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Unraveling the spectrum of microvascular kidney transplant rejection

Boek - Dissertatie

Despite the development of potent immunosuppressive therapy, rejection remains the most frequent cause of premature graft loss after kidney transplantation, highlighting the need to better understand the underlying mechanisms. The current paradigm of kidney transplant rejection distinguishes between T-cell mediated rejection (TCMR), referring to tubulointerstitial inflammation caused by alloreactive T lymphocytes, and antibody-mediated rejection (ABMR), referring to microvascular inflammation caused by donor-specific antibodies (HLA-DSA). However, several observations challenge this paradigm. A particularly challenging problem is microvascular inflammation. Considered as a consequence of antibody-mediated injury, microvascular inflammation is one the most important predictors for late graft failure. However, a significant proportion of patients with microvascular inflammation do not have circulating HLA-DSA. Currently, microvascular inflammation in absence of HLA-DSA is not recognized by the international Banff classification, which puts this phenotype at risk of becoming a blind spot, delaying further diagnostic and therapeutic steps. This doctoral project aimed at unravelling the phenotypes and mechanisms of microvascular inflammation. Our first objective was to provide a precise characterization of microvascular inflammation through a series of epidemiological and molecular studies. In our second objective, we obtained new insights into the etiology and pathophysiology of microvascular inflammation. In the first part of this dissertation, we performed a critical evaluation of the changes in the Banff classification for antibody-mediated rejection, a diagnostic tool for allograft rejection that is implemented in routine clinical practice. We found that the prognostic performance of the current Banff classification decreased in comparison to previous versions, due to the removal of the "suspicious for ABMR" category, which consisted mainly of HLA-DSA negative microvascular inflammation cases. From this, we conclude that a binary classification of ABMR oversimplifies the heterogeneity of this disease. Next, we sought to clarify whether microvascular inflammation in the absence of HLA-DSA should be considered as a separate molecular entity. Interestingly, there were no differences in the molecular landscape of HLA-DSA negative compared to HLA-DSA positive microvascular inflammation. Despite this transcriptional similarity, outcome after microvascular inflammation was worse for HLA-DSA positive patients, suggesting that the underlying stimulus is different in HLA-DSA negative patients. The transcriptional profiling of microvascular inflammation illustrated a clear distinction from other phenotypes, suggesting that molecular transcripts could be used as a diagnostic tool. To facilitate the implementation in clinical practice, we developed and validated a rigid prediction model to measure molecular classifiers for ABMR and TCMR incorporating as few transcripts as necessary. A 2-gene classifier was derived and validated for ABMR, consisting of PLA1A and GNLY, and for TCMR, consisting of IL12RB1 and ARPC1B. The ABMR and TCMR molecular scores were not only relevant for diagnostic purposes, but associated with allograft prognosis, identifying kidney transplants at risk for graft failure that did not fulfill the histological criteria for ABMR and TCMR. The second part of this dissertation aimed at increasing mechanistic insight in the causes of microvascular inflammation. By microarray analysis and deconvolution methods, we elucidated the pathways and repertoire of graft-infiltrating immune cells that distinguish ABMR from TCMR. A specific association was found between microvascular inflammation and NK cell infiltration, which was also a cardinal predictor for allograft failure. The conclusions of this study placed NK cells, a previously underappreciated cell type in rejection, at the forefront of ABMR research, and identified a potential therapeutic target to improve outcome after kidney transplantation. Having identified a central role for NK cells, and the conundrum of HLA-DSA negative microvascular inflammation, we further developed the hypothesis that missing self, an antibody-independent NK cell stimulus, could be involved in the onset of microvascular inflammation. This hypothesis was recently suggested by in vitro and animal studies, but had not yet been evaluated in larger clinical cohorts. In a population-based study, we demonstrated that missing self cumulatively increased the risk of microvascular inflammation after kidney transplantation, independent from other NK cell stimuli, such as HLA-DSA or CMV disease. Complementing a growing body of evidence, these results strongly indicate that microvascular inflammation related to missing self can be considered a form of rejection that is distinct from ABMR and TCMR. Finally, we aimed to uncover the deep phenotypical and spatial profile of immune cell infiltration in rejection. Using single cell RNA sequencing, we constructed a comprehensive cellular atlas of the human kidney allograft in stable and pathological conditions, hereby identifying the specific expression of pro-inflammatory pathways in CD16bright NK cells and non-classical monocytes in microvascular inflammation. We derived gene expression signatures to study the specific associations between leukocyte types and rejection phenotypes in an unbiased microarray cohort. Interestingly, we noted important variability in the cellular composition of the immune infiltrate in biopsies with similar light microscopic appearances. As most of this heterogeneity remained unexplained, our study indicated a level of complexity beyond the current dichotomous paradigm of rejection. Instead, we argue that it would be more appropriate to adopt a diagnostic spectrum of rejection that acknowledges the potential concurrence of several alloimmune stimuli and heterogeneity in the resulting immune cell activation.
Jaar van publicatie:2022
Toegankelijkheid:Open