Ultraviolet a radiation suppresses an established immune response: implications for sunscreen design.

The ultraviolet radiation present in sunlight is the primary cause of nonmelanoma skin cancer and has been implicated in the development of cutaneous malignant melanoma. In addition, ultraviolet is immune suppressive and the suppression induced by ultraviolet radiation has been identified as a risk factor for skin cancer induction. Ultraviolet also suppresses the immune response to infectious agents. In most experimental models, ultraviolet is applied to immunologically naive animals prior to immunization. Of equal concern, however, is the ability of sunlight to suppress established immune reactions, such as the recall reaction in humans, which protects against microbial infections. Here we demonstrate that solar-simulated ultraviolet radiation, applied after immunization, suppresses immunologic memory and the elicitation of delayed-type hypersensitivity. Further, we found that wavelengths in the ultraviolet A region of the solar spectrum were critical for inducing immune suppression. Ultraviolet A (320-400 nm) radiation was as effective as solar-simulated ultraviolet A + B (290-400 nm) in suppressing the elicitation of an established immune response. Irradiation with ultraviolet AI (340-400 nm) had no effect. Supporting a critical role for ultraviolet A in ultraviolet-induced immune suppression was the observation that applying a sunscreen that contained an ultraviolet B only filter had no protective effect, whereas, a sunscreen containing both ultraviolet A and ultraviolet B filters totally blocked ultraviolet-induced immune suppression. These data suggest that sunlight may depress the protective effect of prior vaccination. In addition, the observation that ultraviolet A is immunosuppressive indicates the need for ultraviolet A protection when designing sun protection strategies.

UV irradiation augments lymphoid malignancies in mice with one functional copy of wild-type p53.

Epidemiological studies have suggested an association between exposure to solar UV radiation and the incidence of lymphoid malignancies, which has increased substantially worldwide during the last two decades. Findings from animal studies have raised the question of whether UV radiation might influence the development of lymphoid malignancies by means of its immunosuppressive effect. In this study, we examined the effect of UV irradiation on the development of lymphoid malignancies in mice with no or only one functional copy of p53. Mice that lack both copies of p53 spontaneously develop high frequency of lymphoid malignancies in the thymus and spleen. p53 heterozygous mice with only one copy of the wild-type allele also develop lymphoid malignancies, but with a much lower frequency and a long latent period. In our study using mice of the C57BL/6 background, only one of the unirradiated mice lacking one copy of p53 (p53(+/-)) spontaneously developed a lymphoid tumor (6%), whereas 88% of UV-irradiated p53(+/-) mice developed lymphoid tumors in the spleen or liver. None of the control or UV-irradiated p53 wild-type mice developed lymphoid tumors during the 60-week observation period. Both UV-irradiated and unirradiated mice lacking both copies of p53 (p53(-/-)) rapidly developed thymic lymphomas and/or lymphoid tumors in spleen or liver. All of the lymphoid tumors tested were of T cell type. The immune responses of the mice to contact sensitization were identical and were suppressed to the same extent by UV irradiation regardless of the genotype. These results indicate that differences in immune reactivity do not account for the different effects of UV radiation on lymphoid malignancies and, in addition, that p53 is not required for generation of T cell-mediated immunity. Interestingly, whereas p53 mutations or loss of heterozygosity did not account for the accelerated development of lymphoid tumors in UV-irradiated p53(+/-) mice, deletions in the p16(INK4a) gene were quite common. These data provide the experimental evidence that UV irradiation induces lymphoid neoplasms in genetically susceptible mice and support the hypothesis that extensive sunlight exposure contributes to the induction of lymphoma in humans.

Persistence of immunogenic pulmonary metastases in the presence of protective anti-melanoma immunity.

We have developed a murine melanoma model that allows us to investigate the mechanisms by which spontaneous, immunogenic melanoma metastases escape immunological destruction in syngeneic mice. In the current study, we tested the hypothesis that loss of immunogenicity is an obligatory step in the persistence of pulmonary metastases. Fragments of syngeneic K1735-M2 tumor were implanted in the outer edge of one pinna per C3H/HeN mouse, and the growing tumors were removed 2-3 weeks later. Two weeks after removal of the tumors, the mice demonstrated effective T-cell-mediated immunity to s.c. challenge with K1735-M2 cells. However, lung metastases appeared in 23% of the immunized mice within 9-12 weeks after the initial tumor implantation. The expression of protective immunity to s.c. tumors required the presence of both CD4+ and CD8+ T cells. The immunized mice had specific CTLs capable of killing both K1735-M2 melanoma cells and the cells of nine independently derived melanoma metastases. Furthermore, K1735-M2 immunization protected these mice from s.c. tumor challenge with all nine metastatic cell lines. Our results demonstrate that the persistence of these metastases within the lung was not attributable to emergence of antigen-loss variants in immunized hosts. Our model provides an approach to investigate other mechanisms by which spontaneous metastases escape from immunological control and an opportunity to improve immunotherapy of melanoma metastases.

CD40-CD40 ligand interactions in vivo regulate migration of antigen-bearing dendritic cells from the skin to draining lymph nodes.

Whereas CD40-CD40 ligand interactions are important for various dendritic cell (DC) functions in vitro, their in vivo relevance is unknown. We analyzed the DC status of CD40 ligand -/- mice using a contact hypersensitivity (CHS) model system that enables multiple functions of DCs to be assessed in vivo. Immunohistochemistry of skin sections revealed no differences in terms of numbers and morphology of dendritic epidermal Langerhans cells (LCs) in unsensitized CD40 ligand -/- mice as compared with wild-type C57BL/6 mice. However, after contact sensitization of CD40 ligand -/- mice, LCs failed to migrate out of the skin and substantially fewer DCs accumulated in draining lymph nodes (DLNs). Furthermore, very few antigen-bearing DCs could be detected in the paracortical region of lymph nodes draining sensitized skin. This defect in DC migration after hapten sensitization was associated with defective CHS responses and decreased cutaneous tumor necrosis factor (TNF)-alpha production and was corrected by injecting recombinant TNF-alpha or an agonistic anti-CD40 monoclonal antibody. Thus, CD40-CD40 ligand interactions in vivo regulate the migration of antigen-bearing DCs from the skin to DLNs via TNF-alpha production and play a vital role in the initiation of acquired T cell-mediated immunity.

The Monodelphis melanoma model: initial report on large ultraviolet A exposures of suckling young.

The objective of this study was to determine whether exposure of early suckling young of the opossum Monodelphis domestica to ultraviolet A (UVA) radiation (320-400 nm) can lead to the development of melanocytic lesions similar to those induced after exposure to ultraviolet B (UVB) radiation (280-320 nm) to total doses as low as 380 J/m2. A total of 576 sucklings received nine exposures of 0.6, 2.6 or 15.5 kJ/m2 per dose (total doses approximately 6, 23 and 140 kJ/m2, respectively) from a Blak Ray lamp source with a narrow range emission at 365 nm. A further 280 sucklings were exposed in the same way to doses of 2.6 kJ/m2 per dose (total approximately 23 kJ/m2) broad-band UVA with visible wavelengths from a Dermalight lamp. Frequency of litter loss following all of the UVA-exposure protocols was similar to that within the same stocks in the colony at large. Only one of the 856 UVA-exposed individuals possessed a melanocytic lesion at the 5 month assessment point. No radiation-induced lesions of any type were evident on the skin of the other animals exposed as sucklings. The affected male was from a group of 70 individuals exposed to the highest total dose (140 kJ/m2) from the Blak Ray light source. The melanocytic hyperplasia was provisionally identified as a potential melanoma but it slowly regressed as the animal aged. We conclude that in the opossum suckling exposure system, the potency of UVA for melanoma induction is extremely low compared with that of UVB. Possible explanations, amenable to further investigations, are given for the low UVA sensitivity of the suckling model compared to the adult exposure model of Ley (Ley, R. D. [1997] Cancer Res. 57, 3682-3684).

Topical treatment with liposomes containing T4 endonuclease V protects human skin in vivo from ultraviolet-induced upregulation of interleukin-10 and tumor necrosis factor-alpha.

Exposing human skin to ultraviolet radiation causes DNA damage, sunburn, immune alterations, and eventually, skin cancer. We wished to determine whether liposomes containing a DNA repair enzyme could prevent any of the acute effects of irradiation when applied after ultraviolet exposure. Fifteen human patients with a prior history of skin cancer were exposed to two minimal erythema doses of ultraviolet radiation on their buttock skin. Liposomes containing T4 endonuclease V or heat-inactivated enzyme were applied immediately and at 2, 4, and 5 h after ultraviolet irradiation. Transmission electron microscopy after anti-T4 endonuclease V-staining and immunogold labeling on biopsies taken at 6 h after ultraviolet exposure revealed that the enzyme was present within cells in the skin. Immunohistochemical DNA damage studies suggested a trend toward improved DNA repair at the active T4 endonuclease V liposome-treated test sites. Although the active T4 endonuclease V liposomes did not significantly affect the ultraviolet-induced erythema response and microscopic sunburn cell formation, they nearly completely prevented ultraviolet-induced upregulation of interleukin-10 and tumor necrosis factor-alpha RNA message and of interleukin-10 protein. These studies demonstrate that liposomes can be used for topical intracellular delivery of small proteins to human skin and suggest that liposomes containing DNA repair enzymes may provide a new avenue for photoprotection against some forms of ultraviolet-induced skin damage.

p53 protects against skin cancer induction by UV-B radiation.

To assess the role of the p53 tumor suppressor gene in skin carcinogenesis by UV radiation, mice constitutively lacking one or both copies of the functional p53 gene were compared to wild-type mice for their susceptibility to UV carcinogenesis. Heterozygous mice showed greatly increased susceptibility to skin cancer induction, and homozygous p53 knockout mice were even more susceptible. Accelerated tumor development in the heterozygotes was not associated with loss of the remaining wild-type allele of p53, as reported for tumors induced by other carcinogens, but in many cases was associated with UV-induced mutations in p53. Tumors arose on the ears and dorsal skin of mice of all three genotypes, and homozygous knockout mice also developed ocular tumors, mainly melanomas. Skin tumors in the p53 knockout mice were predominately squamous cell carcinomas and were associated with premalignant lesions resembling actinic keratoses, whereas those in the heterozygous and wild-type mice were mainly sarcomas. These results demonstrate the importance of p53 in protecting against UV-induced cancers, particularly in the eye and epidermis.

Fas ligand: a sensor for DNA damage critical in skin cancer etiology.

DNA-damaged cells can either repair the DNA or be eliminated through a homeostatic control mechanism termed "cellular proofreading." Elimination of DNA-damaged cells after ultraviolet radiation (UVR) through sunburn cell (apoptotic keratinocyte) formation is thought to be pivotal for the removal of precancerous skin cells. Sunburn cell formation was found to be dependent on Fas ligand (FasL), a pro-apoptotic protein induced by DNA damage. Chronic exposure to UVR caused 14 of 20 (70 percent) FasL-deficient mice and 1 of 20 (5 percent) wild-type mice to accumulate p53 mutations in the epidermis. Thus, FasL-mediated apoptosis is important for skin homeostasis, suggesting that the dysregulation of Fas-FasL interactions may be central to the development of skin cancer.

Direct comparison of DNA damage, isomerization of urocanic acid and edema in the mouse produced by three commonly used artificial UV light sources.

Exposure to sunlight can result in a number of harmful effects, including sunburn, erythema, premature aging of the skin, immune suppression and skin cancer. Studies designed to understand the underlying mechanisms often depend upon the use of artificial sources of UV radiation. Unfortunately, conclusions from different laboratories using different lamps often conflict, and it is entirely possible that the different spectra of sunlights used in each may be a source of conflict. To minimize confounding variables, we employed two of the more commonly used UV light sources, fluorescent sunlamps, such as the FS-40 and Kodacel-filtered FS-40 sunlamps, and a xenon arc solar simulator and compared, in one series of standardized experiments, the effects of each light source on DNA damage, urocanic acid isomerization and edema formation. The dose-response curves, calculated by linear regression or curve fitting were compared. The data indicate that DNA damage and urocanic acid isomerization were more sensitive to shorter wavelengths of UV than longer wavelengths, and the biological endpoint of edema most closely correlated with the induction of DNA damage. The results emphasize the dominance of shorter wavelengths within the UV spectrum in damaging biological tissues, even when the solar simulator, which contains significant amounts of UVA, was used and demonstrate that each light source has a characteristic pattern of induction of biochemical and biological endpoints.

Inhibition of solar simulator-induced p53 mutations and protection against skin cancer development in mice by sunscreens.

We demonstrated previously that p53 mutations can be detected in ultraviolet B-irradiated mouse skin months before the gross appearance of skin tumors and that applying sun protection factor 15 sunscreens to mouse skin before each Kodacel-filtered FS40 sunlamp irradiation resulted in the reduction of such mutations. To determine whether there is an association between reduction of ultraviolet-induced p53 mutations by sunscreens and protection against skin cancer using an environmentally relevant light source, we applied sunscreens (sun protection factors 15-22) on to the shaved dorsal skin of C3H mice 30 min before each exposure to 4.54 kJ ultraviolet B (290-400 nm) radiation per m2 from a solar simulator. Control mice were treated 5 d per wk with ultraviolet only or vehicle plus ultraviolet. p53 mutation analysis indicated that mice exposed to ultraviolet only or vehicle plus ultraviolet for 16 wk (cumulative exposure to 359 kJ ultraviolet B per m2) developed p53 mutations at a frequency of 56%-69%, respectively, but less than 5% of mice treated with sunscreens plus ultraviolet showed evidence of p53 mutations. More importantly, 100% of mice that received a cumulative dose of 1000 kJ ultraviolet B per m2 only, or vehicle plus ultraviolet B developed skin tumors, whereas, the probability of tumor development in all the mice treated with the sunscreens plus 1000 kJ ultraviolet B per m2 was 2% and mice treated with sunscreens plus 1500 kJ ultraviolet B per m2 was 15%. These results demonstrate that the sunscreens used in this study not only protect mice against ultraviolet-induced p53 mutations, but also against skin cancers induced with solar-simulated ultraviolet. Because of this association, we conclude that inhibition of p53 mutations is a useful early biologic endpoint of photoprotection against an important initiating event in ultraviolet carcinogenesis.

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.

Molecular regulation of UVB-induced cutaneous angiogenesis.

We determined whether cutaneous angiogenesis induced by exposure of mice to ultraviolet-B (UVB) radiation is associated with an imbalance between positive and negative angiogenesis-regulating molecules. Unshaved C3H/HeN mice were exposed to a single dose (15 kJ per m2) of UVB. At various times, the mice were killed, and their external ears were processed for routine histology and immunohistochemistry. Antibodies against proliferating cell nuclear antigen and bromodeoxyuridine identified dividing cells. Antibodies against CD31/ PECAM-1 identified endothelial cells, and antibodies against basic fibroblast growth factor (bFGF), vascular endothelial growth factor/vascular permeability factor, and interferon-beta (IFN-beta) identified angiogenesis-regulating molecules. Epidermal hyperplasia was documented by 48 h and reached a maximum on day 7 after exposure to UVB. The expression of bFGF increased by 24 h, whereas the expression of IFN-beta decreased by 72 h after exposure to UVB. The expression of vascular endothelial growth factor/vascular permeability factor increased slightly after irradiation. The altered balance between bFGF and IFN-beta was associated with increased endothelial cell proliferation (bromodeoxyuridine + CD31 + cells) within existing blood vessels, leading to telangiectasia and new blood vessels. UV-induced epidermal hyperplasia and cutaneous angiogenesis were highest in IFN-alpha/beta receptor knockout mice. These results demonstrate that in response to UVB radiation, dividing keratinocytes produce a positive angiogenic molecule (bFGF) but not a negative angiogenic molecule (IFN-beta), and that this altered balance is associated with enhanced cutaneous angiogenesis.

Cellular target of UVB-induced DNA damage resulting in local suppression of contact hypersensitivity.

Experimental data are reviewed that lend support to the hypothesis that formation of DNA damage is the initiation event of local suppression of contact hypersensitivity (CHS) after exposure to ultraviolet (UV) radiation and that the antigen-presenting cell (APC) is an important target for this DNA damage.

Inhibition of UV-induced p53 mutations by sunscreens: implications for skin cancer prevention.

Ultraviolet (UV) radiation is a potent human carcinogen and it induces skin cancer in experimental animals. Recent studies have shown that unique mutations in the p53 tumor suppressor gene contribute to the development of human and mouse UV-induced skin cancers. Such mutations are also found in sun-damaged skin and actinic keratosis, suggesting that p53 mutations arise early during UV skin carcinogenesis. Our studies have shown that p53 mutations can be detected in UV-irradiated mouse skin months before the gross appearance of skin tumors, suggesting that p53 mutations can serve as a surrogate early biologic endpoint in skin cancer prevention studies. Indeed, application of sun protection factor 15 sunscreens to mouse skin before each UV irradiation resulted in an 88-92% reduction in the number of p53 mutations. Because p53 mutations represent an early essential step in photocarcinogenesis, these results imply that inhibition of this event may protect against skin cancer development.

Origin and characteristics of ultraviolet-B radiation-induced suppressor T lymphocytes.

Cutaneous exposure to low dose (2 kJ/m2) ultraviolet B radiation impairs the induction of contact hypersensitivity (CHS) responses to haptens applied to UV-irradiated skin and induces hapten-specific suppressor T lymphocytes (Ts). Cells collected from the draining lymph nodes of UV-irradiated, FITC-sensitized mice have impaired Ag-presenting activity and induce Ts cells upon injection into syngeneic recipients. This study investigates whether Ts cells originate in the UV-irradiated donor mice or are induced in lymph node cell recipients and the mechanism of suppression. Using congenic mice, we determined that the Ts cells in recipient animals were derived from T cells in the draining lymph nodes of the UV-irradiated donors. Cell lines and clones established from unirradiated and UV-irradiated, FITC-sensitized mice were CD4+, CD8-, TCR-alpha/beta+, MHC restricted, and hapten specific. The T cells proliferated in response to APC sensitized in vivo, but not to APC coupled in vitro with FITC. Cell lines from unirradiated mice were Th1 like, producing large amounts of IFN-gamma, but little IL-4 or IL-10, whereas cloned Ts cells from UV-irradiated mice produced IL-10, but no IL-4 or IFN-gamma. Ts cells blocked APC functions and IL-12 production in vitro. Injection of 5 x 10(4) cloned Ts cells into untreated recipients suppressed the induction of CHS. These results suggest that UV radiation can induce a distinct T regulatory type 1-like Ts population that may block the activation of Th1 cell-mediated immune responses.

p53 Mutations in hairless SKH-hr1 mouse skin tumors induced by a solar simulator.

In this study, we investigated whether the spectrum of p53 mutations in skin tumors induced in hairless SKH-hr1 mice by a solar simulator (290-400 nm) are similar to those found in skin tumors induced in C3H mice by UV radiation from unfiltered (250-400 nm) and Kodacel-filtered (290-400 nm) FS40 sunlamps. Analysis of tumor DNA for p53 mutations revealed that 14 of 16 (87.5%) SkH-hr1 skin tumors induced by the solar simulator contained mutations. Single C-->T transitions at dipyrimidine sequences located on the nontranscribed DNA strand were the most predominant type of p53 mutation. Remarkably, 52% of all p53 mutations in solar simulator-induced SKH-hr1 skin tumors occurred at codon 270, which is also a hotspot in C3H skin tumors induced by unfiltered and Kodacel-filtered FS40 sunlamps. However, T-->G transversions, which are hallmarks of UVA-induced mutations, were not detected in any of the solar simulator-induced skin tumors analyzed. These results demonstrate that the p53 mutation spectra seen in solar simulator-induced SKH-hr1 skin tumors are similar to those present in -unfiltered and Kodacel-filtered FS40 sunlamp-induced C3H skin tumors. In addition, our data indicate that the UVA present in solar simulator radiation does not play a role in the induction of p53 mutations that contribute to skin cancer development.

Health risks.

The health risks associated with ozone depletion will principally be those due to increased ultraviolet B (UV-B) radiation in the environment, i.e., increased damage to the eyes, the immune system, and the skin. Some new risks may also be introduced with the increased use of alternatives to the ozone-depleting substances (ODSs). Quantitative risk estimates are available for some of the UV-B-associated effects, e.g., cataract and skin cancer; however, the data are insufficient to develop similar estimates for effects such as immunosuppression and the toxicity of alternatives. Ocular damage from UV exposures includes effects on the cornea, lens, iris, and associated epithelial and conjunctival tissues. The most common acute ocular effect of environmental ultraviolet radiation (UVR) is photokeratitis. Also known as snowblindness in skiers, this condition also occurs in other outdoor recreationists. Chronic eye conditions likely to increase with ozone depletion include cataract, squamous cell carcinoma, ocular melanoma, and a variety of corneal/conjunctival effects, e.g., pterygium and pinguecula. Suppression of local (at the site of UV exposure) and systemic (at a distant, unexposed site) immune responses to a variety of antigens has been demonstrated in both humans and animals exposed to UV-B. In experiments with animals these effects have been shown to worsen the course/outcome of some infectious diseases and cancers. There is reasonably good evidence that such immunosuppression plays a role in human carcinogenesis; however, the implications of such immunosuppression for human infectious diseases are still unknown. In light-skinned populations, exposure to solar UVR appears to be the most important environmental risk factor for basal and squamous cell carcinomas and cutaneous melanoma. Originally it was believed that total accumulated exposure to UVR was the most important environmental factor in determining risk for these tumors. Recent information now suggests that only squamous cell carcinoma risk is related to total exposure. In the cases of both basal cell carcinoma and melanoma, new information suggests that increases in risk are tied to early exposures (before about age 15), particularly those leading to severe sunburns. Testing of a number of the chlorofluorocarbon (CFC) alternatives indicates that most of these chemicals have low acute toxicity, and low to moderate chronic toxicity. Some chemicals that were originally proposed as alternatives have been dropped from consideration because these tests raised concerns about toxicity and/or manufacturing difficulties. In one instance, high accidental occupational exposure was associated with liver damage, underlining the need for care in the use of these substitutes. Recent quantitative risk estimates have been developed for cataract, melanoma, and all skin cancers combined. These estimates indicate that under the Montreal Adjustments, cataract and skin-cancer incidence will peak mid-century at additional incidences of just under 3 per 100,000 and about 7 per 100,000, respectively.

Immune response associated with nonmelanoma skin cancer.

It is now clear that UV radiation causes nonmelanoma skin cancer in at least two ways: by causing permanent changes in the genetic code and by preventing immunologic recognition of mutant cells. These are interacting rather than separate mechanisms. Damage to DNA results in disregulation of cellular proliferation and initiates immune suppression by stimulating the production of suppressive cytokines. These cytokines contribute to the loss of immunosurveillance. Ultraviolet radiation has both local and systemic immunosuppressive effects. Locally, it depletes and alters antigen-presenting LC at the site of UV irradiation. Systemic suppression results when Ts cells are induced, by altered LC, by inflammatory macrophages that enter the skin following UV irradiation, or by the action of cytokines. Damage to DNA appears to be one of the triggering events in inducing systemic immunosuppression via the release of immunosuppressive cytokines and mediators. Immunologic approaches to treating skin cancers so far have concentrated on nonspecifically stimulating immune cells that infiltrate these tumors, but induction of specific immune responses against these tumors with antitumor vaccines has received little attention as yet. Preventive measures include sun avoidance and the use of sunscreens to prevent DNA damage by UV light. Future strategies may employ means to reverse UV-induced immunosuppression by using anti-inflammatory agents, biologicals that accelerate DNA repair or prevent the generation of immunosuppressive cytokines, and specific immunotherapy with tumor antigens. New approaches for studying the immunology of human skin cancers are needed to accelerate progress in this field.

The inhibition of antigen-presenting activity of dendritic cells resulting from UV irradiation of murine skin is restored by in vitro photorepair of cyclobutane pyrimidine dimers.

Exposing skin to UVB (280-320 nm) radiation suppresses contact hypersensitivity by a mechanism that involves an alteration in the activity of cutaneous antigen-presenting cells (APC). UV-induced DNA damage appears to be an important molecular trigger for this effect. The specific target cells in the skin that sustain DNA damage relevant to the immunosuppressive effect have yet to be identified. We tested the hypothesis that UV-induced DNA damage in the cutaneous APC was responsible for their impaired ability to present antigen after in vivo UV irradiation. Cutaneous APC were collected from the draining lymph nodes of UVB-irradiated, hapten-sensitized mice and incubated in vitro with liposomes containing a photolyase (Photosomes; Applied Genetics, Freeport, NY), which, upon absorption of photoreactivating light, splits UV-induced cyclobutane pyrimidine dimers. Photosome treatment followed by photoreactivating light reduced the number of dimer-containing APC, restored the in vivo antigen-presenting activity of the draining lymph node cells, and blocked the induction of suppressor T cells. Neither Photosomes nor photoreactivating light alone, nor photoreactivating light given before Photosomes, restored APC activity, and Photosome treatment did not reverse the impairment of APC function when isopsoralen plus UVA (320-400 nm) radiation was used instead of UVB. These controls indicate that the restoration of APC function matched the requirements of Photosome-mediated DNA repair for dimers and post-treatment photoreactivating light. These results provide compelling evidence that it is UV-induced DNA damage in cutaneous APC that leads to reduced immune function.

Sunlight and skin cancer: inhibition of p53 mutations in UV-irradiated mouse skin by sunscreens.

UV-induced mutations in the p53 tumor suppressor gene play an essential role in skin cancer development. We report here that such mutations can be detected in UV-irradiated mouse skin months before the gross appearance of skin tumors. Application of SPF-15 sunscreens to mouse skin before each UV irradiation nearly abolished the frequency of p53 mutations. These results indicate that p53 mutation is an early event in UV skin carcinogenesis and that inhibition of this event may serve as an early end point for assessing protective measures against skin cancer development.