UV-induced Suppression Research Articles

Measurement of In Vivo Sunscreen Immune Protection Factors in Humans

This study investigates the level of protection provided by sunscreens against solar-simulated UV radiation-induced immunosuppression in humans. The in vivo immune protection factors (IPF) of two broad-spectrum sunscreens were determined by assessing their ability to prevent UV-induced suppression of nickel contact hypersensitivity (CHS) in 15 nickel-allergic volunteers. Each volunteer was irradiated on unprotected skin of the back with different doses of UV daily for 4 days. Multiples of these UV doses were concurrently delivered to sunscreen-treated sites on the contralateral back. Nickel patches were then applied to both irradiated sites and adjacent, unirradiated control sites. Nickel-induced erythema at each site was measured 72 h later with a reflectance spectrometer. Comparison of the nickel reactions of irradiated and unirradiated skin revealed linear UV dose-responses for immunosuppression in both unprotected and sunscreen-treated skin. The minimum level of immunosuppression that can be reliably detected with this method is 20%. Therefore, the UV dose that reduces mean nickel CHS by 20% is the minimal immune suppression dose (MISD). Sunscreen IPF were determined by dividing the mean MISD of sunscreen-treated skin by that of unprotected skin. The sunscreens, with sun protection factors of 9 and 24, had IPF of 6.5 and > 25, respectively.

A critical role for Fas ligand in the active suppression of systemic immune responses by ultraviolet radiation.

Induction of antigen-specific suppression elicited by environmental insults, such as ultraviolet (UV)-B radiation in sunlight, can inhibit an effective immune response in vivo and may contribute to the outgrowth of UV-induced skin cancer. Although UV-induced DNA damage is known to be an initiating event in the immune suppression of most antigen responses, the underlying mechanism(s) of such suppression remain undefined. In this report, we document that Fas ligand (FasL) is critical for UV-induced systemic immune suppression. Normal mice acutely exposed to UV exhibit a profound suppression of both contact hypersensitivity and delayed type hypersensitivity (DTH) reactions and the development of transferable antigen-specific suppressor cells. FasL-deficient mice exposed to UV lack both transferable suppressor cell activity and primary suppression to all antigens tested, with the exception of the DTH response to allogeneic spleen cells. Interestingly, suppression of this response is also known to occur independently of UV-induced DNA damage. Delivery of alloantigen as protein, rather than intact cells, restored the requirement for FasL in UV-induced immune suppression of this response. These results substantiate that FasL/Fas interactions are essential for systemic UV-induced suppression of immune responses that involve host antigen presentation and suggest an interrelationship between UV-induced DNA damage and FasL in this phenomenon. Collectively, our results suggest a model whereby UV-induced DNA damage disarms the immune system in a manner similar to that observed in immunologically privileged sites.

Protection against UV-induced systemic immunosuppression in mice by a single topical application of the antioxidant vitamins C and E

Purpose: Reactive oxygen species are involved in UV-induced suppression of the immune system. Topical treatment with the antioxidant vitamins C (l-ascorbic acid, ASC) and E (d-alpha-tocopherol, TOC) can support the endogenous antioxidant defence system and prevent immunosuppression. Materials and methods: Mice were topically treated with a single dose of ASC, TOC or a combination and irradiated with UVB. Then systemic immunosuppression was measured using a model based on the induction of a contact hypersensitivity response to dinitrofluorobenzene. To investigate the mechanism of protection, cis-urocanic acid-induced immunosuppression was investigated in a different contact hypersensitivity model measuring local immunosuppression. The levels of ASC and TOC in the epidermis were determined by HPLC. Results: Both ASC and TOC prevented UV-induced suppression of the contact hypersensitivity response. TOC was effective at doses of 2.5 to 10nmol/cm2 and ASC at 0.5 to 5 mu mol/cm2. At the highest dose, the response in the ASC-treated mice was no longer significantly different from that in the positive control group. Contrary to expectations, combinations of the two compounds did not provide additional protection. The experiments with ASC or TOC against immunosuppression by cis-urocanic acid also yielded protection, but this was less efficient than against UV. The concentrations of ASC and TOC in the epidermis were so low that UVB absorption could be excluded as the cause of the protection. Conclusions: ASC and TOC can be used to prevent systemic UVinduced immunosuppression. They are effective at relatively low doses after a single topical application prior to the irradiation.

Measurement of In Vivo Sunscreen Immune Protection Factors in Humans

This study investigates the level of protection provided by sunscreens against solar-simulated UV radiation-induced immunosuppression in humans. The in vivo immune protection factors (IPF) of two broad-spectrum sunscreens were determined by assessing their ability to prevent UV-induced suppression of nickel contact hypersensitivity (CHS) in 15 nickel-allergic volunteers. Each volunteer was irradiated on unprotected skin of the back with different doses of UV daily for 4 days. Multiples of these UV doses were concurrently delivered to sunscreen-treated sites on the contralateral back. Nickel patches were then applied to both irradiated sites and adjacent, unirradiated control sites. Nickel-induced erythema at each site was measured 72 h later with a reflectance spectrometer. Comparison of the nickel reactions of irradiated and unirradiated skin revealed linear UV dose–responses for immunosuppression in both unprotected and sunscreen-treated skin. The minimum level of immunosuppression that can be reliably detected with this method is 20%. Therefore, the UV dose that reduces mean nickel CHS by 20% is the minimal immune suppression dose (MISD). Sunscreen IPF were determined by dividing the mean MISD of sunscreen-treated skin by that of unprotected skin. The sunscreens, with sun protection factors of 9 and 24, had IPF of 6.5 and >25, respectively.

Effect of UV Exposure and β-Carotene Supplementation on Delayed-Type Hypersensitivity Response in Healthy Older Men

Objectives: The aims of this study were to determine if ultraviolet light (UV) is immunosuppressive in healthy older males, if β-carotene (βC) supplementation could prevent any observed UV-induced immunosuppression, and to compare these effects with those observed previously in younger men.Methods: The study was a placebo-controlled, randomized trial that employed a 2×2 factorial design. Healthy older men (mean age 65.5 years) received 30 mg βC or placebo daily throughout the 47-day trial, while on a low carotenoid diet. After 28 days, half of each group received 12 suberythemic exposures to UV over a 16-day period. Delayed-type hypersensitivity (DTH) tests and plasma carotenoid assays were performed at baseline, pre-UV and post-UV time points, with DTH testing performed on an area of skin protected from UV exposure.Results: UV exposure resulted in significantly suppressed DTH response in the placebo group but not in the βC-UV group. While there was no significant interaction between βC supplementation and UV on DTH response, there was a significant inverse relationship between final plasma βC concentration and extent of UV-induced suppression of DTH response. A similar correlation existed among subjects not exposed to UV.Conclusions: Suberythemic UV exposure was immunosuppressive, as measured by DTH response, in healthy older men as in younger men. Higher plasma βC was significantly associated with maintenance of DTH response, although the extent of protective effect of βC appeared less than previously observed in younger subjects. The attenuated effect of βC in the older UV-exposed subjects may have resulted in part from muted plasma βC responses to βC supplementation and/or higher plasma vitamin E levels than those of younger men. The finding that stronger DTH responses were associated with higher plasma βC concentrations in both UV and non-UV subjects further supports a role for this nutrient in immunomodulation.

Interleukin 12 Breaks Ultraviolet Light Induced Immunosuppression by Affecting CD8+ Rather Than CD4+T Cells

Recent studies showed that injection of interleukin (IL)-12 prevents ultraviolet (UV) light mediated suppression of contact hypersensitivity and breaks UV-induced hapten specific tolerance. UV-mediated suppression can be adoptively transferred by injecting splenocytes from UV-irradiated mice; however, suppression is not transferable when donor mice are treated with IL-12 after UV-irradiation. This study was performed to elucidate the mechanisms by which IL-12 counteracts this immunosuppression. To characterize the cells transferring suppression, depletion studies were performed revealing that UV-induced suppression is transferred via CD8+ T cells. To investigate whether IL-12 counteracts UV-induced suppression by either inhibiting the development of CD8+ suppressor T cells or inducing CD4+ effector T cells, splenocytes from mice, which were IL-12 treated and sensitized through UV-exposed skin, were depleted from CD4+ T cells and transferred into naive mice that were subsequently sensitized. Whereas transfer of splenocytes from UV-irradiated mice inhibited sensitization of recipients, no inhibition was observed after transfer of splenocytes from UV-exposed and IL-12 treated mice. Recipients that received CD4 depleted spleen cells from UV-exposed and IL-12 treated donors, were still fully sensitizable. IL-12 also blocked transfer of UV-induced suppression when it was injected into UV-exposed donor animals at a time point when suppressor cells had already developed. CD4 depletion of such splenocytes did not result in a loss of the reconstitutive effect of IL-12. This suggests that IL-12 may break UV-induced tolerance not by inducing CD4+ effector T cells, but rather by inhibiting or inactivating suppressor T cells belonging to the CD8 subtype.

Protection of UV-induced suppression of skin contact hypersensitivity: a common feature of flavonoids after oral administration?

In this study we investigated the effect of the dietary ingredients fruit and vegetable, green tea phenol extract (GTP) and the specific flavonoid components quercetin and chrysin on the UV-induced suppression of the contact hypersensitivity (CHS) response to picryl chloride (PCl). The SKH-1 mice were fed with test diet from 2 or 4 weeks before and during the UV irradiation (daily, 95 mJ/cm2) and tested for the CHS ear-swelling response 10 weeks after the onset of the irradiation. For the CHS, mice were immunized with PCl by epicutaneous application on nonirradiated sites. Four days after sensitization all mice were challenged on both sides of each ear by topical application of one drop PCl. In addition, from mice fed with the fruit and vegetable mixture the number of Langerhans cells (LC) were scored in the skin and from mice fed with quercetin, quercetin levels in plasma were measured at week 11 after the start of UV irradiation. It was found that fruit and vegetable (19% in the diet), GTP (0.1% and 0.01% in the drinking water), quercetin (1% in the diet) and chrysin (1% and 0.1% in the diet), prevented statistically significantly the UV-induced suppression of CHS to PCl. In the skin of mice fed with fruit and vegetables combined with UV irradiation the number of LC were comparable to the control mice, whereas the number of LC were significantly diminished in mice treated with UV only. This protective effect on the presence of LC in the epidermis after UV irradiation, which was also observed in a previous study with quercetin, may play a role in the prevention of UV-induced immunosuppression by the flavonoids tested. In conclusion, we found protection of flavonoids against UV-induced effects on CHS, which may be a common feature of most flavonoids.

Genetic Variation in Low-Dose UV-Induced Suppression of Contact Hypersensitivity and in the Skin Photocarcinogenesis Response.

Two of the major cutaneous consequences of ultraviolet (UV) radiation exposure are immunosuppression and the development of skin cancer. This study examined whether these effects are genetically determined. Suppression of contact hypersensitivity by local, low-dose UV radiation was examined in what have been termed "UV-susceptible" and "UV-resistant" strains of mice. C3H/HeJ mice ("UV resistant") were resistant to the adverse effects of low-dose UV radiation when normal doses of hapten were applied to UV-irradiated skin; however, they were sensitive when the amount of hapten used for sensitization was reduced. A similar effect was observed in BALB/c mice ("UV resistant") and when the hapten was dimethylbenz(a)anthracene, thus indicating that the genetic variation was not strain or hapten specific. Despite the fact that some strains were sensitive and some were resistant to low-dose UV radiation when high doses of hapten were employed, all strains initially sensitized to hapten through UV-irradiated skin were found to be unresponsive when rechallenged on normal skin, no matter what the initial sensitizing dose of hapten was. To determine whether other biologic effects of UV also exhibited genetic variation, C3H/HeN and C3H/HeJ mice were compared for susceptibility to UVB-induced skin cancer formation. C3H/HeJ mice developed significantly more tumors than C3H/HeN mice when subjected to a single dose of UV radiation followed by repeated exposure to the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate. These studies provide strong evidence that genetic factors influence individual susceptibility to the biologic effects of UV radiation.

Broad-Spectrum Sunscreens Provide Greater Protection against Ultraviolet-Radiation–Induced Suppression of Contact Hypersensitivity to a Recall Antigen in Humans

This study investigates the extent to which sunscreens protect humans from ultraviolet (UV)-radiation-induced immunosuppression. In the presence of solar-simulated UV, three sunscreens with differing UVA transmission were assessed for their ability to protect the contact hypersensitivity (CHS) response to nickel of 16 nickel-allergic subjects. The sunscreens contained 2-ethylhexyl para-methoxycinnamate (cinnamate), cinnamate with oxybenzone, or cinnamate with zinc oxide, respectively. All had sun protection factors of 10 and hence inhibited UV erythema to similar extents. Volunteers were irradiated on their backs with suberythemal UV daily for 5 d after application of the sunscreens and their base lotion to different sites. Nickel-containing patches were then applied to both UV-treated sites and adjacent, unirradiated control sites. Erythema caused by nickel CHS at each site was quantitated 72 h later with a reflectance erythema meter. In comparison of the nickel reactions of irradiated and unirradiated skin, there was 35% mean immunosuppression in unprotected UV-treated skin. Significant immunosuppression also occurred at sites irradiated through the narrow-spectrum cinnamate-only sunscreen but was prevented by the two broad-spectrum sunscreens. To determine whether UV-induced suppression of the nickel response is specific for cell-mediated immunity or reflects suppression of nonspecific inflammation, a further 16 subjects were patch-tested with a skin irritant, sodium lauryl sulfate (SLS), following a sunscreen and irradiation protocol identical to that of the nickel volunteers. UV had no significant effect on SLS responses. We conclude that nickel patch testing is a valid means of assessing UV-induced immunosuppression in humans and that even with suberythemal UV, immune protection was provided only by sunscreens filtering both UVA and UVB.

In situ action spectra suggest that DNA damage is involved in ultraviolet radiation-induced immunosuppression in humans.

The mixed epidermal cell lymphocyte reaction (MECLR) is a commonly used method to study the immunomodulatory effects of UV radiation. The in vitro action spectrum for the MECLR showed that the UV-induced suppression of the MECLR responses is associated with UV-induced DNA damage. To investigate whether in vivo DNA damage also leads to the abrogation of the MECLR, in situ action spectra were made for the MECLR and the induction of thymine dimers (T < > T). Human skin, obtained from plastic surgery, was exposed to monochromatic light of 254, 297, 302 and 312 nm. After irradiation, epidermal cells were isolated and used as stimulator cells in the MECLR or processed for flow cytometric detection of T < > T. On the basis of dose-response curves for each wavelength, the action spectra for suppression of the MECLR and the induction of T < > T were calculated. These spectra showed close similarities, suggesting that, also in situ, UV-induced DNA damage is involved in the UV-induced suppression of the MECLR. Both action spectra showed a small decline from 254 nm to 302 nm, followed by a steep decline to 312 nm. These data show that, in situ, UVC can efficiently induce DNA damage and modulate cutaneous immune responses.

Sunscreens prevent local and systemic immunosuppression of contact hypersensitivity in mice exposed to solar-simulated ultraviolet radiation

Ultraviolet (UV) irradiation causes the immunosuppression of contact hypersensitivity (CH) responses in animals and humans. There are conflicting reports regarding the effectiveness of sunscreens in preventing UV-induced suppression of both local-type CH (induced by the application of the contact sensitizer directly to UV-exposed skin) and systemic-type CH (induced by the application of the contact sensitizer to an unirradiated skin site 3 days after UV exposure). The purposes of this study were as follows: 1. 1. to derive solar simulator UV dose-response curves for the induction of local and systemic CH suppression in C3H mice; 2. 2. to establish minimum immune suppression doses (MISDs) for local and systemic CH; 3. 3. to determine the local and systemic immune protection capacity of two commercial sunscreen lotions with labeled sun protection factors (SPFs) of 4 and 8. Dose-response curves for the induction of local and systemic CH suppression were derived by exposing groups of mice to a range of full-spectrum UV doses (0.37–21.4 kJ m −2) on two consecutive days delivered from a filtered 1000 W xenon arc lamp solar simulator. The MISDs, defined as the lowest dose tested to cause approximately 50% suppression of the normal CH response, were obtained from the dose-response curves. Although the local and systemic immunosuppression dose-response curves were not statistically different, the MISD for local suppression of CH (1.35 kJ m −2) was about fivefold lower than that for systemic CH suppression (6.76 kJ m −2). The MISD was used as the endpoint to determine sunscreen immune protection levels. Both sunscreens, applied at 2 mg cm −2, provided immune protection against the induction of local and systemic CH suppression in mice exposed to an effective UV dose of 1 MISD giventhrough the sunscreen, i.e. 4 MISD to SPF 4 sunscreen-protected mice and 8 MISD to SPF 8 sunscreen-protected mice mounted CH responses that were significantly greater than those elicited in unprotected mice exposed to 1 MISD of solar-simulated UV radiation. The calculated immune protection factors for these sunscreens exceeded the level of protection predicted by their labeled SPFs, i.e. the local immune protection factor of both sunscreens was 15 and the systemic immune protection factors were 8 for the SPF4 sunscreen and 15 for the SPF 8 sunscreen. Our data show that these two sunscreens provide levels of immune protection which exceed the levels predicted by their labeled SPFs in immunoprotection tests conducted in mice exposed to a relevant MISD of UV radiation from a source emitting a UV power spectrum similar to that of sunlight.

The effect of oral quercetin on UVB-induced tumor growth and local immunosuppression in SKH-1

The effects of quercetin (4%) on UVB-induced carcinogenesis and immunosuppression were studied in hairless SKH-1 mice exposed daily to suberythemal UVB for 12/13 and 16/17 weeks. Macroscopic and microscopic examinations showed that quercetin did not affect the onset and growth of UVB-induced non-melanoma skin tumors. Quercetin prevented the UV-induced suppression of the contact hypersensitivity (CHS) and the reduction of the percentage of CD8-positive cells in spleen and lymph nodes. Other immunological parameters were not affected. Thus, the results indicate that oral intake of a high dose of quercetin does not prevent UVB-induced carcinogenesis, although it restores the skin-associated CHS response.

Molecular aspects of UVB-induced immunosuppression.

Ultraviolet light can affect the immune system locally as well as systemically leading to an impaired resistance to neoplastic cells and/or infections. Prior to the biological effect, UVB must be absorbed by a chromophore in the skin where it will give a signal that can lead to an altered immune response in the skin or elsewhere. These altered immune responses may be constituted by alteration in among others: cytokine profile, growth factors and costimulatory signals. Several hypotheses about the identity of the photoreceptor have been put forward. One photoreceptor in the skin is urocanic acid (UCA), that can isomerize from the trans- to the cis-isomer. The cis-isomer has immunosuppressive properties. Another photoreceptor is DNA that also efficiently absorbs UV wavelengths. After absorption the structure of the DNA molecule is altered. This alteration might lead to gene activation responsible for the immunotoxic outcome (altered gene expression). It has been demonstrated that the formation of DNA photoproducts by UV light is associated with the activation of many genes. Several studies indicate that UV-induced DNA damage, in the form of cyclobutyl pyrimidine dimers plays a role in UV-induced suppression of the immune system locally as well as systemically. In mice that were injected with liposomes containing the excision repair enzyme T4 endonuclease UVB-induced dimers were removed more efficiently as compared to control mice. In these mice UV-induced immunosuppression was prevented. Pilot studies by Kripke et al. indicated that the release of IL-IO and TNF alpha that are both induced by DNA damage might be involved. In preliminary studies with mice that were deficient with respect to DNA repair lower doses of UV were needed for the induction of immunosuppression as compared to their normal littermates. These studies indicate that altered gene expression plays a pivotal role in UVB-induced immunosuppression. In addition to a role for UCA and DNA in UV-induced immunosuppression it is postulated recently that signal transduction (EGF-receptor mediated upregulation of phospholipase A2) and transcription factors (NF kappa B, p91) are involved in UV-induced immunomodulation.

Does exposure to UV radiation induce a shift to a Th-2-like immune reaction?

In addition to being the primary cause of skin cancer, UV radiation is immune suppressive and there appears to be a link between the ability of UV to suppress the immune response and induce skin cancer. Cytokines made by UV-irradiated keratinocytes play an essential role in activating immune suppression. In particular, we have found that keratinocyte-derived interleukin (IL)-10 is responsible for the systemic impairment of antigen-presenting cell function and the UV-induced suppression of delayed-type hypersensitivity (DTH). Antigen presentation by splenic adherent cells isolated from UV-irradiated mice to T helper-1 type T (Th1) cells is suppressed, whereas antigen presentation to T helper-2 type T (Th2) cells is enhanced. The enhanced antigen presentation to Th2 cells and the impaired presentation to Th1 cells can be reversed in vivo by injecting the UV-irradiated mice with monoclonal anti-IL-10 antibody. Furthermore, immune suppression can be transferred from UV-irradiated mice to normal recipients by adoptive transfer of T cells. Injecting the recipient mice with anti-IL-4 or anti-IL-10 prevents the transfer of immune suppression, suggesting the suppressor cells are Th2 cells. In addition, injecting UV-irradiated mice with IL-12, a cytokine that has been shown to be the primary inducer of Th1 cells, and one that prevents the differentiation of Th2 cells in vivo, reverses UV-induced immune suppression. These findings support the hypothesis that UV exposure activates IL-10 secretion, which depresses the function of Th1 cells, while enhancing the activity of Th2 cells.

Dietary fat modulates immunoresponsiveness in UV-irradiated mice.

Previous studies have shown that a high level of dietary lipid (corn oil) exacerbates UV-carcinogenic expression in hairless mice. Furthermore, it was demonstrated that this effect occurs at the postinitiation, or promotion, stage of UV-carcinogenesis--a stage believed to be modulated immunologically. Thus, we sought to examine the influence of dietary lipid on specific immune parameters at various times within a UV-carcinogenic protocol, with the purpose of detecting potential relationships to UV carcinogenesis. Hairless mice were fed either a high- (12%, wt/wt, corn oil) or low-fat (0.75%, wt/wt, corn oil) diet for 2 weeks prior to start of the UV or experimental protocols. Animals were sensitized to dinitrochlorobenzene (DNCB) hapten and delayed-type hypersensitivity (DTH) was assessed. Delayed-type hypersensitivity was significantly suppressed (P = 0.01) in the high-fat group, even before UV irradiation. Although both groups exhibited UV-induced suppression of this response, the high-fat group was totally suppressed after 3 weeks of UV, whereas the low-fat group exhibited reactivity through week 8. The splenic T-lymphocyte (Thy 1.2+) population had declined by about 50% at the time of UV termination (11 weeks). Dietary lipid exerted no apparent influence upon this T-cell population. However, after 6 weeks of UV, I-J+ cells (a marker shown to be acquired adaptively by suppressor T lymphocytes) began to increase. By week 15 (4 weeks post-UV) I-J+ cells had increased by about 65% in the high-fat group, twice the % increase that occurred in the low-fat group. When UV-induced tumors were transplanted to recipient animals receiving various periods (0, 6, 11 weeks) of UV irradiation, no significant differences in median tumor rejection times between the two dietary groups occurred at 0 or 6 weeks. After 11 weeks of UV, the low-fat group exhibited a tumor rejection time that was comparable to that of nonirradiated animals, i.e. 21 days. However, median tumor rejection time for the high-fat group was greater than 63 days, significantly (P = 0.01) longer than that of the low-fat group. Thus, suppression of tumor rejection by high fat occurred at a time when high fat had been shown to exacerbate carcinogenic expression and when I-J+ cells had markedly increased. These data demonstrate that level of dietary lipid modulates immunoresponsiveness in UV-irradiated animals and is compatible with the thesis that immune suppression may account for the exacerbation of carcinogenic expression elicited by high dietary fat.

THE ACTION SPECTRA FOR UV‐INDUCED SUPPRESSION OF MLR and MECLR SHOW THAT IMMUNOSUPPRESSION IS MEDIATED BY DNA DAMAGE

Ultraviolet-B (UVB, 280-320 nm) radiation can promote the induction of skin cancer by two mechanisms: damage of epidermal DNA and suppression of the immune system, allowing the developing tumor to escape immune surveillance. The mixed lymphocyte reaction (MLR) and the mixed epidermal cell lymphocyte reaction (MECLR) are commonly used methods to study the immunosuppressive effects of UVB radiation. To obtain a better understanding of the mechanism by which UVB radiation decreases the alloactivating capacity of in vitro-irradiated cells, action spectra for the MLR and MECLR were determined. Suspensions of peripheral blood mononuclear cells or epidermal cells were irradiated with monochromatic light of 254, 297, 302 or 312 nm and used as stimulator cells in the MLR or MECLR. Using dose-response curves for each wavelength, the action spectra were calculated. Both MLR and MECLR action spectra had a maximum at 254 nm and a relative sensitivity at 312 nm that was a thousand times lower than at 254 nm. Strikingly, the action spectra corresponded very closely to the action spectra that were found by Matsunaga et al. (Photochem. Photobiol. 54, 403-410, 1991) for the induction of thymine dimers and (6-4)photoproducts in irradiated calf thymus DNA solutions, strongly suggesting that the UV-induced abrogation of the MLR and MECLR responses is mediated by UV-induced DNA damage. Furthermore, the action spectra for the MLR and MECLR were similar, suggesting that they share a common mechanism for UV-induced suppression.

A Monoclonal Antibody to cis-Urocanic Acid Prevents the Ultraviolet-Induced Changes in Langerhans Cells and Delayed Hypersensitivity Responses in Mice, Although Not Preventing Dendritic Cell Accumulation in Lymph Nodes Draining the Site of Irradiation and Contact Hypersensitivity Responses

Ultraviolet B (UVB) irradiation of C3H mice causes suppression of delayed hypersensitivity and contact hypersensitivity (CH) to antigens encountered following exposure, and is accompanied by a reduction in Langerhans cell (LC) numbers in the epidermis, loss of epidermal antigen-presenting cell function, and accumulation of dendritic cells in lymph nodes draining the site of irradiation. Various photoreceptors and mediators of these changes have been proposed, one of which is cis-urocanic acid (cis-UCA) formed from the naturally occurring trans-UCA in the epidermis on UV irradiation. A monoclonal antibody that reacts with cis-UCA has become available recently and has been used in this study to clarify the role of UCA. Pretreatment of C3H mice with the monoclonal antibody abrogated the UVB-induced and cis-UCA-induced reduction in epidermal LC numbers. It also prevented the UV-induced suppression of epidermal antigen-presenting cell ability as measured by the mixed skin lymphocyte response. However, it had no effect on the accumulation of dendritic cells in lymph nodes draining the site of UV exposure. With regard to hypersensitivity responses, it did not prevent UV-induced suppression of CH to oxazolone at a range of concentrations but it restored to normal the UV-suppressed delayed hypersensitivity to herpes simplex virus, if administered before exposure. Thus cis-UCA is involved in some UV-induced changes in murine skin but not in others, where alternative mediators, such as tumor necrosis factor-alpha, may be more important.

Roles of CD4+ and CD8+ cells in UVB-induced suppression of splenic alloantigen-presenting function.

Whole body ultraviolet light (UV) radiation causes systemic immunosuppression. Splenic antigen-presenting cell (APC) activity is decreased by UV radiation. To determine whether splenic CD4+ or CD8+ cells are involved in the UV-induced depression of splenic alloantigen-presenting function, we investigated the effect of in vivo UV radiation on the splenic stimulatory function in allogeneic mixed lymphocyte reaction in mice after the elimination of CD4+ or CD8+ cells by administering anti-CD8 or anti-CD4 Ab. Ab-treated and non-treated mice were exposed to UVB radiation (2.5 kJ/m2) twice or eight times. Two exposures to UVB radiation significantly suppressed the splenic alloantigen-presenting function of mice previously treated with anti-CD4 Ab, but hardly affected that of anti-CD8 Ab-treated or non-treated mice 2 days after the last radiation. On the other hand, eight exposures to UVB radiation suppressed this function in all mice. FACS analysis revealed that the UV-induced suppression is not associated with a significant decrease in the number of IA+ cell, as stimulator cells. It is suggested that CD4+ cells are somewhat preventive of the UV-induced depression of splenic allo-antigen-presenting function.

Effect of Topical Sunscreens on the UV-Radiation–Induced Suppression of the Alloactivating Capacity in Human Skin In Vivo

Exposure of mice or humans to solar or artificial ultraviolet radiation (UV) has been shown to induce a number of changes in the immune system that may influence their susceptibility to skin tumors. The protective effect of sunscreens on these changes is not clear. Thirty-two patients with a variety of dermatoses routinely undergoing treatment with standard UVB (n = 19) or PUVA (n = 13) therapy were studied. One of the two tested sunscreens or its vehicle was applied to the right flexor forearm immediately prior to each total-body UV exposure. Epidermal sheets were obtained by the suction-blister method from the left flexor forearm before treatment and from both flexor forearms after 4 weeks of photo- or photochemotherapy and used as stimulator epidermal cells (EC) in the mixed epidermal cell-lymphocyte reaction (MECLR). After 4 weeks of either UVB or PUVA therapy the MECLR responses on EC from both arms were markedly decreased. Neither the tested sunscreens nor their vehicles prevented the UV-induced suppression of the alloactivating capacity. The failure of sunscreens to protect against the UV-induced suppression of the alloactivating capacity could be explained in two ways. First, the energy not absorbed by the sunscreen could be sufficient to induce suppression of the alloactivating capacity. An alternative explanation could be systemic immune suppression by UV. In order to discriminate between these possibilities only the right forearms of 10 healthy volunteers, treated with a sunscreen or its vehicle, were irradiated with UVB during 4 weeks. In this manner systemic immune suppression by UVB could be excluded. This experiment resulted in a similar suppression of the MECLR responses, as induced by total body UVB irradiation, without any protection by the sunscreen. Apparently, the UV dose not absorbed by the sunscreen was capable to induce suppression of the alloactivating capacity. Our results indicate that people protected from sunburn by sunscreens may be exposed to UV for a long period of time, and thereby subject themselves to its immunosuppressive action.

DIETARY HISTIDINE INCREASES MOUSE SKIN UROCANIC ACID LEVELS AND ENHANCES UVB‐INDUCED IMMUNE SUPPRESSION OF CONTACT HYPERSNSITIVITY

Urocanic Acid (UCA) exists in mammalian skin primarily as the trans isomer and is photoisomerized to cis UCA upon UVB absorption. Our previous studies indicated that the photoisomerization of UCA is the initiating event in UBV-induced suppression of cell-mediated immunity (tUCA----cUCA----immune suppression). The purpose of this study was to verify the role of UCA in UV-induced immune suppression of contact hypersensitivity (CHS) in BALB/c mice. Since UCA is a metabolite of the amino acid L-histidine, we reasoned that increased dietary levels of histidine should raise skin tUCA levels. If skin tUCA is the UVB photoreceptor for immune suppression, this increase should enhance UV-induced suppression of CHS. HPLC analysis of skin from BALB/c mice given a histidine-rich diet (10%) showed that the total amount of UCA is significantly higher in these animals than in mice fed a normal diet. Further, levels of suppression of CHS of 3% and 49% in control fed mice, induced by 4.8 and 7.2 kJ/m2 UVB were significantly increased to 21% and 71% respectively in histidine-fed animals at these same UVB doses. These findings provide additional support for the UCA model for immune suppression, and provide the first evidence that UV-induced immune suppression can be enhanced by a dietary component, L-histidine.