Differential roles of T-cell subsets in regulation of ultraviolet radiation induced cutaneous photocarcinogenesis

Ultraviolet (UV) radiation, in particular the midwavelength range (UVB; 290-320 nm), is one of the most significant risk factors for the development of nonmelanoma skin cancer. UVB radiation-induced immunosuppression, which occurs in both humans and laboratory animals, contributes to their pathogenesis. However, there are conflicting reports on the relative role of CD4(+) and CD8(+) T cells in UVB induced skin cancer. The purpose of this study was to delineate the contribution of these two cell subpopulations to UVB induced immunosuppression and tumor development using C3H/HeN (WT), CD4 knockout (CD4(-/-) ) and CD8 knockout (CD8(-/-) ) mice. We observed that UVB induced skin carcinogenesis was retarded in terms of number of tumors per group, tumor volume and percentage of mice with tumors, in mice deficient in CD4(+) T cells compared with wild-type mice, whereas significantly greater (P < 0.05) numbers of tumors occurred in CD8(-/-) mice. These results indicate that, CD4(+) T cells promote tumor development while CD8(+) T cells have the opposite effect. Further, we found that CD4(+) T cells from tumor-bearing mice produced interleukin (IL)-4, IL-10, and IL-17 whereas CD8(+) T cells produced interferon-γ. Manipulation of T-cell subpopulations that are induced by UVB radiation could be a means of preventing skin cancers caused by this agent.     

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

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

Inflammation Due to Voriconazole‐induced Photosensitivity Enhanced Skin Phototumorigenesis in Xpa‐knockout Mice

Voriconazole is an antifungal agent and used as a prophylactic measure, especially in immunocompromised patients. However, there have been several reports of its adverse reactions, namely photosensitivity with intense inflammatory rashes and subsequent skin cancer development. To assess the effects of photosensitizing drugs voriconazole and hydrochlorothiazide (HCTZ ) on the enhancement of UV ‐induced inflammatory responses and UV ‐induced tumorigenesis, we utilized Xpa ‐knockout mice, which is DNA repair‐deficient and more susceptible to UV ‐induced inflammation and tumor development than wild‐type mice. Administration of voriconazole prior to broadband UVB exposure significantly upregulated multiple inflammatory cytokines compared with the vehicle‐ or HCTZ ‐administered groups. Voriconazole administration along with chronic UVB exposure produced significantly higher number of skin tumors than HCTZ or vehicle in Xpa ‐knockout mice. Furthermore, the investigation of UVB ‐induced DNA damage using embryonic fibroblasts of Xpa ‐knockout mice revealed a significantly higher 8‐oxo‐7,8‐dihydroguanine level in cells treated with voriconazole N‐oxide, a voriconazole‐metabolite during UV exposure. The data suggest that voriconazole plus UVB ‐induced inflammatory response may be related to voriconazole‐induced skin phototumorigenesis.     

The Effects of Ultraviolet Eye Irradiation on Dextran Sodium Sulfate‐Induced Ulcerative Colitis in Mice

Ultraviolet (UV) eye irradiation denatures the cells of the intestine. This study examined the action of UVA and UVB on dextran sodium sulfate (DSS)‐induced ulcerative colitis. We produced a mouse model of ulcerative colitis by administering DSS for 5 days and irradiated the eye with UVB or UVA for each day of the DSS treatment period. DSS‐induced ulcerative colitis was deteriorated by the UVB eye irradiation. Conversely, the symptoms improved with UVA eye irradiation. The levels of adrenocorticotropic hormone (ACTH), corticotropin‐releasing hormone (CRH), urocortin 2, interleukin (IL)‐18, IL‐6 and histamine in the blood increased after the UVB eye irradiation of DSS‐treated mice (UVB/DSS‐treated mice). In contrast, the β‐endorphin level in the blood of the UVA/DSS‐treated mice increased and the levels of urocortin 2, tumor necrosis factor (TNF)‐α and histamine decreased. Furthermore, in the colon, the expression of melanocortin‐2 receptors (MC2R) increased in the UVB/DSS‐treated mice, while the expression of μ‐opioid receptors increased in the UVA/DSS‐treated mice. When an ACTH inhibitor was administered, UVB eye irradiation caused the deterioration of DSS‐treated ulcerative colitis, while the effect of UV eye irradiation disappeared with a μ‐opioid receptor antagonist. These results suggested that UV eye irradiation plays an important role in DSS‐induced ulcerative colitis.     

UVB exposure impairs immune responses after hepatitis B vaccination in two different mouse strains

Ultraviolet light exposure can impair immune responses that are not restricted to the exposed skin but is also found at other sites, i.e. systemic immunosuppression. Therefore, we investigated the UV-induced modulating effects on vaccination against hepatitis B in a mouse model. Two different mouse strains, BALB/c and C57B1/ 6, were vaccinated intramuscularly against hepatitis B. Mice were exposed to different doses of ultraviolet B (UVB) for five consecutive days on shaved back skin before the vaccination. Vaccination against hepatitis B induced cellular (delayed-type hypersensitivity [DTH] and lymphocyte stimulation test) as well as humoral immune responses in both mouse strains. The DTH responses in C57BB1/6 mice were statistically significantly higher compared with BALB/c mice. UVB exposure induced a dose-dependent suppression of cellular immunity in both strains of mice. C57B1/6 mice seemed to be more susceptible to this suppression. Anti-hepatitis B surface antibodies (total-Ig) were only marginally suppressed after UVB exposure. IgG2a and interferon-gamma levels, both indicators for Th1 immune response, were suppressed in both mouse strains after UVB exposure. In summary, UVB exposure induced a dose-dependent suppression of both cellular and humoral immune responses after hepatitis B vaccination, although the suppressive effects on humoral immunity were limited to IgG2a production. Susceptibility to UVB-induced immunomodulation depended on the strain of mice and their predilection for developing different T cell responses.     

Dietary Feeding of Opuntia Humifusa Inhibits UVB Radiation‐Induced Carcinogenesis by Reducing Inflammation and Proliferation in Hairless Mouse Model

It has been validated that ultraviolet B (UVB) irradiation induced both squamous and basal cell carcinomas, as a tumor initiator and promoter. Opuntia humifusa is a member of the Cactaceae family which has been demonstrated in our previous study to have a chemopreventive effect in 7, 12‐dimethylbenz[a]anthracene and 12‐O‐tetradecanoylphorbol‐13‐acetate induced skin carcinogenesis models. Therefore, this study was designed to determine the protective effects of O.humifusa against photocarcinogenesis. O. humifusa was administrated to mice as a dietary feeding, following exposure to UVB radiation (180 mJ/cm²) twice a week of 30 weeks for skin tumor development in hairless mice. Dietary O.humifusa inhibited UVB‐induced epidermal hyperplasia, infiltration of leukocytes, level of myeloperoxidase and the levels of proinflammatory cytokines, tumor necrosis factor‐ α (TNF‐α), interleukin‐1β (IL‐1β) and interleukin‐6 (IL‐6), in UVB exposed skin. Also, O.humifusa significantly inhibited both protein and mRNA expression level of cyclooxygenase‐2 (COX‐2), nitric oxide synthase (iNOS), proliferating cell nuclear antigen (PCNA) and cyclin D1 compared to the non‐O.humifusa treated group. Collectively, these results suggest that O.humifusa could inhibit photocarcinogenesis in mouse skin and that protective effect is associated with the inhibition of not only UVB‐induced inflammatory responses involving COX‐2, iNOS and proinflammatory cytokines, but also the down‐regulation of UVB‐induced cellular proliferation.     

Fisetin Inhibits UVB‐induced Cutaneous Inflammation and Activation of PI3K/AKT/NFκB Signaling Pathways in SKH‐1 Hairless Mice

Solar ultraviolet B (UVB) radiation has been shown to induce inflammation, DNA damage, p53 mutations and alterations in signaling pathways eventually leading to skin cancer. In this study, we investigated whether fisetin reduces inflammatory responses and modulates PI3K/AKT/NFκB cell survival signaling pathways in UVB‐exposed SKH‐1 hairless mouse skin. Mice were exposed to 180 mJ cm^?2 of UVB radiation on alternate days for a total of seven exposures, and fisetin (250 and 500 nmol) was applied topically after 15 min of each UVB exposure. Fisetin treatment to UVB‐exposed mice resulted in decreased hyperplasia and reduced infiltration of inflammatory cells. Fisetin treatment also reduced inflammatory mediators such as COX‐2, PGE₂ as well as its receptors (EP1–EP4) and MPO activity. Furthermore, fisetin reduced the level of inflammatory cytokines TNFα, IL‐1β and IL‐6 in UVB‐exposed skin. Fisetin treatment also reduced cell proliferation markers as well as DNA damage as evidenced by increased expression of p53 and p21 proteins. Further studies revealed that fisetin inhibited UVB‐induced expression of PI3K, phosphorylation of AKT and activation of the NFκB signaling pathway in mouse skin. Overall, these data suggest that fisetin may be useful against UVB‐induced cutaneous inflammation and DNA damage.     

Effects and Mechanism of Nicotinamide Against UVA‐ and/or UVB‐mediated DNA Damages in Normal Melanocytes

Melanoma incidences are increasing rapidly, and ultraviolet (UV) radiation from the sun is believed to be its major contributing factor. UV exposure causes DNA damage in skin which may initiate cutaneous skin cancers including melanoma. Melanoma arises from melanocytes, the melanin‐producing skin cells, following genetic dysregulations resulting into hyperproliferative phenotype and neoplastic transformation. Both UVA and UVB exposures to the skin are believed to trigger melanocytic hyperplasia and melanomagenesis. Melanocytes by themselves are deficient in repair of oxidative DNA damage and UV‐induced photoproducts. Nicotinamide, an active form of vitamin B3 and a critical component of the human body's defense system has been shown to prevent certain cancers including nonmelanoma skin cancers. However, the mechanism of nicotinamide's protective effects is not well understood. Here, we investigated potential protective effects and mechanism of nicotinamide against UVA‐ and/or UVB‐ induced damage in normal human epidermal melanocytes. Our data demonstrated an appreciable protective effect of nicotinamide against UVA‐ and/or UVB‐ induced DNA damage in melanocytes by decreasing both cyclobutane pyrimidine dimers and 8‐hydroxy‐2′‐deoxyguanosine levels. We found that the photoprotective response of nicotinamide was associated with the activation of nucleotide excision repair genes and NRF2 signaling. Further studies are needed to validate our findings in in vivo models.     

Crosstalk Among UV‐Induced Inflammatory Mediators,DNA Damage and Epigenetic Regulators Facilitates Suppression of the Immune System

The suppression of the immune system by overexposure to ultraviolet (UV) radiation has been implicated in the initiation and progression of photocarcinogenesis. Numerous changes occur in the skin on UVB exposure, including the generation of inflammatory mediators, DNA damage, epigenetic modifications, and migration and functional alterations in the antigen‐presenting dendritic cells. Although each of these alterations can elicit a cascade of events that have the potential to modulate immune sensitivity alone, there is emerging evidence that there is considerable crosstalk between these cascades. The development of an understanding of UV‐induced changes in the skin that culminate in UV‐induced immunosuppression, which has been implicated in the risk of nonmelanoma skin cancer, as a network of events has implications for the development of more effective chemopreventive strategies. In the current review article, we discuss the evidence of interactions between the various molecular targets and signaling mechanisms associated with UV‐induced immunosuppression.     

Inhibition of UV-induced immune suppression and interleukin-10 production by plant oligosaccharides and polysaccharides

Application of Aloe barbadensis poly/oligosaccharides to UV-irradiated skin prevents photosuppression of delayed-type hypersensitivity (DTH) responses in mice. We tested the hypothesis that these carbohydrates belong to a family of biologically active, plant-derived polysaccharides that can regulate responses to injury in animal tissues. C3H mice were exposed to 5 kJ/m2 UVB from unfiltered FS40 sunlamps and treated with between 1 pg and 10 micrograms tamarind xyloglucans or control polysaccharides methylcellulose or dextran in saline. The mice were sensitized 3 days later with Candida albicans. Tamarind xyloglucans and purified Aloe poly/oligosaccharides prevented suppression of DTH responses in vivo and reduced the amount of interleukin (IL)-10 observed in UV-irradiated murine epidermis. Tamarind xyloglucans were immunoprotective at low picogram doses. In contrast, the control polysaccharides methylcellulose and dextran had no effect on immune suppression or cutaneous IL-10 at any dose. Tamarind xyloglucans and Aloe poly/oligosaccharides also prevented suppression of immune responses to alloantigen in mice exposed to 30 kJ/m2 UVB radiation. To assess the effect of the carbohydrates on keratinocytes, murine Pam212 cells were exposed to 300 J/m2 UVB radiation and treated for 1 h with tamarind xyloglucans or Aloe poly/oligosaccharides. Treatment of keratinocytes with immunoprotective carbohydrates reduced IL-10 production by approximately 50% compared with the cells treated with UV radiation alone and completely blocked suppressive activity of the culture supernatants in vivo. The tamarind xyloglucans also blocked UV-activated phosphorylation of SAPK/JNK protein but had no effect on p38 phosphorylation. These results indicate that animals, like plants, may use carbohydrates to regulate responses to environmental stimuli.     

Photo‐co‐carcinogenesis of Topically Applied Retinyl Palmitate in SKH‐1 Hairless Mice

Cosmetic products that contain retinyl palmitate are popular as antiaging skin treatments; however, recent studies suggest a risk for enhanced skin tumor development with topical retinyl palmitate applications and exposure to solar ultraviolet radiation (UVR). In this study, we investigated the potential of retinyl palmitate to enhance UVR‐induced photo‐co‐carcinogenesis. Groups of 36 male and 36 female SKH‐1 hairless mice were exposed to simulated solar light (SSL) and treated with the control cream or creams containing retinyl palmitate, 5 days per week for 40 weeks. Other groups of mice were exposed to SSL and received no cream treatment or received cream treatments and were exposed to ultraviolet‐A or ultraviolet‐B. Mice were monitored for the development of skin tumors, and the incidences and multiplicities of squamous cell neoplasia were determined by histopathology. In both the absence and presence of SSL, mice administered the control cream developed skin tumors earlier and had higher incidences and multiplicities of skin squamous cell neoplasms than mice that received no cream treatment. Compared to the control cream groups, mice exposed to SSL and administered the retinyl palmitate creams demonstrated earlier onsets of skin tumors and had increased incidences and multiplicities of squamous cell skin neoplasms.     

Requirement for Metalloproteinase‐dependent ERK and AKT Activation in UVB‐induced G1‐S Cell Cycle Progression of Human Keratinocytes

UVB (280–315 nm) in natural sunlight represents a major environmental challenge to the skin and is clearly associated with human skin cancer. Here we demonstrate that low doses of UVB induce keratinocyte proliferation and cell cycle progression of human HaCaT keratinocytes. Different from UVA, UVB irradiation induced extracellular signal‐regulated kinase (ERK) and AKT activation and their activation are both required for UVB‐induced cell cycle progression. Activation of epidermal growth factor receptor (EGFR) was observed after UVB exposure and is upstream of ERK/AKT/cyclin D1 pathway activation and cell cycle progression following UVB radiation. Furthermore, metalloproteinase (MP) inhibitor GM6001 blocked UVB‐induced ERK and AKT activation, cell cycle progression, and decreased the EGFR phosphorylation, demonstrating that MPs mediate the EGFR/ERK/AKT/cyclin D1 pathways and cell cycle progression induced by UVB radiation. In addition, ERK or AKT activation is essential for EGFR activation because ERK or AKT inhibitor blocks EGFR activation following UVB radiation, indicating that EGFR/AKT/ERK pathways form a regulatory loop and converge into cell cycle progression following UVB radiation. Identification of these signaling pathways in UVB‐induced cell cycle progression of quiescent keratinocytes as a process mimicking tumor promotion in vivo will facilitate the development of efficient and safe chemopreventive and therapeutic strategies for skin cancer.     

Platelet-activating factor does not mediate UVB-induced local immune suppression

The lipid mediator Platelet-activating factor (PAF) and oxidized glycerophosphocholine PAF agonists produced by UVB have been demonstrated to play a pivotal role in UVB-mediated systemic immunosuppression. Importantly, employing the ability of distant UVB irradiation to inhibit contact hypersensitivity (CHS) responses to the chemical antigen dinitrofluorobenzene (DNFB) to an area of unirradiated murine skin, we and others have demonstrated that UVB-mediated systemic immunosuppression was only observed in PAF-R expressing wild type (WT) mice and not in PAF-R-knockout (Pafr-/-) mice. As it is not known if PAF is involved in UVB-mediated local immunosuppression, these studies compared local UVB on CHS responses in WT versus Pafr-/- mice. We demonstrate that the application of DNFB onto UVB-exposed (locally) area of mouse skin resulted in a similar significant inhibition of subsequent CHS responses in both WT and Pafr-/- mice compared to sham-irradiated control mice. Furthermore, the expression of langerin, a marker for the presence of Langerhans cells was substantially reduced equally in the epidermal ears of UVB-irradiated WT and Pafr-/- mice compared to their respective sham control groups. These findings indicate that the PAF-R is not involved UVB-induced local immunosuppression.     

Resistance of a Lizard (the Green Anole, Anolis carolinensis; Polychridae) to Ultraviolet Radiation–induced Immunosuppression¶

The green anole (Anolis carolinensis) is the most northerly distributed of its Neotropical genus. This lizard avoids a winter hibernation phase by the use of sun basking behaviors. Inevitably, this species is exposed to high doses of ambient solar ultraviolet radiation (UVR). Increases in terrestrial ultraviolet-B (UV-B) radiation secondary to stratospheric ozone depletion and habitat perturbation potentially place this species at risk of UVR-induced immunosuppression. Daily exposure to subinflammatory UVR (8 kJ/m2/day UV-B, 85 kJ/m2/day ultraviolet A [UV-A]), 6 days per week for 4 weeks (total cumulative doses of 192 kJ/m2 UV-B, 2.04 x 10(3) kJ/m2 UV-A) did not suppress the anole's acute or delayed type hypersensitivity (DTH) response to horseshoe crab hemocyanin. In comparison with the available literature UV-B doses as low as 0.1 and 15.9 kJ/m2 induced suppression of DTH responses in mice and humans, respectively. Exposure of anoles to UVR did not result in the inhibition of ex vivo splenocyte phagocytosis of fluorescein labeled Escherichia coli or ex vivo splenocyte nitric oxide production. Doses of UV-B ranging from 0.35 to 45 kJ/m2 have been reported to suppress murine splenic/peritoneal macrophage phagocytosis and nitric oxide production. These preliminary studies demonstrate the resistance of green anoles to UVR-induced immunosuppression. Methanol extracts of anole skin contained two peaks in the ultraviolet wavelength range that could be indicative of photoprotective substances. However, the resistance of green anoles to UVR is probably not completely attributable to absorption by UVR photoprotective substances in the skin but more likely results from a combination of other factors including absorption by the cutis and absorption and reflectance by various components of the dermis.     

Characteristics of the immunosuppression induced by cutaneous photodynamic therapy: persistence, antigen specificity and cell type involved

Relatively little is known about the immunosuppression induced in mice which have received cutaneous photodynamic therapy (PDT). Consequently, experiments were undertaken using mice which received dorsal PDT using Photofrin as the photosensitizer in an attempt to characterize the overall nature of the immunosuppression. Photoirradiation of mice at various times after injection indicated there was no correlation between photosensitivity and immunosuppression. The suppression was found to be adoptively transferable and antigen specific suggesting the generation of suppressor cells. Selective cell depletions prior to adoptive transfer indicated a CD4+ T cell to be responsible for the immunosuppression. Interestingly, using allogeneic spleen cells, no effect on the delayed type hypersensitivity (DTH) response was found. The results indicate that the suppression induced by cutaneous PDT, with the exception of the lack of DTH suppression, is similar to that induced by UVB irradiation but unlike that reported using laser PDT of the peritoneal cavity. This suggests that not only the type of photoirradiation but also the site of photoirradiation might determine the character of the induced immunosuppression.     

Baicalin Protects Keratinocytes from Toll‐like Receptor‐4 Mediated DNA Damage and Inflammation Following Ultraviolet Irradiation

UVB radiation contributes to both direct and indirect damage to the skin including the generation of free radicals and reactive oxygen species (ROS), inflammatory responses, immunosuppression and gene mutations, which can ultimately lead to photocarcinogenesis. A plant‐derived flavonoid, baicalin, has been shown to have antioxidant, anti‐inflammatory and free radical scavenging activities. Previous studies from our laboratory have shown that in murine skin, Toll‐like receptor‐4 (TLR4) enhanced both UVB‐induced DNA damage and inflammation. The aim of this study was to investigate the efficacy of baicalin against TLR4‐mediated processes in the murine keratinocyte PAM 212 cell line. Our results demonstrate that treating keratinocytes with baicalin both before and after UV radiation (100 mJ cm^?2) significantly inhibited the level of intracellular ROS and decreased cyclobutane pyrimidine dimers and 8‐Oxo‐2′‐deoxyguanosine (8‐oxo‐dG)—markers of DNA damage. Furthermore, cells treated with baicalin demonstrated an inhibition of TLR4 and its downstream signaling molecules, MyD88, TRIF, TRAF6 and IRAK4. TLR4 pathway inhibition resulted in NF‐κB inactivation and down‐regulation of iNOS and COX‐2 protein expression. Taken together, baicalin treatment effectively protected keratinocytes from UVB‐induced inflammatory damage through TLR pathway modulation.     

A critical role for dermal mast cells in cis-urocanic acid-induced systemic suppression of contact hypersensitivity responses in mice

Many studies have implicated cis-urocanic acid (cis-UCA) in UVB-induced immunomodulation. The strongest evidence came from studies in mice whereby a cis-UCA antibody blocked UVB-induced suppression of delayed-type hypersensitivity responses. Furthermore, in several studies, the cis-UCA antibody at least partially reversed UVB suppression of contact hypersensitivity responses. Previous reports suggested that cis-UCA was immunomodulatory through its effects on keratinocytes, Langerhans cells, fibroblasts, T lymphocytes, natural killer cells and monocytes/macrophages. As dermal mast cells were recently demonstrated to be critical to UVB-induced systemic suppression of certain delayed-type and contact hypersensitivity responses, we investigated whether they were involved in the processes by which cis-UCA was immunomodulatory. Not only was there a correlation between dermal mast cell prevalence and the degree of susceptibility of different strains of mice to the immunomodulatory effects of cis-UCA, there was also a functional link. Mast cell-depleted Wf/Wf mice were rendered susceptible to immunomodulation by cis-UCA injected subcutaneously only after their dorsal skin had been reconstituted with bone marrow-derived mast cell precursors. These studies suggest that mast cells are critical to the processes by which cis-UCA suppresses systemic contact hypersensitivity responses to the hapten, trinitrochlorobenzene, in mice.     

IL-10 does not play a role in cutaneous Photofrin photodynamic therapy-induced suppression of the contact hypersensitivity response.

Photodynamic therapy (PDT) treatment of both malignant and benign skin diseases has proven to be effective, and its use is increasing worldwide. However, preclinical studies using murine models have shown that PDT of the skin inhibits cell-mediated immune reactions, as measured by the suppression of the contact hypersensitivity (CHS) reaction. We have previously demonstrated that PDT enhances IL-10 expression in treated skin, and that the kinetics of induction of IL-10 is similar to the kinetics of suppression of systemic CHS reactions by cutaneous PDT. In the following report we have expanded upon these studies to demonstrate that cutaneous PDT, using Photofrin, induces elevated levels of systemic IL-10 that persist for at least 28 days following treatment. The increase in systemic IL-10 correlates to a prolonged suppression of CHS of at least 28 days following cutaneous PDT. IL-10 has been implicated as the causative agent in the suppression of cell-mediated immune reactions by UVB and transdermal PDT. However, in the studies reported here we demonstrate that the suppression of CHS by cutaneous PDT occurs via an IL-10 independent mechanism, as administration of anti-IL-10 antibodies had no effect on the ability of PDT to induce CHS suppression. These results were further confirmed using IL-10 knockout (KO) mice. Cutaneous PDT of IL-10 KO mice resulted in CHS suppression that was not significantly different from suppression induced in wild-type mice. Thus, it appears as though IL-10 does not play a role in CHS suppression by cutaneous PDT. Suppression of cell-mediated immune reactions by UVB and transdermal PDT is reversible by IL-12, which is critical for the development of these reactions. We show that administration of exogenous IL-12 is also able to reverse CHS suppression induced by cutaneous PDT, suggesting that whereas suppression of cell-mediated immune reactions by UVB, transdermal PDT and cutaneous PDT occurs via different mechanisms, a common regulatory point exists.