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B Introduction: b Treatment options for relapsed or refractory ( I r i / I r i ) B-cell non-Hodgkin lymphomas (B-NHL) have broadened towards T-cell engaging therapies, including CD19-targeting chimeric antigen receptor T-cells (CD19-CAR) and bispecific antibodies (CD19-BsAb). B Methods: b Aiming to identify lymphoma cell-inherent mechanisms that impair response to T-cell engaging therapy, we quantified the in-vitro response of 46 B-NHL cell lines to 3 SP rd sp generation CD19-CAR and CD19-BsAb. PROTEOMIC PROFILING IDENTIFIES GRANZYME B INHIBITOR SERPIN B9 AS MEDIATOR OF RESISTANCE TO CAR T-CELL AND BISPECIFIC ANTIBODY TREATMENT IN NODAL B-CELL LYMPHOMA. [Extracted from the article]

Abstract: Relapse after anti-CD19 chimeric antigen receptor (CD19-CAR) occurs in a substantial proportion of patients with lymphoid malignancies. We assessed the potential benefits of co-administering CD20-targeting bispecific antibodies (CD20-BsAbs) with CD19-CAR T cells with the aim of enhancing immunotherapeutic efficacy. Addition of CD20-BsAbs to cocultures of CD19-CARs and primary samples of B-cell malignancies, comprising malignant B cells and endogenous T cells, significantly improved killing of malignant cells and enhanced the expansion of both endogenous T cells and CD19-CAR T cells. In an immunocompetent mouse model of chronic lymphocytic leukemia, relapse after initial treatment response frequently occurred after CD19-CAR T-cell monotherapy. Additional treatment with CD20-BsAbs significantly enhanced the treatment response and led to improved eradication of malignant cells. Higher efficacy was accompanied by improved T-cell expansion with CD20-BsAb administration and led to longer survival with 80% of the mice being cured with no detectable malignant cell population within 8 weeks of therapy initiation. Collectively, our in vitro and in vivo data demonstrate enhanced therapeutic efficacy of CD19-CAR T cells when combined with CD20-BsAbs in B-cell malignancies. Activation and proliferation of both infused CAR T cells and endogenous T cells may contribute to improved disease control., (© 2024 American Society of Hematology. Published by Elsevier Inc. All rights are reserved, including those for text and data mining, AI training, and similar technologies.)

Abstract: The T-box transcription factor T-bet is known as a master regulator of the T-cell response but its role in malignant B cells has not been sufficiently explored. Here, we conducted single-cell resolved multi-omics analyses of malignant B cells from patients with chronic lymphocytic leukemia (CLL) and studied a CLL mouse model with a genetic knockout of Tbx21. We found that T-bet acts as a tumor suppressor in malignant B cells by decreasing their proliferation rate. NF-κB activity, induced by inflammatory signals provided by the microenvironment, triggered T-bet expression, which affected promoter-proximal and distal chromatin coaccessibility and controlled a specific gene signature by mainly suppressing transcription. Gene set enrichment analysis identified a positive regulation of interferon signaling and negative control of proliferation by T-bet. In line, we showed that T-bet represses cell cycling and is associated with longer overall survival of patients with CLL. Our study uncovered a novel tumor suppressive role of T-bet in malignant B cells via its regulation of inflammatory processes and cell cycling, which has implications for the stratification and therapy of patients with CLL. Linking T-bet activity to inflammation explains the good prognostic role of genetic alterations in the inflammatory signaling pathways in CLL.

B cell acute lymphoblastic leukemia (B-ALL) is characterized by an accumulation of malignant precursor cells. Treatment consists of multiagent chemotherapy followed by allogeneic stem cell transplantation in high-risk patients. In addition, patients bearing the BCR-ABL1 fusion gene receive concomitant tyrosine kinase inhibitor (TKI) therapy. On the other hand, monoclonal antibody therapy is increasingly used in both clinical trials and real-world settings. The introduction of rituximab has improved the outcomes in CD20 positive cases. Other monoclonal antibodies, such as tafasitamab (anti-CD19), obinutuzumab (anti-CD20) and epratuzumab (anti-CD22) have been tested in trials (NCT05366218, NCT04920968, NCT00098839). The efficacy of monoclonal antibodies is based, at least in part, on their ability to induce antibody-dependent cellular cytotoxicity (ADCC). Combination treatments, e.g., chemotherapy and TKI, should therefore be screened for potential interference with ADCC. Here, we report on in vitro data using BCR-ABL1 positive and negative B-ALL cell lines treated with rituximab and TKI. NK cell activation, proliferation, degranulation, cytokine release and tumor cell lysis were analyzed. In contrast to ATP site inhibitors such as dasatinib and ponatinib, the novel first-in-class selective allosteric ABL myristoyl pocket (STAMP) inhibitor asciminib did not significantly impact ADCC in our settings. Our results suggest that asciminib should be considered in clinical trials.

The redirection of T cells has emerged as an attractive therapeutic principle in B cell non-Hodgkin lymphoma (B-NHL). However, a detailed characterization of lymphoma-infiltrating T cells across B-NHL entities is missing. Here we present an in-depth T cell reference map of nodal B-NHL, based on cellular indexing of transcriptomes and epitopes, T cell receptor sequencing, flow cytometry and multiplexed immunofluorescence applied to 101 lymph nodes from patients with diffuse large B cell, mantle cell, follicular or marginal zone lymphoma, and from healthy controls. This multimodal resource revealed quantitative and spatial aberrations of the T cell microenvironment across and within B-NHL entities. Quantitative differences in PD1 + TCF7 - cytotoxic T cells, T follicular helper cells or IKZF3 + regulatory T cells were linked to their clonal expansion. The abundance of PD1 + TCF7 - cytotoxic T cells was associated with poor survival. Our study portrays lymphoma-infiltrating T cells with unprecedented comprehensiveness and provides a unique resource for the investigation of lymphoma biology and prognosis., (© 2024. The Author(s).)

Ex vivo drug response profiling is a powerful tool to study genotype-drug response associations and is being explored as a tool set for precision medicine in cancer. Here we conducted a prospective non-interventional trial to investigate feasibility of ex vivo drug response profiling for treatment guidance in hematologic malignancies (SMARTrial, NCT03488641 ). The primary endpoint to provide drug response profiling reports within 7 d was met in 91% of all study participants (N = 80). Secondary endpoint analysis revealed that ex vivo resistance to chemotherapeutic drugs predicted chemotherapy treatment failure in vivo. We confirmed the predictive value of ex vivo response to chemotherapy in a validation cohort of 95 individuals with acute myeloid leukemia treated with daunorubicin and cytarabine. Ex vivo drug response profiles improved ELN-22 risk stratification in individuals with adverse risk. We conclude that ex vivo drug response profiling is clinically feasible and has the potential to predict chemotherapy response in individuals with hematologic malignancies beyond clinically established genetic markers., (© 2023. The Author(s).)

Large-scale compound screens are a powerful model system for understanding variability of treatment response and discovering druggable tumor vulnerabilities of hematological malignancies. However, as mostly performed in a monoculture of tumor cells, these assays disregard modulatory effects of the in vivo microenvironment. It is an open question whether and to what extent coculture with bone marrow stromal cells could improve the biological relevance of drug testing assays over monoculture. Here, we established a high-throughput platform to measure ex vivo sensitivity of 108 primary blood cancer samples to 50 drugs in monoculture and coculture with bone marrow stromal cells. Stromal coculture conferred resistance to 52% of compounds in chronic lymphocytic leukemia (CLL) and 36% of compounds in acute myeloid leukemia (AML), including chemotherapeutics, B-cell receptor inhibitors, proteasome inhibitors, and Bromodomain and extraterminal domain inhibitors. Only the JAK inhibitors ruxolitinib and tofacitinib exhibited increased efficacy in AML and CLL stromal coculture. We further confirmed the importance of JAK-STAT signaling for stroma-mediated resistance by showing that stromal cells induce phosphorylation of STAT3 in CLL cells. We genetically characterized the 108 cancer samples and found that drug-gene associations strongly correlated between monoculture and coculture. However, effect sizes were lower in coculture, with more drug-gene associations detected in monoculture than in coculture. Our results justify a 2-step strategy for drug perturbation testing, with large-scale screening performed in monoculture, followed by focused evaluation of potential stroma-mediated resistances in coculture., (© 2023 by The American Society of Hematology. Licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0), permitting only noncommercial, nonderivative use with attribution. All other rights reserved.)

Somatic structural variants (SVs) are widespread in cancer, but their impact on disease evolution is understudied due to a lack of methods to directly characterize their functional consequences. We present a computational method, scNOVA, which uses Strand-seq to perform haplotype-aware integration of SV discovery and molecular phenotyping in single cells by using nucleosome occupancy to infer gene expression as a readout. Application to leukemias and cell lines identifies local effects of copy-balanced rearrangements on gene deregulation, and consequences of SVs on aberrant signaling pathways in subclones. We discovered distinct SV subclones with dysregulated Wnt signaling in a chronic lymphocytic leukemia patient. We further uncovered the consequences of subclonal chromothripsis in T cell acute lymphoblastic leukemia, which revealed c-Myb activation, enrichment of a primitive cell state and informed successful targeting of the subclone in cell culture, using a Notch inhibitor. By directly linking SVs to their functional effects, scNOVA enables systematic single-cell multiomic studies of structural variation in heterogeneous cell populations., (© 2022. The Author(s).)

Cancer heterogeneity at the proteome level may explain differences in therapy response and prognosis beyond the currently established genomic and transcriptomic-based diagnostics. The relevance of proteomics for disease classifications remains to be established in clinically heterogeneous cancer entities such as chronic lymphocytic leukemia (CLL). Here, we characterize the proteome and transcriptome alongside genetic and ex-vivo drug response profiling in a clinically annotated CLL discovery cohort (n = 68). Unsupervised clustering of the proteome data reveals six subgroups. Five of these proteomic groups are associated with genetic features, while one group is only detectable at the proteome level. This new group is characterized by accelerated disease progression, high spliceosomal protein abundances associated with aberrant splicing, and low B cell receptor signaling protein abundances (ASB-CLL). Classifiers developed to identify ASB-CLL based on its characteristic proteome or splicing signature in two independent cohorts (n = 165, n = 169) confirm that ASB-CLL comprises about 20% of CLL patients. The inferior overall survival in ASB-CLL is also independent of both TP53- and IGHV mutation status. Our multi-omics analysis refines the classification of CLL and highlights the potential of proteomics to improve cancer patient stratification beyond genetic and transcriptomic profiling., (© 2022. The Author(s).)

The tumour microenvironment and genetic alterations collectively influence drug efficacy in cancer, but current evidence is limited and systematic analyses are lacking. Using chronic lymphocytic leukaemia (CLL) as a model disease, we investigated the influence of 17 microenvironmental stimuli on 12 drugs in 192 genetically characterised patient samples. Based on microenvironmental response, we identified four subgroups with distinct clinical outcomes beyond known prognostic markers. Response to multiple microenvironmental stimuli was amplified in trisomy 12 samples. Trisomy 12 was associated with a distinct epigenetic signature. Bromodomain inhibition reversed this epigenetic profile and could be used to target microenvironmental signalling in trisomy 12 CLL. We quantified the impact of microenvironmental stimuli on drug response and their dependence on genetic alterations, identifying interleukin 4 (IL4) and Toll-like receptor (TLR) stimulation as the strongest actuators of drug resistance. IL4 and TLR signalling activity was increased in CLL-infiltrated lymph nodes compared with healthy samples. High IL4 activity correlated with faster disease progression. The publicly available dataset can facilitate the investigation of cell-extrinsic mechanisms of drug resistance and disease progression., (© 2022 The Authors. Published under the terms of the CC BY 4.0 license.)

Application of anti-thymocyte globulin (ATG) is a widely used strategy for the prevention of graft-versus-host disease (GvHD). As vitamin D3 serum levels are also discussed to affect hematopoietic stem cell transplantation (HSCT) outcome and GvHD development, we analysed a possible interplay between ATG treatment and serum levels of 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 in 4 HSCT cohorts with different vitamin D3 supplementation. ATG is significantly associated with higher serum level of 1,25-dihydroxyvitamin D3 around HSCT (day -2 to 7, peri-transplant), however only in patients with adequate levels of its precursor 25-hydroxyvitamin D3. ATG exposure had no impact on overall survival in patients supplemented with high dose vitamin D3, but was associated with higher risk of one-year treatment-related mortality (log rank test p=0.041) in patients with no/low vitamin D3 supplementation. However, the difference failed to reach significance applying a Cox-model regression without and with adjustment for baseline risk factors (unadjusted P=0,058, adjusted p=0,139). To shed some light on underlying mechanisms, we investigated the impact of ATG on 1,25-Dihydroxyvitamin D3 production by human dendritic cells (DCs) in vitro. ATG increased gene expression of CYP27B1 , the enzyme responsible for the conversion of 25-hydroxyvitamin D3 into 1,25-dihydroxyvitamin D3, which was accompanied by higher 1,25-dihydroxyvitamin D3 levels in ATG-treated DC culture supernatants. Our data demonstrate a cooperative effect of 25-hydroxyvitamin D3 and ATG in the regulation of 1,25-dihydroxyvitamin D3 production. This finding may be of importance in the context of HSCT, where early high levels of 1,25-dihydroxyvitamin D3 levels have been shown to be predictive for lower transplant related mortality and suggest that vitamin D3 supplementation may especially be important in patients receiving ATG for GvHD prophylaxis., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Matos, Peter, Weich, Peuker, Schoenhammer, Roider, Ghimire, Babl, Decking, Güllstorf, Kröger, Hammon, Herr, Stark, Heid, Renner, Holler and Kreutz.)

Bispecific antibodies (BsAbs) can induce long-term responses in patients with refractory and relapsed B-cell lymphoma. Nevertheless, response rates across patients are heterogeneous, and the factors determining quality and duration of responses are poorly understood. To identify key determinants of response to BsAbs, we established a primary, autologous culture model allowing us to mimic treatment with CD3xCD19 and CD3xCD20 BsAbs within the lymph node microenvironment ex vivo. T cell-mediated killing of lymphoma cells and proliferation of T cells varied significantly among patients but highly correlated between BsAbs targeting CD20 or CD19. Ex vivo response to BsAbs was significantly associated with expansion of T cells and secretion of effector molecules (eg, granzyme B, perforin) but not with expression of T-cell exhaustion (eg, PD1, TIM3) or activation markers (eg, CD25, CD69) or formation of intercellular contacts. In addition, we identified a distinct phenotype of regulatory T cells that was linked to ex vivo response independently from T-cell frequency at baseline. High expression levels of Aiolos (IKZF1), ICOS, and CXCR5 were positively associated with ex vivo response, whereas strong expression of Helios (IKZF2) had an unfavorable impact on ex vivo response to BsAbs. We further showed that lenalidomide, nivolumab, and atezolizumab improved ex vivo response to BsAbs by potentiating T-cell effector functions. In summary, our ex vivo study identified a distinct regulatory T-cell phenotype as a potential contributor to treatment failure of BsAbs and suggests drug combinations of high clinical relevance that could improve the efficacy of BsAbs., (© 2021 by The American Society of Hematology. Licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0), permitting only noncommercial, nonderivative use with attribution. All other rights reserved.)

Genome-wide association studies identified a single-nucleotide polymorphism (SNP) affecting the transcription factor Eomesodermin (EOMES) associated with a significantly increased risk to develop chronic lymphocytic leukemia (CLL). Epigenetic analyses, RNA sequencing, and flow cytometry revealed that EOMES is not expressed in CLL cells, but in CD8 + T cells for which EOMES is a known master regulator. We thus hypothesized that the increased CLL risk associated with the EOMES SNP might be explained by its negative impact on CD8 + T-cell-mediated immune control of CLL. Flow cytometry analyses revealed a higher EOMES expression in CD8 + T cells of CLL patients compared to healthy individuals, and an accumulation of PD-1 + EOMES + CD8 + T cells in lymph nodes rather than blood or bone marrow in CLL. This was in line with an observed expansion of EOMES + CD8 + T cells in the spleen of leukemic Eµ-TCL1 mice. As EOMES expression was highest in CD8 + T cells that express inhibitory receptors, an involvement of EOMES in T-cell exhaustion and dysfunction seems likely. Interestingly, Eomes-deficiency in CD8 + T cells resulted in their impaired expansion associated with decreased CLL control in mice. Overall, these observations suggest that EOMES is essential for CD8 + T-cell expansion and/or maintenance, and therefore involved in adaptive immune control of CLL., (© 2021. The Author(s).)

Most non-Hodgkin's lymphomas grow exclusively in the lymph node compartment protected by the tumor microenvironment. To better understand the cellular heterogeneity and the complex interaction between malignant and non-malignant cells, experiments with primary, patient-derived samples are often indispensable. Here, we describe a time-efficient but gentle protocol to process human lymph node samples. This protocol avoids enzymatic or mechanical stress and was optimized for the purpose of generating single-cell suspension suitable for delicate assays, such as single-cell RNA sequencing. For complete details on the use and execution of this protocol, please refer to Roider et al. (2020)., Competing Interests: The authors declare no competing interests., (© 2021 The Author(s).)

The transcription factor eomesodermin (EOMES) promotes interleukin (IL)-10 expression in CD4 + T cells, which has been linked to immunosuppressive and cytotoxic activities. We detected cytotoxic, programmed cell death protein-1 (PD-1) and EOMES co-expressing CD4 + T cells in lymph nodes (LNs) of patients with chronic lymphocytic leukemia (CLL) or diffuse large B-cell lymphoma. Transcriptome and flow cytometry analyses revealed that EOMES does not only drive IL-10 expression, but rather controls a unique transcriptional signature in CD4 + T cells, that is enriched in genes typical for T regulatory type 1 (T R 1) cells. The T R 1 cell identity of these CD4 + T cells was supported by their expression of interferon gamma and IL-10, as well as inhibitory receptors including PD-1. T R 1 cells with cytotoxic capacity accumulate also in Eµ-TCL1 mice that develop CLL-like disease. Whereas wild-type CD4 + T cells control TCL1 leukemia development after adoptive transfer in leukopenic Rag2 -/ - T cells failed to do so. We further show that T + T cells failed to do so. We further show that T R T cells showed impaired antileukemia activity. Altogether, our data demonstrate that EOMES is indispensable for the development of IL-10-expressing, cytotoxic T + T cells showed impaired antileukemia activity. Altogether, our data demonstrate that EOMES is indispensable for the development of IL-10-expressing, cytotoxic T R 1 cells, which accumulate in LNs of CLL patients and control TCL1 leukemia in mice in an IL-10R-dependent manner., (© 2021. The Author(s).)

The serum level of 25-hydroxyvitamin-D3 is accepted as marker for a person's vitamin D status but its role for the outcome of allogeneic hematopoietic stem cell transplantation (HSCT) is controversially discussed. The impact of 1,25-dihydroxyvitamin-D3 on HSCT outcome, however, has never been studied. In a discovery cohort of 143 HSCT patients we repeatedly (day -16 to 100) measured 1,25-dihydroxyvitamin-D3 and in comparison the well-established marker for serum vitamin D status 25-hydroxyvitamin-D3. Only lower 1,25-dihydroxyvitamin-D3 levels around HSCT (day -2 to 7, peritransplant) were significantly associated with higher 1-year treatment-related mortality (TRM) risk (Mann-Whitney U test, P = 0.001). This was confirmed by Cox-model regression without and with adjustment for baseline risk factors and severe acute Graft-versus-Host disease (aGvHD; unadjusted P = 0.001, adjusted P = 0.005). The optimal threshold for 1,25-dihydroxyvitamin-D3 to identify patients at high risk was 139.5 pM. Also in three replication cohorts consisting of altogether 365 patients 1,25-dihydroxyvitamin-D3 levels below 139.5 pM had a 3.3-fold increased risk of TRM independent of severe aGvHD compared to patients above 139.5 pM (Cox-model unadjusted P < 0.0005, adjusted P = 0.001). Our data highlight peritransplant 1,25-dihydroxyvitamin-D3 levels but not the commonly monitored 25-hydroxyvitamin-D3 levels as potent predictor of 1-year TRM and suggest to monitor both vitamin D metabolites in HSCT patients.

The sterile alpha motif and histidine-aspartic domain-containing protein 1 (SAMHD1) has been demonstrated to predict the response to high-dose cytarabine consolidation treatment in acute myeloid leukemia patients. Here, we evaluated SAMHD1 as potential biomarker for the response to high-dose cytarabine in mantle cell lymphoma (MCL) patients. We quantified SAMHD1 protein expression and determined the mutation status in patients of the MCL Younger and Elderly trials (n = 189), who had received high-dose cytarabine- or fludarabine-based polychemotherapy. Additionally, we quantified SAMHD1 expression in B cell lymphoma cell lines and exposed them to cytarabine, fludarabine, and clinically relevant combinations. Across both trials investigated, SAMHD1 mutations had a frequency of 7.1% (n = 13) and did not significantly affect the failure-free survival (FFS, P = .47). In patients treated with high-dose cytarabine- or fludarabine-containing regimes, SAMHD1 expression was not significantly associated with FFS or complete remission rate. SAMHD1 expression in B cell lymphoma cell lines, however, inversely correlated with their in vitro response to cytarabine as single agent (R = .65, P = .0065). This correlation could be reversed by combining cytarabine with other chemotherapeutics, such as oxaliplatin and vincristine, similar to the treatment regime of the MCL Younger trial. We conclude that this might explain why we did not observe a significant association between SAMHD1 protein expression and the outcome of MCL patients upon cytarabine-based treatment., (© 2020 UICC.)

Small extracellular vesicles (sEVs) are nanoparticles responsible for cell-to-cell communication released by healthy and cancer cells. Different roles have been described for sEVs in physiological and pathological contexts, including acceleration of tissue regeneration, modulation of tumor microenvironment, or premetastatic niche formation, and they are discussed as promising biomarkers for diagnosis and prognosis in body fluids. Although efforts have been made to standardize techniques for isolation and characterization of sEVs, current protocols often result in co-isolation of soluble protein or lipid complexes and of other extracellular vesicles. The risk of contaminated preparations is particularly high when isolating sEVs from tissues. As a consequence, the interpretation of data aiming at understanding the functional role of sEVs remains challenging and inconsistent. Here, we report an optimized protocol for isolation of sEVs from human and murine lymphoid tissues. sEVs from freshly resected human lymph nodes and murine spleens were isolated comparing two different approaches-(1) ultracentrifugation on a sucrose density cushion and (2) combined ultracentrifugation with size-exclusion chromatography. The purity of sEV preparations was analyzed using state-of-the-art techniques, including immunoblots, nanoparticle tracking analysis, and electron microscopy. Our results clearly demonstrate the superiority of size-exclusion chromatography, which resulted in a higher yield and purity of sEVs, and we show that their functionality alters significantly between the two isolation protocols.

Tumour heterogeneity encompasses both the malignant cells and their microenvironment. While heterogeneity between individual patients is known to affect the efficacy of cancer therapy, most personalized treatment approaches do not account for intratumour heterogeneity. We addressed this issue by studying the heterogeneity of nodal B-cell lymphomas by single-cell RNA-sequencing and transcriptome-informed flow cytometry. We identified transcriptionally distinct malignant subpopulations and compared their drug-response and genomic profiles. Malignant subpopulations from the same patient responded strikingly differently to anti-cancer drugs ex vivo, which recapitulated subpopulation-specific drug sensitivity during in vivo treatment. Infiltrating T cells represented the majority of non-malignant cells, whose gene-expression signatures were similar across all donors, whereas the frequencies of T-cell subsets varied significantly between the donors. Our data provide insights into the heterogeneity of nodal B-cell lymphomas and highlight the relevance of intratumour heterogeneity for personalized cancer therapy.

Hairy cell leukemia is a rare B-cell malignancy that is characterized by an indolent course. It was initially described as a distinct entity in 1958. Before the establishment of modern treatment, median survival was only 4 years. Since then, major advances in the treatment and understanding of the biology and genomic landscape of hairy cell leukemia have been made. This review summarizes the present understanding of hairy cell leukemia with particular focus on the development of novel and targeted approaches to treatment., Competing Interests: No competing interests were disclosed.No competing interests were disclosed.No competing interests were disclosed.No competing interests were disclosed.No competing interests were disclosed.

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., Competing Interests: The authors declare no conflict of interest.