Partridge L1-10 Flashcards
What is immunology?
Study of the immune system. Provides defence against infection, distinguishes between self and non self and recognises danger signals.
What are the two types of the immune system?
Innate and adaptive
What are characteristics of innate immunity?
Broad specificity, not affected by prior contact. Immediate response is rapid and within hours.
What are characteristics of adaptive immunity?
Highly specific and enhanced by prior contact. Response is slow and takes days to weeks.
What is involved in innate immunity?
Barriers (physical and chemical), soluble proteins (complement, interferons), local and systemic responses (inflammation, fever), leukocytes.
What is involved in adaptive immunity?
B and T lymphocytes. B cells respond by secreting soluble antibodies in humoral immunity. T cells develop into cytotoxic T cells that kill infected host cells (important in viral infections) or helper T cells which secrete cytokines acting on other cells, in cell mediated immunity. Both B and T cells develop into memory cells.
How is infection recognised in adaptive immunity?
B and T lymphocytes express specific antigens receptors.
How is infection recognised in innate immunity?
Pattern recognition receptors recognise pathogen associated molecular patterns on cells of innate immune system. PAMPs include LPS or peptidoglycan and are conserved due to being essential for survival.
What is the clonal selection hypothesis?
B lymphocytes have a preformed receptor on the surface that recognises a particular microbe. Upon contact with an antigen it undergoes clonal selection where the parent divide as clones. Those that recognise “self” are deleted early in development and so they cannot cause harm to body. Healthy lymphocytes can then go onto become plasma cells and memory cells.
What is primary lymphoid tissue?
Where lymphocytes reach maturity and acquire their specific receptors (bone marrow + thymus).
What is secondary lymphoid tissue?
Where mature lymphocytes are stimulated by antigens.
What do dendritic cells in the lymph node?
Presents microbe to T cell and if it has the right antigen it will differentiate into Helper or Cytotoxic. Same with B cells but these will form plasma cells.
What do the elements of the innate immune system do?
Barriers = prevent establishment of infection.
Preformed mediators = Proteins with broad specificity that damage pathogens, induce inflammation or recruit innate immune cells.
Innate immune cells = Recognise and activated by pathogen, eliminated pathogen, cell communication and activate/steer adaptive immune response.
How do mechanical barriers prevent infection?
Secretes chemicals, anti microbial and commensals to create an unfavourable environment for pathogens.
What are examples of preformed mediators?
Lysozyme. Antimicrobial peptides. Complement.
What is lysozyme?
Present in secretions such as tears, saliva and mucous. Breaks a bond in peptidoglycan so more active against gram +ve bacteria where peptidoglycan is exposed.
What are antimicrobial peptides?
Such as defensins which are evolutionary ancient. Produced by epithelial cells and neutrophils. Cationic meaning they insert into pathogenic membranes and disrupt lipid bilayers.
What is complement?
Discovered as a heat – sensitive substance that complemented antibodies in killing bacteria. There are >20 soluble proteins found in the blood and other bodily fluids. Components usually inter but activated by presence of pathogens or antibody bound to the pathogen. Proenzyme is activated on by infection forming a working enzyme which can further go on to form other proenzymes and enzymes.
What is the classical pathway for complement activation?
C1,4,2,3,5,6,7,8,9
What do activated complement components act as?
Proteases so act on one another to generate a large and smaller fragment. C3 -> C3b + C3a. Where b denotes the bigger fragment and a denotes the smaller fragment. Cleavage of C3 exposes a reactive thioester in C3b which can covalently bind to adjacent proteins.
What are the three pathways for complement activation?
Classical = C1,4,2,3,5,6,7,8,9.
Mannose binding lectin pathway = MBL binding to mannose.
Alternative pathway = LPS to factor B to factor D to factor P.
What is the alternative pathway for complement activation?
If C3b generated binds to LPS on a pathogen surface, factor B binds. Factor B is cleaved by factor D -> C3 convertase (C3bBb). The C3bBb convertase is stabilised by factor protein P (properdin). This became known as the alternative pathway. C3b generated by classical/MBL pathway can also bind factor B.
What are the three major biological activities of complement?
- C5a (C3a) recruits phagocytes and induces inflammation. They act as chemo attractants and anaphylatoxins. Phagocytes have C5a and C3a receptors and undergo chemotaxis in response to C5a/C3a peptide.
- C3b promotes opsonisation. Pathogens coated with C3b peptides are recognised by phagocytosis with C3b receptors, facilitating binding and phagocytosis.
- C5b – C9 form the Membrane Attack Complex, leading to lysis. Activation of C5b – C9 results in C9 polymerisation, forming a pore in the membrane, disrupting it and killing the pathogen. This is important in the killing of gram negative bacteria.
Why is complement inactivation important?
To prevent host damage
How is complement inactivated?
Components are rapidly hydrolysed in the fluid phase. Soluble and membrane bound regulator proteins also aid regulation -> C1 inhibitor inactivates C1 ; Factor H inhibits alternative pathway by competing with Factor B ; carboxypeptidase N inactivates C3a and C5a ; CD59 on host cells binds C9, preventing MAC formation.
What diseases are caused by deficiencies in complement inhibitors?
age related macular degeneration which causes blindness in the elderly (lack of Factor H) and paroxysmal nocturnal hemoglobinuria which is a type of anaemia caused by MAC formation on RBC (lack CD59)
What are the characteristics of eosinophils?
Few in blood but also beneath mucous surfaces. Contain receptors for C3b, IgG, IgA and IgE. Release toxic proteins and free radicals to defend against multicellular parasites. Play role in allergy and asthma
What are the characteristics of basophils?
Very few in blood containing receptors for C3a, C5a and IgE. They release heparin and histamine defending against parasites with a role in allergy.
What are the characteristics of mast cells?
Restricted to tissues and protect mucosal surfaces. Contain receptors for C3a, C5a and IgE. Release histamine etc.
What is the role of sentinel cells?
Such as macrophage and dendritic cells. Come into first contact with a pathogen and defend against parasites.
What are the characteristics of macrophages?
Derived from monocytes during infection. Long lived and act as sentinel cells. Phagocytose 100 bacteria per cell containing receptors for C3b, IgG and IgA. Produce pro inflammatory mediators and can present antigen to T lymphocytes.
What are characteristics of dendritic cells?
Found in skin and lymphoid tissues. Take up foreign material by phagocytosis. Digest foreign material and display fragments on cell surface. Specialised for presenting antigen to T cells. Constitutively express high level of Major histocompatibilty type 2 (MHC II) proteins.
What is the sequence of events for phagocytosis?
- Bacterium binds to the surface of phagocytic cell. Antibody or complement can aid binding.
- Phagocyte pseudopods extend and engulf organism.
- Invagination of phagocyte membrane traps organism within a phagosome.
- Lysosome fuses and deposits enzymes into phagosome. Enzymes cleave macromolecules and generate ROS destroying species.
What is oxygen dependent killing in phagocytosis?
Done by an “oxidative burst” where a transient increase in oxygen consumption occurs following phagocytosis due to an activation of a membrane bound NADPH oxidase.
What are the characteristics of NK cells?
Kill infected host cells by recognising altered self due to changes in MHC I proteins. Contains receptors for IgG (allowing killing of antibody coated infected host cells).
How does NK cells kill infected host cells?
Activated NK cells produce a pore forming protein called perforin which inserts into the membrane of the infected host cells. Granule contents (granzymes) are released into target cell to activate the apoptosis pathway. Target cells then undergo apoptosis.
What do pattern recognition receptors recognise in pathogens during innate immune responses?
- Microbe associated molecular patterns (MAMPs) = Shared by many microbes and distinct from self. Critical for survival/function of pathogens and so are often conserved.
- Damage associated molecular patterns (DAMPs) = Host components released during injury and cell damage.
What are classes of PRRs?
- Soluble receptors such as mannose binding lectin
- Membrane receptors such as lectin receptors which bind carbs, chemotactic receptors and toll like receptors
- Cytoplasmic receptors like NOD receptors.
What do chemotactic receptors recognise?
Chemoattractant
What are toll like receptors?
Sensors that signal the presence of microbial components - upon signal of infection, a change is induced in gene expression. They can be on the cell surface or endosomal but always membrane associated.
What is the structure of toll like receptors?
The extracellular domain of TLR3 has a horseshoe shape formed by leucine-rich repeats. The inner surface has βsheet structure and forms the ligand binding domain. Ligand induced dimerization triggers signalling.
What are the two cytoplasmic receptors in innate immunity?
Nod like receptors and RIG-I-like receptors
What are NOD like receptors?
Large group of cytoplasmic receptors that recognise bacterial components. Signal expression of pro inflammatory cytokines to trigger assembly of inflammasomes - a multi subunit complex that cleaves inactive cytokine precursors into active cytokines.
What are RIG-I-like receptors?
Viral sensors that detect viral RNA produced within host cells and signal interferon expression.
What are the four classic signs of inflammation?
- Release of inflammatory mediators (Redness)
- Dilation of local blood vessels (Swelling)
- Increased permeability and blood flow (Heat)
- Immune cell migration into inflammatory site and stimulation of nerve endings (Pain)
What are examples of inflammatory mediators?
- Lipid mediators like prostaglandins in pain reaction
- Vasoactive amines like histamine to stimulate blood vessel dilation
- Chemo-attractants to aid phagocyte movement
- Complement proteins
- Cytokines
What are the two types of inflammation?
Acute - Generally beneficial in dealing with infection, comes and leaves rapidly
Chronic - Caused by chronic infection like TB and can be damaging
Why are cytokines crucial in controlling immune responses?
Change cell behaviour or gene expression. Most act locally but can have systemic effects by acting on cytokine receptors. Can cause damage if left unchecked
What are the families of cytokines?
- IL-1 family = Produced as inactive precursors, cleaved by inflammasomes
- Haematopoietic = Includes factors involved in leukocyte differentiation
- Interferons = Used in viral response
- TNF family = Tumour necrosis factor used in inflammation
- Chemokines = Involved in cell movement
What are interferons?
Intruder alert cytokines produced through viral infection - interfere with viral replication.
What are the two types of interferons?
- Type 1 such as IFN-α and IFN-β. These induce expression of interferon stimulated genes with some cells such as dendritic being specialised for this.
- Type 2 which modulate immune responses
What are the characteristics of Type I interferons?
Induce resistance to viral replication in ALL cells by inducing expression of endoribonuclease that degrades viral RNA and protein kinase, inhibiting protein translation. Increase MHC I expression in ALL cells. Activate NK cells to kill virally infected cells. Induce chemokines to recruit lymphocytes.
What are the characteristics of Type II interferons?
Made by neutrophils, NK cells and T cells. Primary role is in adaptive immunity to increase expression of MHC I and MHC II. IFN-gamma is made by T helper cells to activate macrophages in response to intracellular pathogens.
What does antibody cleavage by papain show?
Two different fragment types. Fragment antigen binding (Fab) or Fragment crystallisable (Fc). The Fc arm interacts with elements of innate immunity (antigen elimination). The Fab arm can bind divalently with the hinge allowing antigen binding at different distances.
What are the five classes of immunoglobulins?
- IgG = Important in secondary responses and main class in serum and tissues
- IgM = Important in primary responses
- IgA = Protects mucosal surfaces
- IgD = ?
- IgE = Present at low levels involved in parasitic protection and allergy
What are the two light chain antibody types?
Kappa and Lambda. These are not class restricted.
What antibody information came about from myeloma protein sequencing?
Antibodies have C and V regions. V binds antigen and C is same for every antibody of given H or L chain.
Also antibodies are comprised of homologous domains. Hypothesized to form a series of globular domains, each stabilised by an intra-chain disulphide bond. The folding pattern of the domains is known as the Immunoglobulin Fold. In the immunoglobulin fold the C domain has 7 beta strands, V domain has 9 beta strands. It is found in all members of the immunoglobulin gene superfamily.
* The variable region domains contain three hypervariable regions. These hypervariable regions dictate which molecules the antibody will interact with. Can be called complementarity determining regions (CDRs).
What are the interactions between antibody and antigen?
Non covalent so includes electrostatic interactions, VDWs, H bonds and hydrophobic interactions. Individually weak but if many form simultaneously the interaction is specific and of high affinity. Specific part of antigen that antibody binds is an epitope.
What receptors do B cells use?
Mainly IgM or IgD. Recognise and bind to antigen, but cannot generate a signal.
How is a signal generated by membrane immunoglobulins?
Membrane immunoglobulins are associated with two other proteins, Igα and Igβ which facilitate signalling. Igα and Igβ contain a single ITAM (Immunoreceptor Tyrosine Activation Motif) in their long cytoplasmic domains that bypass the bilayer. This sequence is phosphorylated upon binding to allow downstream signalling.
What causes antibody diversity at a structural level?
Variations in the sequence and length of CDRs are the main determinants of antibody diversity. CDR3 tends to be most variable in length and sequence. Heavy chain generally contributes more to antigen binding and is more variable than the light chain
What causes antibody diversity at genetic level?
Somatic recombination and mutation of a limited number of inherited gene segments.
What was the Dreyer + Bennett hypothesis for genetic antibody diversity?
Immunoglobulins are encoded by a separate C region and multiple V region genes
What was the Tonnegawa hypothesis for genetic antibody diversity?
Immunoglobulin genes are rearranged during B cell development which was proved using mouse embryos. Embryo pattern (germline) found in all cells except those of B lineage so it is only B cells that undergo this gene rearrangement.
How is adaptive diversity generated?
Variable, joining and diversity segments of the germline are present in multiple copies of the genome. Rearrangement of light and heavy chain genes occurs during B cell differentiation causing permanent changes in DNA. Intervening sequences are removed by RNA processing.
What is involved in the mechanism of recombination?
Lymphocyte specific recombinases and conserved recognition signal sequences (RSSs). RSSs are conserved heptamer and nonamers separated by 12/23 random nucleotides and guide rearrangement of V,D,J segments.
What is the V(D)J recombinase?
A complex of several enzymes required for somatic V region gene recombination. Includes normal DNA cleavage/repair enzymes and RAG1-RAG2 protein complex which is a specialised endonuclease expressed in developing lymphocytes and terminal deoxy nucleotide transferase.
What is the RAG1-RAG2 complex?
A transposase that is essential in adaptive immunity development. Has endonuclease activity and cleaves DNA. Cleaved DNA is repaired to form the coding joint (V+J segments are now next to each other) and signal joint (intervening DNA) excised.
What is the process of junctional diversity?
1.RAG1-RAG2 complex recognises and aligns the RSSs adjacent to the gene segments to be joined.
2. Two ssDNA breaks are made close to the RSSs.
3. Free 3’-OH attacks phosphodiester bond on other strand of DNA to create hairpin at segments to be joined and a flush ds break at RSS boundary.
4 – 7. Other proteins bind to repair the joints, but this process is imprecise, with nucleotides added or subtracted. DNA hairpins are cleaved at random, symmetrically (4) or asymmetrically (5). For V-D-J joining of the H chain, nucleotides can be added by terminal deoxynucleotide transferase (TdT) (6). Unpaired overhangs are filled in by DNA polymerase (7) or may be excised by an exonuclease.
8. DNA ligase joins the nicked and repaired hairpins to form the “coding joint”. (The blunt ends formed at Stage 3 are ligated to form the “signal joint” and this DNA is typically excised).
What are the four methods of diversity?
- Multiple copies of each V region gene segments.
- Heavy x Light chain combination
- Recombination is imprecise leading to junctional diversity. Nucleotides may be lost or added contributing to diversity of CDR3.
- Somatic mutation of V regions following antigen activation.
What is required for somatic hypermutation?
Activated induced cytidine deaminase (AID). Mutations can be introduced throughout V regions but in mature B cells, mutations appear to be clustered in CDRs (CDR1, CDR2 and CDR3).
What are the main functions of somatic hypermutation?
add diversity but the main function includes affinity maturation where higher affinity receptors selected as immune response proceeds “survival of the fittest”.
Class switching can also occur. This is where the same recombined V region associates with different C region genes e.g. IgM -> IgG, IgA etc. Antigen specificity is retained but different localisation / effector are functions induced meaning it is a flexible response to pathogens.
What is the mechanism of action of activation induced cytidine deaminase?
Only acts on ssDNA. AID is expressed in activated B lymphocytes. Activity triggers DNA repair pathways. The repair pathways are error-prone leading to differential mutation outcomes. Mismatch and base excision repair cause somatic hypermutation. Ss nicks to Ds nicks cause class switching.
What is hyper-IgM syndrome Type 2?
An immunodeficiency syndrome caused by AID mutation. Patients only make IgM and make too much of it to make up for the lack of other antibodies. Means no memory response, no parasitic protection etc.
What are the two types of T lymphocytes?
- CD4 +ve - Augment an immune response
- CD8 +ve - Specifically kill infected cells.
T lymphocyte receptors are only expressed on membranes and not as soluble proteins.
What is the structure of a T cell receptor?
Broadly Fab like structure so able to bind antigen. Extracellular domains are homologous to variable and constant region of immunoglobulins. Each V region contains 3 CDRs.
What is involved in the T cell receptor complex?
α and β subunits (TCR) CD3 subunits (ε, δ and γ) and ζ. Required for optimal cell surface expression and signalling. CD3 subunits contain ITAMs (Immunoreceptor Tyrosine Activation Motifs) in their cytoplasmic regions which then allows for downstream signalling.
Where are the genes for T cell receptors found?
Chromosome 14 and 7 in humans. Alpha chain is similar to light chain with VJC regions and Beta chain is similar to heavy chain with VDJC regions.
What is needed for T cells to recognise antigen?
Antigen presentation by major histocompatibility proteins. Encoded by chromosome 6 and are polymorphic.
What information about MHC was given through inbred mouse experiments?
Originally it was unknown whether there was: 2 receptors on T cells, 1 for antigen and 1 for MHC or if there was 1 receptor that recognises both.
X ray crystallography in 1987 showed there was an unknown peptide antigen bound as part of the structure. These studies eventually showed that MHC binds the peptide and TCR recognises the complex of peptide + self-MHC. CDR1 and CDR2 bind self MHC (germline encoded) and CDR3 binds peptide (variation introduced by junctional diversity).
What are the two classes of MHC?
- MHC I = expressed by all nucleated cells. Present peptides derived from endogenous proteins to cytotoxic (CD8) T cells. Endogenous means made within the cell where the cell displaying the protein has to synthesise it itself.
- MHC II = expressed by certain leucocytes such as dendritic cells, B cells and macrophages. Present peptides derived from exogenous proteins to helper (CD4) T cells.
What peptides does MHC I bind?
Peptides 8-10 a.a. long. N and C termini of peptides bind to invariant sites at ends of groove. 2 or 3 anchor residues on the peptides bind to specificity pockets formed by polymorphic residues.
What peptides does MHC II bind?
Peptides 13-18 a.a. long. Peptide backbone interacts with conserved residues that line the groove. Anchor residues on the peptide bind to specificity pockets formed by polymorphic residues.
How are antigens presented in MHC I?
Peptides transported to ER by ATP-hydrolysis driven transporter, TAP (transporter associated with antigen presentation). This is driven by ATP hydrolysis. Peptides are then loaded onto MHCI in the ER. MHCI-peptide transported to cell surface for recognition by cytotoxic T cell.
Uses a proteasome.
What is a proteosome?
A multi subunit complex that breaks down misfolded proteins. An immunoproteasome is induced by interferons and more efficient at making proteins of the right length to bind to MHC proteins.
How are antigens presented in MHC II?
The antigen is taken up by phagocytosis or endocytosis. Acidification in vesicles promotes unfolding and proteolysis. Peptides associate with MHC II in the endocytic compartment. MHC II – peptide transported to cell surface for recognition by helper T cell.
What is it called when dendritic cells present exogenous peptide (MHC I) to cytotoxic T cells?
. This is called cross presentation. This allows presentation to cytotoxic T cells without dendritic cells themselves being infected. It is important in cytotoxic T cell responses to many tumours.
What is required for T cell activation?
In addition to the TCR complex (αβ +CD3 + ζ chain), co-receptors are also required to 1) stabilise the interaction and 2) facilitate signalling. CD4 and CD8 act as co-receptors for the TCR complex. Both contain immunoglobulin-like domains.
What are functions of MHC proteins?
- Graft rejection
- Antigen presentation to T cells
- T cell activation
- Development of T cell repertoire/tolerance in thymus
- Self / non-self recognition as NK cells detect alterations in MHCI
- Association with certain autoimmune diseases.
What is antigen independent responses?
Antibody genes undergo rearrangement; “naïve” B cells expressing membrane IgM +/- IgD are generated. Occurring in the bone marrow.
What is antigen dependent responses?
B cells activated by antigen divide and differentiate into plasma cells secreting soluble antibody. Occurring in the secondary lymphoid tissue.
What are the characteristics of IgG?
Monomeric with a molecular weight of 150,000. It is the main antibody in tissues and blood, important in secondary or memory responses. Subclasses include: IgG1, IgG2, IgG3 IgG4 differ mainly in length and number of disulphides of hinge region. The subclasses are named after their abundance in the blood with IgG1 being the most common and IgG4 being the least. Can activate complement and binds Fc receptors on phagocytes and NK cells. Can cross the placenta by binding FcRn on trophoblast and has a long serum half-life of 20-24 days. A big hinge region is used to separate the two functions of antibodies and can better activate innate immune mechanisms.
What is FcRn?
The neonatal receptor for IgG. Present on trophoblast, permitting transfer of maternal IgG antibodies - protects foetus and new-born. FcRn is also present in adults in gut, liver and endothelial cells – binds and recycles IgG, preventing excretion (improves half-life)
What are characteristics of IgM?
Pentamer with molecular weight of 970,000. It is usually serum restricted with no defined hinge region. Instead it has a functional hinge that can confer flexibility on the molecule. It has a low affinity but high avidity due to so many binding sites, can bind 10 antigens. High valency and so a good agglutinator of particulate antigen and can activate complement very easily. It is important in primary antibody responses.
What are characteristics of IgA?
Monomer / polymeric with two subclasses including IgA1 and IgA in primates. At mucosal surfaces, secretory IgA is found with an IgA dimer + J chain + secretory component. Has high valency so can bind four antigens at once with rapid catabolism occurring. Present in milk but does not activate complement. Monomer, but not secretory IgA – instead a dimer. Has a passive role in preventing adhesion / infection. Undergoes immune exclusion and stops pathogens from getting into the body in the first place. Does not activate complement due to the wrong binding sites being present and in the secretory version, Fc receptor binding site in the traditional sense is masked by the secretory component. Can instead bind Fc receptor in the gut on phagocytes.
Poly-Immunoglobulin receptor - binds polymeric IgA/IgM and is a member of the immunoglobulin gene superfamily. Transport system allows secretion of IgA (and IgM) into lumen. Bacteria that penetrate mucosa can be transported back to lumen.
What are the characteristics of IgD?
Monomeric with a molecular weight of 184,000 – so high due to a long glycosylated hinge. Is <1% in the serum with an unknown function. Present as antigen receptor on many B lymphocytes, together with IgM. Produced by B cells/plasma cells in upper respiratory tract; interacts with receptors on basophils, inducing antimicrobial, inflammatory and B cell stimulatory factors.
What are characteristics of IgE?
Monomeric with a molecular weight of 190,000. Has no defined hinge with only trace amounts in serum (0.0003%). Binds with high affinity to FcR on mast cells and basophils. Important in allergy with a role in immune defence against large extracellular parasites.
What are the biological role of immunoglobins?
- Label pathogens for elimination or destruction
- Immobilise pathogens (IgM)
- Agglutinate particles (IgM, IgA)
- Form “immune complexes” with soluble antigen
- Block binding of pathogens to host cells (IgG, IgA)
- Neutralize toxins (IgG, IgA)
What are Fc effector functions?
- Invoke destruction of labelled pathogens
- Activate complement (IgM, IgG)
- Bind Fc receptors on leukocytes surfaces (IgG, IgA, IgE)
Effector mechanisms that operate will depend on a) Site and type of infection b) Stage of immune response (primary or secondary)
What does complement activation require?
Requires Ag/Ab, C1, C2 and C4 in the classical pathway. C1 = C1q + C1r + C1s or C1qr2s2. C1s and C1r are serine proteases. C1q must interact with two Fc regions for activation occur. IgM is a much more potent activator of complement than IgG due to being a pentamer and so it is easier for C1q to bind two Fc regions.
What can complement activation result in?
Phagocyte recruitment, inflammation, opsonisation and direct bacterial killing.
What do Fc receptors do?
These uptake immune complexes – both the soluble antigen and antibody. They can cause opsonisation through phagocytosis and destruction of antibody coated pathogens. FcRs can also release lysosomal contents which is called frustrated phagocytosis and used for pathogens too big to phagocytose. Fc receptors on NK cells can mediate antibody-dependent cell-mediated cytotoxicity (ADCC).
What are thymus independent antigens?
e.g. bacterial polysaccharides = Induce a rapid response and production of IgM antibodies. Memory cells are not generated.
What are thymus dependent antigens?
e.g. proteins = Require T cells for differentiation of B cells into plasma cells. Long-lived memory B cells may also be generated. Responses can involve somatic hyper-mutation and class switching.
What three signals are required for Naive T cell production?
Recognition of MHC + peptide + co-receptor (CD4/8) → SIGNAL 1. Recognition of CO-STIMULATORY molecule(s) → SIGNAL 2. Cytokines convert activated T cells into different subsets → (SIGNAL 3).
What is the best characterised co stimulatory molecule?
B7 expressed by dendritic cells, macrophages, B cells. Interacts with CD28 inducing expression of IL-2 and IL-2 receptor. IL-2 acts in autocrine fashion on CD4+ve T helper cells; also required for CD8+ve cytotoxic T cell activation. Other cytokines direct T cell differentiation into different subsets of CD4+ve T effector cells (Signal 3).
What are the five subsets of T cells (not including naive - T0)?
TH1. TH2. TH17. TFH. Tregs
What does TH1 subset of T cells do?
Produce IL-2, γ-interferon and TNF. Activate macrophages → inflammation (Classic CELL-MEDIATED immunity). Induce B cells to make IgG1 and IgG3 (opsonizing) antibodies. Important for the development of cytotoxic T cells.
What does TH2 subset of T cells do?
Produce IL-4, IL-5 and IL-13. Activate eosinophils and mast cells. Induce B cells to make IgE (promotes mast cell degranulation).
What does TH17 subset of T cells do?
Produce IL-17 and IL-22. Activates epithelial cells, fibroblasts. Pro-inflammatory, especially at mucosal surfaces. Recruit neutrophils to sites of infection.
What does TFH subset of T cells do?
Found in lymphoid “follicles”. Help naïve B cells differentiate into plasma cells and memory cells. Promote somatic hyper-mutation + class switch. Contact dependent (requires interaction between co-stimulatory molecules CD154 and CD40), but also produce IL-21.
What does Tregs subset of T cells do?
Heterogeneous group that suppress immune responses. Stimulated by TGF-β.
How do cytotoxic T cells infect target cells and cause apoptosis?
Proteases (granzymes) from cytotoxic T cell enter target cell via perforin channel. Fas ligand induces clustering of fas (“death receptor”) on target cell. Fas pathway may be important in down-regulating immune responses. Causes activation of caspase cascade which causes apoptosis.
What are characteristics of CD8 T cells?
Specific = only infected cells bearing antigen are killed. Efficient = Granzymes are pre-formed so a single T cell can kill 100s of infected cells. Clean = Enzymes formed during apoptosis degrade viral DNA and destroy non-viral pathogens.
What was Strachan’s hygiene hypothesis (1989)?
Children brought up on farms, or from large families, are less prone to develop allergy. Early, repeated childhood infections may be protective. Insufficient exposure to certain types of infection (“dirt”) skews TH1/TH2 balance towards TH2? BUT there is a negative correlation between helminth infections and allergic disease.
What was Van den Biggelar’s Counter regulation hypothesis (2000)?
Childhood infection protects against allergy by promoting IL-10 production (Treg ↑, TH1 ↓ and TH2↓). Infection with microbes or larger parasites plays a critical role in driving immune regulation e.g. promotes formation of Treg, IL-10. Human immune system and “Old friends” co-evolved. May also explain rise in autoimmune disease (TH1/TH17-driven).
What are γδ T cells?
Generated earlier in development than αβ T cells. Often found at mucosal epithelium. Less diverse, recognise a broader range of antigens. Do not appear to require processing or presentation by MHCI/II. Can make cytokines particularly pro-inflammatory cytokines e.g. IL-17, IFN-γ, TNF but also cytotoxic. Can act as antigen presenting cells to αβ T cells. Role in various bacterial, viral and parasitic infections (mycobacteria, flu, HIV, malaria) and also in cancer. May “bridge” innate and adaptive immunity.
How is immune response terminated?
Mechanisms of down-regulation: Tregs, cytotoxic T cells. Inhibitory “immune checkpoints” expressed on lymphocytes e.g. CTLA-4 (induced on activated T cells) binds with high avidity to B7. CTLA-4 engagement inhibits T cell activation. Inactivation can also occur through phosphorylation of lymphocyte receptors with immuno-receptor tyrosine inhibitory motifs (ITIM).
