Modulation of the immune system by UV radiation: more than just the effects of vitamin D?

Humans obtain most of their vitamin D through the exposure of skin to sunlight. The immunoregulatory properties of vitamin D have been demonstrated in studies showing that vitamin D deficiency is associated with poor immune function and increased disease susceptibility. The benefits of moderate ultraviolet (UV) radiation exposure and the positive latitude gradients observed for some immune-mediated diseases may therefore reflect the activities of UV-induced vitamin D. Alternatively, other mediators that are induced by UV radiation may be more important for UV-mediated immunomodulation. Here, we compare and contrast the effects of UV radiation and vitamin D on immune function in immunopathological diseases, such as psoriasis, multiple sclerosis and asthma, and during infection.

Topically applied 1,25-dihydroxyvitamin D3 enhances the suppressive activity of CD4+CD25+ cells in the draining lymph nodes.

The immunomodulatory effects of vitamin D have been described following chronic oral administration to mice or supplementation of cell cultures with 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), the active form of vitamin D. In this study, topically applied 1,25(OH)(2)D(3), enhanced the suppressive capacity of CD4(+)CD25(+) cells from the draining lymph nodes. The effects of topical 1,25(OH)(2)D(3) were compared with those of UVB irradiation, which is the environmental factor required for 1,25(OH)(2)D(3) production in skin. CD4(+) cells from the skin-draining lymph nodes (SDLN) of either 1,25(OH)(2)D(3)-treated or UVB-irradiated mice had reduced capacity to proliferate to Ags presented in vitro, and could suppress Ag-specific immune responses upon adoptive transfer into naive mice. This regulation was lost upon removal of CD4(+)CD25(+) cells. Furthermore, purified CD4(+)CD25(+) cells from the SDLN of 1,25(OH)(2)D(3)-treated or UVB-irradiated mice compared with equal numbers of CD4(+)CD25(+) cells from control mice had increased capacity to suppress immune responses in both in vitro and in vivo assay systems. Following the sensitization of recipient mice with OVA, the proportion of CD4(+)Foxp3(+) cells of donor origin significantly increased in recipients of CD4(+)CD25(+) cells from the SDLN of 1,25(OH)(2)D(3)-treated mice, indicating that these regulatory T cells can expand in vivo with antigenic stimulation. These studies suggest that 1,25(OH)(2)D(3) may be an important mediator by which UVB-irradiation exerts some of its immunomodulatory effects.

Effect of both ultraviolet B irradiation and histamine receptor function on allergic responses to an inhaled antigen.

Exposure of skin to UVB radiation (290-320 nm) modulates the immune system, with most studies showing a suppression of Th1-driven immune responses. This study investigated the effects of UVB on Th2-associated immune responses using a murine model of allergic respiratory inflammation. C57BL/6, histamine receptor-1 knockout (H1RKO), and histamine receptor-2 knockout (H2RKO) mice were exposed to a single 4 kJ/m(2) dose of UVB (twice a minimal edemal dose) on shaved dorsal skin 3 days before intranasal sensitization with papain, a cysteine protease homologue of the dust mite allergen Der p 1. H1RKO mice demonstrated enhanced papain-specific inflammatory responses in the lung-draining lymph nodes (LDLNs), whereas the responses of H2RKO mice closely mimicked those of C57BL/6 mice. UVB irradiation 3 days before sensitization reduced in vitro papain-specific proliferation of LDLN cells of C57BL/6 and H1RKO mice but not H2RKO mice 24 h after challenge. The regulatory effect of UVB was transferred by adoptive transfer of unfractionated LDLN cells from UVB-irradiated, papain-sensitized C57BL/6 and H1RKO donor mice in naive recipients of the corresponding strain that were subsequently sensitized and challenged with papain. Additionally, UVB exposure suppressed papain-induced IL-5 and IL-10 production in vitro by LDLN cells from H1RKO mice but not from C57BL/6 mice or H2RKO mice. The results of this study demonstrate systemic immunomodulation of responses to intranasally delivered Ag by UVB irradiation and implicate a role for the H2 receptor in UVB-induced suppression of Ag-specific responses in the draining lymph nodes.

CD4+ T cells in lymph nodes of UVB-irradiated mice suppress immune responses to new antigens both in vitro and in vivo.

The mechanisms by which erythemal UVB irradiation modulates systemic immune responses to antigens applied to non-irradiated sites are poorly understood. In this study, regulatory CD4+ T cells were identified in the skin-draining lymph nodes (SDLNs) of UVB-irradiated, but otherwise naive mice. A transgenic mouse strain (DO11.10) was utilized in which the majority of CD4+ T cells expressed the ovalbumin (OVA(323-339)) T-cell receptor. Thus, T-cell responses could be examined following erythemal UVB irradiation without further antigen sensitization. CD4+ T cells from the SDLNs of UVB-irradiated mice had significantly reduced capacity to respond to presentation of the OVA(323-339) peptide in vitro. Transfer of CD4+ T cells from the SDLNs of UVB-irradiated antigen-naive mice significantly reduced both OVA sensitization and contact hypersensitivity responses to an experimental hapten in the recipient mice. Depletion of CD4+CD25+ cells abrogated this UVB-suppressive effect in the in vitro proliferation assay. There was also a significant increase in the proportion of CD4+CD25+Foxp3+ cells in the SDLNs of UVB-irradiated mice. The potential of these regulatory cells poised to regulate responses to incoming antigens at distant non-irradiated sites broadens the biological impact of UVB irradiation of skin on immunity.

Mast cells in photodamaged skin: what is their role in skin cancer?

In this review, changes that we have observed in the prevalence of mast cells in human sun-exposed skin, and their potential immunoregulatory role, are discussed. More specifically, in a study of Australian volunteers, the prevalence of dermal mast cells was increased in back-of-hand skin, anecdotally the most sun-exposed site of the body, but not in skin from buttock, inner arm or shoulder. By histological analysis of back-of-hand skin, there was a significant correlation between dermal mast cell prevalence and elastin content suggesting increased mast cell prevalence with photodamage. We hypothesised that these mast cells were immunomodulatory upon activation by sun exposure. However, no link was found between dermal mast cell prevalence in hand skin and the presence of basal cell carcinomas. Finally, we discuss other roles for increased numbers of mast cells in UV-exposed photodamaged skin.

Primary defect in UVB-induced systemic immunomodulation does not relate to immature or functionally impaired APCs in regional lymph nodes.

UVB irradiation of the shaved dorsal skin of mice can cause both local and systemic suppression of contact hypersensitivity responses; the former demonstrated by administration of the sensitizing Ag/hapten to the irradiated site and the latter by its administration at least 72 h later to distal unirradiated sites. The immunological basis of systemic immunomodulation is not clear. When haptens (trinitrochlorobenzene, FITC) were administered to the shaved ventral skin 4 days after irradiation (8 kJ/m(2)) to the shaved dorsum of BALB/c mice, CD11c(+)/FITC(+) cells in the skin-draining lymph nodes from control and irradiated mice produced on a per cell basis similar levels of IL-12 and PGE(2) were phenotypically mature and efficient at presenting FITC to lymphocytes from FITC-sensitized mice. Ag presentation by FACS-sorted CD11c(+) lymph node cells isolated 4 days after UVB irradiation was as efficient as were cells from unirradiated mice at presentation in vitro of an OVA peptide (OVA(323-339)) to CD4(+) cells from OVA-TCR-transgenic DO11.10 mice. Further, IFN-gamma levels were increased in the cultures containing CD11c(+) cells from UVB-irradiated mice, suggesting that inflammation may precede downstream immunosuppression. These results suggest that the primary cause of reduced contact hypersensitivity responses in mice in which UV irradiation and the sensitizing Ag are applied to different sites several days apart must originate from cells other than CD11c(+) APCs that directly or by production of soluble mediators (IL-12, PGE(2)) affect cellular responses in the nodes of UVB-irradiated mice.

Regulation of wheal and flare by tea tree oil: complementary human and rodent studies.

When applied 20 min after injection of histamine into human forearm skin, tea tree oil (TTO) reduces the developing cutaneous vascular response. In this study, the effect of TTO on inflammatory microvascular changes was dissected at the base of an experimental blister on rat skin. 1,8-Cineole, representing 2% of TTO, reduced vascular changes induced by sensory neuropeptides released when the distal portion of a cut sciatic nerve was electrically stimulated. The pre-terminal modulatory effect of 1,8-Cineole was confirmed in tests in sensory-denervated rats. Terpinen-4-ol (approximately 40% TTO) reduced substance P-induced microvascular changes and protein extravasation by a direct nitric oxide-mediated effect on the microvasculature, without sensory nerve involvement. alpha-Terpineol (3% of TTO) regulated both pre- and post-sensory nerve terminals. In human skin, terpinen-4-ol applied 10 min after histamine injection, but not alpha-terpineol or 1,8-cineole, regulated the developing wheal and flare suggesting that the histamine-induced responses in humans (at the dose used in this study, 50 microL of 330 microM histamine) are in large part determined by histamine directly affecting the vasculature via post-terminal-mediated events. The underlying strength of these studies is the use of a well-established rat physiologic model to differentiate the mechanism of regulation of microvascular changes by modulatory agents.

Centrifugation facilitates transduction of green fluorescent protein in human monocytes and macrophages by adenovirus at low multiplicity of infection.

Due to their phagocytic and poorly proliferative nature, it has been difficult to transfect human monocytes and macrophages. Adenoviral vectors have recently allowed transduction of a high percentage of human macrophages, but only after CSF upregulation of the integrins, alphavbeta3 or alphavbeta5, during culture for 48 h, a time allowing significant monocyte to macrophage differentiation. In our hands, after 24-h incubation with M-CSF (20 ng/ml) and a further 24-h incubation with an adenoviral vector encoding green fluorescent protein (AdV-GFP) [multiplicity of infection (MOI)=50:1], only 35% of CD14-positive cells express GFP. We report that centrifugation of these cells with AdV-GFP at 2000 x g for 1 h at 37 degrees C significantly enhanced the number of cells expressing GFP (to 65%) and the level of GFP expression per transduced cell (fivefold). The viability of the cells was not compromised (<5 % CD14-positive cells were 7-aminoactinomycin D (7AAD)-positive after 24 h AdV-GFP exposure at MOI=50:1). Centrifugation allowed efficient transduction of monocytes and macrophages with an MOI at least tenfold lower than otherwise required and did not activate the transduced cells or affect their ability to produce TNFalpha or IL-1beta in response to lipopolysaccharide (LPS). This methodology was also suitable for transducing large numbers of in vitro monocyte-derived macrophages (MDMac) and macrophages isolated from synovial fluids with up to 75-80% of CD14-positive cells transduced after 24-h exposure to AdV-GFP (50:1) and centrifugation (2000 x g). This methodology should provide significant expression of transgenes in human monocytes and macrophages.

Susceptibility to basal cell carcinoma is associated with high dermal mast cell prevalence in non-sun-exposed skin for an Australian populations.

In a Danish population, basal cell carcinoma (BCC) patients have a higher dermal mast cell prevalence in buttock skin than controls. This finding was supported by a functional link in mice between histamine-staining dermal mast cells and the extent of susceptibility to UV-B-induced systemic immunomodulation. It was important to confirm that this association was maintained in an Australian population with very different ancestry and sun exposure patterns. Australian BCC patients (n = 26) had significantly higher densities of mast cells in the dermis of buttock skin than control subjects (n = 25) (P = 0.0003, Mann-Whitney U-test). However, this correlation was lost at the sun-exposed site of the hand (P = 0.547, Mann-Whitney U-test). To further evaluate whether a relationship exists between dermal mast cell prevalence in sun-exposed skin and incidence of BCC in a larger study, biopsies of dorsal hand skin were obtained from an age-stratified random sample of 166 Queensland subjects, together with the 51 South Australian subjects, and dermal mast cell prevalence was quantified. Older subjects (over the median age of 42 years) had a greater incidence of BCC development (P = 0.0001, chi-square test) and significantly higher mast cell densities in hand skin (P = 0.0001, chi-square test) than younger subjects. However, mast cell density in sun-exposed hand skin was not significantly associated with BCC incidence. Finally, cellular expression of c-kit correlated with mast cell prevalence in non-sun-exposed skin, thereby implicating the stem cell factor-c-kit axis in the intrinsic mechanisms that regulate prevalence. These results show that high prevalence of dermal mast cells in buttock skin but not hand is associated with BCC development in an Australian population.

S100A8 induction in keratinocytes by ultraviolet A irradiation is dependent on reactive oxygen intermediates.

Cutaneous exposure to ultraviolet (UV) A (320-400 nm) results in the formation of damaging reactive oxygen intermediates, which are implicated as mediators of DNA damage, apoptosis, and photoaging. S100A8 is a low-molecular-weight calcium-binding protein, highly sensitive to oxidation. In this study, UVA-induced S100A8 expression by keratinocytes was investigated. UVA (50-100 kJ per m2) strongly induced S100A8 in differentiated keratinocytes in the epidermis of BALB/c mice. Similarly, S100A8 mRNA and monomeric protein were significantly upregulated in PAM212 cells (a murine keratinocyte cell line) in response to 10 kJ per m2 UVA 24 h after irradiation. Although S100A9 associates with S100A8 in neutrophils and abnormally differentiated keratinocytes (human psoriasis), in this study it was not coinduced with keratinocyte S100A8. Dorsal application of 4-hydroxy-tempo (a superoxide dismutase-mimicking agent) to mice concentration-dependently reduced UVA-induced S100A8 expression. Incubation of PAM212 cells with superoxide dismutase and catalase during UVA irradiation also abrogated S100A8 induction. These results suggest that UVA-induced S100A8 is expressed by keratinocytes in response to generation of reactive oxygen intermediates.

Mast cells, neuropeptides, histamine, and prostaglandins in UV-induced systemic immunosuppression.

There is a direct correlation between dermal mast cell prevalence in dorsal skin of different mouse strains and susceptibility to UVB-induced systemic immunosuppression; highly UV-susceptible C57BL/6 mice have a high dermal mast cell prevalence while BALB/c mice, which require considerable UV radiation for 50% immunosuppression, have a low mast cell prevalence. There is also a functional link between the prevalence of dermal mast cells and susceptibility to UVB- and cis-urocanic acid (UCA)-induced systemic immunosuppression. Mast cell-depleted mice are unresponsive to UVB or cis-UCA for systemic immunosuppression unless they are previously reconstituted at the irradiated or cis-UCA-administered site with bone marrow-derived mast cell precursors. cis-UCA does not stimulate mast cell degranulation directly. Instead, in support of studies showing that neither UVB nor cis-UCA was immunosuppressive in capsaicin-treated, neuropeptide-depleted mice, cis-UCA-stimulated neuropeptide release from sensory c-fibers which, in turn, could efficiently degranulate mast cells. Studies in mice suggested that histamine, and not tumor necrosis factor alpha (TNF-alpha), was the product from mast cells that stimulated downstream immunosuppression. Histamine receptor antagonists reduced by approximately 60% UVB and cis-UCA-induced systemic immunosuppression. Indomethacin administration to mice had a similar effect which was not cumulative with the histamine receptor antagonists. Histamine can stimulate keratinocyte prostanoid production. We propose that both histamine and prostaglandin E(2) are important in downstream immunosuppression; both are regulatory molecules supporting the development of T helper 2 cells and reduced expression of type 1 immune responses such as a contact hypersensitivity reaction.

Increased dermal mast cell prevalence and susceptibility to development of basal cell carcinoma in humans.

Exposure to ultraviolet B (UVB) radiation (280-320 nm) is the primary etiologic factor associated with the development of basal cell carcinoma (BCC). The outgrowth of these keratinocyte-derived skin lesions is enhanced by the ability of UVB to impair an immune response that would otherwise eliminate them. Studies in a range of inbred mouse strains as well as mast cell-depleted mice reconstituted with mast cell precursors support a functional link between histamine-staining dermal mast cells and the extent of susceptibility to UVB-induced systemic immunomodulation. Humans, like mouse strains, display variations in dermal mast cell prevalence. In a study of Danish and South Australian BCC patients and control subjects, one 4-mm punch biopsy of non-sun-exposed buttock skin was sampled from each participant. This skin site was investigated to avoid any changes in mast cell prevalence caused by sun exposure. Two sections (4 microm) per biopsy were immunohistochemically stained for detection of histamine-containing dermal mast cells. Computer-generated image analysis evaluated dermal mast cell prevalence in both sections by quantifying the total number of mast cells according to the total dermal area (expressed as mast cells per square millimeter). This technique enabled us to detect heterogeneity of dermal mast cell prevalence in buttock skin between individuals and provided evidence of an association between high dermal mast cell prevalence and BCC development in two diverse populations. We hypothesize that mast cells function in humans, as in mouse strains, by initiating immunosuppression following UV irradiation and, thereby, allowing a permissive environment for the development of BCC. Thus, a high dermal mast cell prevalence as demonstrable in buttock skin is a significant predisposing factor for development of BCC in humans.

IFN-gamma downregulates interleukin-4 functional activity on monocytes by multiple mechanisms.

Interleukin-4 (IL-4) has potent anti-inflammatory properties on monocytes and suppresses lipopolysaccharide (LPS)-induced tumor necrosis factor-alpha (TNF-alpha) and IL-1beta production. Culture with interferon (IFN-gamma) alters human monocyte responses to IL-4 by multiple mechanisms. As previously published, IFN-gamma reduced IL-4-activated signal transducer and activator of transcription-6 (STAT-6). This correlated with an inability of IL-4 to suppress LPS-induced TNF-alpha but not IL-1beta production. A second mechanism, apparent some 48 h after exposure to IFN-gamma, involved a significant suppression of IL-4 receptor (IL-4R) expression at the cell surface, and this correlated with the loss of additional functional responses to IL-4, including IL-4-induced suppression of LPS-induced IL-1beta production. This study identified a further role of IFN-gamma on IL-4 responses, including reduced IL-4R surface expression by human monocytes. Increased release of soluble gammac from IFN-gamma-treated monocytes provides an additional mechanism by which IFN-gamma may control the functional activity of IL-4. This study characterizes further the opposing effects of the type 1 and type 2 cytokine regulatory systems.

Endogenous interferon-alpha production by differentiating human monocytes regulates expression and function of the IL-2/IL-4 receptor gamma chain.

In vitro monocyte-derived macrophages (MDMac) and synovial fluid macrophages from inflamed joints differ from monocytes in their responses to interleukin 4 (IL-4). While IL-4 can suppress LPS-induced interleukin beta (IL-beta) and tumour necrosis factor alpha (TNF-alpha) production by monocytes, IL-4 can suppress LPS-induced IL-1 beta, but not TNFalpha production by the more differentiated cells. Recently we reported a correlation between the ability of IL-4 to regulate TNFalpha production by monocytes and the expression of the IL-4 receptor gamma chain or gamma common (gamma c chain). Like MDMac, interferon alpha (IFNalpha)-treated monocytes expressed less IL-4 receptor gamma c chain, reduced levels of IL-4-activated STAT6 and IL-4 could not suppress LPS-induced TNFalpha production. In addition, like monocytes and MDMac, IFNalpha-treated monocytes expressed normal levels of the IL-4 receptor alpha chain and IL-4 significantly suppressed LPS-induced IL-1 beta production. With addition of IFNalpha-neutralizing antibodies, the ability of IL-4 to suppress LPS-induced TNFalpha production with prolonged monocyte culture was restored. Detection of IFNalpha in synovial fluids from inflamed joints further implicates IFNalpha in the inability of IL-4 to suppress TNFalpha production by synovial fluid macrophages. This study identifies a mechanism for the differential expression of gamma c and varied responses to IL-4 by human monocytes compared with MDMac.

Nerve growth factor, neuropeptides, and mast cells in ultraviolet-B-induced systemic suppression of contact hypersensitivity responses in mice.

The induction of systemic immunosuppression following ultraviolet B radiation exposure has been linked with the release of inflammatory and immunomodulatory mediators by cells of the epidermis and dermis. Nerve growth factor has not previously been linked with ultraviolet-B-induced immunosuppressive effects. Nerve growth factor antibodies abrogated ultraviolet-B-induced systemic suppression of contact hypersensitivity responses in BALB/C mice. Subcutaneous injection of nerve growth factor (20 microg per mouse) into dorsal skin 5 d before hapten sensitization on ventral skin suppressed contact hypersensitivity responses in mast-cell-replete but not Wf/Wf mast-cell-depleted mice. Nerve growth factor injected 24 h prior to challenge was not able to suppress the efferent phase of the contact hypersensitivity response. Subcutaneous injection of nerve growth factor (20 microg per mouse) did not suppress contact hypersensitivity responses in capsaicin-pretreated (neuropeptide-depleted) BALB/c mice, and thus sensory c-fibers are necessary for nerve-growth-factor-mediated systemic suppression of contact hypersensitivity responses. Increased concentrations of nerve growth factor within epidermal keratinocytes 8 h after ultraviolet B irradiation were confirmed immunohistochemically. These findings support a role for keratinocyte-derived nerve growth factor via its action on sensory c-fibers, and subsequent release of neuropeptides to mediate mast cell degranulation in systemic suppression of contact hypersensitivity responses in mice following ultraviolet B exposure.