Transplantation Flashcards
L1 Transplatation & Tolerance
LO
immunology behind transplant rejection
different types of rejection
difficulty with preventing rejection
the promise of transplant tolerance
what is the most transplanted organ globally?
kidney
What are the 3 different types of transplant?
- autograft
- isograft
- allograft
What is an autograft /autologous transplant?
patient’s own tissue
skin graft eg major burns, wounds
What is an isograft transplant?
donor and recipient are identical twins
(rarer)
What is an allograft transplant?
genetically non-identical recipient of same species
(most common type)
Which antigens in man are the most polymorphic in biology?
MHC (HLA in man)
What is the main molecular target of rejection?
HLA which determines the major histocompatibility complex
How many genetic combinations are possible with the HLA gene?
pic p544
What does HLA incompatibility initiate and drive?
rejection
allografts can be subject to immune mediated rejection. what does the immune system see on those allografts?
differences on MHC
MHC is fundamental to the activation of what cells?
t cells
minor antigens (MiHC) can also stimulate rejection, give an example of this?
HY male antigen stimulates a response in female recipients seen if you use boy girl twins
how many combinations of MHC class II are there?
> 2 billion
what cells are most important for rejection?
T cells
naive T cells primed by what 2 types of DC (dendritic cells)?
donor
recipient
do cells or antibodies reject transplants?
cells
what effect does depletion of t cells have on allograft rejection?
rejection would not occur
name 3 innate immune cells that would be involved in the rejection process that are found in the blood?
NK, macrophage and neutrophils
outline the chain of events that lead to acute rejection?
- ischaemia/ reperfusion injury
- innate immune attack of transplants
- dendritic cell trafficking for initiating adaptive immunity
- t cell response to transplant via pathways of recognition
- t cell subsets and mechanisms -> graft rejection
what is meant by ischaemia?
blood supply to new organ lost
ischaemia/ reperfusion injury results in hypoxia, when this is detected by oxygen sensing receptors on stromal cells what is produced as a result?
free radicals
name 2 pro inflammatory cytokines that are release when necrosis occurs?
(ischaemia/ reperfusion injury + surgical trauma)
TNFa and IL1 (/6)
give 3 things that are upregulated to drive the rejection process. inflammatory cytokines?
MHC, chemokine and cytokine expression
(due to TNFa and IL1)
in ischaemia and reperfusion injury endothelial cells become activated and are the first point of contact between donor organ and recipient blood. What happens to these cells when they are activated?
increased permeability (vasodilate, easier for cells to pass barrier -> tissue)
upregulation of adhesion molecules = more sticky
MHC, chemokine, cytokine expression upregulated and endothelial cells activated leading to..?
graft ‘flagged’ as an inflammatory target
although we add buffer and put the transplants in ice, they are not…
quiescent.
still have reactive bits of tissue
what happens during ischaemia/reperfusion injury that leads to an innate response?
formation of ROS, MHC upregulation, TNF and IL upregulation.
increased permeability.
flagging
What is reperfusion injury?
tissue damage caused when blood supply returns to the tissue after a period of ischemia or lack of oxygen
how do the innate cells contibute to rejection
although they dont cause rejection themselves they exacerbate inflammation by production of cytokines and chemokines which recruit DC/Tcells.
what are the properties of an immature DC?
sample environment. low MHC, cytokines, costimulatory molecules, inflammatory chemokine receptors, poor t cell stimulators
what are the properties of a mature DC?
opposite to immature and express lymphoid chemokine receptors
what stimulates dc maturation? 3
TLR,
inflamm cytokines,
costimulatory molecules (CD40)
where are TLR
immune cells, endothelial and stromal cells
how do neutrophils cause graft damage
ROS, proteolytic enzymes
how do NK cause graft damage
perforin granzyme, FAS-FAS-L
how do macrophages cause graft damage
ROS, proteolytic enzymes
role of chemoattractants: CXCL9 and 10?
recruiting T-cells and NK cells to sites of infection/ inflammation and enhancing their cytotoxic activity against infected or abnormal cells.
role of chemoattractant: CCL3-5?
recruitment of monocytes, macrophages, and T-cells to sites of inflammation or injury.
what cell secretes tnfa, IL-1b, IL-12. IL-18? ()
macrophages produce TNF-alpha and IL-18,
DC produce IL-12, and
macrophages and dendritic cells produce IL-1beta.
once transplant done with donor tissue, then may get activation of innate immune cells due to what?
inflammation
what do activated T cells, and D cells facilitate further? after innate response?
further immune cell recruitment
DC have 2 origins, name them
donor tissue
in recipient
mature or immature DC :
highly pinocytic, low levels of MHC, costimulatory molecules, cytokine secretion, poor stimulators of t cell response
immature
dendritic cell trafficking occurs for initiating the adaptive immune system. what 2 states do they exist in?
mature and immature
to get immature DC to sites of inflammation they express inflammtory chemokine receptors, once they sample the environement they get exposed to the stimulation. Give 3 thingsthat can facilitate this?
TLR, costimulatory molecules and inflammatory cytokines
once immature DC cells mature what characterstics do they have?
poorly pinocytic, high levels of MHC, costimulatory molecules and cytokine secretion, express lymphoid chemokine receptors
are mature or immature DCs potent stimulators of the t cell response?
mature
full process of DC trafficking for adaptive immunity
- innate response
- DAMPs
- donor cells mature w/ alloantigen
- migrate to lymphoid
- stimulation of immune cells
6.immature cells back into graft
- they acquire alloantigen
- they go back to the resident lymphoid tissue
where do mature DC cells migrate to, to stimulate an immune response?
recipient lymphoid tissue
recipient DC cells traffick into the donor graft and require antigens, mostly MHC, and are subjected to inflammation and DAMPS. what happens to them as a result of this?
mature, upregulate lymphoid receptors and exit the graft and make their way to recipient lymphoid tissue
what are the 3 different pathways that t cells can become activated to alloantigens?
direct, indirect and semi direct
what is the direct pathway of alloantigen recognition
(predominant pathway of stimulating T cells)
donor MHC presented by donor DC recognised by T cell
what is the indirect pathway of alloantigen recognition
donor MHC acquired by recipient APC/ DC
what is the semidirect pathway of alloantigen recognition
recipient APC presenting intact donor MHC plus donor peptide
which t cell alloantigen recognition pathway recognises:
intact donor MHC + peptide complexes presented by donor APC?
direct
which t cell alloantigen recognition pathway involves a recipient APC presenting donor MHC?
indirect
what does precursor frequency (or T cells) tell you?
% of T cells able to recognise alloantigen in these diff pathwyas and be stimulated by recognition of MHC
how does the semi direct pathway happen - 4 steps?
- donor graft releases exosomes
- these are picked up by recipient DC
- binds to cell surface
- presentation of intact MHC complex
acute rejection in transplantation is predominantly mediated by Th1?
true
what is meant by cross dressed DC in terms of semi direct pathway of T cell alloantigen recognition?
exosomes released from donor graft containing intact donor MHC I and II peptides which are presented by the recipient lymphoid tissue
in transplantation what do Th1 cells predominantly mediate?
cytotoxicity (IL-2 too)
delayed type hypersensitivity (IRN-y too)
what role do TH2 cells play in transplant rejection?
humoral antibodies and eosinophils
what role do th17 cells play in transplant rejection?
enhance neutrophil response
is the overriding immune response mediated by th1, th2, th17 or treg cells?
th1
what cell are
th1, th2, th17 or treg cells derived from?
naive CD4 T cell
what must be given to prevent chronic rejection
immunosuppressants
in 7-14 days
chronic rejection happens within first year of transplant.
diffuse concentric intimal thickening of vessels leads to what?
rgadual dec in graft function
when does chronic rejection occur?
months- years after transplant
why does lumen of blood vessel get smaller in chronic rejection?
intimal hyperplasia
-> smooth muscle cells migrate due to inflammation from adventitia into lumen… gets smaller
… then organ starved of O2 blood
what increases chronic rejection? 3
(contribution of alloantigen to chronic rejection)
acute rejection,
inadequate immunosuppression,
HLA mismatch
do lesions associated with chronic rejection contain immune cells or not?
yes
antibodies to HLA and complement deposition on endothelium may be present in chronic rejection T/F?
true
what nonimmunogenic factors contribute to chronic rejection?
older donor, hypertension, chronic effects of cyclosporin/tacrolimus, ischaemic injury
what is hyperacute rejection?
rejection in mins to hours due to a preformed antibody which recognises HLA of donor
what are the 3 types of rejection?
acute: 7-14 days, typ in first yr
chronic: gradual, over 1 year
hyper acute: minutes-hours
hyperacute rejection is not mediated by T cells, but instead: antibody, why?
T cells: take time
process of hyperacute rejection
(thrombolytic cascade)
- antibody
- complement activation
neutrophil accumulation and activation
- endothelial cell damage
- platelet adhesion and fibrin
- thrombosis
- no blood flow
- necrosis
primary target of hyperacute rejection?
HLA expressed on endothelial cells
what are some causes of hyperacute rejection
due to pre-formed abs
req prior sensitisation due to:
- previous blood transfusion/transplant/pregnancy
(increased antibodies)
if someone has blood type A, which antibodies will they have?
which antigen?
anti-b
A antigen (the one on surface)
if someone has blood type B, which antibodies will they have?
which antigen?
anti-A
B antigen (on surface)
if someone has blood type AB, which antibodies will they have and what blood can they accept?
none, universal acceptors: A and B antigens
if someone has blood type O, which antibodies do they have and to what blood groups can they donate?
anti a and b,
no antigens - universal donors
when to screen blood group in process of transplant, to stop hyperacute rejection?
Before
why is blood type important in transplantation?
AB sugars are present on the endothelium
how can you identify if someone might have a hyperacute rejection?
assays to determine whether recipient has particular antibodies
what is the cytotoxic assay (cytotoxic cross-match)?
lytic anti-donor abs detected by culturing recipient serum with donor cells.
how can you prevent hyperacute rejection?
remove abs:
- plasmapheresis,
- B cell removal by rituximab,
- splenectomy
- IVIg
ADAs can be detected by what?2
luminex or flow cytometry
ADAs can be detected by what?2
luminex or flow cytometry
T/F:
current immunosuppression is not specific for transplanted organ?
true
why doesnt B cell removal by rituximab deplete plasma cells?
only stops new ones being formed because plasma cells do not have CD20
what is a xenogenic transplant
from a different species
why are current immunosuppressive agents not specific for transplanted organ?
dont distinguish between protective responses against pathogens and destructive responses against grafts
alloreactive T cells are simply a subset of what cell pool?
normal T cells
current immunosuppression not specific thus leads to immune complications such as… 2
increased risk of
- infection
- malignancy
What are the 3 non-immune complications of immunosuppression?
nephrotoxic
HTN
hyperlipidaemia
immunosuppressives: expensive and unpleasant.
taken for how long in these px?
for life
What is the problem with current immunosuppression?
It is not specific for the transplanted organ
What are alloreactive T cells?
T cells that react against non-self MHC molecules
What is transplant tolerance?
Targeting only T cells that react against the transplanted organ, allowing for allograft acceptance without continuous immunosuppression
What is the pathway to tolerance in adults?
Regulatory T cells suppress effector T cells, leading to tolerance induction
are there more or less of: regulatory, and effector moleculaes in rejection?
less effector
more regulatory
how does proportion of effector and regulatory cells change in tolerance?
less effector T cells
more regulatory T cells
What are the two signals required for T cell activation?
Signal 1: MHC-peptide binding
(int of TCR w MHC + peptide)
Signal 2: costimulation
What happens when T cells receive only Signal 1?
No response or rejection
What happens when T cells receive both Signal 1 and Signal 2?
Optimal activation or rejection
What happens when T cells receive neither Signal 1 nor Signal 2?
No response or rejection
What happens when T cells receive Signal 1 but not Signal 2?
Incorrect activation leading to tolerance
What is the result of blocking costimulation?
Tolerance, as demonstrated in a skin graft experiment
alloantigen-reactive Treg needed to facilitate tolerance T/F?
true
transplantation= successful therapy for end stage organ failure due to what
immunosuppressive drugs
what is a xenogenic transplant
from a different species eg pig
what initiates and drives rejection?
HLA incompatibility
also minor antigens (MiHC) eg H-Y male antigen stim response in female recipients
what specifically rejects transplant?
cells not antibodies
What are the physiological ligands for TLRs in transplantation?
Damage associated molecular patterns (DAMPs).
what are the pre formed antibodies to pig directed against?
Galactose α1-3 galactose (α Gal) epitopes
What is the purpose of generating α Gal knockout pigs?
To prevent xenogeneic responses
What is the purpose of generating transgenic pigs that express molecules that inhibit the formation of the MAC complex and/or coagulation?
To prevent xenogeneic responses
What are the 3 potential outcomes of T cell activation?
Inactivation (anergy),
activation induced cell death,
conversion to Treg
What are the current issues with immunosuppressive drugs?
Serious side effects causing significant mortality and morbidity
