Increased cyclooxygenase expression and thymine dimer formation after repeated exposures of humans to low doses of solar simulated radiation

The impact of repeated doses of solar simulated radiation (SSR) has not been evaluated, particularly to determine if photoadaptation and photoprotection develop over time. In this study, erythema, pigmentation, cyclooxygenase (COX)-1 and 2 expression and thymine dimer (dTT) formation were evaluated in the skin of irradiated subjects of phototype II or III. Groups of 7-10 volunteers were whole-body irradiated with a low dose of SSR on each of 10 consecutive days followed by a single erythemal ultraviolet B (UVB) dose on a small body area, or irradiated only with the single erythemal UVB dose on a small body area, or irradiated with the low dose of SSR on each of 30 consecutive days, or were unirradiated. Erythema and pigmentation were measured 24 h after the final SSR or UVB, and skin biopsies collected for the assessment of COX(+) cells and dTT(+) nuclei. The repeated SSR exposures induced a small increase in pigmentation without erythema, and were slightly protective against the erythemal effects of the subsequent high UVB dose. The number of COX-1(+) and 2(+) cells increased as a result of 10-days SSR and rose still further after 30-days SSR, indicating that photoadaptation had not developed. The SSR exposures did not result in any protection against the further increase in COX-1 and 2 expression caused by the erythemal UVB dose. In contrast, for dTT formation, the repeated SSR exposures led to a limited degree of both photoadaptation and photoprotection.     

Suberythemal ultraviolet B radiation alters the expression of cell cycle-related proteins in the epidermis of human subjects without leading to photoprotection

Background  Deregulation of the cell cycle proteins is one of the critical factors leading to cutaneous carcinogenesis.
Objectives  To monitor the expression of cell cycle proteins in the epidermis of subjects after repeated exposure to ultraviolet (UV) B radiation, and to test for the development of photoprotection by subsequent irradiation with a single erythemal UVB dose.
Methods  A total of 26 healthy volunteers were divided into four groups: group 1 ( n  =   9) were given whole-body UVB irradiation for 10 consecutive days with 0·7 minimal erythema dose (MED), group 2 ( n  =   9) were irradiated as in group 1 followed 24 h later by a single UVB dose of 3 MED on buttock skin, group 3 ( n  =   4) were irradiated with a UVB dose of 3 MED on buttock skin, and group 4 ( n  =   4) were not irradiated. Skin biopsies were collected 24 h after the final irradiation and stained for cyclins A, B1, D1, and p16, p18, p21, p27, p53, pRB, Bax and Bcl-2.
Results  The expression of cyclin D1, p18 and p21 was significantly higher in groups 1 and 2 compared with the nonirradiated group 4 controls and, in group 2, the expression of pRB, p53 and Bax was also increased. In group 3, only p53 and Bax proteins were significantly elevated compared with group 4. The expression of cyclin D1, p16, p18, p27, pRB and Bcl-2 was higher in group 2 compared with group 3.
Conclusions  Suberythemal UVB radiation was sufficient to cause changes in the expression of several epidermal cell cycle proteins. When tested by irradiation with a single erythemal UVB dose following the repeated exposures, no photoprotection against the UV-induced alteration in cell cycle protein expression was apparent.     

Lack of effect of repeated suberythemal ultraviolet-B exposures on human blood dendritic subtypes

Background/purpose: Dendritic cells (DC) play a major role in the afferent immune response. They are found as a minor cell population in the blood as three main subtypes that can be distinguished phenotypically: plasmacytoid DC (PDC), and myeloid DC1 and 2 (MDC1 and 2). The aim of the study was to examine the effect of repeated whole-body suberythemal ultraviolet (UV) B irradiation on the percentages of DC subsets in the blood and skin, and to test for photoadaptation by the subsequent administration of a local erythemal UV dose. Methods: Thirty subjects in each group were irradiated with either 0.7 personal minimal erythema dose (MED) UVB daily for 10 days (whole body), or for the 10 days followed by a single three MED UVB exposure of a local body site, or with the single three MED UVB exposure of a local body site only. Blood was collected before and after the exposures and the percentage of DC and DC subtypes assessed by flow cytometry. Skin biopsies were collected at the same times, and the number and position of the DC subsets examined by immunofluorescent microscopy. Results: The whole-body repeated UVB irradiations did not result in a change in the blood DC (BDC) or the subsets percentages in the blood, except that there was a small but significant rise in the percentage of the MDC2 subset. No alteration occurred following the local erythemal UVB exposure. The total number of BDC in the skin was small, with the PDC being located mainly in the dermis and the myeloid subtypes mainly in the epidermis. No change in cutaneous numbers or distribution was revealed following the irradiation protocols. Conclusions: Repeated whole-body suberythemal UVB irradiation does not cause a change in BDC or BDC subsets in the blood or skin, except for a small increase in the percentage of MDC2 in the blood. Local erythemal UVB irradiation does not alter the BDC subsets in blood or skin.     

Effect of sunscreen application on UV-induced thymine dimers

BACKGROUND: Exposure to UV radiation is a major risk factor for skin cancer, including malignant melanoma. Photocarcinogenesis is caused largely by mutations at sites of incorrectly repaired DNA photoproducts, of which the most common is the thymine dimer. Over the past decade, controversy has arisen over the use of sunscreens to prevent UV-induced skin cancer. OBJECTIVES: To determine if daily application of a broad-spectrum sunscreen with a sun protection factor (SPF) of 15 protects human skin against UV-induced DNA damage as determined by the formation of thymine dimers after repeated exposures to simulated solar light and, if so, to determine whether daily applications are required to achieve this protective effect. METHODS: Over 4 consecutive days, an SPF 15 sunscreen was applied homogeneously to each of 4 buttock sites of 18 women 30 minutes before exposure to 2 minimum erythemal doses of UV radiation. Of these 4 sites, 1 was treated with SPF 15 daily, and the remaining 3 were treated on 3 of the 4 irradiation days, skipping application on day 2, 3, or 4. A fifth site served as the untreated control and was also not irradiated. The number of cells per square millimeter positive for thymine dimer formation was determined using immunohistochemical and image analyses. RESULTS: There was no significant difference in thymine dimer formation between nonirradiated and irradiated skin when application of sunscreen preceded each irradiation. However, when sunscreen application was omitted even once prior to irradiation, a statistically significant increase in thymine dimer formation was apparent. At 48 hours after irradiation of unprotected skin, 50% of epidermal dimers present 24 hours after irradiation had been removed; at 72 hours, more than 75% were gone. CONCLUSIONS: Our study indicates that regular use of a broad-spectrum sunscreen is effective in preventing a major form of UV-induced DNA damage. Irregular and inadequate use of sunscreen during exposure to UV radiation results in thymine dimer formation, which may lead to mutation and subsequent cancer development.     

Epidermal p53 response and repair of thymine dimers in human skin after a single dose of ultraviolet radiation: effects of photoprotection

A cellular p53 response, DNA repair enzymes and melanin pigmentation are important strategies utilized by skin keratinocytes against impairment caused by ultraviolet radiation (UVR). In this study a double-immunofluorescence technique was used to investigate UVR-induced thymine dimers and p53 protein simultaneously. Four healthy volunteers were irradiated on both sides of their buttock skin with a single dose of solar-simulating UVR. One side was pretreated with a topical sunscreen. Biopsies from different time-points were immunostained for visualization of thymine dimers, p53 and proliferation. One single physiological dose of UVR generated widespread formation of thymine dimers throughout the epidermis 4h after irradiation. The level of thymine dimers decreased over time and was followed by a p53 response in the same cells. A late proliferative response was also found. The formation of thymine dimers, the p53 response and the late proliferative response were partially blocked by topical sunscreen. Large inter-individual differences in the kinetics of thymine dimer formation and repair as well as in the p53 response were evident in both sunscreen-protected and unprotected skin.     

窄波UVB对大鼠表皮的损伤及遮光剂保护作用的研究

目的：探讨不同照射剂量的窄波UVB对表皮细胞DNA的损伤，以及遮光剂的保护作用。方法：对大鼠表皮照射不同剂量的紫外线，对部分表皮在照射前应用遮光剂，免疫组化以及HE染色观察表皮细胞中胸腺嘧啶二聚体和晒伤细胞产生情况。结果：正常表皮细胞未发现嘧啶二聚体和晒伤细胞，随着紫外线照射剂量的增加，嘧啶二聚体和晒伤细胞有增加的趋势，应用遮光剂可明显减少这种损伤作用。结论：紫外线照射剂量越大，对表皮细胞的潜在致癌作用越强，遮光剂对表皮细胞有明显保护作用。     

Topical calcineurin inhibitors decrease the production of UVB-induced thymine dimers from hairless mouse epidermis

BACKGROUND: An increased incidence of ultraviolet-light-related skin tumours is a well-known problem in patients undergoing posttransplantation immunosuppression with systemic calcineurin inhibitors such as cyclosporine A or tacrolimus. UV-related carcinogenesis as a consequence of long-term treatment of sun-exposed sites with topical calcineurin inhibitors is therefore of theoretical concern. RESULTS: In this study, we show that tacrolimus acts as a UVB filter when incorporated into liposome membranes. In hairless mice pretreated with 1% pimecrolimus cream, 0.1% tacrolimus ointment or vehicle, the amount of epidermal thymine dimers, measured 1 h after 1 J/cm2 of UVB irradiation, was decreased by 89, 84 and 47%, respectively, as compared to untreated mice. Forty-eight hours after UVB irradiation, 97, 89 and 93% of epidermal thymine dimer levels were removed in pimecrolimus-, tacrolimus- or vehicle-treated mice, respectively. In contrast, 69% of thymine dimers, originally present in much higher amounts than in treated mice, were removed from untreated controls. UVB-induced apoptosis was less pronounced in treated mice. CONCLUSION: Taken together, these results suggest that topical calcineurin inhibitors prevent DNA photodamage due to a filter effect of both vehicle and active components, whereas they do not affect the clearance of DNA photoproducts.     

Role of p38 MAPK in UVB-induced inflammatory responses in the skin of SKH-1 hairless mice

The p38 mitogen-activated protein kinase (MAPK) signaling pathway is activated by numerous inflammatory mediators and environmental stresses. We assessed the effects of ultraviolet B (UVB) on the p38 MAPK pathway and determined whether cyclooxygenase (COX)-2 expression is downstream of this kinase in the skin of UVB-irradiated SKH-1 mice. SKH-1 mice were irradiated with a single dose of UVB (360 mJ per cm2), and activation of the epidermal p38 MAPK pathway was assessed. UVB-induced phosphorylation of p38 MAPK occurred in a time-dependent manner. Phosphorylation of MAPK-activated protein kinase-2 (MAPKAPK-2) also was detected and correlated with an increase in its kinase activity. Phosphorylation of heat shock protein 27 (HSP27), a substrate for MAPKAPK-2, also was detected post-irradiation. Oral administration of the p38 inhibitor, SB242235, prior to UVB irradiation, blocked activation of the p38 MAPK cascade, and abolished MAPKAPK-2 kinase activity and phosphorylation of HSP27. Moreover, SB242235 inhibited expression of the pro-inflammatory cytokines interleukin (IL)-6 and KC (murine IL-8) and COX-2. Our data demonstrate that UVB irradiation of murine skin activates epidermal p38 MAPK signaling and induces a local pro-inflammatory response. Blockade of the p38 MAPK pathway may offer an effective approach to reducing or preventing skin damage resulting from acute solar radiation.     

Contrasting effects of an ultraviolet B and an ultraviolet A tanning lamp on interleukin-6, tumour necrosis factor-α and intercellular adhesion molecule-1 expression

BACKGROUND: Recent studies have demonstrated that a tanning lamp emitting predominantly ultraviolet (UV) A induces significant yields of the type of potentially mutagenic DNA damage that are associated with the onset of skin cancer (i.e. cyclobutane pyrimidine dimers). UV-induced immunosuppression is also an important event leading to skin cancer. OBJECTIVES: To the modulation of key immunological molecules following exposure to a broad-spectrum UVB lamp and a predominantly UVA-emitting tanning lamp using model in vitro systems. METHODS: We compared secretion and mRNA expression of interleukin (IL)-6 and tumour necrosis factor (TNF)-alpha in normal human epidermal keratinocytes, and interferon (IFN)-gamma-induced intracellular adhesion molecule (ICAM)-1 in normal human fibroblasts irradiated in vitro with a broad-spectrum UVB lamp or with a Philips 'Performance' tanning lamp. RESULTS: With broad-spectrum UVB irradiation, upregulation of IL-6 and TNF-alpha mRNA was detected 6 h after irradiation, and a dose-dependent increase of cytokines in the supernatants of irradiated cells was found 24 h after irradiation. In contrast, there was no cytokine secretion and little evidence for mRNA upregulation following exposure to a tanning lamp. When cells were exposed first to broad-spectrum UVB, then the tanning lamp, UVB-induced cytokine secretion was inhibited, although mRNA levels were upregulated to a level close to that observed with UVB alone. By using a Schott WG 320 nm filter to attenuate the level of UVB relative to UVA emitted by the tanning lamp, the inhibition of cytokine secretion was shown to be associated with UVA exposure. Both UV sources inhibited IFN-gamma-induced ICAM-1 mRNA expression in a dose-dependent fashion. By using a Schott WG 335 nm filter, inhibition of ICAM-1 mRNA expression by the tanning lamp was shown to be associated with UVB exposure. CONCLUSIONS: These results suggest that UV sources emitting different levels of UVA and UVB have differential effects on the modulation of different immunoregulatory molecules, and indicate that there are potential interactions between these wavelengths.     

UVB radiation-mediated inhibition of contact hypersensitivity reactions is dependent on the platelet-activating factor system

Through its ability to both induce immunosuppression and act as a carcinogen, UVB radiation plays a major role in cutaneous malignancies. Recent studies have indicated that UVB-mediated inhibition of delayed-type hypersensitivity reactions is mediated, in part, by the lipid mediator platelet-activating factor (PAF). The objective of this study was to further define the mechanism by which UVB inhibits contact hypersensitivity (CHS) reactions. UVB irradiation resulted in an inhibition of subsequent CHS to the chemical DNFB in wild-type, but not in PAF-R-deficient mice. UVB-mediated inhibition of CHS was also blocked by a cyclooxygenase-2 (COX-2) inhibitor or a neutralizing antibody directed against IL-10. UVB irradiation upregulated IL-10 mRNA levels in lymph nodes and spleen only to significant levels in PAF-R-expressing mice. Bone marrow transplantation studies demonstrated that UVB-mediated immunomodulatory effects were dependent on PAF-R-positive bone marrow. These studies suggest that UVB irradiation results in epidermal production of PAF agonists, which then act on PAF-R-positive bone marrow-derived cells to upregulate IL-10 through COX-2-generated prostaglandins.     

Acute effects of UVB radiation on the proliferation and differentiation of keratinocytes

PURPOSE: The effects of UVB radiation on the proliferation and differentiation of epidermal keratinocytes were investigated with respect to timing, dosage, and repeated exposures. METHODS: Nine healthy volunteers were placed into three subgroups and exposed to UVB radiation on buttock skin using a Waldmann UV 800 unit fitted with Philips TL-20W/12 fluorescent lamps. Three volunteers were given 2 MED of UVB and biopsied at: pre-exposure, 24, 48 and 72 h after UVB exposure. For three volunteers, 1 MED, 2 MED, 3 MED of UVB were applied. After 48 h, biopsies were taken from non-irradiated and irradiated sites. Finally, three volunteers received 1 MED of UVB daily for 5 days, and the non-irradiated and irradiated sites were biopsied 48 h after the final exposure. The expression of proliferation and differentiation markers by keratinocytes were detected by immunohistochemical staining, and the results were analysed quantitatively by image analysis. RESULTS: The expression of proliferation and differentiation markers was observed prominently 48 h after irradiation. Higher doses of UVB caused an increase in proliferation and differentiation marker expression. Repeated exposures potentiated the effect of UVB radiation. CONCLUSION: UVB irradiation concomitantly promotes epidermal proliferation and differentiation. Responses were maximal 48 h after irradiation. This effect of UVB increases linearly according to dose and repetition.     

Solar-simulated skin adaptation and its effect on subsequent UV-induced epidermal DNA damage

Repeated skin exposure to ultraviolet radiation leads to increased tolerance for erythema. Whether this tolerance is accompanied by a significant protection against epidermal DNA injury has never been thoroughly investigated. In a first set of experiments we irradiated 25 healthy volunteers three times a week for 3 wk using solar-simulating tanning lamps. In addition, all individuals were exposed to a (challenge) dose of three times the initial minimal erythema dose on a small area of skin before the first and after the final exposure. On both occasions, cyclobutane pyrimidine dimers were quantified in biopsies. As expected, repeated ultraviolet exposures resulted in increased epidermal pigmentation and thickness. The ultraviolet sensitivity for erythema decreased on average by 75%. The cyclobutane pyrimidine dimer formation was reduced on average by 60%. In a second set of experiments, with a group of 13 subjects, DNA repair kinetics were assessed. Within a period of 5 d after a single, slightly erythemal dose (1.2 minimal erythema dose), levels of cyclobutane pyrimidine dimer and p53-expressing cells were determined in skin biopsies. Both markers of DNA damage were elevated upon the single ultraviolet exposure and returned to background levels after 3-4 d. This information is important when trying to minimize the risk of DNA damage accumulation after repeated exposures during a tanning course.     

Effect of infrared radiation A on photoaged hairless mice harboring eumelanin and pheomelanin in the epidermis

Infrared radiation A (IRA) is absorbed by melanin and generates heat. Therefore, the effect of IRA could be well analyzed using skin, which contains melanin in the epidermis. Hairless mice harboring epidermal melanocytes that produce eumelanin, pheomelanin, or non‐melanin were generated by backcrossing K14‐stem cell factor mice, recessive yellow mice, and then albino hairless mice. High‐dose IRA was irradiated over 18 weeks after the establishment of photoaged mice by irradiation with ultraviolet B (UVB) three times a week for 14 weeks. Tumor formation was assessed every week. The formation of cyclobutane pyrimidine dimer and apoptotic cells by the irradiation of IRA and UVB was evaluated. Repetitive irradiation of IRA did not promote tumor formation in all types of mice. Pre‐irradiation of IRA to UVB, but not post‐irradiation, accelerated the elimination of cyclobutane pyrimidine dimers and enhanced apoptosis; these effects were most obvious in eumelanin‐producing mice. Real‐time polymerase chain reaction analysis showed downregulation of FLICE (cellular caspase 8)‐like inhibitory protein and B‐cell lymphoma‐extra large and upregulation of Bcl‐2‐associated X protein by UVB, but further enhancement of these molecules by pre‐irradiation of IRA was not observed. These results indicate that IRA does not confer the promotion of UVB‐induced carcinogenesis in photoaged mice harboring epidermal melanocytes and that photochemical reaction between IRA and melanin might be involved in the induction of apoptosis and the elimination of cyclobutane pyrimidine dimers by UVB. The enhancement of apoptosis by pre‐irradiation of IRA to UVB might be induced by mechanisms other than the modification of the mRNA expression of FLICE (cellular caspase 8)‐like inhibitory protein, B‐cell lymphoma‐extra large, and Bcl‐2‐associated X.     

IL-4 expression by neutrophils in psoriasis lesional skin upon high-dose UVB exposure

BACKGROUND: Upon a single high dose of UVB irradiation of psoriatic lesional skin, IFN-gamma expression is decreased, whereas IL-4 expression is enhanced. A similar type 1 to type 2 shift was found in dermal T cells derived from irradiated lesional skin as compared to unexposed lesional psoriatic skin. We have found recently that the IL-4 protein detected in situ upon UVB exposure of normal skin was not associated with T cells but with infiltrating neutrophils. OBJECTIVE: To determine which cell types express IL-4 in psoriatic skin after UVB irradiation. METHODS: Skin biopsies were obtained from healthy controls and psoriasis patients before and after local UVB exposure. Double immunohistochemical stainings were performed to determine the identity of IL-4-expressing cells. RESULTS: In the irradiated skin of both healthy controls and patients, IL-4-positive cells coexpressed elastase and CD15, but not CD3. CONCLUSION: IL-4-expressing cells found in psoriatic skin after a single high-dose UVB exposure appeared to be neutrophils.     

Narrowband ultraviolet B radiation suppresses contact hypersensitivity

Background/purpose: A main mechanism responsible for the efficacy of narrowband ultraviolet (UV)B is thought to be the induction of apoptosis in pathogenetically relevant cells. Narrowband UVB therapy, however, generally induces a relatively long remission period. Recently, evidence that UVB radiation induces regulatory T (Treg) cells was reported. Based on these findings, we examined whether narrowband UVB suppresses contact hypersensitivity (CHS) by inducing Treg cells. Methods: The shaved abdomens of C3H/HeN mice were irradiated with broadband or narrowband UVB. CHS was defined as an ear‐swelling response. To examine whether tolerance can be induced by adoptive transfer, lymph node cells from UVB‐irradiated mice were injected into naïve mice before sensitization and CHS challenge. Results: Narrowband UVB exposure dose dependently suppressed CHS. Significant suppression was observed at doses between 1000 and 3000 mJ/cm² (P<0.05). The suppressive effect achieved with 1000 mJ/cm² narrowband UVB was very similar to the effect achieved with 100 mJ/cm² broadband UVB. The suppressive effects on CHS were transferred to naïve mice by the injection of lymph node cells from tolerant mice. Conclusion: Narrowband UVB induced local and systemic suppression of CHS. In addition, narrowband UVB induces tolerance to CHS and the suppressive effects were transferable to naïve mice.     

UVB致成纤维细胞损伤及两种中药的保护作用研究

目的观察中波紫外线(UVB)辐射后成纤维细胞(FB)DNA光产物环丁烷嘧啶二聚体(CPD s)产生和清除情况及表没食子儿茶素没食子酸酯(EGCG)和黄芩甙的干预作用。方法以30,60,90 m J/cm2UVB照射FB并予EGCG及黄芩甙干预处理,采用免疫细胞化学法在照光后不同时间检测CPD s的产生和清除情况。结果细胞损伤程度随照光剂量加大而加重;30 m J/cm2UVB照射后细胞CPD s生成量在辐射后1 h左右达到高峰,同时细胞也开始清除CP-D s,辐射后4 h内清除速率较快,4 h后清除速率逐渐降低,至24 h基本清除CPD s;EGCG和黄芩甙处理UVB辐射的细胞CPD s少于单纯照光组(P<0.05)。结论UVB辐射可以导致FB的DNA损伤而产生光产物CPD s;细胞损伤程度显示剂量依赖性;细胞自身有修复能力;EGCG和黄芩甙均可降低UVB辐射所致的光产物水平。     

Whole-body UVB irradiation during allogeneic hematopoietic cell transplantation is safe and decreases acute graft-versus-host disease

Depletion of host Langerhans cells (LCs) prevents cutaneous graft-versus-host disease (GvHD) in mice. We analyzed whether UVB irradiation is tolerated during the course of human allogeneic hematopoietic cell transplantation and whether depletion of LCs by broadband UVB could improve GvHD outcome. A total of 17 patients received six whole-body UVB irradiations with 75% of the individually determined minimal erythemal dose after conditioning with a reduced intensity protocol. LCs, dermal dendritic cells (DCs), and macrophages were analyzed before and after UVB irradiation by immunohistochemical analysis. Circulating blood cells and serum factors were analyzed in parallel. In striking contrast to previous data, our irradiation protocol was well tolerated in all patients. UVB treatment decreased the number of LCs and also affected dermal DCs. UVB-treated patients also had significantly higher 25-hydroxyvitamin D3 serum levels and higher numbers of circulating CD4+ FoxP3+ regulatory T cells. Strikingly, nine out of nine patients with complete LC depletion (<1 LC per field) developed only grade I GvHD or no GvHD up to day 100. Our results strongly suggest that prophylactic UVB irradiation post transplant is safe and should be further explored as a clinical strategy to prevent acute (skin) GvHD.     

Repeated ultraviolet exposure affords the same protection against DNA photodamage and erythema in human skin types II and IV but is associated with faster DNA repair in skin type IV

We have investigated the photoprotective properties of induced pigmentation using erythema and epidermal DNA photodamage as endpoints. Previously unexposed buttock skin of 12 young, healthy adults (six skin type II and six skin type IV) was exposed daily (Monday to Friday) for 2 wk (days 1-12) with 0.65 minimal erythema dose of solar simulated radiation. Mean skin type IV minimal erythema dose was 1.8-fold greater than for skin type II. Compared to skin type II, solar simulated radiation treatments produced less erythema and more tanning in skin type IV. To assess DNA photodamage, biopsies were taken and prepared for paraffin sections that were stained with a monoclonal antibody for thymine dimers. Thymine dimers were quantified by image analysis. The single exposure data (0.65 and 2 minimal erythema dose) showed that DNA damage was related to physical dose (J per cm2) independent of skin type. Our data also showed that DNA photodamage accumulates in both skin types with repeated, suberythemal doses of solar simulated radiation. On day 12, there were more thymine dimers in skin type IV than skin type II, again indicating that physical rather than biologic dose determines the level of DNA damage. Comparisons on days 12 and 19, however, showed a much greater loss of thymine dimers in skin type IV, suggesting better thymine dimer repair. Protection factors for erythema and thymine dimers were calculated and shown to be about 2 in both skin types. This provides further indirect evidence that DNA is a chromophore for erythema, but also suggests that a tan may not be the major factor in natural photoprotection.