Module 8 - Neuroimmunology Flashcards
L8.1 - Describe microglial cells as the innate immune cells of CNS parenchyma
Mircoglia sense threat to the CNS (whan activated by threat, they change their morphology)
They have homeostatic and immune survailance functions
They’re territorial and found in the brain and spinal cord
There is a 1:5 micro-glia to neuron relationship
There is a turnover in cells during the lifespan
They’re yoke-sac derived
They also phagocytose dead stuff during development (apoptotic cells)
L8.1 - Describe development and distribution of microglia and non-parenchymal macrophages
They come from the yoke sac (not part of the germ layers, it’s like a shell and give nutrients) and go to the liver to the brain. The development and maintenance of the microglia depends on their CSF1 receptors (without it you will get microglia pathology - ligands are CSF1 or Il-34). They’re separate from other immune cells due to the BBB. They have a different morphology depending on their location. As they migrate into the CNS, they will proliferate (early pre-natal period). Later on when they mature, they will change morphology to be ramified (have branches).
The non-parenchymal macrophages develop from the bone marrow (in the skull) and can migrate to the meninges, perivascular spaces or the chloride plexus (extra-parenchymal macrophages). Meningeal microphages might come from the skull bone-marrow. Bone-marrow derived macrophages are known as anti-inflammatory, while blood derived ones are pro-inflammatory.
L8.1 - Describe microglial contributions to development and homeostasis in adulthood and aging
Prenatal: Phagocytose neural progenitor cells and axons and release signaling factors to support development
Postnatal: Help forming myelin, help with the synaptic plasticity and pruning. They produce IGF-1. Synaptic pruning is mediated by the complement releasing “eat me” signals (C1q, C3 and fractalkine), which the microglia can sense.
During adulthood, the microglia take part in the homeostasis by phagocyte the apoptotic cells. Microglia will survey the brain in homeostasis and can help synaptic scaling by releasing TNF. They contact synapses to survey them by changing the location of their processes and they express almost all types of NT receptors to understand the signaling going on. They attract to active synapses, specifically those with NMDA activation, as that modulates ATP. They will modulate their interaction with neurons if exposed to PAMPs and DAMPs. The adult brain has less heterogeneity than in development, though high heterogeneity is seen in specific areas during infection/inflamation.
With aging, there is less mobility of microglia processes due to actin down-regulation. Microglia decrease ability to be involved in cellular adhesion and axonal guidance due to changes in gene expression. Their sensome changes, meaning that their ability to sense dangerous pathogens change. Through aging, we become more sensitive to external antigens and less to the endogenous ones.
L8.1 - Explain basic aspects of the microglial response to neural injury
When we have injury, we have morphological changes. They’re normally ramified, but can become activated when encountering a pathogen (foreign and harms the body) or antigen (not per say toxic, it’s a surface protein of cells). They can become reactive (more activated) when necessary. As an alternative to reactive microglia, active microglia can become rod microglia or perineuronal microglia (aggregate around injured neurons). They all become the phagocytotic microglia in the end (this is only morphology and not function). The type of morphological change that occurs is dictated by the type of neuronal injury (depending on if it’s a soma, axons, dendrites issue)
As microglia become active in response to pathogens, they upregulate their markers and expression of MHC antigen presentation. They proliferate and upregulate pattern recognition receptors, cytokines, chemokines, complement proteins and ROS/NO
L8.2 - Describe innate immunity
Innate immunity is present constantly and helps get rid of most bacteria. If the innate immune system can’t handle it, the adaptive immune system is triggered, but only through co-activation. The innate immune system consists of the epithelia barrier, phagocytes, dendritic cells (antigen presenting), complement proteins and NK cells.
L8.2 - Describe antigen presenting cells
Antigen presenting cells are mainly dendritic cells, macrophages (including microglia) and B cells. They capture the antigen, go to the lymph nodes and present them to the lymphocytes (T or B cells). B cells might immediately recognize the antigen or can be presented them through macrophages or dendritic cells. It is manly dendritic cells that present antigens to T cells. Only lymphocytes will express antigen-specific receptors.
For an antigen to be presented, we need 1) the MHC, 2) the peptide from the antigen and 3) co-stimulation
L8.2 - Describe costimulation
Costimultion is performed by APCs and is divided into 2 signals. The first signal is the antigen being presented (through the MHC?) and the second is the APC reacting to the microbe (APC being active), which is happening as the innate immune system is active. Costimultion is necessary for T cell activation and is why any nuclear cell that can express the MHC type 1 isn’t an APC.
L8.2 - Describe the role of MHC
The MHC (Major Histocompatibility Complex) has to bind peptide fragments from a pathogen and display them to T-cells. It’s polygenic (has many different genes to make it up) and polymorphic (each gene has many variants), meaning that each individual won’t have the same MHC. Not all T cells respond to the same MHC or peptide, so there has to be a match with both for effects. The MHC is part of an ATC. The MHC has 2 subtypes: type 1 (expressed in all nucleated cells and attract CD8+ T cells (cytotoxic)) and type 2 (mainly on APCs including microglia and attract CD4+ T cells (helper)).
Each MHC molecule can present only one peptide at a time, because there is only one binding cleft, but each MHC molecule is capable of presenting many different peptides. For MHC to be present on the surface of the APC, a peptide needs to be bound to it.
L8.2 - Describe how T cell differentiation to distinct effector functions is regulated
T lymphocytes are responsible for cell-mediated immunity. They come from the bone marrow, but mature in the Thymus. Naïve T-cells can be triggered by APCs to become effector cells.
When an antigen binds to a dendritic cell (macrophages more important for helper T cells), the dendritic cell goes to the lymphnodes (guided by the chemokines it secretes). As they travel, they mature to express the MHC and co-stimulators. Depedning on if the T cell is CD4+ or CD8+, it can be activated into either a helper T cell (CD4+) or a cytotoxic t cell (CD8+). NB! For the activated T cells (now effector cell) to produce an effect, they require a secondary encounter with the antigen in the origin of the APC (see cartoon below or slide 12).
Proteins in the cytosol of any nucleated cell are processed in proteolytic complexes called proteasomes and displayed by class I MHC molecules (attracts CD8+ T cells), whereas extracellular proteins that are internalized by specialized APCs (dendritic cells, macrophages, B cells) are processed in late endosomes and lysosomes and displayed by class II MHC molecules (attract CD4+ T cells).
Some dendritic cells can present ingested antigens on class I MHC molecules to CD8 + T lymphocytes (cross-presentation)
CD4+ T cells are called helper T cells because they help B lymphocytes to produce antibodies and help phagocytes to destroy ingested microbes. They secrete cytokines that activate B cells, macrophages, and other cell types, thereby mediating the helper function of this lineage.
CD8+ T lymphocytes are called cytotoxic T lymphocytes (CTLs) because they kill cells harboring intracellular microbes.
Effector T lymphocytes are short lived and die as the antigen is eliminated.
L8.2 - To explain migration of immune cells and how they enter tissues
Immune cells can be activated in the lymph nodes (T cells at least), which will make them migrate to the area of initial infection. While they can roam freely in the PNS, the CNS is protected by the BBB, making it harder for activated T-cells to enter the space. Chemokines are what drives T cell entry to the CNS (can be secreted by astrocytes), which is otherwise immune privileged. This means that activated T-cells don’t have easy access even if there is some level of immunosurveillance going on.
To cross the endothelia, neutrophils more paracellularly (in tight junctions) and lymphocytes go transcellularly (through the endothelia cells).
The regions in which they can enter the CNS is typically: 1) into the sub arachnoid through the meningeal venule, 2) into the sub arachnoid through choroid plexus (blood-CSF-barrier) or 3) into the post capillary venules through the BBB.
L8.2 - To explain the role of B lymphocytes and complement in the CNS
B cells produce antibodies in response to pathogens, which can be proteins, carbohydrates, nucleic acids, and lipids (much more sensitive than T cells). They produce humoral immunity. B cells express membrane-bound antibodies that serve as the receptors that recognize antigens and initiate the process of activation of the B cells. If an antigen binds to these, antibodies will be secreted with the same antigen specificity as the membrane receptors of the B cell. B cells is both a lymphocyte and an APC, which makes them able to activate themselves.
B cells in the CNS is assisted by helper T cells to make high affinity antibodies
B cells can up or downregulate inflammatory responses
Antibodies can activate the complement (innate immune system – a collection of proteins), which promotes phagocytoses of microbiomes, inflammation and lysis of microbiomes.
L8.5 - Explain migration of immune cells and how they enter tissues
For the CNS, it can either be direct (e.g. though the nose or during brain surgery), where the antigen is present in the brain, where the antigen or antigen soluble will be drained to the lymphnodes through the ISF being exchanged with the CSF, which is drained to the lymphnodes. There we have T cell activation, which have to re-enter the brain.
In the indirect way as well, where there is a systemic infection (in your whole body). The infection is spreading to the CNS from the PNS (e.g. through the blood). Here the T cells are triggered in the lymphnodes by the PNS APCs being present in the lymphnodes. Here the T cells will still travel to the brain to get rid of the infection in the brain (it’s only an afferent arm and not efferent)
T cells will then have to enter the brain to find the infection through the 3 pathways below.
The parenchymal post-capillary way and the subarachnoid vessel way are guarded more strongly by the endothelia than the cloroid plexus (here it’s perforated). For the subarachnoid case, T cells from the menegies migrate into the CSF, where they meet other cells and penetrate the glial limitance to the brain.
L8.5 - Describe the local T cell response and its regulation during an acute, transient infection
The T cell is initially attracted due to inflammation and is non-specific. If they encounter an antigen, more T cells will come, which are antigen specific.
In cases like the Lymphocytic Choriomeningitis Virus, we see that CD8 T cells are very present and are both the reason for viral control and the pathology (keeps virus in check, but also causes inflammation that can be harmful).
T cells play an important role in the adaptive phase of infection. Here we see the same chemokine production of CXCL10 in astrocytes as in the innate phase, but the CXCL10 will cause the CD8 T cells to produce INF gamma (type 2), which will further up regulate CXCL10. INF gamma (type 2) also up regulates SOCKs, which will suppress cytokines and therefore modulate the immune response.
L8.5 - Describe how induced cytokine/chemokine networks regulate local inflammation
Cytokines and chemokines are very relevant to the T-cell mediated inflammation. They can have many receptors, and only blocking CXC3R (chemocine receptor) or CXCL10 (chemokine) will reduce lethal inflammation in cases of the Lymphocytic Choriomeningitis Virus.
Activation of PRRs or INF beta (type 1) can lead to IRF7 being activated downstream, which is important for cytokine signaling. INF type 1 can be triggered without it, but not including the cytokines.
We also known that INF type 2 can upregulate SOCS (Suppressor of cytokine signaling), which will help decrease inflammation.
In the innate phase, the virus is sensed by the PRR (pattern recognition receptors), which upregulates interferon type 1 (IFN - alpha/beta). They do so through either IRF3 or 7 in the CNS (in the PNS IRF7 is needed). Upregulation of IFN upregulates CXCL10 through STAT2 activation in astrocytes. As CXCL10 can trigger more interferon type 1, this spirals.
Virus PRR INF type 1 (IRF3/7 mediated) STAT2 CXCL10 INF type 1
NB: INF type 1 is the master-switch of inflammation
L8.7 - Describe genetic evidence that innate immunity is involved in AD
One example of genetic evidence is that TREM 2 is one of the genes related to the increase risk of AD, which is a micro glial surface receptor, which can lead to phagocytosis of Amyloid beta and cytokine release if activated. In mutated cases, which as ability is negatively affected, leading to Amyloid beta aggregation. As the microglia is part of the innate immune system, a dysfunctional TREM 2 gene will lead to the microglia not effectively phagocytizing/taking up the Amyloid beta and might have a change in how it modulates inflammation in response to the beta-amyloid.
Other genes that can be mutated as an AD risk gene are:
CD33 Blocks microglia clearance of beta amyloid – mutation upregulates CD33 receptors and blocks phagocytosis
TLR (toll-like receptor): TLR2 and TLR4 bind beta amyloid and stimulate cytokines for inflammation and phagocytosis
CR1 and CR3 (complement receptors): CR3 deficiency of this gene can lead to less interaction between microglia and beta amyloid (improves function) and upregulate enzymes that can break down beta amyloid. CR1 might be overactivated by microglia, leading to loss of synapses
L8.7 - Explain how the disease process can affect microglial functions in AD
In normal states, microglia will briefly release cytokine, clear beta amyloid, and the return to a homeostatic resting state. However, in a state where clearance isn’t possible, more cytokines are produced, which further impairs the microglia’s ability to clear the beta amyloid. The impaired clearance can cause a change in the microglia into damage associate microglia (DAMs). While DAMs are able to promote the removal of beta amyloid, they’re also neurotoxic, leading to neurodegeneration.
Age, genetics and epigenetics can work together to cause glial dysfunction. Glial dysfunction is also seen in AD and leads to decreased AB clearance, leading to amyloid deposits, which can lead to oxidative stress, inflammation, neurofibrillary tangles and neurodegeneration. Microglia become neurotoxic and can’t phagocytose. We also known that losing in the entorhinal cortex leads to atrophy and neuronal loss.
L8.4 - Explain the processes underlying inflammation and demyelination in multiple sclerosis (MS)
MS is an autoimmune disease that causes demyelination and inflammation in the CNS.
The cause of MS is unknown, but it seems more prevalent in people with the HLA-DR2 haplotype and those who have had Epstein-barr virus (EBV) (The antigen from EBV is so similar to the myelin basic protein, that the immune system can be confused).
Inflammation is seen prior to the disease onset and is seen to a higher extend in the relapse remitting cases of MS. In the relapse-remitting case, the inflammation response seems to be caused by the infiltration of lymphocytes and macrophages through the BBB, CD8+ cells being the most prevalent. They enter the CNS, as they have been activated by antigens in the lymph nodes. While MS is an autoimmune disease with several auto-antigens, it seems that the T-cells are attacking the myelin and causing inflammation. This leads to demyelination and axonal damage. The lymphocytes also seem to block AP propagation down the axon, leading to affected neuronal signaling. Microglia is triggered and remove myelin debris and promote re-myelination. If remyelination is not possible, the axons will re-distribute its ion channels, which, if maladaptive, will promote neurodegeneration.
The re-activation of the lymphocytes in the brain will cause the attraction of more immune cells. The B-cells release cytokines to attract more immune cells, also increasing inflammation.
The relapse remission shows these episodes of increased inflammation and demyelination, which can be seen a plaques (areas of de-myelination) or MRI activity.
When relapse remitting progress to secondary progressive the inflammation decreases and axonal damage increase. In primary progressive, we have less inflammation than in relapse remitting, but more than in secondary progressive. Inflammation goes down as most myelin has been removed.
In the progressive cases, we observe that the cell types mediating inflammation changes from lymphocytes to the innate immune system of the brain (microglia and astrocytes).
In progressive MS, there is both immune depended processes and immune in depended processes, that will lead to axonal injury and neurodegeneration. These are:
1. Inflammatory disease for decades: damage to mitochondrial DNA -> ROS -> microchondrial damage
2. Accumulation of ions due to chronic MS lesions -> promote oxidative stress
3. Excitotoxicity -> more influx of Ca2+ -> cytotoxicity
