Host programmed death ligand 1 is dominant over programmed death ligand 2 expression in regulating graft-versus-host disease lethality

The impact of PD-1/PD-L1, CTLA-4, TIM-3 and LAG-3 immune checkpoint receptor expression in the development of acute graft versus host disease (aGVHD) and disease recurrence after allogeneic hematopoietic stem cell transplantation

The immune checkpoint receptors play a crucial role in managing the transplantation outcome including development of acute graft versus host disease (aGVHD) and disease recurrence following allogeneic hematopoietic stem cell transplantation (allo-HSCT) is well established. This study aimed to investigate the expression of immune checkpoint receptors, including PD-1/PD-L1, CTLA-4, TIM-3, and LAG-3 in donors, as well as changes in their expression during the first 90 days (day 30 and day 90) post-HLA-matched allo-HSCT, concerning the development of aGVHD and disease relapse. Forty-one donor/recipient pairs were included in this study. The relative expression of immune checkpoint receptors was measured using the SYBR Green Real-Time PCR method. There was no significant relationship between the expression of PD-1/PD-L1, CTLA-4, TIM-3, and LAG-3 immune checkpoint receptors in donors and the occurrence of aGVHD and disease relapse. Additionally, alterations in the expression of these receptors during the initial 90 days post-transplantation did not correlate with aGVHD development. However, patients exhibiting elevated PD-L1 levels at day 90 had an increased risk of disease recurrence post-allo-HSCT (*P = 0.027). This study is the first to demonstrate that high PD-L1 expression in the peripheral blood at day 90 after allo-HSCT is associated with an increased rate of post-transplantation relapse.

The influence of immune checkpoint blockade on the outcomes of allogeneic hematopoietic stem cell transplantation

The principle of immune checkpoint blockade therapy is based on the activation of T cells. Immune checkpoint inhibitors (ICIs), such as anti-PD-1/PD-L1 and anti-CTLA-4 antibodies, have demonstrated effectiveness in treating solid tumors by reinvigorating the immune system to recognize and eliminate malignant cells. In recent years, ICIs have shown promise in certain patients with relapsed or refractory lymphoma and myeloid malignancies. Allogeneic hematopoietic stem cell transplant (allo-HCT) currently remains the only curative immunotherapy option for eligible patients with these hematologic malignancies. An increasing number of patients with indications for allo-HCT have received treatment with ICIs either before the procedure or as a therapy for relapse after allo-HCT. Nevertheless, initial reports suggest that patients exposed to immune checkpoint inhibitors either before or after allo-HCT are at an increased risk of developing severe graft-versus-host disease and other immune-related adverse events, likely due to the persistent effects of immune checkpoint blocking. Maximizing therapeutic benefits while minimizing side effects of the combination of checkpoint blockade immunotherapy and allo-HCT is an active area of research aimed at improving the prognosis of relapsed or refractory hematologic malignancies. However, there is still a lack of rational design strategies to optimize the combined use of these two different types of immunotherapies. In this review, we addressed the scientific rationale behind ICIs for treating lymphoma and myeloid malignancies. We also summarized the evidence supporting the use of ICIs as salvage therapy before and after allo-HCT. Additionally, we offered insights into current approaches for preventing and treating graft-versus-host disease and other immune-related adverse events during the procedure.

Allogeneic transplantation after immunotherapy for relapsed/refractory non-Hodgkin lymphoma: a comparison with a historical cohort

BACKGROUND AIMS

New immunotherapy drugs, such as bispecific T-cell engager antibodies, checkpoint inhibitors and antibody-drug conjugates, are commonly used as salvage therapy for patients with non-Hodgkin lymphoma relapsing after chimeric antigen receptor (CAR) T-cell therapy. Nevertheless, their potential long-term effects on the outcome of allogeneic stem cell transplantation (Allo-SCT) are not well known.

METHODS

We retrospectively analyzed the outcomes of 27 relapsed/refractory non-Hodgkin lymphoma patients receiving Allo-SCT after immunotherapy in the pre-CAR T-cell therapy era and compared them with a historical cohort of 28 subjects undergoing Allo-SCT after conventional therapy.

RESULTS

The two cohorts had similar outcomes in terms of graft-versus-host disease/relapse-free survival (4 years, 59% versus 46%), overall survival (4 years, 77% versus 44%), non-relapse mortality (4 years, 19% versus 22%) and acute (6 months, 15% versus 21%) and chronic (4 years, 18% versus 24%) graft-versus-host disease. Of note, the cumulative incidence of relapse was lower after immunotherapy (4 years, 4% versus 14%), although significance was not reached. The cumulative incidence of cytomegalovirus and fungal infection did not differ among the two cohorts.

CONCLUSIONS

Consolidation with Allo-SCT is a safe and curative option for patients achieving disease response after new immunotherapy drugs that could represent a desirable salvage strategy for patients relapsing after CAR T-cell therapy.

Anti-PD-1 combined with hypomethylating agent and CAG regimen bridging to allogeneic hematopoietic stem cell transplantation: a novel strategy for relapsed/refractory acute myeloid leukemia

Introduction

The prognosis of relapsed/refractory acute myeloid leukemia (r/rAML) is dismal, and allogeneic hematopoietic stem cell transplant (allo-HSCT) is a potential cure. Combining anti-PD-1, hypomethylating agent (HMA), and CAG (cytarabine, aclarubicin/idarubicin, granulocyte colony-stimulating factor) regimen has showed primary efficacy in r/rAML. However, pre-transplant exposure to anti-PD-1 may lead to severe graft-versus-host disease (GVHD). This preliminary study aimed to evaluate the safety and efficacy of allo-HSCT in r/rAML patients receiving the anti-PD-1+HMA+CAG regimen.

Methods

Fifteen r/rAML patients (12 related haploidentical donors [HIDs], 2 matched siblings, 1 unrelated donor) received this regimen and subsequent peripheral blood HSCT.

Results

Four patients with HIDs received a GVHD prophylaxis regimen consisted of Anti-thymocyte globulin and a reduced-dose of post-transplant cyclophosphamide. The median follow-up was 20.9 months (range, 1.2-34.2). The cumulative incidences of acute GVHD grade 2-4 and grade 3-4 were 40% and 13.3%, respectively. The 2-year incidence of moderate-to-severe chronic GVHD, non-relapse mortality, and relapse were 10%, 22.3%, and 22.5%, respectively. The 2-year overall survival and GVHD-free/relapse-free survival rates were 54% and 48.6%, respectively. No death or relapse was observed in the PTCy group.

Conclusion

The anti-PD-1+HMA+CAG regimen bridging to allo-HSCT for r/r AML was tolerable with promising efficacy. GVHD prophylaxis with PTCy for HID-HSCT showed preliminary survival advantage.

Increased PD-1 expression of bone marrow T-cells in acute myeloid leukaemia patients after stem cell transplantation, and its association with overall survival

BACKGROUND

Immune checkpoints are involved in mechanisms by which tumours escape from the host immune system. Our aim was to evaluate acute myeloid leukaemia (AML) patients to determine expression levels of checkpoint molecules according to diagnosis and treatments, and to identify optimal candidates for checkpoint blockade.

METHODS

Bone marrow (BM) samples were obtained from 279 AML patients at different disease status and from 23 controls. Flow cytometric analyses of PD-1 and PD-L1/PD-L2 expression were performed.

RESULTS

Programmed death-1 (PD-1) expression levels on CD8+ T-cells at AML diagnosis were increased compared to controls. PD-L1 and PD-L2 expression levels on leukaemic cells at diagnosis were significantly higher in secondary AML than in de novo AML. PD-1 levels on CD8+ and CD4+ T-cells after allo-SCT were significantly higher than those at diagnosis and after CTx. PD-1 expression on CD8+ T-cells increased in the acute GVHD group than in the non-GVHD group. The overall survival of patients with high PD-1 expression on CD8+ T-cells was significantly shorter than that of patients with low PD-1 expression.

CONCLUSIONS

In conclusion, patients who underwent allo-SCT exhibited high PD-1 expression, suggesting that allo-SCT increases PD-1 expression on T-cells, and the patients with high PD-1 expression on CD8+ T-cells after allo-SCT showed the poor prognosis. For these patients, PD-1 blockade could be an immunotherapeutic strategy.

Genetic and pharmaceutical targeting of HIF1α allows combo-immunotherapy to boost graft vs. leukemia without exacerbation graft vs. host disease

Despite potential impact on the graft vs. leukemia (GVL) effect, immunotherapy targeting CTLA-4 and/or PD-1 has not been successfully combined with bone marrow transplant (BMT) because it exacerbates graft vs. host disease (GVHD). Here, using models of GVHD and leukemia, we demonstrate that targeting hypoxia-inducible factor 1α (HIF1α) via pharmacological or genetic approaches reduces GVHD by inducing PDL1 expression on host tissue while selectively inhibiting PDL1 in leukemia cells to enhance the GVL effect. More importantly, combination of HIF1α inhibition with anti-CTLA-4 antibodies allows simultaneous inhibition of both PDL1 and CTLA-4 checkpoints to achieve better outcomes in models of mouse and human BMT-leukemia settings. These findings provide an approach to enhance the curative effect of BMT for leukemia and broaden the impact of cancer immunotherapy.

The therapeutic potential of PD-1/PD-L1 pathway on immune-related diseases: Based on the innate and adaptive immune components

Currently, immunotherapy targeting programmed cell death 1 (PD-1) or programmed death ligand 1 (PD-L1) has revolutionized the treatment strategy of human cancer patients. Meanwhile, PD-1/PD-L1 pathway has also been implicated in the pathogenesis of many immune-related diseases, such as autoimmune diseases, chronic infection diseases and adverse pregnancy outcomes, by regulating components of the innate and adaptive immune systems. Given the power of the new therapy, a better understanding of the regulatory effects of PD-1/PD-L1 pathway on innate and adaptive immune responses in immune-related diseases will facilitate the discovery of novel biomarkers and therapeutic drug targets. Targeting this pathway may successfully halt or potentially even reverse these pathological processes. In this review, we discuss recent major advances in PD-1/PD-L1 axis regulating innate and adaptive immune components in immune-related diseases. We reveal that the impact of PD-1/PD-L1 axis on the immune system is complex and manifold and multi-strategies on the targeted PD-1/PD-L1 axis are taken in the treatment of immune-related diseases. Consequently, targeting PD-1/PD-L1 pathway, alone or in combination with other treatments, may represent a novel strategy for future therapeutic intervention on immune-related diseases.

Donor T cell STAT3 deficiency enables tissue PD-L1-dependent prevention of graft-versus-host disease while preserving graft-versus-leukemia activity

STAT3 deficiency (STAT3-/-) in donor T cells prevents graft-versus-host disease (GVHD), but the impact on graft-versus-leukemia (GVL) activity and mechanisms of GVHD prevention remains unclear. Here, using murine models of GVHD, we show that STAT3-/- donor T cells induced only mild reversible acute GVHD while preserving GVL effects against nonsusceptible acute lymphoblastic leukemia (ALL) cells in a donor T cell dose-dependent manner. GVHD prevention depended on programmed death ligand 1/programmed cell death protein 1 (PD-L1/PD-1) signaling. In GVHD target tissues, STAT3 deficiency amplified PD-L1/PD-1 inhibition of glutathione (GSH)/Myc pathways that regulate metabolic reprogramming in activated T cells, with decreased glycolytic and mitochondrial ATP production and increased mitochondrial ROS production and dysfunction, leading to tissue-specific deletion of host-reactive T cells and prevention of GVHD. Mitochondrial STAT3 deficiency alone did not reduce GSH expression or prevent GVHD. In lymphoid tissues, the lack of host-tissue PD-L1 interaction with PD-1 reduced the inhibition of the GSH/Myc pathway despite reduced GSH production caused by STAT3 deficiency and allowed donor T cell functions that mediate GVL activity. Therefore, STAT3 deficiency in donor T cells augments PD-1 signaling-mediated inhibition of GSH/Myc pathways and augments dysfunction of T cells in GVHD target tissues while sparing T cells in lymphoid tissues, leading to prevention of GVHD while preserving GVL effects.

Successful restoration of checkpoint inhibitors efficacy after allogeneic hematopoietic cell transplant for classic Hodgkin lymphoma patients

BACKGROUND

Classic Hodgkin lymphoma (cHL) is a highly-curable disease. However, relapses after bone marrow transplant are challenging especially relapses after allogeneic transplant.

METHODS

A retrospective chart review of the institution transplant database to summarize the safety and efficacy of checkpoint inhibitors (CPIs) use for cHL relapses postallo-HCT in patients who already failed to derive sustained benefit from CPIs received prior to allo-HCT.

RESULTS

Six cases were identified and reviewed. All patients received and failed to derive sustained benefit from CPIs and brentuximab vedotin preallo-HCT. The median age at the time of allo-HCT was 28.6 years (IQR 23.6-34.2), the median number of lines received prior to allo-HCT was 6.5 (range 5-9). The median duration of CPI therapy prior to allo-HCT was 8.1 months (IQR 6.7-12.9). The median time between the discontinuation of CPI and allo-HCT was 5.78 months (IQR 3.15-15.8). The median time to progression postallo-HCT was 5.75 months (IQR 2.6-11.7). The median time between allo-HCT and re-challenge with a CPI was 7.6 months (IQR 3.2-28.6). The median time of follow up after starting postallo-HCT CPIs was 16 months (IQR 7.25-25.75). Five out six patients responded and two patients developed GvHD.

CONCLUSION

Our report shows preserved efficacy without any new safety signals by using CPIs postallo-HCT despite using and having failed to derive sustained benefit from CPIs preallo-HCT.

Immunopathological insights into villitis of unknown etiology on the basis of transplant immunology

Villitis of unknown etiology (VUE) is an inflammatory disease characterized by the infiltration of maternal CD8 +T cells into the placental villi. Although the pathogenesis of VUE is still debated, dysregulation of the immune system appears to be an important factor in the development of the disease. Interaction of maternal T cells with the fetal antigens seems to be the trigger for the VUE onset. In this context, graft vs host disease (GVHD) and allographic rejection seem to share similarities in the VUE immunopathological mechanism, especially those related to immunoregulation. In this review, we compared the immunological characteristics of VUE with allograft rejection, and GVHD favoring a better knowledge of VUE pathogenesis that may contribute to VUE therapeutics strategies in the future.

PD-1 Checkpoint Blockade in Patients for Acute Myeloid Leukemia after HSCT Relapse Resulted in Severe GVHD and sHLH

Treatment with immune checkpoint inhibitors (ICI) such as carrizumab leads to immune-mediated adverse effects including severe acute graft versus host disease (aGVHD) and secondary hemophagocytic syndrome (sHLH). Herein, we present two cases where aGVHD and sHLH developed after ICI administration, which was treated using methylprednisolone (MP). They developed high-grade fever complicated with liver dysfunction and diarrhea 1 day after ICI administration. Treatment with MP does not alleviate symptoms because of steroid resistance. Hyperbilirubinemia, rash with blisters, and watery diarrhea showed severe aGVHD. Hyperferritinemia, hypertriglyceridemia, and cytopenias suggested the diagnosis of HLH and met the criteria for sHLH diagnosis. They were thus administered intravenous high-dose MP, methotrexate (MTX), basiliximab, ruxolitinib, etc, which resolved these symptoms.

Depletion of exhausted alloreactive T cells enables targeting of stem-like memory T cells to generate tumor-specific immunity

Some hematological malignancies such as multiple myeloma are inherently resistant to immune-mediated antitumor responses, the cause of which remains unknown. Allogeneic bone marrow transplantation (alloBMT) is the only curative immunotherapy for hematological malignancies due to profound graft-versus-tumor (GVT) effects, but relapse remains the major cause of death. We developed murine models of alloBMT where the hematological malignancy is either sensitive [acute myeloid leukemia (AML)] or resistant (myeloma) to GVT effects. We found that CD8+ T cell exhaustion in bone marrow was primarily alloantigen-driven, with expression of inhibitory ligands present on myeloma but not AML. Because of this tumor-independent exhaustion signature, immune checkpoint inhibition (ICI) in myeloma exacerbated graft-versus-host disease (GVHD) without promoting GVT effects. Administration of post-transplant cyclophosphamide (PT-Cy) depleted donor T cells with an exhausted phenotype and spared T cells displaying a stem-like memory phenotype with chromatin accessibility present in cytokine signaling genes, including the interleukin-18 (IL-18) receptor. Whereas ICI with anti-PD-1 or anti-TIM-3 remained ineffective after PT-Cy, administration of a decoy-resistant IL-18 (DR-18) strongly enhanced GVT effects in both myeloma and leukemia models, without exacerbation of GVHD. We thus defined mechanisms of resistance to T cell-mediated antitumor effects after alloBMT and described an immunotherapy approach targeting stem-like memory T cells to enhance antitumor immunity.

Targeting Glycolysis in Alloreactive T Cells to Prevent Acute Graft-Versus-Host Disease While Preserving Graft-Versus-Leukemia Effect

Alloreactive donor T cells undergo extensive metabolic reprogramming to become activated and induce graft-versus-host disease (GVHD) upon alloantigen encounter. It is generally thought that glycolysis, which promotes T cell growth and clonal expansion, is employed in this process. However, conflicting data have been reported regarding the requirement of glycolysis to induce T cell-mediated GVHD due to the lack of T cell-specific treatments using glycolysis inhibitors. Importantly, previous studies have not evaluated whether graft-versus-leukemia (GVL) activity is preserved in donor T cells deficient for glycolysis. As a critical component affecting the clinical outcome, it is necessary to assess the anti-tumor activity following treatment with metabolic modulators in preclinical models. In the present study, we utilized T cells selectively deficient for glucose transporter 1 (Glut1^T-KO), to examine the role of glycolysis exclusively in alloreactive T cells without off-targeting effects from antigen presenting cells and other cell types that are dependent on glycolysis. We demonstrated that transfer of Glut1^T-KO T cells significantly improved acute GVHD outcomes through increased apoptotic rates, impaired expansion, and decreased proinflammatory cytokine production. In addition to impaired GVHD development, donor Glut1^T-KO T cells mediated sufficient GVL activity to protect recipients from tumor development. A clinically relevant approach using donor T cells treated with a small molecule inhibitor of glycolysis, 2-Deoxy-D-glucose ex vivo, further demonstrated protection from tumor development. These findings indicate that treatment with glycolysis inhibitors prior to transplantation selectively eliminates alloreactive T cells, but spares non-alloreactive T cells including those that protect against tumor growth. The present study has established a definitive role for glycolysis in acute GVHD and demonstrated that acute GVHD can be selectively prevented through targeting glycolysis.

Real-world outcomes of autologous and allogeneic hematopoietic stem cell transplantation for relapsed/refractory Hodgkin lymphoma in the era of novel therapies: a Canadian perspective

BACKGROUND

Despite high cure rates with frontline therapy for Hodgkin lymphoma (HL), about 30% of patients will relapse or have primary refractory disease (R/r). Autologous hematopoietic stem cell transplantation (AHSCT) is the standard of care for R/r disease and alloHSCT is a curative option for patients in second relapse. Novel agents are being incorporated for the treatment of R/r HL, such that the optimal timing of transplant is currently being challenged. Additionally, because access to these new agents varies amongst transplant centers, we sought to offer a Canadian perspective to the treatment of R/r HL and demonstrate the utility and effectiveness of both AHSCT and alloHSCT for the treatment of R/r HL.

METHODS

This single-center retrospective study examined outcomes in 89 consecutive patients with R/r HL treated with AHSCT between January 2007 and December 2019. A total of 17 patients underwent alloHSCT either as a tandem auto-allo approach or as salvage therapy.

RESULTS

With a median follow-up of 5.0 years, the estimated 5-year PFS and OS for patients undergoing AHSCT were 57.5% (95% CI 45.2-68.0) and 81.3% (95% CI 70.0-88.8), respectively. Corresponding figures for patients who underwent alloHSCT were 76.5% (95% CI 48.8-90.4) and 82.4% (95% CI 54.7-93.9). NRM at 100 days and 5 years was 0% and 9.4% post AHSCT and 0% and 5.9% post alloHSCT. Cumulative incidence (CI) of acute GVHD at day +100 was 35.3% (95% CI 17.7 - 62.3) and CI of chronic GVHD at 1 year was 23.5% (95% CI 6.9 - 45.8).

CONCLUSIONS

AHSCT and alloHSCT can both provide robust and prolonged disease control and new agents should be used as a bridge to improve the curative potential of these definitive cellular therapies.

Treatment of Cutaneous Squamous Cell Carcinoma with Immune Checkpoint Inhibitors in Special Populations

Cutaneous squamous cell carcinoma (cSCC) may develop in patients with dysregulated immune activation (pre-existing autoimmune diseases or immunosuppression due to hematopoietic/solid organ transplant recipients), patients with a compromised immune function (long-term immunosuppression), and patients carrying chronic viral infections, or those affected by lymphoproliferative diseases. It should be also considered that patients presenting with immunosuppression have a high incidence of cSCC (65-250-times higher than general population), highlighting the central role played by the immune system in the development of cSCC. All these cases must be considered as "special populations" for treatment with immune checkpoint inhibitors (ICIs), as the safety and activity of these drugs have not been studied on these specific cases, since these patients were excluded from clinical trials leading to approval of ICIs. It is therefore important to gain as much information as possible from the analysis of real-life data, to derive an indication to be adopted in everyday clinical setting. Moreover, therapeutic alternatives other than ICIs are scarce, mainly consisting in chemotherapy and anti-EGFR agents, whose activity is lower than immunotherapy and whose toxicity (particularly with chemotherapy) are not sustainable by this frail population. Here, we describe the current evidence of treatment with ICIs in special populations and conclude that it is necessary to find a balance between treatment risks (toxicities) and benefits (efficacy), as well as engaging a multidisciplinary team of experts to thoroughly manage and treat these patients.

T-Cell Metabolism in Graft Versus Host Disease

Allogeneic-hematopoietic stem cell transplantation (allo-HSCT) represents the only curative treatment option for numerous hematological malignancies. Elimination of malignant cells depends on the T-cells' Graft-versus-Tumor (GvT) effect. However, Graft-versus-Host-Disease (GvHD), often co-occurring with GvT, remains an obstacle for therapeutic efficacy. Hence, approaches, which selectively alleviate GvHD without compromising GvT activity, are needed. As already explored for autoimmune and inflammatory disorders, immuno-metabolic interventions pose a promising option to address this unmet challenge. Being embedded in a complex regulatory framework, immunological and metabolic pathways are closely intertwined, which is demonstrated by metabolic reprograming of T-cells upon activation or differentiation. In this review, current knowledge on the immuno-metabolic signature of GvHD-driving T-cells is summarized and approaches to metabolically interfere are outlined. Furthermore, we address the metabolic impact of standard medications for GvHD treatment and prophylaxis, which, in conjunction with the immuno-metabolic profile of alloreactive T-cells, could allow more targeted interventions in the future.

Recent Metabolic Advances for Preventing and Treating Acute and Chronic Graft Versus Host Disease

The therapeutic efficacy of allogeneic hematopoietic stem cell transplantation (allo-HSCT) is limited by the development of graft-versus-host disease (GVHD). In GVHD, rigorous pre-conditioning regimen resets the immune landscape and inflammatory milieu causing immune dysregulation, characterized by an expansion of alloreactive cells and a reduction in immune regulatory cells. In acute GVHD (aGVHD), the release of damage- and pathogen- associated molecular patterns from damaged tissue caused by the conditioning regimen sets the stage for T cell priming, activation and expansion further exacerbating tissue injury and organ damage, particularly in the gastrointestinal tract. Studies have shown that donor T cells utilize multiple energetic and biosynthetic pathways to mediate GVHD that can be distinct from the pathways used by regulatory T cells for their suppressive function. In chronic GVHD (cGVHD), donor T cells may differentiate into IL-21 producing T follicular helper cells or tissue resident T helper cells that cooperate with germinal center B cells or memory B cells, respectively, to produce allo- and auto-reactive antibodies with subsequent tissue fibrosis. Alternatively, donor T cells can become IFN- γ/IL-17 cytokine expressing T cells that mediate sclerodermatous skin injury. Patients refractory to the first line standard regimens for GVHD treatment have a poor prognosis indicating an urgent need for new therapies to restore the balance between effector and regulatory immune cells while preserving the beneficial graft-versus-tumor effect. Emerging data points toward a role for metabolism in regulating these allo- and auto-immune responses. Here, we will discuss the preclinical and clinical data available on the distinct metabolic demands of acute and chronic GVHD and recent efforts in identifying therapeutic targets using metabolomics. Another dimension of this review will examine the changing microbiome after allo-HSCT and the role of microbial metabolites such as short chain fatty acids and long chain fatty acids on regulating immune responses. Lastly, we will examine the metabolic implications of coinhibitory pathway blockade and cellular therapies in allo-HSCT. In conclusion, greater understanding of metabolic pathways involved in immune cell dysregulation during allo-HSCT may pave the way to provide novel therapies to prevent and treat GVHD.

Harnessing Treg Homeostasis to Optimize Posttransplant Immunity: Current Concepts and Future Perspectives

CD4⁺CD25⁺Foxp3⁺ regulatory T cells (Tregs) are functionally distinct subsets of mature T cells with broad suppressive activity and have been shown to play an important role in the establishment of immune tolerance after allogeneic hematopoietic stem cell transplantation (HSCT). Tregs exhibit an activated phenotype from the stage of emigration from the thymus and maintain continuous proliferation in the periphery. The distinctive feature in homeostasis enables Tregs to respond sensitively to small environmental changes and exert necessary and sufficient immune suppression; however, on the other hand, it also predisposes Tregs to be susceptible to apoptosis in the inflammatory condition post-transplant. Our studies have attempted to define the intrinsic and extrinsic factors affecting Treg homeostasis from the acute to chronic phases after allogeneic HSCT. We have found that altered cytokine environment in the prolonged post-HSCT lymphopenia or peri-transplant use of immune checkpoint inhibitors could hamper Treg reconstitution, leading to refractory graft-versus-host disease. Using murine models and clinical trials, we have also demonstrated that proper intervention with low-dose interleukin-2 or post-transplant cyclophosphamide could restore Treg homeostasis and further amplify the suppressive function after HSCT. The purpose of this review is to reconsider the distinctive characteristics of post-transplant Treg homeostasis and discuss how to harness Treg homeostasis to optimize posttransplant immunity for developing a safe and efficient therapeutic strategy.

Allogeneic transplantation after PD-1 blockade for classic Hodgkin lymphoma

Anti-PD-1 monoclonal antibodies yield high response rates in patients with relapsed/refractory classic Hodgkin lymphoma (cHL), but most patients will eventually progress. Allogeneic hematopoietic cell transplantation (alloHCT) after PD-1 blockade may be associated with increased toxicity, raising challenging questions about the role, timing, and optimal method of transplantation in this setting. To address these questions, we assembled a retrospective cohort of 209 cHL patients who underwent alloHCT after PD-1 blockade. With a median follow-up among survivors of 24 months, the 2-year cumulative incidences (CIs) of non-relapse mortality and relapse were 14 and 18%, respectively; the 2-year graft-versus-host disease (GVHD) and relapse-free survival (GRFS), progression-free survival (PFS), and overall survival were 47%, 69%, and 82%, respectively. The 180-day CI of grade 3-4 acute GVHD was 15%, while the 2-year CI of chronic GVHD was 34%. In multivariable analyses, a longer interval from PD-1 to alloHCT was associated with less frequent severe acute GVHD, while additional treatment between PD-1 and alloHCT was associated with a higher risk of relapse. Notably, post-transplant cyclophosphamide (PTCy)-based GVHD prophylaxis was associated with significant improvements in PFS and GRFS. While awaiting prospective clinical trials, PTCy-based GVHD prophylaxis may be considered the optimal transplantation strategy for this patient population.

Synchronizing the use of allogeneic hematopoietic cell transplantation in checkpoint blockade therapy for Hodgkin lymphoma

INTRODUCTION

The use of checkpoint blockade therapy (CBT) has shown impressive results for the treatment of relapsed/refractory Hodgkin lymphoma (cHL). The impact of CBT depends on the reversal of an exhausted T-cell immune phenotype and a consequential increase in the immunological, anti-tumor effect derived from a patient's adaptive immunity. As most patients with classical Hodgkin lymphoma will relapse during or after this treatment, clinicians often provide consolidation with allogeneic hematopoietic cell transplantation (alloHCT) in fit patients. However, the mechanisms responsible for CBT efficacy can also be those that increase the risk of immunological complications after alloHCT.

AREAS COVERED

We carried out in-depth research on the current medical literature to report and discuss the mechanism of action of CBT within a cHL setting; clinical results of CBT in cHL setting pre-alloHCT and post-alloHCT; interactions between CBT and alloHCT; and further clinical considerations.

EXPERT OPINION

Checkpoint blockade therapy is an effective strategy for relapsed/refractory cHL. Its use is associated with higher immunological toxicities when administered before or after alloHCT. Whenever alloHCT is planned, clinicians should follow international recommendations such as using post-transplant cyclophosphamide GVHD prophylaxis.

Graft-versus-host disease: a disorder of tissue regeneration and repair

Regenerative failure at barrier surfaces and maladaptive repair leading to fibrosis are hallmarks of graft-versus-host disease (GVHD). Although immunosuppressive treatment can control inflammation, impaired tissue homeostasis leads to prolonged organ damage and impaired quality of life. In this Spotlight article, we review recent research that addresses the critical failures in tissue regeneration and repair that underpin treatment-resistant GVHD. We highlight current interventions designed to overcome these defects and provide our assessment of the future therapeutic landscape.

Myelodysplastic syndrome and immunotherapy novel to next in-line treatments

Patients with Myelodysplastic syndromes (MDS) have few therapy options for sustainable responses in the frontline setting, and even less after hypomethylating agent (HMA) failure in relapsed and refractory setting. The only potential cure is an allogeneic hematopoietic stem cell transplant which is an unrealistic option for the majority of MDS patients. Immunotherapy with checkpoint inhibition, CAR-T cells, and vaccine therapy few have shown promise in a variety cancer and have now been tested in patients with MDS. Most trials have focused on AML patients and included small numbers of MDS patients. Until now, a dedicated review of immunotherapy outcomes in MDS patients has been lacking. Thus, herein we review outcomes of MDS patients after immunotherapies on a variety of clinical trials reported to date.

Deletion of AMPK minimizes graft-versus-host disease through an early impact on effector donor T cells

Allogeneic hematopoietic stem cell transplantation is a viable treatment for multiple hematologic diseases, but its application is often limited by graft-versus-host disease (GVHD), where donor T cells attack host tissues in the skin, liver, and gastrointestinal tract. Here, we examined the role of the cellular energy sensor AMP kinase (AMPK) in alloreactive T cells during GVHD development. Early posttransplant, AMPK activity increased more than 15-fold in allogeneic T cells, and transplantation of T cells deficient in both AMPKα1 and AMPKα2 decreased GVHD severity in multiple disease models. Importantly, a lack of AMPK lessened GVHD without compromising antileukemia responses or impairing lymphopenia-driven immune reconstitution. Mechanistically, absence of AMPK decreased both CD4⁺ and CD8⁺ effector T cell numbers as early as day 3 posttransplant, while simultaneously increasing regulatory T cell (Treg) percentages. Improvements in GVHD resulted from cell-intrinsic perturbations in conventional effector T cells as depletion of donor Tregs had minimal impact on AMPK-related improvements. Together, these results highlight a specific role for AMPK in allogeneic effector T cells early posttransplant and suggest that AMPK inhibition may be an innovative approach to mitigate GVHD while preserving graft-versus-leukemia responses and maintaining robust immune reconstitution.

A human CD137×PD-L1 bispecific antibody promotes anti-tumor immunity via context-dependent T cell costimulation and checkpoint blockade

Immune checkpoint inhibitors demonstrate clinical activity in many tumor types, however, only a fraction of patients benefit. Combining CD137 agonists with these inhibitors increases anti-tumor activity preclinically, but attempts to translate these observations to the clinic have been hampered by systemic toxicity. Here we describe a human CD137xPD-L1 bispecific antibody, MCLA-145, identified through functional screening of agonist- and immune checkpoint inhibitor arm combinations. MCLA-145 potently activates T cells at sub-nanomolar concentrations, even under suppressive conditions, and enhances T cell priming, differentiation and memory recall responses. In vivo, MCLA-145 anti-tumor activity is superior to immune checkpoint inhibitor comparators and linked to recruitment and intra-tumor expansion of CD8 + T cells. No graft-versus-host-disease is observed in contrast to other antibodies inhibiting the PD-1 and PD-L1 pathway. Non-human primates treated with 100 mg/kg/week of MCLA-145 show no adverse effects. The conditional activation of CD137 signaling by MCLA-145, triggered by neighboring cells expressing >5000 copies of PD-L1, may provide both safety and potency advantages.

Vulnerability to reservoir reseeding due to high immune activation after allogeneic hematopoietic stem cell transplantation in individuals with HIV-1

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is the only medical intervention that has led to an HIV cure. Whereas the HIV reservoir sharply decreases after allo-HSCT, the dynamics of the T cell reconstitution has not been comprehensively described. We analyzed the activation and differentiation of CD4⁺ and CD8⁺ T cells, and the breadth and quality of HIV- and CMV-specific CD8⁺ T cell responses in 16 patients with HIV who underwent allo-HSCT (including five individuals who received cells from CCR5Δ32/Δ32 donors) to treat their underlying hematological malignancy and who remained on antiretroviral therapy (ART). We found that reconstitution of the T cell compartment after allo-HSCT was slow and heterogeneous with an initial expansion of activated CD4⁺ T cells that preceded the expansion of CD8⁺ T cells. Although HIV-specific CD8⁺ T cells disappeared immediately after allo-HSCT, weak HIV-specific CD8⁺ T cell responses were detectable several weeks after transplant and could still be detected at the time of full T cell chimerism, indicating that de novo priming, and hence antigen exposure, occurred during the time of T cell expansion. These HIV-specific T cells had limited functionality compared with CMV-specific CD8⁺ T cells and persisted years after allo-HSCT. In conclusion, immune reconstitution was slow, heterogeneous, and incomplete and coincided with de novo detection of weak HIV-specific T cell responses. The initial short phase of high T cell activation, in which HIV antigens were present, may constitute a window of vulnerability for the reseeding of viral reservoirs, emphasizing the importance of maintaining ART directly after allo-HSCT.

Immune checkpoint inhibitors in lymphoma: challenges and opportunities

Immune checkpoint inhibitors (ICIs) are immunomodulatory antibodies that intensify the host immune response, thereby leading to cytotoxicity. The primary targets for checkpoint inhibition have included cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), programmed cell death receptor-1 (PD-1) or programmed cell death ligand-1 (PD-L1). ICIs have resulted in a change in treatment landscape of various neoplasms. Among hematologic malignancies, ICIs have been most successful in certain subtypes of lymphomas such as classic Hodgkin lymphoma (cHL) and primary mediastinal B-cell lymphoma (PMBCL). However, there have been several challenges in harnessing the host immune system through ICI use in other lymphomas. The underlying reasons for the low efficacy of ICI monotherapy in most lymphomas may include defects in antigen presentation, non-inflamed tumor microenvironment (TME), immunosuppressive metabolites, genetic factors, and an overall lack of predictive biomarkers of response. In this review, we outline the existing and ongoing studies utilizing ICI therapy in various lymphomas. We also describe the challenges leading to the lack of efficacy with ICI use and discuss potential strategies to overcome those challenges including: chimeric antigen receptor T-cell therapy (CAR-T therapy), bispecific T-cell therapy (BiTE), lymphocyte activation gene-3 (LAG-3) inhibitors, T-cell immunoglobulin and mucin-domain containing-3 (TIM-3) inhibitors, vaccines, promotion of inflammatory macrophages, indoleamine 2,3-dioxygenase 1 (IDO1) inhibitors, DNA methyltransferase inhibitors (DNMTi) and histone deacetylase inhibitors (HDACi). Tumor mutational burden and interferon-gamma release assays are potential biomarkers of ICI treatment response beyond PD-L1 expression. Further collaborations between clinicians and scientists are vital to understand the immunopathology in ICI therapy in order to improve clinical outcomes.

Current Concepts and Advances in Graft-Versus-Host Disease Immunology

Worldwide, each year over 30,000 patients undergo an allogeneic hema-topoietic stem cell transplantation with the intent to cure high-risk hematologic malignancy, immunodeficiency, metabolic disease, or a life-threatening bone marrow failure syndrome. Despite substantial advances in donor selection and conditioning regimens and greater availability of allograft sources, transplant recipients still endure the morbidity and mortality of graft-versus-host disease (GVHD). Herein, we identify key aspects of acute and chronic GVHD pathophysiology, including host/donor cell effectors, gut dysbiosis, immune system and cytokine imbalance, and the interface between inflammation and tissue fibrosis. In particular, we also summarize the translational application of this heightened understanding of immune dysregulation in the design of novel therapies to prevent and treat GVHD.

Case Report: Combination Therapy With PD-1 Blockade for Acute Myeloid Leukemia After Allogeneic Hematopoietic Stem Cell Transplantation Resulted in Fatal GVHD

Background: Azacitidine is commonly used in the treatment of relapsed acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) after allogeneic hematopoietic stem cell transplantation (allo-HSCT), but the effectiveness of this monotherapy is still very low. A possible mechanism of resistance to hypomethylating agents (HMAs) is the upregulation of the expression of inhibitory checkpoint receptors and their ligands, making the combination of HMAs and immune checkpoint blockade therapy a rational approach. Although the safety of anti-programmed cell death protein (PD)-1 antibodies for patients with post-allo-HSCT remains a complicated issue, the preliminary clinical result of combining azacitidine with anti-PD-1 antibodies is encouraging; however, the safety and efficacy of this approach need further investigation. Case Presentation: We reported a case of treated secondary (ts)-AML in a patient who received tislelizumab (an anti-PD-1 antibody) in combination with azacitidine. The patient relapsed after allo-HSCT and was previously exposed to HMAs-based therapy. The patient received tislelizumab for compassionate use. After the combination treatment, the patient achieved complete remission with incomplete hematologic recovery, negative minimal residual disease (MRD) by flow cytometry (FCM), and negative Wilms' tumor protein 1 (WT1). However, the patient successively developed serious immune-related adverse events (irAEs) and graft vs. host disease (GVHD) and eventually died from complications of GVHD. Conclusion: To our knowledge, this is the first case to report the combined use of tislelizumab and azacitidine to treat relapsed AML posttransplantation. This report highlights the safety concerns of using an anti-PD-1 antibody in combination with azacitidine after allo-HSCT, especially the risk of GVHD, and provides a basis for future studies.

The Role of Immune Checkpoint Molecules for Relapse After Allogeneic Hematopoietic Cell Transplantation

Immune checkpoint molecules represent physiological brakes of the immune system that are essential for the maintenance of immune homeostasis and prevention of autoimmunity. By inhibiting these negative regulators of the immune response, immune checkpoint blockade can increase anti-tumor immunity, but has been primarily successful in solid cancer therapy and Hodgkin lymphoma so far. Allogeneic hematopoietic cell transplantation (allo-HCT) is a well-established cellular immunotherapy option with the potential to cure hematological cancers, but relapse remains a major obstacle. Relapse after allo-HCT is mainly thought to be attributable to loss of the graft-versus-leukemia (GVL) effect and hence escape of tumor cells from the allogeneic immune response. One potential mechanism of immune escape from the GVL effect is the inhibition of allogeneic T cells via engagement of inhibitory receptors on their surface including PD-1, CTLA-4, TIM3, and others. This review provides an overview of current evidence for a role of immune checkpoint molecules for relapse and its treatment after allo-HCT, as well as discussion of the immune mediated side effect graft-vs.-host disease. We discuss the expression of different immune checkpoint molecules on leukemia cells and T cells in patients undergoing allo-HCT. Furthermore, we review mechanistic insights gained from preclinical studies and summarize clinical trials assessing immune checkpoint blockade for relapse after allo-HCT.

Successful Use of Nivolumab in a Patient with Head and Neck Cancer After Allogeneic Bone Marrow Transplantation

Recently, programmed cell death 1(PD-1) inhibitors have shown a significant curative effect in the treatment of most solid cancers and some hematological malignancies. The effects of PD-1 inhibitors in recurrent head and neck squamous cell carcinoma (HNSCC) have also been confirmed. However, there is a lack of reliable clinical evidence to confirm the safety and efficacy of PD-1 inhibitors in patients after allogeneic hematopoietic stem cell transplantation, especially when the patient has a second primary cancer. Generally, graft-versus-host disease (GVHD) is unpredictable among these patients. Here we report the case of a patient who successfully used nivolumab without any GVHD or other immune-related adverse events for HNSCC after allogeneic bone marrow transplantation because of the Philadelphia chromosome-positive T cell acute lymphoblastic leukemia.

A multicenter phase 1 study of nivolumab for relapsed hematologic malignancies after allogeneic transplantation

Programmed cell death-1 (PD-1)/programmed death ligand-1 blockade may potentially augment graft-vs-tumor effects following allogeneic hematopoietic cell transplantation (alloHCT), but retrospective studies of anti-PD-1 therapy reported substantial toxicity from graft-versus-host-disease (GVHD). Here, we report the results of a prospective clinical trial of PD-1 blockade for relapsed hematologic malignancies (HMs) after alloHCT (NCT01822509). The primary objective in this phase 1 multicenter, investigator-initiated study was to determine maximum tolerated dose and safety. Secondary objectives were to assess efficacy and immunologic activity. Patients with relapsed HMs following alloHCT were eligible. Nivolumab was administered every 2 weeks until progression or unacceptable toxicity, starting with a 1-mg/kg cohort, with planned deescalation based on toxicity to a 0.5-mg/kg cohort. Twenty-eight patients were treated (n = 19 myeloid, n = 9 lymphoid). Median age was 57 years (range 27-76), and median time from alloHCT to enrollment was 21 months (range 5.6-108.5). Two of 6 patients treated at 1 mg/kg experienced dose-limiting toxicity (DLT) from immune-related adverse events (irAEs). Twenty-two patients were treated at 0.5 mg/kg, and 4 DLTs occurred, including 2 irAEs and 2 with fatal GVHD. The overall response rate in efficacy-evaluable patients was 32% (8/25). With a median follow-up of 11 months, the 1-year progression-free survival and overall survival were 23% and 56%, respectively. In this first prospective clinical trial of an anti-PD-1 antibody for post-alloHCT relapse, GVHD and irAEs occurred, requiring dose deescalation, with only modest antitumor activity. Further studies of anti-PD-1 therapy post-alloHCT may require specific toxicity mitigation strategies. This trial was registered at www.clinicaltrials.gov as #NCT01822509.

Ruxolitinib Inhibits IFNγ Licensing of Human Bone Marrow Derived Mesenchymal Stromal Cells

Ruxolitinib is a JAK2/JAK1 inhibitor that blocks the inflammatory JAK-STAT signaling pathway. Ruxolitinib has been demonstrated to be effective in the treatment of steroid-resistant acute graft-versus-host disease (GVHD). Ruxolitinib's effect on inflammatory cells of hematopoietic origin is known. However, its effect on nonhematopoietic cell types with immune-modulating and antigen-presenting cell competency plausibly involved in pathogenesis of GVHD has not been explored. Mesenchymal stromal cells (MSCs) are CD45^- nonhematopoietic cells of the bone marrow with immune modulatory functions in vivo. MSCs' immunobiology largely depends on their responsiveness to IFNγ. We aimed to define the effect of ruxolitinib on the immunobiology of MSCs that are modulated by IFNγ. Human bone marrow derived MSCs, peripheral blood mononuclear cells (PBMCs), and primary bone marrow aspirates were analyzed for their sensitivity to ruxolitinib-mediated blocking of IFNγ-induced STAT-1 phosphorylation and downstream effector molecules, utilizing Western blot, flow cytometry, secretome analysis, and phosflow techniques. IFNγ-induced cytostatic effects on MSCs are reversed by ruxolitinib. Ruxolitinib inhibits IFNγ and secretome of activated peripheral PBMC-induced STAT-1 phosphorylation on human bone marrow derived MSCs. In addition, ruxolitinib inhibits IFNγ-induced pro-GVHD pathways on MSCs, which includes HLAABC(MHCI), HLADR(MHCII), CX3CL1, and CCL2. IFNγ-induced immunosuppressive molecules IDO and PDL-1 were also inhibited by ruxolitinib on MSCs. Comparative analysis with PBMCs has demonstrated that MSCs are as equal as to HLADR⁺ PBMC populations in responding to ruxolitinib-mediated inhibition of IFNγ-induced STAT-1 phosphorylation. Ex vivo analysis of human marrow aspirates has demonstrated that ruxolitinib blocks IFNγ-induced STAT-1 phosphorylation in CD45^+/-HLADR^+/- populations at different levels, which is depending on their sensitivity to IFNγ responsiveness. These results inform the hypothesis that ruxolitinib's immune-modulatory effects in vivo may pharmacologically involve marrow and tissue-resident MSCs. Ruxolitinib affects the immunobiology of MSCs equivalent to professional HLADR⁺ antigen presenting cells, which collectively mitigate GVHD.

Impaired T- and NK-cell reconstitution after haploidentical HCT with posttransplant cyclophosphamide

Administration of posttransplant cyclophosphamide (PTCy) has significantly expanded the number of patients undergoing HLA-haploidentical hematopoietic cell transplantation (haplo-HCT). To examine immune reconstitution in these patients, we monitored T- and natural killer (NK)-cell recovery in 60 patients receiving bone marrow or peripheral blood stem cell (PBSC) grafts after haplo-HCT with PTCy and 35 patients receiving HLA-matched donor PBSC grafts with standard graft-versus-host disease (GVHD) prophylaxis. Compared with HLA-matched recipients, early T-cell recovery was delayed in haplo-HCT patients and skewed toward effector memory T cells with markedly reduced naive T cells. We found higher regulatory T (Treg)-cell/conventional T (Tcon)-cell ratios early after HCT and increased PD-1 expression on memory T cells. Within the haplo-HCT, patients who did not develop chronic GVHD (cGVHD) had higher PD-1 expression on central and effector memory CD4+ Treg cells at 1 month after transplant. These findings suggest an immunologic milieu that promotes immune tolerance in haplo-HCT patients. NK cells were decreased early after haplo-HCT with preferential expansion of immature CD56brightCD16- NK cells compared with matched donor transplants. One month after transplant, mass cytometry revealed enrichment of immature NK-cell metaclusters with high NKG2A, low CD57, and low killer-cell immunoglobulin-like receptor expression after haplo-HCT, which partially recovered 3 months post-HCT. At 2 months, immature NK cells from both groups were functionally impaired, but interleukin-15 priming corrected these defects in vitro. Increased immature/mature NK-cell ratios were associated with cytomegalovirus reactivation and increased incidence of cGVHD after haplo-HCT. These homeostatic imbalances in T- and NK-cell reconstitution after haplo-HCT reveal opportunities for early immune-based interventions to optimize clinical outcomes.

Role of PD-L1 in Gut Mucosa Tolerance and Chronic Inflammation

The gastrointestinal (GI) mucosa is among the most complex systems in the body. It has a diverse commensal microbiome challenged continuously by food and microbial components while delivering essential nutrients and defending against pathogens. For these reasons, regulatory cells and receptors are likely to play a central role in maintaining the gut mucosal homeostasis. Recent lessons from cancer immunotherapy point out the critical role of the B7 negative co-stimulator PD-L1 in mucosal homeostasis. In this review, we summarize the current knowledge supporting the critical role of PD-L1 in gastrointestinal mucosal tolerance and how abnormalities in its expression and signaling contribute to gut inflammation and cancers. Abnormal expression of PD-L1 and/or the PD-1/PD-L1 signaling pathways have been observed in the pathology of the GI tract. We also discuss the current gap in our knowledge with regards to PD-L1 signaling in the GI tract under homeostasis and pathology. Finally, we summarize the current understanding of how this pathway is currently targeted to develop novel therapeutic approaches.

PTCy ameliorates GVHD by restoring regulatory and effector T-cell homeostasis in recipients with PD-1 blockade

Graft-versus-host disease (GVHD) after allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a significant cause of morbidity and mortality. Regulatory T cells (Tregs) are critical mediators of immune tolerance after allo-HSCT. Clinical studies have indicated that programmed cell death 1 (PD-1) blockade before allo-HSCT involves a risk of severe GVHD. However, the mechanisms underlying GVHD induction resulting from PD-1 blockade remain unclear. We investigated the impact of PD-1 expression of donor T cells on T-cell reconstitution and GVHD using murine models. We first demonstrated that inhibition of PD-1 signaling induced aggressive expansion of CD4+ conventional T cells; however, Tregs could not maintain expansion because of high susceptibility to apoptosis, resulting in discordant immune recovery and subsequent development of severe GVHD. We then evaluated the impact of posttransplantation cyclophosphamide (PTCy) on abnormal T-cell reconstitution after PD-1 blockade. PTCy efficiently ameliorated GVHD after transplantation from a PD-1-/- donor and extended overall survival by safely regulating the proliferation and apoptosis of T-cell subsets. Notably, in the first 2 weeks after administration of PTCy, Tregs regained their ability to continuously proliferate, resulting in well-balanced reconstitution of donor T-cell subsets. In conclusion, the influence of PD-1 blockade differed within T-cell subsets and caused unbalanced reconstitution of T-cell subsets, resulting in severe GVHD. PTCy successfully restored T-cell homeostasis and ameliorated GVHD induced by PD-1-/- donor T cells. These findings may help explain the pathophysiology behind the observation that PTCy may mitigate the incidence and impact of GVHD associated with prior exposure to PD-1 blockade.

T cell exhaustion and a failure in antigen presentation drive resistance to the graft-versus-leukemia effect

In hematopoietic cell transplants, alloreactive T cells mediate the graft-versus-leukemia (GVL) effect. However, leukemia relapse accounts for nearly half of deaths. Understanding GVL failure requires a system in which GVL-inducing T cells can be tracked. We used such a model wherein GVL is exclusively mediated by T cells that recognize the minor histocompatibility antigen H60. Here we report that GVL fails due to insufficient H60 presentation and T cell exhaustion. Leukemia-derived H60 is inefficiently cross-presented whereas direct T cell recognition of leukemia cells intensifies exhaustion. The anti-H60 response is augmented by H60-vaccination, an agonist αCD40 antibody (FGK45), and leukemia apoptosis. T cell exhaustion is marked by inhibitory molecule upregulation and the development of TOX+ and CD39-TCF-1+ cells. PD-1 blockade diminishes exhaustion and improves GVL, while blockade of Tim-3, TIGIT or LAG3 is ineffective. Of all interventions, FGK45 administration at the time of transplant is the most effective at improving memory and naïve T cell anti-H60 responses and GVL. Our studies define important causes of GVL failure and suggest strategies to overcome them.

Immune-Checkpoint Blockade Therapy in Lymphoma

Tumor cells use immune-checkpoint pathways to evade the host immune system and suppress immune cell function. These cells express programmed cell-death protein 1 ligand 1 (PD-L1)/PD-L2, which bind to the programmed cell-death protein 1 (PD-1) present on cytotoxic T cells, trigger inhibitory signaling, and reduce cytotoxicity and T-cell exhaustion. Immune-checkpoint blockade can inhibit this signal and may serve as an effective therapeutic strategy in patients with solid tumors. Several trials have been conducted on immune-checkpoint inhibitor therapy in patients with malignant lymphoma and their efficacy has been reported. For example, in Hodgkin lymphoma, immune-checkpoint blockade has resulted in response rates of 65% to 75%. However, in non-Hodgkin lymphoma, the response rate to immune-checkpoint blockade was lower. In this review, we evaluate the biology of immune-checkpoint inhibition and the current data on its efficacy in malignant lymphoma, and identify the cases in which the treatment was more effective.

Metabolic Pathways in Alloreactive T Cells

Allogeneic hematopoietic stem cell transplantation (aHSCT) is a curative therapy for a range of hematologic illnesses including aplastic anemia, sickle cell disease, immunodeficiency, and high-risk leukemia, but the efficacy of aHSCT is often undermined by graft-versus-host disease (GVHD), where T cells from the donor attack and destroy recipient tissues. Given the strong interconnection between T cell metabolism and cellular function, determining the metabolic pathways utilized by alloreactive T cells is fundamental to deepening our understanding of GVHD biology, including its initiation, propagation, and potential mitigation. This review summarizes the metabolic pathways available to alloreactive T cells and highlights key metabolic proteins and pathways linking T cell metabolism to effector function. Our current knowledge of alloreactive T cell metabolism is then explored, showing support for glycolysis, fat oxidation, and glutamine metabolism but also offering a potential explanation for how these presumably contradictory metabolic findings might be reconciled. Examples of additional ways in which metabolism impacts aHSCT are addressed, including the influence of butyrate metabolism on GVHD resolution. Finally, the caveats and challenges of assigning causality using our current metabolic toolbox is discussed, as well as likely future directions in immunometabolism, both to highlight the strengths of the current evidence as well as recognize some of its limitations.

Programmed death 1 ligand (PD-L1) in solid cancers after allogeneic hematopoietic stem cell transplantation: a retrospective analysis by the Nagasaki Transplant Group

Programmed death 1 ligand (PD-L1) is an immunomodulatory molecule expressed by cancer cells, and it has been widely demonstrated to inhibit host antitumor responses. The aim of the present study was to identify clinicopathological features associated with PD-L1 expression in the secondary solid cancers of patients after allogeneic hematopoietic stem cell transplantation. In this database of 530 patients who received allo-HSCT between 1990 and 2017, 15 developed solid cancers with a median interval of 3487 days after transplantation. Three patients had 2 different solid cancers. Eighteen solid cancer cases were identified. A multivariate analysis showed that chronic graft-versus-host disease (GVHD) was associated with an increased risk of solid cancer. The presence of chronic GVHD was observed in 8 out of 18 cases at the diagnosis of secondary malignancies. PD-L1 expression levels in cancers were significantly higher in patients with active chronic GVHD than in those without chronic GVHD (P = 0.020). Five cases of cancer that developed in the involved organs of chronic GVHD showed 30% or higher PD-L1 positivity. The present results revealed distinct PD-L1 expression in the secondary solid cancers of post-transplant patients with chronic GVHD.

PD-L1 siRNA-mediated silencing in acute myeloid leukemia enhances anti-leukemic T cell reactivity

Acute myeloid leukemia (AML) is an immune-susceptible malignancy, as demonstrated by its responsiveness to allogeneic stem cell transplantation (alloSCT). However, by employing inhibitory signaling pathways, including PD-1/PD-L1, leukemia cells suppress T cell-mediated immune attack. Notably, impressive clinical efficacy has been obtained with PD-1/PD-L1 blocking antibodies in cancer patients. Yet, these systemic treatments are often accompanied by severe toxicity, especially after alloSCT. Here, we investigated RNA interference technology as an alternative strategy to locally interfere with PD-1/PD-L1 signaling in AML. We demonstrated efficient siRNA-mediated PD-L1 silencing in HL-60 and patients' AML cells. Importantly, WT1-antigen T cell receptor⁺ PD-1⁺ 2D3 cells showed increased activation toward PD-L1 silenced WT1⁺ AML. Moreover, PD-L1 silenced AML cells significantly enhanced the activation, degranulation, and IFN-γ production of minor histocompatibility antigen-specific CD8⁺ T cells. Notably, PD-L1 silencing was equally effective as PD-1 antibody blockade. Together, our study demonstrates that PD-L1 silencing may be an effective strategy to augment AML immune-susceptibility. This provides rationale for further development of targeted approaches to locally interfere with immune escape mechanisms in AML, thereby minimizing severe toxicity. In combination with alloSCT and/or adoptive T cell transfer, this strategy could be very appealing to boost graft-versus-leukemia immunity and improve outcome in AML patients.

Immune checkpoint inhibitors in malignant lymphoma: Advances and perspectives

Classical Hodgkin lymphoma (cHL) has been identified with universal genetic alterations of chromosome 9p24.1, which contains PD-L1/PD-L2 genes. The amplification of 9p24.1 is associated with the increased expression of PD-L1 and PD-L2 on RS cells, which promotes their immune evasion, and subsequently makes cHL sensitive to PD-1 blockade. Several PD-1 inhibitors have shown significant efficacies with overall response rate (ORR) of 70%-90% in relapse/refractory (r/r) cHL and have acquired the approvals for this indication. Recently, more and more studies are conducted to investigate PD-1 blockade in earlier disease course and in combination with neo-agents or chemotherapy. Unlike cHL, non-Hodgkin lymphoma (NHL) consists of numerous subtypes harboring highly biological heterogeneity. Only a few subtypes have been shown to have genetic alteration of9p24.1 including primary mediastinal B cell lymphoma (PMBL), gray zone lymphoma (GZL) with features intermediate between diffuse large B cell lymphoma (DLBCL) and cHL, primary central nervous system lymphoma (PCNSL) and primary testicular lymphoma (PTL). Epstein-Barr virus (EBV)-associated lymphomas have a virally mediated overexpression of PD-L1, also making them sensitive to PD-1 blockade. Therefore, PD-1 inhibitors are less effective in most r/r NHL than in r/r cHL. Further understanding of the biological features of NHL and immune checkpoint inhibitors (ICPi) combined therapy is the research focus in the future. In this review, we outlined the recent progress of ICPi in lymphoma originating from clinical studies.

Relapsing/refractory HL after autotransplantation: which treatment?

For advanced-stage Hodgkin lymphoma (HL), front-line chemotherapy, alone or in combination with radiotherapy, leads to 5-year progression-free survival (PFS) rates and freedom-from-treatment failure (FFTF) rates of 70-85%, regardless of the chemotherapy regimen applied. Patients with HL experiencing disease progression during or within 3 months of front-line therapy (primary refractory) and patients whose disease relapses after a complete response have a second chance of treatment. The standard of care for relapsed or refractory HL is second-line chemotherapy followed by autologous stem cell transplantation (ASCT), which can induce long-term remission in approximately 40-50% of patients. However, HL recurrence occurs in about 50% of patients after ASCT, usually within the first year, and represents a significant therapeutic challenge. Allogeneic transplantation from HLA-matched donors represents the standard of care for patients with HL relapsing after- or refractory to ASCT.

First Report of Severe Acute Graft-Versus-Host Disease After Allogeneic Stem Cell Transplant in a Patient With Myelodysplastic Syndrome Treated With Atezolizumab: Literature Review

Checkpoint inhibitors have become a widely used and available immunotherapy option for treating a variety of malignancies, including hematological malignancies. Patients receiving these therapies may go on to receive a curative allogeneic hematopoietic stem cell transplant (allo-HSCT). This presents a clinical challenge as the safety and efficacy of HSCT is not well reported in this subset of patients and residual programmed death-ligand 1 inhibition could potentially enhance allogeneic T-cell responses, improving the graft-versus-tumor effect, but also increasing the incidence and severity of immune complications such as graft-versus-host disease (GVHD). Here, this report includes a detailed literature review summarizing all available data on HSCT outcomes in the setting of using checkpoint inhibitor therapy pre-transplant. Moreover, we report a case of acute GVHD after allo-HSCT in a patient with high-risk myelodysplastic syndrome who received prior atezolizumab therapy, highlighting the importance of further research into this specific population in order to improve transplant-related outcomes.

Ipilumumab for hepatocellular cancer in a liver transplant recipient, with durable response, tolerance and without allograft rejection

Organ transplant recipients are not routinely included in clinical trials, and as a result there is a paucity of data to guide clinicians in the treatment of malignancies in this unique patient population. This is a case report and focused review of the treatment of hepatocellular carcinoma in patients with orthotopic liver transplants. We describe a single patient's treatment over a period of 4 years from the time of diagnosis to submission of this case report. We submit evidence that the anti-CTLA-4 antibody ipilimumab can produce a durable response, with a tolerable adverse event profile and without associated allograft rejection.

T cell metabolism in graft-versus-host disease

Graft-versus-host disease (GVHD) is a major source of morbidity and mortality following allogeneic hematopoietic stem cell transplant (allo-HSCT), one of the most effective approaches to treat hematopoietic malignancies.¹ However, current prophylaxis regimens and treatments that reduce the detrimental effect of acute GVHD can be offset by increased incidence in opportunistic infections and relapse of the primary malignancy.² In addition, the majority of the approaches that inhibit T cell responses are non-specific, resulting in the inhibition of both alloreactive T cells and protective T cells from the donor. Therefore, there is an increase in the demand to develop novel approaches that selectively target alloreactive T cells. One potential means to address this issue is to take advantage of the unique metabolic profile of activated T cells.

Distinct Regulatory and Effector T Cell Metabolic Demands during Graft-Versus-Host Disease

Despite graft-versus-host disease (GVHD) prophylactic agents, the success and wider utilization of allogeneic hematopoietic stem cell transplantation (allo-HSCT) is limited by GVHD. Increasing donor graft regulatory T cell (Treg):effector T cell (Teff) ratios can substantially reduce GVHD in cancer patients, but pre-HSCT conditioning regimens and GVHD create a challenging inflammatory environment for Treg stability, persistence, and function. Metabolism plays a crucial role in T cell and Treg differentiation, and development of effector function. Although glycolysis is a main driver of allogeneic T cell-driven GVHD, oxidative phosphorylation is a main driver of Treg suppressor function. This review focuses on recent advances in our understanding of Treg metabolism in the context of GVHD, and discusses potential therapeutic applications of Tregs in the prevention or treatment of GVHD in cancer patients.

Donor and host B7-H4 expression negatively regulates acute graft-versus-host disease lethality

B7-H4 is a negative regulatory B7 family member. We investigated the role of host and donor B7-H4 in regulating acute graft-versus-host disease (GVHD). Allogeneic donor T cells infused into B7-H4-/- versus WT recipients markedly accelerated GVHD-induced lethality. Chimera studies pointed toward B7-H4 expression on host hematopoietic cells as more critical than parenchymal cells in controlling GVHD. Rapid mortality in B7-H4-/- recipients was associated with increased donor T cell expansion, gut T cell homing and loss of intestinal epithelial integrity, increased T effector function (proliferation, proinflammatory cytokines, cytolytic molecules), and reduced apoptosis. Higher metabolic demands of rapidly proliferating donor T cells in B7-H4-/- versus WT recipients required multiple metabolic pathways, increased extracellular acidification rates (ECARs) and oxygen consumption rates (OCRs), and increased expression of fuel substrate transporters. During GVHD, B7-H4 expression was upregulated on allogeneic WT donor T cells. B7-H4-/- donor T cells given to WT recipients increased GVHD mortality and had function and biological properties similar to WT T cells from allogeneic B7-H4-/- recipients. Graft-versus-leukemia responses were intact regardless as to whether B7-H4-/- mice were used as hosts or donors. Taken together, these data provide new insights into the negative regulatory processes that control GVHD and provide support for developing therapeutic strategies directed toward the B7-H4 pathway.