Transplantation of polarized type 2 donor T cells reduces mortality caused by experimental graft-versus-host disease

Milestones in acute GVHD pathophysiology

In the past 65 years, over 25 000 referenced articles have been published on graft-versus-host disease (GVHD). Although this included clinically orientated papers or publications on chronic GVHD, the conservative estimate of scientific publications still contains several thousands of documents on the pathophysiology of acute GVHD. Thus, summarizing what we believe are prominent publications that can be considered milestones in our knowledge of this disease is a challenging and inherently biased task. Here we review from a historical perspective what can be regarded as publications that have made the field move forward. We also included several references of reviews on aspects we could not cover in detail.

T Helper Cell Lineage-Defining Transcription Factors: Potent Targets for Specific GVHD Therapy?

Allogenic hematopoietic stem cell transplantation (allo-HSCT) represents a potent and potentially curative treatment for many hematopoietic malignancies and hematologic disorders in adults and children. The donor-derived immunity, elicited by the stem cell transplant, can prevent disease relapse but is also responsible for the induction of graft-versus-host disease (GVHD). The pathophysiology of acute GVHD is not completely understood yet. In general, acute GVHD is driven by the inflammatory and cytotoxic effect of alloreactive donor T cells. Since several experimental approaches indicate that CD4 T cells play an important role in initiation and progression of acute GVHD, the contribution of the different CD4 T helper (Th) cell subtypes in the pathomechanism and regulation of the disease is a central point of current research. Th lineages derive from naïve CD4 T cell progenitors and lineage commitment is initiated by the surrounding cytokine milieu and subsequent changes in the transcription factor (TF) profile. Each T cell subtype has its own effector characteristics, immunologic function, and lineage specific cytokine profile, leading to the association with different immune responses and diseases. Acute GVHD is thought to be mainly driven by the Th1/Th17 axis, whereas Treg cells are attributed to attenuate GVHD effects. As the differentiation of each Th subset highly depends on the specific composition of activating and repressing TFs, these present a potent target to alter the Th cell landscape towards a GVHD-ameliorating direction, e.g. by inhibiting Th1 and Th17 differentiation. The finding, that targeting of Th1 and Th17 differentiation appears more effective for GVHD-prevention than a strategy to inhibit Th1 and Th17 cytokines supports this concept. In this review, we shed light on the current advances of potent TF inhibitors to alter Th cell differentiation and consecutively attenuate GVHD. We will focus especially on preclinical studies and outcomes of TF inhibition in murine GVHD models. Finally, we will point out the possible impact of a Th cell subset-specific immune modulation in context of GVHD.

Frontline-matched sibling donor transplant of aplastic anemia patients using primed versus steady-state bone marrow grafts

Priming donors with G-CSF before BM harvest is reported to improve engraftment and GvHD in recipients. These effects are highly desirable when transplanting patients with non-neoplastic hematologic diseases, particularly AA patients. Here we retrospectively report the outcomes of 39 AA patients receiving a primed BM graft from MSD to 43 patients receiving a steady-state BM graft from MSD, otherwise transplanted using a uniform transplant platform. The graft had higher TNC and CD34 cell concentrations in the primed group (p < 0.001), and that was reflected in higher TNC and CD34 doses per kilogram of recipient in the primed group (p = 0.004 and 0.03, respectively). The OS for primed BM graft recipients was 97.4% and 78.9% for the steady-state BM graft recipients, p-value = 0.01. The cumulative incidence of death without GF was 2.6% in the primed group and 16.3% in the steady-state group, p-value = 0.03. There was no difference in GvHD incidence between the two groups. We confirm that priming improved the TNC and CD34 graft concentration and cell dose; this evidence along with other reported studies constitute reasonable evidence to prove that BM priming improve engraftment. We observed no increase in GvHD using primed BM graft.

Graft-versus-host disease, but not graft-versus-leukemia immunity, is mediated by GM-CSF-licensed myeloid cells

Allogeneic hematopoietic cell transplantation (allo-HCT) not only is an effective treatment for several hematologic malignancies but can also result in potentially life-threatening graft-versus-host disease (GvHD). GvHD is caused by T cells within the allograft attacking nonmalignant host tissues; however, these same T cells mediate the therapeutic graft-versus-leukemia (GvL) response. Thus, there is an urgent need to understand how to mechanistically uncouple GvL from GvHD. Using preclinical models of full and partial MHC-mismatched HCT, we here show that the granulocyte-macrophage colony-stimulating factor (GM-CSF) produced by allogeneic T cells distinguishes between the two processes. GM-CSF drives GvHD pathology by licensing donor-derived phagocytes to produce inflammatory mediators such as interleukin-1β and reactive oxygen species. In contrast, GM-CSF did not affect allogeneic T cells or their capacity to eliminate leukemic cells, retaining undiminished GvL responses. Last, tissue biopsies and peripheral blood mononuclear cells from patients with grade IV GvHD showed an elevation of GM-CSF-producing T cells, suggesting that GM-CSF neutralization has translational potential in allo-HCT.

Humanized mouse models in transplantation research

The interest in the use of humanized mouse models for research topics like Graft versus Host Disease (GvHD), allograft studies and other studies to the human immune system is growing. The design of these models is still improving and enables even more complicated studies to these topics. For researchers it can be difficult to choose the best option from the current pool of available models. The decision will depend on which hypothesis needs to be tested, in which field of interest, and therefore 'the best model' will differ from one to another. In this review, we provide a guide to the most common available humanized mouse models, with regards to different mouse strains, transplantation material, transplantation techniques, pre- and post-conditioning and references to advantages and disadvantages. Also, an evaluation of experiences with humanized mouse models in studies on GvHD and allograft rejection is provided.

Kinetics of T helper subsets and associated cytokines correlate well with the clinical activity of graft-versus-host disease

Background

CD4⁺interferon (IFN)-γ⁺ T cell (Th1) and CD4⁺interleukin (IL)-4⁺ T cell (Th2) polarizations are crucial in the pathogenesis of graft-versus-host disease (GVHD). However, this hypothesis is largely based on animal experiments of Parent-into-F1 GVHD model. The causal relationship between kinetics of Th1, Th2 and associated cytokines and the clinical activity of GVHD in a real world situation remains unknown.

Methodology

Peripheral blood was collected every week prospectively from Day 0 to Day 210 (patients without GVHD) or Day 300 (patients with chronic GVHD) after allogeneic peripheral blood stem cell transplantation in consecutive 27 patients. The frequencies of Th1 and Th2 within CD4⁺ T cells were determined by flow cytometry and pplasma IFN-γ, IL-12, IL-4, and IL-10 were determined by ELISA.

Principal Findings

Kinetics of Th1, Th2 frequency, and the plasma IL-10 and IFN-γ more commonly coincided with, rather than predicted, the activity of GVHD. These markers are significantly higher when acute or chronic GVHD developed. The kinetics of IL-10 is especially correlated well with the activity of GVHD during clinical course of immunosuppressive treatment. For patients with hepatic GVHD, there is a positive correlation between plasma IL-10 levels and the severity of hepatic injury. The frequency of Th2 is also significant higher in acute GVHD and tends to be higher in chronic GVHD. Interestingly, there is a very good positive correlation between the frequency of Th1 and Th2 (r = 0.951, p<0.001). The plasma level of IL-4 and IL-12 are not associated with the activity of GVHD.

Conclusions

The frequency of Th1, Th2 within CD4⁺ T cells and plasma IL-10 and IFN-γ are good biomarkers of GVHD. Plasma IL-10 can also be used to monitor the therapeutic responsiveness. Furthermore, both Th1 and Th2 likely contribute to the pathogenesis of GVHD.

Host CD25+CD4+Foxp3+ regulatory T cells primed by anti-CD137 mAbs inhibit graft-versus-host disease

CD25(+)CD4(+)Foxp3(+) regulatory T cells (Tregs) play a pivotal role in the maintenance of self-tolerance and regulation of immune responses. Previous studies have demonstrated that CD137 signals can promote proliferation and survival of Tregs in vitro. Here, we show that in vivo CD137-induced expansion of Tregs in naive mice was dependent upon IL-2 secreted by memory T cells. Tregs primed by anti-CD137 mAbs had a higher immunosuppressive capacity. Preconditioning with anti-CD137 mAbs significantly inhibited graft-versus-host disease (GVHD) in the C57BL/6 → (C57BL/6 × DBA/2) F1 acute GVHD model. In this disease model, a high proportion of host Tregs remained long-term in the recipient spleen, whereas donor hematopoietic cells replaced other host bone marrow-derived cells. Transient depletion of Tregs before transfer of donor cells completely abrogated the inhibitory effect of anti-CD137 mAbs on GVHD. In addition, adoptive transfer of anti-CD137-primed Tregs ameliorated GVHD. Our results demonstrate that it is possible to enhance the survival and/or the immunosuppressive activity of host Tregs in nonmyeloablative GVHD, and that 1 way of accomplishing this is through the prophylactic use of anti-CD137 mAbs in nonmyeloablative GVHD.

Prevention of GVHD while sparing GVL effect by targeting Th1 and Th17 transcription factor T-bet and RORγt in mice

Allogeneic hematopoietic cell transplantation (HCT) is effective therapy for hematologic malignancies through T cell-mediated GVL effects. However, HCT benefits are frequently offset by the destructive GVHD, which is also induced by donor T cells. Naive Th can differentiate into Th1 and Th17 subsets and both can mediate GVHD after adoptive transfer into an allogeneic host. Here we tested the hypothesis that blockade of Th1 and Th17 differentiation is required to prevent GVHD in mice. T cells with combined targeted disruption of T-bet and RORγt have defective differentiation toward Th1 and Th17 and skewed differentiation toward Th2 and regulatory phenotypes, and caused ameliorated GVHD in a major MHC-mismatched model of HCT. GVL effects mediated by granzyme-positive CD8 T cells were largely preserved despite T-bet and RORγt deficiency. These data indicate that GVHD can be prevented by targeting Th1 and Th17 transcription factors without offsetting GVL activity.

Regulatory T cells and IL-17-producing cells in graft-versus-host disease

Graft-versus-host disease (GvHD), a major complication following allogeneic hematopoietic stem cell transplantation, is mediated by donor-derived T cells. On activation with alloantigens expressed on host antigen-presenting cells, naive CD4+ T cells differentiate into T-helper cell subsets of effector T cells expressing distinct sets of transcriptional factors and cytokines. Classically, acute GvHD was suggested to be predominantly related to Th1 responses. However, we now face a completely different and complex scenario involving possible roles of newly identified Th17 cells as well as Tregs in GvHD. Accumulating data from experimental and clinical studies suggest that the fine balance between Th1, Th2, Th17 and Tregs after transplantation may be an important determinant of the severity, manifestation and tissue distribution of GvHD. Understanding the dynamic process of reciprocal differentiation of regulatory and T-helper cell subsets as well as their interactions will be important in establishing novel strategies for preventing and treating GvHD.

Low doses of natural killer T cells provide protection from acute graft-versus-host disease via an IL-4-dependent mechanism

CD4(+) natural killer T (NKT) cells, along with CD4(+)CD25(+) regulatory T cells (Tregs), are capable of controlling aberrant immune reactions. We explored the adoptive transfer of highly purified (> 95%) CD4(+)NKT cells in a murine model of allogeneic hematopoietic cell transplantation (HCT). NKT cells follow a migration and proliferation pattern similar to that of conventional T cells (Tcons), migrating initially to secondary lymphoid organs followed by infiltration of graft-versus-host disease (GVHD) target tissues. NKT cells persist for more than 100 days and do not cause significant morbidity or mortality. Doses of NKT cells as low as 1.0 × 10(4) cells suppress GVHD caused by 5.0 × 10(5) Tcons in an interleukin-4 (IL-4)-dependent mechanism. Protective doses of NKT cells minimally affect Tcon proliferation, but cause significant reductions in interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α) production by donor Tcons and in skin, spleen, and gastrointestinal pathology. In addition, NKT cells do not impact the graft-versus-tumor (GVT) effect of Tcons against B-cell lymphoma-1 (BCL-1) tumors. These studies elucidate the biologic function of donor-type CD4(+)NKT cells in suppressing GVHD in an allogeneic transplantation setting, demonstrating clinical potential in reducing GVHD in HCT.

Early onset of acute GVHD indicates worse outcome in terms of severity of chronic GVHD compared with late onset

Acute GVHD (aGVHD) is an important risk factor for predicting the incidence or severity of chronic GVHD (cGVHD). Transplant outcome can be influenced by the onset time of aGVHD in patients who have received allogeneic PBSC transplants (PBSCTs). The medical records of 134 patients who survived more than 3 months after myeloablative allogeneic PBSCT were retrospectively reviewed. In all, 38 patients (28.4%) developed grade II-IV aGVHD before day +28 (early aGVHD) and 25 patients (18.7%) after day +28 (late aGVHD). The 5-year cumulative incidence of cGVHD was 78.9% in the early-aGVHD group and 56.6% in the late-aGVHD group (P=0.034). The 5-year OS was 51.0% for the early-aGVHD and 80.8% for the late-aGVHD group (P=0.406). Infection was the primary cause of death for the early-aGVHD group (51.4 vs 16.7%, P=0.017), whereas relapse of the primary disease was higher among the patients with late aGVHD, although this was statistically insignificant (58.3 vs 25.7%, P=0.309). In a multivariate analysis, early aGVHD was identified as a risk factor for developing cGVHD (hazard ratio (HR) 2.278, P=0.004). The development of aGVHD early after allogeneic PBSCT increased the risk of cGVHD and infection-related death rate when compared with the late onset of aGVHD.

Immunomodulatory effects of mesenchymal stem cells involved in favoring type 2 T cell subsets

Graft-vs.-host disease (GVHD) caused by immunologic activated cells remains a real problem in human allogeneic hematopoetic stem cell transplantation. Mesenchymal stem cells (MSCs) play some important roles on immunomodulatory. We developed a parent-into-F1 model of acute GVHD to evaluate the mechanisms involved in immunological mediated damage and the immunomodulatory effect of the MSCs on GVHD. The recipients, BABL/cxC57BL/6 (H-2(bxd)) F1 mice, received 8.5Gy total-body gamma irradiation ((6)(0)C(O)), then rescued with C57BL/6 (H-2(b)) mice (donors) bone marrow cells and induced acute GVHD by adding donor splenocytes. The MSCs culture-expanded from C57BL/6 (H-2(b)) mice were infused to recipients simultaneity in the experimental group. The severity of GVHD was evaluated by histopathologic examination of target organs including liver, intestine, and claw skin and a clinical manifestation scoring system. We analyzed the distribution of peripheral blood T cell subsets of recipients by flow cytometry and measured the expression of CXCR3 on activated T cells in target organs by immunohistochemistry staining. Our results suggested the tissue damage initiated by GVHD was significantly alleviated in the MSCs treated mice, and the proportion of type 2 T cells in peripheral blood was higher in the MSCs treated mice than in the control group. Although the overall survival rate did not significantly improved in the mice with MSCs infusion, the immunomodulatory effect of MSCs was possibly related to favor type 2 T cell subsets and decrease chemokine receptor CXCR3 expression on activated T cells.

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

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

IL-17 contributes to CD4-mediated graft-versus-host disease

CD4(+) interleukin-17 (IL-17)(+) T cells (Th17 cells) have been implicated in allograft rejection of solid organs and several autoimmune diseases. However, the functional role of Th17 cells in the development of acute graft-versus-host disease (GVHD) has not been well-characterized. We detected significant numbers of alloreactive CD4(+) donor T cells expressing IL-17, IL-17F, or IL-22 in the lymphoid organs of recipients of an allogeneic bone marrow transplant. We found no differences in GVHD mortality or graft-versus-tumor (GVT) activity between wild type (WT) and IL-17(-/-) T-cell recipients. However, upon transfer of murine IL-17(-/-) CD4(+) T cells in an allogeneic BMT model, GVHD development was significantly delayed behind recipients of WT CD4(+) T cells, yet overall GVHD mortality was unaffected. Moreover, recipients of IL-17(-/-) CD4(+) T cells had significantly fewer Th1 cells during the early stages of GVHD. Furthermore, we observed a decrease in the number of IFN-gamma-secreting macrophages and granulocytes and decreased production of proinflammatory cytokines (interferon [IFN]-gamma, IL-4, and IL-6) in recipients of IL-17(-/-) CD4(+) T cells. We conclude that IL-17 is dispensable for GVHD and GVT activity by whole T cells, but contributes to the early development of CD4-mediated GVHD by promoting production of proinflammatory cytokines.

IFN-gamma and Fas ligand are required for graft-versus-tumor activity against renal cell carcinoma in the absence of lethal graft-versus-host disease

To determine the mechanisms of graft-versus-tumor (GVT) activity in the absence of graft-versus-host disease (GVHD) against a solid tumor, we established two allogeneic bone marrow transplantation models with a murine renal cell carcinoma (RENCA). The addition of 0.3 x 10(6) donor CD8(+) T cells to the allograft increased the survival of tumor-bearing mice without causing GVHD. The analysis of CD8(+) T cells deficient in cytotoxic molecules demonstrated that anti-RENCA activity is dependent on IFN-gamma and Fas ligand (FasL), but does not require soluble or membrane-bound TNF-alpha, perforin, or TRAIL. Recipients of IFN-gamma(-/-) CD8(+) T cells are unable to reject RENCA compared with recipients of wild-type CD8(+) T cells and, importantly, neither group develops severe GVHD. IFN-gamma(-/-) CD8(+) T cells derived from transplanted mice are less able to kill RENCA cells in vitro, while pretreatment of RENCA cells with IFN-gamma enhances class I and FasL expression and rescues the lytic capacity of IFN-gamma(-/-) CD8(+) T cells. These results demonstrate that the addition of low numbers of selected donor CD8(+) T cells to the allograft can mediate GVT activity without lethal GVHD against murine renal cell carcinoma, and this GVT activity is dependent on IFN-gamma and FasL.

Prevention of Graft-Versus-Host Disease by Intra-Bone Marrow Injection of Donor T Cells

We have recently found that intra-bone marrow-bone marrow transplantation (IBM-BMT) can be used to prevent graft-versus-host disease (GvHD), even when intensive donor lymphocyte infusion (DLI) is carried out. In the present study, in conjunction with IBM-BMT, allogeneic splenic T cells as DLI were also injected into the bone marrow cavity of lethally irradiated (8.5 Gy) recipients. The extent of GvHD was compared with that of recipients that had received allogeneic IBM-BMT plus i.v. injection of allogeneic T cells (intravenous DLI [IV-DLI]). GvHD in recipients treated with allogeneic IBM-BMT plus IBM-DLI was far milder than in those treated with allogeneic IBM-BMT plus IV-DLI. This was confirmed macroscopically and histopathologically. The frequency of regulatory T cells (Tregs) detected as CD4(+)CD25(+) and CD4(+)Foxp3(+) cells was significantly higher in recipients treated with IBM-BMT plus IBM-DLI than in those treated with IBM-BMT plus IV-DLI. Donor-derived helper T (Th) cells polarized to Th2 type in recipients treated with IBM-BMT plus IBM-DLI, whereas Th1 cells were dominant in recipients treated with IBM-BMT plus IV-DLI. Furthermore, the production of transforming growth factor-beta and hepatocyte growth factor from bone marrow stromal cells was enhanced after IBM-DLI. Thus, IBM-BMT plus IBM-DLI seem to preferentially induce Tregs and Th2, resulting in the prevention of GvHD. Disclosure of potential conflicts of interest is found at the end of this article.

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.

Inflammatory cytokines and acute graft-versus-host disease after reduced-intensity conditioning allogeneic stem cell transplantation

This study investigated the role of inflammatory cytokines in acute graft-versus-host disease (aGVHD) incidence and severity in 113 patients who underwent reduced-intensity conditioning (RIC) allogeneic stem cell transplantation (allo-SCT). Among all tested cytokines in the first 3 months after allo-SCT, only interleukin-12 p70 (IL-12p70) levels in the first month were significantly associated with grades II to IV aGVHD development (P < .001). IL-12p70 levels were directly correlated with aGVHD severity grade (P < .001). Before aGVHD onset, blood monocytes, the main precursor pool of IL12p70-secreting dendritic cells, recovered more rapidly in patients with grades II to IV aGVHD (P = .005). Similarly, at the effector level, there was a more robust reconstitution of naive CD3+CD4+CD45RA+CD27+ T cells in patients developing grades II to IV aGVHD (P = .006). In multivariate analysis, IL-12p70 level measured in the first month was the strongest predictive factor for aGVHD development (P < .001). These findings, reconstituting a T(H)1 loop, support a model in which aGVHD reflects a type 1 alloreaction after RIC allo-SCT.

Anti-CD25 monoclonal antibody (basiliximab) for prevention of graft-versus-host disease after haploidentical bone marrow transplantation for hematological malignancies

Haploidentical donors are available for most patients who need allografts but do not have matched donors. However, GVHD, rejection, delayed immune reconstitution, and infections have been significant barriers. We designed a haploidentical BMT protocol focusing on prevention of GVHD and rejection. A total of 53 leukemic patients underwent haploidentical G-CSF-primed BMT without ex vivo T-cell depletion. GVHD prophylaxis consisted of antithymocyte globulin, cyclosporine, methotrexate, and mycophenolate mofetil. In all, 38 patients (the CD25 group) received additional anti-CD25 monoclonal antibody basiliximab. The results were compared to 15 patients who did not receive basiliximab. All patients achieved trilineage engraftment with full-donor chimerism. The incidence of acute II-IV GVHD was 11% in the CD25 group vs 33% in the control group (P=0.046). The overall incidence of extensive chronic GVHD was 15%. T, B, and NK cells recovered within 12 months post transplant. The disease-free survival at 2 years was 53% with a median follow-up of 31 months. In conclusion, G-CSF primed haploidentical BMT along with sequential immunosuppressive agents as described here deserves further study.

Absence of inducible costimulator on alloreactive T cells reduces graft versus host disease and induces Th2 deviation

Inducible costimulator (ICOS) is expressed on activated and memory T cells and is involved in the regulation of cytokine production. We studied the role of ICOS on alloreactive T cells in graft versus host disease (GVHD) and determined that ICOS expression was up-regulated on alloreactive T cells in recipients of an allogeneic hematopoietic stem cell transplantation (allo-HSCT) with GVHD. We compared ICOS-/- T cells with wild-type (WT) T cells in 2 GVHD models. In both models, recipients of ICOS-/- T cells demonstrated significantly less GVHD morbidity and mortality, which was associated with less intestinal and hepatic GVHD but increased cutaneous GVHD. In addition, recipients of ICOS-/- donor T cells displayed a slight decrease in graft versus leukemia (GVL) activity. Further analysis of alloreactive ICOS-/- T cells showed no defect in activation, proliferation, cytotoxicity, and target organ infiltration. Recipients of ICOS-/- T cells had decreased serum levels of interferon-gamma (IFN-gamma), while interleukin-4 (IL-4) and IL-10 levels were increased, suggesting that alloreactive ICOS-/- T cells are skewed toward T helper-2 (Th2) differentiation. These data suggest a novel role for ICOS in the regulation of Th1/Th2 development of activated T cells. In conclusion, alloreactive ICOS-/- donor T cells induce less GVHD due to a Th2 immune deviation while GVL activity is slightly diminished.

A possible association between the presence of interleukin-4–secreting cells and a reduction in the risk of acute graft-versus-host disease

OBJECTIVE

We monitored cytokine-secreting cells using an enzyme-linked immunospot (ELISPOT) assay in a prospective study to assess the cytokine network after transplantation.

PATIENTS AND METHODS

Peripheral blood mononuclear cells were collected from 23 patients who received allogeneic stem cell transplantation, from before the preconditioning regimen to 56 days after transplantation. The number of interleukin-4 (IL-4), interferon-gamma (IFN-gamma), and tumor necrosis factor-alpha (TNF-alpha)-secreting cells were measured by ELISPOT assay. For IL-4 and IFN-gamma, in vitro stimulation with phorbol 12-myristate 13-acetate and phytohemagglutinin was performed.

RESULTS

The frequency of IL-4-secreting cells was significantly higher in five patients receiving peripheral blood stem cell transplantation (PBSCT) than that in 18 patients who received bone marrow transplantation (BMT). Based on IFN-gamma and TNF-alpha release, there was a trend toward a decrease in the number of cytokine-secreting cells in PBSCT compared with BMT. Furthermore, patients who did not develop acute graft-vs-host disease (GVHD, n=5) showed a significantly higher number of IL-4-secreting cells compared with those who developed acute GVHD (n=18). Both IFN-gamma-secreting cells and TNF-alpha-secreting cells showed a trend to increase in number in patients with acute GVHD. In patients who received reduced-intensity stem cell transplantation (n=7) compared with conventional stem cell transplantation (n=16), there was a large number of cytokine-secreting cells detected by IL-4 and IFN-gamma release.

CONCLUSIONS

These results are consistent with the hypothesis that IL-4-producing cells inhibit the development of acute GVHD. In addition, the increased percentage of IL-4-secreting cells may be responsible for the unexpected low incidence of acute GVHD in PBSCT, despite the presence of large numbers of mature T cells in the donor infusion.

Stimulation of Host NKT Cells by Synthetic Glycolipid Regulates Acute Graft-versus-Host Disease by Inducing Th2 Polarization of Donor T Cells

NKT cells are a unique immunoregulatory T cell population that produces large amounts of cytokines. We have investigated whether stimulation of host NKT cells could modulate acute graft-vs-host disease (GVHD) in mice. Injection of the synthetic NKT cell ligand alpha-galactosylceramide (alpha-GalCer) to recipient mice on day 0 following allogeneic bone marrow transplantation promoted Th2 polarization of donor T cells and a dramatic reduction of serum TNF-alpha, a critical mediator of GVHD. A single injection of alpha-GalCer to recipient mice significantly reduced morbidity and mortality of GVHD. However, the same treatment was unable to confer protection against GVHD in NKT cell-deficient CD1d knockout (CD1d(-/-)) or IL-4(-/-) recipient mice or when STAT6(-/-) mice were used as donors, indicating the critical role of host NKT cells, host production of IL-4, and Th2 cytokine responses mediated by donor T cells on the protective effects of alpha-GalCer against GVHD. Thus, stimulation of host NKT cells through administration of NKT ligand can regulate acute GVHD by inducing Th2 polarization of donor T cells via STAT6-dependent mechanisms and might represent a novel strategy for prevention of acute GVHD.

The effect of extracorporeal photopheresis on intracellular cytokine expression in chronic cutaneous graft-versus-host disease

BACKGROUND

Cytokines derived from T helper (Th)1 lymphocytes are thought to be involved in the pathogenesis of graft-versus-host disease (GVHD) and extracorporeal photopheresis (ECP) has been reported to affect Th1/Th2 lymphocyte ratios. It may also influence the balance of cytotoxic Tcells (Tc1/Tc2).

OBJECTIVES

This study was formulated to assess the effect of ECP on the cytokine profiles of peripheral blood (PB) lymphocytes from patients with chronic GVHD.

PATIENTS AND METHODS

Nine patients were studied. Peripheral blood was sampled at baseline and between 3 and 4 months of therapy when clinical effects are demonstrable. Intracellular cytokine production was assessed in vitro by stimulating PB lymphocytes with phorbol-12-myristate 13-acetate (PMA), inhibiting cytokine release and staining with fluorescein-labelled monoclonal antibodies to interleukin (IL)-2, interferon gamma (IFN-gamma) and IL-4. Flow cytometry analysis gave the absolute number and the percentage of cells expressing a particular cytokine within each lymphocyte subset.

RESULTS

Absolute counts of CD3, CD4, CD8, CD19 and CD16+ cells per microlitre were recorded before and after ECP. There was a small but non-significant reduction in all subsets after 3 months of ECP. The percentage of cells expressing IL-2 and IFN-gamma rose following ECP in both the CD4 and CD8 subsets. However, only the percentage of CD4 cells expressing IFN-gamma reached statistical significance (P = 0.02; 95% confidence interval, CI 0.6-15.6). There were no significant changes in the percentage of CD4 cells expressing IL-4.

CONCLUSIONS

Our findings appear to be inconsistent with current theories regarding the pathogenesis of GVHD as increased production of Th1 or Tc1 cytokines might be expected to exacerbate GVHD. However, chronic GVHD is characterized by a relative deficiency of IL-2 and IFN-gamma producing cells compared with other patients post-bone marrow transplantation (BMT). This indicates that Th1 and Tc1 cytokines are depleted in chronic GVHD. Thus, by reducing disease activity, ECP could allow cytokine production by these cells to recover. This indicates that the therapeutic effect of ECP is mediated by a different mechanism, and that the changes observed in this study are epiphenomena.

Immunoablative reduced-intensity stem cell transplantation: potential role of donor Th2 and Tc2 cells

Allogeneic reduced-intensity stem cell transplantation (RISCT) decreases regimen-associated morbidity and mortality, but it is unfortunately still constrained by the same immune T-cell reactions that limit myeloablative transplantation, including graft rejection, graft-versus-host disease (GVHD), and suboptimal graft-versus-leukemia (GVL) or graft-versus-tumor (GVT) effects. Graft rejection is mediated by host T cells, whereas GVHD and GVL/GVT effects are initiated by donor T cells, and to this extent, future advances in RISCT will likely benefit from an ability to modulate both donor and host T-cell immunity. As a step in this direction, we have developed a RISCT approach that first involves chemotherapy-induced host T-cell ablation, and second involves administration of allogeneic inocula enriched for donor CD4(+) Th2 and CD8(+) Tc2 T-cell subsets that in murine studies mediate reduced GVHD. In a pilot clinical trial, "immunoablative" RISCT with human leukocyte antigen (HLA)-matched related allografts resulted in rapid and complete donor chimerism and GVL effects early post-transplant, with GVHD being the primary toxicity. Using this immunoablative RISCT approach, we are now evaluating the feasibility and safety of augmenting allografts with additional donor CD4(+) Th2 cells that are generated in vitro via CD3/CD28 costimulation in the presence of interleukin (IL)-4. We review the biology of host and donor T-cell immunity during allogeneic RISCT and discuss the strategies of host immunoablation and donor Th2 and Tc2 cell therapy as potential means to improve the clinical results in RISCT.

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.

G-CSF-primed haploidentical marrow transplantation without ex vivo T cell depletion: an excellent alternative for high-risk leukemia

Based on our encouraging results of G-CSF-primed HLA-matched related marrow transplants for high-risk leukemia, we extended the study from matched related to haploidentical transplants using G-CSF primed marrow and sequential immunosuppressants to prevent both graft-versus-host disease (GVHD) and host-versus-graft rejection (HVGR). Fifteen high-risk leukemia patients, who needed urgent transplantation but lacked an HLA-matched donor, underwent G-CSF-primed haploidentical marrow transplantation without ex vivo T cell depletion. Donors were given G-CSF (Lenograstim) at 3-4 microg/kg/day for 7 days prior to marrow harvest. GVHD and HVGR prophylaxis were combined in the sequential usage of cyclosporin A, methotrexate, anti-thymocyte globulin and mycophenolate mofetil. All patients established sustained trilineage engraftment at a median of 19 days and 21 days for neutrophil and platelets respectively. G-CSF priming significantly increased CD34(+) and CFU-GM cells, reduced total lymphocytes and reversed the CD4(+)/CD8(+) ratio in the donor marrow. The incidence of grade II-IV acute GVHD was 33.3%. Nine patients survived more than a year with a Karnofsky performance status of 100%. Estimated overall disease-free survival at 2 years was 60 +/- 7%. In conclusion, using G-CSF priming marrow grafts along with sequential immunosuppressants provided an excellent alternative for the treatment of high-risk hematological malignancy in patients who lack matched donors.

Development of allogeneic hematopoietic stem cell transplantation (HSCT)

The field of BMT has entered a new phase based on insights into basic molecular and cellular biology. The mechanism of cure mediated by allogeneic HSCT is now believed to be at least partially mediated by cellular attack against tumor-associated antigens. Better understanding of these cellular targets and the means of targeting them specifically will allow a more precise application of cellular therapy than is currently possible. This in turn may make it possible to design therapies that are increasingly selective without the collateral toxicities of GVHD, infection, and direct organ damage that currently are the limiting features of allogeneic HSCT as practiced until recently. However, it remains a challenge to demonstrate that disease-control using reduced intensity preparative regimens is at least comparable to that achieved by traditional myeloablative HSCT. This will require carefully controlled studies in each the major diseases that have shown sensitivity to the immune-mediated effects of HSCT. The chapters below will highlight progress towards the goals of elucidating the role of NST.

The flavonoid Kaempferol suppresses the graft-versus-host reaction by inhibiting type 1 cytokine production and CD8+ T cell engraftment

We have here examined the immunoregulatory effects of a natural flavonoid, Kaempferol, on T cells. Kaempferol suppressed IFN-gamma and IL-2 production but not that of IL-4 by T cells and shifted the Th1/Th2 balance into the Th2 phenotype. In C57BL/6 anti-BDF1 MLC, Kaempferol inhibited the development and expansion of type 1 effector CD8+ T cells and diminished allospecific CTL activity. Based on these in vitro results, we evaluated the effects of Kaempferol treatment on the development of C57BL/6-into-BDF1 acute graft-versus-host disease (GVHD). Brief administration of Kaempferol led to early recovery from body weight loss, increased survival, and reduced tissue injury in the liver and large intestine. Kaempferol treatment activated Th2 cells and the engraftment of donor cells and also diminished GVHD-associated antihost CTL activity. Thus, our study indicates that Kaempferol has the capacity to modulate the Th1/Th2 balance and could be useful for the treatment of cell-mediated immune diseases, such as acute GVHD.

Donor lymphocyte infusions to treat hematologic malignancies in relapse after allogeneic blood or marrow transplantation

BACKGROUND

Patients with hematologic malignancies in relapse after allogeneic bone marrow transplantation can be treated by infusing leukocytes from the original stem cell donor.

METHODS

The published literature on donor lymphocyte infusion (DLI) was reviewed.

RESULTS

DLI induces complete remissions in the majority of patients with chronic myeloid leukemia (CML) in early-stage relapse and in less than 30% of patients with relapsed acute leukemia, myelodysplasia, and multiple myeloma. DLI-induced remissions of chronic phase CML are durable, but as many as half of patients with other diseases ultimately relapse. Complications of DLI include acute and chronic graft-vs-host disease (GVHD) and aplasia, which induce profound immunosuppression and susceptibility to opportunistic infections. There is a strong correlation of GVHD and disease response.

CONCLUSIONS

Novel methods of augmenting the antitumor efficacy of DLI and of dissociating the graft-vs-leukemia effect from GVHD are needed. These studies will require an improved understanding of the cellular and molecular mechanisms of alloreactivity and the development of novel agents to control the nature and intensity of the alloimmune response.

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

Graft-versus-host disease (GVHD) is a major cause of morbidity and mortality of allogeneic stem cell transplantation. Strategies to control GVHD while maintaining graft versus leukemia (GVL) include herpes simplex virus thymidine kinase (HSV-tk) gene transduction of donor T cells followed by treatment with ganciclovir (GCV). Alternatively, GVHD and GVL may be mediated by distinct processes. In this regard, whether cytokine polarization occurs and to what degrees various subsets of cytokine-producing T cells mediate GVHD or GVL has been an active area of research using cytokine or cytokine antibody infusion or genetically deficient mice. This study takes a different approach that allows simultaneous investigation into both the mechanisms underlying GVHD reactions and the efficacy of HSV-tk suicide gene-based T-cell deletion. A source of donor T cells, splenocytes from mice transgenic for HSV-tk controlled by elements of either the interleukin-2 (IL-2) or IL-4 promoters (IL-2-tk and IL-4-tk, respectively) was used, thus allowing investigation into the roles of T1 and T2 cells in ongoing GVHD reactions. To assess treatment rather than prevention of GVHD, GCV was started at peak disease. Remarkably, treatment at this late time point rescued mice from the clinical effects of GVHD caused by T cells expressing either transgene. Thus, both T1 and T2 cells play an important role in clinical GVHD in a minor histocompatibility antigen-mismatched setting. In addition, because clinical disease was reversible even at its maximum, these observations provide controlled evidence that this strategy of treating ongoing GVHD could be effective clinically.

Calcium signaling inhibits interleukin-12 production and activates CD83(+) dendritic cells that induce Th2 cell development

Mature dendritic cells (DCs), in addition to providing costimulation, can define the Th1, in contrast to the Th2, nature of a T-cell response through the production of cytokines and chemokines. Because calcium signaling alone causes rapid DC maturation of both normal and transformed myeloid cells, it was evaluated whether calcium-mobilized DCs polarize T cells toward a Th1 or a Th2 phenotype. After human monocytes were cultured for 24 hours in serum-free medium and granulocyte-macrophage colony-stimulating factor to produce immature DCs, additional overnight culture with either calcium ionophore (CI) or interferon gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha), and soluble CD40L resulted in phenotypically mature DCs that produced interleukin-8 (IL-8) and displayed marked expression of CD80, CD86, CD40, CD54, CD83, DC-LAMP, and RelB. DCs matured by IFN-gamma, TNF-alpha, and soluble CD40L were additionally distinguished by undetectable CD4 expression, marked secretion of IL-12, IL-6, and MIP-1beta, and preferential ability to promote Th1/Tc1 characteristics during T-cell sensitization. In contrast, DCs matured by CI treatment were distinguished by CD4 expression, modest or absent levels of IL-12, IL-6, and MIP-1beta, and preferential ability to promote Th2/Tc2 characteristics. Calcium signaling selectively antagonized IL-12 production by mature DCs activated with IFN-gamma, TNF-alpha, and soluble CD40L. Although the activation of DCs by calcium signals is largely mediated through calcineurin phosphatase, the inhibition of IL-12 production by calcium signaling was independent of this enzyme. Naturally occurring calcium fluxes in immature DCs, therefore, negatively regulate Dc1 differentiation while promoting Dc2 characteristics and Th2/Tc2 polarization. Calcium-mobilized DCs may have clinical usefulness in treating disease states with excessive Th1/Tc1 activity, such as graft-versus-host disease or autoimmunity.

Bone marrow cell graft engineering: from bench to bedside

Bone marrow transplantation (BMT) has the potential to treat hemoglobinopathies (sickle cell and thalassemia) autoimmunity (diabetes, lupus, multiple sclerosis, rheumatoid arthritis, Crohn's colitis) and enzyme deficiency states. Graft versus host disease (GVHD) is a major complication and limitation to the therapeutic application of BMT. There have been many clinical trials and experimental animal models that have attempted to control GVHD through the engineering of the donor bone marrow cells (BMC). Historically, several methods have demonstrated effectiveness in controlling GVHD; however they were also associated with a marked increase in the rate of graft failure. Highly purified hematopoietic stem cells (HSC) engraft quite readily in genetically-matched recipients while they do not engraft as easily in MHC-disparate recipients. The numbers of HSC must be increased 100-200 fold in order to overcome the allogeneic barrier. We were the first to phenotypically and to functionally characterize a novel cell in the bone marrow that enables engraftment of highly purified HSC in allogeneic recipients. The discovery of graft facilitating cell populations has resulted in the restoration of the engraftment-potential of purified HSC between genetically-disparate individuals. The addition of facilitating cells (FC) to T cell-depleted BMC grafts results in allogeneic engraftment without GVHD or graft failure. New strategies of BMC engineering that retain FC and HSC but avoid GVHD have allowed successful engraftment in mismatched and older recipients. These techniques have expanded the therapeutic potential of BMT to virtually every candidate as well as to non-malignant diseases in which the morbidity associated with conventional BMT could not be accepted. This article reviews the transition of the FC technology from bench to bedside and discuss the potentially broad-reaching applications of BMT and mixed chimerism.

Tumor necrosis factor-alpha neutralization reduces lung injury after experimental allogeneic bone marrow transplantation

BACKGROUND

Idiopathic pneumonia syndrome (IPS) is a frequent and potentially fatal complication of bone marrow transplantation (BMT). We have previously shown that experimental IPS is associated with increased levels of lipopolysaccaride (LPS) and tumor necrosis factor-alpha (TNFalpha) in the bronchoalveolar lavage (BAL) fluid, and that administration of LPS to animals with extensive graft versus host exacerbated underlying lung injury (Blood 1996; 88: 3230).

METHODS

Lethally irradiated CBA mice received BMT from allogeneic (B10.BR) or syngeneic (CBA) donors. The role of TNFalpha in the exacerbation of pulmonary toxicity caused by LPS injection and in the evolution of IPS after allogeneic BMT was examined by neutralizing TNFalpha after BMT using a soluble binding protein (rhTNFR:Fc).

RESULTS

Five weeks after BMT, administration of rhTNFR:Fc dramatically reduced mortality and prevented the exacerbation of lung injury caused by LPS administration. This protective effect was associated with preservation of pulmonary function and with marked reductions of cells, neutrophils, and LPS in the BAL fluid of treated animals. TNFalpha neutralization from week 4 to 6 after allogeneic BMT effectively halted the progression of systemic GVHD and significantly reduced, but did not prevent lung injury that developed during the treatment period.

CONCLUSIONS

We conclude that TNFalpha is central to early LPS induced toxicity in this model and is a significant, but not the exclusive contributor to the development of IPS after allogeneic BMT.

Th2 and Tc2 cells in the regulation of GVHD, GVL, and graft rejection: considerations for the allogeneic transplantation therapy of leukemia and lymphoma

Allogeneic stem cell transplantation (SCT) represents a curative treatment option for patients with leukemia and lymphoma. T lymphocytes contained in the allograft mediate a graft-versus-leukemia (GVL) effect and prevent graft rejection; however, T cells also initiate graft-versus-host disease (GVHD). Identification of T cell populations which mediate a GVL effect and prevent rejection with reduced GVHD will likely improve transplantation outcome. T cells exist in four functionally-defined populations, the CD4+, Th1/Th2 and CD8+, Tc1/Tc2 subsets. Th1-type CD4 cells primarily secrete type I cytokines (IL-2 and IFN-gamma), whereas Th2 cells secrete type II cytokines (IL-4, IL-5, and IL-10). Similarly, the CD8+ Tc1 and Tc2 cells differentially secrete the type I and type II cytokines, respectively. In addition to cytokine secretion, Tc1 and Tc2 populations mediate cytolytic effects, with Tc1 cells utilizing both perforin- and fas-based killing pathways, whereas Tc2 cells primarily utilize perforin-mediated cytolysis. In murine transplantation models of graft rejection, GVHD, and GVL effects, we have evaluated such functional T cell subsets for their ability to differentially mediate and regulate transplantation responses. These studies demonstrate that donor Th2 cells do not initiate acute GVHD, and can regulate the GVHD mediated by unmanipulated donor T cells without impairing alloengraftment. Additional experiments have shown that allospecific donor Tc2 cells result in reduced GVHD, and mediate a significant GVL effect. Thirdly, we have demonstrated that non-host reactive Tc2 cells with veto-like activity can potently abrogate marrow rejection independent of GVHD. Together, these results demonstrate that functionally-defined donor Th2 and Tc2 populations play an important role in the regulation of GVHD, the prevention of graft rejection, and the mediation of GVL effects, and suggest that utilization of Th2 and Tc2 cells in clinical allogeneic SCT may have potential for improving treatment outcome.

Immunobiology of allogeneic peripheral blood mononuclear cells mobilized with granulocyte-colony stimulating factor

The use of mobilized peripheral blood (PB) stem cells for autologous transplantation initially generated much enthusiasm because of enhanced engraftment in comparison to marrow stem cells and avoidance of general anesthesia for the donor. Its application to the allogeneic setting seemed inevitable. For obvious ethical reasons, allogeneic donors are mobilized with cytokines only, mainly granulocyte colony-stimulating factor (G-CSF). Results from preliminary studies suggest that in comparison to standard bone marrow transplants, outcomes such as engraftment, host-versus-graft reaction, graft-versus-host disease, graft-versus-leukemia and immunological reconstitution may be different. Surprisingly, G-CSF, previously recognized as a late acting lineage-specific factor for neutrophil production, not only disrupts homeostasis between stem cells and their microenvironment, but also induces significant quantitative and qualitative changes in the accessory cell compartment, affecting lymphocytes, monocytes, natural killer, dendritic, and stromal cells. Furthermore, mobilization of huge numbers of non-professional antigen presenting cells (CD34+ stem cells) amplifies the tolerizing potential of PB stem cell grafts. Thus, G-CSF mobilization provides PB transplants with different immunobiologic properties in comparison to standard bone marrow grafts. Whether these immunobiologic differences will lead to better transplant outcomes remains to be shown through much awaited results of large randomized clinical trials.

Th1 and Th2 mediate acute graft-versus-host disease, each with distinct end-organ targets

STAT4 and STAT6 are transcription factors that play crucial roles in responding to IL-12 and IL-4, respectively. STAT4 gene knockout (STAT4(-/-)) mice have markedly reduced Th1 responses and enhanced Th2 responses. STAT6(-/-) mice show the inverse phenotype. We compared the ability of bone marrow transplantation (BMT) with the inclusion of spleen cells from STAT6(-/-), STAT4(-/-), and wild-type (WT) mice to produce graft-versus-host disease (GVHD) in lethally irradiated MHC-mismatched recipients. Acute GVHD mortality was more rapid when induced by cells from STAT6(-/-) mice than when induced by STAT4(-/-) cells. However, cells from STAT4(-/-) and STAT6(-/-) donors both induced delayed GVHD mortality compared with WT controls, or compared with combined STAT4(-/-) and STAT6(-/-) cells, indicating a contribution of both Th1 cells and Th2 cells to acute GVHD. Recipients of STAT6(-/-) BMT showed evidence of acute GVHD with severe diarrhea and marked weight loss. Recipients of STAT4(-/-) BMT showed signs of GVHD with only initial transient weight loss and later development of severe skin GVHD. Histopathology showed that Th2 responses were required for the induction of both hepatic and severe skin GVHD. In contrast, both Th1 cells and Th2 cells were capable of causing intestinal pathology of GVHD. Our studies demonstrate an additive role for Th1 and Th2 cells in producing acute GVHD, and suggest a cytokine-directed approach to treating end-organ manifestations of GVHD.

Tolerance and hematopoietic stem cell transplantation 50 years after Burnet's theory

OBJECTIVE

In 1949, the original formulation of Burnet's theory on the mechanisms responsible for the capacity of the immune system to discriminate between foreign antigens (i.e., the "non-self") and the cells of its own body (i.e., the "self") was published. Since then, further refinements and reconsiderations of the basic concepts underlying the achievement of a state of tolerance toward a certain antigen have been reported. Here, we attempt to analyze critically new clinical and experimental strategies aimed at inducing alloantigen-specific unresponsiveness.

DATA SOURCES

The data discussed in this review are drawn from articles and abstracts published in journals covered by the Science Citation Index and Medline.

STATE OF THE ART

Induction of tolerance toward alloantigens still remains one of the most elusive goals of clinical immunology. Until now, nonspecific immunosuppressive drugs have been used to successfully perform both solid organ and hematopoietic stem cell transplantation. However, using this approach, patients given an allograft are exposed to the threat of infections, tumors, and other side effects. Moreover, in solid organ transplant recipients, permanent tolerance toward the graft's alloantigens is never achieved. Recently, considerable progress has been made in expanding our knowledge of transplant tolerance. The traditional model of central tolerance, derived from Burnet's concept, has been complemented by knowledge of mechanisms of peripheral tolerance.

CONCLUSIONS

New experimental and therapeutic trials based on the blockade of costimulatory molecules, as well as on generation and infusion of either regulatory or nonimmunogenic cells, have been recently proposed for inducing alloantigen-specific tolerance.The achievements obtained in understanding the mechanisms of unresponsiveness toward non-self antigens are fundamental prerequisites for successful allogeneic transplants, and they could open a new exciting era of specific, immunosuppressive therapies.

Granulocyte-colony stimulating factor mobilizes T helper 2-inducing dendritic cells

Peripheral blood stem cells (PBSC) obtained from granulocyte-colony stimulating factor (G-CSF)-mobilized donors are increasingly used for allogeneic transplantation. Despite a 10-fold higher dose of transplanted T cells, acute graft-versus-host disease (GVHD) does not develop in higher proportion in recipients of PBSC than in recipients of marrow. T cells from G-CSF-treated experimental animals preferentially produce IL-4 and IL-10, cytokines characteristic of Th2 responses, which are associated with diminished GVHD-inducing ability. We hypothesized that G-CSF-mobilized PBSC contain antigen-presenting cells, which prime T-lymphocytes to produce Th2 cytokines. Two distinct lineages of dendritic cells (DC) have been described in humans, DC1 and DC2, according to their ability to induce naive T-cell differentiation to Th1 and Th2 effector cells, respectively. We have used multicolor microfluorometry to enumerate DC1 and DC2 in the peripheral blood of normal donors. G-CSF treatment with 10 to 16 μg/kg per day for 5 days increased peripheral blood DC2 counts from a median of 4.9 × 106/L to 24.8 × 106/L (P = .0009), whereas DC1 counts did not change. Purified DC1, from either untreated or G-CSF treated donors, induced the proliferation of allogeneic naive T cells, but fresh DC2 were poor stimulators. Tumor necrosis factor- (TNF-)-activated DC1 induced allogeneic naive T cells to produce IFN-γ, which is typical of Th1 responses, whereas TNF--activated DC2 induced allogeneic naive T cells to produce IL-4 and IL-10, which are typical of Th2 responses. PBSC transplants contained higher doses of DC2 than marrow transplants (median, 2.4 × 106/kg versus 0.5 × 106/kg) (P = .006), whereas the dose of DC1 was comparable. Thus, it is conceivable that transplantation of G-CSF-stimulated PBSC does not result in overwhelming acute GVHD because the graft contains predominantly Th2-inducing DC. Adoptive transfer of purified DC2 may be exploited to induce immune deviation after transplantation of hematopoietic stem cells or organ allografts.

Granulocyte-colony stimulating factor mobilizes T helper 2-inducing dendritic cells

Peripheral blood stem cells (PBSC) obtained from granulocyte-colony stimulating factor (G-CSF)-mobilized donors are increasingly used for allogeneic transplantation. Despite a 10-fold higher dose of transplanted T cells, acute graft-versus-host disease (GVHD) does not develop in higher proportion in recipients of PBSC than in recipients of marrow. T cells from G-CSF-treated experimental animals preferentially produce IL-4 and IL-10, cytokines characteristic of Th2 responses, which are associated with diminished GVHD-inducing ability. We hypothesized that G-CSF-mobilized PBSC contain antigen-presenting cells, which prime T-lymphocytes to produce Th2 cytokines. Two distinct lineages of dendritic cells (DC) have been described in humans, DC1 and DC2, according to their ability to induce naive T-cell differentiation to Th1 and Th2 effector cells, respectively. We have used multicolor microfluorometry to enumerate DC1 and DC2 in the peripheral blood of normal donors. G-CSF treatment with 10 to 16 μg/kg per day for 5 days increased peripheral blood DC2 counts from a median of 4.9 × 106/L to 24.8 × 106/L (P = .0009), whereas DC1 counts did not change. Purified DC1, from either untreated or G-CSF treated donors, induced the proliferation of allogeneic naive T cells, but fresh DC2 were poor stimulators. Tumor necrosis factor- (TNF-)-activated DC1 induced allogeneic naive T cells to produce IFN-γ, which is typical of Th1 responses, whereas TNF--activated DC2 induced allogeneic naive T cells to produce IL-4 and IL-10, which are typical of Th2 responses. PBSC transplants contained higher doses of DC2 than marrow transplants (median, 2.4 × 106/kg versus 0.5 × 106/kg) (P = .006), whereas the dose of DC1 was comparable. Thus, it is conceivable that transplantation of G-CSF-stimulated PBSC does not result in overwhelming acute GVHD because the graft contains predominantly Th2-inducing DC. Adoptive transfer of purified DC2 may be exploited to induce immune deviation after transplantation of hematopoietic stem cells or organ allografts.

Tumor necrosis factor- alpha production to lipopolysaccharide stimulation by donor cells predicts the severity of experimental acute graft-versus-host disease

Donor T cell responses to host alloantigen are known predictors for graft-versus-host disease (GVHD); however, the effect of donor responsiveness to an inflammatory stimulus such as lipopolysaccharide (LPS) on GVHD severity has not been investigated. To examine this, we used mouse strains that differ in their sensitivity to LPS as donors in an experimental bone marrow transplant (BMT) system. Lethally irradiated (C3FeB6)F1 hosts received BMT from either LPS-sensitive (LPS-s) C3Heb/Fej, or LPS-resistant (LPS-r) C3H/ Hej donors. Mice receiving LPS-r BMT developed significantly less GVHD as measured by mortality and clinical score compared with recipients of LPS-s BMT, a finding that was associated with significant decreases in intestinal histopathology and serum LPS and TNF-alpha levels. When donor T cell responses to host antigens were measured, no differences in proliferation, serum IFN-gamma levels, splenic T cell expansion, or CTL activity were observed after LPS-r or LPS-s BMT. Systemic neutralization of TNF-alpha from day -2 to +6 resulted in decreased intestinal pathology, and serum LPS levels and increased survival after BMT compared with control mice receiving Ig. We conclude that donor resistance to endotoxin reduces the development of acute GVHD by attenuating early intestinal damage mediated by TNFalpha. These data suggest that the responsiveness of donor accessory cells to LPS may be an important risk factor for acute GVHD severity independent of T cell responses to host antigens.

Differential effects of the absence of interferon-gamma and IL-4 in acute graft-versus-host disease after allogeneic bone marrow transplantation in mice

Graft-versus-host disease (GVHD), in which immunocompetent donor cells attack the host, remains a major cause of morbidity after allogeneic bone marrow transplantation (BMT). To understand the role of cytokines in the pathobiology of GVHD, we used cytokine knockout (KO) mice as a source of donor T cells. Two different MHC-disparate strain combinations were examined: BALB/c (H2(d)) donors into lethally irradiated C57BL/6 (H2(b)) recipients or C57BL/6 (H2(b)) donors into B10.BR (H2(k)) recipients. Donor cells were from mice in which either the interferon-gamma (IFN-gamma) or the IL-4 gene was selectively disrupted to understand the role of these cytokines in acute GVHD. In both strain combinations the same pattern was noted with regard to GVHD onset and morbidity. All mice exhibited the classic signs of acute GVHD: weight loss with skin, gut, and liver pathology resulting in morbidity and mortality. Surprisingly, donor cells obtained from mice lacking IFN-gamma gave rise to accelerated morbidity from GVHD when compared with cells from wild-type control donors. Similar results were obtained using normal donors when neutralizing antibodies to IFN-gamma were administered immediately after the BMT. These results suggest that IFN-gamma plays a role in protection from acute GVHD. In marked contrast, cells obtained from IL-4 KO mice resulted in protection from GVHD compared with control donors. Splenocytes from IFN KO mice stimulated with a mitogen proliferated to a significantly greater extent and produced more IL-2 compared with splenocytes obtained from IL-4 KO or control mice. Additionally, there was increased IL-2 production in the spleens of mice undergoing GVHD using IFN-gamma KO donors. These results therefore indicate, with regard to the TH1/ TH2 cytokine paradigm, the absence of a TH1-type cytokine can be deleterious in acute GVHD, whereas absence of a TH2 cytokine can be protective.

Host Reactive Donor T Cells Are Associated With Lung Injury After Experimental Allogeneic Bone Marrow Transplantation

Noninfectious lung injury is common after allogeneic bone marrow transplantation (BMT), but its association with acute graft-versus-host disease (GVHD) is unclear. Using a murine BMT system where donor and host differ by multiple minor histocompatibility (H) antigens, we investigated the nature of lung injury and its relationship both to systemic GVHD and host-reactive donor T cells. Lethally irradiated CBA hosts received syngeneic BMT or allogeneic (B10.BR) T-cell–depleted (TCD) bone marrow (BM) with and without the addition of T cells. Six weeks after BMT, significant pulmonary histopathology was observed in animals receiving allogeneic BMT compared with syngeneic controls. Lung damage was greater in mice that received allogeneic T cells and developed GVHD, but it was also detectable after TCD BMT when signs of clinical and histologic acute GVHD were absent. In each setting, lung injury was associated with significant alterations in pulmonary function. Mature, donor (Vβ6+and Vβ3+) T cells were significantly increased in the broncho-alveolar lavage (BAL) fluid of all allogeneic BMT recipients compared with syngeneic controls, and these cells proliferated and produced interferon-γ (IFN-γ) to host antigens in vitro. These in vitro responses correlated with increased IFN-γ and tumor necrosis factor-α (TNF-α) in the BAL fluid. We conclude that alloreactive donor lymphocytes are associated with lung injury in this allogeneic BMT model. The expansion of these cells in the BAL fluid and their ability to respond to host antigens even when systemic tolerance has been established (ie, the absence of clinical GVHD) suggest that the lung may serve as a sanctuary site for these host reactive donor T cells. These findings may have important implications with regard to the evaluation and treatment of pulmonary dysfunction after allogeneic BMT even when clinical GVHD is absent.

Host Reactive Donor T Cells Are Associated With Lung Injury After Experimental Allogeneic Bone Marrow Transplantation

Noninfectious lung injury is common after allogeneic bone marrow transplantation (BMT), but its association with acute graft-versus-host disease (GVHD) is unclear. Using a murine BMT system where donor and host differ by multiple minor histocompatibility (H) antigens, we investigated the nature of lung injury and its relationship both to systemic GVHD and host-reactive donor T cells. Lethally irradiated CBA hosts received syngeneic BMT or allogeneic (B10.BR) T-cell–depleted (TCD) bone marrow (BM) with and without the addition of T cells. Six weeks after BMT, significant pulmonary histopathology was observed in animals receiving allogeneic BMT compared with syngeneic controls. Lung damage was greater in mice that received allogeneic T cells and developed GVHD, but it was also detectable after TCD BMT when signs of clinical and histologic acute GVHD were absent. In each setting, lung injury was associated with significant alterations in pulmonary function. Mature, donor (Vβ6+and Vβ3+) T cells were significantly increased in the broncho-alveolar lavage (BAL) fluid of all allogeneic BMT recipients compared with syngeneic controls, and these cells proliferated and produced interferon-γ (IFN-γ) to host antigens in vitro. These in vitro responses correlated with increased IFN-γ and tumor necrosis factor-α (TNF-α) in the BAL fluid. We conclude that alloreactive donor lymphocytes are associated with lung injury in this allogeneic BMT model. The expansion of these cells in the BAL fluid and their ability to respond to host antigens even when systemic tolerance has been established (ie, the absence of clinical GVHD) suggest that the lung may serve as a sanctuary site for these host reactive donor T cells. These findings may have important implications with regard to the evaluation and treatment of pulmonary dysfunction after allogeneic BMT even when clinical GVHD is absent.

CD8+ T cells of Tc2 phenotype mediate a GVL effect and prevent marrow rejection

Donor T cells mediate both beneficial and detrimental immune reactions in the setting of allogeneic BMT (alloBMT). T cells mediate the GVL effect and prevent marrow rejection, but also induce GVHD. In an attempt to favorably influence the balance of these allogeneic responses, we have evaluated the effect of donor CD4+, Th1/Th2 and CD8+, Tc1/Tc2 functional T cell subsets in murine marrow transplantation models. Our studies have identified the CD8+ Tc2 population (which is a cytolytic effector secreting the type II cytokines IL-4, IL-5, and IL-10) as a subset capable of mediating the GVL effect and preventing marrow rejection with reduced GVHD. We have also shown that the Tc2 subset can be generated in humans. These studies indicate that administration of donor CD8+ T cells of Tc2 phenotype represents a strategy for improving allo BMT outcome.