Multiple sclerosis (MS) is a progressive currently incurable, immune-mediated disease of the central nervous system (CNS). Both genetic and environmental factors contribute to MS disease risk, largely through effects on immune pathways [reviewed in Ref. (1, 2)]. Increasing MS incidence, prevalence and mortality with increasing latitude, seasonal effects in relapse rates, and association of MS risk in observational studies with less past sun exposure (1) have led to a focus on insufficient vitamin D as a major risk factor. This was further supported by season of birth effects for MS risk (3). However, studies in humans and mice by us and others [reviewed in Ref. (4, 5)] suggest that immune regulation by UV radiation (UVR) is by both vitamin D-dependent and vitamin D-independent pathways. Our epidemiological work (6, 7) indicates that vitamin D and sun exposure are additive independent risk factors for MS development. A Swedish study has supported these findings (8). Further, direct effects of sun exposure on MRI measures of neurodegeneration in MS, independently of vitamin D, have been reported (9). Thus, associations of 25(OH)D with MS disease progression cannot assume causality of vitamin D and the efficacy of supplementation in the MS disease process even if this is an effective way to raise 25(OH)D levels.

These findings suggest that multiple molecules, including vitamin D, produced in the skin in response to UVR exposure, may contribute to the latitude gradient in incidence, prevalence, and mortality of MS. The question then becomes which strategy might we use to reduce the incidence of MS? Should it be vitamin D supplementation (surely the easiest option with proven safety), or is UVB exposure preferable as it will stimulate the production of vitamin D as well as the multiple other immune-modifying molecules (see below). Here, we introduce the Phototherapy for Clinically Isolated Syndrome (PhoCIS) Trial, which was designed to give individuals at high risk of MS the benefits of both vitamin D and other molecules produced in UVR-exposed skin. Recruitment of participants is ongoing, but here we provide the background and justification for the trial.

The PhoCIS trial examines the effects of narrow band UVB therapy on immune and inflammatory markers of disease, as well as MRI, in patients diagnosed with CIS within the last 120 days (ACTRN12614000185662). The protocol has received approval from the University of Western Australia Human Research Ethics Committee (2014-02-083), and informed consent has been obtained from each participant. All participants receive sufficient vitamin D supplementation to achieve serum 25(OH)D levels of ≥80 nmol/L, as standard care. Half of the participants are randomized to receive the narrowband UVB phototherapy, in addition to vitamin D supplementation. The systemic immune-suppressing effect of UVR exposure has been extensively demonstrated in humans (10, 11). For example, a single sub-erythemal exposure of either 0.25 or 0.5 minimal erythemal dose (the dose of erythemally weighted UVR that causes a just perceptible reddening of the skin) of UVR suppressed contact hypersensitivity responses by 50 and 80%, respectively, of volunteers with Fitzpatrick-defined human skin types I (always burns, never tans) and II (usually burns, tans with difficulty) (12, 13). Human and murine studies suggest a prominent role in UVR-induced immune suppression for dendritic cells (DCs) and induced T-regulatory and B-regulatory cells, culminating in reduced T and B cell responses [reviewed in Ref. (4, 5, 14)].

Supplementation of patients with CIS with oral vitamin D would be simpler and easier to manage. With respect to prevention of MS, there have been two previous studies. The first was a randomized controlled trial in Iran of 30 optic neuritis patients given 50,000 IU vitamin D₃ per week for 12 months (24). There were significantly fewer cases of a second demyelinating attack measured on MRI in the treated group. The second study was a retrospective case–control study in Norway of self-reported consumption of cod liver oil (5 mL contains 400 IU vitamin D), during childhood, adolescence, and adulthood by MS adults diagnosed for <10 years (25). A significant reduction of risk for cod liver oil consumption was recorded only for intake during adolescence. Thus, there is some but limited evidence of a potential benefit of vitamin D supplementation for the prevention of MS and the two reported studies require verification. In Australia, the PrevANZ study is examining the effects of oral vitamin D supplementation on individuals with CIS with the end point of progression to MS by MRI criteria (ACTRN12612001160120). A similar trial has been established in France (D-Lay-MS, NCT01817166).

The majority of published studies have investigated the treatment of MS with vitamin D; these were recently detailed in a Rapid Response Report by the Canadian Agency for Drugs and Technologies in Health (26). The authors evaluated the clinical effectiveness of vitamin D supplementation in four systematic reviews (27–30), eight randomized controlled trials (24, 31–37), and three non-randomized studies (25, 38, 39). The authors of the CADTH Rapid Report concluded that “the outcomes of vitamin D supplementation for MS were heterogeneous, conflicting and inconsistent, with no effect on disability scores and relapse rates. Further, there were both positive and negative results for immunologic factors, imaging studies, and functional outcomes.” Other studies suggest that ongoing inflammation, as in MS, may hinder systemic increases in 25(OH)D levels with supplementation (40).

The precursor of vitamin D in keratinocytes, 7-dehydrocholesterol, absorbs UVB photons to initiate a pathway of vitamin D production, followed by hydroxylations in the liver, kidney, and immune and epithelial cells, and ultimately the biosynthesis of 1,25-dihydroxy vitamin D [1,25(OH)₂D], the active form of vitamin D (41). Unless supplemented, humans obtain up to 80% of their vitamin D by sun exposure. Many recent reviews have covered the immunomodulatory properties of 1,25(OH)₂D (42), particularly with reference to regulation of cells proposed important to the development of MS. Genetic variation of the Vitamin D regulating genes, CYP27B1 and CYP24A1, have also been identified as risk factors (2). Many of the properties of 1,25(OH)₂D have been detected in vitro with isolated cell populations and using concentrations of 1,25(OH)₂D that are much higher than systemic levels, including pharmacological doses. However, immune cells have the enzymes to convert vitamin D or 25(OH)D to 1,25(OH)₂D, and local concentrations, at the level of the cells, may be much higher than systemic levels (43). Generally, there is evidence that 1,25(OH)₂D can stimulate the generation of tolerogenic DCs (44), and enhance both the number and function of T-regulatory cells (45). Direct effects on all immune cell types have been reported, which is supported by their expression of the vitamin D receptor. There are also many effects of vitamin D on human immune cells that are still not fully understood, particularly as responses by human and murine cells can vary. For example, 25(OH)₂D can control T cell antigen receptor signaling and activation of human but not murine T cells (46). Furthermore, trials of vitamin D supplementation for multiple conditions have generally been disappointing (30, 47, 48) and have raised questions about the immunomodulatory abilities of vitamin D in vivo.

UV radiation-induced vitamin D may also regulate the development of MS by its effects on VDR-expressing non-immune cells in the CNS. In one study of CIS patients, each 25 nmol/L increase in 25(OH)D level was significantly associated with 7.8 mL higher gray matter volume, and there was a trend for an inverse relationship over 12 months between 25(OH)D levels and new brain lesions and clinical relapses (49). As vitamin D maternal deficiency has been associated with altered development of multiple organs before birth (50), it is possible that vitamin D may positively regulate re-myelination programs.

Several chromophores in skin for UVB photons have been implicated in vitamin D-independent pathways: trans-urocanic acid in the stratum corneum, DNA, RNA, lipids and tryptophan of keratinocytes, and antigen-presenting cells [reviewed in Ref. (4, 5, 10, 11)]. All may initiate pathways involved in signaling from skin to immune cells in draining lymph nodes and tissues beyond. It has been shown that T-regulatory cells are induced locally but then migrate to sites of inflammation, including the CNS, for regulation of experimental autoimmune encephalomyelitis (EAE), the murine model of MS (22). Epidermal Langerhans cells have been implicated in the immune regulation of narrowband UVB phototherapy (51). Importantly, sub-erythemal amounts of UVR, as in narrowband UV phototherapy, can suppress both local and systemic immunity, measured functionally by reduced cell-mediated immune responses. A recent advance in understanding the immune regulation by sub-erythemal UVR has come from studies of keratinocyte-derived host defense peptides, previously called antimicrobial peptides. These peptides have modest antimicrobial activity but rather pleiotropic immunoregulatory properties (52). They are produced in sub-erythemal UV-irradiated skin in the absence of any inflammation (53); they include cathelicidin LL-37 and the α- and β-defensins (52). As shown in a murine model, β-defensins via induction of T-regulatory cells can prevent and mitigate EAE (54). Recent studies have also implicated UVR in the maintenance of a pool of B-regulatory cells in the periphery that can prevent an autoimmune attack on the CNS (55). Narrow band UVB irradiation can also increase peripheral T-regulatory cell numbers and their suppressive function in patients with polymorphic light eruption (56).

Further advances to understanding the long-term effects of UVR exposure have come from studies of myeloid progenitors in the bone marrow of UV-irradiated mice (57–59). Immune cells in peripheral organs, including DCs that are the most important cells in initiating immunity, are constantly being replaced by bone marrow-derived hemopoietic cells. We have shown that a single irradiation of skin with erythemal UVR, or multiple sub-erythemal exposures, can suppress contact hypersensitivity responses at distant skin sites for between 1 and 3 months, and suggests bone marrow involvement per se (58, 60). Using chimeric mice engrafted with bone marrow from UV-irradiated mice, UV irradiation of skin alters the differentiation program of myeloid progenitors in bone marrow, so terminally differentiated DCs and macrophages are less immunogenic and more regulatory (58, 60). The reduced immunogenicity (confirmed in skin and airways) causes an attenuated ability of bone marrow-derived DCs to respond to inflammatory signals and associated antigens, and to prime new immune responses (58, 60). Adoptive transfer of DCs differentiated from the bone marrow of UV-irradiated mice and loaded in vitro with antigen can reduce inflammatory challenge responses in mice already sensitized to that antigen (58, 60), a scenario that can be likened to injected DCs suppressing ongoing autoimmune disease, such as MS. Importantly, the effect of sub-erythemal UVR to skin on bone marrow myeloid progenitors is by a vitamin D-independent, prostaglandin E₂ (PGE₂)-dependent process (57), with an epigenetic modification of an early myeloid progenitor in the bone marrow proposed. Involvement of UVR, bone marrow-differentiated DCs, and PGE₂ in MS pathogenesis is supported by the identification of the gene encoding PTGER4, a PGE₂ receptor that is an intermediary for the UV effects on DC progenitors in the bone marrow, as an MS risk factor in genome-wide association studies (2). We propose that in patients with CIS, exposure to narrow band UVB will similarly alter the immunogenic properties of circulating DCs and their precursor cells toward a regulatory profile. As UVB exposure can simulate innate immunity (61), we do not anticipate detrimental effects of the narrowband UVB phototherapy on host responses to infection.

Association of lower 25(OH)D levels with increased risk of MS and greater relapse rate, and the plausibility of 1,25(OH)₂D as a biochemical modulator of MS pathogenic processes, has encouraged multiple trials of vitamin D supplementation. However, the inconclusive outcomes to date of these trials encourage a fresh approach to re-interpret the reported associations. As 25(OH)D levels are generally a measure of recent sun exposure, we propose that the source of 25(OH)D on MS prevention is investigated. The PhoCIS trial has been established for individuals with CIS who are administered narrowband UVB phototherapy 24 times over 8 weeks and assessed neurologically and immunologically for 12 months. There are multiple immunomodulatory agents, in addition to vitamin D, produced in UVB-irradiated skin; we propose that by giving individuals at high risk of developing MS narrowband UVB phototherapy that they will gain the benefits of both vitamin D and other molecules implicated in UVB-induced immunoregulation.

PH, AK, RL, DB, WC, and DN conceived the idea to perform the trial and contributed to trial design. JC is the dermatologist in the team; AJ is the trial co-ordinator. All the authors contributed to drafting the manuscript and have approved the final version of the manuscript.

The authors declare no competing interests that are of direct relevance to the current research.