1. Towards epitope matching in kidney allocation
Frans Claas
Eurotransplant Reference Laboratory
Department of Immunohematology and Blood Transfusion
Leiden University Medical Center, Leiden, the Netherlands
EFI meeting Kos, May 12, 2016

2. HLA matching is beneficial for kidney graft survival

3. Especially de novo production of donor specific antibodies (DSA) after transplantation is associated with poor graft survival.
Lachmann et al. Transplantation 2009

4. Number of different HLA proteins
The enormous polymorphism of HLA makes selection of a fully HLA matched donor very difficult.
Number of different HLA proteins
HLA-A
HLA-B
HLA-C
HLA-DR
HLA-DQ
HLA-DP
2313
3011
1985
1850
5995
480
Many patients must be transplanted with a HLA mismatched donor.
However, not every HLA mismatch leads to an antibody response and graft rejection.
Challenges for future organ allocation:
Prediction of the non-immunogenic HLA mismatches, which do not lead to antibody formation in an individual patient?
Prediction of the outcome of a virtual crossmatch ( i.e. DSA: yes or no) in sensitized patients, without the need to test beforehand the antibody reactivity against all these antigens?

5. HLA antigens share antibody epitopes
Immunization by pregnancy: antibodies induced by HLA-A2 react also with HLA-B17.

6. Molecular analysis of HLA alleles enables the prediction of antibody epitopes and their immunogenicity
HLA-A2
HLA-A68
HLA-B27
HLA-B35
HLA-B51
HLA-B44 6

7. Polymorphic structures are often shared between different HLA alleles

8. Every HLA antigen carries an unique set of epitopes but the individual epitopes can also be present on other HLA antigens
HLA antigen HLA antigen HLA antigen HLA antigen 4
Tambur & Claas 2015

9. Principles of HLAMatchmaker:
HLAMatchmaker algorithm predicts the immunogenicity of an HLA alloantigen.
Principles of HLAMatchmaker:
Antigen has many potentially immunogenic epitopes (triplets/eplets) but some of these are shared with one or more of the patients’own HLA molecules.
2. Patient will not make antibodies to epitopes
present on the own HLA antigens and therefore:
3. Polymorphism of an HLA mismatch should be
considered in the context of the HLA type of the
potential antibody producer. 9
Duquesnoy, Human Immunol. 2002

10. Number of foreign "epitopes” on the same HLA-B51 mismatch for:
Immunogenicity of a specific HLA mismatch is different for individual patients.
Number of foreign "epitopes” on the same HLA-B51 mismatch for:
Patient A Patient B Patient C Patient D Patients E
many quiet some few few no

11. HLA triplet matching prevents sensitization after graft rejection.
% patients with DSA in CDC
Number of triplets mismatched
Dankers et al. Transplantation,2005 11

12. Both the number of triplet and eplet mismatches of an HLA mismatch predicts antibody reactivity in luminex..
Antibody reactivity in highly sensitized renal transplant candidates. 2
Kosmoliaptsis et al. 2008

13. Class II HLA Epitope Matching—less antibodies and better outcome
Kaplan–Meier de novo DSA free survival curves. Panel (A) shows DR dnDSA free survival split by HLA‐DRβ1/3/4/5 epitope mismatch quartiles. Panel (B) shows DQ dnDSA free survival split by HLA‐DQα1/β1 epitope mismatch quartiles. Panel (C) shows DR dnDSA free survival split by an optimal mismatch cutoff of 10 mismatches for HLA‐DRβ1/3/4/5 and in Panel (D) an optimal mismatch cutoff of 17 for HLA‐DQα1/β1. dnDSA, de novo donor‐specific antibody; HLA, human leukocyte antigen.
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Wiebe et al. AJT, 2013

14. HLAMatchmaker is useful to determine the immunogenicity of an HLA mismatch
The number of triplet/eplet mismatches predicts the chance that a HLA mismatch will lead to de novo antibody formation
Note: A triplet/eplet is not the actual epitope recognized by the
antibodies in a (highly) sensitized patient.
Knowledge of the complete antibody epitope is essential for an
optimal virtual crossmatch.

15. Six contact sites between antibody and antigens.
(complementarity determining regions (CDR)
H3 binds to few amino acids and determines specificity, other CDR’s stabilize the binding and determine the affinity

16. Simple antibody reactivity: An antibody induced by 62GE reacts with all 62GE carrying alleles.
Immunizing eplet
Amino acids in the environment of the eplet
Crucial position for antibody reactivity:
Duquesnoy et al. 2014

17. More complex antibody reactivity: an antibody induced by 144TKR reacts only with a proportion of the 144TKR carrying alleles.
Immunizing eplet
Amino acids around the eplet are relevant
Reactivity is specific for 144TKR+151H
Crucial positions for antibody reactivity:
Duquesnoy, Mulder et al. 2015

18. Definition of actual epitopes is work in progress.

19. Future matching strategies will be based on "epitope” matching.
Less complicated and …epitope matching prevents antibody formation
More than 10,000 HLA class I alleles.
Polymorphism can be explained by about crucial epitopes.
Enables prediction of acceptable mismatches.
Knowledge of the actual epitopes recognized by existing antibodies enables prediction of a negative cross match in immunized patients.