Migration of Langerhans cells into the epidermis of human skin grafted into nude mice.

In a previous study, it was demonstrated that human Langerhans cells (LC) are preserved in human skin grafted onto a nude mouse. Moreover, although it was observed that mouse LC of the host invade skin grafts from allogeneic mouse or rat, they do not penetrate in human skin grafts. In most of the human skin equivalent systems produced in vitro, LC appear to be lost. The present study was designed to investigate whether the mouse LC will repopulate a human skin equivalent. For this purpose, two different systems of skin equivalent have been grafted into the nude mouse. They were composed of human keratinocytes deposited on dead human dermis, or on lattice composed of human fibroblasts embedded in type I collagen. At different times after grafting, the presence of LC in the transplants was assayed either by indirect immunofluorescence or by electron microscopy. Indirect immunofluorescence was performed on frozen sections or on epidermal sheets with anti-Ia, anti-HLA-DR, or OKT6 antibodies. It was observed that, at 2 months after grafting, Ia(+) HLA-DR(-) OKT6(-) cells are present in grafted human epidermis. Moreover, LC with typical Birbeck granules are also detected by electron microscopy. It could be concluded, from this study, that mouse LC can repopulate human epidermis devoid of human LC.     

Effects of pimecrolimus versus triamcinolone on Langerhans cells after UV exposure

Abstract: Background/Purpose: Pimecrolimus is a topical immunomodulator for atopic dermatitis. Concerns regarding malignancy risk resulted in its black box warning in 2006. The purpose of this study is to determine the effects of pimecrolimus on Langerhans cells (LC), mediators of the cutaneous immunity UV‐irradiated skin. Methods: A RCT was conducted investigating pimecrolimus 1% cream vs triamcinolone 0.1% cream on UV‐irradiated epidermal LC on 20 healthy volunteers. Punch biopsies were stained with antibodies to CD1a, HLADR and CD83. Results: Triamcinolone caused more depletion in UV‐irradiated CD1a⁺ epidermis relative to pimecrolimus treatment. (P = 0.030). Using HLA‐DR as a pan‐marker for APCs, pimecrolimus caused marginally less depletion than triamcinolone (P = 0.013). Using anti‐CD83 as a maturation marker, UV‐irradiated skin treated with pimecrolimus showed more mature LC than skin treated with triamcinolone (P = 0.00090). Conclusion: UV‐induced changes in LC are minimally affected by pimecrolimus, compared with triamcinolone.     

Do OKT6 positive Langerhans cells play a role in the suppression of ultraviolet-induced carcinogenesis?

Langerhans cells (LC) in epidermis are antigen presenting cells. LC may play a role in immune surveillance system and are considered to suppress development of ultraviolet (UV) induced skin cancers. We studied effect of UVB irradiation to LC of xeroderma pigmentosum (XP) and normal subjects by using OKT6 monoclonal antibody. When 3 minimal erythema dose (MED) of UVB were irradiated, density of OKT6 positive LC of XP began to decrease 6 hours after irradiation, and showed the least numbers on day 2 and returned completely to the pre-irradiation level on day 14. Further, after 3 MED irradiation, LCs of both normal subjects became the least on day 3 and returned to the pre-irradiation level on day 14. In XP variant and normal subjects, the number of LC in chronic sun-exposed skin decreased significantly in a similar way comparing to that of non-exposed skin. These results suggest that epidermal LC may not play an essential role in prevention of UV-induced tumor development.     

Further evidence for the self-reproducing capacity of Langerhans cells in human skin

The limited number of Langerhans cells (LC) in the epidermis is one of the main reasons for the technical difficulties in resolving the question of LC kinetics. In the present paper, we describe a method to evaluate the LC replication potential in epidermis. The procedure is based on the specific incorporation of bromodeoxyuridine (BrdU), a thymidine analogue, into the DNA during the S-phase of the cell cycle. Mice, bearing human skin grafts, were injected s.c. every 6 h for up to 17 days with BrdU. At different times, the incorporated BrdU as well as the human epidermal LC were revealed on skin sections using anti-BrdU and OKT-6 monoclonal antibodies, respectively. After 6 h, 4.9% of the LC were labeled with BrdU. Then, the number of OKT-6(+) BrdU(+) cells increased in a linear manner and achieved 34% at 120 h, 67% at 240 h, and 94% at 400 h during the course of continuous labeling procedures. Based on this result we calculated a total cell cycle time of 392 h (16.3 days) and 12 h for the S-phase for human epidermal LC. Applying this technique, we were able to show also that 48 h after local treatment with 12-O-tetradecanoylphorbol-13-acetate or after stripping, the number of BrdU-labeled LC was considerably increased. Furthermore, after i.p. injection of colchicine in the nude mouse, human epidermal LC undergoing mitosis were evidenced by electron microscopy in the graft. From these results we conclude that the LC are actively cycling--therewith a self-reproducing cell population in human epidermis.     

Effect of depletion of epidermal dendritic cells on the induction of contact sensitivity in the guinea-pig

Guinea pig skin was depleted of Langerhans cells (LC) as assessed by ATPase and Ia staining using several techniques. The LCs were depleted either by tape-stripping or exposure of the animals to UV-B or UV-C radiation. Guinea-pigs were sensitized to 2,4-dinitrochlorobenzene (DNCB) by application of the sensitizer to the epidermis depleted of LC. Minimally suppressed contact reactions were found in animals exposed to both wavelengths of radiation, but this was shown to be a systemic rather than a local effect. Tape-stripping did not alter the degree of contact sensitivity when guinea-pigs were sensitized with a large dose of DNCB. When a non-sensitizing dose of DNCB was applied to the ear depleted of LC by tape-stripping, contact sensitivity resulted. Although the depletion of LCs was 97% following UV-B, 93% with UV-C and 78% after tape-stripping, at no time were LCs completely absent from the epidermis.     

Ultrastructural changes in epidermal Langerhans cells and melanocytes in response to ultraviolet irradiation, in Australians of Aboriginal and Celtic descent

The effects of exposure to small doses of artificial ultraviolet radiation (UVR) on the ultrastructure of epidermal Langerhans cells (LC) and melanocytes were studied in two groups of Australian subjects, one of Aboriginal and the other of Celtic descent. UV exposure induced an apparent depletion of LC in the epidermis of both groups. However, LC depletion in the Aboriginal subjects was associated with apoptosis, whereas organelle and membrane disruption in the LC of Celtic subjects suggested a reduction by direct cellular damage. LC in Aboriginal epidermis tended to become relocated at more superficial levels following UV exposure, and their Birbeck granules became more numerous. LC in Celtic epidermis appeared to become relocated in a basal location and contained fewer Birbeck granules. The central lamina of the Birbeck granules in Aboriginal LC, which was more electron-dense than that in Celtic subjects prior to UV treatment, was temporarily lost following treatment, while the ultrastructure of Birbeck granules in Celtic LC was unchanged. LC and 'indeterminate cells' in intimate association with lymphocyte-like cells occurred in the basal layer of Celtic epidermis 5 days after exposure. These complexes were not observed in Aboriginal epidermis although isolated lymphocyte-like cells were observed in the same location. Melanocytes in Aboriginal epidermis contained greater numbers of melanosomes than those in Celtic epidermis throughout the experiment. Inactive epidermal melanocytes in Celtic subjects initially responded to UV exposure with a slight increase in melanosome content followed by a substantial further increase, whereas active melanocytes in the Aboriginal subjects showed the opposite response. The implications of the different responses of LC and melanocytes in the two groups, in relation to immunological function of the epidermis and the marked racial difference in the incidence of skin cancer, are discussed. Cancer of the skin, particularly basal and squamous cell carcinoma, occurs primarily in people with fair skin who burn easily following exposure to ultraviolet radiation (UVR). In contrast, the incidence of skin cancer in inherently dark-skinned people is low. Melanin is synthesized by melanocytes in response to UVR and is thought to protect epidermal cells against damage to their genetic material by absorbing UVR and thereby reducing its penetration into the skin. Thus darkly pigmented skin is more resistant to the effects of UVR.(ABSTRACT TRUNCATED AT 400 WORDS)     

Role of Langerhans cells in epidermotropism of T cells

Summary Certain T lymphocytes display a specific affinity for the epidermis (epidermotropism). Recent studies have suggested that Ia⁺ Langerhans cells (LCs) are possible targets for the epidermotropism. A variety of self-Ia-reactive cloned T cells were tested for their ability to migrate into the epidermis following intradermal inoculation into the footpads of syngeneic mice. Clone BB5 was chosen as representative of the epidermotropic T cells. We investigated whether the depletion of Ia⁺ LCs from the epidermis by tape-stripping could alter the migration of BB5 cells into the epidermis. The epidermal invasion of BB5 cells was markedly impaired in those mice whose LCs were depleted by 95% after repetitive tape-stripping. Because production of epidermal-derived thymocyte activating factor (ETAF) by the epidermal cells was augmented after repetitive tape-stripping, the diminished migration of BB5 cells into tape-stripped epidermis did not result from a decrease in ETAF production which is thought to attract T cells chemotactically. These results suggest that Ia⁺ LCs may play an inductive role in the preferential migration of T cells into the epidermis.     

Melanocytes and Langerhans cells in aged versus young skin before and after transplantation onto nude mice

Previous studies have demonstrated decreased numbers of melanocytes and Langerhans cells (LC) in aged skin. In the present study, we employed dopa and indirect immunoperoxidase techniques in epidermal sheets to determine the fate of melanocytes and LC of aged versus young donors after skin transplantations onto nude mice. The detection of positive homologous leucocytic antibody reaction of degeneration (HLA-DR) of LC indicates an age-associated reduction in sun-protected thigh skin in aged versus young subjects (263 +/- 63 versus 589.25 +/- 142.643, p less than 0.001). The mean number of LC four weeks after transplantation remained almost constant. Prior to skin engraftment, a decreased number of melanocytes was found in aged versus young epidermis (160.77 +/- 51.7 versus 255.83 +/- 81.2, respectively, p less than 0.05). A significantly increased number of melanocytes was noted four weeks following engraftment in epidermis from aged (307.44 +/- 174, p less than 0.05) and young human donors (402.16 +/- 139, p less than 0.02). The marked increase in density of dopa-positive melanocytes following engraftment onto nude mice may indicate the existence of circulating factors in nude mice that perhaps both stimulates and enhances proliferation and activity of these cells.     

The steady-state turnover of murine epidermal Langerhans cells

We have investigated the steady-state turnover of murine epidermal Langerhans cells (LCs) using an X-irradiation model, 3H-thymidine autoradiography and cultured epidermal sheet explants, and by assessing the LC population in normal mice. The LC density after whole-body irradiation without any cutaneous shielding was not significantly different from that in skin shielded during whole-body irradiation (P > 0.05), indicating that the additional irradiation to the skin did not contribute to a decrease in LC density. In both instances, the LC number gradually decreased in a linear fashion. The results indicate that epidermal LCs continuously leave the epidermis and are continually replaced by circulating precursor cells from the bone marrow at a steady rate. Autoradiographic studies after a pulse injection of 3H-thymidine showed a labelling index of 0.013%, indicating that local mitosis is not an important contributor to the maintenance of the epidermal LC population. Although local X-irradiation resulted in temporary reduction of LC density, epidermal sheet explant culture obtained immediately after local X-irradiation showed no difference in LC density as compared with control unirradiated skin, indicating that the decrease in LC density was not due to significant LC destruction. From these data, we calculated that the half-life of murine LCs in the epidermis is approximately 9 days.     

Oxygen intermediates are involved in ultraviolet radiation-induced damage of Langerhans cells

Experiments were conducted to determine whether ultraviolet (UV) radiation exerts its effect through the generation of oxygen intermediates on Langerhans cells (LC). Guinea pigs were exposed to one single dose of UVB (0.9-2.7J/cm2), and biopsy specimens were taken 5 days after the irradiation. The population of LC was evaluated using ATPase-stained epidermal sheets. These exposures reduced the number of LC to 20-25% of the original density. On the other hand, superoxide dismutase (SOD) (0.02-0.2 mg), a scavenger of superoxide anion, which had been injected intradermally just before UV radiation, significantly prevented the depletion of LC, although not completely (37-40% of the original density). The injection immediately after the exposure was still significantly effective, but less so. Other scavengers of oxygen intermediates including catalase, D-mannitol, and L-histidine revealed no detectable effect. A single exposure of UVB at doses of 0.3-0.6 J/cm2 did not deplete the ATPase-positive LC. However, the same dose of UVB reduced the number of LC to 70%, when exposed after the injection of an SOD inactivator, diethyldithiocarbamate, possibly due to inactivation of physiologically existing SOD. These observations indicate that oxygen intermediates such as superoxide anion or its subsequent species are generated by UV radiation exposure and damage the epidermal LC.     

Long-term effects of local ionizing radiation treatment on Langerhans cells in mouse footpad epidermis

Langerhans cells (LC) were studied with ADPase histochemistry in sheets of hind footpad epidermis from groups of CBA/H mice. Single, local 20-Gy doses of 250-kV x-rays were administered to the right hind feet of the mice when they were 3-4 months old, and LC were counted at intervals ranging from 2 to 24 months later. In unirradiated mice, aged 5-19 months, the mean density of LC in footpad was 1521-1617 cells/mm2. It dropped to 1137 +/- 86 cells/mm2 (mean +/- SE) in untreated 28-month-old mice. At times from 2-15 months after irradiation, normal mean densities of LC were present in footpad epidermis. On average, LC numbers were subsequently reduced to 1078 +/- 65/mm2 by 19 months after irradiation (71% of the cells in age-matched controls) and to 789 +/- 53/mm2 by 24 months (59% of the cells in age-matched controls). Loss of cells was focal. Chronic radiation-induced fibrosis and damage to circulatory function in skin may have contributed to impaired replacement of LC from bone marrow precursors. The possibility that late radiation-related depletion of the LC population permits development of skin tumors as a delayed consequence of exposure to ionizing radiation is discussed.     

Low-dose ultraviolet-B irradiation depletes human epidermal Langerhans cells

We have examined the effects of low-dose monochromatic UVB irradiation (295±5 nm), biologically equivalent to that generally incident on the skin during a 12-session sun-bed course, on the expression of the CDla epidermal Langerhans cell surface marker in human skin in vivo. In five subjects, 1.5 minimal erythema doses (MEDs) at 295 nm depleted its expression by 50%. In five further subjects, a single 1.5 MED dose, 1.5 MEDs in 10 equal fractions on alternate days, and a single 1.5 MED dose at one-tenth the previously used irradiance, delivered to separate sites, also led to variable but significant depletion of CD la expression of around-30–50%. Thus, low-dose UVB irradiation, whether received rapidly or slowly, appears significantly and approximately equally to deplete human epidermal Langerhans cell numbers as measured by CDla expression.     

The sensitivity of Langerhans cells to simulated solar radiation in basal cell carcinoma patients

The role of Langerhans cells (LC) in host resistance against the induction and growth of nonmelanoma skin cancers is still obscure. The purpose of this study was to investigate the sensitivity of LC to simulated solar radiation in patients with basal cell carcinoma (BCC). Thirty-four patients (31-74 years old) with at least one histologically diagnosed BCC on a sun-exposed area and 21 healthy volunteers (29-62 years old) were included in the study. Patients and control subjects were given 10 graded doses of simulated solar UV radiation (10-75 mJ/cm2) on the lower back using a 12S solar simulator with a WG 320 filter. Twenty-four hours later, the minimal erythema dose (MED) was determined and shave biopsies were taken from the site given 1.25 X MED and from adjacent, unirradiated skin. Epidermal sheets were stained for LC using the ATPase method. The mean value of the MED of the BCC patients was 25 +/- 2 mJ/cm2 and that of controls was 29 +/- 3 mJ/cm2 (p greater than 0.05). The number of ATPase+ LC was significantly decreased (p less than 0.05), and their morphology was altered in the irradiated skin of nearly all individuals. However, there was no significant difference in the average reduction of LC in the patients (32% +/- 3%) compared with that of control subjects (32% +/- 4%). The depletion of LC ranged from 0% to 74% in different individuals, all of whom were given 1.25 MED. Furthermore, no correlation was found between the percentage decrease in ATPase+ cells and the dose of UV radiation required to produce erythema. Our results indicate that the ability of UV radiation to cause erythema was unrelated to the magnitude of its effects on LC number or morphology. Second, the morphologic alterations of LC in BCC patients after UV irradiation do not differ from those observed in normal individuals. Third, as a group, patients with BCC do not have a significantly lower MED than cancer-free subjects.     

Pimecrolimus does not affect Langerhans cells in murine epidermis

BACKGROUND: Langerhans cells (LCs) function as specialized antigen-presenting cells in the epidermis, and therefore play a critical role in cutaneous immunological reactions. Topical treatment with corticosteroids is associated with a decrease in epidermal LC number and antigen-presenting capacity in laboratory animals and humans. OBJECTIVES: To examine whether pimecrolimus, a nonsteroidal inflammatory cytokine inhibitor recently introduced for the topical treatment of atopic dermatitis, differs from corticosteroids in effects on LCs. METHODS: Groups of BALB/c mice were treated twice daily on one to five consecutive days on the inner surface of the right ear with 10 micro L of ethanolic solutions of the test compounds at their clinically used concentrations (1% pimecrolimus, 0.1% betamethasone-17-valerate, 1% hydrocortisone and 0.05% clobetasol propionate) or with the vehicle (controls) alone. At selected time points after the treatment epidermal sheets were prepared and examined histomorphometrically for LCs immunolabelled with antibodies to major histocompatibility complex (MHC) class II and DEC 205, and adenosine diphosphatase staining. RESULTS: No changes in number or morphology of LCs were observed in epidermal sheets of mice treated for 5 days with pimecrolimus. In contrast, an almost complete depletion of LCs was observed in skin samples treated with hydrocortisone, betamethasone or clobetasol. Even a single-day treatment schedule with hydrocortisone, betamethasone or clobetasol caused a significant reduction in MHC class II+ LCs, by 31%, 62% and 87%, respectively. CONCLUSIONS: It is therefore unlikely that topically applied pimecrolimus affects epidermal LCs, in contrast to corticosteroids.     

Migration of Langerhans cells from carcinogen-treated sheep skin.

To define the mechanism(s) of carcinogen depletion of Langerhans cells (LC) from skin, the migration of LC from the skin to the regional lymph node was examined in carcinogen-treated, antigen-treated, and control sheep. This was assessed by cannulation of afferent lymphatic vessels that drain the treated areas of skin or the efferent lymphatic draining the regional lymph node. Cells draining from test or control skin were continuously collected and enumerated by indirect immunofluorescence and flow cytometry using specific anti-CD1 monoclonal antibodies. There was a marked increase in the rate of LC migration in the 8 h following the application of the contact sensitizing antigen trinitrochlorobenzene (TNCB). The chemical carcinogen 7,12-dimethylbenz(a)anthracene (DMBA) triggered a tenfold-greater migration of LC compared with TNCB--with the peak response at 5 d. After DMBA treatment LC were also detected in the efferent lymph of the regional lymph node. It is concluded that the depletion of LC from carcinogen-treated skin is due to the increased LC migration and not carcinogen-induced cell death.     

Langerhans cells in the physiopathology of atopic dermatitis

INTRODUCTION. Atopic dermatitis (AD), allergic rhino-conjunctivities and allergic asthma constitute the classical triad of atopic diathesis attended, in many cases, by high serum IgE levels. While the pathophysiology of IgE-mediated allergic respiratory diseases is now better understood, the pathophysiological significance of atopic phenomena in the genesis and control of AD is still far from being clear. Numerous clinical and laboratory data point to a pathophysiological relation between IgE-mediated reactions and AD, but no one yet knows by which mechanism this interaction takes place. Some recent studies suggest that Langerhans cells might well be the missing link. THE LANGERHANS CELLS. Langerhans cells (LC) are dendritic epidermal cells originating in the bone marrow and supposedly belonging to the monocyte lineage. Their circulating precursors, the mechanism of their migration into the epidermis and their relationship with other dendritic cells, such as the interdigitating follicular cells, are controverted. LC express numerous surface markers, such as class I and II HLA, CD1a, CD4 and receptors for complement and IgE Fc fragments. Under normal conditions, LC do not express IgE receptors. Ultrastructurally, LC are characterized by the presence of Birbeck granules in their cytoplasm. Among the presumed functions of LC in the skin, the best documented is the presentation of antigens to T lymphocytes in allergic contact dermatitis. LANGERHANS CELLS IN ATOPIC DERMATITIS. Quantitative studies. Modern immunohistological methods based on the reactivity of monoclonal anti-CD1a antibodies have given results that are sometimes conflicting due to differences in the quantification techniques utilized. However, morphometric enumeration of LC on cryostat sections have shown that their number is about the same in AD and in normal skin. PRESENCE OF IgE BEARING LANGERHANS CELLS IN ATOPIC DERMATITIS. The presence of IgE molecules on the LC surface has been demonstrated in subjects with AD. It must be noted that in atopic subjects IgE bearing Lc are only found in patients with high serum IgE levels. They are absent in asthma patients without eczema, irrespective of their serum IgE levels. Daily applications of corticosteroids on AD lesions result in a decrease of anti-IgE markers on LC after one week and in their complete disappearance after 2 weeks. IN ATOPIC DERMATITIS LANGERHANS CELLS EXPRESS A RECEPTOR SPECIFIC TO Fc FRAGMENTS OF IgE. The exact nature of the receptor for IgE expressed in situ in AD patients is still conjectural. Some authors have been able to demonstrate that the binding of IgE molecules by LC isolated from the skin of atopic patients is inhibited by a monoclonal antibody directed against the low affinity receptor (Fc epsilon R2) of eosinophils and macrophages. This strongly suggests that certain factors induce the expression by LC of an Fc epsilon R2 receptor. IN VITRO INDUCTION OF IgE RECEPTORS ON NORMAL LANGERHANS CELLS...     

Suppressor cell activation and enhanced skin allograft survival after tumor promotor but not initiator induced depletion of cutaneous Langerhans cells

During chemical carcinogenesis Langerhans cells (LC) are depleted from the epidermis, disrupting the normal immunological functions of the skin. Tumor promotors but not initiators, have been shown to deplete adenosine triphosphatase (ATPase)-positive LC from the skin and therefore the cutaneous immune system may be impaired during tumor promotion but not initiation. The present study shows that the tumor promotor 12-O-tetradecanoylphorbol 13-acetate (TPA) but not the initiator urethane depletes Ia-positive LC from BALB/c murine ear epidermis, and beta-glucuronidase-positive LC from C57BL mouse tail skin. Sensitization with 2,4-dinitrofluorobenzene (DNFB) through urethane-treated skin resulted in a normal contact sensitivity response when the mice were challenged 5 days later. In contrast, tolerance resulted from sensitization through TPA-treated skin as a result of the generation of suppressor cells. In addition, TPA but not urethane-treated C57BL mouse tail skin survived for an extended time when grafted onto histoincompatible BALB/c mice. Therefore, impairment of the normal immunological functions of skin resulted from treatment with the tumor promotor TPA but not the tumor initiator urethane, which suggests that a loss of LC during tumor promotion may impair immunological protection against skin tumors.     

Lymphocyte subsets and Langerhans cells/indeterminate cells in erythema multiforme

A peroxidase-antiperoxidase study using monoclonal antibodies directed against T and B lymphocytes and Langerhans cells/indeterminate cells (LC/IC) was undertaken in order to understand more clearly the changes observed in erythema multiforme. At the various stages of development, from normal skin to target lesions, the quantity of inflammatory cells differed, but in each case the number of T8+ (cytotoxic/suppressor) cells was greater than the number of T4+ (helper/inducer) cells in the epidermis, whereas the latter exceeded the former in the dermis. Concomitant with the initial epidermis changes, there was an increase in the number of T6+ (LC/IC) cells in the upper and lower epidermis. With slight to moderate basal unit destruction, the number of LC/IC in the upper epidermis exceeded those in the lower epidermis. With severe basal unit destruction, there was a loss of LC/IC in the lower epidermis as detected by T6 reactivity. In fully formed blisters, the LC/IC in the upper half of the epidermis were decreased in parallel with the degree of epidermal necrosis. The character of the lymphocytic inflammatory infiltrate and redistribution in LC/IC are similar to those findings described in allergic contact dermatitis. The clinical, histologic, and immunopathologic changes in erythema multiforme appear to be due in part to cellular immune mechanisms with the lymphocyte as the predominant effector cell, and our data suggest a possible role for LC/IC in this disorder.     

Isoflavonoid photoprotection in mouse and human skin is dependent on metallothionein

Previous studies report that selected topical isoflavonoids are immunoprotective in both mice and humans, when applied following UV irradiation. Isoflavonoids have documented antioxidant activity, but their mechanism of immunomodulation remains unclear. This study examines whether photoimmunoprotection by the isoflavonoids might result from their interaction with one cutaneous antioxidant known to modulate UV photodamage, metallothionein (MT). In mice bearing a null mutation for MT-I and -II, we found that immunoprotection by the isoflavonoid 4',7-dihydroxyisoflavane (equol) against solar-simulated UV radiation (SSUV) or exogenous cis-urocanic acid was abrogated. Topical equol did not activate MT expression in normal mouse skin, but markedly enhanced the increase in MT expression in murine epidermis following SSUV irradiation. Normal human skin, unlike murine, expressed MT in the basal epidermis. Following SSUV irradiation, topical application of the related synthetic isoflavonoid NV-07alpha to human skin also markedly enhanced epidermal MT expression. The NV-07alpha has been reported previously to protect humans against the UV suppression of Mantoux reactions. Thus, epidermal MT expression appears to protect against photoimmunosuppression in both human and mouse skin. We speculate that equol and its related derivative NV-07alpha may activate the MT gene synergistically with SSUV, to produce the enhanced immunoprotective effect.