Dynamic establishment and maintenance of the human intestinal B cell population and repertoire following transplantation

It is unknown how intestinal B cell populations and B cell receptor (BCR) repertoires are established and maintained over time in humans. Following intestinal transplantation (ITx), surveillance ileal mucosal biopsies provide a unique opportunity to map the dynamic establishment of gut lymphocyte populations. Using polychromatic flow cytometry that includes HLA allele group-specific mAbs distinguishing donor from recipient cells along with high throughput BCR sequencing, we tracked the establishment of recipient B cell populations and BCR repertoire in the allograft mucosa of ITx recipients. We confirm the early presence of naïve donor B cells in the circulation and, for the first time, document the establishment of recipient B cell populations, including B resident memory cells, in the intestinal allograft mucosa. Recipient B cell repopulation of the allograft was most rapid in infant (<1 year old)-derived allografts and, unlike T cell repopulation, did not correlate with rejection rates. While recipient memory B cell populations were increased in graft mucosa compared to circulation, naïve recipient B cells remained detectable in the graft mucosa for years. Comparisons of peripheral and intra-mucosal B cell repertoires in the absence of rejection revealed increased BCR mutation rates and clonal expansion in graft mucosa compared to circulating B cells, but these parameters did not increase markedly after the first year post-transplant. Furthermore, clonal mixing between the allograft mucosa and the circulation was significantly greater in ITx recipients, even years after transplantation, than in healthy control adults. Collectively, our data demonstrate intestinal mucosal B cell repertoire establishment from a circulating pool, a process that continues for years without evidence of establishment of a stable mucosal B cell repertoire.

Origin, phenotype and autoimmune potential of T cells in human immune system mice receiving neonatal human thymus tissue

Robust human immune system (HIS) mice are created using human fetal thymus tissue and hematopoietic stem cells (HSCs). A HIS mouse model using neonatal human thymus tissue and umbilical cord blood (CB) HSCs (NeoHu) was recently described. We improved the model by removing the native murine thymus, which can also generate human T cells, and demonstrated definitively the capacity of human T cells to develop in a grafted neonatal human thymus. Human T cells derived from the neonatal thymus tissue appeared in peripheral blood early post-transplantation and CB-derived T cells appeared later. Naïve T cells were demonstrated in peripheral blood but effector memory and T peripheral helper phenotypes predominated later, in association with development of autoimmunity in some animals. Treatment of thymus grafts with 2-deoxyglucose (2-DG) increased the proportion of stem cells derived from injected HSCs, delayed onset of autoimmune disease, reduced early T cell reconstitution, and reduced effector/memory T cell conversion. Younger neonatal human thymus tissue was associated with improved T cell reconstitution. While the NeoHu model bypasses the need for fetal tissue, it has yet to demonstrate equivalent reconstitution to fetal tissue, though 2-DG can improve results by removing native thymocytes prior to transplantation.

T1D patient-derived hematopoietic stem cells are programmed to generate Tph, Tfh, and autoimmunity-associated B cell subsets in human immune system mice

Interactions between B cells and CD4⁺ T cells play a central role in the development of Type 1 Diabetes (T1D). Two helper cell subsets, follicular (Tfh) and peripheral (Tph) helper T cells, are increased in patients with T1D but their role in driving B cell autoimmunity is undefined. We used a personalized immune (PI) mouse model to generate human immune systems de novo from hematopoietic stem cells (HSCs) of patients with T1D or from healthy controls (HCs). Both groups developed Tfh and Tph-like cells, and those with T1D-derived immune systems demonstrated increased numbers of Tph-like and Tfh cells compared to HC-derived PI mice. T1D-derived immune systems included increased proportions of unconventional memory CD27^-IgD^- B cells and reduced proportions of naïve B cells compared to HC PI mice, resembling changes reported for patients with systemic lupus erythematosus. Our findings suggest that T1D HSCs are genetically programmed to produce increased proportions of T cells that promote the development of unconventional, possibly autoreactive memory B cells. PI mice provide an avenue for further understanding of the immune abnormalities that drive autoantibody pathogenesis and T1D.

Reduced Follicular Regulatory T Cells in Spleen and Pancreatic Lymph Nodes of Patients With Type 1 Diabetes

In the attempt to understand the origin of autoantibody (AAb) production in patients with and at risk for type 1 diabetes (T1D), multiple studies have analyzed and reported alterations in T follicular helper (Tfh) cells in presymptomatic AAb⁺ subjects and patients with T1D. Yet, whether the regulatory counterpart of Tfh cells, represented by T follicular regulatory (Tfr) cells, is similarly altered is still unclear. To address this question, we performed analyses in peripheral blood, spleen, and pancreatic lymph nodes (PLN) of organ donor subjects with T1D. Blood analyses were also performed in living AAb^- and AAb⁺ subjects. While negligible differences in the frequency and phenotype of blood Tfr cells were observed among T1D, AAb^-, and AAb⁺ adult subjects, the frequency of Tfr cells was significantly reduced in spleen and PLN of T1D as compared with nondiabetic control subjects. Furthermore, adoptive transfer of Tfr cells delayed disease development in a mouse model of T1D, a finding that could indicate that Tfr cells play an important role in peripheral tolerance and regulation of autoreactive Tfh cells. Together, our findings provide evidence of Tfr cell alterations within disease-relevant tissues in patients with T1D, suggesting a role for Tfr cells in defective humoral tolerance and disease pathogenesis.

Role of the thymus in spontaneous development of a multi-organ autoimmune disease in human immune system mice

We evaluated the role of the thymus in development of multi-organ autoimmunity in human immune system (HIS) mice. T cells were essential for disease development and the same T cell clones with varying phenotypes infiltrated multiple tissues. De novo-generated hematopoietic stem cell (HSC)-derived T cells were the major disease drivers, though thymocytes pre-existing in grafted human thymi contributed if not first depleted. HIS mice with a native mouse thymus developed disease earlier than thymectomized mice with a thymocyte-depleted human thymus graft. Defective structure in the native mouse thymus was associated with impaired negative selection of thymocytes expressing a transgenic TCR recognizing a self-antigen. Disease developed without direct recognition of antigens on recipient mouse MHC. While human thymus grafts had normal structure and negative selection, failure to tolerize human T cells recognizing mouse antigens presented on HLA molecules may explain eventual disease development. These new insights have implications for human autoimmunity and suggest methods of avoiding autoimmunity in next-generation HIS mice.

Negative selection of human T cells recognizing a naturally-expressed tissue-restricted antigen in the human thymus

During T cell development in mice, thymic negative selection deletes cells with the potential to recognize and react to self-antigens. In human T cell-dependent autoimmune diseases such as Type 1 diabetes, multiple sclerosis, and rheumatoid arthritis, T cells reactive to autoantigens are thought to escape negative selection, traffic to the periphery and attack self-tissues. However, physiological thymic negative selection of autoreactive human T cells has not been previously studied. We now describe a human T-cell receptor-transgenic humanized mouse model that permits the study of autoreactive T-cell development in a human thymus. Our studies demonstrate that thymocytes expressing the autoreactive Clone 5 TCR, which recognizes insulin B:9-23 presented by HLA-DQ8, are efficiently negatively selected at the double and single positive stage in human immune systems derived from HLA-DQ8⁺ HSCs. In the absence of hematopoietic expression of the HLA restriction element, negative selection of Clone 5 is less efficient and restricted to the single positive stage. To our knowledge, these data provide the first demonstration of negative selection of human T cells recognizing a naturally-expressed tissue-restricted antigen. Intrathymic antigen presenting cells are required to delete less mature thymocytes, while presentation by medullary thymic epithelial cells may be sufficient to delete more mature single positive cells. These observations set the stage for investigation of putative defects in negative selection in human autoimmune diseases.

Siplizumab selectively depletes effector memory T cells and promotes a relative expansion of alloreactive regulatory T cells in vitro

Siplizumab, a humanized anti-CD2 monoclonal antibody, has been used in conditioning regimens for hematopoietic cell transplantation and tolerance induction with combined kidney-bone marrow transplantation. Siplizumab-based tolerance induction regimens deplete T cells globally while enriching regulatory T cells (Tregs) early posttransplantation. Siplizumab inhibits allogeneic mixed-lymphocyte reactions (MLRs) in vitro. We compared the impact of siplizumab on Tregs versus other T cell subsets in HLA-mismatched allogeneic MLRs using PBMCs. Siplizumab predominantly reduced the percentage of CD4⁺ and CD8⁺ effector memory T cells, which express higher CD2 levels than naïve T cells or resting Tregs. Conversely, siplizumab enriched proliferating CD45RA^- FoxP3^HI cells in MLRs. FoxP3 expression was stable over time in siplizumab-containing cultures, consistent with enrichment for bona fide Tregs. Consistently, high-throughput TCRβ CDR3 sequencing of sorted unstimulated and proliferating T cells in MLRs revealed selective expansion of donor-reactive Tregs along with depletion of donor-reactive CD4⁺ effector/memory T cells in siplizumab-containing MLRs. These results indicate that siplizumab may have immunomodulatory functions that may contribute to its success in tolerance-inducing regimens. Our studies also confirm that naïve in addition to effector/memory T cells contribute to the allogeneic MLR and mandate further investigation of the impact of siplizumab on alloreactive naïve T cells.

Reduced positive selection of a human TCR in a swine thymus using a humanized mouse model for xenotolerance induction

BACKGROUND

Tolerance-inducing approaches to xenotransplantation would be optimal and may be necessary for long-term survival of transplanted pig organs in human patients. The ideal approach would generate donor-specific unresponsiveness to the pig organ without suppressing the patient's normal immune function. Porcine thymus transplantation has shown efficacy in promoting xenotolerance in humanized mice and large animal models. However, murine studies demonstrate that T cells selected in a swine thymus are positively selected only by swine thymic epithelial cells, and therefore, cells expressing human HLA-restricted TCRs may not be selected efficiently in a transplanted pig thymus. This may lead to suboptimal patient immune function.

METHODS

To assess human thymocyte selection in a pig thymus, we used a TCR transgenic humanized mouse model to study positive selection of cells expressing the MART1 TCR, a well-characterized human HLA-A2-restricted TCR, in a grafted pig thymus.

RESULTS

Positive selection of T cells expressing the MART1 TCR was inefficient in both a non-selecting human HLA-A2- or swine thymus compared with an HLA-A2+ thymus. Additionally, CD8 MART1 TCRbright T cells were detected in the spleens of mice transplanted with HLA-A2+ thymi but were significantly reduced in the spleens of mice transplanted with swine or HLA-A2- thymi. [Correction added on October 15, 2019, after first online publication: The missing superscript values +, -, and bright have been included in the Results section.] CONCLUSIONS: Positive selection of cells expressing a human-restricted TCR in a transplanted pig thymus is inefficient, suggesting that modifications to improve positive selection of cells expressing human-restricted TCRs in a pig thymus may be necessary to support development of a protective human T-cell pool in future patients.

Crossreactive public TCR sequences undergo positive selection in the human thymic repertoire

We investigated human T-cell repertoire formation using high throughput TCRβ CDR3 sequencing in immunodeficient mice receiving human hematopoietic stem cells (HSCs) and human thymus grafts. Replicate humanized mice generated diverse and highly divergent repertoires. Repertoire narrowing and increased CDR3β sharing was observed during thymocyte selection. While hydrophobicity analysis implicated self-peptides in positive selection of the overall repertoire, positive selection favored shorter shared sequences that had reduced hydrophobicity at positions 6 and 7 of CDR3βs, suggesting weaker interactions with self-peptides than unshared sequences, possibly allowing escape from negative selection. Sharing was similar between autologous and allogeneic thymi and occurred between different cell subsets. Shared sequences were enriched for allo-crossreactive CDR3βs and for Type 1 diabetes-associated autoreactive CDR3βs. Single-cell TCR-sequencing showed increased sharing of CDR3αs compared to CDR3βs between mice. Our data collectively implicate preferential positive selection for shared human CDR3βs that are highly cross-reactive. While previous studies suggested a role for recombination bias in producing "public" sequences in mice, our study is the first to demonstrate a role for thymic selection. Our results implicate positive selection for promiscuous TCRβ sequences that likely evade negative selection, due to their low affinity for self-ligands, in the abundance of "public" human TCRβ sequences.

Posttransplant Hemophagocytic Lymphohistiocytosis Driven by Myeloid Cytokines and Vicious Cycles of T-Cell and Macrophage Activation in Humanized Mice

Hemophagocytic lymphohistiocytosis (HLH) has recently been increasingly reported as an important complication after stem cell transplantation, in line with the increase in the number of HLA-mismatched transplantation. Although previous clinical studies have shown an elevation of inflammatory cytokines in patients with HLH after hematopoietic stem cell transplantation, as well as those after viral infection or autoimmune disease, the disease pathogenesis remains poorly understood. Here we explored this issue in humanized mice with functional human lymphohematopoietic systems, which were constructed by transplantation of human CD34⁺ cells alone, or along with human fetal thymus into NOD/SCID/γc^−/− (NSG) or NSG mice carrying human SCF/GM-CSF/IL-3 transgenes (SGM3). In comparison with humanized NSG (huNSG) mice, huSGM3 mice had higher human myeloid reconstitution and aggressive expansion of human CD4⁺ memory T cells, particularly in the absence of human thymus. Although all huNSG mice appeared healthy throughout the observation period of over 20 weeks, huSGM3 mice developed fatal disease characterized by severe human T cell and macrophage infiltrations to systemic organs. HuSGM3 mice also showed severe anemia and thrombocytopenia with hypoplastic bone marrow, but increased reticulocyte counts in blood. In addition, huSGM3 mice showed a significant elevation in human inflammatory cytokines including IL-6, IL-18, IFN-α, and TNF-γ, faithfully reproducing HLH in clinical situations. Our study suggests that posttransplant HLH is triggered by alloresponses (or xenoresponses in our model), driven by myeloid cytokines, and exacerbated by vicious cycles of T-cell and macrophage activation.

β-Cell Replacement in Mice Using Human Type 1 Diabetes Nuclear Transfer Embryonic Stem Cells

β-Cells derived from stem cells hold great promise for cell replacement therapy for diabetes. Here we examine the ability of nuclear transfer embryonic stem cells (NT-ESs) derived from a patient with type 1 diabetes to differentiate into β-cells and provide a source of autologous islets for cell replacement. NT-ESs differentiate in vitro with an average efficiency of 55% into C-peptide-positive cells, expressing markers of mature β-cells, including MAFA and NKX6.1. Upon transplantation in immunodeficient mice, grafted cells form vascularized islet-like structures containing MAFA/C-peptide-positive cells. These β-cells adapt insulin secretion to ambient metabolite status and show normal insulin processing. Importantly, NT-ES-β-cells maintain normal blood glucose levels after ablation of the mouse endogenous β-cells. Cystic structures, but no teratomas, were observed in NT-ES-β-cell grafts. Isogenic induced pluripotent stem cell lines showed greater variability in β-cell differentiation. Even though different methods of somatic cell reprogramming result in stem cell lines that are molecularly indistinguishable, full differentiation competence is more common in ES cell lines than in induced pluripotent stem cell lines. These results demonstrate the suitability of NT-ES-β-cells for cell replacement for type 1 diabetes and provide proof of principle for therapeutic cloning combined with cell therapy.

A model for personalized in vivo analysis of human immune responsiveness

Studies of human immune diseases are generally limited to the analysis of peripheral blood lymphocytes of heterogeneous patient populations. Improved models are needed to allow analysis of fundamental immunologic abnormalities predisposing to disease and in which to assess immunotherapies. Immunodeficient mice receiving human fetal thymus grafts and fetal CD34(+) cells intravenously produce robust human immune systems, allowing analysis of human T cell development and function. However, to use humanized mice to study human immune-mediated disorders, immune systems must be generated from adult hematopoietic cells. Here, we demonstrated robust immune reconstitution in mice with hematopoietic stem cells (HSCs) aspirated from bone marrow of adults with type 1 diabetes (T1D) and healthy control volunteers. In these humanized mice, cryopreservation of human leukocyte antigen allele-matched fetal thymic tissue prevented allogeneic adult HSC rejection. Newly generated T cells, which included regulatory T cells (T(regs)), were functional and self-tolerant and had a diverse repertoire. The immune recognition of these mice mimicked that of the adult CD34(+) cell donor, but the T cell phenotypes were more predominantly "naïve" than those of the adult donors. HSCs from T1D and control donors generated similar numbers of natural T(regs) intrathymically; however, peripheral T cells from T1D subjects showed increased proportions of activated or memory cells compared to controls, suggesting possible HSC-intrinsic differences in T cell homeostasis that might underlie immune pathology in T1D. This "personalized immune" mouse provides a new model for individualized analysis of human immune responses that may provide new insights into not only T1D but also other forms of immune function and dysfunction as well.

Conditional deletion of tumor necrosis factor receptor-associated factor 6 (TRAF6) in medullary thymic epithelial cells leads to the development of classical autoimmune hepatitis in mice (123.40)

Autoimmune hepatitis (AIH) is an enigmatic disease in which loss of tolerance towards self-antigens is suspected among the causes; animal models are limited, however. TRAF6 is an important signaling adaptor protein that plays a critical role in T cell tolerance by regulating medullary thymic epithelial cell (mTEC) development. mTECs in turn regulate central tolerance by ectopically expressing self-antigens and eliminating autoreactive T cells during development. In this report, conditional knockout mice lacking TRAF6 expression in thymic epithelial cells (TRAF6ΔTEC) were generated using FoxN1-cre transgenic mice (FoxN1 expression is confined to mTECs). TRAF6 deletion inhibited mTEC development and the production of mature mTECs in the thymus. This correlated with spontaneous liver-specific autoimmunity and pathognomonic features of chronic autoimmune hepatitis including interphase and lobular hepatitis, T cell and plasma cell infiltration, elevated serum transaminases and IgG, and production of anti-nuclear autoantibodies against the livers of TRAF6ΔTEC animals. A significant increase in fibrosis was also detected in TRAF6ΔTEC mice compared to littermate controls. Our data indicate that mTECs play an intrinsic role in central tolerance, and their depletion leads to AIH. This model of spontaneous AIH represents an exciting new tool to investigate autoantigen-specific T and B cell responses and regulatory mechanisms underlying the disease.

Human natural regulatory T cell development, suppressive function, and postthymic maturation in a humanized mouse model

CD4(+) regulatory T cells (Tregs) control adaptive immune responses and promote self-tolerance. Various humanized mouse models have been developed in efforts to reproduce and study a human immune system. However, in models that require T cell differentiation in the recipient murine thymus, only low numbers of T cells populate the peripheral immune systems. T cells are positively selected by mouse MHC and therefore do not function well in an HLA-restricted manner. In contrast, cotransplantation of human fetal thymus/liver and i.v. injection of CD34(+) cells from the same donor achieves multilineage human lymphohematopoietic reconstitution, including dendritic cells and formation of secondary lymphoid organs, in NOD/SCID mice. Strong Ag-specific immune responses and homeostatic expansion of human T cells that are dependent on peripheral human APCs occur. We now demonstrate that FOXP3(+)Helios(+) "natural" Tregs develop normally in human fetal thymic grafts and are present in peripheral blood, spleen, and lymph nodes of these humanized mice. Humanized mice exhibit normal reversal of CD45 isoform expression in association with thymic egress, postthymic "naive" to "activated" phenotypic conversion, and suppressive function. These studies demonstrate the utility of this humanized mouse model for the study of human Treg ontogeny, immunobiology and therapy.