Cytomegalovirus-Specific Cytotoxic T Lymphocytes Can Be Efficiently Expanded from Granulocyte Colony-Stimulating Factor–Mobilized Hemopoietic Progenitor Cell Products Ex Vivo and Safely Transferred to Stem Cell Transplantation Recipients to Facilitate Immune Reconstitution

Uncontrolled cytomegalovirus (CMV) reactivation after allogeneic hematopoietic stem cell transplantation causes significant morbidity and mortality. Adoptive transfer of CMV-specific cytotoxic T lymphocytes (CTLs) is a promising therapy to treat reactivation and prevent viral disease. In this article, we describe the generation of clinical-grade CMV-specific CTLs directly from granulocyte colony-stimulating factor–mobilized hemopoietic progenitor cell (G-HPC) products collected for transplantation. This method requires less than 2.5% of a typical G-HPC product to reproducibly expand CMV-specific CTLs ex vivo. Comparison of 11 CMV CTL lines generated from G-HPC products with 52 CMV CTL lines generated from nonmobilized peripheral blood revealed similar expansion kinetics and phenotype. G-HPC–derived CTLs produced IFN-γ after reexposure to CMVpp65 antigen and exhibited CMV-directed cytotoxicity but no alloreactivity against transplantation recipient–derived cells. Seven patients received CMV-specific CTL lines expanded from G-HPC products in a prophylactic adoptive immunotherapy phase I/II clinical trial. Use of G-HPC products will facilitate integration of CTL generation into established quality systems of transplantation centers and more rapid inclusion of T cell therapies into routine clinical care.     

The impact of early viral infections and graft-versus-host disease on immune reconstitution following paediatric stem cell transplantation

Viral infections and graft‐versus‐host disease (GVHD) render an impact on both the clinical and immunological recovery following allogeneic hematopoietic stem cell transplantation (HSCT). We studied the recuperation of the immune defence after transplant in the paediatric setting and assessed the impact of early (<100 days post‐HSCT) viral [cytomegalovirus (CMV), Ebstein‐Barr virus (EBV) and adenovirus] reactivations/infections and GVHD. Fifty‐one paediatric recipients of HSCT were enrolled. T cell recovery was evaluated on lymphocyte subpopulations using flow cytometry and functionally by measuring T cell excision circles (TRECs) and through the analysis of T lymphocyte responses to mitogens. B cell recovery was studied by flow cytometry and functionally by ELISPOT. Acute and mild chronic GVHD allowed for a brisk recovery of both cellular and humoral immunity while moderate to severe chronic graft‐versus‐host disease (cGVHD) associated with a significant, tampering effect on the immunological recovery after transplant. In the former group, the early viral reactivations/infections seemingly linked with a delayed recovery of T lymphocytes and low TRECs values. Moderate to severe cGVHD appears to associate with an impaired immunological recovery after HSCT. Early viral infections linked with prolonged T cell immunodeficiency and thymic dysfunction may be indicative of the presence of subclinical GVHD.     

Cytotoxic t cells in cytomegalovirus infection: HLA-restricted T-lymphocyte and non-T-lymphocyte cytotoxic responses correlate with recovery from cytomegalovirus infection in bone-marrow-transplant recipients

We studied 58 recipients of bone-marrow transplants to evaluate immune responses to cytomegalovirus infection. Such infection developed in 43 patients; it was fatal in 12, nonfatal in 23, and present at death from other causes in eight. All patients had low or absent cytomegalovirus-specific cytotoxic lymphocyte activity before the onset of infection. Cytomegalovirus-specific cytotoxic responses developed in all survivors, whereas only two patients with fatal infection had even low-level cytomegalovirus-specific cytotoxic responses. Natural and antibody-dependent killer-cell activities were depressed both before and during infection in patients with fatal infections, but not in those who survived. The outcome of the infection did not correlate with the nature of the underlying disease, the type of transplant received, the pretransplantation cytomegalovirus-antibody status, or lymphocyte-proliferation responses to cytomegalovirus antigens or concanavalin A. The correlation between effective virus-specific cytotoxic response and recovery from infection indicates that these effector cells probably mediate recovery from cytomegalovirus infection.     

Fatal ongoing human cytomegalovirus reactivation during high‐dose melphalan and autologous stem cell transplantation

Human cytomegalovirus (HCMV) reactivation can cause a wide range of complications in hematopoietic stem cell transplant recipients, ranging from pneumonia to graft failure. Although reactivations are usually seen in the early post‐transplant period, ongoing and untreated HCMV reactivation at the time of high‐dose chemotherapy and autologous stem cell support is an exceedingly rare circumstance whose consequences remain largely unknown. This case report describes a patient who underwent high‐dose melphalan and autologous transplantation with unknown active HCMV replication. J. Med. Virol. 81:857–860, 2009. © 2009 Wiley‐Liss, Inc.     

Augmentation of virus-specific immunity after hematopoietic stem cell transplantation by adoptive T-cell therapy

Adoptive transfer of virus-specific T cells offers the potential for accelerating reconstitution of antigen-specific immunity after allogeneic hematopoietic stem cell transplantation. However, the logistics of producing virus-specific T cells and the risk of inducing graft-versus-host disease has limited their application. We developed a relatively simple system employing cytomegalovirus lysate-pulsed, monocyte-derived dendritic cells as stimulator cells, requiring only a single blood draw from the donor. We treated 16 patients with these T-cell lines, administered after the detection of human cytomegalovirus (HCMV) DNA by polymerase chain reaction. Massive in vivo expansions of HCMV-specific cytotoxic T lymphocytes (3-5 log) were observed within days of adoptive transfer. In eight cases viral titers were decreasing within 5 days and antiviral drug therapy was not required. The T-cell receptor CDR3 lengths of HCMV-specific cytotoxic T lymphocytes expanding in vivo were identical to those of the transferred cells. A low incidence of late CMV reactivation was seen (2/14 assessable patients compared with 45/72 historical controls, p = 0.001) and no significant toxicities were observed. Our findings indicate that application of cell lines generated in relatively short-term in vitro cultures is both feasible and effective in a clinical environment. This simple in vitro methodology should allow widespread application of adoptive transfer of virus-specific T cells.     

Antiviral T-cell therapy

Serious viral infections are a common cause of morbidity and mortality after allogeneic stem cell transplantation. They occur in the majority of allograft recipients and are fatal in 17–20%. These severe infections may be prolonged or recurrent and add substantially to the cost, both human and financial, of the procedure. Many features of allogeneic stem cell transplantation contribute to this high rate of viral disease. The cytotoxic and immunosuppressive drugs administered pretransplant to eliminate the host hematopoietic/immune system and any associated malignancy, the delay in recapitulating immune ontogeny post-transplant, the immunosuppressive drugs given to prevent graft versus host disease (GvHD), and the effects of GvHD itself, all serve to make stem cell transplant recipients vulnerable to disease from endogenous (latent) and exogenous (community) viruses, and to be incapable of controlling them as quickly and effectively as most normal individuals.     

Amelioration of graft versus host disease by galectin-1

Graft versus host disease is a significant cause of morbidity and mortality following allogeneic hematopoietic stem cell transplantation. Galectin-1, a mammalian lectin that modulates T cell function and apoptosis, has been shown to be immunomodulatory in animal models of autoimmune disease. We investigated the efficacy of galectin-1 in a murine model of graft versus host disease and found that 68% of galectin-1-treated mice survived, compared to 3% of vehicle-treated mice. Galectin-1-treated animals also had reduced inflammatory infiltrates in tissues compared to animals treated with vehicle alone. Galectin-1 did not affect engraftment of donor hematopoietic cells. However, galectin-1-treated animals demonstrated increased cellularity in bone marrow and spleen with increased numbers of splenic B cells and CD4 T cells compared to those animals treated with vehicle alone. Galectin-1 treatment also significantly improved reconstitution of normal splenic architecture following transplant. Production of type I cytokines interleukin-2 (IL-2) and interferon-gamma was reduced in splenocytes derived from galectin-1-treated transplanted mice when compared to animals treated with vehicle alone, while production of the type II cytokines, IL-4 and IL-10, was similar between the two groups of animals. Although splenocytes from galectin-1-treated transplanted animals responded to both third party antigens and leukemic challenge, host alloreactivity was significantly reduced when compared to cells from vehicle-treated animals. These results demonstrate that galectin-1 therapy is capable of increasing survival and suppressing the graft versus host immune response without compromising engraftment or immune reconstitution following allogeneic hematopoietic stem cell transplant.     

异基因造血干细胞移植治疗24例重型再生障碍性贫血北大核心

背景:重型再生障碍性贫血病情重、病死率高,需快速恢复造血功能,目前异基因造血干细胞移植为一线治疗方案,同胞全相合异基因造血干细胞移植为首选,单倍体相合造血干细胞移植作为替代治疗方案也取得了较好的效果。目的:探讨异基因造血干细胞移植(包括同胞全相合造血干细胞移植及单倍体相合造血干细胞移植)治疗重型再生障碍性贫血的临床疗效。方法:回顾性分析2015年4月至2021年7月于徐州市中心医院接受异基因造血干细胞移植治疗24例重型再生障碍性贫血患者的临床资料,其中接受同胞全相合造血干细胞移植8例,接受单倍体相合造血干细胞移植16例。24例重型再生障碍性贫血患者预处理方案为氟达拉滨、环磷酰胺、抗淋巴细胞球蛋白方案。同胞全相合造血干细胞移植采用环孢素联合短程甲氨蝶呤预防移植物抗宿主病,单倍体相合造血干细胞移植在此基础上增加吗替麦考酚酯。结果与结论:①24例重型再生障碍性贫血患者中有2例患者预处理期间死于严重感染,其余22例均达造血重建;中性粒细胞植入中位时间为12.5(10-18)d,血小板植入中位时间为14.5(10-26)d;②22例植入成功患者发生急性移植物抗宿主病8例(36%),Ⅲ/Ⅳ度急性移植物抗宿主病2例,慢性移植物抗宿主病累计发生4例(18%),Ⅲ/Ⅳ度慢性移植物抗宿主病1例;③18例患者存活,6例患者死亡,5年预计总生存率为74%;④结果表明,异基因造血干细胞移植为重型再生障碍性贫血的有效治疗手段,同胞全合供者作为首选,无同胞全相合供者时可选择单倍体相合供者作为替代。     

Tolerance induction in HLA disparate living donor kidney transplantation by facilitating cell-enriched donor stem cell Infusion: The importance of durable chimerism

Successful solid organ transplantation currently requires the life-long use of medications to suppress the immune system in order to prevent transplant rejection. Drug-based immunosuppression significantly increases the risk of infection and cancer, as well as being very costly. Development of new therapies to minimize or eliminate entirely the need for anti-rejection drugs is of great interest to the transplant community. Therapeutic cell transfer for the control of the human immune system represents a compelling approach to reduce or eliminate the need for anti-rejection drugs. Establishment of durable hematopoietic chimerism through hematopoietic stem cell transplantation (HSCT) has been shown in preclinical models and patients to lead to donor specific tolerance. However, the application HSCT is limited by the potential toxicity of conditioning regimens, the risk of graft versus host disease (GVHD) and the challenge of HLA mismatching. In this review we describe the clinical outcomes and science behind a CD8⁺/TCR^? facilitating cell-based hematopoietic stem cell transplant approach (termed FCRx) to induce tolerance to mismatched renal allografts while minimizing the risk of graft-versus-host GVHD and achieving avoidance of long-term immunosuppressant drugs in living donor kidney transplant recipients.     

Cytomegalovirus in Bone Marrow Cells Correlates with Cytomegalovirus in Peripheral Blood Leukocytes

In allogeneic hematopoietic stem cell transplant recipients with bone marrow (BM) suppression, cytomegalovirus (CMV) pp65 antigenemia and DNA were detectable in peripheral blood leukocytes (PBL) and BM cells. A relationship between CMV infection of PBL and BM cells has been found.     

Minor histocompatibility antigen-specific cytotoxic T lymphocytes generated with dendritic cells from DLA-identical littermates.

Donor cytotoxic T lymphocytes (CTL) specific for minor histocompatibility antigens (mHA) mediate the graft-versus-host effect whereas host mHA-specific CTL mediate graft rejection in the setting of major histocompatibility complex identical allogeneic hematopoietic stem cell transplantation. Development of a large animal model from which mHA-specific CTL can be isolated would accelerate translation in clinical studies to improve control of the graft-versus-host effect as well as prevention of graft rejection in sensitized hosts. The aims of the current study were to isolate mHA-specific CTL from dog leukocyte antigen-identical littermate nonsensitized recipients before transplantation, from stable mixed hematopoietic chimeras, and from dogs sensitized to mHA after graft rejection. Donor dendritic cells (DCs) were cultured from bone marrow-derived CD34(+) cells and were used to stimulate recipient T lymphocytes on days 1, 10, and 20 of CTL culture. We reliably generated and expanded mHA-specific CTL ex vivo from sensitized dogs that were given a donor-specific blood transfusion to boost immune recall after graft rejection after a nonmyeloablative transplantation. The mHA-specific cytotoxicity measured by (51)Cr release assay was enriched from less than 5% in the starting population of sensitized peripheral blood mononuclear cells to a median of 63% after 4 weeks in CTL culture. The expanded mHA-specific CTLs were not tissue-specific: hematopoietic cells, fibroblast, and stromal cell lines were lysed in an mHA-specific manner. Allogeneic DCs, but not peripheral blood mononuclear cells, were necessary for stimulating ex vivo expansion of mHA-specific CTL. We were unable to generate mHA-specific CTL from nonsensitized dogs before transplantation, from previously sensitized dogs but without recent recall immunization, or from stable mixed hematopoietic chimeras. We conclude that after recent in vivo sensitization, large-scale ex vivo expansion of mHA-specific CTL was feasible using allogeneic DCs.     

High Incidence of Early Human Herpesvirus-6 Infection in Children Undergoing Haploidentical Manipulated Stem Cell Transplantation for Hematologic Malignancies

Human herpesvirus-6 (HHV-6) infection is increasingly recognized among allogeneic hematopoietic stem cell transplantation (HSCT) recipients, with 30% at risk of reactivation in the haploidentical setting. It has been associated with encephalitis, acute graft-versus-host disease, and graft failure. Here we report 2 cohorts of pediatric haploidentical manipulated HSCT in which, despite many differences, HHV-6 reactivation and disease occurred with very high incidence compared with data reported in the literature and represented the main early post-transplant infectious complication compared with other viral, bacterial, or fungal infections.The 2 cohorts were recruited at the pediatric transplant centers of Perugia (n?=?13), Barcelona (n?=?10), and Madrid (n?=?15). All patients received myeloablative conditioning regimens and 2 different types of ex vivo graft manipulation: CD34⁺ selection and regulatory T cell/conventional T cell infusion in 13 patients and CD45RA T cell depletion in 25 patients. Antiviral prophylaxis was acyclovir in 33 and foscarnet in 5 patients. HHV-6 DNAemia was checked by quantitative or qualitative PCR. In vitro experiments demonstrated that donor CD4⁺ T cells are the reservoir of HHV-6 and suggested a role of the graft composition in both transplant settings (rich in CD4⁺ T cells) in the high rate of HHV-6 infections.All patients presented very early HHV-6 DNAemia after transplantation, and although viremic, 9 patients (24%) developed symptomatic limbic encephalitis. All patients responded to antiviral treatment, and none died of infection, although 2 experienced long-term neurologic sequelae (22%). Moreover, 6 patients presented organ involvement in absence of other causes: 1 hepatitis, 1 pneumonia, 2 gastroenteritis, and 2 multiorgan involvement(1 encephalitis, pneumonia, and gastritis; 1 pneumonia and enteritis). Incidences of other viral, bacterial, and fungal diseases were lower in both cohorts. In vitro, HHV-6 was found to infect only CD4⁺ fraction of the graft. Co-culturing CD4⁺ T cells with CD56⁺ natural killer (NK) cells eliminated the virus, demonstrating the main role of NK cells in the antiviral immune response.All 38 pediatric patients undergoing these manipulated haploidentical HSCTs showed HHV-6 reactivation, and 14 of 38 developed HHV-6 disease with similar features in terms of timing, morbidity, response to treatment, and outcome. The graft composition in both transplant platforms, rich in CD4⁺ T cells and poor in NK cells, seems to play a key role. HHV-6 DNAemia surveillance was useful to diagnose and treat preemptively HHV-6 infection.     

Review: HLA loss and detection in the setting of relapse from HLA-mismatched hematopoietic cell transplant

HLA loss in hematologic malignancies is rare at diagnosis but may occur in leukemic cells at disease relapse. Although HLA mismatched hematopoietic cell transplant (HCT) is useful in exploiting graft vs. leukemia (GvL) effects, alloreactive T cells may exert immune pressure on leukemic cells, leading to immune escape. Roughly 10–30% of relapses after HLA mismatched hematopoietic cell transplant (HCT) involve loss of recipient-specific HLA genes, thereby rendering leukemic cells resistant to donor GvL effects. The use of alloreactive T cells, i.e., donor lymphocyte infusions (DLI) to control relapse following HCT will not be effective in the context of recipient-specific HLA loss and will instead carry all the risk of graft versus host disease (GvHD). Additionally, the source of the lost HLA, maternal or paternal, is important in informing the best donor choice for rescue HCT. The growing body of evidence, together with developments in laboratory testing for genomic loss of HLA, has highlighted the importance of routine testing for HLA loss in patients who relapse post HLA-mismatched HCT. Assessment of HLA loss allows transplant centers to make quick decisions regarding the most appropriate therapies and/or alternative donor selection for rescue HCT.     

Allogeneic Th1 Cells Home to Host Bone Marrow and Spleen and Mediate IFNγ-Dependent Aplasia

Bone marrow graft failure and poor graft function are frequent complications after hematopoietic stem cell transplantation and result in significant morbidity and mortality. Both conditions are associated with graft-versus-host disease (GVHD), although the mechanism remains undefined. Here we show, in 2 distinct murine models of GVHD (complete MHC- and class II-disparate) that mimic human peripheral blood stem cell transplantation, that Th1 CD4⁺ cells induce bone marrow failure in allogeneic recipients. Bone marrow failure after transplantation of allogeneic naïve CD4⁺ T cells was associated with increased CD4⁺ Th1 cell development within bone marrow and lymphoid tissues. Using IFNγ-reporter mice, we found that Th1 cells generated during GVHD induced bone marrow failure after transfers into secondary recipients. Homing studies demonstrated that transferred Th1 cells express CXCR4, which was associated with accumulation within bone marrow and spleen. Allogeneic Th1 cells were activated by radiation-resistant host bone marrow cells and induced bone marrow failure through an IFNγ-dependent mechanism. Thus, allogeneic Th1 CD4⁺ cells generated during GVHD traffic to hematopoietic sites and induce bone marrow failure via IFNγ-mediated toxicity. These results have important implications for prevention and treatment of bone marrow graft failure after hematopoietic stem cell transplantation.     

Antigen-specific T-lymphocyte function after cord blood transplantation.

It has not been possible to determine the singular contribution of naive T lymphocytes to antigen-specific immunity after hematopoietic stem cell transplantation (HSCT), because of the confounding effects of donor-derived antigen-specific T lymphocytes present in most hematopoietic stem cell (HSC) products. Because umbilical cord blood contains only naive T lymphocytes, we longitudinally evaluated the recipients of unrelated cord blood transplantation (UCBT) for the presence of T lymphocytes with specificity for herpesviruses, to determine the contribution of the naive T lymphocytes to antigen-specific immune reconstitution after HSCT. Antigen-specific T lymphocytes were detected early after UCBT (herpes simplex virus on day 29; cytomegalovirus on day 44; varicella zoster virus on day 94). Overall, 66 of 153 UCBT recipients developed antigen-specific T lymphocytes to 1 or more herpesviruses during the evaluation period. The likelihood of developing antigen-specific T lymphocyte function was not associated with immunophenotypic T lymphocyte reconstitution, transplant cell dose, primary disease, or acute and chronic graft-versus-host disease. These results indicate that naive T lymphocytes present in the HSC inoculum can contribute to the generation of antigen-specific T-lymphocyte immunity early after transplantation.     

造血干细胞移植后出血性膀胱炎的相关因素分析

目的观察造血干细胞移植后出血性膀胱炎的发病情况,探讨其发病危险因素和防治策略。方法回顾分析2006年1月～2009年12月期间88例因恶性血液病行造血干细胞移植患者的临床资料,治疗过程中均进行常规的出血性膀胱炎和移植物抗宿主病预防,分析移植后出血性膀胱炎发生的特点及相关因素。结果共14例患者发生移植后出血性膀胱炎(15.9%),其中5例人类白细胞抗原配型全相合,12例伴有其他部位移植物抗宿主病表现,发生巨细胞病毒感染阳性6人,EB病毒阳性5人。结论在采取充分的预防措施后,出血性膀胱炎仍有发生,而受者CMV感染是造血干细胞移后发生出血性膀胱炎的重要危险因素,而与其他各项因素均无显著相关性。     

Hematopoietic Stem Cell Transplantation: Current Status of Old Issues

Since 1968 hematopoietic stem cell transplantation (HSCT) has progressed from an experimental to standard therapeutic procedure. There are many obstacles to the successful outcome of HSCT procedures. Some of these obstacles are lack of healthy histocompatible donors, graft versus host disease, graft rejection and infections. Many advances have been made to overcome these obstacles with significant success. However, these issues and associated problems continue to persist at different levels as the field evolves with expanding indications for HSCT, use of alternative sources for hematopoietic stem cells and alternative transplant procedures. Newer interventions have allowed us to overcome some of these obstacles.     

Identification of new Th peptides from the cytomegalovirus protein pp65 to design a peptide library for generation of CD4 T cell lines for cellular immunoreconstitution

CD8 and CD4 lymphocytes control cytomegalovirus (CMV) infection in immunocompetent individuals, while patients with defective cellular immunity are prone to endogenous reactivation of latent CMV or, like seronegative subjects, prone to primary infection. Administration of CMV-specific CD8 lymphocytes was beneficial for immunocompromised hemopoietic stem cell (HSC) graft recipients. Since CD4 cells contribute to expansion of cytotoxic T lymphocytes (CTL), we defined new T(h) peptides on the immunodominant protein pp65 recognized by CD4 cells from HLA-typed subjects, in the perspective of complementing CTL administration with CMV-specific T(h) cells. Screening by ELISPOT on CD4 and CD8 subsets using overlapping peptides identified 10 novel CD4 peptides. To simplify procedures to generate T cell lines, we used a CD4 peptide library for T cell stimulation instead of ill-defined viral lysates, without the requirement of dendritic cells. This library stimulated CMV-specific CD4 cells. In fact, peptide-induced CD4 cells responded to pp65 and to the viral lysate. These cells were also devoid of alloreactivity after one stimulation cycle. Since Good Manufacturing Procedure-grade peptides can be synthesized, culture conditions are simplified and alloreactivity is rapidly lost, these procedures based on peptide stimulation can facilitate implementation of adoptive reconstitution of CD4 responses in immunocompromised patients also in the case when the HSC allodonor is available for generation of the T cell line.     

Flow cytometric analysis of virus-specific T lymphocytes: practicability of detection of HCMV-specific T lymphocytes in whole blood in patients after stem cell transplantation

The detection and quantification of specific T lymphocytes against human cytomegalovirus (HCMV) has proven an important laboratory marker in the monitoring of patients after stem cell transplantation (SCT). In these patients HCMV infections may cause severe disease and death. However, the determination of HCMV-specific T lymphocytes may be limited by lymphopenia occurring after transplantation. We evaluated a commercial test kit for the reliable determination of HCMV-specific T lymphocyte development in lymphopenic patients after stem cell transplantation. Using a whole blood protocol for the flow cytometric detection of antigen-specific CD4(+) T-helper and CD8(+) cytotoxic T lymphocytes this test kit measures intracellular cytokine production after stimulation with HCMV antigen. The measurement of HCMV-specific T lymphocytes was feasible when at least 3,000 CD4(+) or 1,000 CD8(+) T cells could be counted by flow cytometry. Detection of HCMV-specific T lymphocytes was possible, on average, 67 (SD+/-61) days after transplantation for CD4(+) cells and 27 (SD+/-13) days for CD8(+) cells, thus being still within the critical time for HCMV reactivation. In conclusion, the use of modern test kits permits the measurement of HCMV-specific T lymphocytes in stem cell transplant recipients and may be included in the HCMV monitoring system after SCT.     

Programmed death-1 is required for systemic self-tolerance in newly generated T cells during the establishment of immune homeostasis

Lymphopenia driven T cell activation is associated with autoimmunity. That lymphopenia does not always lead to autoimmunity suggests that control mechanisms may exist. We assessed the importance of the co-inhibitory receptor programmed death-1 (PD-1) in the control of lymphopenia-driven autoimmunity in newly generated T cells vs. established peripheral T cells and in thymic selection. PD-1 was not required for negative selection in the thymus or for maintenance of self tolerance following transfer of established PD-1^?/? peripheral T cells to a lymphopenic host. In contrast, PD-1 was essential for systemic self tolerance in newly generated T cells under lymphopenic conditions, as PD-1^?/? recent thymic emigrants (RTE), generated after transfer of PD-1^?/? hematopoietic stem cell (HSC) precursors or thymocytes into lymphopenic adult Rag^?/? recipients, induced a rapidly lethal multi-organ inflammatory disease. Disease could be blocked by using lymph node deficient recipients, indicating that lymphopenia driven PD-1^?/? T cell activation required access to sufficient lymph node stroma. These data suggested that PD-1^?/? mice themselves might be substantially protected from autoimmunity because their T cell repertoire is first generated early in life, a period naturally deficient in lymph node stroma. Consistent with this idea, neonatal Rag^?/? recipients of PD-1^?/? HSC were resistant to disease. Thus, a critical role of PD-1 resides in the control of RTE in lymphopenia. The data suggest that PD-1 and a paucity of lymphoid stroma cooperate to control autoimmunity in newly generated T cells. Clinical therapies for autoimmune disease employing lymphoablation and hematopoietic stem cell transplantation will need to take into account functional polymorphisms in the PD-1 pathway, if the treatment is to ameliorate rather than exacerbate autoimmunity.