Massive Activation-Induced Cell Death of Alloreactive T Cells With Apoptosis of Bystander Postthymic T Cells Prevents Immune Reconstitution in Mice With Graft-Versus-Host Disease

Spatial proteomics reveals phenotypic and functional differences in T cell and macrophage subsets during villitis of unknown etiology

Villitis of unknown etiology (VUE) is a prevalent inflammatory pathology of the placenta characterized by infiltration of maternal T cells and accumulation of fetal macrophages into chorionic villi. VUE is associated with a variety of adverse clinical outcomes, including fetal growth restriction and fetal demise. Evaluation of the phenotypic and functional differences between two immune cell types associated with this pathology, namely T cells and macrophages, was completed to gain a deeper understanding of the immuno-pathogenesis of VUE. GeoMx Digital Spatial Profiling was performed on placental tissue from 4 high grade VUE cases and 4 controls with no underlying pathology. Placental tissues were fluorescently labeled with CD3 and CD68 antibodies and oligo-conjugated antibodies against 48 protein targets. Overall, T cells in VUE exhibited upregulated markers of activation, memory, and antigen experience compared to controls and were altered based on placental location (villi vs. decidua). Additionally, villous macrophages in VUE upregulated costimulatory and major histocompatibility complex class I and II molecules compared to controls and macrophage subtypes in the decidua. Data herein provides new mechanistic insights into T cell and macrophage biology in VUE which contribute to this abnormal immune response to pregnancy.

Strain-specific antibody therapy prevents cytomegalovirus reactivation after transplantation

Cytomegalovirus infection is a frequent and life-threatening complication that significantly limits positive transplantation outcomes. We developed preclinical mouse models of cytomegalovirus reactivation after transplantation and found that humoral immunity is essential for preventing viral recrudescence. Preexisting antiviral antibodies decreased after transplant in the presence of graft-versus-host disease and were not replaced, owing to poor reconstitution of donor B cells and elimination of recipient plasma cells. Viral reactivation was prevented by the transfer of immune serum, without a need to identify and target specific antigenic determinants. Notably, serotherapy afforded complete protection, provided that the serum was matched to the infecting viral strain. Thus, we define the mechanisms for cytomegalovirus reactivation after transplantation and identify a readily translatable strategy of exceptional potency, which avoids the constraints of cellular therapies.

An insight into the role of the death receptor CD95 throughout pregnancy: Guardian, facilitator, or foe

The prototype death receptor CD95 (Fas) and its ligand, CD95L (FasL), have been thoroughly studied due to their role in immune homeostasis and elimination of infected and transformed cells. The fact that CD95 is present in female reproductive cells and modulated during embryogenesis and pregnancy has raised interest in its role in immune tolerance to the fetoplacental unit. CD95 has been shown to be critical for proper embryonic formation and survival. Moreover, altered expression of CD95 or its ligand causes autoimmunity and has also been directly involved in recurrent pregnancy losses and pregnancy disorders. The objective of this review is to summarize studies that evaluate the mechanisms involved in the activation of CD95 to provide an updated global view of its effect on the regulation of the maternal immune system. Modulation of the CD95 system components may be the immune basis of several common pregnancy disorders.

Nrf2 regulates CD4+ T cell-induced acute graft-versus-host disease in mice

Nuclear factor erythroid-derived 2-like 2 (Nrf2) is a ubiquitously expressed transcription factor that is well known for its role in regulating the cellular redox pathway. Although there is mounting evidence suggesting a critical role for Nrf2 in hematopoietic stem cells and innate leukocytes, little is known about its involvement in T-cell biology. In this study, we identified a novel role for Nrf2 in regulating alloreactive T-cell function during allogeneic hematopoietic cell transplantation (allo-HCT). We observed increased expression and nuclear translocation of Nrf2 upon T-cell activation in vitro, especially in CD4+ donor T cells after allo-HCT. Allo-HCT recipients of Nrf2 -/- donor T cells had significantly less acute graft-versus-host disease (GVHD)-induced mortality, morbidity, and pathology. This reduction in GVHD was associated with the persistence of Helios+ donor regulatory T cells in the allograft, as well as defective upregulation of the gut-homing receptor LPAM-1 on alloreactive CD8+ T cells. Additionally, Nrf2 -/- donor CD8+ T cells demonstrated intact cytotoxicity against allogeneic target cells. Tumor-bearing allo-HCT recipients of Nrf2 -/- donor T cells had overall improved survival as a result of preserved graft-versus-tumor activity and reduced GVHD activity. Our findings characterized a previously unrecognized role for Nrf2 in T-cell function, as well as revealed a novel therapeutic target to improve the outcomes of allo-HCT.

IL-7 Is the Limiting Homeostatic Factor that Constrains Homeostatic Proliferation of CD8+ T Cells after Allogeneic Stem Cell Transplantation and Graft-versus-Host Disease

Immune reconstitution after allogeneic hematopoietic stem cell transplantation relies primarily on homeostatic proliferation (HP) of mature T lymphocytes, but this process is typically impaired during graft-versus-host disease (GVHD). We previously showed that low IL-7 levels combined with lack of dendritic cell (DC) regeneration constrain CD4⁺ T cell HP during GVHD. However, it is not clear whether these alterations to the peripheral CD4⁺ T cell niche also contribute to impair CD8⁺ T cell regeneration during GVHD. We found that IL-7 therapy was sufficient for restoring CD8⁺ T cell HP in GVHD hosts while forcing DC regeneration with Flt3-L had only a modest effect on CD8⁺ T cell HP in IL-7 treated mice. Using bone marrow chimeras, we showed that HP of naïve CD8⁺ T cells is primarily regulated by MHC class I on radio-resistant stromal cells, yet optimal recovery of CD8⁺ T cell counts still requires expression of MHC class I on both radio-resistant and radio-sensitive hematopoietic cells. Thus, IL-7 level is the primary limiting factor that constrains naïve CD8⁺ T cell HP during GVHD, and accessibility of MHC class I on stromal cells explains how IL-7 therapy, as a single agent, can induce robust CD8 ⁺ T cell HP in the absence of DCs.

Host-Derived CD70 Suppresses Murine Graft-versus-Host Disease by Limiting Donor T Cell Expansion and Effector Function

Allogeneic hematopoietic cell transplantation (allo-HCT) is a potentially curative treatment for hematologic and immunologic diseases. However, graft-versus-host disease (GVHD) may develop when donor-derived T cells recognize and damage genetically distinct normal host tissues. In addition to TCR signaling, costimulatory pathways are involved in T cell activation. CD27 is a TNFR family member expressed on T cells, and its ligand, CD70, is expressed on APCs. The CD27/CD70 costimulatory pathway was shown to be critical for T cell function and survival in viral infection models. However, the role of this pathway in allo-HCT is previously unknown. In this study, we have examined its contribution in GVHD pathogenesis. Surprisingly, Ab blockade of CD70 after allo-HCT significantly increases GVHD. Interestingly, whereas donor T cell- or bone marrow-derived CD70 plays no role in GVHD, host-derived CD70 inhibits GVHD as CD70^-/- hosts show significantly increased GVHD. This is evidenced by reduced survival, more severe weight loss, and increased histopathologic damage compared with wild-type hosts. In addition, CD70^-/- hosts have higher levels of proinflammatory cytokines TNF-α, IFN-γ, IL-2, and IL-17. Moreover, accumulation of donor CD4⁺ and CD8⁺ effector T cells is increased in CD70^-/- versus wild-type hosts. Mechanistic analyses suggest that CD70 expressed by host hematopoietic cells is involved in the control of alloreactive T cell apoptosis and expansion. Together, our findings demonstrate that host CD70 serves as a unique negative regulator of allogeneic T cell response by contributing to donor T cell apoptosis and inhibiting expansion of donor effector T cells.

A Novel Xenogeneic Graft-Versus-Host Disease Model for Investigating the Pathological Role of Human CD4+ or CD8+ T Cells Using Immunodeficient NOG Mice

Graft-versus-host disease (GVHD) is a major complication of allogenic bone marrow transplantation and involves the infiltration of donor CD4⁺ and/or CD8⁺ T cells into various organs of the recipient. The pathological role of human CD4⁺ and CD8⁺ T cells in GVHD remains controversial. In this study, we established two novel xenogeneic (xeno)-GVHD models. Human CD4⁺ or CD8⁺ T cells were purified from peripheral blood and were transplanted into immunodeficient NOD/Shi-scid IL2rg^null (NOG) mice. Human CD8⁺ T cells did not induce major GVHD symptoms in conventional NOG mice. However, CD8⁺ T cells immediately proliferated and induced severe GVHD when transferred into NOG mice together with at least 0.5 × 10⁶ CD4⁺ T cells or into NOG human interleukin (IL)-2 transgenic mice. Human CD4⁺ T cell-transplanted NOG mice developed skin inflammations including alopecia, epidermal hyperplasia, and neutrophilia. Pathogenic T helper (Th)17 cells accumulated in the skin of CD4⁺ T cell-transplanted NOG mice. Further, an anti-human IL-17 antibody (secukinumab) significantly suppressed these skin pathologies. These results indicate that pathogenic human Th17 cells induce cutaneous GVHD via IL-17-dependent pathways. This study provides fundamental insights into the pathogenesis of xeno-GVHD, and these humanized mouse models may be useful as preclinical tools for the prevention of GVHD.

T-bet Promotes Acute Graft-versus-Host Disease by Regulating Recipient Hematopoietic Cells in Mice

Beyond its critical role in T cells, T-bet regulates the functions of APCs including dendritic cells and B cells, as well as NK cells. Given that recipient APCs are essential for priming allogeneic T cells and recipient NK or T cells are able to reject allogeneic donor cells, we evaluated the role of T-bet on the host in acute graft-versus-host disease (GVHD) using murine models of allogeneic bone marrow transplantation. T-bet(-/-) recipients developed significantly milder GVHD than their wild type counterparts in MHC-mismatched or CD4-dependent minor histocompatibility Ag-mismatched models. Allogeneic donor T cells, in particular, CD4 subset, significantly reduced IFN-γ production, proliferation and migration, and caused less injury in liver and gut of T-bet(-/-) recipients. We further observed that T-bet on recipient hematopoietic cells was primarily responsible for the donor T cell response and pathogenicity in GVHD. T-bet(-/-) dendritic cells expressed higher levels of Trail, whereas they produced lower levels of IFN-γ and IL-12/23 p40, as well as chemokine CXCL9, resulting in significantly higher levels of apoptosis, less priming, and infiltration of donor T cells. Meanwhile, NK cells in T-bet(-/-) hosts partially contribute to the decreased donor T cell proliferation. Furthermore, although T-bet on hematopoietic cells was required for GVHD development, it was largely dispensable for the graft-versus-leukemia effect. Taken together with our previous findings, we propose that T-bet is a potential therapeutic target for the control of GVHD through regulating donor T cells and recipient hematopoietic cells.

Murine allogeneic CD19 CAR T cells harbor potent antileukemic activity but have the potential to mediate lethal GVHD

Acute lymphoblastic leukemia (ALL) persisting or relapsing following bone marrow transplantation (BMT) has a dismal prognosis. Success with chimeric antigen receptor (CAR) T cells offers an opportunity to treat these patients with leukemia-redirected donor-derived T cells, which may be more functional than T cells derived from patients with leukemia but have the potential to mediate graft-versus-host disease (GVHD). We, together with others, have previously demonstrated tumor-specific T-cell dysfunction in the allogeneic environment. Here, we studied CAR T-cell function following BMT using an immunocompetent murine model of minor mismatched allogeneic transplantation followed by donor-derived CD19-CAR T cells. Allogeneic donor-derived CD19-CAR T cells eliminated residual ALL with equal potency to those administered after syngeneic BMT. Surprisingly, allogeneic CAR T cells mediated lethal acute GVHD with early mortality, which is atypical for this minor mismatch model. We demonstrated that both allogeneic and syngeneic CAR T cells show initial expansion as effector T cells, with a higher peak but rapid deletion of allogeneic CAR T cells. Interestingly, CAR-mediated acute GVHD was only seen in the presence of leukemia, suggesting CAR-target interactions induced GVHD. Indeed, serum interleukin (IL)-6 was elevated only in the presence of both leukemia and CAR T cells, and IL-6 neutralization ameliorated the severity of GVHD in a delayed donor lymphocyte infusion model. Finally, allogeneic CD4(+) CAR T cells were responsible for GVHD, which correlated with their ability to produce IL-6 upon CAR stimulation. Altogether, we demonstrate that donor-derived allogeneic CAR T cells are active but have the capacity to drive GVHD.

Acute GVHD results in a severe DC defect that prevents T-cell priming and leads to fulminant cytomegalovirus disease in mice

Viral infection is a common, life-threatening complication after allogeneic bone marrow transplantation (BMT), particularly in the presence of graft-versus-host disease (GVHD). Using cytomegalovirus (CMV) as the prototypic pathogen, we have delineated the mechanisms responsible for the inability to mount protective antiviral responses in this setting. Although CMV infection was self-limiting after syngeneic BMT, in the presence of GVHD after allogeneic BMT, CMV induced a striking cytopathy resulting in universal mortality in conjunction with a fulminant necrotizing hepatitis. Critically, GVHD induced a profound dendritic cell (DC) defect that led to a failure in the generation of CMV-specific CD8(+) T-cell responses. This was accompanied by a defect in antiviral CD8(+) T cells. In combination, these defects dramatically limited antiviral T-cell responses. The transfer of virus-specific cells circumvented the DC defects and provided protective immunity, despite concurrent GVHD. These data demonstrate the importance of avoiding GVHD when reconstructing antiviral immunity after BMT, and highlight the mechanisms by which the adoptive transfer of virus-specific T cells overcome the endogenous defects in priming invoked by GVHD.

Flow cytometric analysis of the graft-versus-Leukemia-effect after hematopoietic stem cell transplantation in mice

Acute Graft-versus-Host-Disease (aGvHD) is one of the major complications following allogeneic hematopoietic stem cell transplantation (HSCT). Although rather helpful, the use of conventional immunosuppressive drugs leads to general immunosuppression and is toxic. The effects of CD4(+) T-cells, in respect to the development of aGvHD, can be altered by administration of antihuman CD4 monoclonal antibodies, here MAX.16H5 IgG1 . This approach must be tested for possible interference with the Graft-versus-Leukemia-Effect (GvL). Thus, in vitro experiments were conducted, exposing P815 leukemic cells to bone marrow and splenocytes from cd4(-/-) -C57Bl/6 mice transgenic for human CD4 and HLA-DR3 (triple transgenic mice, [TTG]) as well as previously irradiated splenocytes from Balb/c(wt) mice. Using flow cytometry, the vitality of the various malignant and graft cells was analyzed over the course of 4 days. The survival rate of P815 cells did not change significantly when exposed to MAX.16H5 IgG1 , neither did the viability of the graft cells. This provides evidence that MAX.16H5 IgG1 does not impair the GvL effect in vitro. Additionally, P815-Balb/c(wt) leukemic mice were transplanted with P815(GFP) cells, bone marrow, and splenocytes from TTG mice with and without MAX.16H5 IgG1 . Without transplantation, P815(GFP) leukemic cells could be detected by flow cytometry in the liver, the bone marrow, and the spleen of recipients. The antibodies prevented aGvHD while leaving the GvL effect intact. These findings indicate no negative effect of MAX.16H5 IgG1 on the GvL effect in vitro and in vivo after HSCT in a murine model.

Graft-versus-host disease impairs vaccine responses through decreased CD4+ and CD8+ T cell proliferation and increased perforin-mediated CD8+ T cell apoptosis

Tumor-targeted vaccines represent a strategy to enhance the graft-versus-leukemia effect after allogeneic blood and marrow transplantation (BMT). We have previously shown that graft-versus-host disease (GVHD) can negatively impact quantitative responses to vaccines. Using a minor histocompatibility Ag-mismatched BMT (B6 → B6 × C3H.SW) followed by adoptive transfer of HY-specific T cells and HY-expressing dendritic cells, we assessed whether GVHD induced by donor lymphocyte infusion (DLI) affects the persistence, proliferation, and survival of vaccine-responding, nonalloantigen reactive T cells. Both CD8(+) and CD4(+) HY-specific T cells undergo less vaccine-driven proliferation in allogeneic recipients with GVHD. Although vaccine-responding CD8(+) T cells show decreased IFN-γ and CD107a production, CD4(+) T cells exhibit increased programmed death 1 and T cell Ig mucin-like domain 3 expression. In addition, the degree of apoptosis in vaccine-responding CD8(+) T cells was higher in the presence of GVHD, but there was no difference in CD4(+) T cell apoptosis. Using Fas ligand-deficient or TRAIL-deficient DLI had no impact on apoptosis of HY-specific T cells. However, perforin-deficient alloreactive DLI induced significantly less apoptosis of vaccine-responding CD8(+) T cells and resulted in enhanced tumor protection. Thus, diminished vaccine responses during GVHD result from impaired proliferation of CD8(+) and CD4(+) T cells responding to vaccination, with an additional contribution from perforin-mediated CD8(+) T cell apoptosis. These results provide important insights toward optimizing vaccine responses after allogeneic BMT.

Prenatal exposure of mice to diethylstilbestrol disrupts T-cell differentiation by regulating Fas/Fas ligand expression through estrogen receptor element and nuclear factor-κB motifs

Prenatal exposure to diethylstilbestrol (DES) is known to cause altered immune functions and increased susceptibility to autoimmune disease in humans. In the current study, we investigated the effect of prenatal exposure to DES on thymocyte differentiation involving apoptotic pathways. Prenatal DES exposure caused thymic atrophy, apoptosis, and up-regulation of Fas and Fas ligand (FasL) expression in thymocytes. To examine the mechanism underlying DES-mediated regulation of Fas and FasL, we performed luciferase assays using T cells transfected with luciferase reporter constructs containing full-length Fas or FasL promoters. There was significant luciferase induction in the presence of Fas or FasL promoters after DES exposure. Further analysis demonstrated the presence of several cis-regulatory motifs on both Fas and FasL promoters. When DES-induced transcription factors were analyzed, estrogen receptor element (ERE), nuclear factor κB (NF-κB), nuclear factor of activated T cells (NF-AT), and activator protein-1 motifs on the Fas promoter, as well as ERE, NF-κB, and NF-AT motifs on the FasL promoter, showed binding affinity with the transcription factors. Electrophoretic mobility-shift assays were performed to verify the binding affinity of cis-regulatory motifs of Fas or FasL promoters with transcription factors. There was shift in mobility of probes (ERE or NF-κB2) of both Fas and FasL in the presence of nuclear proteins from DES-treated cells, and the shift was specific to DES because these probes failed to shift their mobility in the presence of nuclear proteins from vehicle-treated cells. Together, the current study demonstrates that prenatal exposure to DES triggers significant alterations in apoptotic molecules expressed on thymocytes, which may affect T-cell differentiation and cause long-term effects on the immune functions.

Immune reconstitution is preserved in hematopoietic stem cell transplantation coadministered with regulatory T cells for GVHD prevention

Recipient-specific regulatory T cells (rsTreg) can prevent graft-versus-host disease (GVHD) by inhibiting donor T-cell expansion after hematopoietic stem cell transplantation (HSCT) in mice. Importantly, in adult humans, because of thymus involution, immune reconstitution during the first months after HSCT relies on the peripheral expansion of donor T cells initially present in the graft. Therefore, we developed a mouse model of HSCT that excludes thymic output to study the effect of rsTreg on immune reconstitution derived from postthymic mature T cells present within the graft. We showed that GVHD prevention with rsTreg was associated with improvement of the limited immune reconstitution compared with GVHD mice in terms of cell numbers, activation phenotype, and cytokine production. We further demonstrated a preserved in vivo immune function using vaccinia infection and third-party skin-graft rejection models, suggesting that rsTreg immunosuppression was relatively specific of GVHD. Finally, we showed that rsTreg extensively proliferated during the first 2 weeks and then declined. In turn, donor Treg proliferated from day 15 on. Taken together, these results suggest that rsTreg GVHD prevention is associated with improved early immune reconstitution in a model that more closely approximates the biology of allogeneic HSCT in human adults.

Alloantigen expression on non-hematopoietic cells reduces graft-versus-leukemia effects in mice

Allogeneic hematopoietic stem cell transplantation (HSCT) is used effectively to treat a number of hematological malignancies. Its beneficial effects rely on donor-derived T cell-targeted leukemic cells, the so-called graft-versus-leukemia (GVL) effect. Induction of GVL is usually associated with concomitant development of graft-versus-host disease (GVHD), a major complication of allogeneic HSCT. The T cells that mediate GVL and GVHD are activated by alloantigen presented on host antigen-presenting cells of hematopoietic origin, and it is not well understood how alloantigen expression on non-hematopoietic cells affects GVL activity. Here we show, in mouse models of MHC-matched, minor histocompatibility antigen-mismatched bone marrow transplantation, that alloantigen expression on host epithelium drives donor T cells into apoptosis and dysfunction during GVHD, resulting in a loss of GVL activity. During GVHD, programmed death-1 (PD-1) and PD ligand-1 (PD-L1), molecules implicated in inducing T cell exhaustion, were upregulated on activated T cells and the target tissue, respectively, suggesting that the T cell defects driven by host epithelial alloantigen expression might be mediated by the PD-1/PD-L1 pathway. Consistent with this, blockade of PD-1/PD-L1 interactions partially restored T cell effector functions and improved GVL. These results elucidate a previously unrecognized significance of alloantigen expression on non-hematopoietic cells in GVL and suggest that separation of GVL from GVHD for more effective HSCT may be possible in human patients.

Engineered regulatory T cells prevent graft-versus-host disease while sparing the graft-versus-leukemia effect after bone marrow transplantation

Regulatory T cells (Tregs) can prevent graft-versus-host disease (GVHD) after allogeneic bone marrow transplantation (BMT). Here we developed a lentivirus-based strategy to ectopically express Foxp3 in mouse CD4(+)CD25(-) T cells. These cells shared similar immunophenotypes and biological features of natural Tregs. Co-injection of engineered Tregs with donor bone marrow cells and splenocytes prevented recipients from lethal GVHD. Furthermore, we showed that graft-versus-leukemia (GVL) effect against EL4/DsRed leukemic cells was maximally preserved while GVHD was minimized during exposure to engineered Tregs in a mouse leukemia model. These findings provide a novel approach to preventing GVHD while maintaining GVL effect during BMT.

Induction of lethal graft-versus-host disease by anti-CD137 monoclonal antibody in mice prone to chronic graft-versus-host disease

Chronic graft-versus-host disease (cGVHD) is an increasingly frequent complication of allogeneic stem cell transplantation. We previously showed that anti-CD137 monoclonal antibody (mAb) can cure advanced cGVHD by inducing activation-induced cell death of donor T cells. In this study, we examined whether administration of anti-CD137 mAb can prevent the development of cGVHD after bone marrow transplantation (BMT) in mice conditioned with total body irradiation (TBI). We used the B10.D2-->Balb/c (H-2(d)) minor histocompatibility antigen-mismatched model, which reflects clinical and pathological symptoms of human cGVHD. A single injection of anti-CD137 mAb was administered immediately after BMT. In contrast to the results obtained from the curing model of cGVHD, anti-CD137 given simultaneously with BMT resulted in lethal GVHD. Histopathologic evaluation revealed inflammation and damage of target organs from acute GVHD (aGVHD) in anti-CD137-treated mice. Anti-CD137-induced lethal aGVHD required host cells, as well as irradiation and mature donor T cells. Apparently, anti-CD137 mAb rapidly induced activation of donor T cells and sustained their activation status under the inflammatory condition triggered by irradiation. When given on day 12 after irradiation and BMT, anti-CD137 mAb could still exacerbate GVHD, but when given on day 30, it could not. Our data demonstrate that anti-CD137 mAb can amplify inflammation induced by host preconditioning, subsequently resulting in lethal aGVHD; thus, alleviating irradiation-induced toxicity is critical to allow the use of anti-CD137 mAb as GVHD prophylaxis.

Breaking of CD8+ T cell tolerance through in vivo ligation of CD40 results in inhibition of chronic graft-versus-host disease and complete donor cell engraftment

In the DBA/2 --> unirradiated (C57BL/6 x DBA/2)F(1) model of chronic graft-vs-host disease (cGVHD), donor CD4(+) T cells play a critical role in breaking host B cell tolerance, while donor CD8(+) T cells are rapidly removed and the remaining cells fall into anergy. Previously we have demonstrated that in vivo ligation of GITR (glucocorticoid-induced TNF receptor-related gene) can activate donor CD8(+) T cells, subsequently converting the disease pattern from cGVHD to an acute form. In this study, we investigated the effect of an agonistic mAb against CD40 on cGVHD. Treatment of anti-CD40 mAb inhibited the production of anti-DNA IgG1 autoantibody and the development of glomerulonephritis. The inhibition of cGVHD occurred because anti-CD40 mAb prevented donor CD8(+) T cell anergy such that subsequently activated donor CD8(+) T cells deleted host CD4(+) T cells and host B cells involved in autoantibody production. Additionally, functionally activated donor CD8(+) T cells induced full engraftment of donor hematopoietic cells and exhibited an increased graft-vs-leukemia effect. However, induction of acute GVHD by donor CD8(+) T cells seemed to be not so apparent. Further CTL analysis indicated that there were lower levels of donor CTL activity against host cells in mice that received anti-CD40 mAb, compared with mice that received anti-GITR mAb. Taken together, our results suggest that a different intensity of donor CTL activity is required for removal of host hematopoietic cells, including leukemia vs induction of acute GVHD.

Graft-versus-host disease causes failure of donor hematopoiesis and lymphopoiesis in interferon-gamma receptor-deficient hosts

The immunopathologic condition known as graft-versus-host disease (GVHD) results from a type I T-cell process. However, a prototypical type I cytokine, interferon-gamma (IFN-gamma), can protect against several manifestations of GVHD in recipients of major histocompatibility complex (MHC)-mismatched hematopoietic cells. We transplanted hematopoietic cells from C3H.SW donors in wild-type (wt) and IFN-gamma-receptor-deficient (IFN-gammaRKO) MHC-matched C57BL/6 recipients. In IFN-gammaRKO recipients, host cells were unresponsive to IFN-gamma, whereas wt donor cells were exposed to exceptionally high levels of IFN-gamma. From an IFN-gamma perspective, we could therefore evaluate the impact of a loss-of-function on host cells and gain-of-function on donor cells. We found that lack of IFN-gammaR prevented up-regulation of MHC proteins on host cells but did not mitigate damage to most target organs. Two salient phenotypes in IFN-gammaRKO recipients involved donor cells: lymphoid hypoplasia and hematopoietic failure. Lymphopenia was due to FasL-induced apoptosis and decreased cell proliferation. Bone marrow aplasia resulted from a decreased proliferation of hematopoietic stem/progenitor cells that was associated with down-regulation of 2 genes negatively regulated by IFN-gamma: Ccnd1 and Myc. We conclude that IFN-gamma produced by alloreactive T cells may entail a severe graft-versus-graft reaction and could be responsible for cytopenias that are frequently observed in subjects with GVHD.

Donor bone marrow type II (non-Valpha14Jalpha18 CD1d-restricted) NKT cells suppress graft-versus-host disease by producing IFN-gamma and IL-4

NKT cells in donor bone marrow (BM) have been demonstrated to protect against graft-vs-host disease (GVHD) following BM transplantation. Murine NKT cells are divided into two distinct subsets based on the invariant Valpha14Jalpha18 TCR expression. However, details of the subset and mechanisms of the BM NKT cells involved in suppressing GVHD have not been clarified. Irradiated BALB/c or C3H/HeN mice administered B6 or Jalpha18(-/-) BM cells show attenuation of GVHD, whereas recipients given CD1d(-/-) BM cells did not show attenuation. Moreover, coinjection of BM non-Valpha14Jalpha18 CD1d-restricted (type II) NKT cells and CD1d(-/-) BM cells suppressed GVHD, whereas coinjection of BM Valpha14Jalpha18 TCR (type I) NKT cells did not. These protective effects on GVHD depended upon IFN-gamma-producing type II NKT cells, which induced the apoptosis of donor T cells. The splenocytes of mice administered BM cells from B6.IL-4(-/-) or Jalpha18(-/-)IL-4(-/-) mice produced lower levels of IL-4 and IL-10 than the splenocytes of mice transplanted with BM cells from B6, B6.IFN-gamma(-/-), Jalpha18(-/-), or Jalpha18(-/-)IFN-gamma(-/-) mice. Taken together, our results show that IFN-gamma-producing BM type II NKT cells suppress GVHD by inducing the apoptosis of donor T cells, while IL-4-producing BM type II NKT cells protect against GVHD by deviating the immune system toward a Th2-type response.

Altered homeostasis of CD4(+) memory T cells in allogeneic hematopoietic stem cell transplant recipients: chronic graft-versus-host disease enhances T cell differentiation and exhausts central memory T cell pool

An increased risk of late infection is a serious complication after allogeneic hematopoietic stem cell transplantation (AHSCT), especially for recipients with defective CD4(+) T cell recovery. Although chronic graft-versus-host disease (cGVHD) negatively influences CD4(+) T cell reconstitution, the mechanisms leading to this defect are not well understood. We found that the proportion of CD27(-) CD4(+) T cells was remarkably increased in ASHCT recipients with cGVHD or with repetitive infectious episodes. Isolated CD27(-) CD4(+) T cells from ASHCT recipients had significantly shortened telomere length, displayed enhanced vulnerability to activation-induced cell death, and showed extremely reduced clonal diversity, when compared with CD27(-) CD4(+) T cells from healthy donors. Also, CD27(+) CD4(+) T cells from AHSCT recipients easily lost their expression of CD27 in response to antigen stimulation regardless of cGVHD status. Taken together, these data indicate that homeostasis of memory CD4(+) T cells from AHSCT recipients is altered, and that they easily transit into CD27(-) effector memory T cells. Increased in vivo T cell stimulation observed in recipients with cGVHD further promotes the transition to effector memory cells, a change that decreases the central memory CD4(+) T cell pool and consequently weakens the recipient's defense against persistently infecting pathogens.

Homeostatic expansion and repertoire regeneration of donor T cells during graft versus host disease is constrained by the host environment

Graft versus host disease (GVHD) typically results in impaired T-cell reconstitution characterized by lymphopenia and repertoire skewing. One of the major causes of inadequate T-cell reconstitution is that T-cell survival and expansion in the periphery are impaired. In this report, we have performed adoptive transfer studies to determine whether the quantitative reduction in T-cell numbers is due to an intrinsic T-cell defect or whether the environmental milieu deleteriously affects T-cell expansion. These studies demonstrate that T cells obtained from animals with graft-versus-host disease (GVHD) are capable of significant expansion and renormalization of an inverted CD4/CD8 ratio when they are removed from this environment. Moreover, these cells can generate complex T-cell repertoires early after transplantation and are functionally competent to respond to third-party alloantigens. Our data indicate that T cells from mice undergoing GVHD can respond to homeostatic signals in the periphery and are not intrinsically compromised once they are removed from the GVHD environment. We thereby conclude that the host environment and not an intrinsic T-cell defect is primarily responsible for the lack of effective T-cell expansion and diversification of complex T-cell repertoires that occurs during GVHD.

FTY720 enhances the activation-induced apoptosis of donor T cells and modulates graft-versus-host disease

FTY720 is a novel immunosuppressant that improves the outcomes after solid organ and bone marrow transplantation (BMT) due to the sequestration of T cells into LN. We tested the hypothesis that the sequestration of donor T cells in LN by FTY720 would enhance their interaction with host APC, thus causing a greater degree of activation-induced apoptosis of alloreactive T cells, and thereby resulting in a reduction of graft-vs.-host disease (GVHD). The short-term administration of FTY720 improved the recipient survival after allogeneic BMT. FTY720 treatment facilitated a rapid contraction of the donor T cell pool in association with an increased degree of apoptosis of donor T cells. The donor T cell reactivity to host alloantigens was diminished in host's LN and adoptive transfer of donor T cells isolated from LN of FTY720-treated recipients of allogeneic BMT induced less severe GVHD in secondary recipients than the transfer from controls. Caspase-dependent apoptosis was involved in this mechanism because FTY720-induced protection was abrogated when a pan-caspase inhibitor was administered. These findings thus demonstrate the presence of a novel mechanism by which FTY720 modulates the allogeneic T cell responses: namely, by the induction of activation-induced apoptosis of alloreactive T cells in LN.

IL-7 in allogeneic transplant: clinical promise and potential pitfalls

Much progress has been made in the field of allogeneic stem cell transplantation. However, one major barrier is the delay in immune recovery that can persist for months post-transplant and results in increased susceptibility to infection and relapse of malignancy. Strategies to improve immune recovery must be balanced with the potential for those therapies to exacerbate graft vs host disease. Interleukin 7 is a member of the gammac cytokine family that is required for T-cell development and maintenance of naïve T-cell populations. In addition, IL-7 plays a major role in the expansion of mature T-cells that occurs during lymphopenia and therapeutic IL-7 can enhance both quantitative and functional immune recovery following T-cell depletion. Thus, this agent holds much promise as an immunorestorative agent and as an adjuvant to vaccines or adoptive immunotherapy. Clinic trials with IL-7 are underway. Murine studies with IL-7 in the allogeneic transplant have demonstrated that the potent immune effects of this agent can also be achieved in this setting. However, these studies have indicated that the potential for IL-7 to worsen GVHD exists and that this effect may abrogate the immune benefits. Thus, careful consideration of how best to incorporate IL-7 into allogeneic trials will be needed if the full potential of this agent is to be realized.

Host-derived Langerhans cells persist after MHC-matched allografting independent of donor T cells and critically influence the alloresponses mediated by donor lymphocyte infusions

Mouse models of minor histocompatibility Ag-mismatched bone marrow transplantation were used to study donor dendritic cell (DC) reconstitution after conditioning, variables influencing the persistence of residual host DCs in different compartments, their phenotype, and their role in governing donor lymphocyte infusion (DLI)-mediated alloresponses. Reconstitution of all splenic DC subsets occurred rapidly after bone marrow transplantation and before T cell reconstitution. However, in contrast to MHC-mismatched chimeras, residual host-derived DCs persisted in the cutaneous lymph nodes (CLNs) of MHC-matched chimeras despite the presence or addition of donor T cells to the graft. The phenotype of these residual host-derived DCs in CLNs was consistent with Langerhans' cells (LCs). We confirmed their skin origin and found near-complete preservation of host-derived LCs in the skin. Host-derived LCs retained their ability to continuously traffic to the CLNs, expressed homogeneously increased levels of costimulatory molecules, and could capture and carry epicutaneously applied Ags. To determine the role of residual host LCs in governing DLI-mediated alloresponses, we administered DLI alone or after topical application of the TLR7 ligand imiquimod, which is known to enhance the LC emigration from the skin. DLI administration resulted in a decrease in host-derived DCs in the CLNs and increased recruitment of donor-derived DCs to the skin, whereas imiquimod augmented their alloreactivity. These results suggest uniqueness of the MHC-matched setting in relation to the persistence of host-derived DCs in the skin and points to a previously unrecognized role of host-derived LCs in the induction of DLI-mediated graft-vs-host alloresponses.

Donor-derived thymic-dependent T cells cause chronic graft-versus-host disease

Chronic graft-versus-host disease (GVHD) is the most common cause of poor long-term outcomes after allogeneic bone marrow transplantation (BMT), but the pathophysiology of chronic GVHD still remains poorly understood. We tested the hypothesis that the impaired thymic negative selection of the recipients will permit the emergence of pathogenic T cells that cause chronic GVHD. Lethally irradiated C3H/HeN (H-2k) recipients were reconstituted with T-cell–depleted bone marrow cells from major histocompatibility complex [MHC] class II–deficient (H2-Ab1−/−) B6 (H-2b) mice. These mice developed diseases that showed all of the clinical and histopathological features of human chronic GVHD. Thymectomy prevented chronic GVHD, thus confirming the causal association of the thymus. CD4+ T cells isolated from chronic GVHD mice were primarily donor reactive, and adoptive transfer of CD4+ T cells generated in these mice caused chronic GVHD in C3H/HeN mice in the presence of B6-derived antigen-presenting cells. Our results demonstrate for the first time that T cells that escape from negative thymic selection could cause chronic GVHD after allogeneic BMT. These results also suggest that self-reactivity of donor T cells plays a role in this chronic GVHD, and improvement in the thymic function may have a potential to decrease chronic GVHD.

Unmanipulated HLA 2–3 Antigen-Mismatched (Haploidentical) Stem Cell Transplantation Using Nonmyeloablative Conditioning

The major problems in human leukocyte antigen (HLA)-mismatched stem cell transplantation (SCT) are graft failure and graft-versus-host disease (GVHD). Less-intensive regimens should be associated with a lower release of inflammatory cytokines and possibly less GVHD. The objective of this study was to investigate whether HLA-haploidentical SCT can be performed using nonmyeloablative conditioning and pharmacologic GVHD prophylaxis, including glucocorticoids. Using conditioning consisting of fludarabine, busulfan, and anti-T-lymphocyte globulin and GVHD prophylaxis consisting of tacrolimus and methylprednisolone (1 mg/kg/day), 26 patients who had hematologic malignancies in an advanced stage or with a poor prognosis underwent transplantation using peripheral blood stem cells from an HLA-haploidentical donor (2-3 antigen mismatches in the graft-versus-host [GVH] direction) without T-cell depletion. All patients except for 1 achieved donor-type engraftment. Rapid hematologic engraftment was achieved (neutrophils > 0.5 x 10(9)/L on day 12 and platelets > 20 x 10(9)/L on day 12), with full donor chimerism achieved by day 14. Fifteen patients did not develop acute GVHD clinically, and only 5 patients developed grade II GVHD. The recovery of CD4+ T cells was delayed compared with that of CD8+ T cells. Sixteen of the 26 patients are alive in complete remission. Four died of transplantation-related causes, and 6 died of progressive disease. These data suggest that nonmyeloablative conditioning, GVHD prophylaxis consisting of tacrolimus and methylprednisolone, and early therapeutic intervention for the GVH reaction allow stable engraftment and effectively suppress GVHD in HLA 2-3 antigen-mismatched SCT.

Role of dioxin response element and nuclear factor-kappaB motifs in 2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated regulation of Fas and Fas ligand expression

We have demonstrated previously that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) up-regulates Fas and FasL in immune cells, although the molecular mechanisms remain unknown. We investigated the regulation of Fas or FasL promoter by TCDD in EL4 T cells using luciferase reporter constructs. We observed 20 +/- 5- and 14 +/- 4-fold induction of promoter activity for Fas and FasL, respectively, after TCDD exposure. The induction of luciferase was significantly reduced (2 +/- 1-fold) in the presence of alpha-naphthoflavone, an aryl hydrocarbon receptor (AhR) antagonist. We noted the presence of a dioxin response element (DRE) and five nuclear factor-kappaB (NF-kappaB) motifs on Fas promoter, and no DRE but two NF-kappaB motifs on FasL promoter. When we investigated the role of DRE and NF-kappaB, we observed varying levels of luciferase induction (9 +/- 2-fold for DRE and 8 +/- 2-fold for NF-kappaBs of Fas promoter and 6 +/- 3-fold for NF-kappaBs of FasL promoter). Mutations in DRE of Fas promoter or NF-kappaBs of FasL promoter led to decreased luciferase induction, further supporting our results. Probes for DRE or NF-kappaB motifs of Fas and/or FasL promoters demonstrated mobility shift in the presence of nuclear extract from TCDD-treated EL4 cells. Furthermore, we observed supershift in mobility when DRE and NF-kappaB probes were incubated in the presence of anti-mouse AhR, and anti-NF-kappaB (RelA/p65 and p50) antibodies, respectively. Administration of TCDD into mice caused significant increase in Fas and FasL transcripts in thymus and liver. These data demonstrate that TCDD regulates Fas and FasL promoters through DRE and/or NF-kappaB motifs via AhR.

IL-17 plays an important role in the development of experimental autoimmune encephalomyelitis

IL-17 is a proinflammatory cytokine that activates T cells and other immune cells to produce a variety of cytokines, chemokines, and cell adhesion molecules. This cytokine is augmented in the sera and/or tissues of patients with contact dermatitis, asthma, and rheumatoid arthritis. We previously demonstrated that IL-17 is involved in the development of autoimmune arthritis and contact, delayed, and airway hypersensitivity in mice. As the expression of IL-17 is also augmented in multiple sclerosis, we examined the involvement of this cytokine in these diseases using IL-17(-/-) murine disease models. We found that the development of experimental autoimmune encephalomyelitis (EAE), the rodent model of multiple sclerosis, was significantly suppressed in IL-17(-/-) mice; these animals exhibited delayed onset, reduced maximum severity scores, ameliorated histological changes, and early recovery. T cell sensitization against myelin oligodendrocyte glycoprotein was reduced in IL-17(-/-) mice upon sensitization. The major producer of IL-17 upon treatment with myelin digodendrocyte glycopritein was CD4+ T cells rather than CD8+ T cells, and adoptive transfer of IL-17(-/-) CD4+ T cells inefficiently induced EAE in recipient mice. Notably, IL-17-producing T cells were increased in IFN-gamma(-/-) cells, while IFN-gamma-producing cells were increased in IL-17(-/-) cells, suggesting that IL-17 and IFN-gamma mutually regulate IFN-gamma and IL-17 production. These observations indicate that IL-17 rather than IFN-gamma plays a crucial role in the development of EAE.

CTLA-4.FasL inhibits allogeneic responses in vivo

CTLA-4.Fas ligand (CTLA-4.FasL), a paradigmatic 'trans signal converter protein (TSCP)', can attach to APC (via CTLA-4 binding to B7) and direct intercellular inhibitory signals to responding T cells (via FasL binding to Fas receptor), converting an activating APC-to-T cell signal into an inhibitory one. Our previous studies established that CTLA-4.FasL inhibits human primary mixed lymphocyte reactions (MLR) and induces alloantigen-specific hyporesponsiveness ex vivo. The present study extends this to an in vivo context. Using splenocytes from MHC-mismatched C57BL/6 and Balb/c mice, we demonstrated that his(6)CTLA-4.FasL, effectively inhibits murine MLR. Moving in vivo, we demonstrated that subcutaneously administered his(6)CTLA-4.FasL modulates the in vivo response of infused allogeneic splenocytes. his(6)CTLA-4.FasL reduces the number of cells in each cell division, and increases the percentage of dead cells in each division. These findings are consistent with an antigen-induced cell death of the alloreactive cells, and bolsters recombinant TCSP promise as a therapeutic for transplantation diseases.

Ex Vivo-Expanded CD4+CD25+Immunoregulatory T Cells Prevent Graft-versus-Host-Disease by Inhibiting Activation/Differentiation of Pathogenic T Cells

CD4+CD25+ immunoregulatory T cells (Tregs) can be administered to inhibit graft-vs-host disease (GVHD) while preserving graft-vs-leukemia activity after allogeneic bone marrow transplantation in mice. Preclinical studies suggest that it is necessary to infuse as many Tregs as conventional donor T cells to achieve a clinical effect on GVHD. Thus, it would be necessary to expand Tregs ex vivo before transplantation. Two strategies have been proposed: expansion of Tregs stimulated by anti-CD3/CD28-coated microbeads for polyclonal activation or by host-type allogeneic APCs for selecting Tregs specific for host Ags. In this study, we describe the mechanisms by which ex vivo-expanded Tregs act on donor T cells to prevent GVHD in mice. We demonstrate that expanded Tregs strongly inhibited the division, expansion, and differentiation of donor T cells, with a more pronounced effect with Tregs specific for host Ags. These latter cells permit the efficient and durable control of GVHD and favor immune reconstitution.

Immune reconstitution following hematopoietic progenitor cell transplantation: challenges for the future

Successful hematopoietic progenitor cell transplantation requires rapid and complete transfer of the donor hematopoietic and immune systems to the host. Whereas the uncontrolled transfer of a nontolerant donor immune system results in GVHD in many cases, strategies which diminish GVHD also diminish immune reconstitution. Thus, the reliable, rapid and safe transfer of immunity from donor to host remains a major challenge for the field. Advances in the understanding of the biology of immune reconstitution have elucidated that thymic-dependent immune reconstitution can restore global immunity, but is especially vulnerable to toxicities associated with transplant. Alternatively, homeostatic peripheral expansion can be exploited for targeted immunity toward pathogens and tumors, but is difficult to manipulate without exacerbating GVHD risk. New translatable strategies are needed to safely augment one or both of these pathways in the setting of allogeneic hematopoietic progenitor cell transplantation.

Alloreactive memory T cells are responsible for the persistence of graft-versus-host disease

Graft-vs-host disease (GVHD) is caused by a donor T cell anti-host reaction that evolves over several weeks to months, suggesting a requirement for persistent alloreactive T cells. Using the C3H.SW anti-C57BL/6 (B6) mouse model of human GVHD directed against minor histocompatibility Ags, we found that donor CD8(+) T cells secreting high levels of IFN-gamma in GVHD B6 mice receiving C3H.SW naive CD8(+) T cells peaked by day 14, declined by day 28 after transplantation, and persisted thereafter, corresponding to the kinetics of a memory T cell response. Donor CD8(+) T cells recovered on day 42 after allogeneic bone marrow transplantation expressed the phenotype of CD44(high)CD122(high)CD25(low), were able to homeostatically survive in response to IL-2, IL-7, and IL-15 and rapidly proliferated upon restimulation with host dendritic cells. Both allogeneic effector memory (CD44(high)CD62L(low)) and central memory (CD44(high)CD62L(high)) CD8(+) T cells were identified in B6 mice with ongoing GVHD, with effector memory CD8(+) T cells as the dominant (>80%) population. Administration of these allogeneic memory CD8(+) T cells into secondary B6 recipients caused virulent GVHD. A similar allogeneic memory CD4(+) T cell population with the ability to mediate persistent GVHD was also identified in BALB/b mice receiving minor histocompatibility Ag-mismatched B6 T cell-replete bone marrow transplantation. These results indicate that allogeneic memory T cells are generated in vivo during GVH reactions and are able to cause GVHD, resulting in persistent host tissue injury. Thus, in vivo blockade of both alloreactive effector and memory T cell-mediated host tissue injury may prove to be valuable for GVHD prevention and treatment.

Emergent autoimmunity in graft-versus-host disease

Donor T-cell recognition of host alloantigens presented by host antigen-presenting cells (APCs) is necessary for the induction of graft-versus-host disease (GVHD), but whether direct alloreactivity is sufficient for the propagation of GVHD is unknown. In this study, we demonstrate that GVHD cannot be effectively propagated through the direct pathway of allorecognition. Rather, donor T-cell recognition of antigens through the indirect pathway is necessary for the perpetuation of GVHD. Furthermore, GVHD results in the breaking of self tolerance, resulting in the emergence of donor T cells that can cause autoimmune disease in syngeneic recipients. Notably, GVHD-induced autoreactivity is donor APC dependent, transferable into secondary hosts, and involves cells of the innate immune system. These results indicate that donor T-cell--mediated pathologic damage during GVHD becomes donor APC dependent and provide a mechanistic explanation for the long-standing observation that GVHD is associated with autoimmune clinical manifestations.

Transplantation of hematopoietic stem cells for induction of unresponsiveness to organ allografts

Although it has been recognized since the early days of Owen and Medawar that engraftment of donor stem cells, induced in utero spontaneously or intentionally neonatally, results in life-long unresponsiveness to donor alloantigens. However, successful induction of transplantation tolerance in adult life still represents an unsolved problem. Engraftment of donor stem cells using conventional modalities involves intensive myeloablative or lymphoablative immunosuppression, which is associated with toxicity and mortality and such methods are not suitable for organ allograft recipients. In this chapter, we present an innovative approach for induction of donor-specific unresponsiveness to bone marrow and organ allografts without myeloablative conditioning. Our methods is based on cyclophosphamide-induced, alloantigen-primed lymphocyte depletion. Cyclophosphamide is administered 1 day following infusion of donor hematopoietic cells, thus eliminating predominantly host T lymphocytes reacting against donor cell challenge, and resulting in relative unresponsiveness to donor alloantigens. Subsequently, life-long tolerance to fully mismatched donor skin allografts can be accomplished by a second infusion of stem cells from the same donor, with donor T cells displacing residual alloreactive host cells that may have escaped deletion. Taken together, we believe that induction of true permanent and specific tolerance to organ allografts using donor hematopoietic cells could become a clinical reality in the foreseeable future.

Selective elimination of alloreactive donor T cells attenuates graft-versus-host disease and enhances T-cell reconstitution

Impaired T-cell immune reconstitution is a major complication after allogeneic bone marrow transplantation (BMT) and is particularly exacerbated in the setting of graft-versus-host disease (GVHD). Conventional approaches to reduce GVHD, such as T-cell depletion or pharmacologic immunosuppression, typically fail to enhance T-cell immunity and often further exacerbate this problem. An alternative strategy to mitigate GVHD severity is the selective elimination of graft-versus-host-reactive donor T cells by using an incorporated thymidine kinase suicide gene. This approach has been shown to effectively reduce GVHD, although the effect of this strategy on T-cell reconstitution is unresolved. We addressed this question in a murine BMT model (C57BL/6 [H-2(b)] --> AKR/J [H-2(k)]) in which donor and recipient differ at major and minor histocompatibility antigens. Lethally irradiated AKR recipients transplanted with T cell-depleted bone marrow plus thymidine kinase-positive T cells followed by post-BMT ganciclovir (GCV) administration had more prompt and complete normalization of the T-cell repertoire than phosphate-buffered saline-treated GVHD control animals. By 60 days after transplantation, mice administered GCV had T-cell repertoires that were virtually indistinguishable from those of mice that underwent transplantation with T cell-depleted bone marrow alone (no GVHD controls) when assayed by T-cell receptor (TCR) spectratyping. In contrast, phosphate-buffered saline-treated animals had persistent skewing in most Vbeta families. T cells obtained from GCV-treated mice also had significantly higher in vitro proliferative responses after posttransplantation inoculation with ovalbumin than GVHD animals, indicating that CD4(+) T-cell responses against a nominal antigen were better preserved in these chimeras. Finally, GCV-treated mice had augmented immune reconstitution in response to exogenous interleukin-7 administration, as evidenced by increased overall spleen cellularity and absolute numbers of T and B cells. This was in contrast to GVHD control animals, which had a blunted response to interleukin-7 administration. These data indicate that GVHD severity can be significantly reduced by selective elimination of alloreactive donor T cells without compromise of T-cell immunity. Moreover, in light of previous studies demonstrating that this strategy can reduce GVHD without loss of alloengraftment and antileukemia reactivity, further examination of this approach in humans seems warranted.

Factors affecting reconstitution of the T cell compartment in allogeneic haematopoietic cell transplant recipients

The factors affecting T cell reconstitution post haematopoietic cell transplantation (HCT) are not well characterised. We carried out a longitudinal analysis of T cell reconstitution in 32 HCT recipients during the first 12 months post transplant. We analysed reconstitution of naïve, memory and effector T cells, their diversity and monitored thymic output using TCR rearrangement excision circles (TRECs). Thymic-independent pathways were responsible for the rapid reconstitution of memory and effector T cells less than 6 months post HCT. Thymic-dependent pathways were activated between 6 and 12 months in the majority of patients with naïve T cell numbers increasing in parallel with TREC levels. Increasing patient age, chronic GVHD and T cell depletion (with or without pretransplant Campath-1H) predicted low TREC levels and slow naïve T cell recovery. Furthermore, increasing patient age also predicted high memory and effector T cell numbers. The effects of post HCT immunosuppression, total body irradiation, donor leucocyte infusions, T cell dose and post HCT infections on T cell recovery were also analysed. However, no effects of these single variables across a variety of different age, GVHD and T cell depletion groups were apparent. This study suggests that future analysis of the factors affecting T cell reconstitution and studies aimed at reactivating the thymus through therapeutic intervention should be analysed in age-, GVHD- and TCD-matched patient groups.

Immunobiology of acute graft-versus-host disease

Graft-versus-host disease (GVHD) has been the primary limitation to the wider application of allogeneic bone marrow transplantation (BMT). The immunobiology of acute GVHD is complex and can be conceptualized to be a three-step process. In step 1, the conditioning regimen (irradiation and/or chemotherapy) leads to the damage and activation of host tissues and induces the secretion of inflammatory cytokines TNF-alpha and IL-1. As a consequence expression of MHC antigens and adhesion molecules is increased, thus enhancing the recognition of host alloantigens by donor T cells. Donor T-cell activation in step 2 is characterized by donor T-cell interaction with host APCs and subsequent proliferation, differentiation, and secretion of cytokines. Cytokines such as IL-2 and IFN-gamma enhance T-cell expansion, induce cytotoxic T cells (CTL) and natural killer (NK) cell responses, and prime additional mononuclear phagocytes to produce TNF-alpha and IL-1. These inflammatory cytokines in turn stimulate production of inflammatory chemokines, thus recruiting effector cells into target organs. In step 3, effector functions of mononuclear phagocytes are triggered via a secondary signal provided by lipopolysaccharide (LPS) that leaks through the intestinal mucosa damaged during step 1. This mechanism may result in the amplification of local tissue injury and further promotion of an inflammatory response, which, together with the CTL and NK components, leads to target tissue destruction in the transplant host.

Host conditioning is a primary determinant in modulating the effect of IL-7 on murine graft-versus-host disease

Interleukin-7 has been shown to enhance T cell reconstitution after allogeneic bone marrow transplantation, in part, by expansion of mature donor T cells, but whether IL-7 also exacerbates graft-vs-host disease (GVHD) remains unresolved. To address this issue, we examined the effect of IL-7 on GVHD induction using a well-defined murine GVHD model (B6-->B6AF1/J). Administration of IL-7 to nonirradiated B6AF1/J recipients of B6 T cells resulted in expansion of splenic donor CD4(+) and CD8(+) T cells and increased GVHD mortality. In contrast, administration of IL-7 on the same schedule failed to increase GVHD mortality in either sublethally or lethally irradiated animals that received graded doses of T cells designed to induce varying degrees of GVHD severity. Moreover, IL-7 failed to increase the number of alloreactive T cells when examined in a murine model (B6-->BALB.B) that allowed for direct quantitation of graft-vs-host-reactive T cells. The combination of irradiation and transplantation of alloreactive donor T cells resulted in significantly increased levels of endogenous splenic IL-7 mRNA when compared with nonirradiated transplanted animals, providing a potential explanation for why exogenous IL-7 did not increase GVHD severity in these mice. We conclude that host conditioning modulates the ability of exogenous IL-7 to exacerbate GVHD and that this occurs through induction of endogenous IL-7 production.

Pathophysiology of acute graft-versus-host disease

Graft-versus-host disease (GVHD) has been the primary limitation to the wider application of allogeneic bone marrow transplantation (BMT). The pathophysiology of acute GVHD is complex and can be conceptualized to be a three-step process based on murine studies. In step 1, the conditioning regimen leads to the damage and activation of host tissues and induces the secretion of inflammatory cytokines. As a consequence, the expression of MHC antigens and adhesion molecules is increased enhancing the recognition of host alloantigens by donor T cells. Donor T-cell activation in step 2 is characterized by donor T cell interaction with host APCs and subsequent proliferation, differentiation and secretion of cytokines. Cytokines such as IL-2 and IFN-gamma enhance T-cell expansion, induce cytotoxic T cells (CTL) and natural killer (NK) cell responses and prime additional mononuclear phagocytes to produce TNF-alpha and IL-1. These inflammatory cytokines in turn stimulate production of inflammatory chemokines, thus recruiting effector cells into target organs. In step 3, effector functions of mononuclear phagocytes are triggered via a secondary signal provided by lipopolysaccharide (LPS) that leaks through the intestinal mucosa damaged during step 1. This mechanism may result in the amplification of local tissue injury and further promotion of an inflammatory response, which, together with the CTL and NK components, leads to target tissue destruction in the transplant host. The following review discusses the three-step process of the pathophysiology of experimental acute GVHD.

The effect on the proliferation and apoptosis of alloreactive T cells of cell dose in a murine MHC-mismatched hematopoietic cell transplantation model

Activation-induced cell death (AICD) of lymphocytes is an apoptotic pathway involved in the control of T-cell homeostasis. The magnitude of graft-vs.-host disease (GVHD) following allogeneic hematopoietic cell transplantation may be attenuated by the enhancement of AICD. The aim of the present study was to clarify the effect of T cell dose upon the fate (proliferation or apoptosis) of individual activated T cells in a murine GVHD model. To this end, we investigated the kinetics of the proliferation and apoptosis of donor T cells in recipient spleens in the early stage of a fully major histocompatibility complex (MHC)-mismatched murine transplantation model from C57BL/6 (H-2(b)) to lethally-irradiated (8.5 Gy) BALB/c (H-2(d)) mice. To track the behavior of alloreactive lymphocytes in vivo, we used the fluorescent cytoplasmic dye carboxyfluorescein diacetate succinimidyl ester in combination with flow cytometry. Engraftment of donor T cells to recipient spleens was almost completed within 24 h after transfer. After that, at higher doses of transferred cells, the donor T cells actively divided for up to 72 h resulting in a 30-fold increase in cell number at the maximum cell dose (2.0 x 10(7)). As the transferred cell dose decreased, the proliferation of T cells tended to be suppressed. At cell doses of 0.5 x 10(7) or less, the proliferation of T cells was profoundly suppressed, ultimately resulting in little proliferation of donor T cells observed from 24 to 72 h at the minimum cell dose (0.1 x 10(7)). The frequency of Annexin-V-positive cells was found to increase gradually as the transferred cell dose decreased. Thus, an increase in apoptotic events appeared to play an important role in the suppression of the proliferation of T cells at lower splenocyte doses. Further analyses revealed that Fas ligand (FasL)-positive T cells were observed exclusively among T cells that divided at least 5 times, and that all of them were positive for Annexin-V, indicating that they were in the process of apoptosis. Together with our finding that the frequency of apoptosis increased with the progression of cell division, these findings strongly suggest that AICD occurred through the Fas/FasL system and that AICD increased as the dose of donor T cells participating in the allogeneic response decreased. When relatively small numbers of T cells are confronted with an excess of antigen, they disappear. This process is called clonal exhaustion-deletion. Our results support the idea that AICD is involved in the process of clonal exhaustion-deletion. Relevant to the clinical aspects of hematopoietic cell transplantation, our findings indicate that AICD may be associated with tolerance induction in T-cell-depleted transplantation from HLA-mismatched donors, in which T cells contaminating marrow grafts do not need to be completely removed for achieving tolerance between donors and recipients. Furthermore, our results indicate that a small change in the quantitative balance between antigens and T cells responding to them leads to a large difference in the fate of T cells activated in response to MHC-incompatible antigens. Thus, the size of the T cell dose is one of the important considerations in tolerance induction, GVHD and rejection.

Tissue distribution of target antigen has a decisive influence on the outcome of adoptive cancer immunotherapy

Adoptive transfer of allogeneic T cells has unmatched efficacy to eradicate leukemic cells. We therefore sought to evaluate in kinetic terms interactions between T cells and allogeneic leukemic cells. T cells primed against the model B6(dom1) minor histocompatibility antigen were adoptively transferred in irradiated B10 (B6(dom1)-positive) and congenic B10.H7(b) (B6(dom1)-negative) recipients, some of which were also injected with EL4 leukemia/lymphoma cells (B6(dom1)-positive). A key finding was that the tissue distribution of the target epitope dramatically influenced the outcome of adoptive cancer immunotherapy. Widespread expression of B6(dom1) in B10 recipients induced apoptosis and dysfunction of antigen-specific T cells. Furthermore, in leukemic B10 and B10.H7(b) hosts, a massive accumulation of effector/memory B6(dom1)-specific T cells was detected in the bone marrow, the main site of EL4 cell growth. The accumulation of effector/memory cells in recipient bone marrow was EL4 dependent, and its kinetics was different from that observed in recipient spleen. We conclude that strategies must be devised to prevent apoptosis of adoptively transferred T cells confronted with a high antigen load and that local monitoring of the immune response at the site of tumor growth may be mandatory for a meaningful assessment of the efficacy of adoptive immunotherapy.

LF 15-0195 immunosuppressive agent enhances activation-induced T-cell death by facilitating caspase-8 and caspase-10 activation at the DISC level

The deoxyspergualin derivative LF 15-0195 has demonstrated some efficacy in animal models of autoimmune and graft-versus-host diseases and is currently tested in clinics. The molecular mechanisms of LF 15-0195 immunosuppressive activity remained unknown. We show that exposure to LF 15-0195 sensitizes Jurkat T cells to apoptosis induced by an agonistic anti-CD95 antibody (CH11 clone) and by the cytokine TNF-related apoptosis-inducing ligand. LF 15-0195 does not demonstrate any significant effect on the postmitochondrial activation of caspases, nor does it modify overall expression of CD95, Fas-associated death domain, and procaspase-8. The compound facilitates the recruitment of these molecules to the death-inducing signaling complex (DISC) and enhances caspase-8 and -10 activation, thus increasing cytochrome c and direct IAP binding with low pI (DIABLO)/Smac mitochondrial release. LF 15-0195 also sensitizes Jurkat T cells to CD3-mediated apoptosis, an in vitro model for activation-induced T-cell death (AICD). LF 15-0195-mediated sensitization to AICD was further confirmed in human peripheral T cells exposed to anti-CD3 antibodies, then cultured in the presence of interleukin-2. In these cells, LF 15-0195 increased apoptosis triggered by either anti-CD95 antibodies or CD3 restimulation, whereas no effect was observed on "passive apoptosis." Finally, in bone marrow recipient mice, LF 15-0195 enhanced allogeneic donor T-cell death, which required a functional CD95 pathway. These results suggest that LF 15-0195 sensitizes T cells to AICD by increasing caspase activation at the DISC level in response to CD95 engagement. This original mechanism, together with LF 15-0195 efficacy in various disease models, makes this compound a promising immunosuppressive drug.