RATIONALE Healing after myocardial infarction involves the biphasic accumulation of inflammatory lymphocyte antigen 6C (Ly-6C)(high) and reparative Ly-6C(low) monocytes/macrophages (Mo/MΦ). According to 1 model, Mo/MΦ heterogeneity in the heart originates in the blood and involves the sequential recruitment of distinct monocyte subsets that differentiate to distinct macrophages. Alternatively, heterogeneity may arise in tissue from 1 circulating subset via local macrophage differentiation and polarization. The orphan nuclear hormone receptor, nuclear receptor subfamily 4, group a, member 1 (Nr4a1), is essential to Ly-6C(low) monocyte production but dispensable to Ly-6C(low) macrophage differentiation; dependence on Nr4a1 can thus discriminate between systemic and local origins of macrophage heterogeneity. OBJECTIVE This study tested the role of Nr4a1 in myocardial infarction in the context of the 2 Mo/MΦ accumulation scenarios. METHODS AND RESULTS We show that Ly-6C(high) monocytes infiltrate the infarcted myocardium and, unlike Ly-6C(low) monocytes, differentiate to cardiac macrophages. In the early, inflammatory phase of acute myocardial ischemic injury, Ly-6C(high) monocytes accrue in response to a brief C-C chemokine ligand 2 burst. In the second, reparative phase, accumulated Ly-6C(high) monocytes give rise to reparative Ly-6C(low) F4/80(high) macrophages that proliferate locally. In the absence of Nr4a1, Ly-6C(high) monocytes express heightened levels of C-C chemokine receptor 2 on their surface, avidly infiltrate the myocardium, and differentiate to abnormally inflammatory macrophages, which results in defective healing and compromised heart function. CONCLUSIONS Ly-6C(high) monocytes orchestrate both inflammatory and reparative phases during myocardial infarction and depend on Nr4a1 to limit their influx and inflammatory cytokine expression.