Adoptively transferred donor IL-17-producing CD4+ T cells augment, but IL-17 alleviates, acute graft-versus-host disease

Macrophages in graft-versus-host disease (GVHD): dual roles as therapeutic tools and targets

Graft-versus-host disease remains one of the most formidable barriers to the complete success of hematopoietic stem cell transplantation that has emerged as the curative approach for many hematopoietic malignancies because it affects quality of life and overall survival. Macrophages are among the important members of the immune system, which perform dual roles in GVHD as both therapeutic tools and targets. This review epitomizes the multifunctional role of macrophages in the pathophysiology of both acute and chronic GVHD. Macrophages play an important role in the early phase of GVHD because of their recruitment and infiltration into target organs. Furthermore, they polarize into two functionally different phenotypes, including M1 and M2. In the case of acute GVHD, most macrophages express the M1 phenotype characterized by the production of pro-inflammatory cytokines that contribute to tissue damage. In contrast, in chronic GVHD, macrophages tend toward the M2 phenotype associated with the repair of tissues and fibrosis. A critical balance among these phenotypes is central to the course and severity of GVHD. Further interactions of macrophages with other lymphocytes such as T cells, B cells, and fibroblast further determine the course of GVHD. Macrophage interaction associated with alloreactive T cells promotes inflammation. This is therefore important in inducing injuries of tissues during acute GVHD. Interaction of macrophages, B cell, fibroblast, and CD4+ T cells promotes fibrosis during chronic GVHD and, hence, the subsequent dysfunction of organs. These are some insights, while several challenges remain. First, the impact of the dominant cytokines in GVHD on the polarization of macrophages is incompletely characterized and sometimes controversial. Second, the development of targeted therapies able to modulate macrophage function without systemic side effects remains an area of ongoing investigation. Future directions involve the exploration of macrophage-targeted therapies, including small molecules, antibodies, and nanotechnology, which modulate macrophage behavior and improve patient outcomes. This underlines the fact that a profound understanding of the dual role of macrophages in GVHD is essential for developing new and more effective therapeutic strategies. Targeting macrophages might represent one avenue for decreasing the incidence and severity of GVHD and improving the success and safety of HSCT.

Preventive hyperbaric oxygen therapy improves acute graft-versus-host disease by activating the Nrf2/HO-1 pathway

Background

Hyperbaric oxygen therapy (HBOT) has been confirmed as an effective and economical therapeutic modality for treating hemorrhagic cystitis (HC), whether induced by infection or acute graft-versus-host disease (aGVHD), in transplant recipients. However, its potential benefits in treating aGVHD remain largely unknown. This study explored the effects of HBOT on aGVHD and its underlying mechanisms.

Methods

The beneficial effects of HBOT on aGVHD were investigated in a murine model. Manifestations, pathological alterations, reactive oxygen species (ROS) levels in target organs, and survival data of the recipient mice were collected. Nuclear factor erythroid-derived 2-related factor 2 (Nrf2) and its downstream enzyme heme-oxygenase 1 (HO-1) expression in mouse samples were assessed via Western blot and immunohistochemistry analyses. ML385, an Nrf2 inhibitor, was used to validate the protective role of Nrf2 in the beneficial effect of HBOT on aGVHD. Furthermore, we initiated a clinical cohort study and collected data from the patients with definite aGVHD before and after HBOT to validate the preclinical conclusions.

Results

We found that HBOT alleviated aGVHD in mice, which was associated with a significantly prolonged overall survival (OS) and reduced pathological injury, whereas Nrf2 inhibition had the opposite effect. HBOT decreased ROS levels and proinflammatory cytokines, including IL-6 and TNF-α, while upregulated Nrf2 and its downstream antioxidant enzyme HO-1. In the clinical cohort study, the incidence of grades 1-3 aGVHD was significantly lower in the combination arm containing HBOT than in the HBOT-free cohort.

Conclusion

Preventive HBOT can mitigate aGVHD by activating the Nrf2/HO-1 signal transduction pathway, suggesting that HBOT may be a feasible approach for both the prevention and treatment of aGVHD.

Clinical trial registration

ClinicalTrials.gov, identifier NCT04502628.

Dimethyl fumarate ameliorates chronic graft-versus-host disease by inhibiting Tfh differentiation via Nrf2

Chronic graft-versus-host disease (cGVHD), characterized by chronic tissue inflammation and fibrosis involving multiple organs, remains a major complication after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Dimethyl fumarate (DMF) is an anti-inflammatory drug approved for the treatment of multiple sclerosis and psoriasis. We previously reported that DMF effectively inhibits acute GVHD (aGVHD) while preserving the graft-versus-leukemia effect. However, the role of DMF in cGVHD progression remains unknown. Here, we found that DMF administration significantly suppresses follicular helper T cell (Tfh) differentiation, and germinal center formation and alleviates disease severity in different murine cGVHD models. Mechanistically, DMF treatment downregulates IL-21 transcription by activation of Nrf2, thus orchestrating Tfh-related gene programs both in mice and humans. The inhibitory role of DMF on Tfh cell differentiation was diminished in Nrf2 deficient T cells. Importantly, the therapeutic potential of DMF in clinical cGVHD has been validated in human data whereby DMF effectively reduces IL-21 production and Tfh cell generation in peripheral blood mononuclear cells from active cGVHD patients and further attenuates xenograft GVHD. Collectively, our findings reveal that DMF potently inhibits cGVHD development by repressing Tfh cell differentiation via Nrf2, paving the way for the treatment of cGVHD in the clinic.

Interleukin-27 Promotes the Generation of Myeloid-derived Suppressor Cells to Alleviate Graft-versus-host Disease

BACKGROUND

Stimulation of myeloid-derived suppressor cell (MDSC) formation represents a potential curative therapeutic approach for graft-versus-host disease (GVHD), which significantly impacts the prognosis of allogeneic hematopoietic stem cell transplantation. However, the lack of an effective strategy for inducing MDSC production in vivo has hindered their clinical application. In our previous study, MDSC expansion was observed in interleukin (IL)-27-treated mice.

METHODS

In this study, we overexpressed exogenous IL-27 in mice using a recombinant adeno-associated virus vector to investigate its therapeutic and exacerbating effects in murine GVHD models.

RESULTS

In our study, we demonstrated that exogenous administration of IL-27 significantly suppressed GVHD development in a mouse model. We found that IL-27 treatment indirectly inhibited the proliferation and activation of donor T cells by rapidly expanding recipient and donor myeloid cells, which act as MDSCs after irradiation or under inflammatory conditions, rather than through regulatory T-cell expansion. Additionally, IL-27 stimulated MDSC expansion by enhancing granulocyte-monocyte progenitor generation. Notably, we verified that IL-27 signaling in donor T cells exerted an antagonistic effect on GVHD prevention and treatment. Further investigation revealed that combination therapy involving IL-27 and T-cell depletion exhibited remarkable preventive effects on GVHD in both mouse and xenogeneic GVHD models.

CONCLUSIONS

Collectively, these findings suggest that IL-27 promotes MDSC generation to reduce the incidence of GVHD, whereas targeted activation of IL-27 signaling in myeloid progenitors or its combination with T-cell depletion represents a potential strategy for GVHD therapy.

Can Serum and Saliva Inflammatory Cytokines Be Considered a Reliable Marker in Chronic Oral Graft-Versus-Host Disease Patients?

Background/Objectives: Chronic graft-versus-host disease (cGVHD) and oral lichen planus (LPO) are chronic inflammatory conditions with similar oral manifestations. This study aimed to assess whether serum and salivary cytokines (IL-1α, IL-6, IL-17) could serve as reliable biomarkers for cGVHD. Methods: A prospective cohort study was conducted involving cGVHD patients, LPO patients, and healthy controls. Cytokine levels in serum and saliva were measured by ELISA and compared across the groups using the Kruskal-Wallis test. Results: IL-17 levels were significantly elevated in the serum of cGVHD patients compared to LPO patients and controls (p < 0.05). However, IL-6 and IL-1α did not show significant differences among the groups. A comparison of salivary samples between the three groups did not reach statistical significance (p > 0.05). Conclusions: This study suggests that IL-17 could be a potential biomarker for cGVHD-related inflammation, warranting further investigation. Salivary samples do not seem to be a reliable biological marker for the diagnosis of cGVHD. The findings underline the need for larger studies to validate these preliminary results.

Pathophysiology and preclinical relevance of experimental graft-versus-host disease in humanized mice

Graft-versus-host disease (GVHD) is a life-threatening complication of allogeneic hematopoietic cell transplantations (allo-HCT) used for the treatment of hematological malignancies and other blood-related disorders. Until recently, the discovery of actionable molecular targets to treat GVHD and their preclinical testing was almost exclusively based on modeling allo-HCT in mice by transplanting bone marrow and splenocytes from donor mice into MHC-mismatched recipient animals. However, due to fundamental differences between human and mouse immunology, the translation of these molecular targets into the clinic can be limited. Therefore, humanized mouse models of GVHD were developed to circumvent this limitation. In these models, following the transplantation of human peripheral blood mononuclear cells (PBMCs) into immunodeficient mice, T cells recognize and attack mouse organs, inducing GVHD. Thereby, humanized mice provide a platform for the evaluation of the effects of candidate therapies on GVHD mediated by human immune cells in vivo. Understanding the pathophysiology of this xenogeneic GVHD is therefore crucial for the design and interpretation of experiments performed with this model. In this article, we comprehensively review the cellular and molecular mechanisms governing GVHD in the most commonly used model of xenogeneic GVHD: PBMC-engrafted NOD/LtSz-PrkdcscidIL2rγtm1Wjl (NSG) mice. By re-analyzing public sequencing data, we also show that the clonal expansion and the transcriptional program of T cells in humanized mice closely reflect those in humans. Finally, we highlight the strengths and limitations of this model, as well as arguments in favor of its biological relevance for studying T-cell reactions against healthy tissues or cancer cells.

Current Advances in Graft-versus-host Disease After Intestinal Transplantation

Graft-versus-host disease (GvHD) remains a potentially fatal complication following intestinal transplant (ITx). Over the past decade, advances in the understanding of the pathophysiology of this complex immunological phenomenon have led to the reassessment of the host systemic immune response and have created a gateway for novel preventive and therapeutic strategies. Although sufficient evidence dictates the use of corticosteroids as a first-line option, the treatment for refractory disease remains contentious and lacks a standardized therapeutic approach. Timely diagnosis remains crucial, and the advent of chimerism detection and immunological biomarkers have transformed the identification, prognostication, and potential for survival after GvHD in ITx. The objectives of the following review aim to discuss the clinical and diagnostic features, pathophysiology, advances in immune biomarkers, as well as therapeutic opportunities in the prevention and treatment of GvHD in ITx.

Influence of Housing Temperature and Genetic Diversity on Allogeneic T Cell-Induced Tissue Damage in Mice

The objective of this study was to determine how housing temperature and genetic diversity affect the onset and severity of allogeneic T cell-induced tissue damage in mice subjected to reduced intensity conditioning (RIC). We found that adoptive transfer of allogeneic CD4+ T cells from inbred donors into sub-lethally irradiated inbred recipients (I→I) housed at standard housing temperatures (ST; 22-24 °C) induced extensive BM and spleen damage in the absence of injury to any other tissue. Although engraftment of T cells in RIC-treated mice housed at their thermo-neutral temperature (TNT; 30-32 °C) also developed similar BM and spleen damage, their survival was markedly and significantly increased when compared to their ST counterparts. In contrast, the adoptive transfer of allogeneic T cells into RIC-treated outbred CD1 recipients failed to induce disease in any tissue at ST or TNT. The lack of tissue damage was not due to defects in donor T cell trafficking to BM or spleen but was associated with the presence of large numbers of B cells and myeloid cells within these tissues that are known to contain immunosuppressive regulatory B cells and myeloid-derived suppressor cells. These data demonstrate, for the first time, that housing temperature affects the survival of RIC-treated I→I mice and that RIC-conditioned outbred mice are resistant to allogeneic T cell-induced BM and spleen damage.

Dendritic cell-derived IL-27 p28 regulates T cell program in pathogenicity and alleviates acute graft-versus-host disease

Interleukin 27 (IL-27), a heterodimeric cytokine composed of Epstein-Barr virus-induced 3 and p28, is a pleiotropic cytokine with both pro-and anti-inflammatory properties. However, the precise role of IL-27 in acute graft-versus-host disease is not yet fully understood. In this study, utilizing mice with IL-27 p28 deficiency in dendritic cells (DCs), we demonstrated that IL-27 p28 deficiency resulted in impaired Treg cell function and enhanced effector T cell responses, corresponding to aggravated aGVHD in mice. In addition, using single-cell RNA sequencing, we found that loss of IL-27 p28 impaired Treg cell generation and promoted IL-1R2⁺TIGIT⁺ pathogenic CD4⁺ T cells in the thymus at a steady state. Mechanistically, IL-27 p28 deficiency promoted STAT1 phosphorylation and Th1 cell responses, leading to the inhibition of Treg cell differentiation and function. Finally, patients with high levels of IL-27 p28 in serum showed a substantially decreased occurrence of grade II-IV aGVHD and more favorable overall survival than those with low levels of IL-27 p28. Thus, our results suggest a protective role of DC-derived IL-27 p28 in the pathogenesis of aGVHD through modulation of the Treg/Teff cell balance during thymic development. IL-27 p28 may be a valuable marker for predicting aGVHD development after transplantation in humans.

siRNA against CD40 delivered via a fungal recognition receptor ameliorates murine acute graft-versus-host disease

Acute graft-versus-host disease (aGvHD) remains a major threat to a successful outcome after allogeneic hematopoietic stem cell transplantation (HSCT). Although antibody-based targeting of the CD40/CD40 ligand costimulatory pathway can prevent aGvHD, side effects hampered their clinical application, prompting a need for other ways to interfere with this important dendritic T-cell costimulatory pathway. Here, we used small interfering RNA (siRNA) complexed with β-glucan allowing the binding and uptake of the siRNA/β-glucan complex (siCD40/schizophyllan [SPG]; chemical modifications called NJA-312, NJA-302, and NJA-515) into Dectin1+ cells, which recognize this pathogen-associated molecular pattern receptor. aGvHD was induced by the transplantation of splenocytes and bone marrow cells from C57BL/6J into CBF1 mice. Splenic dendritic cells retained Dectin1 expression after HSCT but showed lower expression after irradiation. The administration of siCD40/SPG, NJA-312, and NJA-302 ameliorated aGvHD-mediated lethality and tissue damage of spleen and liver, but not skin. Multiple NJA-312high injections prevented aGvHD but resulted in early weight loss in allogeneic HSCT mice. In addition, NJA-312 treatment caused delayed initial donor T and B-cell recovery but resulted in stable chimerism in surviving mice. Mechanistically, NJA-312 reduced organ damage by suppressing CCR2+, F4/80+, and IL17A-expressing cell accumulation in spleen, liver, and thymus but not the skin of mice with aGvHD. Our work demonstrates that siRNA targeting of CD40 delivered via the PAMP-recognizing lectin Dectin1 changes the immunological niche, suppresses organ-specific murine aGvHD, and induces immune tolerance after organ transplantation. Our work charts future directions for therapeutic interventions to modulate tissue-specific immune reactions using Pathogen-associated molecular pattern (PAMP) molecules like 1,3-β-glucan for cell delivery of siRNA.

Upregulation of HIF-1α contributes to complement activation in transplantation-associated thrombotic microangiopathy

Transplantation-associated thrombotic microangiopathy (TA-TMA) is a severe complication of haematopoietic stem cell transplantation (HSCT). Complement activation is involved in the development of TA-TMA. However, the underlying mechanism is unclear. Therefore, 21 samples of TA-TMA and 1:1 matched controls were measured for hypoxia-inducible factor-1α (HIF-1α) and complement protein. The mechanism was investigated both in vitro and in vivo. In this study, we found that levels of HIF-1α were significantly higher in TA-TMA patients than that in non-TA-TMA controls. Upregulation of HIF-1α induced an increase in membrane-bound complement C3 and dysfunction of human umbilical vein endothelial cells (HUVECs) in vitro. Increasing HIF-1α in vivo led to C3 and C5b-9 deposition in the glomerular endothelial capillary complex, thrombocytopenia, anaemia, and increased serum lactate dehydrogenase (LDH) levels in wild-type (WT) but not in C3^-/- mice subjected to HSCT. High platelet aggregation in peripheral blood and CD41-positive microthrombi in the kidney were also found in dimethyloxallyl glycine (DMOG)-treated mice, recapitulating the TA-TMA phenotype seen in patients. Comprehensive analysis, including DNA array, luciferase reporter assay, chromatin immunoprecipitation (ChIP)-seq, and quantitative polymerase chain reaction (PCR), revealed that HIF-1α interacted with the promoter of complement factor H (CFH) to inhibit its transcription. Decreased CFH led to complement activation in endothelial cells.

Gene expression profile of chronic oral graft-versus-host disease

Among the complications observed after allogeneic hematopoietic stem cell transplantation, graft-versus-host disease (GVHD) is the primary cause of post-transplant mortality. The oral cavity is the second most affected organ target in chronic GVHD. Tissue damage results from the upregulation of inflammatory mediators, which play a critical role in the immunopathogenesis of the disease. This case series observational study aims to evaluate the participation of cytokines, chemokines, transcription factors, and heat shock proteins in the pathogenesis of oral GVHD (oGVHD), describing the mRNA expression of 28 genes selected. Peripheral blood mononuclear cells were isolated from six participants with oGVHD and two without GVHD, and relative expression of transcripts with established roles as inflammatory mediators was determined in triplicate using the human RT² Profiler™ PCR Array. The gene expression levels in the group with oGVHD were mainly up-regulated compared to those without GVHD. PBMC from oGVDH expressed consistently higher IFN-γ, TNF, IL-1β, CCL2, HSP60 (HSPD1) and HSP90 (HSP90B1). These results can provide a basis for developing new molecular diagnostics and targets therapies for the clinical management of oGVHD.

Teleost CD4+ helper T cells: Molecular characteristics and functions and comparison with mammalian counterparts

CD4⁺ helper T cells play key and diverse roles in inducing adaptive immune responses in vertebrates. The CD4 molecule, which is found on the surfaces of CD4⁺ helper T cells, can be used to distinguish subsets of helper T cells. Teleosts are the oldest living species with bona-fide CD4 coreceptors. Although some components of immune systems of teleosts and mammals appear to be similar, many physiological differences are represented between them. Previous studies have shown that two CD4 paralogs are present in teleosts, whereas only one is present in mammals. Therefore, in this review, the CD4 molecular structure, expression profiles, subpopulations, and biological functions of teleost CD4⁺ helper T cells were summarized and compared with those of their mammalian counterparts to understand the differences in CD4 molecules between teleosts and mammals. This review provides suggestions for further studies on the CD4 molecular function and regulatory mechanism of CD4⁺ helper T cells in teleost fish and will help establish therapeutic strategies to control fish diseases in the future.

Donor γδT Cells Promote GVL Effect and Mitigate aGVHD in Allogeneic Hematopoietic Stem Cell Transplantation

Disease relapse and graft-versus-host disease (GVHD) are the major complications affecting the outcomes of allogeneic hematopoietic stem cell transplantation (allo-HSCT). While the functions of αβT cells are extensively studied, the role of donor γδT cells in allo-HSCT is less well defined. Using TCRδ^-/- donors lacking γδT cells, we demonstrated that donor γδT cells were critical in mediating graft-versus-leukemia (GVL) effect during allo-HSCT. In the absence of donor γδT cells, IFN-γ production by CD8⁺ T cells was severely impaired. Vγ4 subset was the major γδT cell subset mediating the GVL effect in vivo, which was partially dependent on IL-17A. Meanwhile, donor γδT cells could mitigate acute GVHD in a murine allo-HSCT model by suppressing CD4⁺ T cell activation and the major γδT cell subset that exerted this protective function was also Vγ4 γδT cells. Therefore, our findings provide evidence that donor γδT cells, especially Vγ4 subset, can enhance GVL effect and mitigate aGVHD during allo-HSCT.

Th2 polarization in target organs is involved in the alleviation of pathological damage mediated by transplanting granulocyte colony-stimulating factor-primed donor T cells

Acute graft-versus-host disease (aGVHD) is caused by allo-activated donor T cells infiltrating target organs. As a regulator of immune function, granulocyte colony-stimulating factor (G-CSF) has been demonstrated to relieve the aGVHD reaction. However, the role of G-CSF-primed donor T cells in specific target organs is still unknown. In this study, we employed a classical MHC-mismatched transplantation mouse model (C57BL/6 into BALB/c) and found that recipient mice transplanted with G-CSF-primed T cells exhibited prolonged survival compared with that of the PBS-treated group. This protective function against GVHD mediated by G-CSF-primed donor T cells was further confirmed by decreased clinical and pathological scores in this aGVHD mouse model, especially in the lung and gut. Moreover, we found that T cells polarized towards Th2 cells and regulatory T cells were increased in specific target organs. In addition, G-CSF treatment inhibited inducible co-stimulator (ICOS) expression and increased the expression of tolerance-related genes in recipient mice. Our study provides new insight into the immune regulatory effects of G-CSF on T cell-mediated aGVHD, especially for its precise regulation in GVHD target organs.

Macrophage regulation of graft-vs-host disease

Hematopoietic stem cell transplantation has become a curative choice of many hematopoietic malignancy, but graft-vs-host disease (GVHD) has limited the survival quality and overall survival of hematopoietic stem cell transplantation. Understanding of the immune cells’ reaction in pathophysiology of GVHD has improved, but a review on the role of macrophages in GVHD is still absent. Studies have observed that macrophage infiltration is associated with GVHD occurrence and development. In this review, we summarize and analyze the role of macrophages in GVHD based on pathophysiology of acute and chronic GVHD, focusing on the macrophage recruitment and infiltration, macrophage polarization, macrophage secretion, and especially interaction of macrophages with other immune cells. We could conclude that macrophage recruitment and infiltration contribute to both acute and chronic GVHD. Based on distinguishing pathology of acute and chronic GVHD, macrophages tend to show a higher M1/M2 ratio in acute GVHD and a lower M1/M2 ratio in chronic GVHD. However, the influence of dominant cytokines in GVHD is controversial and inconsistent with macrophage polarization. In addition, interaction of macrophages with alloreactive T cells plays an important role in acute GVHD. Meanwhile, the interaction among macrophages, B cells, fibroblasts, and CD4+ T cells participates in chronic GVHD development.

PRMT5 regulates T cell interferon response and is a target for acute graft-versus-host disease

Acute graft-versus-host disease (aGVHD) is a T cell-mediated immunological disorder and the leading cause of nonrelapse mortality in patients who receive allogeneic hematopoietic cell transplants. Based on recent observations that protein arginine methyltransferase 5 (PRMT5) and arginine methylation are upregulated in activated memory T cells, we hypothesized that PRMT5 is involved in the pathogenesis of aGVHD. Here, we show that PRMT5 expression and enzymatic activity were upregulated in activated T cells in vitro and in T cells from mice developing aGVHD after allogeneic transplant. PRMT5 expression was also upregulated in T cells of patients who developed aGVHD after allogeneic hematopoietic cell transplant compared with those who did not develop aGVHD. PRMT5 inhibition using a selective small-molecule inhibitor (C220) substantially reduced mouse and human allogeneic T cell proliferation and inflammatory IFN-γ and IL-17 cytokine production. Administration of PRMT5 small-molecule inhibitors substantially improves survival, reducing disease incidence and clinical severity in mouse models of aGVHD without adversely affecting engraftment. Importantly, we show that PRMT5 inhibition retained the beneficial graft-versus-leukemia effect by maintaining cytotoxic CD8+ T cell responses. Mechanistically, we show that PRMT5 inhibition potently reduced STAT1 phosphorylation as well as transcription of proinflammatory genes, including interferon-stimulated genes and IL-17. Additionally, PRMT5 inhibition deregulates the cell cycle in activated T cells and disrupts signaling by affecting ERK1/2 phosphorylation. Thus, we have identified PRMT5 as a regulator of T cell responses and as a therapeutic target in aGVHD.

PD-L1 Ameliorates Murine Acute Graft-Versus-Host Disease by Suppressing Effector But Not Regulatory T Cells Function

There is increasing evidence that interaction between programmed death 1 (PD-1) and its ligands PD-1 (PD-L1) plays a critical role in the pathology of acute graft-versus-host disease (aGVHD). However, the role of PD-L1 in the development of aGVHD has been controversial in recent mouse studies. In this study, we carried out studies in a murine aGVHD model to clarify the role of PD-L1 in aGVHD pathogenesis. We found that systemic overexpression of PD-L1 by hydrodynamic gene transfer (HGT) method in vivo ameliorates aGVHD-induced lethality in mice. Systemic overexpression of PD-L1 inhibits the donor T cells activation, effector memory status, as well as Th1 and Th17 cells responses in vivo. In addition, PD-L1 Ig treatment significantly suppressed T cells' proliferation, promoted T cells' apoptosis, and reduced pro-inflammatory cytokines expression by effector T cells in vitro in the stimulation of anti-CD3/CD28 and allogeneic dendritic cells. However, we found that PD-L1 overexpression did not affect Treg cells' differentiation in vivo and in vitro, depletion of Treg cells in PD-L1 HGT recipients did not aggravate aGVHD mortality. Therefore, our results demonstrated that systemic treatment with PD-L1 protein ameliorates aGVHD by suppressing effector but not regulatory T cell function. Our findings suggest that systemic treatment with PD-L1 may be a potential strategy to prevent or ameliorate aGVHD.

G-CSF-induced macrophage polarization and mobilization may prevent acute graft-versus-host disease after allogeneic hematopoietic stem cell transplantation

Macrophages (MΦs) are an important immune cell population that are essential for tissue homeostasis and disease pathogenesis. MΦs are now classified as either M1, which produce pro-inflammatory cytokines, or M2, which produce antiinflammatory cytokines. The impact of granulocyte colony-stimulating factor (G-CSF) on MΦs in humans is unclear. Moreover, little is known about the association between MΦ subsets in allografts and the occurrence of acute graft-versus-host disease (aGVHD) in patients who undergo allogeneic hematopoietic stem cell transplantation (allo-HSCT). In the current study, we found that the M1/M2 ratio was markedly decreased in both G-CSF-treated bone marrow (post-BM) and G-CSF-treated peripheral blood from healthy donors. Post-BM MΦs exhibited reduced migration and increased phagocytosis. Moreover, post-BM MΦs reduced the percentage of Th1 and Tc1 lineages and increased the percentage of Th2, Tc2, and Treg lineages. Patients who received BM grafts with a higher M1/M2 ratio exhibited a higher incidence of grade 2-4 aGVHD. In summary, our data indicate that G-CSF decreases the M1/M2 ratio in BM grafts from healthy donors, which may contribute to preventing the occurrence of grade 2-4 aGVHD in patients after allo-HSCT.

IL-17C Mitigates Murine Acute Graft-vs.-Host Disease by Promoting Intestinal Barrier Functions and Treg Differentiation

Acute graft-vs.-host disease (aGVHD) is one of the major complications and results in high mortality after allogeneic hematopoietic stem cell transplantation (allo-HSCT). IL-17C is involved in many inflammatory immune disorders. However, the role of IL-17C in aGVHD remains unknown. Here we demonstrated that IL-17C deficiency in the graft significantly promoted alloreactive T cell responses and induced aggravated aGVHD compared with wildtype donors in a fully MHC-mismatched allo-HSCT model. In contrast, IL-17C overexpression ameliorated aGVHD. IL-17C deficiency increased intestinal epithelial permeability and elevated inflammatory cytokine production, leading to an enhanced aGVHD progression. Tregs was reduced in recipients of IL-17C^−/− graft, whilst restored after IL-17C overexpression. Decreased Treg differentiation was abrogated after neutralizing IFN-γ, but not IL-6. Moreover, depletion of Tregs diminished the protective effect of IL-17C. Of note, patients with low IL-17C expression displayed higher aGVHD incidence together with poor overall survival, thereby IL-17C could be an independent risk factor for aGVHD development. Our results are the first demonstrating the protective role of IL-17C in aGVHD by promoting intestinal barrier functions and Treg differentiation in a MHC fully mismatched murine aGVHD model. IL-17C may serve as a novel biomarker and potential therapeutic target for aGVHD.

The Regulatory Role of IFN-γ on the Proliferation and Differentiation of Hematopoietic Stem and Progenitor Cells

The replenishment of all blood cell lineages is hierarchically organized by the process of hematopoiesis, which is based on the differentiation pathways of hematopoietic stem and progenitor cells (HSPCs). Due to the ability to balance between self-renewal and differentiation, hematopoietic stem cells (HSCs) can generate the appropriate cell type that is required by the immune system and peripheral blood in response to physiological or pathological conditions. Numerous studies have shown that some proinflammatory cytokines contribute to the regulation of the various hematopoietic compartments. Of these, IFN-γ is a type II interferon primarily produced by T cells and natural killer cells, and plays a major role in the defense against invading pathogens and transformed cancer cells; moreover, a growing amount of research indicates that it exerts negative or positive regulatory effect on hematopoiesis. Although IFN-γ is a widely regarded negative regulator of HSC proliferation, it also participates in some chronic infections or hematological malignancies that induce bone marrow failure. Recent studies have demonstrated unexpected effects of IFN-γ, including the promotion of HSC formation and the stimulation of myelopoiesis. Here, we review the direct and indirect effects of IFN-γ on hematopoiesis, as well as the underlying signaling mechanisms of how IFN-γ modulates the self-renewal, cell cycle entry, and proliferation of HSCs. Next, we describe how IFN-γ affects different stages of the lineage differentiation from HSCs. Finally, we discuss the relationship between IFN-γ and compensatory extramedullary hematopoiesis, as well as some related clinical diseases.

Inhibition of Acute Graft-versus-Host Disease with Retention of Graft-versus-Tumor Effects by Dimethyl Fumarate

Acute graft-versus-host disease (aGVHD) remains a clinical challenge and a major source of morbidity and mortality following allogeneic hematopoietic stem cell transplantation (allo-HSCT). Dimethyl fumarate (DMF), an activator of Nrf2, has been shown to have anti-inflammatory and immunomodulatory properties without significant immunosuppression. We therefore hypothesized that DMF could be potentially harnessed for the treatment of aGVHD with retention of graft-versus-tumor effect. In this study, we showed that DMF significantly inhibited alloreactive T cell responses in vitro in mixed lymphocyte reaction assay. Administration of DMF significantly alleviated the severity, histological damage, and the overall mortality of aGVHD in an MHC-mismatched aGVHD model. DMF administration reduced the activation and effector function of donor T cells in vitro and in vivo. In addition, DMF treatment upregulated antioxidant enzymes heme oxygenase-1 and glutathione S-transferase-α1 expressions. Furthermore, DMF treatment markedly increased the frequencies of Treg cells. Depletion of CD25⁺ cells in DMF recipients aggravated aGVHD mortality compared with IgG control recipients. DMF could promote Treg cell differentiation in a dose dependent manner by upregulating TGF-β expression in vitro. Most importantly, DMF administration preserved graft-versus-leukemia effect after bone marrow transplantation. In conclusion, our findings demonstrated DMF as a promising agent for the prevention of aGVHD after allo-HSCT.