The intestine contains the largest pool of macrophages in the body (Bain et al., 2018). Intestinal macrophages can be classified according to their tissue location into lamina propria macrophages, submucosa macrophages and muscularis macrophages (figure 4). Another macrophage population is found in the mucosa-associated lymphoid tissues (MALT), namely the Peyer's Patches and the mesenteric lymph nodes (Viola et al., 2020a).
Intestinal macrophages are first established before birth from yolk sac or fetal liver precursors. Shortly after birth most embryo derived macrophages are rapidly replaced by adult monocyte derived cells (Gross et al., 2015), mainly in the lamina propria. However, a population of long-lived self-maintaining macrophages remains deeper in the gut wall and is associated with blood vessels and enteric and myenteric neurons of the submucosa and the muscularis externa (Viola et al. 2020a). Long-lived embryonic derived macrophages are also located in close proximity of Peyer's patches and Paneth cells (Ruder et al., 2020). Incoming Ly6Chigh monocytes differentiate into mature gut macrophages by upregulation of MHCII and CX3CR1. Fully matured tissue-resident gut macrophages express F4/80, CD64, CD163 and CD206 (figure 5) (Viola et al., 2020). Classical dendritic cells (cDC) also located in the lamina propria can be differentiated from macrophages by the absence of the macrophage markers F4/80, CD64 and CX3XR1. Three main cDC subsets subsets have been identified in mouse and human intestinal lamina propia: CD103pos CD11bneg cDCs of type 1, CD103pos CD11bpos cDCs of type 2 and CD103neg CD11bpos cDCs of type 2 (Sun et al., 2020).
Lamina propria macrophages are positioned in the first line of defence to respond to bacteria and food antigens that breach the intestinal barrier and are thus crucial for maintaining the balance between pathogen defence and oral tolerance (Ruder et al., 2020). Lamina propria macrophages are highly phagocytic and express high levels of MHCII. Lamina propria macrophages exhibit functions to preserve intestinal epithelial integrity, such as pathogenic clearance and phagocytosis of dead cells (Viola et al., 2020a).
Muscularis macrophages are transcriptionally and morphologically different and are characterised by up-regulation of CD163, Retnla and Mrc1 (Viola et al., 2020). Moreover, they express M2-associated genes like Arg1 and Chil3 (Ruder et al., 2020). Muscularis macrophages have been shown to regulate the neuronal development of the enteric nervous system and to directly interact with the smooth muscle cells (Viola et al., 2020a).
Submucosa macrophages are also specialized in enteric neuron and blood vessel support (Viola et al., 2020a). Submucosal neuron-associated macrophages are self-maintaining and upregulate genes, which have been reported to be enriched in microglia, including Fcrls, Mef2a, Hexb and Gpr3 (Viola et al., 2020b).