The mannose receptor (CD206) identifies a population of colonic macrophages in health and inflammatory bowel disease

To understand the contribution of mononuclear phagocytes (MNP), which include monocyte-derived intestinal macrophages, to the pathogenesis of inflammatory bowel disease (IBD), it is necessary to identify functionally-different MNP populations. We aimed to characterise intestinal macrophage populations in patients with IBD. We developed 12-parameter flow cytometry protocols to identify and human intestinal MNPs. We used these protocols to purify and characterize colonic macrophages from colonic tissue from patients with Crohn’s disease (CD), ulcerative colitis (UC), or non-inflamed controls, in a cross-sectional study. We identify macrophage populations (CD45⁺CD64⁺ HLA-DR⁺) and describe two distinct subsets, differentiated by their expression of the mannose receptor, CD206. CD206⁺ macrophages expressed markers consistent with a mature phenotype: high levels of CD68 and CD163, higher transcription of IL-10 and lower expression of TREM1. CD206⁻ macrophages appear to be less mature, with features more similar to their monocytic precursors. We identified and purified macrophage populations from human colon. These appear to be derived from a monocytic precursor with high CCR2 and low CD206 expression. As these cells mature, they acquire expression of IL-10, CD206, CD63, and CD168. Targeting the newly recruited monocyte-derived cells may represent a fruitful avenue to ameliorate chronic inflammation in IBD.

Intestinal cDC1 drive cross-tolerance to epithelial-derived antigen via induction of FoxP3+CD8+ Tregs

Although CD8⁺ T cell tolerance to tissue-specific antigen (TSA) is essential for host homeostasis, the mechanisms underlying peripheral cross-tolerance and whether they may differ between tissue sites remain to be fully elucidated. Here, we demonstrate that peripheral cross-tolerance to intestinal epithelial cell (IEC)-derived antigen involves the generation and suppressive function of FoxP3⁺CD8⁺ T cells. FoxP3⁺CD8⁺ T_reg generation was dependent on intestinal cDC1, whose absence led to a break of tolerance and epithelial destruction. Mechanistically, intestinal cDC1-derived PD-L1, TGFβ, and retinoic acid contributed to the generation of gut-tropic CCR9⁺CD103⁺FoxP3⁺CD8⁺ T_regs Last, CD103-deficient CD8⁺ T cells lacked tolerogenic activity in vivo, indicating a role for CD103 in FoxP3⁺CD8⁺ T_reg function. Our results describe a role for FoxP3⁺CD8⁺ T_regs in cross-tolerance in the intestine for which development requires intestinal cDC1.

Guardians of the epithelium: macrophages protect against toxic fungal derivatives

A recent paper in Cell proposes a new role for macrophages in the distal colonic mucosa, namely the generation of balloon-like processes (BLPs) that sample luminal contents and protect epithelial cells from the toxic effects of fungal metabolites absorbed during this process. Here Allan Mowat and Calum Bain discuss the implications of these novel findings for intestinal physiology and macrophage biology, highlighting how they extend our understanding of how tissue resident macrophages can adapt precisely to the physiological needs of individual anatomical niches.

Publisher Correction: Delphine Parrott, a pioneer of T cell biology

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Mechanisms of innate immune activation by gluten peptide p31-43 in mice

Celiac disease (CD) is an immune-mediated enteropathy triggered by gluten in genetically susceptible individuals. Innate immunity contributes to the pathogenesis of CD, but the mechanisms remain poorly understood. Although previous in vitro work suggests that gliadin peptide p31-43 acts as an innate immune trigger, the underlying pathways are unclear and have not been explored in vivo. Here we show that intraluminal delivery of p31-43 induces morphological changes in the small intestinal mucosa of normal mice consistent with those seen in CD, including increased cell death and expression of inflammatory mediators. The effects of p31-43 were dependent on MyD88 and type I IFNs, but not Toll-like receptor 4 (TLR4), and were enhanced by coadministration of the TLR3 agonist polyinosinic:polycytidylic acid. Together, these results indicate that gliadin peptide p31-43 activates the innate immune pathways in vivo, such as IFN-dependent inflammation, relevant to CD. Our findings also suggest a common mechanism for the potential interaction between dietary gluten and viral infections in the pathogenesis of CD.

The lymph nodes draining the small intestine and colon are anatomically separate and immunologically distinct

Dendritic cells (DCs) in the small intestine (SI) and colon are fundamental to direct intestinal immune responses; they migrate to the mesenteric lymph nodes (MLNs) and prime T cells. We demonstrate anatomical segregation of lymphatic drainage from the intestine, specifically that DCs from the SI and colon migrate to different nodes within the MLN, here called the sMLN and cMLN. As a consequence, different frequencies of DC subsets observed in the SI and colon are reflected among the DCs in the sMLN and cMLN. Consistent with the SI's function in absorbing food, fed antigen is presented in the sMLN, but not in the cMLN. Furthermore, the levels of expression of CCR9 and α4β7 are increased on T cells in the sMLN compared with the cMLN. DCs from the cMLN and colon are unable to metabolize vitamin A to retinoic acid (RA); thus, DCs may contribute to the differential expression of tissue homing markers observed in the sMLN and cMLN. In summary, the sMLN and cMLN, and the DCs that migrate to these LNs are anatomically and immunologically separate. This segregation allows immune responses in the SI and colon to be controlled independently.

CCR2(+)CD103(-) intestinal dendritic cells develop from DC-committed precursors and induce interleukin-17 production by T cells

The identification of intestinal macrophages (mφs) and dendritic cells (DCs) is a matter of intense debate. Although CD103(+) mononuclear phagocytes (MPs) appear to be genuine DCs, the nature and origins of CD103(-) MPs remain controversial. We show here that intestinal CD103(-)CD11b(+) MPs can be separated clearly into DCs and mφs based on phenotype, gene profile, and kinetics. CD64(-)CD103(-)CD11b(+) MPs are classical DCs, being derived from Flt3 ligand-dependent, DC-committed precursors, not Ly6C(hi) monocytes. Surprisingly, a significant proportion of these CD103(-)CD11b(+) DCs express CCR2 and there is a selective decrease in CD103(-)CD11b(+) DCs in mice lacking this chemokine receptor. CCR2(+)CD103(-) DCs are present in both the murine and human intestine, drive interleukin (IL)-17a production by T cells in vitro, and show constitutive expression of IL-12/IL-23p40. These data highlight the heterogeneity of intestinal DCs and reveal a bona fide population of CCR2(+) DCs that is involved in priming mucosal T helper type 17 (Th17) responses.

Lymph-borne CD8α+ dendritic cells are uniquely able to cross-prime CD8+ T cells with antigen acquired from intestinal epithelial cells

Cross-presentation of cellular antigens is crucial for priming CD8(+) T cells, and generating immunity to intracellular pathogens--particularly viruses. It is unclear which intestinal phagocytes perform this function in vivo. To address this, we examined dendritic cells (DCs) from the intestinal lymph of IFABP-tOVA 232-4 mice, which express ovalbumin in small intestinal epithelial cells (IECs). Among lymph DCs (LDCs) only CD103(+) CD11b(-) CD8α(+) DCs cross-present IEC-derived ovalbumin to CD8(+) OT-I T cells. Similarly, in the mesenteric lymph nodes (MLNs), cross-presentation of IEC-ovalbumin was limited to the CD11c(+) MHCII(hi) CD8α(+) migratory DCs, but absent from all other subsets, including the resident CD8α(hi) DCs. Crucially, delivery of purified CD8α(+) LDCs, but not other LDC subsets, into the MLN subcapsular lymphatic sinus induced proliferation of ovalbumin-specific, gut-tropic CD8(+) T cells in vivo. Finally, in 232-4 mice treated with R848, CD8α(+) LDCs were uniquely able to cross-prime interferon γ-producing CD8(+) T cells and drive their migration to the intestine. Our results clearly demonstrate that migrating CD8α(+) intestinal DCs are indispensable for cross-presentation of cellular antigens and, in conditions of inflammation, for the initial differentiation of effector CD8(+) T cells. They may therefore represent an important target for the development of antiviral vaccinations.

Intestinal macrophages and dendritic cells: what's the difference?

Mononuclear phagocytes (MPs) in the murine intestine, comprising dendritic cells (DCs) and macrophages (Mϕs), perform disparate yet complementary immunological functions. Functional analyses of these distinct MP subsets have been complicated by the substantial overlap in their surface phenotypes. Here, we review recent findings that have enabled more accurate definition of these MP subsets. We discuss these recent advances in the context of the current understanding of the functions of DCs and Mϕs in the maintenance of intestinal homeostasis, and how their functions may alter when homeostasis is disrupted.

Oral tolerance to food protein

Oral tolerance is the state of local and systemic immune unresponsiveness that is induced by oral administration of innocuous antigen such as food proteins. An analogous but more local process also regulates responses to commensal bacteria in the large intestine and, together, mucosally induced tolerance appears to prevent intestinal disorders such as food allergy, celiac disease, and inflammatory bowel diseases. Here we discuss the anatomical basis of antigen uptake and recognition in oral tolerance and highlight possible mechanisms underlying the immunosuppression. We propose a model of stepwise induction of oral tolerance in which specialized populations of mucosal dendritic cells and the unique microenvironment of draining mesenteric lymph nodes combine to generate regulatory T cells that undergo subsequent expansion in the small intestinal lamina propria. The local and systemic effects of these regulatory T cells prevent potentially dangerous hypersensitivity reactions to harmless antigens derived from the intestine and hence are crucial players in immune homeostasis.

Differences in the Kinetics, Amplitude, and Localization of ERK Activation in Anergy and Priming Revealed at the Level of Individual Primary T Cells by Laser Scanning Cytometry

One of the potential mechanisms of peripheral tolerance is the unresponsiveness of T cells to secondary antigenic stimulation as a result of the induction of anergy. It has been widely reported that antigenic unresponsiveness may be due to uncoupling of MAPK signal transduction pathways. However, such signaling defects in anergic T cell populations have been mainly identified using immortalized T cell lines or T cell clones, which do not truly represent primary Ag-specific T cells. We have therefore attempted to quantify signaling events in murine primary Ag-specific T cells on an individual cell basis, using laser-scanning cytometry. We show that there are marked differences in the amplitude and cellular localization of phosphorylated ERK p42/p44 (ERK1/2) signals when naive, primed and anergic T cells are challenged with peptide-pulsed dendritic cells. Primed T cells display more rapid kinetics of phosphorylation and activation of ERK than naive T cells, whereas anergic T cells display a reduced ability to activate ERK1/2 upon challenge. In addition, the low levels of pERK found in anergic T cells are distributed diffusely throughout the cell, whereas in primed T cells, pERK appears to be targeted to the same regions of the cell as the TCR. These data suggest that the different consequences of Ag recognition by T cells are associated with distinctive kinetics, amplitude, and localization of MAPK signaling.

A role for dendritic cells in the priming of antigen-specific CD4+ and CD8+ T lymphocytes by immune-stimulating complexes in vivo

Immune-stimulating complexes (ISCOMS) are adjuvant vectors which are unusual in being able to prime both CD4(+) and CD8(+) T cells by parenteral and mucosal routes. However, their mode of action is unclear and to define better the cellular interactions involved we have studied the ability of ISCOMS containing ovalbumin (OVA) to prime TCR transgenic CD4(+) or CD8(+) T cells in vivo. Immunization with OVA ISCOMS caused activation and clonal expansion of CD4(+) and CD8(+) T cells in the T cell areas of the draining lymph nodes, followed by the migration of both CD4(+) and CD8(+) T cells into the B cell follicle. The T cells were primed to proliferate and secrete IFN-gamma after re-stimulation in vitro with the appropriate OVA peptide and CD8(+) T cell priming occurred in the absence of CD4(+) T cells. Increasing the number of dendritic cells (DC) in vivo with flt3 ligand augmented the expansion and activation of the OVA-specific T cells, particularly CD8(+) T cells. These studies indicate DC play a central role in the priming of both CD4(+) and CD8(+) T cells in vivo, and suggest that an ability to target DC may allow ISCOMS to be powerful vaccine vectors for stimulating protective immunity.

CTA1-DD-immune stimulating complexes: a novel, rationally designed combined mucosal vaccine adjuvant effective with nanogram doses of antigen

Mucosally active vaccine adjuvants that will prime a full range of local and systemic immune responses against defined antigenic epitopes are much needed. Cholera toxin and lipophilic immune stimulating complexes (ISCOMS) containing Quil A can both act as adjuvants for orally administered Ags, possibly by targeting different APCs. Recently, we have been successful in separating the adjuvant and toxic effects of cholera toxin by constructing a gene fusion protein, CTA1-DD, that combines the enzymatically active CTA1-subunit with a B cell-targeting moiety, D, derived from Staphylococcus aureus protein A. Here we have extended this work by combining CTA1-DD with ISCOMS, which normally target dendritic cells and/or macrophages. ISCOMS containing a fusion protein comprising the OVA(323-339) peptide epitope linked to CTA1-DD were highly immunogenic when given in nanogram doses by the s.c., oral, or nasal routes, inducing a wide range of T cell-dependent immune responses. In contrast, ISCOMS containing the enzymatically inactive CTA1-R7K-DD mutant protein were much less effective, indicating that at least part of the activity of the combined vector requires the ADP-ribosylating property of CTA1. No toxicity was observed by any route. To our knowledge, this is the first report on the successful combination of two mechanistically different principles of adjuvant action. We conclude that rationally designed vectors consisting of CTA1-DD and ISCOMS may provide a novel strategy for the generation of potent and safe mucosal vaccines.

Oral tolerance

The ability of the mucosal immune system to distinguish between harmful and harmless antigens is essential for mounting protective immune responses and preventing the induction of mucosal pathology yet the basis for this remains unclear. As fed antigen can also exert systemic effects understanding oral tolerance and priming will also have important consequences for therapy and vaccination. Here we will not only review the increasing amount of information about the potential mechanisms of oral tolerance and priming but also attempt to shed some light on how differences in the uptake and handling i.e. 'the journey' of orally administered antigen may promote these mechanisms.

Preparation of immune stimulating complexes (ISCOMs) as adjuvants

Purified proteins are often poorly immunogenic and in such cases the induction of primary immune responses requires use of an adjuvant. The immune stimulating complex (ISCOM) has a unique ability to provoke a full range of immune response to protein antigens, after both parenteral and oral immunization. This unit describes techniques for incorporating proteins into the ISCOM structure, a process that requires the presence of exposed hydrophobic regions on the protein. The basic protocol outlines a method for preparation of ISCOMs containing inherently nonhydrophobic proteins, to which palmitic acid has been attached covalently. Two alternate protocols are given that do not require covalent modification of the protein. In the first, hydrophobic groups are revealed by acid treatment of the protein. The second describes preparation of ISCOMs containing integral membrane proteins that therefore possess a hydrophobic transmembrane domain.

Induction of oral tolerance in the primed immune system: influence of antigen persistence and adjuvant form

Oral tolerance is being promoted as a therapy for autoimmune diseases and therefore will need to be functional in a primed immune system. In previous studies, we found that although primed mice could be tolerized by feeding ovalbumin (OVA), the degree of the tolerance and its effects on individual components of the systemic immune response were more limited than that found in naive animals. Here we increased the dose and frequency of antigen feeding in an attempt to extend the effects of oral tolerance in primed mice and to understand why its effects are limited under these conditions. Increasing the amounts of OVA fed, up to a single dose of 400 mg, or using multiple feeds of 5 x 5 or 5 x 25 mg OVA, did not radically alter the extent of tolerance, with DTH responses, antigen-specific proliferation, and IL5 and IFN-gamma production still being tolerized, but antibody responses remaining generally resistant. The deficient tolerance in primed mice could not be overcome by waiting for maximum clonal expansion to wane and was not influenced by persistent release of antigen from a depot adjuvant. We conclude that the resistance of primed mice to oral tolerance may be due to the fact that antigen-experienced T cells may be inherently resistant to induction of tolerance, or that the microenvironment of the primed immune system inhibits the delivery of tolerogenic signals to antigen-specific T cells.

Basic mechanisms and clinical implications of oral tolerance

Oral tolerance is the physiologic mechanism that prevents hypersensitivity to food proteins and probably to commensal bacteria. It has also attracted attention as a means of administering therapy for autoimmune and inflammatory diseases. Although evidence indicates that both clonal inactivation and active regulatory mechanisms may play a role and that the induction of these may be determined selectively by the feeding regimen used to induce tolerance, the exact mechanisms of oral tolerance remain unclear. Here, we discuss recent evidence that fed antigens are presented to CD4(+) T cells by antigen-presenting cells (APCs) that lack costimulatory activity, resulting in partial activation of T cells followed by a state of unresponsiveness. This seems to occur in many tissues of the immune system but may be particularly important in the draining mesenteric lymph node. Resting dendritic cells may be the predominant population of APCs involved in oral tolerance, and conditions that activate dendritic cells allow the induction of productive immunity. Conventionally, presentation of antigen in the absence of costimulation is thought to induce T-cell anergy, but evidence now indicates that anergic T cells can also act as regulatory cells via the production of inhibitory mediators or via cognate interactions with APCs and other T cells. We discuss how an ability to deactivate APCs may explain bystander suppressor activity in oral tolerance, and we consider how the production of transforming growth factor-beta or interleukin-10 by Th3 or T regulatory 1 cells may contribute to tolerance in vivo. We speculate that both the production of inhibitory mediators and the delivery of suppression via cognate interactions may be properties of otherwise "anergic" T cells.

Normal induction of oral tolerance in the absence of a functional IL-12-dependent IFN-gamma signaling pathway

There is considerable evidence that regulatory cytokines play an important role in mediating the systemic tolerance found after oral administration of protein Ags. Although most existing work has focused on cytokines such as IL-4, IL-10, and TGF-beta, recent evidence from TCR transgenic systems suggests that the induction of oral tolerance is accompanied by priming of Ag-specific IFN-gamma production. IFN-gamma has also been implicated as a mediator of T cell tolerance in other models in vivo and in vitro, including that induced by aerosol administration of protein. We show here that feeding tolerogenic doses of OVA primes for IFN-gamma production in the spleen of mice with a normal T cell repertoire. However, depleting IFN-gamma at the time of feeding OVA had no effect on the induction of tolerance. In addition, tolerance was induced normally in both IFN-gamma receptor knockout (IFN-gammaR-/-) and IL-12 p40 knockout (IL-12-/-) mice. This was the case for all components of the systemic immune response and also with a variety of feeding protocols, including those believed to induce distinct regulatory mechanisms. We conclude that IL-12-dependent IFN-gamma-mediated regulation does not play an essential role in oral tolerance.

Immune-stimulating complexes induce an IL-12-dependent cascade of innate immune responses

The development of subunit vaccines requires the use of adjuvants that act by stimulating components of the innate immune response. Immune-stimulating complexes (ISCOMS) containing the saponin adjuvant Quil A are potential vaccine vectors that induce a wide range of Ag-specific responses in vivo encompassing both humoral and CD4 and CD8 cell-mediated immune responses. ISCOMS are active by both parenteral and mucosal routes, but the basis for their adjuvant properties is unknown. Here we have investigated the ability of ISCOMS to recruit and activate innate immune responses as measured in peritoneal exudate cells. The i.p. injection of ISCOMS induced intense local inflammation, with early recruitment of neutrophils and mast cells followed by macrophages, dendritic cells, and lymphocytes. Many of the recruited cells had phenotypic evidence of activation and secreted a number of inflammatory mediators, including nitric oxide, reactive oxygen intermediates, IL-1, IL-6, IL-12, and IFN-gamma. Of the factors that we investigated further only IL-12 appeared to be essential for the immunogenicity of ISCOMS, as IL-6- and inducible nitric oxide synthase knockout (KO) mice developed normal immune responses to OVA in ISCOMS, whereas these responses were markedly reduced in IL-12KO mice. The recruitment of peritoneal exudate cells following an injection of ISCOMS was impaired in IL-12KO mice, indicating a role for IL-12 in establishing the proinflammatory cascade. Thus, ISCOMS prime Ag-specific immune responses at least in part by activating IL-12-dependent aspects of the innate immune system.

Oral vaccination with immune stimulating complexes

There is a need for non-living adjuvant vectors which will induce a full range of local and systemic immune responses to orally administered purified antigens. Here we describe our experience with lipophilic immune stimulating complexes (ISCOMS) containing the saponin adjuvant Quil A. When given orally, ISCOMS containing the model protein antigen ovalbumin (OVA) induce a wide range of systemic immune responses, including Th1 and Th2 CD4 dependent activity, class I MHC restricted cytotoxic T-cell responses and local production of secretory IgA antibodies. More recent results indicate that ISCOMS may act partly by enhancing the uptake of protein from the gut. In addition, intraperitoneal injection of ISCOMS recruits and activates many components of the innate immune system. including neutrophils, macrophages, and dendritic cells. In parallel, there is increased production of nitric oxide (NO), reactive oxygen intermediates (ROI), interleukins (IL) 1, 6, 12, and gamma interferon (gammaIFN). Of these factors, only IL12 is essential for the immunogenicity of ISCOMS in vivo, as mucosal and systemic responses to ISCOMS are reduced in IL12KO mice, but not in IL4KO, IL6KO, inducible NO synthase (iNOS) KO, or gammaIFN receptor KO mice. We propose that ISCOMS act by targetting antigen and adjuvant to macrophages and/or dendritic cells. This pathway may be amenable to exploitation for vaccine development, especially if combined with another vector with a different mucosal adjuvant profile, such as cholera toxin.

Expanding dendritic cells in vivo enhances the induction of oral tolerance

The intestine is under perpetual challenge from both pathogens and essential nutrients, yet the mucosal immune system is able to discriminate effectively between harmful and innocuous Ags. It is likely that this selective immunoregulation is dependent on the nature of the APC at sites where gut Ags are processed and presented. Dendritic cells (DC) are considered the most potent of APC and are renowned for their immunostimulatory role in the initiation of immune responses. To investigate the role of DC in regulating the homeostatic balance between mucosal immunity and tolerance, we treated mice with Flt3 ligand (Flt3L), a growth factor that expands DC in vivo, and assessed subsequent systemic immune responsiveness using mouse models of oral tolerance. Surprisingly, mice treated with Flt3L to expand DC exhibited more profound systemic tolerance after they were fed soluble Ag. Most notably, tolerance could be induced in Flt3L-treated mice using very low doses of Ag that were ineffective in control animals. These findings contrast with the generally accepted view of DC as immunostimulatory APC and furthermore suggest a pivotal role for DC during the induction of tolerance following mucosal administration of Ag.

Expanding Dendritic Cells In Vivo Enhances the Induction of Oral Tolerance

The intestine is under perpetual challenge from both pathogens and essential nutrients, yet the mucosal immune system is able to discriminate effectively between harmful and innocuous Ags. It is likely that this selective immunoregulation is dependent on the nature of the APC at sites where gut Ags are processed and presented. Dendritic cells (DC) are considered the most potent of APC and are renowned for their immunostimulatory role in the initiation of immune responses. To investigate the role of DC in regulating the homeostatic balance between mucosal immunity and tolerance, we treated mice with Flt3 ligand (Flt3L), a growth factor that expands DC in vivo, and assessed subsequent systemic immune responsiveness using mouse models of oral tolerance. Surprisingly, mice treated with Flt3L to expand DC exhibited more profound systemic tolerance after they were fed soluble Ag. Most notably, tolerance could be induced in Flt3L-treated mice using very low doses of Ag that were ineffective in control animals. These findings contrast with the generally accepted view of DC as immunostimulatory APC and furthermore suggest a pivotal role for DC during the induction of tolerance following mucosal administration of Ag.

Immune stimulating complexes as mucosal vaccines

There is a need for non-living adjuvant vectors that will allow a full range of local and systemic immune responses to orally administered purified antigens. Here we describe our experience with lipophilic immune-stimulating complexes (ISCOMs) containing the saponin adjuvant Quil A. When given orally, ISCOMs containing the model protein antigen ovalbumin (OVA) induce a wide range of systemic immune responses, including Th1 and Th2 CD4-dependent activity, serum IgG antibodies and class I MHC-restricted cytotoxic T cell responses. In addition, there is local production of secretory IgA antibodies in the intestine itself, as well as priming of CD4 and CD8 T cell responses in the draining lymphoid tissues. Preliminary results indicate that the mucosal adjuvant properties of ISCOMs may reflect their ability to deliver antigen combined with the pro-inflammatory properties of Quil A in a particulate form. Of the many inflammatory mediators induced, interleukin-12, derived from dendritic cells and/or macrophages, appears to be of central importance. These results indicate that ISCOMs may prove to be useful mucosal vaccine vectors with functions which are distinct from existing vectors of this type.

Immune responses to dietary antigens: oral tolerance

The concept of immunologically mediated tolerance to food antigens through exposure to mucosal antigen has been the subject of continuous scientific debate. After a decline in interest in the mid-1980s, oral tolerance has again attracted the attention of immunologists. Here, Stephan Strobel and Allan Mowat discuss how this central immunological principle has potential new therapeutic applications for the treatment of autoimmune, inflammatory and possibly food-allergic diseases.

Preservation of mucosal and systemic adjuvant properties of ISCOMS in the absence of functional interleukin-4 or interferon-gamma

Adjuvants are a critical component of non-viable vaccine vectors, particularly for those to be used via mucosal routes. Although most adjuvants act by inducing local inflammatory responses, the molecular basis of many of these effects is unclear. Here we have investigated whether interleukin-4 (IL-4) and interferon-gamma (IFN-gamma) are required for the induction of local and systemic immune responses by oral and parenteral administration of ovalbumin (OVA) in immune stimulating complexes (ISCOMS), a potent mucosal adjuvant vector. Our results show that after oral or systemic immunization with OVA ISCOMS, IL-4 knockout (IL4KO) and IFN-gamma receptor knockout (IFN-gamma RKO) mice develop an entirely normal range of immune responses including delayed-type hypersensitivity (DTH), serum immunoglobulin G (IgG) antibodies, T-cell proliferation and cytokine production, class I major histocompatibility complex (MHC)-restricted cytotoxic T lymphocyte (CTL) activity and intestinal IgA antibodies. These responses were of a similar magnitude to those found in the wild-type mice, indicating that the immunogenicity of ISCOMS is not influenced by the presence of IL-4 or IFN-gamma and emphasizing the potential of ISCOMS as widely applicable mucosal adjuvants.

Immunological consequences of intervention in established immune responses by feeding protein antigens

The usual result of feeding protein antigens to naive animals is the induction of profound immunological unresponsiveness and this is currently being exploited to treat inflammatory disease. Because the most useful therapeutic application of feeding antigen would be to suppress established disease, the aim of this study was to compare the immunological basis of oral tolerance induced by feeding a model antigen to naive and primed animals. We show that feeding 2-200 mg ovalbumin (OVA) to mice 7 days after immunisation with OVA in adjuvant produces dose-dependent suppression of delayed-type hypersensitivity (DTH), T cell proliferation, and both TH1 and TH2 cytokines, although serum IgG levels were unaffected. Feeding OVA before immunisation suppressed all these responses. Although feeding up to 8 days after immunisation could suppress some subsequent responses, tolerance was induced much more effectively when antigen was fed in the first 4 days after immunisation. Tolerance in primed mice was intact in IL-4-/- mice, indicating that it was not caused by selective upregulation of TH2 cells in vivo. We conclude that oral administration of protein antigen can inhibit ongoing responses by all effector T cell subsets, but the exact consequences, and therefore possibly the mechanisms, are different from those induced by tolerising naive mice. These findings may have important implications for designing therapeutic regimes exploiting oral tolerance.

Neutralizing IL-12 during induction of murine acute graft-versus-host disease polarizes the cytokine profile toward a Th2-type alloimmune response and confers long term protection from disease

Injection of parental spleen cells into BDF1 mice results in a graft-vs-host disease (GVHD), the nature of which is critically dependent on the parental haplotype. B6-->BDF1 mice develop a Th1-mediated immunosuppressive lethal GVHD, whereas DBA/2-->BDF1 mice develop a Th2-dependent chronic GVHD, characterized by autoantibody production and glomerulonephritis. In this study we show that neutralizing endogenous IL-12 for a brief period during the initiation of acute GVHD in B6-->BDF1 mice not only confers long term protection from the acute disease, but also permits full repopulation of the recipient with donor B6 lymphocytes. Antibody-treated animals showed normal T cell proliferation in response to Con A stimulation and remained healthy throughout the study. Splenocytes from such mice showed reduced in vitro production of IFN-gamma and enhanced production of IL-5 and IL-10, suggesting a permanent switch from a Th1 to a Th2 cytokine response, comparable to that associated with chronic GVHD in DBA/2-->BDF1 mice. In contrast to DBA/2-->BDF1 mice, however, anti-IL-12-treated B6-->BDF1 mice displayed only mild B cell hyper-responsiveness, as evidenced by a modest increase in serum IgG and IgE levels and moderate levels of anti-dsDNA Abs. Importantly, however, anti-IL-12-treated B6-->BDF1 mice showed no evidence of immune complex-mediated glomerulonephritis. These results demonstrate that neutralizing IL-12 is an effective means of preventing acute GVHD and does not result in the development of chronic GVHD, which might otherwise limit its application.

Neutralizing IL-12 during induction of murine acute graft-versus-host disease polarizes the cytokine profile toward a Th2-type alloimmune response and confers long term protection from disease

Injection of parental spleen cells into BDF1 mice results in a graft-vs-host disease (GVHD), the nature of which is critically dependent on the parental haplotype. B6-->BDF1 mice develop a Th1-mediated immunosuppressive lethal GVHD, whereas DBA/2-->BDF1 mice develop a Th2-dependent chronic GVHD, characterized by autoantibody production and glomerulonephritis. In this study we show that neutralizing endogenous IL-12 for a brief period during the initiation of acute GVHD in B6-->BDF1 mice not only confers long term protection from the acute disease, but also permits full repopulation of the recipient with donor B6 lymphocytes. Antibody-treated animals showed normal T cell proliferation in response to Con A stimulation and remained healthy throughout the study. Splenocytes from such mice showed reduced in vitro production of IFN-gamma and enhanced production of IL-5 and IL-10, suggesting a permanent switch from a Th1 to a Th2 cytokine response, comparable to that associated with chronic GVHD in DBA/2-->BDF1 mice. In contrast to DBA/2-->BDF1 mice, however, anti-IL-12-treated B6-->BDF1 mice displayed only mild B cell hyper-responsiveness, as evidenced by a modest increase in serum IgG and IgE levels and moderate levels of anti-dsDNA Abs. Importantly, however, anti-IL-12-treated B6-->BDF1 mice showed no evidence of immune complex-mediated glomerulonephritis. These results demonstrate that neutralizing IL-12 is an effective means of preventing acute GVHD and does not result in the development of chronic GVHD, which might otherwise limit its application.

The anatomical basis of intestinal immunity

The lymphoid tissues associated with the intestine are exposed continuously to antigen and are the largest part of the immune system. Many lymphocytes are found in organised tissues such as the Peyer's patches and mesenteric lymph nodes, as well as scattered throughout the lamina propria and epithelium of the mucosa itself. These lymphocyte populations have several unusual characteristics and the intestinal immune system is functionally and anatomically distinct from other, peripheral compartments of the immune system. This review explores the anatomical and molecular basis of these differences, with particular emphasis on the factors which determine how the intestinal lymphoid tissues discriminate between harmful pathogens and antigens which are beneficial, such as food proteins or commensal bacteria. These latter antigens normally provoke immunological tolerance, and inappropriate responses to them are responsible for immunopathologies such as food hypersensitivity and inflammatory bowel disease. We describe how interactions between local immune cells, epithelial tissues and antigen-presenting cells may be critical for the induction of tolerance and the expression of active mucosal immunity. In addition, the possibility that the intestine may act as an extrathymic site for T-cell differentiation is discussed. Finally, we propose that, under physiological conditions, immune responses to food antigens and commensal bacteria are prevented by common regulatory mechanisms, in which transforming growth factor beta plays a critical role.

Mechanisms of oral tolerance

Oral tolerance is the specific immunological unresponsiveness induced by feeding antigen. Although it is an obstacle to oral vaccination, it is probably the mechanism that prevents intestinal hypersensitivity reactions to food antigens and may provide a novel strategy for the treatment of a range of inflammatory disorders. Feeding antigen can provide stable and long-lasting tolerance of a wide range of immune responses to a variety of antigens. However, the mechanisms of oral tolerance and the major factors that influence them remain controversial.

Lymphocytes from orally tolerized mice display enhanced susceptibility to death by apoptosis when cultured in the absence of antigen in vitro

The mechanism responsible for the induction of immunological tolerance by oral administration of soluble antigen remains unclear. Here we show that, when cultured in vitro in the absence of antigen, lymphocytes from mice tolerized with a single feed of 25 mg of ovalbumin display an enhanced mortality in comparison with cells from immunized control animals. This increased cell death affects both CD4+ and CD8+ T-lymphocyte subsets, and morphological and flow cytometric analyses suggest that it occurs via apoptosis. All of the changes associated with the propensity of tolerant cells to die by apoptosis in vitro are reduced by the inclusion of the tolerizing antigen in the cultures. These results suggest that tolerance to dietary proteins is accompanied by functional changes in T lymphocytes that render them susceptible to apoptosis. This mechanism may underlie the profound and permanent tolerance to food antigens found under physiological conditions and may provide a useful basis for immunotherapy.

IL-12 is a central mediator of acute graft-versus-host disease in mice

Distinct forms of graft-vs-host disease (GVHD) occur in (C57B1/6 x DBA/2)F1 (BDF1) mice inoculated with either C57B1/6 or DBA/2 parental spleen cells, and it has been suggested that this reflects differential activation of CD4+ Th cell subsets. Transfer of B6 cells produces an acute GVHD, during which an early period of lymphoid hyperplasia precedes immunosuppression, weight loss, and mortality, and a Th1 pattern of cytokines is produced. Conversely, transfer of DBA/2 cells induces a chronic GVHD, in which no weight loss or mortality is observed, but an autoimmune, SLE-like GVHD develops in association with a Th2 pattern of cytokines. Recent work indicates that IL-12 plays a central role in the polarization of Th cell-dependent responses, and here we have examined its role in polarizing GVHD, by administering or depleting IL-12 during the afferent phase of both the acute and chronic forms of GVHD in BDF1 mice. In vivo neutralization of endogenous IL-12 ameliorated acute GVHD, in association with reduced splenic NK cell activity, IFN-gamma production, immunosuppression, weight loss, and mortality. Conversely, administration of exogenous murine rIL-12 exacerbates this disease and converts the chronic GVHD into a lethal acute GVHD-like syndrome. These results indicate that IL-12 plays an important role in the development of acute, but not chronic, GVHD and suggest that differential production of IL-12 early in the disease may underlie these distinct outcomes of the GVHD in BDF1 mice injected with different parental cells.

IL-12 is a central mediator of acute graft-versus-host disease in mice

Distinct forms of graft-vs-host disease (GVHD) occur in (C57B1/6 x DBA/2)F1 (BDF1) mice inoculated with either C57B1/6 or DBA/2 parental spleen cells, and it has been suggested that this reflects differential activation of CD4+ Th cell subsets. Transfer of B6 cells produces an acute GVHD, during which an early period of lymphoid hyperplasia precedes immunosuppression, weight loss, and mortality, and a Th1 pattern of cytokines is produced. Conversely, transfer of DBA/2 cells induces a chronic GVHD, in which no weight loss or mortality is observed, but an autoimmune, SLE-like GVHD develops in association with a Th2 pattern of cytokines. Recent work indicates that IL-12 plays a central role in the polarization of Th cell-dependent responses, and here we have examined its role in polarizing GVHD, by administering or depleting IL-12 during the afferent phase of both the acute and chronic forms of GVHD in BDF1 mice. In vivo neutralization of endogenous IL-12 ameliorated acute GVHD, in association with reduced splenic NK cell activity, IFN-gamma production, immunosuppression, weight loss, and mortality. Conversely, administration of exogenous murine rIL-12 exacerbates this disease and converts the chronic GVHD into a lethal acute GVHD-like syndrome. These results indicate that IL-12 plays an important role in the development of acute, but not chronic, GVHD and suggest that differential production of IL-12 early in the disease may underlie these distinct outcomes of the GVHD in BDF1 mice injected with different parental cells.

Inactivation of Th1 and Th2 cells by feeding ovalbumin

Several different mechanisms have been implicated in oral tolerance to protein antigens, depending on the nature and dose of antigen used and the species under study. Here, we have investigated the basis of unresponsiveness in a well-established model of oral tolerance in mice fed 25 mg ovalbumin (OVA). Our results show that CD8+ T-cell activity is suppressed by feeding OVA and that these cells are not required for the induction of tolerance. CD4+ T cells are essential for tolerance to occur, but both Th1 and Th2 cell-dependent functions are tolerized equally in OVA-fed mice. Peripheral lymph node cells from tolerized mice rapidly undergo apoptosis when cultured in vitro but produce substantial amounts of transforming growth factor beta (TGFbeta) in response to OVA. The appearance of tolerance in vivo is preceded by a transient phase of T-cell priming, and we propose that this model of oral tolerance reflects partial activation of T cells by fed antigen, leading to selective production of TGFbeta and consequent inactivation of all effector T cells. These findings indicate that the active suppression and clonal anergy identified previously in mice with oral tolerance may not be mutually exclusive phenomena.

Regulatory activity of endogenous and exogenous transforming growth factor beta in experimental intestinal immunopathology

Acute graft-versus-host disease (GvHD) is an inflammatory disorder associated with generalised damage to epithelial tissues, including the gastrointestinal tract. There is increasing evidence that this pathology is due to the effects of cytokines on epithelial cell proliferation and differentiation. However, it is unclear whether factors derived from immune cells act directly on epithelial cells or via other mediators whose principal role is to regulate cell growth under normal or diseased conditions. We show here that the increased crypt cell turnover and lymphocytic infiltration which occurs in the jejunum of mice with graft-versus-host reaction (GvHR) is accompanied by decreased enterocyte expression of transforming growth factor beta 2. Administration of exogenous TGF beta inhibits the crypt hyperplasia of GvHR and reduces systemic manifestations of GvHR such as increased splenic natural killer (NK) cell activity. In parallel, neutralisation of endogenous TGF beta by monoclonal antibody exacerbates both the proliferative and inflammatory components of intestinal and systemic GvHR. Thus, the immune system may induce epithelial pathology at least in part by altering the production of endogenous TGF beta. This cytokine may therefore prove a useful focus for therapeutic intervention in immunopathologies such as GvHD.

Induction of Th1 and Th2 CD4+ T cell responses by oral or parenteral immunization with ISCOMS

We examined the ability of oral or parenteral immunization with immune stimulating complexes containing ovalbumin (ISCOMS-OVA) to prime T cell proliferative and cytokine responses. A single subcutaneous immunization with ISCOMS-OVA primed potent antigen-specific proliferative responses in the draining popliteal lymph node, which were entirely dependent on the presence of CD4+ T cells. CD8+ T cells did not proliferate in vitro even in the presence of the appropriate peptide epitope and exogenous interleukin (IL)-2. Primed popliteal lymph node cells produced IL-2, IL-5 and interferon (IFN)-gamma, but not IL-4 when restimulated with OVA in vitro. Serum antigen-specific IgG1 and IgG2a antibody responses were also primed by subcutaneous immunization with ISCOMS-OVA, confirming the stimulation of both Th1 and Th2 cells in vivo. Spleen cells from subcutaneously primed mice produced a similar pattern of cytokines, indicating that disseminated priming had occurred. Oral immunization with ISCOMS-OVA also primed local antigen-specific proliferative responses in the mesenteric lymph node and primed an identical pattern of systemic cytokine responses in the spleen. The ability of ISCOMS to prime both Th1 and Th2 CD4+ T cell responses may be central to their potent adjuvant activities and confirm the potential of ISCOMS as future oral vaccine vectors.

T helper 2 cells are subject to high dose oral tolerance and are not essential for its induction

Oral administration of aqueous protein Ag results in profound immunologic tolerance, and it has been suggested previously that this reflects selective activation of Th subsets. Here we show that the induction of oral tolerance by feeding a single high dose of OVA to mice significantly reduces the production of both Th1- and Th2-dependent cytokines and is accompanied by a marked reduction of specific Abs of both the IgG2a and IgG1 isotypes in vivo. Oral tolerance was also induced normally in IL-4-deficient mice. These results indicate that both subsets of the Th cell are equally susceptible to the induction of tolerance with a single high dose of Ag delivered via the oral route and that this phenomenon does not require Th2 cells.

T helper 2 cells are subject to high dose oral tolerance and are not essential for its induction

Oral administration of aqueous protein Ag results in profound immunologic tolerance, and it has been suggested previously that this reflects selective activation of Th subsets. Here we show that the induction of oral tolerance by feeding a single high dose of OVA to mice significantly reduces the production of both Th1- and Th2-dependent cytokines and is accompanied by a marked reduction of specific Abs of both the IgG2a and IgG1 isotypes in vivo. Oral tolerance was also induced normally in IL-4-deficient mice. These results indicate that both subsets of the Th cell are equally susceptible to the induction of tolerance with a single high dose of Ag delivered via the oral route and that this phenomenon does not require Th2 cells.

Polarization of Th-cell responses: a phylogenetic consequence of nonspecific immune defence?

CD4+ T helper (Th)-cell responses are polarized into Th1 and Th2 types. Here, Paul Garside and Allan McI. Mowat propose that this is an evolutionary consequence of the relationship between specific adaptive immunity, individual components of the nonspecific inflammatory response and different types of infection. This concept has important implications both for understanding protective immunity and for vaccine design and delivery.

CD4+ but not CD8+ T cells are required for the induction of oral tolerance

It has been suggested that oral tolerance is mediated by CD8+ T lymphocytes, but the functional properties of these cells are unclear. Here we show that the induction of tolerance by feeding mice ovalbumin (OVA) does not prime antigen-specific class I MHC-restricted cytotoxic T cells in vivo. Indeed, such responses are markedly suppressed in mice fed OVA, and the induction of oral tolerance is abolished by depletion in vivo of CD4+ but not CD8+ T cells. These results indicate that CD8+ lymphocytes are unlikely to play a major role in the induction of oral tolerance and are the first demonstration that specific cytotoxic responses to an exogenous antigen can be suppressed by feeding antigen.

A role for IL-4 in immunologically mediated enteropathy

A number of clinical enteropathies are associated with a local cell-mediated immune (CMI) response, and experimental evidence indicates that cytokines are responsible for the intestinal pathology. We show here that depletion of IL-4 using MoAb or a soluble form of the IL-4 receptor (IL-4R) prevents the jejunal manifestations of a proliferative form of murine graft-versus-host reaction (GVHR) characterized by crypt hyperplasia and recruitment of intraepithelial lymphocytes (IEL). Depletion of IL-4 did not prevent the appearance of villus atrophy in a destructive model of GVHR, and had no effect on any indices of systemic immunity. These results indicate that IL-4 may play a selective role in mediating proliferative aspects of intestinal immunopathology, and suggest that this cytokine may provide a useful target for immunotherapy.

Differential cytokine production associated with distinct phases of murine graft-versus-host reaction

Previous studies which demonstrated that cytokines such as interferon-gamma (IFN-gamma) and tumour necrosis factor-alpha (TNF-alpha) are essential for the development of graft-versus-host reaction (GvHR) did not establish whether the individual cytokines were responsible for distinct features of the disease. In this report, we show that IFN-gamma production is associated with the early proliferative phase of the disease, whereas the late, destructive phase correlates with production of TNF-alpha. These studies may assist in the development of specific immunotherapies aimed at individual aspects of immunologically mediated disease.

Induction of mucosal and systemic immune responses by immunization with ovalbumin entrapped in poly(lactide-co-glycolide) microparticles

We have examined the range of mucosal and systemic immune responses induced by oral or parenteral immunization with ovalbumin (OVA) entrapped in poly(D,L-lactide-co-glycolide) (PLG) microparticles. A single subcutaneous immunization with OVA-PLG primed significant OVA-specific IgG and delayed-type hypersensitivity (DTH) responses. The DTH responses were of similar magnitude to those obtained using immunostimulating complexes (ISCOMS) as a potent control adjuvant, although ISCOMS stimulated higher serum IgG responses. Both vectors also primed OVA-specific in vitro proliferative responses in draining lymph node cells following a single immunization and strong OVA-specific CTL responses were found after intraperitoneal (i.p.) immunization. ISCOMS were more efficient in inducing cytotoxic T lymphocytes (CTL), requiring much less antigen and only ISCOMS could stimulate primary OVA-specific CTL responses in the draining lymph nodes. Multiple oral immunizations with OVA in PLG microparticles or in ISCOMS resulted in OVA-specific CTL responses and again ISCOMS seemed more potent as fewer feeds were necessary. Lastly, multiple feeds of OVA in PLG microparticles generated significant OVA-specific intestinal IgA responses. This is the first demonstration that PLG microparticles can stimulate CTL responses in vivo and our results highlight their ability to prime a variety of systemic and mucosal immune responses which may be useful in future oral vaccine development.

Immune-stimulating complexes as adjuvants for inducing local and systemic immunity after oral immunization with protein antigens

Orally active synthetic vaccines containing purified antigens would have many benefits for immunizing against systemic and mucosal diseases. However, several factors have limited the development of such vaccines, including the poor immunogenicity of purified proteins and their usual ability to induce tolerance when given orally. Here, we show that incorporation of ovalbumin (OVA) into immune-stimulating complexes (ISCOMS) containing saponin prevents the induction of oral tolerance in mice. In parallel, the spleen and mesenteric lymph node of mice fed OVA ISCOMS are primed for class I major histocompatibility complex (MHC)-restricted cytotoxic T-cell activity which recognizes physiologically processed epitopes on OVA. Oral immunization with OVA ISCOMS also stimulates high secretory IgA antibody responses in the intestine itself, as well as serum IgG antibodies. None of these active immune responses are detectable in mice fed OVA alone. Despite the potent priming of mucosal priming by OVA ISCOMS, re-exposure to antigen does not induce the intestinal immunopathology found in other systems after the breakdown of oral tolerance. Thus, ISCOMS have several unique properties as vectors for oral immunization and could provide a basis for future mucosal vaccines.

A role for interleukin-1 alpha in immunologically mediated intestinal pathology

Interleukin-1 (IL-1) is an important mediator of inflammation and has been implicated in several forms of immunopathology. Here we have investigated whether IL-1 plays a role in the enteropathy which occurs during a graft-versus-host reaction (GVHR) in mice. Non-irradiated (CBA x BALB/c) F1 mice with GVHR had increased production of IL-1 and treatment with rabbit anti-IL-1 alpha antibodies abolished the crypt hyperplasia and significantly reduced the parallel increase in crypt length which occurs in the jejunum. Antibody treatment had no effect on the accompanying increase in intraepithelial lymphocyte (IEL) counts or on the splenomegaly. Recombinant IL-1 itself produced villus atrophy, crypt hyperplasia and increased IEL counts in normal mice and stimulated the proliferation of an intestinal epithelial cell line in vitro. We propose that IL-1 plays an effector role in immunologically mediated enteropathy, either via direct effects on epithelial cells or secondary to an action on other, stromal cells in the mucosa.