The Immune System in Action (12-17) Flashcards
After the immature B cell has formed, what happens next in B cell development?
B cells are tested for reactivity to self antigens, called B cell tolerance
occurs in the spleen in two stages:
1. T1 - B cells are checked for autoreactivity, if they receive a strong signal if they encounter self antigens they are deleted, B cell peripheral tolerance
2. T2 - ?
B cells move through white pulp between T1 and T2
In what stages does the T cell develop?
occurs in thymus, called ‘thymocytes’
leave bone marrow and travels through blood by following chemokine signals
stages of development are defined by expression of CD molecules:
- 4 stages of double negative expression (DN1-4)
- 1 double positive stage (DP)
- then express either CD4 or CD8
after DN3 is where gamma delta T cells can arise
alpha chain recombination occurs after DN4
positive selection takes place after DP so only T cells w good receptors remain
after SP, negative selection occurs to filter out autoreactivity
Describe beta selection in T cell development.
Occurs between DN3 and DN4
- pTalpha (pre alpha receptor) joins rearranged beta chain, forming pre T cell receptor
- pTα is required for expression of the rearranged β chain on the cell surface and complex with CD3 (needed to signal)
- Expression of the pre-TCR is enough to induce signalling. Ligand binding is not required.
Describe positive selection in T cell development?
occurs during DP stage
it is the selection of cells w functional TCRs able to intercts w self MHC-peptide
only T cells able to interact w MHC and signal through their TCR are useful
a lot of the alpha chains generated by gene rearrangement will not be able to make functional TCRs, determines MHC restriction
this is done by cortical thymus epithelial cells (cTEC) present peptides to DP cells both in MHC I and II
Reminder: What cells do CD4+ and CD8+ T cells develop into?
CD4+: Th or Treg cells (if intermediate affinity for MHC)
CD8+: CTL
Describe negative selection in T cell development.
occurs during SP stage
deletion or modification of autoreactive CD8 and CD4 cells in thymus
recognition of cell would lead to autoimmunity
done by SP T cells interacting w:
- DCs: present self antigens from blood/lymph
- mTECs (medullary thymic epithelial cells): present self antigens from tissues areound the body, tissue restricted antigens (TRAs)
high affinity binding = apoptosis
intermediate affinity binding = Treg
low affinity binding = survival
negative selection is central mechanism for central tolerance of T cells
How are mTECs able to present antigens from elsewhere in the body?
AIRE (autoimmune regulator) protein drives expression of tissuerestricted antigens (TRAs) in mTECs
What are the two arms of tolerance?
central tolerance: occurs in generative lymphoid organs during development of the immune system (embryonic/neonatal)
immature lymphocytes specific for self antigens undergo:
- deletion (apoptosis): T and B cells
- receptor editing: B cells
- Differentiation into Treg cells: CD4 T cells
peripheral tolerance: occurs in peripheral tissues
mature self-reactive lymphocytes that escape central tolerance are
- rendered nonresponsive (anergy)
- deleted
- suppressed by Tregs
peripheral tolerance prevents reactivity to antigens which will not be present in thymus such as food and commensal microbes
is a failsafe for mature and self reactive T cells that escape central tolerance
Peripheral tolerance: How is anergy induced in T cells?
- absence of costimulation from APC eg reactive to self peptide
- engagement of inhibitory receptors
Peripheral tolerance: How are T cells deleted?
mitochondrial pathway
- repeated signalling downstream of TCR without costimulation induces BIM/BID, leading to cytochrome c release from mitochondria -> activation of caspases and apoptosis
Fas/FasL
- Fas expression induced in T cells. binding of FasL to Fas activates caspases
Peripheral tolerance: How are T cells suppressed by Tregs?
contact-dependent mechanisms: CTLA-4
1. blocks B7 (CD80/86) APC so not available for effector T cells to bind to CD28
2. removes B7 from APC
3. CTLA-4 binding to B7 sends inhibitory signals to APC -> less antigen presentation -> less production of cytokines by APC
cytokine-mediated mechanisms
1. production of suppressive cytokines (IL-10, TGFbeta)
2. consumption of available IL-2 by CD25 on surface so other T cells cant use it
What is FOXP3?
TF needed for Treg differentiation and function
What are two types of Tregs?
Tregs are generated
- in the thymus ((n)atural Tregs): generated during negative selection
- in the periphery ((i)nduced Tregs): generated in the presence of TGFbeta
What CD receptors do Tregs express on their surface?
CD25 - high affinity IL-2 receptor
CTLA-4 - inhibitory receptor
What is sex bias in the context of autoimmunity?
higher incidence of autoimmunity in women
- hormonal differences
- x-inactivation
- impact of pregnancy on immune system, perhaps
How can infection trigger autoimmunity?
through activation of APCs during somatic hypermutation
APC presenting self antigen at time of infection can cause self reactive T cell to become activated
through molecular mimicry
a microbe that has the same antigen as a self antigen
eg production of antibodies against streptococcal cell wall cross react w heart valve tissue (carditis) or skin tissue (erythema marginatum)
How can physical trauma cause autoimmunity?
there are immunologically privileged sites eg eyes, brain, testis, and developing fetus
trauma to blood-site barriers can cause autoimmunity by allowing lymphocytes to enter
What is Rituximab?
an anti CD20 molecule that depletes B cells developed for B cell-lymphomas
approved for treatment against:
- rheumatoid arthritis
- ANCA-associated vasculitis
- pemphigus vulgaris
not specfic for autoreactive B cells
more vulnerable to infections/cancer
List some properties of gamma delta T cells.
adaptive lymphocytes, but innate like properties
limited RAG-dependent TCR diversity
confined to skin and epithelial barriers, without recirculation
not MHC restricted, can directly recognise Ag
functions still being clarified
Reminder: What signals are involved in T cell activation?
- TCR-peptide MHC engagement
- co-stimulation eg CD28:CD80/86 (B7.1/B7.2)
- cytokines eg IL-12, IL-4
all from APC
type of cytokine produced decides what T cell the t cell differentiates into
type of cytokine produced depends on type of challenge encountered
What is the major co-stimulatory signal provided by in T cells
CD28
effective at mediating tolerance
IL-2 produced by activated APCs and is part of positive feedback loop which keeps cell alive and allows clonal expansion
How do CD4+ T cells initiate responses to diverse pathogens?
Th1 produces inflammatory cytokines which activate macrophages for intracellular pathogens which cant be reached by complement or ab - engulf entire infected cell
Th17 produces IL-17 which activates neutrophils which is good at killing extracellular pathogens such as bacteria and fungi
Th2 activates eosinophils for removal of helminths and ectoparasites and viruses (Abs)
What TFs specify CD4 T cell fate?
Tbet ->
Gata3 ->
Rorγt -> Th17
Foxp3 -> Treg
How do CD4 T cells help B cells?
- recognising peptide in MHC class II
- deliver cytokines such as IL-4 and IFN-γ
- surface ligation of B cell CD40 by T cell CD40L
occurs in lymph node
How does activation of macrophages by Th1 eliminate pathogens?
- iNOS in macrophages kills intracellular pathogens
- causes tissues inflammation
- raise of temperature reduces viral replication
What is interesting about the interaction between Th1 and Th2?
mutually antagonistic
inhibit each other, if one is present more than the other, that ones response will become dominant
IL-10 indirectly inhibits Th1
IFN-gamma inhibits Th2
What are some properties of Tregs?
defined as CD4+CD25+ which express Foxp3
mice deficient in CD25, IL-2, CTLA-4, Foxp3, IL-10 or TGF-beta develop severe immunopathology
Tregs suppress all functional CD4 subsets - IL-10 and TGF-beta
What are CTLs?
cytotoxic T cells
CD8+, only one type of CD8 T cell
require CD8+ cells require IL-2, need CD4+ T cell help
essential for defence against viruses
immune surveillance against tumours
graft rejection
How do T cells kill?
perforins create membrane pores and CTLs release granule contents into target cell
granules specifically target so not released into general environment as they are v powerful, ensure they wont kill healthy cells
forms junction, actin moves back so granules pushed forward into cell
Fas/FasL-mediated killing
FasL on CTL interacts w Fas on target cell
causes apoptosis
What are proto-oncogenes?
genes, if mutated or incorrectly regulated, contribute to cancer progression
protein products stimulate cell growth and division
eg growth factors and their receptors
positive regulation
mutated versions are known as oncogenes
What are tumour suppressor genes?
protein products stop cell growth and promotes apoptosis
prevent unwanted proliferation of damaged cells
eg p53 - induces death in cells w DNA damage
negative regulation
What is tumour vascularisation?
tumours induce angiogenesis, the formation of new blood vessels directly into the tumour to be supplied w oxygen and nutrients
allows benign tumour to become malginant
Is susceptibility to cancer purely genetic?
no, incidence of cancers is higher in immunosuppressed patients
If cancers present self antigens, how does immune system respond?
tumour specific antigens
- present on tumour cells but not normal cells
- viral proteins of cause by oncogenic virus
- mutated versions of normal proteins eg tumour suppressors
- chromosomal rearrangements creating proteins which are fusions of two genes
tumour associated antigens
- antigens found on normal cells but at higher levels on tumours
- antigens found on normal cells at different stages of development eg onco-fetal or testis antigens
What is a common method used by tumours to escape immune responses?
loss of MHC I
no MHC I prevents CD8 T cells from initiating a cytotoxic response
can be overcome by NK cells, which detect downregulated MHC on cells
upregulation of stress proteins such as MIC-A and MIC-B push NK cells towards activation
detection of MHC I reduces expression of stress proteins
What role do macrophages play in cancer suppression?
- activate other immune cells via cytokine release eg IL-12
- induce apoptosis
- phagocytose cancerous cells
however macrophages can become tumour-associated (M2-like TAMs)
- suppress immune response via other cytokines such as TGF and IL-10
- promote invasion and metastasis
- provide growth factors
- promote angiogenesis
How can antibodies be used as a therapeutic?
Neutralisation – prevent receptor ligation
Activation – trigger receptor ligation
Antibody dependant cellular cytotoxicity - depletion
Antibody-drug conjugates -targeted delivery of treatment
How can monoclonal antibody (Mab) treatments be used to target tumours?
direct targeting
- block growth factor receptors
eg Trastuzumab
- deplete tumour cells expressing tumour associated antigens
eg rituximab
target the tumour immune response
- block checkpoint inhibitors
- enhance BiTEs to enhance T cell function
What are BiTEs?
fusion proteins consisting of two single chain variable fragments (scFv) derived from different antibodies
one binds to CD3 and the other a tumour cell which draws T cell and tumour closer together and induces T cell signalling
will result in directed tumour lysis and help to drive anti-tumour response
What are CAR T cells?
have the TcR replaced w chimeric fusion protein consisting of various domains from BcR, TcR, and costimulatory molecules to create a super receptor to enhance T cell activation
can also have CAR NK and CAR M cells
What are the levels of IgM and IgG during primary and secondary response?
IgM peaks earlier than IgG, but IgG levels are much higher than IgM in secondary and tertiary responses
Is memory a property of the system or is it carried by cells that have an enhanced functions?
both
cells are same but over period of time, increase of affinity: property of system
somatic hypermutation inserts point mutation in B regions: enhanced function
What are some characteristics of memory B cells?
express switched isotypes (A, G, E), some remain IgM which have special function, “stem cells of immune system”
express somatically mutated Ig V genes
express higher affinity Ig
lower activation thresholds
increased levels of adhesion molecules (CD44, LFA-1)
Where are memory B cells generated and activated?
generated in secondary lymphoid tissues
activated in germinal centre
Bcl6 TF crucial for GC development
Blimp1 necessary for plasma cell development
activated B cells create short lived B cells or proliferate to GCs, where they can differentiate into long-lived plasma cells or memory B cells
What is the structure of a GC?
formation is at border between B cell and T cell zones in follicles , both are necessary for proliferation
two zones
- proliferating cells
- cells which present antigen so necessary for maturation
initiation of GC reaction
- antigen: localisation of immune complexes onto follicular dendritic cells (FDC)
- signals from T cells: memory is characteristic of T dependent responses
What occurs in germinal centres?
- clonal expansion of antigen-specific B cells
- affinity maturation
– somatic hypermutation
– antigenic selection
both are crucial components of memory B cell formation
What cells are present in GCs?
centroblasts - B cells activated and rapidly divided
centrocytes - B cells with receptors with higher affinity
helper T cells
FDCs - explained in another card
What are FDCs?
follicular dendritic cells
have chains of beads along dendritic process - iccosomes, which contain electron dense antigens
presented to B cells which internalise them
do not present to T cells
intermediary APC required for presentation to T cells
b cell that are not selected by antigen on FDC soon die and are taken up by tangible body macrophages
What are some characteristics of memory T cells?
- change in the isoform of CD45 expressed
- changes in expression of adhesion olecules eg CD44, ICAM-1, as well as chemokine receptors
- altered migration pathways
- lower activation thresholds
- clonal selection may lead to affinity maturation
- no isotype switching
- no somatic mutation
- dont require germinal centres or any morphological distinct site
- dont mutate receptors
How do T cells migrate and enter lymph nodes?
- naive T cells enter lymph nodes from the blood by crossing high endothelial venules (HEV)
- changes in adhesion molecules and chemokine receptors allow entry in both inflamed and non-inflamed tissues
- hence many memory cells enter lymph nodes from the tissues via afferent lymphatics
- some memory cells still enter lymph nodes across HEV
What are homing potentials of T cells?
expression of different adhesion molecules and chemokine receptors changes what T cells can enter
each tissue has a different combination code to enter
naive and memory T express CCR7 so can enter lymph nodes
effector memory T cells do not express CCR7, instead enter skin or mucosa
What are two theories of memory T cell differentiation?
- divergent
two different conditions = two different cells (effector or memory) - linear
eg naive differentiates into effector, then if conditions are right, differentiates into memory
entry into memory pool may simply be a result of resistance to or escape from activation induced cell death (AICD)
is a memory cell inherently long lived or does a memory cell require external stimuli for long term survival?
B and T memory cells require external signals
What are the properties of cytokine effects?
pleiotropy - multiple biological effects on different cell types
redundancy - different cytokines have similar effects on same cells
synergy - combined effects greater than additive effect of each cytokine
antagonism - effect of one cytokine inhibits effect of another
What are some properties of cytokine receptors?
- extracellular and intracellular domains
- 2 or 3 receptor subunits (alpha, beta, gamma)
- one is cytokine-specific, the other is shared
- shared subunits result in redundancy
- cytoplasmic tail of receptor subunits transduces signals
How are cytokine signals transduced?
binding of a cytokine to the receptor induces phosphorylation by a kinase
leads to recruitment of specific STATS
type of STAT contributes to type of response
How can cytokine receptors be negatively controlled?
- degradation of signal transduction elements
- receptor antagonists block receptors
- soluble receptor or antibodies bind away from cell
- altered receptors cannot signal
What is the function of IL-3 and IL-5?
produced by Th2
IL-3: stimulates growth and differentiation of mast cells and eosinophils
IL-5:
1. stimulates growth and differentiation of eosinophils
2. pleiotropic - also switches B cells to IgA
What is the function of GM-CSF?
granulocyte macrophage colony stimulating factor
produced by Th17
essential for differentiation of granulocytes, macrophages and dendritic cells
What is the function of IL-1?
produced by macrophages
pro-inflammatory
vasodilation
blood vessels become leaky, allowing more
immune cells into site of infection
systemic effect: act on hypothalamus to raise temperature - fever
IL-1 primarily made by macrophages but can be made by sentinel cells (DCs and fibroblasts)
What is Anakinra?
IL-1Ralpha
IL-1 has destructive potential, can lead to joint pain and degradation so is closely regulated
What is the function of IL-6?
predominantly made h macrophages
main trigger of acute phase response in the liver
acts w IL-23 to promote differentiation of Th17
increased levels in rheumatoid arthritis patients, treated w Tocilizumab antibody therapy which blocks IL-6 receptor
What is the function of TNFalpha?
predominantly made by macrophages, but also Th1 cells
tumour necrosis factor
acute effects
- septic shock response to bacterial endotoxins. if every cell produces TNFalpha, every blood vessel become leaky and blood leaks into tissues → clots → organ failure
- anti-TNFalpha mAbs abolish spetic shock eg infliximab
chronic effects
- causes consumption of fat reserves
- wasting in malignancy and parasite infection
- involved in rheumatoid arthritis
What is the function of IL-12 and IL-23?
produced by Th1 and Th17 respectively
both cytokines share the p40 subunit, but drive alternative inflammatory T cell subsets
What is the function of IL-17?
produced by Th17
important in defense against extracellular bacteria and fungi
acts to recruit and expand neutrophils by secreting GM-CSF; phagocytes which clear tissues of extracellular pathogens
What is the function of IL-4 and IL-13?
produced by Th2
used to remove parasites
- induce hyper-proliferation of epithelial cells
- airway mucus, fibrosis
What is the function of interferons?
- block viral replication
- alpha and beta (type 1 IFNs) are important anti-viral molecules
- gamma (type 2) is v powerful and pleiotropic cytokine. produced by Th1
–on macrophages, enhance NO
killing of intracellular pathogens,
also raises IL-1 production
– on T cells, recruits more Th1
(inflammatory)
– On b cells, increases IgG_2a and
IgG_3 class switch
-
What is the difference between type 1 and type 2 interferons?
only difference is that type 1 are stable in acidic conditions and type 2 are not
What is the function of IL-10?
- produced by Tregs
- indirectly inhibits Th1 cells
- antagonises IFN-gamma - anti-inflammatory
- inhibits proliferation and function of T cells
- reduces macrophage activation eg IL-1
- prevents runaway inflammatory reactions
blocking of IL-1- causes earlier death but parasiteamia is reduced
this is due to higher necrosis rate of organs
What is the function of TGF-beta?
- produced by Th0
- blocks cell proliferation
- inhibits inflammatory cytokine
production - induces more Tregs
released in an inactive form, latency associated peptide (LAP) needs to be cleaved to activate it
integrins expressed on surface of cells cleave LAP
What is the function of IL-2?
major T cell growth factor, promotes T cell proliferation in any cells bearing an IL-2R
can act in an autocrine manner
needed by effector T cells and by Tregs
knockout mice die due to lack of Tregs
