L7 - Immunological Tolerance COPY

Question 1

WHAT IS IMMUNOLOGICAL TOLERANCE

i) name three things the immune system is tolerant to
ii) what mechanisms does immuno tolerance refer to?

Answer 1

i) imm sys is tolerant to self, harmless antigens (food/enviro ag) and commensal microbiotia
ii) lack of immunological reactivity is induced and maintained

Question 2

T CELL/B CELL RECOGNITION OF ANTIGEN

i) how do CD8 and CD4 T cells reconise antigen?
ii) which co receptor is found on T cells?
iii) what type of protein do T cells recognise?
iv) what happens when B cells recognise non self antigens? which cells does this involve?

Answer 2

i) T cell recognise antigen using the T cell receptor
ii) t cells also have CD3 co-receptor
iii) T cells recognised processed antigen eg peptide
iv) B cell recog non self antigens then produce antibodies from plasma cells

Question 3

B AND T CELL FORMATION

i) where is this induced? give two examples
ii) where is it maintained?
iii) where does B cell production and maturation take place?
iv) where does T cell production and maturation take place?
v) which immature thymocytes are positive selected? which T cells die by apoptosis?
vi) which cells support different stages of T and B cell dev?

Answer 3

i) induced in central lymphoid organs such as bone marrow and thymus
ii) maintained in the periphery
iii) B cell prod and maturation in bone marrow
iv) T cells prod in BM and mature in thymus

v) pos selection of cells whose receptor binds MHC
- death by apop of cells that dont interact with MHC

vi) stromal cells

Question 4

SELF TOLERANCE

i) what makes the primary reprtoire of lymphocytes so big?
ii) in the germ line - are there more variable regions of constant region genes?
iii) what determines the selection of gene segments? does this happen in B, T cells or both?

Answer 4

i) lots of combinatorial diversity (random rearrangement of genes)
ii) more variable region genes than constant region genes
iii) random somatic gene rearrangment determines selection of gene segments which is common to both B and T cells

Question 5

GENE REARRANGEMENT IN T CELLS

i) activation of which genes allows reassortment of gene segments of TCRs? what does this result in?
ii) which organ screens for the most useful receptors produced? where do these cells then go?

Answer 5

i) activation of RAG genes allows reassort of TCRs
- result in lots of TCRs being produced by random rearrangement

ii) the thymus screens for the receptors that are useful and then they move to the periphery

Question 6

B CELL DEVELOPMENT IN THE BONE MARROW

i) what type of B cell has VJ recombination?
ii) which B cell has VDJ recombination?
iii) which Ig does an immature B cell express?
iv) which two Igs does a mature B cell expres?
v) where do mature B cells migrate from and to?
vi) which three things define each stage of development?

Answer 6

i) pro B cell = VJ recomb
ii) pre B cell = VDJ recomb
iii) immature B cell has IgM expression only
iv) mature B cell has IgM and IgD expression
v) mature B cells from BM > periphery
vi) rearrange of Ig heavy/light chains, expression of surface Ig and expression of adhesion molecules/cytokine receptors

Question 7

TCR DNA REARRANGEMENT

i) where do TCR genes undergo DNA rearrangement?
ii) are the alpha and beta chains of TCR on the same or seperate chromosomes?
iii) what happens to the alpha and beta chains to make random receptors? what happens next?

Answer 7

i) DNA rearr in the thymus
ii) alpha and beta chains are found on seperate chromosomes
iii) alpha and beta chains are randomly spliced together then there is VDJ recomb

Question 8

B CELL SOMATIC RECOMBINATION

i) which chain of the BCR undergoes VJ rearrangement?
ii) which chain undergoes VDJ?
iii) where does this take place?

Answer 8

i) light chain - VJ
ii) heavy chain - VDJ
iii) takes place in the bone marrow

Question 9

B CELL SELF TOLERANCE INDUCTION

i) where is the B cell tested for recognition of antigen?
ii) what would the B cell recognise for apoptosis to be induced? what is the name of this process?
iii) in relation to antibody cross linking - what happens can happen to an immature B cell recognises soluble self antigen? what is this called?
iv) what happens in receptor editing?

Answer 9

i) B cell is tested in the bone marrow for recognition
ii) if B cell recognises self ag on bone marrow stromal cells or cytokines > apoptosis = deletion
iii) if immature B cell recog soluble self antigen > no cross linking and no activation of B cell = ANERGY (silences B cell)
iv) receptor editing is when B cell recog self then the BCR is rearranged to make a new one

Question 10

T CELLS CENTRAL SELF TOLERANCE INDUCTION

i) what are the three types of T cell receptors that will be included in the immature repertoire? what will happen to each of these?
ii) what do T cells need to have to complete maturation and form the peripheral T cell pool?
iii) what are the two key properties of naiive T cells? what happens if they dont have these properties? what % of T cells does this happen to?

Answer 10

i) harmful (self antigen recog) - neg selec
- useless (dont recog MHC) - neglect
- useful (recog MHC and not self) - pos selec

ii) T cells need to have a receptor with appropriate affinity for the peptide presented by MHC complexes to complete maturation

iii) naiive T cells = self MHC restricted (recognise MHC) and self tolerant (dont recognise self)
- if they dont meet this criteria then they die by apoptosis
- happens to 98% of cells in thymus

Question 11

THYMUS

i) what is it absolutely required for?
ii) what syndrome occurs in children where they dont have a thymus? what happens to T cells in these children?
iii) when is the thymus fully developed? when does it increase in size?
iv) what happens to the thymus with age? when is this complete by?
v) why doesnt reduced production of T cells completely impair immunity? (2)
vi) what is immunosenescence?

Answer 11

i) differentiation of immature precursors into mature T cells

ii) Di George syndrome = no thymus
- do not have mature T cells

iii) thymus fully developed before birth and increases in size during puberty

iv) with age it atrophies and fat replaces areas where thymocytes used to be
- complete degen by age 30 but residual thymic activity persist

v) there is some ability to produce new T cells and once established, the T cell repertoire is long lived
vi) immunosenescence - progressive deterioration of immune responses mainly associated with age

Question 12

T CELL DEVELOPMENT

i) in which defined microenvironment does it occur?
ii) what provides the microenvironment for T cell development and selection? what is the name of maturing T cells?
iii) what state are T cells in when they have travelled from the blood into the microenviro? which region do they enter?
iv) what happens to the immature T cells in the cortex of the thymus? what type of selection occurs here?
v) what happens to the T cells in the medulla of the thymus? what type of selection occurs here?

Answer 12

i) occurs in thymic microenvironment

ii) the thymic stroma provides the microenvironment
- consists of epithelial cells and connective tissue

iii) T cells that are maturing in the thymus = thymocytes
- enter the subcapsular region

iv) immature T cells that enter the cortex are double negative and then become double positive (CD4/8/3)
- undergo positive selection in the cortex (are they senstive to MHC?)

v) in the medulla of the thymus the thymocytes mature
- undergo negative selection > get rid of cells that recognise self antigen

Question 13

POSITIVE SELECTION

i) what must thymocytes recognise to be positively selected?
ii) what is the state of the TCR when it comes from the bone marrow?
iii) somatic rearragement of which two genes occurs in the thymus? what do the thymocytes then express? which part of the thymus does this occur in?
iv) thymocytes that recognise self MHC on which cells survive? which two processes do they then undergo?
v) what happens to cells that dont recog MHC?

Answer 13

i) thymocytes must recognise self MHC - required for T cell function
ii) when TCR comes from bone marrow it is double negatuve

iii) somatic rearrange of genes encoding for beta and alpha chains of the TCR
- thymocytes then express CD4 and CD8
- occurs in the cortex of the thymus

iv) thymocytes that recog self MHC on surface of cortical eptihelial cells survive
- then undergo survival > differentiation > maturation

v) those that dont express MHC on cortical ep cells in thymus undergo cell death via apoptosis

Question 14

NEGATIVE SELECTION

i) what have these cells previously undergone?
ii) which thymocytes are removed by negative selection?
iii) where does negative selection occur?
iv) which cells are used to present antigen to the thymocytes? what type of antigen do they present?
v) what type of binding allows thymocytes to live and persist? which type of binding will cause death by apoptosis? what may this kind of binding pre dispose to?

Answer 14

i) positive selection
ii) thymocytes that recognise self antigens that are presented by self MHC and bind too strongly are removed
iii) negative selection occurs in the medulla of the thymus
iv) dendritic cells and macrophages at the corticomedullary junction present self peptide to the thymocytes

v) modest binding to the antigen allows thymocytes to live
- if the thymocytes bind too strongly they will die by apop (neg selection) > may predispose to autoimmunity in the periphery

Question 15

TOLERANCE TO ANTIGENS EXPRESSED IN THE THYMUS

i) which transcription factor is expressed at high levels in thymic medullary epithelial cells? what does it allow the expression of in the thymus?
ii) what do mutations in this transcription factor lead to?
iii) what does KO mouse of this transcription factor cause? (2)
iv) what is the sole purpose of expression of lots of antigens in the thymus?
v) what is the name for this kind of gene expression?

Answer 15

i) high level expression of AIRE (autoimmune regulator)
- allows expression of a range of tissue specific antigens in the thymus

ii) mutations in AIRE > APECED aka autoimmune polyendocrine syndrome (APS-1)
iii) mouse KO of AIRE > failure to express many self antigens in the thymus and expression of lots of autoantibodies
iv) sole purpose of lots of antigens being expressed is for screening of thymocytes
v) promiscuous gene expression

Question 16

TOLERANCE TO EXTRA THYMIC ANTIGENS

i) what happens to T cells that have a TCR that reacts too strongly to proteins expressed in the thymus?
ii) name two sources of antigens which may not be expressed in the thymus but the immune system needs tolerance to
iii) by what mechanism is tolerance induced and maintained outside the thymus?
iv) what happens in autoimmunity/allergy?
v) what is split tolerance

Answer 16

i) they are deleted
ii) not expressed in thymus - hormones encountered at puberty and food allergens
iii) induc and main tolerance outside thymus = peripheral tolerance
iv) autoimm/allergy > breakdown of peripheral tolerance and the immune system response to self or enviro antigen

v) split tolerance is where the B cell will mount an immune response to an antigen in one area but is tolerant in another area
- if help is provided eg injecting antibody then the B cell will mount a response

Question 17

MECHANISMS OF PERIPHERAL TOLERANCE

i) what happens in ignorance?
ii) what is clonal anergy?
iii) what happens in supression?
iv) what happens in clonal exhaustion?

Answer 17

i) ignorance > lymphocytes fail to recognise or respond because they dont ever see the antigen
ii) clonal anergy > binding of an antigen makes the lymphocyte unresponsive
iii) supression > interac with supressor cells/anti inflammatory cytokines which inhibit lymphocyte responsiveness
iv) clonal exhaust > continued stimulation by persistent antigen may wear out cells and they stop responding

Question 18

CLONAL IGNORANCE

i) what happens? do the cells die, become anergic or neither?
ii) give two situations where T cells may ignore antigen
iii) what is required for foreign graft survival? where are these placed?
iv) what may happen if sequestered antigen is released? give an eg of when this can happen

Answer 18

i) T cells fail to recognise or respond to self antigens in the periphery - they neither die or become anergic

ii) ignore antigen if:
- antigens are anatomically sequestered eg hid from the immune system so T cells cant reach the antigen
- immunologically priveliged sites eg eye, testis, uterus, placenta - T cells dont contact here

iii) foreign graft survival requires immune privelidge site
- can be placed in anatomically sequestered sites

iv) release of sequestered antigen can cause autoimmunity
- can happen if anti sperm antibodies get out in vasectomy

Question 19

IMMUNE PRIVILEGE IN THE EYE CHAMBER

i) what normally happens to antigens at this site?
ii) what may happen if there is trayma to one eye?
iii) what condition may arise due to trauma in one eye? what mediates this reaction (2)
iv) which symptom may be seen (or not seen….)? which eyes would this affect?

Answer 19

i) antigens are not normally exposed to the immune system
ii) trauma to one eye can release sequestered intraocular protein antigens

iii) trauma > sympathetic opthalmia
- intraocular antigens are released and move to LNs where they activate T cells
- effector T cells then return via the blood stream and attack antigen in both eyes

iv) can cause blindness in both damaged and undamaged eyes

Question 20

INDUCTION OF ANERGY

i) what is it? when does it happen?
ii) presentation with which negative inhibitor of T cell activation also induces anergy?

Answer 20

i) anergy refers to lack of response from a B cell to an antigen which can happen when antigen is presented without costimulation
ii) presence of negative inhibitor CTLA-4

Question 21

CD28 AND CTLA-4

i) what does CD28 do? what does CTLA-4 do?
ii) which one is used to switch off the response after a T cell has been activated?
iii) what normally happens when an antigen is presented on an APC expressing co-stimulators?
iv) what do KO CTLA-4 mice and heterozygous mut in humans result in?

Answer 21

i) CD28 is a stimulatory signal for full activation of a T cell
CTLA-4 switches off response once T cell has been activat

ii) CTLA-4 switches off response
iii) normally when antigen is present with co-stim eg B7 (binds CD28) > effector T cell prolif and differentiation
iv) KO CTLA-4 or mut results in immune dysregulation eg lymphoproliferation/multi organ inflamm (lots of cell activation)

Question 22

REMOVING THE BRAKES ON THE IMMUNE RESPONSE

i) what needs to be removed for a T cell to be activated? what could be used to increase T cell activation?
ii) in what situation could this blocker be useful? what can also be used?
iii) what has been shown to lead to tumour regression in mice?

Answer 22

i) need to remove CTLA-4 for activation
- antibody to CTLA-4 can be used to inc activation

ii) antibod to CTLA-4 to improve response to cancer
- can also use PD1

iii) CTLA-4 antibody

Question 23

TREGS AND SUPRESSION

i) what do Tregs do? what is this critical for?
ii) what effect can Tregs have on APCs?
iii) what is absence of Tregs associated with in mice and humans?
iv) what is constituitively expressed on T reg cells? how does this work to limit T effector expansion? what does depletion of Treg cells lead to in this context
v) which transcription factor is critical for Treg activity and dev? what does a mutation in the gene coding for this TF cause?

Answer 23

i) Tregs supress the activation of effector responses
- critical for regulating homeostasis and tolerance to self antigens

ii) Tregs can supress presentation of antigen from APCs
iii) absence of Tregs > aggresive autoimmunity

iv) Tregs constit express CD25 (IL-2 receptor)
- consumes IL-2 to limit the expansion of T effectors (Il-2 is a key cytokine for T effector expansion)
- depletion leads to autoimmunity due to lots of Teff expan

v) FOXP3 TF is critical for Treg activity/development
- mutation in this gene causes IPEX (fatal autoimmune disorder - systemic autoimm in first year of life)

Question 24

CLINICAL APPLICATIONS OF TREGS

i) what may Tregs be able to promote?
ii) what effect may they have in autoimm disease?
iii) what may they do in transplantation/GvHD, allergy
iv) what effect may they have in cancer patients?

Answer 24

i) promote tolerance by inhibiting T cell response
ii) strengthen or re-estab self tolerance in autoimm disease
iii) induce tolerance to non self antigens in transplant/GVHD/allergy
iv) may induce tumour immunity in cancer patients

Question 25

ACTIVATION INDUCED CELL DEATH

i) what happens?
ii) what is the normal response to stimulation of T cells? when may this be seen and what does it result in?
iii) in what situation may clonal exhaustion occur? what may be expressed on exhausted T cells? (2)
iv) what are expressed on T cells that triggers apoptosis? what would this cause in context of a virus?

Answer 25

i) repeated stimulation of T lymphocytes by persistent antigens results in death by apop of the activated (T) cell

ii) normal response is seen in acute infection
- expression of survival proteins allows T cell activation when presented to an antigen

iii) clonal exhaustion can occur in chronic infection
- exhausted T cells can express inhibitory receptors such as CTLA-4 and PD-1

iv) T cells can express Fas and Fas ligand which triggers apoptosis

Question 26

ANTIGEN PROPERTIES

i) which molec weight/aggregation favours tolerance? which mw favour immunogenicity?
ii) what dosage favours tolerance? what dosage favours immunogenicity?
iii) give three routes of admin that can activate T cells to secrete TGFb? what type of T cells does this activate?
iv) what does food need to interact with before oral tolerance?
v) induction of which type of cells is most important for oral tolerance?

Answer 26

i) smaller, soluble, non aggregated > tolerance
larger, aggregated, complex > immuno

ii) very small or very large dose > tolerance
intermed dose > immuno

iii) oral, intra-tracheal and orbital exposure can activate T cells to secerete TGFb > T regs
iv) food needs to interact with gut associated lymphoid tissue in intesine before oral tolerance
v) induction of T regs is most important for oral tolerance

Question 27

THERAPEUTIC USE OF ORAL TOLERANCE

i) which antigen can be used in multiple sclerosis?
ii) which antigen can be used in RA?
iii) which antigen can be used in T1DM

Answer 27

i) myelin basic protein
ii) type II collagen
iii) insulin

Question 28

HYPOSENSITISATION IMMUNOTHERAPY

i) what does this involve?
ii) how may allergen be administered? (2) what type of allergy could this be useful in?
iii) how can sensitisation to food antigen occur?

Answer 28

i) using small amounts of allergens eg food and pollen to induce antigen specific tolerance

ii) admin allergen oral/subling
- could be useful in peanut allergy

iii) sensitisation to food can occur through cutaneous exposure