Multiple sclerosis (MS) disease risk is associated with reduced sun-exposure. This study assessed the relationship between measures of sun exposure (vitamin D [vitD], latitude) and MS severity in the setting of two multicenter cohort studies ( n NationMS = 946, n BIONAT = 990). Additionally, effect-modification by medication and photosensitivity-associated MC1R variants was assessed. High serum vitD was associated with a reduced MS severity score (MSSS), reduced risk for relapses, and lower disability accumulation over time. Low latitude was associated with higher vitD, lower MSSS, fewer gadolinium-enhancing lesions, and lower disability accumulation. The association of latitude with disability was lacking in IFN-β-treated patients. In carriers of MC1R :rs1805008(T), who reported increased sensitivity toward sunlight, lower latitude was associated with higher MRI activity, whereas for noncarriers there was less MRI activity at lower latitudes. In a further exploratory approach, the effect of ultraviolet (UV)-phototherapy on the transcriptome of immune cells of MS patients was assessed using samples from an earlier study. Phototherapy induced a vitD and type I IFN signature that was most apparent in monocytes but that could also be detected in B and T cells. In summary, our study suggests beneficial effects of sun exposure on established MS, as demonstrated by a correlative network between the three factors: Latitude, vitD, and disease severity. However, sun exposure might be detrimental for photosensitive patients. Furthermore, a direct induction of type I IFNs through sun exposure could be another mechanism of UV-mediated immune-modulation in MS., Competing Interests: Competing interest statement: H.W. received honoraria and consultation fees from Bayer Healthcare, Biogen, Fresenius Medical Care, GlaxoSmithKline, GW Pharmaceuticals, Merck Serono, Novartis, Sanofi Genzyme, and Teva Pharma. K.L. is coinventor on patent no. US 2016/0158322 A1 entitled “NDP-MSH for treatment of inflammatory and/or neurodegenerative disorders of the CNS.” A.S. received speaker honoraria and/or travel compensation for activities with Almirall Hermal GmbH, Biogen, Merck, Novartis, Roche and Sanofi Genzyme, none related to this work. R.G. serves on scientific advisory boards for Teva Pharmaceutical Industries Ltd. (Teva), Biogen Idec, Bayer Schering Pharma, and Novartis; has received speaker honoraria from Biogen Idec, Teva, Bayer Schering Pharma, and Novartis; serves as editor for Therapeutic Advances in Neurological Diseases, and is on the editorial boards of Experimental Neurology and the Journal of Neuroimmunology; and receives research support from Teva, Biogen Idec, Bayer Schering Pharma, Genzyme, Merck Serono, and Novartis, none related to this work. L.K. receives honoraria for lecturing and travel expenses for attending meetings from Biogen, Merck, Sanofi Genzyme, Novartis and Roche; her research is funded by the German Ministry for Education and Research (BMBF), Deutsche Forschungsgemeinschaft (DFG), Interdisciplinary Center for Clinical Studies (IZKF) Muenster, Biogen, Merck, and Novartis. M.A.F. received honoraria for consultation and travel expenses from Biogen, Merck KGaA, Novartis, and Roche; his research is funded by the BMBF, DFG, Landesforschungsförderung Hamburg, Gemeinnützige Hertie-Stiftung, and Else Kröner-Fresenius-Stiftung. S.G.M. receives honoraria for lecturing, and travel expenses for attending meetings from Almirall, Amicus Therapeutics Germany, Bayer Health Care, Biogen, Celgene, Diamed, Genzyme, MedDay Pharmaceuticals, Merck Serono, Novartis, Novo Nordisk, ONO Pharma, Roche, Sanofi-Aventis, Chugai Pharma, QuintilesIMS, and Teva; his research is funded by the BMBF, DFG, Else Kröner Fresenius Foundation, German Academic Exchange Service, Hertie Foundation, IZKF Muenster, German Foundation Neurology and Almirall, Amicus Therapeutics Germany, Biogen, Diamed, Fresenius Medical Care, Genzyme, Merck Serono, Novartis, ONO Pharma, Roche, and Teva. R.H. has received grant support from Biogen, Genzyme-Sanofi, Merck Serono, Novartis, Teva, and personal fees from Actelion, Biogen, Genzyme-Sanofi, Medday, Merck Serono, Novartis, Roche, and Teva. T.K. has received travel expenses and personal compensation from Bayer Healthcare, Teva Pharma, Merck, Novartis Pharma, Sanofi-Aventis/Genzyme, Roche, and Biogen, as well as grant support from Bayer Schering AG, Novartis, and Chugai Pharma. U.Z. has received grants from DFG, BMBF, the European Research Council, Bristol Myers Squibb, Janssen Pharmaceuticals NV, Servier, and Biogen Idec, and personal consultation fees from Pfizer GmbH, Bayer Vital GmbH, and CorTec GmbH. F.Z. received funds for scientific consultation or research by DFG, BMBF, the Pharmaceutical Management Science Association, Novartis, Octapharma, Merck Serono, ONO Pharma, Biogen, Genzyme, Celgene, Roche, Sanofi Aventis. A.S.-M. received travel expenses and research support from Novartis. C.C.G. received speaker honoraria and travel expenses for attending meetings from Biogen, Euroimmun, Genzyme, MyLan, Novartis Pharma GmbH, and Bayer Health Care, none related to this study; her work is funded by Biogen, Novartis, the BMBF (01Gl1603A), the DFG (GR3946/3-1, SFB Transregio 128 A09), the European Union (Horizon2020, ReSToRE), the IZKF, and the Innovative Medizinische Forschung. D.B. receives lectures, congress invitation, board participation with Bayer, Biogen, Medday, Merck, Novartis, Roche, Sanofi Genzyme, and Teva. B.T. received personal speaker honoraria and consultancy fees as a speaker and advisor from Alexion, Bayer Healthcare, Biogen, CSL Behring, GILEAD, GRIFOLS, Merck Serono, Novartis, Octapharma, Roche, Sanofi Genzyme, Teva, and UCB Pharma; his university received unrestricted research grants from Biogen-Idec, Novartis, Teva, Bayer Healthcare, CSL-Behring, GRIFOLS, Octapharma, Sanofi Genzyme, and UCB Pharma. F.T.B. has received, through his institution, grant support and/or travel reimbursement to attend scientific meetings from Actelion, Bayer, Biogen, Merck, Novartis, Sanofi Genzyme, and Teva; he has received personal honoraria for speaking or advisory board activities from Actelion, Bayer, Biogen, Merck, Novartis, Roche, and Sanofi Genzyme; his research is funded, in part, by the DFG and the BMBF. C.W. has received institutional support from Novartis, Sanofi Genzyme, Biogen, and Roche. H.-P.H. has received fees for consulting, speaking, and serving on steering committees from Bayer Healthcare, Biogen, GeNeuro, MedImmune, Merck, Novartis, Opexa, Receptos Celgene, Roche, Sanofi Genzyme, CSL Behring, Octapharma, and Teva, with approval by the Rector of Heinrich-Heine University. H.T. received speaker honoraria from Bayer, Biogen, Fresenius, Genzyme, Merck, Novartis, Roche, Siemens, and Teva; he serves as section editor for the Journal of Neurology, Psychiatry, and Brain Research; H.T. and his institution receives research support from the BMBF, DMSG, Fresenius, Genzyme, Merck, and Novartis, none related to this work. A.B. has received personal compensation from Merck, Biogen, Bayer Vital, Novartis, Teva, Roche, Celgene, Alexion, and Sanofi Genzyme, and grants for congress trips and participation from Biogen, Teva, Novartis, Sanofi Genzyme, Celgene, and Merck, none related to this work. C.L. received an endowed professorship supported by the Novartis foundation, has received consulting and speaker’s honoraria from Biogen-Idec, Bayer Schering, Novartis, Sanofi Genzyme, and Teva; and has received research scientific grant support from Merck-Serono and Novartis. B.W. received research grants and/or honoria from Merck Serono, Biogen, Teva, Novartis, Sanofi Genzyme, Bayer Healthcare, Roche, and research grants from the Dietmar Hopp Foundation, the Klaus Tschira Foundation, the BMBF, and the DFG. U.K.Z. received honoraria for lecturing and/or travel expenses for attending meetings from Almirall, Alexion, Bayer, Biogen, Celgene, Merck Serono, Novartis, Octapharma, Roche, Sanofi Genzyme, and Teva, as well as grant support from the BMBF and European Union, none related to this study; his research is funded by the BMBF, German Ministry of Economy, DFG, and European Union. S.B. has received honoria and compensation for travel from Biogen Idec, Merck Serono, Novartis, Sanofi Genzyme, and Roche. M.S. has received honoraria for scientific lectures or consultancy from Alexion, Bayer Healthcare, Biogen, CSL Behring, Grifols, MedDay, Merck-Serono, Novartis, Roche, Sanofi Genzyme, Takeda, and Teva; his institution received research support from Biogen, Sanofi-Genzyme, and Merck-Serono. R.A.L. received personal compensation from Celgene, Bayer Healthcare, Teva Pharma, Merck, Novartis Pharma, Sanofi-Aventis/Genzyme, Roche, and Biogen, as well as grant support from Novartis and Merck. F.W. served on the scientific advisory board of Genzyme, Merck Serono, and Novartis; he has received travel funding and/or speaker honoraria from Bayer, Biogen, Teva, Merck Serono, Novartis, and Pfizer; he is employed by Sana and received research support from the BMBF, Merck Serono, and Novartis. F.P. reported other support from Bayer, Novartis, Biogen-Idec, Teva, Sanofi-Aventis/Genzyme, Merck Serono, Alexion, Chugai, MedImmune, Roche, and Shire outside the submitted work; he has a post as academic editor for PLoS One and is associate editor for Neurology: Neuroimmunology & Neuroinflammation; he has scientific advisory board membership for Novartis and consulting fees from Sanofi Genzyme, Biogen-Idec, MedImmune, Shire, and Alexion; and research support from Bayer, Novartis, Biogen Idec, Teva, Sanofi-Aventis/Genzyme, Alexion, Merck Serono, the German Research Council, Werth Stiftung, German Ministry of Education and Research, Arthur Arnstein Stiftung Berlin, EU FP7 Framework Program, Arthur Arnstein Foundation Berlin, Guthy Jackson Charitable Foundation, and National Multiple Sclerosis of the United States. B.H. has served on scientific advisory boards for Novartis; he has served as data monitoring and safety committee member for AllergyCare and TG therapeutics; he or his institution have received speaker honoraria from Desitin; he holds part of two patents, one for the detection of antibodies against KIR4.1 in a subpopulation of MS patients and one for genetic determinants of neutralizing antibodies to interferon. All conflicts are not relevant to the topic of the study. F.L. has served on the advisory board of Roche and received travel funding from Teva. M.M. received research support from Merck Serono and Novartis as well as travel expenses for attending meetings from Merck Serono. K.L.-H. has received research support (to Technische Universität München) from Novartis, honoraria from Novartis and F. Hoffmann-La Roche, and compensation for travel expenses from Merck Serono., (Copyright © 2021 the Author(s). Published by PNAS.)