Dendritic cells (DCs) and monocyte-derived macrophages (M s) are key components of intestinal immunity. However, the lack of surface markers differentiating M s from DCs has hampered understanding of their respective functions. Here, we demonstrate that, using CD64 expression, M s 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 M s and we delineate a developmental pathway in the healthy intestine that leads from newly extravasated Ly-6C hi monocytes to intestinal M s. 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 M s endowed with phagocytic activity and the ability to produce inducible nitric oxide synthase. Eur. J. Immunol. 2012. 42: 3150-3166 HIGHLIGHTS 3151 IntroductionThe intestinal lamina propria (LP) contains cells that express high levels of CX 3 CR1, the receptor for the fractalkine chemokine [1,2]. Based on their monocytic origin and on their inability to migrate to the mesenteric lymph nodes (MLNs) such CX 3 CR1 hi cells have been defined as macrophages (M s) [1][2][3]. CX 3 CR1 hi M s contribute to the intestinal LP homeostasis through the production of anti-inflammatory cytokines and the clearance of commensal bacteria that breach the epithelial barrier [4]. In contrast, during intestinal inflammation, microenvironmental signals promote the differentiation of extravasated monocytes into proinflammatory M s with the ability to produce interleukin (IL)-12, IL-23, tumor necrosis (TNF)-α and inducible nitric oxide synthase (iNOS) [5][6][7]. However, little is known about the developmental trajectories that lead extravasated monocytes to either antior proinflammatory intestinal M s. This is primarily due to the fact that a surface marker permitting unequivocal identification of M s within the intestine and their distinction from dendritic cells (DCs) is lacking. The interstitial DCs (Int-DCs) present throughout the LP derive from blood precursors known as pre-DCs [2]. Under steady-state conditions, the Int-DCs found in the intestinal LP induce oral tolerance by carrying antigens originating from food or from harmless bacteria to the MLNs [8,9]. The CD103 + Int-DCs found in the steady-state LP have the selective ability to express aldehyde dehydrogenase (ALDH) and thereby produce retinoic acid (RA). As a result, upon migration to MLNs they trigger the differentiation of naive CD4 + T cells specific for food and microbiota antigens into induced Foxp3 + regulatory T (iTreg) cells [10][11][12][13]. In contrast, the Int-DCs that develop in inflamed LP upon exposure to pathogens lose their capacity to generate iTreg cells and, upon migration to the MLNs, trigger the differentiation of naive, antigen-responsive CD4 + T cells into T helper type 1 (Th1) cells that are specific for the invading patho...

Langerhans cells (LCs) constitute a subset of dendritic cells (DCs) that express the lectin langerin and that reside in their immature state in epidermis. Paradoxically, in mice permitting diphtheria toxin (DT)–mediated ablation of LCs, epidermal LCs reappeared with kinetics that lagged behind that of their putative progeny found in lymph nodes (LNs). Using bone marrow (BM) chimeras, we showed that a major fraction of the langerin+, skin-derived DCs found in LNs originates from a developmental pathway that is independent from that of epidermal LCs. This pathway, the existence of which was unexpected, originates in the dermis and gives rise to langerin+ dermal DCs (DDCs) that should not be confused with epidermal LCs en route to LNs. It explains that after DT treatment, some langerin+, skin-derived DCs reappear in LNs long before LC-derived DCs. Using CD45 expression and BrdU-labeling kinetics, both LCs and langerin+ DDCs were found to coexist in wild-type mice. Moreover, DT-mediated ablation of epidermal LCs opened otherwise filled niches and permitted repopulation of adult noninflammatory epidermis with BM-derived LCs. Our results stress that the langerin+ DC network is more complex than originally thought and have implications for the development of transcutaneous vaccines and the improvement of humanized mouse models.