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Follicular lymphoma (FL) is an indolent lymphoma of mature B-cells but may transform to a more aggressive histology, most commonly diffuse large B cell lymphoma. Recurrent mutations associated with transformation have been identified; however, biological predictors to guide initial therapy have remained elusive. We hypothesized that clonal heterogeneity and patient-specific immune responses would contribute to variable clinical outcomes and that understanding the complexity of the entire tumor "ecosystem" would allow more individualized matching of patients with specific therapies. In prior ASH meetings, we presented preliminary analyses of B and T cell-focused phenotypic profiling of 155 newly diagnosed pre-treatment FL biopsy samples at single cell resolution by mass cytometry (CyTOF). These prior analyses unexpectedly revealed two distinct evolutionary trajectories which were independently reflected in both B and T cell compartments. One trajectory expectedly involved germinal center B cells (GCB); however, the other was more related to naïve/memory B-cells (NMB). Interestingly, cluster co-occurrence analysis suggested that the GCB and NMB trajectories were mutually exclusive of another and tended not to be found within the same tumor despite their high prevalences (χ 2 = 29.8, DF=1, p=4.8e-8; χ 2 test). Clustering analysis based on relative abundances of T cell subsets revealed 4 distinct immune patterns: Group 1 was characterized by naive T cells; Group 2 by T follicular helper (Tfh) cells; Group 3 by CD4+ regulatory T (Treg) and CD8 effector memory cells (CD8EM); and Group 4 by a diverse complement of naive, memory, and differentiated effector subsets. We report here further analyses, now incorporating DNA mutational and clinical outcome information. Tumors were parsed into 3 types based on the phenotype of the majority (>50%) of tumor cells present in the diagnostic biopsy: Type A tumors dominated by GCB cells (28% of samples), type B tumors dominated by NMB cells (18% of samples), and type nonA/nonB tumors dominated by neither GCB nor NMB cells (54% of samples). Type A tumors were significantly enriched for mutations in EZH2, TNFRSF14, and MEF2B, while no significant mutational associations were seen in type B and nonA/nonB tumors. Type B was significantly associated with increased risk of transformation, and when combined with a measure of intratumoral phenotypic diversity ("Entropy"), we found that type B tumors with high (above median) Entropy, representing 8.5% of all cases, exhibited a hazard ratio (HR) of 5.9 for transformation risk in comparison to all others combined (log-rank p We also investigated survival outcomes using a sub-cohort of 108 patients who had received bendamustine + rituximab (BR) as their primary therapy. Despite the association of type B tumors with transformation risk, patients with type nonA/nonB tumors exhibited the poorest outcomes as measured by disease-specific survival (DSS; 5yr survival 78% vs 98% for all others combined, log-rank p=0.0241). Combining type nonA/nonB with high Entropy defined 20% of patients with significantly shorter DSS (HR 5.3, log-rank p=0.0019). In multivariate analysis, type nonA/nonB and high risk FLIPI score were significant (p=0.038 and 0.035, respectively), while high Entropy trended with inferior DSS (HR 2.8, 95% CI 0.76-10; p=0.123). Taken together, these data support that CyTOF-defined phenotypic subtypes of FL and intratumoral phenotypic diversity identify clinically significant subgroups at initial diagnosis and compare favorably against FLIPI score in predicting both risk of transformation and inferior DSS. Figure 1 Figure 1. Disclosures Freeman: Incyte: Honoraria; Seattle Genetics: Honoraria; Sanofi: Honoraria, Speakers Bureau; Bristol Myers Squibb: Honoraria, Speakers Bureau; Amgen: Honoraria; Janssen: Honoraria, Speakers Bureau; Celgene: Honoraria; Teva: Research Funding; Abbvie: Honoraria; Roche: Research Funding. Sehn: Genmab: Consultancy; Novartis: Consultancy; Debiopharm: Consultancy. Savage: BMS: Consultancy, Honoraria, Other: Institutional clinical trial funding; Seattle Genetics: Consultancy, Honoraria; Astra-Zeneca: Consultancy, Honoraria; AbbVie: Consultancy, Honoraria; Merck: Consultancy, Honoraria, Other: Institutional clinical trial funding; Takeda: Other: Institutional clinical trial funding; Roche: Research Funding; Servier: Consultancy, Honoraria; Beigene: Other: Institutional clinical trial funding; Genentech: Research Funding. Craig: Bayer: Consultancy. Scott: Abbvie: Consultancy; AstraZeneca: Consultancy; Incyte: Consultancy; Celgene: Consultancy; NanoString Technologies: Patents & Royalties: Patent describing measuring the proliferation signature in MCL using gene expression profiling.; BC Cancer: Patents & Royalties: Patent describing assigning DLBCL COO by gene expression profiling--licensed to NanoString Technologies. Patent describing measuring the proliferation signature in MCL using gene expression profiling. ; Rich/Genentech: Research Funding; Janssen: Consultancy, Research Funding. Steidl: Trillium Therapeutics: Research Funding; Bristol-Myers Squibb: Research Funding; AbbVie: Consultancy; Seattle Genetics: Consultancy; Curis Inc.: Consultancy; Bayer: Consultancy; Epizyme: Research Funding.

Diffuse large B-cell lymphoma (DLBCL) is the most common histologic subtype of non-Hodgkin lymphoma and is notorious for its clinical heterogeneity. Patient outcomes can be predicted by cell-of-origin (COO) classification, demonstrating that the underlying transcriptional signature of malignant B-cells informs biological behavior in the context of standard combination chemotherapy regimens. In the current study, we used mass cytometry (CyTOF) to examine tumor phenotypes at the protein level with single cell resolution in a collection of 27 diagnostic DLBCL biopsy specimens from treatment naive patients. We found that malignant B-cells from each patient occupied unique regions in 37-dimensional phenotypic space with no apparent clustering of samples into discrete subtypes. Interestingly, variable MHC class II expression was found to be the greatest contributor to phenotypic diversity. Within individual tumors, a subset of cases showed multiple phenotypic subpopulations, and in one case, we were able to demonstrate direct correspondence between protein-level phenotypic subsets and DNA mutation-defined subclones. In summary, CyTOF analysis can resolve both intertumoral and intratumoral heterogeneity among primary samples and reveals that each case of DLBCL is unique and may be comprised of multiple, genetically distinct subclones. © 2019 International Society for Advancement of Cytometry.

T-cell acute lymphoblastic leukemia (T-ALL) can affect any age group; however, disease outcome is worse in older patients. Aging has been described to be accompanied by an increase in the body's proinflammatory status, a phenomenon referred as "inflammaging". By analysing two T-ALL transcriptome datasets, we have found that inflammation pathways are upregulated in older patients, whereas younger patients present upregulation of cell cycle pathways, suggesting that inflammaging is also recapitulated in T-ALL. Here, we hypothesize that T-ALL oncogenes can cooperate with specific cytokines/stimuli and their signaling pathways to create distinct leukemic phenotypes. To test this, human cord blood CD34+ cells were transduced with different T-ALL oncogenes and grown in co-culture with stromal cells under different pro-inflammatory conditions. We observed that most of the transduced cells had their cell growth negatively affected by the proinflammatory conditions, although some oncogene combinations showed more tolerance to these conditions than others. Activated NOTCH1+TLX3 transduced cells represented the only gene combination that showed significantly higher cell growth in the presence of interferon-gamma (IFNγ) when compared to non-transduced cells in the same culture. IL-15 and IFNγ potentiated differentiation arrest observed with certain oncogene combinations. Normal uncommitted T-cell progenitors (CD7+CD5+CD44+CD1a-) responded to IL-15 via STAT5 and AKT phosphorylation, and this response was enhanced by chronic treatment with IL-15. Our results suggest that proinflammatory conditions can alter the effect of certain oncogenes on T-cell progenitors and influence leukemogenesis, potentially leading to distinct biological behaviors of resulting leukemias. These findings may lead to identification of cytokine dependencies associated with particular T-ALL genetic subtypes.

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignancy with diverse oncogenic drivers, the relative contributions of which remain obscure. Enforced expression of select oncogenes is sufficient to generate T-ALL in mice, however, mouse models are limited in their value for understanding human disease. We recently developed the first “synthetic” model of human T-ALL by lentiviral transduction of normal human CD34+ cord blood progenitors with a combination of four known T-ALL oncogenes: NOTCH1, LMO2, TAL1, and BMI1 (NLTB). These cells expand robustly in culture and produce aggressive, serially transplantable T-ALL in immunodeficient mice. Additionally, we found that LTB alone fails to perform in vitro or in vivo, highlighting an essential role of NOTCH1. To determine the minimal complement of oncogenes required to generate de novo human T-ALL, we executed a “leave-one-out” and “leave-two-out” strategy. We scored the following oncogene combinations in vitro and in vivo: NLB, NLT, NTB, NL, NT, and NB. We found that the various oncogene combinations yielded a spectrum of aberrant phenotypes affecting cell differentiation and proliferation, and a subset produced aggressive leukemias in mice. We also performed RNA-seq on non-leukemogenic and preleukemic cell populations, and fully transformed leukemic cells to define gene expression programs necessary for cellular transformation. Our approach allows us to attribute specific gene programs and cellular phenotypes to each oncogene individually and in combination. Our synthetic approach is flexible, reproducible, and experimentally tractable, allowing functional testing of individual genetic variants and can also serve as a customizable platform for testing of targeted therapeutics.

Mechanistic studies in human cancer have relied heavily on established cell lines and genetically engineered mouse models, but these are limited by in vitro adaptation and species context issues, respectively. More recent efforts have utilized patient-derived xenografts (PDX); however, as an experimental model these are hampered by their variable genetic background, logistic challenges in establishing and distributing diverse collections, and the fact they cannot be independently reproduced. We report here a completely synthetic, efficient, and highly reproducible means for generating T-cell acute lymphoblastic leukemia (T-ALL) de novo by lentiviral transduction of normal CD34+ human cord blood (CB) derived hematopoietic progenitors with a combination of known T-ALL oncogenes. Transduced CB cells exhibit differentiation arrest and multi-log expansion when cultured in vitro on OP9-DL1 feeders, and generate serially transplantable, aggressive leukemia when injected into immunodeficient NSG mice with latencies as short as 80 days (median 161 days, range 79-321 days). RNA-seq analysis of synthetic CB leukemias confirmed their reproducibility and similarity to PDX tumors, while whole exome sequencing revealed ongoing clonal evolution in vivo with acquisition of secondary mutations that are seen recurrently in natural human disease. The in vitro component of this synthetic system affords direct access to "pre-leukemia" cells undergoing the very first molecular changes as they are redirected from normal to malignant developmental trajectories. Accordingly, we performed RNA-seq and modified histone ChIP-seq on nascently transduced CB cells harvested from the first 2-3 weeks in culture. We identified coordinate upregulation of multiple anterior HOXB genes (HOXB2-B5) with contiguous H3K27 demethylation/acetylation as a striking feature in these early pre-leukemia cells. Interestingly, we also found coordinate upregulation of these same HOXB genes in a cohort of 264 patient T-ALLs (COG TARGET study) and that they defined a subset of patients with significantly poorer event-free survival (Log-rank p-value = 0.0132). Patients in the "HOXB high" subgroup are distinct from those with ETP-ALL, but are enriched within TAL1, NKX2-1, and "unknown" transcription factor genetic subgroups. We further show by shRNA-mediated knockdown that HOXB gene expression confers growth advantage in nascently transduced CB cells, established synthetic CB leukemias, and a subset of established human T-ALL cell lines. Of note, while there is prior literature on the role of HOXA genes in AML and T-ALL, and of HOXB genes in normal HSC expansion, this is the first report to our knowledge of a role for HOXB genes in human T-ALL despite over 2 decades of studies relying mostly on mouse leukemia and cell line models. The synthetic approach we have taken here allows investigation of both early and late events in human leukemogenesis and delivers an efficient and reproducible experimental platform that can support functional testing of individual genetic variants necessary for precision medicine efforts and targeted drug screening/validation. Further, since all tumors including PDXs continue to evolve during serial propagation in vivo, synthetic tumors represent perhaps the only means by which we can explore early events in cellular transformation and segregate their biology from confounding effects of multiple and varied secondary events that accumulate in highly "evolved" samples. Disclosures Steidl: Seattle Genetics: Consultancy; Tioma: Research Funding; Bristol-Myers Squibb: Research Funding; Roche: Consultancy; Juno Therapeutics: Consultancy; Nanostring: Patents & Royalties: patent holding.

Summary Acute leukemias are aggressive malignancies of developmentally arrested hematopoietic progenitors. We sought here to explore the possibility that changes in hematopoietic stem/progenitor cells during development might alter the biology of leukemias arising from this tissue compartment. Using a mouse model of acute T cell leukemia, we found that leukemias generated from fetal liver (FL) and adult bone marrow (BM) differed dramatically in their leukemia stem cell activity with FL leukemias showing markedly reduced serial transplantability as compared to BM leukemias. We present evidence that this difference is due to NOTCH1-driven autocrine IGF1 signaling, which is active in FL cells but restrained in BM cells by EZH2-dependent H3K27 trimethylation. Further, we confirmed this mechanism is operative in human disease and show that enforced IGF1 signaling effectively limits leukemia stem cell activity. These findings demonstrate that resurrecting dormant fetal programs in adult cells may represent an alternate therapeutic approach in human cancer.