Selective T-cell subset ablation demonstrates a role for T1 and T2 cells in ongoing acute graft-versus-host disease: a model system for the reversal of disease

CD4+CD8+ T-Lymphocytes in Xenogeneic and Human Graft-versus-Host Disease

Mechanisms driving acute graft-versus-host disease (aGVHD) onset in patients undergoing allogeneic hematopoietic stem cell transplantation (allo-HSCT) are still poorly understood. To provide a detailed characterization of tissue-infiltrating T lymphocytes (TL) and search for eventual site-specific specificities, we developed a xenogeneic model of aGVHD in immunodeficient mice. Phenotypic characterization of xenoreactive T lymphocytes (TL) in diseased mice disclosed a massive infiltration of GVHD target organs by an original CD4⁺CD8⁺ TL subset. Immunophenotypic and transcriptional profiling shows that CD4⁺CD8⁺ TL comprise a major PD1⁺CD62L^−/+ transitional memory subset (>60%) characterized by low level expression of cytotoxicity-related transcripts. CD4⁺CD8⁺ TL produce high IL-10 and IL-13 levels, and low IL-2 and IFN-γ, suggestive of regulatory function. In vivo tracking of genetically labeled CD4⁺ or CD8⁺ TL subsequently found that CD4⁺CD8⁺ TL mainly originate from chronically activated cytotoxic TL (CTL). On the other hand, phenotypic profiling of CD3⁺ TL from blood, duodenum or rectal mucosa in a cohort of allo-HSCT patients failed to disclose abnormal expansion of CD4⁺CD8⁺ TL independent of aGVHD development. Collectively, our results show that acquisition of surface CD4 by xenoreactive CD8⁺ CTL is associated with functional diversion toward a regulatory phenotype, but rule out a central role of this subset in the pathogenesis of aGVHD in allo-HSCT patients.

Memory T cells: A helpful guard for allogeneic hematopoietic stem cell transplantation without causing graft-versus-host disease

Graft-versus-host disease (GVHD) is a major complication of allogeneic hematopoietic stem cell transplantation (AHSCT) and the major cause of nonrelapse morbidity and mortality of AHSCT. In AHSCT, donor T cells facilitate hematopoietic stem cell (HSC) engraftment, contribute to anti-infection immunity, and mediate graft-versus-leukemia (GVL) responses. However, activated alloreactive T cells also attack recipient cells in vital organs, leading to GVHD. Different T-cell subsets, including naïve T (T_N) cells, memory T (T_M) cells, and regulatory T (T_reg) cells mediate different forms of GVHD and GVL; T_N cells mediate severe GVHD, whereas T_M cells do not cause GVHD, but preserve T-cell function including GVL. In addition, metabolic reprogramming controls T-cell differentiation and activation in these disease states. This minireview focuses on the role and the related mechanisms of T_M cells in AHSCT, and the potential manipulation of T cells in AHSCT.

Immune Regulation and Oxidative Stress Reduction by Preimplantation Factor following Syngeneic or Allogeneic Bone Marrow Transplantation

Bone marrow transplantation (BMT), a well-established treatment for hematological diseases, is frequently hampered by graft-versus-host disease (GVHD) and/or by infections due to delay in immune restoration. Prelmplantation Factor (PIF) is an embryo-derived peptide whose physiological function is to regulate local and systemic immunity and promote transplant acceptance. Synthetic PIF’s effectiveness to regulate immune response following BMT was herein examined in murine model. PIF administration reduced GVHD following allogenic BMT, decreased skin, liver, and colon inflammation and down regulated GVHD-associated gene expression in the liver. iNOS gene expression was reduced both in liver and colon. In syngeneic BMT, PIF administration reduced proinflammatory genes expression and promoted mice weight recovery up to two months after transplantation. PIF immune-regulatory effects were mediated via interaction with monocytes, resulting in decreased iNOS expression and NO secretion in-vitro. Overall, we demonstrate that by regulating immune response after BMT, PIF reduces inflammation and oxidative stress, leading to transplant success.

Proteasome inhibition and allogeneic hematopoietic stem cell transplantation: a review

The proteasome and its associated ubiquitin protein modification system have proved to be an important therapeutic target in the treatment of multiple myeloma and other cancers. In addition to direct antitumor effects, proteasome inhibition also exerts strong effects on nonneoplastic immune cells. This indicates that proteasome inhibition, through the use of agents like bortezomib, could be used therapeutically to modulate immune responses. In this review we explore the emerging data, both preclinical and clinical, highlighting the importance of proteasome targeting of immunologic responses, primarily in the context of allogeneic hematopoietic stem cell transplantation (HSCT), both for the control of transplant-related toxicities like acute and chronic graft-versus-host disease (aGVHD, cGHVHD), and for improved malignant disease control after allogeneic HSCT.

Chronic graft-versus-host disease (GVHD) in children

Five-year survival rates for childhood cancer now exceed 80% and with the significant progress made by the transplant community in developing less toxic conditioning regimens and in the treatment of posttransplant complications, allo-hematopoietic stem cell transplantation (HSCT) contributes significantly to that population of long-term survivors. In this context, the acute and long-term toxicities of chronic graft-versus-host disease (cGVHD) have an ever-increasing effect on organ function, quality of life, and survival; patients and families who initially felt great relief to be cured from the primary disease, now face the challenge of a chronic debilitating illness for which preventative and treatment strategies are suboptimal. Hence, the development of novel strategies that reduce and or control cGVHD, preserve graft-versus-tumor effects, facilitate engraftment and immune reconstitution, and enhance survival after allo-HSCT represents one of the most significant challenges facing physician-scientists and patients.

Mikulicz's disease with severe thrombocytopenia following autologous stem cell transplantation in a multiple myeloma patient

We report the first case of Mikulicz's disease (MD) occurring 2 years after autologous peripheral blood stem cell transplantation (PBSCT) for multiple myeloma (MM). A 70-year-old man developed bilateral enlargement of parotid and submandibular glands. The patient had previously received 2 courses of autologous PBSCT for IgG-kappa type MM, and had been stable for 2 years. This salivary gland enlargement was initially felt to represent a recurrence of MM, since along with gland swelling, IgG was also elevated. However, repeated biopsy of the left submandibular gland revealed chronic sclerosing sialadenitis rather than plasmacytoma. Results of salivary gland scintigraphy, serological testing, and absence of sicca symptoms also supported the diagnosis of MD. Concurrently, the patient developed severe thrombocytopenia (0.8 x 10(4)/microl). Bone marrow biopsy showed abundant megakaryocytes, suggesting enhanced platelet destruction. After high-dose steroid and immunoglobulin therapy, the platelet count gradually returned to normal with complete resolution of the salivary gland enlargement. No apparent signs of MM recurrence were documented during these clinical events.

Selective effects of cyclosporine a on Th2-skewed dendritic cells matured with viral-like stimulus by means of toll-like receptors

Successful prevention of allograft rejection and graft-versus-host disease with immunosuppression depends on controlled balance of Th1 and Th2 immune responses to establish tolerance and fight infection. Here, we have analyzed the effects of cyclosporine A (CsA) on the differentiation and functions of dendritic cells (DC2) that induce Th2 T cells. DC2 were differentiated from monocytes in the presence of CsA and were matured with viral or bacterial agonists (poly[I:C] or lipopolysaccharide). DC2 differentiation was not affected by CsA. In contrast, cytokine responses were altered with inhibition of interleukin-10 production in poly(I:C)-matured DC2. Surprisingly, interleukin-10 secretion by immature DC2 was increased after CsA treatment. Internalization was impaired in treated DC2, and CsA decreased the T-cell proliferative capacity of DC2 matured with poly(I:C), but not with lipopolysaccharide. In conclusion, CsA altered T-cell activating functions of DC2 with, notably, a regulatory phenotype for immature DC2 and opposite effects on poly(I:C)-matured cells.

Effector memory CD4+ T cells mediate graft-versus-leukemia without inducing graft-versus-host disease

Much of the efficacy of allogeneic hematopoietic stem cell transplantation (alloSCT) in curing hematologic malignancies is due to a graft-versus-leukemia (GVL) effect mediated by donor T cells that recognize recipient alloantigens on leukemic cells. Donor T cells are also important for reconstituting immunity in the recipient. Unfortunately, donor T cells can attack nonmalignant host tissues and cause graft-versus-host disease (GVHD). We previously reported that donor CD4(+) effector memory T cells (T(EMs)) do not cause GVHD but transfer functional T-cell memory. In the present work, we demonstrate in an MHC-mismatched model that CD4(+) T(EMs) (unprimed to recipient antigens) mediate GVL against clinically relevant mouse models of chronic phase and blast crisis chronic myelogenous leukemia, without causing GVHD. By creating gene-deficient leukemias and using perforin-deficient T cells, we demonstrate that direct cytolytic function is essential for T(EM)-mediated GVL, but that GVL is retained when killing via FasL, TNF-alpha, TRAIL, and perforin is individually impaired. However, T(EM)-mediated GVL was diminished when both FasL and perforin pathways were blocked. Taken together, our studies identify T(EMs) as a clinically applicable cell therapy for promoting GVL and immune reconstitution, particularly in MHC-mismatched haploidentical alloSCTs in which T cell-depleted allografts are commonly used to minimize GVHD.

Pretransplant treatment of donors with immunomodulators to control graft-versus-host disease (GVHD) in transplant recipients

OBJECTIVE

Prevention of graft-versus-host disease (GVHD) by pretransplant donor treatment with known immunomodulators like complete Freund's adjuvant (CFA) and synthetic oligo-deoxynucleotides expressing CpG motifs (CpG).

METHODS

Induction of GVHD by inoculation of C57BL/6 (C57) splenocytes into sublethally irradiated (BALB/c x C57BL/6) F1 (F1) mice. Splenocytes were derived from either naive C57 mice or from C57 mice that were treated previously with the immunomodulators.

RESULTS

Inoculation of CFA or CpG into C57 mice led to an increase in the total number of spleen cells and resulted in activation of immunoregulatory cells that significantly suppressed mixed allogeneic lymphocyte reaction in vitro. CFA-treated C57 splenocytes led to GVHD-related death in only 14 out of 61 F1 recipients while the remaining 47 mice survived without disease for more than 200 days. Pretransplant treatment of donor C57 mice with GpG emulsified in incomplete Freund's adjuvant resulted in 19/20 GVHD-free survivors of sublethally irradiated F1 mice for more than 200 days. In contrast, naive C57 splenocytes injected into sublethally irradiated F1 recipients induced severe GVHD, which resulted in the death of 77/78 recipient mice (median of survival was 16 days).

CONCLUSION

Our results suggest that adjuvant-induced immunoregulation of donor cells prior to allogeneic cell therapy may augur a new strategy that will bring the benefits of safe cellular immunotherapy aiming to eradicate malignant and nonmalignant pathological cells while avoiding or minimizing the risk of GVHD.

Graft-versus-host disease

Allogeneic haematopoietic stem-cell transplantation (SCT) is a curative therapy for haematological malignancies and inherited disorders of blood cells, such as sickle-cell anaemia. Mature alphabeta T cells that are contained in the allografts reconstitute T-cell immunity and can eradicate malignant cells in the recipient. Unfortunately, these T cells recognize the recipient as 'non-self' and employ a wide range of immune mechanisms to attack recipient tissues in a process known as graft-versus-host disease (GVHD). The full therapeutic potential of allogeneic haematopoietic SCT will not be realized until approaches to minimize GVHD, while maintaining the positive contributions of donor T cells, are developed. This Review focuses on research in mouse models pursued to achieve this goal.

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.

Optimization of allogeneic transplant conditioning: not the time for dogma

Numerous reduced-intensity conditioning regimens for allogeneic hematopoietic cell transplantation are currently being explored, primarily in older patients and in individuals with comorbid conditions who are not eligible for conventional myeloablative conditioning regimens. There is agreement that these approaches have reduced early transplant-related (non-relapse) toxicity and mortality. It is unclear, however, whether these strategies improve long-term survival. Furthermore, as most trials with reduced-intensity regimens have enrolled older patients and patients with comorbid conditions, it is not appropriate to compare the results of these trials to those obtained with more conventional approaches. It remains to be determined whether younger patients, and patients without comorbid conditions, will derive significant long-term benefits from reduced-intensity regimens when compared to conventional strategies. It may be that the different approaches are complementary and in the end will preferentially serve specific patient populations based on age, comorbid conditions and malignancy type. To determine the role of reduced-intensity approaches, controlled prospective trials are needed, with enrolled patients being stratified according to comorbid conditions, disease characteristics, pre-transplant therapy and source of stem cells, at a minimum.

Shared biology of GVHD and GVT effects: Potential methods of separation

The difficult separation of clinical graft-versus-tumor (GVT) effects from graft-versus-host disease (GVHD) reflects their shared biology. Experimental approaches to mediate GVT effects while limiting GVHD include: (1) allograft T cell depletion followed by immune enhancement; (2) modulation of T cell dose or T cell subset composition; (3) donor lymphocyte infusion; (4) reduced-intensity host preparation; (5) modulation of Th1/Th2 and Tc1/Tc2 cell balance; (6) cytokine therapy or neutralization; (7) T regulatory cell therapy; (8) co-stimulatory pathway modulation; (9) chemokine pathway modulation; (10) induction of antigen-specific T cells; (11) alloreactive NK cell therapy; and (12) targeted pharmaceutical inhibition of proteosome, mammalian target of rapamycin, and histone deacetylase pathways. Clearly, a multitude of approaches exist that hold promise for separating GVT effects from GVHD. Future success in this endeavor will require a strong commitment towards translational research and continued advances in cell, vaccine, cytokine, monoclonal antibody, and targeted molecular therapy.

Ex vivo rapamycin generates donor Th2 cells that potently inhibit graft-versus-host disease and graft-versus-tumor effects via an IL-4-dependent mechanism

Rapamycin (sirolimus) inhibits graft-vs-host disease (GVHD) and polarizes T cells toward Th2 cytokine secretion after allogeneic bone marrow transplantation (BMT). Therefore, we reasoned that ex vivo rapamycin might enhance the generation of donor Th2 cells capable of preventing GVHD after fully MHC-disparate murine BMT. Using anti-CD3 and anti-CD28 costimulation, CD4+ Th2 cell expansion was preserved partially in high-dose rapamycin (10 microM; Th2.rapa cells). Th2.rapa cells secreted IL-4 yet had reduced IL-5, IL-10, and IL-13 secretion relative to control Th2 cells. BMT cohorts receiving wild-type (WT) Th2.rapa cells, but not Th2.rapa cells generated from IL-4-deficient (knockout) donors, had marked Th2 skewing post-BMT and greatly reduced donor anti-host T cell alloreactivity. Histologic studies demonstrated that Th2.rapa cell recipients had near complete abrogation of skin, liver, and gut GVHD. Overall survival in recipients of WT Th2.rapa cells, but not IL-4 knockout Th2.rapa cells, was constrained due to marked attenuation of an allogeneic graft-vs-tumor (GVT) effect against host-type breast cancer cells. Delay in Th2.rapa cell administration until day 4, 7, or 14 post-BMT enhanced GVT effects, moderated GVHD, and improved overall survival. Therefore, ex vivo rapamycin generates enhanced donor Th2 cells for attempts to balance GVHD and GVT effects.

Kinetics of dendritic cell chimerism and T cell chimerism in allogeneic hematopoietic stem cell recipients

Dendritic cells (DC) as potent antigen-presenting cells (APC) and T cells as effector cells play an essential role in the pathophysiology of both graft-versus-host (GvH) and graft-versus-leukemia (GvL) reactions after transplantation. Therefore, we determined the kinetics of DC and T-cell chimerism establishment after allogeneic hematopoietic cell transplantation (AHCT) in a group of 144 patients, using fluorescence-activated cell sorting (FACS) or magnetic cell sorting (MACS) followed by FISH or STR-PCR analysis for chimerism evaluation. In all, three cell lines investigated (CD3(+) T cells, CD11c(+) DC1 and CD123(+) DC2), we found a rapid and consistent establishment of complete donor chimerism (CDC) in over 70% of all patients during the first 6 weeks after AHCT. The rate of patients with CDC increased significantly over time within the first year after transplantation. A related donor (P=0.004) as well as an underlying lymphatic leukemia (P=0.03) were found to be significantly associated with development of MC in T cells. No significant correlation between DC or T cell chimerism and GvHD or relapse was detected. Our results thus demonstrate a fast and stable CDC in DC1, DC2 and T cells after AHCT that continuously increases over time in nearly all patients.

Chronic graft-versus-host disease after granulocyte colony-stimulating factor-mobilized allogeneic stem cell transplantation: the role of donor T-cell dose and differentiation

The use of granulocyte colony-stimulating factor (G-CSF)-mobilized peripheral blood as a source of stem cells has resulted in a high incidence of severe chronic graft-versus-host disease (cGVHD), which compromises the outcome of clinical allogeneic stem cell transplantation. We have studied the effect of G-CSF on both immune complex and fibrotic cGVHD directed to major (DBA/2 --> B6D2F1) or minor (B10.D2 --> BALB/c) histocompatibility antigens. In both models, donor pretreatment with G-CSF reduced cGVHD mortality in association with type 2 differentiation. However, after escalation of the donor T-cell dose, scleroderma occurred in 90% of the recipients of grafts from G-CSF-treated donors. In contrast, only 11% of the recipients of control grafts developed scleroderma, and the severity of hepatic cGVHD was also reduced. Mixing studies confirmed that in the presence of high donor T-cell doses, the severity of scleroderma was determined by the non-T-cell fraction of grafts from G-CSF-treated donors. These data confirm that the induction of cGVHD after donor treatment with G-CSF is dependent on the transfer of large numbers of donor T cells in conjunction with a putatively expanded myeloid lineage, providing a further rationale for the limitation of cell dose in allogeneic stem cell transplantation.

A single-platform approach using flow cytometry and microbeads to evaluate immune reconstitution in mice after bone marrow transplantation

The monitoring of immune reconstitution in murine models of HC transplantation, using accurate and automated methods, is necessary in view of the recent developments of hematopoietic cell (HC) transplantation (including reduced intensity conditioning regimens) as well as emerging immunological concepts (such as the involvement of dendritic cells or regulatory T cells). Here, we describe the use of a single-platform approach based on flow cytometry and tubes that contain a defined number of microbeads to evaluate absolute blood cell counts in mice. This method, previously used in humans to quantify CD34+ stem cells or CD4+ T cells in HIV infected patients, was adapted for mouse blood samples. A CD45 gating strategy in this "lyse no wash" protocol makes it possible to discriminate erythroblasts or red blood cell debris from CD45+ leukocytes, thus avoiding cell loss. Tubes contain a lyophilized brightly fluorescent microbead pellet permitting the acquisition of absolute counts of leukocytes after flow cytometric analysis. We compared this method to determine absolute counts of circulating cells with another method combining Unopette reservoir diluted blood samples, hemocytometer, microscopic examination and flow cytometry. The sensitivity of this single-platform approach was evaluated in different situations encountered in allogeneic HC transplantation, including immune cell depletion after different conditioning regimens, activation status of circulating cells after transplantation, evaluation of in vivo cell depletion and hematopoietic progenitor mobilization in the periphery. This single-platform flow cytometric assay can also be proposed to standardize murine (or other mammalian species) leukocyte count determination for physiological, pharmacological/toxicological and diagnostic applications in veterinary practice.

Kinetics of in vivo elimination of suicide gene-expressing T cells affects engraftment, graft-versus-host disease, and graft-versus-leukemia after allogeneic bone marrow transplantation

Suicide gene therapy is one approach being evaluated for the control of graft-vs-host disease (GVHD) after allogeneic bone marrow transplantation (BMT). We recently constructed a novel chimeric suicide gene in which the entire coding region of HSV thymidine kinase (HSV-tk) was fused in-frame to the extracellular and transmembrane domains of human CD34 (DeltaCD34-tk). DeltaCD34-tk is an attractive candidate as a suicide gene in man because of the ensured expression of HSV-tk in all selected cells and the ability to rapidly and efficiently purify gene-modified cells using clinically approved CD34 immunoselection techniques. In this study we assessed the efficacy of the DeltaCD34-tk suicide gene in the absence of extended ex vivo manipulation by generating transgenic animals that express DeltaCD34-tk in the peripheral and thymic T cell compartments using the CD2 locus control region. We found that DeltaCD34-tk-expressing T cells could be purified to near homogeneity by CD34 immunoselection and selectively eliminated ex vivo and in vivo when exposed to low concentrations of GCV. The optimal time to administer GCV after allogeneic BMT with DeltaCD34-tk-expressing transgenic T cells was dependent on the intensity of the conditioning regimen, the leukemic status of the recipient, and the dose and timing of T cell infusion. Importantly, we used a controlled graft-vs-host reaction to promote alloengraftment in sublethally irradiated mice and provide a graft-vs-leukemia effect in recipients administered a delayed infusion of DeltaCD34-tk-expressing T cells. This murine model demonstrates the potential usefulness of DeltaCD34-tk-expressing T cells to control GVHD, promote alloengraftment, and provide a graft-vs-leukemia effect in man.

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.

Graft-versus-leukemia in a retrovirally induced murine CML model: mechanisms of T-cell killing

The graft-versus-leukemia (GVL) effect, mediated by donor T cells, has revolutionized the treatment of leukemia. However, effective GVL remains difficult to separate from graft-versus-host disease (GVHD), and many neoplasms are GVL resistant. Murine studies aimed at solving these problems have been limited by the use of leukemia cell lines with limited homology to human leukemias and by the absence of loss-of-function leukemia variants. To address these concerns, we developed a GVL model against murine chronic-phase chronic myelogenous leukemia (mCP-CML) induced with retrovirus expressing the bcr-abl fusion cDNA, the defining genetic abnormality of chronic-phase CML (CP-CML). By generating mCP-CML in gene-deficient mice, we have studied GVL T-cell effector mechanisms. mCP-CML expression of Fas or tumor necrosis factor (TNF) receptors is not required for CD8-mediated GVL. Strikingly, maximal CD4-mediated GVL requires cognate interactions between CD4 cells and mCP-CML cells as major histocompatibility complex-negative (MHC II(-/-)) mCP-CML is relatively GVL resistant. Nevertheless, a minority of CD4 recipients cleared MHC II(-/-) mCP-CML; thus, CD4 cells can also kill indirectly. CD4 GVL did not require target Fas expression. These results suggest that CPCML's GVL sensitivity may in part be explained by the minimal requirements for T-cell killing, and GVL-resistance may be related to MHC II expression.

Importance of minor histocompatibility antigen expression by nonhematopoietic tissues in a CD4+ T cell-mediated graft-versus-host disease model

Minor histocompatibility antigens with expression restricted to the recipient hematopoietic compartment represent prospective immunological targets for graft-versus-leukemia therapy. It remains unclear, however, whether donor T cell recognition of these hematopoietically derived minor histocompatibility antigens will induce significant graft-versus-host disease (GVHD). Using established bone marrow irradiation chimeras across the multiple minor histocompatibility antigen-disparate, C57BL/6-->BALB.B combination, we studied the occurrence of lethal GVHD mediated by CD4+ T cells in recipient mice expressing only hematopoietically derived alloantigens. Even substantial dosages of donor C57BL/6 CD4+ T cells were unable to elicit lethal GVHD when transplanted into [BALB.B-->C57BL/6] chimeras. Instead, chimeric mice displayed transient cachexia with reduced target-tissue injury over time, reflecting an early, limited, graft-versus-host response. On the other hand, the importance of minor histocompatibility antigens derived from nonhematopoietic tissues was demonstrated by the finding that [C57BL/6-->BALB.B] chimeric mice succumbed to C57BL/6 CD4+ T cell-mediated GVHD. These data suggest that severe acute CD4+ T cell-mediated GVHD across this minor histocompatibility antigen barrier depends on the expression of nonhematopoietically rather than hematopoietically derived alloantigens for maximal target-tissue infiltration and injury.

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.

Memory CD4+ T cells do not induce graft-versus-host disease

Graft-versus-host disease (GVHD) remains a major cause of morbidity and mortality in allogeneic stem cell transplantation (alloSCT). Donor T cells that accompany stem cell grafts cause GVHD by attacking recipient tissues; therefore, all patients receive GVHD prophylaxis by depletion of T cells from the allograft or through immunosuppressant drugs. In addition to providing a graft-versus-leukemia effect, donor T cells are critical for reconstituting T cell-mediated immunity. Ideally, immunity to infectious agents would be transferred from donor to host without GVHD. Most donors have been exposed to common pathogens and have an increased precursor frequency of memory T cells against pathogenic antigens. We therefore asked whether memory CD62L-CD44+ CD4+ T cells would induce less GVHD than unfractionated or naive CD4+ T cells. Strikingly, we found that memory CD4 cells induced neither clinical nor histologic GVHD. This effect was not due to the increased number of CD4+CD25+ regulatory T cells found in the CD62L-CD44+ fraction because memory T cells depletion of these cells did not cause GVHD. Memory CD4 cells engrafted and responded to antigen both in vivo and in vitro. If these murine results are applicable to human alloSCT, selective administration of memory T cells could greatly improve post-transplant immune reconstitution.

Autoimmune hepatitis following allogeneic PBSCT from an HLA-matched sibling

A 7-year-old boy with acute lymphoblastic leukemia (ALL) in second remission received an allogeneic PBSCT from his HLA-matched sister. Acute grade II graft-versus-host disease (GVHD) resolved with corticosteroids. Chronic GVHD in the skin and oral mucosa at around day 60 responded to corticosteroids and cyclosporin A. At 6 months after the transplant, he developed hepatic dysfunction with elevated serum transaminases and gamma-globulin. Liver biopsy revealed chronic inflammation with lymphocytes and plasma cells in portal areas without destruction of bile ducts, suggesting autoimmune hepatitis. While rare, autoimmune hepatitis should be considered a potential long-term complication in patients with hepatic dysfunction in the late post-transplant phase.

Pretreatment of donors with interleukin-18 attenuates acute graft-versus-host disease via STAT6 and preserves graft-versus-leukemia effects

Interleukin-18 (IL-18) is a unique cytokine that modulates both T(H)1/T(H)2 responses, but its ability to modulate diseases through induction of T(H)2 cytokines is unclear. It has been shown to play an important role in allogeneic bone marrow transplantation (BMT). Because immune responses of allogeneic BM donors may affect acute graft-versus-host disease (GVHD), we investigated the effect of pretreating BM transplant donors with IL-18 on the severity of acute GVHD using a well-characterized experimental BMT model (BALB/c-->B6). Pretreatment of allogeneic BM transplant donors with IL-18 significantly improved survival (80% vs 0%; P <.001), and reduced clinical, biochemical, and pathologic indices of acute GVHD in BM transplant recipients. IL-18 pretreatment was associated with reduced interferon gamma (IFN-gamma) and greater IL-4 secretion by donor T cells after BMT. Acute GVHD mortality was reduced when IL-18 was administered to donors deficient in IFN-gamma and signal transducer and activator of transcription 4 (STAT4) but not STAT6 signaling molecules, suggesting a critical role for STAT6 signaling in IL-18's protective effect. IL-18 treatment did not alter donor CD8(+) cytotoxic T-lymphocyte (CTL) activity and preserved graft-versus-leukemia (GVL) effects after allogeneic BMT (70% vs 10%; P <.01). Together these data illustrate that pretreatment of donors with IL-18 prior to allogeneic BMT attenuates acute GVHD in a STAT6-dependent mechanism while preserving GVL effects.

New strategies for prevention and treatment of graft-versus-host disease and for induction of graft-versus-leukemia effects

Graft-versus-host disease (GVHD) continues to be a problem in allogeneic hemopoietic stem cell transplantation; however, our understanding of the basic pathophysiology of GVHD has improved. Although not all data obtained from murine or other animal models can be extrapolated to the clinic, there are leads that deserve to be pursued. The skin, intestinal tract, and liver are the 3 major target organs of GVHD and share the feature of presenting a barrier to the "environment" of the host. There is evidence that the damage inflicted to these organs, the epithelial and endothelial cells in particular, by the conditioning regimen causes a release of various cytokines and a penetration of endotoxin into the systemic circulation. According to these observations, the nonimmunologic aspects of GVHD have been likened to an inflammatory process. If this characterization is valid, blocking these nonspecific inflammatory changes would ameliorate GVHD without interfering with the graft-versus-leukemia (GVL) reaction. In fact, one study has shown a substantial amelioration of GVHD with a molecule that directly blocks endotoxin. Clinical data also suggest that patients with organ dysfunction early after transplantation that is presumed to be treatment related may benefit from preemptive interventions aimed at controlling GVHD. Furthermore, there is growing evidence that the mechanisms involved in GVHD may differ from organ to organ (for example, Fas/Fas-ligand interactions in the liver versus tumor necrosis factor alpha/receptor interactions in the intestinal tract), and from a therapeutic point of view, the time of onset of clinical GVHD may be important in choosing the appropriate therapy. Thus, combinations of interventions chosen and timed appropriately may be more effective in preventing and managing GVHD than are the standard across-the-board approaches that have been used so far. Such a strategy may also be successful in maintaining a GVL effect and possibly in incorporating direct antileukemic therapy, such as the use of cytotoxic T-cells directed at minor histocompatibility antigens, without increasing the risk of GVHD. The development of nonmyeloablative conditioning regimens and the observations on GVHD kinetics and the progression or eradication of leukemia with that strategy are likely to add new insights into how one can optimally combine various modalities to achieve engraftment, prevent GVHD, and at the same time maintain a GVL effect.

Cytokines, viruses, and graft-versus-host disease

Proinflammatory cytokines released by host tissues during conditioning treatment and interferon gamma released from donor T cells play a major role in acute graft-versus-host disease (GVHD). In the past year the interaction of cytokines has been elucidated further. Host antigen-presenting cells play a key role in the induction of allogeneic recognition. Their activity is modulated by cytokines such as flt3-ligand, viruses, and donor T cells. Expansion of donor T cells is crucial for the pathogenesis of acute GVHD. Cytokines of the T helper 1 response-IFN-gamma, IL-12, and IL-18-regulate the expansion of donor and host cells via the induction of Fas and FasL and subsequent apoptosis. However TNF-alpha, FasL, and IL-1 also cause damage to target cells. Cytokine and receptor gene polymorphism has an impact on the activity of both host and donor cell activation. Genetic factors, conditioning treatment, lipopolysaccharides (LPS) from gram-negative microorganisms, viral infections, and donor T cells determine the activity level of host antigen-presenting cells and macrophages, which have an impact on acute GVHD and other complications of allogeneic stem cell transplantation.

Role of dendritic cells in graft-versus-host disease

A major barrier to successful allogeneic hematopoietic stem cell transplantation is graft-versus-host disease (GVHD). Until recently, the role of antigen presentation in the development of this disorder was unknown. The experimental finding that recipient antigen-presenting cells (APCs) were required for the development of CD8(+) T cell-dependent GVHD has led to a fundamental reappraisal of our ideas concerning the pathogenesis of this disease. Following transplantation, the origin (donor or recipient), number, lineage, and function of APCs within the recipient are altered significantly. Studies that test the influence of each of these factors upon graft-versus-host responses, including graft-versus-tumor responses, are beginning to emerge and suggest that APCs, such as dendritic cells, constitute a potential target for therapeutic manipulation.