The cytostatic deoxycytidine analog cytarabine (ara-C) is the most active agent available against acute myelogenous leukemia (AML). Together with anthracyclines, ara-C forms the backbone of AML treatment for children and adults. In AML, both the cytotoxicity of ara-C in vitro and the clinical response to ara-C therapy are correlated with the ability of AML blasts to accumulate the active metabolite ara-C triphosphate (ara-CTP), which causes DNA damage through perturbation of DNA synthesis. Differences in expression levels of known transporters or metabolic enzymes relevant to ara-C only partially account for patient-specific differential ara-CTP accumulation in AML blasts and response to ara-C treatment. Here we demonstrate that the deoxynucleoside triphosphate (dNTP) triphosphohydrolase SAM domain and HD domain 1 (SAMHD1) promotes the detoxification of intracellular ara-CTP pools. Recombinant SAMHD1 exhibited ara-CTPase activity in vitro, and cells in which SAMHD1 expression was transiently reduced by treatment with the simian immunodeficiency virus (SIV) protein Vpx were dramatically more sensitive to ara-C-induced cytotoxicity. CRISPR-Cas9-mediated disruption of the gene encoding SAMHD1 sensitized cells to ara-C, and this sensitivity could be abrogated by ectopic expression of wild-type (WT), but not dNTPase-deficient, SAMHD1. Mouse models of AML lacking SAMHD1 were hypersensitive to ara-C, and treatment ex vivo with Vpx sensitized primary patient-derived AML blasts to ara-C. Finally, we identified SAMHD1 as a risk factor in cohorts of both pediatric and adult patients with de novo AML who received ara-C treatment. Thus, SAMHD1 expression levels dictate patient sensitivity to ara-C, providing proof-of-concept that the targeting of SAMHD1 by Vpx could be an attractive therapeutic strategy for potentiating ara-C efficacy in hematological malignancies.

© F e r r a t a S t o r t i F o u n d a t i o norder to decrease the lag time for CTL transfer. Among other things, interferon-gamma capture of virus-specific CTL and allogeneic CTL banks containing CTL against the most common HLA haplotypes have been used with some success. 16,22,23 Our group has tried separation of peptide-specific T cells with the help of magnetic beads and MHC multimers. 17,18 Even though this has shown promising results, none of these strategies is yet part of standard clinical practice.In this retrospective analysis we analyzed possible risk factors associated with the development of PTLD after SCT at our center. The earlier part of the cohort of patients was previously included in the studies by Sundin et al. and Omar et al. 1,24 We found several factors that were independently associated with an increased risk of PTLD. Some of the factors seem to act synergistically, which adds additional risk of PTLD in the patients. Moreover, in spite of improved prophylaxis and rituximab use, the long-term survival of affected patients is poor. Methods PatientsIn total 1021 patients who underwent SCT at Karolinska University Hospital in Huddinge, Stockholm, between 1996 and 2011 were included in this retrospective analysis of risk factors for and clinical outcome of PTLD. This study was approved by the regional ethical committee in Stockholm (n. 425/97). A review of patients' charts and the clinical database identified 40 cases of verified PTLD. The characteristics of patients with and without PTLD are displayed in Table 1. Diagnosis and treatment of Epstein-Barr virus-associated post-transplant lymphoproliferative diseaseThe diagnosis of PTLD was made according to the histological criteria reported for B-cell lymphoproliferative states following transplantation. 25 After July 2005 (total of 446 individuals) all patients considered to be at high risk of developing PTLD were screened weekly or biweekly for EBV during the first 3 months post-SCT. In patients at low risk, quantitative polymerase chain reaction (PCR) analysis was performed if EBV reactivation was suspected. From July 2005 all patients were treated with rituximab if the EBV load was >10 000 copies/mL. Thirty-five of the patients were treated with rituximab. More details are available in the Online Supplementary Methods section.Conditioning regimen, graft-versus-host disease prophylaxis and stem-cell source RIC was used in 402 patients, while myeloablative conditioning was given to 619 patients. Antithymocyte globulin (ATG) was given to 705 patients as part of the conditioning with the last dose on day -1. ATG was used in all patients with an unrelated or mismatched donor and in all patients with a non-malignant disease, independently of the type of donor. A few patients with a sibling donor treated with RIC (n=44) were also given ATG. The graft source was bone marrow in 361 cases, peripheral blood in 608 and umbilical cord blood in 52. For more details regarding conditioning regimens, GVHD prophylaxis and stem-cell source see th...

CD8+ T cell exhaustion is a hallmark of many cancers and chronic infections. In mice, T cell factor 1 (TCF-1) maintains exhausted CD8+ T cell responses, whereas thymocyte selection-associated HMG box (TOX) is required for the epigenetic remodeling and survival of exhausted CD8+ T cells. However, it has remained unclear to what extent these transcription factors play analogous roles in humans. In this study, we mapped the expression of TOX and TCF-1 as a function of differentiation and specificity in the human CD8+ T cell landscape. Here, we demonstrate that circulating TOX+ CD8+ T cells exist in most humans, but that TOX is not exclusively associated with exhaustion. Effector memory CD8+ T cells generally expressed TOX, whereas naive and early-differentiated memory CD8+ T cells generally expressed TCF-1. Cytolytic gene and protein expression signatures were also defined by the expression of TOX. In the context of a relentless immune challenge, exhausted HIV-specific CD8+ T cells commonly expressed TOX, often in clusters with various activation markers and inhibitory receptors, and expressed less TCF-1. However, polyfunctional memory CD8+ T cells specific for cytomegalovirus (CMV) or Epstein-Barr virus (EBV) also expressed TOX, either with or without TCF-1. A similar phenotype was observed among HIV-specific CD8+ T cells from individuals who maintained exceptional immune control of viral replication. Collectively, these data demonstrate that TOX is expressed by most circulating effector memory CD8+ T cell subsets and not exclusively linked to exhaustion.