Hydrogen peroxide mediates UV-induced impairment of antigen presentation in a murine epidermal-derived dendritic cell line

Protection from Ultraviolet Damage and Photocarcinogenesis by Vitamin D Compounds

Exposure of skin cells to UV radiation results in DNA damage, which if inadequately repaired, may cause mutations. UV-induced DNA damage and reactive oxygen and nitrogen species also cause local and systemic suppression of the adaptive immune system. Together, these changes underpin the development of skin tumours. The hormone derived from vitamin D, calcitriol (1,25-dihydroxyvitamin D₃) and other related compounds, working via the vitamin D receptor and at least in part through endoplasmic reticulum protein 57 (ERp57), reduce cyclobutane pyrimidine dimers and oxidative DNA damage in keratinocytes and other skin cell types after UV. Calcitriol and related compounds enhance DNA repair in keratinocytes, in part through decreased reactive oxygen species, increased p53 expression and/or activation, increased repair proteins and increased energy availability in the cell when calcitriol is present after UV exposure. There is mitochondrial damage in keratinocytes after UV. In the presence of calcitriol, but not vehicle, glycolysis is increased after UV, along with increased energy-conserving autophagy and changes consistent with enhanced mitophagy. Reduced DNA damage and reduced ROS/RNS should help reduce UV-induced immune suppression. Reduced UV immune suppression is observed after topical treatment with calcitriol and related compounds in hairless mice. These protective effects of calcitriol and related compounds presumably contribute to the observed reduction in skin tumour formation in mice after chronic exposure to UV followed by topical post-irradiation treatment with calcitriol and some, though not all, related compounds.

Defense potential of secondary metabolites in medicinal plants under UV-B stress

Ultraviolet-B (UV-B) radiation has, for many decades now, been widely studied with respect to its consequences on plant and animal health. Though according to NASA, the ozone hole is on its way to recovery, it will still be a considerable time before UV-B levels reach pre-industrial limits. Thus, for the present, excessive UV-B reaching the Earth is a cause for concern, and UV-B related human ailments are on the rise. Plants produce various secondary metabolites as one of the defense strategies under UV-B. They provide photoprotection via their UV-B screening effects and by quenching the reactive oxygen- and nitrogen species produced under UV-B influence. These properties of plant secondary metabolites (PSMs) are being increasingly recognized and made use of in sunscreens and cosmetics, and pharma- and nutraceuticals are gradually becoming a part of the regular diet. Secondary metabolites derived from medicinal plants (alkaloids, terpenoids, and phenolics) are a source of pharmaceuticals, nutraceuticals, as well as more rigorously tested and regulated drugs. These metabolites have been implicated in providing protection not only to plants under the influence of UV-B, but also to animals/animal cell lines, when the innate defenses in the latter are not adequate under UV-B-induced damage. The present review focuses on the defense potential of secondary metabolites derived from medicinal plants in both plants and animals. In plants, the concentrations of the alkaloids, terpenes/terpenoids, and phenolics have been discussed under UV-B irradiation as well as the fate of the genes and enzymes involved in their biosynthetic pathways. Their role in providing protection to animal models subjected to UV-B has been subsequently elucidated. Finally, we discuss the possible futuristic scenarios and implications for plant, animal, and human health pertaining to the defense potential of these secondary metabolites under UV-B radiation-mediated damages.

Effect of laser thermal injury on Langerhans cells in mouse and hairless guinea pig epidermis

To examine the effect of laser thermal injury on Langerhans cells (LC) within the epidermis, the dorsal skin of mice and hairless guinea pigs was exposed to varying levels of laser irradiation using a thulium laser at a wavelength of 2.0 μm. At 6, 24 and 48 h post irradiation, animals were euthanized, skin samples prepared for histology and the epidermis obtained and stained by major histocompatibility complex-II staining (mice) or ATPase assay (hairless guinea pigs) for the enumeration of LC. Mouse skin exhibited histological evidence of thermal damage at 24 h post irradiation at even the lowest dose (0.14 W) and decreases in the numbers of epidermal LC were observed at all doses and decreases were proportional to dose. In contrast, hairless guinea pig skin only showed consistent histological evidence of thermal damage at the highest dose of irradiation (0.70 W) at 24 and 48 h post irradiation and exhibited a statistically significant decrease in numbers of epidermal LC only at this dose. Thus, epidermal LC depletion occurred in the skin of both mice and hairless guinea pigs in response to laser treatment and the magnitude of depletion directly correlated with the extent of thermal damage both within and between species.

The relevance of the vitamin D endocrine system (VDES) for tumorigenesis, prevention, and treatment of non-melanoma skin cancer (NMSC): Present concepts and future perspectives

Solar UV (UV)-B-radiation exerts both beneficial and adverse effects on human health. On the one hand, it is the most important environmental risk factor for the development of non-melanoma skin cancer [NMSC; most importantly basal (BCC) and squamous (SCC) cell carcinomas], that represent the most common malignancies in Caucasian populations. On the other hand, the human body's requirements of vitamin D are mainly achieved by UV-B-induced cutaneous photosynthesis. This dilemma represents a serious problem in many populations, for an association of vitamin D-deficiency and multiple independent diseases including various types of cancer has been convincingly demonstrated. In line with these findings, epidemiologic and laboratory investigations now indicate that vitamin D and its metabolites have a risk reducing effect for NMSC. Potential mechanisms of action include inhibition of the hedgehog signaling pathway (BCC) and modulation of p53-mediated DNA damage response (SCC). As a consequence of these new findings it can be concluded that UV-B-radiation exerts both beneficial and adverse effects on risk and prognosis of NMSC. It can be assumed that many independent factors, including frequency and dose of UV-B exposure, skin area exposed, and individual factors (such as skin type and genetic determinants of the skin`s vitamin D status and of signaling pathways that are involved in the tumorigenesis of NMSC) determine whether UV-B exposure promotes or inhibits tumorigenesis of NMSC. Moreover, these findings may help to explain many of the differential effects of UV-B radiation on risk of NMSC, including variation in the dose-dependent risk for development of SCC in situ (actinic keratosis, AK), invasive SCC, and BCC. In this review, we analyze the relevance of the vitamin D endocrine system (VDES) for tumorigenesis, prevention, and treatment of NMSC and give an overview of present concepts and future perspectives.

Photoimmunology--illuminating the immune system through photobiology

We review the field of photoimmunology with emphasis on immunosuppression induced by ultraviolet B radiation. Recent studies have focused on UVB-induced alterations in epidermal Langerhans cell function, resulting in a shift from Th1 to Th2 phenotype and the activation of regulatory T cells as the source of IL-10 that is central to this form of immunosuppression.

UV-induced immune suppression and photocarcinogenesis: chemoprevention by dietary botanical agents

Studies of immune-suppressed transplant recipients and patients with biopsy-proven skin cancer have confirmed that ultraviolet (UV) radiation-induced immune suppression is a risk factor for the development of skin cancer in humans. UV radiation suppresses the immune system in several ways. The UVB spectrum inhibits antigen presentation, induces the release of immunosuppressive cytokines, and elicits DNA damage that is a molecular trigger of UV-mediated immunosuppression. It is therefore important to elucidate the mechanisms underlying UV-induced immunosuppression as a basis for developing strategies to protect individuals from this effect and subsequent development of skin cancer. Dietary botanicals are of particular interest as they have been shown to inhibit UV-induced immune suppression and photocarcinogenesis. In this review, we summarize the most recent investigations and mechanistic studies regarding the photoprotective efficacy of selected dietary agents, including, green tea polyphenols, grape seed proanthocyanidins and silymarin. We present evidence that these chemopreventive agents prevent UVB-induced immunosuppression and photocarcinogenesis through: (i) the induction of immunoregulatory cytokine interleukin (IL)-12; (ii) IL-12-dependent DNA repair; and (iii) stimulation of cytotoxic T cells in the tumor microenvironment. The new information regarding the mechanisms of action of these agents supports their potential use as adjuncts in the prevention of photocarcinogenesis.

Protective effect of the isoflavone equol against DNA damage induced by ultraviolet radiation to hairless mouse skin

Equol, an isoflavonoid metabolite produced from the dietary isoflavone daidzein by the gut microflora in mammals, has been found to protect not only against ultraviolet (UV) radiation-induced cutaneous inflammation and photoimmune suppression, but also have anti-photocarcinogenic properties in mice. Because the state of DNA damage has been correlated with suppression of the immune system and photocarcinogenesis, we have therefore examined the potential of equol to offer protection from solar-simulated UV (SSUV) radiation-induced DNA damage in hairless mice by the immunohistochemical approach using monoclonal antibody specific for cyclobutane pyrimidine dimers (CPDs; H3 antibody). Topical application of 20 µM equol lotion, which was applied both before and after SSUV significantly reduced the number of CPDs. This reduction was evident immediately after SSUV exposure, at 1 h after exposure, and at 24 h after exposure, revealing 54%, 50%, and 26% reduction in CPDs, respectively. When the same concentration was applied for 5 consecutive days after SSUV exposure, there was no significant difference in the reduction of CPDs immediately after SSUV irradiation or at 1 hour afterwards, but there were significant reductions of 23% and 42% at 24 and 48 h after SSUV exposure, respectively. Despite apparently reducing the number of CPDs post-SSUV, topically applied equol did not appear to increase the rate of dimer removal. To conclude, equol applied topically prior to SSUV irradiation offers protection against CPD formation in hairless mice, possibly by acting as a suncreen and thus inhibiting DNA photodamage.

Mechanisms of UV-induced immunosuppression

Ultraviolet radiation (UV), in particular the UVB range, suppresses the immune system in several ways. UVB inhibits antigen presentation, induces the release of immunosuppressive cytokines and causes apoptosis of leukocytes. UVB, however, does not cause general immunosuppression but rather inhibits immune reactions in an antigen-specific fashion. Application of contact allergens onto UV-exposed skin does not cause sensitization but induces antigen-specific tolerance since such an individual cannot be sensitized against the very same allergen later, although sensitization against other allergens is not impaired. This specific immunosuppression is mediated by antigen-specific suppressor/ regulatory T cells. UVB-induced DNA damage is a major molecular trigger of UV-mediated immunosuppression. Reduction of DNA damage mitigates UV-induced immunosuppression. Likewise interleukin-12 which exhibits the capacity to reduce DNA damage can prevent UV-induced immunosuppression and even break tolerance. Presentation of the antigen by UV-damaged Langerhans cells in the lymph nodes appears to be an essential requirement for the development of regulatory T cells. Studies addressing the molecular mechanisms underlying UV-induced immunosuppression will contribute to a better understanding how UV acts as a pathogen but on the other hand can be also used as a therapeutic tool.

Ultraviolet Immunosuppression: Mechanisms and Consequences

It is well recognized that exposure to solar radiation has several detrimental consequences, both acute and chronic. The suppression of immune functions remains one of the most intriguing phenomena brought about by ultraviolet (UV) radiation. This concept has challenged experts from various disciplines including dermatology, immunology, and photobiology. Although controversies exist regarding the mechanisms involved, the consensus is that UV immune suppression contributes significantly to the growth of cutaneous malignancies--both melanoma and nonmelanoma skin cancer. It is therefore a critical issue to be addressed in the context of developing and using sun protection strategies.

Inhibition of the elicitation phase of contact hypersensitivity by thymidine dinucleotides is in part mediated by increased expression of interleukin-10 in human keratinocytes

The production of immunomodulatory cytokines such as interleukin-10 (IL-10) from keratinocytes and other target cells in the skin plays a crucial role in UV-induced immunosuppression. Substantial evidence supports an association between DNA damage and immunomodulation. It is also known that small DNA fragments such as thymidine dinucleotides (pTpT) can mimic several UV-induced effects, including inhibition of the induction phase of the contact hypersensitivity response and up-regulation of tumor necrosis factor-alpha (TNF-alpha). To determine whether pTpT also induces IL-10 secretion by keratinocytes, and by inference whether IL-10 production after UV irradiation is a response to DNA damage, we compared the effects of pTpT with those of UV irradiation on primary human keratinocyte cultures. Subconfluent cultures of primary human keratinocytes were treated either with 10 micro M or 100 micro M pTpT or diluent alone, or exposed to solar-simulated light (100 J/m2 of UVB) or sham irradiated. An increase in IL-10 mRNA expression was observed 6-24 h after irradiation and at 24-48 h after treatment with pTpT. Detection of secreted IL-10 protein coincided with up-regulation of IL-10 gene expression at 48 and 72 h as determined by ELISA. Conditioned media from human keratinocytes treated with pTpT, like that from irradiated cells, significantly inhibited lymphocyte proliferation in the allogeneic-mixed lymphocyte reaction (MLR) assay. To determine whether pTpT mimics the suppressive influence of UVB on the elicitation phase of contact hypersensitivity, believed to result largely from IL-10 release, we compared the effects of topical application of pTpT with those of UVB irradiation on C57Bl/6 mice sensitized with dinitrofluorobenzene. Sensitized mice treated with pTpT or UVB irradiation showed markedly suppressed elicitation of ear-swelling responses. These results demonstrate that increased keratinocyte IL-10 mRNA level and IL-10 protein release are among the effects of pTpT and support the hypothesis that pTpT treatment triggers many of the biologic effects of UV irradiation by mimicking UV-induced DNA damage. Finally, regardless of mechanism, the data suggest that topical treatment with pTpT may provide a novel means of suppressing contact hypersensitivity or other lymphocyte-mediated reactions in skin.

Nuclear receptor peroxisome proliferator-activated receptor alpha (PPARalpha) is expressed in resting murine lymphocytes. The PPARalpha in T and B lymphocytes is both transactivation and transrepression competent

Peroxisome proliferator-activated receptors (PPARs) are transcription factors that belong to the nuclear hormone receptor superfamily. PPARalpha and PPARgamma ligands have been demonstrated to exert anti-inflammatory activities in macrophages by repressing the activities of several transcription factors. PPARgamma is expressed in T lymphocytes and may play a role in cytokine production, cellular proliferation, and susceptibility to apoptosis. Herein, we demonstrate that T and B lymphocytes constitutively express PPARalpha. PPARalpha represents the predominant isoform expressed in lymphocytes, whereas PPARgamma dominates in all cell types of the myeloid lineage. PPARalpha expression was down-regulated following T-cell activation while PPARgamma expression increased under the same activating conditions. PPARalpha expression in T cells may be regulated by microenvironmental factors, because Peyer's patch T cells expressed far greater levels of PPARalpha than T cells isolated from peripheral lymphoid organs. Exposure to specific ligand determined that PPARalpha in lymphocytes can effectively transactivate a peroxisome proliferator response element reporter construct. PPARalpha's ability to regulate endogenous genes, however, required treatment with histone deacetylase inhibitors. Finally, ligand activation of lymphocyte PPARalpha antagonized NF-kappaB. Our observation that a functional PPARalpha exists within T cells and B lymphocytes suggests an expanding role for this nuclear receptor in cells of the immune system.

Biological consequences of cyclobutane pyrimidine dimers

In the skin many molecules may absorb ultraviolet (UV) radiation upon exposure. In particular, cellular DNA strongly absorbs shorter wavelength solar UV radiation, resulting in various types of DNA damage. Among the DNA photoproducts produced the cyclobutane pyrimidine dimers (CPDs) are predominant. Although these lesions are efficiently repaired in the skin, this CPD formation results in various acute effects (erythema, inflammatory responses), transient effects (suppression of immune function), and chronic effects (mutation induction and skin cancer). The relationships between the presence of CPD in skin cells and the subsequent biological consequences are the subject of the present review.

Higher sensitivity in induction of apoptosis in fibroblast cell lines derived from LEC strain rats to ultraviolet B radiation

When lung fibroblast cell lines from LEC and WKAH rats were irradiated with ultraviolet B (UVB) and assayed for colony formation, LEC rat cells showed a higher sensitivity than did WKAH rat cells. The LEC rat cells were approximately 1.5-fold more sensitive to UVB radiation than were the WKAH rat cells in terms of D37 values, which are the doses of UVB required to reduce cell survival to 37%. When the rat cells were irradiated with UVB in the presence of 0.5 M dimethyl sulfoxide (DMSO), which efficiently scavenges free radicals such as hydroxyl radicals, no significant difference was observed between the survival curves of either LEC or WKAH rat cells irradiated with UVB in the presence of 0.5 M DMSO and those irradiated with UVB in the absence of DMSO. Therefore, formation of free radicals may not be involved in cell death induced by UVB radiation. Flow cytometry showed that the percentage of apoptotic cells in the LEC rat cell population increased with post-incubation time after UVB radiation. The proportion of apoptotic cells in the UVB-irradiated LEC rat cell population increased as the dose of UVB was increased. In contrast, no significant proportion of apoptotic cells was observed in the UVB-irradiated WKAH rat cell population. These results showed a higher sensitivity in induction of apoptosis by UVB radiation in LEC rat cells than in WKAH rat cells.

UVA exposure affects UVB and cis-urocanic acid-induced systemic suppression of immune responses in Listeria monocytogenes-infected Balb/c mice

Ultraviolet radiation can inhibit immune responses locally as well as systemically. Such effects have been measured in animals and humans exposed to ultraviolet B (wavelength 280-315 nm) (UVB) and ultraviolet A (315-400 nm) (UVA). The precise wavelength dependence is important for the identification of possible molecular targets and for assessments of risk of different artificial UV sources and solar UV. In such analyses, it is commonly assumed that radiation energy from each wavelength contributes to the effect independent of the other wavelengths. Here we show that this assumption does not hold good. In the present study, it was investigated whether exposure to broadband UVA or longwave ultraviolet A 1 (340-400 nm) (UVA 1) prior to the standard immunosuppressive UVB protocol might modulate the immunosuppressive effects induced by UVB. Preexposure to broadband UVA or longwave UVA 1, 1 day prior to the standard immunosuppressive UVB protocol, inhibited the UVB-induced suppression of delayed type hypersensitivity (DTH) to Listeria monocytogenes significantly. This effect was not associated with restoring the number of interleukin (IL-12)-positive cells in the spleen. Since isomerization of trans-urocanic acid (UCA) into the immunosuppressive cis-UCA isomer plays a crucial role in UVB-induced immunomodulation, in a second set of experiments it was investigated whether immunosuppression induced by cis-UCA might also be downregulated by preexposure to UVA. Animals were exposed to broad-band UVA or longwave UVA 1 prior to application of an immunosuppressive dose of cis- or trans-UCA as a control. Both UVA and UVA 1 appear to inhibit the cis-UCA-induced systemic immunosuppression (DTH and IL-12) to L. monocytogenes. These studies clearly show that UVA radiation modulates both UVB and cis-UCA-induced immunomodulation. In general, our studies indicate that both broadband UVA and longwave UVA 1 could induce modulation of UVB and cis-UCA-induced immunomodulation. As sunlight contains both UVA and UVB radiation the balance between these two radiations apparently determines the net immunomodulatory effect.

Myeloperoxidase is involved in H2O2-induced apoptosis of HL-60 human leukemia cells

We examined the mechanism of H(2)O(2)-induced cytotoxicity and its relationship to oxidation in human leukemia cells. The HL-60 promyelocytic leukemia cell line was sensitive to H(2)O(2), and at concentrations up to about 20-25 micrometer, the killing was mediated by apoptosis. There was limited evidence of lipid peroxidation, suggesting that the effects of H(2)O(2) do not involve hydroxyl radical. When HL-60 cells were exposed to H(2)O(2) in the presence of the spin trap alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN), we detected a 12-line electron paramagnetic resonance spectrum assigned to the POBN/POBN(.) N-centered spin adduct previously described in peroxidase-containing cell-free systems. Generation of this radical by HL-60 cells had the same H(2)O(2) concentration dependence as initiation of apoptosis. In contrast, studies with the K562 human erythroleukemia cell line, which is often used for comparison with the HL-60, and with high passaged HL-60 cells (spent HL-60) studied under the same conditions failed to generate POBN(.). Cellular levels of antioxidant enzymes superoxide dismutase, glutathione peroxidase, and catalase did not explain the differences between these cell lines. Interestingly, the K562 and spent HL-60 cells, which did not generate the radical, also failed to undergo H(2)O(2)-induced apoptosis. Based on this we reasoned that the difference in H(2)O(2)-induced apoptosis might be due to the enzyme myeloperoxidase. Only the apoptosis-manifesting HL-60 cells contained appreciable immunoreactive protein or enzymatic activity of this cellular enzyme. When HL-60 cells were incubated with methimazole or 4-aminobenzoic acid hydrazide, which are inhibitors of myeloperoxidase, they no longer underwent H(2)O(2)-induced apoptosis. Hypochlorous acid stimulated apoptosis in both HL-60 and spent HL-60 cells, indicating that another oxidant generated by myeloperoxidase induces apoptosis and that it may be the direct mediator of H(2)O(2)-induced apoptosis. Taken together these observations indicate that H(2)O(2)-induced apoptosis in the HL-60 human leukemia cell is mediated by myeloperoxidase and is linked to a non-Fenton oxidative event marked by POBN(.).

Transcription-coupled and global genome repair differentially influence UV-B-induced acute skin effects and systemic immunosuppression

Exposure to UV-B radiation impairs immune responses in mammals by inhibiting especially Th1-mediated contact hypersensitivity and delayed-type hypersensitivity. Immunomodulation is not restricted to the exposed skin, but is also observed at distant sites, indicating the existence of mediating factors such as products from exposed skin cells or photoactivated factors present in the superficial layers. DNA damage appears to play a key role, because enhanced nucleotide excision repair (NER) strongly counteracts immunosuppression. To determine the effects of the type and genomic location of UV-induced DNA damage on immunosuppression and acute skin reactions (edema and erythema) four congenic mouse strains carrying different defects in NER were compared: CSB and XPC mice lacking transcription-coupled or global genome NER, respectively, as well as XPA and TTD/XPD mice carrying complete or partial defects in both NER subpathways, respectively. The major conclusions are that 1) transcription-coupled DNA repair is the dominant determinant in protection against acute skin effects; 2) systemic immunomodulation is only affected when both NER subpathways are compromised; and 3) sunburn is not related to UV-B-induced immunosuppression.

Apoptosis in dendritic cell biology

Apoptosis or programmed cell death regulates many aspects in immunological homeostasis and, thus, controls the initiation, magnitude, duration, and termination of immune responses. Recent studies on dendritic cells (DC), including Langerhans cells (LC), have reinforced this concept by documenting that these antigen presenting cells express surface receptors and ligands that are known to mediate apoptotic cell death and that they are highly susceptible to apoptotic signals. In this review article, four major topics concerning apoptosis in the biology of DC will be overviewed: (a) molecular mechanisms of apoptosis; (b) DC apoptosis induced by various stimuli; (c) regulation of DC apoptosis; and (d) cross-priming and cross-tolerance induced by DC ingesting apoptotic bodies.

Shreedhar V, Ullrich SE, Cox P, Kripke ML. HindIII Liposomes Suppress Delayed-Type Hypersensitivity Responses In Vivo and Induce Epidermal IL-10 In Vitro

Considerable evidence suggests that ultraviolet-B (UV-B) radiation suppresses certain immune responses through the induction of cyclobutane pyrimidine dimers in DNA. To determine whether induction of other forms of DNA damage in the skin mimicked the immunosuppressive effects of UV-B radiation, we produced double-strand breaks in the DNA of epidermal cells with HindIII restriction endonuclease encapsulated in liposomes. Application of these liposomes, but not liposomes containing inactive HindIII or an irrelevant endonuclease, to the skin of C3H mice suppressed the induction of delayed-type hypersensitivity responses to Candida albicans and alloantigen and induced IL-10 production in the epidermis. Treatment of the Pam212 murine keratinocyte cell line with these liposomes in vitro induced immunosuppressive activity and IL-10 in culture supernatants. Unlike UV-B irradiation, however, HindIII in liposomes failed to induce suppressor T cell activity in vivo or in vitro. We conclude that double-strand breaks in DNA of epidermal cells can induce immunosuppression and up-regulate the production of immunomodulatory cytokines; however, either DNA damage alone does not account for all the immunosuppressive properties of UV-B irradiation, or cyclobutane pyrimidine dimers differ qualitatively from double-strand breaks in their biologic consequences. These studies raise the possibility that drugs causing DNA damage may induce cytokine dysregulation and immune suppression in addition to cytotoxicity.

The mechanism of N-acetylcysteine photoprotection is not related to dipyrimidine photoproducts

Topical application of N-acetylcysteine prior to UVB irradiation of BALB/c mice has previously been shown to inhibit systemic suppression of the contact hypersensitivity response. Formation of cis-urocanic acid, however, is not affected. Besides urocanic acid, UV-induced DNA damage has been held responsible for the initiation of suppression of the contact hypersensitivity response. Therefore, the possible inhibitory effect of N-acetylcysteine on UVB-induced cyclobutane pyrimidine dimers and pyrimidine (6-4) pyrimidone photoproducts has been investigated. No effect on the photoproducts studied is observed, suggesting that N-acetylcysteine exerts its photoprotective effect during the post-initiation phase of photoimmunosuppression.

Topical application of alpha-tocopherol modulates the antioxidant network and diminishes ultraviolet-induced oxidative damage in murine skin

The aim of this study was to investigate the effects of topical alpha-tocopherol application on epidermal and dermal antioxidants and its ability to prevent ultraviolet (UV)-induced oxidative damage. Hairless mice received topical applications of alpha-tocopherol 24 h before a single, acute UV irradiation (10 x minimal erythemal dose). The four major antioxidant enzymes (catalase, superoxide dismutase, glutathione reductase and glutathione peroxidase), hydrophilic and lipophilic antioxidants, and lipid hydroperoxides, markers of oxidative damage, were assayed in both epidermis and dermis of hairless mice. Topical alpha-tocopherol treatment increased dermal superoxide dismutase activity by 30% (P < 0.01) and protected epidermal glutathione peroxidase and superoxide dismutase from depletion after UV irradiation. Total and reduced glutathione levels in the epidermis increased by 50% after the topical treatment (P < 0.05), as did dermal ascorbate levels (by 40%: P < 0.01). The topical treatment increased alpha-tocopherol levels both in the epidermis (62-fold) and the dermis (22-fold: P < 0.001 in each layer). Furthermore, alpha-tocopherol treatment significantly reduced the formation of epidermal lipid hydroperoxides after UV irradiation (P < 0.05). These results demonstrate that topical administration of alpha-tocopherol protects cutaneous tissues against oxidative damage induced by UV irradiation in vivo, and suggest that the underlying mechanism of this effect involves the up-regulation of a network of enzymatic and non-enzymatic antioxidants.

Photodynamic therapy inhibits cell adhesion without altering integrin expression

Adhesion is a primordial cell function that, among others, regulates inflammation, metastasis, and tissue repair. To understand how these events could be affected by photodynamic therapy (PDT), we studied the effects of PDT on human foreskin fibroblast (HFF) adhesion to bovine collagen type I, human vitronectin or fibronectin. PDT, using benzoporphyrin derivative monoacid ring A (verteporfin) as the photosensitizer, inhibited cell adhesion in a drug dose-dependent manner, with no significant difference among matrices. The drug dose that killed 90% of cells within 20 h post-treatment inhibited HFF adhesion by 55%-68%. However, 45 min following PDT, a time period corresponding to that of the adhesion assay, HFF membrane integrity remained unaltered. In addition, cell surface expression of integrins was not modified for at least 2h following PDT. Western blots of cell lysates, using the anti-phosphotyrosine 4G10 monoclonal antibody, revealed that PDT prevented the adhesion-induced phosphorylation of 110-130 kDa proteins. Immunoblots of cell lysates immunoprecipitated with antibodies to focal adhesion kinase suggested that its phosphorylation was suppressed by PDT. These results demonstrate that PDT inhibits cell adhesion and affects integrin signalling without modifying cell membrane integrity or integrin expression.

Vitamin C abrogates the deleterious effects of UVB radiation on cutaneous immunity by a mechanism that does not depend on TNF-alpha

Acute low-dose treatment of murine skin with ultra violet B (UVB) light impairs induction of contact hypersensitivity (CH) to dinitrofluorobenzene (DNFB) in certain inbred strains of mice (termed UVB-susceptible), but not in others (termed UVB-resistant), and promotes tolerance. These deleterious effects of ultraviolet radiation (UVR) are mediated in part by TNF-alpha, which is released from UVR-exposed epidermal and dermal cells. Because UVR damage to skin has also been ascribed in part to the generation of reactive oxygen intermediates (ROIs) such as superoxide anion (O2-), hydrogen peroxide (H2O2), hydroxyl radical (OH-), and singlet oxygen ((1)O2), we investigated whether vitamin C (ascorbic acid), which can nullify ROIs, prevents the deleterious effects of UVR on the cutaneous immune system. We found that epicutaneous application of vitamin C (10% L-ascorbic acid solution) abrogated the deleterious effects of acute low-dose UVR on induction of CH and prevented the induction of tolerance. Vitamin C, however, did not reverse the effects of TNF-alpha on CH induction and tolerance. These results indicate that (i) ROIs generated intracutaneously by UVR contribute to the impaired ability of exposed skin to support the induction of CH and to promote the induction of tolerance and (ii) these effects are not dependent on TNF-alpha.

The inhibition of antigen-presenting activity of dendritic cells resulting from UV irradiation of murine skin is restored by in vitro photorepair of cyclobutane pyrimidine dimers

Exposing skin to UVB (280-320 nm) radiation suppresses contact hypersensitivity by a mechanism that involves an alteration in the activity of cutaneous antigen-presenting cells (APC). UV-induced DNA damage appears to be an important molecular trigger for this effect. The specific target cells in the skin that sustain DNA damage relevant to the immunosuppressive effect have yet to be identified. We tested the hypothesis that UV-induced DNA damage in the cutaneous APC was responsible for their impaired ability to present antigen after in vivo UV irradiation. Cutaneous APC were collected from the draining lymph nodes of UVB-irradiated, hapten-sensitized mice and incubated in vitro with liposomes containing a photolyase (Photosomes; Applied Genetics, Freeport, NY), which, upon absorption of photoreactivating light, splits UV-induced cyclobutane pyrimidine dimers. Photosome treatment followed by photoreactivating light reduced the number of dimer-containing APC, restored the in vivo antigen-presenting activity of the draining lymph node cells, and blocked the induction of suppressor T cells. Neither Photosomes nor photoreactivating light alone, nor photoreactivating light given before Photosomes, restored APC activity, and Photosome treatment did not reverse the impairment of APC function when isopsoralen plus UVA (320-400 nm) radiation was used instead of UVB. These controls indicate that the restoration of APC function matched the requirements of Photosome-mediated DNA repair for dimers and post-treatment photoreactivating light. These results provide compelling evidence that it is UV-induced DNA damage in cutaneous APC that leads to reduced immune function.

UV-induced cutaneous photobiology

Ultraviolet radiation (UVR) present in sunlight is a major environmental factor capable of affecting human health and well being. The organ primarily affected by UVR is the skin, which is composed of a variety of different cell types. Here, UVR is needed for production of active vitamin D as well as producing undesirable effects such as sunburn, premature cutaneous photoaging, and promoting skin cancer development. Depending on the radiation dose, UVR influences virtually every cutaneous cell type investigated differently. Since the end of the nineteenth century, sun exposure has been known to induce skin cancer, which is now the human malignancy with the most rapidly increasing incidence. In several experimental models, mid-range UVR has been demonstrated to be the major cause of UV-induced cutaneous tumors. The stratospheric ozone layer protecting the terrestrial surface from higher quantum energy solar radiation is being damaged by industrial activities resulting in the possibility of increased UVR exposure in the future. Investigations in the field of experimental dermatology have shown that within the skin an immunosurveillance system exists that may be able to detect incipient neoplasms and to elicit a host responses against it. This article reviews the literature on studies designed to investigate the effects of UVR on cutaneous cellular components, with special focus on the immune system within the skin and the development of UV-induced cancer.

A novel mechanism of glucocorticoid-induced immune suppression: the inhibiton of T cell-mediated terminal maturation of a murine dendritic cell line

Working with the murine epidermal-derived dendritic cell (DC) line XS52, we have observed previously that antigen-specific interaction with T cells stimulates their "terminal maturation" into fully professional DC. In this study we examined the impact of dexamethasone (DEX) on this T cell-induced event. When added to cocultures of XS52 DC and the KLH-specific Th1 clone HDK-1 in the presence of antigen, DEX at relatively low concentrations (10(-9)-10(-7) M) prevented substantially or completely each of the changes that typify terminal maturation, including (a) secretion of relatively large amounts of IL-1beta, IL-6, and TNFalpha; (b) loss of CD115 (colony-stimulating factor-1 receptor) expression and proliferative responsiveness to colony-stimulating factor-1; and (c) elevated expression of CD86 (B7-2). XS52 cells also underwent terminal maturation upon exposure to lipopolysaccharide alone, and DEX also inhibited effectively each of the same changes, indicating that DC can serve as the direct target of DEX. By contrast, DEX inhibited XS52 DC-stimulated IL-2 secretion by HDK-1 T cells, but not other changes that accompany T cell activation, including the secretion of IFNgamma and TNFalpha and the elevated expression of CD25, CD28, and CD44. These results reveal a new immunosuppressive mechanism of glucocorticoid action, that is, direct inhibition of T cell-mediated terminal maturation by DC.

UVB radiation interrupts cytokine-mediated support of an epidermal-derived dendritic cell line (XS52) by a dual mechanism

We have established long-term dendritic cell lines from the epidermis of newborn mice. These cell lines (XS series) proliferate maximally in response to granulocyte/macrophage-colony stimulating factor, as well as to CSF-1, which is produced by skin-derived NS fibroblast lines and by keratinocytes (albeit in smaller amounts). The purpose of this study was to examine the impact of UVB radiation on CSF-1-mediated interaction of dendritic cells with fibroblasts and keratinocytes. Exposure of NS cells to UVB radiation (unfiltered FS20 sunlamp) decreased CSF-1 production at mRNA and protein levels. Both changes occurred in a dose-dependent fashion, with 50 J/m2 causing a significant reduction. UVB radiation also downregulated CSF-1 mRNA expression by Pam 212 keratinocytes. UVB exposure of XS cells diminished the surface expression of CSF-1 receptors, with 50 J/m2 causing a significant reduction. Thus, UVB radiation interrupts CSF-1-mediated cell-cell interaction by a dual mechanism: downregulating CSF-1 production and abrogating CSF-1 receptor expression. Importantly, granulocyte/macrophage-colony stimulating factor receptor expression by XS cells was also inhibited by UVB radiation, once again, with 50 J/m2 producing significant inhibition. We propose that the resulting CSF-1 deficiency in epidermal microenvironment and unresponsiveness by dendritic cells to relevant growth factors may contribute to UVB-mediated loss of resident epidermal dendritic cells (i.e., Langerhans cells) in skin.

Expression of the common cytokine receptor gamma chain by murine dendritic cells including epidermal Langerhans cells

The common cytokine receptor gamma chain (gamma c) is an indispensable component of interleukin (IL)-2, IL-4, IL-7, IL-9, and IL-15 receptors, and its expression has been detected in several leukocyte populations, including T cells, B cells, monocytes, natural killer cells, and neutrophils. The purpose of this study was to determine whether gamma c receptors are expressed by dendritic cells (DC). Constitutive gamma c mRNA expression was observed by reverse transcription polymerase chain reaction and/or Northern blotting for: (a) Ia+ epidermal Langerhans cells (LC), (b) 4F7+ splenic DC, (c) granulocyte/macrophage colony-stimulated factor-propagated bone marrow-derived DC, and (d) the epidermal-derived DC line, XS52, which retains important functions of epidermal LC. Exposure of XS52 cells to recombinant IL-4 induced a rapid up-regulation of c-myc mRNA expression, and this IL-4-dependent signaling was blocked almost completely by anti-gamma c monoclonal antibody (mAb) TUGm2 in a soluble form. Moreover, c-myc up-regulation was inducible in XS52 cells by the same mAb in an immobilized form. These results imply that molecules recognized by this antibody (i.e. gamma c receptors) are expressed on XS52 cell surfaces. We thus conclude that DC express functional gamma c receptors, which then mediate cytokine-dependent regulation of DC functions.

Phenotypic and functional heterogeneity among murine epidermal-derived dendritic cell clones

We have established recently long-term dendritic cell lines from the epidermis of newborn BALB/c mice. These lines, termed XS series, resembled epidermal resident Langerhans cells or their progenitors in terms of surface phenotype, antigen-presenting capacity, and growth factor requirement. We examined in this study the degree of clonal heterogeneity among XS cells with respect to each of these features. Twelve stable clones were established by limiting dilution microculture from 8-10-week-old cultures of the XS52 or XS20 line. Despite the uniform expression of CD45, these clones varied substantially in their expression of Ia, B7-1, and B7-2 molecules. They also varied significantly in their relative efficiency in activating T cells. Finally, remarkable clone-to-clone heterogeneity was also observed in their growth factor responsiveness; some clones responded equally well to granulocyte macrophage-colony-stimulating factor-1, whereas others responded preferentially to one or the other of these factors. We propose that the observed clonal heterogeneity in XS cells reflects possible heterogeneity in the state of maturation and mitotic potential among the starting populations, i.e., skin-associated dendritic cells in newborn mice.

UVB-dependent modulation of epidermal cytokine network: roles in UVB-induced depletion of Langerhans cells and dendritic epidermal T cells

The epidermis of mice consists of three cellular components, i.e., keratinocytes, Langerhans cells (LC), and dendritic epidermal T cells (DETC). Each epidermal subpopulation produces a different set of cytokines, thereby forming a unique cytokine milieu. These cytokines, in turn, support the survival and growth of LC and DETC and regulate their immunological functions. LC and DETC play important, but distinct, effector roles in protective immunity against antigens that are generated in or penetrate into the epidermis. Acute or chronic exposure of mice to ultraviolet B (UVB) radiation is known to impair this cutaneous immunity, as evidenced by the failure to induce T cell-mediated immune reactions, by the generation of antigen-specific immunological unresponsiveness, and by the development of skin cancers. Importantly, these changes are associated with reduced densities of LC and DETC in UVB-exposed skin, suggesting that the deficiency in these epidermal leukocytes may account for some of the deleterious influences of UVB radiation on skin. Here I will review the recent advance in our understanding of the mechanisms by which UVB radiation may deplete LC and DETC from epidermis. More specifically, I will discuss the following possibilities: a) UVB-mediated suppression of the production of relevant growth factors for LC and DETC, b) UVB-induced abrogation of surface expression of growth factor receptors, and c) UVB-triggered apoptotic cell death in epidermal leukocytes.