The major function of humoral immunity in the gastrointestinal tract is to neutralize luminal microbes, and this function is mediated mainly by IgA produced by plasma cells in the lamina propria and transported across the mucosal epithelium into the lumen. Smaller quantities of IgG and IgM are also secreted into the gut lumen. Within the lumen, the antibodies bind to microbes and toxins and neutralize them by preventing their binding to host cells. This form of humoral immunity, sometimes called secretory immunity, has evolved to be particularly prominent in mammals. Studies in mice indicate that IgA responses are made to antigens expressed on only a small fraction of all the commensal species in the gut, and these are largely bacteria in the small intestine.
IgA is produced in higher amounts than any other antibody class. It is estimated that a normal 70-kg adult secretes about 2 g of IgA per day, which accounts for 60% to 70% of the total production of antibodies. This tremendous output of IgA comes from the large number of IgA-producing plasma cells in the GALT, which by some estimates account for 80% of all the antibody-producing plasma cells in the body (Fig. 1). Because IgA synthesis occurs mainly in mucosal lymphoid tissue and most of the locally produced IgA is efficiently transported into the mucosal lumen, this isotype constitutes less than one-quarter of the antibody in plasma and is a minor component of systemic humoral immunity compared with IgG.
Fig1. Immunoglobulin A (IgA)-secreting plasma cells in the intestine. The abundance of IgA-producing plasma cells (green) in colon mucosa compared with IgG-secreting cells (red) is shown by immunofluorescence staining. IgA that is being transcytosed by the crypt epithelial cells is visualized in their cytoplasm. From Brandtzaeg P. The mucosal immune system and its integration with the mammary glands. J Pediatr. 2010;156(suppl 1):S8–S16.
Several unique properties of the gut environment result in selective development of IgA-secreting cells that stay in the gastrointestinal tract or, if they enter the circulation, home back to the lamina propria of the intestines. The result is that IgA secreting cells efficiently accumulate next to the epithelium that will take up the secreted IgA and transport it into the lumen.
The abundance of intestinal plasma cells that produce IgA is due in part to selective induction of IgA class switching in B cells in GALT and mesenteric lymph nodes. IgA class switching in the gut can occur by T-dependent and T-independent mechanisms (Fig. 2). In both T-dependent and T-independent IgA class switching, two essential steps are cytokine-induced transcription through the IgA gene locus, which opens up access to the enzymes needed for switch recombination, and the induced expression of activation-induced deaminase (AID), the enzyme that mediates switch recombination. TGF-β is the major cytokine required for both T-dependent and T-independent IgA class switching in the gut and in other mucosal tissues. TGF-β is secreted in a latent form and must be activated by interacting with certain integrins, including αv β8. In the gut, latent TGF-β is produced by epithelial cells and DCs in GALT, and GALT DCs express the αv β8 integrin, which activates the TGF-β. In T-dependent IgA class switching, AID is induced in B cells by CD40 signaling, which is activated by CD40-ligand on T follicular helper (Tfh) cells binding to CD40 on B cells that present antigen to the Tfh cells (see Fig. 2A). In T-independent IgA class switching, AID expression in B cells is induced by the cytokines APRIL (a proliferation-inducing ligand) and BAFF (B cell–activating factor), which are structurally related to CD40L and bind to a CD40-related receptor on B cells called TACI (transmembrane activator and calcium-modulating cyclophilin-ligand interactor). APRIL and BAFF are produced by GALT DCs, and intestinal epithelial cells produce APRIL in response to TLR ligands made by commensal bacteria. Intestinal epithelial cells also produce TSLP in response to TLR signals, and TSLP stimulates additional APRIL production by GALT DCs. TLR ligands made by commensal bacteria in the gut also increase the expression of inducible nitric oxide synthase in DCs, leading to nitric oxide production. Nitric oxide is thought to promote both T-dependent and T-independent IgA class switching by enhancing the expression of TGF-β receptors and APRIL. Intestinal B-cell IgA production is at least partly dependent on the vitamin A metabolite all-trans retinoic acid, which is made by intestinal epithelial cells and GALT DCs, although the mechanisms by which retinoic acid promotes IgA production are not known. Retinoic acid is also important in B-cell homing to the gut, as discussed earlier. There is an abundance of TGF-β and retinoic acid within the GALT and mesenteric lymph nodes compared with nonmucosal lymphoid tissues such as spleen- and skin draining lymph nodes, largely accounting for the propensity of B cells in the GALT to switch to IgA production.
Fig2. Immunoglobulin A (IgA) class switching in the gut. IgA class switching in the gut occurs by both T-dependent and T-independent mechanisms. (A) In T-dependent IgA class switching, dendritic cells (DCs) in the subepithelial dome of Peyer’s patches capture microbial antigens delivered by microfold (M) cells and present processed peptides to naive CD4+ T cells, leading to differentiation of T follicular helper cells that engage in cognate interactions with IgM+ B cells that have also taken up and processed the microbial antigen. B-cell class switching to IgA is stimulated through T-cell CD40 ligand (CD40L) binding to B-cell CD40, together with transforming growth factor-β (TGF-β). This T cell–dependent pathway generates B cells that produce high affinity IgA antibodies, and which have a gut-homing phenotype imprinted by DC-derived retinoic acid (RA). (B) T-independent IgA class switching involves DC activation of IgM+ B cells, including B-1 cells. Toll-like receptor (TLR) ligand–activated DCs secrete cytokines that induce IgA class switching, including B cell–activating fac tor (BAFF), A proliferation-inducing ligand (APRIL), and TGF-β. The T cell–independent pathway yields relatively low-affinity IgA antibodies to intestinal bacteria. The molecular mechanisms of class switching are described in Chapter 12. NO, Nitric oxide; PAMP, pathogen-associated molecular pattern.
IgA production in the gastrointestinal tract is further enhanced by selective gut-homing properties of IgA producing cells that arise in GALT and mesenteric lymph nodes (see Fig. 3). Some of the IgA that is transported across the intestinal epithelium may be produced by plasma cells that differentiated and remained within underlying GALT follicles. However, IgA-secreting plasma cells are widely dispersed in the lamina propria of the gastrointestinal tract, not just in lymphoid follicles. As discussed earlier, activated B cells that underwent class switching into IgA-producing cells in the GALT and mesenteric lymph nodes may enter the systemic circulation and then selectively home back to the intestinal lamina propria, where they may reside as plasma cells.
Fig3. Homing properties of intestinal lymphocytes. The gut-homing properties of effector lymphocytes are imprinted in the lymphoid tissues, where they have undergone differentiation from naive precursors. Dendritic cells in gut-associated lymphoid tissues (GALT), including Peyer’s patches and mesenteric lymph nodes, are induced by cytokines such as thymic stromal lymphopoietin (TSLP) and other factors to express retinaldehyde dehydrogenase (RALDH), which converts dietary vitamin A into retinoic acid. When naive B or T cells are activated by antigen in GALT, they are exposed to retinoic acid produced by the dendritic cells, and this induces the expression of the chemokine receptor CCR9 and the integrin α4 β7 on the differentiated plasmablasts and effector T cells. The effector lymphocytes enter the circulation and home back into the gut lamina propria because the chemokine CCL25 (the ligand for CCR9) and the mucosal addressin cell adhesion molecule 1 (MAdCAM -1) (the ligand for α4 β7 ) are displayed on lamina propria venular endothelial cells.
Secreted IgA is transported via transcytosis through epithelial cells into the intestinal lumen by an Fc receptor called the poly-Ig receptor (Fig. 4). The IgA secreted by plasma cells in the lamina propria is in the form of a dimer that is held together by the coordinately produced J chain, which is covalently bound by disulfide bonds to the Fc regions of the α heavy chains of two IgA molecules. Mucosal plasma cells produce abundant J chain, more than plasma cells in nonmucosal tissues, whereas serum IgA is usually a monomer lacking the J chain. The dimeric IgA must be transported from the lamina propria across the epithelium into the lumen. This function is mediated by the poly-Ig receptor, an integral membrane glycoprotein with five extracellular Ig domains. IgM produced by lamina propria plasma cells is also a polymer (pentamer) associated covalently with the J chain, and the poly-Ig receptor also transports IgM into intestinal secretions. This is why this receptor is called the poly-Ig receptor. This receptor is synthesized by mucosal epithelial cells and is expressed on the basal and lateral surfaces of epithelial cells. Its production can be increased by inflammatory stimuli.
Fig4. Transport of immunoglobulin A (IgA) across epithelial cells. IgA is produced by plasma cells in the lamina propria of mucosal tissue and binds to the poly-Ig receptor at the base of an epithelial cell. The complex is transported across the epithelial cell, and the bound IgA is released into the lumen by proteolytic cleavage. The process of transport across the cell, from the basolateral to the luminal surface in this case, is called transcytosis.
Dimeric IgA (and pentameric IgM) secreted by plasma cells in the lamina propria bind to the poly-Ig receptor on the basolateral surface of mucosal epithelial cells through a domain of the J chain (see Fig. 4). The antibody-receptor complex is endocytosed into the epithelial cell, and unlike other endosomes that typically traffic to lysosomes, poly-Ig receptor–containing vesicles are directed to and fuse with the apical (luminal) plasma membrane of the epithelial cell. On the apical cell surface, the poly-Ig receptor is proteolytically cleaved; its transmembrane and cytoplasmic domains are left attached to the epithelial cell; and the extracellular domain of the receptor, carrying the IgA molecule, is released into the intestinal lumen. The cleaved part of the poly-Ig receptor, called the secretory component, remains associated with the dimeric IgA in the lumen. It is thought that the bound secretory component protects IgA (and IgM) from degradation by proteases present in the intestinal lumen, and these antibodies are therefore able to serve their function of neutralizing microbes and toxins in the lumen.
In addition to specifically binding microbes, glycans in the secretory component of IgA can bind to bacteria and reduce their motility, thereby preventing them from reaching the epithelial barrier. Patients with selective IgA deficiency often present with gastrointestinal and respiratory infections. Beyond a role in protective immunity, IgA may also facilitate small intestinal colonization by “helpful” commensals, especially Bacteroides species. One mechanism by which secreted IgA may promote survival of certain bacterial species is that the glycans on the Fc regions and the secretory piece serve as car bon sources for the microbes. Antibody responses to antigens encountered by ingestion are typically dominated by IgA, and secretory immunity is the mechanism of protection induced by oral vaccines such as the polio vaccine.
IgG is present in intestinal secretions at levels equal to those of IgM but lower than those of IgA. In some mucosal secretions (i.e., in the rectum, genitourinary tract, and airways), IgG levels are quite high. The transport of IgG into mucosal secretions may be mediated by transcytosis via the neonatal Fc receptor (FcRn).
IgA produced in lymphoid tissues in the mammary gland is secreted into colostrum and mature breast milk through poly-Ig receptor–mediated transcytosis and mediates passive mucosal immunity in breast-fed children. The human lactating mammary gland contains a large number of IgA-secreting plasma cells, and the mammary gland epithelium can store large quantities of secretory IgA. The plasma cells in the breast may originate in various MALTs. They home to the breast because most IgA plasmablasts express CCR10, regardless of the lymphoid tissues in which they were generated, and the breast tis sues express CCL28, the chemokine that binds CCR10. During breast-feeding, a child ingests a significant quantity of maternal IgA, which provides broad polymicrobial protection in the infant’s gut. Moderate amounts of IgG and IgM are also secreted into breast milk and contribute to the passive immunity of breast-fed children. Many epidemiologic studies have shown that breast-feeding significantly reduces the risk for diarrheal disease and sepsis, especially in developing countries, and this correlates with the presence of secretory IgA in breast milk specific for enterotoxic species of bacteria including Escherichia coli and Campylobacter
