Analysis of T-Cell Receptor Repertoire in Transplantation: Fingerprint of T Cell-mediated Alloresponse

T cells play a key role in determining allograft function by mediating allogeneic immune responses to cause rejection, and recent work pointed their role in mediating tolerance in transplantation. The unique T-cell receptor (TCR) expressed on the surface of each T cell determines the antigen specificity of the cell and can be the specific fingerprint for identifying and monitoring. Next-generation sequencing (NGS) techniques provide powerful tools for deep and high-throughput TCR profiling, and facilitate to depict the entire T cell repertoire profile and trace antigen-specific T cells in circulation and local tissues. Tailing T cell transcriptomes and TCR sequences at the single cell level provides a full landscape of alloreactive T-cell clones development and biofunction in alloresponse. Here, we review the recent advances in TCR sequencing techniques and computational tools, as well as the recent discovery in overall TCR profile and antigen-specific T cells tracking in transplantation. We further discuss the challenges and potential of using TCR sequencing-based assays to profile alloreactive TCR repertoire as the fingerprint for immune monitoring and prediction of rejection and tolerance.

Identification and tracking of alloreactive T cell clones in Rhesus Macaques through the RM-scTCR-Seq platform.

T cell receptor clonotype tracking is a powerful tool for interrogating T cell mediated immune processes. New methods to pair a single cells transcriptional program with its T cell receptor (TCR) identity allow monitoring of T cell clonotype-specific transcriptional dynamics. While these technologies have been available for human and mouse T cells studies, they have not been developed for Rhesus Macaques, a critical translational organism for autoimmune diseases, vaccine development and transplantation. We describe a new pipeline, RM-scTCR-Seq, which, for the first time, enables RM specific single cell TCR amplification, reconstitution and pairing of RM TCRs with their transcriptional profiles. We apply this method to a RM model of GVHD, and identify and track in vitro detected alloreactive clonotypes in GVHD target organs and explore their GVHD driven cytotoxic T cell signature. This novel, state-of-the-art platform fundamentally advances the utility of RM to study protective and pathogenic T cell responses.

The Effects of Interferons on Allogeneic T Cell Response in GVHD: The Multifaced Biology and Epigenetic Regulations

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a potentially curative therapy for hematological malignancies. This beneficial effect is derived mainly from graft-versus-leukemia (GVL) effects mediated by alloreactive T cells. However, these alloreactive T cells can also induce graft-versus-host disease (GVHD), a life-threatening complication after allo-HSCT. Significant progress has been made in the dissociation of GVL effects from GVHD by modulating alloreactive T cell immunity. However, many factors may influence alloreactive T cell responses in the host undergoing allo-HSCT, including the interaction of alloreactive T cells with both donor and recipient hematopoietic cells and host non-hematopoietic tissues, cytokines, chemokines and inflammatory mediators. Interferons (IFNs), including type I IFNs and IFN-γ, primarily produced by monocytes, dendritic cells and T cells, play essential roles in regulating alloreactive T cell differentiation and function. Many studies have shown pleiotropic effects of IFNs on allogeneic T cell responses during GVH reaction. Epigenetic mechanisms, such as DNA methylation and histone modifications, are important to regulate IFNs’ production and function during GVHD. In this review, we discuss recent findings from preclinical models and clinical studies that characterize T cell responses regulated by IFNs and epigenetic mechanisms, and further discuss pharmacological approaches that modulate epigenetic effects in the setting of allo-HSCT.

Genetic Manipulation Resulting in Decreased Donor Chondroitin sulfate Synthesis Mitigates Gvhd Following Allogeneic Hematopoietic Cell Transplantation in a Murine Model.

Introduction: Severe acute graft versus host disease (GVHD) represents a major risk associated with allogeneic hematopoietic cell transplantation (HSCT). Both acute and progressive GVHD following HSCT are in large part attributable to responses of donor T cells to host allo-antigens. Alloreactive T cell activation can be modulated by signals from the tissue microenvironment (TME). Glycosaminoglycans (GAGs) in the TME or modifying T cell surface proteoglycans are known to regulate T cell function. However, the roles of GAGs in acute GVHD following allogeneic HSCT remain to be elucidated, since a suitable murine model has not been available. We previously established a unique mouse model (T1KO), in which knockout of the chondroitin sulfate (CS) N-acetylgalactosaminyltransferase-1 (T1) gene - encoding the rate-limiting CS-synthesizing enzyme - decreases CS production. We also reported a role for CS in murine hematopoietic stem cells. In the present study, we focus on donor T cell CS levels to assess the role of T cell CS in acute GVHD following allogeneic HSCT. We were able to mitigate the clinical features of GVHD in a murine model using T1KO donors. Methods: Eight- to 12-week-old T1KO mice were generated from a C57BL/6N (WT) strain as donors (H-2b) for allogeneic bone marrow transplantation (BMT). Donor BM cells were then transplanted into eight- to 12-week-old BALB/c recipients (H-2d). Prior to transplantation, recipients were irradiated at a dose of 7 Gy. They then received 5 x 106 BM cells and splenocytes of identical genotype, as well as low (5 x 105), intermediate (1 x 106), or high (4 x 106) doses of CD90.2+ cells from either WT or T1KO donors. For five weeks following BMT, survival was monitored daily and clinical GVHD scores (incorporating body weight, activity, fur condition, alopecia, and tortoiseshell-like mucosae) were calculated three times per week. Results: The peripheral blood CD4+/CD8+ T cell ratio of T1KO mice was equivalent to that of WT mice. Following allogeneic BMT, histopathologic analysis confirmed onset of acute recipient GVHD on day 49 when donors were WT mice. As a result, significant lymphocyte infiltration was observed in target organs (liver, colon, and skin). High-dose transplantation of splenocytes from T1KO rather than WT mice significantly prolonged recipient median survival (27.0 versus 20.5 days, p = 0.02). A similar trend was observed for low- and intermediate-dose transplantation (low: 53.0 versus 29.0 days, p = 0.09; intermediate: 50.5 versus 30.5 days, p = 0.13. Similarly, a significant improvement in day 11 GVHD score was observed following high-dose splenocyte transplantation when donors were T1KO mice (mean scores: 1.00 versus 2.75, p < 0.01). However, no significant differences in GVHD score were observed following low- and intermediate-dose transplantation. Taken together, our results suggest that donor T cells producing lower CS levels alleviate acute GVHD following allogeneic HSCT in a murine model. Conclusion: Alloreactive T cell activation following allogeneic HSCT may be down-regulated by lower CS levels, thereby mitigating GVHD severity. Figure 1 Disclosures No relevant conflicts of interest to declare.

Permissive HLA-DPB1 mismatches in HCT depend on immunopeptidome divergence and editing by HLA-DM

In hematopoietic cell transplantation (HCT), permissive HLA-DPB1 mismatches between patients and their unrelated donors (UD) are associated with improved outcomes compared to non-permissive mismatches, but the underlying mechanism is incompletely understood. Here we used mass spectrometry, T-cell receptor-beta (TCRb) deep sequencing, and cellular in vitro models of alloreactivity to interrogate the HLA-DP immunopeptidome and its role in alloreactive T cell responses. We find that permissive HLA-DPB1 mismatches display significantly higher peptide repertoire overlaps compared to their non-permissive counterparts, resulting in lower frequency and diversity of alloreactive TCRb clonotypes in healthy individuals and transplanted patients. Permissiveness can be reversed by the absence of the peptide editor HLA-DM, or the presence of its antagonist HLA-DO, through significant broadening of the peptide repertoire. Our data establish the degree of immunopeptidome divergence between donor and recipient as the mechanistic basis for the clinically relevant permissive HLA-DPB1 mismatches in HCT, and show that permissiveness is dependent on HLA-DM-mediated peptide editing. Its key role for harnessing T-cell alloreactivity to HLA-DP highlights HLA-DM as a potential novel target for cellular and immunotherapy of leukemia.

Non-Hematopoietic Lymphoid Stromal Cells Prime Alloreactive CD4+ T Cells in Acute Graft-Versus-Host Disease

Allogeneic T cell priming is considered as an essential event determining the outcome of allogeneic hematopoietic stem cell transplantation (allo-HCT), ideally triggering anti-leukemic responses (GvL effect) or, at worst, causing life-threatening acute graft-versus-host disease (aGvHD). During acute GvHD initiation, alloreactive T cells are activated by host antigen presenting cells (APCs), rapidly expand and subsequently exert tissue damage. Recently, it was discovered that absence of host hematopoietic APCs does not prevent acute GvHD, suggesting a crucial role of non-hematopoietic APCs for priming alloreactive T cells (Toubai et al., Blood 2012, Li et al., J Immunol. 2012). Furthermore, it was even suggested that in the absence of professional APCs allogeneic CD4+ T cells can be activated in the lamina propria by MHC class II expressing myofibroblasts (Koyama et al., Nat Med 2012). As exact location and identity of host non-hematopoietic APCs triggering alloreactive T cell responses are essential to dissect the priming of GvHD-inducing vs. GvL-mediating allogeneic T cells, we investigated the role of lymph node stromal cells (LNSCs) in the initiation phase of aGvHD and their potential role as non-hematopoietic APCs. Employing allo-HCT mouse models in combination with flow cytometry and advanced microscopy techniques, we explored early alloreactive T cells activation first in a myeloablatively conditioned MHC major mismatch allo-HCT setting (FVB→B6). Under these conditions, CD4+ and CD8+ T cells activation and proliferation occurred exclusively in secondary lymphoid organs (SLOs) and not in the intestinal lamina propria early after allo-HCT within the first three days after allo-HCT. To study non-hematopoietic antigen presentation early after allo-HCT, we generated bone marrow B6.MHCIIΔ/Δ→B6.WT chimeras that lacked MHC II expression in the host hematopoietic compartment. Subsequent allo-HCT of these chimeras revealed activation and expansion of allogenic donor CD4+ T-cells exclusively in SLOs and not the lamina propria in the first three days after transplantation. Next, we generated recipient mice that selectively lacked MHCII expression either in CD11c expressing cells or under the control of the Vav1-promoter in all hematopoietic cells. In both type of recipients, allogenic donor CD4+ T-cells were activated within 3 days after allo-HCT in SLOs and no other tissues. After irradiation of B6.WT mice we observed LNSCs upregulate co-stimulatory receptors early after irradiation (24 and 72 hours), in vitro and in vivo suggesting that they may act as active non-hematopoietic APCs. Next, we performed mixed lymphocyte reactions (MLRs) of irradiated APCs derived from TgVav1-Cre+MHCIIΔ/Δmice with alloreactive CD4+ T cells from FVB mice. Here, we observed in the total absence of hematopoietic MHCII antigen presentation reduced but still pronounced activation of alloreactive CD4+ T cells. Therefore, in these transgenic allo-HCT models, hematopoietic cells in the SLOs were dispensable for alloreactive CD4+ T cell activation in the initiation phase of aGvHD. To ascertain that SLOs are the exclusive priming sites of alloreactive CD4+ T cells, we transplanted mesenteric lymph nodes (mLNs) from a B6.CD11c.DTR mice, depleted of CD11c+ cells into B6.MHCIIΔ/Δmice. After successful engraftment of donor mLNs in these mice, allo-HCT revealed these as unique priming sites in MHCII deficient hosts for alloreactive CD4+ T cells, which differentiated into CD44hiCD62Llow effector T cells that were detectable in the transplanted mLNs. Conclusively, these results indicate that specialized non-hematopoietic lymph node stromal cells prime alloreactive CD4+ T cell within the SLOs early after allo-HCT. Pinpointing the molecular pathways in these non-hematopoietic APCs within SLOs that trigger alloreactive T cell responses may prove fruitful for selective therapeutic intervention after allo-HCT. Disclosures No relevant conflicts of interest to declare.

Resilience of T cell-intrinsic dysfunction in transplantation tolerance

Following antigen stimulation, naïve T cells differentiate into memory cells that mediate antigen clearance more efficiently upon repeat encounter. Donor-specific tolerance can be achieved in a subset of transplant recipients, but some of these grafts are rejected after years of stability, often following infections. Whether T cell memory can develop from a tolerant state and whether these formerly tolerant patients develop antidonor memory is not known. Using a mouse model of cardiac transplantation in which donor-specific tolerance is induced with costimulation blockade (CoB) plus donor-specific transfusion (DST), we have previously shown that systemic infection with Listeria monocytogenes (Lm) months after transplantation can erode or transiently abrogate established tolerance. In this study, we tracked donor-reactive T cells to investigate whether memory can be induced when alloreactive T cells are activated in the setting of tolerance. We show alloreactive T cells persist after induction of cardiac transplantation tolerance, but fail to acquire a memory phenotype despite becoming antigen experienced. Instead, donor-reactive T cells develop T cell-intrinsic dysfunction evidenced when removed from the tolerant environment. Notably, Lm infection after tolerance did not rescue alloreactive T cell memory differentiation or functionality. CoB and antigen persistence were sufficient together but not separately to achieve alloreactive T cell dysfunction, and conventional immunosuppression could substitute for CoB. Antigen persistence was required, as early but not late surgical allograft removal precluded the acquisition of T cell dysfunction. Our results demonstrate transplant tolerance-associated T cell-intrinsic dysfunction that is resistant to memory development even after Lm-mediated disruption of tolerance.