CD8+ T cells specific for an immunodominant SARS-CoV-2 nucleocapsid epitope cross-react with selective seasonal coronaviruses.

Efforts are being made worldwide to understand the immune response to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus responsible for the coronavirus disease 2019 (COVID-19) pandemic, including the impact of T cell immunity and cross-recognition with seasonal coronaviruses. Screening of SARS-CoV-2 peptide pools revealed that the nucleocapsid (N) protein induced an immunodominant response in HLA-B7+ COVID-19-recovered individuals that was also detectable in unexposed donors. A single N-encoded epitope that was highly conserved across circulating coronaviruses drove this immunodominant response. In vitro peptide stimulation and crystal structure analyses revealed T cell-mediated cross-reactivity toward circulating OC43 and HKU-1 betacoronaviruses but not 229E or NL63 alphacoronaviruses because of different peptide conformations. T cell receptor (TCR) sequencing indicated that cross-reactivity was driven by private TCR repertoires with a bias for TRBV27 and a long CDR3ß loop. Our findings demonstrate the basis of selective T cell cross-reactivity for an immunodominant SARS-CoV-2 epitope and its homologs from seasonal coronaviruses, suggesting long-lasting protective immunity.

SARS-CoV-2-specific T cells generated for adoptive immunotherapy are capable of recognizing multiple SARS-CoV-2 variants.

Adoptive T-cell immunotherapy has provided promising results in the treatment of viral complications in humans, particularly in the context of immunocompromised patients who have exhausted all other clinical options. The capacity to expand T cells from healthy immune individuals is providing a new approach to anti-viral immunotherapy, offering rapid off-the-shelf treatment with tailor-made human leukocyte antigen (HLA)-matched T cells. While most of this research has focused on the treatment of latent viral infections, emerging evidence that SARS-CoV-2-specific T cells play an important role in protection against COVID-19 suggests that the transfer of HLA-matched allogeneic off-the-shelf virus-specific T cells could provide a treatment option for patients with active COVID-19 or at risk of developing COVID-19. We initially screened 60 convalescent individuals and based on HLA typing and T-cell response profile, 12 individuals were selected for the development of a SARS-CoV-2-specific T-cell bank. We demonstrate that these T cells are specific for up to four SARS-CoV-2 antigens presented by a broad range of both HLA class I and class II alleles. These T cells show consistent functional and phenotypic properties, display cytotoxic potential against HLA-matched targets and can recognize HLA-matched cells infected with different SARS-CoV-2 variants. These observations demonstrate a robust approach for the production of SARS-CoV-2-specific T cells and provide the impetus for the development of a T-cell repository for clinical assessment.

Pretransplant Cytomegalovirus-Specific Cellular Immunity and Risk of Viral Reactivation Following Lung Transplantation: A Prospective Cohort Study.

Cytomegalovirus (CMV) remains a significant burden in lung transplant recipients. Deficiencies in T-cell immunity posttransplant increase the risk of CMV-associated complications. However, it is not clear if underlying poor pretransplant immunity increases risk. To assess this, we recruited 39 prospective lung transplant patients and performed QuantiFERON-CMV on their peripheral blood. More than a third of prospective CMV-seropositive transplant recipients were CMV non-immune reactive (CMV-NIR) pretransplant. CMV-NIR status was associated with a significantly higher incidence of CMV reactivation posttransplant, demonstrating that dysfunctional CMV immunity in prospective lung transplant recipients is associated with an increased risk of viral reactivation posttransplant.

GVHD prevents NK-cell-dependent leukemia and virus-specific innate immunity.

Allogeneic bone marrow transplantation (allo-BMT) is a curative therapy for hematological malignancies, but is associated with significant complications, principally graft-versus-host disease (GVHD) and opportunistic infections. Natural killer (NK) cells mediate important innate immunity that provides a temporal bridge until the reconstruction of adaptive immunity. Here, we show that the development of GVHD after allo-BMT prevented NK-cell reconstitution, particularly within the maturing M1 and M2 NK-cell subsets in association with exaggerated activation, apoptosis, and autophagy. Donor T cells were critical in this process by limiting the availability of interleukin 15 (IL-15), and administration of IL-15/IL-15Rα or immune suppression with rapamycin could restore NK-cell reconstitution. Importantly, the NK-cell defect induced by GVHD resulted in the failure of NK-cell-dependent in vivo cytotoxicity and graft-versus-leukemia effects. Control of cytomegalovirus infection after allo-BMT was also impaired during GVHD. Thus, during GVHD, donor T cells compete with NK cells for IL-15 thereby inducing profound defects in NK-cell reconstitution that compromise both leukemia and pathogen-specific immunity.

Pirfenidone ameliorates murine chronic GVHD through inhibition of macrophage infiltration and TGF-ß production.

Allogeneic hematopoietic stem cell transplantation is hampered by chronic graft-versus-host disease (cGVHD), resulting in multiorgan fibrosis and diminished function. Fibrosis in lung and skin leads to progressive bronchiolitis obliterans (BO) and scleroderma, respectively, for which new treatments are needed. We evaluated pirfenidone, a Food and Drug Administration (FDA)-approved drug for idiopathic pulmonary fibrosis, for its therapeutic effect in cGVHD mouse models with distinct pathophysiology. In a full major histocompatibility complex (MHC)-mismatched, multiorgan system model with BO, donor T-cell responses that support pathogenic antibody production are required for cGVHD development. Pirfenidone treatment beginning one month post-transplant restored pulmonary function and reversed lung fibrosis, which was associated with reduced macrophage infiltration and transforming growth factor-ß production. Pirfenidone dampened splenic germinal center B-cell and T-follicular helper cell frequencies that collaborate to produce antibody. In both a minor histocompatibility antigen-mismatched as well as a MHC-haploidentical model of sclerodermatous cGVHD, pirfenidone significantly reduced macrophages in the skin, although clinical improvement of scleroderma was only seen in one model. In vitro chemotaxis assays demonstrated that pirfenidone impaired macrophage migration to monocyte chemoattractant protein-1 (MCP-1) as well as IL-17A, which has been linked to cGVHD generation. Taken together, our data suggest that pirfenidone is a potential therapeutic agent to ameliorate fibrosis in cGVHD.

Corruption of dendritic cell antigen presentation during acute GVHD leads to regulatory T-cell failure and chronic GVHD.

Chronic graft-versus-host disease (cGVHD) is a major cause of late mortality following allogeneic bone marrow transplantation (BMT) and is characterized by tissue fibrosis manifesting as scleroderma and bronchiolitis obliterans. The development of acute GVHD (aGVHD) is a powerful clinical predictor of subsequent cGVHD, suggesting that aGVHD may invoke the immunologic pathways responsible for cGVHD. In preclinical models in which sclerodermatous cGVHD develops after a preceding period of mild aGVHD, we show that antigen presentation within major histocompatibility complex (MHC) class II of donor dendritic cells (DCs) is markedly impaired early after BMT. This is associated with a failure of regulatory T-cell (Treg) homeostasis and cGVHD. Donor DC-restricted deletion of MHC class II phenocopied this Treg deficiency and cGVHD. Moreover, specific depletion of donor Tregs after BMT also induced cGVHD, whereas adoptive transfer of Tregs ameliorated it. These data demonstrate that the defect in Treg homeostasis seen in cGVHD is a causative lesion and is downstream of defective antigen presentation within MHC class II that is induced by aGVHD.

Spatiotemporal Characterization of the Cellular and Molecular Contributors to Liver Fibrosis in a Murine Hepatotoxic-Injury Model.

The interplay between the inflammatory infiltrate and tissue resident cell populations invokes fibrogenesis. However, the temporal and mechanistic contributions of these cells to fibrosis are obscure. To address this issue, liver inflammation, ductular reaction (DR), and fibrosis were induced in C57BL/6 mice by thioacetamide administration for up to 12 weeks. Thioacetamide treatment induced two phases of liver fibrosis. A rapid pericentral inflammatory infiltrate enriched in F4/80(+) monocytes co-localized with SMA(+) myofibroblasts resulted in early collagen deposition, marking the start of an initial fibrotic phase (1 to 6 weeks). An expansion of bone marrow-derived macrophages preceded a second phase, characterized by accelerated progression of fibrosis (>6 weeks) after DR migration from the portal tracts to the centrilobular site of injury, in association with an increase in DR/macrophage interactions. Although chemokine (C-C motif) ligand 2 (CCL2) mRNA was induced rapidly in response to thioacetamide, CCL2 deficiency only partially abrogated fibrosis. In contrast, colony-stimulating factor 1 receptor blockade diminished C-C chemokine receptor type 2 [CCR2(neg) (Ly6C(lo))] monocytes, attenuated the DR, and significantly reduced fibrosis, illustrating the critical role of colony-stimulating factor 1-dependent monocyte/macrophage differentiation and linking the two phases of injury. In response to liver injury, colony-stimulating factor 1 drives early monocyte-mediated myofibroblast activation and collagen deposition, subsequent macrophage differentiation, and their association with the advancing DR, the formation of fibrotic septa, and the progression of liver fibrosis to cirrhosis.

Autophagy is required for stem cell mobilization by G-CSF.

Granulocyte colony-stimulating factor (G-CSF) is widely used clinically to prevent neutropenia after cytotoxic chemotherapy and to mobilize hematopoietic stem cells (HSCs) for transplantation. Autophagy, a process of cytoplasmic component recycling, maintains cellular homeostasis and protects the cell during periods of metabolic stress or nutrient deprivation. We have observed that G-CSF activates autophagy in neutrophils and HSCs from both mouse and human donors. Furthermore, G-CSF-induced neutrophil and HSC mobilization is impaired in the absence of autophagy. In contrast, autophagy is dispensable for direct HSC mobilization in response to the CXCR4 antagonist AMD3100. Altogether, these data demonstrate an important role for G-CSF in invoking autophagy within hematopoietic and myeloid cells and suggest that this pathway is critical for ensuring cell survival in response to clinically relevant cytokine-induced stress. These findings have direct relevance to HSC transplantation and the increasing clinical use of agents that modulate autophagy.

Lung parenchyma-derived IL-6 promotes IL-17A-dependent acute lung injury after allogeneic stem cell transplantation.

Idiopathic pneumonia syndrome (IPS) is a relatively common, frequently fatal clinical entity, characterized by noninfectious acute lung inflammation following allogeneic stem cell transplantation (SCT), the mechanisms of which are unclear. In this study, we demonstrate that immune suppression with cyclosporin after SCT limits T-helper cell (Th) 1 differentiation and interferon-γ secretion by donor T cells, which is critical for inhibiting interleukin (IL)-6 generation from lung parenchyma during an alloimmune response. Thereafter, local IL-6 secretion induces donor alloantigen-specific Th17 cells to preferentially expand within the lung, and blockade of IL-17A or transplantation of grafts lacking the IL-17 receptor prevents disease. Studies using IL-6(-/-) recipients or IL-6 blockade demonstrate that IL-6 is the critical driver of donor Th17 differentiation within the lung. Importantly, IL-6 is also dysregulated in patients undergoing clinical SCT and is present at very high levels in the plasma of patients with IPS compared with SCT recipients without complications. Furthermore, at the time of diagnosis, plasma IL-6 levels were higher in a subset of IPS patients who were nonresponsive to steroids and anti-tumor necrosis factor therapy. In sum, pulmonary-derived IL-6 promotes IPS via the induction of Th17 differentiation, and strategies that target these cytokines represent logical therapeutic approaches for IPS.

Tc17 cells are a proinflammatory, plastic lineage of pathogenic CD8+ T cells that induce GVHD without antileukemic effects.

IL-17-producing cells are important mediators of graft-versus-host disease (GVHD) after allogeneic stem cell transplantation (SCT). Here we demonstrate that a distinct CD8(+) Tc17 population develops rapidly after SCT but fails to maintain lineage fidelity such that they are unrecognizable in the absence of a fate reporter. Tc17 differentiation is dependent on alloantigen presentation by host dendritic cells (DCs) together with IL-6. Tc17 cells express high levels of multiple prototypic lineage-defining transcription factors (eg, RORγt, T-bet) and cytokines (eg, IL-17A, IL-22, interferon-γ, granulocyte macrophage colony-stimulating factor, IL-13). Targeted depletion of Tc17 early after transplant protects from lethal acute GVHD; however, Tc17 cells are noncytolytic and fail to mediate graft-versus-leukemia (GVL) effects. Thus, the Tc17 differentiation program during GVHD culminates in a highly plastic, hyperinflammatory, poorly cytolytic effector population, which we term "inflammatory iTc17" (iTc17). Because iTc17 cells mediate GVHD without contributing to GVL, therapeutic inhibition of iTc17 development in a clinical setting represents an attractive approach for separating GVHD and GVL.

Cross-dressing by donor dendritic cells after allogeneic bone marrow transplantation contributes to formation of the immunological synapse and maximizes responses to indirectly presented antigen.

The stimulation of naive donor T cells by recipient alloantigen is central to the pathogenesis of graft-versus-host disease after bone marrow transplantation (BMT). Using mouse models of transplantation, we have observed that donor cells become "cross-dressed" in very high levels of recipient hematopoietic cell-derived MHC class I and II molecules following BMT. Recipient-type MHC is transiently present on donor dendritic cells (DCs) after BMT in the setting of myeloablative conditioning but is persistent after nonmyeloablative conditioning, in which recipient hematopoietic cells remain in high numbers. Despite the high level of recipient-derived alloantigen present on the surface of donor DCs, donor T cell proliferative responses are generated only in response to processed recipient alloantigen presented via the indirect pathway and not in response to cross-dressed MHC. Assays in which exogenous peptide is added to cross-dressed MHC in the presence of naive TCR transgenic T cells specific to the MHC class II-peptide combination confirm that cross-dressed APC cannot induce T cell proliferation in isolation. Despite failure to induce T cell proliferation, cross-dressing by donor DCs contributes to generation of the immunological synapse between DCs and CD4 T cells, and this is required for maximal responses induced by classical indirectly presented alloantigen. We conclude that the process of cross-dressing by donor DCs serves as an efficient alternative pathway for the acquisition of recipient alloantigen and that once acquired, this cross-dressed MHC can assist in immune synapse formation prior to the induction of full T cell proliferative responses by concurrent indirect Ag presentation.

Modification of T cell responses by stem cell mobilization requires direct signaling of the T cell by G-CSF and IL-10.

The majority of allogeneic stem cell transplants are currently undertaken using G-CSF mobilized peripheral blood stem cells. G-CSF has diverse biological effects on a broad range of cells and IL-10 is a key regulator of many of these effects. Using mixed radiation chimeras in which the hematopoietic or nonhematopoietic compartments were wild-type, IL-10(-/-), G-CSFR(-/-), or combinations thereof we demonstrated that the attenuation of alloreactive T cell responses after G-CSF mobilization required direct signaling of the T cell by both G-CSF and IL-10. IL-10 was generated principally by radio-resistant tissue, and was not required to be produced by T cells. G-CSF mobilization significantly modulated the transcription profile of CD4(+)CD25(+) regulatory T cells, promoted their expansion in the donor and recipient and their depletion significantly increased graft-versus-host disease (GVHD). In contrast, stem cell mobilization with the CXCR4 antagonist AMD3100 did not alter the donor T cell's ability to induce acute GVHD. These studies provide an explanation for the effects of G-CSF on T cell function and demonstrate that IL-10 is required to license regulatory function but T cell production of IL-10 is not itself required for the attenuation GVHD. Although administration of CXCR4 antagonists is an efficient means of stem cell mobilization, this fails to evoke the immunomodulatory effects seen during G-CSF mobilization. These data provide a compelling rationale for considering the immunological benefits of G-CSF in selecting mobilization protocols for allogeneic stem cell transplantation.

Differing phagocytic capacities of accessory and main olfactory ensheathing cells and the implication for olfactory glia transplantation therapies.

The rodent olfactory systems comprise the main olfactory system for the detection of odours and the accessory olfactory system which detects pheromones. In both systems, olfactory axon fascicles are ensheathed by olfactory glia, termed olfactory ensheathing cells (OECs), which are crucial for the growth and maintenance of the olfactory nerve. The growth-promoting and phagocytic characteristics of OECs make them potential candidates for neural repair therapies such as transplantation to repair the injured spinal cord. However, transplanting mixed populations of glia with unknown properties may lead to variations in outcomes for neural repair. As the phagocytic capacity of the accessory OECs has not yet been determined, we compared the phagocytic capacity of accessory and main OECs in vivo and in vitro. In normal healthy animals, the accessory OECs accumulated considerably less axon debris than main OECs in vivo. Analysis of freshly dissected OECs showed that accessory OECs contained 20% less fluorescent axon debris than main OECs. However, when assayed in vitro with exogenous axon debris added to the culture, the accessory OECs phagocytosed almost 20% more debris than main OECs. After surgical removal of one olfactory bulb which induced the degradation of main and accessory olfactory sensory axons, the accessory OECs responded by phagocytosing the axon debris. We conclude that while accessory OECs have the capacity to phagocytose axon debris, there are distinct differences in their phagocytic capacity compared to main OECs. These distinct differences may be of importance when preparing OECs for neural transplant repair therapies.

Autophagy and haematopoietic stem cell transplantation.

Allogeneic haematopoietic stem cell transplantation (HSCT) represents the only curative therapy for the majority of bone marrow-derived cancers. Unfortunately, HSCT can result in serious complications such as graft-versus-host disease, graft failure and infection. In the last decade, there have been major advances in the understanding of the role of autophagy in many diseases and cellular processes. Recent findings have demonstrated a crucial role for autophagy in haematopoietic stem cell survival and function, antigen presentation, T-cell differentiation and response to cytokine stimulation. Given the critical requirement for each of these processes in HSCT and subsequent complications, it is surprising that the contribution of autophagy to HSCT per se is relatively unexplored. In addition, the increasing use of autophagy-modulating drugs in the clinic further highlights the need to understand the role of autophagy in allogeneic HSCT. This review will cover established and implicated roles of autophagy in HSCT, suggesting this pathway as an important therapeutic target for improving transplant outcomes.

Promoting regulation via the inhibition of DNAM-1 after transplantation.

Donor T cells play pivotal roles in graft-versus-host disease (GVHD) and graft-versus-leukemia (GVL) effects following bone marrow transplantation (BMT). DNAX accessory molecule 1 (DNAM-1) is a costimulatory and adhesion molecule, expressed mainly by natural killer cells and CD8(+) T cells at steady state to promote adhesion to ligand-expressing targets and enhance cytolysis. We have analyzed the role of this pathway in GVHD and GVL. The absence of DNAM-1 on the donor graft attenuated GVHD in major histocompatibility complex (MHC)-mismatched and MHC-matched BMT following conditioning with lethal and sublethal irradiation. In contrast, DNAM-1 was not critical for GVL effects against ligand (CD155) expressing and nonexpressing leukemia. The effects on GVHD following myeloablative conditioning were independent of CD8(+) T cells and dependent on CD4(+) T cells, and specifically donor FoxP3(+) regulatory T cells (Treg). The absence of DNAM-1 promoted the expansion and suppressive function of Treg after BMT. These findings provide support for therapeutic DNAM-1 inhibition to promote tolerance in relevant inflammatory-based diseases characterized by T-cell activation.

Induced regulatory T cells promote tolerance when stabilized by rapamycin and IL-2 in vivo.

Natural regulatory T cells (nTregs) play an important role in tolerance; however, the small numbers of cells obtainable potentially limit the feasibility of clinical adoptive transfer. Therefore, we studied the feasibility and efficacy of using murine-induced regulatory T cells (iTregs) for the induction of tolerance after bone marrow transplantation. iTregs could be induced in large numbers from conventional donor CD4 and CD8 T cells within 1 wk and were highly suppressive. During graft-versus-host disease (GVHD), CD4 and CD8 iTregs suppressed the proliferation of effector T cells and the production of proinflammatory cytokines. However, unlike nTregs, both iTreg populations lost Foxp3 expression within 3 wk in vivo, reverted to effector T cells, and exacerbated GVHD. The loss of Foxp3 in iTregs followed homeostatic and/or alloantigen-driven proliferation and was unrelated to GVHD. However, the concurrent administration of rapamycin, with or without IL-2/anti-IL-2 Ab complexes, to the transplant recipients significantly improved Foxp3 stability in CD4 iTregs (and, to a lesser extent, CD8 iTregs), such that they remained detectable 12 wk after transfer. Strikingly, CD4, but not CD8, iTregs could then suppress Teff proliferation and proinflammatory cytokine production and prevent GVHD in an equivalent fashion to nTregs. However, at high numbers and when used as GVHD prophylaxis, Tregs potently suppress graft-versus-leukemia effects and so may be most appropriate as a therapeutic modality to treat GVHD. These data demonstrate that CD4 iTregs can be produced rapidly in large, clinically relevant numbers and, when transferred in the presence of systemic rapamycin and IL-2, induce tolerance in transplant recipients.

Identification and expansion of highly suppressive CD8(+)FoxP3(+) regulatory T cells after experimental allogeneic bone marrow transplantation.

FoxP3(+) confers suppressive properties and is confined to regulatory T cells (T(reg)) that potently inhibit autoreactive immune responses. In the transplant setting, natural CD4(+) T(reg) are critical in controlling alloreactivity and the establishment of tolerance. We now identify an important CD8(+) population of FoxP3(+) T(reg) that convert from CD8(+) conventional donor T cells after allogeneic but not syngeneic bone marrow transplantation. These CD8(+) T(reg) undergo conversion in the mesenteric lymph nodes under the influence of recipient dendritic cells and TGF-ß. Importantly, this population is as important for protection from GVHD as the well-studied natural CD4(+)FoxP3(+) population and is more potent in exerting class I-restricted and antigen-specific suppression in vitro and in vivo. Critically, CD8(+)FoxP3(+) T(reg) are exquisitely sensitive to inhibition by cyclosporine but can be massively and specifically expanded in vivo to prevent GVHD by coadministering rapamycin and IL-2 antibody complexes. CD8(+)FoxP3(+) T(reg) thus represent a new regulatory population with considerable potential to preferentially subvert MHC class I-restricted T-cell responses after bone marrow transplantation.

Immune insufficiency during GVHD is due to defective antigen presentation within dendritic cell subsets.

Alloreactivity after transplantation is associated with profound immune suppression, and consequent opportunistic infection results in high morbidity and mortality. This immune suppression is most profound during GVHD after bone marrow transplantation where an inflammatory cytokine storm dominates. Contrary to current dogma, which avers that this is a T-cell defect, we demonstrate that the impairment lies within conventional dendritic cells (cDCs). Significantly, exogenous antigens can only be presented by the CD8(-) cDC subset after bone marrow transplantation, and inflammation during GVHD specifically renders the MHC class II presentation pathway in this population incompetent. In contrast, both classic and cross-presentation within MHC class I remain largely intact. Importantly, this defect in antigen processing can be partially reversed by TNF inhibition or the adoptive transfer of donor cDCs generated in the absence of inflammation.

Autophagy prevents graft failure during murine graft-versus-host disease.

ABSTRACT: Autophagy is an intracellular survival process that has established roles in the long-term survival and function of hematopoietic stem cells (HSC). We investigated the contribution of autophagy to HSC fitness during allogeneic transplantation and graft-versus-host disease (GVHD). We demonstrate in vitro that both tumor necrosis factor and IL-1ß, major components of GVHD cytokine storm, synergistically promote autophagy in both HSC and their more mature hematopoietic progenitor cells (HPC). In vivo we demonstrate that autophagy is increased in donor HSC and HPC during GVHD. Competitive transplant experiments demonstrated that autophagy-deficient cells display reduced capacity to reconstitute the hematopoietic system compared to wild-type counterparts. In a major histocompatibility complex-mismatched model of GVHD and associated cytokine dysregulation, we demonstrate that autophagy-deficient HSC and progenitors fail to establish durable hematopoiesis, leading to primary graft failure and universal transplant related mortality. Using several different models, we confirm that autophagy activity is increased in early progenitor and HSC populations in the presence of T-cell-derived inflammatory cytokines and that these HSC populations require autophagy to survive. Thus, autophagy serves as a key survival mechanism in HSC and progenitor populations after allogeneic stem cell transplant and may represent a therapeutic target to prevent graft failure during GVHD.

Type I-IFNs control GVHD and GVL responses after transplantation.

Although the effects of type II-IFN (IFN-γ) on GVHD and leukemia relapse are well studied, the effects of type I-interferon (type I-IFN, IFN-α/ß) remain unclear. We investigated this using type I-IFN receptor-deficient mice and exogenous IFN-α administration in established models of GVHD and GVL. Type I-IFN signaling in host tissue prevented severe colon-targeted GVHD in CD4-dependent models of GVHD directed toward either major histocompatibility antigens or multiple minor histocompatibility antigens. This protection was the result of suppression of donor CD4(+) T-cell proliferation and differentiation. Studies in chimeric recipients demonstrated this was due to type I-IFN signaling in hematopoietic tissue. Consistent with this finding, administration of IFN-α during conditioning inhibited donor CD4(+) proliferation and differentiation. In contrast, CD8-dependent GVHD and GVL effects were enhanced when type I-IFN signaling was intact in the host or donor, respectively. This finding reflected the ability of type I-IFN to both sensitize host target tissue/leukemia to cell-mediated cytotoxicity and augment donor CTL function. These data confirm that type I-IFN plays an important role in defining the balance of GVHD and GVL responses and suggests that administration of the cytokine after BM transplantation could be studied prospectively in patients at high risk of relapse.