1. PRACTICAL TRANSPLANT IMMUNOLOGY
Dr Nisar Anwar
FRCP(Edin),Accredited Renal Physician (UK)
Professor & Head : Department Of Nephrology
KTH, Peshawar

2. 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.

3. 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).

4. 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.

5. 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 5

6. 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.

7. MHC STRUCTURE

9. MHC CHARACTERISTICS Polymorphism Genes are co-dominantly expressed
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

10. 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

11. 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 11

12. 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 12

14. 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 14

15. Identifying MHC Polymorphisms (‘tissue typing’)
Formerly determined by antibodies against MHC molecules
HLA typing
MLR
Now by DNA testing: allele-specific PCR, sequencing 15

16. Tissue Typing(or HLA-typing)
Used to identify HLA molecules on cells
Ab against HLA1
+ complement
Recipient
Donor
Cells die, appear blue

17. 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

19. 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.

20. 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 self-MHC + 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”)

22. 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 co-stimulatory 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. 

23. DIRECT PRESENTATION IN GRAFT REJECTION
Blue: host cell; pink: graft cell see also Abbas et al. Fig

24. Semi-direct Antigen Presentation the Membrane Patch Pathway
Host CD8+
cytotoxic cell
Host CD8+ T cell
Allogeneic
Donor Cell
Class I
Shed
membrane
with donor MHC
Intact donor MHC
Host APC 24

25. 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)

26. INDIRECT PRESENTATION IN GRAFT REJECTION

27. Allorecognition: Indirect Pathways
CD8+
cytotoxic cell
CD4+ T cell
IL-2
Class I
Shed
Allogeneic
MHC
(donor class I-derived
peptide presented
by host
class II molecules)
Allogeneic
Donor Cell
Donor MHC molecules are
taken up and
processed by host
antigen presenting cell
Host antigen presenting cell 27

28. 3 Signals for T Cell Responses
APC Ag
signal 2
costimulation B7
CD28
cytokines
IL etc
signal 1
IL-2
signal 3
T cell
activation
G S
M G2
replication
key genes
e.g.IL-2, CD40L
Expression of effector activity 28

29. Three-Signal Model
Halloran, N Eng J Med, 2004;351:3715

30. 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.

31. CNIs
Halloran, N Eng J Med, 2004;351:3715

32. CALCINEURIN PATHWAY

33. 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.

34. Corticosteroids
Corticosteroids
Halloran, N Eng J Med, 2004;351:3715

37. Mycophenolates
Halloran, N Eng J Med, 2004;351:3715

38. m-TOR inhibitors
Halloran, N Eng J Med, 2004;351:3715

40. 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.

41. LEUKOCYTE RECRUITMENT

42. 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 42

43. 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 43

44. © 2007 New Science Press Ltd new-science-press.com
‘isohemagglutinins’
Figure The ABO blood cell antigen system Top: schematic representation of the carbohydrate structures corresponding to the A, B and H antigens. The H antigen is modified by the addition of a single sugar residue by the allelic glycosyltransferase enzymes that create the A and B antigens. The H antigen is present on the surface of A, B, AB and O individuals, as indicated in the lower panel, because not all H antigens are modified by the A or B glycosyltransferase. The H antigen is only recognized by antibodies made in rare individuals who lack enzymes that make this oligosaccharide structure, which is present on glycolipids and glycoproteins of red blood cells and some other cell types including endothelial cells. Also shown in the lower panel is the antibody to the A and B antigens present in individuals of different blood types.
ABO compatibility between donor and recipient is crucial to avoid rapid graft rejection
© 2007 New Science Press Ltd new-science-press.com 44

46. ABO Incompatible Renal Transplantation
Plasmapharesis
IVIG
CMV-IVIG
Splenectomy
Pre Transplant Immunosuppression

47. TYPES OF TRANSPLANT GRAFT REJECTION
Antibody-mediated rejection (AMR)
Hyperacute rejection
Acute or delayed AMR
Cellular rejection
‘Chronic’ rejection 47