Ultraviolet radiation and the contact hypersensitivity reaction in mice

Impact of Skin Exposure to Benzo[a]pyrene in Rat Model: Insights into Epidermal Cell Function and Draining Lymph Node Cell Response

The skin is a direct target of the air pollutant benzo[a]pyrene (BaP). While its carcinogenic qualities are well-studied, the immunotoxicity of BaP after dermal exposure is less understood. This study examines the immunomodulatory effects of a 10-day epicutaneous BaP application, in environmentally/occupationally relevant doses, by analyzing ex vivo skin immune response (skin explant, epidermal cells and draining lymph node/DLN cell activity), alongside the skin's reaction to sensitization with experimental hapten dinitrochlorobenzene (DNCB). The results show that BaP application disrupts the structure of the epidermal layer and promotes immune cell infiltration in the dermis. BaP exposure led to oxidative stress in epidermal cells, characterized by decreased reduced glutathione and increased AHR and Cyp1A1 expression. Production and gene expression of proinflammatory cytokines (TNF, IL-1β) by epidermal cells decreased, while IL-10 response increased. Decreased spontaneous production of IFN-γ and IL-17, along with unchanged IL-10, was observed in DLC cells, whereas ConA-stimulated production of these cytokines was elevated. Local immunosuppression caused by BaP application seems to reduce the skin's response to an additional stimulus, evidenced by decreased effector activity of DLN cells three days after sensitization with DNCB. These findings provide new insight into the immunomodulatory effects and health risks associated with skin exposure to BaP.

Sex Differences in Photoprotective Responses to 1,25-Dihydroxyvitamin D3 in Mice Are Modulated by the Estrogen Receptor-β

Susceptibility to photoimmune suppression and photocarcinogenesis is greater in male than in female humans and mice and is exacerbated in female estrogen receptor-beta knockout (ER-β-/-) mice. We previously reported that the active vitamin D hormone, 1,25-dihydroxyvitamin D3 (1,25(OH)2D), applied topically protects against the ultraviolet radiation (UV) induction of cutaneous cyclobutane pyrimidine dimers (CPDs) and the suppression of contact hypersensitivity (CHS) in female mice. Here, we compare these responses in female versus male Skh:hr1 mice, in ER-β-/-/-- versus wild-type C57BL/6 mice, and in female ER-blockaded Skh:hr1 mice. The induction of CPDs was significantly greater in male than female Skh:hr1 mice and was more effectively reduced by 1,25(OH)2D in female Skh:hr1 and C57BL/6 mice than in male Skh:hr1 or ER-β-/- mice, respectively. This correlated with the reduced sunburn inflammation due to 1,25(OH)2D in female but not male Skh:hr1 mice. Furthermore, although 1,25(OH)2D alone dose-dependently suppressed basal CHS responses in male Skh:hr1 and ER-β-/- mice, UV-induced immunosuppression was universally observed. In female Skh:hr1 and C57BL/6 mice, the immunosuppression was decreased by 1,25(OH)2D dose-dependently, but not in male Skh:hr1, ER-β-/-, or ER-blockaded mice. These results reveal a sex bias in genetic, inflammatory, and immune photoprotection by 1,25(OH)2D favoring female mice that is dependent on the presence of ER-β.

Daily very low UV dose exposure enhances adaptive immunity, compared with a single high-dose exposure. Consequences for the control of a skin infection

Ultraviolet radiation (UVr) promotes several well-known molecular changes, which may ultimately impact on health. Some of these effects are detrimental, like inflammation, carcinogenesis and immunosuppression. On the other hand, UVr also promotes vitamin D synthesis and other beneficial effects. We recently demonstrated that exposure to very low doses of UVr on four consecutive days [repetitive low UVd (rlUVd)] does not promote an inflammatory state, nor the recruitment of neutrophils or lymphocytes, as the exposure to a single high UV dose (shUVd) does. Moreover, rlUVd reinforce the epithelium by increasing antimicrobial peptides transcription and epidermal thickness. The aim of this study was to evaluate the adaptive immune response after shUVd and rlUVd, determining T-cell and B-cell responses. Finally, we challenged animals exposed to both irradiation procedures with Staphylococcus aureus to study the overall effects of both innate and adaptive immunity during a cutaneous infection. We observed, as expected, a marked suppression of T-cell and B-cell responses after exposure to an shUVd but a novel and significant increase in both specific responses after exposure to rlUVd. However, the control of the cutaneous S. aureus infection was defective in this last group, suggesting that responses against pathogens cannot be ruled out from isolated stimuli.

CYP11A1 in skin: an alternative route to photoprotection by vitamin D compounds

Topical 1,25-dihydroxyvitamin D (1,25D) and other vitamin D compounds have been shown to protect skin from damage by ultraviolet radiation (UVR) in a process that requires the vitamin D receptor. Yet, while mice which do not express the vitamin D receptor are more susceptible to photocarcinogenesis, mice unable to 1α-hydroxylate 25-hydroxyvitamin D to form 1,25D do not show increased susceptibility to UVR-induced skin tumors. A possible explanation is that an alternative pathway, which does not involve 1α-hydroxylation, may produce photoprotective compounds from vitamin D. The cholesterol side chain cleavage enzyme CYP11A1 is expressed in skin and produces 20-hydroxyvitamin D3 (20OHD) as a major product of vitamin D3. We examined whether topical 20OHD would affect UVR-induced DNA damage, inflammatory edema or immune suppression produced in Skh:hr1 mice. Photoprotection by 20OHD at 23 or 46pmol/cm(2) against cyclobutane pyrimidine dimers (DNA lesions) after UVR in mice was highly effective, up to 98±0.8%, (p<0.001) and comparable to that of 1,25D. Sunburn edema measured as skinfold thickness 24h after UVR was also significantly reduced by 20OHD (p<0.001). In studies of contact hypersensitivity (CHS), which is suppressed by UVR, topical application of 20OHD to mice protected against UVR-induced immunosuppression (p<0.05), similar to the effect of 1,25D at similar doses (46±0.6% protection with 20OHD, 44±0.5% with 1,25D). Both UVR-induced DNA damage and immunosuppression contribute to increased susceptibility to UVR-induced skin tumors. This study indicates a potentially anti-photocarcinogenic role of the naturally occurring vitamin D metabolite, 20OHD, which does not depend on 1α-hydroxylation for generation. This article is part of a Special Issue entitled '17th Vitamin D Workshop'.

Differential photoprotective effects of 1,25-dihydroxyvitamin D3 and a low calcaemic deltanoid

We have previously demonstrated that the active vitamin D hormone, 1α,25-dihydroxyvitamin D3 (1,25(OH)(2)D(3)) and a cis-locked non-genomic analogue, protect skin cells from ultraviolet radiation (UV)-induced skin cell loss, DNA damage, immunosuppression and skin carcinogenesis. Herein, we used a low-calcaemic analogue, 1α-hydroxymethyl-16-ene-24,24-difluoro-25-hydroxy-26,27-bis-homovitamin D3 (QW), which has some transactivating capacity and is approximately 80-100 times less calcaemic than 1,25(OH)(2)D(3). QW (0.1-10 nM) significantly (p < 0.05-0.01) reduced UV-induced DNA lesions (CPD) in skin fibroblasts and keratinocytes and reduced cell death after UV exposure. Moreover, both 1,25(OH)(2)D(3) and QW (1 nM) were equally effective in significantly (p < 0.01) increasing levels of tumour suppressive p53 in cultured human keratinocytes at 3 and 6 h after UV exposure. In a hairless mouse model, both 1,25(OH)(2)D(3) and QW (22.8 ρmol cm(-2)) reduced UV-immunosuppression from 13.7 ± 1.3% to 0.1 ± 1.1% (p < 0.01) and 5.4 ± 1.5% (p < 0.01) respectively. When tested alongside 1,25(OH)(2)D(3) in a murine model of skin carcinogenesis. QW (22.8 ρmol cm(-2)) was not as effective as 1α,25(OH)(2)D(3) or a cis-locked analogue in reducing tumour formation or inhibiting tumour progression. It is possible that the dose required for QW to be effective as an anti-photocarcinogenesis agent in vivo is higher than for protection against the acute effects of UV exposure, but the dissociation between clear acute photo-protective effects and limited long term photoprotection is as yet unexplained.

Hypoxia inducible factor-1α contributes to UV radiation-induced inflammation, epidermal hyperplasia and immunosuppression in mice

Hypoxia inducible factor-1α (HIF-1α), a ubiquitous inducible oxygen-sensing transcription factor, promotes cell survival under hypoxic conditions, including the early pre-angiogenic period of tumorigenesis, and is known to contribute to many malignancies. However HIF-1α can also be activated by inflammatory mediators, and can activate inflammation-modulating proteins itself, including heme oxygenase-1 (HO-1) and the cytokine IL-6. Recently HIF-1α was reported to be induced by UVB (290-320 nm) radiation in cultured human keratinocytes, acting as a stress protein associated with the release of reactive oxygen species. In this in vivo murine study we demonstrate that HIF-1α protein is an early responder to UV radiation in the skin, and its activation can be attenuated by treating mice with its post-translational inhibitor, YC-1. Treatment with YC-1 following UV-irradiation of mice has revealed the involvement of HIF-1α in UV-induced inflammation, IL-6 production, and epidermal hyperplasia. In addition, upregulated cutaneous HIF-1α was found to be an important factor in the UV-suppression of T cell-mediated immunity, measured by contact hypersensitivity (CHS). The mechanism remains unclear, however it did not appear to involve the immunosuppressive cutaneous photoproduct cis-urocanic acid, but HIF-1α induction was inhibited by irradiation with photoimmune protective UVA (320-400 nm), implicating a negative correlation between the two stress proteins, HIF-1α and the photoimmune protective UVA responder HO-1.

Gender differences in UV-induced inflammation and immunosuppression in mice reveal male unresponsiveness to UVA radiation

Immunosuppression attributed mainly to the UVB (290-320 nm) waveband is a prerequisite for skin cancer development in mice and humans. The contribution of UVA (320-400 nm) is controversial, but in mice UVA irradiation has been found to antagonise immunosuppression by UVB. In other studies of photoimmune regulation, protection mediated via oestrogen receptor-β signalling was identified as a normal endogenous defence in mice, and was shown to depend on UVA irradiation. A gender bias in photoimmune responsiveness was thus suggested, and is tested in this study by comparing the UV-induced inflammatory and immune responses in male and female hairless mice. We report that male mice, which show greater skin thickness than females, developed a less intense but slower resolving sunburn inflammatory oedema, correlated with reduced epidermal expression of pro-inflammatory IL-6 than females following solar simulated UV (SSUV, 290-400 nm) exposure. On the other hand, the contact hypersensitivity reaction (CHS) was more severely suppressed by SSUV in males, correlated with increased epidermal expression of immunosuppressive IL-10. Exposure to the UVB waveband alone, or to cis-urocanic acid, suppressed CHS equally in males and females. However, whereas UVA irradiation induced immunoprotection against either UVB or cis-urocanic acid in females, this protection was significantly reduced or abrogated in males. The results indicate that males are compromised by a relative unresponsiveness to the photoimmune protective effects of UVA, alone or as a component of SSUV. This could explain the known gender bias in skin cancer development in both mice and humans.

In vivo UVA irradiation of mouse is more efficient in promoting pulmonary melanoma metastasis than in vitro

BACKGROUND

We have previously shown in vitro that UVA increases the adhesiveness of mouse B16-F1 melanoma cells to endothelium.We have also shown in vivo that UVA exposure of C57BL/6 mice, i.v. injected with B16-F1 cells, increases formation of pulmonary colonies of melanoma. The aim of the present animal study was to confirm the previously observed in vivo UVA effect and to determine whether in vitro UVA-exposure of melanoma cells, prior the i.v. injection, will have an enhancing effect on the pulmonary colonization capacity of melanoma cells. As a second aim, UVA-derived immunosuppression was determined.

METHODS

Mice were i.v. injected with B16-F1 cells into the tail vein and then immediately exposed to UVA. Alternatively, to study the effect of UVA-induced adhesiveness on the colonization capacity of B16-F1 melanoma, cells were in vitro exposed prior to i.v. injection. Fourteen days after injection, lungs were collected and the number of pulmonary nodules was determined under dissecting microscope. The UVA-derived immunosuppression was measured by standard contact hypersensitivity assay.

RESULTS AND DISCUSSION

Obtained results have confirmed that mice, i.v. injected with B16-F1 cells and thereafter exposed to UVA, developed 4-times more of melanoma colonies in lungs as compared with the UVA non-exposed group (p < 0.01). The in vitro exposure of melanoma cells prior to their injection into mice, led only to induction of 1.5-times more of pulmonary tumor nodules, being however a statistically non-significant change. The obtained results postulate that the UVA-induced changes in the adhesive properties of melanoma cells do not alone account for the 4-fold increase in the pulmonary tumor formation. Instead, it suggests that some systemic effect in a mouse might be responsible for the increased metastasis formation. Indeed, UVA was found to induce moderate systemic immunosuppression, which effect might contribute to the UVA-induced melanoma metastasis in mice lungs.

Interdependence between heme oxygenase-1 induction and estrogen-receptor-beta signaling mediates photoimmune protection by UVA radiation in mice

Previous studies have found that signaling by the estrogen receptor-beta (Er-beta) attenuated solar-simulated UV radiation (SSUV)-induced immunosuppression. This study seeks evidence for a common mechanism for this immunoprotection for both Er-beta signaling and irradiation with the UVA waveband. In Skh:hr-1 hairless mice, the immunoprotection afforded by UVA exposure against subsequent UVB or cis-urocanic acid suppression of contact hypersensitivity (CHS) was abrogated by treatment with the antiestrogen, ICI 182,780. Furthermore, in normal C57BL mice, UVA enrichment of UVA/UVB sources provided protection against UVB-suppressed CHS and upregulated epidermal IL-10 expression, but this protection was inhibited in Er-beta-/- mice. These observations indicated that the immunoprotective response to UVA was dependent on Er-beta signaling. As earlier studies have established that UVA photoimmune protection depends on the induction of the stress enzyme, heme oxygenase (HO)-1, its activity was examined relative to Er-beta. Immunoprotection against SSUV by 17-beta-estradiol was prevented by inhibiting HO enzyme activity; immunoprotection against cis-urocanic acid by carbon monoxide (HO product) was prevented by ICI 182,780. In addition, the HO-1 gene was unresponsive to UVA induction in Er-beta-/- mice. Therefore, HO-1 inducibility and Er-beta signaling are interdependent requisite responses to the UVA waveband for its immunoprotective action against UVB exposure.

The hairless mouse in skin research

The hairless (Hr) gene encodes a transcriptional co-repressor highly expressed in the mammalian skin. In the mouse, several null and hypomorphic Hr alleles have been identified resulting in hairlessness in homozygous animals, characterized by alopecia developing after a single cycle of relatively normal hair growth. Mutations in the human ortholog have also been associated with congenital alopecia. Although a variety of hairless strains have been developed, outbred SKH1 mice are the most widely used in dermatologic research. These unpigmented and immunocompetent mice allow for ready manipulation of the skin, application of topical agents, and exposure to UVR, as well as easy visualization of the cutaneous response. Wound healing, acute photobiologic responses, and skin carcinogenesis have been extensively studied in SKH1 mice and are well characterized. In addition, tumors induced in these mice resemble, both at the morphologic and molecular levels, UVR-induced skin malignancies in man. Two limitations of the SKH1 mouse in dermatologic research are the relatively uncharacterized genetic background and its outbred status, which precludes inter-individual transplantation studies.

Radiation sources providing increased UVA/UVB ratios induce photoprotection dependent on the UVA dose in hairless mice

In studies involving mice in which doses of UVA (320-400 nm) and UVB (290-320 nm) radiation were administered alone or combined sequentially, we observed a protective effect of UVA against UVB-induced erythema/edema and systemic suppression of contact hypersensitivity. The UVA immunoprotection was mediated by the induction of the stress enzyme heme oxygenase-1 (HO-1) in the skin, protection of the cutaneous Th1 cytokines interferon-gamma (IFN-gamma) and IL-12 and inhibition of the UVB-induced expression of the Th2 cytokine IL-10. In this study, we seek evidence for an immunological waveband interaction when UVA and UVB are administered concurrently to hairless mice as occurs during sunlight exposure in humans. A series of spectra providing varying ratios of UVA/UVB were developed, with the UVA ratio increased to approximately 3.5 times the UVA component in solar simulated UV (SSUV). We report that progressively increasing the UVA component of the radiation while maintaining a constant UVB dose resulted in a reduction of both the erythema/edema reaction and the degree of systemic immunosuppression, as measured as contact hypersensitivity. The UVA-enhanced immunoprotection was abrogated in mice treated with a specific HO enzyme inhibitor. UVA-enhanced radiation also upregulated the expression of cutaneous IFN-gamma and IL-12 and inhibited expression of both IL-6 and IL-10, compared with the activity of SSUV. The results were consistent with the previously characterized mechanisms of photoprotection by the UVA waveband alone and suggest that the UVA component of solar UV may have beneficial properties for humans.

Unexpected effects of UVB in IL-10 transgenic mice: normalization of contact hypersensitivity response

Solar radiation in the UVB range is absorbed primarily by the epidermal DNA where characteristic photodamage results in altered immune responses and mutagenic lesions. UVB exposure of the skin results in a profound upregulation of the anti-inflammatory cytokine, IL-10 and suppression of contact hypersensitivity (CHS). Given that IL-10 is produced after UVB exposure, and that antibodies against IL-10 have been shown to reverse UVB-induced immune suppression, we hypothesized that IL-10 transgenic mice would show an enhanced immune suppression in response to UVB. Using an IL-10 transgenic mouse model (IL-10tg), we examined the CHS response in unexposed animals and those exposed to UVB. Unexposed IL-10tg animals showed a diminished CHS response compared to wild-type. Surprisingly, however, when IL-10tg animals were exposed to UVB, the CHS response was not further suppressed, but rather was restored to the level observed in unexposed wild-type animals.

Isoflavonoid Photoprotection in Mouse and Human Skin Is Dependent on Metallothionein

Previous studies report that selected topical isoflavonoids are immunoprotective in both mice and humans, when applied following UV irradiation. Isoflavonoids have documented antioxidant activity, but their mechanism of immunomodulation remains unclear. This study examines whether photoimmunoprotection by the isoflavonoids might result from their interaction with one cutaneous antioxidant known to modulate UV photodamage, metallothionein (MT). In mice bearing a null mutation for MT-I and -II, we found that immunoprotection by the isoflavonoid 4',7-dihydroxyisoflavane (equol) against solar-simulated UV radiation (SSUV) or exogenous cis-urocanic acid was abrogated. Topical equol did not activate MT expression in normal mouse skin, but markedly enhanced the increase in MT expression in murine epidermis following SSUV irradiation. Normal human skin, unlike murine, expressed MT in the basal epidermis. Following SSUV irradiation, topical application of the related synthetic isoflavonoid NV-07alpha to human skin also markedly enhanced epidermal MT expression. The NV-07alpha has been reported previously to protect humans against the UV suppression of Mantoux reactions. Thus, epidermal MT expression appears to protect against photoimmunosuppression in both human and mouse skin. We speculate that equol and its related derivative NV-07alpha may activate the MT gene synergistically with SSUV, to produce the enhanced immunoprotective effect.

The Effect of Ultraviolet Radiation on Human Viral Infections

Exposure to UV radiation is recognized to suppress cell-mediated immunity and therefore could adversely affect the course of a viral infection. Rodent models of viral infection confirm this possibility but the situation in human subjects is not so clear, apart from two exceptions. These are herpes simplex, in which sunlight exposure can cause reactivation, and certain papillomavirus types in which sunlight exposure can lead to the development of nonmelanoma skin cancer. In both cases, there are UV response elements in the viral genomes that alter the normal interactions between the viruses and the host following exposure, and UV-induced effects on the immune response occur in addition. These complex mechanisms are discussed, and the situation regarding UV radiation and viral exanthems plus other viruses, including the retroviruses, summarized. Finally viral vaccination is considered in the context of UV exposure and the importance of the host's genetic background emphasized. Further research is required to evaluate whether sunlight can significantly affect the resistance to common viral infections and vaccines.

Suppression of contact hypersensitivity after repeated exposures of humans to low doses of solar simulated radiation

Although it is generally recognised that UV radiation (UVR) can induce suppression of contact hypersensitivity (CHS) in human subjects, most protocols to date have not tested the effect of low daily doses of solar simulated radiation (SSR). In the present study, healthy individuals, divided into four groups each consisting of approximately 34 subjects, were whole-body irradiated with 1.2 standard erythema doses of SSR for 2, 10 or 30 consecutive days, or were unirradiated. They were sensitised with diphenylocyclopropenone (DPCP) on one exposed body site 24 h after the final UVR. The occurrence and severity of the primary allergic response were noted, and both parameters were shown to be significantly lowered in the group irradiated for 30 days compared with the unirradiated group. Elicitation of CHS was undertaken 3 weeks after the sensitisation, using a range of concentrations of DPCP on a UV-protected body site. The extent of the CHS at 48 h was assessed by the clinical score, by an erythema meter and by histological examination of a biopsy taken from the site challenged with one selected concentration of DPCP. Although erythema and pigmentation did not differ between the groups, a significant negative correlation was found between the clinical CHS score and the number of days of UV exposure, at the lowest challenge dose of DPCP. In addition a significant negative correlation was revealed between the intensity of spongiosis (intraepidermal oedema and vesicles, as evaluated by histology) and the number of days of UV exposure. Thus small daily doses of SSR induce suppression of CHS in human subjects and the effect is cumulative, indicating that there is no adaptation to the immunomodulating effects of UVR, at least over the test period of 30 days.