SIgA–Shigella Immune Complexes Interact with Dectin-1 and SIGNR3 to Differentially Regulate Mouse Peyer's Patch and Mesenteric Lymph Node Dendritic Cell's Responsiveness

Engineered Secretory Immunoglobulin A provides insights on antibody-based effector mechanisms targeting Clostridiodes difficile

Secretory (S) Immunoglobin (Ig) A is the predominant mucosal antibody, which mediates host interactions with commensal and pathogenic microbes, including Clostridioides difficile. SIgA adopts a polymeric IgA structure that is bound by secretory component (SC). Despite significance, how SIgA supports diverse effector mechanisms is poorly characterized and SIgA-based therapies nonexistent. We engineered chimeric (c) SIgAs, in which we replaced SC domain D2 with a single domain antibody or a monomeric fluorescent protein, allowing us to investigate and enhance SIgA effector mechanisms. cSIgAs exhibited increased neutralization potency against C. difficile toxins, promoted bacterial clumping and cell rupture, and decreased cytotoxicity. cSIgA also allowed us to visualize and/or quantify C. difficile morphological changes and clumping events. Results reveal mechanisms by which SIgA combats C. difficile infection, demonstrate that cSIgA design can modulate these mechanisms, and demonstrate cSIgA's adaptability to modifications that might target a broad range of antigens and effector mechanisms.

Secretory IgA-ETEC F5 Immune Complexes Promote Dendritic Cell Differentiation and Prime T Cell Proliferation in the Mouse Intestine

Although secretory IgA (SIgA) is the dominant antibody in mucosal secretions, the capacity of the SIgA-antigen complex to prime the activation of dendritic cells (DCs) and T cells in the intestinal epithelium is not well understood. To this end, the SIgA-ETEC F5 immune complexes (ICs) were prepared via Ni-NTA pull-down. After injecting the ICs into the intestines of SPF BALB/c mice, most ICs were observed in the Peyer's patch (PP). We established a microfold (M) cell culture model in vitro for transport experiments and the inhibition test. To evaluate the priming effect of mucosal immunity, we employed the DC2.4 stimulation test, T lymphocyte proliferation assays, and cytokine detection assays. We found that the ICs were taken up via clathrin-dependent endocytosis through M cells. The high expression of costimulatory molecules CD86, CD80, and CD40 indicated that the ICs promoted the differentiation and maturation of DC2.4 cells. The stimulation index (SI) in the complex group was significantly higher than in the control group, suggesting that the ICs stimulated the proliferation of primed T cells. The secretion of some cytokines, namely TNF-α, IFN-γ, IL-2, IL-4, IL-5, and IL-6, in spleen cells from the immunized mice was upregulated. These results indicate that ETEC F5 delivery mediated by SIgA in PPs initiates mucosal immune responses.

Luminal bacteria coated with IgA and IgG during intestinal inflammation as a new and abundant stimulus for colonic macrophages

In the gut, secretory immunoglobulin A is the predominant humoral response against commensals, although healthy hosts also produce microbiota-specific IgG antibodies. During intestinal inflammation, the content of IgG in the lumen increases along with the proportion of commensal bacteria coated with this antibody, suggesting signalling through the IgG-CD64 axis in the pathogenesis of inflammatory bowel diseases. In this work, we evaluated day by day the frequency of faecal bacteria coated with IgA and IgG during the development of DSS colitis. We studied the phenotype and phagocytic activity of F4/80⁺ CD64⁺ colonic macrophages, as well as the production of cytokines and nitric oxide by lamina propria or bone marrow-derived macrophages after stimulation with IgA⁺ , IgG⁺ and IgA⁺ IgG⁺ bacteria. We found that the percentage of faecal IgA⁺ IgG⁺ double-coated bacteria increased rapidly during DSS colitis. Also, analysis of the luminal content of mice with colitis showed a markedly superior ability to coat fresh bacteria. IgA⁺ IgG⁺ bacteria were the most potent stimulus for phagocytic activity involving CD64 and Dectin-1 receptors. IgA⁺ IgG⁺ bacteria observed during the development of DSS colitis could represent a new marker to monitor permeability and inflammatory progression. The interaction of IgA⁺ IgG⁺ bacteria with CD64⁺ F4/80⁺ macrophages could be part of the complex cascade of events in colitis. Interestingly, after stimulation, CD64⁺ colonic macrophages showed features similar to those of restorative macrophages that are relevant for tissue repair and healing.

Microbiota-antibody interactions that regulate gut homeostasis

Immunoglobulin A (IgA) is the most abundant antibody at mucosal surfaces and has been the subject of many investigations involving microbiota research in the last decade. Although the classic functions of IgA include neutralization of harmful toxins, more recent investigations have highlighted an important role for IgA in regulating the composition and function of the commensal microbiota. Multiple reviews have comprehensively covered the literature that describes recent, novel mechanisms of action of IgA and development of the IgA response within the intestine. Here we focus on how the interaction between IgA and the microbiota promotes homeostasis with the host to prevent disease.

Mucosal vaccines: Strategies and challenges

Mucosal immunization has potential benefits over conventional parenteral immunization, eliciting immune defense in both mucosal and systemic tissue for protecting from pathogen invasion at mucosal surfaces. To provide a first line of protection at these entry ports, mucosal vaccines have been developed and hold a significant promise for reducing the burden of infectious diseases. However, until very recently, only limited mucosal vaccines are available. This review summarizes recent advances in selected aspects regarding mucosal vaccination, including appropriate administration routes, reasonable formulations, antigen-sampling and immune responses of mucosal immunity, and the strategies used to improve mucosal vaccine efficacy. Finally, the challenges of developing successful mucosal vaccines and the potential solutions are discussed.

The Role of the Polymeric Immunoglobulin Receptor and Secretory Immunoglobulins during Mucosal Infection and Immunity

The gastrointestinal tract houses millions of microbes, and thus has evolved several host defense mechanisms to keep them at bay, and prevent their entry into the host. One such mucosal surface defense is the secretion of secretory immunoglobulins (SIg). Secretion of SIg depends on the polymeric immunoglobulin receptor (pIgR), which transports polymeric Ig (IgA or IgM) from the basolateral surface of the epithelium to the apical side. Upon reaching the luminal side, a portion of pIgR, called secretory component (SC) is cleaved off to release Ig, forming SIg. Through antigen-specific and non-specific binding, SIg can modulate microbial communities and pathogenic microbes via several mechanisms: agglutination and exclusion from the epithelial surface, neutralization, or via host immunity and complement activation. Given the crucial role of SIg as a microbial scavenger, some pathogens also evolved ways to modulate and utilize pIgR and SIg to facilitate infection. This review will cover the regulation of the pIgR/SIg cycle, mechanisms of SIg-mediated mucosal protection as well as pathogen utilization of SIg.