Monocytes and macrophages in heart valves: Uninvited guests or critical performers?

Monocytes and macrophages are critical components of the myeloid niche of the innate immune system. In addition to traditional roles as phagocytes, this subsection of innate immunity has been implicated in its ability to regulate tissue homeostasis and inflammation across diverse physiological systems. Emergence of discriminatory features within the monocyte/macrophage niche within the last 5 years has helped to clarify specific function(s) of the subpopulations of these cells. It is becoming increasingly aware that these cells are likely implicated in valve development, disease, and tissue engineering outcomes. This review seeks to use current literature and opinions to show the diverse roles and potential contributions of this niche throughout valvulogenic processes, adult homeostatic function, valve disease mechanisms, and tissue engineering approaches.