Transplantation Immunology Flashcards
define xenograft
transplantation from one species to another
why transplant?
treatment of variety of medical conditions, such as end-stage organ failure, chronic diseases, malignancies
T or F. Transplantation is more expensive than dialysis for patients while improving quality of life
F! it’s less expensive and improves quality of life
sources of stem cell transplants
bone marrow
peripheral blood
umbilical cord blood
trnasplantation antigens
MHC = HLA (proteins)
ABO blood group antigens = carb structures on surface of RBCs & endothelium; indivs produce natural antibodies to non-self ABO antigens
minor histocompatibility antigens
- immunogenic peptides derived from polymorphic self proteins
- autoantigens, male-specific antigens (H-Y)
cell surface proteins that present peptides of endogenous/exogenous antigens to T cells
MHC/HLA
6 clinically relevant HLA loci
HLA-A, -B, -C, -DR, -DQ, -DP
Class I HLA
HLA - A,-B,-C
function of HLA
presentation of peptides to T cells
why is HLA so diverse?
needed so we are able to deal with all pathogens that may be encountered
- HLA molecules highly polymorphic
- co-dominant expression of parental HLA
T or F. HLA polymorphism is focused around the peptide binding region
T
HLA polymorphism allows for the presentation of different repertoires of ____________ to __ cells
peptides; T
why not HLA match every donor?
due to HLA diversity it is difficult to find 0 MM donor
demand of organs higher than available supply
on average, 250 Canadians die each year while waiting
god graft survival is possible without matching using modern immunosuppressive drugs
how do B cells identify HLA antigens?
- recognize epitopes on surface of antigens
- various molecular compositions (proteins, lipids, carbs, DNA, etc.)
- soluble as well as membrane-associated antigens
how do T cells recognize HLA antigens
epitopes buried within antigens (must be broken down first to fragments)
peptide:MHC molecule complex
two branches of antibody-mediated rejection
complment-mediated cytotoxicity
antibody-dependent-cell-mediated toxicity
C4d role in antibody-mediated rejection
byproduct of complement cascade
- deposited in cell membrane
- used clinically to assess whether pt is having Ab-mediated rejection
- can stain for it = green colour
pre-formed antibodies present that bind to the graft endothelium right after trnasplantation
hyperacute rejection
- occurs within mins/hrs
- RARE bc we screen for Abs
acute cellular rejection
T cells destroys the graft parenchyma (and vessels) by toxicity and inflammatory rxns
- takes weeks (early) or delayed (months <1 yr)
acute humoral rejection
antibodies damage graft vasculature
- early (weeks; often memory response!) or delayed (months; memory response vs de novo antibodies)
T or F. Immunological memory also applies to HLA antigens
T
T or F. The majority of transplantation rxns we see now are chronic rejections
T
Define chronic rejections
- caused by both humoral and cellular immunity
- intimal thickening and fibrosis of graft vessels as well as graft atrophy
- time = late (months to years)
HLA inheritance
Mendelian
- 1 haplotype from mother
- 1 haplotype from father
- antigens are codominantly expressed = homozygous or heterozygous
T or F. HLA diversity is an evolutionary advantage
T!
T or F. Homozygous individuals feature more HLA types
F! Heterozygotes do = target more antigens and on average better protected (selective advantage)
T or F. Hosts with rare HLAs better protected (selective advantage)
T! HLA helps eliminate pathogens
T or F. HLA-mismatched couples more fertile
T
T or F. HLA presents a challenge in transplantation
T
T or F. ONLY donor-derived HLA peptides that are different from self-HLA peptides will be recognized as foreign = immune response
T!
describe the direct pathway of allorecognition by intact donor HLA
intact dono HLA presented to recipient T cells by donor APCs
on of the leading causes of graft loss
allograft rejection
induction therapy
administered pre-translpant or in the peri-transplant period
maintenance therapy
basal immunosuppressive drugs given continuously long term
rescue therapy
therapies given during rejection episodes
the different immunosuppressive drugs over time
induction therapy
maintenance therapy
rescue therapy
types of immunosuppressive drugs
glucocorticosteroids
small molecule drugs
protein-based/biologic drugs
examples of glucocorticosteroids
prednisone - maintenance therapy
methylprednisolone - resuce therapy
broad spectrum immunosuppressive and anti-inflammatory effects
glucocorticosteroids
these drugs act intracellularly, mainly used for maintenance therapy, and inhibit pathways downstream of cell activation
small molecules drugs
focus of smalll molecule drigs
mainly on inhibiting cell activation and/or proliferation
calcineurin inhibitors (CNIs)
cyclosporine, tacrolimus (small molecules)
- inhibition of transcription factors required for growth factor production and expression of growth factor receptors
- decreases B and T cell activation
- most commonly used as maintenance therapy
purine synthesis inhibitors
azathioprine, mycophenolate mofetil (MMF) (small molecules)
- inhibition of DNA synthesis
- decreased B and T cell clonal expansion
- commonly used along CNIs as maintenance therapy
mTOR inhibitors
mammalian target of rapamycin
- sirolumus/rapamycin, evrolimus
(small molecules)
- inhibits mTOR
- limits response to growth factors (anti-neoplastic)
- decreases T and B cell clonal expansion
- becoming more common as maintenance therapy in CNI sparing protocols
describe trough levels
the lowest concentration reached by a drug before next dose is administered
descibe protein based or biologic drugs
- mainly for induction and rescue therapy
- act extracellularly
- receptor-ligand or antibodies
advantages of protein-based/biologic drugs
high specificity, particularly monoclonal Abs and fusion proteins
low toxicity, a least from the agent itself
long half-life in circulation, same as IgG molecules (~3 weeks)
disadvantages of protein-based/biologic drugs
some interfere with antibody detection assays and/or flow cytometry XM
- cost!
processes to remove antibodies
plasmapheresis
IVIg
plasmapheresis
process that filters the blood and removes harmful antibodies
Intravenous immunoglobulin
pooled immunoglobulins (IgG) from the plasma of approx. a thousand or more blood donors
inhibition of
- Ab production
- complement activation
- immune cell activation
class I HLA
A, B, C
- polymorphic monomor w B2 microglobulin
- all nucleated cells + PLTs
- expression levels vary = high in lymphs
class II HLA
DR, DQ, DP
- heterodimer of polymorphic a and B chains
- APCs only
- can be upregulated on other cells
alloantigen recognition by T cells (3)
direct
indirect
semi-direct pathway
describe the indirect pathway of allorecognition
- peptides from donor HLA
- presented to recipient T cells by recipient APCs
- like how adaptive immune responses to foreign peptides
NOTE: only donor-derived HLA peptides different from self HLA will be recognized as foreign
describe the direct pathway of allorecognition
- APCs are from donor; travelled with organ when transplanted or activated endothelial cells (upregulated HLA)
- intact donor HLA presented to recipient T cells by donor APCs
- unique to transplantation
- greater immune response than indirect pathway
describe the semi-direct pathway of allorecognition
- donor cells can secrete exosomes/vesicles; also happens with inflammation or when cells are damaged; can be in circulation = have HLA molecules in surface; can morph with recipient APCs (direct presentation but with recipient APCs)
antibody cross-reactivity
individuals exposd to just one antigen that contains an epitope… can make Abs to all antigen with that same epitope
two main types of allograft rejection
T-cell mediated rejection (TCMR)
antibody-mediated rejection (AMR)
this is the first to occur if there is rejection
T-cell mediated rejection
what happens in T cell-mediated rejection
donor and recipient APCs migrate to secondary lymphoid organs
APCs meet T cells there
effector T cells migrate to graft
describe antibody-mediated rejection
- donor and recipient APCs migrate to 2ry lymphoid organs
- APCs meet T cells there
- T cells provide help to B cells
- plasma cells develop
- plama cells home to bone marrow
- HLA Ab production
-HLA Ab enters blood and reaches graft - effector T cells migrate to graft
two types of antibody-mediated rejection
complement-mediated and
antibody-dependent cell-mediated cytotoxicity (ADCC)
___ deposition is commonly observed in microvascular endothelium in aggressive ABMR
C4d; antibody-mediated rejection
this type of rejection is very rare nowadays!
hyperacute rejection
how to test patients for antibodies before transplantation
- cells from donor
- sera from patients
= flow cytometry; if strong rxn = don’t transplant bc there might be Abs there
highly sensitized patient registry in kidney transplantation: for patients with cPRA > ___%
95%
- deceased donor HLA entered into national registry
- kidneys may be offered across Canada
T or F. There is no current assay that tests for memory B cells
T
danger of memory in a transplant
when pts tested for Abs, memory B cells don’t come up
so when transplantation occurs (after crossmatch is “ok), it can incite an immune response = previously undetected Ab may elicit a memory response
maybe not exposed to exact antigen from donor but could have been exposed to HLA antigen from donor that shared same epitope as the antigen that elicited response
diagnosis of rejection
- initially non-specific = malaise, lethargy, apathy, weakness, discomfort, low creatinine clearance/proteinuria (kidney), etc.
- Dx based on biopsy, blood test, and HLA Ab testing
T or F. Chronic rejections can be asymptomatic
T
HLA sensitization events
pregnancy
blood transfusion
previous transplants
cross-reactivity
any human tissue allografts (ex: ortho surgeries)
issues with pre-transplant HLA antibody testing to preventing rejection
only addresses issues with pre-existing DSA/ hyperacute AMR
does NOT prevent development of new antibodies to the transplant
Mechanisms of immunosuppression/ how does immunosuppression inhibit undesirable immunologic activity?
depletion of cells involved in immune response
inhibition of pathways required for the induction of an immune response
blocking of receptors
broad spectrum immunosuppressive and anti-inflammatory effects
glucocorticosteroids
glucocorticosteroids mechanism
- interact with GRs found on most body cells = internalized by cell
- down-reg of costim and adhesion molecules
- donw-reg of inflammatory cytokines (IL-12…), up-reg of anti-inflam (IL-10)
this is measured for small molecule drug levels
trough
protein-based/biologic drugs (depleting Abs)
depleting Abs
rabbit polyclonal anti-thymocyte globulin (rATG)
=> polyclonal Abs to thymocytes; inducton therapy
Alemtuzumab (Campath)
=> monoclonal Abs to CD52; expressed on lymphocytes; induction therapy
Rituximab
=> monoclonal Abs to CD20, expressed on B cells (NOT on plasma cells); desensitization or rescue therapy
protein-based/biologic drugs (non-depleting Abs)
Basiliximab
- Ab to IL-2 recptor alpha chain = CD25
- CD25 expressed on activated T cells = deceased cell activation
- does not cause T cell depletion
- induction therapy
protein-based/biologic drugs (fusion protein)
CTLA-4 immunoglobulin
- Fc fragment of a human IgG1 Ig linked to extracell domain of CTLA-4
- blocks signal 2
- improper T cells activation
- maintenance therapy
- once/6 wks
- not commonly used
advantages of protein-based/biologic drugs
- high specificity (monoclonal and fusion esp.)
- low toxicity, at least from agent itself
- long half-life in circulation = same as IgG (~3 wks)
disadvantages of protein-based/biologic drugs
some interfere with Ab detection assays and/or FCXM
EXPENSIVE!
antibody removal methods
- plasmapheresis: filters blood and removes harmful Abs
- IVIg: pooled IgG from plasma of ~1000+ donors
> inhibits antibody production, complement activation, immune cell activation
Bortezomib
- novel; protease inhibitor; small molecule
- induce apoptosis of active plasma cells due to high amount of proteins produced by plasma cell
- build up of misfolded proteins = apoptosis
- rescue therapy
Eculizumab
- humanized monoclonal anti-C5; novel
- prevents complement activation from classical/alt pathways by neutralizing C5
- rescue therapy
transplantation tolerance
absence of a destructive immune response to an allograft in an otherwise competent immune system = NO IMMUNOSUPPRESSION
describe central tolerance
immunological tolerance to self antigens that is established while lymphocytes are developing in central lymphoid organs
- Thymus: eliminate T cells with high affinity for self peptide:MHC complex
- BM: eliminate B cells with high affinity for self molecules
describe peripheral tolerance
immunological tolerance acquired by mature lymphocytes in the peripheral tissues
