Differential activation profiles of multiple transcription factors during dendritic cell maturation.

Immature dendritic cells (DC) at the environmental interfaces, such as the skin, constantly survey the tissue for the emergence of microbial products and pro-inflammatory mediators. Upon recognition of such "danger" signals, they undergo dynamic reprogramming of gene expression and functions, the process known as DC maturation, which plays critical roles in both innate and adaptive immune responses. Although DC have been shown to discriminate different maturation stimuli by expressing stimulus-specific signature genes and unique phenotypic and functional properties, underlying mechanisms for this extraordinary plasticity remain relatively unclear. We hypothesized that DC might activate unique sets of transcription factors (TF) upon sensing different stimuli. To test this hypothesis, we transduced a mouse epidermal-derived DC line XS106 to express the luciferase reporter gene under the control of each of 15 different cis-enhancer elements. The resulting DC panels were then exposed to 14 different microbial, endogenous, environmental, and pharmacological agents that produced unique maturational changes. This approach allowed systematic determination of TF activation profiles in DC. Our results revealed striking diversity, with different classes of stimuli triggering preferential activation of distinct sets of TF. We propose that differential TF usage represents a previously unrecognized mechanism regulating the direction of DC maturation.

Keratinocyte ATP release assay for testing skin-irritating potentials of structurally diverse chemicals.

Irritant dermatitis represents innate inflammatory responses to toxic chemicals. We have reported recently that ATP released from chemically injured keratinocytes may serve as a causative mediator for irritant dermatitis. In this study, we examined whether ATP release from keratinocytes would serve as a reliable readout for predicting skin irritating potentials of structurally diverse compounds. A vast majority (19/20) of the tested compounds, i.e., strong and weak irritant chemicals selected from the literature, induced rapid (<10 min) and significant (P<0.05) ATP release from Pam 212 keratinocytes. Two compounds caused no detectable skin inflammation in our standard mouse model, documenting relatively high sensitivity (false negative rate of 0/18) and specificity (false positive rate of 1/20) of our ATP release assay. Selected compounds, primarily those containing phenol residues or hydrophobic hydrocarbon chains, triggered rapid (<10 min) and robust leakage of a fluorescence probe from liposomes, suggesting that lipid bilayers serve as one, but not the only, target moiety on keratinocytes. Not only do our data support the pathogenic role for keratinocyte-derived ATP in irritant dermatitis, they also form the basis for a formal validation study to evaluate the utility of the keratinocyte-based in vitro assay in screening environmental and industrial chemicals.

Local thermal injury elicits immediate dynamic behavioural responses by corneal Langerhans cells.

Langerhans cells (LCs) represent a special subset of immature dendritic cells (DCs) that reside in epithelial tissues at the environmental interfaces. Although dynamic interactions of mature DCs with T cells have been visualized in lymph nodes, the cellular behaviours linked with the surveillance of tissues for pathogenic signals, an important function of immature DCs, remain unknown. To visualize LCs in situ, bone marrow cells from C57BL/6 mice expressing the enhanced green fluorescent protein (EGFP) transgene were transplanted into syngeneic wild-type recipients. Motile activities of EGFP(+) corneal LCs in intact organ cultures were then recorded by time lapse two-photon microscopy. At baseline, corneal LCs exhibited a unique motion, termed dendrite surveillance extension and retraction cycling habitude (dSEARCH), characterized by rhythmic extension and retraction of their dendritic processes through intercellular spaces between epithelial cells. Upon pinpoint injury produced by infrared laser, LCs showed augmented dSEARCH and amoeba-like lateral movement. Interleukin (IL)-1 receptor antagonist completely abrogated both injury-associated changes, suggesting roles for IL-1. In the absence of injury, exogenous IL-1 caused a transient increase in dSEARCH without provoking lateral migration, whereas tumour necrosis factor-alpha induced both changes. Our results demonstrate rapid cytokine-mediated behavioural responses by LCs to local tissue injury, providing new insights into the biology of LCs.

Hair follicles serve as local reservoirs of skin mast cell precursors.

Several leukocyte populations normally reside in mouse skin, including Langerhans cells and gammadelta T cells in the epidermis and macrophage and mast cells in the dermis. Interestingly, these skin resident leukocytes are frequently identified within or around hair follicles (HFs), which are known to contain stem cell populations that can generate the epidermal architecture or give rise to the melanocyte lineage. Thus, we reasoned that HFs might serve as a local reservoir of the resident leukocyte populations in the skin. When vibrissal follicles of adult mice were cultured in the presence of stem cell factor (SCF), interleukin 3 (IL-3), IL-7, granulocyte-macrophage colony-stimulating factor, and Flt3 ligand, CD45+/lineage-/c-kit+/FcepsilonRI+ cells became detectable on the outgrowing fibroblasts in 10 days and expanded progressively thereafter. These HF-derived leukocytes showed characteristic features of connective tissue-type mast cells, including proliferative responsiveness to SCF, metachromatic granules, mRNA expression for mast cell proteases-1, -4, -5, and -6, and histamine release on ligation of surface IgE or stimulation with substance P or compound 48/80. These results, together with our findings that HFs contain c-kit+ cells and produce SCF mRNA and protein, suggest that HFs provide a unique microenvironment for local development of mast cells.

Generation and function of reactive oxygen species in dendritic cells during antigen presentation.

Although reactive oxygen species (ROS) have long been considered to play pathogenic roles in various disorders, this classic view is now being challenged by the recent discovery of their physiological roles in cellular signaling. To determine the immunological consequence of pharmacological disruption of endogenous redox regulation, we used a selenium-containing antioxidant compound ebselen known to modulate both thioredoxin and glutaredoxin pathways. Ebselen at 5-20 micro M inhibited Con A-induced proliferation and cytokine production by the HDK-1 T cell line as well as the LPS-triggered cytokine production by XS52 dendritic cell (DC) line. Working with the in vitro-reconstituted Ag presentation system composed of bone marrow-derived DC, CD4(+) T cells purified from DO11.10 TCR-transgenic mice and OVA peptide (serving as Ag), we observed that 1) both T cells and DC elevate intracellular oxidation states upon Ag-specific interaction; 2) ebselen significantly inhibits ROS production in both populations; and 3) ebselen at 5-20 micro M inhibits DC-induced proliferation and cytokine production by T cells as well as T cell-induced cytokine production by DC. Thus, Ag-specific, bidirectional DC-T cell communication can be blocked by interfering with the redox regulation pathways. Allergic contact hypersensitivity responses in BALB/c mice to oxazolone, but not irritant contact hypersensitivity responses to croton oil, were suppressed significantly by postchallenge treatment with oral administrations of ebselen (100 mg/kg per day). These results provide both conceptual and technical frameworks for studying ROS-dependent regulation of DC-T cell communication during Ag presentation and for testing the potential utility of antioxidants for the treatment of immunological disease.