Acceptable mismatches based on structural epitopes on HLA molecules

1
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.

4
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.

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Still a low chance for a higly sensitized patient
to be transplanted.
patients transplanted
Waiting time (months)
6
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.
Analysis of sera from highly sensitized patients

• NIMA NIPA
• Antibodies 46 72
• A.M 43 6

Plt 0.001
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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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Difficult to identify acceptable mismatches
A2
B8
Cw6
A1
B7
Cw3
Is HLA-A2 an acceptable mismatch?
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SAL Single antigen expressing cell line
A2
A2
A2
A2
A2
A2
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SALs validated and shown to be useful for
determination of acceptable mismatches.
HLA-A HLA-B
HLA-C
A0101 B0702
B4403 Cw0102 A0201
B1402
B4501 Cw0303 A0301
B 1501 B4601
Cw0304 A1101
B2705 B4901
Cw0401 A2402 B3501
B5501
Cw0602 A2601 B3801

Cw0801 A3101 B3901

Cw1202 A3201 B4001

Cw1402 A3303 B4002

Cw1502 A6901 B4402
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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.

anti-HLA-A2
HLA-A1
HLA-A2
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Many polymorphic sites, some of them shared
between HLA alleles.
HLA-A2
HLA-A68
HLA-B27
HLA-B44
HLA-B51
HLA-B35
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Structural Immunogenicity of HLA-B51
Polymorphic Residues on B51
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Structural Immunogenicity of HLA-B51
Polymorphic Residues on B51
This polymorphism should be considered in the
context of self HLA epitopes of the antibody
producer
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Structural Immunogenicity of HLA-B51
For A2,A68 B27,B44
Polymorphic Residues on B51
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Structural Immunogenicity of HLA-B51
For A2,A68 B27,B44
For A2,A68 B35,B44
Polymorphic Residues on B51
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Structural Immunogenicity of HLA-B51
For A2,A68 B27,B44
For A2,A24 B7,B8
For A2,A68 B35,B44
Polymorphic Residues on B51
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HLAMatchmaker is based on this principle
a computer algorithm developed by René
Duquesnoy
Donor HLA-A,B mismatches are defined by triplets
of amino acid residues (epitopes) on antibody
accessible 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.
a1
a1
a2
peptide
b2M
a2
a3
Top view
Side view
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The number of triplet mismatches predicts HLA
antibody production after renal allograft
rejection
of patients with donor specific HLA antibodies
triplet mismatches
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.

A1 A2 B7 B8
Self-triplet mismatches on basis of own HLA type
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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.

A1 A2 B7 B8 AM A3 AM B14
More self-triplet mismatches on basis of
combination of acceptable mismatches and own HLA
type Consequence more acceptable mismatches
can be found
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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 DR4
• A26, A31 B27, B51 DR4
• A24, A25 B27, B51 DR4
  A24, A26 B27, B51 DR4
• A24, A31 B44, B51 DR4
• A24, A31 B27, B44 DR4
• A25, A31 B44, B51 DR4
• A26, A31 B44, B51 DR4
• A25, A31 B27, B44 DR4
• A26, A31 B27, B44 DR4
• A24, A25 B44, B51 DR4
• A24, 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

• HLAMatchmaker is of benefit for the
  identification of acceptable mismatches for
  highly sensitized patients.
• Increased chance to be transplanted
• Excellent graft survival

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.