IgA production requires B cell interaction with subepithelial dendritic cells in Peyer’s patches

A recipe for intestinal lgA Our guts are teeming with microbes, some friendly and others not. Plasma cells in the gut secrete immunoglobulin A (IgA), which helps to keep the peace with resident commensal bacteria and fights pathogens. B cell isotype switching to IgA occurs in lymphoid tissues called Peyer's patches. Reboldi et al. studied the cellular processes that guide B cells toward making IgA in mice. B cells took an unexpected journey from Peyer's patches follicles to the intestinal mucosa to interact with specialized IgA-triggering dendritic cells. The B cells then migrated back to the follicles to become IgA-producing B cells. Science, this issue p. 10.1126/science.aaf4822 Multiple cellular and molecular cues in the intestine instruct B cells to switch to producing immunoglobulin A. INTRODUCTION Secretory immunoglobulin A (IgA) is made by intestinal plasma cells and has roles both in protection from gut pathogens and in maintaining homeostasis of intestinal commensals. Peyer’s patches (PPs)—the major organized lymphoid tissues of the small intestine, numbering 100 to 200 in humans and 6 to 12 in mice—are the dominant source of IgA-producing cells. A number of molecular factors have been identified that promote B cell switching from IgM to IgA, the best defined being transforming growth factor–β (TGFβ). TGFβ is made in a latent form and must be activated before it can induce TGFβ receptor (TGFβR) signaling. In this study, we explore the requirements for B cell IgA switching in PPs, concentrating on the location where it takes place and the key cell types involved. RATIONALE Mice deficient in the chemokine receptor CCR6 had been reported to mount poor IgA responses, but the mechanism responsible was unclear. The CCR6 ligand, CCL20, is abundant in the subepithelial dome (SED) of the PP, and one thought was that CCR6 was required for positioning dendritic cells (DCs) in the SED. However, CCR6 was known to be expressed by B cells and to be up-regulated following B cell activation. In this study, we have pursued the hypothesis that CCR6 is required within B cells to promote migration events and cellular interactions in the SED necessary for PP IgA responses. RESULTS Using bone marrow (BM) chimera and cell transfer approaches, we find that CCR6 expression in PP B cells is necessary for their efficient switching to IgA and for production of intestinal IgA against cholera toxin and commensal bacteria. Loss- and gain-of-function approaches establish that intrinsic CCR6 expression is necessary and sufficient for B cells to access the SED. CCR6 is up-regulated on pre–germinal center (GC) B cells in a CD40-dependent manner, and a transfer model indicates a more prominent role for CCR6 in T cell–dependent than in T cell–independent IgA responses. PP pre-GC B cells are shown to express IgA germline transcripts and activation-induced cytidine deaminase (AID), consistent with IgA switching initiating in this compartment. Using intravital two-photon microscopy, we find that B cells within the SED undergo prolonged interactions with DCs. Using BM chimera experiments and blocking reagents, we establish that SED DCs are dependent on the cytokine lymphotoxin-α1β2 (LTα1β2). RORγt+ innate lymphoid cells (ILCs) are identified as a necessary source of this cytokine. Deficiency in LTβR-dependent DCs or RORγt-dependent ILCs results in reduced IgA+ B cell frequencies in PPs. Reciprocally, transgenic overexpression of LTα1β2 increases SED DCs and IgA switching. We then examined how the SED DCs augment IgA switching and found that they abundantly expressed αvβ8, an integrin that has an established role in converting TGFβ from its latent to its active state. Experiments with Itgb8f/f Cd11c-Cre mice and with an αvβ8 blocking antibody established that DC αvβ8 expression was necessary for PP IgA switching. In vitro experiments provided further evidence that DC αvβ8 could directly activate TGFβ during DC–B cell interactions and showed that LTβR and retinoic acid signaling promote αvβ8 expression on DCs. CONCLUSION Our study defines a role for the PP SED as a niche that supports events necessary for IgA switching, in particular the induction of TGFβ activation, and it provides an example of a DC–B cell interaction acting to guide B cell fate. By defining a network of interactions required for IgA switching, this study identifies approaches that could be used to augment IgA responses while also defining sites for defects that could underlie IgA deficiency, the most common immune deficiency syndrome in humans. B cell and dendritic cell distribution in mouse Peyer’s patch. Image is a cross-sectional view of a single PP dome and the neighboring villous epithelium of the small intestine. The 7-µm frozen section was stained to detect naïve and pre-GC B cells (IgD, blue) that occupy the follicle and SED; dendritic cells (CD11c, green) that are abundant in the SED, the interfollicular T zone, and the intestinal lamina propria; T cells (CD8, red) that are present in the interfollicular T zone, the lamina propria, and in the epithelium; and nuclei (DAPI, gray). Red staining also occurred nonspecifically in association with the epithelium, and this was most prominent for the follicle-associated epithelium that overlies the SED. The follicle-associated epithelium is the site of intestinal antigen delivery into the PP. The dark (IgD-negative) oval-shaped structure within the follicle is a GC. Immunoglobulin A (IgA) induction primarily occurs in intestinal Peyer’s patches (PPs). However, the cellular interactions necessary for IgA class switching are poorly defined. Here we show that in mice, activated B cells use the chemokine receptor CCR6 to access the subepithelial dome (SED) of PPs. There, B cells undergo prolonged interactions with SED dendritic cells (DCs). PP IgA class switching requires innate lymphoid cells, which promote lymphotoxin-β receptor (LTβR)–dependent maintenance of DCs. PP DCs augment IgA production by integrin αvβ8-mediated activation of transforming growth factor–β (TGFβ). In mice where B cells cannot access the SED, IgA responses against oral antigen and gut commensals are impaired. These studies establish the PP SED as a niche supporting DC–B cell interactions needed for TGFβ activation and induction of mucosal IgA responses.