Immune regulation by polysaccharides: implications for skin cancer.

UV radiation causes sunburn, premature aging of the skin and is the major environmental carcinogen for squamous cell and basal cell skin cancer in humans. Besides causing mutations in DNA, UV radiation contributes to carcinogenesis by suppressing immune responses to highly antigenic, newly arising neoplasms. Strategies aimed at preventing UV-induced immune suppression, the mechanism of action of the agents used, and the significance of immune protection for prevention of skin cancer are reviewed. This review focuses on the use of plant polysaccharides to prevent immune damage triggered by UV radiation, an approach that goes beyond absorption of UV radiation by sunscreens as a means of reducing tissue damage. The efficacy and mechanism of action of these agents in preserving T cell-mediated immunity to model antigens in human beings and in laboratory animals are discussed.     

Effect of UV radiation on the neonatal skin immune system- implications for melanoma

The neonatal immune environment and the events that occur during this time have profound effects for the adult period. While protective immune responses can develop, the neonatal immune system, particularly the skin immune system (SIS), tends to promote tolerance. With this information we undertook a number of studies to identify unique aspects of skin during the neonatal period. Proteomics revealed proteins uniquely expressed in neonatal, but not adult, skin (e.g. Stefin A, peroxiredoxins) and these may have implications in the development of SIS. Vitamin D was found to have a modulating role on SIS and this was apparent from the early neonatal period. Exposure of the neonatal skin to UV radiation altered the microenvironment resulting in the generation of regulatory T cells, which persisted in adult life. As the development of UV radiation-induced melanoma can occur following a single high dose (equivalent to burning in adults) to transgenic mice (hepatocyte growth factor/scatter factor or TPras) during the neonatal period, the early modulating events which lead to suppression may be relevant for the development of UV radiation-induced human melanoma. Any attempt to produce effective melanoma immunotherapy has to accommodate and overcome these barriers. Margaret Kripke's pioneering work on UV-induced immunosuppression still remains central to the understanding of the development of melanoma and how it frequently escapes the immune system.     

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.     

Nicotinamide Prevents Ultraviolet Radiation-induced Cellular Energy Loss

UV radiation is carcinogenic by causing mutations in the skin and also by suppressing cutaneous antitumor immunity. We previously found nicotinamide (vitamin B3) to be highly effective at reducing UV-induced immunosuppression in human volunteers, with microarray studies on in vivo irradiated human skin suggesting that nicotinamide normalizes subsets of apoptosis, immune function and energy metabolism-related genes that are downregulated by UV exposure. Using human adult low calcium temperature keratinocytes, we further investigated nicotinamide’s effects on cellular energy metabolism. We found that nicotinamide prevented UV-induced cellular ATP loss and protected against UV-induced glycolytic blockade. To determine whether nicotinamide alters the effects of UV-induced oxidative stress posttranslationally, we also measured UV-induced reactive oxygen species (ROS). Nicotinamide had no effect on ROS formation, and at the low UV doses used in these studies, equivalent to ambient daily sun exposure, there was no evidence of apoptosis. Hence, nicotinamide appears to exert its UV protective effects on the skin via its role in cellular energy pathways.     

The consequences of UV-induced immunosuppression for human health

Exposure to UV radiation can cause suppression of specific immune responses. The pathways leading to the down-regulation are complex, starting from the absorption of UV photons by chromophores in the skin and ending with local and systemic changes in immune mediators, the generation of T and B regulatory cells and inhibition of effector and memory T cell activation. The consequences for human health are thought to be both beneficial and adverse. The former are illustrated by protection against polymorphic light eruption, and possible protection against T cell-mediated autoimmune diseases and asthma. The latter are illustrated by skin cancer, cutaneous lupus erythematosus and infectious diseases including vaccination. Many outstanding questions remain in this rapidly developing and controversial area, not least what advice to give the general public regarding their sun exposure. While considerable advances have been made in the development of strategies that preserve the health benefits of sunlight exposure and decrease its detrimental effects, further research is required before optimal levels of protection are achieved.     

Evaluation of an Economical Sunlamp that Emits a Near Solar UV Power Spectrum for Conducting Photoimmunological and Sunscreen Immune Protection Studies

Expense and inconvenience have restricted the use of the filtered xenon are lamp (solar simulator) as a UV source for conducting large-scale animal studies. Because sunscreen immunoprotective levels are significantly affected by the UV power spectrum of the source it is imperative that a solar simulating source be used for accurate measurements of sunscreen protection levels that are relevant to human UV exposures from sunlight. However, relatively inexpensive sunlamps, e. g. the UVA-340, that emit a UV power spectrum similar to that of a solar simulator are available. Unlike FS-type UVB sunlamps, which have a significant amount of effective immunosuppressive nonsolar UV energy at wavelengths below 295 nm, the immunosuppression effectiveness spectrum of UVA-340 sunlamps was nearly identical to that of a solar simulator. The purpose of this study was to evaluate this sunlamp for conducting photoimmunological and sunscreen immune protection studies. Groups of C3H mice were exposed to a range of UVA-340 sunlamp doses (0.25 KJ/m² to 20.0 KJ/m²) to establish a dose-response curve and determine the minimum immune suppression dose (MISD) for induction of local-type suppression of contact hypersensitivity (CH). The MISD, defined as the lowest UV dose given to produce ～50% suppression of the CH response in mice, was determined to be 1.0 kJ/m² for UVA-340 sunlamps. Immune protection tests on four marketed sunscreen lotions (sun protection factors [SPF] 4, 8, 15 and 30) were then conducted with UVA-340 sunlamps using MISD as the endpoint. The immune protection factors for these sunscreens were equivalent to the level of protection predicated by their labeled SPF. These results are similar to those we have previously obtained using a solar simulator. We conclude from these data that the immunosuppressive effects of UVA-340 sunlamps are similar to those of a solar simulator; however, further studies are needed to determine if UVA-340, or similar, sunlamps are a viable alternative to the solar simulator for conducting large-scale animal experiments that require a relevant UV solar spectrum.     

Viability of the antigen determines whether DNA or urocanic acid act as initiator molecules for UV-induced suppression of delayed-type hypersensitivity

UV radiation suppresses the immune response, and UV-induced immune suppression contributes to UV-induced photocarcinogenesis. For UV-induced immune suppression to occur, electromagnetic energy (i.e. UV radiation) must be converted to a biological signal. Two photoreceptors have been identified in the skin that serves this purpose, epidermal DNA and trans-urocanic acid (UCA). Although compelling evidence exists to support a role for each pathway (UV-induced DNA damage or photoisomerization of UCA) in UV-induced immune suppression, it is not clear what determines which photoreceptor pathway is activated. To address this question, we injected UV-irradiated mice with a monoclonal antibody with specificity for cis-UCA or applied liposomes containing DNA repair enzymes to the skin of UV-irradiated mice. The effect that each had on UV-induced suppression of delayed-type hypersensitivity was measured. We asked whether the light source used (FS-40 sunlamps vs solar-simulated UV radiation) altered whichever pathway of immune suppression was activated. Different doses of UV radiation and the viability of the antigen were also considered. Neither the dose of UV nor the light source had any influence on determining which pathway was activated. Rather, we found that the viability of the antigen was the critical determinant. When live antigens were used, UV-induced immune suppression was blocked with monoclonal anti-cis-UCA but not with T4 endonuclease V-containing liposomes. The reverse was observed when formalin-fixed or killed antigens were used. Our findings indicate that antigen viability dictates which photoreceptor pathway predominates after UV exposure.     

Green tea polyphenols: DNA photodamage and photoimmunology.

Green tea is a popular beverage consumed worldwide. The epicatechin derivatives, which are commonly called 'polyphenols', are the active ingredients in green tea and possess antioxidant, anti-inflammatory and anti-carcinogenic properties. Studies conducted by our group on human skin have demonstrated that green tea polyphenols (GTP) prevent ultraviolet (UV)-B-induced cyclobutane pyrimidine dimers (CPD), which are considered to be mediators of UVB-induced immune suppression and skin cancer induction. GTP treated human skin prevented penetration of UV radiation, which was demonstrated by the absence of immunostaining for CPD in the reticular dermis. The topical application of GTP or its most potent chemopreventive constituent (-)-epigallocatechin-3-gallate (EGCG) prior to exposure to UVB protects against UVB-induced local as well as systemic immune suppression in laboratory animals. Additionally, studies have shown that EGCG treatment of mouse skin inhibits UVB-induced infiltration of CD11b+ cells. CD11b is a cell surface marker for activated macrophages and neutrophils, which are associated with induction of UVB-induced suppression of contact hypersensitivity responses. EGCG treatment also results in reduction of the UVB-induced immunoregulatory cytokine interleukin (IL)-10 in skin as well as in draining lymph nodes, and an elevated amount of IL-12 in draining lymph nodes. These in vivo observations suggest that GTPs are photoprotective, and can be used as pharmacological agents for the prevention of solar UVB light-induced skin disorders associated with immune suppression and DNA damage.     

Ultraviolet Spectral Energy Differences Affect the Ability of Sunscreen Lotions to Prevent Ultraviolet-Radiation-Induced Immunosuppression*

Acute exposure to UV radiation causes immunosuppression of contact hypersensitivity (CH) responses. Past studies conducted with unfiltered sunlamps emitting nonsolar spectrum UV power (wavelengths below 295 nm) or using excessive UV doses have suggested sunscreens may not prevent UV-induced immunosuppression in mice. This study was thus designed to evaluate critically the effects of different UV energy spectra on the immune protection capacity of sunscreen lotions. Minimum immune suppression doses (MISD), i.e. the lowest UV dose to cause~50% suppression of the CH response to dinitrofluorobenzene in C3H mice, were established for three artificial UV sources. The MISD for each UV source was 0.25 kJ/m² for unfiltered FS20 sunlamps (FS), 0.90 kJ/m² for Kodacel-filtered FS20 sunlamps (KFS), which do not emit UV power at wavelengths <290 nm, and 1.35 kJ/m² for a 1000 W filtered xenon arc lamp solar simulator. Using MISD as baseline, sunscreens with labeled sun protection factors (SPF) of 4, 8, 15 and 30 were tested with each UV source to establish their relative immune protection factors. The immune protection factor of each sunscreen exceeded its labeled SPF in tests conducted with the solar simulator, which has a UV power spectrum (295–400 nm) similar to that of sunlight. Conversely, sunscreen immune protection factors were significantly less than the labeled SPF in tests conducted with FS and KFS. Comparison of the immunosuppression effectiveness spectra showed that relatively small amounts of nonsolar spectrum UV energy, i.e. UVC (200–290 nm) and/or shorter wavelength UVB (between 290 and 295 nm), produced by FS and KFS contributes significantly to the induction of immunosuppression. For example, 36.3% and 3.5% of the total immunosuppressive UV energy from FS and KFS, respectively, lies below 295 nm. Sunscreen absorption spectra showed that transmission of immunosuppressive UV energy below 295 nm for FS was at least eight-fold higher than that for KFS. Compared to the solar simulator UV spectrum the transmission of nonsolar immunosuppressive UV energy through sunscreens was >15-fold higher for FS and ≥1.5-fold higher for KFS. These data demonstrate that relevant evaluations of sunscreen immune protection can only be obtained when tests are conducted with UV sources that produce UV power spectra similar to that of sunlight and UV doses are employed that are based on established MISD.     

Sunscreens Protect from UV-Promoted Squamous Cell Carcinoma in Mice Chronically Irradiated with Doses of UV Radiation Insufficient to Cause Edema

Previously we reported that the broad-spectrum sunscreen microfine titanium dioxide (MTD) could completely protect C3H/HeJ mice from UV radiation-induced immunosuppression to a contact sensitizer. In contrast, 2-ethylhexyl p-methoxycinnamate (2-EHMC), a UVB-absorbing sunscreen, only partially protected the skin immune system. In this study we investigated further this differential protection of the skin immune system by comparing the ability of 2-EHMC and MTD to protect these mice from the promotion phase of tumorigenesis. The mice were initiated using a single subcarcinogenic dose of 7,12-dimethylbenz(a)anthracene (DMBA) followed by promotion with chronic low-dose solar-simulated UV radiation for 32 weeks. We used doses of UV insufficient to cause edema in order to simulate daily human exposure to solar UV radiation. Mice were observed for the appearance of squamous cell carcinomas for 48 weeks. The DMBA-initiation alone and DMBA-initiated, sunscreen-treated groups did not develop tumors. Ultraviolet alone induced the appearance of tumors in 46% of mice at week 48 and therefore some tumors were initiated by UV. Initiation with DMBA prior to UV irradiation enhanced tumorigenesis such that 87% of mice at week 48 had tumors. Both 2-EHMC and MTD completely protected these mice from UV-induced promotion as well as from complete carcinogenesis despite the different UV-absorption spectra of the sunscreens and their differential abilities to protect from UV-induced immunosuppression. Furthermore, we have shown that, if UV exposure is not increased to compensate for tolerance to edema, protection from tumorigenesis is afforded by sunscreens.     

The effect of chronic ultraviolet radiation on the human immune system

A single or a limited number of UVR exposures is recognized to suppress cell-mediated immunity in human subjects. The complex pathway leading from the absorption of photons by chromophores in the skin to the generation of T regulatory cells has been, at least partially, elucidated. However, the effect of repeated UV exposures on immune responses and associated mediators is not well studied, particularly to assess whether they lead, first, to the development of photoprotection so that these immune changes are reduced or no longer occur, and, secondly, to the development of photoprotection against the normal downregulation of immunity induced by a high UV dose. For almost all the parameters evaluated in this review--epidermal DNA damage/erythema, urocanic acid, Langerhans and dendritic cells, natural killer cells, macrophages, mast cells, contact and delayed hypersensitivity responses--none, aside from epidermal DNA damage/erythema and macrophage phagocytic activity, show convincing evidence of photoadaptation or, where appropriate, photoprotection. It is concluded that repeatedly irradiating individuals with UVR is likely to continue to result in downregulation of immunity.     

Proanthocyanidins from Grape Seeds Inhibit UV–Radiation‐Induced Immune Suppression in Mice: Detection and Analysis of Molecular and Cellular Targets

Ultraviolet (UV)–radiation‐induced immunosuppression has been linked with the risk of skin carcinogenesis. Approximately, 2 million new cases of skin cancers, including melanoma and nonmelanoma, diagnosed each year in the USA and therefore have a tremendous bad impact on public health. Dietary phytochemicals are promising options for the development of effective strategy for the prevention of photodamaging effects of UV radiation including the risk of skin cancer. Grape seed proanthocyanidins (GSPs) are such phytochemicals. Dietary administration of GSPs with AIN76A control diet significantly inhibits UV‐induced skin tumor development as well as suppression of immune system. UV‐induced suppression of immune system is commonly determined using contact hypersensitivity (CHS) model which is a prototype of T–cell‐mediated immune response. We present evidence that inhibition of UV‐induced suppression of immune system by GSPs is mediated through: (i) the alterations in immunoregulatory cytokines, interleukin (IL)‐10 and IL‐12, (ii) DNA repair, (iii) stimulation of effector T cells and (iv) DNA repair‐dependent functional activation of dendritic cells in mouse model. These information have important implications for the use of GSPs as a dietary supplement in chemoprevention of UV‐induced immunosuppression as well as photocarcinogenesis.     

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.     

Suppression of Delayed and Contact Hypersensitivity Responses in Mice Have Different UV Dose Responses

Although acute exposure to UV radiation suppresses the induction of delayed-type (DTH) and contact (CHS) hypersensitivity in mice, it is not clear whether the photo-biological mechanism(s) involved in suppressing these closely related immune reactions is the same. A careful examination of the UV dose responses and wavelength dependencies involved in suppressing CHS and DTH may provide important insights into the mechanisms involved. We compared the UV dose-response curves for suppressing four closely related immune reactions, local and systemic suppression of CHS to dinitrofluorobenzene, systemic suppression of DTH to Candida albicans and systemic suppression of DTH to alloantigen using three different UV spectra (FS40 sunlamps, Kodacel-filtered FS40 sunlamps and solar-simulated light). For each immune response studied, the amount of UVB radiation required to induce 50% immune suppression was lowest when FS40 sunlamps were used, highest with solar-simulated light and intermediate when Kodacel-filtered FS40 sunlamps were used, but the differences observed were not statistically significant. The UV dose-response curves for immune suppression differed significantly depending on the assay used, the site of antigenic sensitization and the antigen used. These findings suggest that the mechanisms by which UV radiation induces immune suppression differ for the four immunological reactions studied.     

The effect of suntan parlor exposure on delayed and contact hypersensitivity

Cutaneous and systemic immune function are believed to play an important role in cutaneous carcinogenesis. We therefore sought to determine whether the suntan parlor radiation sources commonly used in the United States cause measurable qualitative suppression of immune function and quantitative alterations in circulating T cell subpopulations. Subjects (n = 22) were recruited and randomly assigned to receive suntan parlor exposures (10 full-body UV exposures over a 2 week period, shielding only the right flexural arm) or no exposure. Baseline circulating T lymphocyte subpopulations (T helper lymphocyte, CD4; T suppressor/cytotoxic lymphocyte, CD8) were measured. Two weeks later (upon completion of UV exposures for those in this group), circulating T cell subpopulations were measured and dinitrochlorobenzene (DNCB) sensitization (in the UV group, on the UV-exposed buttock) was performed. Subsequent DNCB elicitation was performed in a bilateral fashion (in the UV group, on the right UV-shielded and the left UV-exposed upper arm). We found that subjects in the UV group demonstrated localized suppression of contact hypersensitivity sensitization and elicitation and also an increase in circulating CD8 cells when compared to the control group (P < or = 0.05). We conclude that suntan parlor exposures, as typically received in this country, suppress contact hypersensitivity and increase the circulating T suppressor/cytotoxic cell number quantity.     

Routine measurement of erythemally effective UV irradiance on inclined surfaces

Measurements of erythemally weighted UV radiation are usually related to a horizontal surface. The radiation is weighted with the sensitivity of the human skin, but the surface of the human body has only few horizontal surfaces. Therefore the UV radiation on inclined surfaces has to be quantified to investigate UV effects on humans. To fulfill this task three fully automatic measuring systems were built to measure the erythemally weighted UV radiation in 27 directions within 2 min. This system measures routinely during the whole day and has now been in operation for nearly three years (in total 2000 measurement days) under any kind of meteorological conditions. The measurements provide the informations needed for further investigations concerning the UV effects on humans. The calibration of the erythemally weighting radiometers was performed in a way to provide reliable UV index measurements for all directions. The results of four exemplary measurement days in summer and winter for clear sky and overcast conditions are presented.     

Is Phototherapy Safe for HIV-Infected Individuals?

Patients infected with human immunodeficiency virus (HIV) have a high prevalence of UV radiation-responsive skin diseases including psoriasis, pruritus, eosinophilic folliculitis and eczemas. On the other hand, UV has been shown to suppress T cell-mediated immune responses and to induce activatin and replication of HIV. These developments have prompted clinicians and investigators to question whether phototherapy is safe for HIV-infected individuals. We have reviewed these issues and hereby provide a summary and critique of relevant laboratory and clinical evidence.     

UV-induced immunosuppression in the balance

Around 1980, experiments with hairless mice showed us that UV-induced actinic keratoses (AK) and ensuing skin carcinomas did not arise independently: the rate of occurrence in one skin area was increased considerably if AKs had already been induced separately in another distant skin area, i.e. a systemic effect. The ground laying work of Margaret Kripke in the 1970s provided a fitting explanation: UV-induced immunosuppression and tolerance toward the UV-induced tumors. From Kripke's work a new discipline arose: "Photoimmunology." Enormous strides were made in exploring and expanding the effects from UV carcinogenesis to infectious diseases, and in elucidating the mechanisms involved. Stemming from concerns about a depletion of the ozone layer and the general impact of ambient UV radiation, the groups I worked in and closely collaborated with explored the anticipated adverse effects of UV-induced immunosuppression on healthy individuals. An important turning point was brought about in 1992 when the group of Kevin Cooper reported that immunosuppression could be induced by UV exposure in virtually all human subjects tested, suggesting that this is a normal and sound physiological reaction to UV exposure. This reaction could actually protect us from illicit immune responses against our UV-exposed skin, such as observed in idiopathic polymorphic light eruption. This premise has fruitfully rekindled the research on this common "sun allergy," affecting to widely varying degrees about one in five Europeans with indoor professions.     

INHIBITION OF THE IMMUNE RESPONSE TO ALLOANTIGEN IN THE RAT BY EXPOSURE TO ULTRAVIOLET RADIATION

Exposure of mice to ultraviolet radiation (UV) followed by alloantigen sensitization can suppress the immune response to that alloantigen. In order to assess the applicability of using UV-induced immunosuppression in organ transplantation, the effectiveness of UV in prolonging the survival of vascularized organ allografts must be determined. Because, for technical reasons, rats are better suited than mice for such experiments, we first wanted to determine whether UV suppresses the immune response of inbred rats to alloantigens. The data presented here demonstrate that exposure of rats to UV (115-129 kJ/m2) prior to alloantigenic sensitization decreases the mixed lymphocyte response to alloantigen. The depression of the proliferative response to alloantigen was selective in that spleen cells from the UV-treated rats could respond to mitogenic stimulation. In contrast to previous results with mice, suppressor cells could not be demonstrated in the spleens of the UV-treated rats. In addition, UV treatment after sensitization inhibited the response to alloantigen. These data suggest that treatment of the recipient with UV before or after alloantigenic sensitization may provide a novel method of inhibiting immune responses to allogeneic antigens.     

UV Radiation and Mouse Models of Herpes Simplex Virus Infection

Orolabial human infections with herpes simplex virus type 1 (HSV-1) are very common; following the primary epidermal infection, the virus is retained in a latent form in the trigeminal ganglia from where it can reactivate and cause a recrudescent lesion. Recrudescences are triggered by various stimuli including exposure to sunlight. In this review three categories of mouse models are used to examine the effects of UV irradiation on HSV infections: these are UV exposure prior to primary infection, UV exposure as a triggering event for recrudescence and UV exposure prior to challenge with virus in mice already immunized to HSV. In each of these models immunosuppression occurs, which is manifest, in some instances, in increased morbidity or an increased rate of recrudescence. Where known, the immunological mechanisms involved in the models are summarized and their relevance to human infections considered.