Multiple sclerosis is a complex immune mediated condition of the central nervous system characterized by myelin loss and progressive neurodegeneration. The risk of developing MS is influenced by both genetic and environmental agents and, among them, several lines of evidence support a role for vitamin D deficiency, Epstein-Barr virus (EBV) infection and smoking in the aetiology of this disease. The aim of this work was to further elucidate how nature and nurture act in the causal cascade leading to MS. In chapter 1, I show that the main genetic factor in adult MS (the HLA-DRB1*1501 allele) plays an equally important role in paediatric cases of MS (PMS) and that EBV negative PMS patients represent a separate entity characterized by lower age at disease onset, lower female to male ratio and a trend towards a lower frequency of the HLA-DRB1*1501 allele. In chapter 2, I provide evidence in support of month of birth having a role on MS risk and T cell production and that vitamin D may underlie this effect. In chapter 3 I demonstrate the presence of a link between vitamin D deficiency and the immune response against EBV, whereby the proportion of EBV seropositive MS patients and controls increases with increasing latitude and high dose vitamin D supplementation appears to reduce the level of antibodies against this virus. In chapter 4, I show that MS associated genetic variants are located in genomic regions that exert a regulatory function and are active in immune cell types. In chapter 5, I illustrate how vitamin D receptor binding is also located within active regulatory regions in immune cells and that this is particularly evident near MS associated genes. Finally, in chapter 6, I use chromatin data on more than 100 different cell types and conclude that MS associated genetic variants are particularly active in T helper, T cytotoxic and B cells. Further work is needed to elucidate how genetic and environmental agents play a role in the cause of MS and to develop effective strategies for disease treatment and prevention.

Multiple sclerosis (MS) is among the most common neuroinflammatory diseases across the globe and is autoimmune mediated in nature. This progressive, highly debilitating disease often leaves individuals wheelchair bound within 15-25 years of onset. MS is characterized by inflammatory lesions that appear in unpredictable locations around the central nervous system. Lesions can be visualized using magnetic resonance imaging (MRI) technology. As neuroinflammation continues and lesions accumulate, patients can experience a wide array of progressively worsening symptoms including but not limited to motor impairments, sensory disturbances, loss of control of bodily functions, and/or neuropathic pain, depending on the location of lesion formations. There are different types of MS, the most common being relapsing-remitting MS (RRMS) seen in about 85% of cases and characterized by periods of symptom remission followed by flare-ups. A large majority of these patients go on to develop secondary progressive MS (SPMS) where neurological damage and patient decline is progressive and continuous. Primary progressive MS (PPMS) is seen in about 10% of cases and is characterized by progressive and continuous patient decline from the outset of disease. Other rarer forms of MS do exist but will not be discussed further. Research aimed at MS is at an all-time high and the timing could not be better: its global incidence and prevalence is climbing. For decades MS has been thought of as a disease caused by dysfunctional CD4+ T-helper 1 (Th1) cells. It is now known that many different cell types contribute to MS pathophysiology. These other cell types include macrophages and dendritic cells of the innate immune system due to their expression of MHC class II molecules that function to activate CD4+ Th1 cells. More recent research has implicated CD8+ T-cells and B-cells in contributing to disease through direct destruction of neural cells that express MHC class I molecules and through the generation of autoantibodies, respectively. While these discoveries are important and provide hope for future breakthrough treatments, there are still enormous gaps in the medical community’s knowledge of what causes MS. The epidemiologic pattern of MS prevalence has for many decades interested scientists and hinted at a potential cause of this disease. MS tends to affect white individuals with genetic ties to northern Europe, but this relationship may not still hold true, as MS incidence and prevalence may be rising faster in black populations compared to other races/ethnicities, at least in the United States. MS occurs nearly 3 times as often in females than in males, and is strongly associated with Epstein-Barr virus (EBV) infection—especially in those that go on to develop infectious mononucleosis (IM). MS prevalence increases markedly in regions north of 40 degrees North latitude or south of 40 degrees South latitude. MS risk also changes depending on body mass index (BMI) considerations, migration history, and in families with a genetic history of the disease. It is well-accepted that MS has a genetic component, the most important of which is the presence of the HLA-DRB1*1501 allele that codes for certain proteins in MHC class II molecules. However, genetics alone are unable to sufficiently account for MS risk as the concordance rate for identical twins with MS is only 25-30%. These well-established findings imply that some unknown environmental factor(s) must be contributing to MS initiation and progression. All of the environmental factors listed above have a common connecting thread that is logically and empirically verifiable: vitamin D. This fat-soluble vitamin can either be endogenously synthesized in the skin after exposure to ultraviolet B (UVB) light or consumed through the diet, the former being of more importance to humans. Epidemiologic patterns suggest a protective role for vitamin D in MS, where low or deficient levels of vitamin D may be a contributor to increased risk for MS. Populations living at greater latitudes, north or south, have significantly greater prevalence of MS which coincides with the reduction of endogenously produced vitamin D in these regions due to a lesser amount of UVB light (and of lower intensity) experienced year-round. Increases in BMI, especially increased adiposity, correlate with increased risk for MS and with prevalence of vitamin D deficiency. Women tend to naturally have greater adiposity than men, thus increasing their risk for MS. Estrogens and vitamin D have been shown to act synergistically to protect against MS, therefore vitamin D deficiency may increase risk for MS in women. Vitamin D is a known immunomodulatory agent that promotes tolerogenic immune states. Vitamin D also offsets many of the harmful effects caused by EBV, among these including repression of B-cell differentiation into plasma cells, reduced MHC II expression, and promotion of B cell apoptosis. This serves to repress deleterious immunoglobulin secretion by B-cells. Vitamin D is also immunologically beneficial as it promotes regulatory T cell function and their expression of protective cytokines, and through its inhibition of inflammatory Th1 and Th17 cell functions. In total, the immunomodulatory mechanisms of vitamin D are important as the immunological states produced by vitamin D are exactly the opposite of those observed in MS patients and MS animal models. Research in vitamin D is gaining attention as the scientific community is quickly discovering that its true physiologic role extends far beyond its classical function as a calcium regulator. Indeed, rapidly evolving research is revealing roles for vitamin D in cardiovascular function and blood pressure regulation, brain development and neurological function, and even in the prevention of certain cancers. However, this thesis will focus on its most well-known function secondary to calcium regulation: immunomodulation and its anti-inflammatory capabilities. The last portion of this thesis will present information advocating for the increase in minimum dietary intake of vitamin D from its current value of 800 IU/day to 5,000 IU/day. While a more than 5-fold increase may seem drastic, the tolerable upper limit is at least 10,000 IU/day even by the most conservative of estimates—the true upper limit is probably around 20,000 IU/day and may even be 50,000 IU/day. The global prevalence of vitamin D deficiency is so extensive that some authors have even considered it a global pandemic: upwards of 50% of the entire world population may be deficient in this crucial vitamin. Increasing vitamin D supplementation is an extremely low risk way to reduce risk for MS and other diseases.