Translational studies in hematopoietic cell transplantation: treatment of hematologic malignancies as a stepping stone to tolerance induction

Tolerance in xenotransplantation

PURPOSE OF REVIEW

This review describes recent progress in tolerance-inducing strategies across xenogeneic immunological barriers as well as the potential benefit of a tolerance strategy for islets and kidney xenotransplantation.

RECENT FINDINGS

Using advanced gene editing technologies, xenotransplantation from multitransgenic alpha-1,3-galactosyltransferase knockout pigs has demonstrated marked prolongation of renal xenograft survival, ranging from days to greater than several months for life-supporting kidneys, and more than 2 years in a heterotopic nonlife-supporting cardiac xenograft model. Continuous administration of multiple immunosuppressive drugs has been required and attempts to taper immunosuppression have been unsuccessful. It appears likely that low levels of T cell dependent antibodies and activation of innate responses are responsible for xenograft loss. Mixed chimerism and thymic transplantation approaches have achieved xenogeneic tolerance in pig-to-mouse models and both have recently been extended to pig-to-baboon models. Encouraging results have been reported, including persistence of macrochimerism, prolonged pig skin graft survival, donor-specific unresponsiveness in vitro and detection of recent T cell emigrants in vivo.

SUMMARY

Although tolerance induction in vivo has not yet been achieved in pig-to-baboon models, recent results are encouraging that this goal will be attainable through genetic engineering of porcine donors.

The Knife's Edge of Tolerance: Inducing Stable Multilineage Mixed Chimerism but With a Significant Risk of CMV Reactivation and Disease in Rhesus Macaques

Although stable mixed-hematopoietic chimerism induces robust immune tolerance to solid organ allografts in mice, the translation of this strategy to large animal models and to patients has been challenging. We have previously shown that in MHC-matched nonhuman primates (NHPs), a busulfan plus combined belatacept and anti-CD154-based regimen could induce long-lived myeloid chimerism, but without T cell chimerism. In that setting, donor chimerism was eventually rejected, and tolerance to skin allografts was not achieved. Here, we describe an adaptation of this strategy, with the addition of low-dose total body irradiation to our conditioning regimen. This strategy has successfully induced multilineage hematopoietic chimerism in MHC-matched transplants that was stable for as long as 24 months posttransplant, the entire length of analysis. High-level T cell chimerism was achieved and associated with significant donor-specific prolongation of skin graft acceptance. However, we also observed significant infectious toxicities, prominently including cytomegalovirus (CMV) reactivation and end-organ disease in the setting of functional defects in anti-CMV T cell immunity. These results underscore the significant benefits that multilineage chimerism-induction approaches may represent to transplant patients as well as the inherent risks, and they emphasize the precision with which a clinically successful regimen will need to be formulated and then validated in NHP models.

Endothelial microparticles carrying hedgehog-interacting protein induce continuous endothelial damage in the pathogenesis of acute graft-versus-host disease

Accumulating evidence suggests that endothelial microparticles (EMPs), a marker of endothelial damage, are elevated in acute graft-versus-host disease (aGVHD), and that endothelial damage is implicated in the pathogenesis of aGVHD, but the mechanisms remain elusive. In this study, we detected the plasma EMP levels and endothelial damage in patients and mice with aGVHD in vivo and then examined the effects of EMPs derived from injured endothelial cells (ECs) on endothelial damage and the role of hedgehog-interacting protein (HHIP) carried by EMPs in these effects in vitro. Our results showed that EMPs were persistently increased in the early posttransplantation phase in patients and mice with aGVHD. Meanwhile, endothelial damage was continuous in aGVHD mice, but was temporary in non-aGVHD mice after transplantation. In vitro, EMPs induced endothelial damage, including increased EC apoptosis, enhanced reactive oxygen species, decreased nitric oxide production and impaired angiogenic activity. Enhanced expression of HHIP, an antagonist for the Sonic hedgehog (SHH) signaling pathway, was observed in patients and mice with aGVHD and EMPs from injured ECs. The endothelial damage induced by EMPs was reversed when the HHIP incorporated into EMPs was silenced with an HHIP small interfering RNA or inhibited with the SHH pathway agonist, Smoothened agonist. This work supports a feasible vicious cycle in which EMPs generated during endothelial injury, in turn, aggravate endothelial damage by carrying HHIP into target ECs, contributing to the continuously deteriorating endothelial damage in the development of aGVHD. EMPs harboring HHIP would represent a potential therapeutic target for aGVHD.

Fingerprints of transplant tolerance suggest opportunities for immunosuppression minimization

HLA incompatible organ transplant tolerance is the holy grail of transplantation. Stable engraftment of an HLA mismatched allograft and life-long tolerance induction, though feasible in highly selected cohorts with depletional protocols, is not ready for generalized application to the entire transplant recipient pool. It has thus been important to harness biomarkers that can uncover mechanisms and tools for monitoring HLA mismatched recipients that develop a state of operational tolerance, during accidental immunosuppression withdrawal secondary to problems of over-immunosuppression (infection or malignancy) or toxicity (mostly cosmetic or cardiovascular). A restricted and unpredictable group of patients can demonstrate a clinical state of operational tolerance, manifested by state of stable graft function of a graft with HLA mismatches between recipient and donor, intact immune responses to third party antigens and no measurable immunosuppression. These patients have served as the basis for the discovery of clinically correlative biomarkers, in distal biofluids (mainly blood), that can define the existing state of operational clinical tolerance. Operationally tolerant patients are rare, as withdrawal of immunosuppression most often results in rejection and graft loss. Nevertheless, operationally tolerant kidney, liver and heart allograft recipients have been reported. The presence of similar biomarker signature profiles in HLA mismatched transplant recipients on immunosuppression, suggests the feasibility of utilizing these biomarkers for educated immunosuppression minimization with a view to retaining immunological quiescence, while reducing the maintenance immunosuppression burden to a "safe" alloimmune threshold. Though clinical operational tolerance is rare, as immunosuppression cessation most often results in increased alloimmunity and rejection, the biomarker profile studies that have harnessed whole genome profiling suggest that the frequency of this state may be ~8% in kidney allograft recipients, and even more frequent in pediatric recipients and in liver transplantation: 25% in adult liver allograft recipients and ~60% in pediatric liver allograft recipients. In this review we discuss putative molecular mechanisms, cellular players and correlative biomarkers that have been developed through clinically associative studies of tolerant and non-tolerant patients. Through mechanisms of carefully constructed and monitored randomized, prospective clinical trials, the transplant community stands at the cusp of improved quality of recipient life through educated immunosuppression minimization.

The evolution of T-cell depletion in haploidentical stem-cell transplantation

T-cell depletion (TCD) can prevent the onset of graft-versus-host disease (GvHD) in animal models of bone marrow transplantation; this manipulation enabled the successful application in the 1980s of T-cell depleted bone marrow (BM) for the treatment of babies with severe combined immune deficiency (SCID). However, in leukaemia patients, implementation of T-cell depletion has been more difficult, especially due to high rate of graft-rejection, leukaemia relapse and delayed immune reconstitution. These hurdles were gradually overcome by modifying the cell composition of the graft, and by reducing the toxicities associated with conditioning protocols. Although no 'gold standard' TCD method exists, T-cell depletion in its modern forms could offer clinical benefit, even for patients with a matched sibling donor.

Mixed chimerism evolution is associated with T regulatory type 1 (Tr1) cells in a β-thalassemic patient after haploidentical haematopoietic stem cell transplantation

In a cohort of β-Thalassemia (β-Thal) transplanted with haploidentical-HSCT we identified one transplanted patient characterized by persistent mixed chimerism (PMC) for several months after HSCT. In this unique β-Thal patient we assessed the donor engraftment overtime after transplantation, the potential loss of the non-shared HLA haplotype, and the presence of CD49b(+)LAG-3(+) T regulatory type 1 (Tr1) cells, previously demonstrated to be associated with PMC after HLA-related HSCT for β-Thal. The majority of the patient's erythrocytes were of donor origin, whereas T cells were initially mostly derived from the recipient, no HLA loss, but an increased frequency of circulating Tr1 cells were observed. For the first time, we showed that when the proportion of residual donor cells decreases, the frequency of CD49b(+)LAG-3(+) Tr1 cells declines, reaching the levels present in healthy subjects. These findings confirm previous results obtained in transplant related settings for β-Thal, and supported the central role of Tr1 cells in promoting and maintaining PMC after allo-HSCT.

Transplantation tolerance and its outcome during infections and inflammation

Much progress has been made toward understanding the mechanistic basis of transplantation tolerance in experimental models, which implicates clonal deletion of alloreactive T and B cells, induction of cell-intrinsic hyporesponsiveness, and dominant regulatory cells mediating infectious tolerance and linked suppression. Despite encouraging success in the laboratory, achieving tolerance in the clinic remains challenging, although the basis for these challenges is beginning to be understood. Heterologous memory alloreactive T cells generated by infections prior to transplantation have been shown to be a critical barrier to tolerance induction. Furthermore, infections at the time of transplantation and tolerance induction provide a pro-inflammatory milieu that alters the stability and function of regulatory T cells as well as the activation requirements and differentiation of effector T cells. Thus, infections can result in enhanced alloreactivity, resistance to tolerance induction, and destabilization of the established tolerance state. We speculate that these experimental findings have relevance to the clinic, where infections have been associated with allograft rejection and may be a causal event precipitating the loss of grafts after long periods of stable operational tolerance. Understanding the mechanisms by which infections prevent and destabilize tolerance can lead to therapies that promote stable life-long tolerance in transplant recipients.

Path to clinical transplantation tolerance and prevention of graft-versus-host disease

Although organ and bone marrow transplantations are life-saving procedures for patients with terminal diseases, the requirement for the lifelong use of immunosuppressive drugs to prevent organ graft rejection and the development of graft versus host disease (GVHD) remain important problems. Experimental approaches to solve these problems, first in preclinical models and then in clinical studies, developed at Stanford during the past 40 years are summarized in this article. The approaches use fractionated radiation of the lymphoid tissues, a procedure initially developed to treat Hodgkin's disease, to alter the immune system such that tolerance to organ transplants can be achieved and GVHD can be prevented after the establishment of chimerism. In both instances, the desired goal was achieved when the balance of immune cells was changed to favor regulatory innate and adaptive immune cells that suppress the conventional immune cells that ordinarily promote inflammation and tissue injury.

Rapid tolerance induction by hematopoietic progenitor cells in the absence of donor-matched lymphoid cells

BACKGROUND

Donor specific hematopoietic cell transplantation has long been recognized for its potential in tolerance induction for subsequently transplanted organs. We have recently published that co-administration of Myeloid Progenitor (MP) and third party Hematopoietic Stem Cells (HSC) can induce MP-specific tolerance for subsequently transplanted organs [1].

METHODS

Mice received an allogeneic HSC and third party MP transplantation simultaneous with placement of a MP-matched skin graft. Variants tested include time of graft placement, MP genotype and source of cells.

RESULTS

Using B10;B6-Rag2(-/-)Il2rg(-/-) mice, we demonstrate that specific tolerance can be induced by MP given simultaneous with the skin graft in the complete absence of MP-donor-matched lymphoid cells. Ex vivo expanded MP function as well as sorted cells in inducing tolerance. In addition we demonstrate that tolerance can be induced by MP in the context of autologous HSC transplantation.

CONCLUSIONS

Our results demonstrate that the previously observed expansion of organ donor matched Treg is not essential for tolerance, and that MP tolerance protocols can be envisioned in most clinical settings, including those involving deceased donor organs.

Treatment sequencing after failure of the first biologic in cost-effectiveness models of psoriasis: a systematic review of published models and clinical practice guidelines

OBJECTIVES

To analyse the treatment sequencing assumptions after failure on a first-line biologic in cost-effectiveness models of treatment of moderate to severe plaque psoriasis, and to compare them with the most recent treatment guidelines.

METHODS

A systematic search of MEDLINE, Embase, EconLit and the Cochrane Library databases used free text and Medical Subject Headings terms including psoriasis, biologic therapies indicated for psoriasis, and all types of economic evaluations. Two researchers performed 2-level abstract screening for articles meeting pre-specified inclusion criteria. Assumptions about treatment pathways after first-line biologic failure in the cost-effectiveness models were analysed. A second systematic search was performed for psoriasis clinical practice guidelines. Sequence assumptions were compared with treatment guideline recommendations.

RESULTS

Of 25 cost-effectiveness modelling studies identified, ten estimated the incremental cost per responder; time horizons varied (12 weeks-18 months) and treatment sequencing was not considered. In 15 studies where treatment sequencing was considered, with time horizons up to 10 years, five studies included only a switch to nonsystemic therapy or best supportive care after first-line biologic failure. Another five of the 15 treatment-pathway studies were available only as abstracts with no details of the sequence assumptions. In five of the 15 studies, first-line biologic failure was followed by second-line biologic monotherapy, one of the recommendations in current treatment guidelines. In only one of these five studies was the efficacy of the second-line biologic adjusted downwards, compared with first-line treatment. Only one of these studies considered dose titration with a first-line biologic and none combination therapy (biologic plus methotrexate or phototherapy) after first-line biologic failure, as recommended in some treatment guidelines.

CONCLUSIONS

Cost-effectiveness models of first-line biologics for moderate to severe plaque psoriasis either do not include subsequent treatment regimens or include only some of the regimens recommended in current treatment guidelines. Results may be sensitive to assumptions about treatment sequencing and the choice and efficacy of subsequent treatment regimens.

Induction of tolerance through mixed chimerism

"Mixed chimerism" refers to a state in which the lymphohematopoietic system of the recipient of allogeneic hematopoietic stem cells comprises a mixture of host and donor cells. This state is usually attained through either bone marrow or mobilized peripheral blood stem cell transplantation. Although numerous treatment regimens have led to transplantation tolerance in mice, the induction of mixed chimerism is currently the only treatment modality that has been successfully extended to large animals and to the clinic. Here we describe and compare the use of mixed chimerism to establish transplantation tolerance in mice, pigs, monkeys, and in the clinic. We also attempt to correlate the mechanisms involved in achieving tolerance with the nature of the tolerance that has resulted in each case.

Mixed chimerism and growth factors augment β cell regeneration and reverse late-stage type 1 diabetes

Type 1 diabetes (T1D) results from an autoimmune destruction of insulin-producing β cells. Currently, islet transplantation is the only curative therapy for late-stage T1D, but the beneficial effect is limited in its duration, even under chronic immunosuppression, because of the chronic graft rejection mediated by both auto- and alloimmunity. Clinical islet transplantation is also restricted by a severe shortage of donor islets. Induction of mixed chimerism reverses autoimmunity, eliminates insulitis, and reverses new-onset but not late-stage disease in the nonobese diabetic (NOD) mouse model of T1D. Administration of gastrin and epidermal growth factor (EGF) also reverses new-onset but not late-stage T1D in this animal model. Here, we showed that combination therapy of induced mixed chimerism under a radiation-free nontoxic anti-CD3/CD8 conditioning regimen and administration of gastrin/EGF augments both β cell neogenesis and replication, resulting in reversal of late-stage T1D in NOD mice. If successfully translated into humans, this combination therapy could replace islet transplantation as a long-term curative therapy for T1D.

Emerging uses for pediatric hematopoietic stem cells

Many new therapies are emerging that use hematopoietic stem and progenitor cells. In this review, we focus on five promising emerging trends that are altering stem cell usage in pediatrics: (i) The use of hematopoietic stem cell (HSC) transplantation, autologous or allogeneic, in the treatment of autoimmune disorders is one. (ii) The use of cord blood transplantation in patients with inherited metabolic disorders such as Hurler syndrome shows great benefit, even more so than replacement enzyme therapy. (iii) Experience with the delivery of gene therapy through stem cells is increasing, redefining the potential and limitations of this therapy. (iv) It has recently been shown that human immunodeficiency virus (HIV) infection can be cured by the use of selected stem cells. (v) Finally, it has long been postulated that HSC-transplantation can be used to induce tolerance in solid-organ transplant recipients. A new approach to tolerance induction using myeloid progenitor cells will be described.

Interactions between NKT cells and Tregs are required for tolerance to combined bone marrow and organ transplants

We used a model of combined bone marrow and heart transplantation, in which tolerance and stable chimerism is induced after conditioning with fractionated irradiation of the lymphoid tissues and anti-T-cell antibodies. Graft acceptance and chimerism required host CD4(+)CD25(+) Treg production of IL-10 that was in-turn enhanced by host invariant natural killer (NK) T-cell production of IL-4. Up-regulation of PD-1 on host Tregs, CD4(+)CD25(-) conventional T (Tcon) cells, and CD8(+) T cells was also enhanced by NKT cell production of IL-4. Up-regulated PD-1 expression on Tregs was linked to IL-10 secretion, on CD8(+) T cells was linked to Tim-3 expression, and on CD4(+) Tcon cells was associated with reduced IFNγ secretion. Changes in the expression of PD-1 were induced by the conditioning regimen, and declined after bone marrow transplantation. In conclusion, NKT cells in this model promoted changes in expression of negative costimulatory receptors and anti-inflammatory cytokines by Tregs and other T-cell subsets in an IL-4-dependent manner that resulted in tolerance to the bone marrow and organ grafts.