A comprehensive definition of the major antibody specificities in polyclonal rabbit antithymocyte globulin

Anti‐thymocyte globulin‐mediated immunosenescent alterations of T cells in kidney transplant patients

Objectives

Kidney transplant (KT) is the most effective treatment for end‐stage renal disease. The immunosuppressant anti‐thymocyte globulin (ATG) has been applied for induction therapy to reduce the risk of acute transplant rejection for patients at high immunological risk. Despite its putative role in replicative stress during immune reconstitution, the effects of ATG on T‐cell immunosenescent changes remain to be understood.

Methods

Phenotypic and functional features of senescent T cells were examined by flow cytometry in 116 healthy controls (HC) and 95 KT patients for comparative analysis according to ATG treatment and CMV reactivation. The TCR repertoire was analysed in peripheral blood mononuclear cells (PBMCs) of KT patients.

Results

T cells of KT patients treated with ATG (ATG⁺) show typical immunosenescent features, accumulation of CD28⁻, CD85j⁺ or CD57⁺ T cells, and imbalance of functional T‐cell subsets, compared with untreated KT patients (ATG⁻). Plasma IL‐15 and CMV‐IgG levels were higher in KT patients than in HCs, and the IL‐15 level positively correlated with the frequency of CD28⁻ T cells in KT patients. ATG⁺ patients had a higher prevalence of CMV reactivation, which is associated with an increased frequency of CD28⁻ T cells. As a result, ATG⁺ patients had expanded CMV‐specific T cells and decreased TCR diversity. However, proliferation, cytokine‐producing capacity and polyfunctionality of T cells were preserved in ATG⁺ patients.

Conclusion

Our findings suggest that ATG treatment contributes to the accumulation of senescent T cells, which may have lifelong clinical implications in KT patients. Thus, these patients require long‐term and comprehensive immune monitoring.

Strategies for Liver Transplantation Tolerance

Liver transplant (LT) recipients require life-long immunosuppression (IS) therapy to preserve allograft function. The risks of chronic IS include an increased frequency of malignancy, infection, renal impairment, and other systemic toxicities. Despite advances in IS, long-term LT outcomes have not been improved over the past three decades. Standard-of-care (SoC) therapy can, in rare cases, lead to development of operational tolerance that permits safe withdrawal of maintenance IS. However, successful IS withdrawal cannot be reliably predicted and, in current prospective studies, is attempted several years after the transplant procedure, after considerable exposure to the cumulative burden of maintenance therapy. A recent pilot clinical trial in liver tolerance induction demonstrated that peri-transplant immunomodulation, using a regulatory T-cell (Treg) approach, can reduce donor-specific alloreactivity and allow early IS withdrawal. Herein we review protocols for active tolerance induction in liver transplantation, with a focus on identifying tolerogenic cell populations, as well as barriers to tolerance. In addition, we propose the use of novel IS agents to promote immunomodulatory mechanisms favoring tolerance. With numerous IS withdrawal trials underway, improved monitoring and use of novel immunomodulatory strategies will help provide the necessary knowledge to establish an active liver tolerance induction protocol for widespread use.

Rabbit ATG/ATLG in preventing graft-versus-host disease after allogeneic stem cell transplantation: consensus-based recommendations by an international expert panel

This collaborative initiative aimed to provide recommendations on the use of polyclonal antithymocyte globulin (ATG) or anti-T lymphocyte globulin (ATLG) for the prevention of graft-versus-host disease (GvHD) after allogeneic hematopoietic stem cell transplantation (HSCT). A comprehensive review of articles released up to October, 2018 was performed as a source of scientific evidence. Fourteen clinically relevant key questions to the domains indication, administration, and post-transplant management were developed and recommendations were produced using the Delphi technique involving a Panel of 14 experts. ATG/ATLG was strongly recommended as part of myeloablative conditioning regimen prior to matched or mismatched unrelated bone marrow or peripheral blood allogeneic HSCT in malignant diseases to prevent severe acute and chronic GvHD. ATG/ATLG was also recommended prior to HLA-identical sibling peripheral HSCT with good but lesser bulk of evidence. In reduced intensity or nonmyeloablative conditioning regimens, ATG/ATLG was deemed appropriate to reduce the incidence of acute and chronic GvHD, but a higher risk of relapse should be taken into account. Recommendations regarding dose, application, and premedication were also provided as well as post-transplant infectious prophylaxis and vaccination. Overall, these recommendations can be used for a proper and safe application of polyclonal ATG/ATLG to prevent GvHD after allogeneic HSCT.

Treatment of Severe Aplastic Anemia with Porcine Anti-Human Lymphocyte Globulin

Aplastic anemia (AA) is a bone marrow failure syndrome characterized by pancytopenia. Decreased numbers of hematopoietic stem cells and impaired bone marrow microenvironment caused by abnormal immune function describe the major pathogenesis of AA. Hematopoietic stem cell transplantation and immunesuppressive therapy are the first-line treatments for AA. Porcine anti-lymphocyte globulin (p-ALG) is a new product developed in China. Several studies have shown that p-ALG exhibited good therapeutic effects in AA.

Myeloid-derived suppressor cells are bound and inhibited by anti-thymocyte globulin

Myeloid-derived suppressor cells (MDSCs) inhibit T cell responses and are relevant to cancer, autoimmunity and transplant biology. Anti-thymocyte globulin (ATG) is a commonly used T cell depletion agent, yet the effect of ATG on MDSCs has not been investigated. MDSCs were generated in Lewis Lung Carcinoma 1 tumor-bearing mice. MDSC development and function were assessed in vivo and in vitro with and without ATG administration. T cell suppression assays, RT-PCR, flow cytometry and arginase activity assays were used to assess MDSC phenotype and function. MDSCs increased dramatically in tumor-bearing mice and the majority of splenic MDSCs were of the polymorphonuclear subset. MDSCs potently suppressed T cell proliferation. ATG-treated mice developed 50% fewer MDSCs and these MDSCs were significantly less suppressive of T cell proliferation. In vitro, ATG directly bound 99.6% of MDSCs. CCR7, L-selectin and LFA-1 were expressed by both T cells and MDSCs, and binding of LFA-1 was inhibited by ATG pre-treatment. Arg-1 and PD-L1 transcript expression were reduced 30–40% and arginase activity decreased in ATG-pretreated MDSCs. MDSCs were bound and functionally inhibited by ATG. T cells and MDSCs expressed common Ags which were also targets of ATG. ATG may be helpful in tumor models seeking to suppress MDSCs. Alternatively, ATG may inadvertently inhibit important T cell regulatory events in autoimmunity and transplantation.

Posttransplant Immune Activation: Innocent Bystander or Insidious Culprit of Posttransplant Accelerated Atherosclerosis

Cardiovascular disease is a major cause of morbidity, disability, and mortality in kidney transplant patients. Cumulative reports indicate that the excessive risk of cardiovascular events is not entirely explained by the increased prevalence of traditional cardiovascular risk factors. Atherosclerosis is a chronic inflammatory disease, and it has been postulated that posttransplant immune disturbances may explain the gap between the predicted and observed risks of cardiovascular events. Although concordant data suggest that innate immunity contributes to the posttransplant accelerated atherosclerosis, only few arguments plead for a role of adaptive immunity. We report and discuss here consistent data demonstrating that CD8⁺ T cell activation is a frequent posttransplant immune feature that may have pro-atherogenic effects. Expansion of exhausted/activated CD8⁺ T cells in kidney transplant recipients is stimulated by several factors including cytomegalovirus infections, lymphodepletive therapy (e.g., antithymocyte globulins), chronic allogeneic stimulation, and a past history of renal insufficiency. This is observed in the setting of decreased thymic activity, a process also found in elderly individuals and reflecting accelerated immune senescence.

Rabbit anti-human thymocyte immunoglobulin for the rescue treatment of chronic antibody-mediated rejection after pediatric kidney transplantation

BACKGROUND

Chronic antibody-mediated rejection (cAMR) is the leading cause of late kidney graft loss, but current therapies are often ineffective. Rabbit anti-human thymocyte immunoglobulin (rATG) may be helpful, but its use is virtually undocumented.

METHODS

Data were analyzed retrospectively from nine pediatric kidney transplant patients with cAMR were treated with rATG (1.5 mg/kg × 5 days) at our center after non-response to pulsed prednisolone, intravenous immunoglobulin, rituximab, and increased immunosuppressive intensity (including switching to belatacept in some cases), with or without bortezomib.

RESULTS

The median time from diagnosis to cAMR was 179 days. rATG was started 5-741 days after diagnosis. Median estimated glomerular filtration rate (eGFR) increased from 40 mL/min/1.73 m² when rATG was started to 62 mL/min/1.73 m² 9 months later (p = 0.039). Four patients showed substantially higher eGFR after 9 months and 2 patients showed a small improvement; eGFR continued to decline in 3 patients after starting rATG. No grafts were lost during follow-up. At last follow-up, donor-specific antibodies (DSAs) were no longer detectable in 4 out of 8 patients for whom data were available, median fluorescence intensity had decreased substantially in 1 out of 8 patients; anti-HLA DQ DSAs persisted in 2 out of 8 patients. No adverse events with a suspected relation to rATG, including allergic reactions, leukocytopenia or infections, were observed in any of the patients.

CONCLUSIONS

In this small series of patients, rATG appears a promising treatment for unresponsive cAMR. Further evaluation, including earlier introduction of rATG, is warranted.

Antithymocyte globulins in renal transplantation-from lymphocyte depletion to lymphocyte activation: The doubled-edged sword

Compelling data suggest that lymphocyte depletion following T cell depleting therapy may induce prolonged CD4 T cell lymphopenia and trigger lymphocyte activation in some patients. These profound and non-reversible immune changes in T cell pool subsets are the consequence of both impaired thymic renewal and peripheral homeostatic proliferation. Chronic viral challenges by CMV play a major role in these immune alterations. Even when the consequences of CD4 T cell lymphopenia have been now well described, recent studies shed new light on the clinical consequences of immune activation. In this review, we will first focus on the mechanisms involved in T cell pool reconstitution after T cell depletion and further consider the clinical consequences of ATG-induced T cell activation and senescence in renal transplant recipients.

Correlation of rabbit antithymocyte globulin serum levels and clinical outcomes in children who received hematopoietic stem cell transplantation from an alternative donor

We analyzed the correlation between rabbit ATG (rATG) serum levels and clinical outcomes in 37 children who received rATG at a total dose of 10 or 15 mg/kg during HSCT conditioning from an alternative donor. Fourteen patients had advanced malignant diseases, 13 had severe AA, and 10 had inherited disorders. Complete engraftment was achieved in all patients, and no rejection occurred. The cumulative incidence of grades II-IV acute GVHD and extensive chronic GVHD was 27% (95% CI, 12.5-39.6%) and 8.1% (95% CI, 0-23.1%), respectively. Multivariate analysis identified lower rATG levels at week 4 as an independent risk factor in the development of grades II-IV acute GVHD (p = 0.037). Serious infections were not observed in any patient following HSCT. No correlation was found between EBV reactivation and rATG levels at week 2 and week 4 after HSCT. Furthermore, no correlation was found between relapse and rATG levels two and four wk post-transplantation. The probability of five-yr OS among patients was 70.3% (95% CI, 59.8-79.2%). Our results suggest that targeted rATG administration may protect patients from severe acute GVHD without increasing the risk of EBV reactivation or relapse.

Immunomodulation with rabbit anti-thymocyte globulin in solid organ transplantation

Rabbit anti-thymocyte globulin’s manifold mechanisms of action may be attribuited to its polyclonal nature. Its T-cell depleting effect on lymphoid cells is well established: Occurring in the blood and secondary lymphoid tissues, depletion proceeds through complement-dependent lysis, opsonization and apoptotic pathways. Clinical studies have shown that rabbit anti-thymocyte globulin’s immunomodulatory effect extends beyond the initial T-cell depletion and up to the period during which lymphocyte populations begin to recover. The drug is able to mediate immunomodulation and graft tolerance by functionally inactivating cell surface receptors involved in antigen recognition, leukocyte trafficking and leukocyte endothelium adhesion. The complex and prolonged immunomodulation induced by this drug contributes to its efficacy in solid organ transplantation, mainly by reducing the incidence of acute graft rejection.

ATG-induced accelerated immune senescence: clinical implications in renal transplant recipients

Persistent ATG-induced CD4(+) T cell lymphopenia is associated with serious clinical complications. We tested the hypothesis that ATG induces accelerated immune senescence in renal transplant recipients (RTR). Immune senescence biomarkers were analyzed at transplant and one-year later in 97 incident RTR -62 patients receiving ATG and 35 receiving anti-CD25 mAb (α-CD25). This consisted in: (i) thymic output; (ii) bone marrow renewal of CD34(+) hematopoietic progenitor cells (CD34(+) HPC) and lymphoid (l-HPC) and myeloid (m-HPC) progenitor ratio; (iii) T cell phenotype; and (iv) measurement of T cell relative telomere length (RTL) and telomerase activity (RTA). Clinical correlates were analyzed with a 3 year follow-up. Thymic output significantly decreased one-year posttransplant in ATG-treated patients. ATG was associated with a significant decrease in l-HPC/m-HPC ratio. Late stage differentiated CD57(+) /CD28(-) T cells increased in ATG-treated patients. One-year posttransplant T cell RTL and RTA were consequently lower in ATG-treated patients. ATG is associated with accelerated immune senescence. Increased frequency of late differentiated CD4(+) T cell frequency at transplantation tended to be predictive of a higher risk of subsequent opportunistic infections and of acute rejection only in ATG-treated patients but this needs confirmation. Considering pretransplant immune profile may help to select those patients who may benefit from ATG to prevent severe infections and acute rejection.

Risk of serious opportunistic infections after solid organ transplantation: interleukin-2 receptor antagonists versus polyclonal antibodies. A meta-analysis

BACKGROUND

We aimed to evaluate and quantify the risk of serious opportunistic infections after induction with polyclonal antibodies versus IL-2 receptor antagonists (IL-2RAs) in randomized clinical trials.

METHODS

PRISMA guidelines were followed and random-effects models were performed.

RESULTS

70 randomized clinical trials (10,106 patients) were selected: 36 polyclonal antibodies (n = 3377), and 34 IL-2RAs (n = 6729). Compared to controls, polyclonal antibodies showed higher risk of serious opportunistic infections (OR: 1.93, 95% CI: 1.34-2.80; p < 0.0001); IL-2RAs were associated with lower risk of serious opportunistic infections (OR: 0.80, 95% CI: 0.68-0.94; p = 0.009). Polyclonal antibodies were associated with higher risk of bacterial (OR: 1.58, 95% CI: 1.00-2.50; p = 0.049) and viral infections (OR: 2.37, 95% CI: 1.60-3.49; p < 0.0001), while IL-2RAs were associated with lower risk of cytomegalovirus (CMV) disease (OR: 0.73, 95% CI: 0.56-0.97; p = 0.032). Adjusted indirect comparison: compared to polyclonal antibodies, IL-2RAs were associated with lower risk of serious opportunistic infections (OR: 0.41, 95% CI: 0.34-0.49; p < 0.0001), bacterial infections (OR: 0.51, 95% CI: 0.39-0.67; p < 0.0001) and CMV disease (OR: 0.58, 95% CI: 0.34-0.98; p = 0.043). Results remained consistent across allografts.

CONCLUSION

The risk of serious opportunistic infections, bacterial infections and CMV disease were all significantly decreased with IL-2RAs compared to polyclonal antibodies.

Polyclonal antithymocyte globulin and cardiovascular disease in kidney transplant recipients

T-lymphocyte activation may contribute to atherosclerosis, the prevalence of which is increased in transplant patients. However, the cardiovascular consequences of polyclonal antithymocyte globulin (ATG)-induced immune modifications, which include alterations in T-cell subsets, are unknown. We conducted a retrospective single-center study to assess whether ATG associates with an increased incidence of atherosclerotic events (CVEs) in kidney transplant patients. Propensity score analysis was performed to address potential confounding by indication. We also tested whether ATG use induces a proatherogenic immune status. Sixty-nine (12.2%) CVEs occurred during follow-up (87±31 months). The cumulative incidence of CVEs was higher in ATG-treated patients (14.7% versus 8.2%; P=0.03). Cox regression analysis revealed that ATG use was an independent risk factor for CVEs (hazard ratio [HR], 2.36; 95% confidence interval [95% CI], 1.35 to 4.13; P=0.003). Results obtained in the propensity score match analysis recapitulated those obtained from the overall cohort (HR, 2.09; 95% CI, 1.11 to 3.98; P=0.02). Late-stage differentiated CD8(+) T cells increased 1 year after transplantation only in ATG-treated patients. More generally, ATG associated with features of immune activation. These modifications increased markedly in patients exposed to cytomegalovirus (CMV). Subanalyses suggest that the effect of ATG on CVEs is restricted to CMV-exposed patients. However, CMV infection associated significantly with CVEs only in ATG-treated patients (HR, 2.07; 95% CI, 1.16 to 3.70; P=0.01). In conclusion, ATG associated with both immune activation and post-transplant CVEs in this cohort. Further studies should precisely determine whether ATG-induced immune activation is the causal link between ATG and CVEs.

A comprehensive and quantitative analysis of the major specificities in rabbit antithymocyte globulin preparations

Antithymocyte globulin (ATG) preparations are used for treatment and prevention of graft rejection episodes, graft versus host disease and aplastic anemia. The immunomodulatory and immuosuppressive properties of ATGs are mediated by their interaction with a large variety of antigens expressed on immune and nonimmune cell populations. We have conducted a comprehensive analysis on antibody specificities contained in rabbit ATGs in clinical use, ATG-Fresenius (ATG-F) and Thymoglobulin (THG). We have used retroviral expression cloning to identify novel ATG antigens and demonstrate that together with ATG antigens described earlier, these molecules account for the majority of ATG antibodies directed to human cells. Moreover, we have employed cell lines engineered to express antigens at high levels to quantify the antibodies directed to each ATG antigen. We have used cell lines expressing the T cell receptor complex, CD2 and CD28 to remove antibodies to these antigens from ATG preparations and demonstrate that this treatment abrogated the ability of ATGs to induce activation and forkhead box P3 expression in T cells. Comprehensive information and differences on the antigens targeted by ATG-F and THG as well as novel approaches to assess their functional properties are the basis for a better understanding of their immunomodulatory capacities and might eventually translate into improved ATG-based regimen.

Lymphodepletional strategies in transplantation

Because lymphocytes were shown to mediate transplant rejection, their depletion has been studied as a mechanism of preventing rejection and perhaps inducing immunologic tolerance. Agents that profoundly deplete lymphocytes have included monoclonal antibodies, cytotoxic drugs, and radiation. We have studied several such agents but focused on antibodies that deplete not only peripheral blood lymphocytes, but also lymph node lymphocytes. Depletion of lymph node T lymphocytes appears to permit peripheral tolerance at least for T cells in animal models. Nevertheless, B-cell responses may be resistant to such approaches, and T memory cells are likewise relatively resistant to depleting antibodies. We review the experimental and clinical approaches to depletion strategies and outline some of the pitfalls of depletion, such as limitations of currently available agents, duration of tolerance, infection, and malignancy. It is notable that most tolerogenic strategies that have been attempted experimentally and clinically include depleting agents even when they are not named as the underlying strategy. Thus, there is an implicitly acknowledged role for reducing the precursor frequency of donor antigen-specific lymphocytes when approaching the daunting goal of transplant tolerance.

Severe anaphylactoid reaction to thymoglobulin in a pediatric renal transplant recipient

Intraoperative administration of thymoglobulin is an integral part of the anti-rejection regimen during organ transplantation. However, its administration may be associated with complications. An anaphylactoid reaction that occurred in a pediatric recipient of a living-related renal transplant, on initiating an intravenous infusion of thymoglobulin, is presented.

Novel cell and gene therapies for HIV

Highly active antiretroviral therapy dramatically improves survival in HIV-infected patients. However, persistence of HIV in reservoirs has necessitated lifelong treatment that can be complicated by cumulative toxicities, incomplete immune restoration, and the emergence of drug-resistant escape mutants. Cell and gene therapies offer the promise of preventing progressive HIV infection by interfering with HIV replication in the absence of chronic antiviral therapy. Individuals homozygous for a deletion in the CCR5 gene (CCR5Δ32) are largely resistant to infection from R5-topic HIV-1 strains, which are most commonly transmitted. A recent report that an HIV-infected patient with relapsed acute myelogenous leukemia was effectively cured from HIV infection after transplantation of hematopoietic stem/progenitor cells (HSC) from a CCR5Δ32 homozygous donor has generated renewed interest in developing treatment strategies that target viral reservoirs and generate HIV resistance in a patient's own cells. Although the development of cell-based and gene transfer therapies has been slow, progress in a number of areas is evident. Advances in the fields of gene-targeting strategies, T-cell-based approaches, and HSCs have been encouraging, and a series of ongoing and planned trials to establish proof of concept for strategies that could lead to successful cell and gene therapies for HIV are under way. The eventual goal of these studies is to eliminate latent viral reservoirs and the need for lifelong antiretroviral therapy.

Lymphocyte depletion in experimental hemorrhagic shock in Swine

BACKGROUND

Hemorrhagic shock results in systemic activation of the immune system and leads to ischemia-reperfusion injury. Lymphocytes have been identified as critical mediators of the early innate immune response to ischemia-reperfusion injury, and immunomodulation of lymphocytes may prevent secondary immunologic injury in surgical and trauma patients.

METHODS

Yorkshire swine were anesthetized and underwent a grade III liver injury with uncontrolled hemorrhage to induce hemorrhagic shock. Experimental groups were treated with a lymphocyte depletional agent, porcine polyclonal anti-thymocyte globulin (PATG) (n = 8) and compared to a vehicle control group (n = 9). Animals were observed over a 3 day survival period. Circulating lymphocytes were examined with FACS analysis for CD3/CD4/CD8, and central lymphocytes with mesenteric lymph node and spleen staining for CD3. Circulating and lung tissue16 infiltrating neutrophils were measured. Circulating CD3 lymphocytes in the blood and in central lymphoid organs (spleen/lymph node) were stained and evaluated using FACS analysis. Immune-related gene expression from liver tissue was quantified using RT-PCR.

RESULTS

The overall survival was 22% (2/9) in the control and 75% (6/8) in the PATG groups, p = 0.09; during the reperfusion period (following hemorrhage) survival was 25% (2/8) in the control and 100% (6/6) in the PATG groups, p = 0.008. Mean blood loss and hemodynamic profiles were not significantly different between the experimental and control groups. Circulating CD3+CD4+ lymphocytes were significantly depleted in the PATG group compared to control. Lymphocyte depletion in the setting of hemorrhagic shock also significantly decreased circulating and lung tissue infiltrating neutrophils, and decreased expression of liver ischemia gene expression.

CONCLUSIONS

Lymphocyte manipulation with a depletional (PATG) strategy improves reperfusion survival in experimental hemorrhagic shock using a porcine liver injury model. This proof of principle study paves the way for further development of immunomodulation approaches to ameliorate secondary immune injury following hemorrhagic shock.

Ex vivo expansion of human Tregs by rabbit ATG is dependent on intact STAT3-signaling in CD4⁺ T cells and requires the presence of monocytes

The addition of low, nondepleting doses of rabbit antithymocyte globulin (ATG) to human peripheral blood mononuclear cells has been shown to expand functional CD4(+) CD25(+) FoxP3(+) regulatory T cells (Tregs) in vitro. This report is the first to elucidate the exact cellular mechanisms of ATG-mediated Treg expansion. CD4(+) T cells require monocytes, but not other antigen presenting cell subsets, to be present in coculture to expand Tregs. However, T cells do not require direct cell-cell contact with monocytes, suggesting the importance of soluble factors. Moreover, ATG initially "reprograms" CD4(+) T cells, but not monocytes, and induces STAT3 and STAT5 signaling in CD4(+) cells. These reprogrammed CD4(+) T cells subsequently secrete GM-CSF and IL-10 only in case of intact STAT3 signaling, which in turn promote the generation of tolerogenic CD14(+) CD11c(+) dendritic cells characterized by enhanced IL-10 and decreased IL-12 production. Treg expansion following ATG treatment is accompanied by enhanced gene expression of both GM-CSF and Bcl-2, but not TGF-β, in peripheral blood mononuclear cells. These results demonstrate that ex vivo expansion of human Tregs by ATG is due to its ability to reprogram CD4(+) T cells in a STAT3-dependent but TGF-β-independent manner, leading to the generation of monocyte-derived dendritic cells with a tolerogenic cytokine profile.

Apoptotic effects of antilymphocyte globulins on human pro-inflammatory CD4+CD28- T-cells

Background

Pro-inflammatory, cytotoxic CD4⁺CD28⁻ T-cells with known defects in apoptosis have been investigated as markers of premature immuno-senescence in various immune-mediated diseases. In this study we evaluated the influence of polyclonal antilymphocyte globulins (ATG-Fresenius, ATG-F) on CD4⁺CD28⁻ T-cells in vivo and in vitro.

Principal Findings

Surface and intracellular three colour fluorescence activated cell sorting analyses of peripheral blood mononuclear cells from 16 consecutive transplant recipients and short-term cell lines were performed. In vivo, peripheral levels of CD3⁺CD4⁺CD28⁻ T-cells decreased from 3.7±7.1% before to 0±0% six hours after ATG-F application (P = 0.043) in 5 ATG-F treated but not in 11 control patients (2.9±2.9% vs. 3.9±3.0%). In vitro, ATG-F induced apoptosis even in CD4⁺CD28⁻ T-cells, which was 4.3-times higher than in CD4⁺CD28⁺ T-cells. ATG-F evoked apoptosis was partially reversed by the broad-spectrum caspase inhibitor benzyloxycarbonyl (Cbz)-Val-Ala-Asp(OMe)-fluoromethylketone (zVAD-fmk) and prednisolon-21-hydrogensuccinate. ATG-F triggered CD25 expression and production of pro-inflammatory cytokines, and induced down-regulation of the type 1 chemokine receptors CXCR-3, CCR-5, CX3CR-1 and the central memory adhesion molecule CD62L predominately in CD4⁺CD28⁻ T-cells.

Conclusion

In summary, in vivo depletion of peripheral CD3⁺CD4⁺CD28⁻ T-cells by ATG-F in transplant recipients was paralleled in vitro by ATG-F induced apoptosis. CD25 expression and chemokine receptor down-regulation in CD4⁺CD28⁻ T-cells only partly explain the underlying mechanism.

Assessment of batch to batch variation in polyclonal antithymocyte globulin preparations

BACKGROUND

Antithymocyte globulins (ATGs) are used to prevent and treat allograft rejection and graft versus host disease. They are purified IgG fractions derived from rabbits immunized with the Jurkat T-cell line (ATG-Fresenius) or thymus cells (Thymoglobulin). Differences not only in the amounts of leukocyte reactive antibodies but also in the antigens targeted by ATGs could potentially affect the clinical efficacy of different batches of these polyclonal antibody preparations.

METHODS

Four batches of ATG-Fresenius and Thymoglobulin were compared regarding their capacity to interact with human leukocytes from healthy donors and kidney transplant recipients. Using flow cytometric assays, we analyzed the reactivity of these ATG preparations with Jurkat cells and with primary leukocytes. In addition, ATGs derived from different batches were probed with a panel of cell lines expressing high levels of ATG antigens. Their ability to mediate complement-mediated lysis of human monocytes and lymphocytes was also compared.

RESULTS

Binding studies to leukocyte antigens and functional analysis pointed to a high conformity among different batches in both ATG preparations.

CONCLUSIONS

From our in vitro data, it can be expected that ATGs derived from different batches will not differ in their clinical efficacy. Furthermore, the methods described in this study allow for a reliable analysis of ATG batches.

Biologics in organ transplantation

The last two decades have witnessed a pandemic in antibody development, with over 600 entering clinical studies and a total of 28 approved by the FDA and European Union. The incorporation of biologics in transplantation has made a significant impact on allograft survival. Herein, we review the armamentarium of clinical and preclinical biologics used for organ transplantation--with the exception of belatacept--from depleting and IL-2R targeting induction agents to costimulation blockade, B-cell therapeutics, BAFF and complement inhibition, anti-adhesion, and anti-cytokine approaches. While individual agents may be insufficient for tolerance induction, they provide possibilities for reduction of steroid or calcineurin inhibitor use, alternatives to rejection episodes refractory to conventional therapies, and specialized immunosuppression for highly sensitized patients.

The COX-2 gene promoter polymorphism -765 delays CD4 T-cell reconstitution after lymphocyte depletion with antithymocyte globulins

Polyclonal antithymocyte globulins (ATG) induce persistent changes in T-lymphocyte subsets characterized by low CD4 T. The mechanisms remain partly unknown. Prostaglandin E(2) (PGE(2)) is involved in lymphocyte homeostasis. Whether PGE(2) may be involved in persistent CD4 T-cell lymphopenia after ATG is unknown. We examined the association between this polymorphism and CD4 T-cell count in 159 renal transplant recipients (RTR) who received ATG. Analysis of these patients identified 6 CC (3.8%), 32 GC (22.6%), and 117 GG (73.6%) genotypes. Patients with the GG genotype had significantly higher serum PGE(2) concentrations, leading us to compare C carriers with GG patients. Carriers of the C allele had lower CD4 T cell count 1 year (235 ± 96 vs 323 ± 227/mm(3); p = 0.022) and 2 years posttransplant (325 ± 79 vs 422 ± 231/mm(3); p = 0.024). In multivariate analysis, the C allele (p = 0.029) conferred an increased risk of posttransplant CD4 T-cell lymphocytopenia. Pretransplant T-cell receptor excision circle levels were lower in C carriers. COX-2 gene promoter polymorphism at position -765 (G → C) is associated with persistent CD4 T-cell lymphopenia after ATG in RTR. This effect is likely to be mediated by the actions of PGE(2) on thymus function and viability.

Thymic function, anti-thymocytes globulins, and cancer after renal transplantation

BACKGROUND

Prolonged CD4 T cell lymphopenia after polyclonal antithymocyte globulins (ATG) is associated with an increased rate of cancers. Here, we examined whether pre-transplant thymic function estimated by TREC levels is predictive of cancer occurrence following ATG treatment.

PATIENTS AND METHODS

The impact of TREC on cancer occurrence was analyzed in 115 consecutive incident renal transplant recipients having received ATG.

RESULTS

Mean follow-up was 7.5±2.6years. After ATG induction, patients with the lowest pre-transplant TREC values had lower post-transplant CD4(+) and CD4(+) CD45RA(+) CD45RO(-) T cell counts, and a higher frequency of T cells with a regulatory phenotype (CD127(+)CD4(+)CD25(+)Foxp3(+)). Log-transformed pre-transplant TREC values were significantly lower in patients who developed cancer after transplantation (p<0.0001). The cumulative incidence of cancer was higher in patients having the lowest pre-transplant TREC values (T1 [low]: 47.4%, T2 [medium]: 12.5%, and T3 [high]: 2.7%; p<0.0001). In multivariate analysis, pre-transplant TREC value was the only predictive factor of cancer (HR, 0.39; 95% CI, 0.16 to 0.97, for one log (TREC/10(6) PBMC); p=0.046).

CONCLUSIONS

Pre-transplant thymic function is associated with an increased rate of post-transplant cancer in patients having received ATG. Omitting ATG in recipients with low pre-transplant TREC values should be considered.

Evolving concepts in the selection of immunosuppression regimen for liver transplant recipients

The introduction of calcineurin inhibitor (CNI) based immunosuppression has revolutionized the field of liver transplantation by dramatically reducing the incidence of acute cellular rejection and prolonging patient and allograft survival. However, the introduction of CNIs has also come at the price of increased patient morbidity, particularly with regard to the well-known nephrotoxic effects of the medications. In an effort to minimize the adverse effects, immunosuppression regimen have evolved to include the use of various induction agents and purine synthesis inhibitors to limit the dose of CNI necessary to achieve low acute cellular rejection rates. Careful assessments of risks and benefits are needed as these newer agents have their own side effect profiles. In addition, the impact of newer immunosuppression regimen on hepatitis C (HCV) recurrence has not been completely elucidated. This review will provide an overview of the most common immunosuppression regimen used in liver transplantation and discuss their impact on acute cellular rejection, patient and allograft survival, and HCV recurrence.

Chemokine receptor 5 knockout strategies

PURPOSE OF REVIEW

Individuals homozygous for a deletion in the chemokine receptor 5 (CCR5) gene (CCR5Δ32) are almost completely resistant to HIV-1 infection. A recent report that transplantation of hematopoietic stem or progenitor cells (HSCs) from a CCR5Δ32 homozygous donor effectively cured an HIV patient has increased interest in the development of strategies that could be used to recreate this phenotype using a patient's own cells. This review will focus on recent developments to disrupt CCR5 expression in both autologous T cells and HSCs.

RECENT FINDINGS

CCR5 expression in HIV-1 target cells can be suppressed by RNA-based gene suppression technologies such as RNA interference, or completely eliminated by zinc finger nuclease (ZFN)-mediated gene disruption. ZFNs bind specifically to a DNA sequence and generate a double-stranded DNA break, whose subsequent repair by the cell's error-prone nonhomologous end-joining pathway can lead to permanent disruption of the gene's open reading frame. Recent developments in humanized mouse models have facilitated preclinical studies that have demonstrated the ability of CCR5-targeted ZFNs to suppress HIV-1 in vivo, when used to modify human T cells or HSCs. The same CCR5 ZFNs are now being evaluated in a phase I clinical trial of ex vivo expanded autologous T cells.

SUMMARY

CCR5 gene knockout in T cells or HSCs by ZFNs effectively suppresses the replication of CCR5-tropic strains of HIV-1 in animal models. ZFNs are currently being evaluated in a phase I clinical trials using ex vivo expanded T cells and HSCs targeted therapies are under development.

Interaction of antithymocyte globulins with dendritic cell antigens

The polyclonal rabbit antithymocyte globulins (ATGs), Thymoglobulin and ATG-Fresenius S, are widely used for prevention and therapy of allograft rejection and graft versus host disease. Dendritic cells (DC) govern immune responses and thus the interaction of ATGs with these cells could potentially contribute to the clinical effects of ATG therapy. Currently there is little information on the DC-antigens targeted by ATGs. In this study we have used a new methodology to identify DC surface antigens recognized by ATGs. By screening an eukaryotic expression library generated from DC with ATGs we could identify several novel ATG antigens including CD81, CD82, CD98, CD99 and CD147. Furthermore, we engineered cells to express previously described ATG antigens and probed them with Thymoglobulin and ATG-Fresenius S. Our results demonstrated strong binding to some but not all of these molecules. We show that previously described antigens and antigens identified in this study account for around 80% of the DC reactivity of ATGs. Analysis of molecules induced by ATG-DC interaction are more in support for an activation of these cells by ATGs than for a specific induction of a tolerogenic DC phenotype.

Rabbit antithymocyte globulin (thymoglobulin): 25 years and new frontiers in solid organ transplantation and haematology

The more than 25 years of clinical experience with rabbit antithymocyte globulin (rATG), specifically Thymoglobulin, has transformed immunosuppression in solid organ transplantation and haematology. The utility of rATG has evolved from the treatment of allograft rejection and graft-versus-host disease to the prevention of various complications that limit the success of solid organ and stem cell transplantation. Today, rATG is being successfully incorporated into novel therapeutic regimens that seek to reduce overall toxicity and improve long-term outcomes. Clinical trials have demonstrated the efficacy and safety of rATG in recipients of various types of solid organ allografts, recipients of allogeneic stem cell transplants who are conditioned with both conventional and nonconventional regimens, and patients with aplastic anaemia. Over time, clinicians have learnt how to better balance the benefits and risks associated with rATG. Advances in the understanding of the multifaceted mechanism of action will guide research into new therapeutic areas and future applications.

Influence of polyclonal antithymocyte globulins on the expression of adhesion molecules of isolated human umbilical vein endothelial cells

OBJECTIVES

Polyclonal antithymocyte globulins (ATGs) are immunosuppressive agents applied for the treatment and prevention of organ rejection after transplantation. ATGs induce complement-mediated cell death in T lymphocytes and decrease leukocyte adhesion. However, little is known about the effects of ATGs on endothelial cells (EC). Our aim was to study the influence of ATGs upon the expression of adhesion molecules on human umbilical vein endothelial cells (HUVECs) after stimulation with tumor necrosis factor (TNF)-alpha.

MATERIAL AND METHODS

HUVECs obtained from umbilical cords were incubated with ATGs before and after 6-hour stimulation with TNF-alpha. The group incubated without ATG served as the controls. Another group was not stimulated with TNF-alpha. By flow cytometry, we analyzed the expression of several adhesion molecules: intercellular adhesion molecule (ICAM)-1, vascular cell adhesion molecule (VCAM), platelet EC adhesion molecule (PECAM), and CD62E. Statistical analysis used analysis of variance.

RESULTS

After TNF-alpha stimulation, the EC surface expression of ICAM-1 and CD62E was reduced, although not significantly, in treated as compared with untreated cells. The expression of ICAM-1 and CD62E was similar in the unstimulated groups. The expression of VCAM, PECAM, CD55, and CD58 was not modified by ATG treatment.

CONCLUSION

Our results demonstrated that ATGs insignificantly reduced the expression of adhesion molecules in HUVECs. The effect of ATGs on stimulated HUVECs remains unclear, probably due to the lack of effector cells.

A novel clinically relevant approach to tip the balance toward regulation in stringent transplant model

BACKGROUND

Regulatory T cells (Tregs) actively regulate alloimmune responses and promote transplantation tolerance. Thymoglobulin, a rabbit polyclonal antithymocyte globulin (ATG), is a widely used induction therapy in clinical organ transplantation that depletes peripheral T cells. However, resistance to tolerance induction is seen with certain T-cell depleting strategies and is attributed to alterations in the balance of naive, memory and Tregs. The exact mechanism of action of ATG and its effects on the homeostasis and balance between Tregs and T-effector-memory cells (Tem) are unknown.

METHODS

A novel antibody reagent, rabbit polyclonal anti-murine thymocyte globulin (mATG), generated by the same process used to manufacture thymoglobulin, was used alone or in combination with CTLA4Ig or sirolimus (SRL) in a stringent fully major histocompatibility complex-mismatched murine skin allograft model to study graft survival and mechanisms involved.

RESULTS

mATG depletes T cells but preferentially spares CD25+ natural Tregs which limit skewing of T-cell repertoire toward Tem phenotype among the recovering T cells. T-cell depletion with mATG combined with CTLA4Ig and SRL synergize to prolong graft survival by tipping the Treg/Tem balance further in favor of Tregs by preserving Tregs, facilitating generation of new Tregs by a conversion mechanism and limiting Tem expansion in response to alloantigen and homeostatic proliferation.

CONCLUSIONS

Simultaneous T-cell depletion with ATG and costimulatory blockade, combined with SRL, synergizes to promote regulation and prolong allograft survival in a stringent transplant model. These results provide the rationale for translating such novel combination therapy to promote regulation in primate and human organ transplantation.

Induction therapy: why, when, and which agent?

The long-term outcome of paediatric transplantation has improved over the last decade with an increase in the armamentarium of immunosuppressive agents. However, the battle against the hostile immune response at the time of and after transplantation continues. Induction therapy can reduce early injury, to optimize the long-term allograft survival. The goal of induction immunosuppression in paediatric transplantation is to permit the use of lower doses of maintenance immunosuppressive agents without increased rates of acute allograft rejection and chronic allograft damage. The aim of this review is to summarize the current literature relating to the use of antibody agents for induction in paediatric solid organ transplantation.

Prolonged CD4 T cell lymphopenia increases morbidity and mortality after renal transplantation

Prolonged CD4 T cell lymphopenia after administration of polyclonal anti-thymocyte globulins increases the rate of posttransplantation morbidity, but whether impaired immune reconstitution affects survival is unknown. We studied the effect of CD4 T cell lymphopenia on survival in 302 consecutive prevalent renal transplant recipients and the role of thymic function in CD4 T cell reconstitution and posttransplantation outcomes in 100 consecutive incident renal transplant recipients. We followed the prevalent cohort for a mean duration of 92 months. Of these 302 patients, 81 (27%) had persistent CD4 T cell counts <300/mm3 and 36 (12%) died during follow-up. We observed a higher death rate in patients with CD4 T cell lymphopenia persisting for >1 year (24.1 versus 7.6%; P < 0.001). Furthermore, in Cox regression analysis, CD4 T cell lymphopenia associated with a nearly five-fold risk for death (adjusted hazard ratio [HR] 4.63; 95% confidence interval [CI] 1.91 to 10.65; P = 0.001). In the incident cohort, we estimated thymic function by T cell receptor excision circles (TRECs) per 150,000 CD3+ cells, which predicted efficient CD4 T cell reconstitution. Higher pretransplantation TREC values associated with lower risks for cancer (adjusted HR 0.39; 95% CI 0.15 to 0.97; P = 0.046) and infection (HR 0.29; 95% CI 0.11 to 0.78; P = 0.013). In summary, prolonged polyclonal anti-thymocyte globulin-induced CD4 T cell lymphopenia is an independent risk factor for death. Determination of pretransplantation thymic function may identify patients at higher risk for CD4 T cell lymphopenia and posttransplantation morbidity, including cancer and infections.