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

Broad-spectrum sunscreens provide better protection from solar ultraviolet-simulated radiation and natural sunlight-induced immunosuppression in human beings

BACKGROUND

It is well established that ultraviolet (UV) radiation induces immunomodulatory effects that may be involved in skin cancer. Recent studies have shown that UVA (320-400 nm) and UVB (290-320 nm) radiation are immunosuppressive. As a result, sunscreens, which mainly absorb UVB, may be less effective in preventing UV radiation-induced immunosuppression than broad-spectrum products.

OBJECTIVE

We sought to study the effects of UVA exposure on human delayed-type hypersensitivity (DTH) response and compare the efficacy of sunscreens having different levels of sun-protection factor (SPF) and UVA protection against both solar-simulated radiation and outdoor real-life sunlight exposure conditions.

METHODS

DTH was assessed using a kit which includes 7 recall antigens that most of the participants encountered during childhood immunization. Evaluation of DTH test response was made 48 hours after test application before and after UV exposure with or without sunscreens.

RESULTS

In unprotected participants, the response to DTH tests was significantly reduced irrespective of UV types of exposure (full-spectrum UVA, long UVA, solar-simulated radiation). A UVB sunscreen failed to protect from solar-simulated radiation-induced immunosuppression. In contrast, a broad-spectrum sunscreen with the same SPF but providing a high protection in the UVA range significantly reduced local UV-induced immunosuppression and prevented the distant effects. In the outdoor study, as compared with DTH responses obtained before sun exposure, no alteration of immune response was detected when the skin was protected by a broad-spectrum sunscreen having a high protection level in the UVA (SPF 25, UVA protection factor 14). Conversely a broad-spectrum sunscreen with lower protection against UVA (SPF 25, UVA protection factor 6) failed to prevent UV-impaired response.

LIMITATIONS

These results have been obtained after repeated exposure. Additional experiments obtained under acute exposure are in progress.

CONCLUSION

These findings clearly demonstrated the role of UVA in the induction of photoimmunosuppression together with the need for sunscreen products providing efficient photoprotection throughout the entire UV spectrum.

Preparation of polymeric nanocapsules containing octyl methoxycinnamate by the emulsification–diffusion technique: Penetration across the stratum corneum

Polymeric nanocapsules (NCs) containing octyl methoxycinnamate (OMC) as lipophilic molecule were prepared, and their in vivo distribution profile through the stratum corneum (SC) was determined by the tape-stripping technique. Penetration degree of OMC formulated in NCs was compared with that obtained for a nanoemulsion (NE), and a conventional oil-in-water (o/w) emulsion (EM). To produce stable cellulose acetate phthalate (CAP) nanocapsules containing the lipophilic sunscreen, a study was conducted to optimize the process of NC preparation based on the emulsification-diffusion technique. NC formation was verified by measuring their density using differential centrifugation. NC density revealed that an OMC (microL)/CAP (mg) ratio of 2.5:1 is optimal for encapsulation. High encapsulation entrapment (>96%) and excellent process efficiency (recovered quantity of NCs in relation with the initial amount of OMC and CAP >99%) were always achieved with this ratio or a higher one. The capsular structure of the NCs was evidenced with a direct SEM technique. NE was prepared by the emulsification-diffusion technique, dissolving a specific quantity of OMC in water-saturated 2-butanone and then, emulsifying with an aqueous solution of PVAL. In vivo percutaneous penetration, evaluated by the tape-stripping technique, demonstrated that NE increased the extent of OMC penetration relative to the penetration achieved by NCs or EM, with relative penetration depths through the SC of 0.86 +/- 0.1, 0.64 +/- 0.11, and 0.57 +/- 0.08, respectively. In the same manner, the accumulation in the skin of OMC was significantly greater with NE than with EM or NCs. OMC penetration depth was strongly dependent upon the size of the colloidal particles and their flexibility.

High Ultraviolet A Protection Affords Greater Immune Protection Confirming that Ultraviolet A Contributes to Photoimmunosuppression in Humans

Solar radiation causes immunosuppression that contributes to skin cancer growth. Photoprotective strategies initially focused on the more erythemogenic ultraviolet B. More recently, the relationship of ultraviolet A and skin cancer has received increased attention. We hypothesized that if ultraviolet A contributes significantly to human ultraviolet-induced immune suppression, then increased ultraviolet A filtration by a sunscreen would better protect the immune system during ultraviolet exposure. Two hundred and eleven volunteers were randomized into study groups and received solar-simulated radiation, ranging from 0 to 2 minimum erythema dose, on gluteal skin, with or without sunscreen, 48 h prior to sensitization with dinitrochlorobenzene. Contact hypersensitivity response was evaluated by measuring the increase in skin fold thickness of five graded dinitrochlorobenzene challenge sites on the arm, 2 wk after sensitization. Clinical scoring using the North American Contact Dermatitis Group method was also performed. Solar-simulated radiation dose-response curves were generated and immune protection factor was calculated using a nonlinear regression model. Significance of immune protection between study groups was determined with the Mann-Whitney-Wilcoxon exact test. The sunscreen with high ultraviolet A absorption (ultraviolet A protection factor of 10, based on the in vivo persistent pigment darkening method) and a labeled sun protection factor of 15 demonstrated better immune protection than the product that had a low ultraviolet A absorption (ultraviolet A protection factor of 2) and a labeled sun protection factor of 15. Nonlinear regression analysis based on skin fold thickness increase revealed that the high ultraviolet A protection factor sunscreen had an immune protection factor of 50, more than three times its sun protection factor, whereas the low ultraviolet A protection factor sunscreen had an immune protection factor of 15, which was equal to its labeled sun protection factor. This study demonstrates that ultraviolet A contributes greatly to human immune suppression and that a broad-spectrum sunscreen with high ultraviolet A filtering capacity results in immune protection that exceeds erythema protection. These results show that high ultraviolet A protection is required to protect against ultraviolet-induced damage to cutaneous immunity.

Effects of UVA radiation on an established immune response in humans and sunscreen efficacy

It is well established that ultraviolet radiation has immunomodulatory effects which may be involved in skin cancer. Recent studies have shown that UVA radiation (320-400 nm) as well as UVB (290-320 nm) is immunosuppressive. This means that sunscreens which mainly absorb UVB (protection against erythema) may be less effective in preventing UVR-induced immunosuppression than broad-spectrum products. We have studied the effects of UVA exposure on the human delayed-type hypersensitivity response (DTH) and compared the efficacy of sunscreens having different levels of UVA protection under both solar-simulated radiation (SSR) chronic exposures or acute exposure and outdoor real-life solar exposure conditions. DTH was assessed using recall antigens. Our studies clearly demonstrate the role of UVA in the induction of photoimmunosuppression together with the need for sunscreen products providing efficient photoprotection throughout the entire UV spectrum. These data suggest that sun protection factor may not be sufficient to predict the ability of sunscreens for protection from UV-induced immune suppression. Determining the level of UVA protection is particularly necessary, because UVA seems to have a relatively low contribution to erythema but is highly involved in immunosuppression.

Efficacy of broad-spectrum sunscreens against the suppression of elicitation of delayed-type hypersensitivity responses in humans depends on the level of ultraviolet A protection

Sunscreens have been designed to protect against sunburn and their efficacy has, therefore, been labeled by the so-called sun protection factor (SPF). Although this value is well determined using a standardized protocol and it affords a good evaluation of the protection against erythema it may be inadequate to provide a relevant measurement of efficacy against other biologic damages. This is particularly true when action spectra and threshold dose are different from those of erythema. In the case of ultraviolet (UV)-induced immune suppression, the action spectrum is not known, so it cannot be asserted that SPF may accurately predict the level of protection against this endpoint. We addressed this issue by measuring in human volunteers the ability of two broad-spectrum SPF 15 sunscreens with different ultraviolet A (UVA) protection levels, to prevent the alteration of the efferent phase of the local delayed-type hypersensitivity (DTH) response to recall antigens (Multitest Pasteur/Mérieux, Lyon, France) after acute solar-simulated UV exposure. We first determined the ultraviolet radiation (UVR) dose needed to induce a significant DTH inhibition in several groups of 15 volunteers. Two minimal erythemal doses (2 MED) were found to be the minimal immunosuppressive dose (MISD). As a result, the immune DTH response is reduced in average by 36%. The lower doses tested (0.5 and 1 MED) were ineffective. Sunscreen-treated groups were exposed to either 1 or 2 MED x SPF doses. As expected, no alteration in DTH response was observed in the groups exposed to 1 MED x SPF whatever the sunscreen applied. In contrast, after exposure to 2 MED x SPF, the DTH response remained unaltered in the group pretreated with the sunscreen product with the higher protection in the UVA range but was significantly suppressed by 55.7% in the group pretreated with sunscreen with a much lower protection in the UVA range. These data suggest that SPF may not be sufficient to predict the ability of sunscreens to protect from UV-induced immune suppression. Determining the level of UVA protection is particularly needed, as UVA seems to have a relatively low contribution to erythema but is highly involved in immunosuppression.

A commercial sunscreen's protection against ultraviolet radiation-induced immunosuppression is more than 50% lower than protection against sunburn in humans

Ultraviolet radiation (280-400 nm)-induced suppression of cutaneous cell-mediated immunity plays an important part in the development of skin cancer. Sunscreens are widely advocated to protect against skin cancer but if they offer insufficient protection against immunosuppression they may inadvertently increase skin cancer risk. This human study evaluated immunoprotection afforded by a commercial sunscreen preparation (labeled sun protection factor 15) offering primarily ultraviolet B (280-320 nm) protection. Indirectly, it also investigated whether ultraviolet A (320-400 nm) plays a part in ultraviolet radiation-induced immunosuppression. Healthy white-skinned volunteers were used (n=119). Ultraviolet radiation exposures were on previously unexposed buttock skin with an ultraviolet radiation source that complied with European recommendations for sunscreen testing. Ultraviolet radiation dose-response curves for sunburn/erythema and suppression of the contact hypersensitivity response were generated either with or without sunscreen in vivo and protection factors were derived for both end-points. The ultraviolet radiation wavelengths transmitted by the sunscreen were determined in vitro and showed that the sunscreen was primarily an ultraviolet B absorber, with relatively poor absorption in the ultraviolet A region. The sun-screen protected against both erythema and immunosuppression but protection against immunosuppression (IPF=4.9, 95% confidence interval: 2.3-10.6) was less than half that for erythema (Ery-PFg=14.2, 95% confidence interval: 10.2-19.8). Failure of the sunscreen to afford comparable protection against both end-points was probably due to immunosuppression by ultraviolet A, a part of the solar spectrum that does not readily cause sunburn. The sunscreen protected against both end-points, which supports the use of sunscreens to reduce immunosuppression but protection against immunosuppression may be improved if sunscreens are formulated to offer equivalent protection against ultraviolet B and ultraviolet A.

Ultraviolet a irradiation of C57BL/6 mice suppresses systemic contact hypersensitivity or enhances secondary immunity depending on dose

Ultraviolet radiation is the most common environmental carcinogen humans are exposed to. It is now known that in order for skin cancers to develop, both genetic damage and immunosuppression is required. Ultraviolet-induced immunosuppression is therefore a key contributor to the development of skin cancer. Little is known about the relative contributions of the different ultraviolet spectra (A and B), however. Therefore detailed ultraviolet dose-response curves for systemic suppression of contact hypersensitivity in two mouse strains were determined to examine the relative contributions of each of these spectral components of sunlight to primary and secondary immunity. Whereas ultraviolet B caused a linear dose-related immunosuppression in both C57BL/6 and Balb/c mice, only C57BL/6 mice were immunosuppressed by medium doses of ultraviolet A. At higher ultraviolet A doses, C57BL/6 mice were protected from immunosuppression, suggesting a genetic predisposition to ultraviolet-A-induced immunomodulation. Surprisingly, we found that, in contrast to primary immunosuppression, low dose ultraviolet A enhanced the secondary immune response, whereas ultraviolet B caused antigen-specific tolerance. When ultraviolet A and ultraviolet B were combined to mimic sunlight (solar-simulated ultraviolet), immunosuppression and tolerance were only observed over a narrow dose range as the memory-enhancing effect of low dose ultraviolet A and the immunoprotective effect of higher dose ultraviolet A prevented the suppressive effects of ultraviolet B. These studies suggest that complex relationships between ultraviolet dose, immunomodulation, spectra, and genetic background are likely to be important for skin cancer induction. We also describe for the first time that low doses of ultraviolet A are able to enhance secondary immunity, which has important implications for vaccination strategies.

Interaction of UVB-absorbing sunscreen ingredients with cutaneous molecules may alter photoimmune protection

Studies of the photoimmunoprotective properties of sunscreens have produced disparate results. In this study in hairless mice, we compared two UVB absorbers, 2-ethylhexyl-p-methoxycinnamate (2-EHMC) and octyl-N-dimethyl-p-aminobenzoate (o-PABA), individually formulated in a common base lotion with a sunburn protection factor of 6. We measured their capacity to protect against suppression of the contact hypersensitivity (CHS) induced by three daily exposures of the dorsum to 6x the minimal erythemal/edematous dose (MED) of solar-simulated UV radiation (SSUV), in comparison with base lotion-treated mice exposed to 3 x 1 MED of SSUV. All treatments produced a similar minimal erythema. CHS was equally suppressed in mice irradiated through o-PABA and base lotion, but the suppression was significantly reduced in mice irradiated through 2-EHMC. Neither UVB absorber inhibited the epidermal photoisomerization to the immunosuppressive mediator, cis-urocanic acid. However, when mice were treated with exogenous cis-urocanic acid topically on the dorsum, but not when injected subcutaneously on the abdomen, suppression of CHS was observed in o-PABA- and base lotion-treated mice, but not in 2-EHMC-treated mice. Thus, the enhanced immunoprotection in mice irradiated through 2-EHMC apparently resulted from the direct inactivation of epidermal cis-urocanic acid by 2-EHMC. We conclude that comparative assessment of photoimmunoprotection by UV absorbers requires SSUV, erythemally matched exposures and consideration of potential interactions with cutaneous molecules.

Broad-spectrum sunscreens provide better protection from the suppression of the elicitation phase of delayed-type hypersensitivity response in humans

It is well established that ultraviolet radiation has immunomodulatory effects that may be involved in skin cancer. Recent studies have shown that ultraviolet A radiation (320-400 nm) as well as ultraviolet B (290-320 nm) is immunosuppressive. This means sunscreens that mainly absorb ultraviolet B (protection against erythema) may be less effective in preventing ultraviolet radiation-induced immunosuppression than broad-spectrum products. We have studied the effects of ultraviolet A exposure on the human delayed-type hypersensitivity response and compared the efficacy of sunscreens having different levels of ultraviolet A protection under both solar-simulated radiation and outdoor real-life solar exposure conditions. Delayed-type hypersensitivity was assessed using recall antigens. In a first study, two groups of volunteers were exposed to ultraviolet A (either full spectrum ultraviolet A or ultraviolet A1) without prior application of sunscreen and they were shown to exhibit significantly reduced delayed-type hypersensitivity responses. In order to compare the efficacy of sunscreens in preventing photoimmunosuppression, three groups of subjects received 10 cumulative exposures to solar-simulated radiation; one group was exposed unprotected and the other two were exposed after being applied either a ultraviolet B or a broad-spectrum sunscreen, each with the same sun protection factor 9, but with different ultraviolet A protection factors 9 and 2. Then, an outdoor study was conducted in which delayed-type hypersensitivity was assessed before and after six daily exposures. Two different groups of subjects were treated with one of two sunscreens having the same sun protection factor 25 but different ultraviolet A-protection factors. In unprotected volunteers, responses to delayed-type hypersensitivity tests were significantly reduced irrespective of ultraviolet exposure conditions (full spectrum ultraviolet A, ultraviolet A1, solar-simulated radiation). The ultraviolet B sunscreen failed to protect from solar- simulated radiation-induced immunosuppression. In contrast, the broad-spectrum sunscreen having the same sun protection factor but providing high protection in the ultraviolet A range significantly reduced local ultraviolet-induced immunosuppression and prevented the distal effects. In the outdoor study, as compared with delayed-type hypersensitivity responses obtained before sun exposure, no alteration of immune response was detected when the skin was protected by broad-spectrum sunscreen sun protection factor 25 and ultraviolet A-protection factor 14. Conversely, a broad-spectrum sunscreen sun protection factor 25 ultraviolet A-protection factor 6 failed to protect against the sun-impaired response. The above studies clearly demonstrate the role of ultraviolet A in the induction of photoimmunosuppression together with the need for sunscreen products providing efficient photoprotection throughout the entire ultraviolet spectrum.

Role for tumour necrosis factor-alpha receptors in ultraviolet-induced skin tumours

The biological effects of tumour necrosis factor (TNF)-alpha are mediated through either the TNFR1 or the TNFR2 receptor. In the present study, the effects of ultraviolet (UV) irradiation on skin pathology and tumour promotion were studied in hairless mice deficient in either the TNFR1 or the TNFR2 receptor. SKH-1 hairless mice were crossed with either TNFR1 knockout (KO) mice or TNFR2 KO mice to develop a strain of hairless mice deficient in either of these receptors. Elastosis and other pathological indications of UVB irradiation were not affected by the loss of either receptor. The absence of either receptor, however, resulted in a highly significant reduction in skin tumours in response to UVB irradiation. Inflammatory cell influx following chronic UV irradiation was virtually eliminated in the TNFR1 KO mice, while the TNFR2 KO mice responded to UV irradiation with the normal increase in inflammatory cells throughout the lower and upper dermis. Contact hypersensitivity responses were eliminated in the TNFR2 KO mice, whereas the TNFR1 KO mice retained normal contact hypersensitivity reactions. These studies suggest that TNF-alpha plays no part in the accumulation of excessive elastin in the skin during chronic UVB exposure. However, there appears to be an important role for TNF-alpha in mediating tumorigenesis, distinct from its role in initiating cutaneous immune responses.

Improved protection against solar-simulated radiation-induced immunosuppression by a sunscreen with enhanced ultraviolet A protection

Ultraviolet radiation-induced immunosuppression is thought to play a part in skin cancer. Several studies have indicated that sunscreens that are designed to protect against erythema failed to give comparable protection against ultraviolet radiation-induced immunosuppression. One possible reason for this discrepancy is inadequate ultraviolet A protection. This study evaluated the level of immunoprotection in mice afforded by two broad-spectrum sunscreens with the same sun protection factor, but with different ultraviolet A protection factors. Both sunscreens contained the same ultraviolet B and ultraviolet A filters, in the same vehicle, but at different concentrations. Solar simulated radiation dose-response curves for erythema, edema, and systemic suppression of contact hypersensitivity were generated and used to derive protection factors for each end-point. The results of three different techniques for determining immune protection factor were compared. A comparison of the two sunscreens showed that the protection factor for erythema in mice was similar to that determined in humans (sun protection factor) but the protection factor for edema in mice was lower. Both sunscreens protected against suppression of contact hypersensitivity but the product with the higher ultraviolet A-protection factor showed significantly greater protection. The three techniques for determining immunoprotection gave very similar results for a given sunscreen, but immune protection factor was always lower than sun protection factor. These data suggest that sun protection factor may not predict the ability of sunscreens to protect the immune system and that a measure of ultraviolet A protection may also be necessary.

Commercial sunscreen lotions prevent ultraviolet radiation-induced depletion of epidermal Langerhans cells in Skh-1 and C3H mice

There is much controversy regarding the ability of sunscreens to prevent ultraviolet (UV)-induced immune suppression. Epidermal Langerhans cells (LC) play a key antigen-presenting role in the afferent limb of the immune system's response to antigens introduced through the skin. It has been suggested that depletion of LC in UV-exposed skin is a critical step toward the induction of immunosuppression by UV radiation. There are a number of disparate reports with inconsistent results concerning the ability of sunscreens to prevent UV-induced depletion of LC. The purpose of this study was to systematically evaluate the ability of sunscreens to prevent UV-induced LC depletion in mice. Epidermal sheets obtained from skin biopsies taken from mice exposed to UV radiation from Kodacel-filtered FS20 sunlamps, which do not emit UV power at wavelengths < 290 nm, were immunoperoxidase stained for LC using a rat monoclonal antibody against mouse Ia (major histocompatibility complex class II antigen). Time course and dose-response curves for LC depletion were generated for Skh-1 and C3H mice. Dose-response curves for acute UV exposure induced depletion of LC in Skh-1 and C3H mice were similar, but not identical. LC density in the skin of Skh-1 mice that received chronic UV exposure (3 days/week for 8 weeks) was reduced by 62% after 2 weeks of exposure, but returned to normal levels by 6 weeks. Five commercial sunscreen lotions with labeled sun protection factors (SPF) of 4, 8, 15, 30 and 45 were tested for their capacity to block UV-induced depletion of LC. LC were depleted approximately 75% in the skin of unprotected or placebo lotion treated Skh-1 mice exposed to UV given on two consecutive days. Conversely, LC depletion was prevented in similarly UV exposed Skh-1 mice protected with a SPF 30 sunscreen. In C3H mice the levels of protection against LC depletion provided by the five sunscreens were proportional to the level of protection predicted by their labeled SPF. Comparisons of dose-response curves showed that significantly higher doses of UV were required for LC depletion and induction of skin edema than for the induction of local suppression of contact hypersensitivity. Thus, at UV doses where sunscreens provide complete protection against immunosuppression of contact hypersensitivity, prevention of LC depletion and skin edema would be expected.