Interleukin-23 secretion by donor antigen-presenting cells is critical for organ-specific pathology in graft-versus-host disease

Pharmacological updates of nifuroxazide: Promising preclinical effects and the underlying molecular mechanisms

Nifuroxazide (NFX) is a safe nitrofuran antibacterial drug used clinically to treat acute diarrhea and infectious traveler diarrhea or colitis. Recent studies revealed that NFX displays multiple pharmacological effects, including anticancer, antioxidant, and anti-inflammatory effects. NFX has potential roles in inhibiting thyroid, breast, lung, bladder, liver, and colon cancers and osteosarcoma, melanoma, and others mediated by suppressing STAT3 as well as ALDH1, MMP2, MMP9, Bcl2 and upregulating Bax. Moreover, it has promising effects against sepsis-induced organ injury, hepatic disorders, diabetic nephropathy, ulcerative colitis, and immune disorders. These promising effects appear to be mediated by suppressing STAT3 as well as NF-κB, TLR4, and β-catenin expressions and effectively decreasing downstream cytokines TNF-α, IL-1β, and IL-6. Our review summarizes the available studies on the molecular biological mechanisms of NFX in cancer and other diseases and it is recommended to translate the studies in experimental animals and cultured cells and repurpose NFX in various diseases for scientific evidence based on human studies.

The Development of Systemic Inflammatory Diseases in Hidradenitis Suppurativa

It is understood that the skin is a peripheral lymphoid tissue that defends against external environmental stimuli. Continuous activation from these factors, on the other hand, promotes persistent inflammation at the local location and, occasionally, tissue damage. Hidradenitis suppurativa (HS) is a typical inflammatory skin disease and becomes a source of numerous inflammatory cytokines due to the chronic intractable repeated inflamed tissues. Because inflammatory cells and cytokines circulate throughout the body from the inflamed organ, it has been hypothesized that HS-mediated skin inflammation impacts the systemic functioning of numerous organs. Recent updates to clinical and experimental investigations revealed that HS has a significant connection with systemic inflammatory disorders. We provide the details and comprehensive molecular mechanisms associated with systemic inflammatory illnesses due to HS.

Leukemia inhibitory factor protects against graft-versus-host disease while preserving graft-versus-leukemia activity

Graft-versus-host disease (GVHD) remains a major complication after allogeneic hematopoietic stem cell transplantation, a widely used therapy for hematologic malignancies and blood disorders. Here, we report an unexpected role of cytokine leukemia inhibitory factor (LIF) in protecting against GVHD development. Administrating recombinant LIF protein (rLIF) protects mice from GVHD-induced tissue damage and lethality without compromising the graft-versus-leukemia activity, which is crucial to prevent tumor relapse. We found that rLIF decreases the infiltration and activation of donor immune cells and protects intestinal stem cells to ameliorate GVHD. Mechanistically, rLIF downregulates IL-12-p40 expression in recipient dendritic cells after irradiation through activating STAT1 signaling, which results in decreased major histocompatibility complex II levels on intestinal epithelial cells and decreased donor T-cell activation and infiltration. This study reveals a previously unidentified protective role of LIF for GVHD-induced tissue pathology and provides a potential effective therapeutic strategy to limit tissue pathology without compromising antileukemic efficacy.

Risk Factors of Graft-Versus-Host Disease in the Iranian Allogeneic Hematopoietic Stem Cell Transplantation: A 10-Year Experience

Background: Graft-versus-host disease (GVHD) is a serious complication associated with allogeneic hematopoietic stem cell transplantation (allo-HSCT). Thus, it is necessary to evaluate the risk factors of GVHD in allo-HSCT. Herein, we studied the effects of some risk factors on GVHD incidence in patients with allo-HSCT. Methods: We retrospectively evaluated the GVHD incidences and risk factors in 199 patients diagnosed with hematological disorders who underwent allo-HSCT in Taleghani hospital, Tehran, Iran, between 2007 and 2017. The univariable and multivariable analyses of time to event data were performed using the Logistic regression model. Computations were performed using SAS, and the level of statistical significance for univariable and multivariable analyses was set at 20% and 10%, respectively. Results: Acute GVHD (aGVHD) was seen in 59 (29.6%) patients, and 18 (9%) patients developed chronic GVHD (cGVHD). The odds of GVHD incidence in male to female transplants was 3.49 times greater than the male-to-male transplantations (CI, 1.16, 11.5; p<0.001). The patients with body mass index (BMI) below 18.5 had 96% lower odds of GVHD incidence compared with those with BMI above 30 (CI, 0.007-0.27; p=0.013). The odds of GVHD incidence in patients who were negative for cytomegalovirus (CMV) antigen was 76% lower than patients with positive CMV antigen (CI, 0.06-0.93; p=0.081). Conclusion: In a nutshell, our results indicated that the donor-recipient gender disparity, the recipient's BMI, and CMV infection/reactivation status might be pivotal risk factors, which should be taken into account for prevention and management of GVHD.

Translational Clinical Strategies for the Prevention of Gastrointestinal Tract Graft Versus Host Disease

Graft versus host disease (GVHD) is the major non-relapse complication associated with allogeneic hematopoietic stem cell transplantation (HSCT). Unfortunately, GVHD occurs in roughly half of patients following this therapy and can induce severe life-threatening side effects and premature mortality. The pathophysiology of GVHD is driven by alloreactive donor T cells that induce a proinflammatory environment to cause pathological damage in the skin, gastrointestinal (GI) tract, lung, and liver during the acute phase of this disease. Recent work has demonstrated that the GI tract is a pivotal target organ and a primary driver of morbidity and mortality in patients. Prevention of this complication has therefore emerged as an important goal of prophylaxis strategies given the primacy of this tissue site in GVHD pathophysiology. In this review, we summarize foundational pre-clinical studies that have been conducted in animal models to prevent GI tract GVHD and examine the efficacy of these approaches upon subsequent translation into the clinic. Specifically, we focus on therapies designed to block inflammatory cytokine pathways, inhibit cellular trafficking of alloreactive donor T cells to the GI tract, and reconstitute impaired regulatory networks for the prevention of GVHD in the GI tract.

T Cell Subsets in Graft Versus Host Disease and Graft Versus Tumor

Allogeneic hematopoietic cell transplantation (allo-HCT) is an essential therapeutic modality for patients with hematological malignancies and other blood disorders. Unfortunately, acute graft-versus-host disease (aGVHD) remains a major source of morbidity and mortality following allo-HCT, which limits its use in a broader spectrum of patients. Chronic graft-versus-host disease (cGVHD) also remains the most common long-term complication of allo-HCT, occurring in reportedly 30-70% of patients surviving more than 100 days. Chronic GVHD is also the leading cause of non-relapse mortality (NRM) occurring more than 2 years after HCT for malignant disease. Graft versus tumor (GVT) is a major component of the overall beneficial effects of allogeneic HCT in the treatment of hematological malignancies. Better understanding of GVHD pathogenesis is important to identify new therapeutic targets for GVHD prevention and therapy. Emerging data suggest opposing roles for different T cell subsets, e.g., IFN-γ producing CD4+ and CD8+ T cells (Th1 and Tc1), IL-4 producing T cells (Th2 and Tc2), IL-17 producing T cells (Th17 and Tc17), IL-9 producing T cells (Th9 and Tc9), IL-22 producing T cells (Th22), T follicular helper cells (Tfh), regulatory T-cells (Treg) and tissue resident memory T cells (Trm) in GVHD and GVT etiology. In this review, we first summarize the general description of the cytokine signals that promote the differentiation of T cell subsets and the roles of these T cell subsets in the pathogenesis of GVHD. Next, we extensively explore preclinical findings of T cell subsets in both GVHD/GVT animal models and humans. Finally, we address recent findings about the roles of T-cell subsets in clinical GVHD and current strategies to modulate T-cell differentiation for treating and preventing GVHD in patients. Further exploring and outlining the immune biology of T-cell differentiation in GVHD that will provide more therapeutic options for maintaining success of allo-HCT.

Retinoic acid-responsive CD8 effector T cells are selectively increased in IL-23-rich tissue in gastrointestinal GVHD

Gastrointestinal (GI) graft-versus-host disease (GVHD) is a major barrier in allogeneic hematopoietic stem cell transplantation (allo-HSCT). The metabolite retinoic acid (RA) potentiates GI-GVHD in mice via alloreactive T cells expressing the RA receptor-α (RARα), but the role of RA-responsive cells in human GI-GVHD remains undefined. Therefore, we used conventional and novel sequential immunostaining and flow cytometry to scrutinize RA-responsive T cells in tissues and blood of patients who had received allo-HSCT and to characterize the impact of RA on human T-cell alloresponses. Expression of RARα by human mononuclear cells was increased after exposure to RA. RARαhi mononuclear cells were increased in GI-GVHD tissue, contained more cellular RA-binding proteins, localized with tissue damage, and correlated with GVHD severity and mortality. By using a targeted candidate protein approach, we predicted the phenotype of RA-responsive T cells in the context of increased microenvironmental interleukin-23 (IL-23). Sequential immunostaining confirmed the presence of a population of RARαhi CD8 T cells with the predicted phenotype that coexpressed the effector T-cell transcription factor T-bet and the IL-23-specific receptor (IL-23R). These cells were increased in GI- but not skin-GVHD tissues and were also selectively expanded in the blood of patients with GI-GVHD. Finally, functional approaches demonstrated that RA predominantly increased alloreactive GI-tropic RARαhi CD8 effector T cells, including cells with the phenotype identified in vivo. IL-23-rich conditions potentiated this effect by selectively increasing β7 integrin expression on CD8 effector T cells and reducing CD4 T cells with a regulatory cell phenotype. In summary, we have identified a population of RA-responsive effector T cells with a distinctive phenotype that is selectively expanded in human GI-GVHD and that represents a potential new therapeutic target.

Novel antibodies that selectively block mouse IL-12 enable the re-evaluation of the role of IL-12 in immune protection and pathology

The dimeric cytokine IL-12 is important in the control of various infections but also contributes to the pathology of certain diseases making it a potential target for therapy. However, its specific inhibition with antibodies is complicated by the fact that its two subunits are present in other cytokines: p40 in IL-23 and p35 in IL-35. This has led to erroneous conclusions like the alleged implication of IL-12 in experimental autoimmune encephalomyelitis (EAE). Here, we report the development of a mouse anti-mouse IL-12 vaccine and the production of monoclonal antibodies (mAbs) that do not react with p40 or p35 (in IL-35) but specifically recognize and functionally inhibit the IL-12 heterodimer. Using one of these mAbs, MM12A1.6, that strongly inhibited IFN-γ production and LPS-induced septic shock after viral infection, we demonstrate the critical role played by IL-12 in the rejection of male skin graft by female C57BL/6 syngeneic recipients and in the clearance of an immunogenic mastocytoma tumor variant by DBA/2 mice, but not in a parent to F1 immune aggression model nor in MOG-induced EAE, which was clearly prevented by anti-p40 mAb C17.8. Given this selective inhibition of IL-12, these mAbs provide new options for reassessing IL-12 function in vivo.

Pathogenic Bhlhe40+ GM-CSF+ CD4+ T cells promote indirect alloantigen presentation in the GI tract during GVHD

Gastrointestinal (GI) tract involvement is the major cause of morbidity and mortality in acute graft-versus-host disease (GVHD), and pathological damage is largely attributable to inflammatory cytokine production. Recently, granulocyte-macrophage colony stimulating factor (GM-CSF) has been identified as a cytokine that mediates inflammation in the GI tract, but the transcriptional program that governs GM-CSF production and the mechanism by which GM-CSF links adaptive to innate immunity within this tissue site have not been defined. In the current study, we identified Bhlhe40 as a key transcriptional regulator that governs GM-CSF production by CD4+ T cells and mediates pathological damage in the GI tract during GVHD. In addition, we observed that GM-CSF was not regulated by either interleukin 6 (IL-6) or IL-23, which are both potent inducers of GVHD-induced colonic pathology, indicating that GM-CSF constitutes a nonredundant inflammatory pathway in the GI tract. Mechanistically, GM-CSF had no adverse effect on regulatory T-cell reconstitution, but linked adaptive to innate immunity by enhancing the activation of donor-derived dendritic cells in the colon and subsequent accumulation of these cells in the mLNs. In addition, GM-CSF promoted indirect alloantigen presentation, resulting in the accumulation of donor-derived T cells with a proinflammatory cytokine phenotype in the colon. Thus, Bhlhe40+ GM-CSF+ CD4+ T cells constitute a colitogenic T-cell population that promotes indirect alloantigen presentation and pathological damage within the GI tract, positioning GM-CSF as a key regulator of GVHD in the colon and a potential therapeutic target for amelioration of this disease.

The IL-12 Cytokine and Receptor Family in Graft-vs.-Host Disease

Allogeneic hematopoietic cell transplantation (allo-HCT) is performed with curative intent for high- risk blood cancers and bone marrow failure syndromes; yet the development of acute and chronic graft-vs.-host disease (GVHD) remain preeminent causes of death and morbidity. The IL-12 family of cytokines is comprised of IL-12, IL-23, IL-27, IL-35, and IL-39. This family of cytokines is biologically distinct in that they are composed of functional heterodimers, which bind to cognate heterodimeric receptor chains expressed on T cells. Of these, IL-12 and IL-23 share a common β cytokine subunit, p40, as well as a receptor chain: IL-12Rβ1. IL-12 and IL-23 have been documented as proinflammatory mediators of GVHD, responsible for T helper 1 (Th1) differentiation and T helper 17 (Th17) stabilization, respectively. The role of IL-27 is less defined, seemingly immune suppressive via IL-10 secretion by Type 1 regulatory (Tr1) cells yet promoting inflammation through impairing CD4⁺ T regulatory (Treg) development and/or enhancing Th1 differentiation. More recently, IL-35 was described as a potent anti-inflammatory agent produced by regulatory B and T cells. The role of the newest member, IL-39, has been implicated in proinflammatory B cell responses but has not been explored in the context of allo-HCT. This review is directed at discussing the current literature relevant to each IL-12-family cytokine and cognate receptor engagement, as well as the consequential downstream signaling implications, during GVHD pathogenesis. Additionally, we will provide an overview of translational strategies targeting the IL-12 family cytokines, their receptors, and subsequent signal transduction to control GVHD.

Dendritic Cell Expression of Retinal Aldehyde Dehydrogenase-2 Controls Graft-versus-Host Disease Lethality

Recent studies have underscored the critical role of retinoic acid (RA) in the development of lineage-committed CD4 and CD8 T cells in vivo. We have shown that under acute graft-versus-host disease (GVHD) inflammatory conditions, RA is upregulated in the intestine and is proinflammatory, as GVHD lethality was attenuated when donor allogeneic T cells selectively expressed a dominant negative RA receptor α that blunted RA signaling. RA can function in an autocrine and paracrine fashion, and as such, the host cell lineage responsible for the production of RA metabolism and the specific RA-metabolizing enzymes that potentiate GVHD severity are unknown. In this study, we demonstrate that enhancing RA degradation in the host and to a lesser extent donor hematopoietic cells by overexpressing the RA-catabolizing enzyme CYP26A1 reduced GVHD. RA production is facilitated by retinaldehyde isoform-2 (RALDH2) preferentially expressed in dendritic cells (DCs). Conditionally deleted RA-synthesizing enzyme RALDH2 in host or to a lesser extent donor DCs reduced GVHD lethality. Improved survival in recipients with RALDH2-deleted DCs was associated with increased T cell death, impaired T effector function, increased regulatory T cell frequency, and augmented coinhibitory molecule expression on donor CD4⁺ T cells. In contrast, retinaldehydrogenase isoform-1 (RALDH1) is dominantly expressed in intestinal epithelial cells. Unexpectedly, conditional host intestinal epithelial cells RALDH1 deletion failed to reduce GVHD. These data demonstrate the critical role of both donor and especially host RALDH2⁺ DCs in driving murine GVHD and suggest RALDH2 inhibition or CYP26A1 induction as novel therapeutic strategies to prevent GVHD.

Inflammatory Cytokine Networks in Gastrointestinal Tract Graft vs. Host Disease

Graft vs. host disease (GVHD) is the major non-relapse complication associated with allogeneic hematopoietic stem cell transplantation (HSCT). Damage to the gastrointestinal (GI) tract from acute GVHD is a particularly serious event that can result in significant morbidity and mortality. Proinflammatory cytokines play a critical role in the pathophysiology of intestinal GVHD, in part by activating donor T cell populations which subsequently induce tissue damage. In this review, we summarize pre-clinical data derived from experimental murine models that have examined the role of inflammatory cytokine pathways that play critical roles in the pathophysiology of GVHD of the GI tract. Specific areas of focus are on STAT 3-dependent cytokines (e.g., IL-6, IL-23, and IL-21), and members of the IL-1 cytokine family, both of which have been shown to induce pathological damage within the GI tract during this disease. We also review established and ongoing efforts to translate these pre-clinical findings into the clinic in an effort to reduce morbidity and mortality due to this complication.

The TLR7 ligand R848 prevents mouse graft-versus-host disease and cooperates with anti-interleukin-27 antibody for maximal protection and regulatory T-cell upregulation

In spite of considerable therapeutic progress, acute graft-versus-host disease still limits allogeneic hematopoietic cell transplantation. We recently reported that mouse infection with nidovirus lactate dehydrogenase elevating virus impairs disease in non-conditioned B6D2F1 recipients of parental B6 spleen cells. As this virus activates TLR7, we tested a pharmacological TLR7 ligand, R848, in this model and observed complete survival if donor and recipients were treated before transplantation. Mixed lymphocyte culture performed 48 h after R848-treatment of normal mice demonstrated that both T-cell allo-responsiveness and antigen presentation by CD11b⁺ and CD8α⁺ dendritic cells were inhibited. These inhibitions were dependent on IFNAR-1 signaling. In the B6 to B6D2F1 transplantation model, R848 decelerated, but did not abrogate, donor T-cell implantation and activation. However, it decreased interferon-gamma, tumor necrosis factor-alpha and interleukin-27 while upregulating active transforming growth factor-beta 1 plasma levels. In addition, donor and recipient Foxp3⁺ regulatory T-cell numbers were increased in recipient mice and their elimination compromised disease prevention. R848 also strongly improved survival of lethally irradiated BALB/c recipients of B6 hematopoietic cells and this also correlated with an upregulation of CD4 and CD8 Foxp3⁺ regulatory T cells that could be further increased by inhibition of interleukin-27. The combination of anti-interleukin-27p28 mono -clonal antibody and R848 showed strong synergy in preventing disease in the B6 to B6D2F1 transplantation model when recipients were sublethally irradiated and this also correlated with upregulation of regulatory T cells. We conclude that R848 modulates multiple aspects of graft-versus-host disease and offers potential for safe allogeneic bone marrow transplantation that can be further optimized by inhibition of interleukin-27.

Novel Concept of CD4-Mediated Activation of Regulatory T Cells for the Treatment of Graft-Versus-Host Disease

Allogeneic hematopoietic stem cell transplantation is the only curative treatment option for several hematological malignancies and immune deficiency syndromes. Nevertheless, the development of a graft-versus-host disease (GvHD) after transplantation is a high risk and a severe complication with high morbidity and mortality causing therapeutic challenges. Current pharmacological therapies of GvHD lead to generalized immunosuppression followed by severe adverse side effects including infections and relapse of leukemia. Several novel cell-based immunomodulatory strategies for treatment or prevention of GvHD have been developed. Herein, thymus-derived regulatory T cells (tTreg), essential for the maintenance of peripheral immunologic tolerance, are in the focus of investigation. However, due to the limited number of tTreg in the peripheral blood, a complex, time- and cost-intensive in vitro expansion protocol is necessary for the production of an efficient cellular therapeutic. We demonstrated that activation of tTreg using the CD4-binding human immunodeficiency virus-1 protein gp120 leads to a substantially increased suppressor activity of tTreg without the need for additional expansion. Gp120-activated tTreg prevent GvHD development in a preclinical humanized mouse model. In addition, gp120 is not only effective in prevention but also in therapy of GvHD by suppressing all clinical symptoms and improving survival of treated mice. These data indicate that tTreg activation by gp120 is a feasible and potent strategy for significant functional improvement of tTreg as cellular therapeutic for GvHD treatment without the need of complicated, time-intensive, and expensive in vitro expansion of isolated tTreg.

STAT3 Expression in Host Myeloid Cells Controls Graft-versus-Host Disease Severity

Professional antigen-presenting cells (APCs) are important modulators of acute graft-versus-host disease (GVHD). Although dendritic cells (DCs) are the most potent APC subset, other myeloid cells, especially macrophages (MFs) and neutrophils, recently have been shown to play a role in the severity of GVHD. The critical molecular mechanisms that determine the functions of myeloid cells in GVHD are unclear, however. Signal transducer and activator of transcription 3 (STAT3) is a master transcription factor that plays a crucial role in regulating immunity, but its role in MF biology and in acute GVHD remains unknown. To determine the impact of myeloid cell-specific expression of STAT3 on the severity of acute GVHD, we used myeloid cell-specific STAT3-deficient LysM-Cre/STAT3^fl/- animals as recipients and donors in well-characterized experimental models of acute GVHD. We found that reduced expression of STAT3 in myeloid cells from the hosts, but not the donors, increased inflammation, increased donor T cell activation, and exacerbated GVHD. Our data demonstrate that STAT3 in host myeloid cells, such as MFs, dampens acute GVHD.

Altered homeostatic regulation of innate and adaptive immunity in lower gastrointestinal tract GVHD pathogenesis

Lower gastrointestinal (GI) tract graft-versus-host disease (GVHD) is the predominant cause of morbidity and mortality from GVHD after allogeneic stem cell transplantation. Recent data indicate that lower GI tract GVHD is a complicated process mediated by donor/host antigenic disparities. This process is exacerbated by significant changes to the microbiome, and innate and adaptive immune responses that are critical to the induction of disease, persistence of inflammation, and a lack of response to therapy. Here, we discuss new insights into the biology of lower GI tract GVHD and focus on intrinsic pathways and regulatory mechanisms crucial to normal intestinal function. We then describe multiple instances in which these homeostatic mechanisms are altered by donor T cells or conditioning therapy, resulting in exacerbation of GVHD. We also discuss data suggesting that some of these mechanisms produce biomarkers that could be informative as to the severity of GVHD and its response to therapy. Finally, novel therapies that might restore homeostasis in the GI tract during GVHD are highlighted.

Combined nifuroxazide and SAT05f therapy reduces graft-versus-host disease after experimental allogeneic bone marrow transplantation

Acute graft-versus-host disease (aGvHD) is the major barrier to the broader use of allogenetic hematopoietic stem cells. However, currently these are no highly specific and efficient drugs. Monotherapy is not sufficient and more efficient and safe therapeutic regimen are urgent need. Studies demonstrated TLR9 and Stat3 signal pathways are critical for antigen-presenting cell maturation and T-cell activation, which are important mediators in aGvHD. Specific block these two critical signal pathways using their inhibitors SAT05f and nifuroxazide may be the novel strategies for aGvHD therapy. The results showed combined therapy significantly decreased the severity of aGvHD and prolonged the survival rate. Furthermore, after treatment, the activation of CD4⁺ effect T cells was reduced, whereas Treg cells was increased, and the cytokine release was inhibited. In conclusion, combined therapy of nifuroxazide with SAT05f may be potential for the prevention or treatment of aGvHD, providing theoretic and experimental basis.

DLL4+ dendritic cells: Key regulators of Notch Signaling in effector T cell responses

Dendritic cells (DCs) are critical regulators of adaptive immune responses. DCs can elicit primary T cell responses at low DC:T cell ratios through their expression of high levels of antigen-presenting molecules and costimulatory molecules. DCs are important for induction of functionally diverse T cell subsets such as CD4⁺ T helper (Th)1 and Th17 cells and effector CD8⁺ T cells able to reside in epithelial tissues. Recent studies begin illuminating the underlying mechanism by which DCs regulate specialized T cell subsets. DCs are composed of subsets that differ in their phenotype, localization and function. DCs expressing high levels of DLL4 (DLL4⁺ DCs), which is a member of Notch ligand family, are newly discovered cells that have greater ability than DLL4^- DCs to promote the generation of Th1 and Th17 CD4⁺ T cells. DLL4 derived from DLL4⁺ DCs is also important for promoting the differentiation and expansion of effector CD8⁺ T cells. Experimental studies have demonstrated that selective deletion of DLL4 in DCs causes impaired antitumor immunity. In contrast, blocking DLL4 leads to dramatic reduction of inflammatory T cell responses and their-mediated tissue damage. We will discuss emerging functional specialization within the DLL4⁺ DC compartment, DLL4⁺ DC biology and the impact of pharmacological modulation of DLL4 to control inflammatory disorders.

A colitogenic memory CD4+ T cell population mediates gastrointestinal graft-versus-host disease

Damage to the gastrointestinal tract is a major cause of morbidity and mortality in graft-versus-host disease (GVHD) and is attributable to T cell-mediated inflammation. In this work, we identified a unique CD4+ T cell population that constitutively expresses the β2 integrin CD11c and displays a biased central memory phenotype and memory T cell transcriptional profile, innate-like properties, and increased expression of the gut-homing molecules α4β7 and CCR9. Using several complementary murine GVHD models, we determined that adoptive transfer and early accumulation of β2 integrin-expressing CD4+ T cells in the gastrointestinal tract initiated Th1-mediated proinflammatory cytokine production, augmented pathological damage in the colon, and increased mortality. The pathogenic effect of this CD4+ T cell population critically depended on coexpression of the IL-23 receptor, which was required for maximal inflammatory effects. Non-Foxp3-expressing CD4+ T cells produced IL-10, which regulated colonic inflammation and attenuated lethality in the absence of functional CD4+Foxp3+ T cells. Thus, the coordinate expression of CD11c and the IL-23 receptor defines an IL-10-regulated, colitogenic memory CD4+ T cell subset that is poised to initiate inflammation when there is loss of tolerance and breakdown of mucosal barriers.

Blockade of interleukin-27 signaling reduces GVHD in mice by augmenting Treg reconstitution and stabilizing Foxp3 expression

Reestablishment of competent regulatory pathways has emerged as a strategy to reduce the severity of graft-versus-host disease (GVHD), and recalibrate the effector and regulatory arms of the immune system. However, clinically feasible, cost-effective strategies that do not require extensive ex vivo cellular manipulation have remained elusive. In the current study, we demonstrate that inhibition of the interleukin-27p28 (IL-27p28) signaling pathway through antibody blockade or genetic ablation prevented lethal GVHD in multiple murine transplant models. Moreover, protection from GVHD was attributable to augmented global reconstitution of CD4⁺ natural regulatory T cells (nTregs), CD4⁺ induced Tregs (iTregs), and CD8⁺ iTregs, and was more potent than temporally concordant blockade of IL-6 signaling. Inhibition of IL-27p28 also enhanced the suppressive capacity of adoptively transferred CD4⁺ nTregs by increasing the stability of Foxp3 expression. Notably, blockade of IL-27p28 signaling reduced T-cell-derived-IL-10 production in conventional T cells; however, there was no corresponding effect in CD4⁺ or CD8⁺ Tregs, indicating that IL-27 inhibition had differential effects on IL-10 production and preserved a mechanistic pathway by which Tregs are known to suppress GVHD. Targeting of IL-27 therefore represents a novel strategy for the in vivo expansion of Tregs and subsequent prevention of GVHD without the requirement for ex vivo cellular manipulation, and provides additional support for the critical proinflammatory role that members of the IL-6 and IL-12 cytokine families play in GVHD biology.

Increased GVHD-related mortality with broad-spectrum antibiotic use after allogeneic hematopoietic stem cell transplantation in human patients and mice

Intestinal bacteria may modulate the risk of infection and graft-versus-host disease (GVHD) after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Allo-HSCT recipients often develop neutropenic fever, which is treated with antibiotics that may target anaerobic bacteria in the gut. We retrospectively examined 857 allo-HSCT recipients and found that treatment of neutropenic fever with imipenem-cilastatin and piperacillin-tazobactam antibiotics was associated with increased GVHD-related mortality at 5 years (21.5% for imipenem-cilastatin-treated patients versus 13.1% for untreated patients, P = 0.025; 19.8% for piperacillin-tazobactam-treated patients versus 11.9% for untreated patients, P = 0.007). However, two other antibiotics also used to treat neutropenic fever, aztreonam and cefepime, were not associated with GVHD-related mortality (P = 0.78 and P = 0.98, respectively). Analysis of stool specimens from allo-HSCT recipients showed that piperacillin-tazobactam administration was associated with perturbation of gut microbial composition. Studies in mice demonstrated aggravated GVHD mortality with imipenem-cilastatin or piperacillin-tazobactam compared to aztreonam (P < 0.01 and P < 0.05, respectively). We found pathological evidence for increased GVHD in the colon of imipenem-cilastatin-treated mice (P < 0.05), but no difference in the concentration of short-chain fatty acids or numbers of regulatory T cells. Notably, imipenem-cilastatin treatment of mice with GVHD led to loss of the protective mucus lining of the colon (P < 0.01) and the compromising of intestinal barrier function (P < 0.05). Sequencing of mouse stool specimens showed an increase in Akkermansia muciniphila (P < 0.001), a commensal bacterium with mucus-degrading capabilities, raising the possibility that mucus degradation may contribute to murine GVHD. We demonstrate an underappreciated risk for the treatment of allo-HSCT recipients with antibiotics that may exacerbate GVHD in the colon.

Translational opportunities for targeting the Th17 axis in acute graft-vs.-host disease

Allogeneic stem cell transplantation (allo-SCT) is a curative therapy for different life-threatening malignant and non-malignant hematologic disorders. Acute graft-vs.-host disease (aGVHD) and particularly gastrointestinal aGVHD remains a major source of morbidity and mortality following allo-SCT, which limits the use of this treatment in a broader spectrum of patients. Better understanding of aGVHD pathophysiology is indispensable to identify new therapeutic targets for aGVHD prevention and therapy. Growing amount of data suggest a role for T helper (Th)17 cells in aGVHD pathophysiology. In this review, we will discuss the current knowledge in this area in animal models and in humans. We will then describe new potential treatments for aGVHD along the Th17 axis.

The IL-33/ST2 axis augments effector T-cell responses during acute GVHD

Interleukin (IL)-33 binding to the receptor suppression of tumorigenicity 2 (ST2) produces pro-inflammatory and anti-inflammatory effects. Increased levels of soluble ST2 (sST2) are a biomarker for steroid-refractory graft-versus-host disease (GVHD) and mortality. However, whether sST2 has a role as an immune modulator or only as a biomarker during GVHD was unclear. We show increased IL-33 production by nonhematopoietic cells in the gastrointestinal (GI) tract in mice post-conditioning and patients during GVHD. Exogenous IL-33 administration during the peak inflammatory response worsened GVHD. Conversely, GVHD lethality and tumor necrosis factor-α production was significantly reduced in il33(-/-) recipients. ST2 was upregulated on murine and human alloreactive T cells and sST2 increased as experimental GVHD progressed. Concordantly, st2(-/-) vs wild-type (WT) donor T cells had a marked reduction in GVHD lethality and GI histopathology. Alloantigen-induced IL-18 receptor upregulation was lower in st2(-/-) T cells, and linked to reduced interferon-γ production by st2(-/-) vs WT T cells during GVHD. Blockade of IL-33/ST2 interactions during allogeneic-hematopoietic cell transplantation by exogenous ST2-Fc infusions had a marked reduction in GVHD lethality, indicating a role of ST2 as a decoy receptor modulating GVHD. Together, these studies point to the IL-33/ST2 axis as a novel and potent target for GVHD therapy.

Various forms of tissue damage and danger signals following hematopoietic stem-cell transplantation

Hematopoietic stem-cell transplantation (HSCT) is the most potent curative therapy for many malignant and non-malignant disorders. Unfortunately, a major complication of HSCT is graft-versus-host disease (GVHD), which is mediated by tissue damage resulting from the conditioning regimens before the transplantation and the alloreaction of dual immune components (activated donor T-cells and recipient’s antigen-presenting cells). This tissue damage leads to the release of alarmins and the triggering of pathogen-recognition receptors that activate the innate immune system and subsequently the adaptive immune system. Alarmins, which are of endogenous origin, together with the exogenous pathogen-associated molecular patterns (PAMPs) elicit similar responses of danger signals and represent the group of damage-associated molecular patterns (DAMPs). Effector cells of innate and adaptive immunity that are activated by PAMPs or alarmins can secrete other alarmins and amplify the immune responses. These complex interactions and loops between alarmins and PAMPs are particularly potent at inducing and then aggravating the GVHD reaction. In this review, we highlight the role of these tissue damaging molecules and their signaling pathways. Interestingly, some DAMPs and PAMPs are organ specific and GVHD-induced and have been shown to be interesting biomarkers. Some of these molecules may represent potential targets for novel therapeutic approaches.

Development and survival of Th17 cells within the intestines: the influence of microbiome- and diet-derived signals

Th17 cells have emerged as important mediators of host defense and homeostasis at barrier sites, particularly the intestines, where the greatest number and diversity of the microbiota reside. A critical balance exists between protection of the host from its own microbiota and pathogens and the development of immune-mediated disease. Breaches of local innate immune defenses provide critical stimuli for the induction of Th17 cell development, and additional cues within these tissues promote Th17 cell survival and/or plasticity. Normally, this results in eradication of the microbial threat and restitution of homeostasis. When dysregulated, however, Th17 cells can cause a range of immune-mediated diseases, whether directed against Ags derived from the microbiota, such as in inflammatory bowel disease, or against self-Ags in a range of autoimmune diseases. This review highlights recent discoveries that provide new insights into ways in which environmental signals impact Th17 cell development and function in the intestines.

Next generation treatment of acute graft-versus-host disease

Despite rapid increase in the utilization of allogeneic hematopoietic stem cell transplantation, non-relapse mortality and sequela from acute graft-versus-host disease (GVHD) remain principle barriers. GVHD involves complex interactions between innate and adaptive immunity, culminating in tissue damage by inflammatory mediators and cellular effectors. Recently, our understanding of the molecular intricacies of GVHD has grown tremendously. New insights into the roles played by novel cytokines, chemokines, intracellular signaling pathways, epigenetics and post-translational modifications of proteins in GVHD biology provide numerous targets that might be therapeutically exploited. This review highlights recent advances and identifies opportunities for reshaping contemporary GVHD therapeutics.

Helper T-cell differentiation in graft-versus-host disease after allogeneic hematopoietic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is an effective therapeutic option for many malignant diseases. However, the efficacy of allo-HSCT is limited by the occurrence of destructive graft-versus-host disease (GVHD). Since allogeneic T cells are the driving force in the development of GVHD, their activation, proliferation, and differentiation are key factors to understanding GVHD pathogenesis. This review focuses on one critical aspect: the differentiation and function of helper T (Th) cells in acute GVHD. We first summarize well-established subsets including Th1, Th2, Th17, and T-regulatory cells; their flexibility, plasticity, and epigenetic modification; and newly identified subsets including Th9, Th22, and T follicular helper cells. Next, we extensively discuss preclinical findings of Th-cell lineages in GVHD: the networks of transcription factors involved in differentiation, the cytokine and signaling requirements for development, the reciprocal differentiation features, and the regulation of microRNAs on T-cell differentiation. Finally, we briefly summarize the recent findings on the roles of T-cell subsets in clinical GVHD and ongoing strategies to modify T-cell differentiation for controlling GVHD in patients. We believe further exploration and understanding of the immunobiology of T-cell differentiation in GVHD will expand therapeutic options for the continuing success of allo-HSCT.

The expression of Th17-associated cytokines in human acute graft-versus-host disease

The role of Th17 cells and Th17-associated cytokines in the development of acute graft-versus-host disease (aGVHD) in clinical allogeneic hematopoietic stem cell transplantation (allo-HSCT) recipients is not well established. In the current study, a cohort of 69 allo-HSCT patients was examined for the percentages of Th17 and FoxP3(+) Treg cells and the expressions of RORγt and FoxP3 in peripheral blood mononuclear cells (PBMCs). The Th17 percentage and RORγt expression were significantly higher, whereas Treg percentage and FoxP3 expression were significantly lower in severe aGVHD (grade 3 to 4) and mild aGVHD (grade 1 to 2) patients than in patients without aGVHD (grade 0) and healthy donors. We then investigated the expressions of Th17-associated cytokines, including TGF-β, IL-6, IL-1β, IL-17, IL-21, IL-22, IL-23, as well as IL-23R in the PBMCs of patients after allo-HSCT. The expressions of IL-17 and IL-22 in CD4(+) T cells were also examined. The results showed that the expressions of IL-6, IL-1β, IL-17, IL-21, IL-23, and IL-23R were all increased, whereas IL-22 expression was decreased in aGVHD patients. The changes were also correlated with the severity of aGVHD. We also investigated the dynamic changes of Th17/Treg cells and Th17-associated cytokines in patients during the onset and resolution of aGVHD. The results demonstrated a reciprocal relationship between Treg and Th17 cells. Th17-associated cytokine expressions, namely IL-17 and IL-23, were closely related to the occurrence and resolution of aGVHD. We conclude that the dynamic balance between the Th17 and FoxP3(+) Treg cells and the changes of Th17-associated cytokines could be the indicators of the disease progression and promising candidates of prognostic biomarkers of aGVHD.

Allogeneic Th1 cells home to host bone marrow and spleen and mediate IFNγ-dependent aplasia

Bone marrow graft failure and poor graft function are frequent complications after hematopoietic stem cell transplantation and result in significant morbidity and mortality. Both conditions are associated with graft-versus-host disease (GVHD), although the mechanism remains undefined. Here we show, in 2 distinct murine models of GVHD (complete MHC- and class II-disparate) that mimic human peripheral blood stem cell transplantation, that Th1 CD4(+) cells induce bone marrow failure in allogeneic recipients. Bone marrow failure after transplantation of allogeneic naïve CD4(+) T cells was associated with increased CD4(+) Th1 cell development within bone marrow and lymphoid tissues. Using IFNγ-reporter mice, we found that Th1 cells generated during GVHD induced bone marrow failure after transfers into secondary recipients. Homing studies demonstrated that transferred Th1 cells express CXCR4, which was associated with accumulation within bone marrow and spleen. Allogeneic Th1 cells were activated by radiation-resistant host bone marrow cells and induced bone marrow failure through an IFNγ-dependent mechanism. Thus, allogeneic Th1 CD4(+) cells generated during GVHD traffic to hematopoietic sites and induce bone marrow failure via IFNγ-mediated toxicity. These results have important implications for prevention and treatment of bone marrow graft failure after hematopoietic stem cell transplantation.

Role of STAT3 in regulatory T lymphocyte plasticity during acute graft-vs.-host-disease

Regulatory T (Treg) lymphocytes are important mediators of the allogeneic immune response, although the mechanisms by which they are controlled are not fully understood. Studies conducted in mice, including a recent article in Immunity by Laurence et al., have shown that STAT3 is an important factor involved in the instability of natural Treg (nTreg) lymphocytes and the generation of induced Treg (iTreg) lymphocytes. The authors used T lymphocytes obtained from Foxp3-GFP reporter mice, which allowed them to track the in vivo fate of the nTreg and iTreg lymphocyte populations in the inflammatory milieu of acute GvHD. They showed that nTreg lymphocytes lose the expression of FoxP3 within this inflammatory environment and that the loss of FoxP3 is, in part, STAT3-dependent. Ultimately, the absence of STAT3 permitted the conversion of transferred naive CD4⁺ T lymphocytes to iTreg lymphocytes, which correlated with a strikingly improved survival rate during GvHD. We herein discuss how the article by Laurence et al. offers a novel mechanism to explain how the inflammatory environment may alter the stability or phenotype of Treg lymphocytes.

A critical role for the retinoic acid signaling pathway in the pathophysiology of gastrointestinal graft-versus-host disease

Damage to the gastrointestinal tract during graft-versus-host disease (GVHD) is one of the major causes of morbidity and mortality in allogeneic hematopoietic stem cell transplant (HSCT) recipients. In the current study, we identified a critical role for the retinoic acid (RA) signaling pathway in the induction and propagation of gastrointestinal GVHD. The administration of exogenous RA significantly increased expression of the gut-homing molecules, CCR9 and α4β7, on donor T cells in mesenteric lymph nodes, and augmented the accumulation of proinflammatory CD4(+) and CD8(+) T cells within the gut mucosa, leading to a selective exacerbation of colonic GVHD and increased overall mortality. Conversely, depletion of RA in recipient mice by vitamin A deprivation resulted in a dramatic reduction of gut-homing molecule expression on donor T cells after HSCT. Significantly, absence of the RA receptor-α on donor T cells markedly attenuated the ability of these cells to cause lethal GVHD. This observation was attributable to a significant reduction in pathological damage within the colon. These findings identify an organ-specific role for RA in GVHD and provide evidence that blockade of the RA signaling pathway may represent a novel strategy for mitigating the severity of colonic GVHD.

Inorganic arsenic impairs differentiation and functions of human dendritic cells

Experimental studies have demonstrated that the antileukemic trivalent inorganic arsenic prevents the development of severe pro-inflammatory diseases mediated by excessive Th1 and Th17 cell responses. Differentiation of Th1 and Th17 subsets is mainly regulated by interleukins (ILs) secreted from dendritic cells (DCs) and the ability of inorganic arsenic to impair interferon-γ and IL-17 secretion by interfering with the physiology of DCs is unknown. In the present study, we demonstrate that high concentrations of sodium arsenite (As(III), 1-2 μM) clinically achievable in plasma of arsenic-treated patients, block differentiation of human peripheral blood monocytes into immature DCs (iDCs) by inducing their necrosis. Differentiation of monocytes in the presence of non-cytotoxic concentrations of As(III) (0.1 to 0.5 μM) only slightly impacts endocytotic activity of iDCs or expression of co-stimulatory molecules in cells activated with lipopolysaccharide. However, this differentiation in the presence of As(III) strongly represses secretion of IL-12p70 and IL-23, two major regulators of Th1 and Th17 activities, from iDCs stimulated with different toll-like receptor (TLR) agonists in metalloid-free medium. Such As(III)-exposed DCs also exhibit reduced mRNA levels of IL12A and/or IL12B genes when activated with TLR agonists. Finally, differentiation of monocytes with non-cytotoxic concentrations of As(III) subsequently reduces the ability of activated DCs to stimulate the release of interferon-γ and IL-17 from Th cells. In conclusion, our results demonstrate that clinically relevant concentrations of inorganic arsenic markedly impair in vitro differentiation and functions of DCs, which may contribute to the putative beneficial effects of the metalloid towards inflammatory autoimmune diseases.

Role of the mycobiome in human acute graft-versus-host disease

A role for gut bacteria in the pathogenesis of graft-versus-host disease (GVHD) has been firmly established; however, the role of Candida spp, which form part of the mycobiome, remains unknown. In a homogenous group of patients who underwent allogeneic stem cell transplantation (SCT), we found a significant impact of Candida colonization on the occurrence of acute GVHD. Patients colonized with Candida spp developed significantly more grade II-IV acute GVHD compared with noncolonized patients (50% vs 32%; P = .03), as well as more gastrointestinal (GI)-GVHD (33% vs 19%; P = .05). Colonization with Candida spp was more frequent in patients bearing the loss-of-function polymorphism Y238X, which results in dectin-1 dysfunction, compared with patients with the wild-type allele (73% vs 31%; P = .002). There was no direct effect of dectin-1 dysfunction on acute GVHD, although it did influence the occurrence of GVHD indirectly through Candida colonization. The exact mechanism of GVHD induction by Candida spp colonization of the mucosa is unknown, but the link might prove to be the induction of Th 17/IL-23 responses through activation of pattern recognition receptors by fungal motifs, including β-d-glucan and mannans. These data indicate a role for the mycobiome in the pathogenesis of GVHD and suggest that altering the mycobiome by antifungal drugs can help ameliorate GI-GVHD. In addition, given that the genetic constitution of patients affects susceptibility to both Candida colonization and GVHD, whether identifying gene polymorphisms will facilitate personalized treatment of SCT recipients remains to be determined.

New ways to separate graft-versus-host disease and graft-versus-tumour effects after allogeneic haematopoietic stem cell transplantation

A major challenge to transplant immunologists and physicians remains the separation of harmful graft-versus-host disease (GvHD) and beneficial graft-versus-tumour (GvT) effects after allogeneic haematopoietic stem cell transplantation. Recent advances in our understanding of the allogeneic immune response provide potential new opportunities to achieve this goal. Three potential new approaches that capitalize on this new knowledge are considered in depth; the manipulation of organ-specific cytokines and other pro-inflammatory signals, the selective manipulation of donor effector T cell migration, and the development of cell-mediated immunosuppressive strategies using donor-derived regulatory T cells. These new approaches could provide strategies for local control of allogeneic immune responses, a new paradigm to separate GvHD and GvT effects. Although these strategies are currently in their infancy and have challenges to successful translation to clinical practice, all have exciting potential for the future.

STAT3 transcription factor promotes instability of nTreg cells and limits generation of iTreg cells during acute murine graft-versus-host disease

Acute graft-versus-host disease (GvHD) is a major cause of mortality in allogeneic bone marrow transplantation (BMT), for which administration of FoxP3(+) regulatory T (Treg) cells has been proposed as a therapy. However, the phenotypic stability of Treg cells is controversial, and STAT3-dependent cytokines can inhibit FoxP3 expression. We assessed whether the elimination of STAT3 in T cells could limit the severity of GvHD. We found STAT3 limited FoxP3(+) Treg cell numbers following allogeneic BMT by two pathways: instability of natural Treg (nTreg) cells and inhibition of induced Treg (iTreg) cell polarization from naive CD4(+) T cells. Deletion of STAT3 within only the nTreg cell population was not sufficient to protect against lethal GvHD. In contrast, transfer of STAT3-deficient naive CD4(+) T cells increased FoxP3(+) Treg cells post-BMT and prevented lethality, suggesting that the consequence of STAT3 signaling may be greater for iTreg rather than nTreg cells during GvHD.

Interleukin-22 protects intestinal stem cells from immune-mediated tissue damage and regulates sensitivity to graft versus host disease

Little is known about the maintenance of intestinal stem cells (ISCs) and progenitors during immune-mediated tissue damage or about the susceptibility of transplant recipients to tissue damage mediated by the donor immune system during graft versus host disease (GVHD). We demonstrate here that deficiency of recipient-derived IL-22 increased acute GVHD tissue damage and mortality, that ISCs were eliminated during GVHD, and that ISCs as well as their downstream progenitors expressed the IL-22 receptor. Intestinal IL-22 was produced after bone marrow transplant by IL-23-responsive innate lymphoid cells (ILCs) from the transplant recipients, and intestinal IL-22 increased in response to pretransplant conditioning. However, ILC frequency and IL-22 amounts were decreased by GVHD. Recipient IL-22 deficiency led to increased crypt apoptosis, depletion of ISCs, and loss of epithelial integrity. Our findings reveal IL-22 as a critical regulator of tissue sensitivity to GVHD and a protective factor for ISCs during inflammatory intestinal damage.

IFN-γ production by allogeneic Foxp3+ regulatory T cells is essential for preventing experimental graft-versus-host disease

It is emerging that CD4+Foxp3+ regulatory T (Treg) cells can produce the proinflammatory cytokine IFN-γ when stimulated in a Th1 cytokine environment. In this study, we report that Foxp3+ Treg cells readily produced IFN-γ in vivo in a highly inflammatory model of graft-versus-host disease (GVHD) and during a Th1-dominated immune response to intracellular bacteria. Moreover, stimulation in vitro via TCR in the presence of IL-12 alone was sufficient to induce IFN-γ production by Treg cells in a dose-dependent manner. Transfer of donor Treg cells can prevent lethal GVHD; therefore, we used this model as a robust readout for in vivo Treg function. Interestingly, >50% of allogeneic donor, but not residual recipient Foxp3+ Treg cells produced IFN-γ after transplantation, suggesting that this cytokine production was alloantigen specific. These IFN-γ producers were stable Foxp3+ Treg cells because methylation analysis of the Foxp3 gene locus of transferred and reisolated Treg cells during GVHD showed a fully demethylated Treg-specific-demethylated region. Next, we addressed whether IFN-γ production was supporting or rather impairing the immunosuppressive function of Treg cells during GVHD. Blocking of IFN-γ with specific mAb completely abolished the beneficial effect of donor Treg cells. We could further show that only wild-type Treg cells, but not Treg cells from IFN-γ-deficient donor mice, prevented GVHD. This indicated that Treg cell-intrinsic IFN-γ production was required for their protective function. In conclusion, our data show that IFN-γ produced by Foxp3+ Treg cells has essential immune-regulatory functions that are required for prevention of experimental GVHD.

Cytokine profiles in various graft-versus-host disease target organs following hematopoietic stem cell transplantation

Previous studies using genetic-deficient murine models suggest that different T-helper subsets may contribute to different types of tissue damages in graft-versus-host disease (GvHD). However, there is limited information available on the distribution of T-helper cytokines in the various GvHD target tissues. In the current study, an acute GvHD murine model was set up to directly assess the in situ cytokine profiles in various GvHD tissue lesions; in addition, we also studied GvHD tissues from patients who had undergone bone marrow transplantation procedures. We observed that interferon-γ (IFN-γ was dominant in murine liver and gastrointestinal tissue lesions, whereas IFN-γ and interleukin 17 (IL-17) were abundant in murine skin lesions. Furthermore, in human GvHD tissues, interleukin 4 (IL-4) and IFN-γ were predominant in liver lesions and colon lesions, respectively, while no specific cytokine was prevalent in human GvHD skin lesions. In addition, a low ratio of CD4(+) T helper (Th) versus CD8(+) T cytotoxic (Tc) cells in human GvHD tissue lesions, especially in the liver, was detected, and this contrasts with the situation in murine GvHD tissues where CD4(+) Th cells were predominant. Dual staining for CD markers and cytokine expression showed that IFN-γ-secreting T cells were enriched in all murine GvHD target tissue lesions, and Tc1 and Tc2 cells were predominant in human GvHD colon and liver sections, respectively. However, IFN-γ(+) Th1, IL-17(+) Th17, IFN-γ(+) Tc1, and IL-17(+) Tc17 cells were slightly more frequent in human skin lesions compared to IL-4(+) Th2 and IL-4(+) Tc2 cells. To sum up, these results suggest that differences in cytokine imbalances may significantly contribute to tissue-specific pathogenesis in GvHD target organs, and CD8(+) Tc cells may play an important role in human GvHD induction.

The IL-17 differentiation pathway and its role in transplant outcome

The limitations of allogeneic transplantation are graft-versus-host disease (both acute and chronic), infection, and relapse. Acute GVHD has traditionally been thought of as a Th1-mediated disease with inflammatory cytokines (eg, interferon [IFN]-γ and tumor necrosis factor [TNF]) and cellular cytolysis mediating apoptotic target tissue damage in skin, gut, and liver. Chronic GVHD has not fit neatly into either Th1 or Th2 (eg, IL-4, IL-13) paradigms. Increasingly, the Th17 pathway of differentiation has been shown to play important roles in acute and chronic GVHD (aGVHD, cGVHD), particularly in relation to skin and lung disease. Here we discuss the IL-17 pathway of T cell differentiation and the accumulating evidence suggesting it represents an important new target for the control of deleterious alloimmune responses.

Notch and inflammatory T-cell response: new developments and challenges

The inflammatory T-cell response is important for protecting the host against infections and tumors. However, dysregulated generation of effector T cells produces high levels of inflammatory cytokines and cytotoxic molecules and may cause inflammatory disorders, such as chronic infections, autoimmune diseases and graft-versus-host disease after allogeneic bone marrow transplantation. Thus, tight regulation of effector T cells is essential. Accumulating evidence indicates that Notch plays critical roles in regulating the differentiation of antigen activated T cells into distinct lineages of effector T cells. Furthermore, significant progress has been made in the development of new methods in order to modulate Notch for disease treatment. In this article, we will discuss recent findings that help to gain insight into the impact of Notch in the regulation of effector T cells and highlight the beneficial effects of modulating Notch in inflammatory diseases.

Tocilizumab for the treatment of steroid refractory graft-versus-host disease

Corticosteroid refractory graft-versus-host disease (GVHD) is one of the major challenges in the management of allogeneic stem cell transplant recipients. Although numerous agents have been employed to treat this patient population, no standardized second-line therapy exists. In this study, we report our experience with the administration of tocilizumab, an anti-interleukin 6 receptor antibody, in the treatment of steroid refractory GVHD. Tocilizumab was administered to 8 patients with refractory acute (n = 6) or chronic GVHD (cGVHD) (n = 2) once every 3 to 4 weeks. The majority of patients with acute GVHD (aGVHD) had grade IV organ involvement of the skin or gastrointestinal tract, whereas both patients with cGVHD had long-standing severe skin sclerosis at the time of treatment. There were no allergic or infusion-related adverse events. Treatment was discontinued in one patient over concerns that tocilizumab may have worsened preexisting hyperbilirubinemia. Several patients also had transient elevations in serum transaminase values. Infections were the primary adverse events associated with tocilizumab administration. Four patients (67%) with aGVHD had either partial or complete responses apparent within the first 56 days of therapy. One patient with cGVHD had a significant response to therapy, whereas the second had stabilization of disease that allowed for a modest reduction in immune suppressive medications. These results indicate that tocilizumab has activity in the treatment of steroid refractory GVHD and warrants further investigation as a therapeutic option for this disorder.

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

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

Abrogation of donor T-cell IL-21 signaling leads to tissue-specific modulation of immunity and separation of GVHD from GVL

IL-21 is a proinflammatory cytokine produced by Th17 cells. Abrogation of IL-21 signaling has recently been shown to reduce GVHD while retaining graft-versus-leukemia/lymphoma (GVL) responses. However, the mechanisms by which IL-21 may lead to a separation of GVHD and GVL remain incompletely understood. In a murine MHC-mismatched BM transplantation model, we observed that IL-21 receptor knockout (IL-21R KO) donor T cells mediate decreased systemic and gastrointestinal GVHD in recipients of a transplant. This reduction in GVHD was associated with expansion of transplanted donor regulatory T cells and with tissue-specific modulation of Th-cell function. IL-21R KO and wild-type donor T cells showed equivalent alloactivation, but IL-21R KO T cells showed decreased infiltration and inflammatory cytokine production within the mesenteric lymph nodes. However, Th-cell cytokine production was maintained peripherally, and IL-21R KO T cells mediated equivalent immunity against A20 and P815 hematopoietic tumors. In summary, abrogation of IL-21 signaling in donor T cells leads to tissue-specific modulation of immunity, such that gastrointestinal GVHD is reduced, but peripheral T-cell function and GVL capacity are retained. IL-21 is thus an exciting target for therapeutic intervention and improvement of clinical transplantation outcomes.