Antigen-bearing dendritic cells from the sublingual mucosa recirculate to distant systemic lymphoid organs to prime mucosal CD8 T cells.

Effector T cells are described to be primed in the lymph nodes draining the site of immunization and to recirculate to effector sites. Sublingual immunization generates effector T cells able to disseminate to the genital tract. Herein, we report an alternative mechanism that involves the recirculation of antigen-bearing dendritic cells (DCs) in remote lymphoid organs to prime T cells. Sublingual immunization with a muco-adhesive model antigen unable to diffuse through lymphatic or blood vessels induced genital CD8 T cells. The sublingual draining lymph nodes were not mandatory to generate these lymphocytes, and antigen-bearing DCs from distant lymph nodes and spleen were able to prime specific CD8 T cells in a time- and dose-dependent manner. This study demonstrates, for the first time, that antigen-bearing DCs originating from the site of immunization recirculate to distant lymphoid organs and provides insights into the mechanism of distant CD8 T-cell generation by sublingual immunization.

Anti-bacterial and anti-toxic immunity induced by a killed whole-cell-cholera toxin B subunit cholera vaccine is essential for protection against lethal bacterial infection in mouse pulmonary cholera model.

The lack of appropriate animal model for studying protective immunity has limited vaccine development against cholera. Here, we demonstrate a pulmonary cholera model conferred by intranasal administration of mice with live Vibrio cholerae. The bacterial components, but not cholera toxin, caused lethal and acute pneumonia by inducing massive inflammation. Intranasal immunization with Dukoral, comprising killed whole bacteria and recombinant cholera toxin B subunit (rCTB), developed both mucosal and systemic antibody responses with protection against the lethal challenge. Either rCTB-free Dukoral or rCTB alone partially protected the mice against the challenge. However, reconstitution of rCTB-free Dukoral with rCTB restored full protection. Parenteral immunization with Dukoral evoked strong systemic immunity without induction of mucosal immunity or protection from the challenge. These results suggest that both anti-bacterial and anti-toxic immunity are required for protection against V. cholerae-induced pneumonia, and this animal model is useful for pre-clinical evaluation of candidate cholera vaccines.

B cell and T cell immunity in the female genital tract: potential of distinct mucosal routes of vaccination and role of tissue-associated dendritic cells and natural killer cells.

The female genital mucosa constitutes the major port of entry of sexually transmitted infections. Most genital microbial pathogens represent an enormous challenge for developing vaccines that can induce genital immunity that will prevent their transmission. It is now established that long-lasting protective immunity at mucosal surfaces has to involve local B-cell and T-cell effectors as well as local memory cells. Mucosal immunization constitutes an attractive way to generate systemic and genital B-cell and T-cell immune responses that can control early infection by sexually transmitted pathogens. Nevertheless, no mucosal vaccines against sexually transmitted infections are approved for human use. The mucosa-associated immune system is highly compartmentalized and the selection of any particular route or combinations of routes of immunization is critical when defining vaccine strategies against genital infections. Furthermore, mucosal surfaces are complex immunocompetent tissues that comprise antigen-presenting cells and also innate immune effectors and non-immune cells that can act as 'natural adjuvants' or negative immune modulators. The functions of these cells have to be taken into account when designing tissue-specific antigen-delivery systems and adjuvants. Here, we will discuss data that compare different mucosal routes of immunization to generate B-cell and T-cell responses in the genital tract, with a special emphasis on the newly described sublingual route of immunization. We will also summarize data on the understanding of the effector and induction mechanisms of genital immunity that may influence the development of vaccine strategies against genital infections.

Mucosal delivery routes for optimal immunization: targeting immunity to the right tissues.

The mucosal immune system exhibits a high degree of anatomic compartmentalization related to the migratory patterns of lymphocytes activated at different mucosal sites. The selective localization of mucosal lymphocytes to specific tissues is governed by cellular "homing" and chemokine receptors in conjunction with tissue-specific addressins and epithelial cell-derived chemokines that are differentially expressed in "effector" tissues. The compartmentalization of mucosal immune responses imposes constraints on the selection of vaccine administration route. Traditional routes of mucosal immunization include oral and nasal routes. Other routes for inducing mucosal immunity include the rectal, vaginal, sublingual, and transcutaneous routes. Sublingual administration is a new approach that results in induction of mucosal and systemic T cell and antibody responses with an exceptionally broad dissemination to different mucosae, including the gastrointestinal and respiratory tracts, and the genital mucosa. Here, we discuss how sublingual and different routes of immunization can be used to generate immune responses in the desired mucosal tissue(s).

Topical immunization strategies.

Research has yielded an abundance of vaccine candidates against mucosal infections, but only few mucosal vaccines have been registered for human use. Extensive research is being carried out to identify new and safe adjuvants for mucosal immunization, novel delivery systems, including live vectors and reporter molecules for tissue- and cell-specific targeting of vaccine antigens. If these candidates are to reach those in need, several lessons from clinical and field research carried out under resource-poor settings must be considered. These lessons include the need to develop new vaccines that can be administered topically onto the skin or to the mucosa, without needles or expensive delivery devices. Such topical vaccines must be able to protect all age groups at risk, be safe and effective in immunocompromised people, and be able to contain epidemics following complex emergencies. The anatomical compartmentalization of immune responses imposes constraints on the selection of topical route(s) of vaccine administration and on strategies for measuring these responses, especially in young infants. Thus, the selection of any particular route of immunization is critical when designing and formulating vaccines against organ-specific infections.

Mucosally induced immunological tolerance, regulatory T cells and the adjuvant effect by cholera toxin B subunit.

Induction of peripheral immunological tolerance by mucosal administration of selected antigens (Ags) ('oral tolerance') is an attractive, yet medically little developed, approach to prevent or treat selected autoimmune or allergic disorders. A highly effective way to maximize oral tolerance induction for immunotherapeutic purposes is to administer the relevant Ag together with, and preferably linked to the non-toxic B subunit protein of cholera toxin (CTB). Oral, nasal or sublingual administration of such Ag/CTB conjugates or gene fusion proteins have been found to induce tolerance with superior efficiency compared with administration of Ag alone, including the suppression of various autoimmune disorders and allergies in animal models. In a proof-of-concept clinical trial in patients with Behcet's disease, this was extended with highly promising results to prevent relapse of autoimmune uveitis. Tolerization by mucosal Ag/CTB administration results in a strong increase in Ag-specific regulatory CD4(+) T cells, apparently via two separate pathways: one using B cells as APCs and leading to a strong expansion of Foxp3(+) Treg cells which can both suppress and mediate apoptotic depletion of effector T cells, and one being B cell-independent and associated with development of Foxp3(-) regulatory T cells that express membrane latency-associated peptide and transforming growth factor (TGF-beta) and/or IL-10. The ability of CTB to dramatically increase mucosal Ag uptake and presentation by different APCs through binding to GM1 ganglioside (which makes most B cells effective APCs irrespective of their Ag specificity), together with CTB-mediated stimulation of TGF-beta and IL-10 production and inhibition of IL-6 formation may explain the dramatic potentiation of oral tolerance by mucosal Ags presented with CTB.

Enteric vaccines for the developing world: a challenge for mucosal immunology.

Enteric infections kill approximately two million children under the age of 5 in developing countries and cause more than four billion disease episodes worldwide each year. In addition, these diseases affect the growth, cognitive function, and quality of life negatively. There is an urgent need for vaccines that induce effective and long-lasting intestinal immunity against diarrheal infections, especially during infancy and early childhood. Yet, most vaccines available are formulated on an empirical basis. To date, arguably, vaccines have done more for immunologists than immunologists have done for vaccines.

Sublingual 'oral tolerance' induction with antigen conjugated to cholera toxin B subunit generates regulatory T cells that induce apoptosis and depletion of effector T cells.

Sublingual (s.l.) immunotherapy has in the last decade emerged as an effective approach to desensitize patients with pollen, food and insect sting allergies. This treatment has recently also attracted interest as a potential modality to control self-reactive T-cell responses associated with autoimmune disorders. Here, we show that s.l. administration of ovalbumin (OVA) conjugated to cholera toxin B subunit (CTB) (OVA/CTB) can efficiently suppress peripheral effector T (Teff) cell responses to OVA in mice that had adoptively received OVA-specific T-cell receptor (TCR) transgenic CD4(+) T cells, and that the suppression was associated with the development of OVA-specific Foxp3(+)CD25(+)CD4(+) regulatory T (Treg) cells as well as with apoptosis (Annexin V(+)) and depletion of OVA-specific Teff cells in peripheral lymph nodes. The induction of Teff cell apoptosis by s.l. OVA/CTB administration was found to be critically dependent on CD25(+) Treg cells but independent of IL-10 production. Our results suggest that s.l administration of a CTB-conjugated antigen can efficiently induce peripheral Teff cell tolerance through the induction of antigen-specific Treg cells that both inhibit Teff cell proliferation and cytokine production and induce Teff cell apoptosis and depletion.

Sublingual tolerance induction with antigen conjugated to cholera toxin B subunit induces Foxp3+CD25+CD4+ regulatory T cells and suppresses delayed-type hypersensitivity reactions.

Although sublingual (s.l.) immunotherapy with selected allergens is safe and often effective for treating patients with allergies, knowledge of the immunological mechanisms involved remains limited. Can s.l. administration of antigen (Ag) induce peripheral immunological tolerance and also suppress delayed-type hypersensitivity (DTH) responses? To what extent can s.l.-induced tolerance be explained by the generation of Foxp3+CD25+CD4+ regulatory T cells (T(reg))? This study addressed these questions in mice and compared the relative efficacy of administering ovalbumin (OVA) conjugated to cholera toxin B (CTB) subunit with administration of the same Ag alone. We found that s.l. administration of a single or even more efficiently three repeated 40-mug doses of OVA/CTB conjugate suppressed T-cell proliferative responses to OVA by cervical lymph node (CLN), mesenteric lymph node (MLN) and spleen cells and concurrently strongly increased the frequency of Ag-specific T(reg) in CLN, MLN and spleen and also transforming growth factor-beta (TGF-beta) levels in serum. The CLN and splenic cells from OVA/CTB-treated BALB/c mice efficiently suppressed OVA-specific T-cell receptor (TCR) transgenic (DO11.10) CD25-CD4+ effector T-cell proliferation in vitro. Further, s.l. treatment with OVA/CTB completely suppressed OVA-specific DTH responses in vivo and T-cell proliferative responses in mice immunized subcutaneously with OVA in Freund's complete adjuvant. The intracellular expression of Foxp3 was strongly increased in OVA-specific (KJ1-26+) CD4+ T cells from OVA/CTB-treated mice. Thus, s.l. administration of CTB-conjugated Ag can efficiently induce peripheral T-cell tolerance associated with strong increases in serum TGF-beta levels and in Ag-specific Foxp3+CD25+CD4+ T(reg) cells.

Nasal administration of Schistosoma mansoni egg antigen-cholera B subunit conjugate suppresses hepatic granuloma formation and reduces mortality in S. mansoni-infected mice.

Granulomatous inflammation in schistosomiasis is a delayed-type hypersensitivity reaction mediated by CD4+ T cells specific for parasite egg antigens (Ags). In an attempt to control T-cell responses leading to excessive harmful inflammation and granuloma formation, especially in the liver, BALB/c mice were intranasally (i.n.) treated with soluble Schistosoma mansoni egg Ags (SEA) conjugated to cholera toxin B subunit (CTB), a mucosa-binding protein with demonstrated capacity to suppress inflammatory T-cell functions after mucosal administration. Treatment with CTB-SEA significantly conjugate a reduced liver granuloma formation in infected mice associated with decreased SEA specific Th1- and Th2-type immune responses by liver leukocytes. Importantly, treatment with CTB-SEA conjugate also significantly reduced the mortality in chronically infected mice. In S. mansoni-infected large-granuloma forming CBA mice, i.n. treatment with purified Sm-p40, the major egg antigen, conjugated to CTB likewise significantly inhibited hepatic egg granuloma formation. A reduction of SEA-driven lymphoproliferation and of interferon (IFN)-gamma, interleukin (IL)-4 and IL-5 production, together with an increase in transforming growth factor (TGF)-beta1 production, were observed in splenic cells from CTB-Sm-p40-treated SEA-sensitized mice, as well as in liver leukocytes from CTB-Sm-p40-treated schistosome-infected mice. These results indicate that mucosal administration of SEA or purified Sm-p40 antigen in conjunction with CTB is highly effective in curtailing immunopathologic manifestations of schistosomiasis in vivo in infected hosts.

Local and systemic immune responses to rectal administration of recombinant cholera toxin B subunit in humans.

The induction of immune responses to rectally administered recombinant cholera toxin B subunit (CTB) in humans was studied. Three immunizations induced high levels of CTB-specific antibody-secreting cells, particular of the immunoglobulin A isotype, in both rectum and peripheral blood. Antitoxin antibody responses in rectal secretions and serum were also found.

Enhanced immunological tolerance against allograft rejection by oral administration of allogeneic antigen linked to cholera toxin B subunit.

A single oral intragastric administration of cholera toxin B subunit (CTB) conjugated to allogeneic thymocytes (ATC, 4 x 10(7) cells) under conditions allowing the CTB to bind the complex to GM1 ganglioside receptors was shown to be efficacious in inducing peripheral T cell tolerance associated with significant suppression of both primary and secondary accelerated rejection of heart allografts when tested in mice. Allogeneic in vivo delayed-type hypersensitivity (DTH), in vitro cytotoxicity responses, and mixed lymphocyte reactions (MLR) by T cells from mesenteric lymph nodes (MLN), popliteal lymph nodes (PLN), and spleen were significantly reduced in mice treated with the CTB-ATC conjugate, as were also the numbers of cells in these organs producing IL-2, IFN-gamma, or IL-4. In contrast, a marked increase in the production of IL-4 in Peyer's patches (PP) and of TGF-beta(1) in PLN was observed. The suppressive potential of T cells from PP and/or MLN after oral treatment with CTB-ATC was further evident by intraperitoneal transfer of such cells from CTB-ATC-treated animals to primed recipients, which led to marked suppression of both allogen-specific DTH and MLR responses. A critical role for PP in inducing peripheral tolerance after oral CTB-ATC treatment was indicated by the absence of tolerance induction in animals whose PP had been destroyed before treatment with CTB-ATC. The results indicate that the protection against allograft rejection by oral treatment with CTB-ATC is mediated by T cells and associated with a strong induction of IL-4 production at mucosal sites and TGF-beta(1) at the effector sites.

Oral administration of cholera toxin B subunit conjugated to myelin basic protein protects against experimental autoimmune encephalomyelitis by inducing transforming growth factor-beta-secreting cells and suppressing chemokine expression.

The efficacy and mechanism of immunosuppression against experimental autoimmune encephalomyelitis (EAE) by oral low-dose administration of myelin basic protein (MBP) conjugated to cholera toxin B subunit (CTB) were investigated in Lewis rats immunized with MBP together with complete Freund's adjuvant 4 days before the start of treatment. Oral treatment with CTB-MBP conjugate gave almost complete protection against disease, an effect that was totally abrogated by including a low dose of cholera holotoxin (CT). The protection by CTB-MBP was associated with a dramatic reduction in the number of leukocytes staining for CD4, CD8, IL-2R or MHC class II in the spinal cord as examined by immunohistochemistry. The mRNA expressions of T(h)1 cytokines IFN-gamma, IL-12 and tumor necrosis factor-alpha, as well as of chemokines monocyte chemotactic protein (MCP)-1 and RANTES in the spinal cord were also reduced by 76-94%, as assessed by in situ hybridization. In contrast, transforming growth factor (TGF)-beta mRNA-expressing cells were strongly increased in the spinal cord from animals treated orally with the CTB-MBP conjugate. In the draining peripheral lymph nodes, the number of MBP-specific TGF-beta mRNA-expressing cells was also increased, whereas there was a decrease in cells expressing T(h)1 or T(h)2 cytokine mRNA. Protection against EAE could be transferred by injection of cells from the mesenteric lymph nodes of animals fed with CTB-MBP into naive animals exposed to encephalitogenic T cells. The results indicate that the protective anti-inflammatory effect by oral treatment with CTB-MBP conjugate is, to a large extent, due to the induction of TGF-beta-secreting suppressive-regulatory T cells and to local down-regulation of MCP-1 and RANTES in the spinal cord.

Differential effect of cholera toxin on CD45RA+ and CD45RO+ T cells: specific inhibition of cytokine production but not proliferation of human naive T cells.

We have studied how cholera toxin (CT) and its non-toxic cell-binding B-subunit (CTB) affect the activation of pure human T cells in an anti-CD3-driven system. CT, as opposed to CTB, strongly suppressed the proliferative responses as well as cytokine production in CD4+ and CD8+ T cells. CT however, had a differential effect on naive and activated/memory T cell subsets. Costimulation through exogenous IL-2 or through CD28 cross-linking rescued the proliferation of CT-treated naive CD45RA+ T cells, but not of activated/memory CD45RO+ cells. IL-2 production and IL-2 receptor expression were markedly reduced by CT in all T cell fractions, i.e. also in CD45RA+ cells which had maintained proliferative responses. However, the proliferative responses of CT-treated CD45RA+ T cells were IL-2-dependent, as shown by blocking experiments using anti-IL-2 antibodies. These results indicate (i) that CTB has no cytostatic effect on human T cells, (ii) that CT affects proliferation and cytokine production by two different signal pathways, and (iii) that CT might interact with a signal pathway generated through or influenced by CD45.

Prolonged oral treatment with low doses of allergen conjugated to cholera toxin B subunit suppresses immunoglobulin E antibody responses in sensitized mice.

BACKGROUND: Oral tolerance is a long recognized method for inducing systemic immunological tolerance. However, large doses of antigen and frequent administrations are often required. By linking the antigen to the nontoxic mucosa-binding B subunit of cholera toxin (CTB), the required amount can be dramatically reduced. We have previously shown that mucosal administration of small amounts of antigens coupled to CTB can suppress peripheral Th1 cell-reactivity and associated inflammatory immunopathology in both naive and systemically-immunized animals. Induction of oral tolerance by repeated feeding of relatively small doses of antigen has, in some cases been shown to involve the generation of regulatory Th2-like CD4+ T cells, and hence could promote rather than suppress type I immunoglobulin (Ig) E-mediated allergic responses. OBJECTIVES: We examined whether oral prophylactic or therapeutic administration of a model allergen coupled to CTB would modulate allergen-specific IgE responses in high IgE responder Balb/c mice. METHODS: Ovalbumin (OVA) was used as a model allergen. Mice were treated perorally with free or CTB-coupled OVA before or after systemic priming with alum-adsorbed OVA. Allergen-specific IgE levels in serum were measured with the passive cutaneous anaphylaxis test at various time-points. RESULTS: Oral administration of a single low dose of CTB-linked OVA, prior to systemic sensitization and challenge with OVA, suppressed allergen-specific serum IgE antibody responses. Treatment with comparable doses of free OVA was much less effective. Most importantly, oral treatment with CTB-OVA conjugate could also suppress an already initiated IgE antibody response, but to achieve such a 'therapeutic effect', administration of multiple low doses of conjugate over a long time was required. Oral treatment with CTB-OVA conjugate could also effectively suppress antigen-specific Th1-mediated delayed-type hypersensitivity. Thus treatment with a CTB-conjugated model allergen can affect a broad range of T-cell-driven immune responses, even in antigen-experienced animals. CONCLUSION: These results may impact on the development of therapeutic vaccines against type I allergies.

Mucosal and systemic antibody responses after peroral or intranasal immunization: effects of conjugation to enterotoxin B subunits and/or of co-administration with free toxin as adjuvant.

The mucosa-binding molecules cholera toxin (CT) from Vibrio cholerae and heat-labile enterotoxin (LT) from Escherichia coli have previously been used as mucosal adjuvants and carriers for many types of antigen. However, since these molecules are toxic and cannot be used in human vaccines, it is important to study whether their non-toxic mucosa-binding B subunits, CTB and LTB, can be used as alternative safe mucosal adjuvants and/or carrier molecules. We have as a model protein antigen used human gammaglobulin (HGG) for admixture with or chemical conjugation to recombinantly produced CTB and LTB, respectively, and measured antigen-specific local secretory IgA antibodies in saponin extracts from intestine and lung tissue by ELISA following intra-nasal (i.n.) or per-oral (p.o.) immunization. The results show that local antibody formation against HGG was increased after immunization with conjugated as compared to free HGG. However, while the conjugates alone gave rise to significant immune responses in the lung and also, to a lesser degree, in the intestine after i.n. immunization, co-administration of a small amount of free CT/LT as adjuvant was needed to induce a significant immune response in the intestine after p.o. immunization. We also found that following i.n. immunization, the addition of CTB to HGG, without coupling, increased the mucosal immune response to some extent, indicating that CTB by itself can work as an adjuvant by the i.n. route of immunization. A striking finding was that, as a carrier, CTB was superior to LTB when the conjugates were used by the oral but not by the i.n. route of immunization. In conclusion, conjugation of an antigen to mucosa-binding molecules such as CTB and/or LTB can dramatically increase their mucosal immunogenicity. This approach may thus be useful in the preparation of mucosal vaccines.

Mucosal immunity and tolerance: relevance to vaccine development.

The mucosal immune system of mammals consists of an integrated network of lymphoid cells which work in concert with innate host factors to promote host defense. Major mucosal effector immune mechanisms include secretory antibodies, largely of immunoglobulin A (IgA) isotype, cytotoxic T cells, as well as cytokines, chemokines and their receptors. Immunologic unresponsiveness (tolerance) is a key feature of the mucosal immune system, and deliberate vaccination or natural immunization by a mucosal route can effectively induce immune suppression. The diverse compartments located in the aerodigestive and genitourinary tracts and exocrine glands communicate via preferential homing of lymphocytes and antigen-presenting cells. Mucosal administration of antigens may result in the concomitant expression of secretory immunoglobulin A (S-IgA) antibody responses in various mucosal tissues and secretions, and under certain conditions, in the suppression of immune responses. Thus, developing formulations based on efficient delivery of selected antigens/tolerogens, cytokines and adjuvants may impact on the design of future vaccines and of specific immunotherapeutic approaches against diseases associated with untoward immune responses, such as autoimmune disorders, allergic reactions, and tissue-damaging inflammatory reactions triggered by persistent microorganisms.

Anatomic segmentation of the intestinal immune response in nonhuman primates: differential distribution of B cells after oral and rectal immunizations to sites defined by their source of vascularization.

We show that the distribution of specific antibodies and antibody-secreting cells in the intestine after oral and rectal immunizations corresponds to the vascularization and lymph drainage patterns of the gut. Oral immunizations induce antibody responses along the parts of the intestine connected to the superior mesenteric vessels and lymph ducts, whereas rectal immunizations induce antibody responses along the parts of the intestine associated with the inferior mesenteric vessels and ducts.