Naive and Stem Cell Memory T Cell Subset Recovery Reveals Opposing Reconstitution Patterns in CD4 and CD8 T Cells in Chronic Graft vs. Host Disease

The impact of regulatory T cells on the graft-versus-leukemia effect

Allogeneic Hematopoietic Stem Cell Transplantation (allo-HSCT) is the only curative therapy for many hematologic malignancies, whereby the Graft-versus-Leukemia (GVL) effect plays a pivotal role in controlling relapse. However, the success of GVL is hindered by Graft-versus-Host Disease (GVHD), where donor T cells attack healthy tissues in the recipient. The ability of natural regulatory T cells (Treg) to suppress immune responses has been exploited as a therapeutical option against GVHD. Still, it is crucial to evaluate if the ability of Treg to suppress GVHD does not compromise the benefits of GVL. Initial studies in animal models suggest that Treg can attenuate GVHD while preserving GVL, but results vary according to tumor type. Human trials using Treg as GVHD prophylaxis or treatment show promising results, emphasizing the importance of infusion timing and Treg/Tcon ratios. In this review, we discuss strategies that can be used aiming to enhance GVL post-Treg infusion and the proposed mechanisms for the maintenance of the GVL effect upon the adoptive Treg transfer. In order to optimize the therapeutic outcomes of Treg administration in allo-HSCT, future efforts should focus on refining Treg sources for infusion and evaluating their specificity for antigens mediating GVHD while preserving GVL responses.

Effect of mTOR Inhibition with Sirolimus on Natural Killer Cell Reconstitution in Allogeneic Stem Cell Transplantation

Sirolimus is an inhibitor of the mammalian target of rapamycin (mTOR) and is emerging as a promising component of graft-versus-host disease (GVHD) prophylaxis regimens in the context of allogeneic hematopoietic stem cell transplantation (HSCT). Multiple studies have explored the clinical benefits of adding sirolimus to GVHD prophylaxis; however, detailed immunologic studies have not yet been carried out in this context. Mechanistically, mTOR is at the center of metabolic regulation in T cells and natural killer (NK) cells and is critical for their differentiation to mature effector cells. Therefore, close evaluation of the inhibition of mTOR in the context of immune reconstitution post-HSCT is warranted. In this work, we studied the effect of sirolimus on immune reconstitution using a biobank of longitudinal samples from patients receiving either tacrolimus/sirolimus (TAC/SIR) or cyclosporin A/methotrexate (CSA/MTX) as conventional GVHD prophylaxis. Healthy donor controls, donor graft material, and samples from 28 patients (14 with TAC/SIR, 14 with CSA/MTX) at 3 to 4 weeks and 34 to 39 weeks post- HSCT were collected. Multicolor flow cytometry was used to perform broad immune cell mapping, with a focus on NK cells. NK cell proliferation was evaluated over a 6-day in vitro homeostatic proliferation protocol. Furthermore, in vitro NK cell responses to cytokine stimulation or tumor cells were evaluated. Systems-level assessment of the immune repertoire revealed a deep and prolonged suppression (weeks 34 to 39 post-HSCT) of the naïve CD4 T cell compartment with relative sparing of regulatory T cells and enrichment of CD69⁺Ki-67⁺HLA-DR⁺ CD8 T cells, independent of the type of GVHD prophylaxis. Early after transplantation (weeks 3 to 4), while patients were still on TAC/SIR or CSA/MTX, we found a relative increase in less-differentiated CD56^bright NK cells and NKG2A⁺CD57^-KIR^- CD56^dim NK cells and a distinct loss of CD16 and DNAM-1 expression. Both regimens led to suppressed proliferative responses ex vivo and functional impairment with preferential loss of cytokine responsiveness and IFN-γ production. Patients who received TAC/SIR as GVHD prophylaxis showed delayed NK cell reconstitution with lower overall NK cell counts and fewer CD56^bright and NKG2A⁺ CD56^dim NK cells. Treatment with sirolimus- containing regimens generated similar immune cell profiles as conventional prophylaxis; however, the NK cell compartment seemed to be composed of slightly more mature NK cells. These effects were also present after the completion of GVHD prophylaxis, suggesting that mTOR inhibition with sirolimus leaves a lasting imprint on homeostatic proliferation and NK cell reconstitution following HSCT.

Epigenetic regulation of T cell lineages in skin and blood following hematopoietic stem cell transplantation

Allogeneic hematopoietic stem-cell transplantation (HSCT) seeks to reconstitute the host's immune system from donor stem cells. The success of HSCT is threatened by complications including leukemia relapse or graft-versus-host-disease (GvHD). To investigate the underlying regulatory processes in central and peripheral T cell recovery, we performed sequential multi-omics analysis of T cells of the skin and blood during HSCT. We detected rapid effector T cell reconstitution, while emergence of regulatory T cells was delayed. Epigenetic and gene-regulatory programs were associated with recovering T cells and diverged greatly between skin and blood T cells. The BRG1/BRM-associated factor chromatin remodeling complex and histone deacetylases (HDACs) were epigenetic regulators involved in restoration of T cell homeostasis after transplantation. In isolated T cells of patients after HSCT, we observed class I HDAC-inhibitors to modulate their dysbalance. The present study highlights the importance of epigenetic regulation in the recovery of T cells following HSCT.

T Cell and Cytokine Dynamics in the Blood of Patients after Hematopoietic Stem Cell Transplantation and Multipotent Mesenchymal Stromal Cell Administration

Multipotent mesenchymal stromal cells (MSCs) are currently under intensive investigation for the treatment and prevention of graft-versus-host disease (GVHD) after allogeneic hematopoietic stem cell transplantation (allo-HSCT), owing to their substantial immunomodulatory properties. The responses of recipients to MSC infusion following allo-HSCT are not yet well understood. T cells are central to the adaptive immune system, protecting the organism from infection and malignant cells. Memory T cells with different phenotypes, gene expression profiles, and functional properties are critical for immune processes regulation. The aim of this study was to study the dynamics of memory T cell subpopulations and cytokines in the blood of allo-HSCT recipients after MSC administration. In clinical trial NCT01941394, patients after allo-HSCT were randomized into 2 groups, one receiving standard GVHD prophylaxis and the other also receiving MSC infusion on the day of leukocyte recovery to 1000 cells/μL (engraftment, day E0). Blood samples of patients from both groups were analyzed on days E0, E+3, and E+30. T cell subpopulations were studied by flow cytometry, and cytokine concentrations were evaluated by the Bio-Plex Pro Human Cytokine Panel. Administration of MSCs to patients on day E0 did not affect the overall dynamics of restoration of absolute numbers and proportions of T and B lymphocytes after 3 and 30 days. At 3 days after MSC injection, only the numbers of CD8⁺ effector cells (CD8⁺_TE, CD8⁺_TM, and CD8⁺_EM) were found to increase significantly. A significant increase in the number of CD4⁺ cells after 30 days compared to day E0 was observed only in patients who received MSCs, indicating faster recovery of the CD4⁺ cell population following MSC injection. An increase in CD8⁺ cell number by day E+30 was significant regardless of MSC administration. To characterize the immune status of patients following allo-HSCT in more detail, changes in the cytokine concentration in the peripheral blood of patients on days E0, E+3, and E+30 after MSC administration were investigated. On day E+30, significant increases in the numbers of CD4⁺_CM and activated CD4⁺CD25⁺ cells were observed. The concentrations of proinflammatory and anti-inflammatory cytokines IL-6, IL-8, IL-17, TNF-α, and IFN-γ were increased significantly in patients injected with MSCs. Analysis of growth factor levels showed that in the group of patients who received MSCs, the concentrations of G-CSF, GM-CSF, PDGFbb, FGFb, and IL-5 increased by day E+30. Among the cytokines involved in regulation of the immune response, concentrations of IL-9, eotaxin, IP-10, MCP-1, and MIP-1a were increased after 30 days irrespective of MSC administration. The administration of MSCs exerts a positive effect on the restoration of T cell subpopulations and immune system recovery in patients after allo-HSCT.

Extracorporeal photopheresis as an immunomodulatory treatment modality for chronic GvHD and the importance of emerging biomarkers

Haematopoietic stem cell transplantation (HSCT) is the treatment of choice for malignant haematological diseases. Despite continuous improvements in pre- and post-transplantation procedures, the applicability of allo-HSCT is limited by life-threatening complications such as graft-versus-host disease (GvHD), engraftment failure, and opportunistic infections. Extracorporeal photopheresis (ECP) is used to treat steroid resistant GvHD with significant success. However, the molecular mechanisms driving its immunomodulatory action, whilst preserving immune function, require further understanding. As ECP is safe to administer with few significant adverse effects, it has the potential for earlier use in the post-HSCT treatment of GvHD. Thus, further understanding the immunomodulatory mechanisms of ECP action may justify more timely use in clinical practice, as well as identify biomarkers for using ECP as first line or pre-emptive GvHD therapy. This review aims to discuss technical aspects and response to ECP, review ECP as an immunomodulatory treatment modality for chronic GvHD including the effect on regulatory T cells and circulating vs. tissue-resident immune cells and consider the importance of emerging biomarkers for ECP response.

Cell-based therapy in prophylaxis and treatment of chronic graft-versus-host disease

Hematopoietic allogeneic stem cell transplantation (allo-SCT) is a curative option for patients with hematological malignancies. However, due to disparities in major and minor histocompatibility antigens between donor and recipient, severe inflammatory complications can occur, among which chronic graft-versus-host disease (cGVHD) can be life-threatening. A classical therapeutic approach to the prevention and treatment of cGVHD has been broad immunosuppression, but more recently adjuvant immunotherapies have been tested. This review summarizes and discusses immunomodulatory approaches with T cells, including chimeric antigen receptor (CAR) and regulatory T cells, with natural killer (NK) cells and innate lymphoid cells (ILCs), and finally with mesenchymal stromal cells (MSC) and extracellular vesicles thereof. Clinical studies and pre-clinical research results are presented likewise.

A Fenton-like cation can improve arsenic trioxide treatment of sclerodermatous chronic Graft-versus-Host Disease in mice

Graft-versus Host Disease (GvHD) is a major complication of hematopoietic stem cell transplant. GvHD is characterized by the chronic activation of immune cells leading to the development of systemic inflammation, autoimmunity, fibrosis and eventually death. Arsenic trioxide (ATO) is a therapeutic agent under clinical trial for the treatment of patients with systemic lupus erythematosus (SLE) and chronic GvHD (cGvHD). This therapy is admittedly rather safe although adverse effects can occur and may necessitate short interruptions of the treatment. The aim of this study was to combine ATO with a divalent cation, to generate a Fenton or Fenton-like reaction in order to potentiate the deletion of activated immune cells through the reactive oxygen species (ROS)-mediated effects of ATO in a mouse model, and thereby enabling the use of lower and safer ATO concentrations to treat patients with cGvHD. In vitro, among the various combinations of divalent cations tested, we observed that the combination of ATO and CuCl2 (copper chloride) induced a high level of oxidative stress in HL-60 and A20 cells. In addition, this co-treatment also decreased the proliferation of CD4+ T lymphocytes during a mixed lymphocyte reaction (MLR). In vivo, in a cGvHD mouse model, daily injections of ATO 2.5 µg/g + CuCl2 0.5 µg/g induce a decrease in lymphocyte activation and fibrosis that was equivalent to that induced by ATO 5 µg/g. Our results show that the addition of CuCl2 improved the effects of ATO and significantly limited the development of the disease. This co-treatment could be a real benefit in human patients to substantially decrease the known ATO side effects and optimize ATO treatment in pathologies characterized by activated cells sensitive to an increase in oxidative stress.

Naive T Cells in Graft Versus Host Disease and Graft Versus Leukemia: Innocent or Guilty?

The outcome of allogeneic hematopoietic cell transplantation (allo-HCT) largely depends on the development and management of graft-versus-host disease (GvHD), infections, and the occurrence of relapse of malignancies. Recent studies showed a lower incidence of chronic GvHD and severe acute GvHD in patients receiving naive T cell depleted grafts compared to patients receiving complete T cell depleted grafts. On the other hand, the incidence of acute GvHD in patients receiving cord blood grafts containing only naive T cells is rather low, while potent graft-versus-leukemia (GvL) responses have been observed. These data suggest the significance of naive T cells as both drivers and regulators of allogeneic reactions. The naive T cell pool was previously thought to be a quiescent, homogenous pool of antigen-inexperienced cells. However, recent studies showed important differences in phenotype, differentiation status, location, and function within the naive T cell population. Therefore, the adequate recovery of these seemingly innocent T cells might be relevant in the imminent allogeneic reactions after allo-HCT. Here, an extensive review on naive T cells and their contribution to the development of GvHD and GvL responses after allo-HCT is provided. In addition, strategies specifically directed to stimulate adequate reconstitution of naive T cells while reducing the risk of GvHD are discussed. A better understanding of the relation between naive T cells and alloreactivity after allo-HCT could provide opportunities to improve GvHD prevention, while maintaining GvL effects to lower relapse risk.

Lower Absolute Lymphocyte Count Before Conditioning Predicts High Relapse Risk in Patients After Haploidentical Peripheral Blood Stem Cell Transplantation With Low Dose Anti-Thymocyte Globulin/Post-Transplant Cyclophosphamide for GvHD Prophylaxis

Standard anti-thymocyte globulin (ATG) weight-based dosing often resulted in highly variable ATG exposure, which had profound effects on relapse and survival, especially in recipients with relatively low absolute lymphocyte count (ALC) before conditioning. Data regarding rabbit ATG pharmacokinetics and pharmacodynamics in the setting of HLA-haploidentical peripheral blood stem cell transplantation (haplo-PBSCT) is lacking. We conducted a retrospective study on 90 consecutive patients who underwent haplo-PBSCT with low dose rabbit ATG (5 mg/kg) plus low dose post-transplant cyclophosphamide (50 mg/kg) based regimen for graft-versus-host disease (GvHD) prophylaxis. We compared serum concentration of ATG and post-transplant results between patients with ALC<500/μl and ALC≥500/μl before conditioning. Patients with ALC<500/μl had higher ATG concentrations, delayed immune reconstitution, lower incidence of grade II-IV acute GvHD (0 vs. 19.42%, P = 0.043), higher risk of Epstein-Barr virus infection within 100 days post-transplant (47.78% vs. 22.22%, P = 0.020) and 1-year relapse rate (33.33% vs.11.59%, P = 0.041), and lower 1-year overall survival (OS) (52.38% vs.79.71%, P = 0.004), 1-year relapse free survival (RFS) (47.62% vs. 75.36% for RFS, P = 0.014), and 1-year GvHD free relapse-free survival (GRFS) (42.89% vs. 65.22%, P = 0.043). ALC<500/μl before conditioning was a significant poor risk factor for relapse, OS, RFS, and GRFS.

Stem cell-like memory T cells: The generation and application

Stem cell-like memory T cells (Tscm), are a newly defined memory T cell subset with characteristics of long life span, consistent self-renewing, rapid differentiation into effector T cells, and apoptosis resistance. These features indicate that Tscm have great therapeutic or preventive purposes, including being applied in chimeric Ag receptor-engineered T cells, TCR gene-modified T cells, and vaccines. However, the little knowledge about Tscm development restrains their applications. Strength and duration of TCR signaling, cytokines and metabolism in the T cells during activation all influence the Tscm development via regulating transcriptional factors and cell signaling pathways. Here, we summarize the molecular and cellular pathways involving Tscm differentiation, and its clinical application for cancer immunotherapy and prevention.

Decreased Naïve T-cell Production Leading to Cytokine Storm as Cause of Increased COVID-19 Severity with Comorbidities

Aging, type 2 diabetes, and male gender are major risk factors leading to increased COVID-19 morbidity and mortality. Thymic production and the export of naïve T cells decrease with aging through the effects of androgens in males and in type 2 diabetes. Furthermore, with aging, recovery of naïve T-cell populations after bone marrow transplantation is delayed and associated with an increased risk of chronic graft vs. host disease. Severe COVID-19 and SARS infections are notable for severe T-cell depletion. In COVID-19, there is unique suppression of interferon signaling by infected respiratory tract cells with intact cytokine signaling. A decreased naïve T-cell response likely contributes to an excessive inflammatory response and increases the odds of a cytokine storm. Treatments that improve naïve T-cell production may prove to be vital COVID-19 therapies, especially for these high-risk groups.

Reconstitution of T Cell Subsets Following Allogeneic Hematopoietic Cell Transplantation

Allogeneic (allo) hematopoietic cell transplantation (HCT) is the only curative treatment option for patients suffering from chemotherapy-refractory or relapsed hematological malignancies. The occurrence of morbidity and mortality after allo-HCT is still high. This is partly correlated with the immunological recovery of the T cell subsets, of which the dynamics and relations to complications are still poorly understood. Detailed information on T cell subset recovery is crucial to provide tools for better prediction and modulation of adverse events. Here, we review the current knowledge regarding CD4+ and CD8+ T cells, γδ T cells, iNKT cells, Treg cells, MAIT cells and naive and memory T cell reconstitution, as well as their relations to outcome, considering different cell sources and immunosuppressive therapies. We conclude that the T cell subsets reconstitute in different ways and are associated with distinct adverse and beneficial events; however, adequate reconstitution of all the subsets is associated with better overall survival. Although the exact mechanisms involved in the reconstitution of each T cell subset and their associations with allo-HCT outcome need to be further elucidated, the data and suggestions presented here point towards the development of individualized approaches to improve their reconstitution. This includes the modulation of immunotherapeutic interventions based on more detailed immune monitoring, aiming to improve overall survival changes.

Co‑transplantation of tonsil‑derived mesenchymal stromal cells in bone marrow transplantation promotes thymus regeneration and T cell diversity following cytotoxic conditioning

Bone marrow (BM) transplantation (BMT) represents a curative treatment for various hematological disorders. Prior to BMT, a large amount of the relevant anticancer drug needed to be administered to eliminate cancer cells. However, during this pre-BMT cytotoxic conditioning regimen, hematopoietic stem cells in the BM and thymic epithelial cells were also destroyed. The T cell receptor (TCR) recognizes diverse pathogen, tumor and environmental antigens, and confers immunological memory and self-tolerance. Delayed thymus reconstitution following pre-BMT cytotoxic conditioning impedes de novo thymopoiesis and limits T cell-mediated immunity. Several cytokines, such as RANK ligand, interleukin (IL)-7, IL-22 and stem cell factor, were recently reported to improve thymopoiesis and immune function following BMT. In the present study, it was found that the co-transplantation of tonsil-derived mesenchymal stromal cells (T-MSCs) with BM-derived cells (BMCs) accelerated the recovery of involuted thymuses in mice following partial pre-BMT conditioning with busulfan-cyclophosphamide treatment, possibly by inducing FMS-like tyrosine kinase 3 ligand (FLT3L) and fibroblast growth factor 7 (FGF7) production in T-MSCs. The co-transplantation of T-MSCs with BMCs also replenished the CD3⁺ cell population by inhibiting thymocyte apoptosis following pre-BMT cytotoxic conditioning. Furthermore, T-MSC co-transplantation improved the recovery of the TCR repertoire and led to increased thymus-generated T cell diversity.

Mesenchymal stromal cells induce regulatory T cells via epigenetic conversion of human conventional CD4 T cells in vitro

Regulatory T cells (Treg) play a critical role in immune tolerance. The scarcity of Treg therapy clinical trials in humans has been largely due to the difficulty in obtaining sufficient Treg numbers. We performed a preclinical investigation on the potential of mesenchymal stromal cells (MSCs) to expand Treg in vitro to support future clinical trials. Human peripheral blood mononuclear cells from healthy donors were cocultured with allogeneic bone marrow‐derived MSCs expanded under xenogeneic‐free conditions. Our data show an increase in the counts and frequency of CD4⁺ CD25^high Foxp3⁺ CD127^low Treg cells (4‐ and 6‐fold, respectively) after a 14‐day coculture. However, natural Treg do not proliferate in coculture with MSCs. When purified conventional CD4 T cells (Tcon) are cocultured with MSCs, only cells that acquire a Treg‐like phenotype proliferate. These MSC‐induced Treg‐like cells also resemble Treg functionally, since they suppress autologous Tcon proliferation. Importantly, the DNA methylation profile of MSC‐induced Treg‐like cells more closely resembles that of natural Treg than of Tcon, indicating that this population is stable. The expression of PD‐1 is higher in Treg‐like cells than in Tcon, whereas the frequency of PDL‐1 increases in MSCs after coculture. TGF‐β levels are also significantly increased MSC cocultures. Overall, our data suggest that Treg enrichment by MSCs results from Tcon conversion into Treg‐like cells, rather than to expansion of natural Treg, possibly through mechanisms involving TGF‐β and/or PD‐1/PDL‐1 expression. This MSC‐induced Treg population closely resembles natural Treg in terms of phenotype, suppressive ability, and methylation profile.

Immune Monitoring After Allogeneic Hematopoietic Cell Transplantation: Toward Practical Guidelines and Standardization

Hematopoietic cell transplantation (HCT) is often a last resort, but potentially curative treatment option for children suffering from hematological malignancies and a variety of non-malignant disorders, such as bone marrow failure, inborn metabolic disease or immune deficiencies. Although efficacy and safety of the HCT procedure has increased significantly over the last decades, the majority of the patients still suffer from severe acute toxicity, viral reactivation, acute or chronic graft-versus-host disease (GvHD) and/or, in case of malignant disease, relapses. Factors influencing HCT outcomes are numerous and versatile. For example, there is variation in the selected graft sources, type of infused cell subsets, cell doses, and the protocols used for conditioning, as well as immune suppression and treatment of adverse events. Moreover, recent pharmacokinetic studies show that medications used in the conditioning regimen (e.g., busulphan, fludarabine, anti-thymocyte globulin) should be dosed patient-specific to achieve optimal exposure in every individual patient. Due to this multitude of variables and site-specific policies/preferences, harmonization between HCT centers is still difficult to achieve. Literature shows that adequate immune recovery post-HCT limits both relapse and non-relapse mortality (death due to viral reactivations and GvHD). Monitoring immune parameters post-HCT may facilitate a timely prediction of outcome. The use of standardized assays to measure immune parameters would facilitate a fast comparison between different strategies tested in different centers or between different clinical trials. We here discuss immune cell markers that may contribute to clinical decision making and may be worth to standardize in multicenter collaborations for future trials.

Mechanisms of Immune Tolerance in Liver Transplantation-Crosstalk Between Alloreactive T Cells and Liver Cells With Therapeutic Prospects

Liver transplantation (LTx) is currently the most powerful treatment for end-stage liver disease. Although liver allograft is more tolerogenic compared to other solid organs, the majority of LTx recipients still require long-term immune suppression (IS) to control the undesired alloimmune responses, which can lead to severe side effects. Thus, understanding the mechanism of liver transplant tolerance and crosstalk between immune cells, especially alloreactive T cells and liver cells, can shed light on more specific tolerance induction strategies for future clinical translation. In this review, we focus on alloreactive T cell mediated immune responses and their crosstalk with liver sinusoidal endothelial cells (LSECs), hepatocytes, hepatic stellate cells (HSCs), and cholangiocytes in transplant setting. Liver cells mainly serve as antigen presenting cells (APCs) to T cells, but with low expression of co-stimulatory molecules. Crosstalk between them largely depends on the different expression of adhesion molecules and chemokine receptors. Inflammatory cytokines secreted by immune cells further elaborate this crosstalk and regulate the fate of naïve T cells differentiation within the liver graft. On the other hand, regulatory T cells (Tregs) play an essential role in inducing and keeping immune tolerance in LTx. Tregs based adoptive cell therapy provides an excellent therapeutic option for clinical transplant tolerance induction. However, many questions regarding cell therapy still need to be solved. Here we also address the current clinical trials of adoptive Tregs therapy and other tolerance induction strategies in LTx, together with future challenges for clinical translation from bench to bedside.