Early blast clearance during sequential conditioning prior to allogeneic stem cell transplantation in patients with acute myeloid leukaemia.

For patients with relapsed or refractory AML, sequential conditioning prior to allogeneic stem cell transplantation (alloSCT) is an established and potentially curative treatment option. Early response to treatment during conditioning indicates chemotherapy-responsive disease and may have prognostic value. We retrospectively evaluated blast clearance on day 5 after melphalan, administered 11 days prior to alloSCT as part of a sequential conditioning in 176 patients with active AML. Overall survival (OS) was 52% (95% confidence interval [CI] 45%-60%), and relapse-free survival (RFS) was 47% (95% CI 40%-55%) at 3 years. Patients who achieved early blast clearance did not show a significant improvement in OS and RFS (OS, hazard ratio [HR] HR 0.75, p 0.19; RFS, HR 0.71, p 0.09, respectively), but had a significantly lower non-relapse mortality rate (HR 0.46, p 0.017). HLA-mismatched donor, older age, adverse genetic risk and higher comorbidity scores were associated with inferior survival outcomes. A high initial blast count was only associated with inferior prognosis in patients receiving chemotherapy-only compared to total body irradiation containing conditioning therapy. These results indicate that for patients transplanted with active AML, sensitivity to chemotherapy might be of less importance, compared to other disease- and transplant-related factors.

Comparable outcomes for TBI-based versus treosulfan based conditioning prior to allogeneic hematopoietic stem cell transplantation in AML and MDS patients.

Allogeneic hematopoietic stem cell transplantation (allo-HCT) is a standard treatment for patients with AML and MDS. The combination of fractionated total body irradiation(8GyTBI/Flu) with fludarabine is an established conditioning regimen, but fludarabine/treosulfan(Flu/Treo) constitutes an alternative in older/comorbid patients. We conducted a retrospective analysis of 215 AML(in CR) and 96 MDS patients undergoing their first allo-HCT between 2011 and 2022, identifying 53 matched Flu/Treo and 8GyTBI/Flu patients through propensity score matching. Median follow-up of survivors was 3.3 years and 4.1 years. For the Flu/Treo group, 1-year non-relapse mortality (2% vs. 10%, p = 0.03) was lower, while 1-year relapse incidence (16% vs. 13%, p = 0.81) was similar. Three-year outcomes, including relapse-free survival and graft-versus-host disease incidence, were comparable (OS: 81% vs. 74%, p = 0.70; RFS: 78% vs. 66%, p = 0.28; chronic GvHD: 34% vs. 36%, p = 0.97; acute GvHD (100 days): 11% vs. 23%, p = 0.11). Multivariable analysis, considering age, ECOG, HCT-CI, and MRD status, revealed no associations with main outcomes. Dose-reduced conditioning with Flu/Treo or 8GyTBI/Flu demonstrated favorable and comparable survival rates exceeding 70% at 3 years with 1-year NRM rates below 10% and low relapse rates in the matched cohort. These data underline the need for further evaluation of TBI and Treo-based conditionings in prospective trials.

Comparison of antibody-based immunotherapeutics for malignant hematological disease in an experimental murine model.

ABSTRACT: Antibody-based immunotherapies have revolutionized leukemia and lymphoma treatment, with animal studies being crucial in evaluating effectiveness and side effects. By targeting the evolutionary conserved Slamf7 immune receptor, which is naturally expressed by the murine multiple myeloma cell line MPC-11, we have developed a syngeneic mouse model for direct comparison of 3 immunotherapies: monoclonal antibodies (mAb), bispecific T-cell engagers (BiTE), and chimeric antigen receptor (CAR) T cells (CART), all targeting Slamf7. Slamf7-BiTE is a bispecific single-chain antibody consisting of α-Slamf7 and α-CD3 Fv fragments joined through a Gly-Ser linker, and Slamf7-CART comprises the α-Slamf7 Fv fragment fused to the msCD8α transmembrane and msCD28, 4-1BB, and CD3ζ intracellular signaling domains. Slamf7-BiTE and Slamf7-CART effectively killed MPC-11 cells in vitro, independently of Slamf7-mediated inhibitory signaling by self-ligation. After chimerizing the constant region of the rat-anti-mouse Slamf7 antibody to mouse Fc-immunoglobulin G2a for enhanced effector functions, Slamf7-mAb triggered antigen-specific antibody-dependent cellular cytotoxicity by binding to Fcγ receptor IV. In vivo, all 3 immunotherapies showed antitumor effects against Slamf7-expressing targets. Unlike Slamf7-mAb, Slamf7-BiTE led to considerable side effects in test animals, including weight loss and general malaise, which were also observed to a lesser extent after Slamf7-CART infusion. In allogeneic transplant, Slamf7-BiTE and Slamf7-CART maintained activity compared with the nontransplant setting, whereas Slamf7-mAb displayed enhanced antimyeloma activity. In summary, our model faithfully replicates treatment efficacy and side effects detected after human immunotherapy. It aids in developing and improving immunotherapies and may help devise novel approaches to mitigate undesired effects in steady state and allogeneic stem cell transplantation.

Multicenter development of a PET-based risk assessment tool for product-specific outcome prediction in large B-cell lymphoma patients undergoing CAR T-cell therapy.

PURPOSE: The emergence of chimeric antigen receptor (CAR) T-cell therapy fundamentally changed the management of individuals with relapsed and refractory large B-cell lymphoma (LBCL). However, real-world data have shown divergent outcomes for the approved products. The present study therefore set out to evaluate potential risk factors in a larger cohort. METHODS: Our analysis set included 88 patients, treated in four German university hospitals and one Italian center, who had undergone 2-[18F]fluoro-2-deoxy-D-glucose positron emission tomography (PET) before CAR T-cell therapy with tisagenlecleucel or axicabtagene ciloleucel. We first determined the predictive value of conventional risk factors, treatment lines, and response to bridging therapy for progression-free survival (PFS) through forward selection based on Cox regression. In a second step, the additive potential of two common PET parameters was assessed. Their optimal dichotomizing thresholds were calculated individually for each CAR T-cell product. RESULTS: Extra-nodal involvement emerged as the most relevant of the conventional tumor and patient characteristics. Moreover, we found that inclusion of metabolic tumor volume (MTV) further improves outcome prediction. The hazard ratio for a PFS event was 1.68 per unit increase of our proposed risk score (95% confidence interval [1.20, 2.35], P = 0.003), which comprised both extra-nodal disease and lymphoma burden. While the most suitable MTV cut-off among patients receiving tisagenlecleucel was 11 mL, a markedly higher threshold of 259 mL showed optimal predictive performance in those undergoing axicabtagene ciloleucel treatment. CONCLUSION: Our analysis demonstrates that the presence of more than one extra-nodal lesion and higher MTV in LBCL are associated with inferior outcome after CAR T-cell treatment. Based on an assessment tool including these two factors, patients can be assigned to one of three risk groups. Importantly, as shown by our study, metabolic tumor burden might facilitate CAR T-cell product selection and reflect the individual need for bridging therapy.

Ibrutinib for First-Line Treatment of Chronic Graft-Versus-Host Disease: Results From the Randomized Phase III iNTEGRATE Study.

PURPOSE: To present primary and final analyses from the randomized, double-blind, placebo-controlled, phase III iNTEGRATE study, which evaluated the safety and efficacy of ibrutinib with prednisone in previously untreated patients with chronic graft-versus-host disease (cGVHD). METHODS: Patients (age ≥ 12 years) with newly diagnosed moderate or severe cGVHD, requiring systemic corticosteroid therapy, and with no prior systemic treatment for cGVHD were randomly assigned 1:1 to receive ibrutinib 420 mg once daily plus prednisone, starting at 1 mg/kg once daily or placebo plus prednisone. The primary end point was response rate at 48 weeks according to 2014 National Institutes of Health Consensus Development Project Criteria. Other end points included event-free survival, duration of response, time to withdrawal of immunosuppressants, improvement in Lee cGVHD Symptom Scale score, overall survival (OS), and safety. RESULTS: Ninety-five and 98 patients enrolled in the ibrutinib-prednisone and placebo-prednisone arms, respectively. At 48 weeks, response rates were 41% (ibrutinib-prednisone) and 37% (placebo-prednisone; P = .54). At 33 months of follow-up, median duration of response was 19 months (ibrutinib-prednisone) and 10 months (placebo-prednisone; P = .10). Median event-free survival was 15 months (ibrutinib-prednisone) and 8 months (placebo-prednisone; hazard ratio, 0.76; 95% CI, 0.54 to 1.1; P = .11). Improvement in overall Lee cGVHD Symptom Scale was 43% (ibrutinib-prednisone) and 31% (placebo-ibrutinib; P = .07). Median OS was not reached in either arm. The 24-month Kaplan-Meier OS estimates were 80% for both arms (hazard ratio, 1.06; 95% CI, 0.59 to 1.90). Grade ≥ 3 serious adverse events occurred in 49% (ibrutinib-prednisone) and 47% (placebo-prednisone) of patients. CONCLUSION: There was no statistical difference observed in the primary and secondary end points with ibrutinib-prednisone treatment. No new safety signals were observed with ibrutinib treatment in previously untreated patients with cGVHD. The primary end point of iNTEGRATE was not met.

Outcome Prediction in Patients With Large B-cell Lymphoma Undergoing Chimeric Antigen Receptor T-cell Therapy.

The introduction of chimeric antigen receptor (CAR) T-cell therapy has led to a fundamental shift in the management of relapsed and refractory large B-cell lymphoma. However, our understanding of risk factors associated with non-response is still insufficient and the search for predictive biomarkers continues. Some parameters measurable on 18F-fluorodeoxyglucose positron emission tomography (PET) may be of additional value in this context. A total of 47 individuals from three German university centers who underwent re-staging with PET prior to CAR T-cell therapy were enrolled into the present study. After multivariable analysis considering tumor characteristics and patient factors that might affect progression-free survival (PFS), we investigated whether metabolic tumor volume (MTV) or maximum standardized uptake value (SUVmax) further improve risk stratification. Their most suitable cut-offs were determined by Cox and logistic regression. Forward selection identified extra-nodal disease as the most predictive factor of those routinely available, and we found it to be associated with significantly inferior overall survival after CAR T-cell treatment (P = 0.012). Furthermore, patients with MTV and SUVmax higher than the optimal threshold of 11 mL and 16.7, respectively, experienced shorter PFS (P = 0.016 and 0.002, respectively). Hence, these risk factors might be useful for selection of individuals likely to benefit from CAR T-cell therapy and their management.

18F-FDG-PET-MRI for the assessment of acute intestinal graft-versus-host-disease (GvHD).

BACKGROUND: Graft versus host disease (GvHD) is a frequent complication of allogeneic stem cell transplantation (alloSCT), significantly increasing mortality. Previous imaging studies focused on the assessment of intestinal GvHD with contrast-enhanced MRI/CT or 18F-FDG-PET imaging alone. The objective of this retrospective study was to elucidate the diagnostic value of a combined 18F-FDG-PET-MRI protocol in patients with acute intestinal GvHD. METHODS: Between 2/2015 and 8/2019, 21 patients with acute intestinal GvHD underwent 18F-FDG-PET-MRI. PET, MRI and PET-MRI datasets were independently reviewed. Readers assessed the number of affected segments of the lower gastrointestinal tract and the reliability of the diagnosis on a 5-point Likert scale and quantitative PET (SUVmax, SUVpeak, metabolic volume (MV)) and MRI parameter (wall thickness), were correlated to clinical staging of acute intestinal GvHD. RESULTS: The detection rate for acute intestinal GvHD was 56.8% for PET, 61.4% for MRI and 100% for PET-MRI. PET-MRI (median Likert-scale value: 5; range: 4-5) offers a significantly higher reliability of the diagnosis compared to PET (median: 4; range: 2-5; p = 0.01) and MRI alone (median: 4; range: 3-5; p = 0.03). The number of affected segments in PET-MRI (rs = 0.677; p <  0.001) and the MV (rs = 0.703; p <  0.001) correlated significantly with the clinical stage. SUVmax (rs = 0.345; p = 0.14), SUVpeak (rs = 0.276; p = 0.24) and wall thickening (rs = 0.174; p = 0.17) did not show a significant correlation to clinical stage. CONCLUSION: 18F-FDG-PET-MRI allows for highly reliable assessment of acute intestinal GvHD and adds information indicating clinical severity.

Fluorodeoxyglucose F 18 for the Assessment of Acute Intestinal Graft-versus-Host Disease and Prediction of Response to Immunosuppressive Therapy.

Graft-versus-host disease (GVHD) is a common complication that increases morbidity and mortality after allogeneic stem cell transplantation (allo-SCT). Fluorodeoxyglucose F 18 (18F-FDG)-positron emission tomography (PET) imaging has been demonstrated to be highly informative for evaluating and mapping of intestinal GVHD. To corroborate and extend existing findings and to investigate whether glucose metabolism assessed by 18F-FDG-PET might be an effective diagnostic tool to predict corticosteroid-refractory acute GVHD and overall survival. In this retrospective analysis, 101 patients with clinically suspected acute intestinal GVHD underwent 18F-FDG-PET between June 2011 and February 2019. Seventy-four of these patients with clinically and/or histologically proven acute intestinal GVHD as well as positive 18F-FDG-PET findings were analyzed in detail to assess the predictive value of 18F-FDG-PET regarding the response to immunosuppressive therapy and survival. Quantitative PET parameters, particularly the maximum standard uptake value (SUVmax), of patients with a fast response (ie, clinical improvement and decreased GVHD activity by at least 1 stage after 1 week of GVHD treatment) or slow/no response (ie, persistent disease activity for more than 1 week or increasing GVHD activity following first-line immunosuppressive therapy) were evaluated. 18F-FDG-PET detected intestinal GVHD with a sensitivity of 93% (95% confidence interval [CI], 85% to 97%) and specificity of 73% (95% CI, 45% to 91%). Patients with a fast response to immunosuppressive therapy had a mean SUVmax of 13.7 (95% CI, 11.0 to 16.5) compared with 7.6 (95% CI, 7.0 to 8.3; P = .005) observed in patients with prolonged or no response. The median overall survival (OS) was 573.0 days (95% CI, 539.5 to 606.5 days) for patients with fast response versus 255 days (95% CI, 161.0 to 349.0 days; P = .009) for patients with slow or no responses. A SUVmax threshold >8.95 applied to 18F-FDG-PET performed within 100 days after transplantation identified patients with a median OS of 390 versus 117 days for patients with SUVmax ≤8.95 (P = .036). SUVmax threshold and donor type were independent factors for OS. Our results indicate that 18F-FDG-PET is highly accurate in identifying patients with acute intestinal GVHD and may predict responses to immunosuppressive therapy as well as survival, particularly when applied within the first 100 days after transplantation. These results provide a strong rationale to integrate PET imaging in future prospective trials evaluating new therapies for acute GVHD.

Blinatumomab or Inotuzumab Ozogamicin as Bridge to Allogeneic Stem Cell Transplantation for Relapsed or Refractory B-lineage Acute Lymphoblastic Leukemia: A Retrospective Single-Center Analysis.

BACKGROUND: Blinatumomab and inotuzumab ozogamicin are now widely used to treat relapsed or refractory B-cell acute lymphoblastic leukemia (r/r B-ALL). PATIENTS AND METHODS: We have reported the clinical course of 34 adult patients with r/r B-ALL receiving blinatumomab or inotuzumab ozogamicin at our institution from 2009 to 2019. RESULTS: Blinatumomab-based salvage therapy was applied for overt r/r B-ALL (n = 13) or minimal residual disease (MRD) positivity (n = 5). Of the 13 patients with r/r B-ALL, 9 (69%; 95% confidence interval [CI], 39%-91%) achieved complete remission (CR), with 78% of CR patients (95% CI, 40%-97%) reaching MRD negativity. MRD negativity was also achieved in all 5 patients treated for MRD positivity. The 1-year overall survival of patients receiving blinatumomab for r/r B-ALL and MRD positivity was 54% (n = 13; 95% CI, 26%-81%) and 80% (n = 5; 95% CI, 44-100), respectively. In the inotuzumab ozogamicin group, all 16 patients were treated for overt r/r B-ALL. The rate of CR was 94% (95% CI, 70%-100%), with 67% (95% CI, 38%-88%) of CR patients reaching MRD negativity. The 1-year OS after the first application of inotuzumab ozogamicin was 46% (95% CI, 18%-74%). Of those patients receiving blinatumomab and inotuzumab ozogamicin as a bridge-to-transplant strategy, 79% and 80%, respectively, proceeded to allogeneic stem cell transplantation. The most frequent drug-specific adverse events were similar to those previously reported, including cytokine release syndrome, capillary leak syndrome, and neurotoxicity for blinatumomab and transplant-associated veno-occlusive disease of the liver for inotuzumab ozogamicin. CONCLUSION: Together with previous observations from phase III clinical trials, these data suggest that blinatumomab and inotuzumab ozogamicin are highly effective salvage regimens in r/r B-ALL.

First-In-Class CD13-Targeted Tissue Factor tTF-NGR in Patients with Recurrent or Refractory Malignant Tumors: Results of a Phase I Dose-Escalation Study.

BACKGROUND: Aminopeptidase N (CD13) is present on tumor vasculature cells and some tumor cells. Truncated tissue factor (tTF) with a C-terminal NGR-peptide (tTF-NGR) binds to CD13 and causes tumor vascular thrombosis with infarction. METHODS: We treated 17 patients with advanced cancer beyond standard therapies in a phase I study with tTF-NGR (1-h infusion, central venous access, 5 consecutive days, and rest periods of 2 weeks). The study allowed intraindividual dose escalations between cycles and established Maximum Tolerated Dose (MTD) and Dose-Limiting Toxicity (DLT) by verification cohorts. RESULTS: MTD was 3 mg/m2 tTF-NGR/day × 5, q day 22. DLT was an isolated and reversible elevation of high sensitivity (hs) Troponin T hs without clinical sequelae. Three thromboembolic events (grade 2), tTF-NGR-related besides other relevant risk factors, were reversible upon anticoagulation. Imaging by contrast-enhanced ultrasound (CEUS) and dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) showed major tumor-specific reduction of blood flow in all measurable lesions as proof of principle for the mode of action of tTF-NGR. There were no responses as defined by Response Evaluation Criteria in Solid Tumors (RECIST), although some lesions showed intratumoral hemorrhage and necrosis after tTF-NGR application. Pharmacokinetic analysis showed a t1/2(terminal) of 8 to 9 h without accumulation in daily administrations. CONCLUSION: tTF-NGR is safely applicable with this regimen. Imaging showed selective reduction of tumor blood flow and intratumoral hemorrhage and necrosis.

Monitoring minimal residual/relapsing disease after allogeneic haematopoietic stem cell transplantation in adult patients with acute lymphoblastic leukaemia.

Relapse after allogeneic haematopoietic stem cell transplantation (SCT) is a major cause of death in patients with acute lymphoblastic leukaemia (ALL). Here, we retrospectively analysed the contributions of lineage-sorted donor cell chimerism (sDCC) and quantitative PCR (qPCR) targeting disease-specific genetic rearrangements to detect minimal residual/relapsing disease (MRD) and predict impending relapse in 94 adult ALL patients after SCT. With a median follow-up of surviving patients (n = 61) of 3.3 years, qPCR and/or sDCC measurements turned positive in 38 patients (40%). Of these, 22 patients relapsed and 16 remained in complete remission. At 3 years, qPCR and/or sDCC positive patients showed an increased incidence of relapse (50% vs. 4%, p < 0.0001), decreased relapse-free survival (RFS, 40% vs. 85%, p < 0.0001), and decreased overall survival (OS, 47% vs. 87%, p 0.004). Both, qPCR and sDCC pre-detected 11 of 21 relapses occurring within the first two years after SCT and, overall, complemented for each other method in four of the relapsing and four of the non-relapsing cases. Patients receiving pre-emptive MRD-driven interventions (n = 11) or not (n = 10) showed comparable median times until relapse, RFS, and OS. In our single centre cohort, qPCR and sDCC were similarly effective and complementary helpful to indicate haematological relapse of ALL after SCT.

Alloantigen expression on malignant cells and healthy host tissue influences graft-versus-tumor reactions after allogeneic hematopoietic stem cell transplantation.

Durable remissions of hematological malignancies regularly observed following allogeneic hematopoietic stem cell transplantation (aHSCT) are due to the conditioning regimen, as well as an immunological phenomenon called graft-versus-leukemia (GVL) or graft-versus-tumor (GVT) effect. The development of GVL is closely linked to graft-versus-host disease (GVHD), the main side effect associated with aHSCT. Both, GVHD and GVL are mediated by donor T cells that are initially activated by antigen-presenting cells that present recipient-derived alloantigens in the context of either matched or mismatched MHC class I molecules. Using murine models of aHSCT we show that ubiquitously expressed minor histocompatibility alloantigens (mHAg) are no relevant target for GVT effects. Interestingly, certain ubiquitously expressed MHC alloantigens augmented GVT effects early after transplantation, while others did not. The magnitude of GVT effects correlated with tumor infiltration by CD8+ cytotoxic T cells and tumor cell apoptosis. Furthermore, the immune response underlying GVHD and GVT was oligoclonal, highlighting that immunodominance is an important factor during alloimmune responses. These results emphasize that alloantigen expression on non-hematopoietic tissues can influence GVT effects in a previously unrecognized fashion. These findings bear significance for harnessing optimal GVL effects in patients receiving aHSCT.

Mafb lineage tracing to distinguish macrophages from other immune lineages reveals dual identity of Langerhans cells.

Current systems for conditional gene deletion within mouse macrophage lineages are limited by ectopic activity or low efficiency. In this study, we generated a Mafb-driven Cre strain to determine whether any dendritic cells (DCs) identified by Zbtb46-GFP expression originate from a Mafb-expressing population. Lineage tracing distinguished macrophages from classical DCs, neutrophils, and B cells in all organs examined. At steady state, Langerhans cells (LCs) were lineage traced but also expressed Zbtb46-GFP, a phenotype not observed in any other population. After exposure to house dust mite antigen, Zbtb46-negative CD64+ inflammatory cells infiltrating the lung were substantially lineage traced, but Zbtb46-positive CD64- cells were not. These results provide new evidence for the unique identity of LCs and challenge the notion that some inflammatory cells are a population of monocyte-derived DCs.

Batf3 maintains autoactivation of Irf8 for commitment of a CD8α(+) conventional DC clonogenic progenitor.

The transcription factors Batf3 and IRF8 are required for the development of CD8α(+) conventional dendritic cells (cDCs), but the basis for their actions has remained unclear. Here we identified two progenitor cells positive for the transcription factor Zbtb46 that separately generated CD8α(+) cDCs and CD4(+) cDCs and arose directly from the common DC progenitor (CDP). Irf8 expression in CDPs required prior autoactivation of Irf8 that was dependent on the transcription factor PU.1. Specification of the clonogenic progenitor of CD8α(+) cDCs (the pre-CD8 DC) required IRF8 but not Batf3. However, after specification of pre-CD8 DCs, autoactivation of Irf8 became Batf3 dependent at a CD8α(+) cDC-specific enhancer with multiple transcription factor AP1-IRF composite elements (AICEs) within the Irf8 superenhancer. CDPs from Batf3(-/-) mice that were specified toward development into pre-CD8 DCs failed to complete their development into CD8α(+) cDCs due to decay of Irf8 autoactivation and diverted to the CD4(+) cDC lineage.

L-Myc expression by dendritic cells is required for optimal T-cell priming.

The transcription factors c-Myc and N-Myc--encoded by Myc and Mycn, respectively--regulate cellular growth and are required for embryonic development. A third paralogue, Mycl1, is dispensable for normal embryonic development but its biological function has remained unclear. To examine the in vivo function of Mycl1 in mice, we generated an inactivating Mycl1(gfp) allele that also reports Mycl1 expression. We find that Mycl1 is selectively expressed in dendritic cells (DCs) of the immune system and controlled by IRF8, and that during DC development, Mycl1 expression is initiated in the common DC progenitor concurrent with reduction in c-Myc expression. Mature DCs lack expression of c-Myc and N-Myc but maintain L-Myc expression even in the presence of inflammatory signals such as granulocyte-macrophage colony-stimulating factor. All DC subsets develop in Mycl1-deficient mice, but some subsets such as migratory CD103(+) conventional DCs in the lung and liver are greatly reduced at steady state. Importantly, loss of L-Myc by DCs causes a significant decrease in in vivo T-cell priming during infection by Listeria monocytogenes and vesicular stomatitis virus. The replacement of c-Myc by L-Myc in immature DCs may provide for Myc transcriptional activity in the setting of inflammation that is required for optimal T-cell priming.

Re(de)fining the dendritic cell lineage.

Dendritic cells (DCs) are essential mediators of innate and adaptive immune responses. Study of these critical cells has been complicated by their similarity to other hematopoietic lineages, particularly monocytes and macrophages. Progress has been made in three critical areas of DC biology: the characterization of lineage-restricted progenitors in the bone marrow, the identification of cytokines and transcription factors required during differentiation, and the development of genetic tools for the visualization and depletion of DCs in vivo. Collectively, these advances have clarified the nature of the DC lineage and have provided novel insights into their function during health and disease.

Compensatory dendritic cell development mediated by BATF-IRF interactions.

The AP1 transcription factor Batf3 is required for homeostatic development of CD8α(+) classical dendritic cells that prime CD8 T-cell responses against intracellular pathogens. Here we identify an alternative, Batf3-independent pathway in mice for CD8α(+) dendritic cell development operating during infection with intracellular pathogens and mediated by the cytokines interleukin (IL)-12 and interferon-γ. This alternative pathway results from molecular compensation for Batf3 provided by the related AP1 factors Batf, which also functions in T and B cells, and Batf2 induced by cytokines in response to infection. Reciprocally, physiological compensation between Batf and Batf3 also occurs in T cells for expression of IL-10 and CTLA4. Compensation among BATF factors is based on the shared capacity of their leucine zipper domains to interact with non-AP1 factors such as IRF4 and IRF8 to mediate cooperative gene activation. Conceivably, manipulating this alternative pathway of dendritic cell development could be of value in augmenting immune responses to vaccines.

Dual actions of Meis1 inhibit erythroid progenitor development and sustain general hematopoietic cell proliferation.

Myeloid ecotropic viral integration site 1 (Meis1) forms a heterodimer with Pbx1 that augments Hox-dependent gene expression and is associated with leukemogenesis and HSC self-renewal. Here we identified 2 independent actions of Meis1 in hematopoietic development: one regulating cellular proliferation and the other involved in megakaryocyte lineage development. First, we found that endogenous Mesp1 indirectly induces Meis1 and Meis2 in endothelial cells derived from embryonic stem cells. Overexpression of Meis1 and Meis2 greatly enhanced the formation of hematopoietic colonies from embryonic stem cells, with the exception of erythroid colonies, by maintaining hematopoietic progenitor cells in a state of proliferation. Second, overexpression of Meis1 repressed the development of early erythroid progenitors, acting in vivo at the megakaryocyte-erythroid progenitor stage to skew development away from erythroid generation and toward megakaryocyte development. This previously unrecognized action of Meis1 may explain the embryonic lethality observed in Meis1(-/-) mice that arises from failure of lymphatic-venous separation and can result as a consequence of defective platelet generation. These results show that Meis1 exerts 2 independent functions, with its role in proliferation of hematopoietic progenitors acting earlier in development from its influence on the fate choice at the megakaryocyte-erythroid progenitor between megakaryocytic and erythroid development.

Zbtb46 expression distinguishes classical dendritic cells and their committed progenitors from other immune lineages.

Distinguishing dendritic cells (DCs) from other cells of the mononuclear phagocyte system is complicated by the shared expression of cell surface markers such as CD11c. In this study, we identified Zbtb46 (BTBD4) as a transcription factor selectively expressed by classical DCs (cDCs) and their committed progenitors but not by plasmacytoid DCs (pDCs), monocytes, macrophages, or other lymphoid or myeloid lineages. Using homologous recombination, we replaced the first coding exon of Zbtb46 with GFP to inactivate the locus while allowing detection of Zbtb46 expression. GFP expression in Zbtb46(gfp/+) mice recapitulated the cDC-specific expression of the native locus, being restricted to cDC precursors (pre-cDCs) and lymphoid organ- and tissue-resident cDCs. GFP(+) pre-cDCs had restricted developmental potential, generating cDCs but not pDCs, monocytes, or macrophages. Outside the immune system, Zbtb46 was expressed in committed erythroid progenitors and endothelial cell populations. Zbtb46 overexpression in bone marrow progenitor cells inhibited granulocyte potential and promoted cDC development, and although cDCs developed in Zbtb46(gfp/gfp) (Zbtb46 deficient) mice, they maintained expression of granulocyte colony-stimulating factor and leukemia inhibitory factor receptors, which are normally down-regulated in cDCs. Thus, Zbtb46 may help enforce cDC identity by restricting responsiveness to non-DC growth factors and may serve as a useful marker to identify rare cDC progenitors and distinguish between cDCs and other mononuclear phagocyte lineages.