Transient lymphopenia breaks costimulatory blockade-based peripheral tolerance and initiates cardiac allograft rejection

Heart transplantation and biomarkers: a review about their usefulness in clinical practice

Advanced heart failure (AdvHF) can only be treated definitively by heart transplantation (HTx), yet problems such right ventricle dysfunction (RVD), rejection, cardiac allograft vasculopathy (CAV), and primary graft dysfunction (PGD) are linked to a poor prognosis. As a result, numerous biomarkers have been investigated in an effort to identify and prevent certain diseases sooner. We looked at both established biomarkers, such as NT-proBNP, hs-troponins, and pro-inflammatory cytokines, and newer ones, such as extracellular vesicles (EVs), donor specific antibodies (DSA), gene expression profile (GEP), donor-derived cell free DNA (dd-cfDNA), microRNA (miRNA), and soluble suppression of tumorigenicity 2 (sST2). These biomarkers are typically linked to complications from HTX. We also highlight the relationships between each biomarker and one or more problems, as well as their applicability in routine clinical practice.

Prevalence and pathogenicity of autoantibodies in patients with idiopathic CD4 lymphopenia

BACKGROUNDIdiopathic CD4 lymphopenia (ICL) is defined by persistently low CD4+ cell counts (<300 cells/μL) in the absence of a causal infection or immune deficiency and can manifest with opportunistic infections. Approximately 30% of ICL patients develop autoimmune disease. The prevalence and breadth of their autoantibodies, however, and their potential contribution to pathogenesis of ICL remain unclear.METHODSWe hybridized 34 and 51 ICL patients' sera to a 9,000-human-proteome array and to a 128-known-autoantigen array, respectively. Using a flow-based method, we characterized the presence of anti-lymphocyte Abs in the whole cohort of 72 patients, as well as the Ab functional capability of inducing Ab-dependent cell-mediated cytotoxicity (ADCC), complement deposition, and complement-dependent cytotoxicity (CDC). We tested ex vivo the activation of the classical complement pathway on ICL CD4+ T cells.RESULTSAll ICL patients had a multitude of autoantibodies mostly directed against private (not shared) targets and unrelated quantitatively or qualitatively to the patients' autoimmune disease status. The targets included lymphocyte intracellular and membrane antigens, confirmed by the detection by flow of IgM and IgG (mostly IgG1 and IgG4) anti-CD4+ cell Abs in 50% of the patients, with half of these cases triggering lysis of CD4+ T cells. We also detected in vivo classical complement activation on CD4+ T cells in 14% of the whole cohort.CONCLUSIONOur data demonstrate that a high prevalence of autoantibodies in ICL, some of which are specific for CD4+ T cells, may contribute to pathogenesis, and may represent a potentially novel therapeutic target.TRIAL REGISTRATIONClinicalTrials.gov NCT00867269.FUNDINGNIAID and National Institute of Arthritis and Musculoskeletal and Skin Diseases of the NIH.

T Cell Repertoire Maturation Induced by Persistent and Latent Viral Infection Is Insufficient to Induce Costimulation Blockade Resistant Organ Allograft Rejection in Mice

CD28:CD80/86 pathway costimulation blockade (CoB) with the CD80/86-specific fusion protein CTLA4-Ig prevents T cell-mediated allograft rejection in mice. However, in humans, transplantation with CoB has been hampered by CoB-resistant rejection (CoBRR). CoBRR has been attributed in part to pathogen-driven T cell repertoire maturation and resultant heterologous alloreactive memory. This has been demonstrated experimentally in mice. However, prior murine models have used viral pathogens, CoB regimens, graft types, and/or antigen systems atypically encountered clinically. We therefore sought to explore whether CoBRR would emerge in a model of virus-induced memory differentiation designed to more closely mimic clinical conditions. Specifically, we examined mouse homologs of clinically prevalent viruses including murine polyomavirus, cytomegalovirus, and gammaherpesvirus 68 in the presence of clinically relevant maintenance CoB regimens using a fully MHC-mismatched, vascularized allograft model. Infected mice developed a significant, sustained increase in effector memory T cells consistent with that seen in humans, but neither developed heterologous alloreactivity nor rejected primarily vascularized heterotopic heart transplants at an increased rate compared with uninfected mice. These results indicate that memory acquisition alone is insufficient to provoke CoBRR and suggest that knowledge of prior latent or persistent viral infection may have limited utility in anticipating heterologous CoB-resistant alloimmunity.

Change in lymphocyte to neutrophil ratio predicts acute rejection after heart transplantation

AIMS

Most immunosuppressive drugs provide targeted immunosuppression by selective inhibition of lymphocyte activation and proliferation. This study evaluated whether a change in the lymphocyte to neutrophil ratio (LNR) is related to acute rejection.

METHODS

In 74 cardiac transplant recipients peripheral blood lymphocyte and neutrophil counts were measured soon after (baseline) and three, six, and 12months after heart transplantation. The primary endpoint was the incidence of acute rejection.

RESULTS

Significant acute rejection after heart transplantation occurred in 20 patients (27%) during a median follow-up of 49.4 [IQR 37.4-61.1] months. LNR significantly increased over time (0.1149±0.1354 at baseline, 0.2330±0.2266 at 3months, 0.2961±0.2849 at 6months, and 0.3521±0.2383 at 12months; P<0.001), especially during the first 3months in the group without acute rejection. The area under the curve of the change in LNR during the first three months (ΔLNR) for acute rejection was 0.565 (95% CI 0.420 to 0.710, P=0.380) on ROC curve analysis. The best cutoff value of Δ LNR to differentiate those with and without acute rejection was ≤0.046 by ROC curve analysis. Kaplan-Meier analysis revealed that the low ΔLNR group (≤0.046) had a significantly higher rate of acute rejection than the high ΔLNR group (>0.046) (37.5% vs. 19.0%, log-rank: P=0.0358). The low ΔLNR for the first 3months was an independent predictor of clinically significant acute rejection after adjusting for cytomegalovirus donor seropositive and recipient seronegative.

CONCLUSIONS

The results of this study suggest that ΔLNR over the first 3months after heart transplantation is a strong and independent predictor of acute rejection after heart transplantation. ΔLNR can be used as an early biomarker for predicting of acute rejection after heart transplantation.

Fifty Shades of Tolerance: Beyond a Binary Tolerant/Non-Tolerant Paradigm

Purpose of review

It has long been considered that tolerance in a transplant recipient is a binary all-or-none state: either the graft is accepted without immunosuppression identifying the recipient as tolerant, or the recipient rejects the graft and is not tolerant. This tolerance paradigm, however, does not accurately reflect data emerging from animal models and patients and requires revision.

Recent Findings

It is becoming appreciated that there may be different gradations in the quality of tolerance based on underlying cellular mechanisms of immunological tolerance, and that individuals may enhance their tolerance by strengthening or combining different cellular mechanisms. Furthermore, evidence suggests that even if tolerance is lost, the loss may be only temporary, and in some circumstances tolerance can be restored.

Summary

Shifting our focus from an all-or-nothing tolerance paradigm to one with many shades may help us better understand how tolerance operates, and how this state may be tracked and enhanced for better patient outcomes.

Delayed Donor Bone Marrow Infusion Induces Liver Transplant Tolerance

BACKGROUND

Nonmyeloablative conditioning followed by donor bone marrow infusion (BMI) to induce tolerance has not been robustly tested in liver transplantation (LT) and may be unsafe at the time of LT. We hypothesized T cell-depleted BMI is effective in inducing tolerance when delayed after LT, resulting in potentially safer future clinical applications.

METHODS

Nonimmunosuppressed syngeneic (Lewis to Lewis) and allogeneic (ACI to Lewis) rat LT transplants were initially performed as controls. Three experimental allogeneic LT groups were treated with tacrolimus (TAC) for 3 to 4 weeks and then underwent: (1) TAC withdrawal alone; (2) nonmyeloablative conditioning (anti-αβTCR mAb + total body irradiation [300 cGy]) followed by TAC withdrawal; (3) Nonmyeloablative conditioning + donor BMI (100 × 10 T cell-depleted bone marrow cells) followed by TAC withdrawal.

RESULTS

All group 1 recipients developed chronic rejection. Group 2 had long-term survival but impaired liver function and high donor-specific antibody (DSA) levels. In contrast, group 3 (conditioning + BMI) had long-term TAC-free survival with preserved liver function and histology, high mixed chimerism and blood/liver/spleen CD4 + CD25 + Foxp3+ regulatory T cells, and low DSA titers, similar to syngeneic grafts. While donor-specific tolerance was observed post-BMI, graft-versus-host disease was not.

CONCLUSIONS

These results support that donor-specific tolerance can be achieved with BMI even when delayed after LT and this tolerance correlates with increased mixed chimerism, regulatory T cell generation, and diminished DSA.

Depletion-Resistant CD4 T Cells Enhance Thymopoiesis During Lymphopenia

Lymphoablation is routinely used in transplantation, and its success is defined by the balance of pathogenic versus protective T cells within reconstituted repertoire. While homeostatic proliferation and thymopoiesis may both cause T cell recovery during lymphopenia, the relative contributions of these mechanisms remain unclear. The goal of this study was to investigate the role of the thymus during T cell reconstitution in adult allograft recipients subjected to lymphoablative induction therapy. Compared with euthymic mice, thymectomized heart allograft recipients demonstrated severely impaired CD4 and CD8 T cell recovery and prolonged heart allograft survival after lymphoablation with murine anti-thymocyte globulin (mATG). The injection with agonistic anti-CD40 mAb or thymus transplantation only partially restored T cell reconstitution in mATG-treated thymectomized mice. After mATG depletion, residual CD4 T cells migrated into the thymus and enhanced thymopoiesis. Conversely, depletion of CD4 T cells before lymphoablation inhibited thymopoiesis at the stage of CD4^- CD8^- CD44^hi CD25⁺ immature thymocytes. This is the first demonstration that the thymus and peripheral CD4 T cells cooperate to ensure optimal T cell reconstitution after lymphoablation. Targeting thymopoiesis through manipulating functions of depletion-resistant helper T cells may thus improve therapeutic benefits and minimize the risks of lymphoablation in clinical settings.

Challenges and opportunities in targeting the CD28/CTLA-4 pathway in transplantation and autoimmunity

INTRODUCTION

T cell activation is a complex process that requires multiple cell signaling pathways, including a primary recognition signal and additional costimulatory signals. One of the best-characterized costimulatory pathways includes the Ig superfamily members CD28 and CTLA-4 and their ligands CD80 and CD86. Areas covered: This review discusses past, current and future biological therapies that have been utilized to block the CD28/CTLA-4 cosignaling pathway in the settings of autoimmunity and transplantation, as well the challenges facing successful implementation of these therapies. Expert opinion: The development of CD28 blockers Abatacept and Belatacept provided a more targeted therapy approach for transplant rejection and autoimmune disease relative to calcineurin inhibitors and anti-proliferatives, but overall efficacy may be limited due to their collateral effect of simultaneously blocking CTLA-4 coinhibitory signals. As such, current investigations into the potential of selective CD28 blockade to block the costimulatory potential of CD28 while exploiting the coinhibitory effects of CTLA-4 are promising. However, as selective CD28 blockade inhibits the activity of both effector and regulatory T cells, an important goal for the future is the design of therapies that will maximize the attenuation of effector responses while preserving the suppressive function of T regulatory cells.

Tracking of TCR-Transgenic T Cells Reveals That Multiple Mechanisms Maintain Cardiac Transplant Tolerance in Mice

Solid organ transplantation tolerance can be achieved following select transient immunosuppressive regimens that result in long-lasting restraint of alloimmunity without affecting responses to other antigens. Transplantation tolerance has been observed in animal models following costimulation or coreceptor blockade therapies, and in a subset of patients through induction protocols that include donor bone marrow transplantation, or following withdrawal of immunosuppression. Previous data from our lab and others have shown that proinflammatory interventions that successfully prevent the induction of transplantation tolerance in mice often fail to break tolerance once it has been stably established. This suggests that established tolerance acquires resilience to proinflammatory insults, and prompted us to investigate the mechanisms that maintain a stable state of robust tolerance. Our results demonstrate that only a triple intervention of depleting CD25⁺ regulatory T cells (Tregs), blocking programmed death ligand-1 (PD-L1) signals, and transferring low numbers of alloreactive T cells was sufficient to break established tolerance. We infer from these observations that Tregs and PD-1/PD-L1 signals cooperate to preserve a low alloreactive T cell frequency to maintain tolerance. Thus, therapeutic protocols designed to induce multiple parallel mechanisms of peripheral tolerance may be necessary to achieve robust transplantation tolerance capable of maintaining one allograft for life in the clinic.

CD4 T Cell Help via B Cells Is Required for Lymphopenia-Induced CD8 T Cell Proliferation

Ab-mediated lymphoablation is commonly used in solid organ and hematopoietic cell transplantation. However, these strategies fail to control pathogenic memory T cells efficiently and to improve long-term transplant outcomes significantly. Understanding the mechanisms of T cell reconstitution is critical for enhancing the efficacy of Ab-mediated depletion in sensitized recipients. Using a murine analog of anti-thymocyte globulin (mATG) in a mouse model of cardiac transplantation, we previously showed that peritransplant lymphocyte depletion induces rapid memory T cell proliferation and only modestly prolongs allograft survival. We now report that T cell repertoire following depletion is dominated by memory CD4 T cells. Additional depletion of these residual CD4 T cells severely impairs the recovery of memory CD8 T cells after mATG treatment. The CD4 T cell help during CD8 T cell recovery depends on the presence of B cells expressing CD40 and intact CD40/CD154 interactions. The requirement for CD4 T cell help is not limited to the use of mATG in heart allograft recipients, and it is observed in nontransplanted mice and after CD8 T cell depletion with mAb instead of mATG. Most importantly, limiting helper signals increases the efficacy of mATG in controlling memory T cell expansion and significantly extends heart allograft survival in sensitized recipients. Our findings uncover the novel role for helper memory CD4 T cells during homeostatic CD8 T cell proliferation and open new avenues for optimizing lymphoablative therapies in allosensitized patients.

Recollective homeostasis and the immune consequences of peritransplant depletional induction therapy

One's cellular immune repertoire is composed of lymphocytes in multiple stages of maturation - the dynamic product of their responses to antigenic challenges and the homeostatic contractions necessary to accommodate immune expansions within physiologic norms. Given that alloreactivity is predominantly a cross-reactive phenomenon that is stochastically distributed throughout the overall T-cell repertoire, one's allospecific repertoire is similarly made up of cells in a variety of differentiation states. As such, the continuous expansion and elimination of activated memory populations, producing a 'recollective homeostasis' of sorts, has the potential over time to alter the maturation state and effector composition of both ones protective and alloreactive T-cell repertoire. Importantly, a T cell's maturation state significantly influences its response to numerous immunomodulatory therapies used in organ transplantation, including depletional antibody induction. In this review, we discuss clinically utilized depletional induction strategies, how their use alters a transplant recipient's cellular immune repertoire, and how a recipient's repertoire influences the clinical effects of induction therapy.

Combination of C-X-C motif chemokine 9 and C-X-C motif chemokine 10 antibodies with FTY720 prolongs the survival of cardiac retransplantation allografts in a mouse model

The upregulation of chemokine genes and the subsequent T-lymphocyte recruitment to the graft are early events in the development of acute cardiac transplant rejection or cardiac allograft vasculopathy. In the present study, a combined immunosuppressive regimen of C-X-C motif chemokine 9 (CXCL9) antibody (Ab), CXCL10 Ab and FTY720 was used in order to reduce the infiltration of memory T lymphocytes and prolong graft survival in a retransplantation murine model. BALB/c donor hearts were transplanted heterotopically into C57BL/6 mice at day 28 after skin transplantation. The mice were divided into four groups: i) Control (normal saline), ii) CXCL9 Ab and CXCL10 Ab [150 μg; once daily (qd); intraperitoneal (ip)], iii) FTY720 (0.2 mg/day; qd; ip) and iv) combined (2 mg/kg/day; qd; ip). Measurements of the median survival time of the cardiac grafts, histological examination, reverse transcription-quantitative polymerase chain reaction analysis, enzyme-linked immunosorbent assay and a mixed lymphocyte reaction were performed. The median graft survival time of the combined group was prolonged (9.3 days) compared with that of the control group (3.5 days) (P<0.001). Histological examination revealed that the combined treatment group graft rejection pathological score was 0.50, while the control group score was 3.62 (P<0.001). In addition, the gene expression level of interleukin (IL)-2 was significantly lower and the levels of IL-10 and transforming growth factor-β (TGF-β) were significantly higher in the combined group compared with those in the control group (P<0.001). Furthermore, the serum concentration levels of IL-2 and interferon-γ (IFN-γ) were significantly lower (P<0.001) and the concentration of IL-10 was significantly higher (P<0.05) in the combined group compared with those in the control group. In the mixed lymphocyte reaction, T-cell proliferation was found to be significantly lower in the combined treatment group than that in the control group (P<0.001). In conclusion, treatment with CXCL9 Ab and CXCL10 Ab or FTY720 reduced the graft infiltration of inflammatory cells, inhibited T-cell proliferation and prolonged graft survival. The combined treatment regimen of CXCL9 Ab, CXCL10 Ab and FTY720 was found to significantly reduce the infiltration of inflammatory cells in the graft and prolong graft survival.

Homeostatic expansion as a barrier to lymphocyte depletion strategies

PURPOSE OF REVIEW

Following lymphodepletion, lymphocytes repopulate the immune space both through enhanced thymopoiesis and proliferation of residual nondepleted peripheral lymphocytes. The term homeostatic proliferation (alternatively homeostatic expansion or lymphopenia-induced proliferation) refers to the latter process. Homeostatic proliferation is especially relevant to reconstitution of the lymphocyte compartment following immunodepletion therapy in transplantation. Repopulating lymphocytes can skew toward an effector memory type capable of inducing graft rejection, autoimmunity, or, in the case of allogeneic bone marrow transplantation, graft versus host disease. Here we review recent studies exploring the biologic mechanisms underlying homeostatic proliferation and explore implications for therapy in transplantation.

RECENT FINDINGS

Two immune-depleting agents, alemtuzumab and rabbit antithymocyte globulin, have been well characterized in their abilities to induce an effector-memory phenotype in repopulating lymphocytes. Additionally, we have gained new understandings of the mechanisms by which the cytokines interleukin-7 and interleukin-15 regulate this process. Recent studies have also explored the functions of noncytokine and signaling molecules in lymphopenia-induced proliferation. Finally, we have seen the promise and limitations of several therapeutic approaches, including recombinant interleukin-7 therapy, CD8-targeted antibodies, and peri-transplant cyclophosphamide, to treat posttransplant lymphopenia and reduce the risks of immune dysregulation following homeostatic proliferation.

SUMMARY

Immune dysfunction following homeostatic proliferation is a special challenge in transplantation. A deeper understanding of the underlying biology has led to a number of promising new therapies to overcome this problem.

Memory T Cells in Transplantation

Following infections and environmental exposures, memory T cells are generated that provide long-term protective immunity. Compared to their naïve T cell counterparts, memory T cells possess unique characteristics that endow them with the ability to quickly and robustly respond to foreign antigens. While such memory T cells are beneficial in protecting their hosts from recurrent infection, memory cells reactive to donor antigens pose a major barrier to successful transplantation and tolerance induction. Significant progress has been made over the past several decades contributing to our understanding of memory T cell generation, their distinct biology, and their detrimental impact in clinical and animal models of transplantation. This review focuses on the unique features which make memory T cells relevant to the transplant community and discusses potential therapies targeting memory T cells which may ameliorate allograft rejection.