Dynamic Changes in Macrophage Activation and Proliferation during the Development and Resolution of Intestinal Inflammation

Macrophages (Mφs) accumulate at sites of inflammation, and, because they can assume several functionally distinct states of activation, they can either drive or restrain inflammatory responses. Once believed to depend on the recruitment of blood monocytes, it is now clear that the accumulation of Mφs in some tissues can result from the proliferation of resident Mφs in situ. However, little is known about the proliferation and activation state of Mφ subsets in the gut during the development and resolution of intestinal inflammation. We show that inflammatory Mφs accumulate in the large intestine of mice during the local inflammatory response to infection with the gastrointestinal nematode parasite Trichuris muris. Classically activated Mφs predominate initially (as the inflammation develops) and then, following worm expulsion (as the inflammation resolves), both the resident and inflammatory populations of Mφs become alternatively activated. A small but significant increase in the proliferation of inflammatory Mφs is seen but only during the resolution phase of the inflammatory response following both worm expulsion and the peak in Mφ accumulation. In contrast to recent studies in the pleural and peritoneal cavities, the proliferation of resident and alternatively activated Mφs does not increase during the inflammatory response. Furthermore, in CCR2(−/−) mice, monocyte recruitment to the gut is impeded, and the accumulation of alternatively activated Mφs is greatly reduced. In conclusion, the recruitment of blood monocytes is the principle mechanism of Mφ accumulation in the large intestine. This study provides a novel insight into the phenotype and behavior of intestinal Mφ during infection-driven inflammation.