Secretory IgA: Designed for Anti-Microbial Defense

Abstract

Prevention of infections by vaccination remains a compelling goal to improve public health. Mucosal vaccines would make immunization procedures easier, be better suited for mass administration, and most efficiently induce immune exclusion - a term coined for non-inflammatory antibody shielding of internal body surfaces, mediated principally by secretory immunoglobulin A (SIgA). The exported antibodies are polymeric, mainly IgA dimers (pIgA), produced by local plasma cells (PCs) stimulated by antigens that target the mucose. SIgA was early shown to be complexed with an epithelial glycoprotein - the secretory component (SC). A common SC-dependent transport mechanism for pIgA and pentameric IgM was then proposed, implying that membrane SC acts as a receptor, now usually called the polymeric Ig receptor (pIgR). From the basolateral surface, pIg-pIgR complexes are taken up by endocytosis and then extruded into the lumen after apical cleavage of the receptor - bound SC having stabilizing and innate functions in the secretory antibodies. Mice deficient for pIgR show that this is the only receptor responsible for epithelial export of IgA and IgM. These knockout mice show a variety of defects in their mucosal defense and changes in their intestinal microbiota. In the gut, induction of B-cells occurs in gut-associated lymphoid tissue, particularly the Peyer's patches and isolated lymphoid follicles, but also in mesenteric lymph nodes. PC differentiation is accomplished in the lamina propria to which the activated memory/effector B-cells home. The airways also receive such cells from nasopharynx-associated lymphoid tissue but by different homing receptors. This compartmentalization is a challenge for mucosal vaccination, as are the mechanisms used by the mucosal immune system to discriminate between commensal symbionts (mutualism), pathobionts, and overt pathogens (elimination).

Keywords: GALT; MALT; NALT; antibodies; commensals; germinal centers; mucosa; pathogens.

Figure 1

Receptor-mediated epithelial export of polymeric IgA (pIgA, mainly dimers) to provide secretory IgA (SIgA) antibodies. At the mucosal surface, SIgA antibodies together with mucus perform immune exclusion of antigens. The epithelial polymeric Ig receptor (pIgR) is expressed basolaterally, mainly in the intestinal crypts (glands), as membrane secretory component (mSC) and mediates external transcytosis of pIgA (and pentameric IgM, not shown). SIgA is released to the lumen with bound SC by apical cleavage of pIgR, in the same manner as unoccupied pIgR (carrying no ligand) is cleaved to provide free SC. Mucosal plasma cells produce abundantly pIgA with incorporated J chain (IgA + J), which is required for high-affinity epithelial binding of the pIgR ligands. Modified from Brandtzaeg (16).

Figure 2

Generation of secretory IgA (SIgA) and free secretory component (SC). SIgA is formed as a hybrid antibody molecule stabilized by a disulfide bridge between the two cell products. The amount of dimeric IgA (pIgA) produced by a plasma cell depends on its level of J-chain expression, which generally is high in mucosal and glandular tissue. Inset (left) shows direct demonstration of abundant cytoplasmic expression of pIgA (p) in most plasmablasts and plasma cells in the parotid gland achieved by in vitro affinity test with free SC on tissue section as described (9), whereas a single cell producing only monomers (m) is seen in this field. On average, ∼50% of SC occurring in various secretions is in a free form (unoccupied by ligand). The immunostained panel is from Brandtzaeg (45).

Figure 3

Relationship between local production of Ig isotypes by gland-associated plasma cells and Ig transfer by secretory epithelium. (A) Compared with the local production in the parotid gland, export of IgA into stimulated secretion is clearly favored over export of IgM (and IgG and IgD), whereas translocation of the two subclasses of IgA appears to handled equally well by the glandular epithelium. (B) Comparison of epithelial translocation of dimeric IgA (pIgA) and pentameric IgM (pIgM) was performed in vitro with polarized MDCK cells transfected with the human polymeric Ig receptor. Cells were incubated with ¹²⁵I-labeled pIgA or pIgM for 2 h at 4°C, washed for 10 min at 4°C, and chased at 37°C for different times as indicated. Translocation is expressed as the cumulative appearance of ¹²⁵I-pIgA and ¹²⁵I-pIgM in the apical medium. Each point represents mean result of three filters for pIgA and pIgM translocation at 50 nM ligand concentration. Adapted from Norderhaug et al. (29).

Figure 4

Synopses of the structural basis for the excellent anti-microbial binding properties of secretory IgA (SIgA). Domain interactions in the formation of SIgA based on data reviewed in Norderhaug et al. (29). Non-covalent interactions are shown between the J chain (J) and the extracellular domain 1 (D1) of bound secretory component (SC), and covalent disulfide bonding is indicated between cysteine 467 or 502 in D5 of bound SC and cysteine 311 in the Cα2 domain of one of the two IgA subunits. Some studies have indicated that there may be two J chains in dimeric IgA (30). Insert to the lower left is from modeling data for dimeric IgA1 based on X-ray and neutron scattering in solutions published in Bonner et al. (63). Note the T-shape of the Fab fragments, allowing for antibody binding to large particles like bacteria. V, variable region; C, constant region; L, light chain; H, heavy chain; Fab, fragment antigen binding; Fc, fragment crystallizable.

Figure 5

Homing properties of human mucosal memory/effector B cells. Putative scheme for compartmentalized migration of B cells from inductive (top) to effector (bottom) sites. Depicted are more or less preferred pathways (graded arrows) presumably followed by mucosal B cells activated in nasopharynx-associated lymphoid tissue (NALT) represented by palatine tonsils and adenoids, bronchus-associated lymphoid tissue (BALT), and gut-associated lymphoid tissue (GALT) represented by Peyer’s patches, appendix, and colonic-rectal isolated lymphoid follicles. The principal homing receptor profiles of the respective B-cell populations, and adhesion/chemokine cues directing extravasation at different effector sites, are indicated (pink and blue panels) – those operating in lactating mammary glands apparently being shared for NALT- and GALT-derived cells. Homing molecules integrating airway immunity with systemic immunity are encircled in red. Adapted from Brandtzaeg (51). MEC, mucosae-associated epithelial chemokine; TECK, thymus-expressed chemokine.

Figure 6

Immune events taking place in the dark and light zones of human germinal center in secondary lymphoid follicle. The “germinal center founder cells” receive their initial stimulation through cognate interaction with activated CD4⁺ helper T cells just outside the lymphoid follicle before they enter it to become proliferating centroblasts forming the dark zone. Affinity maturation of sIgM as part of the B-cell receptor is achieved after somatic hypermutation by competition for antigen presented in ICs on FDCs. Antigen taken up by the B cells is further presented in a cognate fashion to T_FH cells, which also receive stimulatory signals from GCDCs. The activated T_FH cells act on the B cells with their cytokines, thereby mediating expansion of high-affinity B-cell clones in the light zone. Further details are discussed in the text. sIg, surface immunoglobulin; bcl-2, anti-apoptotic B-cell lymphoma protein; IC, immune complex; FDC, follicular dendritic cell; GCDC, germinal center dendritic cell; HLA-II, HLA class II molecule; TCR, T cell receptor; T_FH, follicular helper T cell; CD40L/CD40, costimulatory molecules.

Figure 7

Average tissue densities of plasmablasts/plasma cells producing different Ig classes at various normal secretory tissue sites as indicated. Representative areas illustrating merge of immunofluorescence staining (see color key) are shown for submandibular gland and colonic lamina propria. Based on published data from the author’s laboratory.

Figure 8

Hypothetical depiction of how the intestinal immune system handles symbionts and potentially pathogenic residents (pathobionts) of the commensal microbiota versus overt exogenous pathogens. Secretory IgA (SIgA) antibody levels against commensal bacteria may go in waves because of epitope drift and shielding of gut-associated lymphoid tissue from antigen uptake. The overall affinity of SIgA antibodies probably increases with age and may be enhanced or reduced against pathobionts during dysbiosis, and particularly raised by persistent stimulation with overt pathogens. One goal of mutualism with commensals is mucosal barrier reinforcement by mechanisms listed such as SIgA export and induction of regulatory T cells, whereas pathogens exhibit various virulence mechanisms to break the barrier.

Figure 9

IgA coating of commensal bacteria may modulate mucosal immunity and metabolism. (A) Direct immunofluorescence staining of bacterial sediment from whole saliva to demonstrate in vivo IgA coating. Contaminating epithelial cell is faintly visualized because of autofluorescence. Numerous cocci (mainly diplococci) – partly adhering to epithelial cell – have bound IgA, which also decorates older cell-wall segments of streptococci forming long chains, whereas new crosswalls formed by growth in vitro after sampling are negative as indicated. Adapted from Brandtzaeg et al. (57) (Original magnification: ×2000). (B) In healthy individuals, IgA⁺ mucosal plasma cells produce dimers with J chain (IgA + J) which are exported to the epithelial surface by the polymeric Ig receptor (see Figure 1). Secretory IgA binds to commensal bacteria, and the IgA-coated bacteria modulate mucosal immunity and homeostasis by delivering signals through innate microbial sensors (red) such as pattern recognition receptors on the epithelial cells. In addition to enhancing innate defense through immune pathways controlled by interferon (IFN) cytokines, these signals also regulate the intake of food lipids through metabolic pathways controlled by the transcription factor Gata4. When IgA is lacking (not shown), the gut epithelium upregulates its expression of IFN-dependent innate defense genes to compensate for the lack of adaptive IgA immunity. This upregulation leads to a downregulation of the genes controlled by Gata4 (196). The resulting gene imbalance impairs the epithelial absorption of lipids, such as cholesterol, resulting in metabolic disorders with reduced leptin levels and fat storage. Modified from Chorny and Cerutti (197).