Acceptable mismatches based on structural epitopes on HLA molecules Toulouse, April 2, 2008

Acceptable mismatches based on structural epitopes on HLA molecules

Toulouse, April 2, 2008

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Highly sensitized patients

• Highly sensitized patients: antibodies against the HLA antigens of more than 85% of the panel.

• Difficult to transplant because cross-match with most donors is positive.

• Obvious solution: HLA identical or compatible donor but only available for a small proportion of the patients.

How to find a suitable donor for these patients?

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Approaches to enhance transplantation of highly sensitized patients

• Do not accept that the patient is sensitized and try to remove the antibodies by:

* plasmapheresis * intravenous immunoglobulins (IVIg) * anti-CD20 antibodies

• Accept that the patient is sensitized and try to stimulate the allocation of cross-match negative donor kidneys to these patients.

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Eurotransplant Kidney Allocation System.

Point system based on different parameters:

• HLA match

• Match prognostic index (extra points for sensitized patients)

• Waiting time

• Regional donor (cold ischemia time)

• Country balance.

Still a low chance for a higly sensitized patient to be transplanted.

0102030405060708090

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0 3 6 9 12 15 18 21 24

ET-KAS

% patients transplanted

Waiting time (months)

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Need for an acceptable mismatch program

• Policy in Eurotransplant is the registration of the non-

acceptable HLA mismatches for sensitized patients to

prevent selection of donors with HLA mismatches

towards which the patient has preformed antibodies.

• Problem: it is impossible to determine all antibody

specificities in highly sensitized patients

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Acceptable Mismatch Program

• Basis: definition of those HLA antigens toward which the

patient did never form antibodies and use this knowledge

for donor selection.

• Original method: look at HLA type of negative panel

donors in screening and extensive antibody screening

against a patient specific panel (donors with a single HLA-A

or –B mismatch), taking advantage of a pool of 20,000 HLA

typed blood donors.

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Antibody screening

• Serum of patient (HLA: A1, A2, B7, B8) is tested against a panel of

HLA typed blood donors.

positive

negative

PRA is 92%

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Antibody screening

• Serum of patient (HLA: A1, A2, B7, B8) is tested against a panel of

HLA typed blood donors.

positive

negative

PRA is 92%

HLA type: A1, A24, B7, B8 acceptable mismatch is A24

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Non-inherited maternal HLA antigens are often acceptable mismatches.

NIMA NIPA

Antibodies 46 72

A.M 43 6

P< 0.001

Analysis of sera from highly sensitized patients

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Very difficult patients

• AM are difficult to determine for highly immunized

patients with rare HLA phenotypes.

• For these patients no suitable blood donors are available

to determine acceptable mismatches or cross-matches

with the few available donors are positive.

• Main problem: most target cells express several

mismatched HLA antigens.

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A1

A2

B7

B8

Cw3

Cw6

Difficult to identify acceptable mismatches

Is HLA-A2 an acceptable mismatch?

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A2

A2

A2

A2

A2

A2

SAL: Single antigen expressing cell line

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SALs validated and shown to be useful for determination of acceptable mismatches.

HLA-A HLA-B HLA-C

A*0101 B*0702 B*4403 Cw*0102

A*0201 B*1402 B*4501 Cw*0303

A*0301 B* 1501 B*4601 Cw*0304

A*1101 B*2705 B*4901 Cw*0401

A*2402 B*3501 B*5501 Cw*0602

A*2601 B*3801 Cw*0801

A*3101 B*3901 Cw*1202

A*3201 B*4001 Cw*1402

A*3303 B*4002 Cw*1502

A*6901 B*4402

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Alternative approach

• Commercial assays including the use of single antigen beads

(Luminex) although the conformation of these molecules may be

different than that of membrane bound HLA molecules.

HLA-A1

HLA-A2

HLA-A3

HLA-B7

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Definition of acceptable mismatches is difficult and is often based on trial and error because our interpretation of the humoral immune

response to HLA is too simple.

HLA-A1 HLA-A2

anti-HLA-A2

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HLA-A2

HLA-B44HLA-B51HLA-B35

HLA-A68 HLA-B27

Many polymorphic sites, some of them shared between HLA alleles.

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Polymorphic Residues on B51

Structural Immunogenicity of HLA-B51

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This polymorphism should be considered in the context of self HLA

epitopes of the antibody producer

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ForA2,A68;B27,B44



21

ForA2,A68;B27,B44

ForA2,A68;B35,B44



22

ForA2,A68;B27,B44

ForA2,A24;B7,B8

ForA2,A68;B35,B44



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HLAMatchmaker is based on this principle a computer algorithm developed by Rena computer algorithm developed by René Duquesnoyé Duquesnoy

Donor HLA-A,B mismatches are defined byDonor HLA-A,B mismatches are defined by triplets of amino acid residues (epitopes) on antibodytriplets of amino acid residues (epitopes) on antibody

accessible sites of HLA moleculesaccessible sites of HLA molecules

HLA-A1

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HLA matching at the triplet level

Donor m.m B18

Patient: B7

Immune system of the recipient recognizes:

A single HLA mismatch or 11 triplet mismatches

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HLA matching at the triplet level

Donor m.m

B18

Patient: B7

B52

A33

.

Immune system of the recipient recognizes:

No triplet mismatches!

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No foreign antibody epitopes on HLA-B18 mismatch for patient, with HLA type HLA-A33, B51, B7.

2M

1

23

2

1

peptide

Top view Side view

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The number of triplet mismatches predicts HLA antibody production after renal allograft rejection

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708090

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0 1 2 3 4 5 6 7 8 9 10 11 12 triplet mismatches

% of patients with donor specific HLA antibodies

Dankers et al. 2005

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Validation of HLA matchmaker for the identification of acceptable mismatches in highly sensitized patients.

Mismatch HLA-A HLA-B

tested AM tested AM

zero-triplet 18 18 54 54

CDC cross-matches confirm theoretical approach

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Additional value of HLAMatchmaker

Also HLA antigens with triplet differences can be identified as

acceptable mismatches. This information can be used for

identification of additional acceptable mismatches.

Self-triplet mismatches on basis of own HLA type

A1

A2

B7

B8

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Additional value of HLAMatchmaker

Also HLA antigens with triplet differences can be identified as

acceptable mismatches. This information can be used for

identification of additional acceptable mismatches.

More self-triplet mismatches on basis of combination of acceptable mismatches and own HLA type:

Consequence: more acceptable mismatches can be found

A1

A2

B7

B8

AM: A3

AM: B14

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Donor selection on basis of acceptable mismatches.

Patient: A24, A31, B27, B51, DR4 (highly sensitized)

AM: A25, A26, B44

Suitable donors: A25, A31; B27, B51; DR4A26, A31; B27, B51; DR4A24, A25; B27, B51; DR4

A24, A26; B27, B51; DR4A24, A31; B44, B51; DR4A24, A31; B27, B44; DR4A25, A31; B44, B51; DR4A26, A31; B44, B51; DR4A25, A31; B27, B44; DR4A26, A31; B27, B44; DR4A24, A25; B44, B51; DR4A24, A26; B44, B51; DR4 etc.

If such donor becomes available within Eurotransplant: mandatory shipment of the kidney to this highly sensitized patient

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Conclusions:

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0 3 6 9 12 15 18 21 24

AM

ET-KAS

HLAMatchmaker is of benefit for the identification of acceptable

mismatches for highly sensitized patients.

• Increased chance to be transplanted:

• Excellent graft survival:

50

60

70

80

90

100

0 6 12 18 24

AM

<5% PRA

5-85% PRA

>85% PRA

% patients transplanted

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Acknowledgements

Marian Witvliet Ilias Doxiadis Arend Mulder

Jon van Rood Guido Persijn Marlies Dankers

René Duquesnoy and the Eurotransplant community.

Documents

Acceptable mismatches based on structural epitopes on HLA molecules Toulouse, April 2, 2008