Epstein-Barr virus infection, B-cell dysfunction and other risk factors converge in gut-associated lymphoid tissue to drive the immunopathogenesis of multiple sclerosis: a hypothesis.

Multiple sclerosis is associated with Epstein-Barr virus (EBV) infection, B-cell dysfunction, gut dysbiosis, and environmental and genetic risk factors, including female sex. A disease model incorporating all these factors remains elusive. Here, we hypothesise that EBV-infected memory B cells (MBCs) migrate to gut-associated lymphoid tissue (GALT) through EBV-induced expression of LPAM-1, where they are subsequently activated by gut microbes and/or their products resulting in EBV reactivation and compartmentalised anti-EBV immune responses. These responses involve marginal zone (MZ) B cells that activate CD4+ T-cell responses, via HLA-DRB1, which promote downstream B-cell differentiation towards CD11c+/T-bet+ MBCs, as well as conventional MBCs. Intrinsic expression of low-affinity B-cell receptors (BCRs) by MZ B cells and CD11c+/T-bet+ MBCs promotes polyreactive BCR/antibody responses against EBV proteins (e.g. EBNA-1) that cross-react with central nervous system (CNS) autoantigens (e.g. GlialCAM). EBV protein/autoantigen-specific CD11c+/T-bet+ MBCs migrate to the meningeal immune system and CNS, facilitated by their expression of CXCR3, and induce cytotoxic CD8+ T-cell responses against CNS autoantigens amplified by BAFF, released from EBV-infected MBCs. An increased abundance of circulating IgA+ MBCs, observed in MS patients, might also reflect GALT-derived immune responses, including disease-enhancing IgA antibody responses against EBV and gut microbiota-specific regulatory IgA+ plasma cells. Female sex increases MZ B-cell and CD11c+/T-bet+ MBC activity while environmental risk factors affect gut dysbiosis. Thus, EBV infection, B-cell dysfunction and other risk factors converge in GALT to generate aberrant B-cell responses that drive pathogenic T-cell responses in the CNS.

Circulating Memory B Cells in Early Multiple Sclerosis Exhibit Increased IgA+ Cells, Globally Decreased BAFF-R Expression and an EBV-Related IgM+ Cell Signature.

Multiple sclerosis (MS) is an immune-mediated inflammatory disease of the central nervous system that results in demyelination of axons, inefficient signal transmission and reduced muscular mobility. Recent findings suggest that B cells play a significant role in disease development and pathology. To further explore this, B cell profiles in peripheral blood from 28 treatment-naive patients with early MS were assessed using flow cytometry and compared to 17 healthy controls. Conventional and algorithm-based analysis revealed a significant increase in MS patients of IgA+ memory B cells (MBC) including CD27+, CD27- and Tbet+ subsets. Screening circulating B cells for markers associated with B cell function revealed a significantly decreased expression of the B cell activation factor receptor (BAFF-R) in MS patients compared to controls. In healthy controls, BAFF-R expression was inversely associated with abundance of differentiated MBC but this was not observed in MS. Instead in MS patients, decreased BAFF-R expression correlated with increased production of proinflammatory TNF following B cell stimulation. Finally, we demonstrated that reactivation of Epstein Barr Virus (EBV) in MS patients was associated with several phenotypic changes amongst MBCs, particularly increased expression of HLA-DR molecules and markers of a T-bet+ differentiation pathway in IgM+ MBCs. Together, these data suggest that the B cell compartment is dysregulated in MS regarding aberrant MBC homeostasis, driven by reduced BAFF-R expression and EBV reactivation. This study adds further insights into the contribution of B cells to the pathological mechanisms of MS, as well as the complex role of BAFF/BAFF-R signalling in MS.

Sex-Specific Environmental Impacts on Initiation and Progression of Multiple Sclerosis.

The immunological mechanisms that contribute to multiple sclerosis (MS) differ between males and females. Females are 2-3 times more likely to develop MS compared to males, however the reason for this discrepancy is unknown. Once MS is established, there is a more inflammatory yet milder form of disease in females whereas males generally suffer from more severe disease and faster progression, neural degradation, and disability. Some of these differences relate to genetics, including genetic control of immune regulatory genes on the X-chromosome, as well as immune modulatory properties of sex hormones. Differences in MS development may also relate to how sex interacts with environmental risk factors. There are several environmental risk factors for MS including late-onset Epstein Barr virus infection, low serum vitamin D levels, low UV radiation exposure, smoking, obesity, and lack of physical activity. Most of these risk factors impact males and females differently, either due to biological or immunological processes or through behavioral differences. In this review, we explore these differences further and focus on how the interaction of environmental risk factors with sex hormones may contribute to significantly different prevalence and pathology of MS in males and females.

Changes in serum neurofilament light chain levels following narrowband ultraviolet B phototherapy in clinically isolated syndrome.

OBJECTIVE: To determine whether serum neurofilament light chain (sNfL) levels are suppressed in patients with the clinically isolated syndrome (CIS) following narrowband ultraviolet B phototherapy (UVB-PT). METHODS: sNfL levels were measured using a sensitive single-molecule array assay at baseline and up to 12 months in 17 patients with CIS, 10 of whom received UVB-PT, and were compared with healthy control (HC) and early relapsing remitting multiple sclerosis (RRMS) group. sNfL levels were correlated with magnetic resonance imaging total lesion volume (LV) determined using icobrain version 4.4.1 and with clinical outcomes. RESULTS: Baseline median sNfL levels were significantly higher in the CIS (20.6 pg/mL, interquartile range [IQR] 13.7-161.4) and RRMS groups (36.6 pg/ml [IQR] 16.2-212.2) than in HC (10.7 pg/ml [IQR] 4.9-21.5) (p = .012 and p = .0002, respectively), and were strongly correlated with T2 and T1 LV at 12 months (r = .800; p = .014 and r = .833; p = .008, respectively) in the CIS group. Analysis of changes in sNfL levels over time in the CIS group showed a significant cumulative suppressive effect of UVB-PT in the first 3 months (UVB-PT -10.6% vs non-UVB-PT +58.3%; p = .04) following which the levels in the two groups converged and continued to fall. CONCLUSIONS: Our findings provide the basis for further studies to determine the utility of sNfL levels as a marker of neuro-axonal damage in CIS and early MS and for assessing the efficacy of new therapeutic interventions such as UVB-PT.

Developing an Online Tool to Promote Safe Sun Behaviors With Young Teenagers as Co-researchers.

Despite education about the risks of excessive sun exposure, teenagers in Australia are sun-seeking, with sunburn common in summer. Conversely, some regular (time-limited) exposure to sunlight (that avoids sunburn) is necessary for vitamin D and healthy bones and other molecules important for immune and metabolic health. New interventions are thus required to better support teenagers to make healthy and balanced decisions about their sun behaviors. This paper describes the development of a prototype online tool-a smartphone app-that aimed to foster safe sun practices in teenagers. We recruited young adolescents (aged 12-13 years, n = 24) as "co-researchers" to provide ongoing input into the nature and design of the online tool. This age group was selected, as it is a critical time when young people transition from primary education, where "SunSmart" behaviors are entrenched in Australian schools, to high school, where risky behaviors emerge. Through a series of interviews and workshops, we codesigned an Apple iOS smartphone app with the co-researchers, leading health promotion professionals, researchers, and app designers. The developed app, Sun Safe, contains educational content relevant to teenagers about safe sun behaviors, complemented by other features requested by co-researchers and stakeholders to help engage young people, including gamified quizzes to test their sun health knowledge, real-time weather data on the UV Index and temperature, a sunscreen application timer, and reminders to check the UV Index. The developed prototype app was rated well by co-researchers, suggesting it is suitable for further feasibility and efficacy testing as an intervention tool to improve knowledge and promote safe sun behaviors by young adolescents.

Demographic and clinical predictors of vitamin D status in pregnant women tested for deficiency in Western Australia.

OBJECTIVE: This study aimed to describe the vitamin D status of pregnant women in Western Australia and identify predictors of deficiency in pregnancy. METHODS: A cross-sectional study was conducted using linked data from statewide administrative data collections. Participants included pregnant women aged 18-44 years who gave birth between 2012 and 2014. RESULTS: The mean 25-hydroxyvitamin D (25[OH]D) concentration was 70.7 nmol L-1 (SD 25.7; range 5-255 nmol L-1 ). Approximately one-fifth of the pregnant women were vitamin D deficient (<50 nmol L-1 ). Maternal age (under 25 years) was identified as an independent risk factor of vitamin D deficiency in addition to known predictors. Only 20% of women were screened within the first 10 weeks of their pregnancy. CONCLUSIONS: In addition to the existing risk factors for deficiency, maternal age was an independent predictor of vitamin D deficiency. There was a large discrepancy between the time of first antenatal visit and screening for vitamin D deficiency. Implications for public health: Our findings support the addition of maternal age (under 25 years) to the current clinical guidelines for targeted screening of 25(OH)D levels in pregnancy and the practical application of screening for vitamin D deficiency at the first antenatal visit.

Higher ultraviolet radiation during early life is associated with lower risk of childhood type 1 diabetes among boys.

Population-level ecological studies show type 1 diabetes incidence is inversely correlated with ambient ultraviolet radiation (UVR) levels. We conducted a nested case-control study using administrative datasets to test this association at the individual level. Cases (n = 1819) were children born in Western Australia (WA) from 1980-2014, diagnosed with type 1 diabetes at ≤ 16 years. Controls (n = 27,259) were randomly selected from all live births in WA, matched to cases by sex and date of birth. Total ambient erythemal ultraviolet radiation (UVR) doses for each trimester of pregnancy and first year of life were estimated for each individual, using daily NASA satellite data that were date- and geographically-specific. Conditional logistic regression tested the association between UVR dose and case-control status. Type 1 diabetes risk was 42% lower in boys of mothers with third-trimester UVR dose in the highest (compared to the lowest) quartile (p = 0.04). Higher UVR in the first year of life was associated with lower type 1 diabetes risk among boys (p = 0.01). UVR dose was not associated with type 1 diabetes risk in girls. Higher UVR in late pregnancy and early life appear to interact with sex-specific factors to lower type 1 diabetes risk among boys in Western Australia.

More Than Effects in Skin: Ultraviolet Radiation-Induced Changes in Immune Cells in Human Blood.

Cells of the skin and circulation are in constant two-way communication. Following exposure of humans to sunlight or to phototherapy, there are alterations in the number, phenotype and function of circulating blood cells. In this review, only data obtained from human studies are considered, with changes induced by UV radiation (UVR) exposure described for phagocytic leukocytes and peripheral blood mononuclear cells plus their component T and B cells, natural killer cells and dendritic cells. These immune modulations illustrate the potential of UVR to have therapeutic effects beyond the skin, and that sunlight exposure is an important environmental influence on human health.

Expression of CYP24A1 and other multiple sclerosis risk genes in peripheral blood indicates response to vitamin D in homeostatic and inflammatory conditions.

Although genetic and epidemiological evidence indicates vitamin D insufficiency contributes to multiple sclerosis (MS), and serum levels of vitamin D increase on treatment with cholecalciferol, recent metanalyses indicate that this vitamin D form does not ameliorate disease. Genetic variation in genes regulating vitamin D, and regulated by vitamin D, affect MS risk. We evaluated if the expression of vitamin D responsive MS risk genes could be used to assess vitamin D response in immune cells. Peripheral blood mononuclear cells (PBMCs) were isolated from healthy controls and people with MS treated with dimethyl fumarate. We assayed changes in expression of vitamin D responsive MS risk (VDRMS) genes in response to treatment with 25 hydroxy vitamin D in the presence or absence of inflammatory stimuli. Expression of CYP24A1 and other VDRMS genes was significantly altered in PBMCs treated with vitamin D in the homeostatic and inflammatory models. Gene expression in MS samples had similar responses to controls, but lower initial expression of the risk genes. Vitamin D treatment abrogated these differences. Expression of CYP24A1 and other MS risk genes in blood immune cells indicate vitamin D response and could enable assessment of immunological response to vitamin D in clinical trials and on therapy.

Metabolic dysfunction induced by a high-fat diet modulates hematopoietic stem and myeloid progenitor cells in brown adipose tissue of mice.

Brown adipose tissue (BAT) may be an important metabolic regulator of whole-body glucose. While important roles have been ascribed to macrophages in regulating metabolic functions in BAT, little is known of the roles of other immune cells subsets, particularly dendritic cells (DCs). Eating a high-fat diet may compromise the development of hematopoietic stem and progenitor cells (HSPCs)-which give rise to DCs-in bone marrow, with less known of its effects in BAT. We have previously demonstrated that ongoing exposure to low-dose ultraviolet radiation (UVR) significantly reduced the 'whitening' effect of eating a high-fat diet upon interscapular (i) BAT of mice. Here, we examined whether this observation may be linked to changes in the phenotype of HSPCs and myeloid-derived immune cells in iBAT and bone marrow of mice using 12-colour flow cytometry. Many HSPC subsets declined in both iBAT and bone marrow with increasing metabolic dysfunction. Conversely, with rising adiposity and metabolic dysfunction, conventional DCs (cDCs) increased in both of these tissues. When compared with a low-fat diet, consumption of a high-fat diet significantly reduced proportions of myeloid, common myeloid and megakaryocyte-erythrocyte progenitors in iBAT, and short-term hematopoietic stem cells in bone marrow. In mice fed the high-fat diet, exposure to low-dose UVR significantly reduced proportions of cDCs in iBAT, independently of nitric oxide release from irradiated skin [blocked using the scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt (cPTIO)], but did not significantly modify HSPC subsets in either tissue. Further studies are needed to determine whether changes in these cell populations contribute towards metabolic dysfunction .

Associations of serum short-chain fatty acids with circulating immune cells and serum biomarkers in patients with multiple sclerosis.

Altered composition of gut bacteria and changes to the production of their bioactive metabolites, the short-chain fatty acids (SCFAs), have been implicated in the development of multiple sclerosis (MS). However, the immunomodulatory actions of SCFAs and intermediaries in their ability to influence MS pathogenesis are uncertain. In this study, levels of serum SCFAs were correlated with immune cell abundance and phenotype as well as with other relevant serum factors in blood samples taken at first presentation of Clinically Isolated Syndrome (CIS; an early form of MS) or MS and compared to healthy controls. There was a small but significant reduction in propionate levels in the serum of patients with CIS or MS compared with healthy controls. The frequencies of circulating T follicular regulatory cells and T follicular helper cells were significantly positively correlated with serum levels of propionate. Levels of butyrate associated positively with frequencies of IL-10-producing B-cells and negatively with frequencies of class-switched memory B-cells. TNF production by polyclonally-activated B-cells correlated negatively with acetate levels. Levels of serum SCFAs associated with changes in circulating immune cells and biomarkers implicated in the development of MS.

The Multiple Roles of Urocanic Acid in Health and Disease.

Trans-urocanic acid (trans-UCA) is synthesized in the skin, liver, and brain. It is a major natural moisturizing factor in skin and maintains its acid pH. In skin, it isomerizes to cis-UCA following exposure to UVR. Both isomers fulfill multiple roles in health and disease. Cis-UCA has immunomodulatory properties linked with several cutaneous diseases such as skin cancer, atopic dermatitis, and urticaria and associates with systemic diseases including multiple sclerosis. The levels of UCA in the skin, brain, urine, and feces reflect some physiological processes and may be disease biomarkers. Both isomers of UCA have therapeutic potential for a range of disorders.

Use of linked administrative and laboratory data to confirm that serum 25(OH)D levels in pregnant women can be predicted from satellite estimates of ultraviolet radiation.

BACKGROUND: Serum 25 hydroxyvitamin D [25(OH)D] levels of pregnant women have been linked to various health outcomes in their offspring. Satellite-derived ultraviolet radiation (UVR) data have been used as a proxy for 25(OH)D levels, as individual-level cohort studies are time-consuming, costly and only feasible for common outcomes. METHODS: Data on 25(OH)D levels from a public laboratory database were linked to data from the Western Australian Midwives' Notification System and daily erythemal UVR dose from NASA satellites. Regression analysis was used to identify the time period prior to venesection where daily UVR dose best predicted 25(OH)D levels. A predictive model was used to validate the use of daily UVR dose as a proxy for personal sun exposure during pregnancy. RESULTS: Data from 19 173 pregnancies in women aged 18-43 years in Western Australia were included. The daily UVR dose averaged over the 90 days before venesection was the strongest UVR predictor of 25(OH)D level (a 5% increase per 1000 J m-2; equal to 3.3 nmol L-1 at the median of 66 nmol L-1). Ethnicity was the strongest predictor of 25(OH)D levels (21% lower in non-Caucasian vs Caucasian: equal to 7.2 nmol L-1 difference). Other significant predictors were gestation, age, year, parity, socio-economic status, remoteness, medical conditions and season. CONCLUSION: NASA-derived erythemal UVR dose in the 90 days prior to venesection is a significant predictor of 25(OH)D levels in pregnant women. Linked administrative data can be used to investigate associations between UVR during pregnancy and health outcomes in offspring.

Narrowband UVB phototherapy reduces TNF production by B-cell subsets stimulated via TLR7 from individuals with early multiple sclerosis.

OBJECTIVES: At the end of a 60-day course of narrowband UVB phototherapy, administered to individuals with early multiple sclerosis, there were changes in the relative proportions of circulating B-cell subsets. This study investigated phototherapy-associated changes to cytokine responses of B cells when exposed to a TLR7 ligand. METHODS: PBMCs from participants of the PhoCIS (Phototherapy for Clinically Isolated Syndrome) trial taken before (day 1) and after phototherapy for 8 weeks (day 60) were incubated with, or without, the TLR7 ligand, R848, for 18 h. Production of TNF and IL-10 in seven B-cell subsets was examined, with cytokine responses in each individual at day 60, adjusted for responses at day 1. Paired PBMCs were from participants administered phototherapy (n = 7) or controls (n = 6). RESULTS: At day 60, significantly fewer B cells, particularly marginal zone-like B cells (CD27+/IgD+), from participants administered phototherapy produced TNF in response to TLR7 stimulation. When responses by seven B-cell subsets were analysed together using multivariate methods, a phototherapy-specific signature was observed. An increased responsiveness from day 1 to day 60 in IgM-only memory B cells (CD27+/IgD-/IgM+) after TLR7 stimulation also predicted slower progression from CIS to MS. Phototherapy was without significant effect on B-cell IL-10 production. CONCLUSIONS: Reduced TNF responses after TLR7 stimulation in marginal zone-like B cells from participants administered phototherapy suggested treatment-associated priming effects that were detected upon subsequent polyclonal B-cell activation. Changes in responsiveness to TLR7 stimulation also suggested that IgM-only memory B cells may be important in conversion from CIS to MS.

Insufficient Sun Exposure Has Become a Real Public Health Problem.

This article aims to alert the medical community and public health authorities to accumulating evidence on health benefits from sun exposure, which suggests that insufficient sun exposure is a significant public health problem. Studies in the past decade indicate that insufficient sun exposure may be responsible for 340,000 deaths in the United States and 480,000 deaths in Europe per year, and an increased incidence of breast cancer, colorectal cancer, hypertension, cardiovascular disease, metabolic syndrome, multiple sclerosis, Alzheimer's disease, autism, asthma, type 1 diabetes and myopia. Vitamin D has long been considered the principal mediator of beneficial effects of sun exposure. However, oral vitamin D supplementation has not been convincingly shown to prevent the above conditions; thus, serum 25(OH)D as an indicator of vitamin D status may be a proxy for and not a mediator of beneficial effects of sun exposure. New candidate mechanisms include the release of nitric oxide from the skin and direct effects of ultraviolet radiation (UVR) on peripheral blood cells. Collectively, this evidence indicates it would be wise for people living outside the tropics to ensure they expose their skin sufficiently to the sun. To minimize the harms of excessive sun exposure, great care must be taken to avoid sunburn, and sun exposure during high ambient UVR seasons should be obtained incrementally at not more than 5-30 min a day (depending on skin type and UV index), in season-appropriate clothing and with eyes closed or protected by sunglasses that filter UVR.

IgG3 + B cells are associated with the development of multiple sclerosis.

OBJECTIVES: Disease-modifying therapies (DMTs) targeting B cells are amongst the most effective for preventing multiple sclerosis (MS) progression. IgG3 antibodies and their uncharacterised B-cell clones are predicted to play a pathogenic role in MS. Identifying subsets of IgG3 + B cells involved in MS progression could improve diagnosis, could inform timely disease intervention and may lead to new DMTs that target B cells more specifically. METHODS: We designed a 31-parameter B-cell-focused mass cytometry panel to interrogate the role of peripheral blood IgG3 + B cells in MS progression of two different patient cohorts: one to investigate the B-cell subsets involved in conversion from clinically isolated syndrome (CIS) to MS; and another to compare MS patients with inactive or active stages of disease. Each independent cohort included a group of non-MS controls. RESULTS: Nine distinct CD20+IgD-IgG3 + B-cell subsets were identified. Significant changes in the proportion of CD21+CD24+CD27-CD38- and CD27+CD38hiCD71hi memory B-cell subsets correlated with changes in serum IgG3 levels and time to conversion from CIS to MS. The same CD38- double-negative B-cell subset was significantly elevated in MS patients with active forms of the disease. A third CD21+CD24+CD27+CD38- subset was elevated in patients with active MS, whilst narrowband UVB significantly reduced the proportion of this switched-memory B-cell subset. CONCLUSION: We have identified previously uncharacterised subsets of IgG3 + B cells and shown them to correlate with autoimmune attacks on the central nervous system (CNS). These results highlight the potential for therapies that specifically target IgG3 + B cells to impact MS progression.

Effects of UVR exposure on the gut microbiota of mice and humans.

Many alterations to the skin microbiome by exposure to UV radiation (UVR) have been postulated and may contribute to the ability of UVR phototherapy to regulate skin inflammatory diseases. Very recently, an effect of sub-erythemal narrowband UVB radiation (311 nm) on the gut microbiome of healthy individuals was reported. The relative abundance of Firmicutes and Proteobacteria increased in faecal samples of those receiving three exposures to narrowband UVB radiation; the Bacteroidetes phyla were reduced by UVB. In mice chronically exposed to sub-erythemal broadband UVR, similar faecal changes in Firmicutes and Bacteroidetes have been reported. Murine studies have allowed a further dissection of the relative ability of UVR and dietary vitamin D to modulate the gut microbiome by analysis of relative bacterial abundance in mice with similar 25-hydroxy vitamin D levels obtained by UVR exposure or from their diet, respectively. The studies of mice recovering from colitis suggested that dietary vitamin D could stimulate greater faecal abundance of Rikenellaceae, whilst exposure to UVR was necessary for changes to the abundance of Lachnospiraceae and Desulfovibrionaceae. Both human and murine studies report that multiple exposures to sub-erythemal UVR can increase the diversity of the gut microbiome, which in turn may be beneficial to the health of the host.

Low-dose UV radiation before running wheel access activates brown adipose tissue.

In previous preclinical studies, low (non-burning) doses of UV radiation (UVR) limited weight gain and metabolic dysfunction in mice fed with a high-fat diet. Here, we explored the effects of low-dose UVR on physical activity and food intake and mechanistic pathways in interscapular brown adipose tissue (iBAT). Young adult C57Bl/6J male mice, housed as individuals, were fed a high-fat diet and exposed to low-dose UVR (sub-oedemal, 1 kJ/m2 UVB, twice-a-week) or 'mock' treatment, with or without running wheel access (2 h, for 'moderate' physical activity) immediately after phototherapy. There was no difference in distance run in mice exposed to UVR or mock-treated over 12 weeks of exposure to running wheels (P = 0.14). UVR (alone) did not significantly affect food intake, adiposity, or signs of glucose dysfunction. Access to running wheels increased food intake (after 10 weeks, P ≤ 0.02) and reduced gonadal white adipose tissue and iBAT mass (P ≤ 0.03). Body weight and hepatic steatosis were lowest in mice exposed to UVR with running wheel access. In the iBAT of mice exposed to UVR and running wheels, elevated Atgl, Cd36, Fasn, Igf1, Pparγ, and Ucp1 mRNAs and reduced CD11c on F4-80 + MHC class II+ macrophages were observed, while renal Sglt2 mRNA levels were increased, compared to high-fat diet alone (P ≤ 0.03). Blood levels of 25-hydroxyvitamin D were not increased by exposure to UVR and/or access to running wheels. In conclusion, when combined with physical activity, low-dose UVR may more effectively limit adiposity (specifically, body weight and hepatic steatosis) and modulate metabolic and immune pathways in iBAT.

Are there differences in immune responses following delivery of vaccines through acutely or chronically sun-exposed compared with sun-unexposed skin?

The majority of human vaccines are administered above the deltoid muscle of the arm, a site that is chronically sun-exposed in many people. It is known that exposure of the skin to the UV wavelengths in sunlight stimulates systemic immunosuppression, an outcome that is associated with reduced immunity to microbial infections in animal models. Here we consider whether immunization of humans through a UV-irradiated skin site will lead to a less effective immune response compared with immunization through an unexposed site. Studies showing that the efficacy of vaccination can be reduced when surrogates of increased levels of sun exposure, such as latitude of residence and season of the year, are considered. Results from a limited number of intervention experiments in humans demonstrate a similar pattern. To provide an explanation for these findings, changes in the number and functional potential of immune cells in chronically sun-exposed compared with unexposed skin are outlined. UV radiation-induced changes to skin cells are also relevant when considering skin sites for administration of immune-tolerizing peptides. The review provides the basis for further research into the effects of acute and chronic UV radiation exposure on skin cells in the context of vaccination.