18 Chapter 1 to infectious mononucleosis, i.e. EBV infection in childhood and young adulthood154. EBV interacts with complement C2 receptors on memory B cells to infect and subsequently immortalise them155-157. There are reports that EBV can infect specific memory B-cells to produce anti-MOG (myelin oligodendrocyte glycoprotein) auto-antibodies158. These autoantibodies induce upregulation of heat shock protein B5 (HSPB5) in oligodendrocytes, which activates microglia159, but also activates HSPB5-reactive memory T-cells160. When these T-cells are activated they in turn start producing IFNγ, a pro-inflammatory cytokine with detrimental effects in MS159, possibly initiating the development of demyelinating lesions in MS. The latitudinal effect on risk of MS, is mainly attributed to sunlight exposure, as well as vitamin D production161. High vitamin D consumption in Norway, or through supplementation in the US military have protective effects against MS162,163. Additionally, there is a genetic risk to developing MS. Risk of developing MS increases 10-25 times for first-degree relatives, compared to the general population, where monozygotic twins have the highest risk if either one twin develops MS164,165. Risk of developing MS has also been linked to human leukocyte antigen (HLA) and is increased in people expressing HLA-DR*1501 common in northern Europeans. This is also supported by an increased risk people homozygous for HLA-DR1501 compared to heterozygous166. Additionally, regional variation revealed many haplotypes of the HLA gene that have either protective or detrimental effects on the risk of developing MS. These HLA haplotypes differ in magnitude of effect which and can either interact with each other or act on their own167. Clinical symptoms The clinical presentation of MS is heterogeneous depending on the presence of lesions and extent of damage affecting the brain but also the spinal cord and optic nerve. The variety in clinical presentation is a consequence of the transient nature of focal attacks of neuroinflammatory lesions in the CNS. The location and timing of the lesions can result in clinical symptoms ranging from visual problems, if lesions arise in the optic nerve, to motor problems, when lesions arise in the motor areas. The first symptoms of MS often include signs of visual problems such as diplopia and optic neuritis, but also sensory disorders and limb weakness. During the early stages of the disease when neuronal reserve is high, pwMS may fully recover from their clinical attacks, recognised as relapsing-remitting forms of the disease (RRMS). Over time, this reserve and ability to repair the damage in the CNS is lost and pwMS develop a more progressive form of the disease: secondary progressive MS (SPMS). SPMS develops in 90% of people with RRMS over the course of 10-15 years after onset. However, up to 10% of people don’t recover from their clinical attacks and have a primary progressive disease course (PPMS) with increasing disease disability. The diagnosis of MS is made according the McDonald criteria168 (Table 3). The McDonald criteria have been extensively revised due to increasing knowledge of the pathology of the disease, and the incorporation of rapidly advances in development of imaging modalities. However, the key requirement of a diagnosis according to the criteria is that people have experienced damage of the CNS that is disseminated in time and space, meaning that there is evidence of CNS damage at 2 or more time points and different areas of the CNS. These criteria are supported by the presence of oligoclonal bands in the CSF.
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