Tissue-Resident Macrophages in the Control of Infection and Resolution of Inflammation.

Macrophage, as an integral component of the immune system and the first responder to local damage, is on the front line of defense against infection. Over the past century, the prevailing view of macrophage origin states that all macrophage populations resided in tissues are terminally differentiated and replenished by monocytes from bone-marrow progenitors. Nonetheless, this theory has been reformed by ground-breaking discoveries from the past decades. It is now believed that tissue-resident macrophages (TRMs) are originated from the embryonic precursors and seeded in tissue prenatally. They can replenish via self-renewal throughout the lifespan. Indeed, recent studies have demonstrated that tissue-resident macrophages should not be classified by the over-simplified macrophage polarization (M1/M2) dogma during inflammation. Moreover, multiple lines of evidence have indicated that tissue-resident macrophages play critical roles in maintaining tissue homeostasis and facilitating tissue repair through controlling infection and resolving inflammation. In this review, we summarize the properties of resident macrophages in the lung, spleen, and heart, and further highlight the impact of TRM populations on inflammation control and tissue repair. We also discuss the potential role of local proliferation in maintaining a physiologically stable TRM pool in response to acute inflammation.