The mucosal immune system: From dentistry to vaccine development

Abstract

The oral cavity is the beginning of the aero-digestive tract, which is covered by mucosal epithelium continuously under the threat of invasion of pathogens, it is thus protected by the mucosal immune system. In the early phase of our scientific efforts for the demonstration of mucosal immune system, dental science was one of major driving forces due to their foreseeability to use oral immunity for the control of oral diseases. The mucosal immune system is divided functionally into, but interconnected inductive and effector sites. Intestinal Peyer's patches (PPs) are an inductive site containing antigen-sampling M cells and immunocompetent cells required to initiate antigen-specific immune responses. At effector sites, PP-originated antigen-specific IgA B cells become plasma cells to produce polymeric IgA and form secretory IgA by binding to poly-Ig receptor expressed on epithelial cells for protective immunity. The development of new-generation mucosal vaccines, including the rice-based oral vaccine MucoRice, on the basis of the coordinated mucosal immune system is a promising strategy for the control of mucosal infectious diseases.

Figure 1.

Coordination between inductive and effector sites for the induction and regulation of antigen-specific mucosal immune responses. Antigens in the lumens of the gastrointestinal tract, nasal cavity, and tear ducts are endocytosed by M cells located on the follicle-associated epithelium (FAE) of the mucosa-associated lymphoid tissues. In the case of gut-associated lymphoid tissue or Peyer’s patches, M cells located in the FAE form the subepithelial dome structure, and antigen-presenting cells such as dendritic cells (DCs) lie immediately beneath the FAE. M-cell–endocytosed antigens are immediately processed by DCs, which transport antigens to underlying T cell zones through molecular interactions such as CCL19–CCR7 and CCL20–CCR6. Antigen-primed T cells support the induction of IgA-committed B cells (IgA⁺ B cells) owing to the biologic influences of transforming growth factor (TGF)-β, IL-2, IL-5, IL-6, IL-10, and the CD40–CD40 ligand (CD40L) interaction. In addition, IgA⁺ B cells acquire mucosal-imprinting molecules, such as CCR9, CCR10, and α4β7 integrin, and subsequently migrate to the effector sites. At the effector sites (e.g., the intestinal lamina propria), IgA⁺ B cells differentiate into plasma cells after stimulation by the IgA-enhancing cytokines IL-5, IL-6, and IL-10, which are secreted by antigen-specific Th2 cells. Dimeric or polymeric IgA secreted from plasma cells is transported to the mucosal surface as secretory IgA (SIgA) through the binding to polymeric Ig receptor expressed on the basal membrane of epithelial cells (ECs).

Figure 2.

Characteristics of antigen-sampling M cells located in the follicle-associated epithelium (FAE) of Peyer’s patches (PPs) and villous epithelium. M cells are preferentially located in the FAE of mucosa-associated lymphoid tissues and characterized by their unique morphologies: an irregular brush border, few microvilli, and decreased glycocalyx. NKM 16-2-4 is a monoclonal antibody specifically recognizing an α(1,2) fucose–containing carbohydrate moiety of murine M cells. When NKM 16-2-4 was applied to a tissue section of PPs, it specifically bound the apical surfaces of M cells in the FAE of the PPs (red cells in the bottom left part of the figure). Furthermore, NKM 16-2-4 recognized another M cell (known as villous M cell) located in the villous epithelium (red cells in the bottom right part of the figure), which is an additional “gateway” cell and has the similar morphologic features as M cell. Some contents of the figure have been adopted and modified from our original article reference 65.

Figure 3.

Specific delivery of vaccine antigen to M cells located in the follicle associated epithelium (FAE) of Peyer’s patches (PPs) induces antigen-specific mucosal IgA and serum IgG responses. NKM 16-2-4 is an M cell-specific monoclonal antibody (mAb) and used as an antigen delivery vehicle to M cells located in FAE of PPs. When mice were orally immunized with the mAb-conjugated vaccine antigen (e.g., botulinum toxoid [BT]) (NKM 16-2-4-BT), the mAb-conjugated vaccine antigen (NKM 16-2-4-BT) was preferentially delivered to M cells in the FAE of PPs. Furthermore, oral immunization with the chimeric form of vaccine antigen NKM 16-2-4-BT induced BT-specific mucosal IgA and serum IgG antibody responses with full neutralizing activity against botulinum toxin. Some contents of the figure have been adopted and modified from our original article reference 62.

Figure 4.

Contributions of fusion science from agriculture, gene engineering, and immunology to rice-based antigen-delivery technology and the potential oral vaccine MucoRice. (A) The gene encoding cholera toxin B subunit (CTB) is transduced into rice seed by means of an Agrobacterium-mediated method. The resultant CTB protein accumulates in the rice protein body (PB), which is a unique protein-storage organelle. (B) When the section of MucoRice-CTB seeds was stained with gold particle-conjugated anti-CTB, electron microscopic analysis showed that MucoRice-CTB preferentially expressed CTB in the protein body (PB)-I and -II of rice seeds (B-1). MucoRice-CTB powder containing 15 µg of CTB and recombinant CTB (rCTB: 15 µg) were treated with or without pepsin (0.5 mg/ml) for 1 hr at 37 ℃ and then subjected to Western blotting analysis. The rCTB protein was easily digested by pepsin (right column with + pepsin), whereas the CTB protein expressed in the protein body of rice (MucoRice-CTB) was resistant to digestive enzymes (left column with + pepsin) (B-2). In addition, MucoRice-CTB was stable at room temperature for years and thus the amount of CT expressed in the rice seeds did not change after 18 months storage period (B-3). (C) Following oral immunization, MucoRice-CTB is delivered effectively to gut mucosal surfaces and is endocytosed by M cells located follicle associated epithelium of Peyer’s patch. (D) Oral administration of MucoRice-CTB in mice effectively elicited CT-neutralizing serum IgG and fecal IgA antibodies and protects mice from the diarrhea induced by orally challenged CT. Some contents of the figure have been adopted and modified from our original article reference 91.