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.

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100

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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Polymorphic Residues on B51

Structural Immunogenicity of HLA-B51

This polymorphism should be considered in the context of self HLA

epitopes of the antibody producer

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ForA2,A68;B27,B44

Polymorphic Residues on B51

Structural Immunogenicity of HLA-B51

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ForA2,A68;B27,B44

ForA2,A68;B35,B44

Polymorphic Residues on B51

Structural Immunogenicity of HLA-B51

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ForA2,A68;B27,B44

ForA2,A24;B7,B8

ForA2,A68;B35,B44

Polymorphic Residues on B51

Structural Immunogenicity of HLA-B51

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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

100

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.