Solar ultraviolet radiation and the risk of infectious disease: summary of a workshop

Exploring the impact of solar radiation on skin microbiome to develop improved photoprotection strategies

The skin microbiome undergoes constant exposure to solar radiation (SR), with its effects on health well-documented. However, understanding SR's influence on host-associated skin commensals remains nascent. This review surveys existing knowledge on SR's impact on the skin microbiome and proposes innovative sun protection methods that safeguard both skin integrity and microbiome balance. A team of skin photodamage specialists conducted a comprehensive review of 122 articles sourced from PubMed and Research Gateway. Key terms included skin microbiome, photoprotection, photodamage, skin cancer, ultraviolet radiation, solar radiation, skin commensals, skin protection, and pre/probiotics. Experts offered insights into novel sun protection products designed not only to shield the skin but also to mitigate SR's effects on the skin microbiome. Existing literature on SR's influence on the skin microbiome is limited. SR exposure can alter microbiome composition, potentially leading to dysbiosis, compromised skin barrier function, and immune system activation. Current sun protection methods generally overlook microbiome considerations. Tailored sun protection products that prioritize both skin and microbiome health may offer enhanced defense against SR-induced skin conditions. By safeguarding both skin and microbiota, these specialized products could mitigate dysbiosis risks associated with SR exposure, bolstering skin defense mechanisms and reducing the likelihood of SR-mediated skin issues.

Keratinocytes Counteract UVB-Induced Immunosuppression in Mice Via HIF-1a Signaling

The transcription factor Hypoxia-Inducible Factor-1alpha (HIF-1a) regulates cellular metabolism under hypoxia but also immune responses and UVB-induced skin reactions. In keratinocytes, HIF-1a is an environmental sensor orchestrating the adaptation to environmental changes. Here, we investigated the role of HIF-1a in keratinocytes for skin reactions to acute and chronic UVB exposure in mice. The function of HIF-1a in keratinocytes under UVB exposure was analyzed in conditional keratinocyte-specific HIF-1a-KO (in short "cKO") mice. cKO mice were hypersensitive to acute high-dose UVB irradiation compared to wildtype (WT), displaying increased cell death and delayed barrier repair. After chronic low-dose UVB treatment, cKO mice also had stronger epidermal damage but reduced infiltration of dermal macrophages and T helper cells compared to WT mice. Irradiated cKO mice revealed accumulation of regulatory lymphocytes in dorsal skin-draining lymph nodes compared to WT and unirradiated mice. This was reflected by augmented IL-10 release of lymph node cells and a weaker contact hypersensitivity reaction to DNFB in UVB-exposed cKO mice compared to WT and unirradiated controls. In summary, we found that keratinocyte-specific HIF-1a expression is crucial for adaptation to UVB exposure and inhibits the development of UVB-induced immunosuppression in mice. Therefore, HIF-1a signaling in keratinocytes could ameliorate photoaging-related skin disorders.

Ultraviolet radiation, vitamin D, and COVID-19

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the causative agent of coronavirus disease 2019 (COVID-19), has become pandemic on March 11^th, 2020. COVID-19 has a range of symptoms that includes fever, fatigue, dry cough, aches, and labored breathing to acute respiratory distress and possibly death. Health systems and hospitals have been completely rearranged since March 2020 in order to limit the high rate of virus spreading. Hence, a great debate on deferrable visits and treatments including phototherapy for skin diseases is developing. In particular, as regards phototherapy very few data are currently available regarding the chance to continue it, even if it may be a useful resource for treating numerous dermatological patients. However, phototherapy has an immunosuppressive action possibly facilitating virus infection. In the context of COVID-19 infection risk it is important to pointed out whether sunlight, phototherapy and in particular ultraviolet radiation (UV-R) constitute or not a risk for patients. In this review we aimed to focus on the relationship between UV-R, sunlight, phototherapy, and viral infections particularly focusing on COVID-19.

Rodent Papillomaviruses

Preclinical infection model systems are extremely valuable tools to aid in our understanding of Human Papillomavirus (HPV) biology, disease progression, prevention, and treatments. In this context, rodent papillomaviruses and their respective infection models are useful tools but remain underutilized resources in the field of papillomavirus biology. Two rodent papillomaviruses, MnPV1, which infects the Mastomys species of multimammate rats, and MmuPV1, which infects laboratory mice, are currently the most studied rodent PVs. Both of these viruses cause malignancy in the skin and can provide attractive infection models to study the lesser understood cutaneous papillomaviruses that have been frequently associated with HPV-related skin cancers. Of these, MmuPV1 is the first reported rodent papillomavirus that can naturally infect the laboratory strain of mice. MmuPV1 is an attractive model virus to study papillomavirus pathogenesis because of the ubiquitous availability of lab mice and the fact that this mouse species is genetically modifiable. In this review, we have summarized the knowledge we have gained about PV biology from the study of rodent papillomaviruses and point out the remaining gaps that can provide new research opportunities.

Engineered nanoparticles of titanium dioxide (TIO2): Uptake and biological effects in a sea bass cell line

With the rapid development of nanotechnology there has been a corresponding increase in the application of titanium dioxide nanoparticles (TiO₂-NPs) in various consumer and industrial products, consequently their potential health hazards and environmental effects are considered an aspect of great concern. In the present study, in order to assess the impact of TiO₂-NPs in the marine environment, the biological effects of TiO₂-NPs on a sea bass cell line (DLEC) were investigated. Cells were exposed for 24 h to different concentrations of TiO₂-NPs (1, 8, 40, 200 and 1000 μg/ml) or co-exposed with CdCl₂ (Cd). The effects of UV light irradiation were also investigated in cells treated with TiO₂-NPs and/or Cd. The internalization of TiO₂-NPs and the morphological cell modifications induced by the treatments were examined by transmission and scanning electron microscopy, this latter coupled with energy dispersive X-ray spectroscopy (EDS) for particle element detection. In addition, the effects of controlled exposures were studied evaluating the cytotoxicity, the DNA damage and the expression of inflammatory genes. Our study indicates that TiO₂-NPs were localized on the cell surface mainly as agglomerates revealed by EDS analysis and that they were uptaken by the cells inducing morphological changes. Photoactivation of TiO₂-NPs and/or co-exposure with Cd affects ATP levels and it contributes to induce acute cellular toxicity in DLEC cells dependent on Ti concentration. The inflammatory potential and the DNA damage, this latter displayed through a caspase-3 independent apoptotic process, were also demonstrated. Overall our data suggest that the interaction of TiO₂-NPs with marine water contaminants, such as cadmium, and the UV irradiation, may be an additional threat to marine organisms.

The Skin Microbiome: Is It Affected by UV-induced Immune Suppression?

Human skin apart from functioning as a physical barricade to stop the entry of pathogens, also hosts innumerable commensal organisms. The skin cells and the immune system constantly interact with microbes, to maintain cutaneous homeostasis, despite the challenges offered by various environmental factors. A major environmental factor affecting the skin is ultraviolet radiation (UV-R) from sunlight. UV-R is well known to modulate the immune system, which can be both beneficial and deleterious. By targeting the cells and molecules within skin, UV-R can trigger the production and release of antimicrobial peptides, affect the innate immune system and ultimately suppress the adaptive cellular immune response. This can contribute to skin carcinogenesis and the promotion of infectious agents such as herpes simplex virus and possibly others. On the other hand, a UV-established immunosuppressive environment may protect against the induction of immunologically mediated skin diseases including some of photodermatoses such as polymorphic light eruption. In this article, we share our perspective about the possibility that UV-induced immune suppression may alter the landscape of the skin’s microbiome and its components. Alternatively, or in concert with this, direct UV-induced DNA and membrane damage to the microbiome may result in pathogen associated molecular patterns (PAMPs) that interfere with UV-induced immune suppression.

Ultraviolet-induced alloantigen-specific immunosuppression in transplant immunity

After the first observation of the immunosuppressive effects of ultraviolet (UV) irradiation was reported in 1974, therapeutic modification of immune responses by UV irradiation began to be investigated in the context immunization. UV-induced immunosuppression is via the action of regulatory T cells (Tregs). Antigen-specific Tregs were induced by high-dose UV-B irradiation before antigen immunization in many studies, as it was considered that functional alteration and/or modulation of antigen-presenting cells by UV irradiation was required for the induction of antigen-specific immunosuppression. However, it is also reported that UV irradiation after immunization induces antigen-specific Tregs. UV-induced Tregs are also dominantly transferable, with interleukin-10 being important for UV-induced immunosuppression. Currently, various possible mechanisms involving Treg phenotype and cytokine profile have been suggested. UV irradiation accompanied by alloantigen immunization induces alloantigen-specific transferable Tregs, which have potential therapeutic applications in the transplantation field. Here we review the current status of UV-induced antigen-specific immunosuppression on the 40^th anniversary of its discovery.

Histopathological Analysis of UVB and IR Interaction in Rat Skin

To determine the chronic skin effects caused by the interaction of infrared and ultraviolet B radiations, male Rattus norvegicus (Wistar) (2 months old) were exposed for 15 days to infrared radiation (600-1500 nm, with a peak at 1000 nm, n = 12) for 30 min (1080 J cm(-2) ) (IRo); to ultraviolet B radiation (peak emission at 313 nm, n = 9) for 90 min (55.08 J cm(-2) ) (UVB); to infrared radiation followed after 90 min by ultraviolet B (n = 6) (IRUVB) and to ultraviolet B followed after 90 min by infrared radiation (n = 9) (UVBIR). Skin samples were collected and histopathological analysis showed the presence of acanthosis, parakeratotic and orthokeratotic hyperkeratosis, intraepidermal pustules, keratin pearls, detachment of epidermis, collagen necrosis, inflammatory infiltrate, vasodilation, basal cell vacuolization and superficial dermis degeneration both in UVB and UVBIR treatments. IRUVB animals showed the same characteristics as above except for parakeratotic hyperkeratosis, keratin pearls and superficial dermis degeneration. To conclude, infrared radiation exposure after ultraviolet B irradiation increases skin damage without protecting the tissue, while infrared radiation exposure before ultraviolet B irradiation showed a protective effect against ultraviolet skin damage.

Characterization of retinoic acid-inducible gene-I (RIG-I) expression corresponding to viral infection and UVB in human keratinocytes

BACKGROUND

Retinoic acid-inducible gene-I (RIG-I) is a cytoplasmic protein that recognizes viral double-stranded RNA to induce the type I interferon (IFN) response. In human keratinocytes, RIG-I is induced by IFN-γ and tumor necrosis factor-α stimulation, and is abundantly expressed in psoriatic keratinocytes of the spinous and basal layers.

OBJECTIVE

This study investigated the effects of extraneous stimuli including viral infection and UVB exposure on RIG-I expression in human keratinocytes.

METHODS

Human skin keratinocytes (HaCaT cells) were stimulated by polyinosinic-polycytidylic acid (poly(I:C)), which mimics viral infection, and UVB exposure. We assessed the expression of RIG-I and IFN-regulatory factor (IRF)-1 in HaCaT cells by RT-PCR and Western blot analysis. Moreover, we investigated the effect of IRF-1 binding site of RIG-I gene promoter on the regulation of RIG-I expression by luciferase promoter assay and electrophoretic mobility shift assay.

RESULTS

Poly(I:C) induced RIG-I expression, while UVB inhibited basal RIG-I expression and the poly(I:C)-induced RIG-I overexpression in HaCaT cells. IRF-1, which binds to a regulatory element located on the RIG-I gene promoter, was required for both inductions of RIG-I expression. IRF-1 expression was enhanced three hours after the poly(I:C) stimulation, consistent with the RIG-I response to poly(I:C), and thereafter was suppressed. Moreover, UVB exposure promptly decreased IRF-1 expression, resulting in decreased IRF-1 protein binding to the RIG-I promoter, and consequently, decreased RIG-I expression.

CONCLUSION

Thus, suppression of RIG-I and IRF-1 expression caused by UVB exposure may partly explain the inhibition of skin-based immune responses, leading to viral infection and recrudescence.

Green tea prevents non-melanoma skin cancer by enhancing DNA repair

Excessive exposure of the skin to solar ultraviolet (UV) radiation is one of the major factors for the development of skin cancers, including non-melanoma. For the last several centuries the consumption of dietary phytochemicals has been linked to numerous health benefits including the photoprotection of the skin. Green tea has been consumed as a popular beverage world-wide and skin photoprotection by green tea polyphenols (GTPs) has been widely investigated. In this article, we have discussed the recent investigations and mechanistic studies which define the potential efficacy of GTPs on the prevention of non-melanoma skin cancer. UV-induced DNA damage, particularly the formation of cyclobutane pyrimidine dimers, has been implicated in immunosuppression and initiation of skin cancer. Topical application or oral administration of green tea through drinking water of mice prevents UVB-induced skin tumor development, and this prevention is mediated, at least in part, through rapid repair of DNA. The DNA repair by GTPs is mediated through the induction of interleukin (IL)-12 which has been shown to have DNA repair ability. The new mechanistic investigations support and explain the anti-photocarcinogenic activity, in particular anti-non-melanoma skin cancer, of green tea and explain the benefits of green tea for human health.

The dark and the sunny sides of UVR-induced immunosuppression: photoimmunology revisited

Immunosuppression induced by solar UVR is regarded as one of the major negative impacts of sunlight on human health. Despite this immunosuppression, bacterial superinfections are rarely observed after UVR exposure. A possible explanation for this seeming paradox may be that although it suppresses T-cell-mediated immune reactions, UVR induces the release of cutaneous antimicrobial peptides--an essential component of the innate immune system. The "sunshine vitamin," vitamin D, also appears to be involved, as UVR suppresses the adaptive but induces the innate immune response. T cells in the skin are the critical cellular mediators of the vast majority of inflammatory dermatoses, and thus probably more harmful than beneficial. Hence, it is tempting to speculate that a certain and constant level of immunosuppression by physiological UVR doses might be beneficial, taming overshooting immune reactions. At the same time, by inducing antimicrobial peptides, these low UVR doses may foster the antibacterial defense. Thus, suppression of the adaptive and induction of the innate immune system by UVR may be components of a physiological protection process. These insights might have effect on the future recommendations for daily sun protection.

Green tea polyphenols prevent UV-induced immunosuppression by rapid repair of DNA damage and enhancement of nucleotide excision repair genes

UV radiation-induced immunosuppression has been implicated in the development of skin cancers. Green tea polyphenols (GTP) in drinking water prevent photocarcinogenesis in the skin of mice. We studied whether GTPs in drinking water (0.1-0.5%, w/v) prevent UV-induced immunosuppression and (if so) potential mechanisms of this effect in mice. GTPs (0.2% and 0.5%, w/v) reduced UV-induced suppression of contact hypersensitivity (CHS) in response to a contact sensitizer in local (58-62% reductions; P < 0.001) and systemic (51-55% reductions; P < 0.005) models of CHS. Compared with untreated mice, GTP-treated mice (0.2%, w/v) had a reduced number of cyclobutane pyrimidine dimer-positive (CPD(+)) cells (59%; P < 0.001) in the skin, showing faster repair of UV-induced DNA damage, and had a reduced (2-fold) migration of CPD(+) cells from the skin to draining lymph nodes, which was associated with elevated levels of nucleotide excision repair (NER) genes. GTPs did not prevent UV-induced immunosuppression in NER-deficient mice but significantly prevented it in NER-proficient mice (P < 0.001); immunohistochemical analysis of CPD(+) cells indicated that GTPs reduced the numbers of UV-induced CPD(+) cells in NER-proficient mice (P < 0.001) but not in NER-deficient mice. Southwestern dot-blot analysis revealed that GTPs repaired UV-induced CPDs in xeroderma pigmentosum complementation group A (XPA)-proficient cells of a healthy person but did not in XPA-deficient cells obtained from XPA patients, indicating that a NER mechanism is involved in DNA repair. This study is the first to show a novel NER mechanism by which drinking GTPs prevents UV-induced immunosuppression and that inhibiting UV-induced immunosuppression may underlie the chemopreventive activity of GTPs against photocarcinogenesis.

Ultraviolet Light Absorption of an Ophthalmic Formulation with Aloe Extracts

Aloin and polysaccharide present in extracts of Aloe arborescens Miller were formulated into a binary solution to protect eyes from bacterial infection and ultraviolet radiation (UVR). The UVR absorption spectrum was recorded from 190 to 440 nm using a UV spectrometer. The physical properties of the product were examined in terms of its appearance, odor, pH, viscosity, density, refractive index, and stability. The binary solution exhibited three absorption peaks in the UVA, B and C regions, respectively. Such UV absorption capability was attributed to the phenolic chromophores pertaining to aloin. The present study suggested that the formulated binary solution has potential application as an UV absorption agent with built-in antimicrobial activity.

25 years of UV-induced immunosuppression mediated by T cells-from disregarded T suppressor cells to highly respected regulatory T cells

For more than 25 years it is known that UV radiation, in particular the UVB range suppresses the immune system. In contrast to conventional immunosuppression by immunosuppressive drugs, UV radiation does not compromise the immune system in a general but rather in an antigen-specific fashion via induction of immunotolerance. This effect is mostly mediated via regulatory T cells (Treg) induced by UV. Several subtypes of UV-induced Treg may exist, the best characterized are those which inhibit contact hypersensitivity. Induction of these Tregs by UV radiation is an active process which requires antigen presentation by UV-damaged but still alive Langerhans cells (LC) in the lymph nodes. UV-induced Treg have recently been characterized as expressing CD4 and CD25 and as releasing upon activation the immunosuppressive cytokine interleukin (IL)-10. Once activated in an antigen-specific manner, they suppress immune responses in a general fashion via the release of IL-10, a phenomenon called bystander suppression. The further phenotypic and functional characterization of these cells will not only contribute to a better understanding of the impact of UV radiation on the immune system but will also determine whether they can be applied in the future therapeutically with the final aim of achieving specific 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.

Interleukin-12 and photocarcinogenesis

UV radiation induces immunosuppression and inflammatory responses, as well as oxidative stress and DNA damage, in skin cells and these various effects have been implicated in melanoma and nonmelanoma skin cancers, i.e., photocarcinogenesis. The cytokine interleukin (IL)-12 has been shown to possess potent antitumor activity in a wide variety of murine tumor models. In this review, we summarize the evidence that IL-12 plays a role in preventing photocarcinogenesis, and present a model of its possible mechanisms of action. Treatment of mice with IL-12 prevents UV-induced immunosuppression in a process mediated by repair of UV-induced damaged DNA. After exposure to the photocarcinogenesis protocol, the development of UV-induced tumors is more rapid and the tumor multiplicity and tumor size are significantly greater in IL-12-deficient or knockout (KO) mice than their wild-type counterparts. IL-12-deficiency in mice enhances the proliferation potential of tumor cells, and this may be one of the reasons for the rapid growth of the tumors and their greater size. The rate of malignant transformation of UV-induced papillomas to carcinomas also is higher in the IL-12 KO mice than in their wild-type counterparts in terms of carcinoma incidence and carcinoma multiplicity. UV-induced DNA damage in the form of cyclobutane pyrimidine dimers (CPDs) and sunburn cells is lower, or repaired more rapidly, in wild-type mice than IL-12 KO mice. The IL-12-associated reduction in UV-specific CPDs is due to induction of DNA repair, and particularly enhancement of nucleotide-excision repair. We suggest that endogenous stimulation of IL-12 may protect the skin from UV-induced immunosuppression, DNA damage, and, ultimately, the risk of photocarcinogenesis. Taken together, this information suggests that augmentation of IL-12 should be considered as a strategy for the prevention and treatment of photocarcinogenesis.

Silymarin inhibits UV radiation-induced immunosuppression through augmentation of interleukin-12 in mice

We have shown previously that silymarin, a plant flavonoid, inhibits UVB-induced photocarcinogenesis in mice. As UVB-induced immunosuppression has been implicated in the development of skin cancer, we investigated whether silymarin can modulate the effects of UVB radiation on the immune system. Treatment of C3H/HeN mice with topically applied silymarin (0.5 or 1.0 mg/cm(2)) or silibinin, a major component of silymarin, markedly inhibited UVB (180 mJ/cm(2))-induced suppression of contact hypersensitivity response in a local model of immunosuppression and had a moderate inhibitory effect in a systemic model of contact hypersensitivity. Silymarin reduced the UVB-induced enhancement of the levels of the immunosuppressive cytokine, interleukin (IL)-10, in the skin and draining lymph nodes and enhanced the levels of the immunostimulatory cytokine, IL-12. Intraperitoneal injection of mice treated with silymarin with an endotoxin-free neutralizing anti-IL-12 antibody abrogated the protective effects of the silymarin against UVB-induced suppression of the contact hypersensitivity response. Furthermore, the treatment of silymarin did not prevent UVB-induced suppression of the contact hypersensitivity response in IL-12 knockout mice but prevented it in their wild-type mice. Moreover, i.p. injection of IL-12 to silymarin-treated or non-silymarin-treated IL-12 knockout mice resulted in an enhanced response to contact hypersensitivity compared with the response in mice that were exposed to either UVB alone or silymarin plus UVB. These data indicate for the first time that silymarin has the ability to protect mice from UVB-induced immunosuppression and that this protective effect is mediated, at least in part, through IL-12.

(-)-Epigallocatechin-3-gallate prevents photocarcinogenesis in mice through interleukin-12-dependent DNA repair

We have shown previously that topical application of (-)-epigallocatechin-3-gallate (EGCG), the major polyphenol of green tea, prevents photocarcinogenesis in mice. EGCG prevents UVB-induced immunosuppression by inducing interleukin-12 (IL-12). As immunosuppression is a risk factor for photocarcinogenesis, we investigated the possibility that EGCG also prevents UVB-induced photocarcinogenesis through an IL-12-dependent DNA repair mechanism. To investigate this possibility, we determined the effects of EGCG on photocarcinogenesis in IL-12 knockout (KO) mice using the formation of cyclobutane pyrimidine dimers (CPD) as an indicator of the extent of UVB-induced DNA damage. Topical application of EGCG (1 mg/cm(2) skin) prevented photocarcinogenesis in wild-type (C3H/HeN) mice in terms of tumor incidence and tumor multiplicity but did not prevent photocarcinogenesis in IL-12 KO mice. UVB-induced DNA damage, as determined by the formation of CPDs and the number of sunburn cells, was resolved more rapidly in the skin of wild-type mice treated with EGCG than untreated control mice. In contrast, the extent of UVB-induced DNA damage and the numbers of sunburn cells were not significantly different in the EGCG-treated IL-12 KO mice and untreated control mice. In addition, treatment of XPA-proficient human fibroblast cells with EGCG promoted repair of UVB-induced CPDs in a dose-dependent manner but not in an XPA-deficient cells, indicating that the nucleotide excision repair mechanism is involved in EGCG-mediated DNA repair. Taken together, these results indicate for the first time that EGCG can prevent photocarcinogenesis through an EGCG-induced IL-12-dependent DNA repair mechanism.

Prevention of ultraviolet radiation-induced immunosuppression by (-)-epigallocatechin-3-gallate in mice is mediated through interleukin 12-dependent DNA repair

PURPOSE

Solar UV radiation-induced immunosuppression is considered to be a risk factor for melanoma and nonmelanoma skin cancers. We previously have shown that topical application of (-)-epigallocatechin-3-gallate (EGCG) prevents UV-induced immunosuppression in mice. We studied whether prevention of UV-induced immunosuppression by EGCG is mediated through interleukin 12 (IL-12)-dependent DNA repair.

EXPERIMENTAL DESIGN

IL-12 knockout (KO) mice on C3H/HeN background and DNA repair-deficient cells from xeroderma pigmentosum complementation group A (XPA) patients were used in this study. The effect of EGCG was determined on UV-induced suppression of contact hypersensitivity and UV-induced DNA damage in the form of cyclobutane pyrimidine dimers (CPD) in mice and XPA-deficient cells using immunohistochemistry and dot-blot analysis.

RESULTS

Topical treatment with EGCG prevented UV-induced suppression of the contact hypersensitivity in wild-type (WT) mice but did not prevent it in IL-12 KO mice. Injection of anti-IL-12 monoclonal antibody to WT mice blocked the preventive effect of EGCG on UV-induced immunosuppression. EGCG reduced or repaired UV-induced DNA damage in skin faster in WT mice as shown by reduced number of CPDs(+) cells and reduced the migration of CPD(+) antigen-presenting cells from the skin to draining lymph nodes. In contrast, this effect of EGCG was not seen in IL-12 KO mice. Further, EGCG was able to repair UV-induced CPDs in XPA-proficient cells obtained from healthy person but did not repair in XPA-deficient cells, indicating that nucleotide excision repair mechanism is involved in DNA repair.

CONCLUSIONS

These data identify a new mechanism by which EGCG prevents UV-induced immunosuppression, and this may contribute to the chemopreventive activity of EGCG in prevention of photocarcinogenesis.

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 radiation--immune response]

Ultraviolet (UV) radiation can exert a variety of biological effects, including induction of skin cancer, premature skin ageing and inhibition of the immune system. The immunosuppressive properties of UV radiation are of major biological and clinical relevance since suppression of the immune system by UV radiation also contributes to the induction of skin cancer. Hence, understanding of the mechanisms by which UV radiation compromises the immune system is of primary importance. UV radiation suppresses the immune system in multiple ways. It inhibits antigen presentation, stimulates the release of immunosuppressive cytokines and induces the generation of lymphocytes of the regulatory subtype. The major molecular target for UV-induced immunosuppression is UV-induced DNA damage. Further elucidation of the mechanisms underlying UV-induced immunosuppression will not only lead to a better understanding of the physiologic and pathologic effects of UV radiation but also contribute to the development of new protective strategies.

Prevention of UV radiation-induced immunosuppression by IL-12 is dependent on DNA repair

The immunostimulatory cytokine IL-12 is able to antagonize immunosuppression induced by solar/ultraviolet (UV) radiation via yet unknown mechanisms. IL-12 was recently found to induce deoxyribonucleic acid (DNA) repair. UV-induced DNA damage is an important molecular trigger for UV-mediated immunosuppression. Thus, we initiated studies into immune restoration by IL-12 to discern whether its effects are linked to DNA repair. IL-12 prevented both UV-induced suppression of the induction of contact hypersensitivity and the depletion of Langerhans cells, the primary APC of the skin, in wild-type but not in DNA repair-deficient mice. IL-12 did not prevent the development of UV-induced regulatory T cells in DNA repair-deficient mice. In contrast, IL-12 was able to break established UV-induced tolerance and inhibited the activity of regulatory T cells independent of DNA repair. These data identify a new mechanism by which IL-12 can restore immune responses and also demonstrate a link between DNA repair and the prevention of UV-induced immunosuppression by IL-12.

Ultraviolet exposure of thymocytes: selective inhibition of apoptosis

PURPOSE

To evaluate selective effects of ultraviolet (UV) irradiation on spontaneous and induced apoptosis in freshly extracted mice thymocytes.

MATERIALS AND METHODS

Cells were exposed to UV radiation with emission peaks of 365 nm (UVA) exposures of 1620-10200 J m(-2), of 312 nm (UVB) exposures of 34-1620 J m(-2) or of 254 nm (UVC) exposures of 1.5-1620 J m(-2), and incubated for 5.5 h with or without hydrocortisone, phorbol-12-myristate-13-acetate or anti-Fas antibody. Additionally, cells were irradiated with gamma-rays (5 Gy) before UVB exposure (408 J m(-2)) at different times. Apoptosis was quantified by DNA fragmentation.

RESULTS

Up to an irradiation of 5000 J m(-2), UVA exposure did not show any effect on thymocyte apoptosis, while at 10200 J m(-2) irradiation, considerable DNA fragmentation was observed. In contrast, UVB and UVC irradiation clearly inhibited natural and cortisone-induced apoptosis. Moreover, UVB inhibited apoptosis triggered by phorbol-12-myristate-13-acetate and gamma-irradiation, but not by anti-Fas antibody.

CONCLUSIONS

The response of mouse thymocytes in culture to UV irradiation strongly depends on the wavelength used. It is suggested that either a survival or an apoptotic pathway occurs depending on the physiological state of the cell, spectral composition of the UV light and cell type. The possible involvement of extracellular signal-regulated kinase and stress-activated protein kinase/c-Jun N-terminal kinase in the apoptotic pathway is discussed.

Skin infections in renal transplant recipients and the relation with solar ultraviolet radiation

BACKGROUND

Ultraviolet radiation (UVR) is an important risk factor for skin cancer in transplant recipients. In view of the potential suppressive effect of UVR on host resistance it was examined whether exposure to UVR was also associated with the occurrence of various skin infections.

METHODS

In a cohort of renal transplant recipients (n = 137), lifetime exposure was assessed by means of a retrospective questionnaire on cumulative sunlight exposure. Diagnosed skin infections since renal transplantation were extracted from the patient's medical charts. Season of diagnosis was regarded as indicative of short-term exposure.

RESULTS

In comparison with winter a high rate of herpes simplex infections was found in spring [rate ratio (RR) = 4.09, 95% confidence interval (CI) 1.2-14.5], and high rates of herpes zoster infections (RR = 1.6, 95% CI: 0.8-3.5) and fungal/yeast infections in summer (RR = 2.1, 95% CI: 1.3-3.4). A higher lifetime exposure (RR = 2.31, 95% CI: 1.04-5.1) and a greater cumulative number of reported sunburns (RR = 2.3, 95% CI: 1.1-5.1) were independently associated with a higher risk of bacterial infections.

CONCLUSIONS

The seasonal association with the occurrence of clinical herpes infections indicates an effect of short-term UVR. Our data suggest that the number of sunburn episodes in the past is also relevant for the susceptibility to certain skin infections.

20 years after--milestones in molecular photobiology

Ultraviolet radiation represents one of the most relevant environmental factors because of its hazardous health effects, which include induction of skin cancer, premature skin aging, and exacerbation of infectious diseases. The biologic effects exerted by ultraviolet radiation have been well characterized by a variety of in vitro and in vivo studies. The events taking place inside the cell during the ultraviolet response, however, remained unclear for quite a long time. Molecular photobiology has increased our knowledge about ultraviolet-induced signal transduction enormously within the last 10 years. For a long time, nuclear DNA has been regarded as the only chromophore for ultraviolet radiation. Today we know that ultraviolet radiation can affect also other molecular targets located in the cytoplasm and at the cell membrane. These targets include cell surface receptors, kinases, phosphatases, and transcription factors. Detailed knowledge about ultraviolet-induced signal transduction will certainly increase our understanding of how ultraviolet radiation exerts its biologic effects and furthermore will provide us with tools to interfere with these pathways, thereby reducing the adverse effects of ultraviolet radiation.

Gamma irradiation-reduced IFN-gamma expression, STAT1 signals, and cell-mediated immunity

The signal transducer and activator of transcription (STAT)1 is a cytoplasmic-transcription factor that is phosphorylated by Janus kinases (Jak) in response to interferon gamma(IFN-gamma). The phosphorylated STAT1 translocates to the nucleus, where it turns on specific sets of IFN-gamma-inducible genes, such as the interferon regulatory factor (IRF)-1. We show here that gamma irradiation reduces the IFN-gamma mRNA expression. The inhibition of the STAT1 phosphorylation and the IRF-1 expression by gamma irradiation was also observed. In contrast, the mRNA levels of IL-5 and transcription factor GATA-3 were slightly induced by gamma irradiation when compared to the non-irradiated sample. Furthermore, we detected the inhibition of cell-mediated immunity by gamma irradiation in the allogenic-mixed lymphocytes' reaction (MLR). These results postulate that gamma irradiation induces the polarized-Th2 response and interferes with STAT1 signals, thereby causing the immunosuppression of the Th1 response.

Downregulation of bcl-xL is relevant to UV-induced apoptosis in fibroblasts

Exposure to ultraviolet light (UV) induces apoptosis in mammalian cells. The caspase group of proteases is required for the apoptosis. This pathway is initiated by a release of cytochrome c from the mitochondria into the cytosol. Several Bcl-2 family proteins can regulate the release of cytochrome c by stabilizing the mitochondrial membrane. Here we show that expression of the endogenous bcl-xL was strongly downregulated in NIH3T3 cells within 2 h after UV-C irradiation, and that of bax was upregulated from 8 h after irradiation. Apoptosis was induced in more than 50% of the NIH3T3 cells 48 h after irradiation. Constitutive overexpression of bcl-xL in NIH3T3 cells protected the UV-induced apoptosis by preventing the loss of mitochondrial membrane potential and the activation of caspase 9. These results suggest that downregulation of Bcl-xL is relevant to UV-induced apoptosis of fibroblasts.

Ultraviolet radiation, resistance to infectious diseases, and vaccination responses

Exposure to ultraviolet (UV) radiation, as in sunlight, can modulate immune responses in animals and humans. This immunomodulation can lead to positive health effects especially with respect to certain autoimmune diseases and allergies. However, UV-induced immunomodulation has also been shown to be deleterious. Experimental animal studies have revealed that UV exposure can impair resistance to many infectious agents, such as bacteria, parasites, viruses, and fungi. Importantly, these effects are not restricted to skin-associated infections, but also concern systemic infections. The real consequences of UV-induced immunomodulation on resistance to infectious diseases are not known for humans. Risk estimations have been performed through extrapolation of animal data, obtained from infection models, to the human situation. This estimation indicated that UV doses relevant to outdoor exposure can impair the human immune system sufficiently to have effects on resistance to infections. To further quantify and validate this risk estimation, data, e.g., from human volunteer studies, are necessary. Infection models in humans are not allowed for ethical reasons. However, vaccination against an infectious disease evokes a similar immune response as the pathogen and thereby provides an opportunity to measure the effect of UV radiation on the immune system and an estimate of the possible consequences of altered resistance to infectious agents. Effects of controlled UVB exposure on immune responses after hepatitis B vaccination have been established in mice and human volunteers. In mice, cellular and Th1-associated humoral immune responses to hepatitis B were significantly impaired, whereas in human volunteers no significant effect of UVB on these responses could be found. Preliminary data indicate that cytokine polymorphisms might be, at least in part, responsible for interindividual differences in immune responses and in susceptibility to UVB-induced immunomodulation. In addition, adaptation to UV exposure needs to be considered as a possible explanation for the difference between mice and humans that was observed in the hepatitis B vaccination model.

A review of studies on the effects of ultraviolet irradiation on the resistance to infections: evidence from rodent infection models and verification by experimental and observational human studies

Recent studies on the immunosuppressive effects of ultraviolet radiation (UVR) and the related resistance to infections in rodents and humans are presented. The waveband dependency of trans-to-cis isomerisation of urocanic acid in the stratum corneum and the role of DNA damage in UVR-induced erythema and immunosuppression were investigated to further elucidate the underlying mechanisms. Furthermore, human experimental studies on UVR-induced immunomodulation were performed. It appeared that the doses needed to suppress various immune parameters in humans (e.g. NK activity, contact hypersensitivity) were higher than those needed in experiments in rodents. Still, extrapolation of experimental animal data to the human situation showed that UVR may impair the resistance to different systemic infections at relevant outdoor doses. In observational human studies we aimed to substantiate the relevance of UVR for infections in humans. It was shown that sunny season was associated with a slightly retarded but clinically non-relevant antibody response to hepatitis B vaccination. Furthermore, sunny season appeared to be associated with a small decline in the number of CD4+ T-helper cells in a cohort of HIV-infected persons and a higher recurrence of herpes simplex and herpes zoster in a cohort of renal transplant recipients. However, in a study among young children a higher exposure to solar UVR was associated with a lower occurrence of upper respiratory tract symptoms. As disentangling the effects of UVR from other relevant factors is often impossible in observational studies, concise quantitative risk estimations for the human situation cannot be given at present.

Ultraviolet light induced injury: immunological and inflammatory effects

This article reviews many of the complex events that occur after cutaneous ultraviolet (UV) exposure. The inflammatory changes of acute exposure of the skin include erythema (sunburn), the production of inflammatory mediators, alteration of vascular responses and an inflammatory cell infiltrate. Damage to proteins and DNA accumulates within skin cells and characteristic morphological changes occur in keratinocytes and other skin cells. When a cell becomes damaged irreparably by UV exposure, cell death follows via apoptotic mechanisms. Alterations in cutaneous and systemic immunity occur as a result of the UV-induced inflammation and damage, including changes in the production of cytokines by keratinocytes and other skin-associated cells, alteration of adhesion molecule expression and the loss of APC function within the skin. These changes lead to the generation of suppressor T cells, the induction of antigen-specific immunosuppression and a lowering of cell-mediated immunity. These events impair the immune system's capacity to reject highly antigenic skin cancers. This review gives an overview of the acute inflammatory and immunological events associated with cutaneous UV exposure, which are important to consider before dealing with the complex interactions that occur with chronic UV exposure, leading to photocarcinogenesis.

Ultraviolet radiation inhibits interleukin-2-induced tyrosine phosphorylation and the activation of STAT5 in T lymphocytes

UV radiation was recently found to hinder interferon-gamma from exerting its biological effects by inhibiting the phosphorylation of signal transducer and activator of transcription (STAT)-1, a crucial signal transducing protein in the interferon-gamma pathway. Because this activity by UV may contribute to its immunosuppressive properties we studied whether this is specific for STAT1 or whether UV also affects other members of the STAT family. STAT5 is crucially involved in signaling of interleukin (IL)-2, enabling up-regulation of the IL-2 receptor alpha chain, an essential component of the high affinity IL-2 receptor. Exposure of the murine T cell line CTLL to IL-2 caused tyrosine phosphorylation of STAT5 that was remarkably reduced when cells were exposed to UV. Accordingly, STAT5 binding activity was significantly impaired in UV-exposed cells. In contrast, IL-2-induced tyrosine phosphorylation of the kinases Jak1 and Jak3 located upstream of STAT5 was not affected by UV. The effect of UV on STAT5 phosphorylation was antagonized by orthovanadate, implying involvement of a phosphatase in this process. Accordingly, up-regulation of the IL-2 receptor alpha chain was reduced in cells that were treated with IL-2 plus UV. Because STAT5-mediated IL-2 effects are vital for normal immune functions, inhibition of STAT5 signaling by UV may contribute to its well known immunosuppressive properties.

Nuclear and cell membrane effects contribute independently to the induction of apoptosis in human cells exposed to UVB radiation

UVB-induced DNA damage is a crucial event in UVB-mediated apoptosis. On the other hand, UVB directly activates death receptors on the cell surface including CD95, implying that UVB-induced apoptosis can be initiated at the cell membrane through death receptor clustering. This study was performed to measure the relative contribution of nuclear and membrane effects in UVB-induced apoptosis of the human epithelial cell line HeLa. UVB-mediated DNA damage can be reduced by treating cells with liposomes containing the repair enzyme photolyase followed by exposure to photoreactivating light. Addition of photolyase followed by photoreactivation after UVB reduced the apoptosis rate significantly, whereas empty liposomes had no effect. Likewise, photoreactivating treatment did not affect apoptosis induced by the ligand of CD95, CD95L. UVB exposure at 4 degrees C, which prevents CD95 clustering, also reduced the apoptosis rate, but to a lesser extent. When cells were exposed to UVB at 4 degrees C and treated with photolyase plus photoreactivating light, UVB-induced apoptosis was almost completely prevented. Inhibition of caspase-3, a downstream protease in the CD95 signaling pathway, blocked both CD95L and UVB-induced apoptosis, whereas blockage of caspase-8, the most proximal caspase, inhibited CD95L-mediated apoptosis completely, but UVB-induced apoptosis only partially. Although according to these data nuclear effects seem to be slightly more effective in mediating UVB-induced apoptosis than membrane events, both are necessary for the complete apoptotic response. Thus, this study shows that nuclear and membrane effects are not mutually exclusive and that both components contribute independently to a complete response to UVB.

The effects of ultraviolet radiation on the human immune system

The adverse outcome of increased ultraviolet (UV) irradiation on human health is currently of concern. While many experiments have been carried out in rodent models, fewer have been designed to test the effects of UV exposure in human subjects. This review concentrates on the modulations induced in the human immune system by UV, and outlines changes in antigen presentation by Langerhans cells and macrophages, in the activities of natural killer cells and T cells, and in cytokine regulation. Precautionary measures which might be taken to help protect people against the immunosuppressive action of UV irradiation are considered.

Use of immunotoxicity data in health risk assessments: uncertainties and research to improve the process

A number of environmental contaminants can suppress immune responses and enhance susceptibility to infectious and/or neoplastic disease. Most of the evidence for immunotoxicity of such contaminants has been obtained from laboratory animal studies and risk assessors must make decisions about risk to the human population based on these studies. Uncertainties associated with this process include determining what level of immune suppression is adverse, extrapolating across species from rodent to human, and across levels of biologic organization from effects on immune function at the cellular level to effects on incidence of disease at the population level, accounting for intra-species variability, and assessing the relationship between effects following acute, subchronic, and chronic exposure. This paper reviews immunotoxicity data that may be applied to the development of risk assessment methods and models designed to reduce some of these uncertainties.

Ultraviolet Light-Induced Immune Tolerance Is Mediated via the Fas/Fas-Ligand System

Hapten sensitization through UV-exposed skin induces tolerance that is mediated via the induction of hapten-specific T suppressor cells. However, the detailed mechanisms underlying tolerance induction remain unclear to date. We show here that the apoptosis-related surface Ag Fas (APO-1, CD95) and its ligand, Fas ligand (FasL) are critically involved, since Fas-deficient lpr mice and FasL-deficient gld mice do not develop UV-induced tolerance. Adoptive transfer experiments revealed that the mediation of tolerance does not require the expression of Fas or FasL by the T suppressor cells but does require the expression of both molecules by the cells of mice receiving the T suppressor cells. To identify the mechanisms involved, the effect of suppressor cells on Ag-presenting dendritic cells (DC) was studied. Coincubation of hapten-pulsed DC with T cells that were obtained from UV-tolerized mice resulted in an enhanced death rate of DC, and this cell death was dependent upon Fas expression. The addition of IL-12, which recently was found to break established tolerance in vivo, prevented DC death. Moreover, IL-12 did not only rescue DC from T suppressor cell-induced death but also from apoptosis induced by rFasL, suggesting that IL-12 may interfere with the Fas/FasL system. Together, these data indicate a crucial role for the Fas/FasL system in UV-induced tolerance, and suggest that UV-induced T suppressor cells may act by inducing the cell death of APCs via the Fas pathway. The ability of IL-12 to break established tolerance may be due to the prevention of DC death induced by T suppressor cells.

Down-regulation of interferon gamma-activated STAT1 by UV light

STAT1 is a cytoplasmic transcription factor that is phosphorylated by Janus kinases (Jak) in response to interferon-gamma (IFNgamma). Phosphorylated STAT1 translocates to the nucleus, where it turns on specific sets of IFNgamma-inducible genes. Here, we show that UV light interferes with tyrosine phosphorylation of STAT1, thereby hindering IFNgamma from exerting its biological effects. This effect is not due to a down-regulation of the IFNgamma receptor because phosphorylation of upstream-located Jak1 and Jak2 was not suppressed by UV light. In contrast, UV light had no effect on the phosphorylation of STAT3, which is activated by the proinflammatory cytokine interleukin 6. The UV light effect on STAT1 phosphorylation could be antagonized by vanadate, indicating at least partial involvement of a protein tyrosine phosphatase. Therefore, this study indicates a mechanism by which UV light can inhibit gene activation and suggests STAT1 as a new extranuclear UV target closely located to the membrane.

Sunscreens offer the same UVB protection factors for inflammation and immunosuppression in the mouse

Many studies report that sunscreens effective against UVR-induced inflammation afford poor protection against immunosuppression. We have studied the relationship between photoprotection of inflammation and immunosuppression with monochromatic UVB (Philips TL01 tubes, lambda max = 311 nm) to remove possible confounding effects of differences in end point action spectra. Dose-response curves for edema and systemic suppression of contact hypersensitivity (CHS) in HRA.HRII-c/+/Skh mice showed that suppression of CHS was more sensitive to UVB irradiation by a factor of 2. The UVB dose-response curve for murine edema was similar to that for human erythema, with threshold doses of 773 mJ x cm(-2) and 632 mJ x cm(-2), respectively. The protection afforded by two UVB filters, octyl dimethyl para-aminobenzoic acid and 2-ethylhexyl-4'-methoxycinnamate, prepared in an identical vehicle, each with the same optical density at 311 nm, was tested in mice. We applied sunscreen to all exposed skin or to transpore tape above the irradiation cages, prior to exposure with 2.8 minimal edema doses. Topical or tape application of both sunscreens protected totally against edema but only partially against immunosuppression, with no significant difference in protection between the two application techniques (p > 0.4). A sunscreen protection factor of 4 in vivo was determined for 2-ethylhexyl-4'-methoxycinnamate for both edema and immunosuppression. Failure of the sunscreens to protect completely against immunosuppression was due to the ability of subedemal doses of UVB to induce substantial immunosuppression and not, as previously suggested, to any skin interaction.