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Burkitt lymphoma (BL) is an aggressive lymphoma type that is currently treated by intensive chemoimmunotherapy. Despite the favorable clinical outcome for most patients with BL, chemotherapy-related toxicity and disease relapse remain major clinical challenges, emphasizing the need for innovative therapies. Using genome-scale CRISPR-Cas9 screens, we identified B-cell receptor (BCR) signaling, specific transcriptional regulators, and one-carbon metabolism as vulnerabilities in BL. We focused on serine hydroxymethyltransferase 2 (SHMT2), a key enzyme in one-carbon metabolism. Inhibition of SHMT2 by either knockdown or pharmacological compounds induced anti-BL effects in vitro and in vivo. Mechanistically, SHMT2 inhibition led to a significant reduction of intracellular glycine and formate levels, which inhibited the mTOR pathway and thereby triggered autophagic degradation of the oncogenic transcription factor TCF3. Consequently, this led to a collapse of tonic BCR signaling, which is controlled by TCF3 and is essential for BL cell survival. In terms of clinical translation, we also identified drugs such as methotrexate that synergized with SHMT inhibitors. Overall, our study has uncovered the dependency landscape in BL, identified and validated SHMT2 as a drug target, and revealed a mechanistic link between SHMT2 and the transcriptional master regulator TCF3, opening up new perspectives for innovative therapies.

Burkitt lymphoma (BL) accounts for most pediatric non-Hodgkin lymphomas, being less common but significantly more lethal when diagnosed in adults. Much of the knowledge of the genetics of BL thus far has originated from the study of pediatric BL (pBL), leaving its relationship to adult BL (aBL) and other adult lymphomas not fully explored. We sought to more thoroughly identify the somatic changes that underlie lymphomagenesis in aBL and any molecular features that associate with clinical disparities within and between pBL and aBL. Through comprehensive whole-genome sequencing of 230 BL and 295 diffuse large B-cell lymphoma (DLBCL) tumors, we identified additional significantly mutated genes, including more genetic features that associate with tumor Epstein-Barr virus status, and unraveled new distinct subgroupings within BL and DLBCL with 3 predominantly comprising BLs: DGG-BL (DDX3X, GNA13, and GNAI2), IC-BL (ID3 and CCND3), and Q53-BL (quiet TP53). Each BL subgroup is characterized by combinations of common driver and noncoding mutations caused by aberrant somatic hypermutation. The largest subgroups of BL cases, IC-BL and DGG-BL, are further characterized by distinct biological and gene expression differences. IC-BL and DGG-BL and their prototypical genetic features (ID3 and TP53) had significant associations with patient outcomes that were different among aBL and pBL cohorts. These findings highlight shared pathogenesis between aBL and pBL, and establish genetic subtypes within BL that serve to delineate tumors with distinct molecular features, providing a new framework for epidemiologic, diagnostic, and therapeutic strategies.

Signals emanating from the B cell receptor (BCR) promote proliferation and survival in diverse forms of B cell lymphoma. Precision medicine strategies targeting the BCR pathway have been generally effective in treating lymphoma, but often fail to produce durable responses in diffuse large B cell lymphoma (DLBCL), a common and aggressive cancer. New insights into DLBCL biology garnered from genomic analyses and functional proteogenomic studies have identified novel modes of BCR signaling in this disease. Herein, we describe the distinct roles of antigen-dependent and antigen-independent BCR signaling in different subtypes of DLBCL. We highlight mechanisms by which the BCR cooperates with TLR9 and mutant isoforms of MYD88 to drive sustained NF-κB activity in the activated B cell-like (ABC) subtype of DLBCL. Finally, we discuss progress in detecting and targeting oncogenic BCR signaling to improve the survival of patients with lymphoma.

Primary central nervous system lymphoma (PCNSL) is confined to the brain, eyes, and cerebrospinal fluid without evidence of systemic spread. Rarely, PCNSL occurs in the context of immunosuppression (eg, posttransplant lymphoproliferative disorders or HIV [AIDS-related PCNSL]). These cases are poorly characterized, have dismal outcome, and are typically Epstein-Barr virus (EBV)-associated (ie, tissue-positive). We used targeted sequencing and digital multiplex gene expression to compare the genetic landscape and tumor microenvironment (TME) of 91 PCNSL tissues all with diffuse large B-cell lymphoma histology. Forty-seven were EBV tissue-negative: 45 EBV- HIV- PCNSL and 2 EBV- HIV+ PCNSL; and 44 were EBV tissue-positive: 23 EBV+ HIV+ PCNSL and 21 EBV+ HIV- PCNSL. As with prior studies, EBV- HIV- PCNSL had frequent MYD88, CD79B, and PIM1 mutations, and enrichment for the activated B-cell (ABC) cell-of-origin subtype. In contrast, these mutations were absent in all EBV tissue-positive cases and ABC frequency was low. Furthermore, copy number loss in HLA class I/II and antigen-presenting/processing genes were rarely observed, indicating retained antigen presentation. To counter this, EBV+ HIV- PCNSL had a tolerogenic TME with elevated macrophage and immune-checkpoint gene expression, whereas AIDS-related PCNSL had low CD4 gene counts. EBV-associated PCNSL in the immunosuppressed is immunobiologically distinct from EBV- HIV- PCNSL, and, despite expressing an immunogenic virus, retains the ability to present EBV antigens. Results provide a framework for targeted treatment.

The role of Fas in germinal center (GC) homeostasis is controversial. Razzaghi et al. show that Fas is a strong cell-intrinsic regulator of the GC and that its loss defines an aggressive subtype of GC-derived lymphoma., Fas is highly expressed on germinal center (GC) B cells, and mutations of FAS have been reported in diffuse large B cell lymphoma (DLBCL). Although GC-derived DLBCL has better overall outcomes than other DLBCL types, some cases are refractory, and the molecular basis for this is often unknown. We show that Fas is a strong cell-intrinsic regulator of GC B cells that promotes cell death in the light zone, likely via T follicular helper (Tfh) cell–derived Fas ligand. In the absence of Fas, GCs were more clonally diverse due to an accumulation of cells that did not demonstrably bind antigen. FAS alterations occurred most commonly in GC-derived DLBCL, were associated with inferior outcomes and an enrichment of Tfh cells, and co-occurred with deficiency in HVEM and PD-L1 that regulate the Tfh–B cell interaction. This work shows that Fas is critically required for GC homeostasis and suggests that loss of Tfh-mediated counterselection in the GC contributes to lethality in GC-derived lymphoma., Graphical Abstract

Background: Follicular lymphoma (FL) shows marked variation in clinical course including spontaneous regression and histologic transformation (HT). Watchful waiting (W&W) is routinely applied to pts with newly diagnosed FL, but monitoring strategies are not standardized. Pts with progression within 1-2y of diagnosis have worse outcomes, but the biologic basis is unclear and biologic-based classifiers are not routinely applied at diagnosis. Circulating tumor DNA (ctDNA) is a highly tumor-specific biomarker that is prognostic in aggressive B-cell lymphomas, but its ability to serially monitor FL remains undefined. We applied a next-generation sequencing assay to identify tumor clonotypes for serial monitoring of peripheral blood in pts with untreated FL as part of an ongoing prospective clinical trial [NCT03190928]. Methods: Pts with grade I-II or 3A FL are eligible if evaluable disease on CT or FDG-PET, age ≥18, ECOG ≤2, no evidence of HT, and no prior systemic therapy. Pts undergo W&W until they meet uniform protocol-defined treatment criteria and remain on study until second-line therapy. Baseline testing includes labs, peripheral blood flow cytometry, BM biopsy/aspirate, CT and FDG-PET scans, and research biopsy. Pt have clinic visits every 4m for 2y, every 6m in years 3-5, then annually. CT scans are every 8m for 2y, then annually. FDG-PET scans are at baseline, at 2y, and any time of suspected progression. Peripheral blood samples including Streck tubes (plasma) and PBMCs are drawn at each clinic visit and stored. For ctDNA analysis, tumor DNA was amplified from FFPE using locus-specific primer sets for the Ig heavy-chain and light-chain loci along with BCL1/BCL2 translocations (Adaptive Biotechnologies). Amplified products were sequenced and tumor clonotypes were identified in plasma and PBMCs. Serial tracking of ctDNA was done in plasma and blinded to clinical outcomes. Results: 77 pts enrolled between July 2017 and July 2021. Median age was 57 (range 24-83) including 14 (18%) low-risk, 29 (38%) intermediate-risk, and 34 (44%) high-risk by FLIPI. Fourteen (18%) pts had stage I-II disease. Forty-three (56%) pts had monoclonal B-cells on peripheral blood flow cytometry. Twenty-nine (38%) pts progressed requiring frontline therapy including 7 (9%) pts with HT. Twenty-five (32%) pts were monitored ≥2y with no progression including 10 (13%) pts with evidence of at least some spontaneous regression by CT. Twenty (26%) pts were on study for Conclusions: ctDNA quantified from plasma in FL mirrors TMTV. Serial monitoring of ctDNA in patients without therapy demonstrated various patterns of fluctuation, including some patients in which ctDNA became undetectable coincident with spontaneous clinical regressions. ctDNA thus provides a non-invasive platform to monitor the natural history of FL, enabling future studies of tumor immune surveillance in this disease. Figure 1 Figure 1. Disclosures Jacob: Adaptive Biotechnologies: Current Employment, Current equity holder in publicly-traded company. Bagaev: BostonGene Corp.: Current Employment, Current holder of stock options in a privately-held company, Patents & Royalties: BostonGene. Meerson: BostonGene: Current Employment, Current holder of stock options in a privately-held company, Patents & Royalties: BostonGene. Postovalova: BostonGene Corp.: Current Employment, Current holder of stock options in a privately-held company, Patents & Royalties: BostonGene. Kudryashova: BostonGene: Current Employment, Current holder of stock options in a privately-held company, Patents & Royalties: BostonGene. Kotlov: BostonGene Corp: Current Employment, Current holder of stock options in a privately-held company, Patents & Royalties. Fowler: BostonGene: Current Employment, Current holder of stock options in a privately-held company.

Introduction: Immune tolerance and evasion plays a significant role in the pathogenesis of EBV+ lymphoproliferative disorders (LPD) and non-Hodgkin lymphomas (NHL). Programmed cell death protein-1 (PD-1) is a signaling molecule on the surface of T-cells that suppresses the cytotoxic effects of T-cells on tumor cells. PD-L1 expression is a marker of poor prognosis in aggressive lymphomas and most EBV+ LPDs demonstrate high levels of PD-L1 expression. Chronic viral infections, such as EBV, also result in T-cell exhaustion that can be reversed by PD-1 blockade. Nivolumab is a fully human IgG4 monoclonal anti-PD-1 antibody which has demonstrated activity and favorable safety in lymphoid malignancies. We hypothesized that PD-1 blockade may reverse the inactivation of tumor-specific effector T-cells and result in anti-tumor responses in EBV+ LPD and NHL. Methods: Relapsed/refractory (R/R) EBV+ LPD and B-cell NHL pts age ≥ 12y with adequate organ function are eligible. Untreated pts are eligible if EBV+ LPD. Exclusions include prior use of PD-1/PD-L1/PD-L2/CD137/CTLA-4 antibodies, prior solid organ transplant and HIV. Pts with immunodeficiency or autoimmune illness are eligible if not requiring steroids or immunosuppression. CNS involvement is permitted if no seizure activity within 4 weeks of study. Nivolumab 480mg IV is given every four weeks for up to 2y. Pts who achieve CR discontinue nivolumab after 1y of treatment. Baseline evaluation includes CT, PET, MRI brain, flow cytometry of peripheral blood and CSF, BM biopsy along with optional tumor biopsy. CT scans are performed after cycles 3, 6, 13 and 19 and end of treatment (EoT). PET is performed after cycles 1, 3 and EoT. Surveillance CT scans are performed q3m for 1y, q6m for yrs 2-5, and annually thereafter. Results: 9 pts, 7 (78%) R/R and 2 (22%) untreated, enrolled between April 2018 and May 2021; 5 (56%) with EBV+ LPD [4 G1-2 lymphomatoid granulomatosis (LYG) and 1 chronic active EBV disease (CAEBV)] and 4 (44%) with EBV+ NHL (all DLBCL, NOS). Median age was 48y (range 30-63) and all pts (100%) had stage III/IV disease. Four pts (44%) had elevated LDH (all DLBCL). Median baseline CD4 and CD8 count (cells/mcL) was 378 (range 99-984) and 86 (range 22-1237), respectively, for LPD and 190 (range 133-255) and 90 (range 9-630), respectively, for NHL. Median EBV VL at baseline (Log10 IU/mL) was 2.55 (range 0-6.78) and 2.53 (range 0-5.33) for LPD and NHL, respectively. Eight (89%) pts had extranodal disease with pulmonary involvement most common in 6 (67%). Median prior therapies were 1 (range 0-1) and 2 (range 1-4) for LPD and NHL pts, respectively. Three (43%) R/R pts were refractory (i.e., Of 9 pts enrolled, 7 were evaluable for response (1 NHL pt died prior to restaging and 1 NHL pt has not yet been restaged). In 6 measurable pts, tumor reduction was observed in 67% (Fig 1A). ORR and CR rate was 57% (4/7) and 43% (3/7), respectively; 60% (3/5) and 40% (2/5) in LPD and 50% (1/2) and 50% (1/2) in NHL. Median TTR was 3.0m with 3 (75%) of 4 responses ongoing from 6.9m to 35.2m after first response (Fig 1B). Most common adverse events (AEs) (% pts) included maculopapular rash (38%), ALT elevation (25%), AST elevation (25%), CPK elevation (25%) and fatigue (25%). One pt discontinued therapy due to G2 immune-mediated myositis that required prolonged steroid therapy. >G3 AEs included AST elevation in 1 (13%) pt with no G4/G5 or serious adverse events. With a median potential follow up of 12.6m, 12-month PFS and OS was 50.8% (95% CI: 15.7-78.1) and 75.0% (95% CI: 31.5-93.1). In LPD pts, 12-month PFS and OS was 80% (95% CI: 20.4-96.9) and 100%. Three (75%) NHL pts progressed and 2 (50%) died of disease progression. One NHL pt stopped therapy due to apparent disease progression after 2 cycles but later developed CR without further therapy and remains in remission 35.2m after stopping therapy. Conclusion: Nivolumab appears safe in pts with EBV+ LPD and NHL without unexpected toxicities. Preliminary clinical activity, including CRs, is noted in pts with EBV+ LPD and NHL. Additional pts are needed for a better assessment of true activity in these rare entities and correlates of response including PD-1/PD-L1 expression and/or 9p24.1 alterations are ongoing and will be presented at the meeting. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare. OffLabel Disclosure: Nivolumab for EBV+ LPD and EBV+ NHL.

Background: Diffuse large B-cell lymphoma (DLBCL) subtypes have differential response to BTK inhibitors (BTKi). Ibrutinib with R-CHOP improves survival in DLBCL subsets, but toxicity is limiting. Precise characterization of BTKi-responsive tumors enhances pt selection. Acalabrutinib (acala) is a BTKi with activity in DLBCL, but the molecular correlates of acala response are unknown. Circulating tumor DNA (ctDNA) is a prognostic biomarker in DLBCL including early changes during chemotherapy. PhasED-Seq is a novel ctDNA method that lowers the error profile of mutation detection by requiring the concordant detection of two separate mutations on an individual cell-free DNA molecule (Kurtz et al. Nat Biotechnol 2021). We employed a response-adapted study of acala for up to 14d prior to frontline therapy for aggressive B-cell lymphoma to determine the molecular profile of BTKi-responsive tumors. We report preliminary results including dynamic changes in ctDNA from this ongoing trial [NCT04002947]. Methods: Pts with untreated aggressive B-cell lymphoma and any HIV status are eligible if age ≥18, ≥stage 2, PS ≤2, and adequate organ function. Pts with PMBL, unmeasurable lesions, or active CNS disease are excluded. Screening includes labs, CT and FDG-PET, BM, and CSF with flow cytometry. Pts first receive acala 100mg twice daily x 14d. Pts with Results: 34 pts enrolled between August 2019 and July 2021 and completed the acala window. Median age was 64 (range 28-85) including 13 (38%) < 60y, 14 (41%) 61-69, and 7 (21%) ≥70y. Three (9%) pts had HIV and 17 (50%) were high-risk by IPI. The median diagnosis to treatment was 22.5d (4-53). IHC subtypes by Hans included 17 (50%) non-GCB, 16 (47%) GCB, and 1 (3%) T-cell/histiocyte-rich large B-cell lymphoma (TRLBCL). Four (12%) pts were high-grade B-cell lymphoma with MYC and BCL2 and/or BCL6 (HGBL-DH). Fifteen (44%) pts responded to acala during the window, while 19 (56%) pts had no response (Figure 1A). Acala responses were seen across DLBCL subtypes including 7 (47%) pts with non-GCB, 7 (47%) pts with GCB, and 1 (7%) pt with TRLBCL. Twenty pts had RNA sequencing to confirm cell-of-origin including 10 responders which included 7 (70%) GCB, 2 (20%) ABC, and 1 (10%) Unclassified. Notably, 13 (86%) BTKi-responsive tumors were CD10 negative and only 2 (18%) CD10+ tumors were BTKi-responsive. ctDNA dynamics strongly correlated with CT response as the log-fold change in ctDNA (hGE/mL) at the end of the window correlated with change on CT (r=0.75, p=0.0013). Remarkably, ctDNA dynamics after only 7d also correlated with change on CT (r=0.82, p=0.00006)(Figure 1B-C). Interestingly, one pt had improved symptoms and a 20-fold drop in ctDNA, but no corresponding CT changes suggesting that ctDNA changes may precede CT changes in some cases. Twenty-nine (85%) pts completed all planned cycles of therapy while 5 pts stopped chemotherapy early due to myelosuppression (n=3), CHF (n=1), and MI (n=1). Toxicity across 156 cycles was mostly hematologic. G3/G4 neutropenia occurred in 50% and 38% of cycles and febrile neutropenia in 10% of cycles. G3/G4 thrombocytopenia occurred in 22% and 12% of cycles. No increase in infections, atrial fibrillation, or bleeding were observed in pts treated with acala. All 27 pts who completed therapy achieved a CR. Two pts (1 acala responder) have relapsed from CR and 1 pt died of an MI. After a median follow-up of 9.2m the estimated 1-year PFS was 84.9% (95% CI: 58-95). Conclusions: Acalabrutinib prior to frontline therapy has activity in GCB, non-GCB, and HGBL-DH: confirmed by gene expression profiling. CD10+ GCB tumors are mostly acala-resistant. Toxicity is mainly hematologic and manageable across age groups including pts with HIV. ctDNA correlates with CT change and may predict response to targeted agents as early as 7 days. Updated clinical results within genetic subtypes will be presented at the meeting. Figure 1 Figure 1. Disclosures Chabon: Foresight Diagnostics: Current Employment, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company. Lurain: CTI Biopharma: Research Funding; EMD-Serrono: Research Funding; Merck: Research Funding; BMS-Celgene: Research Funding; Janssen: Research Funding. Bagaev: BostonGene Corp.: Current Employment, Current holder of stock options in a privately-held company, Patents & Royalties: BostonGene. Postovalova: BostonGene Corp.: Current Employment, Current holder of stock options in a privately-held company, Patents & Royalties: BostonGene. Meerson: BostonGene: Current Employment, Current holder of stock options in a privately-held company, Patents & Royalties: BostonGene. Kudryashova: BostonGene: Current Employment, Current holder of stock options in a privately-held company, Patents & Royalties: BostonGene. Kotlov: BostonGene Corp: Current Employment, Current holder of stock options in a privately-held company, Patents & Royalties. Fowler: BostonGene: Current Employment, Current holder of stock options in a privately-held company. Kurtz: Roche: Consultancy; Foresight Diagnostics: Consultancy, Current holder of stock options in a privately-held company; Genentech: Consultancy. Alizadeh: CAPP Medical: Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company; Forty Seven: Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company; Foresight Diagnostics: Consultancy, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company; Roche: Consultancy, Honoraria; Janssen Oncology: Honoraria; Celgene: Consultancy, Research Funding; Gilead: Consultancy; Bristol Myers Squibb: Research Funding; Cibermed: Consultancy, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company.

Background: Mantle cell lymphoma (MCL) is a biologically and clinically heterogenous B-cell lymphoma that is generally incurable with standard therapies. Novel targeted agents can disrupt key survival pathways in MCL such as regulation of apoptosis (BCL2: venetoclax), B-cell receptor signaling (BTK: ibrutinib), and NF-κB survival pathways (IRF4/SPIB: lenalidomide). As monotherapy, these agents fail to induce deep responses and doublet/triplet regimens often require continuous or maintenance therapy. ViPOR has been shown to be safe and active in non-MCL NHL pts without significant tumor lysis syndrome (TLS) (Melani et al. Blood. 2020; 136(Supplement 1):44-45). We hypothesized that combining agents that target multiple survival pathways with ViPOR will leverage efficacy and time-limited, cyclic dosing will limit toxicities in MCL. Methods: Relapsed/refractory (R/R) and untreated MCL pts with adequate organ function were eligible. In R/R MCL, a phase I "3+3" design was used to determine the maximum tolerated dose (MTD) of 2 dose-levels of dose-escalated venetoclax (200mg and 400mg) PO D2-14 (starting C2) in combination with fixed-dose ibrutinib 560mg PO D1-14, prednisone 100mg PO D1-7, obinutuzumab 1000mg IV D1-2, and lenalidomide 15mg PO D1-14. A phase II expansion in untreated MCL was included at the MTD. ViPOR q21d x 6C was given without maintenance or consolidation. All pts were admitted C2 for 12d venetoclax escalation and TLS monitoring. All pts received TLS prophylaxis (ppx) with IVFs and allopurinol as well as PCP and G-CSF ppx. VTE ppx was per investigator discretion. Baseline CT, PET, BM, and tumor biopsy was performed with CT scans after cycles 1, 2, 4, and 6 and PET after cycle 6 or at time of suspected complete response (CR). Surveillance CT was performed q3m for 1y, q4m x 1y, q6m x 1y, then annually x 2y. Plasma for ctDNA was collected at baseline, prior to each treatment cycle, at each follow-up visit, and at disease progression. Results: 11 pts have been enrolled and treated; 9 (82%) R/R in dose-escalation and 2 (18%) untreated in dose-expansion. Median age was 71y (range 57-79) with 73% >65y and 64% male. Low, intermediate, and high-risk MIPI occurred in 18%, 64%, and 18% of pts, respectively. Stage IV disease was seen in 91%, with BM involvement in 73%, extranodal disease in 82%, and both in 64%. Disease bulk >5cm occurred in 45% of pts. Blastoid morphology, Ki-67 >30%, and TP53 IHC >50% occurred in 27%, 36%, and 18% of pts, respectively. Median prior therapies in R/R pts was 3 (range 1-4) with 44% receiving prior BTKi, 11% receiving prior CAR-T, and 78% refractory (i.e., No dose-limiting toxicities (DLTs) occurred in 9 evaluable pts in the dose-escalation cohort; thus, venetoclax 400mg was used in expansion. G3-4 heme AEs (% cycles) included neutropenia (13%), anemia (11%), and thrombocytopenia (9%). No cases of febrile neutropenia occurred across 46 total cycles. G3-4 non-heme AEs (% pts) included hypokalemia in 3 (33%) pts as well as fatigue, hypomagnesemia, elevated bilirubin, atrial fibrillation, lung infection, and syncope in 1 (11%) pt each. No laboratory or clinical TLS occurred. Dose reductions and delays occurred in 5% and 15% of cycles, respectively. Of 11 pts enrolled, 10 are evaluable for response (1 pt has not yet been restaged) with an overall response rate (ORR) and CR rate of 100% (10/10) and 80% (8/10), respectively (Fig 1A). Of 8 pts who have completed therapy, all 8 (100%) have achieved CR, including all 4 post-BTKi pts, 1 post-CAR-T pt, and 6 refractory pts. With a median potential f/u of 5.2m, median TTR and DOR was 0.7m and not reached, respectively, with 9 (90%) responses ongoing ranging from 0.3m to 14.5m after first response (Fig 1B). One pt with R/R blastoid MCL relapsed in the CNS 9.9m after initial response. Median PFS and OS were both not reached with 10 (91%) pts alive and progression-free and 1 relapse and death from progression at 11.9m. Conclusion: ViPOR is safe in MCL without significant TLS or DLTs using a 12d venetoclax ramp-up on C2 and venetoclax 400mg was taken forward in expansion. Most common G3-4 AEs were hematologic with no febrile neutropenia observed when given with G-CSF ppx. High preliminary activity is noted in MCL pts with fixed-duration ViPOR x 6C, including CRs in refractory, post-BTKi, and post-CAR-T pts. Molecular and ctDNA analyses are ongoing and will be presented at the meeting. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare. OffLabel Disclosure: Off-label use of ViPOR in relapsed/refractory and untreated MCL.

Background: Secondary CNS B-cell lymphomas (SCNSL) are aggressive lymphomas with a very poor prognosis. Genetic subtypes of DLBCL with CNS tropism are enriched for chronic active B-cell receptor signaling and may respond to BTK inhibition (BTKi). CLL, MCL, and transformed lymphomas can also involve the CNS and are BTKi responsive. TEDDI-R achieves durable remissions in relapsed/refractory primary DLBCL of the CNS (PCNSL) but the profile of SCNSL tumors that are ibrutinib-responsive is unknown. We present preliminary results from a response-adapted trial of ibrutinib with TEDD-R in SCNSL. Methods: Pts with aggressive B-cell lymphomas with secondary CNS involvement are eligible if age ≥18 and adequate organ function. Pts must have relapsed after frontline therapy or can be untreated if brain parenchyma involved. Prior BTKi is allowed, but HIVinfection and EBV+ lymphomas are excluded. Baseline tests include brain MRI, FDG-PET brain and body, CSF with flow cytometry, Ommaya, and eye exam. Pts receive isavuconazole starting at 200mg BID x 3d prior to ibrutinib to prevent fungal infections associated with TEDDI-R and then 200mg daily unless ibrutinib stopped. Pts first receive ibrutinib 560mg daily x 14d in a window. If ≥20% reduction after ibrutinib, pts receive TEDDI-R for 4 cycles every 21d with IT cytarabine. Pts with Results: 16 pts with a median age 67 (range 40-79) enrolled between June 2019 and July 2020. 15 (94%) pts had DLBCL comprising 9 (60%) non-GCB, 5 (33%) GCB, and 1 (7%) transformed from MZL. One pt had plasmablastic lymphoma. Eight (50%) pts had a MYC-rearrangement including 4 (25%) with both MYC and BCL2 or BCL6 rearrangements. Eight (50%) pts had isolated CNS disease and 8 (50%) had synchronous CNS and peripheral disease. All pts relapsed after a median of 2 (range 1-4) prior therapies and all (100%) pts received prior anthracycline. Seven pts (44%) had prior CNS prophylaxis and 8 pts (50%) had prior HD-MTX based salvage therapy. Toxicity was evaluated across 35 cycles. G3 and G4 neutropenia occurred in 49% and 29% of cycles, respectively, while febrile neutropenia occurred in 9% of cycles. The median (range) duration of neutropenia was 6 (1-13) days. Five (14%) cycles were complicated by ≥G3 infection, but no opportunistic infections (including Aspergillus) were observed. One pt developed a bacterial infection during cycle 1 and died. G3 and G4 thrombocytopenia occurred in 34% and 23% of cycles, respectively, and 1 pt developed G3 hematuria. G3 mucositis occurred in 9% of cycles and palmar-plantar-erythrodysesthesia led to dose reductions of liposomal doxorubicin in 5 (36%) pts. Of 15 pts who completed the 14d ibrutinib window and were evaluable, 8 (53%) were ibrutinib-responsive and 7 (47%) were ibrutinib-resistant (Figure 1). Clinical responses were concordant across anatomic compartments; of 8 pts with both CNS and peripheral disease, 5 (63%) responded to ibrutinib in both compartments while 3 (37%) did not respond in either compartment. All 8 ibrutinib responders had a non-GCB phenotype and six (75%) achieved CR. One died of treatment-related toxicity after a PR and 1 is still on therapy. Only two (29%) pts with ibrutinib-resistant tumors achieved PR and none have achieved CR. After a median follow-up of 5.1m, a landmark analysis starting after the ibrutinib window demonstrated the PFS for pts with ibrutinib-responsive compared to ibrutinib-resistant tumors was not reached vs. 0.9m (95% CI: 0.1-2m)(p=0.002)(Figure 2). Conclusions: Patients with SCNSL tumors that are ibrutinib-responsive achieve a high rate of complete response to TEDDI-R in both CNS and peripheral disease. Patients with tumors that are ibrutinib-resistant also respond poorly to TEDD-R. Toxicity is mainly hematologic, and no Aspergillus infections have occurred with the use of isavuconazole prophylaxis. Updated clinical results from this ongoing study (NCT03964090) and will be presented at the meeting. Disclosures No relevant conflicts of interest to declare. OffLabel Disclosure: ibrutinib for use in secondary cns lymphoma as part of a clinical trial

Introduction Follicular lymphoma (FL) has a highly variable clinical course. Chemoimmunotherapy can induce durable remissions, but ~20% relapse early. The PI3K pathway is central to FL biology and multiple PI3K inhibitors (PI3Ki) are approved for FL, but the molecular profile of tumors most sensitive to PI3Ki is unknown. Further, PI3Ki are dosed indefinitely which contributes to toxicity and cost. Copanlisib is an IV inhibitor of PI3Kα and δ isoforms with high activity in relapsed FL. We hypothesized that patients with FL tumors most sensitive to PI3Ki will achieve deep and durable remissions after a fixed duration of copanlisib-based therapy. Here we report preliminary results of a response-adapted study using copanlisib and rituximab as frontline therapy for FL. Methods Pts with untreated grade 1-2, 3A FL, ≥stage 2, and any tumor burden are eligible if they meet criteria for need of systemic therapy that includes symptoms, increasing size of nodes, critical organ involvement, or impending organ compromise. No prior systemic therapy other than radiation is permitted. Frozen or archival tissue is required. Eligibility includes age ≥18 and adequate organ function unless involved by FL. Active HIV, CMV, Hep B or C, and autoimmune conditions requiring therapy are excluded. All pts receive PCP prophylaxis. Pts first receive copanlisib 60mg on days 1, 8, and 15 of a 28-day window to test activity of monotherapy. On-treatment tumor biopsies are optional after the window. Following the window, pts receive 6 cycles of copanlisib 60mg on days 1, 8, and 15 of a 28 day cycle along with rituximab 375mg weekly x 4 then on day 1 of each cycle. Streck tubes for circulating tumor DNA (ctDNA) are collected weekly during the window, after each cycle, and during surveillance. FDG-PET and CT scans are performed at baseline, after the window, and after cycles 3 and 6 to determine response. Patients with complete response (CR) after 6 cycles stop therapy. Patients with partial response (PR) after 6 cycles receive another 6 cycles of combination therapy with the copanlisib reduced to day 1 and 15 of each cycle. Non-responding patients will receive standard chemotherapy. The primary endpoint is the overall rate of CR with secondary endpoints of safety and duration of CR. Exploratory objectives include identification of a signature that predicts PI3Ki response. Results Ten pts have been enrolled and completed the copanlisib window. Median age was 50y (range, 28-77) including 3 (30%) over age 70. Seven (70%) pts had high-risk FLIPI scores ≥3 and two (20%) pts had Grade 3A FL. Comorbid conditions included prediabetes in 4 (40%) and hypertension in 4 (40%). All 10 (100%) pts had tumor reductions during copanlisib monotherapy with a median reduction of 41% (range 16-62%) (Figure 1). Four pts have completed 6 cycles of copanlisib and rituximab and all 4 (100%) have responded including 2 (50%) who achieved CR. In the two pts who achieved a PR after 6 cycles, one had a 90% tumor reduction and one had only persistent minimal residual disease in the bone marrow by flow cytometry. Toxicity was evaluated in 10 pts across 58 cycles and the most common have included rash (50%), diarrhea (50%) and mucositis (40%) which have all been G1 or G2 and successfully managed with supportive care and did not recur with subsequent cycles. One pt developed grade 3 neutropenia that responded to growth factors and did not recur. Four pts had dose delays due to rash (N=3) and lung infection (N=1). One pt had copanlisib reduced to 45mg due to recurrent rash and elevated liver tests. No pt has discontinued therapy. One pt required oral diabetic medications during therapy that were stopped after therapy completed. One pt had asymptomatic PCP pneumonia diagnosed during the copanlisib window prior to starting prophylaxis that was successfully treated while therapy continued. Conclusion Copanlisib is highly active in untreated FL and the first 10 (100%) patients all had tumor reductions after the first cycle of copanlisib monotherapy, including patients with high tumor bulk. Combination of copanlisib and rituximab can induce complete responses after only 6 cycles and without indefinite therapy. The safety profile includes rash and diarrhea that respond to supportive care and lessen on subsequent cycles. Updated clinical results from this ongoing trial (NCT03789240) along with the molecular profile of FL tumors will be presented at the meeting. Disclosures No relevant conflicts of interest to declare.

Background: Primary DLBCL of the CNS (PCNSL) relies on chronic active B-cell receptor (BCR) signaling. Ibrutinib targets BCR signaling through BTK inhibition (BTKi), which may also impair innate immunity. We showed that ibrutinib and temozolomide, etoposide, liposomal doxorubicin, dexamethasone, rituximab (TEDDI-R) induces durable remissions in relapsed/refractory PCNSL but 7 (39%) pts developed Aspergillus infections without fungal prophylaxis. Newer triazoles are effective against Aspergillus but inhibit ibrutinib clearance through CYP3A4. Isavuconazole has less effect on CYP3A4 and less hepatotoxicity than voriconazole. We hypothesized that ibrutinib and isavuconazole could be safely co-administered in TEDDI-R and ameliorate the risk of Aspergillus while maintaining efficacy. We studied escalating doses of ibrutinib in TEDDI-R with isavuconazole to determine the safety profile, ibrutinib PK, and clinical activity in relapsed/refractory PCNSL. Methods: Pts with relapsed/refractory PCNSL, age ≥18, ECOG PS ≤2, and adequate organ function were enrolled. Previous BTKi, HIV, EBV+, and pregnancy were excluded. Pts had baseline MRI brain, FDG-PET brain and body, Ommaya placed, CSF with flow cytometry, and eye exam. Isavuconazole 200mg BID x 3d started prior to ibrutinib then 200mg daily. Three dose levels of ibrutinib (280mg, 420mg, 560mg) were given continuously through each cycle. Pts received up to 6 cycles of TEDDI-R with IT cytarabine. No one received maintenance or consolidation. If a DLT occurred in the first 3 pts at a given ibrutinib dose level, 3 more pts were treated before escalating. Full safety and PK data was reviewed after two dose levels prior to escalating. An expansion of 10 pts was planned at the highest ibrutinib dose level to confirm safety and clinical activity. Surveillance for fungal infections included chest CT mid-cycle 1 and after each cycle along with Beta-D glucan and aspergillus galactomannan in blood and CSF. Brain MRI was performed after cycles 1, 2, 4, and 6 to determine response and screen for CNS Aspergillus. All remissions by MRI were confirmed with FDG-PET and CSF analysis. Surveillance brain MRI were q3m for 1y, q4m x 1y, q6m x 1y, then annually. Primary objective was to identify the highest dose of ibrutinib safely co-administered with isavuconazole in TEDDI-R that achieves adequate PK concentrations. Results: 13 relapsed/refractory PCNSL pts enrolled between 11/2018 and 06/2020. 10 (77%) pts were male and the median age was 65 (range 46-77), including 3 pts ≥age 70. 13 (100%) pts had prior high-dose MTX, and 2 (15%) pts had prior autologous stem cell transplant (ASCT). Three evaluable pts received ibrutinib 280mg, 3 pts received ibrutinib 420mg, and 6 pts received ibrutinib 560mg. One pt in the 280mg cohort was not evaluable. Toxicity was evaluated in 13 pts across 49 cycles and the toxicity was mainly hematologic. G3 and G4 neutropenia occurred in 45% and 37% of cycles, respectively, while febrile neutropenia occurred in 8% of cycles. The median (range) duration of neutropenia was 4.5 (1-12) days. One pt with prior ASCT stopped after 4 cycles due to myelosuppression. Four (8%) cycles were complicated by ≥G3 infection, but no opportunistic infections (including Aspergillus) were observed. G3 and G4 thrombocytopenia occurred in 22% and 8% of cycles, respectively, and 1 pt developed melena with no overt GI bleeding. ≥G3 mucositis occurred in 6% of cycles and 1 patient stopped therapy after 5 cycles due to recurrent mucositis. Palmar-plantar-erythrodysesthesia led to dose reductions of liposomal doxorubicin in 9 (69%) pts, but only 1 G3 event occurred. Twelve pts were evaluable for response, and 11 (92%) pts have responded and all after receiving only 1 cycle (Figure 1). All 8 (100%) pts who have completed at least 4 cycles have achieved CR and the other 4 remain on therapy. Six (75%) pts who achieved CR remain in remission while 2 (25%) pts relapsed within 3 months of stopping therapy. After a median potential f/u of 5.2 months, the 1-year PFS is estimated at 60.0% (95% CI, 12.6-88.2) and the OS is 100%. Conclusions: Ibrutinib 560mg was safely co-administered with isavuconazole in TEDDI-R for relapsed/refractory PCNSL. No DLTs were observed, no cases of Aspergillus occurred, and no new safety signals. The first 8 (100%) patients who have completed therapy achieved complete response. Updated clinical results from this ongoing study (NCT02203526) will be presented at the meeting. Disclosures Lai: Abbvie: Consultancy; Agios: Consultancy; Jazz: Speakers Bureau; Macrogenics: Consultancy; Astellas: Speakers Bureau.

Background: Aggressive B-cell non-Hodgkin lymphoma (NHL) can be cured with chemoimmunotherapy; however, those who fail primary therapy and those with indolent NHL are rarely curable. Targeted agents can disrupt key survival pathways in NHL such as regulation of apoptosis (BCL2: venetoclax), B-cell receptor signaling (BTK: ibrutinib), and NF-κB survival pathways (IRF4/SPIB: lenalidomide). These agents are active as monotherapy but fail to induce deep responses and require continuous therapy. Also, genetically defined subtypes of NHL that best respond to these targeted agents are undefined. Synergistic cytotoxicity has been shown with these targeted therapies and corticosteroids in DLBCL cell lines. We hypothesized that combining agents that target multiple survival pathways will leverage efficacy and time-limited, cyclic dosing will limit toxicities. Methods: Relapsed/refractory (R/R) B-cell NHL pts, excluding MCL and CLL/SLL, with adequate organ function were eligible. A phase I "3+3" design was used to determine the maximum tolerated dose (MTD) of 4 dose-levels (DLs) of dose-escalated venetoclax (200mg, 400mg, 600mg, and 800mg) PO D2-14 (starts cycle 2 for DL1) in combination with fixed-dose ibrutinib 560mg PO D1-14, prednisone 100mg PO D1-7, obinutuzumab 1000mg IV D1-2, and lenalidomide 15mg PO D1-14. A phase II expansion in R/R DLBCL and FL was included at the MTD. Up to 6 cycles of ViPOR every 21-days was given without maintenance. TLS and PCP prophylaxis was given to all pts and VTE prophylaxis and G-CSF use was per investigator discretion. Baseline CT, PET, BM and tumor biopsy was performed with CT scans after cycles 1, 2, 4, and 6 and PET after cycle 6 or at time of suspected CR. Surveillance CT was performed q3m for 1y, q4m x 1y, q6m x 1y, then annually x 2y. Results: 53 pts were enrolled and treated; 17 in dose-escalation and 36 in dose-expansion. NHL subtypes included DLBCL (23), FL (19), HGBCL "double-hit" (9), and MZL (2). Of 32 aggressive pts, 34% transformed from indolent NHL. Median age was 57y (range 29-83) with stage III/IV disease in 89%, elevated LDH in 68%, and >2 EN sites in 57%. Median prior therapies was 3 (range 1-9) with 45% of pts refractory (i.e. A single dose-limiting toxicity (DLT) of G3 intracranial hemorrhage occurred at DL1 with concomitant enoxaparin and ASA. No other DLTs occurred and venetoclax 800mg was used in expansion. Heme AEs (% cycles) were most common and included thrombocytopenia (23%), neutropenia (23%) and anemia (7%). G-CSF was used in 92% of pts and 89% of cycles with only 3 (6%) cases of febrile neutropenia. Non-heme AEs (% pts) were mainly G1-2 and included diarrhea (67%), hypokalemia (56%), nausea (52%), and rash (42%). Most common G3-4 non-heme AEs included hypokalemia (19%), diarrhea (8%), and a.fib/flutter (6%). G4 TLS occurred in 1 pt with HGBCL after the first venetoclax dose and was successfully treated without further TLS upon continued treatment. Dose reductions and delays occurred in 8% and 9% of cycles, respectively. Of 53 total patients, 51 completed 1C of therapy with restaging CT and tumor reduction occurred in 90% of pts overall (Fig 1A). Of 44 pts who are now off therapy, 43 were evaluable for response with an ORR of 70% and 49% CR, with responses across all DLs and NHL subtypes. In 27 pts with aggressive NHL, ORR was 56% with 37% CR. Based on DLBCL subtype by IHC, ORR and CR rate was 62% (8/13) and 54% (7/13) in non-GCB and 50% (7/14) and 21% (3/14) in GCB DLBCL, respectively. In 16 pts with indolent NHL, ORR was 94% with 69% CR. ORR and CR rate was 52% (11/21) and 29% (6/21) in refractory pts and 86% (19/22) and 68% (15/22) in relapsed pts, respectively. ORR was 40% with 30% CR in 10 patients who failed prior CAR-T and completed ViPOR therapy. With a median potential f/u of 13m, median TTR and DOR was 0.8m and NR, respectively, with 25 (69%) of 36 responses ongoing. 5 pts relapsed after CR, including 2 non-GCB at 3m and 6m, 1 HGBCL at 5m, 1 FL at 6m, and 1 MZL at 16m. Median PFS and OS was 9m and NR, respectively; 20m and NR in indolent NHL, 3m and 13m in GCB, and 7m and 13m in non-GCB DLBCL (Fig 1B). Conclusions: ViPOR is safe without unexpected toxicities observed. Most common AEs were hematologic with rare febrile neutropenia and no severe infections observed when given with G-CSF prophylaxis. ViPOR induces durable CRs without maintenance therapy, including refractory and post CAR-T pts. Molecular analyses are ongoing and will be presented at the meeting. Disclosures Portell: Infinity: Research Funding; Roche/Genentech: Consultancy, Research Funding; Xencor: Research Funding; Kite: Consultancy, Research Funding; TG Therapeutics: Research Funding; AbbVie: Research Funding; Pharmacyclics: Consultancy; Janssen: Consultancy; Amgen: Consultancy; Bayer: Consultancy; BeiGene: Consultancy, Research Funding; Acerta/AstraZeneca: Research Funding. OffLabel Disclosure: Off-label use of the combination of venetoclax, ibrutinib, prednisone, obinutuzumab and lenalidomide in relapsed/refractory B-cell non-Hodgkin lymphoma.

Summary Tumors frequently subvert major histocompatibility complex class I (MHC-I) peptide presentation to evade CD8+ T cell immunosurveillance, though how this is accomplished is not always well defined. To identify the global regulatory networks controlling antigen presentation, we employed genome-wide screening in human diffuse large B cell lymphomas (DLBCLs). This approach revealed dozens of genes that positively and negatively modulate MHC-I cell surface expression. Validated genes clustered in multiple pathways including cytokine signaling, mRNA processing, endosomal trafficking, and protein metabolism. Genes can exhibit lymphoma subtype- or tumor-specific MHC-I regulation, and a majority of primary DLBCL tumors displayed genetic alterations in multiple regulators. We established SUGT1 as a major positive regulator of both MHC-I and MHC-II cell surface expression. Further, pharmacological inhibition of two negative regulators of antigen presentation, EZH2 and thymidylate synthase, enhanced DLBCL MHC-I presentation. These and other genes represent potential targets for manipulating MHC-I immunosurveillance in cancers, infectious diseases, and autoimmunity.

The importance of understanding the genetic and biochemical basis of B cell receptor (BCR) survival signaling in diffuse large B cell lymphoma (DLBCL) is underscored by the recent clinical success of agents that target the BCR pathway. DLBCL is composed of multiple distinct molecular subtypes with divergent clinical outcomes. The activated B cell-like (ABC) subtype is the most aggressive form of DLBCL and is often resistant to standard chemotherapies. ABC DLBCL expresses numerous genes found in antigen-activated B cells, and genetic and pharmacologic studies have demonstrated that ABC DLBCL tumors are addicted to NF-κB activity. The origins of this NF-κB activity remained obscure until RNA interference screens established that the majority of ABC DLBCL cell lines rely on expression of BCR components and downstream signaling effectors for NF-κB activation. Pharmacological inhibition with ibrutinib of Bruton’s tyrosine kinase (Btk), a kinase that is required for BCR signaling to engage NF-κB, is selectively toxic for ABC DLBCL tumors; a finding that has now been translated to the clinic. These novel targets not only offer a promising new therapy options for ABC DLBCL, but also demonstrate the value of a deep molecular understanding of oncogenic signaling pathways.

Expression of the activating transcription factor 3 (ATF3) gene is induced by Toll-like receptor (TLR) signaling. In turn, ATF3 protein inhibits the expression of various TLR-driven proinflammatory genes. Given its counter-regulatory role in diverse innate immune responses, we defined the effects of ATF3 on neutrophilic airway inflammation in mice. ATF3 deletion was associated with increased lipopolysaccharide (LPS)-driven airway epithelia production of CXCL1, but not CXCL2, findings concordant with a consensus ATF3-binding site identified solely in the Cxcl1 promoter. Unexpectedly, ATF3-deficient mice did not exhibit increased airway neutrophilia after LPS challenge. Bone marrow chimeras revealed a specific reduction in ATF3−/− neutrophil recruitment to wild-type lungs. In vitro, ATF3−/− neutrophils exhibited a profound chemotaxis defect. Global gene expression analysis identified ablated Tiam2 expression in ATF3−/− neutrophils. TIAM2 regulates cellular motility by activating Rac1-mediated focal adhesion disassembly. Notably, ATF3−/− and ATF3-sufficient TIAM2 knockdown neutrophils, both lacking TIAM2, exhibited increased focal complex area, along with excessive CD11b-mediated F-actin polymerization. Together, our data describe a dichotomous role for ATF3-mediated regulation of neutrophilic responses: inhibition of neutrophil chemokine production but promotion of neutrophil chemotaxis.

Diffuse large B-cell lymphoma (DLBCL) is an aggressive cancer of aberrant B-lymphocytes. Although a portion of DLBCL is curable with standard immunochemotherapy, patients who fail this treatment have a poor prognosis. Recently, cancer genomics has paved the way for better understanding of the genetic basis of lymphoma pathogenesis. Characterization of point mutations and structural alterations has uncovered novel molecular targets for lymphoma therapy and provided a comprehensive view of lymphoma development. By performing multiplatform genomic analysis of DLBCL biopsy samples, we have identified KLHL14 as a recurrent target of somatic mutations in activated B-cell-like (ABC) DLBCL biopsies (10.8% of patients). KLHL14 contains a BTB (broad complex, tramtrack, and bric a brac) domain that can potentially mediate dimerization and binding to Cullin3 (CUL3)-a essential scaffold component of the Cullin-RING-based E3 ubiquitin ligase complexes. KLHL14 also contains kelch repeats that can form a B-propeller tertiary structure that can serve as a substrate-binding domain. KLHL14 is highly expressed in B-cells but is found at low levels in non-immune tissues. Deficiency of KLHL14 in mice leads to embryonic lethality while KLHL14 heterozygous mice show reduction of B-1a cells, suggesting a role for KLHL14 in B-cell homeostasis. Importantly, KLHL14 mutations are highly enriched in tumors belonging to the recently defined MCD (MYD88L265P/CD79B mutation) genetic subtype of DLBCL, the subset of ABC DLBCLs. Somatic mutations primarily localize to the N-terminus of the protein in the BTB domain and BACK (BTB and C-terminal Kelch) domain. However, the impact of these mutations as well as the molecular function of KLHL14 is largely unknown. To investigate the biological effect of KLHL14 loss of function, we used an inducible CRISPR/Cas9 system to delete KLHL14 in ABC DLBCL cell lines and monitored cell growth. Ablation of KLHL14 resulted in an increase in cell proliferation and survival, supporting a role for KLHL14 as a tumor suppressor. Next, we performed a multiplatform -omic analysis (proteomics, phosphoproteomics, ubiquitinomics, high-throughput sequencing) to explore the signaling networks and interactome of KLHL14. Whereas ectopic expression of wild-type KLHL14 altered the dynamics of tyrosine phosphorylation and ubiquitylation events in ABC DLBCL lines, KLHL14 lymphoma-associated mutant alleles had little if any effect, suggesting that they are loss-of-function variants. Gene expression profiling by RNA-sequencing revealed that KLHL14-inactivated cells have a higher NF-kB target gene expression than wild-type cells. Thus, tumor-associated inactivating mutations of KLHL14 depend on a subset of essential NF-kB-related oncoproteins for their survival and this might contribute to the proliferative advantage of DLBCL. In summary, we have uncovered a tumor suppressive function of KLHL14 and found that KLHL14 mutants promote ABC DLBCL survival by increasing NF-kB activity. These findings suggest that tumors with KLHL14 inactivating mutations may serve as a marker of resistance to anti-NF-kB treatment and provide the basis for treating MCD subtype patients with downstream NF-kB pathway inhibitors in the clinical settings. Disclosures Staudt: Nanostring: Patents & Royalties.

After neonatal HTLV-I infection through breast feeding, approximately 5% of HTLV-I carriers eventually develop Adult T-Cell Leukemia/Lymphoma (ATLL) with a latency of ~50 years, suggesting that acquired genetic and epigenetic changes in cellular genes act in concert with HTLV-I to initiate and maintain oncogenic transformation. We and others have recently utilized next generation sequencing technology to identify mutated genes that could be pivotal in the pathogenesis of ATLL. However, due to the complexity of genomic/epigenetic alteration in the ATLL genome, the identification of indispensable genes for proliferation and/or survival of ATLL cells remains a formidable challenge. To discover essential regulatory networks that are required for the proliferation and survival of ATLL cells, we performed a pooled shRNA screen in 8 ATLL cell lines using a library enriched for shRNAs targeting lymphoid regulatory factors and discovered that two BATF3 shRNAs and one IRF4 shRNA were highly toxic for all ATLL lines, but had little if any effect in other T cell and B cell lines. It is recently shown that a transcriptional complex of Irf4 and Batf binds to AP1-IRF composite (AICE) DNA motifs and plays key roles in the differentiation and function of certain mouse helper T cell subsets. A close paralogue of Batf, Batf3, is an indispensable transcription factor in a mouse dendritic cell subset, but also appears to play a redundant role with Batf in the differentiation of TH2 cells and can substitute for Batf in Batf knockout T cells. Our observations from shRNA screening suggested that IRF4 and BATF3 may cooperate to drive a transcriptional program that is essential for ATLL viability. We next used genome-wide chromatin precipitation (ChIP-seq) to identify the loci that are bound by BATF3 and IRF4. The set of binding peaks and the associated genes in IRF4 and BATF3 ChIP-seq intersected significantly. By integrating the ChIP-seq and gene expression profiling data of shBATF3- and shIRF4-ATLL cells, we defined a set of 68 BATF3-IRF4 direct target genes. Gene set enrichment analysis using gene expression profiling data from primary T cell lymphomas demonstrated that BATF3-IRF4 direct target genes were significantly enriched among genes that are more highly expressed in ATLL than in peripheral T-cell lymphoma, not otherwise specified (PTCL-NOS), suggesting that the BATF3 and IRF4 cooperatively regulate transcription in primary ATLL cells. HBZ is unique among HTLV-I viral proteins in being maintained in expression in all ATLL cases, suggesting that it may help maintain the malignant phenotype. Given that BATF3 and IRF4 are essential regulators in ATLL, we hypothesized possible relationship between HBZ and BATF3-IRF4 complex. We defined HBZ direct target genes by integrating the ChIP-seq and gene expression profiling data of HBZ-knockout ATLL cell lines by CRISPR/Cas9. Notably we discovered that BATF3 was among these. BATF3 mRNA and protein expression decreased following HBZ inactivation. The above considerations suggested that pharmacologic inhibition of the BATF3-IRF4 regulatory network might be a means to attack the HBZ oncogenic program therapeutically. ChIP-seq analysis of two enhancer marks, H3K27ac and BRD4, identified super-enhancers at the BATF3 locus in two ATLL cell lines. The small molecule JQ1 prevents the BET-protein BRD4 from interacting with chromatin, which is required for the function of super-enhancers. JQ1 treatment reduced BATF3 mRNA and protein levels in all ATLL lines tested, correlating with the eviction of BRD4 from the BATF3 super-enhancer. MYC mRNA and protein expression was also broadly downmodulated by JQ1. JQ1 treatment was consistently toxic for all ATLL cell lines tested at dose ranges that killed cell line models of T-ALL and DLBCL, which are known to rely on BET-proteins. In a dose-dependent manner, JQ1 also reduced the viability of primary ATLL samples and downregulated their expression BATF3 and MYC mRNA. Finally, we treated mouse xenograft models of ATLL with the BET-protein inhibitor CPI-203, a JQ1 analog with superior bioavailability in mice. In two different xenograft models, we observed significant tumor regression or growth inhibition, without evidence of systemic toxicity. Our study demonstrates that the HTLV-I virus exploits a regulatory module that can potentially be attacked therapeutically with BET protein inhibitors. Disclosures Yu: Celgene Corporation: Employment.

The activated B-cell-like (ABC) subtype of diffuse large B-cell lymphoma (DLBCL) relies on chronic active B-cell receptor (BCR) signaling. BCR pathway inhibitors induce remissions in a subset of ABC DLBCL patients. BCR microclusters on the surface of ABC cells resemble those generated following antigen engagement of normal B cells. We speculated that binding of lymphoma BCRs to self-antigens initiates and maintains chronic active BCR signaling in ABC DLBCL. To assess whether antigenic engagement of the BCR is required for the ongoing survival of ABC cells, we developed isogenic ABC cells that differed solely with respect to the IgH V region of their BCRs. In competitive assays with wild-type cells, substitution of a heterologous V region impaired the survival of three ABC lines. The viability of one VH4-34(+) ABC line and the ability of its BCR to bind to its own cell surface depended on V region residues that mediate the intrinsic autoreactivity of VH4-34 to self-glycoproteins. The BCR of another ABC line reacted with self-antigens in apoptotic debris, and the survival of a third ABC line was sustained by reactivity of its BCR to an idiotypic epitope in its own V region. Hence, a diverse set of self-antigens is responsible for maintaining the malignant survival of ABC DLBCL cells. IgH V regions used by the BCRs of ABC DLBCL biopsy samples varied in their ability to sustain survival of these ABC lines, suggesting a screening procedure to identify patients who might benefit from BCR pathway inhibition.

Juvenile rheumatoid arthritis (JRA), also known as juvenile idiopathic arthritis (JIA), includes the most common chronic autoimmune arthropathies of childhood. These two nomenclatures for classification include components representing the major subclasses of disease. The chromosomal regions and the genes involved in these complex genetic traits are being elucidated, with findings often specific for a particular disease subtype. With the advent of new SNP technologies, progress is being made at an ever-increasing pace. This review discusses the difficulties of deciphering the genetic components in complex disorders, while demonstrating the similarities that JRA shares with other autoimmune disorders. Particular emphasis has been placed on positive findings either for candidate genes that have been replicated independently in JRA/JIA, or findings in JRA for which consistent results have been reported in other forms of autoimmunity.

T follicular helper (Tfh) cells are specialized CD4 helper T cells that signal B cells to produce high affinity antibodies and provide long-term humoral immunity. Identifying genes that govern Tfh differentiation and function could provide targets for improving vaccines and therapies for autoimmunity. However, Tfh differentiation is only partially understood, and not fully recapitulated in vitro. To discover novel Tfh-regulating genes, we developed a CRISPR-based functional genetic system to mutate genes in primary mouse T cells, and measured the effects on Tfh cells in vivo. Transgenic Cas9-expressing mouse CD4 T cells were transduced in vitro with an optimized retroviral vector encoding an sgRNA and GFP. With a guide targeting Tcf7 (encoding a Tfh-essential transcription factor Tcf1), >90% cells could be transduced; up to >80% of GFP+ cells were Tcf1neg by flow and mutational analyses. To evaluate the functional consequence of CRISPR editing, we adoptively transferred virus-specific CD4 T cells that were transduced with either a control or Tcf7 sgRNA construct into wild-type hosts. After viral challenge, control T cells differentiated into both Tfh and non-Tfh cells, while very few Tcf7-edited T cells became Tfh cells. Similarly, guides against the Tfh master transcription factor Bcl6 abrogated Tfh differentiation in vivo. Next, we constructed sgRNA libraries of up to 400 guides, targeting up to 80 genes, including ones implicated in primary immunodeficiencies and druggable targets. NGS analysis of T cells post-viral challenge revealed depletion of multiple guides targeting known Tfh-required genes in the Tfh vs non-Tfh populations, validating this approach. We are currently following up on potential hits from these screens.

Cancer genome and transcriptome sequencing can identify novel oncogenes and tumor suppressors, discover distinct cancer subtypes, and predict therapeutic responses. Analysis of the coding genome of diffuse large B cell lymphoma (DLBCL) has identified various genetic mutations in the ABC and GCB molecular subtypes. Notably, ABC DLBCLs have recurrent activating mutations involving the canonical NF-kB pathway. Although an oncogenic role for constitutive NF-kB activity has been demonstrated in ABC DLBCL, the molecular mechanisms of various oncogenic mutations are still elusive. To understand the pathogenesis of DLBCL, we used high-throughput exome and transcriptome sequencing of more than 500 DLBCL biopsies. To enable functional screening of mutant alleles identified by exome-seq and RNA-seq, we are constructing an inducible retroviral expression library of over 200 open reading frames (ORFs) of genes mutated in DLBCL. We have cloned the mutant and wildtype forms of these genes together with a unique 26-base-pair 'bar code' to facilitate screening with high-throughput sequencing. We have transduced a subset of this ORF library into DLBCL cell lines, induced ORF expression by doxycycline, and FACS sorted into high and low populations based on expression the known NF-κB target gene CD83. We then performed barcode sequencing to assay relative enrichment of ORFs that confer higher or lower NF-κB activity. ORF screening revealed that MYD88L265P, CARD11L232LI, both well-characterized gain-of-function mutants, were among the highest ranking genes for induction of NF-κB activity. Interestingly, we observed enrichment for USP7D271E in NF-κB high populations, though these genes have not been shown to have a tumorigenic role in DLBCL. Ubiquitin Specific Protease-7 (USP7) is a regulator of NF-κB transcriptional activity, in part by de-ubiquitinating p65. To test how USP7 mutations contribute to NF-κB activity, we engineered the ABC DLBCL cell line TMD8 to express an NF-κB reporter consisting of the NF-κB transcriptional response element (TRE) fused to GFP. NF-κB activity upon overexpression of wildtype or mutant USP7 was monitored by flow cytometry (FACS). We observed that the mutated form of USP7 significantly increased NF-κB activity over that seen with wildtype, suggesting this is a gain-of-function mutant. We next used the CRISPR gene-targeting system to probe the function of USP7 genetically. Single guide RNAs (sgRNAs) targeting the USP7 coding regions were coexpressed in TMD8 cells with the endonuclease Cas9. SgRNA-expressing, GFP+ cells were monitored over time among total live cells by flow cytometry, and a relative reduction of the GFP+ population was observed in the sgUSP7 population. These results suggest that USP7 may function as an oncogene in DLBCL. This ongoing work demonstrates the efficacy of a high-throughput ORF expression screen to characterize mutations found in the genomic landscape of the DLBCL. Disclosures No relevant conflicts of interest to declare.

Cdc42 of the Rho GTPase family has been implicated in cell actin organization, proliferation, survival, and migration but its physiological role is likely cell-type specific. By a T cell-specific deletion of Cdc42 in mouse, we have recently shown that Cdc42 maintains naïve T cell homeostasis through promoting cell survival and suppressing T cell activation. Here we have further investigated the involvement of Cdc42 in multiple stages of T cell differentiation. We found that in Cdc42(-/-) thymus, positive selection of CD4(+)CD8(+) double-positive thymocytes was defective, CD4(+) and CD8(+) single-positive thymocytes were impaired in migration and showed an increase in cell apoptosis triggered by anti-CD3/-CD28 antibodies, and thymocytes were hyporesponsive to anti-CD3/-CD28-induced cell proliferation and hyperresponsive to anti-CD3/-CD28-stimulated MAP kinase activation. At the periphery, Cdc42-deficient naive T cells displayed an impaired actin polymerization and TCR clustering during the formation of mature immunological synapse, and showed an enhanced differentiation to Th1 and CD8(+) effector and memory cells in vitro and in vivo. Finally, Cdc42(-/-) mice exhibited exacerbated liver damage in an induced autoimmune disease model. Collectively, these data establish that Cdc42 is critically involved in thymopoiesis and plays a restrictive role in effector and memory T cell differentiation and autoimmunity.

Inhibitory anti-cytokine mAbs are used to treat cytokine-mediated disorders. Recently, however, S4B6, an anti-IL-2 mAb that blocks IL-2 binding to IL-2Rα, a receptor component that enhances affinity but is not required for signaling, was shown to enhance IL-2 agonist effects in vivo. We evaluated how S4B6 enhances IL-2 effects and whether a similar mechanism allows mAbs to IL-4 to enhance IL-4 effects. Induction of T cell proliferation by IL-2/S4B6 complexes did not require complex dissociation and was IL-2Rα independent. S4B6 increased IL-2 agonist effects by increasing in vivo half-life, not by focusing IL-2 onto cells through Fc receptors. In contrast to IL-2/S4B6 complexes, anti-IL-4 mAb enhancement of in vivo IL-4 effects required IL-4/anti-IL-4 mAb complex dissociation. Thus, agonist effects observed with high doses of anti-IL-2 mAb are most likely only applicable for mAbs that maintain cytokine half-life without blocking binding to receptor signaling components.

Pediatric arthritis is a heterogeneous group of chronic arthropathies that are influenced by complex genetic and perhaps environmental factors. Interacting genetic traits may one day be identified that provide the basis for predicting disease risk and other characteristics such as course, age of onset, and disease severity. The purpose of this review is to describe the recent progress towards identifying the multiple genes related to pediatric arthritis and understand how they relate to each other and to disease pathology.Candidate gene studies are by far the most widely reported type of genetic studies to date for juvenile arthritis with only one genome-wide screen for juvenile rheumatoid/idiopathic arthritis published. Particular attention is paid to studies of candidate genes with potential immunological roles and those associated with other forms of autoimmunity.Genomic studies may perhaps one day provide information to allow future classification systems of childhood arthritis to include molecular biomarkers as a complement to clinical observations, as well as understand how these genes or proteins relate to each other and to disease pathogenesis.

Abstract 560 More than 50% of patients diagnosed with B or T-cell leukemia and lymphoma will fail current treatment protocols. This highlights the urgent need for new and improved therapies. Since the transcription factor Growth factor independent-1 (Gfi1) plays an important role in lymphoid differentiation, we explored whether it might be a suitable target for therapy. Using mouse models in which T-cell leukemia can be induced by transgenic expression of mutated forms of Notch1, by injection of the carcinogen N-Ethyl–nitrosourea (ENU) or infection with a Murine Moloney Leukemia Virus, we found that Gfi1 knockout mice had a significantly lower incidence and a longer latency period of T-ALL. To verify whether targeting Gfi1 would be a novel approach to treat B- or T-cell lymphoma, we used Mx1Cre Gfi1fl/fl mice. In these mice, injection of polyinosinic-polycytidylic acid (pIpC) activates the Mx1 promoter driven Cre expression, which ultimately leads to the deletion of the floxed Gfi1 alleles. As controls, we used Gfi1fl/fl mice, which lack the Cre recombinase and thus still express Gfi1 after pIpC injection. To elicit a T- or B-cell lymphoma, we used ENU injection combined with expression of a mutated Notch1 transgene for T-ALL, or transgenic over-expression of c-Myc for B-cell leukemia (Eμ-Myc). Using in-vivo ultrasound supported imaging, we observed complete regression of tumour masses when Gfi1 was eliminated in Mx1Cre Gfi1fl/fl mice, curing the mice of the either the T or B-cell malignancies. Strikingly, this effect was observed in the absence of any other treatment regimen. To explore the mechanisms underlying this phenomenon, we explanted tumor samples from mice, in which Gfi1 expression was either maintained or deleted and performed gene expression arrays. A comparative analysis of the array data demonstrated that loss of Gfi1 affects pathways of key importance for leukemia such as metabolism, cell cycle progression, basal transcription and apoptosis but also the response to DNA damage. It has been shown previously for non hematological malignancies that oncogenic transformation in conjuction with dysregulated cell cycle induces DNA strand breaks (DSBs), which leads to an increased p53 dependent apoptotic response in tumours compared to non-transformed cells. This forces the tumor cells to counteract this effect – for instance by selection for the loss of p53. Consistent with this concept, we noted that leukemic cells from our tumor models displayed a greater amount of DSBs and also higher rates of spontaneous apoptosis than normal cells. Interestingly, the number of apoptotic cells was further increased in those tumors where Gfi1 had been deleted. We hypothesized that Gfi1 protects leukemia cells against DSB induced apoptosis. To test this hypothesis, we irradiated in Gfi1+/+ and Gfi1−/− thymocytes, which induces DSB in thymocytes. In line with our hypothesis we found that Gfi1−/− thymocytes showed increased rates of apoptosis compared to irradiated Gfi1+/+ thymocytes and that loss of Gfi1 led to an increased induction of pro-apoptotic genes such as Bax, Noxa and Puma after irradiation. Since Bax, Noxa and Puma are all p53 target genes, we investigated a possible link between Gfi1 and p53 and found that (I) Gfi1 binds to p53, (II) that Gfi1 inhibits the transcription of p53 target genes and (III) that Gfi1 occupies p53 target gene at the same sites as p53. In summary, Gfi1 antagonizes p53 function and loss of Gfi1 sensitizes cells to p53-mediated apoptosis. Next, we used different human T-ALL cell lines and treated these cells either with sh-RNA lentivirus or morpholinos to abrogate GFI1 expression. In all cases, down-regulation of GFI1 expression led to increased apoptosis and impeded growth of human leukemia cells. Finally, we transplanted leukemic cells of T-ALL patients into NOD-Scid, IL2Rnull (NSG) mice, waited for the leukemic cells to engraft, and then injected GFI1 specific- or control morpholinos. While mice treated with control morpholino died of leukemia, the animals treated with GFI1-specific morpholinos survived showing a significant reduction of human leukemic cells in the blood, bone marrow and spleen, even with samples from patients who did not respond to first line therapy. Since morpholinos have received approval for use in humans, our data suggest that targeting GFI1 in human T-ALL patients may be a promising therapeutic target and a feasible way to complement current T-ALL treatment regimens. Disclosures: No relevant conflicts of interest to declare.

Abstract 2174 Growth factor independent-1 (Gfi1) is a zinc finger transcriptional repressor protein originally identified in a rodent model of T-cell leukemia. Gfi1 deficient mice have defects in T cell development and a moderate loss of thymic cellularity. In Drosophila, orthologs of Notch1 and Gfi1 cooperate to specify embryo sensory organ precursors. Given the established requirement for Notch1 in T cell specification and development as well as the functional relationship of Notch and Gfi1 orthologs in Drosophila genetics, we investigated the ability of Gfi1 and Notch to cooperate in T-cell development. Utilizing transgenic mice in which the expression of Cre recombinase is controlled by the proximal Lck promoter (LckCre) to both activate intracellular Notch1 (ICN) while simultaneously deleting Gfi1, we demonstrate that T cells overexpressing ICN require Gfi1 for their survival and proper integration of ICN signaling. First, we validated our approach by showing that Lck-Cre-mediated deletion of Gfi1 alleles (Gfi1flox/-) or activation of ICN expression (Rosa26lox-stop-loxICN ires eGFP, “RosaICN”) lead to expected phenotypes. We next examined the consequences of ICN activation with simultaneous deletion of Gfi1. Whereas inducible deletion of Gfi1 alone decreases thymic cellularity by ∼4-fold, Gfi1 deletion coupled with ICN activation leads to complete thymic involution with a 14-fold reduction in total T cell numbers (p Disclosures: No relevant conflicts of interest to declare.

Abstract 13 Mutations in ELANE, GFI1, G6PC3, and HAX1 account for the genetic defects in many patients with severe congenital neutropenia (SCN). Here we utilized next generation sequencing technology to characterize genome-wide transcriptome activity and chromatin structure states associated with normal and SCN–affected hematopoietic precursor cells. We profiled cells from Gfi1−/−, Gfi1P2A, LysM-Cre Mcl1flox and wild type mice. Growth factor independent-1 (Gfi1) is a zinc finger transcription repressor required for murine and human granulopoiesis. In Gfi1−/− bone marrow cells, the genetic lack of Gfi1 protein blocks granulopoietic programming. In Gfi1P2A knockin mice the Gfi1 protein is made, but the substitution of alanine for proline (P2A) inhibits the function of the SNAG transcriptional repressor domain; resulting in neutropenia. Mcl1 is a Bcl2 family member required for the survival of multiple hematopoietic lineage cells. LysM-Cre Mcl1flox mice are neutropenic due to myeloid-lineage-specific Cre-mediated deletion of Mcl1 and subsequent apoptosis of neutrophil progenitors after the metamyelocyte stage. Other hematopoietic lineages in LysM-Cre-Mcl1flox mice are normal. Thus, all three models are neutropenic; two lack a functional lineage specifying transcription factor while one lacks a survival factor. Bone marrow cells from each of these mouse models were processed through Miltenyi AutoMacs lineage-depletion. Total RNA was extracted and subjected to paired-end deep RNA-sequencing using Illumina Truseq library preparation methods. Reads were aligned to the reference mouse mm9 genome and Refseq and ENSEMBL gene transcript models using Tophat, Bowtie, and Cufflinks analysis suite. Both gene and transcript model specific gene expression level were estimated using the fragments per kilobase of exon model per million mapped reads (FKPM) approach. Using gene-level analyses, a representative group of 2,186 genes were identified that exhibited relatively high level expression (>50th %ile by FPKM in at least one sample type) and differential gene expression relative to WT sample profiles in at least one model. First, our data indicate that the SNAG domain is critical for most Gfi1 associated functions. The SNAG domain associates with the Lysine specific demethylase-1 (Lsd1), an enzyme which removes histone 3 lysine 4 dimethyl (H3K4me2) marks. Chromatin immunoprecipitation revealed increased H3K4me2 marks in many genes deregulated in Gfi1−/− and Gfi1P2A Lin- bone marrow cells. Moreover, the group of genes overexpressed in Gfi1−/− and Gfi1P2A Lin- bone marrow cells was functionally associated with a variety of signal transduction and transcriptional control programs responsible for the development and differentiation of myeloid and lymphoid lineages, whereas genes that were commonly downregulated were associated with erythroid development. Many of the overexpressed immune myeloid genes were the targets of the ETS family of transcription factors and included a rich overrepresentation of genes associated with vesicle/lysosome/ and granule formation. Furthermore, using statistical and clustering-based analyses, approximately 70% of the dysregulated genes exhibited similar over-expression and under-expression in all three models. The large scale similarity of these patterns and the disparate genetic and biological mechanism of neutropenia in the three models suggests an underlying signature of the bone marrow progenitor activation in response to neutropenia. To confirm these findings in a different setting, we next examined the gene expression of human CD34+ cells from several ELANE mutant SCN patients and a single GFI1N382S patient, compared to normal CD34+ cells. Notably, all patients received G-CSF. In fact, a significant number of genes differentially expressed between ELANE and normal CD34+ cells were commonly deregulated in the GFI1 patient sample. Thus, human samples also demonstrate a signature associated with bone marrow progenitor activation in response to neutropenia. Our results reveal not only the transcriptional circuits controlled by Gfi1, but also cell autonomous transcriptional circuits underlying neutropenia. Disclosures: No relevant conflicts of interest to declare.

Abstract 3156 T cell acute lymphoblastic leukemia (T-ALL) is one of the most common childhood cancers associated with mutations in NOTCH1. The Growth factor independent-1 (Gfi1) transcriptional repressor gene was originally discovered as a common target of Moloney murine leukemia virus (MMLV) proviral insertion in murine T-ALL. In fact, the Gfi1 locus is the most frequently activated gene in MMLV-induced T cell leukemia. Therefore, we investigated whether the most commonly activated gene in MMLV-induced murine T-ALL, Gfi1, could collaborate with the most commonly activated gene in human T-ALL, NOTCH1. Here, we show that GFI1 expression is associated with Notch signaling in human T-ALL (p'0.0003). Functionally, Gfi1 collaborates with Notch-induced murine T-ALL by accelerating an already rapid disease model (p=0.03) without altering the lymphoblastic nature of the disease. Furthermore, inducible deletion of Gfi1 is counter-selected in both Notch-driven retroviral and transgenic mouse models of T-ALL; whereas, constitutive absence of Gfi1 completely prevents transgenic Notch-induced T-ALL (p≤0.04). However, T-ALL tumors can form in Gfi1-/- animals using either ENU-mutagenesis or MMLV-infection, yet tumor formation is delayed (p≤0.02, p≤0.03 respectively). This suggests that Gfi1 deletion does not prevent the formation of the T-ALL initiating cell and that Gfi1 might be absolutely required for Notch-induced T-ALL. Most striking is that Gfi1 is required for T-ALL maintenance in vitro and in vivo. Using three separate Tal1-initiated murine T-ALL cell lines, the overexpression of the Gfi1 dominant-negative, Gfi1N382S, was quickly and completely counter-selected. As Gfi1 has previously been found to regulate pro-apoptotic genes in T cells, we attempted to rescue the above loss of function phenotype by overexpressing the anti-apoptotic factor Bcl2. Notably, counter-selection of Gfi1N382 is not observed or is significantly delayed in all three cell lines. In vivo, inducible deletion of Gfi1 leads to both mutagen- or Notch-induced tumor regression as measured by ultrasound. In fact, levels of Gfi1 expression directly correlate to tumor regression and disease free survival of T-ALL. Finally, targeting Gfi1 enhances the efficacy of radiation therapy and bone marrow transplantation. Deletion of Gfi1 sensitizes T-ALL tumors and T cells to p53-dependent apoptosis after exposure to DNA-damaging agents such as radiation, Etoposide or Daunorubicin by de-repression of the pro-apoptotic Gfi1 target gene Bax. These data extend the role of Gfi1 to human T-ALL and suggest that T-ALL is dependent upon Gfi1 to repress pro-apoptotic genes for tumor survival, ultimately highlighting a new therapeutic target in the fight against lymphoid malignancies. Disclosures: No relevant conflicts of interest to declare.