Toll-like receptor 4 ligation enforces tolerogenic properties of oral mucosal Langerhans cells

Dendritic cells: bridging innate and adaptive immunity in atopic dermatitis

Much knowledge has been gained about the multifaceted functions of dendritic cells (DCs). The central role of various DC subtypes as bridges between innate and adaptive immunity has become more and more evident. However, a high number of differences exist in the expression of pattern-recognition receptors, the first sensors of the innate immune system, in particular Toll-like receptors (TLRs) by distinct DC subtypes (including myeloid and plasmacytoid DCs), their maturation stage, and tissue distribution, as well as state of health or disease. Furthermore, a plethora of variations in human and murine model systems have to be considered. This review sheds some light on this complex and rapidly growing field. It summarizes the most recent findings and deals with the role of TLR-expressing DCs as promoters of chronic inflammatory immune responses in patients with atopic dermatitis, as well as tolerogenic pathways. Therefore TLR-bearing DCs represent promising targets, which might help to improve tolerance induction during immunotherapeutic approaches in the future.

Sublingual grass pollen immunotherapy is associated with increases in sublingual Foxp3-expressing cells and elevated allergen-specific immunoglobulin G4, immunoglobulin A and serum inhibitory activity for immunoglobulin E-facilitated allergen binding to B cells

BACKGROUND

The mechanisms of sublingual immunotherapy (SLIT) are less well understood than those of subcutaneous immunotherapy (SCIT).

OBJECTIVES

To determine the effects of grass-pollen SLIT on oral mucosal immune cells, local regulatory cytokines, serum allergen-specific antibody subclasses and B cell IgE-facilitated allergen binding (IgE-FAB).

METHODS

Biopsies from the sublingual mucosa of up to 14 SLIT-treated atopics, nine placebo-treated atopics and eight normal controls were examined for myeloid dendritic cells (mDCs) (CD1c), plasmacytoid dendritic cells (CD303), mast cells (AA1), T cells (CD3) and Foxp3 using immunofluorescence microscopy. IL-10 and TGF-beta mRNA expression were identified by in situ hybridization. Allergen-specific IgG and IgA subclasses and serum inhibitory activity for binding of allergen-IgE complexes to B cells (IgE-FAB) were measured before, during and on the completion of SLIT.

RESULTS

Foxp3(+) cells were increased in the oral epithelium of SLIT- vs. placebo-treated atopics (P=0.04). Greater numbers of subepithelial mDCs were present in placebo-treated, but not in SLIT-treated, atopics compared with normal controls (P=0.05). There were fewer subepithelial mast cells and greater epithelial T cells in SLIT- compared with placebo-treated atopics (P=0.1 for both). IgG(1) and IgG(4) were increased following SLIT (P<0.001). Peak seasonal IgA(1) and IgA(2) were increased during SLIT (P<0.05). There was a time-dependent increase in serum inhibitory activity for IgE-FAB in SLIT-treated atopics.

CONCLUSIONS

SLIT with grass pollen extract is associated with increased Foxp3(+) cells in the sublingual epithelium and systemic humoral changes as observed previously for SCIT.

How tolerogenic dendritic cells induce regulatory T cells

Since their discovery by Steinman and Cohn in 1973, dendritic cells (DCs) have become increasingly recognized for their crucial role as regulators of innate and adaptive immunity. DCs are exquisitely adept at acquiring, processing, and presenting antigens to T cells. They also adjust the context (and hence the outcome) of antigen presentation in response to a plethora of environmental inputs that signal the occurrence of pathogens or tissue damage. Such signals generally boost DC maturation, which promotes their migration from peripheral tissues into and within secondary lymphoid organs and their capacity to induce and regulate effector T cell responses. Conversely, more recent observations indicate that DCs are also crucial to ensure immunological peace. Indeed, DCs constantly present innocuous self- and nonself-antigens in a fashion that promotes tolerance, at least in part, through the control of regulatory T cells (Tregs). Tregs are specialized T cells that exert their immunosuppressive function through a variety of mechanisms affecting both DCs and effector cells. Here, we review recent advances in our understanding of the relationship between tolerogenic DCs and Tregs.

The role of costimulatory molecules in allergic disease and asthma

The prevalence of allergic diseases has increased rapidly in recent years. It is well established that the deleterious allergic response is initiated by T-cell recognition of major histocompatibility class II-peptide complexes at the surface of antigen-presenting cells. While this first signal gives antigen specificity to the adaptive immune response, a second nonspecific costimulatory signal is required by T cells to become fully activated. This signal is provided by interactions between antigen-presenting cells and T cells through molecules borne at the surfaces of the two cell types. Depending on the type of molecules involved, this secondary signal can promote the development of an inflammatory allergic reaction or may favor immune regulation. Several molecules of the B7 family (CD80, CD86, PD-1, ICOS, CTLA-4) and tumor necrosis factor receptor family (OX40, CD30, 4-1BB, Fas, CD27, CD40) play an important role in delivering costimulatory signals in early and late phases of allergic response. Therefore, costimulatory molecules involved in promotion or prevention of allergic immune responses are potential targets for the development of novel therapeutic approaches. This review aims to recapitulate our current understanding of the relationship between allergic diseases and costimulatory molecules.

Therapeutic manipulation of immune tolerance in allergic disease

Immune tolerance - the adaptation of the immune system to external antigens or allergens - might be therapeutically manipulated to restore normal immunity in conditions such as allergy, asthma and autoimmune diseases. The field of allergen-specific immunotherapy is experiencing exciting and novel developments for the treatment of allergic and autoimmune diseases, and recent insights into the reciprocal regulation and counter-balance between different T-cell subsets is foreseen to facilitate new strategies for immunointervention. This Review highlights current knowledge of immunomodulatory therapies for the manipulation of immune tolerance and highlights recent approaches to improve allergen-specific immunotherapy for the treatment of allergic diseases.

Immune tolerance in allergy

Research on the mechanisms of immune regulation in allergy and asthma has shown substantial progress in recent years and has led to a variety of allergen-specific therapeutic and preventive approaches. Studies on the area of allergen-specific immunotherapy (allergen-SIT) have provided substantial knowledge on the mechanisms of allergic disease with novel developments for treatment and prevention. Several studies have demonstrated that increased numbers and the activation of allergen-specific T regulatory cells correlate with successful allergen-SIT. Particularly, targeting of the molecular mechanisms of immune tolerance and reciprocal regulation of effector and regulatory T-cell subsets are foreseen for the development of new strategies for immune intervention. This review focuses on current knowledge of immune tolerance to allergens in healthy immune response to allergens and allergen-SIT.

Mechanisms of sublingual immunotherapy

Allergen-specific sublingual immunotherapy is now recognized to be an efficacious and well-tolerated treatment for allergic rhinitis. Emerging treatment strategies are also aimed at the primary treatment of allergic asthma, particularly allergy to house dust mites. Knowledge of the exact mechanisms of action of sublingual immunotherapy is at a basic level, although there appear to be similarities to the immunological changes seen in subcutaneous immunotherapy. An improved understanding should allow the development of more effective treatment programs and widen the potential use of this form of immunotherapy. This review discusses the possible mechanism of action of sublingual immunotherapy, including data from animal and clinical studies, while comparing this with the current understanding of subcutaneous immunotherapy.

Sublingual immunotherapy: what have we learnt from the 'big trials'?

PURPOSE OF REVIEW

Recent systematic reviews support the use of sublingual immunotherapy for allergic rhinoconjunctivitis in adults, whereas data in children have been less convincing. The present review evaluates three recent 'definitive' trials in adults and one in children.

RECENT FINDINGS

Two large independent randomized controlled trials of grass allergen tablets have confirmed the efficacy of sublingual immunotherapy in adults with seasonal allergic rhinoconjunctivitis. Effects were both allergen dose-dependent and time-dependent. Tablets were well tolerated and equally effective in monosensitized compared with polysensitized patients and in patients with peak seasonal asthma (patients with perennial asthma were specifically excluded). Local side effects were common but largely self-limiting and not bothersome. There were no serious treatment-related adverse events. Results were similar in magnitude to those observed in a comparable study of subcutaneous immunotherapy using an alum-based vaccine. A trial of sublingual drops in children with hayfever in a primary care setting was negative, although these results could not be generalized.

SUMMARY

Sublingual immunotherapy represents an effective and well tolerated treatment for seasonal allergic rhinoconjunctivitis in adults. Current ongoing paediatric trials and evaluation of long-term effects in adults will further define its role in therapy.

The balance between immunity and tolerance: the role of Langerhans cells

Langerhans cells are immature skin-homing dendritic cells that furnish the epidermis with an immune surveillance system, and translate information between the internal and external milieu. Dendritic cells, in particular Langerhans cells, are gaining prominence as one of the potential principal players orchestrating the decision between immunity and tolerance. Langerhans cells capture aberrant self-antigen and pathogen-derived antigen for display to the efferent immune response. Recent evidence suggests redundancy in the antigen-presenting function of Langerhans cells, with dermal dendritic subsets capable of fulfilling an analogous role. There is mounting evidence that Langerhans cells can cross-prime T cells to recognize antigens. Langerhans cells are proposed to stimulate T regulatory cells, and are implicated in the pathogenesis of cutaneous T cell lymphoma.The phenotype of Langerhans cells, which may be tolerogenic or immunogenic, appears to depend on their state of maturity, inciting immunogen and cytokine environment, offering the potential for manipulation in immunotherapy.

Stimulation of human CD4⁺ T lymphocytes via TLR3, TLR5 and TLR7/8 up-regulates expression of costimulatory and modulates proliferation

The cells of innate and adaptive immunity, although activated by different ligands, engage in cross talk to ensure a successful immune outcome. Toll-like receptors (TLRs) are key components of the innate immune system and have the ability to detect microbial infection and trigger host defence responses. Otherwise, human T lymphocytes are able to produce most TLRs. Thus, we analyze the capability of some TLR ligands to modulate the function of highly-purified CD4⁺ T cells. We found that agents acting via TLRs (poly I:C, a TLR3 ligand; flagellin, a TLR5 ligand; and R848, a TLR7/8 ligand) are able to regulate the expression of costimulatory molecules both on purified antigen presenting cells and on purified T lymphocytes. Moreover, the activation mediated by TLRs determines a kinetic expression of B7-family members such as through an inhibition of T lymphocytes delayed proliferation. These findings suggest a functional role of some invading microorganisms in regulating acquired immunity.

New insights into mechanisms of immunoregulation in 2007

Substantial progress in understanding the mechanisms of immune regulation in allergic diseases and asthma has been made during the last year. In asthma, rhinitis, and atopic dermatitis the immune system is activated by allergens, autoantigens, and components of superimposed infectious agents. Immune regulation in the lymphatic organs and in the tissue has an important role in the control and suppression of allergic disease in all stages of the inflammatory process, such as cell migration to tissues, cells gaining an inflammatory and tissue-destructive phenotype in the tissues, and their interaction with resident tissue cells to augment the inflammation. After the discovery of regulatory T cells, the importance of their unique suppressive capacity was strongly emphasized for the suppression of effector T-cell responses. However, it seems that all 3 subsets of effector T(H)1, T(H)2, and T(H)17 cells, as well as regulatory T cells, regulate each other at the level of transcription, major cytokines, and surface molecules. This review highlights key advances in immune regulation that were published in the Journal of Allergy and Clinical Immunology.

Evaluation of subcutaneous versus mucosal (intranasal) allergen-specific rush immunotherapy in experimental feline asthma

Rush immunotherapy (RIT) is effective for the treatment of experimental feline allergic asthma. In humans, the safety profile of immunotherapy is improved by delivering allergen by a mucosal route. We hypothesized that mucosal (intranasal) RIT would have similar efficacy to subcutaneous RIT with improved safety. Twelve cats sensitized and challenged with Bermuda grass allergen (BGA) were randomized to receive subcutaneous (SC) or intranasal (IN) RIT. Increasing doses of BGA (20-200 microg) were administered over 24h followed by 200 microg BGA weekly as maintenance. Adverse reactions were recorded. Clinical respiratory scores after BGA aerosol challenge, bronchoalveolar lavage fluid (BALF) % eosinophils, and cytokine concentrations were measured before RIT (day 1) and at months 1, 3 and 6 (M1, M3, M6). More adverse events were recorded with SC RIT (n=12) compared with IN RIT (n=6). Respiratory scores were lower by M6 compared with D1 in both the groups. The % BALF eosinophils declined significantly after RIT for both groups (mean+/-SEM, SC RIT D1 62+/-12, M6 9+/-4; IN RIT D1 54+/-9, M6 14+/-6). The BALF IL-4:IFN-gamma ratio significantly decreased over time in the IN RIT group (mean+/-SEM, D1 2.4+/-0.2, M6 1.0+/-0.2). While both protocols decreased eosinophilic airway inflammation, the SC RIT protocol did not cause life-threatening adverse events and demonstrated more consistent resolution of clinical signs after allergen challenge. Either protocol could be considered for the treatment of feline allergic asthma.

[Specific immunotherapy for allergic rhinitis. Current methods and innovative developments]

Apart from allergen avoidance and pharmaceutical medication, specific immunotherapy (SIT) represents the third most important mainstay in a balanced therapeutic concept for the treatment of allergic rhinitis. For subcutaneous immunotherapy (SCIT) there is evidence for therapeutic clinical effectiveness, duration of the therapeutic result, prevention of bronchial asthma, reduced generation of additional co-sensitization and safety of the procedure. The present guidelines on allergen-specific immunotherapy (SIT) were established by the German allergy societies in 2006. Another option for specific immunotherapy in future is the sublingual route (SLIT). This method might play an important role as in-vitro studies indicated that oral Langerhans cells are potential target cells involved in the tolerance-induction of SLIT. However, questions concerning SLIT and prevention of bronchial asthma, reduced generation of co-sensitization and long-term effects have not yet been fully clarified. Furthermore, this paper gives an overview on recent innovative strategies in SCIT and SLIT, such as cluster and rush schemes, pre-seasonal therapy, recombinant allergen extracts and additive therapy with adjuvants or IgE-specific antibodies, high-dosis SLIT preparation or allergen tablets.

Oral dendritic cells mediate antigen-specific tolerance by stimulating TH1 and regulatory CD4+ T cells

BACKGROUND

A detailed characterization of oral antigen-presenting cells is critical to improve second-generation sublingual allergy vaccines.

OBJECTIVE

To characterize oral dendritic cells (DCs) within lingual and buccal tissues from BALB/c mice with respect to their surface phenotype, distribution, and capacity to polarize CD4(+) T-cell responses.

METHODS

In situ analysis of oral DCs was performed by immunohistology. Purified DCs were tested in vitro for their capacity to capture, process, and present the ovalbumin antigen to naive CD4(+) T cells. In vivo priming of ovalbumin-specific T cells adoptively transferred to BALB/c mice was analyzed by cytofluorometry in cervical lymph nodes after sublingual administration of mucoadhesive ovalbumin.

RESULTS

Three subsets of oral DCs with a distinct tissue distribution were identified: (1) a minor subset of CD207(+) Langerhans cells located in the mucosa itself, (2) a major subpopulation of CD11b(+)CD11c(-) and CD11b(+)CD11c(+) myeloid DCs at the mucosal/submucosal interface, and (3) B220(+)120G8(+) plasmacytoid DCs found in submucosal tissues. Purified myeloid and plasmacytoid oral DCs capture and process the antigen efficiently and are programmed to elicit IFN-gamma and/or IL-10 production together with a suppressive function in naive CD4(+) T cells. Targeting the ovalbumin antigen to oral DCs in vivo by using mucoadhesive particles establishes tolerance in the absence of cell depletion through the stimulation of IFN-gamma and IL-10-producing CD4(+) regulatory T cells in cervical lymph nodes.

CONCLUSION

The oral immune system is composed of various subsets of tolerogenic DCs organized in a compartmentalized manner and programmed to induce T(H)1/regulatory T-cell responses.

The immune privilege of the oral mucosa

Despite high bacterial colonization and frequent allergen contact, acute inflammatory and allergic reactions are rarely seen in the oral mucosa. Therefore we assert that immune tolerance predominates at this site and antigen presenting cells, such as dendritic cells and different T cell subtypes, serve as key players in oral mucosal tolerance induction. In this article we describe the mechanisms that lead to tolerance induced in the oral mucosa and how they differ from tolerance induced in the lower gastrointestinal tract. Furthermore we discuss ways in which novel nonparenteral approaches for immune intervention, such as allergen-specific immunotherapy applied by way of the sublingual route, might be improved to target the tolerogenic properties of the sophisticated oral mucosal immune network.