Blaise Corthésy

Role of secretory IgA in infection and maintenance of homeostasis.

An important activity of mucosal surfaces is the production of antibodies (Abs) referred to as secretory immunoglobulin A (SIgA) that serve as a first line of defense to repel pathogenic microorganisms and provide a finely tuned balance to guarantee controlled survival of essential commensal bacteria. By excluding bacteria from the epithelial cell, SIgA participates in the cross-talk between the host and its intestinal content, ensuring appropriate homeostasis under normal conditions. Besides the classical view of immune exclusion function, SIgA Abs exhibit the striking feature to adhere to gastrointestinal M cells residing in the follicle-associated epithelium in organized structures called Peyers patches. Selective binding of SIgA results in transport across the microfold (M) cells, a process that facilitates the association of the Ab with dendritic cells (DCs) located in the underlying subepithelial dome region of Peyers patches. Limited entry of free SIgA and SIgA-coated bacteria via this pathway is crucial to the modulation of local immune responses in an environment that limits the onset of pro-inflammatory circuits. Such a mechanism would ensure homeostasis by allowing antigen recognition under neutralized conditions and by avoiding tissue dissemination, two features that endow SIgA with non-inflammatory properties in the mucosal environment.

Recombinant immunoglobulin A: powerful tools for fundamental and applied research

The use of monoclonal antibodies has become routine in research and diagnostic laboratories but the potential level of antibodies in use in public health and medical applications is still far from its maximum. From a clinical perspective, topical immunotherapy of mucosal surfaces with monoclonal antibodies can block entry and transmission of bacteria, viruses, fungi and parasites that infect humans, and defeat some key strategies, evolved by many pathogens, to evade the host immune system. The chief antibody at mucosal surfaces is secretory immunoglobulin A (SIgA), a multi-polypeptide complex originating from two cell types. The recent design of heterologous expression systems, coupled with modern biotechnology processes, should form a sound basis for studying the functional properties of SIgAs and evaluate their value as biotherapeutics. Here, we discuss the principles underlying mucosal immunity and review the application of recombinant SIgA to the dissection of mechanisms in passive and active protection at mucosal surfaces.

Recognition of Gram-positive Intestinal Bacteria by Hybridoma- and Colostrum-derived Secretory Immunoglobulin A Is Mediated by Carbohydrates

Humans live in symbiosis with 1014 commensal bacteria among which >99% resides in their gastrointestinal tract. The molecular bases pertaining to the interaction between mucosal secretory IgA (SIgA) and bacteria residing in the intestine are not known. Previous studies have demonstrated that commensals are naturally coated by SIgA in the gut lumen. Thus, understanding how natural SIgA interacts with commensal bacteria can provide new clues on its multiple functions at mucosal surfaces. Using fluorescently labeled, nonspecific SIgA or secretory component (SC), we visualized by confocal microscopy the interaction with various commensal bacteria, including Lactobacillus, Bifidobacteria, Escherichia coli, and Bacteroides strains. These experiments revealed that the interaction between SIgA and commensal bacteria involves Fab- and Fc-independent structural motifs, featuring SC as a crucial partner. Removal of glycans present on free SC or bound in SIgA resulted in a drastic drop in the interaction with Gram-positive bacteria, indicating the essential role of carbohydrates in the process. In contrast, poor binding of Gram-positive bacteria by control IgG was observed. The interaction with Gram-negative bacteria was preserved whatever the molecular form of protein partner used, suggesting the involvement of different binding motifs. Purified SIgA and SC from either mouse hybridoma cells or human colostrum exhibited identical patterns of recognition for Gram-positive bacteria, emphasizing conserved plasticity between species. Thus, sugar-mediated binding of commensals by SIgA highlights the currently underappreciated role of glycans in mediating the interaction between a highly diverse microbiota and the mucosal immune system.

Potentiation of polarized intestinal Caco-2 cell responsiveness to probiotics complexed with secretory IgA.

The precise mechanisms underlying the interaction between intestinal bacteria and the host epithelium lead to multiple consequences that remain poorly understood at the molecular level. Deciphering such events can provide valuable information as to the mode of action of commensal and probiotic microorganisms in the gastrointestinal environment. Potential roles of such microorganisms along the privileged target represented by the mucosal immune system include maturation prior, during and after weaning, and the reduction of inflammatory reactions in pathogenic conditions. Using human intestinal epithelial Caco-2 cell grown as polarized monolayers, we found that association of a Lactobacillus or a Bifidobacterium with nonspecific secretory IgA (SIgA) enhanced probiotic adhesion by a factor of 3.4-fold or more. Bacteria alone or in complex with SIgA reinforced transepithelial electrical resistance, a phenomenon coupled with increased phosphorylation of tight junction proteins zonula occludens-1 and occludin. In contrast, association with SIgA resulted in both enhanced level of nuclear translocation of NF-κB and production of epithelial polymeric Ig receptor as compared with bacteria alone. Moreover, thymic stromal lymphopoietin production was increased upon exposure to bacteria and further enhanced with SIgA-based complexes, whereas the level of pro-inflammatory epithelial cell mediators remained unaffected. Interestingly, SIgA-mediated potentiation of the Caco-2 cell responsiveness to the two probiotics tested involved Fab-independent interaction with the bacteria. These findings add to the multiple functions of SIgA and underscore a novel role of the antibody in interaction with intestinal bacteria.

The Role of Secretory Immunoglobulin A in the Natural Sensing of Commensal Bacteria by Mouse Peyer's Patch Dendritic Cells

Background: Intestinal commensal bacteria are not ignored by the mucosal immune system, yet the mechanisms ensuring homeostatic communication are poorly defined. Results: Coating of commensals by SIgA mediates their targeting to Peyers patch dendritic cells. Conclusion: SIgA is important for the natural dynamic host-microbiota dialogue. Significance: Beyond pathogens, immune surveillance function of SIgA applies to the control of commensal bacteria. The mammalian gastrointestinal (GI) tract harbors a diverse population of commensal species collectively known as the microbiota, which interact continuously with the host. From very early in life, secretory IgA (SIgA) is found in association with intestinal bacteria. It is considered that this helps to ensure self-limiting growth of the microbiota and hence participates in symbiosis. However, the importance of this association in contributing to the mechanisms ensuring natural host-microorganism communication is in need of further investigation. In the present work, we examined the possible role of SIgA in the transport of commensal bacteria across the GI epithelium. Using an intestinal loop mouse model and fluorescently labeled bacteria, we found that entry of commensal bacteria in Peyers patches (PP) via the M cell pathway was mediated by their association with SIgA. Preassociation of bacteria with nonspecific SIgA increased their dynamics of entry and restored the reduced transport observed in germ-free mice known to have a marked reduction in intestinal SIgA production. Selective SIgA-mediated targeting of bacteria is restricted to the tolerogenic CD11c+CD11b+CD8− dendritic cell subset located in the subepithelial dome region of PPs, confirming that the host is not ignorant of its resident commensals. In conclusion, our work supports the concept that SIgA-mediated monitoring of commensal bacteria targeting dendritic cells in the subepithelial dome region of PPs represents a mechanism whereby the host mucosal immune system controls the continuous dialogue between the host and commensal bacteria.

Role of secretory IgA in the mucosal sensing of commensal bacteria

While the gut epithelium represents the largest mucosal tissue, the mechanisms underlying the interaction between intestinal bacteria and the host epithelium lead to multiple outcomes that remain poorly understood at the molecular level. Deciphering such events may provide valuable information as to the mode of action of commensal and probiotic microorganisms in the gastrointestinal environment. Potential roles of such microorganisms along the privileged target represented by the intestinal immune system include maturation processes prior, during and after weaning, and the reduction of inflammatory reactions in pathogenic conditions. As commensal bacteria are naturally coated by natural and antigen-specific SIgA in the gut lumen, understanding the consequences of such an interaction may provide new clues on how the antibody contributes to homeostasis at mucosal surfaces. This review discusses several aspects of the role of SIgA in the essential communication existing between the host epithelium and members of its microbiota.

Secretory immunoglobulin A: well beyond immune exclusion at mucosal surfaces

At mucosal surfaces, secretory IgA (SIgA) antibodies serve as the first line of defense against microorganisms through a mechanism called immune exclusion that prevents interaction of neutralized antigens with the epithelium. In addition, SIgA plays a role in the immune balance of the epithelial barrier through selective adhesion to M cells in intestinal Peyer’s patches. This mediates the transepithelial retro-transport of the antibody and associated antigens from the intestinal lumen to underlying gut-associated organized lymphoid tissue. In Peyer’s patches, SIgA-based immune complexes are internalized by underlying antigen-presenting cells, leaving the antigen with masked epitopes, a form that limits the risk of overwhelming the local immune protection system with danger signals. This translates into the onset of mucosal and systemic responses associated with production of anti-inflammatory cytokines and limited activation of antigen-presenting cells. In the gastrointestinal tract, SIgA exhibits thus pr...At mucosal surfaces, secretory IgA (SIgA) antibodies serve as the first line of defense against microorganisms through a mechanism called immune exclusion that prevents interaction of neutralized antigens with the epithelium. In addition, SIgA plays a role in the immune balance of the epithelial barrier through selective adhesion to M cells in intestinal Peyer’s patches. This mediates the transepithelial retro-transport of the antibody and associated antigens from the intestinal lumen to underlying gut-associated organized lymphoid tissue. In Peyer’s patches, SIgA-based immune complexes are internalized by underlying antigen-presenting cells, leaving the antigen with masked epitopes, a form that limits the risk of overwhelming the local immune protection system with danger signals. This translates into the onset of mucosal and systemic responses associated with production of anti-inflammatory cytokines and limited activation of antigen-presenting cells. In the gastrointestinal tract, SIgA exhibits thus properties of a neutralizing agent (immune exclusion) and of an immunopotentiator inducing effector immune responses in a noninflammatory context favorable to preserve local homeostasis.

Intragastric and Intranasal Administration of Lactobacillus paracasei NCC2461 Modulates Allergic Airway Inflammation in Mice.

Introduction. Preclinical and clinical evidences for a role of oral probiotics in the management of allergic diseases are emerging. Aim. We aimed at testing the immunomodulatory effects of intranasal versus intragastric administration of Lactobacillus paracasei NCC2461 in a mouse model of allergic airway inflammation and the specificity of different probiotics by comparing L. paracasei NCC2461 to Lactobacillus plantarum NCC1107. Methods. L. paracasei NCC2461 or L. plantarum NCC1107 strains were administered either intragastrically (NCC2461) or intranasally (NCC2461 or NCC1107) to OVA-sensitized mice challenged with OVA aerosols. Inflammatory cell recruitment into BALF, eotaxin and IL-5 production in the lungs were measured. Results. Intranasal L. paracasei NCC2461 efficiently protected sensitized mice upon exposure to OVA aerosols in a dose-dependent manner as compared to control mice. Inflammatory cell number, eotaxin and IL-5 were significantly reduced in BALF. Intranasal supplementation of L. paracasei NCC2461 was more potent than intragastric application in limiting the allergic response and possibly linked to an increase in T regulatory cells in the lungs. Finally, intranasal L. plantarum NCC1107 reduced total and eosinophilic lung inflammation, but increased neutrophilia and macrophages infiltration. Conclusion. A concerted selection of intervention schedule, doses, and administration routes (intranasal versus intragastric) may markedly contribute to modulate airway inflammation in a probiotic strain-specific manner.

N-Glycans on secretory component

Human beings live in symbiosis with billions of microorganisms colonizing mucosal surfaces. The understanding of the mechanisms underlying this fine-tuned intestinal balance has made significant processes during the last decades. We have recently demonstrated that the interaction of SIgA with Gram-positive bacteria is essentially based on Fab-independent, glycan-mediated recognition. Results obtained using mouse hybridoma- and colostrum-derived secretory IgA (SIgA) consistently show that N-glycans present on secretory component (SC) play a crucial role in the process. Natural coating may involve specific Gram-positive cell wall components, which may explain selective recognition at the molecular level. More widely, the existence of these complexes is involved in the modulation of intestinal epithelial cell (IEC) responses in vitro and the formation of intestinal biofilms. Thus, SIgA may act as one of the pillars in homeostatic maintenance of the microbiota in the gut, adding yet another facet to its multiple roles in the mucosal environment.

agglutinating secretory IgA preserves intestinal epithelial cell integrity during apical infection by Shigella flexneri

ABSTRACT Shigella flexneri, by invading intestinal epithelial cells (IECs) and inducing inflammatory responses of the colonic mucosa, causes bacillary dysentery. Although M cells overlying Peyers patches are commonly considered the primary site of entry of S. flexneri, indirect evidence suggests that bacteria can also use IECs as a portal of entry to the lamina propria. Passive delivery of secretory IgA (SIgA), the major immunoglobulin secreted at mucosal surfaces, has been shown to protect rabbits from experimental shigellosis, but no information exists as to its molecular role in maintaining luminal epithelial integrity. We have established that the interaction of virulent S. flexneri with the apical pole of a model intestinal epithelium consisting of polarized Caco-2 cell monolayers resulted in the progressive disruption of the tight junction network and actin depolymerization, eventually resulting in cell death. The lipopolysaccharide (LPS)-specific agglutinating SIgAC5 monoclonal antibody (MAb), but not monomeric IgAC5 or IgGC20 MAbs of the same specificity, achieved protective functions through combined mechanisms, including limitation of the interaction between S. flexneri and epithelial cells, maintenance of the tight junction seal, preservation of the cell morphology, reduction of NF-κB nuclear translocation, and inhibition of proinflammatory mediator secretion. Our results add to the understanding of the function of SIgA-mediated immune exclusion by identifying a mode of action whereby the formation of immune complexes translates into maintenance of the integrity of epithelial cells lining the mucosa. This novel mechanism of protection mediated by SIgA is important to extend the arsenal of effective strategies to fight against S. flexneri mucosal invasion.