Lee, Kelvin P.

Abstract Abstract 4919 Waldenstrom's macroglobulinemia (WM) is characterized by the presence of lymphoplasmacytic cells in the bone marrow and the secretion of IgM monoclonal antibody in the serum. Several conventional therapies are available but the disease remains incurable. Recently, bortezomib (a proteasomal inhibitor) has shown promising anti-WM activity with enhanced responses when combined with traditional therapies. Resistance to bortezomib therapy is an important event that is associated with continued treatment. In order to understand the mechanism of bortezomib resistance in WM we exposed BCWM.1 (a known WM cell line) in vitro to increasing concentrations of bortezomib over prolonged periods of time and isolated the bortezomib resistant clone (BCWM.1/BR). This clone was compared with its parent wild type cell line (BCWM.1/WT). Investigation to understand the susceptibility of BCWM.1/Br cells to various therapeutic agents showed that these cells are resistant to many of the agents such as melaphalan, fludarabine or doxorubicin. Interestingly, verapamil, a broad spectrum inhibitor of multidrug resistance, failed to reverse the resistance induced by bortezomib indicating that bortezomib resistance is not because of an activation of multidrug resistance in these cells. While BCWM.1/WT cells showed an IC50 of 7.8nM when treated for 72h with bortezomib, the BCWM.1/BR cells were not inhibited at any concentration of the compound up to 100nM. Furthermore, the cells with the acquired resistance showed a 4 fold increase in the proteasomal activity as measured by the release of a fluorescent product (7-Amino-4-methylcoumarin (AMC)) from its peptide substrate, suc-LLVY-AMC. Biochemical analysis further revealed that many of the proteasomal components are altered in BCWM.1/BR cells as compared to their parental control cells. Interestingly, protein levels of two of the proteasomal catalytic subunits, PSMB5 and PSMB9 are upregulated in resistant cells suggesting a reason for the enhanced proteasomal activity of these cells. The resistant cells showed an altered gene expression profile that indicates a transformation of the parental wild type cell line to acquire resistance. A comparative analysis of the signal transduction pathways operated in these cells showed that many of the activation and cell survival pathways that are present in BCWM.1 cells are inhibited in the resistant cells. For example, BCWM.1 cells show a constitutive activation of AKT and ERK1/2 which are inhibited in the resistant cells thus making them insensitive to the inhibitors of these pathways. Similarly, HSP27 which was earlier shown to contribute to bortezomib induced resistance was completely inhibited in BCWM.1 resistant cells. Interestingly, there is an increase in Bcl-2 protein in BCWM.1/BR cells as compared to WT cells indicating that the resistant cells might be dependent on Bcl-2 family for their survival. Inhibition of Bcl-2 induced potent apoptosis in BCWM.1/BR cells. Thus the results presented here indicate that acquired bortezomib resistance in BCWM.1 cells alters their proteasomal activity, cellular signaling pathways to make them resistant to many of the known therapies but these cells retain the Bcl-2 mediated pathway for targeting thus inhibitors of Bcl-2 may be used in therapy against bortezomib-resistant WM. Disclosures Chanan-Khan: Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Millennium: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Immunogen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau.

Abstract Abstract 2861 Poster Board II-837 Bcl-2 protein family has the unique capability to balance between the cell survival and death by regulating the expression of its individual members. AT-101 is a BH3 mimetic and a potent inducer of noxa and puma, the natural ligands of BH3 family proteins. It is known to bind and inhibit the anti-apoptotic functions of Bcl-2 family members Bcl-2 and Bcl-XL and Mcl-1. In vitro it has been shown to induce apoptosis in several tumor models systems including multiple myeloma. In this report we investigated the effect of AT-101 on a model cell line, BCWM.1 (a known WM cell line, BCWM.1/WT), representing Waldenström Macroglobulinemia. This disease is characterized by the presence of lymphoplasmacytic cells in the bone marrow and the secretion of IgM monoclonal protein into the serum. Several conventional therapies are available but the disease remains incurable. Therefore there remains a need to develop new therapies for this orphan disease. Recently, bortezomib (a proteasomal inhibitor) has shown promising anti-WM activity with enhanced responses when combined with traditional therapies. But continued treatment with bortezomib result in the development of resistance in the clinic. We developed an in vitro model of bortezomib resistance from BCWM.1 (hereafter referred as BCWM.1/BR). These cells also developed cross resistance to conventional therapies used for WM such as fludarabine and doxorubicin. Biological characterization of this cell line demonstrated Bcl-2 as a potentially important therapeutic target. We therefore assessed the effect of AT-101 to identify preclinically if this could be a potential clinical strategy in future. AT-101 induced a dose and time dependent inhibition in the viability of both BCWM.1/WT as well as BCWM.1/BR cells. Cell death was observed at as low as 1uM concentration of AT-101 and at 10uM a maximum of 50-70% death was observed by 24h. While BCWM.1/WT cells showed a significant death at 12h, treatment with AT-101 induced cell death in BCWM.1/BR cells as early as 6h. These results indicate that AT-101 induced a potent and quick inhibition in viability in BCWM.1/BR cells as compared to their parental wild type cells. Investigation into the mechanism of cell death showed that AT-101 induced apoptosis in a mitochondrial dependent pathway in these cells. A comparative analysis of the signal transduction pathways operated in BCWM.1/WT and BCWM.1/BR cells showed that many of the cellular activation and survival pathways such as AKT, ERK1/2 that are present in BCWM.1 cells are inhibited in the resistant cells. Interestingly, BCWM.1/BR cells expressed a fivefold increase in the Bcl-2 protein as compared to BCWM.1/WT cells suggesting a Bcl-2 dependent survival of these cells in the absence of other cellular activation and survival signals. Increased susceptibility of BCWM.1/BR cells to AT-101 thus can be understood to be a direct consequence of an increased expression of Bcl-2 and a dependence of the resistant cells on Bcl-2 family of anti-apoptotic proteins for their survival. Results presented in this report suggest that AT-101 has a unique therapeutic potential against Waldenström Macroglobulinemia that is independent of resistance to bortezomib. These observations highlight bcl-2 as a potential target, and AT-101 as possible therapeutic avenue for WM patients. Disclosures: Chanan-Khan: Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Millennium: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Immunogen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau.

Abstract Waldenström Macroglobulinemia (WM) is a disorder of lymphoplasmacytoid cells that inhabit lymph nodes and the bone marrow. WM cells are characterized by secretion of monoclonal pentameric IgM. These cells are CD19+, CD20+, CD22+, CD38+, CD138+/- and phenotypically resemble IgM plasmablasts or plasma cells. In addition, 91% of WM cases carry an activating mutation of MyD88 (L265P). Mature resting B cells can be driven to differentiate to IgM secreting plasmablasts and plasma cells with similar phenotypes using the TLR4 ligand lipopolysaccharide (LPS). We have demonstrated that LPS (+ cytokine)-differentiated cells become Bcl-xL dependent during this process, rendering them sensitive to the Bcl-xL/Bcl-2 inhibitor ABT-737. For this reason, we hypothesized that activation of MyD88 in WM cells could drive Bcl-xL dependence in a similar manner conferring ABT-737 sensitivity. We treated three WM cell lines, BCWM.1, MWCL-1 and RPCI-WM1 which all harbor the MyD88 (L265P) mutation with ABT-737. We found varying levels of resistance to ABT-737 with an IC50 > 2 μM for all three lines as compared with the ABT-737 sensitive multiple myeloma cell line MM.1s which has an IC50 of 0.4 μM. The RPCI-WM1 cell line was the most insensitive to ABT-737-induced apoptosis with no apoptosis above baseline up to 1.6 μM of drug. Since the WM cell lines were not sensitive to direct inhibition of intrinsic survival regulators, we then examined the sensitivity of these cell lines to other activators of the intrinsic apoptosis pathway. Two of the three cell lines were moderately sensitive to bortezomib with IC50 ≈ 5 nM as compared with the sensitive multiple myeloma cell line MM.1s with an IC50 of 2 nM. The RPCI-WM1 cell line was insensitive to bortezomib with no apoptosis above baseline up to 20 nM bortezomib. Similarly, we found that two of the cell lines were moderately sensitive to arsenic trioxide with an IC50 ≈ 6 μM as compared with the multiple myeloma cell line MM1.s (IC50 ≈ 4 μM). The RPCI-WM1 cell line was insensitive to ATO as well with an IC50 > 20 μM. Given the lack of sensitivity of the three WM cell lines we tested to Bcl-xL/Bcl-2 inhibition with ABT-737 treatment, and that RPCI-WM1 appears insensitive to multiple inducers of intrinsic apoptosis, we examined the expression levels of Bcl-2 family members in these cells. Both BCWM.1 cells and MWCL-1 cells expressed Bim mRNA at very low levels with MWCL-1 expressing no detectable Bim at the protein level. Surprisingly, more moderate levels of Bim were detected in RPCI-WM1 cells. These findings were confirmed at the mRNA level by qRT-PCR. Bcl-xL and Mcl-1 were detectable in all three lines at moderate levels while Bcl-2 which was only expressed at significant levels in MWCL-1 cells and undetectable in BCWM.1 cells. We examined the expression levels of the Bax and Bak in these cells and remarkably there was no detectable Bax and very small amounts of Bak protein in RPCI-WM1 cells. Consistent with a defect in gene expression, Bax mRNA was also low in RPCI-WM1. This was not due to copy number variation, as determined by array-CGH in both the initial patient isolate and the established cell line. Additionally, no loss of Bax, Bak or Bim (Bcl2l11) was observed in SNP array analysis of 46 patients with WM. Interestingly, Bak mRNA levels in RPCI-WM1 were similar to the other WM lines, suggesting a defect in translation or post-translational regulation is responsible for the low protein expression. These results lead us to conclude that these WM cell lines are not sensitive to Bcl-xL/ Bcl-2 inhibition despite activation of MyD88. We have further shown that there are multiple and distinct differences in Bcl-2 family protein expression that lead to this insensitivity. While low levels of Bim combined with expression of Mcl-1 confer resistance to ABT-737 in MWCL-1 and BCWM.1, the lack of Bax and Bak confers resistance to intrinsic apoptotic stimuli in general in RPCI-WM1. Moreover, the loss of Bax and Bak protein expression occur through distinct mechanisms. These WM cell lines demonstrate that sensitivity to agents that kill through the intrinsic apoptotic pathway may vary within a disease that is characterized by a single activating mutation and suggests that additional heterogeneous events regulate the expression of Bcl-2 family proteins in WM. Disclosures: Leleu: CELGENE: Honoraria; JANSSEN: Honoraria. Lonial:Millennium: Consultancy; Celgene: Consultancy; Novartis: Consultancy; BMS: Consultancy; Sanofi: Consultancy; Onyx: Consultancy. Boise:Onyx Pharmaceuticals: Consultancy.

Abstract Abstract 132 Multiple myeloma (MM), an incurable neoplasia of terminally differentiated plasma cells, are critically dependent on their interactions with bone marrow stromal cells (BMSC) for essential survival signals, growth and immunosuppressive factors. Very little is known about the specific BM cell type or the molecular elements in these interactions, an understanding of which could provide novel targets that could be interdicted to enhance conventional chemotherapy. A potential MM surface protein that could be involved in these interactions is CD28, based on its known pro-survival role in T cells. Clinical studies have shown that expression of CD28 in multiple myeloma highly correlates (p=0.006) with myeloma tumoral expansion. Moreover, CD28+ MM cells invariably express the CD28 ligand CD86. A survival role for MM-CD28 might involve interactions with BM cells that express B7 (CD80/CD86) such as dendritic cells (DCs, that are known to be closely associated with MM cells in the BM) or with CD86+ MM cells themselves. We had previously shown (ASH2008, #I-769) that blocking CD28-CD86 interactions between myeloma cells with high affinity B7 ligand CTLA4Ig (Abatacept®) sensitized myeloma cells to chemotherapy. Now we show that myeloma cells co-cultured with myeloid DCs in vitro derive both direct and indirect survival signals from DCs, and this can be partially blocked by commercially available reagents. Our data show that flow cytometric analysis of mononuclear cells (MNC) from BM aspirates of myeloma patients with increased CD138+ plasma cell populations (9-58%), show an increased CD11b+ (myeloid) population (20-37%) as well, which is in contrast to healthy transplant donor controls (12-15% CD11b+, 4–6% CD138+). Moreover, a larger fraction (11-47%) of the myeloma CD138+ plasma cells expressed CD28 compared to healthy control (3.3-7.7%). Also, when we analyzed gene expression datasets (NCBI #GSE5900 and GSE4204) from plasma cells (PC) of normal donors, monoclonal gammopathy of undetermined significance (MGUS), smoldering myeloma (SM) and newly diagnosed multiple myeloma (MM), we found a progressive increase in patients showing CD28 expression with increasing severity of disease (normal<MGUS<SM<MM) (Fig 1A). When we sorted the highest scoring MM group (n=538) into 8 genetic subgroups as defined earlier, CD28 expression was found to peak in the MF subgroup (typically associated with poor survival in myeloma patients) (Zhan et al. 2006, Blood 108, pp. 2020) relative to total population (p<0.0001) (Fig 1B). Antibody mediated activation of MM-CD28 over 48 hrs increased viability of myeloma cell line MM.1S cultured under serum starvation (3.7%) or with drugs ATO (1.9%), melphalan (18%) or dexamethasone (3.3%) to 66%, 21%, 33% and 11% respectively. Viability of MM.1S cells or primary CD138+ plasma cells (isolated from myeloma BM aspirates) cultured under serum starvation was enhanced >3 fold (p<0.001) when co-cultured with monocyte derived DCs, and in MM.1S this was partially reversed when either MM-CD28 or DC-B7 was blocked (Fig 2). Similar protection of MM.1S was also observed against a gradient of dexamethasone or melphalan. CD28 activation was accompanied by rapid tyrosine phosphorylation of CD28, association of p85 (PI3K), activation of Vav-1 and increase in CD28 associated tyrosine kinase activity, as shown by immunoprecipitation, western and kinase activity assays. We had previously shown that MM-CD28 interaction drive DC production of pro-survival factor IL-6 and immunosuppressive factor IDO via DC-B7 “backsignaling” (ASH2008 #I-769). Now we show that MM induced DC production of IL-6 (8 ng/ml) was partially inhibited in presence of CD28 blocking αCD28(Fab) fragments (3 ng/ml) or with protein kinase C (PKC) inhibitor Bisindolylmaleimide-I (2.1ng/ml). Activity of the immunosuppressive enzyme IDO in these co-cultures was completely inhibited in the presence of a novel IDO inhibitor from Incyte corporation, and this helped partially reverse IDO mediated suppression of T-cell proliferation in proliferation assays using co-culture supernatants. In conclusion, our data characterizes CD28-B7 pathway and DCs in the BM as vital for myeloma survival and also as possible targets to include in future strategies in the treatment of myeloma. FIGURE 1 FIGURE 1. FIGURE 2 FIGURE 2. Disclosures: Boise: University of Chicago: Patents & Royalties.

Significance While CD8 + T cells are essential for antitumor immunity, tumors often evade CD8 + T cell surveillance by immunosuppression. Recent study has shown that tumor-induced β-catenin activation in DCs suppresses CD8 + T cell immunity by inhibiting cross-priming, suggesting that activation of β-catenin in DCs might be a key mechanism tumors use to achieve immunosuppression. This report identified mTOR/IL-10 signaling as a previously unidentified mechanism for β-catenin–dependent inhibition of cross-priming. Surprisingly, our study also revealed that β-catenin in DCs was required for CD8 + T cell maintenance post-clonal expansion, suggesting that β-catenin exerts opposite functions during different stages of CD8 + T cell responses. Based on these findings, we have demonstrated selectively manipulating β-catenin signaling as a feasible strategy to improve DC vaccine efficacy.

Key PointsCD28 delivers a pro-survival signal to MM cells via regulation of PI3K/Akt, FoxO3a, and Bim. Blockade of CD28:CD80/CD86 in vivo resensitizes MM cells to chemotherapy and significantly reduces tumor burden.

Abstract Interactions between the malignant plasma cells of multiple myeloma and stromal cells within the bone marrow microenvironment are essential for myeloma cell survival, mirroring the same dependence of normal bone marrow-resident long-lived plasma cells on specific marrow niches. These interactions directly transduce prosurvival signals to the myeloma cells and also induce niche production of supportive soluble factors. However, despite their central importance, the specific molecular and cellular components involved remain poorly characterized. We now report that the prototypic T cell costimulatory receptor CD28 is overexpressed on myeloma cells during disease progression and in the poor-prognosis subgroups and plays a previously unrecognized role as a two-way molecular bridge to support myeloid stromal cells in the microenvironment. Engagement by CD28 to its ligand CD80/CD86 on stromal dendritic cell directly transduces a prosurvival signal to myeloma cell, protecting it against chemotherapy and growth factor withdrawal-induced death. Simultaneously, CD28-mediated ligation of CD80/CD86 induces the stromal dendritic cell to produce the prosurvival cytokine IL-6 (involving novel cross-talk with the Notch pathway) and the immunosuppressive enzyme IDO. These findings identify CD28 and CD80/CD86 as important molecular components of the interaction between myeloma cells and the bone marrow microenvironment, point to similar interaction for normal plasma cells, and suggest novel therapeutic strategies to target malignant and pathogenic (e.g., in allergy and autoimmunity) plasma cells.