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Editorials
Learning the next-generation sequencing alphabet of immune reconstitution: factors determining CD8+ T-cell receptor α-chain repertoire dynamics after hematopoietic stem cell transplantation
Esteban Arrieta-Bolaños1 and Katharina Fleischhauer1,2
1Institute for Experimental Cellular Therapy, University Hospital Essen and 2German Cancer Consortium, Heidelberg, Germany
E-mail: KATHARINA FLEISCHHAUER - katharina.fleischhauer@uk-essen.de doi:10.3324/haematol.2018.209130
In this issue of Haematologica, Link-Rachner et al.1 report their findings on CD8+ T-cell receptor-alpha (TRα) chain dynamics in patients after hematopoietic stem cell transplantation (HSCT) using next-generation sequencing (NGS) in relation to different treatment plat- forms. Their study aimed at unraveling the effect of post- transplant T-cell-depleting immunosuppressive therapy (namely anti-thymocyte globulin, ATG, and post-trans- plant cyclophosphamide, PTCy) and degree of HLA matching on the TRα diversity of naïve and memory CD8+ T-cell repertoires reconstituting the patient’s peripheral blood in the first six months after transplanta- tion. Furthermore, the authors attempted to determine the extent to which the donor’s TRα repertoire influences the post-transplant repertoires in the respective patients.
The TR and its huge variability is one of the pillars of adaptive immunity. Healthy, diverse TR repertoires in nor- mal individuals contain millions of different clones with unique TRs,2 and provide the immune system with an arsenal of highly specific, yet also cross-reactive cells to fight off pathogens and malignant cells. Until recently, this extreme diversity limited a detailed and deep analysis of TR repertoires in healthy and pathological conditions, with most available techniques focusing on broad reper- toire alterations, and extensive, cumbersome T-cell cloning required to investigate specific complementarity- determining region 3 (CDR3) variants. The advent of NGS-based high-throughput analysis of TRs has revolu- tionized the field of immune repertoire analysis.3 TR NGS can now provide qualitative and quantitative information on hundreds of thousands of different T-cell clones direct- ly from a single blood or tissue sample.
Hematopoietic stem cell transplantation is a field in which TR analysis is of extreme interest, both for medical and biological reasons. Patients undergoing allogeneic HSCT see the partial or nearly total elimination of their own hematopoietic system with radio- and/or chemotherapy followed by its replacement with that of a donor. In most cases, HSCT is the only curative therapy for the underlying disease. However, HSCT poses several risks for the patient, many of which derive from the abla- tion of their bone marrow and the concomitant risk of infection and pathogen reactivation. In addition, the new hematopoietic system can induce graft-versus-host dis- ease (GvHD) associated with tissue and organ damage and, in some cases, death.4 T-cell reconstitution dynamics is central to these post-transplant immune processes and represents an area of intense research in the HSCT field. High-throughput TR NGS has thus quickly attracted researchers eager to use its power to study post-HSCT T-
cell clonal dynamics, its relationship to transplant-related factors, and its role in transplant complications.5
The study by Link-Rachner et al. contains a number of noteworthy aspects. Contrary to most previous reports, the researchers focus on the TRα chain. Most published TR analyses have studied the TRb chain, probably because it is considered more diverse due to the added combinatorial potential conferred by the D segment, but also perhaps on account of the fact that TR NGS method- ologies for this chain are more extended. However, both the α and the b chains contribute to TR specificity and, because of the order of the recombination events during T-cell maturation and development, one mature T cell can express two different functional α chains, both of which can pair with the cell’s b chain, forming two different TR heterodimers.6 Indeed, this happens in approximately 10% of the T cells in peripheral blood, and TRα CDR3 diversity has actually been observed to be 1.2-2.4 times greater than TRb in T-cell subsets from a single individ- ual.7 Importantly, these ‘dual’ T cells have been associated with increased autoimmune and alloreactive capacities,8 as well as with acute9 and chronic10 GvHD. Hence, analy- sis of TRα chains in HSCT is of special interest, and this study should encourage researchers to pay more attention to this locus in future analyses.
Furthermore, the authors designed their study in order to perform a comparison of TR dynamics between differ- ent clinical platforms. By including in their analysis patients transplanted from HLA-matched and -mis- matched unrelated donors treated or not treated with ATG, haploidentical donors treated with PTCy, and relat- ed donors with no T-cell depletion (TD), the authors pro- vide valuable insights into how these clinical platforms differ in terms of T-cell and TR dynamics during immune reconstitution at different time points in the first six months after HSCT. Despite the limited sample size, each of the groups was analyzed separately, leading to some interesting observations. First, the data suggest that TD has a stronger effect on the reconstitution of naïve than of memory T cells in terms of numbers and TRα diversity; both were significantly lower in patients treated with TD (Figure 1). Considering that diversity is deemed a hall- mark of a ‘healthy’ T-cell repertoire,11 this raises questions as to ways in which one could improve TR diversity in patients undergoing HSCT platforms with in vivo TD, as already suggested in the context of TRαb-depleted grafts.12
In addition, the data from Link-Rachner et al. provide some intriguing new insights into the relative contribu- tion of the donor’s memory and naïve T-cell repertoires to
haematologica | 2019; 104(3)