Chapter 17 Transplantation
General Features
graft rejection: rejection of recipient’s tissue by donor’s immune system
due to genetic differences in MHC (major & minor)
polymorphism: many different forms of MHC in population
each person has six different types of class I MHC and class II MHC (codominant expression)
successful organ transplantation depends on: 1. MHC matching; 2. immunsuppressive therapy
Classification of Grafts According to Their Source
isografts: grafts between identical twins
autografts: grafts within same individual
allografts: grafts between members of the same species
xenografts: grafts across species
Iso (or auto) grafts
no need for immunosuppression
no Graft vs. Host Disease (no rejection)
Allografts
match MHC
immunosuppression
Xenografts
porcine heart grafts
concern with risk of retroviral species crossing species to human
Classification of Graft Rejection
Hyperacute rejection
minutes to hours
preformed antibody mediated
Individuals at high risk: multiparous women, previous graft rejection
Xenograft

natural antibodies to carbs on pig organs
Complement activation, stimulation of coagulation cascade, thrombosis & rapid graft failure
Tx: graft removal
Delayed accelerated rejection
1-3 days post transplant
antibody/complement-mediated activation of graft endothelium

Acute rejection
most common type of allograft rejection
weeks
T cell mediated
If rejection is suspected a tissue biopsy is performed looking for immune cell infiltration and/or inflammation
Tx: increase immunosuppressive therapy

increased risk of infection, malignancy, and drug toxicity
Type 1 cytokine production (DTH)
Chronic rejection
weeks/months/years
fibroblast growth factor; endothelial growth factor

fibrosis and hyperproliferation of connective tissue
does not respond to treatment
type 2 cytokine production
Genetics of Transplantation
Major histocompatibility complex
HLA in humans (MHC in mice)
The fewer the number of mismatched loci, the greater the likelihood that the graft will be accepted
Minor histocompatibility complex
differ between individuals
can lead to graft rejection that is as strong as those occurring with MHC differences
currently we can’t measure MiHC and matching for MHC does not imply matching of MHC
Major histocompatibility typing in organ transplantation
matching MHC between donor and recipient
methods for determining compatibility between donor and recipient are serological, mixed lymphocyte reaction, and molecular techniques
Serological techniques
looking for a negative reaction
if HLA antibody recognizes the RBC there will be lysis by complement and a dye is used to detect lysis = positive reaction
Disadvantage: does not necessarily indicate compatibility
Mixed lymphocyte reaction (MLR)
measures CD4+ T cell activation because these cells are key players in graft rejection

cells from donor and recipient are cultured together. Proliferation of recipient
CD4+ T cells only occurs when they see a class II MHC difference on donor cells
proliferation is measured by radioactive isotype
Disadvantage: requires 72-96 hrs
Molecular techniques
RFLP

DNA fragments compared after specific enzyme digestion
Number of disparities does not predict the severity of rejection between donor/recipient
PCR (amplify MHCI and MHCII to compare alleles)
Immunology of Graft Rejection
mediated by activation of CD4+ or CD8+ T cells, macrophages, neutrophils, and the vascular endothelium
early after transplantation, ischemia-reperfusion damage induces chemokine & cytokine secretion by donor graft cells
increase in vascular permeability and alters expression of adhesion molecules on leukocytes and endothelium
several days later, monocytes, neutrophils, CD4+ and CD8+ T cells infiltrate the graft
monocytes  macs by IFN

(also enhances phagocytosis by macs)
macrophages and neutrophils secrete toxic products that lead to tissue damage
CD4+ T cells differentiate to Th1 cells secreting type 1 cytokines
CTL matures and leads to cell lysis of graft
Graft invasion by CD4+ T cells
when T cells leave the circulation and enter a graft, alloantigen on APC is recognized by T cell (DIRECT recognition)
recipient APC with self-MHC class II presents alloantigen

INDIRECT recognition (particularly for CD4+ T cells)
Antigenic stimuli that activate CD4+ T cells
higher frequency of T cells activated after allostimulation
review Direct and indirect recognition
Role of CD4+ T cells in graft rejection
IFN

stimulates monocytes, macrophages, and NKs
IL-2 aids in differentiation of a pCTL to a CTL
IL-4, IL-5 growth/differentiation of B cells
Role of macrophages in graft rejection
secrete cytokines and chemokines that enhance the inflammatory response
activated macs

IL-12

activation of NKs, differentiation of CD4+ T cells to
Th1
IL-1, TNF

increase expression of adhesion molecules, fever

Role of vascular endothelial cells
Mac derived TNF, IL-1, IL-8 induce expression and higher affinity of integrins on endothelial cells and leukocytes
Leukocytes that enter the tissue secrete metalloproteinases which break the basement membrane resulting in increased extravasation of additional leukocytes
Role of CD8+ T cells in graft rejection
DIRECT recognition of graft

donor cell with alloMHCI is recognized by recipient’s CD8+ T cell
INDIRECT allo-recognition in CD8+ T cells
Role of B cells
preformed antibodies have role in hyperacute rejection

complement destruction of graft
xenoantibody against

1,3 galactosyl linkage of porcine carbohydrate antigens

IC formed on blood vessel wall

activation of the classical pathway
 complement activation on endothelial cells lining graft blood vessels, activation of the coagulation cascade, thrombosis, and graft loss
Tissue Differences in Clinical Transplantation
Corneal transplant

immunologically privileged site, don’t require immunosuppression
Heart transplants

high incidence of atherosclerosis in the years following successful transplantation
Liver transplants

resistant to rejection once any early acute rejection episodes pass, and long term survival is similar for both well-matched and unmatched tissues
Kidney transplant

immunosuppressive therapy for life
Pancreas transplants

islet grafts
Bone transplants

avascular

no problem with immune rejection
Bone marrow transplants

graft versus host disease
Graft Versus Host Disease
donor T cells reject host tissue
donor CD8+ and/or CD4+ T cells are activated when they interact with host cells expressing class I and/or class II MHC
skin sloughing, diarrhea, inflammation of the lungs, liver and kidneys
deplete donor T cells and give patients IL-3, GM-CSF to speed up restoration of the lymphohematopoietic system from donor stem cells
treatment for leukemia or lymphoma
graft vs leukemia effect

Immunosuppression in Transplantation
Nonspecific immunosuppression
cyclosporine A and FK506

block production of IL-2
rapamycin

synergistic with cyclosporine
prednisone

suppress activation of macs and release of INF
 
inhibit antigen presentation
azathioprine

blocks cell division and clonal expansion of activated cells
anti-CD3 antibodies

suppress activity of all T cells
anti-CD4 antibody
Specific immunosuppression (tolerance)
deliberate infusion of donor cells in addition to organ allograft which led to prolonged survival of the graft