Inflammation in Multiple Sclerosis Flashcards
What is MS?
- It is a slow, progressive CNS disease characterized by the destruction ot the myelin sheath around axons in any part of the CNS
- The major hallmark of MS is breadown of the myelin sheath by immune cells
- This disrupts the transmission of action potentials, thus leading to failure of nerve cell communication
What is the epidemiology of MS?
- Most common cause of chronic neurologic disability
- Increasing incidence over the past century
- Usually starts between 20-40 years
- Women affected twice as often as men
- Northern European more common
- Incidence of 1:1000
What are the causes of MS (genetic and environmental)
- Multifactorial disease – no single cause
- Combination of genetic predisposition, infectious agent and environmental factors thought to induce an abnormal immune response towards the self-antigen myelin, so MS is an autoimmune disease
Genetic factors:
• Risk assessment studies performed in Canada appear to confirm that genetic, and not environmental factors are primarily responsible for the familial clustering of cases
• However, an intriguing association with the month of birth was found in the Canadian cases, reflecting perhaps an interaction between genes and the environment during gestation or shortly after birth.
• Concordant siblings tend to share age of symptom onset rather than year, suggesting genetic effect
• Twin studies indicate 20-30% concordance
• The HLA-DRB1 gene is the strongest genetic factor identified as influencing MS:
− The DRB1*1501 allele is consistent across nearly all populations
− Exact mechanism by which it influences susceptibility remains undefined, but are likely related to the physiological functions of the HLA molecules in the immune response → antigen binding and presentation
− The identification for the true predisposing gene within the HLA region has been held back by the extensive linkage disequilibrium → genes for the complement proteins, TNF, HSP - collectively known as the MHC-III region
− Estimated to account for 17-60% of the genetic susceptibility
Role of the environment:
• Clusters or outbreaks used to illustrate potential environmental affects
• Common viruses including measles, mumps, rubella, EBV, HSV, VZV plausible infectious agents relating to pathology
• Nutritional and dietary factors
• exposure to animals, minerals, chemicals, metals, organic solvents
What are the symptoms of MS?
They vary depending on the brain region (in brackets):
• Result from the interruption of muyelinated tracts in the CNS
• Weakness or diminished dexterity in one or more limbs (spinal cord)
• Sensory disturbance (cerebrum)
• Monocular vision loss (optic nerve)
• Double vision (optic nerve)
• Gait instability (cerebellum)
• Ataxia
As the disease worsens: • Bladder dysfunction • Fatigue • Heat sensitivity • Cognitive defects (cerebrum) → memory loss, impaired attention, problem-solving difficulties, slowed information processing, • Depression experienced in 60% of patients (cerebrum) • Suicide 7.5 fold more common • Hemifacial weakness or pain • Vertigo (brain stem) • Brief tonic spasms (spinal cord)
How is MS diagnosed?
- Neurological examination
- Lumbar puncture (CSF) → electrophoresis shows oligoclonal IgG, mononuclear cells
- Blood tests
- Reduced evoked potential (measures speed and strength of conduction along the optic nerve)
- MRI → most important diagnostic tool nowadays. Reveals number and location of inflammatory lesions as white dots. The white dots represent an influx of immune cells, and hence a leakage of the BBB
What are the clinical patterns of MS?
• Has a heterogenous disease pattern
• Relapsing remitting → 75%. This is recurrent attacks of neurological dysfunction, but patients fully recover from a relapse
− Relapses are associated with the development of new focal lesions
− 6/7 new lesions thought to be silent
− Even symptomatic lesions do not in most cases produce permanent disability, as remissions are the rule.
− Nevertheless, the development of large number of early lesions is associated with greater risk of disability later on
• Secondary progressive → In time, RR may progress where, after relapsing and remitting several times, you get a sustained build up of disability, incomplete recovery from relapses and a gradual progression of the condition.
• Primary progressive → 15%. Here, you never have recovery.
→ The risk of transitioning to progressive is 2.5% each year.
− Suggested that once a certain pathological threshold is reached, patients progress along an irreversible neurodegenerative pathway.
What are the pathological findings of MS?
- Macroscopically, lesions are detectable by eye on post-mortem material
- Post-mortem MRI assists in lesion finding when not visible by eye, and correlate with neuropathology
- Histological staining for myelin indicates the level of demyelination
→ MS generally considered to be a white matter disease, but increasing interest in the involvement of grey matter
− Cortical demyelination seems to be rare in early MS, suggesting cortical pathology follows the white matter disease
− However, emerging consensus that cortical lesions and atrophy are major contributors to the patholog
General microscopic findings:
• Perivascular infiltrates (lymphocytes and monocyte derived macrophages)
• Demyelination by activated macrophages → phagocytosis the myelin
• Destruction of oligodendrocytes
• Hypertrophy of reactive astrocytes (leading to scar formation)
• Axonal damage and loss
• BBB damage
Is there neuronal damage in MS?
- The traditional neuropathological view of MS highlights myelin loss as the key event leading to pathology
- However, axonal damage appears to take place in every newly formed lesions, and the cumulative axonal loss is considered now to be the reason for progressive neurological disability
- Pathological studies indicate as many as 70% of axons are lost in patients with advanced
- It is unclear whether demyelination is a pre-requisite
- However, demyelination results in reduced support for the axons as well as redistribution of ion channels, destabilization of axon membrane potentials and conduction block – axons can initially adapt to this but eventually retrograde degeneration occurs
Is there BBB damage in MS?
• Has been shown that in the early stage of lesion development, the function of the BBB is impaired
• Serum proteins (fibrinogen) seen in the CNS of patients before cellular influx occurs
• After infiltration, monocytes accumulate in the perivascular space
• There are BM alterations → IHC for laminin shows much higher expression in MS
• It has thus been concluded that:
− The function of the BBB is impaired
− BBB dysfunction may lead to cellular influx
− BM alterations exist
• Interventions at the BBB may lead to new therapeutics
What models are used to determine what causes the damage in MS? And what have they shown?
- In vitro models for BBB (permeability, tight junctions)
- In vitro models that mimic the inflammatory phase of the disease
- In vitro cell migration models
- The most frequently used in vivo model of MS is the experimental allergic encephalomyelitis (EAE) rat → reflects the inflammatory phase of the disease
- involve acute immunization with CNS homogenate or myelin proteins in adjuvants
- EAE could be adoptively transferred with myelin-sensitised T cells.
EAE clinical symptom score:
- Loss of tail tonus
- Unsteady gait
- Partial hind limb paralysis
- Paralysis of hind limbs and diaphragm
- Death
• Peak clinical symptoms are around 14 days post immunization, after which animals recover
• To distinguish BBB leakage and cellular infiltration during EAE, MRI is used against:
− Gadolinum-DPTA (vascular leakage marker)
− Ultrasmall particles of ion oxide (USPIO) – accumulate in macrophages
GdDTPA leakage in EAE:
• Leakage is visible as white dots
• Visible from day 11 (onset of the disease)
• These results indicate that BBB leakage is already present at the onset of the disease
USPIO accumulation in the brain during EAE:
• Accumulation is at a maximum 14 days post immunization, and declines by day 17 when the animals have recovered
• These particles are present in the infiltrating monocytes
What are the role of monocytes migrating through the endothelium in MS?
- Monocyte derived macrophages are crucial for dmyelination and disease progression.
- Monocyte migration through the endothelial layer consists of tethering, rolling, firm adhesion, transmigration and then infiltration
- In order to allow transmigration. the endothelial layer has to move apart to allow passage
- Recent advances have indicated the importance of cell migration for disease progression – the use of Natalizumab, which blocks the a4b1 integrin, limits cell entry and the formation of new lesions.
To examine the roll of the endothelium in this:
• A brain endothelium cell line expressing GFP-occludin was generated
• Was established that monocytes traffic towards the junction, and have the ability to open them
→ In the context of tight junctions, MMPs may play an important role:
MMPs and cerebral inflammation:
MMPs:
• Degrade the ECM → one of its targets is occluding
• Secreted by endothelial cells and inflammatory cells
• Secreted as pro-enzymes, activated by other proteases
• Activity is highly regulated by cell-cell interaction
• Co-localise with tight junction proteins
• When we inhibit MMP, we inhibit transjuctional transmigration.
Monocyte migration through the brain endothelium is a critical event in neuroinflammaton in MS:
• Monocyte migration can be inhibited by IFNb, MMP inhibitors, Lovastatin, anti-oxidants → studies have suggested that ROS are the most dominant pro-inflammatory agents secreted by activated monocytes.
• Monocyte adhesion enhances BBB permeability
− When we add monocytes, BBB permeability incrraeses
− When we add the anti-oxidant lipoic acid, permeability decreases
→ Indicates a role for ROS produced by macrophages in enhancing the BBB permeability
• It was observed that occludin protein was displaced upon exposure to ROS
The model is:
• Monocyte attachment to the endothelial cells results in ROS production
• ROS can activate intracellular signaling cascades involving RhoA, PI3Kinase and PKB/AKT
• This leads to cytoskeletal destabilization that opens up the BBB
Other than monocyte-mediated, what are the other immunological processes occuring in MS?
In EAE, T lymphocytes also mediate the pathology:
• Components of the myelin sheath share amino acid homologies with proteins of measles, influenza, herpes, papilloma and other viruses
• Infection with these pathogens, through molecular mimicry, can therefore result in the activation of myelin-specific T cells and drive a misguided response
➢ Sequence homology to hepatitis B virus
➢ Structural homology to EBV
• Once activated, T cells express integrins to mediate binding ot the BBB → one of these is VLA4
• As the activated T cells migrate acoss the BBB, they express MMPs to lyse the basal lamina
• After transversing the BBB, pathogenic T cells are reactivated by fragments of myelin antigens
• Primed CD4+ T cells are engaged by CD11c expressing APCs. Reactivation involves additional release of pro-inflammatory cytokines that stimulate increased chemotaxis, further BBB opening, resulting in additional waves of inflammatory cell recruitment.
• Myelin-specific B cell activation and antibody responses appear to be necessary for full development of demyelination → in most patients, this can be detected by oligoclonal IgG in the CSF
• Suggested that B cell activation occurs primarily in the meningeal spaces
• Antibodies may participate in myelin and axonal destruction through opsonisaiton and phagocytosis, complement fixation or ADCC.
Causes of the neurodegeneration:
• May be mediated directly by toxic products from microglia, macrophages and CD8+ T lymphocytes
• Indeed, neurons and sxons express MHC
• Resident microglia activated in the neuroinflamamtory process are likely to cause CNS tissue injury through the release of NO, ROS, complement, proteases, cytokines and eicosanoids.
− Excessive NO has been linked to neurological symptoms as a result of direct injury
− NO also mediates the excitatory effects of glutamate → this coupled with excess glutamate release by microglia and decreased AMPA receptors is toxic
Is there evidence of an attempt of remyelination and repair in MS?
- A characteristic of many MS lesions is the presence of large numbers of oligodendryocyte precursor cells
- These progress to the vicinity of surviving axons but fail to remyelinate
- The development of practical strategies to promote reconstitution of functional myelin from this locally available precursor pool represents a strategy for therapy
- Remyelination dependent on the transcription factor Olig-1
- Human OPCs engrafted in myelin-deficient mice restores myelination
- Schwann cells represent another source of cell replacement therapy
- However, the failure of remyelination seems to be the result of an inhospitable environment within the lsion as oppoised to lack of myelin precursor cells – therefore seems unlikely that cells alone will prove sufficient.
What therapy is available for MS?
Antioxidant Therapy
• The importance of ROS indicates that anti-oxidant therapy may prove useful in limiting neuroinflammation
• A new avenue is looking at flavonoids
− Over 4000 plant derived compounds found in fruit, veg and plant-derived products
− Anti-oxidant, anti-inflamamtory, anti-tumour and anti-viral
• A number of these have been tested in EAE:
− Flavonoids inhibit acute EAE → reduces monocyte infiltration, demyelination and axonal damage
− Luteolin inhibits chronic EAE before onset
− Luteolin inhibits chronic EAE after onset
Other Therapy:
Immunosuppressive:
• Generally anti-inflammatory, aiming to dampen the immune response
• Corticosteroids
• IFN-b → antagonizes IFN-y mediated MHC upregulation on APCs, alters proinflammatory cytokine profile, blocks migration across the endothelia
• Copaxone
Novel therapies:
• Also aim to limit the inflammatory response
• Natalizumab – an anti-VLA4 → aims to limit neutrophil influx
• Anti-CD20 (Rituximab) → limits B cell activation
• Anti-CD52 → limits immune activation
• Anti-CD4 antibody has not been successfully translated to humans
• Glatiramer Acetate → polymer mimic of parts of myelin basic protein. May saturate MHC.
• Statins
• Canabinoids
• Estrogens
Briefly describe the sequence of events that leads to the pathology of MS
- An infection by a virus or bacteria
- Antigen gets into the blood stream, and is digested by a macrophage –APC
- The macrophage displays the antigen on the MHC
- MHC is recognized by T cell
- The activated T cells cross the BBB → mediates both by factors from macrophages and T cells
- Immune cells release inflammatory mediators within the brain
- One in the CNS, the T cells encounter myelin-basic protein antigen presented by microglia
• Molecular mimicry with the virus they were activated against. - T cells are reactivated to secrete more cytokines that stimulate microglial and astrocyte activation, and recruit additional inflammatory cells and induce antibody production
- Loss of myelin sheath
- Damage to axons
- Some early, but not necessarily successful, attempt at remyeination
- High proportion of lesion bdcomes scar due to astrogliosis
