The Peyer's Patch Mononuclear Phagocyte System at Steady State and during Infection

Abstract

The gut represents a potential entry site for a wide range of pathogens including protozoa, bacteria, viruses, or fungi. Consequently, it is protected by one of the largest and most diversified population of immune cells of the body. Its surveillance requires the constant sampling of its encounters by dedicated sentinels composed of follicles and their associated epithelium located in specialized area. In the small intestine, Peyer's patches (PPs) are the most important of these mucosal immune response inductive sites. Through several mechanisms including transcytosis by specialized epithelial cells called M-cells, access to the gut lumen is facilitated in PPs. Although antigen sampling is critical to the initiation of the mucosal immune response, pathogens have evolved strategies to take advantage of this permissive gateway to enter the host and disseminate. It is, therefore, critical to decipher the mechanisms that underlie both host defense and pathogen subversive strategies in order to develop new mucosal-based therapeutic approaches. Whereas penetration of pathogens through M cells has been well described, their fate once they have reached the subepithelial dome (SED) remains less well understood. Nevertheless, it is clear that the mononuclear phagocyte system (MPS) plays a critical role in handling these pathogens. MPS members, including both dendritic cells and macrophages, are indeed strongly enriched in the SED, interact with M cells, and are necessary for antigen presentation to immune effector cells. This review focuses on recent advances, which have allowed distinguishing the different PP mononuclear phagocyte subsets. It gives an overview of their diversity, specificity, location, and functions. Interaction of PP phagocytes with the microbiota and the follicle-associated epithelium as well as PP infection studies are described in the light of these new criteria of PP phagocyte identification. Finally, known alterations affecting the different phagocyte subsets during PP stimulation or infection are discussed.

Keywords: IgA; M cells; Peyer’s patch; bacterial and viral infections; dendritic cells; macrophages; microbiota; mucosal immunity.

Figure 1

The Peyer’s patch (PP) mononuclear phagocyte system (MPS). The PP MPS encompasses two large families of cells based on their origin, the common DC precursor (CDP)-derived and the monocyte-derived phagocytes. The CDP-derived cells comprise CD11c^hi conventional DC (cDC) and CD11c^int plasmacytoid DC. Among cDC, cDC1 are CD8α⁺ but SIRPα⁻ whereas cDC2 are SIRPα⁺ but CD8α⁻. cDC2 exist in several stages of differentiation among which the two extremes are the so-called double negative (DN) cDC2, which do not express CD11b, and the CD11b⁺ cDC2. CD11b⁺ cDC2 derive from DN cDC2 through the upregulation of CCR7, CD11b, EpCAM, JAM-A, and MHCII. CDP-derived cells are mainly located in the T cell zones, i.e., interfollicular regions (IFR), at the exception of DN cDC2, which transit through the subepithelial dome (SED). cDC excel in helper T cell priming but are poorly phagocytic. On the contrary, CD11c^hi monocyte-derived cells are strongly phagocytic. They also display a broad range of antimicrobial defense mechanisms. CD11c^hi monocyte-derived cells encompass two main subsets based on their phenotype, lifespan, and ability to prime T cells: macrophages (MF) and the monocyte-derived dendritic cell (DC) termed LysoDC. LysoDC are CD4⁻MHCII^hi short-lived SED-located DC with helper T cell priming ability. CD11c^hi MF, also called LysoMac, are CD4⁺MHCII^lo long-lived cells without any helper T cell priming ability. TIM-4⁻ LysoMac are mainly located in the SED whereas TIM-4⁺ LysoMac are mainly located in the IFR. A third type of MF, termed tingible-body macrophages, reside in the germinal center (GC) of the follicle (F) where they are involved in apoptotic B cell removal. Unlike other PP MF, they do not express CD11c. Although shown on the monocyte-derived cell part of the diagram, it is currently unknown whether they truly derive from monocytes or whether they self-renew from embryonic precursors. Adapted from Ref. (39).

Figure 2

Anatomic localization of Peyer’s patch (PP) phagocyte subpopulations. Origin and shape of each PP phagocyte subset is displayed on the right. Color codes correspond to colors displayed in pie charts. The latter show in each region of the dome the distribution of PP phagocyte subsets at the exception of plasmacytoid DC (pDC). FAE, follicle-associated epithelium; SED, subepithelial dome; F, follicle; IFR, interfollicular region; DAV, dome-associated villus; MF, macrophages. Adapted from Ref. (40).

Figure 3

Location of dome double negative (DN) and CD11b⁺ cDC2 based on JAM-A and CCR7 expression. (A) Left panel: normalized mean relative expression ± SD of F11r (JAM-A) in dome conventional DC (cDC) subsets. Mid-panel: identification of four developmental stages of dome cDC2 based on CD11b and MHCII surface expression. Stage I, CD11b⁻MHCII^lo; Stage II, CD11b⁻MHCII^int; Stage III, CD11b^loMHCII^hi; Stage IV, CD11b^intMHCII^hi. Right panel: mean fluorescence intensity of JAM-A in the four developmental stages of dome cDC2. JAM-A expression increases from stage I (DN cDC2) to stage IV (CD11b⁺ cDC2). Lower panel: confocal microscopy projection of a Zbtb46-GFP^−/+ mouse Peyer’s patch (PP) section stained for EGFP (green), CD11c (red), JAM-A (orange), and collagen IV (magenta). Higher magnifications of the numbered boxed area are shown on the right. cDC (CD11c⁺GFP⁺ cells) are mainly located in the IFR. However, some of them are located in the SED with a progressive decrease in numbers while reaching the upper part of the dome. Like LysoDC, they can penetrate into the follicle-associated epithelium (FAE). Subepithelial cDC2 (boxed area 1–4) express no or faint levels of JAM-A (stage I or II of dome cDC2; DN cDC2) whereas interfollicular cDC2 (boxed area 5 and 6) express it (stage III or IV of dome cDC2; CD11b⁺ cDC2). (B) Left panel: normalized mean relative expression ± SD of Ccr7 in dome cDC subsets. Right panel: confocal microscopy projection of a Zbtb46-GFP^−/+ mouse PP section stained for EGFP (green), CD11c (red), and CCR7 (orange). Higher magnifications of the numbered boxed area are shown on the right. Subepithelial cDC2 (boxed area 1) do not express CCR7 (DN cDC2) whereas interfollicular cDC2 (boxed area 2 and 3) do (CD11b⁺ cDC2). Parts of (A,B) are adapted from Ref. (40).

Figure 4

Location of plasmacytoid DC (pDC) in Peyer’s patch (PP). (A) Left panel: normalized mean relative expression ± SD of Bst2 in intestinal phagocytes based on Immgen database (139) and on the PP phagocyte microarray data deposited to NCBI GEO under accession numbers GSE94380 and GSE65514 (39, 40). Expression of Bst2 by LysoDC and LysoMac is shown. Right panel: in PP, monocyte-derived cells (CD11c^hiB220⁻lysozyme⁺ in blue), i.e., LysoDC and LysoMac, express lower levels of BST2 than pDC (CD11c^intB220⁺lysozyme⁻ in red). (B) Confocal microscopy projection of CX₃CR1-EGFP^−/+ mouse PP sections stained for EGFP (green), CD11c (red), lysozyme (yellow), CD4 (blue), and BST2 (magenta). Upper panel: BST2 is strongly expressed by cells of the dome-associated villus (DAV) but only weakly by LysoDC and LysoMac (CX₃CR1⁺CD11c⁺lysozyme⁺ cells) in the subepithelial dome (SED). A single putative pDC (arrow) strongly stained for BST2 is located in the SED. Lower panel: unlike the SED, the IFR is enriched in pDC. (A) is adapted from Ref. (39).

Figure 5

Expression of defined markers by Peyer’s patch (PP) phagocyte subpopulations. (A–D) Normalized mean relative expression ± SD of Il22ra2, Ltbr, Naip1, Naip2, Naip5, Il1b, Il18, Ftl1, and Lamp1 in intestinal phagocytes based on Immgen database (139) and on the PP phagocyte microarray data deposited to NCBI GEO under accession numbers GSE94380 and GSE65514 (39, 40). (A) In the gut, Il22ra2 [IL-22 binding protein (IL-22BP)] is mainly expressed by LysoDC and TIM-4⁻ LysoMac. (B) In PP, Ltbr (lymphotoxin β receptor) is mainly expressed by LysoDC, LysoMac and, to a lesser extent, conventional DC (cDC). (C) Enrichment of some members of the NAIP/NLRC4 inflammasome pathway (Naip1, Naip2, Naip5, Il1b, and Il18) in LysoDC and LysoMac. Note that Naip1 is only expressed by dome monocyte-derived cells. (D) Left panel: in the gut, LysoDC and LysoMac express higher levels of Ftl1 (ferritin light chain) and Lamp1 than other phagocytes. Right panel: labeling of a PP section shows enrichment of ferritin and LAMP1 expression in LysoDC and LysoMac of the subepithelial dome (SED) and of the follicle (F). Inserts: higher magnification of the boxed area showing one LysoDC strongly stained for ferritin and LAMP1.

Figure 6

Involvement of LysoDC trans-M cell dendrites in the sampling of Salmonella typhimurium. (A) A CX₃CR1-deficient LysoDC (GFP in place of CX₃CR1 in green; CD11c in red; lysozyme in yellow) extends a dendrite through the follicle-associated epithelium (FAE) to reach the lumen. In addition to CX₃CR1, CD11c, and lysozyme, LysoMac (*) stain for CD4 (blue). (B) 2 h postoral infection, a Salmonella (red) is taken up by a LysoDC (green) extending a dendrite into an uninfected FAE. (C) M cell transcellular pores (arrowheads), through which trans-M cell dendrites (CD11c in blue) cross the FAE, are highlighted by circular holes in the UEA-I cell surface staining. (D) By correlative scanning electron microscopy (SEM), Salmonella (second and last panels in red, large arrows) are located at the periphery of a protrusion (pseudocolored in blue) arising from an M cell. Circular holes (thin arrowheads) in the UEA-I cell surface staining indicate the presence of M cell transcellular pores. (B,C) are adapted from Ref. (43), (D) is adapted from Ref. (110).