Pathophysiology of multiple sclerosis Domina Petric, MD

Epidemiology I. 8.11.2018

Epidemiology Lack of vitamin D, increased smoking! The further north or south from the equator, the higher the prevalence of MS. Prevalence in 2013 globally was 33/100000. Lack of vitamin D, increased smoking! MS is commonly diagnosed between 20 years and 40 years, but it can affect younger and older individuals. 8.11.2018

Types of MS II. 8.11.2018

Types of MS Relapsing-remitting MS (RRMS) is characterized by relapses followed by remission (85%). Secondary progressive MS (SPMS) develops over time following diagnosis of RRMS. Primary progressive MS (PPMS) is characterized by gradual continuous neurologic deterioration (8-10%). Progressive relapsing MS (PRMS) is similar to PPMS, but with overlapping relapses (5%). 8.11.2018

Genetic studies III. 8.11.2018

Genetic studies The risk of developing MS is increased if a first-degree relative suffers from the disease. The lifetime incidence of MS is 0,1% in the normal population. Siblings of MS patients carry a lifetime risk of 3%, but if they are twins, risk is up to 25%. 8.11.2018

Genetic studies HLA-DR2 HLA-DR4 Risk alleles for MS: IL2Rα, IL7R and CD58 genes. The expression of the related gene products on leukocytes suggests a role of these molecules in the immune-mediated pathologic cascade of MS. HLA-DR2 HLA-DR4 8.11.2018

Infectious agents and MS IV. 8.11.2018

Infectious agents and MS Borrelia burgdorferi Chlamydia pneumoniae Human herpes virus 6 (HHV-6) Epstein-Barr virus (EBV) ? 8.11.2018

Immunopathophysiology V. 8.11.2018

Immunopathophysiology MS is not just a disease of the immune system, but equally involves factors contributed by the central nervous system (CNS). Immune cells residing in the CNS get activated following damage to CNS tissue. Immune CNS cells (microglial cells) upregulate MHC class I and II molecules and cell surface co- stimulatory molecules. 8.11.2018

Immunopathophysiology Microglia secretes cytokines and chemokines, paving entry for T (CD4, CD8) cells, B cells, monocytes, macrophages and dendritic (DC)-like cells into CNS lesions. Infiltrating immune cells secrete pro- inflammatory cytokines, nitric oxide and matrix metalloproteinases, leading to destruction of the myelin sheath. 8.11.2018

Immunopathophysiology Myelin-reactive auto-T cells (MRAT) cross the blood brain barrier (BBB). Their migration into CNS initiates an inflammatory cascade: demyelination of the CNS, axonal damage. MRAT cells reside in the perivenous demyelinating lesions: inflammatory demyelinated plaques situated within white matter. 8.11.2018

Immunopathophysiology MS lesions: white matter inside the visual neuron, basal ganglia, brain stem and spinal cord. Two main steps of MS: Myelin sheath damage: formation of lesions in the CNS. Inflammation: destruction of the neuron tissue. 8.11.2018

Immunopathophysiology Damage of oligodendrocytes and destruction of myelin sheath leads to breakdown of the nerve axon and loss of neuronal function. Demyelination increases the inflammatory activation processes leading to damage of BBB and stimulation of macrophage activation and oxidative stress pathways. 8.11.2018

Immunopathophysiology The white matter lesions include myelin breakdown and infiltration of monocytes, B cells, T cells and DC. Microglia and macrophages are the main innate immune cells present in MS lesions. Microglia and macrophages (M&M) act together with T and B cells, or directly cause neuroinflammatory tissue damage. 8.11.2018

Natural killer T (NKT) cells NKT cells share properties of both T cells and NK cells. NKT cells recognize glycolipid antigens presented in complex with the MHC class I-like molecule, CD1d. Type I: invariant NKT (iNKT) cells. Type II: variant NKT (vNKT) cells. 8.11.2018

iNKT cells iNKT cells express cell surface markers characteristic of activated or memory T cells (CD25, CD44, CD69). Activation of iNKT cells is via binding to α-Gal-Cer- CD1d complex: secretion of cytokines pro-inflammatory immune responses anti-inflammatory immune responses 8.11.2018

iNKT cells secrete IL-4, IL-13: stimulation of CD4+ T cells to differentiate into anti- inflammatory Th2 cells (IL-4, IL-10 producers), which inhibit Th17, Th1 and CD8+ T cells in the CNS. IL-2, tumor growth factor beta (TGF-β): stimulation of the production of T regulatory (Treg) cells (IL-10, TGF-β producers), which inhibit Th17, Th1 and CD8+ T cells in the CNS. IL-4, IL-10, IL-13, interferon gamma (IFN-γ) and GM-CSF: activation of suppressive myeloid derived suppressor cells (MDCs), DC and macrophages. MDCs, DC and macrophages secrete IL-10 to activate Treg cells and suppress Th17, Th1 and CD8+ T cells in the CNS. 8.11.2018

iNKT cells iNKT cells play a role in protecting the host against pathogens, tumors, autoimmunity. iNKT cells are involved in tissue rejection, ischemia reperfusion injury and obesity related diabetes. Deficiency or dysfunction of iNKT cells is linked to the development of autoimmune diseases. iNKT cell number is decreased in patients with MS. 8.11.2018

Mucosal-associated invariant T cells (MAIT) MAIT cells are a subset of T cells of the innate immune system to defend against microbial infections. These cells are present in the liver, lungs, mucosa and blood. MAIT cells make up to 25% of CD8 T cells in healthy individuals. They support adaptive immune responses. 8.11.2018

Mucosal-associated invariant T cells (MAIT) The MHC class I-like molecule presents microbial antigens and vitamin B metabolites to MAIT cells: ACTIVATION. In MS, MAIT cells are highly present at the sites of demyelination. MAIT cells secrete pro-inflammatory Th1 cytokines (IFN-γ, TNF-α) and activate Th17 cells (IL-17 and IL-22 cytokines). 8.11.2018

Mucosal-associated invariant T cells (MAIT) MAIT cells have been noted in white matter inflammatory lesions. MAIT cells are decreased in blood of patients with RRMS. 8.11.2018

Regulatory T cells (Tregs) Tregs are a subset of CD4+ T cells that modulate immunity, maintain tolerance against self-antigens and prevent autoimmunity. Tregs are primarily characterized as Fox3+CD25+CD4+ and are anti-inflammatory (IL-10 secretion). 8.11.2018

Regulatory T cells (Tregs) In patients with MS the requency of Fox3+CD25+CD4+ Treg cells does not differ to those in healthy individuals, but the function of these cells is impaired. mRNA and protein levels of Foxp3 are impaired in Treg cells of patients in RRMS, but normalized during SPMS. Impaired functionality of Treg cells is primarily observed in the early stages of MS, but not in their chronic stage: CAUSATIVE ROLE. 8.11.2018

Macrophages M1 macrophages are pro-inflammatory and primarily secrete IL-1, IL-6, IL-12, TNF-α, iNOS and MCP-1. They stimulate adaptive immune responses. M2 macrophages are anti-inflammatory and primarily secrete IL-1 receptor antagonists: IL-4, IL-10, TGF-β. 8.11.2018

Macrophages Macrophages are highly present in active demyelinating and early re-myelinating lesions. Both M1 macrophages and intermediate subtype (M1/M2, CD40+, CD206+) are present. 8.11.2018

Microglia M1 microglia cells are pro-inflammatory and express CD40, CD74, CD86 and CCR7. M2 microglia cells are anti-inflammatory and express mannose receptor (CD206) and CCL22. In MS brain lesions an intermediate microglia phenotype is present: CD40, CD74, CD86, CCL22, but not CD206 markers. 8.11.2018

T helper cells CD4 cells or T helper (Th) cells recognize short 9-17 amino acid peptides presented on the surface of antigen presenting cells (APC) in complex with MHC class II. CD4 T cells differentiate into distinct Th cells depending on the cytokine secretion profiles. 8.11.2018

IL secretion by T helper cells Th1 cells are pro-inflammatory: high levels of IL-2, IL-12, TNF-α and IFN-γ. Th2 cells are anti-inflammatory: IL-4, IL-5, IL-6, IL-10, IL-13, IL-25. Th17 cells are pro-inflammatory: high levels of IL-17A, IL-17F, IL-21, IL-22, IL-24, IL-26 and low levels of IL-9, IFN-γ. 8.11.2018

IL secretion by T helper cells Th22 cells are a combination of Th1, Th2, Th17 phenotype and secrete IL-13, IL22 and TNF-α. Th9 cells secrete IL-9. Th1, Th9 and Th17 cells are key contributors to MS: increasing inflammation within the milieu of the myelin site. 8.11.2018

T helper cells Th1 cells and their pro-inflammatory cytokine products are present in high levels within the demyelinating axon and CNS lesions. Th17 cells induce an inflammatory milieu in patients with MS. IL-17A is present at high levels in CNS lesions, cerebrospinal fluid and in serum of patients with MS. 8.11.2018

T helper cells Th17 cells express high levels of CCR6 which binds to the ligand CCL20 on vascular endothelial cells, enabling their entry through the blood brain barrier. IL-17 interferes with the remyelination process. 8.11.2018

T helper cells IL-27 is secreted by macrophages, dendritic cells and microglia cells. Th1 cells are induced by IL-27. Th2, Th17 and Treg cells are inhibited by IL-27. Tr1 (T cells that secrete IL-10) are induced in the presence of IL-27. Circulating plasma IL-27 levels were significantly higher in patients with RRMS. 8.11.2018

CD8 T cells Classical CD8 T cells or cytotoxic T cells (Tc1 cells) recognize short antigenic 7-9-mer peptide epitopes presented on the surface of APC in complex with MHC class I. In MS there is a genetic association with HLA-A3. In MS, CD8 T cells are noted in high numbers, much higher than CD4 T cells (ratio 10:1 for CD8 T cells). 8.11.2018

IL secretion by CD8 T cells Classical Tc1 cells: IFN-γ. Tc2: IL-4. Tc10: IL-10. Tc17: IL-17. Tc21: IL-21. Tc 22: IL-22. 8.11.2018

CD8 T cells MHC class I is highly expressed within MS lesions. CD8 T cells, that are present in both acute and chronic MS lesions, secrete high levels of IL-17 (Tc17 CD8 T cells). In peripheral blood of patients with SPMS and RRMS there are elevated levels of Tc1 and Tc17 cells. Tc21 cells are increased in the remisssion phase of RRMS. 8.11.2018

B cells In patients with MS there are oligoclonal bands (OCB) present in cerebrospinal fluid and brain parenchyma (95% of patients). OCB is a product of clonally expanded B cells and IgG synthesis. In MS plaques plasma cells are noted in large numbers. 8.11.2018

Dendritic cells (DC) DCs (professional APC) process and present antigenic peptide epitopes on their surface in complex with MHC class I or II: CD4 or CD8 T cell stimulation. Activation of DCs in the periphery leads to T cells specific to myelin epitopes activation and induction of pro-inflammatory cytokines: entry through the BBB into the CNS. In the CNS resident APC and T cells are further activated: demyelination, motor deficits. 8.11.2018

Dendritic cells (DC) In patients with MS, DCs are present within inflamed lesions, cerebrospinal fluid and in the circulation. DCs produce high levels of TNF-α, IFN-γ and IL-6. The expression of co-stimulatory molecules, CD40 and CD80 on DCs, is increased in RRMS and SPMS patients: activated pro-inflammatory state of DCs. 8.11.2018

Myeloid derived suppressor cells (MDSC) MDSC are myeloid progenitors. They have strong immunosuppressive properties. Their major role is in tumor development and chronic inflammation having immune suppressive effects. In patients with MS, failure of MDSCs to suppress autoimmune T cells probably happens, as a result of disease progression. 8.11.2018

Literature Korn, Thomas. (2009). Pathophysiology of multiple sclerosis. Journal of neurology. 255 Suppl 6. 2-6. 10.1007/s00415-008-6001-2. Dargahi N, Katsara M, Tselios T et al. Multiple Sclerosis: Immunopathology and Treatment Update. Brain Sci. 2017;7:78. 8.11.2018