Symposium 7.2 – Recent advances in kidney transplantation: focus on immunity

Symposium Summary

Written by Jasna Trbojevic-Stankovic
All the speakers reviewed and approved the contents

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NON-HLA incompatibilities: what’s new?

Rainer Oberbauer, Austria

Human leukocyte antigen (HLA) molecules are expressed on almost all nucleated cells, representing the major molecules that initiate graft rejection. HLA matching reduces the risk of rejection, contributes to better graft function and longer graft survival, and permits reduced immunosuppression. HLA matching has had the greatest clinical impact in kidney and bone marrow transplantation. Nevertheless, finding a well-matched donor may not be possible in all cases and often prolongs the waiting time.

Figure 1. The quantitative approach to HLA matching

In approximately one-half of histologic antibody-mediated rejection (ABMR) cases circulating HLA donor-specific antibodies (DSA) are not detectable by the current methodology at the time of biopsy. Three main reasons have been identified for failing to detect HLA-DSAs: failure to type all eleven HLA loci in donor and recipient, absorption of HLA-DSA by the allograft, and HLA-DSA directed against a very public epitope expressed on many single antigen beads (SABs) diluting down the mean fluorescence intensity (MFIs) of any single bead. Nevertheless, ABMR histology is not solely determined by the histomolecular presentation but is also predicted by the underlying etiologic factor irrespective of HLA-DSA status. Thus, it appears that non-HLA immunity has a much stronger role in clinical transplantation than previously thought and is primarily associated with chronic graft loss. Recently published works suggest a broader concept of immunological non-self that goes beyond HLA incompatibility and expands the current concept of polymorphic non-self epitopes on cell surface molecules from HLA to non-HLA targets.

A quantitative concept of B cell epitope, developed to assess immunological risk, showed superiority in terms of prediction of development of ABMR compared to HLA antigen mismatch. Eplets represent the central parts of an epitope, consisting of a few amino acids located on the antibody-accessible site of the HLA-molecule, thus presenting the smallest functional unit of the antibody-antigen binding site and deciding the antibody specificity. To provide large cohorts to investigate the genetic architecture of comorbidities that may impact graft and recipient life expectancy, the International Genetics & Translational Research in Transplantation Network has been assembled. This consortium delivered pioneering insights into the genetic architecture of transplant-related outcomes across a range of different solid-organ transplant studies. Namely, genetic mismatch of non-HLA haplotypes coding for transmembrane or secreted proteins are associated with an increased risk of functional graft loss independently of HLA incompatibility. As in HLA alloimmunity, donor-specific alloantibodies can be identified against genotype-derived non-HLA epitopes.

The genetic heterogeneity of HLA and non-HLA alleles prevents further optimization of transplantation outcomes through histocompatibility matching. Tolerance induction by mixed chimerism without toxic conditioning and with a low risk of graft versus host disease is a visionary but realistic goal. The ongoing Vienna Trex study is expected to shed more light on this topic.

Targeting immuno-inflammation in kidney transplantation: a new weapon against organ fibrosis

Gianluigi Zaza, Italy

Kidney allograft fibrosis or interstitial fibrosis (jointly to tubular atrophy, IF/TA) is a complex, dynamic and progressive process characterized by deep remodelling, production, and deposition of extracellular matrix (ECM) resulting in disruption of the tissue architecture that leads to organ failure. IF/TA is detectable in about 40% of kidney allografts at 3-6 months and increases to about 65% at two years. For more than two decades calcineurin inhibitors (CNI) nephrotoxicity has been considered the main cause of allograft fibrosis. However, in the last few years, thanks to the introduction of new biomolecular technologies, inflammation in scarred and fibrotic parenchymal areas has been recognized as a pivotal element able to accelerate the onset and development of the allograft chronic damage.

The intra-graft inflammation may drive the development of fibrotic damage, a process characterized by four distinct phases. In the first, “trigger phase”, tubular and glomerular cells produce pro-inflammatory cytokines, which facilitate the recruitment of new interstitial mononuclear cells. Then, M1 macrophages release cytokines and chemokines with pro-fibrotic potentials, whereas M2 macrophages release TGF-β which induces the expression of pro-fibrotic genes. During the second “activation phase”, myofibroblasts, fibroblasts, fibrocytes, bone marrow-derived cells, epithelial cells, endothelial cells, and pericytes are activated by pro-fibrotic cytokines and growth factors secreted by lymphocytes upon injury of the endothelium. The third, “formation phase”, is characterized by the excessive production and deposition of interstitial matrix and formation of collagen fibers enhancing the pro-fibrotic network. Lastly, during the “progression phase”, the collagen matrix is susceptible to proteolysis, thus making the fibrosis potentially reversible. As fibrosis progresses, the fibrotic kidney matrix becomes a cause of irreversibility and stabilization. During stabilization, matrix proteins are induced by enzymes, which make them rigid and resistant to proteolysis.

A large number of innovative pharmacological agents have been proposed to slow down the progression of allograft fibrosis, including pirfenidone, THR-123, pentoxifylline, tranilast, neutralizing antibodies against different isoforms of TGF-β and anti-TNF-α monoclonal antibodies.

Figure 2. Schematic representation of the major biological elements involved in kidney allograft fibrosis

There is also a close link between oxidative stress, dysfunctional mitochondria, inflammation, and kidney allograft fibrosis. Furthermore, oxidative stress plays a key role in chronic CNI toxicity. Among the most important biological elements involved in CNI toxicity is NADPH oxidase 2, a protein that generates superoxide reactive oxygen species whose synthesis is mediated by TGF-B. There are a large number of antioxidants available as dietary supplements on the market able to reduce mitochondrial dysregulation/ oxidative stress and the consequent activation of the pro-fibrotic machinery. Unfortunately, these antioxidants are not able to reach adequate mitochondrial concentrations. However, there are several molecules currently under development synthesized by the conjugation of antioxidants with the triphenylphosphonium lipophilic cation (TPP) that can cross biological membranes and, thanks to their positive charge, enter the mitochondria.

A series of studies are currently underway aimed at analysing and identifying new elements potentially involved in the extensive pro-fibrotic pathway of the transplanted kidney (such as heparanase). Key elements of the immune-inflammatory machinery associated with fibrosis may represent good therapeutic candidates, but additional studies are still needed.

Immunologic monitoring and biomarkers in kidney transplant recipients

Chan-Duck Kim, Rep. of South Korea,

Transplant biopsy has always been the gold standard for assessing the immune response to a kidney allograft. However, the procedure is not without risk, it is unable to predict rejection, and is only diagnostic once rejection has already commenced. Fortunately, there is a wide range of novel methods and biomarkers for post-transplant immunologic monitoring which are divided into antigen-specific (i.e. anti-HLA antibodies, non-HLA DSA, MLR, and ELISPOT) and non-antigen-specific assays (i.e. ATP measurement, soluble CD30, flow cytometry, gene expression monitoring and OMICs study of biomarkers).

Detection of a circulating anti-HLA antibody is now a widely used immunologic monitoring assay in the clinical setting. The presence of anti-HLA DSAs is considered a diagnostic criterion for antibody-mediated rejection (AMR). According to the Transplantation Society (TTS) Consensus guideline, the desensitized, high-risk patients should be monitored by DSA and protocol biopsy in the first 3 months after kidney transplantation (KT). Intermediate-risk patients, with a history of DSA but currently negative, should be monitored for DSA within the first month and if DSAs are present, a biopsy should be performed. Low-risk patients (non-sensitized, first KT) should be screened for DSA at least once in 3-12 months after KT. If DSA is present, a biopsy should be performed.

OMICs study is also a highly effective tool in the hunt for biomarkers as many biological molecules are expressed during the process of acute rejection which can be detected by this procedure. The candidate molecules for the ideal biomarker should be detectable early in the allograft rejection process and be able to differentiate rejection from other causes of allograft dysfunction.

Figure 3. Potential biomarkers for acute rejection, chronic allograft dysfunction, and IF/TA

To achieve transplantation tolerance, the “Holy Grail” of transplantation, it is first necessary to have a reliable and reproducible method for detecting a biomarker that can identify recipients in whom tolerance is likely to occur. The ideal tool for clinical monitoring should be non-invasive, inexpensive, reproducible, and accessible to clinicians and patients. Currently, there is no optimal immunological monitoring method, but promising advancements have been achieved over the past few years. With the development of these technologies, understanding the strengths and weaknesses of each test will allow clinicians to integrate new monitoring methods with a clinical assessment to achieve the best long-term outcomes in transplant recipients.

Further reading

Granata S, Benedetti C, Gambaro G, Zaza G. Kidney allograft fibrosis: what we learned from latest translational research studies. J Nephrol. 2020;33(6):1201-1211. doi:10.1007/s40620-020-00726-z

Van Linthout S, Miteva K, Tschöpe C. Crosstalk between fibroblasts and inflammatory cells. Cardiovasc Res. 2014;102(2):258-269. doi:10.1093/cvr/cvu062

Liu Y. Cellular and molecular mechanisms of renal fibrosis. Nat Rev Nephrol. 2011;7(12):684-696. doi:10.1038/nrneph.2011.149

Zepeda-Orozco D, Kong M, Scheuermann RH. Molecular Profile of Mitochondrial Dysfunction in Kidney Transplant Biopsies Is Associated With Poor Allograft Outcome. Transplant Proc. 2015;47(6):1675-1682. doi:10.1016/j.transproceed.2015.04.086

Djamali A, Reese S, Hafez O, et al. Nox2 is a mediator of chronic CsA nephrotoxicity. Am J Transplant. 2012;12(8):1997-2007. doi:10.1111/j.1600-6143.2012.04081.x

Townamchai N, Safa K, Chandraker A. Immunologic monitoring in kidney transplant recipients. Kidney Res Clin Pract. 2013;32(2):52-61. doi:10.1016/j.krcp.2013.04.002

Lim JH, Lee CH, Kim KY, et al. Novel urinary exosomal biomarkers of acute T cell-mediated rejection in kidney transplant recipients: A cross-sectional study. PLoS One. 2018;13(9):e0204204. doi:10.1371/journal.pone.0204204

Ko EJ, Seo JW, Kim KW, et al. Phenotype and molecular signature of CD8+ T cell subsets in T cell- mediated rejections after kidney transplantation. PLoS One. 2020;15(6):e0234323.

Filippone EJ, Farber JL. Histologic Antibody-Mediated Kidney Allograft Rejection in the Absence of Donor Specific HLA Antibodies. Transplantation. 2021 (ahead of print). doi: 10.1097/TP.0000000000003797.

Reindl-Schwaighofer R, Heinzel A, Gualdoni GA, Mesnard L, Claas FHJ, Oberbauer R. Novel insights into non-HLA alloimmunity in kidney transplantation. Transpl Int. 2020;33(1):5-17. doi: 10.1111/tri.13546.

International Genetics & Translational Research in Transplantation Network (iGeneTRAiN). Design and Implementation of the International Genetics and Translational Research in Transplantation Network. Transplantation. 2015;99(11):2401-12. doi: 10.1097/TP.0000000000000913.

Reindl-Schwaighofer R, Heinzel A, Kainz A; iGeneTRAiN consortium. Contribution of non-HLA incompatibility between donor and recipient to kidney allograft survival: genome-wide analysis in a prospective cohort. Lancet. 2019;393(10174):910-917. doi: 10.1016/S0140-6736(18)32473-5.