Graft-versus-host-disease-associated lymphoid hypoplasia and B cell dysfunction is dependent upon donor T cell-mediated Fas-ligand function, but not perforin function

Deciphering bone marrow engraftment after allogeneic stem cell transplantation in humans using single-cell analyses

BACKGROUNDDonor cell engraftment is a prerequisite of successful allogeneic hematopoietic stem cell transplantation. Based on peripheral blood analyses, it is characterized by early myeloid recovery and T and B cell lymphopenia. However, cellular networks associated with bone marrow engraftment of allogeneic human cells have been poorly described.METHODSMass cytometry and CITE-Seq analyses were performed on bone marrow cells 3 months after transplantation in patients with acute myelogenous leukemia.RESULTSMass cytometric analyses in 26 patients and 20 healthy controls disclosed profound alterations in myeloid and B cell progenitors, with a shift toward terminal myeloid differentiation and decreased B cell progenitors. Unsupervised analysis separated recipients into 2 groups, one of them being driven by previous graft-versus-host disease (R2 patients). We then used single-cell CITE-Seq to decipher engraftment, which resolved 36 clusters, encompassing all bone marrow cellular components. Hematopoiesis in transplant recipients was sustained by committed myeloid and erythroid progenitors in a setting of monocyte-, NK cell-, and T cell-mediated inflammation. Gene expression revealed major pathways in transplant recipients, namely, TNF-α signaling via NF-κB and the IFN-γ response. The hallmark of allograft rejection was consistently found in clusters from transplant recipients, especially in R2 recipients.CONCLUSIONBone marrow cell engraftment of allogeneic donor cells is characterized by a state of emergency hematopoiesis in the setting of an allogeneic response driving inflammation.FUNDINGThis study was supported by the French National Cancer Institute (Institut National du Cancer; PLBIO19-239) and by an unrestricted research grant by Alexion Pharmaceuticals.

Influence of Housing Temperature and Genetic Diversity on Allogeneic T Cell-Induced Tissue Damage in Mice

The objective of this study was to determine how housing temperature and genetic diversity affect the onset and severity of allogeneic T cell-induced tissue damage in mice subjected to reduced intensity conditioning (RIC). We found that adoptive transfer of allogeneic CD4+ T cells from inbred donors into sub-lethally irradiated inbred recipients (I→I) housed at standard housing temperatures (ST; 22-24 °C) induced extensive BM and spleen damage in the absence of injury to any other tissue. Although engraftment of T cells in RIC-treated mice housed at their thermo-neutral temperature (TNT; 30-32 °C) also developed similar BM and spleen damage, their survival was markedly and significantly increased when compared to their ST counterparts. In contrast, the adoptive transfer of allogeneic T cells into RIC-treated outbred CD1 recipients failed to induce disease in any tissue at ST or TNT. The lack of tissue damage was not due to defects in donor T cell trafficking to BM or spleen but was associated with the presence of large numbers of B cells and myeloid cells within these tissues that are known to contain immunosuppressive regulatory B cells and myeloid-derived suppressor cells. These data demonstrate, for the first time, that housing temperature affects the survival of RIC-treated I→I mice and that RIC-conditioned outbred mice are resistant to allogeneic T cell-induced BM and spleen damage.

Ambivalent role of FasL in murine acute graft-versus-host-disease

Fas ligand is increased in several immune-mediated diseases, including acute graft-versus-host disease, a donor cell-mediated disorder post-hematopoietic stem cell transplantation. In this disease, Fas ligand is involved in T-cell-mediated damage to host tissues. However, the role of its expression on donor non-T cells has, so far, never been addressed. Using a well-established CD4- and CD8-mediated graft-versus-host disease murine model, we found that precocious gut damage and mice mortality are increased with a graft of donor T- and B-depleted bone marrow cells devoid of Fas ligand as compared with their wild-type counterparts. Interestingly, serum levels of both soluble Fas ligand and IL-18 are drastically reduced in the recipients of Fas ligand-deficient grafts, indicating that soluble Fas ligand stems from donor bone marrow-derived cells. In addition, the correlation between the concentrations of these 2 cytokines suggests that IL-18 production arises through a soluble Fas ligand-driven mechanism. These data highlight the importance of Fas ligand-dependent production in IL-18 production and in mitigating acute graft-versus-host disease. Overall, our data reveal the functional duality of Fas ligand according to its source.

Meibomian Gland Dysfunction: A Route of Ocular Graft-Versus-Host Disease Progression That Drives a Vicious Cycle of Ocular Surface Inflammatory Damage

PURPOSE

To investigate the role of aggressive meibomian gland dysfunction (MGD) in the immune pathogenesis of ocular graft-vs-host disease (GVHD).

METHODS

In mice, an allogeneic GVHD model was established by transferring bone marrow (BM) and purified splenic T cells from C57BL/6J mice into irradiated C3-SW.H2b mice (BM+T). Control groups received BM only. Mice were scored clinically across the post-transplantation period. MGD severity was categorized using the degree of atrophy on harvested lids. Immune disease was analyzed using flow cytometry of tissues along with fluorescent tracking of BM cells onto the ocular surface. In humans, parameters from 57 patients with ocular GVHD presenting to the Duke Eye Center were retrospectively reviewed. MGD was categorized using the degree of atrophy on meibographs. Immune analysis was done using high-parameter flow cytometry on tear samples.

RESULTS

Compared with BM only, BM+T mice had higher systemic disease scores that correlated with tear fluid loss and eyelid edema. BM+T had higher immune cell infiltration in the ocular tissues and higher CD4+-cell cytokine expression in draining lymph nodes. BM+T mice with worse MGD scores had significantly worse corneal staining. In patients with ocular GVHD, 96% had other organs affected. Patients with ocular GVHD had abnormal parameters on dry eye testing, high matrix metalloproteinase-9 positivity (92%), and abundance of immune cells in tear samples. Ocular surface disease signs were worse in patients with higher MGD severity scores.

CONCLUSIONS

Ocular GVHD is driven by a systemic, T-cell-dependent process that causes meibomian gland damage and induces a robust form of ocular surface disease that correlates with MGD severity. NOTE: Publication of this article is sponsored by the American Ophthalmological Society.

Recipient Tregs: Can They Be Exploited for Successful Hematopoietic Stem Cell Transplant Outcomes?

Human and mouse CD4⁺FoxP3⁺ T cells (Tregs) comprise non-redundant regulatory compartments which maintain self-tolerance and have been found to be of potential therapeutic usefulness in autoimmune disorders and transplants including allogeneic hematopoietic stem cell transplantation (allo-HSCT). There is substantial literature interrogating the application of donor derived Tregs for the prevention of graft versus host disease (GVHD). This Mini-Review will focus on the recipient's Tregs which persist post-transplant. Although treatment in patients with low dose IL-2 months post-HSCT are encouraging, manipulating Tregs in recipients early post-transplant is challenging, in part likely an indirect consequence of damage to the microenvironment required to support Treg expansion of which little is understood. This review will discuss the potential for manipulating recipient Tregs in vivo prior to and after HSCT (fusion proteins, mAbs). Strategies that would circumvent donor/recipient peripheral blood harvest, cell culture and ex-vivo Treg expansion will be considered for the translational application of Tregs to improve HSCT outcomes.

Hematopoietic Dysfunction during Graft-Versus-Host Disease: A Self-Destructive Process?

Graft-versus-host disease (GvHD) is a major complication of allogeneic hematopoietic (stem) cell transplantation (HCT). Clinically, GvHD is associated with severe and long-lasting hematopoietic dysfunction, which may contribute to the high mortality of GvHD after HCT. During GvHD, excessive immune activation damages both hematopoietic stem and progenitor cells and their surrounding bone marrow niche, leading to a reduction in cell number and functionality of both compartments. Hematopoietic dysfunction can be further aggravated by the occurrence—and treatment—of HCT-associated complications. These include immune suppressive therapy, coinciding infections and their treatment, and changes in the microbiome. In this review, we provide a structured overview of GvHD-mediated hematopoietic dysfunction, including the targets in the bone marrow, the mechanisms of action and the effect of GvHD-related complications and their treatment. This information may aid in the identification of treatment options to improve hematopoietic function in patients, during and after GvHD.

Impaired Hematopoiesis after Allogeneic Hematopoietic Stem Cell Transplantation: Its Pathogenesis and Potential Treatments

Impaired hematopoiesis is a serious complication after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Bone marrow aplasia and peripheral cytopenias arise from primary and secondary graft failure or primary and secondary poor graft function. Chimerism analysis is useful to discriminate these conditions. By determining the pathogenesis of impaired hematopoiesis, a timely and appropriate treatment can be performed. Hematopoietic system principally consists of hematopoietic stem cells and bone marrow microenvironment termed niches. Abnormality in hematopoietic stem and progenitor cells and/or abnormality in the relevant niches give rise to hematological diseases. Allo-HSCT is intended to cure each hematological disease, replacing abnormal hematopoietic stem cells and bone marrow niches with hematopoietic stem cells and bone marrow niches derived from normal donors. Therefore, treatment for graft failure and poor graft function after allo-HSCT is required to proceed based on determining the pathogenesis of impaired hematopoiesis. Recent progress in this area suggests promising treatment manipulations for graft failure and poor graft function.

Efficient treatment of murine acute GvHD by in vitro expanded donor regulatory T cells

Acute graft-versus-host disease (aGvHD) is a frequent complication after allogeneic bone marrow/stem cell transplantation (BMT/SCT) induced by co-transplanted alloreactive conventional donor T cells. We previously demonstrated that the adoptive transfer of donor CD4⁺CD25⁺Foxp3⁺ regulatory T cells (Treg) at the time of BMT prevents aGvHD in murine models. Yet, the therapeutic potential of donor Treg for the treatment of established aGvHD has not yet been studied in detail. We now used in vitro expanded phenotypically and functionally stable murine Treg to explore their therapeutic efficacy in haploidentical aGvHD models. Upon transfer donor Treg ameliorate clinical and histologic signs of aGvHD and significantly improve survival. They migrate to lymphoid as well as aGvHD target organs, predominantly the gastrointestinal tract, where they inhibit the proliferation of conventional T cells, reduce the influx of myeloid cells, and the accumulation of inflammatory cytokines. Successfully treated animals restore aGvHD-induced tissue damage in target organs and lymphoid tissues, thereby supporting lymphocyte reconstitution. The therapeutically applied Treg population survives long term without conversion into pathogenic effector T cells. These results demonstrate that donor Treg not only prevent aGvHD, but are also efficacious for the treatment of this life-threatening BMT complication.

Inhibition of the IRE-1α/XBP-1 pathway prevents chronic GVHD and preserves the GVL effect in mice

Hematopoietic stem cell transplantation (HCT) is a curative procedure for hematological malignancies, but chronic graft-versus-host disease (cGVHD) remains a major complication after allogeneic HCT. Because donor B cells are essential for cGVHD development and B cells are sensitive to endoplasmic reticulum (ER) stress, we hypothesized that the IRE-1α/XBP-1 pathway is required for B-cell activation and function and for the development of cGVHD. To test this hypothesis, we used conditional knock-out mice deficient of XBP-1 specifically in B cells. Recipients transplanted with donor grafts containing XBP-1-deficient B cells displayed reduced cGVHD compared with controls. Reduction of cGVHD correlated with impaired B-cell functions, including reduced production of anti-double-stranded DNA immunoglobulin G antibodies, CD86, Fas, and GL7 surface expression, and impaired T-cell responses, including reduced interferon-γ production and follicular helper T cells. In a bronchiolitis obliterans cGVHD model, recipients of transplants containing XBP-1-deficient B cells demonstrated improved pulmonary function correlated with reduced donor splenic follicular helper T cells and increased B cells compared with those of wild-type control donor grafts. We then tested if XBP-1 blockade via an IRE-1α inhibitor, B-I09, would attenuate cGVHD and preserve the graft-versus-leukemia (GVL) effect. In a cutaneous cGVHD model, we found that prophylactic administration of B-I09 reduced clinical features of cGVHD, which correlated with reductions in donor T-cell and dendritic cell skin infiltrates. Inhibition of the IRE-1α/XBP-1 pathway also preserved the GVL effect against chronic myelogenous leukemia mediated by allogeneic splenocytes. Collectively, the ER stress response mediated by the IRE-1α/XBP-1 axis is required for cGVHD development but dispensable for GVL activity.

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.

Impaired bone marrow B-cell development in mice with a bronchiolitis obliterans model of cGVHD

Chronic graft-versus-host disease (cGVHD) causes significant morbidity and mortality in patients after allogeneic bone marrow (BM) or stem cell transplantation (allo-SCT). Recent work has indicated that both T and B lymphocytes play an important role in the pathophysiology of cGVHD. Previously, our group showed a critical role for the germinal center response in the function of B cells using a bronchiolitis obliterans (BO) model of cGVHD. Here, we demonstrated for the first time that cGVHD is associated with severe defects in the generation of BM B lymphoid and uncommitted common lymphoid progenitor cells. We found an increase in the number of donor CD4⁺ T cells in the BM of mice with cGVHD that was negatively correlated with B-cell development and the frequency of osteoblasts and Prrx-1-expressing perivascular stromal cells, which are present in the B-cell niche. Use of anti-DR3 monoclonal antibodies to enhance the number of donor regulatory T cells (T_regs) in the donor T-cell inoculum ameliorated the pathology associated with BO in this model. This correlated with an increased number of endosteal osteoblastic cells and significantly improved the generation of B-cell precursors in the BM after allo-SCT. Our work indicates that donor T_regs play a critical role in preserving the generation of B-cell precursors in the BM after allo-SCT. Approaches to enhance the number and/or function of donor T_regs that do not enhance conventional T-cell activity may be important to decrease the incidence and severity of cGVHD in part through normal B-cell lymphopoiesis.

Chronic graft-versus-host disease: biological insights from preclinical and clinical studies

With the increasing use of mismatched, unrelated, and granulocyte colony-stimulating factor-mobilized peripheral blood stem cell donor grafts and successful treatment of older recipients, chronic graft-versus-host disease (cGVHD) has emerged as the major cause of nonrelapse mortality and morbidity. cGVHD is characterized by lichenoid changes and fibrosis that affects a multitude of tissues, compromising organ function. Beyond steroids, effective treatment options are limited. Thus, new strategies to both prevent and treat disease are urgently required. Over the last 5 years, our understanding of cGVHD pathogenesis and basic biology, born out of a combination of mouse models and correlative clinical studies, has radically improved. We now understand that cGVHD is initiated by naive T cells, differentiating predominantly within highly inflammatory T-helper 17/T-cytotoxic 17 and T-follicular helper paradigms with consequent thymic damage and impaired donor antigen presentation in the periphery. This leads to aberrant T- and B-cell activation and differentiation, which cooperate to generate antibody-secreting cells that cause the deposition of antibodies to polymorphic recipient antigens (ie, alloantibody) or nonpolymorphic antigens common to both recipient and donor (ie, autoantibody). It is now clear that alloantibody can, in concert with colony-stimulating factor 1 (CSF-1)-dependent donor macrophages, induce a transforming growth factor β-high environment locally within target tissue that results in scleroderma and bronchiolitis obliterans, diagnostic features of cGVHD. These findings have yielded a raft of potential new therapeutics, centered on naive T-cell depletion, interleukin-17/21 inhibition, kinase inhibition, regulatory T-cell restoration, and CSF-1 inhibition. This new understanding of cGVHD finally gives hope that effective therapies are imminent for this devastating transplant complication.

Novel Scoring Criteria for the Evaluation of Ocular Graft-versus-Host Disease in a Preclinical Allogeneic Hematopoietic Stem Cell Transplantation Animal Model

Ocular complications occur after transplantation in 60% to 90% of chronic graft-versus-host disease (GVHD) patients and significantly impair vision-related quality of life. Ocular surface inflammation and dry eye disease are the most common manifestations of ocular GVHD. Ocular GVHD can be viewed as an excellent preclinical model that can be studied to understand the immune pathogenesis of this common and debilitating disease. A limitation of this is that only a few experimental models mimic the ocular complications after hematopoietic stem cell transplantation (HSCT) and have focused on the acute GVHD process. To address this issue, we used a preclinical animal model developed by our group where ocular involvement was preceded by systemic GVHD to gain insight regarding the contributing immune mechanisms. Employing this "matched unrelated donor" model enabled the development of clinical scoring criteria, which readily identified different degrees of ocular pathology at both the ocular surface and adnexa, dependent on the level of conditioning before HSCT. As far as we are aware, we report for the first time that these clinical and immune responses occur not only on the ocular surface, but they also heavily involve the lid margin region. In total, the present study reports a preclinical scoring model that can be applied to animal models as investigators look to further explore GVHD's immunologic effects at the level of the ocular surface and eyelid adnexa compartments. We speculate that future studies will use this clinical scoring index in combination with what is recognized histologically and correlated with serum biomarkers identified in chronic/ocular GVHD.

Bone Marrow GvHD after Allogeneic Hematopoietic Stem Cell Transplantation

The bone marrow is the origin of all hematopoietic lineages and an important homing site for memory cells of the adaptive immune system. It has recently emerged as a graft-versus-host disease (GvHD) target organ after allogeneic stem cell transplantation (alloHSCT), marked by depletion of both hematopoietic progenitors and niche-forming cells. Serious effects on the restoration of hematopoietic function and immunological memory are common, especially in patients after myeloablative conditioning therapy. Cytopenia and durable immunodeficiency caused by the depletion of hematopoietic progenitors and destruction of bone marrow niches negatively influence the outcome of alloHSCT. The complex balance between immunosuppressive and cell-depleting treatments, GvHD and immune reconstitution, as well as the desirable graft-versus-tumor (GvT) effect remains a great challenge for clinicians.

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.

Recruitment of Donor T Cells to the Eyes During Ocular GVHD in Recipients of MHC-Matched Allogeneic Hematopoietic Stem Cell Transplants

PURPOSE

The primary objective of the present study was to identify the kinetics and origin of ocular infiltrating T cells in a preclinical model of graft-versus-host disease (GVHD) that induces eye tissue damage.

METHODS

Graft-versus-host disease was induced using an major histocompatibility complex (MHC)-matched, minor histocompatibility-mismatched hematopoietic stem cell transplant (HSCT) model. This approach, which utilized congenic and EGFP-labeled donor populations, mimics a matched, clinically unrelated donor (MUD) cell transplant. Systemic and ocular GVHD were assessed at varying time points using clinical examination, intravital microscopy, immune phenotype via flow cytometric analyses, and immunohistochemical staining.

RESULTS

Following transplant, we observed characteristic changes in GVHD-associated immune phenotype as well as clinical signs present in recipients post transplant. Notably, the kinetics of the systemic changes and the ocular damage paralleled what is observed clinically, including damage to the cornea as well as the conjunctiva and lacrimal gland. Importantly, the infiltrate contained predominantly donor CD4 as well as CD8 T cells with an activated phenotype and macrophages together with effector cytokines consistent with the presence of a TH1 alloreactive population.

CONCLUSIONS

Overall, the findings here unequivocally demonstrated that donor T cells compose part of the corneal and ocular adnexa infiltrate in animals undergoing ocular GVHD. In total, the results describe a novel and promising preclinical model characterized by both systemic and ocular changes as detected in significant numbers of patients undergoing GVHD following allo-HSCT, which can help facilitate dissecting the underlying immune mechanisms leading to damage associated with ocular GVHD.

Apoptosis Susceptibility Prolongs the Lack of Memory B Cells in Acute Leukemic Patients After Allogeneic Hematopoietic Stem Cell Transplantation

Long-term survival after allogeneic hematopoietic stem cell transplantation requires intact immunosurveillance, which is hampered by lymphoid organ damage associated with conditioning therapy, graft-versus-host disease, and immunosuppression. Our study aimed to identify the mechanisms contributing to sustained low memory B cell numbers after transplantation. Peripheral B and T cell subset recovery and functional marker expression were investigated in 35 acute leukemic patients up to 1 year after transplantation. Apoptosis of B cells after CD40/TLR-9, CD40/BCR, and CD40/BCR/TLR-9-dependent stimulation and drug efflux capacity were analyzed. One half of the patients suffered from infections after day 180. All patients had strongly diminished CD27(+) memory B cells despite already normalized total B cell numbers and fully recovered CD27(-)IgD(-) memory B cells, putatively of extra-follicular origin. Circulating memory follicular helper T cells were reduced in the majority of patients as well. Naïve B cells exhibited a decreased expression of CXCR5, which mediates follicular B cell entry. Additionally, a lower HLA-DR expression was found on naïve B cells, impairing antigen presentation. Upon CD40/TLR-9-dependent activation, B cells underwent significantly increased apoptosis paralleled by an aberrant up-regulation of Fas-L on activated T cells and Fas on resting B cells. Significantly increased B cell apoptosis was also observed after CD40/BCR and CD40/BCR/TLR-9-dependent activation. Drug efflux capacity of naïve B cells was diminished in cyclosporin A-treated patients, additionally contributing to an apoptosis-prone phenotype. We conclude that B cell survival and migration and T cell communication defects are contributing candidates for an impaired germinal center formation of memory B cells after allogeneic hematopoietic stem cell transplantation. Follow-up studies should evaluate effectiveness of revaccinations on the cellular level and should address the long-term sequelae of B cell defects after transplantation.

The complex and central role of interferon-γ in graft-versus-host disease and graft-versus-tumor activity

Allogeneic hematopoietic cell transplantation (allo-HCT) is increasingly being performed to treat patients with hematologic malignancies. However, separating the beneficial graft-versus-tumor (GVT) or graft-versus-leukemia effects from graft-versus-host disease (GVHD) has been difficult and remains a significant challenge toward improving therapeutic efficacy and reducing toxicity of allo-HCT. GVHD is induced by donor T cells that also mediate potent anti-tumor responses. However, despite the largely shared effector mechanisms, extensive animal studies have demonstrated the potential of dissociating the GVT effect from GVHD. Also in many clinical cases, long-term remission was achieved following allo-HCT, without significant GVHD. A better mechanistic understanding of the immunopathophysiology of GVHD and GVT effects may potentially help to improve allo-HCT as well as maximize the benefit of GVT effects while minimizing GVHD. In this article, we review the role of IFN-γ in regulation of alloresponses following allo-HCT, with a focus on the mechanisms of how this cytokine may separate GVHD from GVT effects.

A radio-resistant perforin-expressing lymphoid population controls allogeneic T cell engraftment, activation, and onset of graft-versus-host disease in mice

Immunosuppressive pretransplantation conditioning is essential for donor cell engraftment in allogeneic bone marrow transplantation (BMT). The role of residual postconditioning recipient immunity in determining engraftment is poorly understood. We examined the role of recipient perforin in the kinetics of donor cell engraftment. MHC-mismatched BMT mouse models demonstrated that both the rate and proportion of donor lymphoid cell engraftment and expansion of effector memory donor T cells in both spleen and BM were significantly increased within 5 to 7 days post-BMT in perforin-deficient (pfn(-/-)) recipients, compared with wild-type. In wild-type recipients, depletion of natural killer (NK) cells before BMT enhanced donor lymphoid cell engraftment to that seen in pfn(-/-) recipients. This demonstrated that a perforin-dependent, NK-mediated, host-versus-graft (HVG) effect limits the rate of donor engraftment and T cell activation. Radiation-resistant natural killer T (NKT) cells survived in the BM of lethally irradiated mice and may drive NK cell activation, resulting in the HVG effect. Furthermore, reduced pretransplant irradiation doses in pfn(-/-) recipients permitted long-term donor lymphoid cell engraftment. These findings suggest that suppression of perforin activity or selective depletion of recipient NK cells before BMT could be used to improve donor stem cell engraftment, in turn allowing for the reduction of pretransplant conditioning.

Comparison of different rabbit ATG preparation effects on early lymphocyte subset recovery after allogeneic HSCT and its association with EBV-mediated PTLD

PURPOSE

Rabbit antithymocyte globulin (ATG) is commonly used before allogeneic hematopoietic stem cell transplantation (allo-HSCT) to prevent graft-versus-host disease. Studies comparing the effect of different ATG preparations and dosages on immune reconstitution and risk for Epstein-Barr virus (EBV)-mediated post-transplant lymphoproliferative disorder (PTLD) are rare.

METHODS

In this retrospective study, we determined T and B cell subsets by flow cytometry after allo-HSCT in children, who received ATG-Genzyme (ATG-G, n = 15), ATG-Fresenius (ATG-F, n = 25) or no-ATG treatment (n = 19). Additionally, PCR-quantified EBV-genome copy counts were correlated with incidence of PTLD.

RESULTS

We could confirm a dose-dependent impairment of CD8(+) and CD4(+) T cell regeneration by ATG-G, including naïve and memory CD4(+) T cells. No differences were seen between the currently applied dosages of 5-10 mg/kg ATG-G and 20-60 mg/kg ATG-F. Significantly delayed T cell subset reconstitution was determined only at high dosages of 20-60 mg/kg ATG-G compared to ATG-F. B cell reconstitution was comparably impaired in ATG-G- and ATG-F-treated patients. Although the incidence of EBV reactivation was similar in both ATG groups, EBV copy counts of >10(4) copies/10(5) peripheral blood mononuclear cells and the occurrence of PTLD were only found in ATG-G-treated patients.

CONCLUSIONS

We conclude that high, but importantly not currently applied low dosages of ATG-G, impair thymic T cell regeneration and memory T cell immunity to a greater extent than ATG-F in pediatric patients. In addition, our results suggest an increased risk for EBV-PTLD when treated with ATG-G. Prospective studies are warranted to compare different ATG preparations with regard to the immune reconstitution and EBV-PTLD.

Engineering of bone marrow cells with fas-ligand protein-enhances donor-specific tolerance to solid organs

Effective immunomodulation to induce tolerance to tissue/organ allografts is attained by infusion of donor lymphocytes endowed with killing capacity through ectopic expression of a short-lived Fas-ligand (FasL) protein. The same approach has proven effective in improving hematopoietic stem and progenitor cell engraftment. This study evaluates the possibility of substitution of immune cells for bone marrow cells (BMC) to induce FasL-mediated tolerance to solid organ grafts. Expression of FasL protein on BMC increased the survival of simultaneously grafted vascularized heterotopic cardiac grafts to 90%, as compared to 30% in recipients of naïve BMC. Similar results were obtained for skin allografts implanted into radiation chimeras at 1 week after bone marrow transplantation. Further reduction of preparative conditioning to busulfan resulted in acceptance of donor skin implanted at 2 weeks after transplantation of naïve and FasL-coated BMC, whereas third-party grafts were acutely rejected. The levels of donor chimerism were in the range of 0.7% to 12% at the time of skin grafting, with higher levels in recipients of FasL-coated BMC. It is concluded that FasL-mediated abrogation of alloimmune responses can be effectively attained with BMC. There is no threshold of donor chimerism, but tolerance to solid organs evolves during the process of donor-host mutual acceptance.

Dysfunction of bone marrow vascular niche in acute graft-versus-host disease after MHC-haploidentical bone marrow transplantation

Acute graft-versus-host disease (aGvHD) is the most common complication of allogeneic hematopoietic stem cell transplantation (HSCT), which is often accompanied by impaired hematopoietic reconstitution. Sinusoidal endothelial cells (SECs) constitute bone marrow (BM) vascular niche that plays an important role in supporting self-renewal capacity and maintaining the stability of HSC pool. Here we provide evidences that vascular niche is a target of aGvHD in a major histocompatibility complex (MHC)–haploidentical matched murine HSCT model. The results demonstrated that hematopoietic cells derived from GvHD mice had the capacity to reconstitute hematopoiesis in healthy recipient mice. However, hematopoietic cells from healthy donor mice failed to reconstitute hematopoiesis in GvHD recipient mice, indicating that the BM niche was impaired by aGvHD in this model. We further demonstrated that SECs were markedly reduced in the BM of aGvHD mice. High level of Fas and caspase-3 expression and high rate of apoptosis were identified in SECs, indicating that SECs were destroyed by aGvHD in this murine HSCT model. Furthermore, high Fas ligand expression on engrafted donor CD4⁺, but not CD8⁺ T cells, and high level MHC-II but not MHC-I expression on SECs, suggested that SECs apoptosis was mediated by CD4⁺ donor T cells through the Fas/FasL pathway.

The allure and peril of hematopoietic stem cell transplantation: overcoming immune challenges to improve success

Since its inception in the mid-twentieth century, the complication limiting the application and utility of allogeneic hematopoietic stem cell transplantation (allo-HSCT) to treat patients with hematopoietic cancer is the development of graft-versus-host disease (GVHD). Ironically, GVHD is induced by the cells (T lymphocytes) transplanted for the purpose of eliminating the malignancy. Damage ensuing to multiple tissues, e.g., skin, GI, liver, and others including the eye, provides the challenge of regulating systemic and organ-specific GVH responses. Because the immune system is also targeted by GVHD, this both: (a) impairs reconstitution of immunity post-transplant resulting in patient susceptibility to lethal infection and (b) markedly diminishes the individual's capacity to generate anti-cancer immunity--the raison d'etre for undergoing allo-HSCT. We hypothesize that deleting alloreactive T cells ex vivo using a new strategy involving antigen stimulation and alkylation will prevent systemic GVHD thereby providing a platform for the generation of anti-tumor immunity. Relapse also remains the major complication following autologous HSCT (auto-HSCT). While GVHD does not complicate auto-HSCT, its absence removes significant grant anti-tumor responses (GVL) and raises the challenge of generating rapid and effective anti-tumor immunity early post-transplant prior to immune reconstitution. We hypothesize that effective vaccine usage to stimulate tumor-specific T cells followed by their amplification using targeted IL-2 can be effective in both the autologous and allogeneic HSCT setting. Lastly, our findings support the notion that the ocular compartment can be locally targeted to regulate visual complications of GVHD which may involve both alloreactive and self-reactive (i.e., autoimmune) responses.

Bone marrow T-cell infiltration during acute GVHD is associated with delayed B-cell recovery and function after HSCT

B-cell immune dysfunction contributes to the risk of severe infections after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Delayed B-cell regeneration is found in patients with systemic graft-versus-host disease (GVHD) and is often accompanied by bone marrow (BM) suppression. Little is known about human BM GVHD. We analyzed the reconstitution kinetics of B-cell subsets in adult leukemic patients within 6 months after allo-HSCT. B-cell deficiency already existed before transplant and was aggravated after transplant. Onset of B-cell reconstitution characterized by transitional B-cell recovery occurred either early (months 2-3) or late (from month 6 on) and correlated highly positively with reverse transcription-polymerase chain reaction quantified numbers of κ-deleting recombination excision circles (KRECs). Delayed recovery was associated with systemic acute GVHD and full-intensity conditioning therapy. Histological analysis of BM trephines revealed increased T-cell infiltration in late recovering patients, which was associated with reduced numbers of osteoblasts. Functionally, late recovering patients displayed less pneumococcal polysaccharide-specific immunoglobin M-producing B cells on ex vivo B-cell activation than early recovering patients. Our results provide evidence for acute BM GVHD in allo-HSCT patients with infiltrating donor T cells and osteoblast destruction. This is associated with delayed B-cell reconstitution and impaired antibody response. Herein, KREC appears suitable to monitor BM B-cell output after transplant.

Inhibition of plasmin attenuates murine acute graft-versus-host disease mortality by suppressing the matrix metalloproteinase-9-dependent inflammatory cytokine storm and effector cell trafficking

The systemic inflammatory response observed during acute graft-versus-host disease (aGVHD) is driven by proinflammatory cytokines, a 'cytokine storm'. The function of plasmin in regulating the inflammatory response is not fully understood, and its role in the development of aGVHD remains unresolved. Here we show that plasmin is activated during the early phase of aGVHD in mice, and its activation correlated with aGVHD severity in humans. Pharmacological plasmin inhibition protected against aGVHD-associated lethality in mice. Mechanistically, plasmin inhibition impaired the infiltration of inflammatory cells, the release of membrane-associated proinflammatory cytokines including tumor necrosis factor-α (TNF-α) and Fas-ligand directly, or indirectly via matrix metalloproteinases (MMPs) and alters monocyte chemoattractant protein-1 (MCP-1) signaling. We propose that plasmin and potentially MMP-9 inhibition offers a novel therapeutic strategy to control the deadly cytokine storm in patients with aGVHD, thereby preventing tissue destruction.

Bone marrow graft-versus-host disease: evaluation of its clinical impact on disrupted hematopoiesis after allogeneic hematopoietic stem cell transplantation

Idiopathic cytopenias are frequently observed in patients after allogeneic hematopoietic stem cell transplantation (allo-HSCT). We have previously reported the effect of graft-versus-host disease (GVHD) on bone marrow (BM) in murine models, indicating that the osteoblast injury mediated by donor T cells was associated with bone marrow suppression and delayed immune reconstitution. In this study, we prospectively evaluated the relevance of these findings in 51 patients. Patients with chronic GVHD manifested the loss of osteoblasts, contributing to cytopenic symptoms (P = .0427 compared with patients without cytopenic symptoms). The loss of osteoblasts was significantly associated with the extensive type of chronic GVHD (P = .012), and flow cytometric analyses revealed lower numbers of CD19(+) B cells and a significantly increased CD4 to CD8 ratio (P = .0002) in these patients. Our data, for the first time to our knowledge, summarize the detailed analyses of the effect of GVHD on BM in the clinical allo-HSCT patients.

Antigen and lymphopenia-driven donor T cells are differentially diminished by post-transplantation administration of cyclophosphamide after hematopoietic cell transplantation

Administration of cyclophosphamide after transplantation (post-transplantation cyclophosphamide, PTC) has shown promise in the clinic as a prophylactic agent against graft-versus-host disease (GVHD). An important issue with regard to recipient immune function and reconstitution after PTC is the extent to which, in addition to diminution of antihost allo-reactive donor T cells, the remainder of the nonhost allo-reactive donor T cell pool may be affected. To investigate PTC's effects on nonhost reactive donor CD8 T cells, ova-specific (OT-I) and gp100-specific Pmel-1 T cells were labeled with proliferation dyes and transplanted into syngeneic and allogeneic recipients. Notably, an intermediate dose (66 mg/kg) of PTC, which abrogated GVHD after allogeneic HSCT, did not significantly diminish these peptide-specific donor T cell populations. Analysis of the rate of proliferation after transplantation illustrated that lymphopenic-driven, donor nonhost reactive TCR Tg T cells in syngeneic recipients underwent slow division, resulting in significant sparing of these donor populations. In contrast, after exposure to specific antigens at the time of transplantation, these same T cells were significantly depleted by PTC, demonstrating the global susceptibility of rapidly dividing T cells after an encounter with cognate antigen. In total, our results, employing both syngeneic and allogeneic minor antigen-mismatched T cell replete models of transplantation, demonstrate a concentration of PTC that abrogates GVHD can preserve most cells that are dividing because of the accompanying lymphopenia after exposure. These findings have important implications with regard to immune function and reconstitution in recipients after allogeneic hematopoietic stem cell transplantation.

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.

Quantitation of hematogones at the time of engraftment is a useful prognostic indicator in allogeneic hematopoietic stem cell transplantation

Quantitation of hematogones at engraftment is useful to predict prognosis of patients treated with allogeneic stem cell transplantation.

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.

Inhibition of IL-32 activation by α-1 antitrypsin suppresses alloreactivity and increases survival in an allogeneic murine marrow transplantation model

Interleukin (IL)-32 was originally identified in natural killer cells and IL-2-activated human T lymphocytes. As T cells are activated in allogeneic transplantation, we determined the role of IL-32 in human mixed lymphocyte cultures (MLCs) and GVHD. In allogeneic MLCs, IL-32 increased two-fold in responding T cells, accompanied by five-fold increases of TNFα, IL-6, and IL-8. After allogeneic hematopoietic cell transplantation, IL-32 mRNA levels in blood leukocytes were statistically significantly higher in patients with acute GVHD (n = 10) than in serial samples from patients who did not develop acute GVHD (n = 5; P = .02). No significant changes in IL-32 levels were present in patients with treated (n = 14) or untreated (n = 8) chronic GVHD, compared with healthy controls (n = 8; P = .5, and P = .74, respectively). As IL-32 is activated by proteinase-3 (PR3), we determined the effect of the serine protease inhibitor α-1 antitrypsin (AAT) on IL-32 levels and showed suppression of IL-32 and T-lymphocyte proliferation in MLCs. In an MHC-minor antigen disparate murine transplant model, preconditioning and postconditioning treatment with AAT resulted in attenuation or prevention of GVHD and superior survival compared with albumin-treated controls (80% vs 44%; P = .04). These findings suggest that AAT modulates immune and inflammatory functions and may represent a novel approach to prevent or treat GVHD.

Roles of CD28, CTLA4, and inducible costimulator in acute graft-versus-host disease in mice

T cells deficient for CD28 have reduced ability to expand and survive, but still cause graft-versus-host disease (GVHD). Inducible costimulator (ICOS), a member of the CD28 family, is expressed on antigen-activated T cells and plays unique roles in T cell activation and effector function. We hypothesized that ICOS contributes to the development of GVHD in the absence of B7:CD28/CTLA4 costimulation. In this study, we evaluated the roles of CD28, CTLA4, and ICOS in the pathogenesis of acute GVHD after myeloablative allogeneic bone marrow transplantation. Unexpectedly, we found that blocking CD28 and CTLA4 signals using the clinically relevant reagent CTLA4-Ig increases the severity of GVHD mediated by CD4(+) T cells, and that such treatment does not add any benefit to the blockade of ICOS. In contrast, selectively blocking CD28 and ICOS, but not CTLA4, prevents GVHD more effectively than blocking either CD28 or ICOS alone. Taken together, these results indicate that CD28 and ICOS are synergistic in promoting GVHD, whereas the CTLA4 signal is required for T cell tolerance regardless of ICOS signaling. Thus, blocking CD28 and ICOS while sparing CTLA4 represents a promising approach for abrogating pathogenic T cell responses after allogeneic bone marrow transplantation.

Rapid reconstitution of antibody responses following transplantation of purified allogeneic hematopoietic stem cells

Allogeneic hematopoietic cell transplantation has broad clinical applications extending from the treatment of malignancies to induction of immunologic tolerance. However, adaptive cellular and humoral immunity frequently remain impaired posttransplantation. Here, recovery of T-dependent and T-independent Ab responses was evaluated in mice transplanted with purified hematopoietic stem cells (HSCs) devoid of the mature immune cells believed to hasten immune recovery. Mixed and full donor chimeras were created by conditioning recipients with sublethal or lethal irradiation, respectively, across different donor/host genetic disparities. By 6 wk posttransplantation, all animals demonstrated robust T-independent Ab responses, and all mixed chimeras and recipients of MHC-matched or haploidentical HSCs with a shared MHC haplotype had T-dependent Ab responses equivalent to those of untransplanted controls. Full chimeras that received fully MHC-disparate HSCs showed delayed T-dependent Ab responses that recovered by 12 wk. This delay occurred despite early reconstitution and proper migration to germinal centers of donor-derived T(follicular helper) (T(FH)) cells. Congenic transplants into T(FH)-deficient CD4(-/-) mice revealed restoration of T-dependent Ab responses by 6 wk, leading us to conclude that MHC disparity caused delay in humoral recovery. These findings, together with our previous studies, show that, contrary to the view that depletion of graft lymphocytes results in poor posttransplant immunity, elimination of immune-suppressing graft-versus-host reactions permits superior immune reconstitution. This study also provides insight into the regeneration of T(FH) cells and humoral immunity after allogeneic HSC transplantation.

Allogeneic T cells impair engraftment and hematopoiesis after stem cell transplantation

Because of the perception that depleting hematopoietic grafts of T cells will result in poorer immune recovery and in increased risk of graft rejection, pure hematopoietic stem cells (HSC), which avoid the potentially lethal complication of graft-versus-host disease (GVHD), have not been used for allogeneic hematopoietic cell transplantation (HCT) in humans. Ideal grafts should contain HSC plus mature cells that confer only the benefits of protection from pathogens and suppression of malignancies. This goal requires better understanding of the effects of each blood cell type and its interactions during engraftment and immune regeneration. Here, we studied hematopoietic reconstitution post-HCT, comparing grafts of purified HSC with grafts supplemented with T cells in a minor histocompatibility antigen (mHA)-mismatched mouse model. Cell counts, composition, and chimerism of blood and lymphoid organs were evaluated and followed intensively through the first month, and then subsequently for up to 1 yr. Throughout this period, recipients of pure HSC demonstrated superior total cell recovery and lymphoid reconstitution compared with recipients of T cell-containing grafts. In the latter, rapid expansion of T cells occurred, and suppression of hematopoiesis derived from donor HSC was observed. Our findings demonstrate that even early post-HCT, T cells retard donor HSC engraftment and immune recovery. These observations contradict the postulation that mature donor T cells provide important transient immunity and facilitate HSC engraftment.

Bone marrow graft-versus-host disease: early destruction of hematopoietic niche after MHC-mismatched hematopoietic stem cell transplantation

Disrupted hematopoiesis and delayed immune reconstitution are life-threatening complications of allogeneic hematopoietic stem cell transplantation (allo-HSCT). Although graft-versus-host disease (GVHD) is a major risk factor for the bone marrow (BM) insufficiency, how GVHD impairs BM hematopoiesis has been largely unknown. We hypothesized that BM stromal niche could be a target of GVHD. In major histocompatibility complex (MHC)-mismatched murine models of GVHD, we have demonstrated the early destruction of osteoblasts that especially affected B-cell lineages. The defective B lymphopoiesis was due to the impaired ability of BM stroma and osteoblasts to support the hematopoiesis, as evidenced by the failure of GVHD-affected BM to reconstitute the hematopoietic cells. The administration of anti-CD4 monoclonal antibody (mAb) ameliorated these effects and improved B lymphopoiesis while preserving graft-versus-tumor effects. Genetic ablation of Fas-Fas ligand signaling also partially restored B lymphopoiesis. Our present study provided evidence of BM GVHD, with the identification of osteoblasts as the main target for GVHD in BM. Moreover, our data showed the potential for mAb therapies to enhance immune reconstitution in vivo for patients undergoing allo-HSCT.

The cytolytic molecules Fas ligand and TRAIL are required for murine thymic graft-versus-host disease

Thymic graft-versus-host disease (tGVHD) can contribute to profound T cell deficiency and repertoire restriction after allogeneic BM transplantation (allo-BMT). However, the cellular mechanisms of tGVHD and interactions between donor alloreactive T cells and thymic tissues remain poorly defined. Using clinically relevant murine allo-BMT models, we show here that even minimal numbers of donor alloreactive T cells, which caused mild nonlethal systemic graft-versus-host disease, were sufficient to damage the thymus, delay T lineage reconstitution, and compromise donor peripheral T cell function. Furthermore, to mediate tGVHD, donor alloreactive T cells required trafficking molecules, including CCR9, L selectin, P selectin glycoprotein ligand-1, the integrin subunits alphaE and beta7, CCR2, and CXCR3, and costimulatory/inhibitory molecules, including Ox40 and carcinoembryonic antigen-associated cell adhesion molecule 1. We found that radiation in BMT conditioning regimens upregulated expression of the death receptors Fas and death receptor 5 (DR5) on thymic stromal cells (especially epithelium), while decreasing expression of the antiapoptotic regulator cellular caspase-8-like inhibitory protein. Donor alloreactive T cells used the cognate proteins FasL and TNF-related apoptosis-inducing ligand (TRAIL) (but not TNF or perforin) to mediate tGVHD, thereby damaging thymic stromal cells, cytoarchitecture, and function. Strategies that interfere with Fas/FasL and TRAIL/DR5 interactions may therefore represent a means to attenuate tGVHD and improve T cell reconstitution in allo-BMT recipients.

The role of B cells in the pathogenesis of graft-versus-host disease

Allogeneic hematopoietic stem cell transplantation is an established treatment modality for malignant and nonmalignant hematologic diseases. Acute and chronic graft-versus-host diseases (GVHDs) are a major cause of morbidity and mortality after allogeneic stem cell transplantation. T cells have been identified as key players in the graft-versus-host reaction and, therefore, most established drugs used against GVHD target T cells. Despite our knowledge on the pathogenesis of the GVH reaction, success of established therapies for prevention and treatment of GHVD is unsatisfactory. Recently, animal and human studies demonstrated that B cells are involved in the immunopathophysiology of acute and chronic GVHD. Early phase clinical trials of B-cell depletion with rituximab have shown beneficial effects on both acute and chronic GVHD. This review summarizes the current experimental and clinical evidence for the involvement of B cells in the pathogenesis of acute and chronic GVHD and discusses the clinical implications for the management of patients undergoing allogeneic stem cell transplantation.

Improved outcome of allogeneic bone marrow transplantation due to breastfeeding-induced tolerance to maternal antigens

Exposure of offspring to noninherited maternal antigens (NIMAs) during pregnancy may have an impact on transplantations performed later in life. Using a mouse model, we recently showed that bone marrow transplantation (BMT) from NIMA-exposed offspring to the mother led to a reduction of graft-versus-host disease (GVHD). Since offspring can also be exposed to NIMAs by breastfeeding after birth, we tested whether breast milk could mediate the tolerogenic NIMA effect. We found that oral exposure to NIMAs by breastfeeding alone was sufficient to reduce GVHD, and that in utero exposure to NIMAs is required for maximum reduction of GVHD. The tolerogenic milk effects disappeared when donor mice were injected with CD25 monoclonal antibodies during the lactation period, suggesting a CD4+CD25+ regulatory T cell–dependent mechanism. Our results suggest a previously unknown impact of breastfeeding on the outcome of transplantation.

Purified hematopoietic stem cell allografts reconstitute immunity superior to bone marrow

Antigen-specific immune responses are impaired after allogeneic hematopoietic cell transplantation (HCT). The events contributing to this impairment include host hematolymphoid ablation and donor cell regeneration, which is altered by pharmacologic immune suppression to prevent graft-versus-host disease (GVHD). A generally accepted concept is that graft T cell depletion performed to avoid GVHD yields poorer immune recovery because mature donor T cells are thought to be the major mediators of protective immunity early post-HCT. Our findings contradict the idea that removal of mature donor cells worsens immune recovery post-HCT. By transplantation of purified hematopoietic stem cells (HSC) compared with bone marrow (BM) across donor and recipient pairs of increasing genetic disparity, we show that grafts composed of the purified progenitor population give uniformly superior lymphoid reconstitution, both qualitatively and quantitatively. Subclinical GVHD by T cells in donor BM likely caused this lympho-depleting GVHD. We further determined in the major histocompatibility complex (MHC)-mismatched pairs, that T cell restricted proliferative responses were dictated by donor rather than host elements. We interpret these latter findings to show the importance of peripheral antigen presentation in the selection and maintenance of the T cell repertoire.

The impact of regulatory T cells on T-cell immunity following hematopoietic cell transplantation

Regulatory T cells (Tregs) prevent graft-versus-host disease (GvHD) by inhibiting the proliferation and function of conventional T cells (Tcons). However, the impact of Tregs on T-cell development and immunity following hematopoietic cell transplantation (HCT) is unknown. Using a murine GvHD model induced by Tcons, we demonstrate that adoptive transfer of Tregs leads to (1) abrogration of GvHD, (2) preservation of thymic and peripheral lymph node architecture, and (3) an accelerated donor lymphoid reconstitution of a diverse TCR-Vbeta repertoire. The resultant enhanced lymphoid reconstitution in Treg recipients protects them from lethal cytomegalovirus (MCMV) infection. By contrast, mice that receive Tcons alone have disrupted lymphoid organs from GvHD and remain lymphopenic with a restricted TCR-Vbeta repertoire and rapid death on MCMV challenge. Lymphocytes from previously infected Treg recipients generate secondary response specific to MCMV, indicating long-term protective immunity with transferred Tregs. Thymectomy significantly reduces survival after MCMV challenge in Treg recipients compared with euthymic controls. Our results indicate that Tregs enhance immune reconstitution by preventing GvHD-induced damage of the thymic and secondary lymphoid microenvironment. These findings provide new insights into the role of Tregs in affording protection to lymphoid stromal elements important for T-cell immunity.

Pathophysiology of acute graft-versus-host disease: recent advances

Allogeneic hematopoietic stem cell transplantation (HSCT) is a potentially curative therapy for many malignant and nonmalignant hematologic diseases. Donor T cells from the allografts are critical for the success of this effective therapy. Unfortunately these T cells not only recognize and attack the disease cells/tissues but also the other normal tissues of the recipient as "foreign" or "nonself" and cause severe, immune-mediated toxicity, graft-versus-host disease (GVHD). Several insights into the complex pathophysiology of GVHD have been gained from recent experimental observations, which show that acute GVHD is a consequence of interactions between both the donor and the host innate and adaptive immune systems. These insights have identified a role for a variety of cytokines, chemokines, novel T-cell subsets (naĩve, memory, regulatory, and NKT cells) and for non-T cells of both the donor and the host (antigen presenting cells, delta T cells, B cells, and NK cells) in modulating the induction, severity, and maintenance of acute GVHD. This review will focus on the immunobiology of experimental acute GVHD with an emphasis on the recent observations.

Immunobiology of allogeneic hematopoietic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) has evolved into an effective adoptive cellular immunotherapy for the treatment of a number of cancers. The immunobiology of allogeneic HSCT is unique in transplantation in that it involves potential immune recognition and attack between both donor and host. Much of the immunobiology of allogeneic HSCT has been gleaned from preclinical models and correlation with clinical observations. We review our current understanding of some of the issues that affect the success of this therapy, including host-versus-graft (HVG) reactions, graft-versus-host disease (GVHD), graft-versus-tumor (GVT) activity, and restoration of functional immunity to prevent transplant-related opportunistic infections. We also review new strategies to optimize the GVT and improve overall immune function while reducing GVHD and graft rejection.

Unexpected role of B and T lymphocyte attenuator in sustaining cell survival during chronic allostimulation

B and T lymphocyte attenuator (BTLA; CD272) can deliver inhibitory signals to B and T cells upon binding its ligand herpesvirus entry mediator. Because CD28, CTLA-4, programmed death-1, and ICOS regulate the development of acute graft-vs-host disease (GVHD), we wished to assess if BTLA also played a role in this T cell-mediated response. In the nonirradiated parental-into-F1 model of acute GVHD, BTLA+/+ and BTLA-/- donor lymphocytes showed equivalent engraftment and expansion during the first week of the alloresponse. Unexpectedly, BTLA-/- donor T cells failed to sustain GVHD, showing a decline in surviving donor cell numbers beginning at day 9 and greatly reduced by day 11. Similarly, inhibition of BTLA-herpesvirus entry mediator engagement by in vivo administration of a blocking anti-BTLA Ab also caused reduced survival of donor cells. Microarray analysis revealed several genes that were differentially expressed by BTLA-/- and BTLA+/+ donor CD4+ T cells preceding the decline in BTLA-/- donor T cells. Several genes influencing Th cell polarization were differentially expressed by BTLA+/+ and BTLA-/- donor cells. Additionally, the re-expression of the IL-7Ralpha subunit that occurs in BTLA+/+ donor cells after 1 wk of in vivo allostimulation was not observed in BTLA-/- donor CD4+ cells. The striking loss of BTLA-/- T cells in this model indicates a role for BTLA activity in sustaining CD4+ T cell survival under the conditions of chronic stimulation in the nonirradiated parental-into-F1 GVHD.

Fas ligand enhances hematopoietic cell engraftment through abrogation of alloimmune responses and nonimmunogenic interactions

Early after transplantation, donor lineage-negative bone marrow cells (lin(-) BMC) constitutively upregulated their expression of Fas ligand (FasL), suggesting an involvement of the Fas/FasL axis in engraftment. Following the observation of impaired engraftment in the presence of a dysfunctional Fas/FasL axis in FasL-defective (gld) donors or Fas-defective (lpr) recipients, we expressed a noncleavable FasL chimeric protein on the surface of donor lin(-) BMC. Despite a short life span of the protein in vivo, expression of FasL on the surface of all the donor lin(-) BMC improved the efficiency of engraftment twofold. The FasL-coated donor cells efficiently blunted the host alloimmune responses in primary recipients and retained their hematopoietic reconstituting potential in secondary transplants. Surprisingly, FasL protein improved the efficiency of engraftment in syngeneic transplants. The deficient engraftment in lpr recipients was not reversed in chimeric mice with Fas(-) stroma and Fas(+) BMC, demonstrating that the host marrow stroma was also a target of donor cell FasL. Hematopoietic stem and progenitor cells are insensitive to Fas-mediated apoptosis and thus can exploit the constitutive expression of FasL to exert potent veto activities in the early stages of engraftment. Manipulation of the donor cells using ectopic FasL protein accentuated the immunogenic and nonimmunogenic interactions between the donor cells and the host, alleviating the requirement for a megadose of transplanted cells to achieve a potent veto effect. Disclosure of potential conflicts of interest is found at the end of this article.

Opposing effects of ICOS on graft-versus-host disease mediated by CD4 and CD8 T cells

ICOS, a CD28 family member expressed on activated CD4(+) and CD8(+) T cells, plays important roles in T cell activation and effector function. Here we studied the role of ICOS in graft-vs-host disease (GVHD) mediated by CD4(+) or CD8(+) T cells in allogeneic bone marrow transplantation. In comparison of wild-type and ICOS-deficient T cells, we found that recipients of ICOS(-/-) CD4(+) T cells exhibited significantly less GVHD morbidity and delayed mortality. ICOS(-/-) CD4(+) T cells had no defect in expansion, but expressed significantly less Fas ligand and produced significantly lower levels of IFN-gamma and TNF-alpha. Thus, ICOS(-/-) CD4(+) T cells were impaired in effector functions that lead to GVHD. In contrast, recipients of ICOS(-/-) CD8(+) T cells exhibited significantly enhanced GVHD morbidity and accelerated mortality. In the absence of ICOS signaling, either using ICOS-deficient donors or ICOS ligand-deficient recipients, the levels of expansion and Tc1 cytokine production of CD8(+) T cells were significantly increased. The level of expansion was inversely correlated with the level of apoptosis, suggesting that increased ability of ICOS(-/-) CD8(+) T cells to induce GVHD resulted from the enhanced survival and expansion of those cells. Our findings indicate that ICOS has paradoxical effects on the regulation of alloreactive CD4(+) and CD8(+) T cells in GVHD.

Molecular monitoring of T-cell chimerism early after allogeneic stem cell transplantation may predict the occurrence of acute GVHD grades II-IV

Mixed chimerism (MC) within CD4+ and CD8+ T cell days 7 and 10 after allogeneic stem cell transplantation (SCT) was compared with the occurrence of acute graft-vs.-host disease (GVHD) in 34 patients after SCT. Acute GVHD was diagnosed in 22 patients within the first 3 months after SCT, 15 of these developed acute GVHD grades II-IV. The difference in the clearance rate of host T cell between the two days were compared. We found a significantly higher risk (p = 0.005) for developing acute GVHD grades II-IV in patients with complete donor CD4+ T-cell chimerism day 7 after SCT together with patients who increased 50% or more in donor CD4+ T cells between days 7 and 10 after SCT. Our data suggest that molecular monitoring of MC early after transplantation may be useful as a diagnostic tool in predicting the occurrence of moderate to severe acute GVHD after SCT.

Graft-versus-lymphoma effects: clinical review, policy proposals, and immunobiology

The indubitable existence of a graft-versus-lymphoma (GVL) effect is difficult to prove directly. This article reviews the difficulties in interpreting the current literature in this field and, with a number of caveats, argues for the existence of a clinically meaningful GVL effect in follicular, mantle cell, small lymphocytic, and Hodgkin lymphomas. The evidence, however, for a potent GVL effect in diffuse large-cell lymphoma and Burkitt lymphoma is not convincing. Policies for allografting in lymphoma are proposed on the basis of this evidence. The immunobiology of GVL effects is discussed--in particular, the expression of HLA class I and II and co-stimulatory molecules on lymphomas that influence the generation of alloreactive T cells--together with future directions in immunotherapy that may help to eradicate chemoresistant disease.

Induction of tolerance using Fas ligand: a double-edged immunomodulator

Apoptosis mediated by Fas ligand (FasL) interaction with Fas receptor plays a pivotal regulatory role in immune homeostasis, immune privilege, and self-tolerance. FasL, therefore, has been extensively exploited as an immunomodulatory agent to induce tolerance to both autoimmune and foreign antigens with conflicting results. Difficulties associated with the use of FasL as a tolerogenic factor may arise from (1) its complex posttranslational regulation, (2) the opposing functions of different forms of FasL, (3) different modes of expression, systemic versus localized and transient versus continuous, (4) the level and duration of expression, (5) the sensitivity of target tissues to Fas/FasL-mediated apoptosis and the efficiency of antigen presentation in these tissues, and (6) the types and levels of cytokines, chemokines, and metalloproteinases in the extracellular milieu of the target tissues. Thus, the effective use of FasL as an immunomodulator to achieve durable antigen-specific immune tolerance requires careful consideration of all of these parameters and the design of treatment regimens that maximize tolerogenic efficacy, while minimizing the non-tolerogenic and toxic functions of this molecule. This review summarizes the current status of FasL as a tolerogenic agent, problems associated with its use as an immunomodulator, and new strategies to improve its therapeutic potential.