Phototherapeutic hardening modulates systemic cytokine levels in patients with polymorphic light eruption

Abstract

The etiopathogenesis of polymorphic light eruption (PLE) has been linked to impaired UV-immunosuppression, Langerhans cell (LC) retention, and an absence of neutrophil infiltration into UV-exposed PLE skin. We have previously shown that photohardening restores the impaired neutrophil responsiveness to the chemoattractants leucotriene B4 and formyl-methionyl-leucyl-phenylalanin in PLE patients. The aim of this study was to investigate whether photohardening modulates baseline chemokine and cytokine levels which would alter chemoresponsiveness and hence immune function in PLE patients. Sixteen PLE patients received photohardening therapy for 4–9 weeks by 311 nm UVB. Plasma samples were taken both before and within 48 h of the penultimate phototherapeutic exposure. Plasma from these 16 patients, 8 non-irradiated PLE patients, and 14 control subjects was analyzed for IL-1β, CXCL8 (IL-8), IL-10, IL-17, TNF, CCL2 (MCP-1), CCL5 (RANTES), CCL11 (eotaxin), and CCL22 (MDC). These cytokines and chemokines were measured in early spring (March to April) and again in late spring (April to June). PLE patients had a significantly elevated level of CCL11 (p = 0.003) and IL-1β (p = 0.002) in early spring (before phototherapy). In late spring, after phototherapy, PLE patients had significantly elevated CCL2 (p = 0.002) and TNF (p = 0.002) but a trend for lowered plasma levels of CXCL8 (p = 0.021). When comparing the cytokine shifts from early to late spring, while healthy controls and non-UV-irradiated PLE patients showed an increase, PLE patients undergoing photohardening exhibited a trend for decrease in IL-1β (p = 0.012). Taken together, our results indicate that photohardening may alter the complex cytokine milieu in PLE, in particular via IL-1β, helping to normalise the pathophysiologic response to subsequent UV exposure.

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Introduction

Polymorphic light eruption (PLE) is the most common form of photodermatosis, with a prevalence of up to 21%, particularly among young women in temperate climates.^1–3 A failure to establish immune suppression after UV exposure, as determined through experimental work in models of contact hypersensitivity,^4,5 and a simultaneous reaction against potential photo-neoantigens,⁶ has been implicated in the pathogenesis of PLE. The resistance to UV-induced immune suppression has been linked to Langerhans cell (LC) retention and an absence of neutrophil infiltration into UV-exposed PLE skin, related to a variety of immunosuppressive cytokines.^2,1

In normal subjects, LC emigration from the epidermis is induced and enhanced by cytokines like TNF and IL-1β,⁷ which are upregulated in human epidermis after UVB irradiation.^8–12 In PLE, Koelgen et al. showed that UVB failed to deplete CD1a+ epidermal LCs, indicating that a defect in UV-induced immune suppression is a key event in these patients.¹³ Upon UV exposure, the appearance of DNA-damaged LC in the lymph nodes is important for the induction of UV-induced regulatory T cells.¹⁴ Comparing the expression of UVB-induced cytokines in the skin of normal individuals with that from PLE patients, Koelgen et al. showed that the skin of PLE patients contained lower levels of cytokines, in particular the LC-migration inducing IL-1 and TNF.¹⁵ However we were unable to confirm this as we found no difference in UV-induced LC migration or cytokine gene expression measured by mRNA levels of TNF, IL-1β, IL-10, and IL-12 between PLE patients and control subjects.¹⁶ In contrast, we found a lack of skin infiltration by CD123+ plasmacytoid dendritic cells upon UV exposure in patients with PLE compared with lupus erythematosus,¹⁷ another photosensitive disease, in which IL-6 is a key cytokine.¹⁸

Concurrently, with the depletion of CD1a+ LCs after UV exposure, CD36+CD11b+CD1− macrophage-like cells expand in the dermis and infiltrate the epidermis.^19,20 CD11b+ macrophage-like cells and neutrophils play an important role in the induction of tolerance and suppression of delayed type hypersensitivity after UVB irradiation.^20–23 The role for neutrophils in UVB-induced skin pathology was supported by a study of Teunissen et al. who showed that UVB irradiation induces a transient appearance of IL-4+ neutrophils in normal human skin and that these cells contribute to the enhanced development of Th2 cells in UVB-irradiated skin.⁹ The UVB-induced IL-4 was of sufficient quantity to be measured in suction blister fluid obtained from UV-irradiated skin and in addition an induction or increase in the levels of TNF, IL-6 and IL-8 was detected in the suction blister fluid of UV-exposed skin.⁹

IL-10 is an immunosuppressive cytokine induced in normal skin upon UVB irradiation that counteracts IL-12 activity thereby inhibiting the activation of Th1 cells.^23–27 IL-10 has been linked to impaired antigen function of LCs²⁷ and is essential for UV immunosuppression.²⁸ IL-10 is predominantly expressed by CD11b+ HLA DR+ macrophages infiltrating normal UVB-exposed human skin^23,24 although newly recruited neutrophils may also contribute.²⁹ However, neutrophils inadequately infiltrate the skin of UV-irradiated PLE patients suggesting that this may be an important mechanism by which UVB irradiation fails to induce immune suppression.³⁰ This is supported by the work of Koelgen et al., who showed that the lack of neutrophil infiltration into the skin of UVB-irradiated PLE patients was associated with a reduced expression of TNF, IL-4, and to a lesser extent, IL-10.¹⁵ They concluded that the reduced expression of neutrophil derived TNF, IL-4, and IL-10 in UVB-irradiated PLE skin is responsible for both reduced LC migration and a failure to suppress Th1 responses in these patients.¹⁵

Leukocyte migration into tissue is initiated by the local release of chemoattracting cytokines such as CXCL8 (previously known as IL-8), activated complement products like anaphylatoxins (C5a), bacterial products such as formyl-methionyl-leucyl-phenylalanin (fMLP), as well as lipid mediators including platelet activating factor (PAF)^31,32 and LTB₄.³³ CXCL8 and CCL3 (previously known as MIP-1α) are potent neutrophil chemoattractants³⁴ while other cytokines and chemokines can mediate the emigration of LCs (TNF, IL-1β) or recruit monocytes and lymphocytes (CCL2, CCL5). CCL2 (previously known as monocyte chemotactic protein-1, MCP-1), is a small inducible cytokine belonging to the CC chemokine family. It is chemoattractive for monocytes, memory T cells, and dendritic cells to sites of tissue injury, infection, and inflammation.^35,36 CCL5 (previously known as RANTES) expression and production is upregulated in UVB-exposed keratinocytes, promoting mast cell accumulation and perpetuating the skin inflammation associated with lupus erythematosus.³⁷ CCL22 (previously known as macrophage derived chemokine (MDC) is another CC chemokine produced by macrophages and dendritic cells that is chemotactic for dendritic cells as well as CCR4-expressing Th2 cells. It is upregulated by Th2 cytokines such as IL-4 as well as prostaglandins, but is downregulated by the Th1 cytokine IFNγ.³⁸ CCL11 (previously known as eotaxin) is chemoattractive for eosinophils, basophils, neutrophils, and macrophages.

TNF and CXCL8 are up-regulated in the epidermis of normal human skin after UVB exposure coinciding with neutrophil accumulation and upregulation of the adhesion molecule E-selectin.⁸ The failure to induce LC emigration and the reduced infiltration by neutrophils within the skin of UVB-exposed PLE patients was not attributable to any differences in expression of UVB-induced proinflammatory and chemotactic cytokines.³⁹ In fact, the aberrant cell migration observed in patients with PLE after UVB irradiation is not attributed to differential expression of CXCL8, CCL2, CCL3, or CCL4 (MIP-1β).³⁹ Moreover, neutrophil chemotactic responses to CXCL8 and C5a were found to be similar in PLE patients compared with healthy controls.³⁰ In contrast, although UVB irradiation caused significant IL-1α increases in the skin of both groups, the levels of IL-1α and IL-1β were two-fold higher in the PLE group. Consequently the ratios of IL-1Ra to both IL-1α and IL-1β were significantly lower in the skin of PLE patients, revealing an amplified early proinflammatory cytokine response in PLE that may explain the abnormal PLE reaction to UVB irradiation.³⁹ However, our conflicting studies found no significant difference in IL-1β mRNA expression in the skin between PLE patients and healthy controls exposed to up to 3 minimal erythema doses (MEDs) of solar simulated UV.¹⁶

Medical photohardening^40–42 significantly improves the UV-induced cell migratory responses in PLE patients restoring both the capacity of UV to deplete LC and recruit neutrophils into the skin.⁴³ Restoring normal immune cell migration is likely to be a key mechanism involved in the beneficial effects of phototherapy in PLE patients. We have previously shown that photohardening restores the impaired neutrophil responsiveness to the chemoattractants LTB₄ and fMLP in PLE patients.⁴⁴ This revealed abnormal neutrophil chemotactic responses as a crucial factor in the pathogenesis of PLE. However, the mechanisms by which photohardening normalises the neutrophil responsiveness remains unknown. We hypothesised that photohardening restores neutrophil trafficking in PLE patients by altering the key cytokines and chemokines responsible for responsiveness and recruitment of neutrophils to the skin. The aim of this study was therefore to investigate the effects of photohardening on TNF, IL-1β, CXCL8, IL-10, IL-17, CCL2, CCL5, CCL11, and CCL22 in PLE patients.

Methods

Setting

The study was conducted at the Research Unit for Photodermatology, Medical University of Graz, Austria. The study protocol (No. 18-116 ex 06/07) was approved by the Ethical committee of the Medical University of Graz. All patients gave written informed consent to study participation. The study was conducted adherent to the Declaration of Helsinki principles.

Participants

Sixteen PLE patients (14 females and 2 males; mean age, 33.2 years; age range, 18–52 years) with mean disease duration of 8.4 years (range, 1–30 years) seeking preventive medical photohardening therapy by 311 nm UVB, eight PLE patients (8 females; mean age, 42.7 years; age range, 21–55 years) with mean disease duration of 13.9 years (range, 2–35 years) not seeking preventive medical photohardening therapy, and 14 control subjects (10 females and 4 males; mean age, 44.9 years; age range, 35–61 years) were enrolled. The skin phototype of the PLE patients was as follows: PLE patients seeking photohardening: 3 skin phototype II, 13 skin phototype III; PLE patients not seeking photohardening: 1 skin phototype II; 7 phototype III. The diagnosis of PLE had been confirmed in each subject by patient's history, histologic findings and/or phototesting procedures.⁴⁵ The inclusion and exclusion criteria of the study have been reported in detail elsewhere.⁴⁴ Two of the sixteen initially enrolled PLE patients of the phototherapy group dropped out of the study early due to personal reasons.

Phototherapy and blood samples

In the PLE patients, the photohardening therapy was performed in the spring of the year. Fourteen patients completed photohardening with 311 nm UVB given 2–3 times per week for 4 to 9 weeks (median, 6 weeks) with a total of 11 to 19 exposures (mean, 17.5) using a Waldmann 7001K cabinet (Waldmann Medizintechnik, Villingen-Schwenningen, Germany) equipped with Philips T01 fluorescent tubes. The mean starting dose was 0.3 J cm⁻² (range, 0.2 to 0.5; depending on Fitzpatrick skin phototype). The mean total cumulative 311 nm UVB dose was 13.5 J cm⁻² (range, 5.7 to 20.6). Plasma from these 14 PLE patients treated with photohardening, 8 non-irradiated PLE patients, and 14 control subjects was analysed for the various cytokines and chemokines listed below. The blood samples were collected in early spring (March to April; i.e. before photohardening) and late spring (April to June; i.e., after photohardening). In patients with phototherapy, blood samples were taken shortly before the start of photohardening and within 48 h of the penultimate phototherapeutic exposure.

Cytokine analysis

For each sample, peripheral blood was collected in acid-citrate-dextrose Vacutainers (BD Biosciences, San Jose, CA, USA) centrifuged, and the plasma harvested. Plasma was aliquoted and stored at −80 °C for batch cytokine analysis. A multiplex suspension bead-array immunoassay (BioPlex analyzer; BioRad Laboratories, Hercules, CA) was used to measure simultaneous the concentration of IL-1β, CXCL8, IL-10, IL-17, TNF, CCL2, CCL11, and CCL22. Plasma samples were run in concordance with the instructions of the kit protocol. Briefly, 50 μl of plasma sample was incubated with antibody-coupled beads. After washing a biotinylated detection antibody was added to the beads, and the reaction mixture was detected by the addition of streptavidin–phycoerythrin. The bead sets were analyzed using a 100-suspension array system (BioPlex 200). Unknown cytokine and chemokine concentrations were calculated by BioPlex Manager software 4.1 using a standard curve derived from reference cytokine concentrations supplied by the manufacturer. A five-parameter model was used to calculate final concentrations (of duplicate measurements), expressed in pg ml⁻¹. According to the manufacturer, the minimum detectable concentrations (pg ml⁻¹) of the factors by overnight protocol were as follows: IL-1β (0.4), CXCL8 (0.2), IL-10 (0.3), IL-17 (0.2), TNF (0.1), CCL2 (0.9), CCL11 (1.2), and CCL22 (3.7). A commercial ELISA (Chemicon/Millipore, Vienna, Austria) was used to measure CCL5 plasma levels (minimum detectable concentration, 1 pg ml⁻¹).

Statistics

Normality of data was confirmed by Kolmogorov Smirnov and Shapiro Wilk tests. Data are presented as median (first quartile–third quartile). Potential differences between patients and controls in continuous variables were tested by the non-parametric Mann–Whitney U test. Wilcoxon tests were used for comparing time point 1 and time point 2 for controls, PLE + PT and PLE − PT. For analysis of more than 2 groups and overall comparison, Kruskal–Wallis tests were performed. A p-value of <0.05 was considered significant. P-values of significance in multiple comparisons were adjusted by Bonferroni–Holm corrections. Statistical analysis was performed by SPSS version 17.0 (SPSS Inc., Chicago, IL).

Results

Bead-array immunoassay studies revealed that, compared to control subjects, PLE patients had significantly higher baseline levels of IL-1β (p = 0.002) and CCL11 (p = 0.003) in early spring before phototherapy (Table 1). Differences in the cytokine levels at this early spring time point were observed between patients that were and those that were not seeking photohardening therapy (IL-1β, IL-17, CCL2, and TNF were all higher while CXCL8 was lower). After Bonferroni adjustment for multiple endpoint testing, these differences were not significant. In late spring, after phototherapy, PLE patients had significantly elevated CCL2 (p = 0.002) and TNF (p = 0.002) but a trend for lowered plasma levels of CXCL8 (p = 0.021) (Table 2). When comparing the cytokine shifts from early to late spring, while healthy controls and non-UV-irradiated PLE patients showed an increase, PLE patients undergoing photohardening displayed a decreasing trend in IL-1β (p = 0.012) (Table 3). ELISA analysis of CCL5 revealed that the levels of this particular chemokine were highly variable with no significant difference between PLE patients and control subjects (data not shown).

Table 1 Cytokine and chemokine levels at the first time point (baseline before photohardening) in normal control subjects vs. PLE patients^a

Cytokine/chemokine	Patient group		P-value
	Control	PLE
a Control, healthy control subjects (n = 14); PLE, patients with polymorphic light eruption (n = 24). Data shown depict median (quartile p25; p75) of cytokine levels in pg ml^−1. Entries in bold represent values of cytokines with significant statistical differences after Bonferroni–Holm adjustment for multiple endpoint testing (p < 0.00625). P-value determined by Mann–Whitney U test.
CCL11 (eotaxin)	0.0 (0.0–21.6)	39.4 (21.9–66.2)	0.003
IL-1β	1.3 (0.5–2.3)	4.4 (1.6–9.1)	0.002
CXCL8 (IL-8)	11.3 (9.6–12.9)	6.1 (3.3–15.3)	0.208
IL-10	0.0 (0.0–0.0)	0.0 (0.0–2.4)	0.229
IL-17	0.0 (0.0–0.5)	0.7 (0.0–18.3)	0.059
CCL2 (MCP-1)	142.5 (95.6–171.0)	173.8 (127.1–253.4)	0.061
TNF	0.0 (0.0–0.0)	0.0 (0.0–2.7)	0.013
CCL22 (MDC)	351.7 (275.8–467.6)	443.2 (302.9–557.6)	0.159

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Table 2 Cytokine and chemokine levels at the second time point in PLE patients with (+PT) or without photohardening (−PT) vs. normal control subjects^a

Cytokine/chemokine	Patient group			P-value
	Control	PLE + PT	PLE − PT
a Control, healthy control subjects (n = 14); PLE + PT, polymorphic light eruption patients with photohardening (n = 14); PLE − PT, polymorphic light eruption patients without photohardening (n = 8). Data shown depict median (quartile p25; p75) of cytokine levels in pg ml^−1. Entries in bold represent values of cytokines with significant statistical differences after Bonferroni–Holm adjustment for multiple endpoint testing (p < 0.00625). P-value determined by global Kruskal–Wallis test for an overall comparison among all groups. b P = 0.009 vs. control; c P = 0.002 vs. control; d P = 0.002 vs. control; and e P = 0.003 vs. PLE + PT, as determined by Mann–Whitney U test.
CCL11 (eotaxin)	28.1 (2.9–56.5)	41.3 (19.0–70.0)	25.5 (10.4–65.9)	0.614
IL-1β	1.7 (1.1–3.5)	4.9 (1.7–7.0)	2.9 (1.7–7.5)	0.332
CXCL8 (IL-8)	11.0 (9.6–13.4)	3.8 (2.1–10.9)^b	11.0 (5.2–29.3)	0.021
IL-10	0.0 (0.0–0.1)	0.0 (0.0–0.0)	0.2 (0.0–2.4)	0.115
IL-17	0.0 (0.0–1.3)	1.5 (0.0–15.6)	0.0 (0.0–56.7)	0.261
CCL2 (MCP-1)	162.2 (133.7–187.4)	254.1 (170.3–374.0)	140.0 (113.0–158.8)	0.002
TNF	0.0 (0.0–0.0)	0.1 (0.0–11.3)	0.0 (0.0–0.0)	0.002
CCL22 (MDC)	381.8 (240.7–700.0)	322.5 (257.3–386.0)	330.5 (256.4–1168.7)	0.747

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Table 3 Comparison of cytokine and chemokine levels between the two time points (pre- and post-photohardening for PLE + PT or early and late spring for the other 2 groups, PLE − PT and control)^a

Cytokine/chemokine	Group	Time point 1	Time point 2	P-value
a Control, healthy control subjects (n = 14); PLE + PT, polymorphic light eruption patients with photohardening (n = 14–16); PLE − PT, polymorphic light eruption patients without photohardening (n = 8). Data shown depict median (quartile p25; p75) of cytokine levels in pg ml^−1. Entries in bold represent values of cytokines with significant statistical differences. P-value comparing time points 1 and 2, by Wilcoxon test.
CCL11 (eotaxin)	PLE + PT	36.6 (21.5–64.3)	41.1 (16.4–60.3)	0.520
	PLE − PT	41.6 (32.5–64.3)	25.5 (10.4–65.9)	0.438
	Control	0.0 (0.0–21.6)	28.1 (2.9–56.5)	0.147
IL-1β	PLE + PT	6.1 (2.6–8.5)	4.2 (1.7–6.5)	0.012
	PLE − PT	2.3 (0.4–9.5)	2.9 (1.7–7.5)	0.813
	Control	1.3 (0.5–2.3)	1.7 (1.1–3.5)	0.266
CXCL8 (IL-8)	PLE + PT	4.6 (2.8–17.1)	3.4 (1.9–6.7)	0.903
	PLE − PT	10.3 (6.1–11.5)	11.0 (5.2–29.3)	0.375
	Control	11.3 (9.6–12.9)	11.0 (9.6–13.4)	0.970
IL-10	PLE + PT	0.0 (0.0–2.4)	0.0 (0.0–0.0)	0.063
	PLE − PT	0.0 (0.0–2.7)	0.2 (0.0–2.4)	0.625
	Control	0.0 (0.0–0.0)	0.0 (0.0–0.1)	0.875
IL-17	PLE + PT	3.5 (0.0–32.4)	1.2 (0.0–6.9)	0.413
	PLE − PT	0.0 (0.0–6.7)	0.0 (0.0–56.7)	0.500
	Control	0.0 (0.0–0.5)	0.0 (0.0–1.3)	0.844
CCL2 (MCP-1)	PLE + PT	195.7 (167.4–328.1)	244.8 (169.4–368.9)	0.376
	PLE − PT	126.5 (122.8–159.6)	140.0 (113.0–158.8)	1.000
	Control	142.5 (95.6–171.0)	162.2 (133.7–187.4)	0.146
TNF	PLE + PT	0.5 (0.0–8.5)	0.1 (0.0–8.5)	0.751
	PLE − PT	0.0 (0.0–0.0)	0.0 (0.0–0.0)	1.000
	Control	0.0 (0.0–0.0)	0.0 (0.0–0.0)	1.000
CCL22 (MDC)	PLE + PT	334.9 (281.6–511.5)	312.4 (246.2–376.2)	0.305
	PLE − PT	510.2 (408.2–638.2)	330.5 (256.4–1168.8)	1.000
	Control	351.7 (275.8–467.6)	381.8 (240.7–700.0)	0.765

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Discussion

This study showed that PLE patients have differences in baseline cytokine and chemokine levels (in particular CCL11 (eotaxin) and IL-1β) compared to healthy control subjects. Moreover, it revealed that photohardening did affect plasma levels of certain immunomodulatory cytokines and chemokines, including IL-1β, CCL2 (MCP-1), TNF, and CXCL8 (IL-8) (Tables 2 and 3). Importantly, there was a trend that the pro-inflammatory IL-1β cytokine level in PLE patients was reduced after hardening therapy (Table 3). Whereas PLE patients had a significantly higher baseline level of CCL11 and IL-1β in early spring (before phototherapy), in late spring (after phototherapy) they had significantly elevated CCL2 and TNF but lowered plasma levels of CXCL8 (Table 2). As depicted in Table 3, there were intrinsic differences in early spring (time point 1) between the group of patients seeking photohardening vs. those not seeking photohardening, for several cytokines, including IL-1β, CXCL8, IL-17, CCL2, TNF, and CCL22. This may have been attributable to differences in the characteristics of the patient groups, including age, severity, and duration of disease. However, the effect of photohardening on IL-1β expression may be crucial in PLE patients since the differentiation of proinflammatory Th17 cells from naive T cells appears to involve signals from cytokines such as transforming growth factor (TGF)-β, IL-6, IL-21, IL-23, as well as IL-1β.^46,47 While IL-17-producing Th17 cells mediate inflammation by stimulating the production of inflammatory cytokines such as TNF and IL-1β, which in turn promote the recruitment of neutrophils and macrophages,⁴⁷ whether they are involved in the pathogenesis of PLE remains to be determined. Our observations of elevated TNF and IL-17 levels in PLE patients compared to normal subjects in early spring (Table 1) suggests that further investigations are warranted.

The question that still remains is how photohardening modulates systemic cytokine levels in PLE patients. One possibility is that this is due to a direct effect on cells, including keratinocytes, dendritic cells, macrophages, and/or mast cells in the skin. Alternatively, photohardening may mediate its efficacy by affecting the vitamin D pathway. Indeed, beyond its effect on bone metabolism, and calcium and phosphorus homeostasis, vitamin D exerts profound immunomodulating effects.⁴⁸ In keratinocytes, a number of genes including TNF, IL-1α, IL-6, CXCL8, IL-10, and CCL5 are regulated by biologically active Vitamin D₃ (calcitriol).⁴⁹ Recent work has indicated that vitamin D may be involved in cellular chemotaxis and migratory potential.^50,51 Whether UV-induced Vitamin D₃ is responsible for the effects we have observed in this study requires further investigation.

Recent results showed that PLE patients have significantly decreased 25-hydroxy-vitamin D3 serum levels, which are below normal limits throughout the year.⁵² Prophylactic UVB-311 nm phototherapy that prevented PLE symptoms significantly increased 25-hydroxy-vitamin D3 serum levels in treated PLE patients. Vitamin D has been suggested as the missing link in UV immune suppression, as at least some effects of UV immunosuppression may be mediated or imitated via the induction of vitamin D synthesis.⁵³ 1,25-Dihydroxy-vitamin D3 exerts profound effects in the immune system, altering cytokine secretion patterns in a way similar to phototherapy⁵⁴, or affecting Tregs.^48,55 It has also been shown to affect T helper cell polarisation by inhibiting the production of the Th1-cytokines IFNγ and IL-12 while simultaneously augmenting Th2 development through IL-4 and IL-10 production.⁵⁶ More recently, the products of Th17 cells have also been shown to be inhibited by 1,25-dihydroxy-vitamin D3.⁵⁷

A role for vitamin D modulation of cytokines is consistent with our recent observation that topical treatment with calcipotriol, a vitamin D analogue, enhanced UV adaption in most PLE patients by approximately 40% with a one step higher value in 10 out of 13 (77%) PLE patients.⁵⁸ Mild UV hardening therapy is reported to result in increased MED,⁴³ and naive MED levels in PLE are often found to be slightly lower than in controls.⁴ It has been suggested that UV hardening or calcipotriol treatment restores the MED differences in PLE patients in part by normalising UV inducible proinflammatory patterns.³⁹ Increased expression of IL-1β in human skin shortly after UV exposure is described in healthy people and PLE patients^16,39,59,60 with a pro-inflammatory (and presumably pathogenic) elevation of IL-1β over the whole dose range (from 0 to 6 MEDs), and an abnormal IL-1Ra/IL-1 ratio in PLE skin upon erythematogenic UV exposure with 3 to 6 MEDs compared to normal subjects.³⁹ Our results demonstrate that baseline IL-1β plasma levels in PLE patients are altered by suberythematogenic photohardening. While vitamin D supplementation can suppress proinflammatory cytokine profiles⁶¹ and reduce IL-1 and TNF in psoriasis lesions,⁶² it remains to be determined whether treatment with vitamin D analogues in PLE patients works by normalising IL-1β levels.

Taken together, our results indicate that photohardening may alter the complex cytokine milieu in PLE patients, rendering it chemotactic for dendritic cells, neutrophils, monocytes, memory T cells, eosinophils, and/or mast cells.^{7,35,36,63–65} We therefore propose that this is one mechanism by which UV-phototherapy normalises the pathophysiologic response to subsequent UV exposure in PLE patients.

Acknowledgements

The authors thank the patients and volunteers for participating in the study. The authors additionally thank Dr Angelika Hofer and Dr Franz J. Legat for their help in patient enrollment, and Isabella Bambach for technical support. This work was supported by the Austrian National Bank Jubilee Funds grant no. 13279 and FWF Austrian Science Fund no. KLI 132-B00.

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Footnotes

† This article is published as part of a themed issue on current topics in photodermatology.

‡ AGW and PW contributed equally to the work.

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