Download HISTOCOMPATIBILITY ANTIGENS

PRACTICAL TRANSPLANT
IMMUNOLOGY
Dr Nisar Anwar
FRCP(Edin),Accredited Renal Physician (UK)
Professor & Head : Department Of Nephrology
KTH, Peshawar
TERMINOLOGY OF TRANSPLANTATION
• Auto graft (e.g. skin grafts for burns, etc.)
• Syngraft (genetically identical twins; some stem cell therapies:
nuclear transfer, iPS approaches)
– – No immunologic rejection for these two types
• Allograft (genetically distinct member of same species; almost
all medical transplants)
• Xenograft (different species; attractive due to shortage of
organs for transplants)
– – Very severe rejection problems.
IMMUNOLOGY OF TRANSPLANTATION
• Major barrier to organ/tissue transplantation is immunological
• Immune system recognizes transplant as foreign and destroys
it
– Memory and specificity have properties of adaptive immune response
– Memory is transferred by T cells (antibodies also can contribute).
HISTOCOMPATIBILITY ANTIGENS
• ABO blood group antigens Pre-existing antibodies to A or B
antigens lead to
– “hyperacute rejection” if not matched properly
• Major histocompatibility antigens
– Encoded by the MHC, originally discovered for their role in skin graft
rejection in mice
• Minor histocompatibility antigens Allelic variants of normal cellular
proteins.
Transplantation antigens
Major Histocompatibility Complex (MHC):
– Class I antigens: constitutively expressed on surface
of most cells
– Class II antigens: expressed on cells of lymphoid
system
– Expression of MHC molecules can be up regulated
by ischemia, etc.
– nomenclature
• HLA (human) class I: A, B, C; class II: DR, DQ
• H-2 (mouse) class I: K, D, L; class II: IA, IE
MHC NOMENCLATURE
• Identified & characterized over 40 years ago
• Antigens were identified using antisera obtained
from multiparous women, new antisera can split
HLA antigens in to narrower specificities
• DNA based techniques are more accurate in
describing alleles
• Frequency of antigens, alleles and combinations
vary considerably
• Most common HLA antigen A2 is found in 50%
of the population.
MHC STRUCTURE
MHC CHARACTERISTICS
• Polymorphism
– Each MHC locus can express any one of hundreds
of different molecules
– Set of different MHC molecules or alleles
expressed on one chromosome is called a
Haplotype
– A Genotype is the sum of two haplotypes
• Genes are co-dominantly expressed
– An individual expresses alleles from both
chromosomes at each locus
INHERITANCE
• Each parental chromosome 6 provides a haplotype or
linked set of MHC genes to the offspring.
• Haplotypes are usually inherited intact from each
parent
• The child carries one representative antigen from each
of class I & class II loci of each parent
• A child is a one haplotype match to each parent
• HLA haplotypes are inherited in a mendelian fashion
• 25% siblings will share two haplotype match, 50% will
share one haplotype and 25% will have zero match
HLA-sensitization
• Exposure to non-self HLA antigens can cause
production of HLA-directed antibodies
• Common causes of HLA-sensitization include
blood transfusions, pregnancies, previous
transplants
• In infants, tissue patches implanted during
cardiac surgery cause sensitization
Does MHC (HLA) ‘Matching’
Prevent Rejection?
• Reduces rejection but there are still ‘minor
histocompatibility antigens’ (MiHA)
• MiHA are probably polymorphisms affecting
peptides in the grooves
• But we cannot MHC-match most grafts: too
much polymorphism, too little time, too few
donors
• Therefore need immunosuppression
Matching and Cross-Matching
• Matching: finding a donor who shares the
HLA antigens of the recipient, to minimize
antigen disparities
– requires donor and recipient antigens to be
identified
• Cross-matching: testing the SERUM of the
recipient for antibodies against the donor
antigens
Identifying MHC Polymorphisms
(‘tissue typing’)
• Formerly determined by antibodies
against MHC molecules
 HLA typing
 MLR
• Now by DNA testing: allele-specific
PCR, sequencing
Tissue Typing(or HLA-typing)
Used to identify HLA molecules on cells
Ab against HLA1
+ complement
Donor
Recipient
Cells die, appear blue
MIXED LYMPHOCYTE REACTION:
Recipient
Donor
+
(Irradiate)
Cell Proliferation
 Strong Proliferation--->High incompatibility
 Weak proliferation--->Low incompatibility
 No proliferation---> 100% compatibility
 Helps to identify any antigenic differences between
donor and recipient
CD4 & CD8 Subsets Of T Cells
• CD 4 or Helper subset of T cells directly bind MHC class II
molecules
• CD 8 or Cytolytic subset of T cells directly bind MHC class I
molecules
• CD 4 cells mediate initial recognition of an allograft
• CD 4 deficient mice are unable to reject grafts while CD 8
deficient mice do reject grafts.
TCR SPECIFICITY AND SELF-MHC
• MHC molecules are highly polymorphic
• During development in the thymus, T cells are selected for
moderate/weak binding to self-MHC + a self-peptide
(“positive selection”)
• During development in the thymus, T cells are killed (or
become regulatory T cells) if they have strong binding to selfMHC + a self-peptide (“negative selection”)
• Do the T cells of one person “see” MHC molecules from
another person? Actually about 1% of T cells bind such MHC
with self peptide strongly--like a microbial antigen/MHC
complex-- and become activated (“alloreactive T cells”)
DIRECT PRESENTATION
• Direct presentation in transplantation (MHC + peptide: MHC
is from graft; peptides are from cellular proteins, graft and/or
host)
• Graft dendritic cell is induced to mature, express costimulatory molecules (B7) and migrate to lymph node by
stress of surgery, etc.
• DC in lymph node activates allo-MHC recognizing CD4+ T
cells and CD8+ T cells
• Activated T cells expand and then home to sites of
inflammation where they find allo-MHC on graft cells and
secrete cytokines/kill graft cells
• This is the critical pathway for CD8 T cell response.
DIRECT PRESENTATION IN GRAFT
REJECTION
• Blue: host cell; pink: graft cell see also Abbas et al. Fig. 10-8.
Semi-direct Antigen Presentation
the Membrane Patch Pathway
Host
CD8+
cytotoxic
cell
Host CD8+ T cell
Class I
Allogeneic
Donor Cell
Shed
membrane
with donor
MHC
Intact donor MHC
Host APC
INDIRECT PRESENTATION
• Indirect presentation in transplantation (MHC + peptide: MHC
is from host, peptide may be
• from graft MHC)
• Dendritic cell pick up antigens (e.g. allo-MHCs), and load
peptides from these allo-MHC onto their own MHC II, migrate
to lymph nodes and activate T cells
• CD4+ T cells are activated and then migrate to sites of
inflammation where they are activated by host macrophages or
DCs presenting graft antigens, leads to inflammatory reaction
and tissue damage.
• The indirect pathway is probably responsible for chronic
rejection (rejection after 1 year)
INDIRECT PRESENTATION IN
GRAFT REJECTION
Allorecognition: Indirect Pathways
CD8+
cytotoxic
cell
CD4+ T cell
IL-2
Class I
Allogeneic
Donor Cell
Shed
Allogeneic
MHC
(donor class I-derived
peptide presented
by host
class II molecules)
Donor MHC molecules are
taken up and
processed by host
antigen presenting cell
Host antigen presenting cell
3 Signals for T Cell Responses
APC
Ag
signal 1
costimulation
signal 2
cytokines
B7
CD28
IL-2
signal 3
T cell
activation
key genes
e.g.IL-2, CD40L
IL- 15
etc
G1
S
replication
M
G2
Expression of
effector activity
Three-Signal Model
Halloran, N Eng J Med, 2004;351:3715
ROLE OF CO-STIMULATORY PATHWAY
• Do Alloreactive T cells need co stimulation?
• Ordinarily T cell immune responses require co-stimulation
(B7/CD28)
• Blocking co stimulation with CTLA4-Ig, “belatacept” protects
against acute graft rejection in kidney patients (phase III
clinical trials published in 2010; FDA approval in 2011)
• Most likely explanation: -ischemia, stress to organ, release of
DAMPs can induce expression of co-stimulators -may explain
why live donor organs have superior outcomes for kidney
transplants compared to cadaver donor organs.
CNIs
Halloran, N Eng J Med, 2004;351:3715
CALCINEURIN PATHWAY
MECHANISM OF IMMUNOSUPPRESSION
BY CYCLOSPORINE
• NF-AT: nuclear factor of activated T cells NF-AT is required
for transcription of the IL-2 gene and some other cytokine
genes
• NF-AT cannot be activated in presence of cyclosporine.
Corticosteroids
Corticosteroids
Halloran, N Eng J Med, 2004;351:3715
Mycophenolates
Halloran, N Eng J Med, 2004;351:3715
m-TOR inhibitors
Halloran, N Eng J Med, 2004;351:3715
ROLE OF B LYMPHOCYTES
• Key role in hyper acute rejection.
• B cells are less important than T cells in initiating an allo
response but antibody formation can play a role in the effecter
phase of graft destruction and chronic rejection.
LEUKOCYTE RECRUITMENT
Transplantation Antigens
ABO system
– ABH antigens are complex carbohydrate
(polysaccharide) structures on surface of many cell
types including graft cells & RBC; genes encode
production of specific glycosyltransferases catalyze
addition of terminal trisaccharide
– nomenclature
•
•
•
•
H antigen: base chain; defines blood type O
A trisaccharide on H chain: blood type A or A1
B trisaccharide on H chain: blood type B
A and B trisaccharides on H chains: blood type AB
The ABO Blood Group Barrier in Organ
Transplantation
• ‘ABO’ antigens: carbohydrate structures expressed
on many tissues and organs, including endothelium
of organ transplants
• Recipient pre-formed ‘natural’ anti-A or anti-B
antibodies to non-self A/B antigens
• Transplantation of ABO-incompatible organs:
‘Hyperacute’ rejection
‘isohemagglutinins’
ABO compatibility between
donor and recipient is crucial to
avoid rapid graft rejection
© 2007 New Science Press Ltd new-science-press.com
ABO Incompatible Renal
Transplantation
•
•
•
•
•
Plasmapharesis
IVIG
CMV-IVIG
Splenectomy
Pre Transplant Immunosuppression
TYPES OF TRANSPLANT GRAFT
REJECTION
• Antibody-mediated rejection (AMR)
– Hyperacute rejection
– Acute or delayed AMR
• Cellular rejection
• ‘Chronic’ rejection