Recipient-specific T-cell repertoire reconstitution in the gut following murine hematopoietic cell transplant

Microbiota dictate T cell clonal selection to augment graft-versus-host disease after stem cell transplantation

Allogeneic T cell expansion is the primary determinant of graft-versus-host disease (GVHD), and current dogma dictates that this is driven by histocompatibility antigen disparities between donor and recipient. This paradigm represents a closed genetic system within which donor T cells interact with peptide-major histocompatibility complexes (MHCs), though clonal interrogation remains challenging due to the sparseness of the T cell repertoire. We developed a Bayesian model using donor and recipient T cell receptor (TCR) frequencies in murine stem cell transplant systems to define limited common expansion of T cell clones across genetically identical donor-recipient pairs. A subset of donor CD4+ T cell clonotypes differentially expanded in identical recipients and were microbiota dependent. Microbiota-specific T cells augmented GVHD lethality and could target microbial antigens presented by gastrointestinal epithelium during an alloreactive response. The microbiota serves as a source of cognate antigens that contribute to clonotypic T cell expansion and the induction of GVHD independent of donor-recipient genetics.

Tissue-specific features of the T cell repertoire after allogeneic hematopoietic cell transplantation in human and mouse

T cells are the central drivers of many inflammatory diseases, but the repertoire of tissue-resident T cells at sites of pathology in human organs remains poorly understood. We examined the site-specificity of T cell receptor (TCR) repertoires across tissues (5 to 18 tissues per patient) in prospectively collected autopsies of patients with and without graft-versus-host disease (GVHD), a potentially lethal tissue-targeting complication of allogeneic hematopoietic cell transplantation, and in mouse models of GVHD. Anatomic similarity between tissues was a key determinant of TCR repertoire composition within patients, independent of disease or transplant status. The T cells recovered from peripheral blood and spleens in patients and mice captured a limited portion of the TCR repertoire detected in tissues. Whereas few T cell clones were shared across patients, motif-based clustering revealed shared repertoire signatures across patients in a tissue-specific fashion. T cells at disease sites had a tissue-resident phenotype and were of donor origin based on single-cell chimerism analysis. These data demonstrate the complex composition of T cell populations that persist in human tissues at the end stage of an inflammatory disorder after lymphocyte-directed therapy. These findings also underscore the importance of studying T cell in tissues rather than blood for tissue-based pathologies and suggest the tissue-specific nature of both the endogenous and posttransplant T cell landscape.

Multimodal immune cell phenotyping in GI biopsies reveals microbiome-related T cell modulations in human GvHD

Acute graft-versus-host disease (aGvHD) is a significant complication after allogeneic hematopoietic stem cell transplantation (aHSCT), but major factors determining disease severity are not well defined yet. By combining multiplexed tissue imaging and single-cell RNA sequencing on gastrointestinal biopsies from aHSCT-treated individuals with fecal microbiome analysis, we link high microbiome diversity and the abundance of short-chain fatty acid-producing bacteria to the sustenance of suppressive regulatory T cells (Tregs). Furthermore, aGvHD severity strongly associates with the clonal expansion of mainly CD8 T cells, which we find distributed over anatomically distant regions of the gut, persistent over time, and inversely correlated with the presence of suppressive Tregs. Overall, our study highlights the pathophysiological importance of expanded CD8 T cell clones in the progression of aGvHD toward more severe clinical manifestations and strongly supports the further development of microbiome interventions as GvHD treatment via repopulation of the gut Treg niche to suppress inflammation.

Exosomes, MDSCs and Tregs: A new frontier for GVHD prevention and treatment

The development of graft versus host disease (GVHD) represents a long-standing complication of allogeneic hematopoietic cell transplantation (allo-HCT). Different approaches have been used to control the development of GVHD with most relying on variations of chemotherapy drugs to eliminate allo-reactive T cells. While these approaches have proven effective, it is generally accepted that safer, and less toxic GVHD prophylaxis drugs are required to reduce the health burden placed on allo-HCT recipients. In this review, we will summarize the emerging concepts revolving around three biologic-based therapies for GVHD using T regulatory cells (Tregs), myeloid-derived-suppressor-cells (MDSCs) and mesenchymal stromal cell (MSC) exosomes. This review will highlight how each specific modality is unique in its mechanism of action, but also share a common theme in their ability to preferentially activate and expand Treg populations in vivo. As these three GVHD prevention/treatment modalities continue their path toward clinical application, it is imperative the field understand both the biological advantages and disadvantages of each approach.

Donor T-Cell Repertoire Profiling in Recipient Lymphoid and Parenchyma Organs Reveals GVHD Pathogenesis at Clonal Levels After Bone Marrow Transplantation in Mice

The diversity and composition of T-cell receptor (TCR) repertoire, which is the result of V, (D), and J gene recombination in TCR gene locus, has been found to be implicated in T-cell responses in autoimmunity, cancer, and organ transplantation. The correlation of T-cell repertoire with the pathogenesis of graft-versus-host disease (GVHD) after allogeneic hematopoietic cell transplantation remains largely undefined. Here, by utilizing high-throughput sequencing of the genes encoding TCRβ-chain, we comprehensively analyzed the profile of T-cell repertoire in recipient lymphoid and GVHD target organs after bone marrow transplantation (BMT) in mice. In lymphoid organs, TCR diversity was narrowed, accompanied with reduced numbers of unique clones while increased accumulation of dominant clones in allogeneic T cells compared to syngeneic T cells. In an individual allogeneic recipient, donor-derived TCR clones were highly overlapped among tissue sites, and the degree of overlapping was increasing from day 7 to 14 after allogeneic BMT. The top clones in peripheral blood, gut, liver, and lungs were highly mutually shared in an allogenic recipient, indicating that blood has the potential to predict dominant clones in these GVHD target organs. T cells in GVHD target organs from allogeneic recipients had fewer overlapped clones with pre-transplant donor T cells compared to those from syngeneic recipients. Importantly, the top 10 clones in allogeneic recipients were not detectable in pre-transplant donor T cells, indicating clonal expansion of rare rearrangements. Interestingly, even starting from the same pool of donor repertoires, T cells had very few overlapped clones between each allogeneic recipient who developed completely different dominant clones. We were only able to trace a single clone shared by three replicate allogeneic recipients within the top 500 clones. Although dominant clones were different among allogeneic recipients, V26 genes were consistently used more frequently by TCR clones in allogeneic than syngeneic recipients. This is the first study to extensively examine the feature of T-cell repertoire in multiple lymphoid and parenchyma organs, which establishes the association between T-cell activation and GVHD pathogenesis at the level of TCR clones. Immune repertoire sequencing-based methods may represent a novel personalized strategy to guide diagnosis and therapy in GVHD.

Advances in single-cell sequencing: insights from organ transplantation

Single-cell RNA sequencing (scRNA-seq) is a comprehensive technical tool to analyze intracellular and intercellular interaction data by whole transcriptional profile analysis. Here, we describe the application in biomedical research, focusing on the immune system during organ transplantation and rejection. Unlike conventional transcriptome analysis, this method provides a full map of multiple cell populations in one specific tissue and presents a dynamic and transient unbiased method to explore the progression of allograft dysfunction, starting from the stress response to final graft failure. This promising sequencing technology remarkably improves individualized organ rejection treatment by identifying decisive cellular subgroups and cell-specific interactions.

GVHD Pathogenesis, Prevention and Treatment: Lessons From Humanized Mouse Transplant Models

Graft-vs-host disease (GVHD) is the most common cause of non-relapse mortality following allogeneic hematopoietic stem cell transplantation (HSCT) despite advances in conditioning regimens, HLA genotyping and immune suppression. While murine studies have yielded important insights into the cellular responses of GVHD, differences between murine and human biology has hindered the translation of novel therapies into the clinic. Recently, the field has expanded the ability to investigate primary human T cell responses through the transplantation of human T cells into immunodeficient mice. These xenogeneic HSCT models benefit from the human T cell receptors, CD4 and CD8 proteins having cross-reactivity to murine MHC in addition to several cytokines and co-stimulatory proteins. This has allowed for the direct assessment of key factors in GVHD pathogenesis to be investigated prior to entering clinical trials. In this review, we will summarize the current state of clinical GVHD research and discuss how xenogeneic HSCT models will aid in advancing the current pipeline of novel GVHD prophylaxis therapies into the clinic.