Acute graft-versus-host disease - Wiley Online Library

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Acute Graft-versus-Host Disease: Pathophysiology, Clinical Manifestations, and Management Daniel Couriel, M.D. Humberto Caldera, M.D. Richard Champlin, M.D. Krishna Komanduri, M.D. Department of Blood and Marrow Transplantation, The University of Texas M. D. Anderson Cancer Center, Houston, Texas.

Hematopoietic stem cell transplantation has evolved as a central treatment modality in the management of different hematologic malignancies. Despite adequate posttransplantation immunosuppressive therapy, acute graft-versus-host disease (GVHD) remains a major cause of morbidity and mortality in the hematopoietic stem cell transplantation setting, even in patients who receive human leukemic antigen (HLA) identical sibling grafts. Up to 30% of the recipients of stem cells or bone marrow transplantation from HLA-identical related donors and most patients who receive cells from other sources (matched-unrelated, non-HLA-identical siblings, cord blood) will develop ⬎ Grade 2 acute GVHD despite immunosuppressive prophylaxis. Thus, GVHD continues to be a major limitation to successful hematopoietic stem cell transplantation. In this review, the authors summarize the most current knowledge on the pathophysiology, clinical manifestations, and management of this potentially life-threatening transplantation complication. Cancer 2004;101:1936 – 46. © 2004 American Cancer Society.

KEYWORDS: hematopoietic stem cell transplantation, immunosuppressive therapy, inflammatory cytokines, T-cell subsets.

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espite adequate posttransplantation immunosuppressive therapy, acute graft-versus-host disease (GVHD) remains a major cause of morbidity and mortality in the hematopoietic stem cell transplantation setting, even in patients who receive human leukemic antigen (HLA) identical sibling grafts. Up to 30% of the recipients of stem cells or bone marrow transplantation from HLA-identical related donors and most patients who receive cells from other sources (matched, unrelated, non-HLA-identical siblings; cord blood) will develop ⬎ Grade 2 acute GVHD despite immunosuppressive prophylaxis.1– 4 The treatment of GVHD also continues to be a challenge. This challenge reflects in great part the need for a better understanding of the pathophysiology and immunology of the disease process. The objective of this review was to offer an overview of our current knowledge regarding the pathophysiology, clinical manifestations, prophylaxis, and management of acute GVHD.

Pathophysiology Address for reprints: Daniel Couriel, M.D., Department of Blood and Marrow Transplantation, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Boulevard, Box 423, Houston, TX 77030-4009; Fax: (713) 794-4902; E-mail: [email protected] Received December 15, 2003; revision received July 23, 2004; accepted July 23, 2004.

The pathophysiology of acute GVHD is described best as a triphasic phenomenon (Fig. 1): The initial phase involves the development of an inflammatory milieu resulting from damage in the host tissues induced by the preparative chemotherapy or radiotherapy regimen. Damaged tissues secrete inflammatory cytokines, including interleukin 1 (IL-1), tumor necrosis factor ␣ (TNF-␣), and interferon ␥.5–7 In a second phase, both recipient and donor antigen-presenting cells (APCs) as well as inflammatory cytokines trigger the activation of

© 2004 American Cancer Society DOI 10.1002/cncr.20613 Published online 15 September 2004 in Wiley InterScience (www.interscience.wiley.com).

Acute Graft-versus-Host Disease/Couriel et al.

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FIGURE 1. Pathophysiology of acute graft-versus-host disease. APC: antigen-presenting cell; CTL: cytotoxic T lymphocyte; GI: gastrointestinal; IL-1, IL-6: interleukin-1 and interleukin-6; IFN: interferon; MNCs: mononuclear cells; NK: natural killer cell; TH1: Thelper cell type 1; TNF: tumor necrosis factor.

donor-derived T cells, which expand and differentiate into effector cells.7 In this activation phase, minor histocompatibility antigens play a central role, particularly in the setting of matched sibling transplantations.8 T-cell activation pathways result in the transcription of genes for cytokines, such as IL-2 and interferon ␥. T cells that produce IL-2 and interferon ␥ are considered to be of the Th1 phenotype, compared with T cells that produce predominantly IL-4, IL-5, IL-10, and IL-13, which define the Th2 phenotype.9 Therefore, Th1 cells mediate acute GVHD, and the differentiation of T cells into this subtype is dictated by the nature of the T-cell-APC interaction.10 –12 IL-2 plays a central role in controlling and amplifying the allogeneic immune response,13 and both IL-2 and its receptor have been and are used as targets for the management of GVHD.14 –16 In the third phase, the effector phase, activated donor T cells mediate cytotoxicity against target host cells through Fas-Fas ligand interactions,17–19 perforin-granzyme B,17 and the production of cytokines, such as TNF-␣.20 TNF-␣ is produced mainly by monocytes and macrophages and secondarily by T lymphocytes and natural killer cells.21 TNF-␣ has been implicated in the pathophysiology of GVHD at several steps in the process, including induction of apoptosis in target tissues through the TNF-␣ receptor; activation of macrophages, neutrophils, eosinophils, B cells, and T cells; stimulating production of additional inflammatory cytokines (IL-1, IL-6, IL-10, IL-12, and TNF-␣

itself); increased expression of HLA; and the facilitation of T-lymphocyte lysis.7,21–23 High levels of TNF-␣ also have been implicated in an increased incidence of GVHD in bone marrow transplantation recipients.20 This allogeneic interaction in the setting of cytokine dysregulation23,24 leads to the tissue damage characteristic of acute GVHD.

Host APCs and GVHD Although it is well established that donor T cells recognizing host minor and major histocompatibility antigens are the major mediators of GVHD, recent studies have defined better the role of APCs and T-cell subsets in GVHD. The importance of residual host APCs in the initiation phase of GVHD recently has been established in murine models. In one such model in which the development of GVHD was dependent on donor CD8-positive (CD8⫹) T cells that recognized recipient minor histocompatibility antigens, it was established that the presence of persistent, recipient-derived APCs was required in the induction phase of GVHD.25 Additional studies have suggested that APCs localized in tissue compartments may be important in organ-specific manifestations of GVHD.26 Although these studies suggest that strategies targeting residual host APCs may decrease the incidence or ameliorate the severity of GVHD, the role of APCs in sustaining expansions of GVHD-mediating T cells and in the setting of clinical GVHD in humans is not yet clear.

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CANCER November 1, 2004 / Volume 101 / Number 9

T-Cell Subsets that Mediate GVHD We recently made additional progress in defining subsets of T cells that may play a role in mediating or regulating alloreactivity. In the setting of experimental infections in murine models, it has been demonstrated that CD4⫹ T cells are crucial for sustaining the expansion of CD8⫹ T cells.27 In CD4⫹ knockout mice, it was shown that, although CD8⫹ T cells initially expanded normally, the numbers of these cells were not sustained due to either deletion or persistence without effector function.28 Other studies have confirmed the importance of CD4⫹ T cells in facilitating the secondary, but not primary, expansion of CD8⫹ T cells.29 A similar process occurs in the expansion of GVHD-mediating T cells, as demonstrated by recent studies of the effects of donor lymphocyte infusions (DLIs) in allogeneic transplantation recipients. It has been shown that the infusion of purified CD4⫹ T cells as DLI; resulted in the expansion of CD8⫹ T cells, suggesting the critical importance of CD4⫹ T cells in regulating the expansion of CD8⫹ T cells that mediate GVHD.30 Further evidence of the importance of CD4⫹ T cells rests in the results from studies that eliminated CD8⫹ T cells from the donor graft. Although it was predicted that this strategy would reduce the incidence of acute GVHD, one recent clinical trial employing this strategy resulted in a greater incidence of periengraftment fever and rash, along with a similar rate of noncutaneous Grade 2– 4 GVHD,31 relative to results in the setting of nondepleted grafts.32 These data suggest that, although CD8⫹ T cells are the likely mediators of GVHD, both CD4⫹ and CD8⫹ T cells play crucial roles in GVHD pathogenesis.

CD4ⴙ/CD25ⴙ Regulatory T Cells and GVHD Recently, substantial attention has been focused on the potential for regulatory T (Treg) cells that may be capable of suppressing alloreactivity in the setting of murine and human allogeneic transplantation. Although a number of putative regulatory T-cell subsets have been identified, a subset of CD4⫹ T cells that coexpress CD25 (the IL-2 receptor ␣ chain) has drawn the most attention. CD4⫹/CD25⫹ Treg cells originally were described as a cellular subset that could prevent autoimmunity after neonatal thymectomy in mice.33 It is believed that CD4⫹/CD25⫹ Treg cells suppress alloreactivity in a contact-dependent fashion, with conflicting evidence suggesting a role for immunosuppressive cytokines secreted by these cells, including IL-10 and transforming growth factor ␤.34 –36 Several studies in murine models have demonstrated that the infusion of donor grafts enriched in CD4⫹/CD25⫹ Treg cells may suppress the incidence of

lethal GVHD and may even facilitate allogeneic transplantation across HLA barriers.37– 40 These data suggest that CD4⫹/CD25⫹ Treg cells are a good candidate for an immunoregulatory population that may be exploited therapeutically to decrease the incidence or severity of GVHD in humans. Unfortunately, CD25 also is up-regulated in the setting of antigen-specific or alloreactive T-cell stimulation. Unfortunately, to our knowledge, no unique surface markers exist currently that may differentiate activated versus regulatory subpopulations of CD4⫹/CD25⫹ T cells. Thus, efforts to eliminate activated CD4⫹/CD25⫹ T cells (which may be involved in mediating or sustaining GVHD) inadvertently may target regulatory T cells with the same phenotype. Two recent studies of human allogeneic transplantation recipients also demonstrated how problematic our current understanding is of CD4⫹/CD25⫹ T cells. In a study in which we examined the frequencies and absolute numbers of CD4⫹/CD25⫹ T cells infused in grafts from 60 matched-sibling allogeneic transplantation donors, we found that the infusion of increased numbers of these cells was associated with a greater incidence of both acute and chronic GVHD in recipients.41 Another study, which examined the frequencies of CD4⫹/ CD25⫹ T cells in the peripheral blood of allogeneic transplantation recipients, also found that these cells were present with greater frequency in patients with chronic GVHD.42 In aggregate, these data suggest that CD4⫹/ CD25⫹ Treg cells may be an important T-cell subpopulation capable of regulating GVHD in recipients. However, they also suggest that we have much more to learn about the characterization and function of CD4⫹/CD25⫹ T-cell subsets; such an understanding should facilitate better therapies aimed at enriching subsets of Treg cells while specifically eliminating GVHD-mediating, activated T cells.

Predictive Factors and Clinical Manifestations HLA disparity is the major factor predisposing patients to acute GVHD.43 The relative importance of the incompatibility of individual HLA antigens on GVHD is controversial.44 Other relevant factors have been identified as predictors of GVHD, including age;45 gender mismatch (female donor to male recipient);45 minor histocompatibility antigens in HLA-matched transplants;8,46 donor age,47 source,48 and dose49 of hematopoietic stem cells (peripheral stem cells more than bone marrow stem cells), the intensity of the preparative regimen;50 and GVHD prophylaxis49 or other forms of graft manipulation, such as T-cell depletion.51 Acute GVHD predominantly affects the skin, the


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TABLE 1 Consensus Criteria for Organ Staging of Acute Graft-versus-Host Diseasea Stage Organ staging 0 I II

III

IV

Skin

Liver

Gut

No rash due to GVHD Maculopapular rash ⬍25% of body surface area without associated symptoms Maculopapular rash or erythema with pruritis or other associated symptoms covering ⱖ25% and ⬍50% of body surface area or localized desquamation Generalized erythroderma or symptomatic macular, popular, or vesicular eruption with bullous formation or desquamation covering ⱖ50% of the body surface area Generalized exfoliative dermatitis or ulcerative dermatitis or bullous formation

Bilirubin ⬍2 mg/dL Bilirubin from 2 mg/dL to ⬍3 mg/dL

None Diarrhea ⬎500–1000 mL/day; nausea and emesis

Bilirubin from 3 mg/dL to ⬍6 mg/dL

Diarrhea from ⬎1000 mL/day to 1500 mL/day; nausea and emesis

Bilirubin from 6 mg/dL to ⬍ 15 mg/dl

Diarrhea ⬎1500 mL/day; nausea and emesis

Bilirubin ⱖ15 mg/dL

Severe abdominal pain with or without ileus

GVHD: graft-versus-host disease. a Adapted from Przepiorka et al, 1995.31

TABLE 2 Overall Clinical Grading of Acute Graft-versus-Host Diseasea Stage Grade

Skin

Liver

Gut

Functional impairmentb

0 (none) 1 (mild) 2 (moderate) 3 (severe) 4 (life threatening)

0 1 to 2 1 to 3 2 to 3 2 to 3

0 0 1 2 to 3 2 to 3

0 0 1 2 to 3 2 to 3

0 0 1 2 2 to 4

a b

Adapted from Przepiorka, et al., 1995.31 Zubrod performance status.

upper and lower gastrointestinal (GI) tract, the liver, and occasionally the eye and oral mucosa. The staging systems are shown on Tables 1 and 2, including the first three systems but not the eye or the oral mucosa.31 Acute GVHD occurs most frequently after engraftment, up to the arbitrary 100-day period that traditionally has defined this acute form of the disease. Our transplantation practice changes (e.g., lower intensity preparative regimens, donor lymphocyte infusions to treat posttransplantation recurrence of malignancy, etc.) as the timing of acute GVHD changes. It is not uncommon to diagnose acute GVHD prior to full neutrophil engraftment of beyond 100 days.52,53 Therefore, acute GVHD is defined better by clinical manifestations than timing. Skin is the organ most often involved in acute GVHD54 and manifests as a maculopapular rash, sometimes pruritic, and sometimes painful. The distribution of the rash is quite characteristic, in that it

typically affects palms and soles initially and later progresses to cheek, ears, neck, trunk, chest, and upper back (Fig. 2). In the more severe forms, the skin involvement is erythrodermic (Stage III) and occasionally demonstrates bullae formation (Stage IV).55 The main sign of GI involvement is diarrhea, with or without upper GI symptoms such as anorexia, nausea, and emesis. The most severe forms of GI GVHD manifest with painful cramping, occasional bleeding, and ultimately ileus (Stage IV).31,56,57 Acute GVHD of the liver manifests with hyperbilirubinemia and increases in alkaline phosphatase58 and, to a lesser degree, transaminase levels.59 On occasion, isolated elevation of alkaline phosphatase levels may be the only manifestation of liver involvement (unpublished data). Biopsy of involved tissues is important in the diagnosis and management of GVHD. Although it has been reported that biopsies lack sensitivity and specificity,60,61 when they are positive, they may be useful in confirming a diagnosis with relatively nonspecific manifestations, predicting outcomes,19 and guiding immunosuppressive therapy. Other diagnostic methods, such as gene microarray,62 are being researched and may be useful adjuncts to histopathology.

Treatment of Acute GVHD Both the treatment and prevention of acute GVHD attempt to disrupt the three-step pathophysiologic cycle of acute GVHD. All current treatments for acute GVHD affect more than one event in this cycle through relatively nonspecific immunosuppressive and antiinflammatory mechanisms. Therefore, we must constantly balance and revisit benefits of ther-

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FIGURE 2. These photographs show a patient with severe acute graft-versus-host disease of the skin.

apy against the two main complications that result from their immunosuppressive and antiinflammatory effects: infection and recurrence of the underlying malignancy. Disease recurrence is secondary to blockade of the beneficial “graft-versus-leukemia” effect,63 which is associated with GVHD. Therefore, the benefits and duration of all of these therapies need to be balanced constantly against their life-threatening complications. The following paragraphs summarize the main treatment approaches for the management of acute GVHD.

Primary Therapy The initial management of acute GVHD usually is comprised of steroids. Steroids, in combination with cyclosporine54,64 or, in our center, tacrolimus,65–71 have been considered to be standard therapy for the initial management of acute GVHD.2,54,64 Their mechanism of action is unclear but most likely is related to suppression of cytokines and lympholytic activities. To our knowledge, numerous dose schedules have been used in multiple clinical trials, but the majority of centers use methylprednisolone (MP) at doses of 2.0 mg/kg. Higher doses also are effective but at the cost of significant side effects and severe catabolic damage, including hyperglycemia, fluid retention, muscular wasting, avascular bone necrosis, and an increased rate of infectious complications.2,54,64,72–74 However, to our knowledge, once a clinical improvement has been noted, there is no consensus regarding the best way of tapering steroids in responding patients, but a

faster taper may result in less steroid-related complications.75 Approximately 50% of patients who are treated with steroids in the initial management of acute GVHD will achieve a partial or complete response to therapy.54 The remainder will require additional “salvage” or “secondary” therapy for steroid-refractory acute GVHD, which carries a dismal prognosis.

Secondary Therapy Currently, to our knowledge, there is no consensus regarding the definition of steroid-refractory acute GVHD or its optimal management. At The University of Texas M. D. Anderson Cancer Center, acute GVHD is considered steroid-refractory when there is no response to MP at 2 mg/kg for 1 week or when there is progressive disease after 72 hours of MP at this dose. Numerous agents have been and continue to be evaluated, unfortunately with uniformly poor outcomes. Antithymocyte globulin (ATG) has been the most common form of immunosuppression used in this setting. Different studies have documented its efficacy74,76,77 but with significant morbidity and mortality, mainly due to infectious complications. Arai et al. at the Johns Hopkins Oncology Center reported 69 patients with steroid-refractory GVHD treated with ATG, with only 4 survivors reported in their series.78 In what to our knowledge is the largest published ATG study published to date, a retrospective analysis was performed by Khoury et al.79 Fiftyeight patients with acute GVHD who failed steroids


therapy were treated with ATG at different dose schedules. Overall, 42% of thos patients had improvement in at least 1 organ. Approximately 90% of the patients died at 40 days, mainly from infections. Pentostatin, a nucleoside analog that inhibits adenosine deaminase, is another immunosuppressant that has been used increasingly for the management of acute GVHD.80 Objective overall responses in up to 67% of patients have been reported,64 but to our knowledge, its effect on overall survival remains unknown. Monoclonal antibodies such as daclizumab, visilizumab, and infliximab have been evaluated in the prevention and treatment of acute GVHD. Daclizumab is a human monoclonal antibody immunoglobulin G1 (IgG1) that incorporates murine complementary-determining regions directed against CD25.81 It is believed that its mechanism of action is the competitive inhibition of the binding of IL-2 to its receptor.82 Przepiorka et al. reported 43 patients with advanced, refractory GVHD who were treated with 2 different schedules of daclizumab at 1 mg/kg.16 Complete responses were noted in up to 47% of those patients with the best schedule, and the majority of those responses occurred in GVHD that was confined to the skin. The overall survival was reported to be 53% in that group, and the main causes of death were GVHD and infectious complications.16 An additional study by Willenbacher et al. confirmed the activity of daclizumab, although its use was associated with an apparent increase in infectious mortality.83 Recently, Lee et al. completed a multicenter, randomized, placebo-controlled trial was designed to determine whether the addition of daclizumab could improve the success of initial steroid therapy for acute GVHD. Unfortunately, a planned interim analysis demonstrated that patients who received daclizumab experienced significantly worsened 100-day survival (77% vs. 94%; P ⫽ 0.02) and 1-year overall survival (29% vs. 60%; P ⫽ 0.002). These findings appropriately led to premature termination of the trial, and the authors concluded that daclizumab should not be added to corticosteroids for the initial treatment of acute GVHD. The increased mortality in the experimental arm was due to GVHD-related mortality as well as increased recurrences. It is noteworthy that individuals in the daclizumab arm were more likely to develop additional sites of GVHD target organ damage, despite the fact that responses at Day ⫹ 42 were similar in both arms (53% for the daclizumab arm vs. 51% for steroids alone) and the fact that chronic GVHD occurred similarly in both arms. These results suggest that studies assessing GVHD outcomes should include primary endpoints beyond the likelihood of

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obtaining a clinical response, and should also demonstrate that responses to a given treatment in the setting of primary therapy may not always be concordant with response in the setting of steroid-refractory disease. Infliximab is a genetically constructed IgG1 murine-human chimeric monoclonal antibody that binds both the soluble subunit and the membrane-bound precursor of TNF-␣,84 a major mediator of the third phase in the pathogenesis of acute GVHD. Infliximab inhibits a broad range of biologic activities of TNF-␣ by blocking the interaction with its receptors. It also may cause lysis of cells that produce TNF-␣.84 The drug has been used with success for the treatment of inflammatory bowel disease and rheumatoid arthritis.85 At The University of Texas M. D. Anderson Cancer Center, we studied the efficacy of infliximab for the treatment of steroid-refractory acute GVHD,86 with a promising response rate of 65% and an overall survival rate of 31% reported. Infliximab was found to be tolerated well in this study, but others have reported an increased incidence of mold infections associated with its use.87 Another humanized monoclonal antibody, visilizumab, is directed against a chain of CD3, the T-cell receptor. Visilizumab selectively induces apoptosis of activated T cells,88 thus predominantly affecting the second (effector) phase in the pathophysiology of GVHD. Seventeen patients with steroid-refractory, acute, severe GVHD and visceral involvement were treated at the Fred Hutchinson Cancer Center in a Phase I/II dose-finding study. Acute GVHD improved in all patients, with six patients achieving a complete response. The median survival for the 11 patients who were treated at the dose level of 3.0 mg/m2 was ⬎ 300 days, and 64% of patients were alive at a median of 1 year. Two of the first seven patients developed Epstein–Barr virus-related posttransplantation lymphoproliferative disorder (PTLD); these patients subsequently received preemptive therapy with rituximab, and there were no further incidents of PLTD. More recently, a toxin fusion, denileukin diftitox, has been evaluated in the treatment of steroid-refractory acute GVHD.89 Denileukin diftitox is a fusion of IL-2 and diphtheria toxin that has with high affinity for the IL-2 receptor, which is present in activated T cells. To date, it has shown a promising response rate of up to 71%, and 33% of patients were alive at a follow-up of 6.0 –24.6 months.89 Further studies currently are underway; but, to our knowledge to date, none of these or other treatment modalities have achieved any improvements in overall survival in patients with steroid-refractory, acute GVHD.

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The Potential for Graft Engineering as GVHD Prophylaxis Our difficulty in moving beyond the less than optimal success of steroid therapy for primary GVHD underscores the need to develop better strategies aimed at preventing GVHD. Recent advances in characterizing, isolating, and manipulating graft subpopulations ex vivo have made it possible to contemplate a future in which we will engineer donor grafts that are less capable of causing GVHD while retaining T-cell responses specific for both tumor antigens and pathogens. Several investigators have developed approaches to deplete90 –98 or anergize99 alloreactive T cells. In one general strategy, cells expressing the CD25 antigen associated with the IL-2 receptor are depleted using a toxin-conjugated monoclonal antibody after the stimulation of donor cells with alloantigenic stimulators.91,94,97,100,101 Early clinical results appear to confirm that this approach may be used to decrease the risk of GVHD in the setting of allogeneic sibling transplantation, despite the infusion of large numbers of T cells within the donor graft.63 Although the preliminary results with immunotoxin-based purging of alloreactive T cells are encouraging, it is important to note that no approved agents currently exist that would enable the wider application of this approach. Thus, additional methods to deplete alloreactive T-cell subpopulations are being investigated. One alternate approach involves the sort-based depletion of proliferating or activated donor T cells that recognize recipient antigens. Godfrey et al. demonstrated that the expression of a fluorescent marker of dividing cells could be utilized to deplete alloreactive, donor-derived T cells from human participants and in a murine model, in which they sharply reduced the incidence of lethal GVHD.98 In preclinical experiments, we have demonstrated that an alternate strategy, wherein alloreactive CD4⫹ T cells expressing unique combinations of surface markers are depleted by sorting, may reduce alloreactivity while leaving a residual T-cell population that is diverse at the molecular level and retains the ability to respond to pathogens, such as cytomegalovirus.102 Ex vivo expansion of Treg cells may prove to be another viable graft-manipulation strategy that may decrease the incidence of GVHD, as discussed previously.36 These latter approaches will need additional preclinical development before they can be applied in targeted trials of allogeneic stem cell transplantation recipients. If they are successful, then they may not only reduce the risk of GVHD in patient groups who currently are candidates for allogeneic transplantation, but they also may broaden the applicability of transplantation to patients who currently lack suitable donors and are ex-

cluded from candidacy because of the high risks of GVHD using available matched or mismatched, unrelated or haploidentical family donors. Recent studies in the mouse have pointed to another potential strategy for reducing the risk of GVHD through graft manipulation. Shlomchik et al. demonstrated that the depletion of naïve T cells in donor murine grafts facilitated allogeneic transplantation without GVHD in a CD4⫹, T cell-dependent model. Additional studies confirmed the viability of this approach in other murine models, although specifics of the model systems differed along with the definition of naïve vs. memory T cells.102,103 The possibility of transferring the portion of the donor graft that contains the memory T cells responsible for donor pathogen-specific (and, possibly, disease-specific) immunity is a compelling one, especially if such transfers result in a markedly reduced incidence of GVHD. However, mice often are kept in environments that are relatively pathogen free, and they may have naïve and memory T-cell compartments that do not completely mirror those in the human allograft. In addition, most mice utilized for transplantation are used at a relatively early stage of life compared with a human donor population that is increasingly older. Because the human memory T-cell repertoire may be relatively broader and, thus, may contain memory T cells capable of cross-recognition of recipient major and minor antigens, additional studies will be needed to confirm whether the depletion of naïve human donor T cells similarly will decrease the risks of GVHD.

Conclusions Acute GVHD is a major cause of early mortality after allogeneic transplantation. In patients who survive acute GVHD, the chronic form of this disease will cause significant morbidity and even death over months and years after transplantation. Once established, acute GVHD is difficult to treat, and the best primary treatments have shown responses of approximately 50%. Once GVHD becomes steroid-refractory, the chances of survival are slim, whereas the possibility of further long-term complications from chronic GVHD are almost always the rule. A number of approaches to engineer grafts containing increased numbers of Treg cells or decreased numbers of donor cells capable of mediating alloreactivity currently are under intense investigation in murine models and in limited human preclinical and clinical trials. Although these approaches hold promise, to our knowledge none are close to emerging as standard methods that we may use routinely to decrease the incidence of GVHD in the future. Therefore, GVHD is the major barrier to successful allogeneic stem cell transplanta-


tion, a potentially curable treatment modality for several malignant and benign hematologic conditions. Additional research efforts should be directed toward prevention, because, once established, the outcome of acute GVHD is dismal.

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