Retinoic acid-loaded liposomes induce human mucosal CD103+ dendritic cells that inhibit Th17 cells and drive regulatory T-cell development in vitro

The active vitamin A metabolite, all-trans-retinoic acid (RA), primes precursor dendritic cells (DCs) into a mucosal phenotype with tolerogenic properties characterized by the expression of integrin CD103. CD103+ DCs can counteract pathogenic Th1 and Th17 in inflammatory bowel disease (IBD) or celiac disease (CD). Tolerogenic manipulation of DCs using nanoparticles carrying tolerogenic adjuvants and disease-specific antigens is a valuable treatment strategy to induce antigen-specific mucosal tolerance in vivo. Here, we investigated the effects of RA-loaded liposomes on human DC phenotype and function, including DC-driven T-cell development, both during the generation of monocyte-derived DCs (moDCs) as well as by priming immature moDCs. RA liposomes drove CD103+ DC differentiation as well as ALDH1A2 expression in DCs. Neutrophil-dependent Th17 cell development was reduced by RA-liposome-differentiated and RA-liposome-primed DCs. Moreover, RA liposome treatment shifted T-cell development toward a Th2 cell profile. Importantly, RA liposomes induced the development of IL-10-producing and FoxP3+ regulatory T cells (Tregs) of various Treg subsets, including ICOS+ Tregs, that were potent inhibitors of bystander memory T-cell proliferation. Taken together, RA-loaded liposomes could be a novel treatment avenue for IBD or CD patients.

Aberrant neutrophil degranulation in hospitalized patients with COVID-19 partially remains for 6 months

Neutrophils are important players in COVID-19, contributing to tissue damage by release of inflammatory mediators, including ROS and neutrophil elastase. Longitudinal studies on the effects of COVID-19 on neutrophil phenotype and function are scarce. Here, we longitudinally investigated the phenotype and degranulation of neutrophils in COVID-19 patients (28 nonhospitalized and 35 hospitalized patients) compared with 17 healthy donors (HDs). We assessed phenotype, degranulation, CXCL8 (IL-8) release, and ROS generation within 8 days, at one or 6 month(s) after COVID-19 diagnosis. For degranulation and ROS production, we stimulated neutrophils, either with ssRNA and TNF or granulocyte-macrophage colony-stimulating factor and N-Formylmethionyl-leucyl-phenylalanine. During active COVID-19, neutrophils from hospitalized patients were more immature than from HDs and were impaired in degranulation and ROS generation, while neutrophils from nonhospitalized patients only demonstrated reduced CD66b+ granule release and ROS production. Baseline CD63 expression, indicative of primary granule release, and CXCL8 production by neutrophils from hospitalized patients were elevated for up to 6 months. These findings show that patients hospitalized due to COVID-19, but not nonhospitalized patients, demonstrated an aberrant neutrophil phenotype, degranulation, CXCL8 release, and ROS generation that partially persists up to 6 months after infection.

Liposomes loaded with vitamin D3 induce regulatory circuits in human dendritic cells

Introduction

Nanomedicine provides a promising platform for manipulating dendritic cells (DCs) and the ensuing adaptive immune response. For the induction of regulatory responses, DCs can be targeted in vivo with nanoparticles incorporating tolerogenic adjuvants and auto-antigens or allergens.

Methods

Here, we investigated the tolerogenic effect of different liposome formulations loaded with vitamin D3 (VD3). We extensively phenotyped monocyte-derived DCs (moDCs) and skin DCs and assessed DC-induced regulatory CD4+ T cells in coculture.

Results

Liposomal VD3 primed-moDCs induced the development of regulatory CD4+ T cells (Tregs) that inhibited bystander memory T cell proliferation. Induced Tregs were of the FoxP3+ CD127low phenotype, also expressing TIGIT. Additionally, liposome-VD3 primed moDCs inhibited the development of T helper 1 (Th1) and T helper 17 (Th17) cells. Skin injection of VD3 liposomes selectively stimulated the migration of CD14+ skin DCs.

Discussion

These results suggest that nanoparticulate VD3 is a tolerogenic tool for DC-mediated induction of regulatory T cell responses.

Vitamin D3 Priming of Dendritic Cells Shifts Human Neutrophil-Dependent Th17 Cell Development to Regulatory T Cells

Vitamin D3 (VD3) is a potential adjuvant for use in tolerogenic vaccine formulations that target dendritic cells (DCs) for the treatment of chronic inflammatory disorders, e.g., autoimmune diseases. These disorders are often associated with enhanced activity of IL-17-producing T helper 17 (Th17) cells which develop in a DC-driven and neutrophil-dependent fashion. Here, we investigated the effect of VD3 on Candida albicans-specific human T-cell differentiation, since C. albicans is a model pathogen for Th17 cell development. VD3 priming of DCs restricted neutrophil-dependent Th17 cell development and neutrophil-independent Th1 cell formation from naive CD4⁺ T cells. In line with this, the production of Th1/Th17-polarizing cytokines IL-12 and IL-23 by DCs was reduced by VD3 priming. Development of both FoxP3⁺CD127_lowCD25⁺ Tregs and IL-10-producing T cells was significantly enhanced in VD3-primed conditions, even in the presence of neutrophils. ICOS⁺ Tregs, major IL-10 producers, CD69⁺FoxP3⁺, and TIGIT⁺FoxP3⁺ Tregs were significantly induced by VD3 priming as well. Our data support the potential use of VD3 as an adjuvant to induce tolerance in the treatment of autoimmune disorders, including those in which neutrophils are involved in pathogenesis, since we show that Treg development is enhanced by VD3 even in the presence of neutrophils, while Th17 cell development is restricted.

Antigen Uptake After Intradermal Microinjection Depends on Antigen Nature and Formulation, but Not on Injection Depth

The skin is an attractive alternative administration route for allergy vaccination, as the skin is rich in dendritic cells (DCs) and is easily accessible. In the skin multiple subsets of DCs with distinct roles reside at different depths. In this study antigen (=allergen for allergy) formulations were injected in ex vivo human skin in a depth-controlled manner by using a hollow microneedle injection system. Biopsies were harvested at the injection site, which were then cultured for 72 h. Subsequently, the crawled-out cells were collected from the medium and analyzed with flow cytometry. Intradermal administration of ovalbumin (OVA, model antigen) solution at various depths in the skin did not affect the migration and maturation of DCs. OVA was taken up efficiently by the DCs, and this was not affected by the injection depth. In contrast, Bet v 1, the major allergen in birch pollen allergy, was barely taken up by dermal DCs (dDCs). Antigens were more efficiently taken up by CD14⁺ dDCs than CD1a⁺ dDCs, which in turn were more efficient at taken up antigen than Langerhans cells. Subsequently, both OVA and Bet v 1 were formulated in cationic and anionic liposomes, which altered antigen uptake drastically following intradermal microinjection. While OVA uptake was reduced by formulation in liposomes, Bet v 1 uptake in dDCs was increased by encapsulation in both cationic and anionic liposomes. This highlights the potential use of liposomes as adjuvant in intradermal allergy vaccine delivery. In conclusion, we observed that antigen uptake after intradermal injection was not affected by injection depth, but varied between different antigens and formulation.

Human milk extracellular vesicles target nodes in interconnected signalling pathways that enhance oral epithelial barrier function and dampen immune responses

Maternal milk is nature's first functional food. It plays a crucial role in the development of the infant's gastrointestinal (GI) tract and the immune system. Extracellular vesicles (EVs) are a heterogeneous population of lipid bilayer enclosed vesicles released by cells for intercellular communication and are a component of milk. Recently, we discovered that human milk EVs contain a unique proteome compared to other milk components. Here, we show that physiological concentrations of milk EVs support epithelial barrier function by increasing cell migration via the p38 MAPK pathway. Additionally, milk EVs inhibit agonist‐induced activation of endosomal Toll like receptors TLR3 and TLR9. Furthermore, milk EVs directly inhibit activation of CD4+ T cells by temporarily suppressing T cell activation without inducing tolerance. We show that milk EV proteins target key hotspots of signalling networks that can modulate cellular processes in various cell types of the GI tract.

Signal Inhibitory Receptor on Leukocytes-1 is highly expressed on lung monocytes, but absent on mononuclear phagocytes in skin and colon

Signal Inhibitory Receptor on Leukocytes-1 (SIRL-1) is expressed on human blood monocytes and granulocytes and inhibits myeloid effector functions. On monocytes, but not granulocytes, SIRL-1 expression is low or absent in individuals with the single nucleotide polymorphism (SNP) rs612529C. The expression of SIRL-1 in tissue and the influence of rs612529 hereon is currently unknown. Here, we used flow cytometry to determine SIRL-1 expression on immune cells in human blood and three barrier tissues; skin, colon and lung. SIRL-1 was expressed by virtually all neutrophils and eosinophils in these tissues. In contrast, SIRL-1 was not expressed by monocyte-derived cells in skin and colon, whereas it was highly expressed by lung classical monocytes. Lung monocytes from individuals with a rs612529C allele had decreased SIRL-1 expression, consistent with the genotype association in blood. Within the different monocyte subsets in blood and lung, SIRL-1 expression was highest in classical monocytes and lowest in nonclassical monocytes. SIRL-1 was not expressed by dendritic cells in blood and barrier tissues. Together, these results indicate that SIRL-1 is differentially expressed on phagocyte subsets in blood and barrier tissues, and that its expression on monocytes is genotype- and tissue-specific. Immune regulation of monocytes by SIRL-1 may be of particular importance in the lung.

Different Interferon Beta Preparations Induce the Same Qualitative Immune Response in Human Skin

Interferon beta (IFNβ) is used as a first-line treatment for multiple sclerosis (MS) and is injected intramuscularly or subcutaneously (s.c.). The subcutaneous route is considered more immunogenic as it is associated with increased antidrug antibody-positive patients. The skin contains dendritic cells (DCs) and it is unclear whether these contribute to immunogenicity. To assess the effect of IFNβ on skin-resident cells, IFNβ was injected intradermally (i.d.) ex vivo using a human skin explant model or s.c. in vivo in MS patients. Ex vivo, intradermal IFNβ injections reduced migration and enhanced surface CD86 expression of dermal DCs, and an increased expression of HLA-DR⁺ was observed in skin biopsies taken after subcutaneous IFNβ injection (in vivo). In both models, IFNβ elevated the expression of several inflammatory cytokines when compared to the control biopsies. Our results show that 3 different IFNβ preparations, normalized in dose and injection site, induce similar immune responses, suggesting that the differences in immunogenicity are likely due to the route and frequency of administration.

Vitamin D3 metabolite calcidiol primes human dendritic cells to promote the development of immunomodulatory IL-10-producing T cells

Vitamin D is recognized as a potent immunosuppressive drug. The suppressive effects of vitamin D are attributed to its physiologically active metabolite 1,25 dihydroxy vitamin D3 (calcitriol), which was shown, to prime dendritic cells (DCs) to promote the development of regulatory T (Treg) cells. Despite the potential benefit in treating autoimmune diseases, clinical application of calcitriol is hindered by deleterious side effects manifested by hypercalcemia and hypercalciuria. Conversely, the physiological precursors of calcitriol, vitamin D3 (cholecalciferol) and its first metabolite 25-hydroxy vitamin D3 (calcidiol) are widely applied in the clinic due to their low calcimic burden. However, the mechanisms by which cholecalciferol and calcidiol may modulate adaptive immunity remain elusive. This prompted us to unravel the immunosuppressive capacity of these precursors by assessing their influence on DC functions and the subsequent polarization of naïve CD4(+) T cells. In this study we show that, whereas cholecalciferol has insignificant effects on DC maturation and cytokine production, it only weakly primed DCs to induce suppressive T cells. However, like calcitriol, calcidiol not only exerted an inhibitory effect on DC maturation and cytokine production, and primed DCs to promote the development of suppressive IL-10-producing Treg cells. Strikingly, in contrast to the population of IL-10-producing Treg cells induced by calcitriol-primed DCs, the IL-10-producing Treg cells induced by calcidiol-primed DCs exhibited sustained IFN-γ production in face of their suppressive capacity. Experiments with the steroid synthesis inhibitor ketoconazole indicated that the immunomodulatory features of the precursors are dependent on their conversion into calcitriol. Collectively, calcidiol is a potent immune modulator, which may be more adequate than calcitriol for the treatment of chronic inflammatory diseases, since it is less hypercalcimic. This may be of particular interest for the treatment of allergic disease, where concurrent suppression and sustained IFN-γ production by Treg cells effectively counterbalance the Th2-dominated immune responses.

Eliminating acute lymphoblastic leukemia cells from human testicular cell cultures: a pilot study

OBJECTIVE

To study whether acute lymphoblastic leukemia (ALL) cells survive in a human testicular cell culture system.

DESIGN

Experimental laboratory study.

SETTING

Reproductive biology laboratory, academic medical center.

PATIENT(S)

Acute lymphoblastic leukemia cells from three patients and testicular cells from three other patients.

INTERVENTION(S)

Acute lymphoblastic leukemia cells were cultured alone or in combination with testicular cells, at various concentrations, in a system that has recently been developed to propagate human spermatogonial stem cells.

MAIN OUTCOME MEASURE(S)

Viability of ALL and testicular cells during culture was evaluated by flow cytometry using markers for live/dead cells. Furthermore, the presence of ALL cells among testicular cells was determined by highly sensitive (1:10,000 to 1:100,000 cells) patient-specific antigen-receptor minimal residual disease polymerase chain reaction. The presence of spermatogonia at the end of culture was determined by reverse transcription-polymerase chain reaction for ZBTB16, UCHL1, and GPR125.

RESULT(S)

The ALL cells cultured separately did not survive beyond 14 days of culture. When cultured together with testicular cells, even at extremely high initial concentrations (40% ALL cells), ALL cells were undetectable beyond 26 days of culture. Reverse transcription-polymerase chain reaction confirmed the presence of spermatogonia at the end of the culture period.

CONCLUSION(S)

Our pilot study shows that the described testicular cell culture system not only allows for efficient propagation of spermatogonial stem cells but also eliminates contaminating ALL cells.

Intradermal application of vitamin D3 increases migration of CD14+ dermal dendritic cells and promotes the development of Foxp3+ regulatory T cells

The active form of vitamin D3 (VitD) is a potent immunosuppressive drug. Its effects are mediated in part through dendritic cells (DCs) that promote the development of regulatory T cells (Tregs). However, it remains elusive how VitD would influence the different human skin DC subsets, e.g., CD1a(+)/langerin(+) Langerhans cells, CD14(+) DDCs and CD1a(+) DDCs upon administration through the skin route in their natural environment. We addressed this issue by intradermal (ID) administration of VitD in a human skin explant system that closely resembles physiological conditions. ID injection of VitD selectively enhanced the migration of CD14(+) DDCs, a subset known for the induction of tolerance. Moreover, ID injection of VitD repressed the LPS-induced T cell stimulatory capacity of migrating DCs. These migrating DCs collectively induced T cells with suppressive activity and abolished IFN-γ productivity. Those induced T cells were characterized by the expression of Foxp3. Thus, we report the novel finding that ID injection of VitD not only modifies skin DC migration, but also programs these DCs in their natural milieu to promote the development of Foxp3(+) Tregs.

Vitamin D3 targets epidermal and dermal dendritic cells for induction of distinct regulatory T cells

BACKGROUND

The vitamin D metabolite 1,25(OH)2D3 (VitD3) is a potent immunosuppressive drug and, among others, is used for topical treatment of psoriasis. A proposed mechanism of VitD3-mediated suppression is priming of dendritic cells (DCs) to induce regulatory T (Treg) cells.

OBJECTIVE

Currently, there is confusion about the phenotype of VitD3-induced Treg cells and the DC-derived molecules driving their development. We investigated Treg cell induction after VitD3 priming of 2 distinct skin DC subsets: Langerhans cells (LCs) and dermal dendritic cells (DDCs).

METHODS

LCs and DDCs primed with VitD3 were cocultured with allogeneic naive T cells. The phenotype and function of the DCs and induced T cells were analyzed.

RESULTS

Both VitD3-primed DC subtypes induced T cells with regulatory activity. Unexpectedly, whereas the Treg cell populations generated by VitD3-primed LCs were CD25(hi)CD127(lo) forkhead box protein 3 (Foxp3)-positive cells, which meet the criteria of classical inducible Treg cells, the T cells developing in response to VitD3-primed DDCs were Foxp3(-) T(R)1 cells expressing IL-10. Inhibition experiments revealed that LC-derived TGF-β is a key factor in the induction of Foxp3(+) Treg cells, whereas DDC-derived IL-10 is important for the induction of IL-10(+) T(R)1 cells.

CONCLUSION

Thus we report the novel finding that distinct but closely related DC subsets are differentially programmed by VitD3 to support development of either TGF-β-dependent Foxp3(+) Treg cells or IL-10-dependent IL-10(+) Treg cells.

Selective C-Rel activation via Malt1 controls anti-fungal T(H)-17 immunity by dectin-1 and dectin-2

C-type lectins dectin-1 and dectin-2 on dendritic cells elicit protective immunity against fungal infections through induction of T_H1 and T_H-17 cellular responses. Fungal recognition by dectin-1 on human dendritic cells engages the CARD9-Bcl10-Malt1 module to activate NF-κB. Here we demonstrate that Malt1 recruitment is pivotal to T_H-17 immunity by selective activation of NF-κB subunit c-Rel, which induces expression of T_H-17-polarizing cytokines IL-1β and IL-23p19. Malt1 inhibition abrogates c-Rel activation and T_H-17 immunity to Candida species. We found that Malt1-mediated activation of c-Rel is similarly essential to induction of T_H-17-polarizing cytokines by dectin-2. Whereas dectin-1 activates all NF-κB subunits, dectin-2 selectively activates c-Rel, signifying a specialized T_H-17-enhancing function for dectin-2 in anti-fungal immunity by human dendritic cells. Thus, dectin-1 and dectin-2 control adaptive T_H-17 immunity to fungi via Malt1-dependent activation of c-Rel.

Fungal infections are a major health threat and the incidence is growing worldwide. There is a need for efficient antifungal vaccines. Adaptive immune responses and in particular T helper cell type 17 (T_H-17) responses are crucial in the defence against fungal infections. Human dendritic cells (DCs) induce T_H-17 responses after interaction with fungi. DCs express C-type lectins dectin-1 and dectin-2 that interact with the carbohydrate structures present in the cell-wall of fungi. It is unclear how signaling by these C-type lectins leads to specific T_H-17 responses. Here we demonstrate that the signaling molecule Malt1 present in the CARD9-Bcl10-Malt1 complex is responsible for T_H-17 induction by selectively activating the NF-κB transcription factor c-Rel, which drives transcription of the T_H-17-polarizing cytokines. Inhibition of either Malt1 or c-Rel prevents T_H-17 induction in response to fungi. Furthermore, we show that the C-type lectin dectin-2 selectively activates c-Rel, signifying a specialized T_H-17-enhancing function for this C-type lectin. Thus, novel vaccination strategies that target dectin-2 or activate Malt1 can induce predominant T_H-17 responses. Since aberrant T_H-17 responses underlie the pathology of atopic dermatitis and various autoimmune diseases, Malt1 is a rational therapeutic target to attenuate anomalous adaptive immune responses.

Aberrant function of peripheral blood myeloid and plasmacytoid dendritic cells in atopic dermatitis patients

BACKGROUND

Dendritic cells (DCs) can act both as innate cells in host defense and as antigen-presenting cells for naive T cells in adaptive immunity. These functions, among others, are determined by the level of production of particular cytokines. Atopic dermatitis (AD) is a chronic inflammatory skin disorder characterized by an initial phase predominated by T(H)2 cytokines that switches into a second, more chronic T(H)1-dominated eczematous phase.

OBJECTIVE

To assess to what extent the AD phenotype is associated with an aberrant phenotype and function of DCs.

METHODS

Classic CD1c(+)/blood DC antigen (BDCA)-1(+) myeloid (m) DCs and CD304(+)/BDCA4(+) plasmacytoid (p) DCs, the natural IFN-producing cells, were isolated from peripheral blood of patients with AD and healthy controls and analyzed for their phenotype and function.

RESULTS

Purified CD1c(+)/BDCA1(+) mDCs from patients with AD showed a selective and dramatic reduction of IL-12p70 and TNF-alpha release. IL-12p70 reduction was attributed to a defective expression of both IL-12p35 and IL-12p40 subunits. Accordingly, mature CD1c(+)/BDCA1(+) mDCs from patients with AD induced considerably less IFN-gamma-producing and more IL-4-producing T(H) cells compared with mDCs from healthy controls. In addition, CD304(+)/BDCA4(+) pDCs from patients with AD produced significantly lower levels of IFN-alpha compared with healthy controls.

CONCLUSION

Myeloid DCs and pDCs from patients with AD show defective IL-12, TNF-alpha, and IFN-alpha production, which may contribute to increased susceptibility to infection and to the maintenance of the T(H)2 cell-mediated allergic state in patients with AD.

Human Keratinocytes Express Functional Toll-Like Receptor 3, 4, 5, and 9

Keratinocytes are continuously in contact with external stimuli and have the capacity to produce several soluble mediators. Pathogen-associated molecular patterns (PAMPs) are recognized, among others, by Toll-like receptors (TLRs). The functional responses of keratinocytes to different PAMPs have not yet been fully established. Here we show that keratinocytes constitutively express TLR1, 2, 3, 4, 5, 6, 9, and 10 mRNA, but not TLR7 and 8. Stimulation of keratinocytes with TLR3, 4, 5, and 9 ligands resulted in differential immune-associated responses. Tumor necrosis factor-alpha, CXC chemokine ligand 8 (CXCL8), CCL2, and C chemokine ligand 20 (CCL20) release was enhanced in response to all PAMPs tested, in a time- and dose-dependent manner. Only TLR9 ligand CpG-oligodeoxynucleotides (ODNs) and TLR3 ligand poly-I:C could additionally induce type I IFNs. CCL27 production was selectively induced by poly-I:C and flagellin, whereas CXCL9 and CXCL10 were exclusively induced by CpG-ODNs and/or poly-I:C. Upregulation of ICAM-1, HLA-DR, HLA-ABC, FasR, and CD40 was mainly observed in response to poly-I:C, flagellin, and lipopolysaccharide. Furthermore, PAMP triggering resulted in the phosphorylation of phosphorylated-IkappaB alpha and in the nucleus translocation of NF-kappaB p65. Altogether, these findings stress an unexpectedly multifaceted role of keratinocytes in innate immunity as evident by their differential, TLR-mediated responses to PAMPs associated with different classes of pathogens.

Opposing roles of blood myeloid and plasmacytoid dendritic cells in HIV-1 infection of T cells: transmission facilitation versus replication inhibition

CD11c(+) myeloid dendritic cells (MDCs) and CD11c(-) CD123(+) plasmacytoid DCs (PDCs) have been identified as main human DC subsets. MDCs are professional antigen-presenting cells for T cells, and include Langerhans cells, dermal DCs, and interstitial DCs. They have been associated with HIV-1 capture and sexual transmission, whereas PDCs play an important role in the innate immune responses to different types of viruses, including HIV-1. To compare the influence of MDCs and PDCs on HIV-1 infection of T cells, we isolated donor-matched MDCs and PDCs from peripheral blood, activated them by adding different maturation-inducing compounds, and cocultured them with T cells and HIV-1. We found that MDCs enhance HIV-1 infection through capture of the virus and subsequent transmission to T cells, and that differently matured MDC subsets have different HIV-1 transmission efficiencies. These differences were not due to soluble factors, viral capture differences, or the expression of integrins ICAM-1, -2, -3, or LFA-1. In contrast, regardless of their state of maturation, PDCs inhibit HIV-1 replication in T cells through the secretion of IFNalpha and an additional, unidentified small molecule. This study shows that the 2 main types of DCs have opposing roles in HIV-1 infection of T cells.

Selective probiotic bacteria induce IL-10-producing regulatory T cells in vitro by modulating dendritic cell function through dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin

BACKGROUND

Lactobacilli are probiotic bacteria that are frequently tested in the management of allergic diseases or gastroenteritis. It is hypothesized that these probiotics have immunoregulatory properties and promote mucosal tolerance, which is in part mediated by regulatory T cells (Treg cells). On the basis of pathogenic or tissue-specific priming, dendritic cells (DC) acquire different T cell-instructive signals and drive the differentiation of naive T H cells into either T H 1, T H 2, or regulatory effector T cells.

OBJECTIVE

We studied in what way different species of lactobacilli prime human DCs for their ability to drive Treg cells.

METHODS

Human monocyte-derived DCs were cultured in vitro with lactobacilli of different species.

RESULTS

Two different species of lactobacilli, Lactobacillus reuteri and Lactobacillus casei , but not Lactobacillus plantarum, prime monocyte-derived DCs to drive the development of Treg cells. These Treg cells produced increased levels of IL-10 and were capable of inhibiting the proliferation of bystander T cells in an IL-10-dependent fashion. Strikingly, both L reuteri and L casei , but not L plantarum , bind the C-type lectin DC-specific intercellular adhesion molecule 3-grabbing non-integrin (DC-SIGN). Blocking antibodies to DC-SIGN inhibited the induction of the Treg cells by these probiotic bacteria, stressing that ligation of DC-SIGN can actively prime DCs to induce Treg cells.

CONCLUSIONS

The targeting of DC-SIGN by certain probiotic bacteria might explain their beneficial effect in the treatment of a number of inflammatory diseases, including atopic dermatitis and Crohn's disease.

Double-stranded RNA-exposed human keratinocytes promote Th1 responses by inducing a Type-1 polarized phenotype in dendritic cells: role of keratinocyte-derived tumor necrosis factor alpha, type I interferons, and interleukin-18

Dendritic cells play a key role in establishing the class of immune response against invading pathogens. Upon engagement with double-stranded RNA, a major bioactive constituent of many virus types, immature dendritic cells develop into type 1 immunostimulatory dendritic cells that promote Th1 responses. Immature dendritic cells reside in the epithelia and are in close contact with keratinocytes. We studied to what extent dendritic cells can also adopt a type 1 immunostimulatory dendritic cell phenotype indirectly, as a result of the interaction with keratinocytes responding to double-stranded RNA. In contrast to supernatants from keratinocytes activated by the combination of tumor necrosis factor alpha and interleukin-1beta, supernatants from keratinocytes activated by synthetic double-stranded RNA, polyriboinosinic polyribocytidylic acid, comprised tumor necrosis factor alpha and type I interferons, which induced maturation of human monocyte-derived immature dendritic cells. In addition, dendritic cells matured in the presence of these supernatants strongly biased the development of Th1 cells from naive Th cells. This bias was dependent on keratinocyte-derived interferon-alpha/beta and interleukin-18, as neutralization of both interferon-alpha/beta and interleukin-18 in the keratinocyte culture supernatant reduced the development of interferon-gamma-producing Th cells. These findings suggest that keratinocytes can contribute to the development of selective Th1/Th2 responses through the induction of maturation and functional polarization of dendritic cells, indicating a novel role for keratinocytes as initiators and regulators of cutaneous T-cell-mediated inflammation. In addition, these results support the concept that, in addition to direct interaction with pathogens, dendritic cells may also be activated and primed by pathogen indirectly, via the effect of resident tissue cells responding to pathogen.