Harnessing regulatory T cells to establish immune tolerance.

Engineered regulatory T (Treg) cells have emerged as precision therapeutics aimed at inducing immune tolerance while reducing the risks associated with generalized immunosuppression. This Viewpoint highlights the opportunities and challenges for engineered Treg cell therapies in treating autoimmune and other inflammatory diseases.

Donor antigen-specific regulatory T cell administration to recipients of live donor kidneys: A ONE Study consortium pilot trial.

Regulatory T cells (Tregs) can inhibit cellular immunity in diverse experimental models and have entered early phase clinical trials in autoimmunity and transplantation to assess safety and efficacy. As part of the ONE Study consortium, we conducted a phase I-II clinical trial in which purified donor antigen reactive (dar)-Tregs (CD4+CD25+CD127lo) were administered to 3 patients, 7 to 11 days after live donor renal transplant. Recipients received a modified immunosuppression regimen, without induction therapy, consisting of maintenance tacrolimus, mycophenolate mofetil, and steroids. Steroids were weaned off over 14 weeks. No rejection was seen on any protocol biopsy. Therefore, all patients discontinued mycophenolate mofetil 11 to 13 months posttransplant, per protocol. An early for-cause biopsy in 1 patient, 5 days after dar-Treg infusion, revealed absence of rejection and accumulation of Tregs in the kidney allograft. All patients had Treg-containing lymphoid aggregates evident on protocol biopsies performed 8 months posttransplant. The patients are now all >6 years posttransplant on tacrolimus monotherapy with excellent graft function. None experienced rejection episodes. No serious adverse events were attributable to Treg administration. These results support a favorable safety profile of dar-Tregs administered early after renal transplant, suggest early biopsy might be an instructive research endpoint and provide preliminary evidence of potential immunomodulatory activity.

Ex vivo generation of regulatory T cells from liver transplant recipients using costimulation blockade.

The potential of adoptive cell therapy with regulatory T cells (Tregs) to promote transplant tolerance is under active exploration. However, the impact of specific transplant settings and protocols on Treg manufacturing is not well-delineated. Here, we compared the use of peripheral blood mononuclear cells (PBMCs) from patients before or after liver transplantation to the use of healthy control PBMCs to determine their suitability for Treg manufacture using ex vivo costimulatory blockade with belatacept. Despite liver failure or immunosuppressive therapy, the capacity for Treg expansion during the manufacturing process was preserved. These experiments did not identify performance or quality issues that disqualified the use of posttransplant PBMCs-the currently favored protocol design. However, as Treg input correlated with output, significant CD4-lymphopenia in both pre- and posttransplant patients limited Treg yield. We therefore turned to leukapheresis posttransplant to improve absolute yield. To make deceased donor use feasible, we also developed protocols to substitute splenocytes for PBMCs as allostimulators. In addition to demonstrating that this Treg expansion strategy works in a liver transplant context, this preclinical study illustrates how characterizing cellular input populations and their performance can both inform and respond to clinical trial design and Treg manufacturing requirements.

Anti-tumor effects of RTX-240: an engineered red blood cell expressing 4-1BB ligand and interleukin-15.

Recombinant agonists that activate co-stimulatory and cytokine receptors have shown limited clinical anticancer utility, potentially due to narrow therapeutic windows, the need for coordinated activation of co-stimulatory and cytokine pathways and the failure of agonistic antibodies to recapitulate signaling by endogenous ligands. RTX-240 is a genetically engineered red blood cell expressing 4-1BBL and IL-15/IL-15Rα fusion (IL-15TP). RTX-240 is designed to potently and simultaneously stimulate the 4-1BB and IL-15 pathways, thereby activating and expanding T cells and NK cells, while potentially offering an improved safety profile through restricted biodistribution. We assessed the ability of RTX-240 to expand and activate T cells and NK cells and evaluated the in vivo efficacy, pharmacodynamics and tolerability using murine models. Treatment of PBMCs with RTX-240 induced T cell and NK cell activation and proliferation. In vivo studies using mRBC-240, a mouse surrogate for RTX-240, revealed biodistribution predominantly to the red pulp of the spleen, leading to CD8 + T cell and NK cell expansion. mRBC-240 was efficacious in a B16-F10 melanoma model and led to increased NK cell infiltration into the lungs. mRBC-240 significantly inhibited CT26 tumor growth, in association with an increase in tumor-infiltrating proliferating and cytotoxic CD8 + T cells. mRBC-240 was tolerated and showed no evidence of hepatic injury at the highest feasible dose, compared with a 4-1BB agonistic antibody. RTX-240 promotes T cell and NK cell activity in preclinical models and shows efficacy and an improved safety profile. Based on these data, RTX-240 is now being evaluated in a clinical trial.

Engineered red blood cells as an off-the-shelf allogeneic anti-tumor therapeutic.

Checkpoint inhibitors and T-cell therapies have highlighted the critical role of T cells in anti-cancer immunity. However, limitations associated with these treatments drive the need for alternative approaches. Here, we engineer red blood cells into artificial antigen-presenting cells (aAPCs) presenting a peptide bound to the major histocompatibility complex I, the costimulatory ligand 4-1BBL, and interleukin (IL)-12. This leads to robust, antigen-specific T-cell expansion, memory formation, additional immune activation, tumor control, and antigen spreading in tumor models in vivo. The presence of 4-1BBL and IL-12 induces minimal toxicities due to restriction to the vasculature and spleen. The allogeneic aAPC, RTX-321, comprised of human leukocyte antigen-A*02:01 presenting the human papilloma virus (HPV) peptide HPV16 E711-19, 4-1BBL, and IL-12 on the surface, activates HPV-specific T cells and promotes effector function in vitro. Thus, RTX-321 is a potential 'off-the-shelf' in vivo cellular immunotherapy for treating HPV + cancers, including cervical and head/neck cancers.

Regulatory cell therapy in kidney transplantation (The ONE Study): a harmonised design and analysis of seven non-randomised, single-arm, phase 1/2A trials.

BACKGROUND: Use of cell-based medicinal products (CBMPs) represents a state-of-the-art approach for reducing general immunosuppression in organ transplantation. We tested multiple regulatory CBMPs in kidney transplant trials to establish the safety of regulatory CBMPs when combined with reduced immunosuppressive treatment. METHODS: The ONE Study consisted of seven investigator-led, single-arm trials done internationally at eight hospitals in France, Germany, Italy, the UK, and the USA (60 week follow-up). Included patients were living-donor kidney transplant recipients aged 18 years and older. The reference group trial (RGT) was a standard-of-care group given basiliximab, tapered steroids, mycophenolate mofetil, and tacrolimus. Six non-randomised phase 1/2A cell therapy group (CTG) trials were pooled and analysed, in which patients received one of six CBMPs containing regulatory T cells, dendritic cells, or macrophages; patient selection and immunosuppression mirrored the RGT, except basiliximab induction was substituted with CBMPs and mycophenolate mofetil tapering was allowed. None of the trials were randomised and none of the individuals involved were masked. The primary endpoint was biopsy-confirmed acute rejection (BCAR) within 60 weeks after transplantation; adverse event coding was centralised. The RTG and CTG trials are registered with ClinicalTrials.gov, NCT01656135, NCT02252055, NCT02085629, NCT02244801, NCT02371434, NCT02129881, and NCT02091232. FINDINGS: The seven trials took place between Dec 11, 2012, and Nov 14, 2018. Of 782 patients assessed for eligibility, 130 (17%) patients were enrolled and 104 were treated and included in the analysis. The 66 patients who were treated in the RGT were 73% male and had a median age of 47 years. The 38 patients who were treated across six CTG trials were 71% male and had a median age of 45 years. Standard-of-care immunosuppression in the recipients in the RGT resulted in a 12% BCAR rate (expected range 3·2-18·0). The overall BCAR rate for the six parallel CTG trials was 16%. 15 (40%) patients given CBMPs were successfully weaned from mycophenolate mofetil and maintained on tacrolimus monotherapy. Combined adverse event data and BCAR episodes from all six CTG trials revealed no safety concerns when compared with the RGT. Fewer episodes of infections were registered in CTG trials versus the RGT. INTERPRETATION: Regulatory cell therapy is achievable and safe in living-donor kidney transplant recipients, and is associated with fewer infectious complications, but similar rejection rates in the first year. Therefore, immune cell therapy is a potentially useful therapeutic approach in recipients of kidney transplant to minimise the burden of general immunosuppression. FUNDING: The 7th EU Framework Programme.

Beta cell-specific CD8+ T cells maintain stem cell memory-associated epigenetic programs during type 1 diabetes.

The pool of beta cell-specific CD8+ T cells in type 1 diabetes (T1D) sustains an autoreactive potential despite having access to a constant source of antigen. To investigate the long-lived nature of these cells, we established a DNA methylation-based T cell 'multipotency index' and found that beta cell-specific CD8+ T cells retained a stem-like epigenetic multipotency score. Single-cell assay for transposase-accessible chromatin using sequencing confirmed the coexistence of naive and effector-associated epigenetic programs in individual beta cell-specific CD8+ T cells. Assessment of beta cell-specific CD8+ T cell anatomical distribution and the establishment of stem-associated epigenetic programs revealed that self-reactive CD8+ T cells isolated from murine lymphoid tissue retained developmentally plastic phenotypic and epigenetic profiles relative to the same cells isolated from the pancreas. Collectively, these data provide new insight into the longevity of beta cell-specific CD8+ T cell responses and document the use of this methylation-based multipotency index for investigating human and mouse CD8+ T cell differentiation.

The Untapped Opportunity and Challenge of Immunometabolism: A New Paradigm for Drug Discovery.

Here, we explore the manipulation of immune cell metabolism as a strategy in target discovery and drug development for immune-mediated diseases. Comparing exploitation of metabolic pathways to kill tumor cells for cancer treatment with the reprogramming of immune cells to treat autoimmune diseases highlights differences that confer several advantages to the latter (including a more favorable therapeutic index and greater target stability). We discuss technological capabilities and gaps, including the challenge of relating in vitro observations to in vivo biology. Finally, we conclude by identifying future opportunities that will move the field forward and accelerate drug discovery.

Extensive intrathecal T cell renewal following hematopoietic transplantation for multiple sclerosis.

A recent study of autologous hematopoietic stem cell transplantation (AHSCT) for active relapsing-remitting multiple sclerosis (RRMS) showed efficacy in preventing disease worsening. However, the immunologic basis for efficacy remains poorly defined. Multiple sclerosis pathology is known to be driven by inflammatory T cells that infiltrate the CNS. Therefore, we hypothesized that the preexisting T cell repertoire in the intrathecal compartment of active RRMS participants was ablated and replaced with new clones following AHSCT. T cell repertoires were assessed using high-throughput TCRß chain sequencing in paired cerebrospinal fluid (CSF) and peripheral blood CD4+ and CD8+ T cells from participants that underwent AHSCT, before and up to 4 years following transplantation. More than 90% of the preexisting CSF repertoire in participants with active RRMS was removed following AHSCT and replaced with clonotypes predominantly generated from engrafted autologous stem cells. Of the preexisting clones in CSF, approximately 60% were also detected in blood before therapy, and concordant treatment effects were observed for clonotypes in both compartments following AHSCT. These results indicate that replacement of the preexisting TCR repertoire in active RRMS is a mechanism for AHSCT efficacy and suggest that peripheral blood could serve as a surrogate for CSF to define mechanisms associated with efficacy in future studies of AHSCT.

Immunosuppression Withdrawal in Liver Transplant Recipients on Sirolimus.

BACKGROUND AND AIMS: As conversion from calcineurin inhibitor to sirolimus (SRL), a mechanistic target of rapamycin inhibitor (mTOR-I), has been shown to enhance immunoregulatory profiles in liver transplant (LT) recipients (LTRs), mTOR-I therapy might allow for increased success of immunosuppression (IS) withdrawal. Our aim was to determine if operational tolerance could be observed in LTRs withdrawn from SRL and if blood/graft tolerance biomarkers were predictive of successful withdrawal. APPROACH AND RESULTS: We performed a prospective trial of SRL monotherapy withdrawal in nonimmune, nonviremic LTRs > 3 years post-LT. SRL was weaned over ~6 months, and biopsies were performed 12 months postweaning or at concern for acute rejection. Twenty-one LTRs consented; 6 were excluded due to subclinical acute rejection on baseline biopsy or other reasons, and 15 underwent weaning (age 61.3 ± 8.8 years; LT to SRL weaning 6.7 ± 3 years). Eight (53%) achieved operational tolerance (TOL). Of the 7 who were nontolerant (non-TOL), 6 had mild acute rejection on biopsy near the end of weaning or at study end; 1 was removed from the trial due to liver cancer recurrence. At baseline preweaning, there were statistically increased blood tolerogenic dendritic cells and cell phenotypes correlating with chronic antigen presentation in the TOL versus non-TOL groups. A previously identified biopsy gene signature accurately predicted TOL versus non-TOL in 12/14 LTRs before weaning. At study end, biopsy staining revealed statistically significant increases in antigen-presenting cell:leukocyte pairings, FOXP3+ /CD4+ T cells, Tbet+ /CD8+ T cells, and lobular dendritic cells in the non-TOL group. CONCLUSIONS: This study evaluated IS withdrawal directly from mTOR-I therapy in LTRs and achieved > 50% operational tolerance. Preweaning gene expression and peripheral blood mononuclear cell profiling may be useful as predictors of successful mTOR-I therapy withdrawal. NCT02062944.

Differential effects of 2-deoxy-D-glucose on in vitro expanded human regulatory T cell subsets.

Regulatory T cells (Tregs) are required for the maintenance of immune tolerance and adoptive Treg infusion therapy has become a promising approach to suppress immune responses in diseases such as autoimmunity and transplant rejection. However, one critical challenge of Treg therapy is the requirement of in vitro expansion of functionally stable Tregs while preventing either the contamination of T effector and/or emergence of unstable pathogenic Tregs. Recent studies showing distinct metabolic requirements of T effectors and Tregs suggest that manipulation of cell metabolism may be an attractive strategy to achieve this goal. Here we show that human thymically derived Tregs (tTregs) and in vitro induced Tregs (iTregs) from naive T cells engage glycolysis equivalently upon activation. However, inhibiting glucose metabolism via 2-deoxy-D-glucose (2DG) has distinct effects on each of these subsets. While 2DG treatment at the onset of activation significantly reduced the proliferation and expression of suppressive molecules such as ICOS and CTLA-4 in tTregs, its effect on FOXP3 expression was small. In contrast, 2DG treatment during iTreg induction modestly decreased their proliferation but strongly reduced both ICOS and FOXP3 expression. Importantly, both Treg subsets became insensitive to 2DG after day 3 post activation with little effect on either proliferation or FOXP3 expression while T conventional Th0 cells showed reduced proliferation under the same conditions. Moreover, 2DG treatment at day 3 did not impair the suppressive capabilities of Treg subsets. Collectively, these findings suggest that there is a distinct temporal requirement of glycolysis in each of the activated human Treg subsets and T conventional cells. Furthermore, 2DG treatment at the onset as a strategy to impair contaminating T effector cell proliferation is unfavorable for optimal Treg generation as well.

Targeting PI3Kδ function for amelioration of murine chronic graft-versus-host disease.

Chronic graft-versus-host disease (cGVHD) is a leading cause of morbidity and mortality following allotransplant. Activated donor effector T cells can differentiate into pathogenic T helper (Th)-17 cells and germinal center (GC)-promoting T follicular helper (Tfh) cells, resulting in cGVHD. Phosphoinositide-3-kinase-δ (PI3Kδ), a lipid kinase, is critical for activated T cell survival, proliferation, differentiation, and metabolism. We demonstrate PI3Kδ activity in donor T cells that become Tfh cells is required for cGVHD in a nonsclerodermatous multiorgan system disease model that includes bronchiolitis obliterans (BO), dependent upon GC B cells, Tfhs, and counterbalanced by T follicular regulatory cells, each requiring PI3Kδ signaling for function and survival. Although B cells rely on PI3Kδ pathway signaling and GC formation is disrupted resulting in a substantial decrease in Ig production, PI3Kδ kinase-dead mutant donor bone marrow-derived GC B cells still supported BO cGVHD generation. A PI3Kδ-specific inhibitor, compound GS-649443, that has superior potency to idelalisib while maintaining selectivity, reduced cGVHD in mice with active disease. In a Th1-dependent and Th17-associated scleroderma model, GS-649443 effectively treated mice with active cGVHD. These data provide a foundation for clinical trials of US Food and Drug Administration (FDA)-approved PI3Kδ inhibitors for cGVHD therapy in patients.

Mitochondrial complex III is essential for suppressive function of regulatory T cells.

Regulatory T cells (Treg cells), a distinct subset of CD4+ T cells, are necessary for the maintenance of immune self-tolerance and homeostasis1,2. Recent studies have demonstrated that Treg cells exhibit a unique metabolic profile, characterized by an increase in mitochondrial metabolism relative to other CD4+ effector subsets3,4. Furthermore, the Treg cell lineage-defining transcription factor, Foxp3, has been shown to promote respiration5,6; however, it remains unknown whether the mitochondrial respiratory chain is required for the T cell-suppression capacity, stability and survival of Treg cells. Here we report that Treg cell-specific ablation of mitochondrial respiratory chain complex III in mice results in the development of fatal inflammatory disease early in life, without affecting Treg cell number. Mice that lack mitochondrial complex III specifically in Treg cells displayed a loss of T cell-suppression capacity without altering Treg cell proliferation and survival. Treg cells deficient in complex III showed decreased expression of genes associated with Treg function, whereas Foxp3 expression remained stable. Loss of complex III in Treg cells increased DNA methylation as well as the metabolites 2-hydroxyglutarate (2-HG) and succinate that inhibit the ten-eleven translocation (TET) family of DNA demethylases7. Thus, Treg cells require mitochondrial complex III to maintain immune regulatory gene expression and suppressive function.

Abatacept Targets T Follicular Helper and Regulatory T Cells, Disrupting Molecular Pathways That Regulate Their Proliferation and Maintenance.

Abatacept is a CTLA-4-Ig fusion protein that binds to the costimulatory ligands CD80 and CD86 and blocks their interaction with the CD28 and CTLA-4 receptors expressed by T cells, therefore inhibiting T cell activation and function. Abatacept has shown clinical efficacy in treating some autoimmune diseases but has failed to show clinical benefit in other autoimmune conditions. The reasons for these disparate results are not clear and warrant further investigation of abatacept's mode of action. Longitudinal specimens from the Immune Tolerance Network's A Cooperative Clinical Study of Abatacept in Multiple Sclerosis trial were used to examine the effects of abatacept treatment on the frequency and transcriptional profile of specific T cell populations in peripheral blood. We found that the relative abundance of CD4+ T follicular helper (Tfh) cells and regulatory T cells was selectively decreased in participants following abatacept treatment. Within both cell types, abatacept reduced the proportion of activated cells expressing CD38 and ICOS and was associated with decreased expression of genes that regulate cell-cycle and chromatin dynamics during cell proliferation, thereby linking changes in costimulatory signaling to impaired activation, proliferation, and decreased abundance. All cellular and molecular changes were reversed following termination of abatacept treatment. These data expand upon the mechanism of action of abatacept reported in other autoimmune diseases and identify new transcriptional targets of CD28-mediated costimulatory signaling in human regulatory T and Tfh cells, further informing on its potential use in diseases associated with dysregulated Tfh activity.

Differential Roles of IL-2 Signaling in Developing versus Mature Tregs.

Although Foxp3+ regulatory T cells (Tregs) require interleukin-2 (IL-2) for their development, it has been unclear whether continuing IL-2 signals are needed to maintain lineage stability, survival, and suppressor function in mature Tregs. We generated mice in which CD25, the main ligand-binding subunit of the IL-2 receptor, can be inducibly deleted from Tregs after thymic development. In contrast to Treg development, we find that IL-2 is dispensable for maintaining lineage stability in mature Tregs. Although continuous IL-2 signaling is needed for long-term Treg survival, CD25-deleted Tregs may persist for several weeks in vivo using IL-7. We also observe defects in glycolytic metabolism and suppressor function following CD25 deletion. Thus, unlike developing Tregs in which the primary role of IL-2 is to initiate Foxp3 expression, mature Tregs require continuous IL-2 signaling to maintain survival and suppressor function, but not to maintain lineage stability.

Early expansion of donor-specific Tregs in tolerant kidney transplant recipients.

Allograft tolerance, in which a graft is accepted without long-term immunosuppression, could overcome numerous obstacles in transplantation. Human allograft tolerance has been intentionally induced across HLA barriers via combined kidney and bone marrow transplantation (CKBMT) with a regimen that induces only transient chimerism. Tregs are enriched early after CKBMT. While deletional tolerance contributes to long-term tolerance, the role of Tregs remains unclear. We have optimized a method for identifying the donor-specific Treg repertoire and used it to interrogate the fate of donor-specific Tregs after CKBMT. We expanded Tregs with several different protocols. Using functional analyses and T cell receptor sequencing, we found that expanding sorted Tregs with activated donor B cells identified the broadest Treg repertoire with the greatest potency and donor specificity of suppression. This method outperformed both alloantigen stimulation with CTLA4Ig and sequencing of CFSElo cells from the primary mixed lymphocyte reaction. In 3 tolerant and 1 nontolerant CKBMT recipients, we sequenced donor-specific Tregs before transplant and tracked them after transplant. Preexisting donor-specific Tregs were expanded at 6 months after CKBMT in tolerant patients and were reduced in the nontolerant patient. These results suggest that early expansion of donor-specific Tregs is involved in tolerance induction following CKBMT.

Cutting Edge: TGF-ß and Phosphatidylinositol 3-Kinase Signals Modulate Distinct Metabolism of Regulatory T Cell Subsets.

Murine Foxp3+ regulatory T cells (Tregs) differentiated in vitro (induced Tregs [iTregs]) in the presence of anti-inflammatory cytokine TGF-ß rely predominantly upon lipid oxidation to fuel mitochondrial oxidative phosphorylation. Foxp3 expression underlies this metabolic preference, as it suppresses glycolysis and drives oxidative phosphorylation. In this study, we show that in contrast to iTregs, thymic-derived Tregs (tTregs), engage in glycolysis and glutaminolysis at levels comparable to effector T cells despite maintained Foxp3 expression. Interestingly, exposure of tTregs to the anti-inflammatory cytokine TGF-ß represses PI3K-mediated mTOR signaling, inhibits glucose transporter and Hk2 expression, and reprograms their metabolism to favor oxidative phosphorylation. Conversely, replicating the effects of inflammation via elevation of PI3K signaling has minimal effects on tTregs but dramatically enhances the glycolysis of normally oxidative iTregs, resulting in reduction of Foxp3 expression. Collectively, these findings suggest both extrinsic and intrinsic factors govern the unique metabolic signature of Treg subsets.

The vimentin intermediate filament network restrains regulatory T cell suppression of graft-versus-host disease.

Regulatory T cells (Tregs) are critical for maintaining immune homeostasis. However, current Treg immunotherapies do not optimally treat inflammatory diseases in patients. Understanding the cellular processes that control Treg function may allow for the augmentation of therapeutic efficacy. In contrast to activated conventional T cells, in which protein kinase C-θ (PKC-θ) localizes to the contact point between T cells and antigen-presenting cells, in human and mouse Tregs, PKC-θ localizes to the opposite end of the cell in the distal pole complex (DPC). Here, using a phosphoproteomic screen, we identified the intermediate filament vimentin as a PKC-θ phospho target and show that vimentin forms a DPC superstructure on which PKC-θ accumulates. Treatment of mouse Tregs with either a clinically relevant PKC-θ inhibitor or vimentin siRNA disrupted vimentin and enhanced Treg metabolic and suppressive activity. Moreover, vimentin-disrupted mouse Tregs were significantly better than controls at suppressing alloreactive T cell priming in graft-versus-host disease (GVHD) and GVHD lethality, using a complete MHC-mismatch mouse model of acute GVHD (C57BL/6 donor into BALB/c host). Interestingly, vimentin disruption augmented the suppressor function of PKC-θ-deficient mouse Tregs. This suggests that enhanced Treg activity after PKC-θ inhibition is secondary to effects on vimentin, not just PKC-θ kinase activity inhibition. Our data demonstrate that vimentin is a key metabolic and functional controller of Treg activity and provide proof of principle that disruption of vimentin is a feasible, translationally relevant method to enhance Treg potency.