HLA in Solid Organ Transplantation

From the early days of solid organ transplantation, it has been well established that the better the HLA match between donor and recipient the better the graft survival. To this day this is still relevant despite the presence of efficient immunosuppression. Unfortunately, due to the high polymorphism of HLA and scarcity of donors, most organ transplantations are performed with at least one or more HLA mismatches.

In case a recipient receives an HLA mismatched donor organ, the immune system of the recipient can recognize the HLA of the donor as foreign and develop antibodies against that HLA. Consequently, the presence of these donor-specific antibodies (DSA) can cause graft damage, which can eventually lead to graft rejection. Therefore, minimizing the risk of developing DSA will improve graft survival and allow for lowering immunosuppressive therapy. The latter is also pivotal as this improves quality of life of recipients as immunosuppressive drugs have their own detrimental side effects. Considering that not every HLA mismatch will lead to DSA formation, and that antibodies are not formed against the whole HLA molecule but rather specific areas of the HLA molecule, it is relevant to look beyond the traditional HLA matching rules.

HLA Molecular Mismatch: Tools to Predict Immunogenecity

Over the years, several approaches have been introduced to predict the immunogenicity of mismatched HLA. These approaches are all based on polymorphic amino acids of HLA molecules and can be used to define molecular mismatches between donor and recipient. These molecular mismatches are based on either eplets (HLAMatchmaker, HLA Eplet Registry), solvent accessible amino acids (HLA-EMMA), electrostatic mismatch score (EMS-2D and EMS-3D, University of Cambridge), or prediction of indirectly recognizable HLA epitopes (T-cell epitopes, PIRCHE). Molecular mismatches between donor and recipient are mismatches defined using one or more of these approaches. Indeed, it has been well established that the number of molecular mismatches strongly associate with development of de novo DSA, graft outcome, and other clinical parameters. Chris Wiebe and his colleagues even proposed a molecular mismatch threshold to indicate whether a recipient has low, intermediate, or high risk of humoral allo-immune response.

Clinical Applications of HLA Molecular Matching

Consequently, the field is interested in using molecular matching in allocation to prevent humoral allo-immune response after transplantation, to predict acceptable HLA for highly sensitized recipients, and assess immunological risk after transplantation to lower immunosuppressive therapy. While using eplets for antibody analysis has become more common, matching recipient and donors based on molecular mismatch scores have not been implemented yet. Currently, only one pilot study has been published that used eplet mismatch threshold in allocation.

For the clinical implementation of molecular matching in transplantation, it is important to note that it is not a numbers game. Although immunological risk generally increases with a higher number of molecular mismatches between donor and recipient, on individual level a recipient with high score is not automatically at risk, and even individuals with a low score may still develop a humoral allo-immune response.

Differential Immunogenicity of HLA Molecular Mismatches and HLA-Antibody Interaction

Humoral allo-immune response depends on crosstalk between B-cells and T-cells during which both cells need to recognize a specific antigen via B-cell receptor (BCR) (antibody) and T-cell receptor (TCR), respectively. It has yet to be established if all molecular mismatches are indeed clinically relevant as it is unclear if all these mismatches are indeed recognized by B- and T-cells. While studies using monoclonal HLA-specific antibodies, adsorption/elution, and site-directed mutagenesis have been performed to verify eplets, these studies are very labor intensive, and restrictive to availability of samples and populations studied. Similarly, for the predicted T-cell epitopes (PIRCHE) it is unknown if these are indeed presented by HLA molecules.

Thereby, not every molecular mismatch is equally immunogenic. Certain molecular mismatches are more immunogenic and will more often initiate humoral allo-immune response than others. In addition, the interaction between HLA and antibody should also be considered. Especially, as the footprint of antibody on HLA molecule covers a larger area than the area considered for example for the definition of eplets. Indicating that also surrounding amino acids, including those that are not located on the surface, can influence the interaction between HLA and antibody. Thereby, the physiochemical properties of the amino acids also play a key role in this interaction and can thus influence the immunogenicity level. Therefore, understanding the interaction between HLA and antibody is pivotal, but currently detailed information is lacking, and so far, only one crystal structure of HLA and antibody has been published.

What We Still Need to Know Before Implementing HLA Molecular Matching

Consequently, several questions need to be addressed before implementing molecular mismatch scores at individual level. For one, what is the relative weight of specific molecular mismatch scores, especially considering that for many mismatches the clinical relevance remains unknown and scores will be population specific? Additionally, what constitutes the best match, a match with a low number of molecular mismatches or a match avoiding the most immunogenic mismatch? And, when is a match considered good enough and how long can a recipient wait for a donor with a more suitable molecular mismatch score?

Future Perspectives for HLA Molecular Matching in Transplantation

Overall, definition of molecular mismatches between donor and recipient have shown promise in predicting the risk of humoral allo-immune responses after transplantation. The immunological risk assessment may serve as biomarker for transplant recipients to determine whether they are eligible for lowering their immunosuppression and may be used for the selection of the most suitable donor in case of multiple donors, as in living donor setting. However, for the implementation of HLA molecular matching of deceased donor and recipient in solid organ transplantation, several pivotal questions remain unanswered. To accomplish this, combined efforts of researchers and clinicians are essential as well as international collaborations. In the end, HLA molecular matching is the future of solid organ transplantation, but the exact implementation needs to be defined by the experts in the field.

 

About the Author

Cynthia Kramer, PhD, Scientist at GenDx

Cynthia Kramer, PhD

Cynthia Kramer has worked in the field of HLA for more than 10 years and has a strong background in transplantation immunology.
During her time as a researcher at department of Immunology at Leiden University Medical Centre, she worked on the definition of
immunogenic HLA molecular mismatches (mainly eplets), the generation and characterization of human recombinant monoclonal
HLA class I & II antibodies, and innovation and improvement of organ allocation based on histocompatibility.

 

Relevant references

  1. Kramer CSM “Towards HLA epitope matching in clinical setting” (dissertations), 2020, Leiden, the Netherlands (ISBN 978-94-6375-988-5)
  2. Duquesnoy RJ, Askar M. HLAMatchmaker: a molecularly based algorithm for histocompatibility determination. V. Eplet matching for HLA-DR, HLA-DQ, and HLA-DP. Hum Immunol. 2007 Jan;68(1):12-25. doi: 10.1016/j.humimm.2006.10.003
  3. Kramer CSM, Koster J, Haasnoot GW, Roelen DL, Claas FHJ, Heidt S. HLA-EMMA: A user-friendly tool to analyse HLA class I and class II compatibility on the amino acid level. HLA. 2020 Jul;96(1):43-51. doi: 10.1111/tan.13883
  4. Kosmoliaptsis V, Chaudhry AN, Sharples LD, Halsall DJ, Dafforn TR, Bradley JA, Taylor CJ. Predicting HLA class I alloantigen immunogenicity from the number and physiochemical properties of amino acid polymorphisms. Transplantation. 2009 Sep 27;88(6):791-8. doi: 10.1097/TP.0b013e3181b4a9ff
  5. Mallon DH, Kling C, Robb M, Ellinghaus E, Bradley JA, Taylor CJ, Kabelitz D, Kosmoliaptsis V. Predicting Humoral Alloimmunity from Differences in Donor and Recipient HLA Surface Electrostatic Potential. J Immunol. 2018 Dec 15;201(12):3780-3792. doi: 10.4049/jimmunol.1800683
  6. Otten HG, Calis JJ, Keşmir C, van Zuilen AD, Spierings E. Predicted indirectly recognizable HLA epitopes presented by HLA-DR correlate with the de novo development of donor-specific HLA IgG antibodies after kidney transplantation. Hum Immunol. 2013 Mar;74(3):290-6. doi: 10.1016/j.humimm.2012
  7. Wiebe C, Kosmoliaptsis V, Pochinco D, Gibson IW, Ho J, Birk PE, Goldberg A, Karpinski M, Shaw J, Rush DN, Nickerson PW. HLA-DR/DQ molecular mismatch: A prognostic biomarker for primary alloimmunity. Am J Transplant. 2019 Jun;19(6):1708-1719. doi: 10.1111/ajt.15177
  8. Kramer CSM, Bezstarosti S, Franke-van Dijk MEI, Vergunst M, Roelen DL, Uyar-Mercankaya M, Voogt-Bakker KH, Heidt S. Antibody verification of HLA class I and class II eplets by human monoclonal HLA antibodies. HLA. 2024 Jan;103(1):e15345. doi: 10.1111/tan.15345
  9. Kosmoliaptsis V, Dafforn TR, Chaudhry AN, Halsall DJ, Bradley JA, Taylor CJ. High-resolution, three-dimensional modeling of human leukocyte antigen class I structure and surface electrostatic potential reveals the molecular basis for alloantibody binding epitopes. Hum Immunol. 2011 Nov;72(11):1049-59. doi: 10.1016/j.humimm.2011.07.303
  10. Meng T, Bezstarosti S, Singh U, Yap M, Scott L, Petrosyan N, Quiroz F, Eps NV, Hui EK, Suh D, Zhu Q, Pei R, Kramer CSM, Claas FHJ, Lowe D, Heidt S. Site-directed mutagenesis of HLA molecules reveals the functional epitope of a human HLA-A1/A36-specific monoclonal antibody. HLA. 2023 Feb;101(2):138-142. doi: 10.1111/tan.14895
  11. Matias EG, Austin DE, Franca KC, Maguire C, Lockhart JS, Lantz CW, Tambur AR. Not All Mismatches Were Created Equal: Empirical Identification of Regions Within HLA-DQ Molecules That Harbor the Highest Immunogenicity. Transplantation. 2026 Jan 15. doi: 10.1097/TP.0000000000005609
  12. Kramer CSM, Israeli M, Mulder A, Doxiadis IIN, Haasnoot GW, Heidt S, Claas FHJ. The long and winding road towards epitope matching in clinical transplantation. Transpl Int. 2019 Jan;32(1):16-24. doi: 10.1111/tri.13362