Citations

Despite the remarkable activity of CD19 directed chimeric antigen receptor T cell (CART19) therapy in the treatment of B cell malignancies, the therapy is limited by the development of severe life-threatening complications such as neurotoxicity (NT) and cytokine release syndrome (CRS). Additionally, durable efficacy following CART19 therapy is not optimal. Emerging literature suggests that inhibitory myeloid cells and their cytokines play an important role in inducing CAR-T cell toxicities and also contribute to the inhibition of their effector functions. Specifically, GM-CSF was identified as a critical cytokine in the development of NT and CRS after CART19 therapy. Neutralization of GM-CSF in preclinical models has been shown to prevent CRS and enhance CART anti-tumor activity through modulation of myeloid cell behavior, resulting in reduction of tumor associated macrophages. In addition to the predominant effect of GM-CSF on myeloid cells, there appears to be a direct effect on CART19 cells. In this study, we aimed to evaluate the direct effect of GM-CSF on CART cells. Our initial finding of enhanced anti-tumor activity of CART19 cells after GM-CSF inhibition suggested a direct effect of GM-CSF on CART cells (Sterner et al. 2019, Blood). In these experiments, a guide RNA (gRNA) targeting exon 3 of GM-CSF in a CRISPR-Cas9 lentiviral vector was used to knock out GM-CSF during CART cell manufacturing. This resulted in a disruption efficiency of approximately 70% of the GM-CSF gene. Using a high tumor burden xenograft model for relapsed acute lymphoblastic leukemia established through the engraftment of the CD19+ luciferase+ NALM6 cell line (1x106 cells intravenously) in immunocompromised NOD-SCID-γ-/- mice, treatment with low doses of GM-CSFk/o CART19 resulted in improved anti-tumor activity and overall survival compared to GM-CSFwt CART19. The lack of myeloid cells in this model pointed to an intrinsic effect of GM-CSF on CAR-T cells. To ensure that this was not related to an off-target effect of the gRNA, whole exome sequencing (WES) of the modified cells was performed. There was no difference in the single nucleotide variants or indel counts between GM-CSFk/o CART19 and GM-CSFwt CART19 (Figure 1A). WES was significant for only two alterations in the exon 3 targeted by the gRNA (Figure 1B). The high efficiency and accuracy of targeting exon 3 of GM-CSF indicated that the improvement in CART function is unlikely related to an off-target effect of the gRNA and suggested the possibility of a direct interaction between GM-CSF and CART cells as a potential mechanism behind the improved anti-tumor activity. To investigate this interaction, we first assessed the expression of GM-CSF receptors on CART cells. While resting CART cells do not express any GM-CSF receptors, our analysis robustly indicates that activated CART cells significantly upregulate both α and β subunits of the GM-CSF receptor. This finding was significant both when CART cells are activated through their T cell receptor with CD3/CD28 beads (Figure 1C) or through the CAR with irradiated NALM6 cells (Figure 1D). Additionally, activated GM-CSFk/o CART19 cells also upregulated GM-CSF receptors, indicating this upregulation is induced by T cell stimulation. These results suggest a direct interaction between GM-CSF and upregulated GM-CSFR on activated CART cells. Having demonstrated that 1) GM-CSF depletion enhances CART19 efficacy in xenograft models in the absence of monocytes and 2) T cell activation increases GM-CSF receptor expression, we sought to uncover the downstream changes resulting from this effect. Transcriptome interrogation of GM-CSFk/o CART19 revealed a distinct signature including a significant inhibition of the Fas death pathway, a known critical pathway in inducing CART cell apoptosis. This suggests a potential mechanism for enhanced CART19 activity following GM-CSF depletion (Figure 1E). In summary, our results strongly indicate that CART cells increase expression of GM-CSF receptor subunits when activated, resulting in modulation of CART cell functions. Furthermore, GM-CSFk/o CART19 revealed a distinct transcriptome signature compared to GM-CSFwt CART19. These results illuminate a novel mechanism for a direct modulatory effect of GM-CSF on activated CART cells. Disclosures Cox: Humanigen: Patents & Royalties. Sterner:Humanigen: Patents & Royalties. Sakemura:Humanigen: Patents & Royalties. Ahmed:Humanigen: Employment. Chappell:Humanigen: Employment. Durrant:Humanigen: Employment. Parikh:Acerta Pharma: Research Funding; MorphoSys: Research Funding; AbbVie: Honoraria, Research Funding; Genentech: Honoraria; Janssen: Research Funding; AstraZeneca: Honoraria, Research Funding; Pharmacyclics: Honoraria, Research Funding; Ascentage Pharma: Research Funding. Kay:MorphoSys: Other: Data Safety Monitoring Board; Infinity Pharmaceuticals: Other: DSMB; Celgene: Other: Data Safety Monitoring Board; Agios: Other: DSMB. Kenderian:Novartis: Patents & Royalties, Research Funding; Tolero: Research Funding; Lentigen: Research Funding; Humanigen: Other: Scientific advisory board , Patents & Royalties, Research Funding; Kite/Gilead: Research Funding; Morphosys: Research Funding.

CD19 directed chimeric antigen receptor T cell (CART) therapy has shown remarkable activity in B cell lymphoma and acute lymphoblastic leukemia leading to the approval of two CART therapies. With the emergence of therapeutic anti-CD19 antibodies for the treatment of B cell malignancies, it remains to be elucidated whether such antibodies would interfere with the ability of CD19 targeting CARTs to exert their anti-tumor effect in a subsequent therapy. To address a part of this question, we investigated the potential for functional interference between the monoclonal anti-CD19 antibody tafasitamab (MOR208) and CD19 directed CART cells (CART19). CART19 cells were generated through lentiviral transduction of healthy donor T cells with a second generation CD19 CAR construct (FMC63-CD8h-CD8TM-41BBζ) which is similar to the construct used for the FDA-approved CART tisagenlecleucel. Tafasitamab, is an Fc-enhanced humanized monoclonal antibody which mediates antibody-dependent cellular toxicity (ADCC), antibody-dependent cellular phagocytosis (ADCP) and direct cytotoxicity. It is currently being studied in phase 2 and 3 clinical trials in diffuse large B-cell lymphoma (DLBCL) in combination with the immunomodulatory agent lenalidomide (L-MIND) and the chemotherapeutic drug bendamustine (B-MIND). As a first step we confirmed the relevance of the tested CD19-positive target cell lines, JEKO (mantel cell lymphoma), Ly7 (DLBCL) and NALM-6 (ALL) based on functional activity of tafasitamab and CART19. In a 24 hours ADCC (tafasitamab titration plus natural killer (NK) cells; Figure 1A) and T cell cytotoxicity assays (CART19, E:T titrations; data not shown) distinct activity was observed for both therapies on all tested cell lines. Secondly, we studied whether the observed CART19 activity may be influenced by tafasitamab in case of a direct CD19 binding competition between tafasitamab and the CAR. To test for such binding competition we incubated the CD19+ cell lines NALM6 or JEKO with up to 100 µg/ml tafasitamab, to saturate the receptors. Subsequent flow cytometry analysis using the FMC63 antibody (carrying the same CD19 binding domain as contained in CART19) failed to detect CD19 expression, indicating a direct binding competition between FMC63 and tafasitamab (Figure 1B). Next, to investigate the potential impact of such binding competition on CART19 cell effector functions, we co-cultured tafasitamab CD19+ JEKO cell line at increasing concentrations of up to 100µg/ml, and then added CART19 cells at different effector to target ratios to the cell culture. The presence of tafasitamab, binding to the CD19 antigen, did not affect important CART cell effector functions such as antigen specific killing (Figure 1C), degranulation (Figure 1D), cytokine production or proliferation of CART19 (Figure 1E). In summary, our studies indicate that CART19 continue to exhibit potent antigen specific effector functions despite presence of tafasitamab and the related competition for CD19 binding. Besides the presented in vitro work the questions of therapeutic sequencing of tafasitamab and CART19 is being studied in xenograft models and will be presented at the meeting. Disclosures Sakemura: Humanigen: Patents & Royalties. Cox:Humanigen: Patents & Royalties. Schanzer:MorphoSys AG: Employment. Endell:MorphoSys AG: Employment, Patents & Royalties. Nowakowski:Selvita: Membership on an entity's Board of Directors or advisory committees; NanoString: Research Funding; MorphoSys: Consultancy, Research Funding; Genentech, Inc.: Research Funding; F. Hoffmann-La Roche Ltd: Research Funding; Curis: Research Funding; Bayer: Consultancy, Research Funding; Celgene: Consultancy, Research Funding. Kay:MorphoSys: Other: Data Safety Monitoring Board; Infinity Pharmaceuticals: Other: DSMB; Celgene: Other: Data Safety Monitoring Board; Agios: Other: DSMB. Kenderian:Novartis: Patents & Royalties, Research Funding; Tolero: Research Funding; Humanigen: Other: Scientific advisory board , Patents & Royalties, Research Funding; Lentigen: Research Funding; Morphosys: Research Funding; Kite/Gilead: Research Funding.

Background: Two-year follow-up from ZUMA-1 trial for axicabtagene ciloleucel (axi-cel) CD19 chimeric antigen receptor T-cell (CART) in aggressive non-Hodgkin lymphoma (NHL) demonstrated that patients (pts) achieving complete remission (CR) as their best response have the longest progression free survival (PFS), while PFS remained poor for others. Majority of the best response are achieved by month 3 post-infusion. Moreover, a subset of pts who do not achieve CR by month 1 may still achieve CR while others will progress within months. Having a clinically available test to predict for durable response can assist clinicians in selecting pts who can be observed for continued response vs. those who may need intervention earlier. Peak CAR-T expansion has been correlated with best clinical response to axi-cel. However there is no clinically available test to quantify CART cells post treatment. Given the prolonged lymphotoxic effect of the lymphodepletion (LD) chemotherapy administered before CART infusion, we hypothesize that the peak expansion of absolute lymphocyte count (ALC, ALCpeak) after infusion likely corresponds to CART expansion. We examined for correlation between ALCpeak and clinical response at month 3 post-infusion for pts who received axi-cel. Methods: Data was collected from pts who received axi-cel between June 2016 and March 2019 at Mayo Clinic Rochester (NCT02348216, NCT03153462, and standard of care). All infused pts were included in the intention to treat analysis for response and event-free survival (EFS). Lugano 2014 criteria were used for clinical response. EFS was defined as the date of infusion until progression or death due to any cause and evaluated using Kaplan Meier curves with log-rank test. Comparison between subgroups was investigated using Fisher's exact test and Wilcoxon test for categorical and continuous variables, respectively. Statistical analysis performed using JMP Pro 14.1. Results: Of the thirty-six NHL pts treated with axi-cel, the median age was 56 yrs (range 26-65); 78% (28) were male; 44% (16) had DLBCL; the median number of prior therapies was 3 (2-6), 47% (17) had stem cell transplant. Nineteen (53%) pts received bridging therapy after leukapheresis. All pts received fludarabine and cyclophosphamide for LD chemotherapy and achieved ALC nadir by the day of CART infusion (median 0.03 x 10^9/L, range 0.01-0.11). As CART expansion were found to peak within the first two weeks in ZUMA-1 trial, we examined ALC trend over 15 days post infusion and found that ALCpeak generally happened within the same timeframe (median ALCpeak day: 9, range 6-15). At 3 months post infusion, 33.3% (12/36) of pts achieved CR, 8.3% (3/36) pts were in partial remission (PR). No correlation was found between CR response and clinical characteristics: age >60 years, stage III/IV, B symptoms, elevated LDH, IPI score 3-4, number of previous treatments, use of bridging therapy, peak CRP and ferritin. However, the ALCpeak was significantly different between CR and non-CR pts (median, range: 1.09, 0.35-2.55; 0.67, 0.15-1.03; p=0.04). Using receiver operator curve (ROC)-derived area under the curve (AUC) estimates, ALCpeak 1.08 x 10^9/L was identified as the best threshold to discriminate patients at different likelihood to achieve CR at month 3. Pts achieving ALCpeak ≥ 1.08 showed a strong increased likelihood of achieving CR at month 3 (OR 2.8; 95% CI, 1.12-6.99; P=0.02). There was no statistical difference between the ALCpeak high and low groups in terms of use of bridging therapy, occurrence of any grade or higher grade cytokine release syndrome or neurotoxicity, or the use of tocilizumab or steroid. Interestingly, for pts who had changes in clinical response from month 1 to month 3, all 3 pts who converted to CR by month 3 had ALCpeak≥1.08, while the 13 of the 15 pts who had progressed by month 3 had ALCpeak<1.08. In addition, EFS was significantly different between pts who had ALCpeak above and below 1.08 (6 months EFS rate 75% vs. 25%, respectively, Figure 1). Conclusions: We found that the ALC expansion occurred in the first 15 days post axi-cel infusion, consistent with the known time frame for CART expansion. Higher ALCpeak level in this timeframe correlated with higher likelihood of achieving CR, and is associated with conversion to CR. This study is limited by the small sample size and limited follow-up, but the results are promising for further prospective investigation in a larger cohort of patients. Disclosures Paludo: Verily Life Sciences: Research Funding; Verily Life Sciences: Research Funding; Celgene: Research Funding; Celgene: Research Funding. Ansell:Mayo Clinic Rochester: Employment; Bristol-Myers Squibb: Research Funding; Seattle Genetics: Research Funding; LAM Therapeutics: Research Funding; Regeneron: Research Funding; LAM Therapeutics: Research Funding; Mayo Clinic Rochester: Employment; Seattle Genetics: Research Funding; Affimed: Research Funding; Affimed: Research Funding; Trillium: Research Funding; Mayo Clinic Rochester: Employment; Trillium: Research Funding; Seattle Genetics: Research Funding; Seattle Genetics: Research Funding; Seattle Genetics: Research Funding; Mayo Clinic Rochester: Employment; Trillium: Research Funding; Mayo Clinic Rochester: Employment; Trillium: Research Funding; Trillium: Research Funding; Mayo Clinic Rochester: Employment; Seattle Genetics: Research Funding; Regeneron: Research Funding; Regeneron: Research Funding; Bristol-Myers Squibb: Research Funding; Affimed: Research Funding; Affimed: Research Funding; Mayo Clinic Rochester: Employment; Seattle Genetics: Research Funding; LAM Therapeutics: Research Funding; Bristol-Myers Squibb: Research Funding; Bristol-Myers Squibb: Research Funding; Regeneron: Research Funding; LAM Therapeutics: Research Funding; Affimed: Research Funding; Seattle Genetics: Research Funding; Bristol-Myers Squibb: Research Funding; Trillium: Research Funding; Regeneron: Research Funding; Affimed: Research Funding; LAM Therapeutics: Research Funding; Bristol-Myers Squibb: Research Funding; Trillium: Research Funding; Mayo Clinic Rochester: Employment; LAM Therapeutics: Research Funding; Regeneron: Research Funding; LAM Therapeutics: Research Funding; Affimed: Research Funding; Trillium: Research Funding; Regeneron: Research Funding; Trillium: Research Funding; LAM Therapeutics: Research Funding; Affimed: Research Funding; Regeneron: Research Funding; Bristol-Myers Squibb: Research Funding; LAM Therapeutics: Research Funding; Affimed: Research Funding; Bristol-Myers Squibb: Research Funding; Mayo Clinic Rochester: Employment; Regeneron: Research Funding; Bristol-Myers Squibb: Research Funding; Seattle Genetics: Research Funding. Bennani:Purdue Pharma: Other: Advisory board; Purdue Pharma: Other: Advisory board; Seattle Genetics: Other: Advisory board; Adicet Bio: Other: Advisory board; Bristol-Myers Squibb: Research Funding; Kite Pharma: Other: Advisory board; Kite Pharma: Other: Advisory board; Adicet Bio: Other: Advisory board; Seattle Genetics: Other: Advisory board; Adicet Bio: Other: Advisory board; Bristol-Myers Squibb: Research Funding; Kite Pharma: Other: Advisory board; Bristol-Myers Squibb: Research Funding; Seattle Genetics: Other: Advisory board; Purdue Pharma: Other: Advisory board. Dingli:alexion: Consultancy; Janssen: Consultancy; Millenium: Consultancy; Karyopharm: Research Funding; Rigel: Consultancy. Kapoor:Celgene: Honoraria; Janssen: Research Funding; Cellectar: Consultancy; Sanofi: Consultancy, Research Funding; Amgen: Research Funding; Takeda: Honoraria, Research Funding; Glaxo Smith Kline: Research Funding. Kenderian:Novartis: Patents & Royalties, Research Funding; Tolero: Research Funding; Humanigen: Other: Scientific advisory board , Patents & Royalties, Research Funding; Lentigen: Research Funding; Morphosys: Research Funding; Kite/Gilead: Research Funding. Kumar:Takeda: Research Funding; Janssen: Consultancy, Research Funding; Celgene: Consultancy, Research Funding.

Introduction : Daratumumab (DARA) is a human CD38 antibody approved in combination with standard-of-care (SoC) regimens in pts with NDMM who are TIE. DARA-based treatments showed significantly improved progression-free survival (PFS) in pts with TIE NDMM in ongoing phase 3 studies ALCYONE (DARA + bortezomib/melphalan/prednisone [D-VMP] vs VMP) and MAIA (DARA + lenalidomide/dexamethasone [D-Rd] vs Rd continuous). An NMA allows estimation of the relative efficacy of regimens not compared in head-to-head trials. A previous NMA examined D-VMP and D-Rd vs comparators (Facon T et al., EHA 2019), excluding the SWOG S0777 study (bortezomib/lenalidomide/dexamethasone [VRd]; Durie BG, et al. Lancet 2017) as it enrolled both transplant-eligible (TE) and TIE pts, and data for pts who are TIE only (~50% [aged ≥65 y and frail]) were unavailable. In this NMA, D-Rd and D-VMP had the highest probabilities of being more effective than reference treatment Rd continuous in terms of PFS. Recently, VRd received EMA approval for treatment of TIE NDMM based on SWOG S0777, but results specific to TIE pts are still unavailable. Recent NMAs of TIE pts (Blommestein HM et al., Haematologica, 2019; Weisel et al., EHA 2019; Cao Y et al., Clin Lymphoma Myeloma Leuk, 2019) have included data from the the intent-to-treat (ITT) population of SWOG S0777, though its inclusion violates the similarity assumption. Here, we present a sensitivity analysis of the previously-conducted NMA including all pts who received VRd in SWOG S0777 for a comprehensive view of the comparative effectiveness of DARA-based treatments in TIE NDMM. Methods: VRd was included as part of a sensitivity analysis of the previous NMA based on the ITT population, reporting PFS and overall survival (OS). The Guyot method was used to estimate OS hazard ratios (HRs) for pts aged ≥65 y (proxy for TIE) from SWOG S0777 (Durie BG et al., ASH 2018). Median follow-up in ALCYONE and MAIA was 27.8 months and 28.0 months, respectively. Based on a systematic literature review conducted through January 2019, both PFS and OS were extracted and synthesized in Bayesian NMAs. Choice of fixed- or random-effects (FE or RE) model was based on lowest deviance information criterion (DIC) and/or presence of heterogeneity in the network. Rd continuous was selected as reference, as it was commonly included in guidelines across regions. For PFS and OS, HR <1 indicates the comparison is not in favor of Rd continuous. In addition to the previous NMA, we also report an NMA sensitivity analysis including only comparators approved by the EMA that are relevant for the European, Middle East and/or Africa (EMEA) region (EMEA NMA). Results: Global NMA: RE model was used for both PFS and OS. Compared with Rd continuous, the PFS HR for VRd in the sensitivity analysis was 0.74 (95% credible interval [CrI], 0.42-1.30), which was similar to SWOG S0777 (0.71; [96% Wald confidence interval [CI], 0.56-0.91]) and higher than HRs for D-Rd (0.55; 95% CrI, 0.31-0.97) and D-VMP (0.58; 95% CrI, 0.20-1.62; Table). For OS, the SWOG S0777 HR estimated by Guyot algorithm for pts aged ≥65 y was 0.80 (95% CI, 0.56-1.16). Compared with Rd continuous, the OS HR for VRd in the sensitivity analysis was 0.80 (95% CrI, 0.51-1.26), which was similar to SWOG S0777 and higher than D-Rd (0.78; 95% CrI, 0.51-1.18). EMEA NMA: FE model was used for both PFS and OS. Compared with Rd continuous, the PFS HR for VRd in the sensitivity analysis was 0.74 (95% CrI, 0.60-0.92), which was higher than HRs of D-Rd (0.55; 95% CrI, 0.43-0.71) and D-VMP (0.64; 95% CrI, 0.41-1.02). Compared with Rd continuous, the OS HR for VRd in the sensitivity analysis was 0.80 (95% CrI, 0.55-1.17), which was higher than for D-Rd (0.78; 95% CrI, 0.56-1.09). These results are consistent with the Global NMA, but differ slightly as the EMEA NMA has fewer evidence-loops (only EMEA SoC regimens were included). Conclusions: This study demonstrated favorable efficacy outcomes for DARA-based regimens vs other relevant frontline options, such as VRd, for pts with NDMM who are TIE. Limitations of the sensitivity analysis include lack of comparative trial data and demographic differences between pts from SWOG S0777 and trials of frontline MM treatments for pts who are TIE included in our network. The majority of SWOG S0777 pts are TE (<65 y), who often have better prognoses than TIE pts (≥65 y), and thus violates the similarity assumption of NMAs. However, DARA-based regimens still showed a comparative clinical advantage. Disclosures Facon: Takeda: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Celgene: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Janssen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Sanofi: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees. San-Miguel:Amgen, Bristol-Myers Squibb, Celgene, Janssen, MSD, Novartis, Roche, Sanofi, and Takeda: Consultancy, Honoraria. Usmani:Amgen, Bristol-Myers Squibb, Celgene, Janssen, Merck, SkylineDX, Takeda: Other: Consultant/Advisor; Amgen Array Biopharma, Bristol-Myers Squibb, Celgene, Janssen, Merck, Pharmacyclics, Sanofi, Takeda: Other: Research Grant; Amgen, Celgene, Janssen, Sanofi, Takeda: Speakers Bureau. Dimopoulos:Sanofi Oncology: Research Funding. Kumar:Janssen: Consultancy, Research Funding; Takeda: Research Funding; Celgene: Consultancy, Research Funding. Mateos:Adaptive: Honoraria; EDO: Membership on an entity's Board of Directors or advisory committees; Pharmamar: Membership on an entity's Board of Directors or advisory committees; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; GSK: Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees. Cavo:Janssen, Celgene, Amgen, Abbvie: Consultancy, Membership on an entity's Board of Directors or advisory committees; Celgene, Janssen, Amgen, BMS, Abbvie, Takeda: Honoraria; Janssen, Celgene: Other: Travel Accommodations; Janssen, Celgene: Speakers Bureau. Heeg:Ingress-Health: Employment. Van Beekhuizen:Ingress-health: Consultancy. Pisini:Janssen: Employment, Equity Ownership. Nair:Janssen: Employment, Equity Ownership. Lam:Janssen: Employment, Equity Ownership. Slavcev:Janssen: Employment, Equity Ownership. He:Janssen: Employment, Equity Ownership.

Introduction: Fecal microbiota transplant (FMT) has been shown to be an effective treatment for recurrent Clostridioides difficile infection (CDI). Immunosuppression is one of the risk factors for CDI. However, most of the trials evaluating efficacy of FMT have excluded immunocompromised patients. Patients with hematological cancers represent a major class of immunocompromised patients. Hence, we performed a meta-analysis to evaluate the efficacy of FMT in patients with hematological cancer for the treatment of recurrent CDI. Methods: A systematic search of Medline, Embase, and Web of Science was performed from January 2000 up to December 2018. Articles included for meta-analysis were case series and case reports that assessed efficacy of FMT in hematological cancer patients were included. Study quality was assessed using the Newcastle-Ottawa scale. The main outcomes were pooled proportions of patients achieving cure after first FMT. The cure rate was defined as resolution of diarrhea without any recurrence. Results: 10 studies (5 case series and 5 case reports) were included in the analysis, making up 29 patients who underwent 38 total FMT procedures. The follow up period ranged from 1- 14 months. Of the included patients, 17 had leukemia, 10 had lymphoma and 2 patients had multiple myeloma and myelodysplastic syndrome each. 7 of these patients had undergone hematopoietic stem cell transplant. The pooled cure rate was 82% [confidence interval (CI) 67-94%] after 1 FMT with no heterogeneity (I2=0%). Minor publication bias was seen on visual inspection of funnel plot. FMT was generally well tolerated by these patients. No serious side effects were reported in any study. One of the studies reported death due to cardiac arrest 5 days after FMT which was thought to be unrelated to the procedure. Mild side effects including mild abdominal pain, transient diarrhea, fecal urgency, constipation and nausea were reported in 5 patients. Conclusion: Based on limited observational data, FMT seems to be an effective and safe modality for management of recurrent CDI in patients with hematological cancers. However, further prospective clinical trials are needed to establish its safety as a first line therapy for recurrent CDI in this population. Figure Disclosures Jamshed: Takeda Pharmaceutical: Honoraria. Khanna:Rebiotix, Inc: Research Funding; Probio Tech, LLC: Consultancy; Facile Therapeutics: Consultancy; Shire, Plc: Consultancy.

Introduction: Daratumumab (DARA) is a human, CD38-targeted, IgG1κ monoclonal antibody approved as monotherapy in heavily pretreated relapsed/refractory multiple myeloma (RRMM) and in combination with standard-of-care regimens for transplant-ineligible NDMM and RRMM. The addition of DARA to standard-of-care regimens in phase 3 studies has consistently demonstrated a near doubling of complete response (CR) rates, tripling of minimal residual disease (MRD)-negativity rates, and reduction in the risk of disease progression or death by ≥44% in patients with transplant-ineligible NDMM or RRMM. In the primary analysis of the phase 3 MAIA study (median follow-up: 28.0 months), a significant progression-free survival (PFS) benefit (median not reached [NR] vs 31.9 months; hazard ratio [HR], 0.56; P<0.001) and a >3-fold increase in MRD-negativity rates (10-5 sensitivity threshold; 24.2% vs 7.3%; P<0.001) were observed for D-Rd vs Rd in patients with transplant-ineligible NDMM (Facon T, N Engl J Med 2019). Here, we report updated efficacy and safety findings from MAIA after 9 months of additional follow-up. Methods: Patients with NDMM ineligible for high-dose chemotherapy and autologous stem cell transplantation due to age ≥65 years or comorbidities were randomly assigned (1:1) to receive Rd ± DARA. Stratification factors included International Staging System stage (ISS [I vs II vs III]), region (North America vs other), and age (<75 vs ≥75 years). All patients received 28-day cycles of Rd (R: 25 mg orally once daily on Days 1-21; d: 40 mg orally on Days 1, 8, 15 and 22). In the D-Rd arm, DARA (16 mg/kg intravenously) was given weekly for Cycles 1-2, bi-weekly for Cycles 3-6, and every 4 weeks thereafter. Patients were treated until disease progression or unacceptable toxicity in both treatment arms. The primary endpoint was PFS. Key secondary endpoints included overall response rate (ORR), MRD-negativity rate (10-5 sensitivity, clonoSEQ® version 2.0), and safety. PFS on the next line of therapy (PFS2), defined as the time from randomization to progression on the next line of therapy or death, was also measured. Results: A total of 737 patients were randomized (D-Rd, N = 368; Rd, N = 369). Patient baseline characteristics were well balanced between the two treatment arms. Median (range) age was 73 (45-90) years, with 44% of patients ≥75 years of age. 27%, 43%, and 29% of all patients were ISS stage I, II, and III, respectively. Among 642 patients evaluable for FISH/karyotyping analysis, 86% had standard and 14% had high cytogenetic risk. After a median follow-up of 36.4 months, median PFS was NR with D-Rd vs 33.8 months with Rd (HR, 0.56; 95% confidence interval [CI], 0.44-0.71; P<0.0001; Figure). The estimated 36-month PFS rate was 68% with D-Rd vs 46% with Rd. The PFS benefit of D-Rd vs Rd was observed in all prespecified subgroups, except for patients with impaired hepatic function. Adding DARA to Rd continued to result in deeper responses with higher rates of ≥CR and ≥very good partial response (VGPR; Table). Median duration of response among responders was NR with D-Rd vs 40.7 months with Rd. Median PFS2 was NR vs 47.3 months with D-Rd vs Rd, respectively (HR, 0.69; 95% CI, 0.53-0.91; P=0.0079); follow up is ongoing. 143 (39%) vs 233 (64%) patients with D-Rd vs Rd, respectively, have discontinued treatment. 85 (23%) patients with D-Rd vs 103 (28%) patients with Rd have discontinued the study due to death. Grade 3/4 treatment-emergent adverse events (TEAEs; D-Rd/Rd) occurring in ≥10% of patients were neutropenia (51%/35%), lymphopenia (15%/11%), pneumonia (15%/9%), anemia (14%/21%), leukopenia (11%/6%), and hypokalemia (10%/10%); grade 3/4 infection rates were 36%/27%. The most common serious TEAE was pneumonia (14%/9%). 9% of patients in the D-Rd arm and 18% of patients in the Rd arm discontinued treatment due to TEAEs. The complete updated data set will be presented at the meeting with additional efficacy endpoints, including MRD-negativity rate. Conclusion: After longer follow up, the addition of DARA to Rd continues to demonstrate a significant PFS benefit and improved rates of deeper and more durable responses vs Rd alone in patients with transplant-ineligible NDMM. The longer follow-up also demonstrated a significant improvement in PFS2 favoring D-Rd, and no new safety concerns were observed. These results continue to support the use of D-Rd in the first line of treatment for transplant-ineligible patients with NDMM. Disclosures Bahlis: Janssen: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Takeda: Consultancy, Honoraria; AbbVie: Consultancy, Honoraria; Celgene: Consultancy, Honoraria. Facon:Janssen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Celgene: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Takeda: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Sanofi: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees. Usmani:Amgen Array Biopharma, Bristol-Myers Squibb, Celgene, Janssen, Merck, Pharmacyclics, Sanofi, Takeda: Other: Research Grant; Amgen, Celgene, Janssen, Sanofi, Takeda: Speakers Bureau; Amgen, Bristol-Myers Squibb, Celgene, Janssen, Merck, SkylineDX, Takeda: Other: Consultant/Advisor. Kumar:Celgene: Consultancy, Research Funding; Janssen: Consultancy, Research Funding; Takeda: Research Funding. Plesner:Oncopeptides: Consultancy; Janssen: Consultancy, Research Funding; Takeda: Consultancy; AbbVie: Consultancy; Celgene: Consultancy; Genmab: Consultancy. Orlowski:Poseida Therapeutics, Inc.: Research Funding. Hulin:celgene: Consultancy, Honoraria; Janssen, AbbVie, Celgene, Amgen: Honoraria. Quach:Sanofi: Research Funding; Karyopharm: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees, Research Funding; GSK: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding. Goldschmidt:ArtTempi: Honoraria; John Hopkins University: Other: Grants and/or provision of Investigational Medicinal Product; BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Grants and/or provision of Investigational Medicinal Product, Research Funding; Novartis: Honoraria, Research Funding; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Grants and/or provision of Investigational Medicinal Product, Research Funding; Dietmar-Hopp-Foundation: Other: Grants and/or provision of Investigational Medicinal Product; Mundipharma: Research Funding; Adaptive Biotechnology: Membership on an entity's Board of Directors or advisory committees; MSD: Research Funding; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other, Research Funding; Amgen: Membership on an entity's Board of Directors or advisory committees, Other: Grants and/or provision of Investigational Medicinal Product, Research Funding; Sanofi: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Grants and/or provision of Investigational Medicinal Product, Research Funding; Molecular Partners: Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees, Research Funding; Chugai: Honoraria, Other: Grants and/or provision of Investigational Medicinal Product, Research Funding. O'Dwyer:GlycoMimetics Inc: Research Funding; Janssen: Membership on an entity's Board of Directors or advisory committees, Research Funding; BMS: Research Funding; Onkimmune: Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Research Funding; AbbVie: Consultancy. Venner:Janssen: Honoraria; J&J: Research Funding; Amgen: Honoraria, Research Funding; Sanofi: Honoraria; Takeda: Honoraria; Celgene: Honoraria. Weisel:GSK: Honoraria; Takeda: Consultancy, Honoraria; Sanofi: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; Juno: Consultancy; Celgene Corporation: Consultancy, Honoraria, Research Funding; Adaptive Biotech: Consultancy, Honoraria; Bristol-Myers Squibb: Consultancy, Honoraria; Amgen: Consultancy, Honoraria, Research Funding. Krevvata:Janssen: Employment. Pei:Janssen: Employment, Equity Ownership. Wang:Janssen: Employment. Van Rampelbergh:Janssen: Employment. Ukropec:Janssen: Employment, Equity Ownership. Uhlar:Janssen: Employment. Kobos:Janssen: Employment. Perrot:Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees; Takeda: Honoraria; Amgen: Honoraria; Sanofi: Honoraria.

Abstract BACKGROUND Efficacy of chimeric antigen receptor T cell (CART) therapy remains limited in solid tumors. Given the heterogeneity of surface receptor expression and immunosuppressive stromal microenvironment, strategies to target tumor neovasculature and tumor stromal cells are needed to help overcome CART inhibition by GBM. Eph receptors are the largest family of receptor tyrosine kinases and are integral to cell adhesion, migration, and axon guidance, during development and homeostasis. EphA3 is a receptor tyrosine kinase which is lowly expressed in adult tissues but is highly expressed in tumor neovasculature and tumor stromal cells in GBM and other solid tumors. EphA3 is over-expressed in up to 40% of GBM samples. We aimed to develop CART cells directed against EphA3 to use in targeting tumor neovasculature and tumor stromal cells in GBM. METHODS we developed a second generation CD28 co-stimulated CAR construct in a third generation lentivirus backbone to generate EphA3 CART cells using the single chain variable fragment of ifabotuzumab, a monoclonal antibody directed against EphA3. Patient derived GBM xenograft cell lines were used in these experiments. RESULTS EphA3 directed CART cells exhibited specific and potent antitumor activity against EphA3+ GBM cell lines with variable transcriptome EphA3 expression indicating its broader applicability in patients with GBM. Killing over 24-hour incubation was significant at low effector: target ratio: 52.5% killing at 1.25:1 against cell lines with 25.05% EphA3 expression and 37.1% killing at 1.25:1and 90% killing at 5:1 ratio against a cell line with 19.28% expression. Conversely, when co-cultured with UTD controls there was significantly lower killing, or growth of tumor cells. CONCLUSION We demonstrate for the first time that targeting EphA3 with CART cells is feasible, efficacious and represents a novel therapeutics strategy to target GBM. Data from in vivo and combinatorial CART experiments will be reported at the meeting.

Abstract Despite its efficacy, chimeric antigen receptor T-cell therapy (CART) is limited by the development of cytokine release syndrome (CRS) and neurotoxicity (NT). While CRS is related to extreme elevation of cytokines and massive T cell expansion, the exact mechanisms for NT have not yet been elucidated. Preliminary studies suggest that NT might be mediated by myeloid cells that cross the blood brain barrier. This is supported by correlative analysis from CART19 pivotal trials where CD14+ cell numbers were increased in the cerebrospinal fluid of patients that developed severe NT (Locke et al, ASH 2017). Therefore, we aimed to investigate the role of GM-CSF neutralization in preventing CRS and NT after CART cell therapy via monocyte control. First, we investigated the effect of GM-CSF blockade on CART cell effector functions. Here, we used the human GM-CSF neutralizing antibody (lenzilumab, Humanigen, Burlingame, California) that has been shown to be safe in phase II clinical trials. Lenzilumab (10 ug/kg) neutralizes GM-CSF when CART19 cells are stimulated with the CD19+ Luciferase+ acute lymphoblastic leukemia (ALL) cell line NALM6, but does not impair CART cell function in vitro. We have found that malignancy associated macrophages reduce CART proliferation. GM-CSF neutralization with lenzilumab results in enhanced CART cell antigen specific proliferation in the presence of monocytes. To confirm this in vivo, NOD-SCID-g-/- mice were engrafted with high disease burdens of NALM6 and treated with low doses of CART19 or control T cells (to induce tumor relapse), in combination with lenzilumab or isotype control antibody. The combination of CART19 and lenzilumab resulted in significant anti-tumor activity and overall survival benefit compared to control T cells (Fig 1A), similar to mice treated with CART19 combined with isotype control antibody, indicating that GM-CSF neutralization does not impair CART cell activity in vivo. This anti-tumor activity was validated in an ALL patient derived xenograft model. Next, we explored the impact of GM-CSF neutralization on CART cell related toxicities in a novel patient derived xenograft model. Here, NOD-SCID-g-/- mice were engrafted with leukemic blasts (1-3x106 cells) derived from patients with high risk ALL. Mice were then treated with high doses of CART19 cells (2-5x106 intravenously). Five days after CART19 treatment, mice began to develop progressive motor weakness, hunched bodies, and weight loss that correlated with massive elevation of circulating human cytokine levels. Magnetic Resonance Imaging (MRI) of the brain during this syndrome showed diffuse enhancement and edema, associated with central nervous system (CNS) infiltration of CART cells and murine activated myeloid cells. This is similar to what has been reported in CART19 clinical trials in patients with severe NT. The combination of CART19, lenzilumab (to neutralize human GM-CSF) and murine GM-CSF blocking antibody (to neutralize mouse GM-CSF) resulted in prevention of weight loss (Fig 1B), decrease in critical myeloid cytokines (Fig 1C-D), reduction of cerebral edema (Fig 1E), enhanced leukemic disease control in the brain (Fig 1F), and reduction in brain macrophages (Fig 1G). Finally, we hypothesized that disrupting GM-CSF through CRISPR/Cas9 gene editing during the process of CART cell manufacturing would result in functional CART cells with reduced secretion of GM-CSF. We designed guide RNA targeting exon 3 of the GM-CSF gene and generated GM-CSFk/o CART19 cells. Our preliminary data suggest that these CARTs produce significantly less GM-CSF upon activation but continue to exhibit similar production of other cytokines and exhibit normal effector functions in vitro (Fig 1H). Using the NALM6 high tumor burden relapse xenograft model as described above, GM-CSFk/o CART19 cells resulted in slightly enhanced disease control compared to CART19 cells (Fig 1I). Thus, modulating myeloid cell behavior through GM-CSF blockade can help control CART mediated toxicities and may reduce their immunosuppressive features to improve leukemic control. These studies illuminate a novel approach to abrogate NT and CRS through GM-CSF neutralization that also potentially enhances CART cell functions. Based on these results, we have designed a phase II clinical trial using lenzilumab as a modality to prevent CART related toxicities in patients with diffuse large B cell lymphoma. Disclosures Ahmed: Humanigen: Employment. Sahmoud:Humanigen: Employment. Durrant:Humanigen: Employment. Russell:Vyriad: Equity Ownership. Kay:Morpho-sys: Membership on an entity's Board of Directors or advisory committees; Pharmacyclics: Membership on an entity's Board of Directors or advisory committees, Research Funding; Tolero Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding; Gilead: Membership on an entity's Board of Directors or advisory committees; Agios Pharm: Membership on an entity's Board of Directors or advisory committees; Cytomx Therapeutics: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees; Infinity Pharm: Membership on an entity's Board of Directors or advisory committees; Acerta: Research Funding. Kenderian:Novartis: Patents & Royalties; Tolero Pharmaceuticals: Research Funding; Humanigen: Research Funding.

Abstract Despite the remarkable outcomes and recent FDA approval of CD19 directed chimeric antigen receptor T (CART19) cell therapy in B cell malignancies, the durable responses in diffuse large B cell lymphoma are less than 40% and CART activity in chronic lymphocytic leukemia (CLL) is further limited. This is thought to be related to loss of CART persistence, poor trafficking to lymph nodes and inhibition by the leukemic microenvironment. Therefore, strategies to enhance CART cell function to overcome these limitations are needed. Recent studies have shown that abnormal expression of the receptor tyrosine kinase (RTK) AXL is associated with poor prognosis in human cancers. AXL signaling is associated with tumor proliferation, survival, metastasis, and drug resistance. Inhibition of AXL RTK with TP-0903, a high affinity AXL inhibitor has been found to induce robust apoptosis of CLL B cells. Based on the significant modulation of T cell functions observed with BTK inhibitor, we examined the role of AXL RTK inhibition with TP-0903 on T cell function in CLL and other B cell malignancies. First, we investigated the effect of AXL inhibition on T cell phenotype in normal donors. When naïve T cells were stimulated with PMA/Ionomycin and cultured with low dose TP0903, cytokine production was favorably altered through the promotion of Th1 and reduction of Th2 cytokines. This was associated with a significant reduction of inhibitory receptors (Fig 1a). Western blot of T cell lysates suggests low dose TP-0903 results in inhibition of LCK. When effector T cells and regulatory T cells (Treg) were treated with TP-0903 for 3 days, there was a preferential reduction of Treg (Fig 1b). Next, we investigated the influence of TP-0903 on CART19 cell phenotype and functions. Here, we used 41BB costimulated, lentiviral-transduced CART cells. Similar to our findings on naïve T cells, TP-0903 treatment led to polarization of CART cells into a Th1 phenotype when T cells were stimulated with the CD19+ mantle cell lymphoma (MCL) cell line JeKo or with leukemic B cells isolated from CLL patients (Fig 1c). TP-0903 treatment also significantly downregulated inhibitory receptors on activated CART cells, including a reduction of canonical cytokines known to be associated with the development of cytokine release syndrome (CRS) (Fig 1c). The combination of CART19 cells and TP-0903 yielded a synergistic antitumor activity against JeKo in vitro, at low E:T ratios (Fig 1d). Western blot of T cell lysates revealed phosphorylation of LCK was remarkably reduced in the presence of TP-0903, suggesting a mechanism for the observed Th1 polarization. We compared the transcriptome of activated CART cells treated with TP-0903 and more than 100 genes were differentially expressed compared to non-treated cells. Among these genes, immune synapse related genes such as cell junction and cell migration related genes were significantly increased in activated CART cells treated with TP-0903. To investigate the effect of AXL RTK inhibition of CART cells with TP-0903 in vivo, we established MCL xenografts through the injection of 1.0x106 of JeKo into NSG mice. A week after the injection of JeKo, mice were treated with either vehicle alone, TP-0903 (20mg/kg/day) alone, 0.5x106 of CART19 alone, or TP-0903 (20mg/kg/day)+0.5x106 of CART19. Three weeks after the treatment, mice were rechallenged with 1.0x106 of JeKo. Mice treated with CART19 and TP-0903 rejected the JeKo tumor challenge while mice previously treated with CART19 alone redeveloped JeKo, suggesting that AXL inhibition enhanced CART cell persistence (Fig 1e). Finally, we validated our preclinical findings in a correlative analyses of Phase I clinical trial of TP-0903 for patients with solid tumors (NCT02729298). Blood T cells from 3 patients were isolated and analyzed before and a week after treatment with TP-0903. Similar to our findings, there was a significant reduction in Tregs, reduction of inhibitory receptors and polarization to a Th1 phenotype. These findings will be further investigated in a planned Phase I clinical trial of TP-0903 in relapsed/refractory CLL (NCT03572634). In summary, we demonstrated for the first time that AXL inhibitior is capable of polarizing T cells into a Th1 phenotype, downregulates inhibitory receptors, reduces CRS associated cytokines and synergizes with CART cells in B cell malignancies. These findings encourage further study of TP-0903 as an enhancer of T cell immunotherapies. Disclosures Mouritsen: Tolero Pharmaceuticals: Employment. Foulks:Tolero Pharmaceuticals: Employment. Warner:Tolero Pharmaceuticals: Employment. Parikh:Janssen: Research Funding; Abbvie: Honoraria, Research Funding; AstraZeneca: Honoraria, Research Funding; MorphoSys: Research Funding; Pharmacyclics: Honoraria, Research Funding; Gilead: Honoraria. Ding:Merck: Research Funding. Kay:Gilead: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees; Pharmacyclics: Membership on an entity's Board of Directors or advisory committees, Research Funding; Infinity Pharm: Membership on an entity's Board of Directors or advisory committees; Tolero Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding; Acerta: Research Funding; Agios Pharm: Membership on an entity's Board of Directors or advisory committees; Cytomx Therapeutics: Membership on an entity's Board of Directors or advisory committees; Morpho-sys: Membership on an entity's Board of Directors or advisory committees. Kenderian:Tolero Pharmaceuticals: Research Funding; Humanigen: Research Funding; Novartis: Patents & Royalties.

Abstract Introduction: Unprecedented clinical outcomes were reported after CD19 chimeric antigen receptor T cell (CART19) therapy and led to their FDA approval in diffuse large B cell lymphoma and in acute lymphoblastic leukemia. However, the complete response rate in chronic lymphocytic leukemia (CLL) after CART19 therapy is much lower, at approximately 20-30%, and the mechanism(s) for this relative lack of success is unclear. The dominant known mechanism(s) that prevent successful CART cell therapy in CLL have been limited to CART expansion and poor persistence. However, potential mechanisms are not limited to the CLL T-cell. Several immune defects have been identified in CLL that result from the complex bi-directional interaction between B-CLL cells and their microenvironment. In CLL the leukemic microenvironment is rich with extracellular vesicles (EVs) secreted by B-CLL cells. There is growing evidence that these vesicles play an important role in intracellular communication by the delivery of growth factors, genetic material and microenvironmentally relevant molecules. Therefore, we aimed to investigate the role and interactions of EVs in the diminished or absent CART response seen in some CLL patients. Methods: EVs were isolated from peripheral blood of 16 patients with untreated CLL at different Rai stages (8 patients had early and 8 had advanced stage disease) and risk profile by FISH (8 patients had low risk and 8 patients had high risk disease, based on the presence of 17p deletion). Cytometry was used to determine size, number of particles per µl, Annexin V and CD19 expression. These variables were correlated to the Rai stage and risk category of the disease. To investigate the impact of EVs on CART cell functions, CART19 cells were stimulated with either CLL EVs alone or in combination with the CD19 positive cell line JeKo1. After coincubation different effector functions were analysed. Results: Two patterns of EVs in CLL patients were identified; a single versus two distinct EV size populations (small [EVssmall]; 50-240nm, median=110nm) and large [EVslarge]; 180-560nm, median = 360nm Fig 1.A). In 25% of patients, EVs were CD19 positive (EVCD19+). CD19 positivity was detected only in patients with the EVslarge (Fig 1.B). The EVs concentration, CD19 expression (EVsCD19+ vs EVsCD19-), or the size (EVssmall vs EVslarge) did not correlate with disease stage (early vs advanced Rai stage) or risk profile of CLL (low vs high risk) although some variation could be seen (Fig 1.C). To investigate our hypothesis that EVs could modulate CART19 function, CART19 cell effector functions were examined in the presence of EVsCD19+, EVsCD19-, EVssmall, or EVslarge. EVs, 1.5x10e5 particles, alone were insufficient to stimulate CART19 cells. However when CART19 cells were stimulated with the CD19 positive cell line JeKo1, their effector functions were reduced only in the presence of EVsCD19+, 50,000 particles, 2.5 x 10e3/ µl, but not EVsCD19- at the same concentration. This included a significant reduction in CART specific killing (Fig 1.D) and a reduction in cytokine production. The impairment of CART cell functions was independent of the size of EVs, i.e. there was no impairment of CART functions with large or small size EVCD19- in co-culture. Summary: We identify CD19 positive large size EVs from patients with CLL and demonstrate that these EVs play a role in the leukemic microenvironment by reducing CART cell activity. Studies are ongoing to define the mechanism(s). Disclosures Parikh: Janssen: Research Funding; AstraZeneca: Honoraria, Research Funding; Pharmacyclics: Honoraria, Research Funding; Gilead: Honoraria; MorphoSys: Research Funding; Abbvie: Honoraria, Research Funding. Kay:Cytomx Therapeutics: Membership on an entity's Board of Directors or advisory committees; Pharmacyclics: Membership on an entity's Board of Directors or advisory committees, Research Funding; Acerta: Research Funding; Infinity Pharm: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees; Agios Pharm: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Morpho-sys: Membership on an entity's Board of Directors or advisory committees; Tolero Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding; Gilead: Membership on an entity's Board of Directors or advisory committees. Kenderian:Tolero Pharmaceuticals: Research Funding; Humanigen: Research Funding; Novartis: Patents & Royalties.