Rabbit antithymocyte globulin inhibits monocyte-derived dendritic cells maturation in vitro and polarizes monocyte-derived dendritic cells towards tolerogenic dendritic cells expressing indoleamine 2,3-dioxygenase

Biologicals in the prevention and treatment of intestinal graft rejection: The state of the art Biologicals in Intestinal Transplantation

Intestinal transplantation is the standard treatment for patients with intestinal failure with severe complications due to parenteral nutrition; however, rejection leads to graft failure in approximately half of both adult and pediatric recipients within 5 years of transplantation. Although intensive immunosuppressive therapy is used in an attempt to reduce this risk, commonly used treatment strategies are generally practice- and/or expert-based, as head-to-head comparisons are lacking. In this ever-developing field, biologicals designed to prevent or treat rejection are used increasingly, with both infliximab and vedolizumab showing potential in the treatment of acute cellular rejection in individual cases and in relatively small patient cohorts. To help advance progress in clinical care, we review the current use of biologicals in intestinal transplantation, and we provide future perspectives to guide this progress.

Anti-thymocyte globulin for treatment of T-cell-mediated allograft rejection

Anti-thymocyte globulin (ATG) is a pivotal immunosuppressive therapy utilized in the management of T-cell-mediated rejection and steroid-resistant rejection among renal transplant recipients. Commercially available as Thymoglobulin (rabbit-derived, Sanofi, United States), ATG-Fresenius S (rabbit-derived), and ATGAM (equine-derived, Pfizer, United States), these formulations share a common mechanism of action centered on their interaction with cell surface markers of immune cells, imparting immunosuppressive effects. Although the prevailing mechanism predominantly involves T-cell depletion via the com plement-mediated pathway, alternate mechanisms have been elucidated. Optimal dosing and treatment duration of ATG have exhibited variance across ran domised trials and clinical reports, rendering the establishment of standardized guidelines a challenge. The spectrum of risks associated with ATG administration spans from transient adverse effects such as fever, chills, and skin rash in the acute phase to long-term concerns related to immunosuppression, including susceptibility to infections and malignancies. This comprehensive review aims to provide a thorough exploration of the current understanding of ATG, encom passing its mechanism of action, clinical utility in the treatment of acute renal graft rejections, specifically steroid-resistant cases, efficacy in rejection episode reversal, and a synthesis of findings from different eras of maintenance immunosuppression. Additionally, it delves into the adverse effects associated with ATG therapy and its impact on long-term graft function. Furthermore, the review underscores the existing gaps in evidence, particularly in the context of the Banff classification of rejections, and highlights the challenges faced by clinicians when navigating the available literature to strike the optimal balance between the risks and benefits of ATG utilization in renal transplantation.

Anti-thymocyte globulin exposure in patients with diffuse cutaneous systemic sclerosis undergoing autologous haematopoietic stem cell transplantation

Introduction

Autologous haematopoietic stem cell transplantation improves event-free survival and lung function and reduces skin thickening in patients with progressive diffuse cutaneous systemic sclerosis. Anti-thymocyte globulin is a key lymphoablative constituent of conditioning protocols and is administered in a weight-based dosage. However, whether anti-thymocyte globulin exposure contributes to response to autologous haematopoietic stem cell transplantation and lymphocyte reconstitution in diffuse cutaneous systemic sclerosis patients is unknown. We aimed to explore the relationship between anti-thymocyte globulin exposure, lymphocyte reconstitution and treatment response in diffuse cutaneous systemic sclerosis patients undergoing autologous haematopoietic stem cell transplantation.

Methods

A retrospective cohort of 15 diffuse cutaneous systemic sclerosis patients undergoing autologous haematopoietic stem cell transplantation was performed. Clinical characteristics and routine laboratory results were retrieved from electronic medical records. Anti-thymocyte globulin concentrations were measured in cryopreserved plasma samples at four time points (day 1 and week 1, 2 and 4) after stem cell reinfusion. Anti-thymocyte globulin exposure was estimated using a validated population pharmacokinetic model.

Results

During a median follow-up of 45 months (interquartile range 19-66), 11 (73%) patients had a treatment response, and 4 (27%) were non-responders. Although all patients received the same weight-based anti-thymocyte globulin dosage, 7.5 mg/kg divided over 3 days, anti-thymocyte globulin exposure varied. Anti-thymocyte globulin exposure was higher in responders than in non-responders (163 AU*day/mL (interquartile range 153-183) and 137 AU*day/mL (interquartile range 101-149), respectively, p = .026). Anti-thymocyte globulin exposure was not correlated with lymphocyte reconstitution or infection rate.

Conclusion

Weight-based dosing of anti-thymocyte globulin results in variable anti-thymocyte globulin exposure and treatment response across individuals.

Antithymocyte Globulin Inhibits CD8+ T Cell Effector Functions via the Paracrine Induction of PDL-1 on Monocytes

BACKGROUND

Antithymocyte globulins (ATG) are T cell-depleting antibodies used in solid organ transplantation for induction therapy in sensitized patients with a high risk of graft rejection. Previously described effects besides the depletion of T cells have suggested additional modes of action and identified further cellular targets.

METHODS

We examined the transcriptional changes arising in immune cells from human blood after ex vivo stimulation with ATG at the single-cell level to uncover additional mechanisms by which ATG regulates T cell activity and effector functions.

FINDINGS

Analysis of the paracrine factors present in the plasma of ATG-treated whole blood revealed high levels of chemokines and cytokines, including interferon-γ (IFN-γ). Furthermore, we identified an increase in the surface expression of the programmed death ligand 1 (PDL-1) on monocytes mediated by the released paracrine factors. In addition, we showed that this induction is dependent on the activation of JAK/STAT signaling via the binding of IFN-γ to interferon-γ receptor 1 (IFN-γR1). Lastly, we demonstrated that the modulation of the immune regulatory axis of programmed cell death protein 1 (PD1) on activated CD8⁺ T cells with PDL-1 found on monocytes mediated by ATG potently inhibits effector functions including the proliferation and granzyme B release of activated T cells.

INTERPRETATION

Together, our findings represent a novel mode of action by which ATG exerts its immunosuppressive effects.

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.

Dendritic Cells Are Critical for the Activation and Expansion of Vδ2+ T Cells After Allogeneic Hematopoietic Transplantation

γδ T cells perform antitumor and antiviral effector functions and are involved in both innate and adaptive immunity. Vδ2⁺ T cells represent the predominant γδ T subset in the peripheral blood of healthy subjects. Vδ2⁺ T cells can be selectively activated and expanded by phosphoantigens (pAgs). Dendritic cells (DCs), as potent antigen-presenting cells, are capable of mediating pAgs–triggered Vδ2⁺ T cells expansion. However, the association between DCs and Vδ2⁺ T cell recovery in the context of hematopoietic stem cell transplantation (HSCT) remains unclear. We previously demonstrated that the recovery of Vδ2⁺ T cells was hampered and inversely correlated with Epstein-Barr virus (EBV) reactivation in patients undergoing haploidentical HSCT (haploHSCT). Whether Vδ2⁺ T cells from haploHSCT recipients can be expanded by stimulation with aminobisphosphonates or pAg–presenting DCs is of particular interest. Herein, we showed that Vδ2⁺ T cells recovered after haploHSCT failed to expand after ex-vivo stimulation with pamidronate. In addition, we found that the recovery of DC subsets was significantly decreased, and the concentration of myeloid DCs (mDCs) correlated significantly with Vδ2⁺ T cell recovery in the setting of allogeneic HSCT. Furthermore, coculture of peripheral lymphocytes from recipients with monocyte-derived and pamidronate-pretreated autologous or allogeneic DCs induced the successful expansion of Vδ2⁺ T cells. Of note, allogeneic DCs from third-party donors stimulated a significantly higher efficiency of Vδ2⁺ T cell expansion than autologous DCs. More importantly, the memory features were well-retained and the cytotoxic cytokines-production capacity was significantly enhanced in the expanded Vδ2⁺ T cells. Taken together, these results suggest that the frequency and function of DCs are critical for the recovery of Vδ2⁺ T cells after allogeneic HSCT. The fact that vigorous expansions of Vδ2⁺ T cells were induced by phosphoantigen-pretreated DCs, especially by allogeneic third-party DCs, provides additional options for the development of individualized immunotherapy strategies that utilize the anti-viral and anti-leukemic effects of γδ T cells in the context of hematopoietic transplantation.

Immune reconstitution and outcomes after conditioning with anti-thymocyte-globulin in unrelated cord blood transplantation; the good, the bad, and the ugly

Unrelated umbilical cord blood transplantation (UCBT) exhibits a low risk of graft-versus-host-disease (GvHD) and has unique potent anti-virus and anti-leukemia effects. Anti-thymocyte globulin (ATG) in the conditioning regimen for UCBT is successful in reducing graft rejection and GvHD. Nevertheless, this beneficial effect of ATG coincides with its detrimental effect on immune reconstitution. The latter directly relates to a high incidence of viral infections and leukemia relapses. ATG has been used in transplant patients for over 30 years. In recent years, the knowledge on the mechanisms of action of ATG and its implementation in the UCBT setting has increased dramatically. Important data became available showing the highly variable pharmacokinetics (PK) of ATG and its consequence on outcome measures. Here, we review the effects of ATG on immune reconstitution and subsequent outcomes after UCBT, and describe the mechanisms causing these effects. We highlight the importance of optimizing ATG exposure before and after UCBT and discuss strategies to maintain the 'good' and overcome the 'bad and ugly' effects of ATG on UCBT outcome.

In vitro immunoregulatory effects of thymoglobulin on human immune cell subpopulations

Thymoglobulin (ATG) is a polyclonal rabbit antibody against human thymocytes used as a T cell-depleting agent in organ transplantation. Its polyclonal character suggests that its effect may go far beyond just T cell depletion. The aim of this study was to further elucidate possible mechanisms underlying the suppressive activity of ATG. For in vitro studies, human peripheral blood mononuclear cells (PBMC) were incubated with ATG or control Ig for various time points. Foxp3⁺ regulatory cells (T_regs) and monocytes were phenotypically analyzed by flow cytometry and functionally tested by in vitro suppression assays. Cytokine levels were determined by quantitative RT- PCR, Multiplex or ELISA techniques. In vitro, the frequencies of Foxp3⁺ T_regs increased when human PBMC were stimulated with ATG as compared with stimulation by rabbit Ig or without stimulation. ATG-treated cells suppressed proliferation of autologous PBMC stimulated with anti-CD3 and anti-CD28 monoclonal antibodies and this suppression could be reversed by exogenous IL-2. The Foxp3⁺ expression dropped down on day 10, which suggests that it is transient. Monocytes and natural killer cells stimulated with ATG down-modulated CD16. Monocytes suppressed the proliferation of autologous PBMC. However, there were not statistically significant differences in IL-10, TNF-α and TGF-β1 secretion by monocytes stimulated with ATG or control rabbit Ig. These findings suggest that ATG has immunomodulatory effects that go beyond T cell depletion and induction of Foxp3⁺ T_regs. The induction of immunosuppressive monocytes might have a protective role in delaying transplant rejection.

Antithymocyte Globulin Induces a Tolerogenic Phenotype in Human Dendritic Cells

Antithymocyte globulin (ATG) is used in the prevention of graft-versus-host disease during allogeneic hematopoietic stem cell transplantation. It is generally accepted that ATG mediates its immunosuppressive effect primarily via depletion of T cells. Here, we analyzed the impact of ATG-Fresenius (now Grafalon^®) on human monocyte-derived dendritic cells (DC). ATG induced a semi-mature phenotype in DC with significantly reduced expression of CD14, increased expression of HLA-DR, and intermediate expression of CD54, CD80, CD83, and CD86. ATG-DC showed an increase in IL-10 secretion but no IL-12 production. In line with this tolerogenic phenotype, ATG caused a significant induction of indoleamine 2,3-dioxygenase expression and a concomitant increase in levels of tryptophan metabolites in the supernatants of DC. Further, ATG-DC did not induce the proliferation of allogeneic T cells in a mixed lymphocyte reaction but actively suppressed the T cell proliferation induced by mature DC. These data suggest that besides its well-known effect on T cells, ATG modulates the phenotype of DC in a tolerogenic way, which might constitute an essential part of its immunosuppressive action in vivo.

Prevention and treatment of liver allograft antibody-mediated rejection and the role of the 'two-hit hypothesis'

PURPOSE OF REVIEW

The review outlines the diagnosis, prevention strategies, and possible treatment options for acute and chronic antibody-mediated rejection (AMR).

RECENT FINDINGS

Although rare, severe acute AMR (aAMR) usually occurs in patients with high mean fluorescence intensity despite serial dilutions or high-titer preformed class I donor-specific alloantibodies (DSA). The diagnosis is suspected when allograft dysfunction occurs with DSA, diffuse C4d staining, and a microvascular injury, and may be aided by the aAMR score. However, the incidence of and treatment approach to combined T-cell-mediated rejection (TCMR) with DSA present and some but not all features of AMR is yet to be determined. Chronic liver allograft AMR is characterized by low-grade chronic inflammation and progressive fibrosis with DSA, the chronic AMR (cAMR) score may facilitate diagnosis. The 'two-hit' hypothesis, whereby a coexistent insult upregulates human leukocyte antigen class II target antigens on the microvascular endothelium, may explain why suboptimal donors with lower sensitization levels might suffer from acute AMR and those with chronic complications (e.g., recurrent original disease) might be more susceptible to chronic AMR. Although treatment algorithms are needed, prevention is preferable and at a minimum includes transfusion minimization, and medication adherence.

SUMMARY

Severe acute AMR is rare but diagnosable, and there is need to determine the incidence of and optimal therapy for less severe combined AMR and TCMR. Chronic AMR is likely more common and of significant relevance to long-term allograft survival improvement. The two-hit hypothesis may help to explain the rarity of both findings and shed insight onto future prevention and treatment strategies.

In Vivo T Cell Depletion with Myeloablative Regimens on Outcomes after Cord Blood Transplantation for Acute Lymphoblastic Leukemia in Children

The inclusion of antithymocyte globulin (ATG) in cord blood transplantation is controversial. We evaluated outcomes according to ATG inclusion in 297 children and adolescents with acute lymphoblastic leukemia (ALL) who received myeloablative total body irradiation-based conditioning and either single-unit (74%) or double-unit (26%) grafts. Ninety-two patients (31%) received ATG and 205 (69%) did not. ATG recipients were more likely to be cytomegalovirus seronegative. The incidences of day 100 grades II to IV acute graft-versus-host disease (GVHD; 30% versus 54%, P = .0002) and chronic GVHD (22% versus 43%, P = .0008) were lower with ATG compared with non-ATG regimens. However, day 100 grades III to IV acute GVHD was comparable (11% versus 17%, P = .15). The 3-year incidences of transplant-related mortality (16% versus 17%, P = .98), relapse (17% versus 27%, P = .12), and leukemia-free survival (66% versus 55%, P = .23) in ATG and non-ATG recipients were similar. There were no differences in viral reactivation between treatment groups (60% versus 58%, P = .83). Therefore, the data suggest that incorporation of ATG with myeloablative conditioning regimens may be useful in reducing the risk of acute and chronic GVHD without any deleterious effect on transplant-related mortality, relapse, or leukemia-free survival in children and adolescents with ALL.

A novel mechanism of anti-T-lymphocyte globulin mediated by fractalkine in renal ischemia-reperfusion injury in rats

BACKGROUND

Ischemia-reperfusion injury (IRI) is among the main challenges in kidney transplantation. It causes delayed graft function and graft loss in long-term follow-up studies. Anti-T lymphocyte globulin (ATG), a common induction immunosuppressive, has been used in kidney transplantation to prevent rejection. Fractalkine (FKN) is among the main chemokines involved in IRI. This study was designed to identify the relationship between ATG and FKN after warm ischemia in rat kidneys.

METHODS

Rats were divided into three groups: Control, IRI+normal saline(NS) and IRI+ATG. After IRI was initiated, rats received a dose of ATG or NS during surgery as well as two more doses at 24 and 48 hours after surgery. All rats were humanely killed at 72 hours.

RESULTS

The concentration of FKN as well as dendritic cells (DC) and macrophages were lower in both peripheral blood and the injured kidney among ATG-treated versus control rats. Additionally cell necrosis, cytoplasmic vacuolization, cast formation, and tubular dilatation were improved among ATG-treated rats. Serum creatinine levels were lower in rats that received ATG.

CONCLUSION

ATG depleted the concentration of FKN, which inhibits migrations of DCs and macrophages into the kidney, and reduces IRI-related pathology.

Regulatory myeloid cells in transplantation

Regulatory myeloid cells (RMC) are emerging as novel targets for immunosuppressive (IS) agents and hold considerable promise as cellular therapeutic agents. Herein, we discuss the ability of regulatory macrophages, regulatory dendritic cells, and myeloid-derived suppressor cells to regulate alloimmunity, their potential as cellular therapeutic agents, and the IS agents that target their function. We consider protocols for the generation of RMC and the selection of donor- or recipient-derived cells for adoptive cell therapy. Additionally, the issues of cell trafficking and antigen (Ag) specificity after RMC transfer are discussed. Improved understanding of the immunobiology of these cells has increased the possibility of moving RMC into the clinic to reduce the burden of current IS agents and to promote Ag-specific tolerance. In the second half of this review, we discuss the influence of established and experimental IS agents on myeloid cell populations. IS agents believed historically to act primarily on T cell activation and proliferation are emerging as important regulators of RMC function. Better insights into the influence of IS agents on RMC will enhance our ability to develop cell therapy protocols to promote the function of these cells. Moreover, novel IS agents may be designed to target RMC in situ to promote Ag-specific immune regulation in transplantation and to usher in a new era of immune modulation exploiting cells of myeloid origin.

The role of Thymoglobulin induction in kidney transplantation: an update

The rabbit antithymocyte globulin Thymoglobulin first became available over 25 yr ago and is the most widely used lymphocyte-depleting preparation in solid organ transplantation. Thymoglobulin targets a wide range of T-cell surface antigens as well as natural killer-cell antigens, B-cell antigens, plasma cell antigens, adhesion molecules and chemokine receptors, resulting in profound, long-lasting T-cell depletion. Randomized studies have established the anti-rejection efficacy of Thymoglobulin in kidney transplantation. Experimental and clinical data suggest that Thymoglobulin administration may ameliorate ischemia reperfusion injury, thus reducing the incidence of delayed graft function (DGF). Studies have demonstrated the benefit of using Thymoglobulin to facilitate immunosuppression minimization, both for corticosteroid and calcineurin inhibitor (CNI) withdrawal or avoidance, with potential improvement in cardiovascular and renal outcomes. The optimal cumulative dose for Thymoglobulin induction is 6-7.5 mg/kg, with vigilant short- and long-term monitoring of hematological status. Induction with Thymoglobulin is now indicated in immunologically high-risk patients, in those at increased risk of DGF and to maintain efficacy in low-risk transplant recipients receiving steroid or CNI minimization or avoidance regimens. We suggest that in future trials Thymoglobulin be tested with costimulation signal blockers and other immunosuppressants with the objective of establishing operational tolerance.

Anti-thymocyte globulin induces neoangiogenesis and preserves cardiac function after experimental myocardial infarction

Rationale

Acute myocardial infarction (AMI) followed by ventricular remodeling is the major cause of congestive heart failure and death in western world countries.

Objective

Of relevance are reports showing that infusion of apoptotic leucocytes or anti-lymphocyte serum after AMI reduces myocardial necrosis and preserves cardiac function. In order to corroborate this therapeutic mechanism, the utilization of an immunosuppressive agent with a comparable mechanism, such as anti-thymocyte globulin (ATG) was evaluated in this study.

Methods and Results

AMI was induced in rats by ligation of the left anterior descending artery. Initially after the onset of ischemia, rabbit ATG (10 mg/rat) was injected intravenously. In vitro and in vivo experiments showed that ATG induced a pronounced release of pro-angiogenic and chemotactic factors. Moreover, paracrine factors released from ATG co-incubated cell cultures conferred a down-regulation of p53 in cardiac myocytes. Rats that were injected with ATG evidenced higher numbers of CD68+ macrophages in the ischemic myocardium. Animals injected with ATG evidenced less myocardial necrosis, showed a significant reduction of infarct dimension and an improvement of post-AMI remodeling after six weeks (infarct dimension 24.9% vs. 11.4%, p<0.01). Moreover, a higher vessel density in the peri-infarct region indicated a better collateralization in rats that were injected with ATG.

Conclusions

These data indicate that ATG, a therapeutic agent successfully applied in clinical transplant immunology, triggered cardioprotective effects after AMI that salvaged ischemic myocardium by down-regulation of p53. This might have raised the resistance against apoptotic cell death during ischemia. The combination of these mechanisms seems to be causative for improved cardiac function and less ventricular remodeling after experimental AMI.

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.

Immunodepletive anti-alpha/beta-TCR antibody in transplantation of composite tissue allografts: Cleveland Clinic research experience

The immunologic characteristics of composite tissue allografts (CTA), which contain skin, lymphoid elements and bone with bone marrow, raise new challenges for transplant immunologists. Owing to the heterogeneity of transplanted tissues in limb or face transplant models, researchers are focusing on the new tolerance-inducing strategies facilitating CTA acceptance. A number of immunosuppressive protocols have been designed to develop tolerance in experimental models; however, only a few protocols have been introduced to clinical transplantation. In this review, based on own experiences, we discuss the major strategies for tolerance induction in limb and face allograft models in experimental studies. This review is focused on tolerance induction strategies by establishment of donor-specific chimerism using different immunomodulatory protocols, including nonselective T-cell depletion with polyclonal antibody antilymphocyte serum and selective inhibition of alphabeta-T-cell receptors on the alloreactive T cells.

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.

Targeting of natural killer cells by rabbit antithymocyte globulin and campath-1H: similar effects independent of specificity

T cell depleting strategies are an integral part of immunosuppressive regimens widely used in the hematological and solid organ transplant setting. Although it is known to induce lymphocytopenia, little is known about the effects of the polyclonal rabbit antithymocyte globulin (rATG) or the monoclonal anti-CD52 antibody alemtuzumab on Natural Killer (NK) cells in detail. Here, we demonstrate that induction therapy with rATG following kidney/pancreas transplantation results in a rapid depletion of NK cells. Treatment of NK cells with rATG and alemtuzumab in vitro leads to impairment of cytotoxicity and induction of apoptosis even at a 10-fold lower concentration (0.1 microg/ml) compared with T and B cells. By generating Fc-parts of rATG and alemtuzumab we illustrate that their ligation to FcgammaRIII (CD16) is sufficient for the significant induction of degranulation, apoptosis and inflammatory cytokine release (FasL, TNFalpha and IFNgamma) exclusively in CD3(-)CD56(dim) NK cells whereas application of rATG and alemtuzumab F(ab) fragments abolishes these effects. These findings are of general importance as our data suggest that NK cells are also mediators of the clinically relevant cytokine release syndrome and that their targeting by therapeutic antibodies should be considered as they are functionally relevant for the effective clearance of opportunistic viral infections and anti-tumor activity posttransplantation.

Treatment of Autoimmune Disease with Rabbit Anti-T Lymphocyte Globulin: Clinical Efficacy and Potential Mechanisms of Action

The rabbit anti-T lymphocyte globulins (rATGs) are immune-suppressive anti-T cell agents with beneficial effects in solid organ and hematological transplantation. The present review evaluates the potential mechanisms of rATGs and their impact on pilot and exploratory studies of diffuse cutaneous systemic sclerosis (scleroderma-SSc), inclusion body myositis (IBM), vasculitis, and type 1 diabetes mellitus (T1DM). The rATGs are associated with improvements in well-defined parameters of clinical autoimmunity: insulin usage, tissue inflammation, and systemic organ functions. Meta-analysis of a retrospective database of SSc, N = 196 and two prospective randomized pilot studies; IBM, N = 11 and T1DM, N = 17 shows a two- to ninefold increase in the relative response to treatments with intravenous infusions of rATG. The rATGs deplete T cells and are associated with increases in the percentage of CD25+ T cell subsets. This may underlie the apparent long-lasting immunomodulation associated with these agents. The future optimization of rATG adjunct therapy requires statistically powered-controlled prospective trials of rATG dose-finding and timing of administration. The potential mechanisms of rATGs:depletion of autoreactive T cells, generation CDCD25+Foxp3+ regulatory T cells (Tregs), and the acquisition of regulatory immune cell functions, need to be examined in patients prior to rATG infusion and at time intervals following rATG treatment to identify those mechanisms relevant to the improvement of their clinical outcome.

Mechanisms of action of antithymocyte globulin: T-cell depletion and beyond

The success of allogeneic stem cell transplantation and solid-organ transplantation owes much to improvements in the immunosuppressive regimens that prevent graft-versus-host disease (GVHD) or suppress allograft rejection. A better understanding of the immune mechanisms underlying induction of immunological tolerance is the key to successful transplantation. Polyclonal antibodies such as antithymocyte globulins (ATG) have been used for decades. The common belief is that ATG efficacy relies on its capacity to deplete T lymphocytes. The aim of this review is to offer an overview of the recent findings that have been demonstrated in ATG's immunomodulatory activity. The polyclonal nature of ATG is reflected in its diverse effects on the immune system: (1) T-cell depletion in blood and peripheral lymphoid tissues through complement-dependent lysis and T-cell activation and apoptosis; (2) modulation of key cell surface molecules that mediate leukocyte/endothelium interactions; (3) induction of apoptosis in B-cell lineages; (4) interference with dendritic cell functional properties; and (5) induction of regulatory T and natural killer T cells. As a consequence, ATG provides multifaceted immunomodulation paving the way for future applications and suggesting that the use of ATG should be included in the immunosuppression therapeutic armamentarium to help reduce the incidence of organ rejection and GVHD.