Tolerance in intestinal transplantation.

Intestinal transplantation (ITx) is highly immunogenic, resulting in the need for high levels of immunosuppression, with frequent complications along with high rejection rates. Tolerance induction would provide a solution to these limitations. Detailed studies of alloreactive T cell clones as well as multiparameter flow cytometry in the graft and peripheral tissues have provided evidence for several tolerance mechanisms that occur spontaneously following ITx, which might provide targets for further interventions. These include the frequent occurrence of macrochimerism and engraftment in the recipient bone marrow of donor hematopoietic stem and progenitor cells carried in the allograft. These phenomena are seen most frequently in recipients of multivisceral transplants and are associated with reduced rejection rates. They reflect powerful graft-vs-host responses that enter the peripheral lymphoid system and bone marrow after expanding within and emigrating from the allograft. Several mechanisms of tolerance that may result from this lymphohematopoietic graft-vs-host response are discussed. Transcriptional profiling in quiescent allografts reveals tolerization of pre-existing host-vs-graft-reactive T cells that enter the allograft mucosa and become tissue-resident memory cells. Dissection of the pathways driving and maintaining this tolerant tissue-resident state among donor-reactive T cells will allow controlled tolerance induction through specific therapeutic approaches.

The Impact of Inflammation on the Immune Responses to Transplantation: Tolerance or Rejection?

Transplantation (Tx) remains the optimal therapy for end-stage disease (ESD) of various solid organs. Although alloimmune events remain the leading cause of long-term allograft loss, many patients develop innate and adaptive immune responses leading to graft tolerance. The focus of this review is to provide an overview of selected aspects of the effects of inflammation on this delicate balance following solid organ transplantation. Initially, we discuss the inflammatory mediators detectable in an ESD patient. Then, the specific inflammatory mediators found post-Tx are elucidated. We examine the reciprocal relationship between donor-derived passenger leukocytes (PLs) and those of the recipient, with additional emphasis on extracellular vesicles, specifically exosomes, and we examine their role in determining the balance between tolerance and rejection. The concept of recipient antigen-presenting cell "cross-dressing" by donor exosomes is detailed. Immunological consequences of the changes undergone by cell surface antigens, including HLA molecules in donor and host immune cells activated by proinflammatory cytokines, are examined. Inflammation-mediated donor endothelial cell (EC) activation is discussed along with the effect of donor-recipient EC chimerism. Finally, as an example of a specific inflammatory mediator, a detailed analysis is provided on the dynamic role of Interleukin-6 (IL-6) and its receptor post-Tx, especially given the potential for therapeutic interdiction of this axis with monoclonal antibodies. We aim to provide a holistic as well as a reductionist perspective of the inflammation-impacted immune events that precede and follow Tx. The objective is to differentiate tolerogenic inflammation from that enhancing rejection, for potential therapeutic modifications. (Words 247).

Chimerism, the Microenvironment and Control of Leukemia.

Transplantation of allogeneic hematopoietic cells faces two barriers: failure of engraftment due to a host versus graft reaction, and the attack of donor cells against the patient, the graft versus host (GVH) reaction. This reaction may lead to GVH disease (GVHD), but in patients transplanted due to leukemia or other malignant disorders, this may also convey the benefit of a graft versus leukemia (GVL) effect. The interplay of transplant conditioning with donor and host cells and the environment in the patient is complex. The microbiome, particularly in the intestinal tract, profoundly affects these interactions, directly and via soluble mediators, which also reach other host organs. The microenvironment is further altered by the modifying effect of malignant cells on marrow niches, favoring the propagation of the malignant cells. The development of stable mixed donor/host chimerism has the potential of GVHD prevention without necessarily increasing the risk of relapse. There has been remarkable progress with novel conditioning regimens and selective T-cell manipulation aimed at securing engraftment while preventing GVHD without ablating the GVL effect. Interventions to alter the microenvironment and change the composition of the microbiome and its metabolic products may modify graft/host interactions, thereby further reducing GVHD, while enhancing the GVL effect. The result should be improved transplant outcome.

Allogeneic Hemopoietic Stem Cell Transplantation for Myelofibrosis: 2021.

The aim of this review is to update the current status of allogeneic hemopoietic stem cell transplants (HSCT) for patients with myelofibrosis (MF). We have first summarized the issue of an indication for allogeneic HSCT, discussing several prognostic scoring systems, developed to predict the outcome of MF, and therefore to identify patients who will benefit of an allogeneic HSCT. Patients with low risk MF are usually not selected for a transplant, whereas patients with intermediate or high risk MF are eligible. A separate issue, is how to predict the outcome of HSCT: we will outline a clinical molecular myelofibrosis transplant scoring system (MTSS), which predicts overall survival, ranging from 90% for low risk patients, to 20% for very high risk patients. We will also discuss transfusion burden and spleen size, as predictors of transplant outcome. The choice of a transplant platform including the conditioning regimen, the stem cell source and GvHD prophylaxis, are crucial for a successful program in MF, and will be outlined. Complications such as poor graft function, graft failure, GvHD and relapse of the disease, will also be reviewed. Finally we discuss monitoring the disease after HSCT with donor chimerism, driver mutations and hematologic data. We have made an effort to make this review as comprehensive and up to date as possible, and we hope it will provide some useful data for the clinicians.

Myeloid-Derived Suppressor Cells in Lung Transplantation.

Myeloid-derived suppressor cells (MDSC) are a heterogeneous group of immune cells from the myeloid lineage. MDSCs expand in pathological situations, such as chronic infection, cancer, autoimmunity, and allograft rejection. As chronic lung allograft dysfunction (CLAD) limits long-term survival after lung transplantation (LTx), MDSCs may play a role in its pathophysiology. We assessed phenotype and frequency of MDSCs in peripheral blood from lung transplant recipients and its relationship to post-transplant complications and immunosuppression. Granulocytic (G)-MDSC were identified and quantified by flow cytometry of blood from 4 control subjects and 20 lung transplant patients (stable n = 6, infection n = 5; CLAD n = 9). G-MDSC functionality was assessed in vitro by their capability to block CD4 and CD8 T cell proliferation. More G-MDSC could be assessed using EDTA tubes compared to heparin tubes (p = 0.004). G-MDSC were increased in stable lung transplant recipients vs. non-transplant controls (52.1% vs. 9.4%; p = 0.0095). The infection or CLAD groups had lower G-MDSCs vs. stable recipients (28.2%p = 0.041 and 33.0%; p = 0.088, respectively), but were not different among CLAD phenotypes. G-MDSC tended to correlate with cyclosporine A and tacrolimus levels (r2 = 0.18; r2 = 0.17). CD4 and CD8 cells proliferation decreased by 50 and 80% if co-cultured with MDSCs (1:6 and 1:2 MDSC:T-cell ratio, respectively). In conclusion, circulating MDSCs are measurable, functional and have a G-MDSC phenotype in lung transplant patients. Their frequency is increased in stable patients, decreased during post-transplant complications, and related to level of immunosuppression. This study may pave the way for further investigations of MDSC in the context of lung transplantation.

Mixed chimerism established by hematopoietic stem cell transplantation is maintained by host and donor T regulatory cells.

Transplantation is an effective treatment of many clinical disorders, but the mechanisms that regulate immunological tolerance are uncertain and remain central to improving patient outcome. Hemopoietic stem cell transplantation (SCT) often establishes "mixed chimerism" in which immune cells from both the donor and patient coexist in vivo in a setting of immunological tolerance. We studied immune function in 69 patients within 2 months following SCT; 37 were fully donor and 32 displayed mixed chimerism. The proportion of T regulatory (Treg) cells was increased during mixed chimerism and comprised equal numbers of donor and host-derived regulatory cells. This was associated with a tolerogenic PD-L1+ profile on dendritic cells. Importantly, effector T cells from patients with mixed chimerism exhibited reduced cytotoxicity against host target cells in vitro, but this was restored following depletion of CD4+ Treg cells. These data show that Treg cells play a major role in sustaining immunological tolerance during mixed chimerism. These insights should help to guide novel interventions to improve clinical transplantation.

Allogeneic Human Double Negative T Cells as a Novel Immunotherapy for Acute Myeloid Leukemia and Its Underlying Mechanisms.

Purpose: To explore the potential of ex vivo expanded healthy donor-derived allogeneic CD4 and CD8 double-negative cells (DNT) as a novel cellular immunotherapy for leukemia patients.Experimental Design: Clinical-grade DNTs from peripheral blood of healthy donors were expanded and their antileukemic activity and safety were examined using flow cytometry-based in vitro killing assays and xenograft models against AML patient blasts and healthy donor-derived hematopoietic cells. Mechanism of action was investigated using antibody-mediated blocking assays and recombinant protein treatment assays.Results: Expanded DNTs from healthy donors target a majority (36/46) of primary AML cells, including 9 chemotherapy-resistant patient samples in vitro, and significantly reduce the leukemia load in patient-derived xenograft models in a DNT donor-unrestricted manner. Importantly, allogeneic DNTs do not attack normal hematopoietic cells or affect hematopoietic stem/progenitor cell engraftment and differentiation, or cause xenogeneic GVHD in recipients. Mechanistically, DNTs express high levels of NKG2D and DNAM-1 that bind to cognate ligands preferentially expressed on AML cells. Upon recognition of AML cells, DNTs rapidly release IFNγ, which further increases NKG2D and DNAM-1 ligands' expression on AML cells. IFNγ pretreatment enhances the susceptibility of AML cells to DNT-mediated cytotoxicity, including primary AML samples that are otherwise resistant to DNTs, and the effect of IFNγ treatment is abrogated by NKG2D and DNAM-1-blocking antibodies.Conclusions: This study supports healthy donor-derived allogeneic DNTs as a therapy to treat patients with chemotherapy-resistant AML and also reveals interrelated roles of NKG2D, DNAM-1, and IFNγ in selective targeting of AML by DNTs. Clin Cancer Res; 24(2); 370-82. ©2017 AACR.

Donor CD19 CAR T cells exert potent graft-versus-lymphoma activity with diminished graft-versus-host activity.

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a potentially curative therapy for hematological malignancies. However, graft-versus-host disease (GVHD) and relapse after allo-HSCT remain major impediments to the success of allo-HSCT. Chimeric antigen receptors (CARs) direct tumor cell recognition of adoptively transferred T cells. CD19 is an attractive CAR target, which is expressed in most B cell malignancies, as well as in healthy B cells. Clinical trials using autologous CD19-targeted T cells have shown remarkable promise in various B cell malignancies. However, the use of allogeneic CAR T cells poses a concern in that it may increase risk of the occurrence of GVHD, although this has not been reported in selected patients infused with donor-derived CD19 CAR T cells after allo-HSCT. To understand the mechanism whereby allogeneic CD19 CAR T cells may mediate anti-lymphoma activity without causing a significant increase in the incidence of GVHD, we studied donor-derived CD19 CAR T cells in allo-HSCT and lymphoma models in mice. We demonstrate that alloreactive T cells expressing CD28-costimulated CD19 CARs experience enhanced stimulation, resulting in the progressive loss of both their effector function and proliferative potential, clonal deletion, and significantly decreased occurrence of GVHD. Concurrently, the other CAR T cells that were present in bulk donor T cell populations retained their anti-lymphoma activity in accordance with the requirement that both the T cell receptor (TCR) and CAR be engaged to accelerate T cell exhaustion. In contrast, first-generation and 4-1BB-costimulated CAR T cells increased the occurrence of GVHD. These findings could explain the reduced risk of GVHD occurring with cumulative TCR and CAR signaling.

Immunothérapie et greffe de cellules souches hématopoïétiques allogéniques.

IMMUNOTHERAPY AND ALLOGENEIC STEM CELLS TRANSPLANTATION: Allogeneic stem cell transplantations represent perfect example of immunotherapy. Its positive aspects are due to the graft versus tumor effect. Unfortunately, this therapeutic advantage is usually associated with graft versus host effects. While the mechanism of these two graft reactions remain unclear, this is possible to modulate these immunologic effects. The type of conditioning regimen, the source of donor and the use of donor cells after the transplantation may influence the toxicity and the tumor response, leading to a better optimization of the procedure. This paper is presenting all the parameters which may contribute to improve allogeneic stem cell transplantations.

Rapid Functional Decline of Activated and Memory Graft-versus-Host-Reactive T Cells Encountering Host Antigens in the Absence of Inflammation.

Inflammation in the priming host environment has critical effects on the graft-versus-host (GVH) responses mediated by naive donor T cells. However, it is unclear how a quiescent or inflammatory environment impacts the activity of GVH-reactive primed T and memory cells. We show in this article that GVH-reactive primed donor T cells generated in irradiated recipients had diminished ability compared with naive T cells to increase donor chimerism when transferred to quiescent mixed allogeneic chimeras. GVH-reactive primed T cells showed marked loss of cytotoxic function and activation, and delayed but not decreased proliferation or accumulation in lymphoid tissues when transferred to quiescent mixed chimeras compared with freshly irradiated secondary recipients. Primed CD4 and CD8 T cells provided mutual help to sustain these functions in both subsets. CD8 help for CD4 cells was largely IFN-γ dependent. TLR stimulation after transfer of GVH-reactive primed T cells to mixed chimeras restored their cytotoxic effector function and permitted the generation of more effective T cell memory in association with reduced PD-1 expression on CD4 memory cells. Our data indicate that an inflammatory host environment is required for the maintenance of GVH-reactive primed T cell functions and the generation of memory T cells that can rapidly acquire effector functions. These findings have important implications for graft-versus-host disease and T cell-mediated immunotherapies.

Δ9-Tetrahydrocannabinol attenuates allogeneic host-versus-graft response and delays skin graft rejection through activation of cannabinoid receptor 1 and induction of myeloid-derived suppressor cells.

Immune cells have been shown to express cannabinoid receptors and to produce endogenous ligands. Moreover, activation of cannabinoid receptors on immune cells has been shown to trigger potent immunosuppression. Despite such studies, the role of cannabinoids in transplantation, specifically to prevent allograft rejection, has not, to our knowledge, been investigated previously. In the current study, we tested the effect of THC on the suppression of HvGD as well as rejection of skin allografts. To this end, we studied HvGD by injecting H-2(k) splenocytes into H-2(b) mice and analyzing the immune response in the draining ingLNs. THC treatment significantly reduced T cell proliferation and activation in draining LNs of the recipient mice and decreased early stage rejection-indicator cytokines, including IL-2 and IFN-γ. THC treatment also increased the allogeneic skin graft survival. THC treatment in HvGD mice led to induction of MDSCs. Using MDSC depletion studies as well as adoptive transfer experiments, we found that THC-induced MDSCs were necessary for attenuation of HvGD. Additionally, using pharmacological inhibitors of CB1 and CB2 receptors and CB1 and CB2 knockout mice, we found that THC was working preferentially through CB1. Together, our research shows, for the first time to our knowledge, that targeting cannabinoid receptors may provide a novel treatment modality to attenuate HvGD and prevent allograft rejection.

Biology of the immunomodulatory molecule HLA-G in human liver diseases.

The non-classical human leukocyte antigen-G (HLA-G), plays an important role in inducing tolerance, through its immunosuppressive effects on all types of immune cells. Immune tolerance is a key issue in the liver, both in liver homeostasis and in the response to liver injury or cancer. It would therefore appear likely that HLA-G plays an important role in liver diseases. Indeed, this molecule was recently shown to be produced by mast cells in the livers of patients infected with hepatitis C virus (HCV). Furthermore, the number of HLA-G-positive mast cells was significantly associated with fibrosis progression. The generation of immune tolerance is a role common to both HLA-G, as a molecule, and the liver, as an organ. This review provides a summary of the evidence implicating HLA-G in liver diseases. In the normal liver, HLA-G transcripts can be detected, but there is no HLA-G protein. However, HLA-G protein is detectable in the liver tissues and/or plasma of patients suffering from hepatocellular carcinoma, hepatitis B or C, or visceral leishmaniasis and in liver transplant recipients. The cells responsible for producing HLA-G differ between diseases. HLA-G expression is probably induced by microenvironmental factors, such as cytokines. The expression of HLA-G receptors, such as ILT2, ILT4, and KIRD2L4, on liver cells has yet to be investigated, but these receptors have been detected on all types of immune cells, and such cells are present in liver. The tolerogenic properties of HLA-G explain its deleterious effects in cancers and its beneficial effects in transplantation. Given the key role of HLA-G in immune tolerance, new therapeutic agents targeting HLA-G could be tested for the treatment of these diseases in the future.

[Th1/2-balance shift during acute graft-versus-host reaction developing against the background of experimental hyperlipidemia].

AIM: Evaluate the effect of experimental hyperlipidemia on the intensity of development of acute graft-versus-host reaction (GVHR) in mice. MATERIALS AND METHODS: Half-allogenic system C57Bl/6 (C57Bl/6 x DBA/2)F1 was used as a laboratory model of acute GVHR. Experimental hyperlipidemia in mice-recipients was induced by repeated administration of poloxamer 407. RESULTS: Lethality in the group of mice with acute GVHR developing against the background of preceding hyperlipidemia was significantly higher (70% at day 50 of GVHR development) compared with control group with acute GVHR (50% lethality at day 50). Such effect on the degree of severity of acute GVHR induced under the conditions of hyperlipidemia is confirmed by a more pronounced destruction of thymus in mice of the group with previously induced hyperlipidemia. CONCLUSION: Preceding hyperlipidemia induced by administration of poloxamer 407 shifts Th1/2- balance in the development of acute GVHR towards Th1. Mechanisms of this effect and possible role of nuclear LXR receptors in regulation of immune reactions are discussed.

Rabbit anti-T-lymphocyte globulin (ATG) persists with differential reactivity in patients' sera after full hematopoetic regeneration from allogeneic stem cell transplantation.

BACKGROUND: Rabbit polyclonal anti-T-lymphocyte Globulin (ATG-F®, Fresenius) is widely used for GvHD prophylaxis in allogeneic stem cell transplantation (SCT). ATG has a wide epitope spectrum and has been shown to react with all compartments of peripheral blood mononuclear cells (PBMNCs). ATG induces apoptosis in all cellular compartments. In this study we investigated the binding of ATG in sera from ten patients treated with rabbit ATG to PBMNCs and subcellular compartments after full hematopoetic regeneration on day 21 post SCT. METHODS: Sera from ten patients treated with unrelated donor allogeneic SCT for hematologic malignancy were collected after full hematopoetic regeneration on day 21 post SCT and incubated with healthy donor PBMNCs. Rabbit ATG on PBMNCs was detected by staining with fluorochrome labeled anti-rabbit IgG antibody. PBMNC compartments were investigated by counterstaining with lineage markers CD4, CD8, CD14 CD20 and CD56. Positive control was the fresh ATG preparation. RESULTS: We found that patient's' sera retained activity towards PBMNCs in all patients, yet at reduced intensity. When cell compartments were analyzed we found a differential pattern of ATG reactivity within sera. The mean percentage of total cells reacting with serum ATG from ten patients compared to fresh ATG (100%) was 44% of CD4 positive and 58% of CD8 positive T-lymphocytes, 41% of CD56 positive NK-cells, 83% of CD20 positive B-lymphocytes and 98% of CD14 positive monocytes. However, inter-individual variations were high with a wide spread around the mean especially for T-lymphocytes. CONCLUSION: We conclude that upon PBMNC regeneration following SCT and immunosuppressive treatment with ATG subpopulations of T-lymphocytes (CD4, CD8) and NK cells (CD56) are selected that lose epitopes recognized by ATG while B-lymphocytes (CD20) and monocytes (CD14) maintain a homogeneity with respect to epitopes recognized by ATG. This may be due to loss of idiotypes reacting with subpopulations of high frequency and turnover. Further studies should investigate the subphenotype of these populations and functional effects of extremely high or low reactivity with one or more compartments in some patients on GvHD and disease outcome.

Biomarker evaluation of face transplant rejection: association of donor T cells with target cell injury.

This series of 113 sequential biopsies of full facial transplants provides findings of potential translational significance as well as biological insights that could prompt reexamination of conventional paradigms of effector pathways in skin allograft rejection. Serial biopsies before, during, and after rejection episodes were evaluated for clinicopathological assessment that in selected cases included specific biomarkers for donor-versus-recipient T cells. Histologic evidence of rejection included lymphocyte-associated injury to epidermal rete ridges, follicular infundibula, and dermal microvessels. Surprisingly, during active rejection, immune cells spatially associated with target cell injury consisted abundantly or predominantly of lymphocytes of donor origin with an immunophenotype typical of the resident memory T-cell subset. Current dogma assumes that skin allograft rejection is mediated by recipient T cells that attack epidermal targets, and the association of donor T cells with sites of target cell injury raises questions regarding the potential complexity of immune cell interactions in the rejection process. A more histopathologically refined and immune-based biomarker approach to assessment of rejection of facial transplants is now indicated.

Trogocytosis as a mechanistic link between chimerism and prenatal tolerance.

In utero hematopoietic cellular transplantation (IUHCT) holds great promise for the treatment of congenital diseases of cellular dysfunction such as sickle cell disease, immunodeficiency disorders and inherited metabolic disorders. However, repeated failures in clinical cases of IUHCT that do not involve an immunodeficiency disease force a closer examination of the fetal immune system. While the mechanisms regulating T cell tolerance have been previously studied, the educational mechanisms leading to NK cell tolerance in prenatal chimeras remain unknown. As a low level of donor cells (1.8%) is required to induce and maintain this tolerance, it is likely that these mechanisms employ indirect host-donor interaction. This report examines donor-to-host MHC transfer (trogocytosis) as an intrinsic mechanism regulating the development and maintenance of NK cell tolerance in prenatal chimeras. The findings demonstrate that phenotypically tolerant host NK cells express low levels of transferred donor MHC antigens during development and later as mature cytotoxic lymphocytes. Further study is needed to understand how the cis-recognition of transferred donor MHC ligand influences the selection and maintenance of tolerant NK cells in prenatal chimeras.

Ex vivo activation of CD56(+) immune cells that eradicate neuroblastoma.

Despite the use of intensive contemporary multimodal therapy, the overall survival of patients with high-risk neuroblastoma is still less than 50%. Therefore, immunotherapy without cross-resistance and overlapping toxicity has been proposed. In this study, we report the development of a novel strategy to specifically activate and expand human CD56(+) (NCAM1) natural killer (NK) immune cells from normal donors and patients with neuroblastoma. Enriched CD56(+) cells from peripheral blood were mixed with CD56(-) fraction at 1:1 ratio and cultured in the presence of OKT3, interleukin (IL)-2, and -15 for five days and then without OKT3 for 16 more days. The final products contained more than 90% CD56(+) cells and could kill neuroblastoma cells effectively that were originally highly resistant to nonprocessed NK cells. Mechanistically, cytolysis of neuroblastoma was mediated through natural cytotoxicity receptor (NCR), DNAX accessory molecule-1 (DNAM-1; CD226), perforin, and granzyme B. Successful clinical scale-up in a good manufacturing practices (GMP)-compliant bioreactor yielded effector cells that in a neuroblastoma xenograft model slowed tumor growth and extended survival without GVHD. Investigation of CD56(+) cells from patients with neuroblastoma revealed a similar postactivation phenotype and lytic activity. Our findings establish a novel and clinically expedient strategy to generate allogeneic or autologous CD56(+) cells that are highly cytotoxic against neuroblastoma with minimal risk of GVHD.

Regulatory T cells inhibit CD8(+) T-cell tissue invasion in human skin graft-versus-host reactions.

BACKGROUND: Regulatory T cells (Tregs) effectively ameliorate graft-versus-host disease (GVHD). The mechanisms underlying Treg therapeutic effect on GVHD are not fully elucidated. This study investigates whether Treg prevention of GVH tissue damage is associated with blocking CD8 effector T-cell tissue invasion, a question not yet addressed in humans. METHOD: Tissue-infiltrating T cells and histopathology scores were detected using an in vitro human GVHD skin explant model, together with immunohistochemistry, cytometric bead array, functional adhesion and migration assays, flow cytometry, and quantitative real-time polymerase chain reaction. RESULTS: Treg intervention during priming significantly decreased effector T-cell infiltration into target tissue (P<0.01) resulting in a striking reduction in the histopathology score of tissue injury (P<0.0001). These results were coupled with reduced CXCR3 and cutaneous lymphocyte antigen expression by effector T cells, together with decreased CXCL10 and CXCL11 expression in target tissue. Treg intervention also impaired the functional interaction of CXCR3 and cutaneous lymphocyte antigen with their specific ligands (P<0.01) and suppressed the secretion of CXCL9, CXCL10, and interferon-γ (P<0.01, P<0.05, and P<0.001, respectively). Late addition of Tregs into the effector phase abolished their ability to suppress effector T-cell tissue invasion, resulting in a total loss of their ability to ameliorate GVH tissue damage. CONCLUSION: Preventing effector T-cell tissue invasion is a critical mechanistic event leading to Treg attenuation of GVH tissue damage. This therapeutic effect is associated with a failure of CD8 T cells to increase tissue homing receptors after allo-stimulation, together with a breakdown of interferon-γ-induced chemoattractant expression in the target tissue.