Oxidative breakdown and conversion of urocanic acid isomers by hydroxyl radical generating systems

cis-Urocanic acid (cis-UCA), formed from trans-urocanic acid (trans-UCA) by photoisomerization, has been shown to mimic suppressive effects of UV on the immune system. It is our hypothesis that UCA oxidation products in the skin play a role in the process of immunosuppression. Recently, both UCA isomers were found to be good hydroxyl radical scavengers and in this context we investigated the formation of products resulting from the interaction of hydroxyl radicals with UCA. Hydroxyl radicals were generated by (1) UV/H(2)O(2) (photooxidation), (2) ferrous ions/H(2)O(2) (Fenton oxidation) and (3) cupric ions/ascorbic acid. Oxidation products were identified by spectrometric methods and assessed by reversed-phase HPLC analysis. The photooxidation of UCA was induced by UV-B and UV-C, but not by UV-A radiation. Photooxidation and Fenton oxidation of trans-UCA, as well as of cis-UCA yielded comparable chromatographic patterns of UCA oxidation products. Several of the formed products were identified. The formation of three identified imidazoles was shown in UV-B exposed corneal layer samples, derived from human skin.

Quantitative analysis of chemokine expression by dendritic cell subsets in vitro and in vivo

Upon maturation, dendritic cells (DCs) have to adjust their chemokine expression to sequentially attract different leukocyte subsets. We used real-time quantitative polymerase chain reaction analysis to study in detail the expression of 12 chemokines involved in the recruitment of leukocytes into and inside secondary lymphoid organs, by DCs in distinct differentiation stages, both in vitro and in vivo. Monocyte-derived immature DCs expressed high levels of DC chemokine 1 (DC-CK1), EBI1-ligand chemokine (ELC), macrophage-derived chemokine (MDC), macrophage-inflammatory protein (MIP)-1alpha, and thymus and activation-regulated chemokine (TARC). Upon maturation, DCs up-regulated the expression of DC-CK1 (60-fold), ELC (7-fold), and TARC (10-fold). Activation of DCs by CD40 ligand further up-regulated the expression of ELC (25-fold). We found that freshly isolated blood DCs expressed only low levels of interleukin-8, lymphotactin, and MIP-1alpha. It is interesting that the chemokine profile expressed by activated CD11c(-) lymphoid-like as well as CD11c(+) myeloid blood DCs mimics that of monocyte-derived DCS: Additionally, purified Langerhans cells that had migrated out of the epidermis expressed a similar chemokine pattern. These data indicate that different DC subsets in vitro and in vivo can express the same chemokines to attract leukocytes.

Exposure to UVB induces accumulation of LFA-1+ T cells and enhanced expression of the chemokine psoriasin in normal human skin

Normal human skin shows preferential (epi)dermal infiltration of CD4+ T cells upon acute UV exposure. To study the mechanism behind this feature we locally exposed healthy volunteers to doses of UV commonly encountered by the population. Expression of integrins on T cells and expression of adhesion molecules on dermal endothelial cells were quantitatively assessed by immunohistochemistry in situ. We also investigated the effects of ultraviolet-B (UVB) exposure on psoriasin and IL-16, two specific chemoattractant factors for CD4+ T cells, at messenger RNA (mRNA) level by semiquantitative reverse transcriptase-polymerase chain reaction and at protein level by immunohistochemistry. We found, at day 2 after exposure to four minimal erythema doses of UVB, predominant accumulation of LFA-1+/CLA-/VLA-4- T cells in the dermis. Concomitantly the expression of ICAM-1, but not that of E-selectin and VCAM-1, was upregulated on dermal endothelial cells. The increase in the number of dermal T cells was not due to proliferation because only 2% of the UVB-induced dermal T cells expressed the marker of proliferation Ki-67. Whereas exposure to 35 J/cm2 of ultraviolet-A (UVA), like UVB, induced a loss of intraepidermal T cells at day 2 after exposure, UVA induced neither any influx of T cells into the dermis nor any adhesion molecule upregulation on endothelial cells. In response to UVB exposure, the expression of psoriasin mRNA, but not of IL-16 mRNA, was upregulated; the expression of psoriasin protein was also found to be upregulated. These results suggest that LFA-1/ICAM-1 pathway and psoriasin are both involved in the accumulation of CD4+ T cells into UVB-irradiated skin, possibly via a recruitment mechanism.

Exposure to UVB induces accumulation of LFA-1+ T cells and enhanced expression of the chemokine psoriasin in normal human skin

Normal human skin shows preferential (epi)dermal infiltration of CD4+ T cells upon acute UV exposure. To study the mechanism behind this feature we locally exposed healthy volunteers to doses of UV commonly encountered by the population. Expression of integrins on T cells and expression of adhesion molecules on dermal endothelial cells were quantitatively assessed by immunohistochemistry in situ. We also investigated the effects of ultraviolet-B (UVB) exposure on psoriasin and IL-16, two specific chemoattractant factors for CD4+ T cells, at messenger RNA (mRNA) level by semiquantitative reverse transcriptase-polymerase chain reaction and at protein level by immunohistochemistry. We found, at day 2 after exposure to four minimal erythema doses of UVB, predominant accumulation of LFA-1+/CLA-/VLA-4- T cells in the dermis. Concomitantly the expression of ICAM-1, but not that of E-selectin and VCAM-1, was upregulated on dermal endothelial cells. The increase in the number of dermal T cells was not due to proliferation because only 2% of the UVB-induced dermal T cells expressed the marker of proliferation Ki-67. Whereas exposure to 35 J/cm2 of ultraviolet-A (UVA), like UVB, induced a loss of intraepidermal T cells at day 2 after exposure, UVA induced neither any influx of T cells into the dermis nor any adhesion molecule upregulation on endothelial cells. In response to UVB exposure, the expression of psoriasin mRNA, but not of IL-16 mRNA, was upregulated; the expression of psoriasin protein was also found to be upregulated. These results suggest that LFA-1/ICAM-1 pathway and psoriasin are both involved in the accumulation of CD4+ T cells into UVB-irradiated skin, possibly via a recruitment mechanism.

Differential modulation of human epidermal Langerhans cell maturation by ultraviolet B radiation

UVB irradiation of the skin causes immunosuppression and Ag-specific tolerance in which Langerhans cells (LC) are involved. We tested the effect of UVB on LC that had migrated out of cultured epidermal sheets derived from the skin that was irradiated ex vivo (200, 400, 800, or 1600 J/m2). Two separate subpopulations of LC were distinguished: large-sized LC with high HLA-DR expression, and HLA-DR-low, small LC. UVB stimulated the maturation of the former LC subset as demonstrated by enhanced up-regulation of CD80, CD86, CD54, CD40, and CD83 and reduced CD1a expression in comparison with unirradiated controls. In contrast, the latter LC exhibited little or no up-regulation of these molecules except for high CD1a expression and high binding of annexin V, indicating that they were apoptotic, although their CD95 expression was relatively low. Stimulation of enriched LC with CD40 ligand-transfected cells and IFN-gamma revealed that the release of IL-1beta, IL-6, IL-8, and TNF-alpha was enhanced by UVB. In comparison with HLA-DR-low LC, HLA-DR-high LC were the principal IL-8 producers as demonstrated by intracellular cytokine staining, and they retained more accessory function. There was no detectable secretion of IL-12 p70, and IL-18 production was neither affected by any stimulus nor by UVB. These results suggest a dual action of UVB on LC when irradiated in situ: 1) immunosuppression by preventing maturation and inducing apoptotic cell death in part of LC, and 2) immunopotentiation by enhancing the up-regulation of costimulatory molecules and the production of proinflammatory cytokines in another part.

Urocanic acid isomers are good hydroxyl radical scavengers: a comparative study with structural analogues and with uric acid

UV-exposure of the epidermis leads to the isomerisation of trans-UCA into cis-UCA as well as to the generation of hydroxyl radicals. This study shows by means of the deoxyribose degradation test that UCA isomers are more powerful hydroxyl radical scavengers than the other 4-(5-)substituted imidazole derivatives, such as histidine, though less powerful than uric acid. UCA, present in relatively high concentrations in the epidermis, may well be a major natural hydroxyl radical scavenger.

Interleukin-17 and interferon-gamma synergize in the enhancement of proinflammatory cytokine production by human keratinocytes

Keratinocytes are influenced by cytokines released by skin-infiltrating T lymphocytes. IL-17 is produced by activated CD4+ T cells and can stimulate epithelial cells. We investigated whether IL-17 could modulate the cytokine production and cell-surface molecule expression of keratinocytes. The effects of IL-17 were compared with those of IFN-gamma, which is also derived from activated T cells and is a strong stimulator for keratinocytes. IL-17 enhanced the mRNA and protein production of the proinflammatory cytokines IL-6 and IL-8 in a concentration-dependent way, and induced a weak expression of intercellular adhesion molecule (ICAM)-1 and HLA-DR. The production of IL-1alpha and IL-15 was not altered. IFN-gamma augmented the production of IL-6, IL-8, and IL-15 and strongly induced both cell-surface molecules. IL-17 and IFN-gamma showed marked synergism in the stimulation of IL-6 and IL-8 protein secretion and, to a lesser extent, in the induction of ICAM-1 and HLA-DR expression. The majority of the CD4+ and CD8+ T cell clones derived from lesional psoriatic skin expressed IL-17 mRNA, suggesting that skin-infiltrating T cells can produce this cytokine. This IL-17 mRNA expression was detectable in T helper cell type 1 and type 2 and did not correlate with the IFN-gamma or IL-4 production. In addition, IL-17 mRNA is detectable in biopsies from lesional psoriatic skin, but not in nonlesional control biopsies. Our study indicates that IL-17 is a proinflammatory cytokine, which could amplify the development of cutaneous inflammation and may support the maintenance of chronic dermatoses, through stimulation of keratinocytes to augment their secretion of proinflammatory cytokines.

Low expression of CD40 and B7 on macrophages infiltrating UV-exposed human skin; role in IL-2Ralpha-T cell activation

After UV exposure of skin, epidermal Langerhans cells (LC) are depleted, whereas CD11b+CD36 CD1a- monocytes/macrophages (UV-Mphi) infiltrate. Different immunological outcomes in vivo are mediated by LC (sensitization) and UV-Mphi (tolerance) which may be related to the distinct T cell activation states that these antigen-presenting cells (APC) induce. We previously demonstrated that CD4+ T lymphocytes activated by UV-Mphi are, in contrast to LC-activated T cells, IL-2Ralpha deficient, and we hypothesize that this differential T cell activation is related to differences in co-stimulatory molecules between UV-Mphi and LC. Using four-color flow cytometry, we found a reduced capacity to up-regulate expression of the important co-stimulatory molecules CD40, B7-1 and B7-2 by UV-Mphi relative to LC. This alteration in co-stimulatory molecule expression was selective, because UV-Mphi express equal levels of ICAM-1 and ICAM-3, and increased levels of LFA-1, relative to LC. After bidirectional signaling with T cells during alloantigen presentation, UV-Mphi still exhibited less CD40 and B7-1 than LC. Addition of IFN-gamma induced CD40 and B7-1 expression on UV-Mphi and restored IL-2Ralpha expression on UV-Mphi-activated T cells but had no effect on IL-2Ralpha on resting or LC-activated T cells. The restoration of IL-2Ralpha expression on UV-Mphi-activated T cells by IFN-gamma was inhibited (67 %, p = 0.005) by addition of neutralizing anti-CD40. Therefore, differences in co-stimulatory molecule expression, in particular CD40, on UV-Mphi and LC are critical in determining the distinct T cell activation induced by these APC.

UVB radiation preferentially induces recruitment of memory CD4+ T cells in normal human skin: long-term effect after a single exposure

Acute, low-doses of ultraviolet (UV)-B radiation affect the immune competent cells of the skin immune system. In this study, we examined the time-dependent changes of the cutaneous T cell population in normal human volunteers following a single local exposure to UV. Solar-simulated UV radiation caused an initial decrease in intraepidermal T cell numbers, even leading to T cell depletion at day 4, whereupon a considerable infiltration of T cells in the epidermis occurred that peaked at day 14. In the dermis the number of T cells was markedly increased at days 2 (peak) and 4 after irradiation, and subsequently declined to the nonirradiated control values at day 10. Double-staining with several T cell markers showed that the T cells, infiltrating the (epi)dermis upon UV exposure, were almost exclusively CD4+ CD45RO+ T cells, expressing an alpha/beta type T cell receptor, but lacking the activation markers HLA-DR, VLA-1, and IL-2R. Application of UVB radiation resulted in similar dynamics of T cells, indicating that the UVB wavelengths within the solar-simulated UV radiation were responsible for the selective influx of CD4+ T cells. In conjunction with UVB-induced alterations in the type and function of antigen-presenting cells (i.e., Langerhans cells and macrophages), the changes of the cutaneous T cell population may also contribute to UVB-induced immunosuppression at skin level in man.

The human hair follicle: a reservoir of CD40+ B7-deficient Langerhans cells that repopulate epidermis after UVB exposure

The ability of skin to maintain its protective structural and functional integrity depends on both resident and circulating cells. Until now, it was thought that dendritic antigen presenting cells of epidermis (Langerhans cells) were replaced by circulating bone marrow derived precursors. Here we show by immunostaining studies of timed biopsies taken from human skin after ultraviolet exposure, that hair follicle is a critical reservoir of Langerhans cells that repopulate epidermis depleted of Langerhans cells by a single four minimal erythema dose of ultraviolet B. Immunostaining with antibodies to thymidine dimers showed that ultraviolet B only penetrated the superficial hair follicle opening, whereas deeper follicle was relatively protected. Langerhans cells migrating from hair follicle into epidermis 72 h after ultraviolet exposure have a partial deficiency of molecules important to T cell costimulation. We used four color flow cytometry to show that Langerhans cells isolated from epidermis 72 h after ultraviolet B can upregulate CD40 but not B7-1 or B7-2 expression in culture, suggesting a different phenotype of hair follicle Langerhans cells. Therefore, the hair follicle is a specialized immune compartment of the skin that serves as an intermediate reservoir of Langerhans cells between bone marrow and epidermis, and that may play a critical role in immune surveillance.

Despite the presence of UVB-induced DNA damage, HLA-DR+ cells from ex vivo UVB-exposed human skin are able to migrate and show no impaired allostimulatory capacity

In this study, we investigated the effect of ultraviolet B radiation on human Langerhans cell function. Normal human skin was irradiated ex vivo with single doses of ultraviolet B. For assessment of T-cell stimulatory function, cells that spontaneously migrated from epidermal sheets were used, whereas full-thickness skin biopsies were used to investigate alterations in migratory properties. The cells migrating from ultraviolet B-exposed epidermal sheets demonstrated a decrease in the percentage of HLA-DR positive Langerhans cells, as well as a reduced capacity to induce proliferation of allogeneic T cells, when compared with cells migrating from nonexposed sheets. When a correction was made for the decreased number of HLA-DR positive Langerhans cells migrating from ultraviolet B-exposed epidermis, however, it appeared that the capacity to induce T-cell proliferation was identical for Langerhans cells migrating from ultraviolet B-exposed and nonexposed epidermis. The presence of ultraviolet B-induced DNA damage could be demonstrated in the Langerhans cells from ultraviolet B-treated skin, indicating that the cells had received significant doses of ultraviolet B. As regards the effect of ultraviolet B on migratory properties of Langerhans cells, we found not only that reduced numbers of CD1a-positive Langerhans cells migrated from the ultraviolet B-exposed full-thickness skin, but also that there was a reduction in CD1a-positive Langerhans cells in the epidermis. This implies that ultraviolet B induces death of Langerhans cells as well as loss of cell surface molecules rather than altering Langerhans cells migration, whereas the Langerhans cells that were still able to migrate fully retained the capacity to activate allogeneic T cells.

Prolonged increase of cis-urocanic acid levels in human skin and urine after single total-body ultraviolet exposures

Cis-urocanic acid (cis-UCA), a mediator of immunosuppression, is formed from trans-UCA upon UV-exposure of the skin. This study describes a liquid chromatographic method for the simultaneous quantification of cis- and trans-UCA in skin, urine and plasma of nonirradiated volunteers. It also describes cis- and trans-UCA kinetics in UV-irradiated volunteers. New procedures to remove interfering substances from urine and plasma are reported. Normal levels of cis-UCA in skin, urine and plasma of nonirradiated volunteers were 0.5 nmol/cm2, 0.03 mumol/mmol creatinine (median 0.00) and undetectable and those of trans-UCA were 17.1 nmol/cm2, 1.36 mumol/ mmol creatinine and 0.5 microM, respectively. Upon single total body UVB (290-320 nm) exposures of 250 J/m2, epidermal cis-UCA levels immediately reached a maximum and returned to basic levels 3 weeks later. The cis-UCA levels in urine reached a maximum in 5-12 h postirradiation and reached baseline values in 8-12 days. Additionally, a single total body UVA (320-400 nm) irradiation of 200 kJ/m2 yielded a similar pattern. The kinetics of cis-UCA in plasma could not be followed due to low concentrations; however, that of skin and urine was informative in relation to solar exposures and phototherapy.

In contrast to their murine counterparts, normal human keratinocytes and human epidermoid cell lines A431 and HaCaT fail to express IL-10 mRNA and protein

In mice, keratinocyte-derived IL-10 is up-regulated by ultraviolet-B (UVB) radiation and plays a major role in UVB-induced immunosuppression. The present study was designed to examine whether a comparable phenomenon can be detected in man. Freshly isolated or cultured normal human keratinocytes (NHK) and keratinocyte cell lines A431 and HaCaT were stimulated with graded doses of UVB (up to 200 J/m2) or with a variety of other stimuli. RNA was extracted at various time points post-stimulation and analysed by reverse transcriptase-polymerase chain reaction (RT-PCR) using four different IL-10-specific primer pairs and RNA from monocytes or T cells as positive controls. We failed to detect IL-10 mRNA in NHK from 40 different donors (breast, abdomen, leg, scalp, foreskin) and in A431 and HaCaT cells, irrespective of the stimulation used and despite successful stimulation. Supernatants of NHK, A431 and HaCaT cultures were negative for IL-10 protein, as tested by four different ELISAs and a bioassay. Murine keratinocytes, stimulated under comparable conditions and tested by the same techniques, displayed a strong expression of IL-10 mRNA and protein. Remarkably, an IL-10 mRNA signal could be detected in NHK after a second round of PCR amplification. Because NHK suspensions are contaminated with Langerhans cells, melanocytes and possibly fibroblasts, we tested pure populations of each individual cell type to determine the origin of this IL-10 mRNA. Our results clearly indicate that NHK, Langerhans cells and fibroblasts fail to express IL-10 and that melanocytes are the principal source of IL-10 mRNA in normal human epidermis.

Solar-simulated ultraviolet irradiation induces selective influx of CD4+ T lymphocytes in normal human skin

The proportion and composition of the human cutaneous CD3+ T lymphocyte population was determined in situ following a single exposure to physiological, erythema-inducing doses of simulated solar radiation, mainly consisting of UV radiation. Biopsies were taken 1, 2 and 7 days after local irradiation of normal volunteers with 1, 2 and 4 MED by a xenonarc lamp and immunohistochemistry was performed on cryostat sections. Ultraviolet radiation caused an initial decrease of intraepidermal CD3+ T-cell numbers or even could lead to T-cell depletion 24 and 48 h postirradiation, and this was followed by an infiltration of T cells in the epidermis as determined 1 week after UV exposure. The number of dermal CD3+ T cells was increased 24 h after irradiation, reached a maximum at 48 h and subsequently declined at day 7, though remained significantly higher than the unirradiated control. Double staining demonstrated that the CD3+ T cells, which immigrated into the (epi)dermis upon UV exposure, coexpressed CD4 but not CD8. Therefore the CD4/CD8 ratio in skin was markedly increased during the first week upon UV exposure. Our time course study shows that UV radiation affects the T-cell population within human skin by depleting the majority of epidermal T cells and initiating a selective influx of CD4+ T cells.

Reduced IL-12 production by monocytes upon ultraviolet-B irradiation selectively limits activation of T helper-1 cells

The capacity of APC to stimulate the proliferation of human peripheral blood T cells decreases upon ultraviolet-B (UVB) irradiation. The aim of this study was to investigate whether all T cell subsets are equally sensitive to this reduced APC function. Established human Th1, Th2, and Th0 clones were stimulated with monocytes in a soluble CD3 mAb-mediated assay that is dependent on the presence of APC. Monocytes were exposed to low nonlethal doses of UVB radiation before coculture with T cells. UVB irradiation inhibited the capacity of monocytes to stimulate the proliferation and IFN-gamma production of Th1 cells in a dose-related fashion. In contrast, UVB-treated monocytes induced normal proliferation and IL-4 production in Th2 cells. Stimulation of Th0 cell proliferation by UVB-irradiated monocytes was normal, but a preferential suppression of IFN-gamma production was observed, thus leading to a more Th2-like cytokine response. The loss of Th1 proliferation upon stimulation with UVB-irradiated monocytes could be overcome by rIL-2; however, IFN-gamma production remained suppressed. IFN-gamma production could be completely restored by rIL-12, whereas the addition of IL-1 beta, TNF-alpha, or indomethacin had no such effect, nor did the addition of mAb to CD28, added to compensate for the reduced B7 expression of UVB-irradiated monocytes. Monocytes exposed to UVB radiation exhibited reduced expression of mRNA for the IL-1 2 subunits p35 and p40 and suppressed production of the IL-12 p70 protein. Our results thus indicate that UVB irradiation of APC selectively impairs Th1-like responses, a phenomenon caused by the UVB-induced suppression of monocyte IL-12 production.

Reduced IL-12 production by monocytes upon ultraviolet-B irradiation selectively limits activation of T helper-1 cells

The capacity of APC to stimulate the proliferation of human peripheral blood T cells decreases upon ultraviolet-B (UVB) irradiation. The aim of this study was to investigate whether all T cell subsets are equally sensitive to this reduced APC function. Established human Th1, Th2, and Th0 clones were stimulated with monocytes in a soluble CD3 mAb-mediated assay that is dependent on the presence of APC. Monocytes were exposed to low nonlethal doses of UVB radiation before coculture with T cells. UVB irradiation inhibited the capacity of monocytes to stimulate the proliferation and IFN-gamma production of Th1 cells in a dose-related fashion. In contrast, UVB-treated monocytes induced normal proliferation and IL-4 production in Th2 cells. Stimulation of Th0 cell proliferation by UVB-irradiated monocytes was normal, but a preferential suppression of IFN-gamma production was observed, thus leading to a more Th2-like cytokine response. The loss of Th1 proliferation upon stimulation with UVB-irradiated monocytes could be overcome by rIL-2; however, IFN-gamma production remained suppressed. IFN-gamma production could be completely restored by rIL-12, whereas the addition of IL-1 beta, TNF-alpha, or indomethacin had no such effect, nor did the addition of mAb to CD28, added to compensate for the reduced B7 expression of UVB-irradiated monocytes. Monocytes exposed to UVB radiation exhibited reduced expression of mRNA for the IL-1 2 subunits p35 and p40 and suppressed production of the IL-12 p70 protein. Our results thus indicate that UVB irradiation of APC selectively impairs Th1-like responses, a phenomenon caused by the UVB-induced suppression of monocyte IL-12 production.

Intercellular adhesion molecule-3 (CD50) on human epidermal Langerhans cells participates in T-cell activation

Three different intercellular adhesion molecules (ICAMs) have been identified acting as ligand for counter-receptor leukocyte-function-associated antigen-1 (LFA-1) (CD11a/CD18). We have recently shown that ICAM-1 (CD54) is present on cultured human epidermal Langerhans cells but not on freshly isolated Langerhans cells, and that this molecule participates in the generation of an antigen-specific T-cell response. ICAM-2 (CD102) was not involved because this molecule is expressed by neither fresh nor cultured Langerhans cells. In this study, the presence of ICAM-3 (CD50) on Langerhans cells was examined. Flow cytofluorometric analysis demonstrated that ICAM-3 is strongly displayed by fresh Langerhans cells, and daily determinations showed that the level of this trypsin-resistant molecule remained nearly unchanged during in vitro culture for up to 4 d, indicating that Langerhans cells constitutively express this molecule. Analysis of RNA extracted from purified cultured Langerhans cells by means of reverse transcriptase-polymerase chain reaction demonstrated the presence of mRNA specific for ICAM-3. Antigen-specific T-cell responses triggered by Langerhans cells were dose-dependently inhibited by anti-ICAM-3 if the antibody was added within the first 16 h of T-cell stimulation. Simultaneous addition of anti-ICAM-1 and anti-ICAM-3 synergistically inhibited T-cell responses, although a total block was never achieved. Pretreatment of Langerhans cells with anti-ICAM-3 resulted in a reduced T-cell response, whereas pretreatment of T cells did not. These results suggest that ICAM-3 on Langerhans cells, like ICAM-1, is functionally involved in the initiation of antigen-specific activation of T cells, but the expression of these two ICAMs on Langerhans cells is differently regulated.

Photoisomerization spectrum of urocanic acid in human skin and in vitro: effects of simulated solar and artificial ultraviolet radiation

Ultraviolet (UV) irradiation of trans-urocanic acid (UCA), a major UV absorbing component of the epidermis, leads to the formation of cis-UCA, which mediates immunosuppressive effects. In this study, the net yield of cis-UCA was measured after the photoisomerization of urocanic acid by narrow UV wavebands (spectral range 295-405 nm), with the irradiation doses related to solar irradiance at sea level. The formation of cis-UCA in Caucasian skin (in vivo), as well as in aqueous solution (in vitro), was determined by HPLC analysis. The same irradiation conditions were met in both components of the study. The in vivo experiments showed high efficiency of cis-UCA formation in the spectral region of 305-341 nm, whereas high efficiency in vitro was found at 305 and 326 nm. At 350 and 363 nm, cis-UCA was formed in vivo, but not in vitro. At longer test wavelengths up to 405 nm, no significant formation of cis-UCA was detectable. The established partition between UVB and UVA at 320 nm is not relevant for the isomerization pattern of UCA. Additional studies revealed substantial cis-UCA formation in human skin by UVA phototherapy lamps. Furthermore, raised levels of 295 nm irradiation doses, a possible effect of stratospheric ozone depletion, were found to increase the cis-UCA yield. Our results demonstrate that the formation of cis-UCA in the skin with common exposures takes place over a broad spectrum range of UVB and UVA, up to at least 363 nm. These findings emphasize the potency of UVA to isomerize UCA, and they may contribute to further elucidation of the effects of phototherapy and sunbathing.

Function of adhesion molecules lymphocyte function-associated antigen-3 and intercellular adhesion molecule-1 on human epidermal Langerhans cells in antigen-specific T cell activation

In addition to the interaction between the TCR and the MHC/Ag complex on the APC, optimal T cell activation also requires interaction between adhesion molecules on the APC and their ligands on T cells. We determined the presence of adhesion molecules on human epidermal Langerhans cells (LC) and their role in Ag-specific T cell activation. Freshly isolated LC did not display ICAM-1 (CD54), ICAM-2, LFA-1 (CD11a), and LFA-3 (CD58), as detected by double-color FACS analysis, using HLA-DR expression for LC identification. Upon culture, LC clearly expressed ICAM-1 and LFA-3, both already detectable after 1 day, reaching a plateau at day 2. ICAM-2 and LFA-1 were undetectable on cultured LC and attempts to induce this expression by different culture conditions remained unsuccessful. mAb against ICAM-1, LFA-1, LFA-3, and CD2, continuously present during culture, inhibited the T cell proliferative response to Candida albicans presented by cultured LC. Pretreatment of LC and/or T cells with mAb indicated that anti-ICAM-1 and anti-LFA-3 inhibited at the LC level, whereas anti-LFA-1 and anti-CD2 inhibited at the T cell level. The mAb-induced inhibition was dose-dependent, but a total blockade of the response was never achieved. Time-course observations revealed that ICAM-1 and LFA-3 on LC only functioned during the initiation phase of T cell activation. Our study demonstrates that both ICAM-1 and LFA-3 on LC considerably contribute to the generation of a T cell response. The high expression of these accessory molecules enable LC, at least in part, to perform their powerful Ag-presenting function.

Function of adhesion molecules lymphocyte function-associated antigen-3 and intercellular adhesion molecule-1 on human epidermal Langerhans cells in antigen-specific T cell activation

In addition to the interaction between the TCR and the MHC/Ag complex on the APC, optimal T cell activation also requires interaction between adhesion molecules on the APC and their ligands on T cells. We determined the presence of adhesion molecules on human epidermal Langerhans cells (LC) and their role in Ag-specific T cell activation. Freshly isolated LC did not display ICAM-1 (CD54), ICAM-2, LFA-1 (CD11a), and LFA-3 (CD58), as detected by double-color FACS analysis, using HLA-DR expression for LC identification. Upon culture, LC clearly expressed ICAM-1 and LFA-3, both already detectable after 1 day, reaching a plateau at day 2. ICAM-2 and LFA-1 were undetectable on cultured LC and attempts to induce this expression by different culture conditions remained unsuccessful. mAb against ICAM-1, LFA-1, LFA-3, and CD2, continuously present during culture, inhibited the T cell proliferative response to Candida albicans presented by cultured LC. Pretreatment of LC and/or T cells with mAb indicated that anti-ICAM-1 and anti-LFA-3 inhibited at the LC level, whereas anti-LFA-1 and anti-CD2 inhibited at the T cell level. The mAb-induced inhibition was dose-dependent, but a total blockade of the response was never achieved. Time-course observations revealed that ICAM-1 and LFA-3 on LC only functioned during the initiation phase of T cell activation. Our study demonstrates that both ICAM-1 and LFA-3 on LC considerably contribute to the generation of a T cell response. The high expression of these accessory molecules enable LC, at least in part, to perform their powerful Ag-presenting function.

Effect of low-dose ultraviolet-B radiation on the function of human T lymphocytes in vitro

Purified peripheral blood human T lymphocytes, derived from normal individuals, were assayed for their susceptibility to low doses of ultraviolet B (UVB) in vitro. Exposure of T cells to graded single doses (range 0-8 mJ/cm2) of UVB resulted in a dose-dependent reduction of viability. This phototoxic effect was not immediately apparent, however, but became manifest 48-72 h subsequent to irradiation. A dose as little as 0.5-1 mJ/cm2 was sufficient to cause 50% mortality. Irradiated T cells showed a reduced ability to proliferate, irrespective of the stimulus used, and a reduced ability to produce cytokines IL-2, IL-4, IL-5, interferon-gamma (IFN-gamma) and tumour necrosis factor-alpha (TNF-alpha). This decreased ability was UVB-dose related and, remarkably, was exactly correlated to phototoxicity. UVB had no effect on CD4 and CD8 expression or their ratio, whereas the expression of IL-2R (CD25) was only slightly reduced. Our data suggest that UVB radiation neither selectively affects Th1 or Th2 nor CD4 or CD8 T cell subsets. The high susceptibility of T cells to UVB might explain, at least in part, the beneficial effect of phototherapy during treatment of certain immunodermatological diseases.

Functional role of adhesion molecules LFA-3 and ICAM-1 on cultured human epidermal Langerhans cells in antigen-specific T-cell activation

Interaction of T-cell antigen receptors with antigen/major histocompatibility complex (MHC) molecule complexes on antigen-presenting cells (APC) leads to T-cell activation. Additional interaction between adhesion molecules on the APC and their ligands on T cells is required for optimal T-cell activation. In vitro cultured human epidermal Langerhans cells (cLC) are powerful APC that express adhesion molecules LFA-3 and ICAM-1. Both molecules on cLC serve a functional role in the generation of a T-cell response.

Dynamic nature and function of epidermal Langerhans cells in vivo and in vitro: a review, with emphasis on human Langerhans cells

Epidermal Langerhans cells (LC) are Birbeck granule-containing bone-marrow-derived cells, which are located mainly in the suprabasal layer of the epidermis. They can be readily identified by their strong expression of CD1a and MHC class II molecules. In addition to these 'classical' properties, an extensive phenotypic profile of normal human LC, summarized in this review, is now available. The powerful capacity of LC to activate T lymphocytes is clearly documented and, to date, LC are recognized as the prominent antigen-presenting cells of the skin immune system. They are generally believed to pick up antigens encountered in the epidermis and to migrate subsequently from the epidermis to the skin-draining lymph nodes. Upon arrival in the paracortex of lymph nodes, the antigen-laden LC transform into interdigitating cells and they present antigen to naive T lymphocytes in a MHC class II-restricted fashion; this results in the generation of antigen-specific immune responses. It has also been demonstrated that transformation of LC into interdigitating cells occurs when LC are cultured in vitro. Both in vivo and in vitro studies have indicated that properties of LC, such as phenotype, morphology and the stimulatory potential to activate T lymphocytes, are dependent on the local microenvironment in which the LC reside. The essential role of LC in the induction of contact allergic skin reactions and skin transplant rejection is well established.

A comparison of the inhibitory effects of immunosuppressive agents cyclosporine, tetranactin, and didemnin B on human T cell responses in vitro

The agents cyclosporine, tetranactin (TN), and didemnin B (DB) were compared for their ability to inhibit proliferative human T cell responses in vitro, using anti-CD3, PHA, alloantigen, or tetanus toxoid as stimuli and using monocytes or Langerhans cells as antigen-presenting cells/accessory cells (APC/AC). We found that all three agents suppressed T cell activation in a dose-dependent fashion, irrespective of the stimulus of APC/AC type used. Both T cells and APC/AC were affected by the drugs. DB appeared to be the most potent suppressive drug (IC50 = 1-4 ng/ml), whereas CsA and TN exerted approximately similar potency (IC50 = 50-60 ng/ml). Remarkably however, DB was toxic at a concentration of 10 ng/ml, which is quite close to the inhibition-inducing dose. No toxicity was observed with CsA and TN at doses up to 5000 ng/ml. The agents TN and DB could interrupt ongoing T cell responses and could block responsiveness to exogenous recombinant IL-2. Expression of IL-2 receptors was slightly inhibited by all three drugs. Expression of MHC class II molecule HLA-D and of adhesion molecules LFA-1, LFA-3, and ICAM-1 was clearly reduced by DB, giving an explanation for the observed inhibition of cluster formation between T cells and APC/AC. Except for a slight reduction of LFA-3 by TN, CsA and TN did not affect the expression of any of these cell surface markers or the formation of clusters. Differences in the effects of CsA, TN, and DB on immune responses in vitro and on the phenotype of T cells and APC/AC suggest that these immunosuppressive drugs have different inhibitory mechanisms.

Inhibitory effect of cyclosporin A on antigen and alloantigen presenting capacity of human epidermal Langerhans cells

The effect of cyclosporin A (CyA) on the capacity of human epidermal Langerhans cells (LC) to stimulate allogeneic T cells or to present antigen to autologous T cells was investigated. Preparations of LC enriched by discontinuous density gradient centrifugation were pulsed for 2 or 16 h with graded doses (5-5000 ng/ml) of CyA prior to co-culture with T cells. Pretreatment of LC with CyA resulted in a dose-dependent decrease of the functional capacity of LC to stimulate T cells. This inhibition (up to 90%), already achieved after a pulse of 2 h, was not due to a cytotoxic effect of the drug and appeared to be reversible. The possibility that CyA exerted its effect indirectly on T cells via release of CyA from LC into the supernatant during co-culture was excluded. The suppression of immunostimulatory function was a direct effect of the drug on LC. CyA did not affect the production by LC of IL-1 or prostaglandin, nor the expression of MHC class II products HLA-D and RFD1 or adhesion molecules ICAM-1 and LFA-3. These results suggest that inhibition of contact allergic skin reactions by CyA may be due in part to an impairment of the function of LC.

Human epidermal Langerhans cells undergo profound morphologic and phenotypical changes during in vitro culture

Morphology, phenotype, and enzyme activity of highly enriched (80%) unlabeled human epidermal Langerhans cells (LC) have been studied, with emphasis on changes during a short-term culture of three days in vitro. All freshly isolated LC contained Birbeck granules and expressed high levels of CD1a, CD1c, and MHC class II molecules HLA-DR, -DP, and -DQ. They have a weak to moderate expression of RFD1, C3biR, Fc gamma R, p 150/95, MHC class I molecules HLA-ABC, and of the adhesion molecules LFA-3 and ICAM-1, whereas no expression of LFA-1 and several monocyte/macrophage markers were detected. Human LC undergo profound changes during in vitro culture. Birbeck granules, C3biR, Fc gamma R, and p 150/95 were completely lost and the expression of CD1a and CD1c was markedly decreased or lost. Expression of molecules that have essential functions in antigen presentation remained present at the same level (MHC class II molecules and ICAM-1) or was markedly enhanced (LFA-3 and MHC class I). Highly remarkable was the dramatically enhanced expression of interdigitating cell marker RFD1. The monocyte/macrophage markers initially absent remained absent and the enzyme activity initially present (including ATPase and nonspecific esterase) remained present. In conclusion, the results in this report stress rapid alterations of human LC during in vitro culture, resulting in transformation into cells that have phenotypical characteristics of potent antigen presenting cells that resemble interdigitating cells.

T-cell receptor gamma delta bearing cells in normal human skin

T-cell antigen receptors (TCR) are divided into common alpha beta and less common gamma delta types. In the murine skin, TCR gamma delta+ cells have been reported to form the great majority of epidermal T lymphocytes. We have examined the relative contribution of TCR alpha beta+ and TCR gamma delta+ cells to the T-cell population in normal human skin. Serial sections of freshly frozen skin specimens were acetone fixed, incubated with anti-CD3, beta F1 (anti-TCR alpha beta), anti-TCR gamma delta-1 and anti-TCR delta 1 (anti-TCR gamma delta) monoclonal antibodies (MoAb), and stained with a highly sensitive method. Over 90% of the T cells of normal human skin are localized around the postcapillary venules of the dermis, while less than 5% are present within the epidermis. In papillary dermis, TCR gamma delta+ cells formed on average 7% (anti-TCR gamma delta-1) or 9% (anti-TCR delta 1) of the total number of CD3+ cells, while TCR alpha beta+ cells constituted up to 80%. In epidermis, these percentages were 18% and 29% for TCR gamma delta+ cells, and up to 60% for TCR alpha beta+ cells. It is concluded that there is no preferential immigration or in situ expansion of TCR gamma delta+ T cells in normal human skin, because the relative percentages found for the TCR alpha beta+ and TCR gamma delta+ populations in skin are comparable to those found in lymphoid organs and peripheral blood. However, the percentage of TCR gamma delta+ cells in epidermis seemed on average higher than in papillary dermis. Therefore, there may still be a difference in migration patterns of TCR gamma delta+ v TCR alpha beta+ cells, but this does not result in their preferential localization in human epidermis. The hypothesis that TCR gamma delta+ T cells have a specialized function in immunosurveillance of epithelia may thus not be valid for human epidermis.

The selective antigen-presenting cell capacity of activated B lymphocytes in HLA-II-restricted responses of CD4+ T lymphocytes

We examined the role of antigen-presenting B lymphocytes using panels of antigen-specific CD4+8-T-lymphocyte clones (TLC). All 19 TLC showed a class II major histocompatibility complex-encoded (HLA-II) restricted proliferation to antigen presented by antigen-presenting cells (APC) from the monocyte fraction of peripheral blood. Only six TLC were effectively activated by antigen presented by autologous B lymphocytes activated by EBV transformation. This failure of B lymphocytes was not due to: (i) a high degree of cell surface sialic acid; (ii) a low expression of the cell surface proteins HLA-II, ICAM-1 or LFA-3 that restrict antigen presentation; (iii) lack of secretion of the cytokine IL-1 or other soluble factors that may be required as secondary signals; or (iv) induction of incomplete T-cell activation resulting in the production of growth factor interleukin-2 (IL-2) or the expression of receptors for IL-2 only. These data suggest the involvement of another cell surface interaction in antigen presentation acting besides the interactions known so far.

Enrichment of unlabeled human Langerhans cells from epidermal cell suspensions by discontinuous density gradient centrifugation

In this report we introduce an alternative procedure for enrichment of human epidermal Langerhans cells (LC) from epidermal cell suspensions of normal skin. By means of discontinuous Ficoll-Metrizoate density gradient centrifugation, a fraction containing high numbers of viable, more than 80% pure LC was recovered, as judged by CD1a expression. The purity of the LC-enriched fraction appeared to be dependent on the percentage LC in the crude epidermal cell suspension. LC enriched by this method retained their accessory and antigen-presenting capacities, as determined in the Concanavalin-A induced T-cell response, in the allogeneic mixed leukocyte reaction and in the antigen-specific T-cell proliferation assay in vitro. The great advantage of this method is that it is simple and rapid and that the isolated LC are unlabeled.

Pathogenesis of HIV and its implications for serodiagnosis and monitoring of antiviral therapy

Human immunodeficiency virus (HIV) is lymphotropic and neurotropic. In vivo clinical and immunological abnormalities develop in a large proportion of long-term HIV antibody seropositive persons. Different stages of HIV infection are marked by expression of HIV genes, production of HIV antibodies, formation of antigen/antibody complexes and clearance of such complexes. Transient HIV antigenemia appearing generally 6-8 wk prior to HIV antibody (HIV-Ab) seroconversion and lasting 3-4 mth is generally seen in acute infection. IgM antibodies predominantly to core proteins may occasionally be detectable when, or just before, IgG antibodies appear. If IgG antibodies to both envelope and core proteins persist in the absence of HIV-Ag the short-term prognosis is relatively good. However, HIV-Ag seroconversion may appear at any time after HIV-Ab seroconversion. Progression to AIDS is strongly associated with declining or absent levels of IgG antibodies to p24. IgG2 and IgG4 antibodies to HIV, which are mainly directed to p24, disappear most dramatically. Titers of antibodies to HIV p24 below 64 are strongly associated with the presence of HIV antigen and a poor clinical outcome. HIV antigen was detected frequently in sera from children in all stages of infection in contrast to adults whose sera were generally HIV-Ag negative when asymptomatic and positive when AIDS was apparent. HIV antigen may be less efficiently detected with the present assays in sera from regions where the prototype strains of HIV (HTLV-III and LAV) are less prevalent, like Central Africa. Persistence of HIV-Ag in cerebrospinal fluid (CSF) appears to be pathognomonic for progressive encephalopathy, particularly in children. Levels of HIV-Ag in serum, and possibly in CSF, can be decreased by nucleoside analogues, such as AZT. This indicates HIV-Ag and possibly antibody to HIV core protein p24 as suitable markers for selecting individuals for antiviral therapy as well as monitoring the efficacy of such therapy.

Nickel-specific T lymphocyte clones derived from allergic nickel-contact dermatitis lesions in man: heterogeneity based on requirement of dendritic antigen-presenting cell subsets

Inflammatory skin T cells were cloned in an antigen-independent way from lesions of patients with experimentally induced nickel-contact dermatitis. In three experiments 7-15% of the CD4+8- T lymphocyte clones (TLC) appeared to be specific for nickel in a proliferation assay. These proliferative response of nickel-specific TLC required the presence of antigen-presenting cells (APC) and were restricted by HLA class II molecules. All TLC recognized nickel presented by APC from epidermal skin. Remarkably, 5 out of 8 nickel-specific TLC exclusively recognized nickel when presented by these skin-specific APC whereas 3 out of these 8 clones could also recognize nickel presented by APC from peripheral blood. The critical APC within the fractions of epidermal cells and peripheral blood appeared to belong to the family of dendritic cells, i.e. Langerhans cells and circulating dendritic cells, respectively.

Antigen-presenting cell function of dendritic cells and macrophages in proliferative T cell responses to soluble and particulate antigens

The capacity of dendritic cells (DC) and macrophages (M phi) to present soluble and particulate antigen was tested in an ovalbumin (OVA)-specific T cell proliferation assay. In a comparative investigation we found that both DC and M phi were able to present soluble OVA, but that only M phi could present insolubilized OVA to T cells. DC were found to be able to present OVA in collaboration with M phi. The failure of DC to present insolubilized OVA is probably caused by their inability to endocytose these antigens. DC appeared not to endocytose substantial amounts of soluble OVA either. In contrast to M phi, antigen presentation by DC is not blocked by lysosomotropic drugs. Taken together, these observations suggest that DC can present soluble protein antigens without intracellular degradation.

Dendritic cells as accessory cells in antigen-specific murine T lymphocyte proliferation in vitro

The role of dendritic cells in antigen-induced murine T lymphocyte activation was studied by addition of purified dendritic cells to purified lymph node T lymphocytes from ovalbumin-primed mice. In the presence of the priming antigen T cells generated an antigen-specific response. The response was at least 3-fold higher with the use of a modified IMDM culture medium. The complete requirement for accessory cells was demonstrated only when nylon wool-purified T lymphocytes were thoroughly depleted of Ia antigen-expressing cells. Dendritic cells as well as peritoneal exudate macrophages were equally effective as antigen-presenting cells.