CD64 distinguishes macrophages from dendritic cells in the gut and reveals the Th1-inducing role of mesenteric lymph node macrophages during colitis

Dendritic cells (DCs) and monocyte-derived macrophages (MFs) are key components of intestinal immunity. However, the lack of surface markers differentiating MFs from DCs has hampered understanding of their respective functions. Here, we demonstrate that, using CD64 expression, MFs can be distinguished from DCs in the intestine of both mice and humans. On that basis, we revisit the phenotype of intestinal DCs in the absence of contaminating MFs and we delineate a developmental pathway in the healthy intestine that leads from newly extravasated Ly-6Chi monocytes to intestinal MFs. We determine how inflammation impacts this pathway and show that T cell-mediated colitis is associated with massive recruitment of monocytes to the intestine and the mesenteric lymph node (MLN). There, these monocytes differentiate into inflammatory MFs endowed with phagocytic activity and the ability to produce inducible nitric oxide synthase. In the MLNs, inflammatory MFs are located in the T-cell zone and trigger the induction of proinflammatory T cells. Finally, T cell-mediated colitis develops irrespective of intestinal DC migration, an unexpected finding supporting an important role for MLN-resident inflammatory MFs in the etiology of T cell-mediated colitis.