EXAM: advanced basic immunology
29. Describe the different CD4+ T cells and the role of these in the immune response
Subsets of effector CD4 T cells are specialized to help different target cells eradicate different classes of pathogens. Unlike CD8 T cells, which act directly on infected target cells to eliminate pathogens, CD4 T cells typically enhance the effector functions of other cells that eradicate pathogens—whether cells of the innate immune system or, in the case of TFH cells, antigen-specific B cells. Fig 9.33 TH1 cells (first panels) produce cytokines, such as IFN-γ, which activate macrophages, enabling them to destroy intracellular microorganisms more efficiently. TH2 cells (second panels) produce cytokines that recruit and activate eosinophils (IL-5) as well as mast cells and basophils (IL-4) and promote enhanced barrier immunity at mucosal surfaces (IL-13) to eradicate helminths. TH17 cells (third panels) secrete IL-17 family cytokines that induce local epithelial and stromal cells to produce chemokines that recruit neutrophils to sites of infection. TH17 cells also produce IL-22, which along with IL-17 can activate epithelial cells at barrier sites to enhance barrier integrity and repair, and produce antimicrobial peptides that kill bacteria. TFH cells (fourth panels) form cognate interactions with naive B cells through linked recognition of antigen and traffic to B-cell follicles, where they promote the germinal center response. TFH cells can produce cytokines characteristic of other subsets that participate in type 1, 2, and 3 immune responses to influence isotype class-switching, but they primarily produce IL-21, which is important for optimal production of high-affinity, class-switched antibodies. Regulatory T cells (right panels) generally suppress naive T-cell responses and can produce immune regulatory cytokines, such as IL-10 and TGF-β, which regulate the response of effector T cells directly or via repression of pro-inflammatory cytokines
24. Describe cytokines with importance for innate immunity
cytokines important for innitiation of the inflammatory response are: -TNF, leukotrines, prostaglandins and lipid mediators produced by macrophages, strong activator of endothelial cell to induce their expression of adhesion molecules and vessel permability -IL-1β (pro-inflammatory, produced by the inflammasome), leads to macrophage activation. -IFN-g: produced by NK cell activate Macrophage cytokines important of the innate response towards viral infections: -Type I interferons (IFN-a and IFN-b): antiviral response. -IL-12: NK cell activation, produced by MØ and DC Colony stimulating factors: -G-CSF, GM-CSF, M-CSF: growth and differentiation of Neutrophils, myelomonocytic (especially DCs), and monocytic lineage cells, respectively.
32. Describe the role of memory T and B cells in the immune response
faster and quantitatively and qualitatively improved host response upon reinfection.
2. Describe antigen processing for MHC class I molecules and the function of these
-MHC class I presents liniarized intracellular peptides to cytotoxic CD8 T cells. -since all cells can potentially become infected with intracellular pathogens, e.g. virus, or become tumorigenic, all cells presents MHC Class I on their surface. -due to spatial constraints in the MHC class one cleft, the peptides presented are usually 9-10aa in length. -a fraction of intracellular proteins are continually being degraded into fragments by the proteasome, and transported into the ER (by TAP and Tapasin). Once loaded in the ER, the MHC:peptide complex is stabilized and will travel through the Golgi apparatus to the cell surface, where the peptide is presented to CD8 T cells. -If the peptide is recognized by the T cell, in the presence of a co-stimulatory signal. the T cell will become activated, proliferate, and ultimately kill infected cells - either by release of their cytotoxic granular content (granzyme B and perforin) or by Fas:FasL induced apoptosis look up: is there a surrogate petide in the cleft prior to antigen loading? elaborate: -MHC class I haplotype -anchor residues, non-covalent interactions -details on loading mechanism -cross presentation by DCs
3. Describe antigen processing for MHC class II molecules and the function of these
-MHC class II presents liniarized extracellular peptides to CD4 T cells. -Professional antigen presenting cells(APCs: Dendritic cells, Macrophages, B cells) have MHC class II on their surface. -in contrast to MHC Class I, there are on spatial constrints in the ends of the MHC class II cleft, thus the peptide presented can be of varying length. however there seems to be a preference for peptides around 15aa in lenght. -peptides from the extracelluar enviroment are taken up, by phagocytosis or pinocytosis, and degraded into smaller fragments in the phagolysosome. meanwhile the MHC class II molecule is formed in the ER and associated with the invariant chain, in travels through to the Golgi and fuses with the phagolysosome. In the phagolysosome the peptide is loaded, by displcing the invariant chain, stabilizing the MHC:peptide complex, which then travels to the cell surface, where the peptide is presented to CD4 T cells. -If the peptide is recognized by a naive T cell, in the presence of a co-stimulatory signal, the T cell will become activated, proliferate, and differentiat into one of the helper T cell subtypes, depending on the nature of the pathogen (i.e. the polerizing cytokine enviroment). look up: elaborate:
7. Describe the different classes of antibodies and their role in the immune system
5 classes of antibodies exists; M, D, G, E, and A. whereas the V region determine the antigen specificity of the Ab, the constant region determines the functions (e.g. to which Fc receptors that Ab is able to bind). the IgM and IgD isoforms are the once that are found in a membrane bound version that makes up the BCR. once a B cell becomes activated, it differentiated into a plasma cell that starts to secrete antibodies (soluable immunoglobulins) instead of having a membrane bound BCR. the first isotype to be produced is the IgM. IgM can form Heptamers and are found both in the circulation and secreted from muscosal membranes. IgMs are often of relatively low affinity, however the heptamere formation increases ths avidity, given the 10 antigen binding sites. The nature of the pathogen/antigen determines which isotype will become the dominant once as the response matures. Allergens will lead to isotype switching to IgE which activated mast cells and basophils. muscosal infections will give rise to an IgA response, a secreted antibodytype with the ability to dimerize. However, most the IgG isotype is the most prevalent antibody isotype found in the circulation. several IgG isotype exist. The function of IgD remains unknows. antibodies work by having to antigen binding sites (Fab) and a Fc part that can bind to cells expressing the right Fc receptor. thus the immune response elicited by an antibody, is dictated by the cell expressing the receptor for the given antibody. e.g. IgE bound the mast cells can lead to histamin release, resulting in the symptoms associated with allergy. Antibodies also serve a function that is independent of the Fc receptors: -neutralization -opsonization (classical compliment pathway) -aggregation for easier expulsion. elaborate: -ADCC
4. Describe antigen presenting cells and their role in immune responses
All cells can present antigens to the immunesystem, and continually do so, in the context of MHC class I. however, a subset of immune cells, the Dendritic cell, macrophages and B cells, a classified as professional antigen presenting cells. these Cells have the ability to sample the extracellular environment, and present their finding in the context of MHC class II. Especially the dendritic cell is impontant in presenting extracelluar peptides to T cells, and continually "drink" or pinocytose the extracellular fluid and present its findings (both in MHC class I and II context, given it's ability of do crosspresentation). once presented in a MHC context, a peptide can by recognized by the T cell receptor. in the absence of a co-stimulatory signal, the specific T cell can have different fates; it can either die, become anergic og become an iTreg, an example of periferal tolerance). If, in addition to antigen recognition (signal 1), a co-stimulatory signal is present (signal 2: B7 (CD80/86) on APC which will interact with CD28 on the T cell) together with stimulatory cytokines (signal 3: IL-2), the Naive T cell will become activated and start to proliferate and differentiate into effector T cells. the APCs aid in all 3 signals.
22. Describe the principles behind central tolerance in lymphocytes
B cell develop all the way from the multipotent hematopoitic stem cell stage to an immature B cell in the Bone marrow and the migrate to the spleen to becoome a fully mature Naive B cell. During development in the bone marrow, the B cells are exposed to self antigens on the bone marrow stromal cells. The B cells, that express a self-reactive BCR, i.e. receive too strong a signal from the Ig-α and Ig-β chains of the BCR, either go back and rearrange its receptor once again (termed receptor editing) or die. Receptor editing can happen multiple times, first at the kappa loci and, if that proves unsuccesful, then at the lambda loci. once a receptor has formed, that do not provide a strong BCR signal, the immature B cell can leave the BM and travel to the spleen, where peripheral tolerance is carried out. T cells undergo somewhat similar development, however important aspects diverge (e.g. the T cells cannot perform receptor editing). early in development, T cell precursors travel to the Thymus. During development, the T cells are first (positively) selected based on their ability to recognize the MHC molecules (too weak a signal from the receptor). Next, the T cells are negatively selected based on their ability to react to self-antigens (autoreactivity), i.e. TCRs that signals to strongly are killed off or become Tregs. once T cells have passed the negative selection step they emerge at mature Naive cells and anter the circulation. elaborate: -during positive selection, how are ability to bind MHC detected? is the TCR fully recombined at this point, or have one the β-chain been formed? -where in the thymus does this happen? are only the thymic stromal cells involved? -when does T cells become anergic? is it part of the periferal tolerance?
20. Describe B cell activation
B cells need to become activated, in order to differentiate into an antibody producing plasma cells. In order to become activated the BCR need to bind its specific antigen, endocytose that antigen (BCR dependent endocytosis), process the antigen and present it in a MHC class II context. The Peptide:MHC molecule will be recognized by a Tfh cell, which provides the B cell with crucial CD40L co-stimulation, and produce IL-21 which stimulate germinal center formation. Other cytokines are involved in directing Ig Isotype switching. -INF-g: IgG2a, IgG3 -IL-4: IgE, IgG1 -TGF-b: IgG2b, IgA -IL-5: IgG1, IgA in mice -IL-13: IgE, IgG1 -IL-17: IgG2a, IgG2b, (IgG3 in mice) -IL-21: IgG class switching (IgG3, IgG1, IgA) The Ig will undergo affinity maturation, and some cells will differentiate into antibody-producing Plasma cells, while others become memory B cells.
30. Describe the role of cytokines in the regulation of the CD4+ T cell response
Fig. 9.34 Cytokines are the principal determinants of alternative programs of CD4 T-cell effector differentiation. Antigen-presenting cells, principally dendritic cells, as well as other innate immune cells can provide various cytokines that induce the development of naive CD4 T cells into distinct subsets. The environmental conditions, such as the exposure to various pathogens, determine which cytokines the innate sensor cells will produce. TH1 cells differentiate in response to sequential IFN-γ and IL-12 signaling, whereas TH2 cells differentiate in response to IL-4. IL-6 produced by dendritic cells acts with transforming growth factor-β (TGF-β) to induce differentiation of TH17 cells, which up-regulate expression of the IL-23 receptor and become responsive to IL-23. Mouse TFH cells also require IL-6 for their development, although it does not appear that human TFH cells do, and it is not currently understood what additional signals might induce their differentiation from naive precursors in either species. When pathogens are absent, the presence of TGF-β and IL-2, and the lack of IL-6, favor the development of peripheral Treg (pTreg) cells.
28. Describe cellular reactions in the innate immune response
I'm not quite sure what they are fishing for here? The cellular reactions can be subdivided into three branches: detection of pathogen (or damage caused by it), destruction of pathogen, and orchestration of downstream inflammatory responses through cytokines. macrophages: sense invading pathogens, rely signals to recruit other effector cells, phagocytosis. dendritic cells: sensing invading pathogens, migrate to lymph nodes to activate T cells. neutrophils: phagocytosis, formation of NETs (nuclear extracellular traps) when the neutrophils die by netosis. the NETs trap phatogen, preventing their spread, and the "messy" way of dying works to amplify the inflammatory response by triggering PRR (extracallular DNA is a DAMP (signal for cellular damage), since is not present under health conditions). mast cells: release histamins that causes vessel dilation (?) eosinophils: basophils: NK cells: resemble CD8 t cells in effector function, but lack the antigen specific TCR. ILCs: ILC1, ILC2 and ILC3s share smililarities with TH1, Th2 and Th17 cells in their cytokine profile, but lack the antigen specific TCR.
1. Describe differences between an innate and adaptive immune response
Innate: -the immune system we are born with. -broad specificities, fast acting (hours), no memory. -consists of our barriers (muscosal and epithelial), AMPs, compliment, and innate immune cells (Neterm-12utrophils, basophils, eosinophils, mast cells, macrophages, dendritic cells). -needed for establishment of inflammation, and thus needed for activation of the adaptive immune response. -non-clonal expression of PRRs (innate cells express multiple different PRRs). Adaptive: -our aquried immunesystem, develops during our lifespan to become more specific and potent. -slower in response (days a week), at least to novel antigen. -has memory -high degree of specificity -the antigen receptors, responsible for the high specificity, are not germline encoded, but arise from somatic recombination. -consists of: T cells, B cells and antibodies. -clonal expression of antigen receptors (only one antigen receptor specificity).
12. Describe the role and function of macrophages in the immune response
Macrophages plays an important role in all phase on an infection. macrophages are found in most tissues, either seeded during embryonic development or differentated from monocytes, and survail the tissue for intruderes. Macrophages have phagocytic receptors (e.g. compliment receptors, Fc receptors, mannose receptors), and upon pathogen encounter, the pathogen can become ingested in a phagosome. the phagosome fuse with a lysosome, to produce a phagolysosome, and the lysosomal content kill of the invader. Pathogen encounter also triggers macrophages to produce pro-inflammatory cytokines (e.g. TNF-a, a potent activator of endothelial cells, but also prostaglandin, leukotrines and platelet activating factor) and chemokines. The cytokines and lipid mediators produced by macrophages stimulate dilation of local blood vessels, induce changes in endothelial cell walls ( upregulation(or shift to a high affinity state) of adhension molecules, which allows for rolling, adhesion, and extravasation of effector cells such as neutrophils, monocytes (that will differentiate to MØ) and later on other granulocytes and lymphocytes. Blood clotting of the small vessels will also occur to prevent further spread of the infection. together these function leads to establishment of inflammation with the cardinal signs: heat, pain, redness, swelling, and loss-of-function. in addition to their phagocytic functions, macrophages belong to the group of professional antigen-presenting cells (APC) and have the ability to present antigens to T cells in both MHC class I and II contexts. (Sensing of PAMPs (such a LPS) via PRRs on the macrophage also lead to upregulation of co-stimulatory molecules(CD80/CD86) and production of fate-determining cytokines, important for lymphocyte activation) - mostly done by DCs though). during the resolution phase, after an infection has been cleared, macrophages also play a role in tissue repair and wound healing (e.g. produced collagenase, fibroblast stimulating factor, and angiogenic factors), and phagocytosis of apoptotic cells. Elaborate: -cytokines and chemokine produced by MØs and their functions -different types of MØs -Wound healing
13. Describe different pathogen-recognizing receptors in the immune system
Pathogen recognition receptors (PRRs) are a class of germ line-encoded receptors that recognize pathogen-associated molecular patterns (PAMPs) and danger associated molecular patterns (DAMPs). they can be divided into -TLRs: membrane bound receptors that surveil the extracellurlar space (plasma membran and endosomes). examples are: TLR-4 which detects LPS, TLR-5 detects flagellin, TLR-3 detects dsRNA (in endosomes). -RLR's: detect cytoplasmic viral RNAs --> lead to type I interferon production. examples: RIG-I detects uncapped dsRNA in the cytosol. -NLR's: cytosolic sensors, resembles TLR in structure having multiple LRR's forming a horseshoe, and a range of other signaling domains. Some NLRs activate NFkB, just like some TLRs, inducing similar response. Other NLR triggers different responses leading to formation of the inflammasome (activation of caspase 1), cell death(pyroptosis, by gasdermin D cleaved by caspase 1, allows for exit of IL-1beta and IL-18), and/or pro-inflammatory cytokine release (IL-1beta, IL-18, cleaved from their profrom by caspase 1). NLR ligands can stem from intracellular infections or released to the cytoplasm from endosomes.
17. Describe the role of the acquired immune response against bacteria
Sometimes the innate immune system response is in itself enough to combat the pathogen, other times adaptive immune system help is necessary. For the adaptive immune system to become activated, the inflammation formed by the innate immune system is of paramount importance. The "reaction" time depends on whether the infection is novel to the host or has been encountered previously, i.e. whether some sort of memory exist and can be re-activeted or a de novo response need to be launched. (draw graph that illustrated the pathogen load over time, in normal conditions, when the adaptive immunesystem is defect, or when the innate immunesystem is defect). elaborate: IL-17, IL-22 leads to production of AMP. what are the function of these, who produces them.
9. Describe the role of T cells in antibody secretion and isotype switching
T helper cells play an important role in B cell activation. upon pathogen encounter, antigen bound to the BCR will provide the first signal for B cell activation. this signal is enhanced by compliment receptors (CD21, CD19) binding to C3b on the microbial surface. The B cell will also internalize antigen, process it, and present it in an MHC class II context to T cells. The Tfh cell will stimulate the B cell via CD40:CD40L interaction, IL-21 (which induced proliferation and survival), and isotype-determining cytokines (IL-4, IL-6, TGF-b, IFN-g). Once activated the B cells can become germinal center B cells (that undergo further proliferation and affinity maturation), plasma cells, or become longlived memory B cells. NB a thymic-independent pathway of B cell activation exists. The B cell still requires a co-stimulatory signal, and in the thymic-independent pathway, this signal is provided by innate receptors such as TLR4 recognizing LPS. look into: -where are B cells located? do they circulate og stay in the lymphoid organs permanently? -what induces the B cell to become a plasma cell? -how is the isotype determined?
15. Describe the role of TLR stimulation of initiating the immune response
TLRs can recognize patterns from bacteria, viruses, fungi, and protozoa. All members are membrane-bound and are either found on the plasma membrane or in the endosome, thus they all sense "the outside" or extracellular environment. TLRs are expressed by many cells including MØs, DCs, B cells, epithelial cells, etc. enabling initiation of antimicrobial or antiviral response (depending on the nature of the PAMP) in various tissues. Signaling by TLRs results in the production of cytokines, chemokines, AMPs, and/or the antiviral type I interferon (IFN-a and IFN-b) response. (It can also induce the upregulation of co-stimulatory molecules, such as CD80/CD86, important for T cell activation). Once a TLR binds its ligand it will induce an antimicrobial/antiviral response in the target cell, partly orchestrated by cytokine and chemokine production (dependent on the cell type). The combination and level of cytokines produced are dependent on the PRR originally activated, ensuring an appropriate response (i.e. cells recruited and activated) in the next phase. -e.g. if the cell is a Macrophage and the PAMP is LPS (binds to TLR-4) the result will be production of ??? -however is the PAMP is of viral origin a Type I interferon response (IFN-alpha and IFN-bate) is initiated, leading to an anti-viral state in surrounding cells (i.e. increased MHC class I expression, decreased protein synthesis, -TLR-1:TLR-2 and TLR-2:TLR-6: lipomannans (mycobacteria), diacyl and triacyl lipoproteins (bacteria), lipoteicacids (gram-positive bacteria), cell-wall beta-glycans (fungi). -TLR-3: ds RNA (virus), poly I:C(commonly used adjuvant) -TLR-4: LPS (Gram-negative bacteria) -TLR-5: flagellin (bacteria) -TLR-7: ssRNA (virus) -TLR-8: ssRNA (virus) -TLR-9: unmethylated CpG DNA (bacteria and DNA viruses) -TLR-10: unknown elaborate: give examples of innate immune activation to different pathogens via TLRs on differnet cell types.
10. Describe the role of co-stimulatory molecules in T and B cell activation
The immunesystem is potent and need to be kept i check to avoid excessive damage of tissues and auto reactivity. Thus T and B cells have developed in a way that requires multiple signals to become activated. For the T cell, in addition to the 1. signal via the TCR, an additional Co-stimulatory signal via CD28 is needed as well as cytokine stimulation to become activated and proliferate. This ensures that the T cell is not activated in the absence of "danger". Does the TCR bind its antigen, in the absence of a "danger" signal it will become a anergic, an i Treg or die. This system is being therapeutically exploited in cancer treatment. The so called Checkpoint inhibitors, such as anti-CTLA block CTLA binding of B7 (CD80/86) allowing more B7 to interact with CD28. The B cells need co-stimulation via CD40:CD40L (?) and isotype determining cytokines (IL-4? IL-21? IL-10? BAFF?).
27. Describe the most important reactions involved in inflammatory response
The inflammatory response (inflammation) occurs when tissues are injured by bacteria, trauma, toxins, heat, or any other cause. The damaged cells (and macrophages) release chemicals including histamine, bradykinin, and prostaglandins. These chemicals, together with TNF produced by tissue resident MØs, induce endothelial cells to upregulate the adhesion molecule expression, cause blood vessels to dilate and leak fluid into the tissues know as edema. the vessel dialation slow the blood flow and allows neutrophils (first responders) and later on other immune cells (monocytes, eosinophils, and adaptive immune cells) to roll along, adhere to, and extravasate through the endothelium. Blood clotting is also induced, by macrophage production of platelet activation factor (PAF), leading to clotting of the small vessel to prevent further spread of the pathogen. The edema allows fluid and plasma protein (complement proteins, antibodies, etc) to enter the tissue, but also have the function of carrying the invading pathogen to the draining lymph node, where the adaptive immune response is primed. Together these reactions are behind the well-known cardinal signs of inflammation; Heat, redness, swelling, pain, and loss of function. fig. 1.10 elaborate: -adhesion molecules -CXCL8 is the chemokine that recruits Neutrophils, basophils and T cells -the mechanism behind fever (Il-1b, TNF, IL-6).
18. Describe the role of the innate immune response against viruses
The plasmacytoid dendritic cells (pcDC) are important in sensing viral infections, they express TLR7 and 9 (recognize ssRNA and CpG DNA, respectively). their main function is to produce Type I interferons (not T cell activation). A type I interferon response will lead to upregulation of MHC class I on neighboring cells, decreased protein synthesis, fever, and recruitment and activation of NK cells (and CD8+ T cells). The type 1 conventional dendritic cell (cDC1) is also mobilized during a viral infection (express various TLRs, including TLR3 (dsRNA)). Conventional DCs are involved in T cell activation (as oppose to the pcDCs), and the cDC1 is mainly activating T cells (produce IL-12, which activated CD8 T cells and Th1 cells) to fight intracellular pathogens. Not all viruses have the ability to infect DCs, thus the DCs have developed an alternate mechanism to present viral peptides in the context of MHC class I, termed cross-presentation. elaborate: -PRRs that recognize viral patterns -which cytokines and chemokines are involved? -DC cross-presentation of viral peptides -
19. Describe the role of the acquired immune response against viruses
Viruses are obligate intracellular parasites that infect and replicate within host cells, thus are hidden from the immune system. however, prior to infection and during spread from one cell to another, the viruses are exposed to the extracellular milieu. The acquired immune response, i.e. antigen specific T cells, B cells and antibodies, are important for combatting the infection. CD8+ T cells are important for killing of virally infected cells by inducing apoptosis (either by release of its apoptosis inducing granular content granzyme B and perforin) or by Fas:FasL interaction. but signal to activate the apoptosome. All cells can, in theory, become infected by viruses, thus all cells express and present peptides in the context of MHC class I. however, not all viruses infect DCs, and how then are the CD8+ T cells activated? answer: the DCs have the ability to do cross presentation, where an extracellular antigen is taken up and presented in an MHC class I context. (this ability is being exploited in cancer therapy, where neoepitope vaccines given to a patient with the aim of arming DCs to activate CD8 T cells to kill oncogenic cells). furthermore, the innate type I interferon response works to increase the level of MHC class I expression on the surface of surrounding cells. B cells and Antibodies are important for neutralizing extracellular viral particles, preventing them from infecting new cells and targeting them for phagocytosis. anti-viral-Antibodies secreted on mucosal surfaces, can potentially prevent the virus from ever entering the body. these antibodyes are usually of the IgM and IgA isotypes, that have the ability for form multimer complexes, thus the ability to aggrate large antigen:antibody complexes which are more easily expelled from e.g. the gastrointestinal tract. an example of the role of the aquired immunesystem are AIDS patients infected with an HIV virus that primarily infects CD4 T cells. this patients most often dies from otherwise harmless infections, due to their inability to combat these infections.
33. Describe the role of the different memory T cells subsets for the immune response
faster and quantitatively and qualitatively improved host response upon reinfection. every reencounter of the same antigen (e.g. a booster vaccine) further develops the immune response to become more potent. memory cells can persist for extended periods without antigen encounter. Memory T cells also have a greater range of surveillance capabilities compared to naive T cells; that is, they are not restricted to searching for antigen in secondary lymphoid tissues. Two general models for memory T-cell development have gained favor (Fig. 11.24). In one model, often referred to as the linear model of memory-cell development, memory T cells arise from effector T cells as the primary immune response subsides. In a second model, often referred to as the branching (or asymmetric division) model of memory-cell development, an antigen-activated naive T cell gives rise to daughter cells that preferentially commit to either the effector- or memory-cell program. subsets of memory T cells: -Central memory T cells (Tcm): circulate the blood, lymph, and secondary lymphoid organs (like naive cells). -Effector memory T cells (Tem): circulate everywhere; blood, lymph, secondary lymphoid organs, and non-lymphoid tissues. -Tissue-resident memory T cells (Trm) only reside in the perifery (i.e. non-lymphoid tissues) and do not appear to migrate.
23. Describe the principles behind peripheral tolerance in lymphocytes
for B cells, peripheral tolerance take place in the circulation and the spleen. Transitional B cells (immature, express IgM on surface), traveling from the BM to the spleen will die off if they encounter sufficient amounts of self-antigens to produce a strong BCR signal. in the spleen, where the immature B cells become fully matured or deselected, they compete for interaction with the follicular dendritic cell (FDC). The FDC provide the B cell with BAFF (a TNF family member) that stimulates B cell survival. In order to become mature B cells, the immature B cells need to be stimulated by the right balance of BAFF and low-level BCR signaling. The low-level signaling does not stem from specific antigen binding, but rather from low-affinity interactions with self-antigens. Cells that have strong BCR signals or lack BAFF stimulation die off. the periferal tolerance selection step for B cells, ensures that B cells specific for self antigens that are not present in the BM, and thus not subjected to central tolerance, are still killed off. T cells in the periphery, which encounter their specific antigen in the absence of co-stimulation, are subjected to periferal tolerance and will either become iTregs, anergic or die off. iTreg have the ability to produce anti-inflammatory cytokines e.g. TGF-b and IL-10, that helps dampen the immune response toward the particular antigen.
5. Describe the mechanisms behind antigen receptor gene recombination
given the repertoire of TCR, BCR and antibody specificities, it's simply impossible for these receptors to be germ-line encoded. the BCR and TCR arise from a process termed somatic recombination, and shares a high degree on similarity. The different elements of the Receptor (constant region (C) and variable region (VDJ) are found in multiple copies in the genome. during lymphocyte development, the elements are recombined to create a unique antigen receptor. For the BCR, the heavy chain are recombined first, by joining of a V, D, and a J segment. during this recombination, catalysed by the RAG protein, nucleotides may be added or deleted. this addition/deletion contributes to the high degree of variability amongst antigen receptors in the (CDR3 region). however the introduction of nucleotides, in 2 out of 3 cases result in a frameshift, making the receptor non-functional. If that is the case, the cell can go back and recombine with a new segment.
8. Describe the maturation and selection of T cells in the thymus
immature T cells arise in the bone marrow, from the common lymphoid progenitor, and travel to the Thymus for maturation. In the thymus the cells enter as double negative cells (DN) --> become double positive (DP) and start expressing the pre-TCR --> then they are positively selected for their ability to recognize MHC molecules (those that bind to strong are killed off, those that do not bind (recombined again?) --> the TCR is formed and the T cells become single positive T cells (CD4 or CD8) and are negatively elected based on their ability to recognize self peptides. once selected the mature naive T cells travels to the circulation and periphery. elaborate: -positive selection -negative selection -fate in the perifery.
11. Describe T cell activation
once mature naive T cells have formed, they leave the thymus and enter the circulation. the T cells, leave the bloodstream at HEV and enter lymphnodes where they scout for their specific antigen, if they do not encounter antigen the T cells leave via the efferent lymphatic vessel and reenter the blood stream where it travels to new lymphnodes. However, if an APC (most likely a DC) is present that displays the specific antigen in the context on an MHC molecule, the TCR receptor will bind. if an infection or other Danger signals are present, the APC will provide the T cell with a co-stimulator signal (B7 (CD80/CD86):CD28) as well as cytokines (IL-2? potentially produced by the T cell itself). Look into: the steps of t cell activation an proliferation in the lymphnode.
25. Describe cytokines with importance for adaptive immunity
the chemokines CCL21 and CXCL13 are important for localising cells to T- and B cell zones in the lymphoid organs, respectively. CXCL8 is important for T cell recruitment to the site of infection. fate specifying cytokines (for CD4+ T cells): -Th1: IFN-g, IL-12 -Th2: IL-4, -Th17: IL-6, IL-23, TGF-b, -Tfh: IL-6? -Treg: IL-2, TGF-b. -IL-2 important during T cell activation (stimulate survival?) -T cell signature cytokines: IFN-g, IL-4, IL-5, IL-13, IL-17, IL-22, IL-21, TGF-b, IL-10 -IL-21: B cell activation, proliferation and class switching. -Isotype determining cytokines: -INF-g -IL-4 -IL-6 -TGF-b
6. Describe the selection of the mature B cell repertoire
the development of B cells start in the bone marrow from the multipotent hematopoeitic stem cell. Some progeny of this stem cell will commit to the lympoid liniage (common lymphoid progenitor) and develop into immature B cells via interactions with bone marrow stromal cells (including IL-7 stimulation). During the developmental phases of pro-b cell (no BCR or pre-BCR expression) --> pre-B cell (Pre-BCR expressed (consist of only heavy chain and a surragate light chain), cells that have successfully recombined their heavy chain proliferate) --> immature B cell (or "transitional B cells". express the recombined light chain (IgM on the surface). have been selected for self-reactivity (central tolerance), further subjected to tolerence selection in the spleen (periferal tolerance). leaves the BM at this point) --> mature naive B cell (IgM and IgD on the surface, the maturation take place in B cell follicles in the spleen). failure to produce a functional heavy chain at one loci, leads to recombination of the other allele. The pre-BCR (successfully combined heavy chain associated with surrogate light chain) is associated with two other protein chains, Igα and Igβ, which signal the B cell to halt heavy-chain gene rearrangement; this drives the transition to the large pre-B-cell stage by inducing proliferation. Failure to produce a functional H-chain leading to a pre-B-cell receptor signal leads to cell death. The progeny of large pre-B cells stop dividing and become small pre-B cells, in which light-chain gene rearrangements commence. Vκ-Jκ rearrangement (see Section 5-2) occurs first, and if unsuccessful (i.e. out-of-frame. can theoretically be done 5 times, due to 5 different Jk segments), Vλ to Jλ rearrangement occurs next. Successful light-chain gene rearrangement results in the production of a light chain that binds the μ chain to form a complete IgM molecule, which is expressed together with Igα and Igβ at the cell surface. Signaling via this surface receptor complex is thought to trigger the cessation of light-chain gene rearrangement. Failure to produce a functional L chain leads to cell death. (fig. 8.5) In the spleen, where periferal tolerance take place, the immature B cells competes for contact (and BAFF stimulation) on the folicular dendritic cell (FDC. which is NOT a dendritic cell, but looks like one). the immature B cells require BAFF:BAFF-R stimulation, in addition to low level signaling from the BCR (not induced by specific antigen recognition, which would elicit a strong signal, but rather thought to be caused by low affinity binding of self-antigens). also the periferal pool of B cells competes for contact to the FDC, and tend to be preferred over most immature B cells.
31. Describe the role of regulatory T cells for the immune system
the role of Tregs is to prevent an immuneresponse in the absense of Danger. at least two types of Tregs exists - thymic Tregs (produced in the Thymus) and periferal Tregs induced in the periphery. the two type appear to have equvivalent functions. Regulatory T cell are formed when a T cell encounters its epitope in the absence co-stimulation and pro-inflammatory cytokine such as IL-6 (and in the presence of IL-2, TGF-b, and Retinoic acid - the two latter are abundant in the gut mucosa). Such epitope could be a self-peptide or a harmless foreign peptide. an example of a harmless foreign peptide could be a peanut allergen, which most people tolerate, however in some individuals peanuts trigger an unwanted hypersensitivity reaction that is potentially lethal. the lethality of the peanut, is not posed by the peanut itself but the elicited immune reaction, speaking to the importance of tolerance. Regulatory T cells work by producing anti-inflammatory cytokines, such as IL-10 and TGF-b. They also express IL-2 Receptor-a (CD25) to soak up IL-2 and use CTLA-4 to stop the activation of nearby CD4 T cells (by scavenging B7 (cd80/CD86) from the APC). . Tregs are induced by the fate-determing cytokine IL-2, which downstream signal transduction via STAT5 leads to the transcription factor FoxP3 activation.
14. Describe the Toll-like receptors (TLR) and the different functions of these
there are 10 human TLR genes (12 in mice, including pseudogenes). TLRs recognize conserved molecular patterns of microbes, often referred to as PAMPs. TLRs can recognize patterns from bacteria, viruses, fungi, and protozoa. the TLRs share a common structure (made of multiple copies of the domain leucin-rich-repeats (LRR) and an intracellular signaling TIR domain), and form either homo- or heterodimers upon ligand binding. All members are membranebound and are either found on the plasma membrane or in endosome, thus they all sense "the outside" or extracellular environment. TLRs are expressed by many cells including MØs, DCs, B cells, epithelial cells, etc. enabling initiation of the antimicrobial/antiviral response in various tissues. Signaling by TLRs results in the production of cytokines, chemokines, AMPs, and/or the antiviral type I interferon (IFN-a and IFN-b) response. Most TLRs signal via MyD88 and lead to the activation of the transcription factor NFkB and interferon regulatory factor(IRF) via a different pathway. However, the exact signaling pathway and ultimately the genes transcribes are determined by the specific TLR that has been activated and the cell by which it is being expressed. -TLR-1:TLR-2 and TLR-2:TLR-6: lipomannans (mycobacteria), diacyl and triacyl lipoproteins (bacteria), lipoteicacids (gram-positive bacteria), cell-wall beta-glycans (fungi). -TLR-3: ds RNA (virus), poly I:C(commonly used adjuvant) -TLR-4: LPS (Gram-negative bacteria) -TLR-5: flagellin (bacteria) -TLR-7: ssRNA (virus) -TLR-8: ssRNA (virus) -TLR-9: unmethyloated CpG DNA (bacteria and DNA viruses) -TLR-10: unknown
16. Describe the role of the innate immune response against bacteria
upon infection with a bacteria, the innate immunesystem is actived within hours. The activation is important for several reasons; Sometimes the innate immune system response is in itself enough to combat the pathogen, other times adaptive immune system help is necessary, and for the adaptive immune system to become activated, the inflammation formed by the innate immune system is of paramount importance. (draw graph that illustrated the pathogen load over time, in normal conditions, when the adaptive immunesystem is defect, or when the innate immunesystem is defect). Once a bacteria has breached the barrier of the human skin or musoca and entered the body, it will be recognized by Pathogen recognizing receptors e.g. TLR4. TLR4 recognize LPS of gram-negative bacteria. The membrane bound TLR4 will signal via the NFkB pathway, which ultimately leads to cytokine and chemokine(?) production. elaborate: -Other PRR -which cells express TLR4 -what cytokine and chemokine are produced and what are their function. -which innate cells are the first ones to be recruited (neutrophils?).
21. Describe the role of B cells in the immune response
B cells main function is to, upon activation, become antibody producing factories that confer humoral protection against pathogens. several antibody isotypes exists, each optimized for a specific class of pathogens.
