A defining feature of resident gut macrophages is their high replenishment rate from blood monocytes attributed to tonic commensal stimulation of this site. with a model of ontogenetic diversity where locally managed subsets coexist with rapidly replaced monocyte-derived populations. Graphical Abstract Open in a separate window Introduction Resident gastrointestinal macrophages are a dominant immune cell type present in gut tissues that are crucial for homeostatic maintenance of this organ and hence its optimal physiological functioning (Bain and Mowat, 2014; Gross et al., 2015; Grainger et al., 2017). Important functions include clearance and sampling of apoptotic cells (Cummings et al., 2016; Schridde et al., 2017), training of epithelial progenitor proliferation in the intestinal crypts (Pull et al., 2005), bactericidal activity with limited inflammatory cytokine production (Smythies et al., 2005), and supporting neuroimmune interactions (Muller et al., 2014; Gabanyi et al., 2016). Inappropriate macrophage activity, particularly potential imbalance between resident- and inflammation-elicited (inflammatory) macrophages, has been implicated in driving pathophysiological complications in the gut. These include inflammatory bowel diseases (Kamada et al., 2008; Bain et al., 2013) and colon cancer (Afik et al., 2016). Delineating the origins and processes that underlie development of resident gut macrophages is usually therefore of high importance to provide novel mechanistic understanding of disease says. For almost half a century, it was commonly believed that tissue-resident macrophages in all bodily organs were continuously renewed from adult bone marrow (BM)-derived circulating blood monocytes (van Furth et al., 1972). More recent research challenged this paradigm, exposing the presence of bona fide tissue-resident macrophages often arising from embryonic or perinatal precursors that are managed locally and independently of blood monocytes at homeostasis (Ginhoux et al., 2010; Hoeffel et ARHGEF11 al., 2012, 2015; Schulz et al., 2012; Hashimoto et al., 2013; Yona et NBQX enzyme inhibitor al., 2013; Sheng et al., 2015; Ginhoux and Guilliams, 2016). These include the brain microglia of the central nervous system (Ginhoux et al., 2010), alveolar macrophages of the lung airspaces (Guilliams et al., 2013; Hashimoto et al., 2013), and Langerhans cells of the skin epidermis (Merad et al., 2002; Hoeffel et al., 2012). Many of these organs are also home to blood monocyteCreplenished macrophage populations that coexist with the locally managed macrophages at homeostasis and can undertake functionally unique activities (Ginhoux and Guilliams, 2016; Goldmann et al., 2016; Kim et al., 2016). The striking exception to this model has been the gastrointestinal tract, the last bastion of the continuous monocyte replenishment model of resident macrophage ontogeny, where all gut resident macrophages are replenished by high turnover from blood monocytes (Bain et al., 2014) with a predicted half-life of 4C6 wk (Jaensson et al., 2008; Ginhoux and Jung, 2014). This unique feature of resident gut macrophages has been attributed to the inflammatory firmness of the gut stimulated by the high commensal burden (Bain et al., 2014; Ginhoux and Jung, 2014). Unlike in other tissues, where multiple phenotypic markers in tandem with unique morphological characteristics or localization (Merad et al., 2002; Guilliams et al., 2013; Tamoutounour et al., 2013; Bain et al., 2016; Scott et al., 2016) have been used to distinguish subsets of locally managed resident macrophages from NBQX enzyme inhibitor those that are monocyte replenished, gut studies have predominantly used relative expression of the chemokine receptor CX3C chemokine receptor 1 (CX3CR1; Jung et al., 2000; Varol et al., 2009; Tamoutounour et al., 2012; Zigmond et al., 2012; Bain et al., 2013, 2014). It therefore remains a distinct possibility that locally managed resident macrophages or slowly monocyte-replenished populations could be present in the gut, but identification has been hampered by an failure to distinguish these cells from the total pool. In this study, using the novel gut macrophage markers Tim-4 and CD4, we found that the adult gut macrophage pool is usually comprised of three similarly sized subsets with unique replenishment rates from blood monocytes. Challenging current assumptions, abundant Tim-4+CD4+ gut-resident macrophages were found to be locally managed impartial of monocytes, whereas the Tim-4CCD4+ populace had a slow replenishment rate from monocytes. Together, these two CD4+ populations accounted for the vast majority of mature macrophages in the gut. Indeed, the only populace with high turnover from monocytes was the Tim-4CCD4C macrophage subset. Supporting the differential requirement for monocyte replenishment, Tim-4+CD4+ macrophages NBQX enzyme inhibitor experienced unique developmental dynamics from early life and dominated in = 2C3 per experiment. (B) Expression of CX3CR1-GFP by P1 monocytes, P2 transitioning monocytes, and Tim-4CCD4C, Tim-4CCD4+, and Tim-4+CD4+ (P3/P4) macrophages from your.