Exaggerated IDO1 expression and activity in Langerhans cells from patients with atopic dermatitis upon viral stimulation: a potential predictive biomarker for high risk of Eczema herpeticum.

BACKGROUND: Atopic dermatitis (AD) is a heterogenous and highly complex disease characterized by an increased microbial colonization. For unknown reasons, a subgroup of patients with AD develops Eczema herpeticum (EH), a severe viral complication due to spreading of herpes simplex virus (HSV). Indoleamine 2,3-dioxygenase (IDO1) is a tryptophan (Trp)-catabolizing enzyme which is assumed to be instrumental in the antibacterial and antiviral defence mechanisms. METHODS: Comparative investigation of the IDO1 expression and activity in freshly isolated monocytes, plasmacytoid DC (pDC) and in vitro-generated Langerhans cells (LC) obtained from AD patients with HSV infections and EH and nonatopic controls. RESULTS: We demonstrate an increase in Trp degradation in the serum of patients during acute EH episodes. Circulating pDC from patients with history of EH display an increased IDO1 expression. An increased Trp degradation is detected in the supernatants of circulating monocytes from AD patients with acute EH. Mature LC from AD patients with history of EH and with acute EH display an increased IDO1 expression and activity, respectively. In LC from patients with history of EH, viral signals induce an exaggerated IDO1 expression and activity. CONCLUSION: IDO1 expression and activity in LC seem to be involved in the pathophysiology of EH in AD and could represent a predictive biomarker for patients with risk to develop EH and other viral complications.

Generation and functional analysis of human TNF-α/iNOS-producing dendritic cells (Tip-DC).

BACKGROUND: A unique type of CD11c(pos) dendritic cells (DC) is abundant in inflamed tissue, for example, in chronic inflammatory skin diseases. Due to their remarkable production of tumor necrosis factor (TNF)-α and inducible NO synthase (iNOS), these cells have been referred to as TNF and iNOS-producing DC (Tip-DC). While Tip-DC have been mainly characterized in murine models of infection, functional data about their human counterpart are lacking. OBJECTIVES: We sought to generate human Tip-DC in vitro und thus provide a new model for the investigation of their phenotype and function. METHODS: We generated human Tip-DC from monocytic precursor cells of healthy individuals, atopic and psoriatic patients using human serum. Resting and stimulated cells were analyzed by flow cytometry, real-time PCR, and by ELISA. INOS activity was measured by fluorometric detection of NO. RESULTS: Tip-DC closely resembled their in vivo counterparts by expressing CD11c, CD86, and CD40 while lacking CD1a, CD1c, or CD207/Langerin. Bacterial stimulation of Tip-DC from healthy donors, atopic dermatitis, or psoriasis patients resulted in a similar increase in iNOS activity and TNF-α production. In kinetic experiments, TNF-α, a putative activator of Tip-DC, could not induce NOS2. Upon bacterial stimulation, TNFA, IL6, IL12B, and IL23A mRNA appeared in a first wave, while IL12A and NOS2 mRNA were up-regulated later on but not blocked by anti-TNF-α agents, implying a biphasic pro-inflammatory response. CONCLUSIONS: We developed a new model for the study of human Tip-DC and provide the first evidence of their pro-inflammatory capacity.

Differential expression of toll-like receptor 2 on dendritic cells from asymptomatic and symptomatic atopic donors.

BACKGROUND: Early microbial exposure may reduce the risk for developing allergies on an atopic genetic background. Toll-like receptors (TLRs) recognize pathogen-associated molecular patterns of microbes and modulate innate and adaptive immunity. Different expression of TLRs in symptomatic and asymptomatic atopic donors may contribute to the development of allergic disease. METHODS: Monocytes and monocyte-derived dendritic cells (DCs) from symptomatic (n = 12) and asymptomatic atopic donors (n = 11), healthy nonatopics (n = 14) and from patients with psoriasis (n = 13) were analyzed for their expression of TLR2, TLR4 and TLR9 by real-time PCR. RESULTS: Monocytes did not show any differences in TLR2, TLR4 and TLR9 expression between the 4 groups. In contrast, DCs from asymptomatic donors showed an enhanced expression of TLR2 over DCs from nonatopics (p = 0.038) and just failed to reach significance when compared to symptomatic atopic patients (p = 0.060). TLR2 expression kinetics from monocytes to monocyte-derived DCs showed sustained expression of TLR2 in DCs only from asymptomatic donors but downregulation in the other groups. In DCs from symptomatic atopic donors, the expression of TLR2 correlated significantly with total IgE values in the serum (p = 0.01994). CONCLUSION: Differential expression and functional regulation of TLR2 expression by DCs from symptomatic and asymptomatic atopic donors may be important for the manifestation of allergic disease. Increased and sustained TLR2 expression on DCs, possibly as a result of an increased exposure to microorganisms or as a mechanism enhancing the sensitivity of microbe detection, may be of functional importance for the maintenance of clinical unresponsiveness toward allergens.

The indoleamine 2,3-dioxygenase (IDO) pathway controls allergy.

A series of recent studies have demonstrated that the immunoregulatory pathway of tryptophan catabolism, initiated by the enzyme indoleamine 2,3-dioxygenase (IDO), is a critical participant in allergic inflammation. Originally known for its regulatory function during pregnancy and during chronic inflammation in tumorigenesis and infection, the activity of IDO seems to positively modify the inflammatory state of atopy or allergy. The tryptophan degradation pathway is important for tolerance induction during systemic allergen immunotherapy. Here, we focus on recent findings that establish the IDO pathway as central to allergic inflammation.

Comparative immunophenotyping of monocytes from symptomatic and asymptomatic atopic individuals.

BACKGROUND: Allergy has at least two components - a genetic predisposition referred to as atopy and the progress from an atopic state to clinically apparent disease. Peripheral blood monocytes are circulating myeloid precursors of antigen-presenting cells. The expression of cell surface proteins on monocytes may therefore witness the disease status and affect the development of allergic disease. METHODS: Monocytes were isolated from atopic individuals with seasonal allergic rhinitis (n = 10), from atopic individuals sensitized to aeroallergens but without any signs of acute disease (n = 11), and from healthy nonatopic donors (n = 21). Detailed comparative phenotypic analysis of CD14(+) and FcepsilonRI(+)CD14(+) monocytes was performed by flow cytometry. RESULTS: CD14(+) monocytes from symptomatic atopic donors showed a significant increase in the cell surface intensity of the integrin adhesion molecule CD11c over monocytes from asymptomatic atopic and nonatopic donors. Asymptomatic atopic individuals showed significantly enhanced expression of FcepsilonRI on the CD14(high)CD16(dim) monocyte subset compared with this subset from symptomatic atopic and nonatopic donors. CONCLUSION: The increase in monocyte surface intensity of the adhesion molecule CD11c may be involved in the manifestation of allergic disease. FcepsilonRI on CD14(high)CD16(dim) monocytes of asymptomatic atopic donors may be of functional importance for the maintenance of clinical unresponsiveness toward allergens.

Asymptomatic atopy is associated with increased indoleamine 2,3-dioxygenase activity and interleukin-10 production during seasonal allergen exposure.

BACKGROUND: Indoleamine 2,3-dioxygenase (IDO) is a tryptophan (TRP)-catabolizing enzyme with regulatory effects on T cells. T cell inhibition is achieved through both TRP depletion and TRP metabolite accumulation in specific local tissue microenvironments. The expression of IDO activity by different types of antigen-presenting cells (APCs) has been shown to play a role in many instances of immunoregulation and tolerance induction. Induction of IDO after the engagement of the high-affinity receptor for IgE, FcepsilonRI, on atopic monocytes has been suggested to regulate T cell responses in atopic disorders. Interleukin-10 (IL-10), a cytokine known for its down-regulatory functions in the immune system, has recently been associated with the stable expression of IDO in mature tolerogenic dendritic cells. OBJECTIVE: This study was devised to understand the role of systemic IDO and IL-10 in the prevention of clinical apparent allergy. METHODS: The concentration of TRP and the break-down product kynurenine were measured by high-performance liquid chromatography in- and off-season in sera from patients with seasonal allergic rhinitis (n=12) and from clinically asymptomatic atopic patients sensitized to specific aeroallergens (n=12). Non-atopic (NA) individuals (n=12) served as control. The concentration of plasma IL-10 was determined in parallel from these donors by ELISA in- and off-season. RESULTS: In clinically unresponsive but aeroallergen-sensitized atopic individuals significantly higher systemic activity of IDO and increased plasma IL-10 levels were found during allergen exposure but not off-season compared to symptomatic atopic individuals with allergic rhinitis and NA individuals. CONCLUSION: Enhanced systemic IDO activity as well as increased systemic levels of IL-10 may contribute to the containment of allergic T cell responses and could be involved in the maintenance of a state of clinical unresponsiveness.

Quantification of indoleamine 2,3-dioxygenase gene induction in atopic and non-atopic monocytes after ligation of the high-affinity receptor for IgE, Fc(epsilon)RI and interferon-gamma stimulation.

Antigen-presenting cells (APCs) are crucial in regulating the outcome of T cell responses. Certain APCs are able to down-regulate T cell proliferation in vitro by inducing the enzyme indoleamine 2,3-dioxygenase (IDO) upon interferon-gamma (IFN-gamma) stimulation. IDO is the rate-limiting enzyme in the catabolism of the essential amino acid tryptophan. A lack of extracellular tryptophan creates environments in which cells become starved for this amino acid. The high-affinity receptor for IgE, Fc(epsilon)RI, is the principal receptor for the binding of specific IgE in type I-mediated allergies. We demonstrated recently that IDO is overexpressed in Fc(epsilon)RI-stimulated monocytes. In the present study, we performed quantification of IDO gene induction after treatment of atopic (Fc(epsilon)RI(high)) and non-atopic (Fc(epsilon)RI(low/-)) monocytes with IgE/anti-IgE and IFN-gamma. By quantitative PCR ELISA, we found IDO molecule induction in atopic monocytes was enhanced about 50-fold over non-atopic monocytes after ligation of Fc(epsilon)RI. Stimulation with IFN-gamma at a concentration of 100 U/ml in culture medium caused an increase in IDO gene copy numbers in atopics of about fourfold over that of non-atopics. This comparative quantification study demonstrates clearly the regulation of IDO gene expression by Fc(epsilon)RI and discloses differences thereof in atopic and non-atopic cells upon inflammatory stimuli.

The central role of FcepsilonRI in allergy.

The high-affinity receptor for immunoglobulin E (IgE), FcepsilonRI, plays a central role in the initiation and control of atopic allergic inflammation. On mast cells and basophils, the function of the receptor is well known and constitutes cellular degranulation and the release of various mediators. FcepsilonRI on antigen-presenting cells (APCs), however, does not lead to degranulation of preformed granula, but has different functions: signal transduction pathways like the activation of NF-kappaB are initiated to induce inflammatory cytokine gene expression. In addition, FcepsilonRI on APCs acts as an allergen-focusing structure and can efficiently amplify allergen presentation in an IgE-dependent manner. Recently, we and others have gained new insight into the regulation and function of FcepsilonRI on APCs, which has shed new light on the modulating effects of the immune system in atopic inflammation.

Antigen-presenting cells and tolerance induction.

T cell tolerance induction to foreign and self-antigens has occupied research since the beginning of the understanding of the immune system. Much controversy still exists on this question even though new methods became available to investigate immunoregulatory mechanisms. Antigen-presenting cells play a pivotal role in transferring information from the periphery of the organism to lymphoid organs. There, they initiate not only the activation of naive T cells but seem to deliver important signals which result in T cell unresponsiveness with antigen-specific tolerance induction.

Antigen-presenting cells in allergy.

The complex interaction of the innate and adaptive immune system requires flexibility and cooperation among various cell types. In this regard, antigen-presenting-cells (APCs) play a pivotal role in transferring information from the periphery of the organism to lymphoid organs, where they initiate the activation of naive T cells. Dendritic cells, Langerhans' cells (LCs), and macrophages are also critical in the induction of allergic inflammation by presenting allergens to T lymphocytes and by contributing to the local recruitment of effector cells. Because of a complex genetic background, atopic individuals exhibit a dysregulation of T cell-mediated immune mechanisms. Attempts to understand the role APCs play in these pathophysiologic conditions are in progress and may allow development of new treatment strategies. In this review we will focus on the biology of APCs and their unique role in the induction and control of allergic inflammation.

Topical tacrolimus (FK506) leads to profound phenotypic and functional alterations of epidermal antigen-presenting dendritic cells in atopic dermatitis.

BACKGROUND: Atopic dermatitis (AD) is a chronic inflammatory skin disease in which antigen-presenting epidermal dendritic cells (DCs), ie, Langerhans cells and the so-called inflammatory dendritic epidermal cells (IDECs) expressing the high-affinity receptor for IgE (FcepsilonRI) may play a significant pathophysiologic role. Therapeutic efficacy of the immunosuppressive macrolide tacrolimus (FK506) in AD has been demonstrated in clinical trials, but little is known of its mode of action. OBJECTIVE: The present study focused on the effects of topical tacrolimus treatment on epidermal CD1a+/FcepsilonRI+ DC populations in lesional AD. METHODS: Immunohistological analysis, epidermal DC phenotyping, and functional studies were performed on skin biopsy specimens from treated and untreated lesional skin of 10 patients with AD participating in a clinical trial with tacrolimus. RESULTS: Untreated lesional skin was characterized by a high proportion of CD1a+ cells, which was largely due to a high proportion of IDECs strongly expressing FcepsilonRI. Epidermal DCs isolated from untreated lesional skin exhibited high stimulatory activity toward autologous T cells, which was strongly reduced while clinical improvement was seen during application of tacrolimus. Concomitantly, a decreased FcepsilonRI expression was observed in both Langerhans cells and IDECs. Finally, topical tacrolimus led to a progressive decrease in the IDEC population within the pool of CD1a+ epidermal DCs and also to a decrease in their CD36 expression, which is indicative of lower local inflammation. CONCLUSION: Epidermal CD1a+ DCs may represent a target for topical tacrolimus in the treatment of AD.