The healing myocardium sequentially mobilizes two monocyte subsets with divergent and complementary functions

Area of Science:

• Immunology
• Cardiovascular Biology
• Regenerative Medicine

Background:

• Monocytes/macrophages are crucial for myocardial infarction (MI) healing, performing functions like debris clearance and tissue remodeling.
• The specific roles and regulatory mechanisms of different monocyte subsets in MI healing remain largely unknown.
• Circulating monocyte heterogeneity suggests distinct contributions to post-ischemic cardiac repair.

Purpose of the Study:

• To investigate the distinct roles of monocyte subsets in the healing process following myocardial infarction (MI) in a mouse model.
• To elucidate the mechanisms by which different monocyte subsets are recruited and function during cardiac ischemic injury.
• To propose a model for monocyte participation in MI healing based on their distinct properties and temporal recruitment.

Main Methods:

• Analysis of monocyte subset dynamics in mouse models of myocardial infarction.
• Assessment of chemokine expression profiles in infarcted hearts over time.
• Investigation of monocyte subset recruitment via specific chemokine receptors (CCR2 and CX3CR1).
• Functional characterization of Ly-6C(hi) and Ly-6C(lo) monocyte subsets in vitro and in vivo.

Main Results:

• Two distinct phases of monocyte involvement in MI healing were identified, characterized by sequential recruitment of Ly-6C(hi) and Ly-6C(lo) monocytes.
• Ly-6C(hi) monocytes dominate early post-MI, exhibiting phagocytic, proteolytic, and inflammatory functions to clear damaged tissue.
• Ly-6C(lo) monocytes predominate later, with reduced inflammation and increased expression of vascular-endothelial growth factor, promoting angiogenesis and collagen deposition.
• Impaired healing was observed in atherosclerotic mice with chronic Ly-6C(hi) monocytosis, highlighting the importance of a balanced monocyte response.

Conclusions:

• Distinct monocyte subsets, Ly-6C(hi) and Ly-6C(lo), play sequential and specialized roles in myocardial infarction healing.
• The temporal and chemokine-mediated recruitment of these subsets is critical for effective cardiac repair and remodeling.
• Understanding these distinct monocyte functions offers potential therapeutic targets for improving outcomes after myocardial infarction.