Boysen, Justin C.

Abstract Abstract 2414 Background: Mesenchymal stromal cells (MSC) derived from normal subjects are known to have immunosuppressive capacity by virtue of inhibiting T- and B-cell activation. A novel subset of T cells, Th17, plays an important role in inflammation and autoimmunity. A recent report demonstrated that normal MSC ameliorates experimental autoimmune encephalomyelitis by inhibiting CD4+ Th17 cells in a chemokine ligand 2-dependent manner (J Immunol. 2009, 182: 5994). It remains unknown if MSC derived from leukemic or cancer patients play a role in Th17 cell differentiation. In particular this would be of interest to study in B-Chronic Lymphocytic Leukemia (CLL) where immunosuppression is evident even in early stage disease. Methods: MSC derived from bone marrow of CLL patients or normal subjects were expanded in vitro as previously described by us (Br J Haematol. 2009, 147:471). CD4+ cells positively selected from normal peripheral blood mononuclear cells were co-cultured with either CLL MSC or normal MSC at a ratio of 50:1 for 3 days with stimulation via CD3/CD28 beads, as well as interleukin-1β (IL-1β; 50 ng/ml). Then phorbol 12-myristate 13-acetate (50 ng/ml) and ionomycin (500 ng/ml) were introduced into the co-culture for 5 hrs in the presence of brefeldin A. Subsequently, cells were stained with CD4-phycoerythrin (PE) and IL-17-Alexa647 using intracellular flow to analyze the percent expression of IL-17 in CD4 + cells. Cytokine production from both CLL MSC and normal MSC as secreted into culture medium (CM) were tested using a commercial multiplex cytokine array (Invitrogen, CA). This array measures the level of 30 different cytokines. Results: Positively selected CD4+ cells from peripheral blood of normal donors contain minimal percentages of Th17 cells (range: 0.48–0.71%). IL-1β stimulation induced increased IL-17 expression (range: 1.05–1.12%). Co-culture of CLL MSC with CD4+ cells induced significantly increased IL-17 expression in the CD4+ T cells (range: 1.16–1.32%). The promoting effect of CLL MSC on these Th17 cells appeared to be mediated by direct contact since the CM of CLL MSC was not able to induce increased IL-17 expression (mean = 0.66%) to a similar level as direct co-culture. When IL-1b was used to stimulate Th17 cell differentiation from CD4+ cells, CLL MSC were able to further promote the level of Th17 cell differentiation (range: 2.01–2.63%), indicating synergistic function for CLL MSC with IL-1β. This latter finding again appeared to be more pronounced for CLL MSC as normal MSC with IL-1β was not able to promote Th17 cell differentiation to a similar degree. To further investigate the mechanism of CLL MSC on Th17 cell differentiation, we assessed the cytokine production for resting CLL MSC and normal MSC compared to cytokine production of CLL and normal MSC stimulated with IL-17. The data from multiplex cytokine arrays revealed that the cytokine profiles were not different between resting CLL and normal MSC; however, when MSC were stimulated with IL-17, there were significant differences between CLL and normal MSC in terms of IL-6 and MCP-1 production (IL-6, CLL vs. normal, 957.9 ± 98 vs. 554.2 ± 92.3 pg/ml, p = 0.01; MCP-1, CLL vs. normal, 787.7 ± 166.9 vs. 330.2 ± 116.5 pg/ml, p = 0.04, n = 7). Since both IL-6 and MCP-1 have been demonstrated to play important roles in Th17 differentiation, we are conducting further studies to dissect the mechanism of CLL MSC in the promotion of Th17 cell differentiation. Conclusions: These results indicate that MSC derived from CLL patients promotes Th17 cell differentiation in vitro, which is in contrast to the previous published suppressive role of normal MSC on Th17 cell differentiation. Recent findings have indeed demonstrated that CLL patients do have high percentage of Th17 cells (Cancer Res. 2009. 69: 5922) when compared to other lymphoproliferative diseases. Given this data we believe that CLL MSC are intrinsically different from normal MSC in terms of immune regulation and cytokine production. This may occur as a result of the bi-directional activation that we found to be present between MSC and CLL leukemic cells (Br. J Haematol. 2009. 147:471). In total, our findings demonstrated that the dynamic interactions between the CLL leukemic cells and MSC appear to influence the Th 17 cell levels. This is of biological and clinical interest in that Th17 cells have the potential to regulate the immune environment to favor tumor proliferation and progression. Disclosures: No relevant conflicts of interest to declare.

Abstract B-cell chronic lymphocytic leukemia (CLL) is an incurable disease and represents a significant health problem in the western world. We and others have reported that primary CLL B-cells spontaneously produce increased levels of proangiogenic basic fibroblast growth factor (bFGF) in vitro and that most CLL plasma contains elevated levels of bFGF. However, the precise role of bFGF in CLL pathobiology is not clearly understood. In this study we investigated the functional implication of the FGF/FGF receptor (FGFR) signaling axis in CLL B-cell biology. We have detected expression of FGFR1 and FGFR3 with comparatively higher levels of the latter receptor tyrosine kinase (RTK), but no or notably low levels of FGFR2/FGFR4, by flow cytometry and Western blot analyses in primary CLL B-cells. This observation was further supported by detection of FGFR1/FGFR3 transcripts in CLL B-cells by semi-quantitative reverse transcriptase polymerase chain reaction. Although both FGFR1 and FGFR3 in CLL B-cells remain as constitutively phosphorylated, we found significantly higher levels of phosphorylation on FGFR3 and thus this latter receptor is likely the predominant RTK of the FGFR family in these leukemic B-cells. Of note, in vitro stimulation of FGFRs with recombinant bFGF was unable to increase total phosphorylation on FGFRs from their constitutive basal levels in CLL B-cells. Further analysis using a bFGF neutralizing antibody suggested that FGFR phosphorylation in CLL B-cells is likely independent of bFGF ligation. We then interrogated the mechanism of how FGFRs were being phosphorylated and/or maintained at the observed constitutive levels of phosphorylation in CLL B-cells. Our previous studies established that Axl is a critical RTK in CLL B-cells since it acts as a docking site for multiple cellular kinases/lipase, an observation supported by earlier literatures in human malignancies. Given this, Axl is likely capable of cross talk with other RTKs including FGFRs to regulate FGFR-signaling in CLL B-cells. Therefore, in an effort to determine whether Axl is functionally associated with FGFR, we examined if these two RTKs exist in the same molecular complex in CLL B-cells. Indeed, immunoprecipitation assays demonstrated that Axl formed a complex with FGFR3 in CLL B-cells, suggesting that Axl is likely functionally linked to the FGFR signaling. In this regard we found that Axl inhibition, using a high-affinity Axl inhibitor (TP-0903; Tolero Pharmaceuticals), resulted in significant reduction of total FGFR phosphorylation in CLL B-cells. Additionally, siRNA-mediated partial depletion of Axl in CLL B-cells reduced total FGFR phosphorylation. In contrast, inhibition of FGFR phosphorylation using a high-affinity FGFR inhibitor could not alter phosphorylation levels on Axl RTK in CLL B-cells. Together, these findings suggest that Axl has a dominant role in the regulation of FGFR signaling in CLL B-cells. To find out if inhibition of FGFR can induce apoptosis in CLL B-cells we used a specific inhibitor for FGFR (TKI-258; Novartis) to treat CLL B-cells. Here we found a substantial level of apoptosis induction in the leukemic B-cells with a mean LD50 dose of ~2.5 μM. Interestingly, Axl inhibition by TP-0903 induced a robust level of apoptosis in CLL B-cells in the nanomolar dose range with a mean LD50 dose of 0.14 mM. Thus Axl inhibition exerts a very robust cytotoxic effect on CLL B-cell survival likely targeting both Axl and FGFR signaling pathways via Axl inhibition. In conclusion, we have detected expression of constitutively active FGFR1 and 3 in primary CLL B-cells and that inhibition of FGFR signaling induces considerable levels of CLL B-cell apoptosis albeit lower than that observed on Axl RTK inhibition. Interestingly, our findings here suggest that Axl forms an active RTK complex with FGFR and that Axl inhibition modifies FGFR phosphorylation levels. Thus it is likely that Axl RTK can regulate FGFR signaling in the CLL B-cells. In total these observations suggest that the finding of robust induction of apoptosis in CLL B-cells is as a result of targeting two signaling pathways with Axl inhibition: Axl and FGFR. These studies further support investigation of Axl inhibition as a way to develop a more effective and efficient therapeutic intervention for CLL patients. Disclosures Warner: Tolero Pharmaceuticals: Employment, Equity Ownership, Patents & Royalties. Kay:Genetech: Research Funding; Pharmacyclics: Research Funding; Hospira: Research Funding; Celgene: Membership on an entity's Board of Directors or advisory committees; Gilead: Membership on an entity's Board of Directors or advisory committees.

Abstract Background: A challenge for novel therapeutic strategies will be the fine tuning of intracellular reactive oxygen species (ROS) signaling in order to effectively deprive cells from ROS-induced tumor promoting events and subsequent tipping the balance to ROS-induced apoptotic signaling. ROS plays a critical role in regulation of the pro-survival receptor tyrosine kinase (RTK) signaling pathways in human cancers. Studies have identified mitochondrial metabolism as the key source for abundant ROS in chronic lymphocytic leukemia (CLL). Unlike in other malignant cells, increased oxidative phosphorylation but not increased aerobic glycolysis has been found in CLL B-cells. While we have detected constitutively phosphorylated/active RTKs like Axl in CLL B-cells, the mechanism of activation remains largely undefined. Here we report that (i) elevated ROS activates the Axl signaling pathway in CLL B-cells and, (ii) mechanism of increased ROS accumulation in the leukemic B-cells. Methods: CLL B- and normal B-cells were purified from peripheral blood of previously untreated CLL patients and normal healthy individuals, respectively using a RossetteSep B-Cell enrichment kit. CLL B-cells were exposed to H2O2 for 5 min and cell lysates were analyzed for activation of RTKs including Axl by immunoprecipitation/Western blots. We also examined expression status of SIRT3, acetylated-superoxide dismutase (SOD)2 and catalase in CLL B-cells by Western blots. Catalase mRNA levels in CLL B-/normal B-cells were determined by qRT-PCR. In some experiments, genomic DNA was isolated from CLL B-/normal B-cells for catalase promoter methylation studies. Finally, accumulation of O2‾ and H2O2 in CLL B- and normal B-cells were measured by flow cytometry after treating the cells withdihydroethidium (DHE) and dichlorohydrofluorescein diacetate (DCFDA), respectively. Results: We found that enforced induction of ROS significantly increases tyrosine phosphorylation levels on multiple cellular proteins in CLL B-cells (Fig. 1). Further analysis finds that increased ROS activates Axl and its downstream targets AKT/Erk1/2 MAPK and the fibroblast growth factor receptor (FGFR) which we recently defined as a downstream target of Axl; while ROS accumulation did not show any significant effect on other RTKs, cMET or IGFR1, in CLL B-cells. Interestingly, the histone deacetylase SIRT3 which activates mitochondrial SOD2 via deacetylation, we found, was overexpressed in CLL B-cells as compared to normal B-cells indicating more efficient conversion of O2‾ into H2O2 in the leukemic B-cells (Fig. 2). However expression of catalase, which converts H2O2 into O2 and H2O, was reduced significantly in CLL B-cells as compared to normal B-cells both at mRNA and protein levels. To delineate the mechanism of reduced expression of catalase in CLL B-cells we studied the human catalase promoter region for potential methylation sites using the method of HpaII restriction enzyme digestion where HpaII is unable to cut DNA (CCGG site) when the internal cytosine is methylated. Initial findings from this approach suggest that while the catalase gene promoter in CLL B-cells may contain methylated cytosines, the catalase gene in normal B-cells may not confer methylation in the promoter region. Next, genomic DNA from CLL B-cells (n=10) and normal B-cells (n=6) was subjected to bisulfite treatment, followed by PCR amplification and sequencing. This approach revealed no significant promoter methylation of the catalase gene in normal B-cells, while moderate to heavily methylated promoter of the catalase gene was detected in CLL B-cells from majority of CLL patients (9 of 10). Further studies are in progress to examine catalase promoter methylation status in a larger cohort of CLL patients and its association with H2O2 accumulation, RTK phosphorylation and risk-factors. Indeed, flow cytometric analysis finds lower levels of O2‾ but accumulation of H2O2 in CLL B-cells as compared to normal B-cells. Taken together, these findings suggest that although SOD2 remains highly active, increased accumulation of H2O2 may occur in CLL B-cells due to epigenetic silencing of the catalase gene. Conclusion: These observations may explain, at least in part, the expression of constitutively active Axl signaling pathway in CLL B-cells that plays a role in CLL B cell survival and may account for the prolonged survival and/or apoptotic resistance of the leukemic B-cells. Disclosures No relevant conflicts of interest to declare.

Abstract Introduction: The use of B-cell receptor (BCR) signal inhibitors-based therapies (e.g., Ibrutinib) for B-chronic lymphocytic leukemia (CLL) was initiated just a few years ago but has rapidly escalated due to their clinical efficacy and relative ease of use. However newer therapeutic approaches are needed due to multiple issues including the continued need to improve complete responses and reduce toxicity profiles. To that end our group has discovered a novel membrane target in the ubiquitous presence of Axl receptor tyrosine kinase (Axl RTK) on CLL B-cells and has reported that the Axl RTK inhibitor TP-0903 is able to induce apoptosis of CLL B-cells at nanomolar doses (Sinha, Clin Cancer Res, 2015). Given this we assessed if TP-0903 would be effective in the induction of apoptosis of leukemic B-cells from CLL patients who are currently on Ibrutinib therapy or whom have relapsed while on Ibrutinib treatment. Methods: Relapsed/refractory CLL patients (n=22) who were placed on Ibrutinib for progressive disease provided blood samples at a median of 3.2 months after Ibrutinib therapy initiation for these studies. We also obtained sequential samples on 8 patients from initial start of ibrutinib therapy and then over a 6 month follow-up period. CLL B-cells from these blood samples were subject to Ficoll separation, purified by using a Rosette Sep B-cell enrichment kit and then studied by flow cytometry to determine Axl RTK expression levels by flow cytometric analysis. Purified CLL B-cells (CD19+/CD5+) were cultured with TP-0903 in vitroat increasing doses (0.01µM - 0.50µM) for 24 hours and the LD50 dose was determined. In addition, 3 CLL patients who had been on Ibrutinib therapy and had a documented relapse were studied in similar fashion using TP-0903. LD50-sensitivity was measured. "LD50-sensitivity" was defined as an LD50 ≤0.50µM and "insensitive" was defined as an LD50 dose >0.50µM. CLL prognostic factors (e.g., FISH, IGHV mutation status, Rai stage, CD38, and CD49d) were evaluated at the time of ibrutinib treatment. Differences in factors between sensitive and insensitive cases were computed using the Kruskal-Wallis test for continuous variables and Chi-square test for categorical variables. Results: Twenty-two CLL patients (5 female, 17 male) were included in the analysis. Fourteen (64%) patients were found to be TP-0903 LD50-sensitive. Axl expression on CLL B-cells for this cohort was heterogeneous with a median of CD19+/CD5+ cells positive for Axl at 69.9% (range of 2.7-91.3%). The sensitive subjects tended to be younger with a median age at Ibrutinib treatment initiation of 62 vs 75.5 years (p=0.004). There were no significant differences in gender, FISH, IGHV mutation status, CD38, CD49d, or Rai stage between the sensitive and insensitive LD50 groups. There were no significant differences in relation to median Axl expression on CLL B-cells (sensitive: 72.6%, range: 2.7-91.3%; insensitive: 41.5%, range: 16.5-83.1%; p=0.35). The median number of treatments prior to initiation of ibrutinib did not differ between sensitivity groups (sensitive: 2.53, range: 8-10; insensitive: 43.5, range 12-20; p=0.2833). Association for ZAP70+ CLL B-cells tended to have more apoptosis induction by TP-0903 (sensitive: 84.6% ZAP70+; insensitive: 42.9% ZAP70+; p=0.052). In 8 CLL patients that were studied sequentially while on Ibrutinib continued to express Axl or increased their Axl expression (n=2) over a 3-6 month follow-up period. Three CLL patients who had relapsed on Ibrutinib were sensitive to TP-0903 with LD50 values of ≤0.50µM. Summary: Here we find that CLL B-cells from over 60% of relapsed CLL patients on Ibrutinib therapy were highly sensitive to the high-affinity Axl inhibitor TP-0903 with induction of apoptosis at nanomolar doses (≤0.50µM). The sensitivity of CLL B-cells to TP-0903 appears to be independent of Axl expression levels and of the known CLL prognostic factors but more evident for younger patients and for ZAP70+ expression status. Given this level of activity for apoptosis induction of CLL B-cells by TP-0903 encourages the further testing of this drug in clinical trials for CLL patients. Disclosures Parikh: Pharmacyclics: Honoraria, Research Funding. Shanafelt:Pharmacyclics: Research Funding; Janssen: Research Funding; Genentech: Research Funding; GlaxoSmithKline: Research Funding; Celgene: Research Funding; Cephalon: Research Funding; Hospira: Research Funding. Warner:Tolero Pharmaceuticals: Employment, Equity Ownership, Patents & Royalties. Bearss:Tolero Pharmaceuticals: Employment, Equity Ownership, Patents & Royalties. Kay:Pharmacyclics: Research Funding; Tolero Pharmaceuticals: Research Funding; Acerta: Research Funding; Gilead: Membership on an entity's Board of Directors or advisory committees; Morpho-Sys: Membership on an entity's Board of Directors or advisory committees; Infinity Pharm: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees.

Abstract Introduction: B-cell chronic lymphocytic leukemia (CLL) is an incurable disease despite that still requires options for treatment despite recent advances with BTK therapy. We previously found that Axl receptor tyrosine kinase (RTK), a member of the TAM (Tyro3, Axl, MER) family of RTKs, plays a critical role in CLL B-cell survival (Blood 2011, 117:1928). Here, we explored the levels of apoptosis induction of CLL B-cells using a novel high-affinity Axl inhibitor as a single agent or in combination with BTK inhibitors. Methods: Expression/activation status of the Tyro3, Axl and MER in CLL B-cells was evaluated by Immunoprecipitation/Western blots. Cells were treated with a high-affinity orally bioavailable Axl inhibitor TP-0903 (Tolero Pharmaceuticals) or FGFR inhibitor TKI-258 (Novartis) with or without the presence of CLL bone marrow stromal cells (BMSCs). Impact of Axl or FGFR signal inhibition on CLL B-cell survival was also evaluated by flow cytometry after staining with annexin V/propidium iodide. Finally, cells were exposed to TP-0903 in combination with BTK inhibitors to determine any synergistic/additive effects of the combination in induction of apoptosis. Combination index (CI) of two agents was calculated using the method of Chou and Talalay, 1984. Results: Although we defined pro-survival role of Axl in CLL B-cells, contribution of other members of the TAM family of RTKs including Tyro3 and MER to prolonged survival of the leukemic B-cells remains largely undefined. In this study we found that CLL B-cells overexpress Tyro3, but not MER, and that Tyro3 is constitutively phosphorylated. Interestingly, immunoprecipitation of Tyro3 from CLL B-cell lysates shows retention of highly active Axl in the immune complex suggesting that these two RTKs either heterodimerize or form a molecular complex in CLL B-cells. Our earlier work found that Axl regulates multiple cell survival signaling pathways including PI3K/AKT and Src and that, Axl inhibition using a first generation Axl inhibitor induced apoptosis in CLL B-cells (Blood 2011, 117:1928). Thus, we hypothesized that targeting Axl using a very high affinity Axl inhibitor would induce even higher levels of apoptosis in CLL B-cells compared to the first generation Axl inhibitor. To address this, we utilized a high-affinity orally bioavailable Axl inhibitor TP-0903 to generate preliminary information on its efficiency to specifically target Axl and the level of apoptosis induction in CLL B-cells. Indeed TP-0903 as a single agent induced a robust apoptotic cell death in CLL B-cells within 24 hours of exposure at nanomolar doses, which are easily achievable in vivo (information provided by Tolero Pharmaceuticals). Interestingly, while TP-0903 effectively reduces phosphorylation on Axl in CLL B-cells, it was unable to target P-Tyro3 suggesting that Axl is likely the predominant RTK of the TAM family in CLL B-cells. A marked reduction of the anti-apoptotic proteins Mcl-1, Bcl-2, XIAP and upregulation of the pro-apoptotic protein BIM in CLL B-cells via activation of its upstream regulator FOXO3a were noted as a result of inhibition of P-Axl and its downstream signaling pathways, P-AKT/P-Src. Importantly, TP-0903 was able to overcome CLL BMSC mediated protection of the leukemic B-cells from apoptosis induction. Given the observed robust induction of apoptosis with TP-0903 and the known impact of BTK inhibitors on CLL patient clinical outcome, we then exposed CLL B-cells to TP-0903 in combination with two BTK inhibitors that included Ibrutinib and a reversible BTK inhibitor TP-4216 (a kind gift from Tolero Pharmaceuticals). Based on the CI values, we found that the in vitro combination of TP-0903 and Ibrutinib showed additive effects in CLL B-cells from 6 of 11 CLL patients, CLL B-cells from the remaining 5 CLL patients showed antagonistic effects (Fig. 1A). However, we observed synergistic effects of TP-0903 in combination with TP-4216 in the induction of apoptosis in CLL B-cells from 2 of 10 patients and additive effects in CLL B-cells from 6 patients (Fig. 1B). Cells from the remaining 2 patients exhibited a slightly antagonisticeffect. Conclusion: In total, these results suggest that TP-0903 is quite effective in inducing robust CLL B-cell apoptosis as a single agent even at a nanomolar concentration and its combination with the reversible BTK inhibitor TP-4216 could be an effective and unique strategy for the treatment of CLL patients. Figure 1 Figure 1. Disclosures Warner: Tolero Pharmaceuticals: Employment, Equity Ownership, Patents & Royalties. Bearss:Tolero Pharmaceuticals: Employment. Shanafelt:Genentech: Research Funding; GlaxoSmithKline: Research Funding; Celegene: Research Funding; Cephalon: Research Funding; Pharmacyclics/Jannsen: Research Funding; Hospira: Research Funding; Polyphenon E Int'l: Research Funding. Kay:Genetech: Research Funding; Pharmacyclics: Research Funding; Hospira: Research Funding; Celgene: Membership on an entity's Board of Directors or advisory committees; Gilead: Membership on an entity's Board of Directors or advisory committees.

Abstract BACKGROUND. The detection of minimal residual disease (MRD) at the level of 0.01%/10-4 or above is a strong independent predictor of reduced progression-free (PFS) and overall survival (OS) in patients with CLL treated with chemoimmunotherapy. Although newer agents such as B-cell receptor pathway inhibitors can result in prolonged survival without achieving complete response, there remains a important role for MRD analysis in assessing therapeutic strategies aimed at disease eradication and cure. This is particularly important in front-line trials for fit patients which now require at least five years of follow-up if PFS is used as an endpoint. The feasibility of using MRD as a surrogate or intermediate endpoint for accelerated approval of new treatments is under review by regulatory agencies but further prospective validation is required. At the same time technology is rapidly evolving and high-throughput sequencing (HTS) technologies now detect MRD at the 0.0001%/10-6 level. It is therefore important to determine the most effective approaches for quantifying MRD that are compatible with previous studies but sufficiently sensitive for current treatments. AIMS. This collaborative project had two objectives. First, to identify the simplest and most flexible flow cytometry panel capable of detecting MRD at the 0.01%/10-4 or lower, that is compatible with published data and independent of instrument/reagent manufacturer. Second, to compare the flow cytometry approach with HTS analysis using the ClonoSEQ assay (Adaptive Biotechnologies, Seattle, WA). METHODS AND RESULTS. A core panel of antibodies for MRD detection was identified by testing an 8-marker combination in 52 samples (27 post-treatment and 25 dilution study) and re-analysing data with serial exclusion of single markers to determine redundancy. A 1-tube core panel of CD19, CD20, CD5, CD43, CD79b, and CD81 was identified and validated against the previously published 2-tube 6-marker and 4-tube 4-marker ERIC panels in 76 samples (19 post-treatment and 57 dilution study). The results showed good concordance (for log-transformed data above the LoQ, linearity=0.977, Pearson correlation co-efficient=0.983, average difference=0.026 log, 95% limit of agreement 0.20log) and a limit of detection of 0.001%/10-5 for the 1-tube core panel. Inter-operator variation was similar to CML MRD monitoring with both experienced operators, or inexperienced cytometrists after ~1 hour of specific education, achieving a 95% limit of agreement less than 0.3log in samples with MRD levels ranging from 0.0001 – 100%. The flow cytometry approach was compared with the ClonoSEQ HTS assay in 109 samples (21 dilution study and 88 post-treatment samples, complete data currently available on 13/88). The assay was applicable to the vast majority CLL patients, often with two clonal markers. There was 94% concordance at the 0.01% (10-4) threshold (15 samples with ≥0.01% CLL by both methods, 14 samples with <0.01% by both methods, 1 sample with 0.03% CLL by HTS and <0.003% CLL by flow cytometry, and 1 sample with 0.005% CLL by HTS and 0.012% by flow cytometry. HTS detected CLL IGH sequences in 22% (7/31) samples with no detectable CLL cells by flow cytometry (i.e. CLL level 0.0001-0.001%, 3/13 patient samples and 4/18 dilution samples). HTS demonstrated a relatively high variability in quantification, as seen in previous studies, but with a clear superiority in the limit of detection and good linearity (linearity=0.905, Pearson correlation co-efficient=0.870, average difference=0.078 log, 95% limit of agreement 1.5 log). CONCLUSIONS. The 1-tube 6-marker flow cytometry core panel is compatible with published studies, manufacturer-independent and flexible, providing directly quantitative results to 0.001%/10-5 without requiring a pre-treatment sample. HTS requires further work to standardise the quantitative analysis and prospective validation but the ClonoSEQ assay is applicable to >95% of CLL patients, does not require viable cells and is extremely sensitive, detecting residual disease in a significant proportion of cases with <0.01% CLL. The results indicate that flow cytometry and HTS are complementary technologies with a combined approach offering the most reliable way of quantifying CLL at the 0.01%/10-4 threshold while allowing higher sensitivity in clinical trials aimed at disease eradication. Disclosures Rawstron: Roche: Honoraria; Biogen Idec: Consultancy; Gilead: Consultancy, Honoraria; Abbvie: Honoraria; BD Biosciences: Intrasure reagent Patents & Royalties; Celgene: Honoraria; GSK: Honoraria. Williamson:Adaptive Biotechnologies: Employment, Equity Ownership. Sanders:Adaptive Biotechnologies: Employment, Equity Ownership. Robins:Adaptive Biotechnologies: Consultancy, Equity Ownership, Patents & Royalties. Hallek:Celgene: Honoraria, Research Funding; Mundipharma: Honoraria, Research Funding; Roche: Honoraria, Research Funding; Janssen: Honoraria, Research Funding; GSK: Honoraria; Gilead: Honoraria. Hillmen:Roche: Honoraria, Research Funding; GSK: Honoraria, Research Funding; Janssen: Honoraria, Research Funding; Pharmacyclics: Honoraria, Research Funding; Gilead: Honoraria, Research Funding.

Abstract Introduction: We demonstrated that elevated levels ofcirculating microvesicles (MV) in CLL activate CLL bone marrow stromal cells (BMSCs) with resultant enhanced vascular endothelial growth factor (VEGF) production, a pro-survival factor for CLL B-cells (Ghosh 2009 Blood 113:5568-74). We also detected a phenotypic shift from predominantly platelet/megakaryocyte-derived MV (CD61+) in early stages towards a more leukemic B-cell derived MV (CD19+) in advanced Rai stages (Ghosh 2010 Blood 115:1755-64). These observations suggest that the MV parameters of their levels and surface phenotypes may predict or be associated with the clinical outcome and/or therapeutic response in a given CLL patient. In this study we therefore designed an approach to being to assess these latter possibilities in regard to MV in CLL. Methods: In vitro stimulation – Purified CLL B-cells were stimulated in vitro using CpG oligonucleotide (n=10) or anti-IgM antibody (n=10) for 72 hours. MV were isolated from the used media, levels measured and then phenotyped by flow cytometry (BD Canto-I) using standard TRUCount beads (BD Biosciences). In vivo (plasma) work – MV were isolated from normal plasma (n=5) or plasma of 17 CLL patients entered onto a previously reported Pentostatin, Cyclophosphamide, Ofatumumab (PCO) clinical trial at their entry to the trial and at 6-months after therapy and then sequentially over a period of 18 months (Shanafelt 2013 Cancer 119:3788-96). We determined the phenotypes and plasma levels of MV as described above. Results Phenotypes of in vitro generated MV: We found that, for the most part, CLL B-cells shed MV upon in vitro stimulation however; some CLL B-cells of unmutated IGHV status, but not mutated status, spontaneously generated MV in culture (Fig. 1). Most interestingly, we found that the surface phenotype of MV generated from CLL B-cells carrying predominantly the CD52 surface marker, not the CD19 or CD5 marker. Interestingly, none of the in vitro generated MV was CD20+, another known marker of CLL B-cells. We found similar phenotypes of the in vitro generated MV by stimulating two different signaling pathways, Toll-like receptor and BCR, in CLL B-cells. Together these results suggest that CLL B-cells generate predominantly CD52+ MV, but not CD19 or CD5, upon in vitro stimulation. MV phenotypes in pre- and post-therapy CLL plasma: Here we focused on the kinetics of MV levels and phenotype in relation to disease outcome for PCO treated CLL. Thus, we found total MV levels were elevated in all patients compared to normal plasma at entry to the PCO trial and then dropped substantially after therapy when compared to the trial entry. Also most CLL patients (n=11) continued to maintain low plasma MV levels until the end of the follow up period (18 months). Interestingly, in a few cases (n=6) total plasma MV levels decreased initially after therapy but then started to rise. These patients are now being followed for disease status in order to determine if rising MV levels are related to a disease relapse. Of interest on flow phenotyping the plasma MV obtained at entry to the PCO trial we detected predominantly CD52+ MV but not CD19+ MV, similar to the findings of the in vitro MV generated from CLL B-cells. Normal plasma (n=5) does not contain significant levels CD52+ MV or CD19+ MV. Together these results suggest that the malignant B-cells shed preferentially CD52+ MV, but not CD19+ MV. We are currently analyzing the MV levels and phenotypes pre- and post-therapy response levels in relation to the patients’ response to therapy in a larger cohort of treated CLL patients (n=60). Conclusion: Current understanding about the phenotypes of MV is based on the concept that MV carry the unique surface marker of the originator cells, e.g., CD19 on B-cell derived MV. However, findings from our in vitro stimulation studies demonstrate that the CD52 but not CD19 is preferentially expressed on CLL B-cell derived MV. We are currently studying the mechanism for such a preferential surface expression on these leukemic derived MV. To our knowledge, this is the first report describing that CLL B-cells generate preferentially CD52+ MV. Our studies are now committed to a more extensive analysis of MV parameters in association with CLL disease progression, depth of clinical response and if the elevation of plasma MV heralds a relapse in responding patients. Figure 1 Figure 1. Disclosures Shanafelt: Genentech: Research Funding; GlaxoSmithKline: Research Funding; Celegene: Research Funding; Cephalon: Research Funding; Pharmacyclics/Jannsen: Research Funding; Hospira: Research Funding; Polyphenon E Int'l: Research Funding. Kay:Genetech: Research Funding; Pharmacyclics: Research Funding; Hospira: Research Funding; Celgene: Membership on an entity's Board of Directors or advisory committees; Gilead: Membership on an entity's Board of Directors or advisory committees.

Background:The expression of the receptor tyrosine kinase (RTK) Axl on cancer cells has been associated with poor prognosis and implicated in disease progression. Inhibition of Axl‐RTK with TP‐0903, a high affinity Axl‐RTK inhibitor induces robust apoptosis of several tumors, including chronic lymphocytic leukemia (CLL). Axl‐RTK inhibition has also been found to induce nonspecific immunomodulatory effects in syngeneic tumor models.Aims:We aimed to investigate the impact and downstream effects of Axl‐RTK inhibition on naïve T cells and chimeric antigen receptor T cells (CART), using CD19 directed CART cells (CART19) as a model.Methods:We utilized either PMA/Ionomycin or the CD19+ mantle cell lymphoma cell line, JeKo as a stimulant for naïve T cells and CART19, respectively. Cells were analyzed for cytokine profile, T cell phenotype, and kinetics after the stimulation in the presence or the absence of the Axl‐RTK inhibitor TP‐0903. Regarding in vivo experiment, a relapse JeKo xenograft model was used. Mice were treated with vehicle alone, TP‐0903 alone, CART19 alone, or TP‐0903 + CART19. Three weeks after the treatment, mice were re‐challenged with JeKo as a CART19 relapse model.Results:Stimulation of naïve T cells in the presence of the Axl‐RTK inhibitor TP‐0903 resulted in a significant reduction of Th2 cytokines and downregulation of inhibitory receptors. The differential expansion of either effector T cells or Tregs in the presence of TP‐0903 resulted in a preferential reduction of Tregs (Fig 1A). Axl‐RTK inhibition of CART19 led to Th1 polarization upon stimulation with the JeKo, as evident by the selective reduction of Th2 cytokine. Additionally, this resulted in a reduction of myeloid attracting cytokines, such as CXCL1, IL‐8, IL‐6, and IL‐10, cytokines implicated in the development cytokine release syndrome and neurotoxicity after CART cell therapy and in adaptive resistance to immunotherapy (Fig 1B). TP‐0903 also significantly downregulated inhibitory receptors on activated CARTs (Fig 1C). In our relapsed in vivo model, mice treated with CART19 + TP‐0903 rejected the JeKo challenge while mice received CART19 alone redeveloped disease, suggesting that Axl‐RTK inhibition enhanced CART cell persistence. Peripheral blood analysis of mice after the rechallenge showed a higher number of circulating T cells and lower level of MCP‐1. Western blotting was performed to investigate the downstream effect of Axl‐RTK inhibition on CART19 and it revealed a relatively selective inhibition of LCK on stimulated CART19. Transcriptome analysis of activated CARTs treated with TP‐0903 revealed >100 genes that were differentially expressed compared to untreated cells. Among these, Th cell and Treg polarization genes were significantly altered in activated CARTs treated with TP‐0903.imageFinally, we validated our findings in phase I clinical trial of TP‐0903 (NCT02729298). T cells isolated from 3 cancer patients showed a significant reduction of Tregs and Th1 polarization. This will be further investigated in a planned phase I clinical trial of TP‐0903 in relapsed/refractory CLL (NCT03572634).Summary/Conclusion:We showed for the first time that Axl‐RTK inhibition polarizes T cells into a Th1 functional phenotype, downregulates inhibitory receptors, and reduces myeloid associated cytokine production by CART19. The combination therapy yielded synergistic cytotoxicity against JeKo in vitro and in a CART19 relapse model. These findings encourage further study of Axl‐RTK inhibition with TP‐0903 as an enhancer of T cell immunotherapies.

Despite the remarkable outcomes 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 strategies to enhance this response are desperately needed. Inhibition of AXL RTK with TP0903, a high-affinity AXL inhibitor has been found to induce robust apoptosis of malignant B cells. Here, we aimed to examine the role of AXL RTK inhibition with TP0903 on T-cell function in B-cell malignancies. First, we investigated the influence of TP0903 on CART19 cell phenotype and functions. Here, we used 41BB costimulated, lentiviral-transduced CART cells. AXL inhibition 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 patients with chronic lymphocytic leukemia (CLL), in the presence of TP0903 (Fig 1A). Exposure of activated CART cells to TP0903 also resulted in significant downregulation of inhibitory receptors on activated CART cells (Fig 1B), a reduction of conical cytokines known to be associated with the development of cytokine release syndrome (CRS) (Fig 1C). To investigate the effect of AXL RTK inhibition of CART cells with TP0903 in vivo, we established MCL xenografts through the injection of 1.0x106 of Jeko into NSG mice. A week later, mice were treated with either vehicle alone, TP0903 (20 mg/kg/day) alone, 0.5x106 of CART19 alone, or TP0903 (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 TP0903 rejected the tumor challenge while mice previously treated with CART19 alone redeveloped MCL, suggesting that AXL inhibition enhanced CART cell persistence (Fig 1D). Finally, we validated our preclinical findings in correlative analyses of phase 1 clinical trial of TP0903 in patients with solid tumors (NCT02729298). 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 1 clinical trial of TP0903 in relapsed/refractory CLL (NCT03572634) In summary, we demonstrated for the first time that AXL inhibition polarizes T cells into a Th1 phenotype, downregulates inhibitory receptors, and synergizes with CART cells in B-cell malignancies. These findings encourage further study of TP0903 as an enhancer of T-cell immunotherapies.

Background: Unprecedented clinical outcomes were reported after CD19 chimeric antigen receptor T-cell (CART19) therapy. However, the complete response rate in chronic lymphocytic leukemia (CLL) is much lower, at approximately 20%–30%. Several immune defects have been identified in CLL that result from the complex interaction between CLL cells and the microenvironment. This leukemic microenvironment is rich with extracellular vesicles (EVs) secreted by B-CLL cells. Here, we aimed to investigate the role of EVs play in the diminished CART response seen in some CLL patients. Methods: EVs were isolated from peripheral blood of 16 patients with untreated CLL at different Rai stages and risk profile by FISH. Cytometry was used to determine size, number of particles per μl, and CD19 expression. To investigate the impact of EVs on CAR T-cell functions, CART19 cells were stimulated with either CLL B cells or the CD19-positive cell line JEKO and different effector functions were analyzed. Results: Two patterns of EVs in CLL patients were identified; a single versus 2 distinct EV populations, characterized by size (small [EVssmall] and large [EVslarge], Fig 1A). In 25% of patients, EVs were CD19 positive (EVCD19+). CD19 positivity was detected only in patients with the EVslarge (Fig 1B). 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). 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 alone were insufficient to stimulate CART19 cells. However when CART19 cells were stimulated with the CD19-positive cell line JEKO, their effector functions were reduced only in the presence of EVsCD19+ but not EVsCD19-. This included a significant reduction in CART-specific killing (Fig 1C) and a reduction in cytokine production. The impairment of CART cell functions was independent of the size of EVs, ie, there was no impairment of CART functions with large or small size EVCD19- in co-culture. Conclusion: 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 CAR T-cell activity. Studies are ongoing to define the mechanism(s).