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

Immunopathogenic mechanisms and modulatory approaches to graft-versus-host disease prevention in acute myeloid leukaemia

Allogeneic haematopoietic stem cell transplantation (HSCT) remains the only potential cure for intermediate to high-risk acute myeloid leukaemia (AML). The therapeutic effect of HSCT is largely dependent on the powerful donor-derived immune response against recipient leukaemia cells, known as graft-versus-leukaemia effect (GvL). However, the donor-derived immune system can also cause acute or chronic damage to normal recipient organs and tissues, in a process known as graft-versus-host disease (GvHD). GvHD is a leading cause of non-relapse mortality in HSCT recipients. There are many similarities and cross talk between the immune pathways of GvL and GvHD. Studies have demonstrated that both processes require the presence of mismatched alloantigens between the donor and recipient, and activation of immune responses centered around donor T-cells, which can be further modulated by various recipient or donor factors. Dissecting GvL from GvHD to achieve more effective GvHD prevention and enhanced GvL has been the holy grail of HSCT research. In this review, we focused on the key factors that contribute to the immune responses of GvL and GvHD, the effect on GvL with different GvHD prophylactic strategies, and the potential impact of various AML relapse prevention therapy or treatments on GvHD.

The Effects of Interferons on Allogeneic T Cell Response in GVHD: The Multifaced Biology and Epigenetic Regulations

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a potentially curative therapy for hematological malignancies. This beneficial effect is derived mainly from graft-versus-leukemia (GVL) effects mediated by alloreactive T cells. However, these alloreactive T cells can also induce graft-versus-host disease (GVHD), a life-threatening complication after allo-HSCT. Significant progress has been made in the dissociation of GVL effects from GVHD by modulating alloreactive T cell immunity. However, many factors may influence alloreactive T cell responses in the host undergoing allo-HSCT, including the interaction of alloreactive T cells with both donor and recipient hematopoietic cells and host non-hematopoietic tissues, cytokines, chemokines and inflammatory mediators. Interferons (IFNs), including type I IFNs and IFN-γ, primarily produced by monocytes, dendritic cells and T cells, play essential roles in regulating alloreactive T cell differentiation and function. Many studies have shown pleiotropic effects of IFNs on allogeneic T cell responses during GVH reaction. Epigenetic mechanisms, such as DNA methylation and histone modifications, are important to regulate IFNs’ production and function during GVHD. In this review, we discuss recent findings from preclinical models and clinical studies that characterize T cell responses regulated by IFNs and epigenetic mechanisms, and further discuss pharmacological approaches that modulate epigenetic effects in the setting of allo-HSCT.

Functional Contributions of Antigen Presenting Cells in Chronic Graft-Versus-Host Disease

Chronic graft-versus-host disease (cGVHD) is one of the most common reasons of late non-relapse morbidity and mortality of patients with allogeneic hematopoietic stem cell transplantation (allo-HSCT). While acute GVHD is considered driven by a pathogenic T cell dominant mechanism, the pathogenesis of cGVHD is much complicated and involves participation of a variety of immune cells other than pathogenic T cells. Existing studies have revealed that antigen presenting cells (APCs) play crucial roles in the pathophysiology of cGVHD. APCs could not only present auto- and alloantigens to prime and activate pathogenic T cells, but also directly mediate the pathogenesis of cGVHD via multiple mechanisms including infiltration into tissues/organs, production of inflammatory cytokines as well as auto- and alloantibodies. The studies of this field have led to several therapies targeting different APCs with promising results. This review will focus on the important roles of APCs and their contributions in the pathophysiology of cGVHD after allo-HSCT.

New insights into the basic biology of acute graft-versus-host-disease

Although allogeneic hematopoietic stem cell transplantation is an important therapy for many hematological and non-hematological diseases, acute graft-versus-host-disease (aGVHD) is a major obstacle to its success. The pathogenesis of aGVHD is divided into three distinct phases which occur largely as the result of interactions between infused donor T cells and numerous cell types of both hematopoietic and non-hematopoietic origin. In light of the disease's immensely complex biology, epigenetics has emerged as a framework with which to examine aGVHD. This review focuses on new findings that clarify the roles specific epigenetic regulators play in T cell-mediated aGVHD development and discusses how their modulation could disrupt that process to beneficial effects. DNA methyltransferases, histone methyltransferases and histone deacetylases are the most closely studied regulators across aGVHD priming, induction and effector phases and have been manipulated using drugs and other methods in both murine models and clinical trials to varying degrees of success. Antigen-presenting cells, effector T cells and memory T cells, among others, are targeted and affected by these regulators in different ways. Finally, our review highlights new directions for study and potential novel targets for modulation to abrogate aGVHD.

The primacy of gastrointestinal tract antigen-presenting cells in lethal graft-versus-host disease

Allogeneic stem cell transplantation is a cornerstone of curative therapy for high-risk and/or advanced hematological malignancies but remains limited by graft-versus-host disease (GVHD). GVHD is initiated by the interaction between recipient antigen-presenting cells (APCs) and donor T cells, culminating in T-cell differentiation along pathogenic type-1 and type-17 paradigms at the expense of tolerogenic regulatory T-cell patterns. Type-1 and type-17 T cells secrete cytokines (eg, granulocyte-macrophage colony-stimulating factor and interferon-γ) critical to the cytokine storm that amplifies expansion of donor APCs and their alloantigen presentation. It has become increasingly clear that pathogenic donor T-cell differentiation is initiated by both professional recipient APCs (eg, dendritic cells [DCs]) and nonprofessional APCs (eg, epithelial and mesenchymal cells), particularly within the gastrointestinal (GI) tract. In the immediate peritransplantation period, these APCs are profoundly modified by pathogen-associated molecular pattern (PAMP)/damage-associated molecular pattern (DAMP) signals derived from conditioning and intestinal microbiota. Subsequently, donor DCs in the GI tract are activated by DAMP/PAMP signals in the colon that gain access to the lamina propria once the mucosal barrier mucosa is compromised by GVHD. This results in donor DC expansion and alloantigen presentation in the colon and subsequent migration into the mesenteric lymph nodes. Here, new donor T cells are primed, expanded, differentiated, and imprinted with gut-homing integrins permissive of migration into the damaged GI tract, resulting in the lethal feed-forward cascade of GVHD. These new insights into our understanding of the cellular and molecular factors initiating GVHD, both spatially and temporally, give rise to a number of logical therapeutic targets, focusing on the inhibition of APC function in the GI tract.

Dendritic Cell Regulation of Graft-Vs.-Host Disease: Immunostimulation and Tolerance

Graft-vs.-host disease (GVHD) remains a significant cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Significant progresses have been made in defining the dichotomous role of dendritic cells (DCs) in the development of GVHD. Host-derived DCs are important to elicit allogeneic T cell responses, whereas certain donor-types of DCs derived from newly engrafted hematopoietic stem/progenitor cells (HSPCs) can amply this graft-vs.-host reaction. In contrast, some DCs also play non-redundant roles in mediating immune tolerance. They induce apoptotic deletion of host-reactive donor T cells while promoting expansion and function of regulatory T cells (Treg). Unfortunately, this tolerogenic effect of DCs is impaired during GVHD. Severe GVHD in patients subject to allo-HSCT is associated with significantly decreased number of circulating peripheral blood DCs during engraftment. Existing studies reveal that GVHD causes delayed reconstitution of donor DCs from engrafted HSPCs, impairs the antigen presentation function of newly generated DCs and reduces the capacity of DCs to regulate Treg. The present review will discuss the importance of DCs in alloimmunity and the mechanism underlying DC reconstitution after allo-HSCT.

HLA discrepancy between graft and host rather than that graft and first donor impact the second transplant outcome

Second allogeneic hematopoietic stem cell transplantation is a curative treatment option for patients with hematologic malignancies. However, it is unclear whether HLA discrepancy between graft and first donor has an impact on the outcome of second transplantation. We retrospectively analyzed 646 patients receiving second transplantation after an initial HLA mismatched transplantation. With regard to graft-versus-host, the one-allele mismatch (1 mismatch) group (SHR, 1.88; 95%CI: 0.79-4.45; P=0.163) and more than one-allele mismatch group (≥ 2 mismatch) (SHR, 1.84; 95%CI, 0.75–4.51; P=0.182) had higher risks of grade III–IV acute graft-versus-host disease (GvHD) compared to the HLA-matched (0 mismatch) group. In contrast, no difference in risk of acute GvHD was found among the 0, 1, and ≥ 2 mismatch group with respect to graft-versus-first donor. With regard to graft-versus-host, the ≥ 2 mismatch group showed a significantly higher risk of treatment-related mortality (SHR, 1.90; 95%CI, 1.04–3.50; P=0.038) compared to the 0 mismatch group, while the risk of relapse was slightly lower in the ≥ 2 mismatch group (SHR, 068; 95%CI, 0.44–1.06; P=0.086). In contrast, with regard to graft-versus-first donor, there were no significant differences in treatment-related mortality or relapse among the three groups. These findings suggested that HLA discrepancy between graft and host induces transplant-related immunological responses in second transplantation leading to an increase in treatment-related mortality, in contrast, the biological effects of HLA discrepancy between graft and first donor on outcome may be negligible.

Alloantigen expression on malignant cells and healthy host tissue influences graft-versus-tumor reactions after allogeneic hematopoietic stem cell transplantation

Durable remissions of hematological malignancies regularly observed following allogeneic hematopoietic stem cell transplantation (aHSCT) are due to the conditioning regimen, as well as an immunological phenomenon called graft-versus-leukemia (GVL) or graft-versus-tumor (GVT) effect. The development of GVL is closely linked to graft-versus-host disease (GVHD), the main side effect associated with aHSCT. Both, GVHD and GVL are mediated by donor T cells that are initially activated by antigen-presenting cells that present recipient-derived alloantigens in the context of either matched or mismatched MHC class I molecules. Using murine models of aHSCT we show that ubiquitously expressed minor histocompatibility alloantigens (mHAg) are no relevant target for GVT effects. Interestingly, certain ubiquitously expressed MHC alloantigens augmented GVT effects early after transplantation, while others did not. The magnitude of GVT effects correlated with tumor infiltration by CD8+ cytotoxic T cells and tumor cell apoptosis. Furthermore, the immune response underlying GVHD and GVT was oligoclonal, highlighting that immunodominance is an important factor during alloimmune responses. These results emphasize that alloantigen expression on non-hematopoietic tissues can influence GVT effects in a previously unrecognized fashion. These findings bear significance for harnessing optimal GVL effects in patients receiving aHSCT.

New Insights into Graft-Versus-Host Disease and Graft Rejection

Allogeneic transplantation of foreign organs or tissues has lifesaving potential, but can lead to serious complications. After solid organ transplantation, immune-mediated rejection mandates the use of prolonged global immunosuppression and limits the life span of transplanted allografts. After bone marrow transplantation, donor-derived immune cells can trigger life-threatening graft-versus-host disease. T cells are central mediators of alloimmune complications and the target of most existing therapeutic interventions. We review recent progress in identifying multiple cell types in addition to T cells and new molecular pathways that regulate pathogenic alloreactivity. Key discoveries include the cellular subsets that function as potential sources of alloantigens, the cross talk of innate lymphoid cells with damaged epithelia and with the recipient microbiome, the impact of the alarmin interleukin-33 on alloreactivity, and the role of Notch ligands expressed by fibroblastic stromal cells in alloimmunity. While refining our understanding of transplantation immunobiology, these findings identify new therapeutic targets and new areas of investigation.

Alloantigen presentation and graft-versus-host disease: fuel for the fire

Allogeneic stem cell transplantation (SCT) is a unique procedure, primarily in patients with hematopoietic malignancies, involving chemoradiotherapy followed by the introduction of donor hematopoietic and immune cells into an inflamed and lymphopenic environment. Interruption of the process by which recipient alloantigen is presented to donor T cells to generate graft-versus-host disease (GVHD) represents an attractive therapeutic strategy to prevent morbidity and mortality after SCT and has been increasingly studied in the last 15 years. However, the immune activation resulting in GVHD has no physiological equivalent in nature; alloantigen is ubiquitous, persists indefinitely, and can be presented by multiple cell types at numerous sites, often on incompatible major histocompatibility complex, and occurs in the context of intense inflammation early after SCT. The recognition that alloantigen presentation is also critical to the development of immunological tolerance via both deletional and regulatory mechanisms further adds to this complexity. Finally, GVHD itself appears capable of inhibiting the presentation of microbiological antigens by donor dendritic cells late after SCT that is mandatory for the establishment of effective pathogen-specific immunity. Here, we review our current understanding of alloantigen, its presentation by various antigen-presenting cells, subsequent recognition by donor T cells, and the potential of therapeutic strategies interrupting this disease-initiating process to modify transplant outcome.

Donor CD4+ Foxp3+ regulatory T cells are necessary for posttransplantation cyclophosphamide-mediated protection against GVHD in mice

Posttransplantation cyclophosphamide (PTCy) is an effective prophylaxis against graft-versus-host disease (GVHD). However, it is unknown whether PTCy works singularly by eliminating alloreactive T cells via DNA alkylation or also by restoring the conventional (Tcon)/regulatory (Treg) T-cell balance. We studied the role of Tregs in PTCy-mediated GVHD prophylaxis in murine models of allogeneic blood or marrow transplantation (alloBMT). In 2 distinct MHC-matched alloBMT models, infusing Treg-depleted allografts abrogated the GVHD-prophylactic activity of PTCy. Using allografts in which Foxp3(+) Tregs could be selectively depleted in vivo, either pre- or post-PTCy ablation of donor thymus-derived Tregs (tTregs) abolished PTCy protection against GVHD. PTCy treatment was associated with relative preservation of donor Tregs. Experiments using combinations of Foxp3(-) Tcons and Foxp3(+) Tregs sorted from different Foxp3 reporter mice indicated that donor Treg persistence after PTCy treatment was predominantly caused by survival of functional tTregs that retained Treg-specific demethylation and also induction of peripherally derived Tregs. Finally, adoptive transfer of tTregs retrieved from PTCy-treated chimeras rescued PTCy-treated, Treg-depleted recipients from lethal GVHD. Our findings indicate that PTCy-mediated protection against GVHD is not singularly dependent on depletion of donor alloreactive T cells but also requires rapidly recovering donor Tregs to initiate and maintain alloimmune regulation.

Antigen-Specific Priming is Dispensable in Depletion of Apoptosis-Sensitive T Cells for GvHD Prophylaxis

Prophylactic approaches to graft versus host disease (GvHD) have employed both phenotypic reduction of T cells and selective elimination of host-primed donor T cells in vitro and in vivo. An additional approach to GvHD prophylaxis by functional depletion of apoptosis-sensitive donor T cells without host-specific sensitization ex vivo showed remarkable reduction in GHD incidence and severity. We address the role and significance of antigen-specific sensitization of donor T cells and discuss the mechanisms of functional T cell purging by apoptosis for GvHD prevention. Host-specific sensitization is dispensable because migration is antigen-independent and donor T cell sensitization is mediated by multiple and redundant mechanisms of presentation of major and minor histocompatibility complex and tissue antigens by donor and host antigen-presenting cells. Our data suggest that potential murine and human GvH effectors reside within subsets of preactivated T cells susceptible to negative regulation by apoptosis prior to encounter of and sensitization to specific antigens.

Depletion of host CCR7(+) dendritic cells prevented donor T cell tissue tropism in anti-CD3-conditioned recipients

We reported previously that anti-CD3 mAb treatment before hematopoietic cell transplantation (HCT) prevented graft-versus-host disease (GVHD) and preserved graft-versus-leukemia (GVL) effects in mice. These effects were associated with downregulated donor T cell expression of tissue-specific homing and chemokine receptors, marked reduction of donor T cell migration into GVHD target tissues, and deletion of CD103(+) dendritic cells (DCs) in mesenteric lymph nodes (MLN). MLN CD103(+) DCs and peripheral lymph node (PLN) DCs include CCR7(+) and CCR7(-) subsets, but the role of these DC subsets in regulating donor T cell expression of homing and chemokine receptors remain unclear. Here, we show that recipient CCR7(+), but not CCR7(-), DCs in MLN induced donor T cell expression of gut-specific homing and chemokine receptors in a retinoid acid-dependent manner. CCR7 regulated activated DC migration from tissue to draining lymph node, but it was not required for the ability of DCs to induce donor T cell expression of tissue-specific homing and chemokine receptors. Finally, anti-CD3 treatment depleted CCR7(+) but not CCR7(-) DCs by inducing sequential expansion and apoptosis of CCR7(+) DCs in MLN and PLN. Apoptosis of CCR7(+) DCs was associated with DC upregulation of Fas expression and natural killer cell but not T, B, or dendritic cell upregulation of FasL expression in the lymph nodes. These results suggest that depletion of CCR7(+) host-type DCs, with subsequent inhibition of donor T cell migration into GVHD target tissues, can be an effective approach in prevention of acute GVHD and preservation of GVL effects.

Inhibition of cathepsin S reduces allogeneic T cell priming but not graft-versus-host disease against minor histocompatibility antigens

Cathepsin (Cathepsin) S, L, and B proteases mediate antigen presentation on major histocompatibility complex (MHC) class II by degrading the invariant chain Ii, which blocks peptide loading. The ability of the Cathepsin S inhibitor LHVS (morpholinurea-leucine-homophenylalanine-vinylsulfone phenyl) to impede antigen presentation has led its development as a therapy for autoimmune diseases. There is substantial evidence that donor T cell recognition of host minor histocompatibility antigens (miHA) and subsequent destruction of host tissue mediates graft-versus-host disease (GVHD). We hypothesized that enzymes involved in antigen presentation may play a role in the development of GVHD. Using the C57BL/6 → BALB.B minor mismatch acute GVHD (aGVHD) model, we found that the cathepsin S activity of spleens from allogenetically transplanted mice were significantly increased 1 week after transplantation compared with syngeneic mice. Although LHVS decreased T cell priming responses against both single OVA antigen and miHA in vitro, LHVS did not reduce the severity of aGVHD. In fact, LHVS exacerbated a CD4(+)-T cell-dependent model of GVHD similar to chronic GVHD. This suggests that cytokines rather than T cells may mediate much of the damage in the aGVHD model and that therapeutics based on inhibition of antigen presentation for GVHD must be approached with caution.

Recipient nonhematopoietic antigen-presenting cells are sufficient to induce lethal acute graft-versus-host disease

The presentation pathways by which allogeneic peptides induce graft-versus-host disease (GVHD) are unclear. We developed a bone marrow transplant (BMT) system in mice whereby presentation of a processed recipient peptide within major histocompatibility complex (MHC) class II molecules could be spatially and temporally quantified. Whereas donor antigen presenting cells (APCs) could induce lethal acute GVHD via MHC class II, recipient APCs were 100-1,000 times more potent in this regard. After myeloablative irradiation, T cell activation and memory differentiation occurred in lymphoid organs independently of alloantigen. Unexpectedly, professional hematopoietic-derived recipient APCs within lymphoid organs had only a limited capacity to induce GVHD, and dendritic cells were not required. In contrast, nonhematopoietic recipient APCs within target organs induced universal GVHD mortality and promoted marked alloreactive donor T cell expansion within the gastrointestinal tract and inflammatory cytokine generation. These data challenge current paradigms, suggesting that experimental lethal acute GVHD can be induced by nonhematopoietic recipient APCs.

Induction of acute GVHD by sex-mismatched H-Y antigens in the absence of functional radiosensitive host hematopoietic-derived antigen-presenting cells

It is currently thought that acute GVHD cannot be elicited in the absence of Ag presentation by radiosensitive host hematopoietic-derived APCs after allogeneic BM transplantation. Because clinical data suggest that sex-mismatched H-Y Ags may be important minor histocompatibility Ags for GVH responses, we directly tested their relevance and ability to initiate GVHD when presented by either the hematopoietic- (host or donor) or the nonhematopoietic-derived APCs. H-Y minor Ag incompatibility elicited both CD4(+) and CD8(+) T-cell driven GVHD lethality. Studies with various well-established BM chimera recipients, in contrast to the current views, have reported that in the absence of functional radiosensitive host hematopoietic-derived APCs, H-Y Ag presentation by either the donor hematopoietic-derived or the host nonhematopoietic-derived APCs is sufficient for inducing GVHD. Our data further suggest that infusion of sufficient numbers of alloreactive donor T cells will induce GVHD in the absence of radiosensitive host hematopoietic-derived APCs.

Graft-versus-leukemia (GVL) against mouse blast-crisis chronic myelogenous leukemia (BC-CML) and chronic-phase chronic myelogenous leukemia (CP-CML): shared mechanisms of T cell killing, but programmed death ligands render CP-CML and not BC-CML GVL resistant

Graft-versus-leukemia (GVL) against chronic-phase chronic myelogenous leukemia (CP-CML) is potent, but it is less efficacious against acute leukemias and blast-crisis chronic myelogenous leukemia (BC-CML). The mechanisms underlying GVL resistance are unknown. Previously, we found that alloreactive T cell targeting of GVL-sensitive bcr-abl-induced mouse CP-CML (mCP-CML) required TCR-MHC interactions and that multiple and redundant killing mechanisms were in play. To better understand why BC-CML is resistant to GVL, we performed a comprehensive analysis of GVL against mouse BC-CML (mBC-CML) induced by the retroviral transfer of the bcr-abl and NUP98/HOXA9 fusion cDNAs. Like human BC-CML, mBC-CML was GVL resistant, and this was not due to accelerated kinetics or a greater leukemia burden. To study T cell recognition and killing mechanisms, we generated a panel of gene-deficient leukemias by transducing bone marrow from gene-deficient mice. T cell target recognition absolutely required that mBC-CML cells express MHC molecules. GVL against both mCP-CML and mBC-CML required leukemia expression of ICAM-1. We hypothesized that mBC-CML would be resistant to some of the killing mechanisms sufficient to eliminate mCP-CML, but we found instead that the same mechanisms were effective against both types of leukemia, because GVL was similar against wild-type or mBC-CML genetically lacking Fas, TRAIL-R, Fas/TRAIL-R, or TNFR1/R2 or when donor T cells were perforin(-/-). However, mCP-CML, but not mBC-CML, relied on expression of programmed death-1 ligands 1 and 2 (PD-L1/L2) to resist T cell killing, because only GVL against mCP-CML was augmented when leukemias lacked PD-L1/L2. Thus, mBC-CML cells have cell-intrinsic mechanisms, distinct from mCP-CML cells, which protect them from T cell killing.

A repertoire-independent and cell-intrinsic defect in murine GVHD induction by effector memory T cells

Effector memory T cells (T(EM)) do not cause graft-versus-host disease (GVHD), though why this is has not been elucidated. To compare the fates of alloreactive naive (T(N)) or memory (T(M)) T cells, we developed a model of GVHD in which donor T cells express a transgene-encoded TCR specific for an antigenic peptide that is ubiquitously expressed in the recipient. Small numbers of naive TCR transgenic (Tg) T cells induced a robust syndrome of GVHD in transplanted recipients. We then used an established method to convert TCR Tg cells to T(M) and tested these for GVHD induction. This allowed us to control for the potentially different frequencies of alloreactive T cells among T(N) and T(M), and to track fates of alloreactive T cells after transplantation. T(EM) caused minimal, transient GVHD whereas central memory T cells (T(CM)) caused potent GVHD. Surprisingly, T(EM) were not inert: they, engrafted, homed to target tissues, and proliferated extensively, but they produced less IFN-γ and their expansion in target tissues was limited at later time points, and local proliferation was reduced. Thus, cell-intrinsic properties independent of repertoire explain the impairment of T(EM), which can initiate but cannot sustain expansion and tissue damage.

Blockade of IL-6-signaling inhibits the pathogenesis of CD4+ T cell-mediated lethal graft-versus-host reaction against minor histocompatibility antigen

Graft-versus-host reaction (GVHR) is considered as a problem in hematopoietic cell transplantation. We found that CD45RB(high) CD62L(+) naïve CD4(+) T cells from wild-type B10D2 (H-2d MMTV6(-)) mice immediately differentiated into effector T cells producing high-levels of various cytokines after the transfer into BALB/c RAG2(-/-) (H-2d MMTV6(+)) mice. The expanded CD4(+) T cells, which have almost TCR Vβ3 chain, recognized the minor antigen of recipient mice and brought typical severe GVHR symptoms such as eyelid irritation, diarrhea, and liver failure. Eventually, all of the recipient mice transferred CD4(+) T cells was dead within 10 days. We demonstrated here that blockade of IL-6 signaling by administration of anti-IL-6 receptor (IL-6R) monoclonal antibody (mAb) remarkably inhibited the CD4(+) T cell-mediated lethal GVHR. In addition, we confirmed that the in vivo injection of anti-IL-6R mAb prevented the generation of effector CD4(+) T cells which produce the inflammatory cytokines such as IFN-γ, TNF-α, and IL-17. These findings indicated that IL-6 was a critical factor in the CD4(+) T cell-dependent acute GVHR induced by a minor-antigen, suggesting that IL-6-mediated signaling pathway would be a strong therapeutic target in T cell-mediated GVHR as well as other diseases including autoimmune and inflammation.

Mesenchymal stromal cells transiently alter the inflammatory milieu post-transplant to delay graft-versus-host disease

BACKGROUND

Multipotent mesenchymal stromal cells suppress T-cell function in vitro, a property that has underpinned their use in treating clinical steroid-refractory graft-versus-host disease after allogeneic hematopoietic stem cell transplantation. However the potential of mesenchymal stromal cells to resolve graft-versus-host disease is confounded by a paucity of pre-clinical data delineating their immunomodulatory effects in vivo.

DESIGN AND METHODS

We examined the influence of timing and dose of donor-derived mesenchymal stromal cells on the kinetics of graft-versus-host disease in two murine models of graft-versus-host disease (major histocompatibility complex-mismatched: UBI-GFP/BL6 [H-2(b)]→BALB/c [H-2(d)] and the sibling transplant mimic, UBI-GFP/BL6 [H-2(b)]→BALB.B [H-2(b)]) using clinically relevant conditioning regimens. We also examined the effect of mesenchymal stromal cell infusion on bone marrow and spleen cellular composition and cytokine secretion in transplant recipients.

RESULTS

Despite T-cell suppression in vitro, mesenchymal stromal cells delayed but did not prevent graft-versus-host disease in the major histocompatibility complex-mismatched model. In the sibling transplant model, however, 30% of mesenchymal stromal cell-treated mice did not develop graft-versus-host disease. The timing of administration and dose of the mesenchymal stromal cells influenced their effectiveness in attenuating graft-versus-host disease, such that a low dose of mesenchymal stromal cells administered early was more effective than a high dose of mesenchymal stromal cells given late. Compared to control-treated mice, mesenchymal stromal cell-treated mice had significant reductions in serum and splenic interferon-γ, an important mediator of graft-versus-host disease.

CONCLUSIONS

Mesenchymal stromal cells appear to delay death from graft-versus-host disease by transiently altering the inflammatory milieu and reducing levels of interferon-γ. Our data suggest that both the timing of infusion and the dose of mesenchymal stromal cells likely influence these cells' effectiveness in attenuating graft-versus-host disease.

Alloantigen expression on non-hematopoietic cells reduces graft-versus-leukemia effects in mice

Allogeneic hematopoietic stem cell transplantation (HSCT) is used effectively to treat a number of hematological malignancies. Its beneficial effects rely on donor-derived T cell-targeted leukemic cells, the so-called graft-versus-leukemia (GVL) effect. Induction of GVL is usually associated with concomitant development of graft-versus-host disease (GVHD), a major complication of allogeneic HSCT. The T cells that mediate GVL and GVHD are activated by alloantigen presented on host antigen-presenting cells of hematopoietic origin, and it is not well understood how alloantigen expression on non-hematopoietic cells affects GVL activity. Here we show, in mouse models of MHC-matched, minor histocompatibility antigen-mismatched bone marrow transplantation, that alloantigen expression on host epithelium drives donor T cells into apoptosis and dysfunction during GVHD, resulting in a loss of GVL activity. During GVHD, programmed death-1 (PD-1) and PD ligand-1 (PD-L1), molecules implicated in inducing T cell exhaustion, were upregulated on activated T cells and the target tissue, respectively, suggesting that the T cell defects driven by host epithelial alloantigen expression might be mediated by the PD-1/PD-L1 pathway. Consistent with this, blockade of PD-1/PD-L1 interactions partially restored T cell effector functions and improved GVL. These results elucidate a previously unrecognized significance of alloantigen expression on non-hematopoietic cells in GVL and suggest that separation of GVL from GVHD for more effective HSCT may be possible in human patients.

Roles of Host Nonhematopoietic Cells in Autoimmunity and Donor Cell Engraftment in Graft-versus-host Disease

Background

Graft-versus-host disease (GVHD) is initiated when alloreactive donor T cells are primed by host APCs to undergo clonal expansion and maturation. Since there is a controversy regarding the role of nonhematopoietic cells in GVHD, we wanted to investigate the influence of MHC disparity on nonhematopoietic cells on the pathogenesis of GVHD in the MHC-haplomismatched C57BL/6 (H-2^b) or DBA/2 (H-2^d)→unirradiated (C57BL/6×DBA/2) F₁(BDF₁; H-2^b/d) murine model of acute GVHD (aGVHD) or chronic GVHD (cGVHD).

Methods

We generated (BDF₁→C57BL/6), (BDF₁→DBA/2), and (BDF₁→BDF₁) chimeras and examined GVHD-related parameters and donor cell engraftment in those chimeras.

Results

Using this experimental system, we found that 1) severe aGVHD across MHC Ag barrier depends on the expression of nonhematopoietically rather than hematopoietically derived alloAgs for maximal GVHD manifestations; 2) host APCs were sufficient to break B cell tolerance to self molecules in cGVHD, whereas host APCs were insufficient to induce autoimmunity in aGVHD; 3) donor cell engraftment was greatly enhanced in the host with MHC-matched nonhematopoietic cells.

Conclusion

Taken together, our results provide an insight into how MHC disparity on GVHD target organs contribute to the pathogenesis of GVHD.

NCI First International Workshop on The Biology, Prevention, and Treatment of Relapse After Allogeneic Hematopoietic Stem Cell Transplantation: Report from the Committee on the Biology Underlying Recurrence of Malignant Disease following Allogeneic HSCT: Graft-versus-Tumor/Leukemia Reaction

The success of allogeneic hematopoietic stem cell transplantation (HSCT) depends on the infusion of benign stem cells as well as lymphocytes capable of participating in a graft-versus-tumor/leukemia (GVL) reaction. Clinical proof of concept is derived from studies showing increased relapse after the infusion of lymphocyte depleted hematopoietic grafts as well as the therapeutic efficacy of donor lymphocyte infusions without chemotherapy to treat relapse in some diseases. Despite this knowledge, relapse after allogeneic HSCT is common with rates approaching 40% in those with high-risk disease. In this review, we cover the basic biology and potential application to exploit adaptive T cell responses, minor histocompatibility antigens, contraction and suppression mechanisms that hinder immune responses, adaptive B cell responses and innate NK cell responses, all orchestrated in a GVL reaction. Optimal strategies to precisely balance immune responses to favor GVL without harmful graft-versus-host disease (GVHD) are needed to protect against relapse, treat persistent disease and improve disease-free survival after HSCT.

Immunization with host-type CD8{alpha}+ dendritic cells reduces experimental acute GVHD in an IL-10-dependent manner

Little is known about the role of active immunization in suppressing undesirable immune responses. Because CD8alpha(+) dendritic cells (DCs) suppress certain immune responses, we tested the hypothesis that immunization of donors with host-derived CD8alpha(+) DCs will reduce host-specific donor T-cell responses. BALB/c T cells from the animals that were immunized with B6 CD8alpha(+) DCs demonstrated, in vitro and in vivo, significantly reduced proliferation and secretion of inflammatory cytokines but showed enhanced secretion of interleukin-10 (IL-10). The responses against third-party and model antigens were preserved demonstrating antigen specificity. The in vivo relevance was further demonstrated by the reduction on graft-versus-host disease (GVHD) in both a major histocompatibility complex-mismatched clinically relevant BALB/c --> B6 model and major histocompatibility complex-matched, minor-mismatched C3H.SW --> B6 model of GVHD. Immunization of the donors that were deficient in IL-10 (IL-10(-/-)) or with CD8alpha(+) DCs from B6 class II (class II(-/-)) failed to reduce T-cell responses, demonstrating (1) a critical role for secretion of IL-10 by donor T cells and (2) a direct contact between the T cells and the CD8alpha(+) DCs. Together, these data may represent a novel strategy for reducing GVHD and suggest a broad counterintuitive role for vaccination strategies in mitigating undesirable immune responses in an antigen-specific manner.

Acute graft-versus-host disease: from the bench to the bedside

During the past decade, progress in basic immunology has been impressive. In parallel, whereas our understanding of the pathophysiology of acute graft-versus-host disease (GVHD) has greatly improved, so has our knowledge of the complexities of the immune system. Much of the immunobiology of acute GVHD has been gleaned from preclinical models and far less from correlations with clinical observations or therapeutic interventions. In this review, we summarize some of the major advances in GVHD pathophysiology, including the translation of these from the bench to the bedside, and discuss preclinical approaches that warrant further exploration in the clinic.

Selective reduction of graft-versus-host disease-mediating human T cells by ex vivo treatment with soluble Fas ligand

Previous work done in our laboratory, using mouse models, showed that soluble Fas ligand (sFasL) can efficiently delete donor anti-host T cells during their activation against irradiated host cells in MLCs. In the mouse models, this ex vivo sFasL treatment abrogated graft-vs-host disease (GVHD) while sparing donor T cells with antitumor reactivity. The present work was performed with human cells, to extend our work toward reduction of clinical GVHD. PBMC responders from a given individual (first party) were stimulated in vitro with irradiated PBMC stimulators from a second person (second party), in the presence of sFasL. In control MLCs without sFasL, alloreacting T cells began to up-regulate Fas (CD95) detectably and became sensitive to Fas-mediated apoptosis by as early as day 1-2. In MLCs containing sFasL, there were greatly reduced numbers of alloreacting CD3(+)CFSE(lo) cells, activation Ag-expressing CD4(hi) and CD8(hi) cells, IFN-gamma-producing CD4(+) and CD8(+) cells, and CD8(+)CD107a(+) CTLs. Furthermore, mice transplanted with the ex vivo sFasL/MLR-treated cells had prolonged time to fatal GVHD in an in vivo xenogeneic GVHD model. Responder cells harvested from primary MLCs containing sFasL had reduced proliferation in response to second party cells, but proliferated in response to CMV Ags, PHA, and third party cells. In addition, sFasL/MLR-treated cell populations contained influenza-specific T cells, CD4(+)FOXP3(+) T cells, and CD4(+)CD25(+) T cells. These data indicate that this ex vivo sFasL/MLR depletion of alloreacting human donor anti-host T cells was efficient and selective.

Plasmacytoid dendritic cells prime alloreactive T cells to mediate graft-versus-host disease as antigen-presenting cells

Dendritic cells (DCs) can be classified into 2 distinct subsets: conventional DCs (cDCs) and plasmacytoid DCs (pDCs). cDCs can prime antigen-specific T-cell immunity, whereas in vivo function of pDCs as antigen-presenting cells remains controversial. We evaluated the contribution of pDCs to allogeneic T-cell responses in vivo in mouse models of graft-versus-host disease (GVHD) after allogeneic hematopoietic stem cell transplantation by an add-back study of MHC-expressing pDCs into major histocompatibility complex-deficient mice that were resistant to GVHD. Alloantigen expression on pDCs alone was sufficient to prime alloreactive T cells and cause GVHD. An inflammatory environment created by host irradiation has the decisive role in maturing pDCs for T-cell priming but this process does not require Toll-like receptor signaling. Thus, functional outcomes of pDC-T-cell interactions depend on the immunologic context of encounter. To our knowledge, these results are the first to directly demonstrate an in vivo pathogenic role of pDCs as antigen-presenting cells in an antigen-specific T cell-mediated disease in the absence of other DC subsets and to provide important insight into developing strategies for tolerance induction in transplantation.

CD8+ but not CD4+ T cells require cognate interactions with target tissues to mediate GVHD across only minor H antigens, whereas both CD4+ and CD8+ T cells require direct leukemic contact to mediate GVL

Whether T-cell antigen receptors (TCR) on donor T cells require direct interactions with major histocompatibility complex class I or class II (MHCI/MHCII) molecules on target cells to mediate graft-versus-host disease (GVHD) and graft-versus-leukemia (GVL) is a fundamental question in allogeneic stem-cell transplantation (alloSCT). In MHC-mismatched mouse models, these contacts were not required for GVHD. However, this conclusion may not apply to MHC-matched, multiple minor histocompatibility antigen-mismatched alloSCT, the most common type performed clinically. To address this, we used wild-type (wt)-->MHCI-/- or wt-->MHCII-/- bone marrow chimeras as recipients in GVHD experiments. For GVL experiments, we used MHCI-/- or MHCII-/- chronic-phase CML cells created by expressing the BCR-ABL cDNA in bone marrow from MHCI-/- or MHCII-/- mice. TCR/MHCI contact was obligatory for both CD8-mediated GVHD and GVL. In contrast, CD4 cells induced GVHD in wt-->MHCII-/- chimeras, whereas MHCII-/- mCP-CML was GVL-resistant. Donor CD4 cells infiltrated affected skin and bowel in wt-->MHCII-/- recipients, indicating that they mediated GVHD by acting locally. Thus, CD4 cells use distinct effector mechanisms in GVHD and GVL: direct cytolytic action is required for GVL but not for GVHD. If these noncytolytic pathways can be inhibited, then GVHD might be ameliorated while preserving GVL.

Organ-derived dendritic cells have differential effects on alloreactive T cells

Dendritic cells (DCs) are considered critical for the induction of graft-versus-host disease (GVHD) after bone marrow transplantation (BMT). In addition to their priming function, dendritic cells have been shown to induce organ-tropism through induction of specific homing molecules on T cells. Using adoptive transfer of CFSE-labeled cells, we first demonstrated that alloreactive T cells differentially up-regulate specific homing molecules in vivo. Host-type dendritic cells from the GVHD target organs liver and spleen or skin- and gut-draining lymph nodes effectively primed naive allogeneic T cells in vitro with the exception of liver-derived dendritic cells, which showed less stimulatory capacity. Gut-derived dendritic cells induced alloreactive donor T cells with a gut-homing phenotype that caused increased GVHD mortality and morbidity compared with T cells stimulated with dendritic cells from spleen, liver, and peripheral lymph nodes in an MHC-mismatched murine BMT model. However, in vivo analysis demonstrated that the in vitro imprinting of homing molecules on alloreactive T cells was only transient. In conclusion, organ-derived dendritic cells can efficiently induce specific homing molecules on alloreactive T cells. A gut-homing phenotype correlates with increased GVHD mortality and morbidity after murine BMT, underlining the importance of the gut in the pathophysiology of GVHD.

Target cells in graft-versus-host disease: implications for cancer therapy

Acute graft-versus-host disease (GVHD) conceptually may be divided into three evolutionary stages: allostimulation, effector cell homing to specific tissues, and cellular targeting and injury. Surprisingly, little is known regarding the targeting stage of GVHD. Recently, we have learned that epithelial target cell injury is mediated by specific subpopulations of effector T cells that may be identified based on Vbeta family expansion during allostimulation. Antibody probes specific for these Vbeta families have permitted precise identification of effector cell homing patterns. In squamous epithelium, allospecific T cells selectively home to basal cell layer subpopulations that express cytokeratin 15 (CK15) and that undergo target cell injury via apoptosis. Interestingly, these target cells coincide with basal layer subpopulations that have properties of epithelial stem cells and that normally express an apoptosis-resistant genomic profile. Accordingly, epithelial cell injury in GVHD appears to involve selective targeting of stem-cell subpopulations via conversion from an anti-apoptotic to a pro-apoptotic phenotype. Understanding of the mechanism(s) of this conversion could facilitate development of translationally relevant approaches to shielding target cells from injury in GVHD. Moreover, determination of how putative apoptosis-resistant stem cells may be rendered vulnerable to immune-mediated targeting has implications potentially relevant to more directed immunotherapeutic approaches focused at elimination of neoplastic (cancer) stem cells.

DC homeostasis in hematopoietic stem cell transplantation

Hematopoietic stem cell transplantation is an important experimental tool and therapeutic modality. Its efficacy and toxicity are both linked to a GvH reaction that is initiated by donor T cells recognizing recipient APC, of which DC are the most potent. In most tissues recipient DC are replaced after transplantation because they turnover rapidly from BM-derived precursors. However, in a number of sites, notably the skin, recipient DC may persist and even self-renew for many months after transplantation. Understanding the homeostasis of different APC populations and how they are related to the induction of alloreactivity may help to improve the therapeutic benefit of transplantation.

Pathophysiology of acute graft-versus-host disease: recent advances

Allogeneic hematopoietic stem cell transplantation (HSCT) is a potentially curative therapy for many malignant and nonmalignant hematologic diseases. Donor T cells from the allografts are critical for the success of this effective therapy. Unfortunately these T cells not only recognize and attack the disease cells/tissues but also the other normal tissues of the recipient as "foreign" or "nonself" and cause severe, immune-mediated toxicity, graft-versus-host disease (GVHD). Several insights into the complex pathophysiology of GVHD have been gained from recent experimental observations, which show that acute GVHD is a consequence of interactions between both the donor and the host innate and adaptive immune systems. These insights have identified a role for a variety of cytokines, chemokines, novel T-cell subsets (naĩve, memory, regulatory, and NKT cells) and for non-T cells of both the donor and the host (antigen presenting cells, delta T cells, B cells, and NK cells) in modulating the induction, severity, and maintenance of acute GVHD. This review will focus on the immunobiology of experimental acute GVHD with an emphasis on the recent observations.

Graft-versus-host disease

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

The role of antigen-presenting cells in triggering graft-versus-host disease and graft-versus-leukemia

After allogeneic blood or bone marrow transplantation, donor T cells interact with a distorted antigen-presenting cell (APC) environment in which some, but not all, host APCs are replaced by APCs from the donor. Significantly, host APCs are required for the priming of acute graft-versus-host disease (GVHD). Donor APCs play a lesser role in the induction of acute GVHD despite their predicted capacity to cross-present host antigens. In contrast, donor APCs may play a role in perpetuating the tissue injury observed in chronic GVHD. Host APCs are also required for maximal graft-versus-leukemia responses. Recent studies have suggested potential strategies by which the continued presence of host APCs can be exploited to prime strong donor immunity to tumors without the induction of GVHD.

Expression of CD27 on Peripheral CD4 + T-Lymphocytes Correlates with the Development of Severe Acute Graft-versus-Host Disease after Allogeneic Bone Marrow Transplantation

Allogeneic immune responses during hematopoietic reconstitution play central roles in beneficial and adverse effects after allogeneic bone marrow transplantation (allo-BMT). Appropriate regulation of the immune responses might improve the outcome of allo-BMT. However, a useful marker for monitoring allogeneic immune responses remains to be established. We enrolled 22 consecutive patients who underwent myeloablative allo-BMT between March 2002 and March 2006 and examined the relationship between CD27 expression on peripheral blood T-lymphocytes, a possible marker for naive/effector phenotypes, and clinical events, especially acute graft-versus-host disease (aGVHD). In 8 patients with aGVHD of grades II to IV, the CD27+/CD27- ratios of CD4+ (but not CD8+) T-lymphocytes were significantly higher after allo-BMT, even at day 21, than the ratios in patients with aGVHD of grade 0 or I and remained high after day 21. In contrast, the ratios were low after day 21 following allo-BMT in 14 patients with aGVHD of grade 0 or I. Moreover, the clinical analysis suggested a relationship between the ratio and aGVHD grade. Thus, we showed that the CD27+/CD27- ratio in CD4+ T-lymphocytes may have value in predicting the development of severe aGVHD and may correlate with clinical symptoms of aGVHD.

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.

Evidence that donor intrinsic response to G-CSF is the best predictor of acute graft-vs-host disease following allogeneic peripheral blood stem cell transplantation

OBJECTIVE

Risk factors of acute graft-vs-host disease (GVHD) following allogeneic bone marrow transplantation have been well described before. In this study, we tested the hypothesis that acute GVHD after allogeneic peripheral blood stem cell (PBSC) transplant might be associated with donors' responsiveness to granulocyte colony-stimulating factor (G-CSF), rather than the dose of CD34(+) cells infused.

PATIENTS AND METHODS

We retrospectively analyzed mobilization and transplant data (demographic characteristics, donor blood cell subsets after G-CSF, graft composition) in 149 consecutive HLA-identical donor/recipient pairs in order to identify acute GVHD risk factors.

RESULTS

In 25% of donors, G-CSF mobilization led to an outstanding response, defined as greater than 117 x 10(6) CD34(+) cells/L. Overall, incidence of grades II-IV acute GVHD was 20.1% (95% CI: 16.6-23.6). Following univariate analysis, the incidence increased significantly in recipients receiving greater than 10 x 10(6) CD34(+) cells/kg (35% vs 15%; p = 0.007), and those transplanted from outstanding mobilizers (41% vs 12%, p < 0.0001). In multivariate analysis, only transplantation from outstanding mobilizers remained significant (p = 0.02). Donor or recipient demographic characteristics and lymphocyte subsets reinfused in the graft had no impact.

CONCLUSIONS

We demonstrate for the first time that donor responsiveness to G-CSF is associated with acute GVHD following PBSC transplantation. If confirmed, this correlation will help to identify recipients who could potentially benefit from improved prophylaxis. As further corollary, decreasing the dose of CD34(+) cells infused is unlikely to prevent acute GVHD. Future studies should focus on the molecular bases of interindividual discrepancies in response to G-CSF.

A crucial role for antigen-presenting cells and alloantigen expression in graft-versus-leukemia responses

Graft-versus-leukemia (GVL) response after allogeneic bone marrow transplantation (BMT) represents one of the most potent forms of immunotherapy against malignant diseases. Antigen-presenting cells (APCs) are crucial for the induction of graft-versus-host disease (GVHD), the most serious complication of allogeneic BMT, but their role in GVL responses is unclear. Using a series of clinically relevant mouse GVL tumor models, we found that APCs and alloantigen expression on tumors are crucial for GVL. Moreover, APCs of host origin predominated in GVL responses although donor APCs contributed as the acuity of tumor burden decreased.

T cell immunity and graft-versus-host disease (GVHD)

Graft-versus-host disease (GVHD), induced by the reaction of donor T cells to recipient histoincompatible antigens, is a serious complication of allogeneic bone marrow transplantation (BMT), resulting in considerable morbidity and mortality. In MHC-disparate BMT, donor T cells directly react with major histocompatibility complex (MHC) antigens, while in MHC-matched BMT, T cells react with minor histocompatibility antigens (miHA) presented by shared MHC molecules. Clinically, acute and chronic GVHD can be distinguished on the basis of the time of onset, clinical manifestations and distinct pathobiological mechanisms. Acute GVHD usually occurs within 2 to 6 weeks following allogeneic BMT and primarily affects the skin, liver and the gastrointestinal tract with T cell infiltration of the epithelia of the skin, mucous membranes, bile ducts and gut. In addition, hair follicle cells, airways, bone marrow, and a variety of other tissue systems can be involved. Acute GVHD occurs in up to 50% of allogeneic HLA-matched and 70% of HLA-disparate BMT recipients despite prophylactic immunosuppressive drugs. Chronic GVHD involves a wider range of organs and clinical manifestations include scleroderma, liver failure, immune complex disease, glomerulonephritis, and autoantibody formation.

Distinct roles for donor- and host-derived antigen-presenting cells and costimulatory molecules in murine chronic graft-versus-host disease: requirements depend on target organ

The application of allogeneic stem cell transplantation (alloSCT) is limited by graft-versus-host disease (GVHD). GVHD can be divided into acute and chronic forms that likely have different requirements for initiation and pathogenesis mechanisms. In prior studies we demonstrated that residual host antigen-presenting cells (APCs) were required to initiate acute GVHD (aGVHD) mediated by CD8 T cells. In contrast, here we demonstrate that either donor or host APCs can initiate CD4-mediated GVHD in a model that has features of chronic GVHD (cGVHD). Both donor and host APCs must provide CD80/86-dependent costimulation to elicit maximal cGVHD, and there is no GVHD when both donor and host lack CD80/86. Finally, we were surprised to find that, although either donor or host APCs are sufficient to stimulate skin cGVHD, donor APCs play a dominant role in intestinal cGVHD. Both CD40 and CD80/86 are critical for donor APC function in intestinal cGVHD, but only CD80/86 is required for skin cGVHD. Thus, there are target-tissue–specific differences in APC requirements. These results identify differences in APC requirements between CD8-mediated aGVHD and CD4-mediated cGVHD. They further highlight donor APCs as additional targets for GVHD therapy.

Minors come of age: minor histocompatibility antigens and graft-versus-host disease

Minor histocompatibility antigens (miHA) are responsible for the occurrence of graft-versus-host disease in the setting of a major histocompatibility complex matched sibling allogeneic stem cell transplantation. These miHA are peptide fragments that are associated with major histocompatibility complex class I or class II antigens. Elegant experiments have led to the molecular characterization of these antigens. Efforts to prevent graft-versus-host disease could be targeted through this pathway by matching for these miHA or by preventing antigen recognition. Alternatively, these miHA could be exploited as targets for a more potent graft-versus-malignancy effect. This area of miHA promises to continue to be an exciting area of continued research.