Monocyte and macrophage function in respiratory viral infections

Pulmonary macrophages, such as tissue-resident alveolar and interstitial macrophages and recruited monocyte-derived macrophages, are the major macrophages present in the lungs during homeostasis and diseased conditions. While tissue-resident macrophages act as sentinels of the alveolar space and play an important role in maintaining homeostasis and immune regulation, recruited macrophages accumulate in the respiratory tract after acute viral infections. Despite sharing similar anatomical niches, these macrophages are distinct in terms of their origins, surface marker expression, and transcriptional profiles, which impart macrophages with distinguished characteristics in physiological and pathophysiological conditions. In this review, we summarize the current view on these macrophage populations, their shared functions, and what makes them distinct from each other in the context of homeostasis and respiratory viral infections.