1.1. MS Pathology and Clinical Disease
MS pathology is characterized by multiple lesions within the CNS [
1]. The pathogenesis of brain lesions remains largely unknown; however, neurodegeneration due to inflammation and immune reaction towards the brain cells is believed to play the central role. Interestingly, brain plaques in MS are predominantly found within the white matter around the lateral ventricles and optic nerve [
2,
3]. Recently, the presence of the brain plaques in the grey matter was demonstrated by Calabrese et al. [
4]. These lesions could be detected early and correlate with the disease severity [
5]. Lesions in periventricular locations along the 4th ventricle, midbrain and cerebellar peduncle are also characteristic for MS, though they are less prevalent [
6,
7]. The close proximity of lesions to the ventricles containing the choroid plexus, a structure of the blood-cerebrospinal fluid (CSF) barrier, suggests a connection between peripheral circulation and brain tissue pathology (
Figure 1).
Several factors (sunlight exposure, serum vitamin D (vitD) level, viral infection and genetic factors) trigger development of autoreactive T lymphocytes in the periphery. T lymphocytes targeting myelin cross the blood-brain barrier (BBB) and reach the neurons in CNS. Within the brain, T lymphocytes trigger inflammation and myelin degradation. Consequently, chronic inflammation and reduced myelin will affect the neuron function, leading to clinical symptoms of Multiple Sclerosis.
This assumption is supported by the finding that activated mononuclear cells, such as lymphocytes, macrophages and dendritic cells, occur within the brain plaques near the BBB [
8,
9,
10,
11]. It appears that MS lymphocytes have a higher ability to cross the BBB when compared to cells from healthy controls [
12]. Leukocyte migration is tightly regulated and involves interaction between cell adhesion molecules (CAMs) on BBB endothelial cells and their ligands on the white blood cells [
13,
14,
15]. The very late antigen 4 (VLA-4) ligand interaction with VCAM appears essential for activated T-cell migration across the BBB in MS, as VLA-4
+ leukocyte infiltration was demonstrated in MS lesions [
16]. Additionally, the therapeutic effect of interferon (IFN) was linked to decreased expression of this ligand on circulating leukocytes in MS [
17]. Therefore, targeting VLA-4 was suggested for treatment of MS, which was shown to be effective in RRMS [
18]. Decreased counts of circulating CD4
+, CD8
+ and CD19
+ lymphocytes together with reduced expression of VLA-4 ligand on activated leukocytes was also linked to therapeutic efficacy of natalizumab, a humanized monoclonal antibody binding to VLA-4 [
19,
20,
21]. Interestingly, leukocyte migration can be inhibited by IFN-β and tissue inhibitor of metalloprotease 1 (TIMP-1) [
22,
23], suggesting that this process is regulated by immune mediators and requires metalloprotease. This assumption was further supported by Prat et al. demonstrating that antibody to CCL2, a potent mononuclear leukocyte chemoattractant [
24], has significantly reduced leukocyte migration in MS [
25].
Currently, the leading role of Th17 and Th1 lymphocytes in pathogenesis of the MS brain plaques is well recognized [
26,
27]. High numbers of Th1 lymphocytes, expressing IL-12 and IFN-γ, are commonly found in the brains of experimental autoimmune encephalomyelitis (EAE) mice, the animal model of MS [
28]. Also, elevated numbers of CD8
+ and CD4
+ lymphocytes producing IL-17 have been found in active brain lesions in MS cases [
29]. Th17 cells can readily cross the BBB, as they produce cytokines, chemokines and express receptors that compromise tissue barrier permeability [
30,
31,
32]. Based on the large body of evidence supporting the role of Th1 and Th17 lymphocytes in MS pathogenesis, it has now been suggested that the Th17 response is more relevant in the early stages of the disease, while the Th1 lymphocytes are important later, supporting local inflammation [
33]. B cells are also involved in MS pathogenesis with studies demonstrating increased B cell counts [
34] and the presence of oligoclonal antibody complex bands in MS CSF [
35]. The most compelling evidence for the role of B cells in MS pathogenesis comes from clinical trials using the CD20 monoclonal antibodies rituximab, ocrelizumab and ofatumumab [
36,
37,
38], which deplete the B cell population. Collected data from these trials demonstrated that treatment with rituximab reduced the number of new lesions and proportion of patients experiencing relapses [
39].
Autoreactive lymphocytes target basic myelin protein, which is a component of the myelin sheets that sheath neurons. Oligodendrocyte apoptosis is detected at the site of demyelinization in MS [
40] resulting in neuroglial activation [
41,
42]. Myelin reactive lymphocytes are found in the circulation and CSF of MS cases as compared to healthy subjects [
43,
44]. The role of myelin-specific leukocytes in MS pathogenesis was confirmed using EAE mice, where injection of myelin-specific CD8
+ cytotoxic lymphocytes led to severe CNS autoimmunity in animals [
45]. This immune response is, however, polyclonal with no clear overlap in antibody or T cell clones between individual MS patients or different studies, nor is it clear whether structural changes to the protein itself are triggers of the immune response that typifies MS or responses to it [
46,
47,
48].
Clinically, several forms of MS are recognized: relapsing remitting MS (RRMS), secondary progressing MS (SPMS), primary progressing MS (PPMS) and progressive relapsing (PRMS) (
Figure 2). In 80–85% of cases, the onset of RRMS is characterized by episodes of neurological disability and recovery [
49]. As the RRMS progresses, 60–70% of cases will gradually worsen with a steady progression of symptoms [
50]. This pattern of the disease is referred to as SPMS. A small group of patients (approximately 10%) will develop PPMS, which is characterized by steady progression of the neurological symptoms without periods of recovery [
51,
52].
Each form of MS differs in the frequency of exacerbations and duration of the remission. Interestingly, it appears that periventricular lesions are more often found in progressive MS as compared to the relapsing form of the disease [
54]. However, the number of lesions and their localization in the spinal cord do not correlate with the form of the disease or disability. Brain lesion localization and their number are related to clinical symptoms and clinical presentation, where the number of demyelination foci increases each time the patient experiences a clinical relapse [
55]. However, it appears that brain inflammation does continue in remission with the number of brain plaques increasing gradually during disease remission. Clinical symptoms of MS vary depending on the localization of the brain lesions, as they affect structures connected specifically to those parts of the CNS [
56]. Spinal cord involvement presents with sensory loss or motor weakness in the body, while damage to the brainstem affects sensation or weakness of the face and diplopia. When inflammation is localized in the optic nerve, signs such as blurry vision, ocular pain and visual loss are typically described [
57].
1.2. Gene-Environment Interaction
Various studies suggest a genetic predisposition to MS. There is a clear difference in risk of developing MS between different ethnic groups. Although diagnosed worldwide, the highest MS incidence rate is registered in Europe [
58]. The highest prevalence (≥200/100,000) is in Scotland, Northern Ireland and within certain populations in Scandinavia and Sicily [
59,
60,
61]. MS prevalence in the British Isles ranged from 96/100,000 to more than 200/100,000, with the highest rates in Scotland and Northern Ireland (7.2 to 12.2 per 100,000) [
62,
63,
64]. It appears that populations of Northern European descent are at higher risk of developing MS. Eight-fold lower rates of MS have been found in Asian and African populations in Norway when compared to the indigenous Sami [
65,
66]. Similarly, in Malta, Northern European immigrants had a 10-times higher MS prevalence than that of Maltese-born individuals [
67]. These data also suggest a genetic predisposition to MS independent of latitude effects.
There is also a wealth of evidence supporting a higher frequency of MS diagnosis in siblings [
68,
69,
70] and individuals closely related to MS probands [
71]. High risk of an MS diagnosis among closely related individuals suggests a link between HLA haplotype and the disease. There is strong support from multiple studies including large genome wide association studies (GWAS) for the HLA allele DRB1*15:01 as the most significant genetic risk factor for MS, particularly for a lowered age of onset [
72]. This allele is also present at very high frequency (14%) in Northern European populations [
73] and 18–19% in Southern European populations [
74]. Its methylation status and co-morbidity with various environmental risk factors for MS are also well described as increasing MS risk [
75,
76].
There have also been at least a further 11 HLA alleles statistically associated with MS risk. A number of large GWAS studies (some including up to 80,000 individuals) have further identified up to 200 non-HLA genetic variants statistically associated with MS risk. These variants are, however, still only calculated to explain about 30% of the genetic risk of MS. It is also worth noting in this respect that many of these studies have been restricted to European ethnicity and that many of these genes have limited support for association and await functional characterization for a contribution to MS phenotype. There have also been a number of studies of rarer gene variants in specific families with high MS risk, some of which have been controversial [
77]. The diagnostic confusion between MS and several rare gene disorders that can have similar presentations is also potentially a confounder in some of these studies [
78,
79].