Loss Of CD4 Research Articles

Abstract P4-06-01: Immunomonitoring of triple negative breast cancer patients undergoing neoadjuvant therapy with durvalumab - Results from the prospectively randomized GeparNuevo trial

Abstract Background: The GeparNuevo trial is a randomized, double-blind, multi-center phase II trial of neoadjuvant therapy in patients with early-stage triple negative breast cancer (TNBC) investigating the role of durvalumab, an anti-PD-L1 antibody, which blocks PD-L1 binding to PD1 and CD80, in addition to standard anthracycline/taxane based chemotherapy (Loibl S et al. ASCO 2018). Methods: In order to determine possible predictive and / or prognostic biomarkers, blood samples were taken before and during the different treatment phases (3 or 4 time points) and evaluated by multicolor flow cytometry by monitoring the absolute cell counts of T cells, B cells and NK cells as well as the frequency, composition and functionality of different immune cell populations using a panel of 35 distinct antibodies. Results: Overall 174 patients were randomized into the GeparNuevo study. 117 patients participated in the window phase of this study and 49 patients provided evaluable material for flow cytometric analyses for material at the above specified time points. Evaluation of the absolute cell count in the whole blood prior and after therapy highlighted a mixed behavior of the total leukocytes. Overall, there was a statistically significant reduction in the lymphocyte count, particularly during the last phase of the treatment (ECdd +/- durvalumab). Further dissection into the different immune populations highlighted an almost complete loss of B cells, which in some patients was also accompanied by a reduction of NK cells, mainly of the CD16+ subset. The loss of CD4+ and CD8+ T cells was less pronounced resulting overall in an enhancement of their percentages within the total lymphocytes. In addition, the different populations have also been evaluated for the expression of PD-L1 activation and exhaustion markers. A pre-specified analysis clearly demonstrated a specific effect of durvalumab during the window phase with mean decreases of 21.7% and 15.0 % in PDL1+ lymphocytes in the CD4+ and CD8+ subgroups, respectively (P<0.001). Currently, all data generated are statistically analyzed focusing on their clinical significance in relation to the treatment received and the pathological complete remission (pCR) of these patients. Conclusion: Using this approach we hope to identify biomarkers, which will allow a better selection of TNBC patients undergoing specific immunotherapies. Final data will be presented at the meeting. The trial and this translational research project were funded by AstraZeneca and Celgene, Germany. Citation Format: Massa C, Schneeweiss A, Karn T, Hanusch CA, Blohmer J-U, Fasching PA, Jackisch C, van Mackelenbergh M, Marmé F, Müller V, Schem C, Stickeler E, Huober J, Weber KE, Untch M, Denkert C, Loibl S, Mueller A, Biehl K, Seliger B. Immunomonitoring of triple negative breast cancer patients undergoing neoadjuvant therapy with durvalumab - Results from the prospectively randomized GeparNuevo trial [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P4-06-01.

HIV-1 and SIV Infection Are Associated with Early Loss of Lung Interstitial CD4+ T Cells and Dissemination of Pulmonary Tuberculosis

SUMMARYLung interstitial CD4+ T cells are critical for protection against pulmonary infections, but the fate of this population during HIV-1 infection is not well described. We studied CD4+ T cells in the setting of HIV-1 infection in human lung tissue, humanized mice, and a Mycobacterium tuberculosis (Mtb)/simian immunodeficiency virus (SIV) nonhuman primate co-infection model. Infection with a CCR5-tropic strain of HIV-1 or SIV results in severe and rapid loss of lung interstitial CD4+ T cells but not blood or lung alveolar CD4+ T cells. This is accompanied by high HIV-1 production in these cells in vitro and in vivo. Importantly, during early SIV infection, loss of lung interstitial CD4+ T cells is associated with increased dissemination of pulmonary Mtb infection. We show that lung interstitial CD4+ T cells serve as an efficient target for HIV-1 and SIV infection that leads to their early depletion and an increased risk of disseminated tuberculosis.

HIV Impacts CD34+ Progenitors Involved in T-Cell Differentiation During Coculture With Mouse Stromal OP9-DL1 Cells

HIV-1 causes the loss of CD4+ T cells via depletion or impairment of their production. The latter involves infection of thymocytes, but the involvement of hematopoietic CD34+ cells remains unclear even though HIV-positive patients frequently manifest myelosuppression. In order to have a closer look at the impact of HIV-1 on T-lineage differentiation, this study utilized the OP9-DL1 coculture system, which supports in vitro T-lineage differentiation of human hematopoietic stem/progenitor cells. In the newly developed in vitro OP9-DL1/HIV-1 model, cord-derived CD34+ cells were infected with CXCR4-tropic HIV-1NL4−3 and cocultured. The HIV-infected cocultures exhibited reduced CD4+ T-cell growth at weeks 3–5 post infection compared to autologous uninfected cocultures. Further assays and analyses revealed that CD34+CD7+CXCR4+ cells can be quickly depleted as early as 1 week after infection of the subset, and this was accompanied by the emergence of rare CD34+CD7+CD4+ cells. A subsequent theoretical model analysis suggested potential influence of HIV-1 on the differentiation rate or death rate of lymphoid progenitor cells. These results indicate that CXCR4-tropic HIV-1 strains may impact the dynamics of CD34+CD7+ lymphoid progenitor cell pools, presumably leading to impaired T-cell production potential.

TLR9 polymorphism correlates with immune activation, CD4 decline and plasma IP10 levels in HIV patients

BackgroundThe mechanism behind HIV mediated immune activation remains debated, although the role of virus replication in this process is increasingly evident. Toll like Receptor 9 (TLR9) has been implicated in HIV mediated immune activation via sensing of viral CpG DNA. Polymorphisms in the TLR9 gene and promoter region including TLR9 1635A/G and 1486C/T have been found to be associated with multiple infectious diseases and cancers.MethodsIn the current study, we looked at the correlation of TLR9 polymorphisms 1635A/G and 1486C/T with key hallmarks of HIV disease in a cohort of 50 HIV infected patients. We analyzed CD4 counts, T cell immune activation characterized by upregulation of CD38 and HLA-DR and upregulation of plasma biomarkers of inflammation like LPS, sCD14, IL-6 and IP10 in the HIV patient cohort and compared it to healthy controls.ResultsWe found that TLR9 1635AA genotype was associated with lower CD4 counts and significantly higher immune activation in both CD4+ and CD8+ T cells. Analysis of HIV associated plasma biomarkers including LPS, sCD14, IL-6 and IP10 revealed a strong correlation between IP10 and immune activation. Interestingly, IP10 levels were also found to be higher in HIV patients with the 1635AA genotype. Furthermore, the TLR9 1486C/T polymorphism that is in linkage disequilibrium with 1635A/G was weakly associated with lower CD4 counts, higher CD8 immune activation and higher IP10 levels.ConclusionsAs TLR9 stimulation is known to induce IP10 production by dendritic cells, our findings provide new insights into HIV mediated immune activation and CD4 loss. TLR9 stimulation by viral CpG DNA may be important to HIV immunopathogenesis and the TLR9 polymorphisms 1635A/G and 1486C/T may be associated with disease progression.

Peripheral T-Cell Lymphoma, not Otherwise Specified (PTCL-NOS).

Peripheral T-cell lymphoma, not otherwise specified (PTCL-NOS) is a World Health Organization (WHO)-defined diagnostic category within the highly heterogeneous group of mature post-thymic T-cell neoplasms. It is the most common subtype of mature post-thymic T-cell neoplasms globally, accounting for up to 35% of PTCL cases in Europe and North America. PTCL-NOS is a diagnosis of exclusion, comprising several disease entities that differ in biology, clinical presentation, and outcome. The diagnosis of PTCL-NOS is made based on the presence of typical histopathological features of lymphoma, an aberrant T-cell immunophenotype, often with a loss of CD5 and CD7, and a clonal T-cell receptor (TCR) gene rearrangement, in the appropriate clinical context. Unlike other types of T-cell lymphoma, recurrent mutations to assist with the diagnosis have not been identified. Patients often present with advanced stage. Prognosis is poor, with a 5-year overall survival (OS) of 20-30%. Anthracycline-based combination chemotherapy remains the most frequently used frontline strategy, with overall response rates (ORR) of 50-60%, and complete response rates (CRR) of 20-30%. Prospective studies with intent-to-treat analyses have shown that consolidation with high-dose chemotherapy and autologous stem cell transplant (ASCT) results in progression-free survivals (PFS) that compare favorably with historical cohorts and may improve OS in selected patient populations. However, randomized data are still lacking. Over the past decade, therapeutic agents approved in the relapsed and refractory setting have produced response rates of up to 33% and median PFS up to 18 months. Overall, outcomes remain poor and there is a dire need for more effective treatments. This review discusses the latest information on the diagnosis and treatment of PTCL-NOS.

Senescent T cell population in Glioblastoma

Background and Hypothesis: Glioblastoma (GBM) is the most common primary brain tumor in adults and has a median overall survival of 20.6 months. Intracranial location, infiltrative growth with near 100% recurrence and the blood brain barrier are challenges which make treating GBM difficult. CD8+ T cells (CTLs) are largely responsible for mediating anti-tumor responses and thus, are an endpoint to most immunotherapies. However, the immunosuppressive tumor microenvironment (TME) hampers T cell effectiveness and causes exhaustion and senescence. Senescent CTLs (CD28-CD57+) can result from DNA damage which causes loss of CD28. Lack of CD28 renders CTLs insensitive to immune signals and result in loss of cytotoxic functions. Thus, we hypothesize that CTLs have lost CD28 expression and are becoming senescent in the TME of GBM patients. 
 Experimental Design or Project Methods: CD8 and CD28 RNA expression data was pulled from TCGA in glioma and normal brain patients and a Pearson’s correlation was conducted. Lymphocytes were isolated from patient tumor brain and blood and using flow cytometry, CD3+ CD8+ T cells were identified and stained for CD28, CD57, PD-1, and TIM-3. 
 Results: RNA sequencing data from TCGA database on CD8 and CD28 expression showed a negative correlation in GBM patients compared to normal patients. Majority of CD8+ lymphocytes from patient brain had lost CD28+ expression compared to blood. 
 Conclusion and Potential Impact: The increased senescent T cell population in GBM suggests that the TME has a mechanism for cytotoxic T cell immunosuppression. Preventing CD8+ T cell senescence in GBM can improve current immunotherapies by promoting activation of tumor infiltrating lymphocytes.

Deep Immunoprofiling of the Bone Marrow Microenvironmental Changes Underlying the Multistep Progression of Multiple Myeloma

Introduction The multistep progression of multiple myeloma from a normal plasma cell to a system with the features of invasive cancer provides a unique opportunity to understand the co-evolution of the malignant clone within its microenvironment. Understanding these changes is becoming increasingly important as we attempt to design early intervention strategies and to precisely leverage emerging immunotherapeutic modalities to prevent and treat disease progression. In this work, we used mass cytometry (CyTOF) to generate a high-resolution map of the BM microenvironment and how it changes during the transition from health through pre-malignancy to disease. This approach allows us to both understand microenvironmental patterns that correlate with rapid disease progression as well as to generate new hypotheses about permissive and protective immune-phenotypes that might reveal novel immunologic drug targets.Methods To understand the immunologic characteristics of monoclonal gammopathy of undetermined significance (MGUS), smoldering multiple myeloma (SMM), newly diagnosed multiple myeloma (NDMM) and relapsed-refractory multiple myeloma (RRMM), we profiled BM aspirates from 79 patients using mass cytometry by time of flight (CyTOF). Furthermore, we compared the BM compartment of pre-malignant, malignant, and relapsed disease states to the BM of healthy donors using a 37-marker pan-immune panel. In this panel, we used antibodies against several immune lineages, tumor antigens, and functional surface markers, including co-stimulatory and co-inhibitory receptors. Cell clusters defined by Citrus analysis of CyTOF data were combined into an evolutionarily optimized decision tree by evtree to identify cluster interactions that strongly partition patient samples.Results During MGUS, when the tumor plasma cells are <10% of BM, there is little evidence of immune dysregulation; the immune compartment of MGUS patients contains normal numbers of innate and adaptive populations. In SMM, there is considerably more heterogeneity, with patients both resembling MGUS/healthy individuals and those that had changes to their immune microenvironment more consistent with newly diagnosed patients. These features include the loss of specific CD4 T cell and B cell subsets (FDR<0.1). The loss of CD4 was the most pronounced in the central memory and effector populations. This reduction in CD4 T cells is important because it diminishes help for CD8 T cell-mediated killing and immune cell maturation. Among B cell subsets, there was a loss in both mature and memory B cells. In comparison of SMM and NDMM samples, there was a clear progression from samples resembling healthy (normal B and CD4 subsets), to loss of only B cell subsets, and finally, loss of both B cell and CD4 subsets as samples diverged from healthy controls (Figure 1). In addition, a supervised machine learning analysis (evtree) identified CD4 effector memory abundance as the top node for partitioning samples into two distinct subtypes based on all available CyTOF markers across the myeloma continuum, with the high and low CD4 effector memory subtypes further subdivided by pre-B/immature B cell abundance (p=0.01), supporting these two cell types as being robust discriminators of the immune microenvironment as disease evolves. Between NDMM and RRMM samples, we observed heterogeneous loss in B cell subsets, including memory and naïve B cells. Interestingly, in RRMM we observed a strong increase in an unidentified population of CD45- cells that are quiescent (FDR<0.1), which may be stem or stromal cells. Available RNA sequencing from matching samples may reveal the lineage and function of these cells that increase during relapse.Conclusions Immune dysregulation is thought to be a major contributor to the progression and outcome of patients with MGUS, SMM, and MM. Using CyTOF, we have begun to benchmark the content of the immune microenvironment across the myeloma continuum. Based on this cross-sectional analysis we hypothesize that it is important to further interrogate whether the losses in the CD4 memory and effector populations we described correlate with outcomes after therapy with either CAR T or T cell engager trials that are currently ongoing, and whether reconstituting these cell types could provide a meaningful treatment strategy. [Display omitted] DisclosuresYoung:Celgene Corporation: Employment, Equity Ownership. Danziger:Celgene Corporation: Employment, Equity Ownership. Fitch:Celgene Corporation: Employment, Equity Ownership. Schmitz:Celgene Corporation: Employment, Equity Ownership. Gockley:Celgene Corporation: Employment. McConnell:Celgene Corporation: Employment. Reiss:Celgene Corporation: Employment, Equity Ownership. Copeland:Celgene Corporation: Employment, Equity Ownership. Newhall:Celgene Corporation: Employment, Equity Ownership. Hershberg:Celgene Corporation: Employment, Equity Ownership, Patents & Royalties. Foy:Celgene Corporation: Employment, Equity Ownership. Ratushny:Celgene Corporation: Employment, Equity Ownership. Dervan:Celgene Corporation: Employment, Equity Ownership. Morgan:Takeda: Consultancy, Honoraria; Janssen: Research Funding; Celgene: Consultancy, Honoraria, Research Funding; Bristol-Myers Squibb: Consultancy, Honoraria.

Target Antigen Downregulation and Other Mechanisms of Failure after Axicabtagene Ciloleucel (CAR19) Therapy

Abstract Introduction: Autologous anti-CD19 chimeric antigen receptor T cells (CAR19) have shown dramatic clinical responses in relapsed-refractory large B cell lymphomas, but more than 50% of patients will have disease progression. Here we characterize the observed mechanisms of treatment failure following Axicabtagene Ciloleucel (Axi-cel) therapy. Methods: Sixty-nine patients with refractory large B cell lymphoma were referred for CAR19 therapy from October 2017 to June 2018. The WHO diagnosis and B cell antigen expression on lymphoma cells were assessed by immunohistochemistry and/or flow cytometry before and at the time of progression. We assessed peripheral blood CAR-T cell numbers at Days 7, 14, 21, and 28 by flow cytometry immunophenotyping and monitored disease response with PET-CT at Day 28, 3 months, and 6 months post-infusion. Results: Twenty-two patients who received CAR19 therapy, including patients with transformed large B cell lymphoma (N =5), diffuse large B cell lymphoma, not otherwise specified (N =11), high grade B cell lymphoma, not otherwise specified (N = 2), primary mediastinal large B cell lymphoma (N = 2), and high grade B cell lymphoma with rearrangement of MYC and BCL2 or BCL6 (N =2). The Day 28 ORR was 86%: 10 patients had complete response, 9 had a partial response, 1 had stable disease, and 2 had progressive disease. There was no statistically significant difference in age, gender, underlying disease, or number of prior treatment regimens between patients who achieved a clinical response versus those who failed therapy. Both patients (2 of 2) with progressive disease at Day 28 showed dim or partial CD19 expression prior to CAR-T infusion but nonetheless demonstrated robust Axi-cel expansion. In contrast, one patient with Day 28 stable disease showed no CAR-T cell expansion despite intact CD19 expression. Overall, there was no statistical difference in relative or absolute CAR+ T cells in patients who responded versus those who did not at Day 28 (Figure 1). Day 90 response was available for 12 patients with either CR or PR at Day 28. Five patients (26%) developed progressive disease, and 4 of 5 underwent repeat biopsy. Of these patients, 2 had complete loss of tumor CD19 (Figure 2) and another had downregulation of CD19 with variable expression of other B cell antigens. Conclusion: Eight of 22 (36%) of patients who underwent CAR19 infusion did not respond or relapsed after Day 28 response. Five patients (62%) who failed therapy had loss or downregulation of CD19, which emphasizes that single target antigen loss is a common mechanism of CAR-T failure. However, lack of CAR-T cell expansion was noted in multiple patients, suggesting that there may be T cell intrinsic causes of treatment failure. Further studies are necessary to help identify and predict which patients will benefit from targeted immunotherapy. Disclosures No relevant conflicts of interest to declare.

Systems Biology Analyses to Delineate Mechanisms of Anti-CD20 Antibody Resistance in Non-Hodgkin Lymphoma (NHL): Influence of BCR Signaling and the Critical Importance of Calcium Polarization

Introduction:CD20 a cell surface protein selectively expressed in mature B cells is involved in regulation of B cell differentiation and is a well-established target for anti-CD20 therapy in the treatment of B cell malignancies. However, therapeutic resistance to anti-CD20 antibodies, including rituximab and obinutuzumab, continues to remain a major obstacle in the treatment for NHL patients. Known mechanisms of anti-CD20 resistance include loss of CD20 expression, apoptotic dysfunction and/or poor immunoreactivity. Further mechanisms, including potential biologic etiologies, which engender resistance to anti-CD20 antibodies remain largely unclear. We sought to delineate the immuno-biological landscape and mechanisms associated with anti-CD20 resistance via systems biology analyses and immune functional studies with rituximab and obinutuzumab resistant (RR & OR) NHL models in a novel single-cell microfluidics platform.Methods:RR or OR cells (SUDHL4 and SUDHL10) were developed by repeated culturing with chronic low dose exposure to antibody (Ab) drugs. Anti-CD20 resistance was biologically characterized by CD20 immunostaining-flow cytometry, gene expression profiling and systems biology analysis (with Ingenuity Pathway Analysis (IPA) and Gene Set Enrichment Analysis (GSEA). Immunological characterization was performed using NK mediated ADCC activity (AcellaTox-Glo), real-time analysis of NK cell-NHL interaction kinetics by microfluidics and cytokine/immune factor secretion using proximity extension assay (Olink immuno-oncology panel). Ionomycin induced release of Calcium assay was performed using Fluo-4 cell-based assay and plate reader.Results: Flow cytometric studies and ADCC assays using anti-CD20 Abs and primary NK cells, exhibited prominent downregulation of CD20 cell surface expression and markedly decreased NK mediated ADCC activity in RR (11%, P=0.0002) and OR (17%, P=0.001) compared with 10mg/ml of rituximab (51%) or obinutuzumab (56%), respectively, in the parental SUDHL4 cells. The basal NK mediated Furthermore, microfluidic analyses revealed decreased cell-cell interactions among NK cells and anti-CD20 resistant NHL cells in the presence of either antibody. Systems biology analysis of the transcriptome with these RR and OR cells showed downregulation of immune signaling mechanisms closely related MAPK, NFkB, mTOR and JAK/STAT pathways, with partial upregulation of PLCγ, B-cell receptor (BCR) signaling (via western blot). Using the Olink 92-plex cytokine assay, we observed hyposecretion of the majority of cytokines including IL-2, IL-6, IL-8, IL-10, TNFα, IFNγ and FASL all CD20 resistant cells (Fig. 1A). Based in part on these biological changes, we hypothesized that loss of CD20 associated with decreased calcium signaling could be the master regulator observed with anti-CD20 resistance. Assessment of ionomycin-induced releasable calcium showed lack of ionomycin releasable calcium in anti-CD20 resistant cells (Fig. 1B). Treatment with the depolarizing agent, veratridine, restored ionomycin releasable calcium levels, reversed BCR signaling, and it was accompanied with significantly decreased cell viability (60%, P<0.0001) in anti-CD20 resistant cells compared with 100% in sensitive cells. In contrast, use of the hyperpolarizing ivermectin resulted in mimicking anti-CD20 resistant upregulation of BTK and anti-CD20 sensitive cells intolerant to decreased calcium levels.Conclusions: RR and OR NHL cells were associated with downregulation of immune signaling pathways, cytokine hyposecretion, and upregulation of BCR signaling. Furthermore, these observed biological changes were associated with decreased intracellular calcium suggesting its role as a master regulator in anti-CD20 resistance. Utilizing depolarizing and hyperpolarizing pharmacologic agents, we were able to restore favorable signaling changes associated with anti-CD20 antibody sensitivity as well as mimic a resistant phenotype in sensitive cells. Additional investigation of the modulation of calcium signaling as a novel mechanism and target for the treatment of anti-CD20 resistant NHL is warranted. [Display omitted] DisclosuresEvens:Pharmacyclics International DMC: Membership on an entity's Board of Directors or advisory committees; Affimed: Consultancy; Bayer: Consultancy; Janssen: Consultancy; Seattle Genetics, Inc.: Membership on an entity's Board of Directors or advisory committees, Research Funding; Acerta: Consultancy; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Abbvie: Consultancy; Tesaro: Research Funding; Novartis: Consultancy.

CAR T Cell Cytotoxicity Is Dependent on Death Receptor-Driven Apoptosis

▪Background: T cells engineered to express chimeric antigen receptors targeting the B-cell antigen CD19 (CART19) have demonstrated impressive results in the treatment of lymphoid cancers. Despite these promising outcomes, a significant subset of patients relapse after initial response. To investigate the molecular pathways that drive relapse, we performed an unbiased, CRISPR/Cas9-mediated genome-wide knockout screen in the acute lymphoblastic leukemia (ALL) cell line Nalm6, and found that loss of CD19 was the primary driver of relapse after initial response. This finding is consistent with clinical observations that antigen loss is a primary driver of late disease recurrence, however it fails to address the molecular etiology of intrinsic resistance, which affects ~50% of patients with non-Hodgkin lymphoma and ~20% of patients with ALL, or of late antigen-independent relapse, which accounts for ~40% of relapses in ALL. Identification of the mechanisms regulating CART19 susceptibility is an essential first step in overcoming resistance to this powerful therapy. We hypothesized that genetic alteration in ALL cells were responsible for mediating intrinsic, CD19-independent resistance. To investigate this, we conducted a genome-wide loss of function screen in a model designed to evaluate intrinsic resistance to CART19.Methods: Using a lentiviral guide RNA (gRNA) library containing four distinct gRNAs targeting each human gene (~80,000 gRNAs total), we enabled genome-wide knockout in Nalm6, whereby each target cell lost function of only one gene. This gene-modified cell pool was then exposed to either CART19 or control T cells at a low effector:target ratio (0.25:1) to model the expected in vivo E:T ratio. At 24h, surviving Nalm6 cells were collected and gRNA from these cells underwent next-generation sequencing. Sequenced samples were processed using three distinct genome-scale knockout screen algorithms (MAGeCK, permutation-based non-parametric analysis and ScreenBeam). This pipeline allowed identification of (i) significantly enriched gRNA, postulated to mediate loss of gene function that confers resistance to CART19, and (ii) significantly depleted guides, postulated to mediate loss of gene function that confers sensitivity to CART19. The role of identified genes was then validated in in vitro and in vivo studies.Results: Analysis of gRNA sequencing data from our screen (Figure 1) revealed that the three genes whose loss of function most significantly promoted resistance to CART19 were BID, FADD and CASP8, all of which are key regulators of death receptor-driven apoptosis. TNFRSF10B, encoding the death receptor TRAIL-R2, was also significantly enriched. Interestingly, amongst the 10 genes whose loss most significantly sensitized to CART19 were TRAF2, BIRC2 and CFLAR, all negative regulators of death receptor activity. Pathway analysis of the top 50 genes (25 enriched, 25 depleted) demonstrated significant enrichment in the death receptor pathway, with a false discovery rate of 3.79x10-7.We proceeded to functionally validate the role of BID and FADD in mediating resistance to CART19 by deleting these genes in Nalm6 using de novo designed gRNAs. Strikingly, BIDKO and FADDKO cells were highly resistant to CART19 cytotoxicity in vitro as compared to wild-type Nalm6. Resistance was evident as early as 6 hours after co-culture and was maintained for at least 7 days. Observed resistance to CART19 directly correlated to fraction of KO cells present, suggesting that gene loss was mechanistically responsible for failed CART19 cytotoxicity. We further evaluated the impact of BID or FADD loss on anti-leukemic activity of CART19 in our Nalm6 xenograft model. We observed that BIDKO or FADDKO significantly impaired the anti-leukemic activity of CART19 in vivo.Conclusions: CART19 can cure select patients with B-cell cancers, while others experience transient or no clinical benefit. Using a genome-wide loss of function screen, we identified that death receptor-associated proteins are centrally involved in regulating CART19 cytotoxicity, and that loss of these molecules leads to intrinsic resistance to CART19. These findings are, to our knowledge, the first to characterize the role of death receptors as critical regulators of CART19 cytotoxicity, and suggest that tumor cell modulation of death receptor signaling may drive both inherent resistance and antigen-independent relapse. DisclosuresJune:Celldex: Consultancy, Membership on an entity's Board of Directors or advisory committees; Novartis Pharmaceutical Corporation: Patents & Royalties, Research Funding; Immune Design: Membership on an entity's Board of Directors or advisory committees; Tmunity Therapeutics: Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties, Research Funding; Novartis Pharmaceutical Corporation: Patents & Royalties, Research Funding; Immune Design: Membership on an entity's Board of Directors or advisory committees; Tmunity Therapeutics: Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties, Research Funding. Gill:Novartis: Research Funding; Carisma Therapeutics: Equity Ownership; Extellia: Consultancy, Membership on an entity's Board of Directors or advisory committees. Ruella:University of Pennsylvania: Patents & Royalties.

Clonal Evolution of a Mutation in PTPRA As a Potential Cause for Resistance Against Anti-CD19 Directed Chimeric Antigen Receptor T-Cell (CAR-T) Therapy with CTL019 in DLBCL Patients

Introduction:Anti-CD 19 directed CAR-T cell therapy has shown very promising effects in treatment of diffuse-large B-cell lymphoma (DLBCL). However, a substantial fraction of patients does not respond to treatment or relapses after approximately three to six months [Schuster, NEJM, 2017; Neelapu, NEJM, 2017]. Resistance mechanisms against anti-CD19 directed CAR-T cell therapy have so far only been described in B-cell acute lymphoblastic leukemia (B-ALL) and involve mutations, splice variants or even loss of CD19 [Sotillo, Cancer Discovery, 2015]. To further elucidate mechanisms of acquired resistance in DLBCL, we obtained matched pair tumor samples from four patients before and after treatment with CTL019 treated within the phase II JULIET trial.Methods:We performed whole exome sequencing in one patient (#UKK2) employing one matched normal sample that has been sequenced at a coverage of 130x and one pre-treatment as well as two post-treatment samples that were all sequenced at a coverage of 160x. Whole exome sequencing data were used to reconstruct clonal evolution of DLBCL after treatment with CTL019 as has been described previously [Herling, Nature Communications, 2018]. Furthermore, we obtained biopsies at relapse or disease progression including matched normal samples from three more patients. Whole exome sequencing from the three missing patients as well as 3´RNA sequencing from all four patients' samples are ongoing and results will be updated for presentation at the meeting.Results:The first analyzed patient, #UKK2, is a 67 year old male patient, who had chemotherapy refractory disease after treatment with R-CHOP, R-DHAP and Pixantrone. He showed predominantly pelvic lesions including infiltration of the iliopsoas muscle. The patient achieved a complete response at month 3 (Figure 1A) after CTL019 transfusion. At month 5 he developed new cutaneous lesions on his left thigh (Figure 1B, posttreatment sample 1) as well as new muscle infiltrations on his right quadriceps femoris muscle (posttreatment sample 2) and rectus abdominis muscle. Pathological examination confirmed relapse of CD19 positive DLBCL in both posttreatment samples. Whole exome sequencing from a biopsy obtained before study entry and the above mentioned posttreament biopsies showed clonal evolution of a single missense mutation in PTPRA (N297D). PTPRA encodes for the receptor protein-tyrosine phosphatase alpha, a receptor phosphatase involved in activation of c-Src via dephosphorylation of an inhibitory tyrosine at position 530 (Y530) [Mustelin, Science Signaling, 2002 and Gut, International Journal of Oncology, 2017]. On the other hand, c-Src is an onco-protein known to cause an invasive phenotype and metastasis in different tumor entities [Ishizawar, Cancer Cell, 2004]. All other mutations observed were either already clonal before CTL019 treatment or gained clonality in only one posttreatment biopsy (Figure 1C).Conclusion:To our knowledge, this is the first report identifying clonal evolution during CD19 redirected CAR-T cell therapy in r/r DLBCL involving PTPRA. We hypothesize that this PTPRA mutation contributes to an invasive phenotype and extranodal lymphoma growth pattern allowing the lymphoma to escape from the pattern allowing the lymphoma to escape from the CAR-T cell attack. Genomic workup of patients with pre- and posttreatment biopsies and functional investigation of this hypothesis is ongoing and will be presented. [Display omitted] DisclosuresBalke-Want:Novartis: Honoraria. Borchmann:Novartis: Consultancy, Honoraria.

Simultaneous Targeting of CD19 and CD22: Phase I Study of AUTO3, a Bicistronic Chimeric Antigen Receptor (CAR) T-Cell Therapy, in Pediatric Patients with Relapsed/Refractory B-Cell Acute Lymphoblastic Leukemia (r/r B-ALL): Amelia Study

Simultaneous Targeting of CD19 and CD22: Phase I Study of AUTO3, a Bicistronic Chimeric Antigen Receptor (CAR) T-Cell Therapy, in Pediatric Patients with Relapsed/Refractory B-Cell Acute Lymphoblastic Leukemia (r/r B-ALL): Amelia Study

Increasing CD28-FOXP3+CD8+ Treg and Senescent CD8+NK2GA+Eomes+ NK-like T Cells in Peripheral Blood of Patients with Multiple Myeloma

Immune dysfunction in patients with multiple myeloma (MM) includes TGF-β induced dendritic cell dysfunction, regulatory T cell (Tregs)/Th17 cell imbalance, accumulation of Tregs and myeloid derived suppressor cells. These tumor-induced dysfunctions may contribute to immune escape and even suppress immune cells introduced in adoptive cellular immunotherapy. CD28 independent T cells of the effector memory (TEM) and CD45RA+ effector memory (TEMRA) population were shown to accumulate with age and contribute to the immunosuppressive tumor microenvironment in several solid tumors and hematological cancers. Especially in the CD8+ population, the loss of CD28 is associated with high cytotoxicity and regulatory function while showing high diversity and defective antigen-induced proliferation. In the CD4 subset, regulatory and senescent T cells were studied extensively, while in the CD8 positive subset their heterogeneity is still not clearly defined. Their potential immunosuppressive role and distribution in healthy individuals (HI) as well as patients with multiple myeloma (MM) remains to be observed. Furthermore, a recently characterized CD8+CD28- NK-like T cell subset showing expression of NK-related inhibitory receptors and TCR independent effector function is potentially of interest in the progression of MM. In the present study, we compared the changes of distribution of CD8+CD25+ and CD8+FOXP3+ regulatory T cells (Treg), CD28-CD57+ senescent T cells (Tsen), and CD8+KIR/NK2GA+EOMES+ NK-like T in peripheral blood (PB) between HI and patients with MM by multicolour flow cytometry (Gating strategy shown in Figure 1A). When comparing 35 HIs (Median age is 54) and 14 MM patients (Median age is 52), it was shown that there is no significant change in the proportion of senescent T cells in CD8 (P = 0.2452), TEM/CD8 (P = 0.1686) and TEMRA/CD8 (P = 0.4861) between HIs and the MM group, while both the CD25+FOXP3+ regulatory T cells of the CD4 population (P = 0.0031) and CD28-FOXP3+ regulatory T cells of the CD8 population (P = 0.0014) were shown to increase. There is no significant difference in the percentages of KIR/NK2GA+EOMES+ in the CD8 T cell and TEMRA/CD8 T cell population between HIs and the MM group. Remarkably, although there was no overall increase in senescent T cell in MM patients, senescent CD8+NK2GA+EOMES+ NK-like T cells increased in MM patients in comparison to HIs (P = 0.0068) (Figure 1B). In conclusion, the increase of regulatory T cells of both the CD4 and CD8 population as well as the increase of senescent NK-like T cells in the CD8 population potentially contributes to cancer progression through creation of suppressive microenvironments. Moreover, we found that regulatory CD8 T cells and CD8 NK-like T cells only contribute to a small part of the overall CD28- senescent T cell pool. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Intrabone Delivery of CD34+ Cells Using an Optimized Delivery System Does Not Enhance Engraftment in a Rhesus Macaque Model of Hematopoietic Stem Cell Transplantation

Introduction: Umbilical cord blood (UCB) grafts are the only option for a significant minority of patients who require hematopoietic stem cell transplantation (HSCT) but lack a suitable related or unrelated donor. While UCB can serve as a suitable ‘off the shelf’ graft for many patients, units with the best HLA match often contain low and sometimes insufficient numbers of CD34+ cells for use in transplantation, particularly for adult patients. Furthermore, UCB grafts contain lower CD34+ cells number compared to BM or PBSC grafts, which leads to longer engraftment times and a higher risk of graft failure. BM and PBSC grafts are usually injected intravenously, homing to the bone marrow after several hours in the circulation. During this time, CD34+ cells are lost in the lungs, liver and spleen with typically < 20% making it to the bone marrow.1 Investigators have sought to overcome the limitation of low cell dose in UCB grafts and loss of CD34+ cells in the circulation by injecting CD34+ cells directly into the bone marrow space. However, we have recently shown that conventional intrabone delivery methods used in investigational trials to transplant UCB, result in low-level retention of hematopoietic progenitor cells in the intrabone space. Recently, we developed an optimized intrabone (OIB) transplant method using computer controlled low pressure and low volume injection (controlled infusion rate <0.2ml/min, total volume <5ml) under CT guidance. Utilizing porcine and rhesus macaque models, we have shown that OIB delivery of CD34+ cells improves intrabone retention, preventing circulation of CD34+ cells through the lungs, which is observed with conventional non-optimized intrabone methods.2 Here we conducted experiments using an autologous myeloablative transplant model in rhesus macaques comparing engraftment of gene-marked CD34+ cells transplanted intravenously, with cells transplanted using OIB delivery.Methods: Rhesus macaques received GCSF and plerixafor mobilization prior to apheresis. Products were CD34+ selected using Miltenyi beads. CD34+ cells were split equally for transduction with lentiviral vectors encoding the reporters GFP and YFP. After myeloablative conditioning with 10Gy total body irradiation, half the graft was injected directly intra-bone using the OIB method, with the other half of the autograft simultaneously being injected intravenously via slow iv push. Peripheral blood samples were measured daily by flow cytometry to assess the proportion of engrafting cells deriving from each source. To address whether OIB transplantation could allow engraftment of low CD34+ cell numbers, as found in UCB units, the cell doses transplanted in one rhesus macaque recipient were reduced to only 0.5 x 106CD34+ cells/kg.Results: CD34+ cells injected intrabone utilizing the OIB method engrafted in all 3 animals. However, flow cytometric analysis gating on GFP vs YFP positive neutrophils showed CD34+ cells injected utilizing OIB delivery did not engraft quicker than IV transplanted cells. Sequential monitoring of neutrophils over 30 days showed the contribution to hematopoiesis of OIB delivered cells was significantly lower than the cells injected IV (Table 1.)Conclusions: We developed a novel intrabone delivery system that optimizes the retention of CD34+ cells into the bone marrow space. Although autologous CD34+ cells injected using this OIB transplant method were capable of engrafting in rhesus macaques that had undergone myeloablative conditioning, they did not engraft faster and contributed less to hematopoiesis than CD34+ cells simultaneously transplanted using conventional IV infusion. These data raise questions over whether intrabone delivery, even when using techniques to optimize intrabone retention, has utility in improving CD34+ cell engraftment.

Immune Profiling of Relapsed or Refractory Multiple Myeloma Patients Treated with Pomalidomide and Low-Dose Dexamethasone in Combination with Daratumumab

Immune Profiling of Relapsed or Refractory Multiple Myeloma Patients Treated with Pomalidomide and Low-Dose Dexamethasone in Combination with Daratumumab

Modeling Sézary Syndrome for Immunophenotyping and Anti-Tumor Effect of Ucart and Long-Acting Interleukin-7 Combination Therapy

Background: Sézary syndrome (SS) is a highly-morbid T cell leukemic lymphoma with no widely-effective treatments and few preclinical models. We demonstrated effective T cell lymphoma therapy with allogeneic gene-edited anti-CD7 CARTs (Cooper et al, Leukemia, 2018). However, SS T cells typically lose CD7 but maintain ubiquitous high CD2 expression. Thus, we generated CD2- and TRAC-deleted anti-CD2 universal CARTs (UCART2) and multiple SS xenograft models (PDXs) as preclinical UCART2 testing platforms. We further tested a stable homodimeric interleukin-7 molecule, the long-acting form of recombinant human interleukin-7 fused with hybrid Fc (rhIL-7-hyFc, NT-I7), to potentiate UCART2 killing of an SS xenograft in vivo.Methods: To generate SS PDX models, we injected NOD scid IL2Rgammanull (NSG) mice expressing SCF, GM-CSF, and IL-3 (NSG-SGM3) with ~2x106 mononuclear cells derived from SS patients. We immunophenotyped SS patient blood and PDX engraftment with two 21-color flow cytometry panels assessing major immune subsets, CTCL, and exhaustion markers (Staser et al, Cytometry A, 2018). To generate UCART2s, we activated human T cells on CD3/CD28 beads, electroporated the T cells with Cas9, a TRAC-targeted gRNA, and a CD2-targeted gRNA followed by viral transduction with an anti-CD2 scFv 3rd generation CAR. For initial UCART2 testing, we injected NSG mice with 5x105 cells from a human Sézary cell line transduced with click beetle red luciferase (HHCBR-GFP) four days prior to UCART2 treatment. Mice were treated with NT-I7 (10mg/kg SC) on days +1, +15 and +29 post UCART2 infusion.Results: SS patient blood showed specific defects in monocyte, monocytic dendritic cell, and natural killer cell differentiation, increased skewing toward granulocytes and non-classical CD16+ monocytes (p<0.01, SS vs. normal PBMCs), and loss of effector memory CD4 cells (8% vs 34%, p<0.001, SS vs. normal PBMCs). SS cells were CD3+CD4+CD2+CD5+CD8- with variable CD7 loss and PD1 gain. Four of six unique human SS samples injected in NSG-SGM3 mice engrafted within ~6 weeks with no signs of xenogeneic GVHD. Following engraftment, SS cells showed near ubiquitous PD1 expression (>90% vs ~20%, p<0.001, SS vs. normal PBMCs), CD7 loss, and increased CD30 and CD26 expression. Immunohistochemistry further revealed atypical CD3+CD4+CD8-CD7- lymphocytes lining the dermo-epidermal junction. Second generation PDXs showed infiltration of the spleens, blood, and bone marrow with CD2+CD7- human cells and developed alopecia, scaling, and subcutaneous and intraperitoneal masses, with immunophenotyping, sequencing, and UCART treatment studies ongoing.To test UCART2's efficacy in killing SS cells in vivo, we injected NSG mice with HHCBR-GFP+ cells. UCART2-treated HHCBR-GFP mice showed dramatically reduced tumor burden as compared to control UCART19-treated HHCBR-GFP mice (BLI; 10^7 vs. 10^11 photon flux/s at 3 weeks, p<0.0001, UCART2 vs. UCART19). Moreover, UCART2-treated HHCBR-GFP mice survived ~40 days as compared to ~21 days in the UCART19 group. Remarkably, UCART2-treated HHCBR-GFP mice receiving NT-I7 showed virtually no tumor burden (maximum 106 photon flux/s vs. 1010 photon flux/s, UCART2+NT-I7 vs. UCART2 only groups) with 100% of UCART2- and NT-I7-treated HHCBR-GFP mice surviving beyond 49 days (Figure 1).Discussion: We describe the generation of physiologically-relevant SS preclinical models, comprehensive immunophenotyping of patient SS samples, clonal SS PDX outgrowth, and the highly effective anti-tumor activity of UCART2- plus NT-I7-mediated killing of SS cells in vivo using an NSG xenograft model. Ongoing studies involve treating primary SS and CTCL PDX models with UCART2 and NT-I7. These preclinical data validate the use of allogeneic “off-the-shelf” adoptive immunotherapy for the treatment of Sézary syndrome, while demonstrating the dramatic enhancement of CART efficacy using a dose-adjustable, clinic-ready long-acting interleukin-7 agonist given in an adjuvant setting. [Display omitted] DisclosuresPark:NeoImmuneTech: Employment. Lee:NeoImmuneTech: Employment. Musiek:Seattle Genetics: Honoraria; Actelion: Other: Scientific Advisory Committee ; Kyowa Kirin: Honoraria.

Emerging Clinical Activity of REGN1979, an Anti-CD20 x Anti-CD3 Bispecific Antibody, in Patients with Relapsed/Refractory Follicular Lymphoma (FL), Diffuse Large B-Cell Lymphoma (DLBCL), and Other B-Cell Non-Hodgkin Lymphoma (B-NHL) Subtypes

Emerging Clinical Activity of REGN1979, an Anti-CD20 x Anti-CD3 Bispecific Antibody, in Patients with Relapsed/Refractory Follicular Lymphoma (FL), Diffuse Large B-Cell Lymphoma (DLBCL), and Other B-Cell Non-Hodgkin Lymphoma (B-NHL) Subtypes

Lack of T-cell-mediated IL-2 and TNFα production is linked to decreased CD58 expression in intestinal tissue during acute simian immunodeficiency virus infection.

For an effective T-cell activation and response, co-stimulation is required in addition to the antigen-specific signal from their antigen receptors. The CD2/CD58 interaction is considered as one of the most important T-cell co-stimulatory pathways for T-cell activation and proliferation, and its role in regulating intestinal T-cell function in acute and chronic SIV -infected macaques is poorly documented. Here, we demonstrated a significant reduction of CD58 expression in both T- and B-cell populations during acute SIV infection along with high plasma viral load and a loss of intestinal CD4+ T cells compared to SIV-uninfected control macaques. The reduction of CD58 expression in T cells was correlated with the reduced expression of T-cell-mediated IL-2 and TNFα production. Together, these results indicate that reduction in the CD2/CD58 interaction pathway in mucosal lymphocytes might play a crucial role in mucosal T-cell dysfunction during acute SIV/HIV infection.

Residual Clonal Plasma Cells Detected By Flow Cytometry at 10-4level within Autologous Stem Cell Grafts Is Associated with Significantly Less Overall Survival

Introduction:High-dose Melphalan followed by autologous hematopoietic cell transplantation (ASCT) compared to conventional chemotherapies has been shown to improve survival and progression free survival (PFS) in eligible patients with multiple myeloma (MM). However cells that are resistant to the high-dose therapy and remaining in the patient and/or PBSC grafts may eventually lead to relapse. Over the past decade, data have been published on the role of circulating plasma cells (CPCs) as a poor prognostic feature at the time of diagnosis and prior to auto HSCT. But these studies have not sought this issue exclusively in PBSC grafts. The aim of this study was to analyze the impact of flow cytometric measurement of residual clonal cells within the apheresis products on outcome following ASCT.Materials and Methods:Patients with a diagnosis of MM who underwent auto HSCT at our center between January 2008- Mart 2018 were prospectively analyzed. PBSC grafts were tested for the presence of abnormal PCs (APC) and the number of normal PCs (NPC) by multi-parameter flow cytometry (FCM). Standard panel was set up with CD138FITC/CD38PE/CD45ECD/CD56PC5, CD45-CD56+PCs were identified as APC if any aberrant expression (including; CD20(loss)CD27(loss)CD28(gain)CD33(loss)CD34(gain)CD81(loss) CD117(gain)) is detected at diagnosis, the corresponding antibody was also added to the panel. Since maintenance was not an approved treatment in myeloma most patients did not receive any. Outcome was determined as response to transplant, PFS and overall survival (OS) in months by Kaplan-Meier analysis using SPSS (IBM SPSS Statistics 21; IBM Corp., Chicago, IL) statistical tool kit.Results: A total of 209 patients with MM with routine assessment for APC in the apheresis product were included in the analysis. Of these patients, 195 who underwent ASCT, met the predetermined criteria for inclusion. There were 81 (41.5%) female and 114 (58.5%) male patients. The median age at diagnosis of MM was 56 years (range, 31-71 years). The interval from the time of diagnosis to ASCT was median 7 months and median follow-up from ASCT was 49 months (range, 3-198 months). Among 195 patients, 36 (18.5%) had evidence of APC contamination in the PBSC grafts ranging between 1x10-4-12.8 x10-4of total cells. Subtypes of MM were similar among those w/wo APC. Seventy-four patients had pre and post ASCT PET-CT imaging done with 28.2% still active lesions post-ASCT. There was no correlation between PET-CR and absence of APC in grafts. Neither was there a statistically significant association between disease response <VGPR at mobilization and the number of APC in the apheresis product. Post-transplant responses were closely associated with pre-transplant responses. A total of 25 and 57 patients relapsed/progressed in respectively 6 and 12 months after ASCT. Post-transplant response was significantly associated with relapse at 6 months (≥VGPR vs. <VGPR; 37.5% vs. 62.5%, p=0.000). Estimated median OS was significantly different between patients w/o APC contamination; 20.4±8.1 and 63.3±3.8, respectively (p=0.000) (Figure 1). There were no differences in OS among the patients achieving VGPR/CR at the time of mobilization and also two months after the ASCT. We performed a subgroup analysis of patients in grouping together VGPR/CR or only CR and among those patients presence of APC was correlated with worse OS (Figure 1). PFS at 2 years were 60.3±4.3% and 75.6±9.7% in patients receiving APC free PBSC grafts compared with grafts with evidence of APC contamination, respectively (p=0.595).Discussion:PostASCT immune response was predictive for OS only when combined with APC results. Our study provides new insight suggesting a survival advantage value of residual APC detected by FCM, even at a low sensitivity level, on OS but not PFS following ASCT. The impact of such analysis may be more relevant on PFS with better assessment of residual APC with next generation flow cytometric/sequencing approaches while taking into account maintenance. [Display omitted] DisclosuresIlhan:Roche: Speakers Bureau; BMS: Speakers Bureau; Alexion: Speakers Bureau; Celgene: Speakers Bureau.

IFITM3-Mediated Regulation of Cell Membrane Dynamics Is Essential for Malignant B-Cell Transformation

Background & Hypothesis: B cell receptor (BCR) signaling and oncogenic tyrosine kinases that mimic BCR-signaling in B-lineage leukemia and lymphoma depend on assembly of membrane proximal signaling complexes. Signalosomes in normal BCR- and oncogene (e.g. BCR-ABL1, RAS-pathway lesions) signal transduction are recruited to phospholipid anchors in lipid rafts. The robustness of these complexes depends on cholesterol accumulation in lipid rafts. Here we identified the interferon-induced transmembrane protein IFITM3 as a central regulator of cholesterol in lipid rafts.Results: IFITM3 is mostly localized to endosomal compartments. By antagonizing VAP-A and oxysterol-binding protein 1 (OSBP1), IFITM3 promotes cholesterol accumulation and solidifies the endosomal membrane. This mechanism is particular important in anti-viral immunity, to “trap”intraluminal viral particles for lysosomal degradation. In B-cells, IFITM3 can translocate to the cell membrane and form a complex with the BCR and its co-receptors CD19, CD81 and CD21. While the functional significance of membrane expression of IFITM3 on B-cells was not known, we found that higher IFITM3 mRNA levels at the time of diagnosis represents a strong predictor of poor clinical outcome for children (COG P9906; P=0.006; n=207) and adults (ECOG E2993; P=0.014; n=215) with B-ALL. In addition, higher than median IFITM3 mRNA levels at the time of diagnosis were associated with a higher risk of relapse and positive MRD status at the end of induction chemotherapy in B-ALL and other B-cell malignancies. Interestingly, IFITM3 is a transcriptional target and strongly repressed by IKZF1 (Ikaros) a potent tumor suppressor in B- ALL and high IFITM3 mRNA levels represents a biomarker for patients with IKZF1-deletion.While its membrane-topology can vary in different cell types, we found that IFITM3 functions as a dual-pass transmembrane protein in tight association with CD19 and the Iga and Igb signaling chains of the BCR in B-ALL and B-cell lymphoma cells. To study the function of Ifitm3 in a model for human pre-B ALL, pre-B cells from Ifitm3-/- mice were transformed with BCR-ABL1 or oncogenic NRASG12D. Strikingly, deletion of IFITM3 resulted in destabilization of lipid rafts, loss of CD19 surface expression and loss of PI3K signaling. Ifitm3-/- leukemia cells could not sustain oncogenic signaling from BCR-ABL1 or oncogenic NRASG12D and failed to initiate fatal leukemia in transplant recipient mice. These changes were paralleled by G0/1 cell cycle arrest (P<0.001), loss of colony formation capacity (P=0.0004) and increased propensity to apoptosis.In mechanistic studies, we identified type II transmembrane topology for IFITM3 at plasma membrane with extracellular C and intracellular N terminus which interacted with CD19, LYN, SYK, PI3K and AKT (see schematic, left). Disruption of endocytic motif (20YEML23) by substitution of Tyr20 to Phe induced IFITM3 gain of function and forced accumulation of IFITM3 on the cell membrane, constitutive CD19-PI3K signaling, intracellular calcium mobilization, homotypic cellular aggregation and massively increased proliferation of pre-B ALL cells (see schematic, right). Conversely, inducible overexpression of IKZF1 transcriptionally silenced IFITM3, resulting in loss of IFITM3 expression, reduction of lipid rafts and impairment of membrane-associated oncogenic signaling. Through Filipin-based cholesterol staining, we found Ifitm3-/- pre-B cells have reduced levels of cholesterol in lipid rafts, which causes disruption of lipid rafts formation, as reflected by decreased levels of ganglioside GM1. Notably, the homeostatic cholesterol fluidity by presence of IFITM3 on plasma membrane was also required for initiation of B- and T cell receptor signaling in mature B- and T cell lymphoma to induce Ca2+ mobilization.Conclusions: These findings identify novel role of the viral immunity IFITM3 surface receptor as a central regulator of cell membrane cholesterol fluidity and critical mediator of sustained oncogenic tyrosine kinase (BCR-ABL1) and RAS (NRASG12D) signaling in B cell malignancies. In promoting cholesterol aggregates in lipid rafts, IFITM3 protects healthy individuals from potentially lethal viral infections, but also enables oncogenic signaling by providing a robust membrane scaffold for tyrosine kinase and RAS-pathway oncogenes. DisclosuresWiita:Sutro Biopharma: Research Funding; TeneoBio: Research Funding.