The controversial role of vitamin D in the skin: immunosuppression vs. photoprotection.

Vitamin D is produced in the skin by ultraviolet (UV) B radiation (290-320 nm). The active metabolite 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] is made systemically by hydroxylation of vitamin D in the liver and the kidney, but also locally in the epidermis, which suggests that 1,25(OH)(2)D(3) may have important functions in the skin. 1,25(OH)(2)D(3) has opposing effects: it can mimic immunosuppressive effects caused by UV irradiation in some models, or reverse UV-induced DNA damage and immunosuppression in other models. 1,25(OH)(2)D(3) exerts effects on Langerhans cells that are characteristic of those associated with UV radiation (UVR)-induced suppression of contact hypersensitivity, and topical application of the vitamin D analogue calcipotriene suppresses contact hypersensitivity in human subjects to a similar extent as UVR. However, 1,25(OH)(2)D(3) decreases DNA damage both in vitro when added to human skin cells in culture before and after UVR, and in vivo when applied to mouse skin after UVR. Furthermore, topical 1,25(OH)(2)D(3) applied to mouse skin after UVR reversed the immunosuppressive effect of UVR in a contact hypersensitivity model. This review will discuss the role of 1,25(OH)(2)D(3) as either a mediator of UVR-induced immune suppression or as a photoprotective molecule against UVR-induced DNA damage and immune suppression.

Suppression of the asthmatic phenotype by ultraviolet B-induced, antigen-specific regulatory cells.

BACKGROUND: Over recent decades, there has been a significant global increase in the prevalence of asthma, an inflammatory disease of the respiratory system. While ultraviolet radiation (UV) has been used successfully in the treatment of inflammatory conditions such as psoriasis, studies of UV-induced regulation of allergic respiratory responses have been rare, and have not analysed in vivo measurements of airway hyperresponsiveness (AHR) or the antigen specificity of the UV-induced effects. OBJECTIVE: To investigate the regulatory properties of erythemal ultraviolet B (UVB) irradiation of the skin and the induction of allergen-induced airway immunity in a murine asthma model, and to examine the mechanisms involved. METHODS: BALB/c mice were exposed to a single erythemal dose of UV 3 days before intraperitonial sensitization (day 0) and boost (day 14) with the antigen, ovalbumin (OVA). Airway-associated, asthma-like responses to aerosolized OVA at day 21 were analysed including (a) AHR measured in vivo, (b) OVA-specific proliferative responses and cytokine production by cells from the lung-draining lymph nodes (LDLN), and (c) inflammatory cells and cytokines in the bronchoalveolar lavage fluid. To determine UVB-induced mechanisms of regulation, LDLN cells from UVB irradiated, OVA-sensitized mice were adoptively transferred into naïve BALB/c mice that were subsequently sensitized and challenged with OVA, or a non-specific antigen. RESULTS: UVB irradiation of skin significantly suppressed AHR to methacholine and OVA-specific responses in the LDLN and in the lung compartment. Reduced OVA-specific responses by LDLN cells from both UVB irradiated mice and mice that received 5 x 10(6) LDLN cells from UVB irradiated, but not from non-irradiated, OVA-sensitized mice suggested that UVB-induced regulatory cells are responsible for many of the asthma-reducing effects of dorsal UVB exposure. CONCLUSION: UVB irradiation of skin suppresses AHR and cellular responses of the airways to respiratory allergens. Further, this study implicates UVB or its downstream mediators as a potential approach to reducing the severity of asthma.

Consumption of untreated tank rainwater and gastroenteritis among young children in South Australia.

BACKGROUND: Tank rainwater is a source of untreated drinking water in Australia and elsewhere. The aim of this study was to determine whether the risk of gastroenteritis among children who drank tank rainwater differed from that of children who drank treated public mains water. METHODS: A cohort study of 1,016 4- to 6-year old children who drank rainwater or treated mains water in rural South Australia was undertaken in 1999. Parents kept a daily diary of their child's gastrointestinal symptoms and water consumption for a period of 6 weeks. Data on respiratory illness and other risk factors for gastroenteritis were also collected. RESULTS: The incidence of gastroenteritis among children was 3.8-5.3 episodes per child-year, but most episodes (60%) lasted just 1 day. No increase in odds of gastroenteritis was observed among children who drank rainwater compared with treated mains water. The adjusted odds ratio for gastroenteritis associated with rainwater consumption compared with mains consumption was 0.84 (95% confidence interval 0.63-1.13). CONCLUSIONS: Gastroenteritis was found to be a significant cause of morbidity among young children. Young children, who were regular consumers of tank rainwater, were at no greater odds of gastroenteritis than those who drank treated public mains water.

Reduction of nickel-induced contact hypersensitivity reactions by topical tea tree oil in humans.

OBJECTIVE AND DESIGN: Whilst the anti-microbial properties of tea tree oil (TTO) are established, the anti-inflammatory effects of TTO in human skin remain largely anecdotal and require evaluation. This study examined the effect of topically applied TTO on nickel-induced contact hypersensitivity reactions in human dorsal skin. TREATMENT: TTO (100%), a 5% TTO lotion, a placebo lotion (no TTO), or 100% macadamia oil were applied at days 3 and 5 after nickel exposure. METHODS: The flare area and erythema index were measured on days 3, 5 and 7. The regulatory effects of TTO were also investigated on the proliferative response to nickel or polyclonal mitogens by peripheral blood mononuclear cells from nickel-sensitive and control subjects. RESULTS: TTO (100%) significantly reduced the flare area and erythema index when compared to the nickel-only sites. With respect to the erythema index, the anti-inflammatory effects were predominantly, but not exclusively, seen in a subgroup of nickel-sensitive subjects with a prolonged development phase of nickel-induced contact hypersensitivity response. The 5% TTO lotion, the placebo lotion and the 100% macadamia oil were all without significant effect. TTO significantly inhibited proliferation to nickel but not to non-specific polyclonal mitogens by peripheral blood mononuclear cells from nickel-sensitive subjects. CONCLUSIONS: Topical application of 100% TTO may have therapeutic benefit in nickel-induced contact hypersensitivity in human skin. The mode of action of TTO requires further investigation, but may be an effect on the antigen presenting cells or the antigen presenting process in nickel-induced contact hypersensitivity, as well as vascular changes associated with this response.

Association between melanoma and dermal mast cell prevalence in sun-unexposed skin.

BACKGROUND: Both exposure to intermittent intense sunlight during childhood and ultraviolet (UV) radiation-induced immunomodulation have been directly associated with melanoma development. In mice, the prevalence of dermal mast cells determines susceptibility to UVB-induced systemic suppression of contact hypersensitivity responses and thus may affect immunological responses to melanoma antigens. OBJECTIVES: To determine the relevance of murine studies of dermal mast cell prevalence to human melanoma pathogenesis. METHODS: The prevalence of mast cells was examined in sun-unexposed buttock skin of 45 melanoma patients and 68 control volunteers who had no history of skin cancer development. Buttock skin was studied because mast cell prevalence is stable with ageing and the confounding effects of environmental UV exposure are minimized. RESULTS: Using tissue immunostaining, the buttock skin from melanoma patients had a significantly higher dermal mast cell prevalence (mean +/- SEM 38 +/- 2 mast cells mm(-2)) than controls (32 +/- 2 mast cells mm(-2)) (P = 0.02). Analysis by binary logistic regression showed that the association between mast cell prevalence and melanoma outcome was not significantly altered by skin phototype. CONCLUSIONS: The immunomodulatory effects of mast cell products in UV-irradiated skin may contribute significantly to the initiation and development of human cutaneous malignant melanoma.

The effect of ultraviolet radiation exposure on the prevalence of mast cells in human skin.

BACKGROUND: Dermal mast cells have been implicated as important effector cells in innate immunity, hypersensitivity responses and ultraviolet (UV)B-induced suppression of cell-mediated immune responses to contact allergens. Humans, like mouse strains, display variations in dermal mast cell prevalence. The factors determining these differences are yet to be fully elucidated. In mice, expression of the receptor for stem cell factor, c-kit, on dermal mast cells correlates with prevalence. OBJECTIVES: To evaluate dermal mast cell prevalence and mast cell c-kit expression in non-sun-exposed and sun-exposed skin in the same donor. METHODS: In 14 subjects, biopsies of skin (4 mm) were sampled from the skin sites of buttock, inner arm, shoulder and back of hand skin and dermal mast cell prevalence quantified. Non-sun-exposed buttock and chronically sun-exposed hand skin were evaluated for mast cell expression of c-kit and elastin content, a feature of photoageing and surrogate marker of UV exposure. RESULTS: The prevalence of dermal mast cells was significantly higher in hand skin than in the three other anatomically different skin sites. Significant correlations were observed in hand but not buttock skin between increasing dermal mast cell densities, extent of elastin content in the papillary dermis and age of the subject. Cellular expression of c-kit correlated with mast cell prevalence in hand skin. However, no relationship was observed in hand skin between c-kit expression, elastin content and age. CONCLUSIONS: The prevalence of mast cells in human skin is altered by factors that are intrinsic (mechanisms regulating c-kit expression) and extrinsic (chronic sun exposure), and the fact that the associations of mast cell prevalence with age is explained by the latter being a correlate of cumulative sun exposure.

Tea tree oil reduces histamine-induced skin inflammation.

BACKGROUND: Tea tree oil is the essential oil steam-distilled from Melaleuca alternifolia, an Australian native plant. In recent years it has become increasingly popular as an antimicrobial for the treatment of conditions such as tinea pedis and acne. OBJECTIVES: To investigate the anti-inflammatory properties of tea tree oil on histamine-induced weal and flare. METHODS: Twenty-seven volunteers were injected intradermally in each forearm (study and control assigned on an alternating basis) with histamine diphosphate (5 microg in 50 microL). Flare and weal diameters and double skin thickness were measured every 10 min for 1 h to calculate flare area and weal volume. At 20 min, 25 microL of 100% tea tree oil was applied topically to the study forearm of 21 volunteers. For six volunteers, 25 microL paraffin oil was applied instead of tea tree oil. RESULTS: Application of liquid paraffin had no significant effect on histamine-induced weal and flare. There was also no difference in mean flare area between control arms and those on which tea tree oil was applied. However, mean weal volume significantly decreased after tea tree oil application (10 min after tea tree oil application, P = 0.0004, Mann-Whitney U-test). CONCLUSIONS: This is the first study to show experimentally that tea tree oil can reduce histamine-induced skin inflammation.

Tea tree oil reduces the swelling associated with the efferent phase of a contact hypersensitivity response.

OBJECTIVE: To examine the anti-inflammatory activities of tea tree oil (TTO) in vivo. METHODS: Mice were sensitized to a chemical hapten, trinitrochlorobenzene, on their ventral skin and 7 days later challenged (or re-exposed) on their dorsal skin with the same hapten. RESULTS: TTO applied 30 min before or up to 7 h after to the same dorsal site as hapten challenge caused a significant reduction in skin swelling after 24 h. TTO reduced oedema but not the influx of inflammatory cells. This finding was supported by the inability of TTO to suppress TNFalpha-induced E-selectin expression by human umbilical vein endothelial cells. TTO did not suppress irritant- or ultraviolet B-induced oedema. CONCLUSION: Topical TTO, specifically the TTO components, terpinen-4-ol and alpha-terpineol can regulate the oedema associated with the efferent phase of a contact hypersensitivity response.

Tea tree oil reduces histamine-induced oedema in murine ears.

OBJECTIVE: To examine the effect of topically applied tea tree oil (TTO) on histamine-induced oedema in the ears of mice. METHODS AND RESULTS: For BALB/c mice, 10 microl undiluted TTO applied immediately after, but not 30 min before intradermal injection of 600 microg histamine in 10 microl, significantly suppressed oedema development. TTO applied after histamine injection also suppressed histamine-induced oedema in C57/BL6 mice. TTO applied immediately after intradermal injection of compound 48/80 (200 microg in 10 microl saline) also significantly reduced ear swelling. TTO suppressed histamine-induced oedema to the same extent in capsaicin-treated (neuropeptide-depleted) and control mice which suggests that TTO does not inhibit histamine-induced oedema by regulating the activity of peripheral sensory neurons. Terpinen-4-ol, the major water-soluble component of TTO, was equivalent in potency to TTO in the suppression of histamine-induced ear swelling. CONCLUSION: Topical application of TTO, and in particular terpinen-4-ol, may be effective in controlling histamine-induced oedema often associated with Type I allergic immediate hypersensitivities.

cis-Urocanic acid stimulates neuropeptide release from peripheral sensory nerves.

Previous studies using an antibody to cis-urocanic acid and mast-cell-depleted mice implicated both cis-urocanic acid and mast cells in the mechanisms by which ultraviolet B light suppresses systemic contact hypersensitivity responses in mice. In the absence of a direct stimulatory effect of cis-urocanic acid on connective tissue mast cells, an indirect association was investigated. A blister induced in the rat hind footpad was used to examine the effects of slowly perfused cis-urocanic acid on cutaneous blood flow. cis-Urocanic acid but not trans-urocanic acid increased microvascular flow by a mechanism largely dependent on the combined activity of the neuropeptides, substance P and calcitonin gene-related peptide. Perfusion of cis-urocanic acid over the base of blisters induced in sensory-neuropeptide-depleted rats did not have any stimulatory effect above that seen with perfusion of cis-urocanic acid together with neuropeptide receptor antagonists in control rats. There was a small direct effect of cis-urocanic acid on microvascular blood flow. As both substance P and calcitonin gene-related peptide could directly degranulate connective tissue mast cells, this study suggests that cis-urocanic acid indirectly activates mast cells via its effects on peripheral terminals of unmyelinated primary afferent sensory nerves. cis-Urocanic-acid-induced neuropeptides may also contribute to ultraviolet-B-induced cutaneous inflammation and alterations to Langerhans cell activity.

The water-soluble components of the essential oil of Melaleuca alternifolia (tea tree oil) suppress the production of superoxide by human monocytes, but not neutrophils, activated in vitro.

OBJECTIVE: To evaluate the regulatory properties of the essential oil of Melaleuca alternifolia (tea tree oil) on the production of oxygen derived reactive species by human peripheral blood leukocytes activated in vitro. MATERIALS AND METHODS: The ability of tea tree oil to reduce superoxide production by neutrophils and monocytes stimulated with N-formyl-methionyl-leucyl-phenylalanine (fMLP), lipopolysaccharide (LPS) or phorbol 12-myristate 13-acetate (PMA) was examined. RESULTS: The water-soluble fraction of tea tree oil had no significant effect on agonist-stimulated superoxide production by neutrophils, but significantly and dose-dependently suppressed agonist-stimulated superoxide production by monocytes. This suppression was not due to cell death. Chemical analysis identified the water-soluble components to be terpinen-4-ol, alpha-terpineol and 1,8-cineole. When examined individually, terpinen-4-ol significantly suppressed fMLP- and LPS- but not PMA-stimulated superoxide production; alpha-terpineol significantly suppressed fMLP-, LPS- and PMA-stimulated superoxide production; 1,8-cineole was without effect. CONCLUSION: Tea tree oil components suppress the production of superoxide by monocytes, but not neutrophils, suggesting the potential for selective regulation of cell types by these components during inflammation.

Sunlight, immunosuppression and skin cancer: role of histamine and mast cells.

1. The development into tumours of skin cells transformed by ultraviolet (UV) B radiation of wavelengths 290-320 nm is enhanced by the ability of UVB to suppress an immune response that would otherwise destroy them. Ultraviolet B-induced immunomodulation may be by multiple mechanisms, but generally manifests in an antigen-presenting cell defect and an altered cytokine environment in the draining lymph nodes. 2. Immune responses to microbial or self-antigens may be dysfunctional by similar mechanisms following UVB exposure. 3. Earliest-acting intermediates in the initiation of UVB-induced immunosuppression are the UVB absorbers (photoreceptors) of the skin, notably DNA resulting in immunoregulatory cytokine production, and trans-urocanic acid (UCA), which, upon isomerization to its cis isomer, signals downstream immunosuppressive events. 4. In mice, dermal mast cells are critical to UVB-induced systemic immunomodulation. In mice, there is a functional link as well as a linear relationship between the prevalence of histamine-staining dermal mast cells and the log of the dose of UVB required for 50% immunosuppression. Studies with histamine receptor antagonists support histamine as the main' product of mast cells involved. Histamine acts in large part via a prostanoid-dependent pathway. 5. Approximately 50% of humans and greater than 90% of patients with non-melanoma skin cancer are UVB susceptible for suppression of a contact hypersensitivity response. Neither cytokine polymorphisms nor UVB-induced levels of cis-UCA in irradiated skin have been linked to UVB susceptibility. Patients with basal cell carcinomas (BCC) have an increased dermal mast cell prevalence in non-sun-exposed buttock skin. We propose that mast cells function in humans, as in mice, by initiating immunosuppression and, thereby, allowing a permissive environment for BCC development.

Characterization of IL-4 receptor components expressed on monocytes and monocyte-derived macrophages: variation associated with differential signaling by IL-4.

The anti-inflammatory effects of IL-4 on activated monocytes differ from those on monocyte-derived macrophages (MDMac). While IL-4 suppresses LPS-induced IL-1beta , IL-12, IL-10 and TNFalpha production by monocytes, IL-4 suppresses only IL-1beta and IL-12 production by MDMac. The U937 and Mono Mac 6 cell lines have similar cytokine responses to IL-4 as monocytes and MDMac, respectively. The IL-4Ralpha and IL-2Rgamma (gammac) chains are well-characterized components of the IL-4 receptor. Cross-linking studies with 125I-IL-4 revealed that for monocytes and U937 cells, the binding of IL-4 to the receptor components was approximately 1:1 for IL-4Ralpha:gammac. In contrast, for MDMac and Mono Mac 6 cells that have a relative reduction in gammac surface expression, the binding of IL-4 to IL-4Ralpha:gammac was approximately 3:1. Furthermore, IL-4 induced IL-4Ralpha chain phosphorylation more rapidly in MDMac and Mono Mac 6 cells than in monocytes and U937 cells. This study identifies a correlation between altered 125I-IL-4 cross-linking to IL-4Ralpha:gammac, IL-4-induced signaling and regulation of pro-inflammatory cytokine production by IL-4.

Terpinen-4-ol, the main component of the essential oil of Melaleuca alternifolia (tea tree oil), suppresses inflammatory mediator production by activated human monocytes.

OBJECTIVE AND DESIGN: To evaluate potential antiinflammatory properties of tea tree oil, the essential oil steam distilled from the Australian native plant, Melaleuca alternifolia. MATERIAL AND METHODS: The ability of tea tree oil to reduce the production in vitro of tumour necrosis factor-alpha (TNFalpha), interleukin (IL)-1beta, IL-8, IL-10 and prostaglandin E2 (PGE2) by lipopolysaccharide (LPS)-activated human peripheral blood monocytes was examined. RESULTS: Tea tree oil emulsified by sonication in a glass tube into culture medium containing 10% fetal calf serum (FCS) was toxic for monocytes at a concentration of 0.016% v/v. However, the water soluble components of tea tree oil at concentrations equivalent to 0.125% significantly suppressed LPS-induced production of TNFalpha, IL-1beta and IL-10 (by approximately 50%) and PGE2 (by approximately 30%) after 40 h. Gas chromatography/mass spectrometry identified terpinen-4-ol (42 %), a-terpineol (3 %) and 1,8-cineole (2%, respectively, of tea tree oil) as the water soluble components of tea tree oil. When these components were examined individually, only terpinen-4-ol suppressed the production after 40 h of TNFalpha, IL-1beta, IL-8, IL-10 and PGE2 by LPS-activated monocytes. CONCLUSION: The water-soluble components of tea tree oil can suppress pro-inflammatory mediator production by activated human monocytes.

Communications: high dermal mast cell prevalence is a predisposing factor for basal cell carcinoma in humans.

Ultraviolet B radiation (280-320 nm) can initiate skin cancer as well as suppress the immune system, thereby preventing the rejection of ultraviolet-B-induced tumors. Recently we reported that there was not only a correlation but also a functional link between dermal mast cell prevalence and susceptibility to ultraviolet-B-induced systemic immunosuppression in multiple strains of mice. In this study, we investigated whether increased dermal mast cell prevalence is a significant predisposing factor for basal cell carcinoma development in humans. In 21 Danes with a history of basal cell carcinoma and 20 control subjects of similar age, sex, skin phototype, and recreational sun exposure over the past 12 mo, dermal mast cell prevalence was quantified on non-sun-exposed buttock skin. We investigated this skin site in order to avoid any changes in mast cell prevalence caused by sun exposure and assumed that the prevalence of mast cells in buttock skin correlated with that at sun-exposed sites at critical times in the development of basal cell carcinomas. Patients with a history of basal cell carcinoma had a significantly higher median dermal mast cell prevalence than control subjects (p = 0.01, Mann-Whitney U ). No correlation was observed between dermal mast cell prevalence and age of basal cell carcinoma patients and control subjects. These results suggest that increased dermal mast cell prevalence is a predisposing factor for basal cell carcinoma development in humans. We hypothesize that mast cells function in humans, as in mice, by initiating immunosuppression and thereby allowing a permissive environment for basal cell carcinoma development.

Mast cells in UV-B-induced immunosuppression.

Degranulating dermal mast cells in UV-B-irradiated skin have been implicated for many years in the mechanisms of irradiation erythema. There is now considerable evidence that dermal mast cells are important to the processes by which both UV-B radiation and cis-urocanic acid (cis-UCA) suppress immune responses to sensitizing antigens applied to non-irradiated/non-cis-UCA-exposed sites. Mast-cell-depleted mice are resistant to the immunosuppressive effects of UV-B radiation and cis-UCA for 'systemic' immunomodulation. However, these mice gain responsiveness if the dorsal skin is reconstituted six weeks prior to irradiation or cis-UCA administration at that site with cultured bone-marrow-derived mast cells from +/+ mice. The molecular triggers for initiating mast-cell degranulation are being actively sought. Evidence suggests that histamine, and not tumour necrosis factor alpha, is the major mast-cell product that signals altered immune responses to sensitizing antigens applied to non-irradiated, non-cis-UCA-exposed sites. Histamine may have multiple roles, but experiments with indomethacin administered to mice have shown that one process involves induction of prostanoid production.

Ultraviolet B-induced suppression of immune responses in interleukin-4-/- mice: relationship to dermal mast cells.

Ultraviolet B radiation is immunosuppressive by multiple mechanisms. In interleukin-4-/- mice, ultraviolet B radiation was not able to suppress delayed-type hypersensitivity or contact hypersensitivity responses when the sensitizing antigen was applied to nonirradiated sites. In contrast, ultraviolet B significantly suppressed contact hypersensitivity responses to haptens applied to irradiated sites in interleukin-4-/- mice. In mast cell depleted Wf/Wf mice, ultraviolet B radiation also significantly suppressed contact hypersensitivity responses to sensitizing antigens applied to irradiated but not to unirradiated sites. In both interleukin-4-/- mice and Wf/Wf mice, the mast cell product, histamine, was immunosuppressive implicating mast cells as the dysfunctional cell in interleukin-4-/- mice. The prevalence of dermal mast cells was similar in wild-type and interleukin-4-/- mice. Dermal mast cells of interleukin-4-/- mice, however, express very low levels of c-kit and did not significantly degranulate in response to ultraviolet B. Ultraviolet radiation induced significant and similar levels of serum interleukin-10 in wild-type and interleukin-4-/- mice. We conclude that interleukin-4 indirectly affects ultraviolet B suppression of contact hypersensitivity and delayed-type hypersensitivity responses to sensitizing antigens applied at sites other than those irradiated by providing a critical differentiative signal for dermal mast cells. This study further emphasizes the central role of mast cells in the initial processes by which ultraviolet B radiation is immunomodulatory for immune responses to sensitizing antigens applied to nonirradiated sites.

Age-related changes in dermal mast cell prevalence in BALB/c mice: functional importance and correlation with dermal mast cell expression of Kit.

Differences in dermal mast cell prevalence for adult mice of different strains have been reported previously. In this study, the dermal mast cell prevalence for BALB/c and C57BL/6 mice at 6 weeks of age was similar but as BALB/c mice matured from 6 to 10 weeks of age, their dermal mast cell prevalence halved. In contrast, there was no significant difference in the dermal mast cell prevalence of 6- and 10-week-old C57BL/6 mice. These differences determined the degree of susceptibility of BALB/c and C57BL/6 mice of different ages to UVB (UV radiation of wavelength 280-320 nm)-induced systemic immunosuppression. Expression of the receptor for stem cell factor, Kit protein, was examined on mast cells under conditions in which the dermal mast cell prevalence varied. A significant correlation was observed between Kit expression by mast cells from adult BALB/c, DBA/2 and C57BL/6 mice and dermal mast cell prevalence. In BALB/c mice, mast cell Kit expression decreased as the mice matured from 6 to 10 weeks of age and correlated with the reduction in dermal mast cell numbers. Kit levels on dermal mast cells from C57BL/6 mice were consistently higher than on mast cells from BALB/c mice although significant reductions in Kit were also measured with ageing from 6 to 10 weeks. We hypothesize that regardless of the extent of Kit expression, the dermal mast cell populations were maximally expanded in C57BL/6 mice. We suggest that BALB/c mice of 6 and 10 weeks of age are useful hosts in which to quantitatively evaluate mast cell involvement in a range of functional assays involving skin.

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.

Differential responses of human monocytes and macrophages to IL-4 and IL-13.

The primary interleukin-4 (IL-4) receptor complex on monocytes (type I IL-4 receptor) includes the 140-kDa alpha chain (IL-4R alpha) and the IL-2 receptor gamma chain, gamma(c), which heterodimerize for intracellular signaling, resulting in suppression of lipopolysaccharide (LPS)-inducible inflammatory mediator production. The activity of IL-13 on human monocytes is very similar to that of IL-4 because the predominant signaling chain (IL-4R alpha) is common to both receptors. In fact, IL-4R alpha with IL-13R alpha1 is designated both as an IL-13 receptor and the type II IL-4 receptor. When the anti-inflammatory activities of IL-4 and IL-13 were investigated on synovial fluid macrophages and compared with the responses by monocytes isolated from the patients at the same time as joint drainage, the response profiles differed with some responses similar in the two cell populations, others reduced on the inflammatory cells. Similar differences were recorded in the response profiles to IL-4 and IL-13 by monocytes and monocytes cultured for 7 days in macrophage colony-stimulating factor (M-CSF) or granulocyte-macrophage CSF (GM-CSF) (monocyte-derived macrophages, MDMac). MDMac have reduced gamma(c) mRNA levels and reduced expression of the functional 64-kDa gamma(c). There was a similar loss of IL-13R alpha1 mRNA on monocyte differentiation. In turn, there was a significant reduction in the ability of IL-4 and IL-13 to activate STAT6. These findings suggest that different functional responses to IL-4 and IL-13 by human monocytes and macrophages may result from reduced expression of gamma(c) and IL-13R alpha1.