ALBERT

Description: Recent studies have demonstrated that β-catenin in DCs serves as a key mediator in promoting both CD4+ and CD8+ T-cell tolerance, although how β-catenin exerts its functions remains incompletely understood. Here we report that activation of β-catenin in DCs inhibits cross-priming of CD8+ T cells by up-regulating mTOR-dependent IL-10, suggesting blocking β-catenin/mTOR/IL-10 signaling as a viable approach to augment CD8+ T-cell immunity. However, vaccination of DC–β-catenin−/− (CD11c-specific deletion of β-catenin) mice surprisingly failed to protect them against tumor challenge. Further studies revealed that DC–β-catenin−/− mice were deficient in generating CD8+ T-cell immunity despite normal clonal expansion, likely due to impaired IL-10 production by β-catenin−/− DCs. Deletion of β-catenin in DCs or blocking IL-10 after clonal expansion similarly led to reduced CD8+ T cells, suggesting that β-catenin in DCs plays a positive role in CD8+ T-cell maintenance postclonal expansion through IL-10. Thus, our study has not only identified mTOR/IL-10 as a previously unidentified mechanism for β-catenin–dependent inhibition of cross-priming, but also uncovered an unexpected positive role that β-catenin plays in maintenance of CD8+ T cells. Despite β-catenin’s opposite functions in regulating CD8+ T-cell responses, selectively blocking β-catenin with a pharmacological inhibitor during priming phase augmented DC vaccine-induced CD8+ T-cell immunity and improved antitumor efficacy, suggesting manipulating β-catenin signaling as a feasible therapeutic strategy to improve DC vaccine efficacy.

Description: Dendritic Cell (DC) differentiation is a complex system involving multiple progenitors with potential to differentiate into a variety of DC subsets. Understanding the mechanisms regulating these differentiation pathways is critical to understanding how defective DCs arise in cancer. Impaired DC differentiation often results in immunosuppressive cells that either hinder immune activation in disease or promote tumor growth and metastasis. We previously established that the serine-threonine kinase Protein Kinase C β isoform II (PKCβII) is required for human DC differentiation from CD34+ progenitor cells and monocytes, and have recently found that murine bone marrow (BM) cells also need it to become fully differentiated and functional DCs. However, the molecular targets of PKCβII in this pathway remain unclear. It is well established that the transcription factors Interferon Regulatory Factors 4 and 8 (IRF4 and IRF8) are also important for DC differentiation. IRF4 is crucial for the development of conventional DCs (mediated by GM-CSF), while IRF8 is crucial for the development of plasmacytoid and CD8α+DCs (mediated by FLT3-L). We hypothesized that a relationship existed between PKCβII and IRF4/8, and investigated the effects of PKC activation on IRF4/8 expression. Using human progenitor cell lines and murine BM cells we found that PKC activation upregulated IRF4 and IRF8 expression, while PKC inhibition downregulated IRF4 and IRF8. PKC inhibition also prevented these cells from differentiating into DCs, as determined by their phenotypic markers, physical characteristics, and T-cell stimulatory activity. However, we found that in progenitor cells GM-CSF (a known PKCβII activator) decreased IRF8 expression while upregulating IRF4 expression. This led us to investigate the differential effects of GM-CSF and FLT3-L on the PKC-IRF relationship. We saw that FLT3-L treatment of murine BM cells caused an upregulation of IRF8 and stimulated DC differentiation, and that DC differentiation and IRF8 upregulation were both lost in the presence of a PKC inhibitor. Using Image Stream analysis we found that FLT3-L treatment of progenitor cells activated PKCβII and PKCα. To determine which PKC(s) mediates the FLT3-L driven upregulation of IRF8, we used PKCα knockout (KO) BM and saw that cells were still able to differentiate into DCs and IRF8 levels were still being upregulated. Thus, PKCβII is the PKC that mediates FLT3-L driven DC differentiation and IRF8 upregulation. To determine what molecules could be acting downstream of PKCβII in regulating IRF4/8, we again used Image Stream analysis and visualized STAT3 and STAT5 translocation into the nucleus. Using murine BM cells we found that STAT3 and IRF8 nuclear localization increased with FLT3-L treatment, while GM-CSF treatment caused increased STAT5 and IRF4 nuclear localization. When looking at human monocytes and the human monocytic progenitor cell lineTHP-1 we saw similar effects: GM-CSF treatment increased STAT5 and IRF4 nuclear localization, while pan-PKC inhibition decreased basal STAT5 and IRF4 nuclear localization. Interestingly, these human monocytes and THP-1 cells had lower nuclear levels of STAT3 and IRF8 following FLT3-L treatment – possibly because these cells are already somewhat committed to the monocyte-derived conventional DC pathway. However, in murine early progenitor cells, after 15 minutes of PKC activation we saw increased STAT3 activation, indicating that PKC-regulated STAT3 activation is playing a role earlier in the differentiation process. To find the progenitor cells immediately effected by PKCβII activation, we used IRF8-eGFP murine BM and saw that PKC activation caused induced IRF8 expression as early as in the multi-potent progenitor cells (MPP2 and MPP3), and this upregulation continued to increase as cells differentiated to CD11b+progenitor cells and GMP. These studies indicate that PKCβII is activated in progenitor cells by either FLT3-L or GM-CSF, causing an upregulation of IRF8 or IRF4, respectively. PKCβII may be acting through STAT5 and STAT3 to induce IRF4 and IRF8, depending on the cytokine treatment. By having a better understanding of how PKCβII regulates the expression of these transcription factors, which are required for DC differentiation, we can manipulate the PKCβII-IRF relationship to drive or impair DC differentiation in pathological settings, and may improve DC-vaccine development. Disclosures: No relevant conflicts of interest to declare.

Description: Bortezomib, a therapeutic agent for multiple myeloma (MM) and mantle cell lymphoma, suppresses proteosomal degradation leading to substantial changes in cellular transcriptional programs and ultimately resulting in apoptosis. Transcriptional regulators required for bortezomib-induced apoptosis in MM cells are largely unknown. Using gene expression profiling, we identified 36 transcription factors that displayed altered expression in MM cells treated with bortezomib. Analysis of a publically available database identified Kruppel-like family factor 9 (KLF9) as the only transcription factor with significantly higher basal expression in MM cells from patients who responded to bortezomib compared with nonresponders. We demonstrated that KLF9 in cultured MM cells was up-regulated by bortezomib; however, it was not through the induction of endoplasmic reticulum stress. Instead, KLF9 levels correlated with bortezomib-dependent inhibition of histone deacetylases (HDAC) and were increased by the HDAC inhibitor LBH589 (panobinostat). Furthermore, bortezomib induced binding of endogenous KLF9 to the promoter of the proapoptotic gene NOXA. Importantly, KLF9 knockdown impaired NOXA up-regulation and apoptosis caused by bortezomib, LBH589, or a combination of theses drugs, whereas KLF9 overexpression induced apoptosis that was partially NOXA-dependent. Our data identify KLF9 as a novel and potentially clinically relevant transcriptional regulator of drug-induced apoptosis in MM cells.

Description: 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.

Description: 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.

Description: Abstract 1862 Poster Board I-887 Background: Biotest AG (Dreieich, Germany) is developing the immunoconjugate BT062, which comprises the anti-CD138 chimerized MAb (nBT062) and the cytotoxic agent maytansinoid (DM4). Once bound to CD138 on a target cell, the conjugate is internalized and releases DM4. At present, CD138 represents one of the most reliable target antigens for identification of multiple myeloma (MM) cells and has been reported to be a highly sensitive and specific diagnostic marker of MM. Preclinical investigations demonstrated significant in vitro and in vivo anti-MM activity of BT062, providing the rationale for the conduct of clinical trials (Ikeda et al., 2009). Objectives: To determine the maximum tolerated dose (MTD), the dose-limiting toxicities (DLTs), pharmacokinetics (PK) and anti-MM activity of increasing doses of BT062 on a repeated single dose schedule once every three weeks in relapsed or relapsed/refractory MM. Clinical response was assessed as per the international working group criteria (Durie et al., 2006). Methods: This is a prospective, open label, dose-escalation multicenter study. Patients aged ≥ 18 years with relapsed or relapsed/refractory MM who have failed previous treatments including an immunomodulating agent and a proteasome inhibitor were eligible to participate. Patients with clinical response (or no evidence of progressive disease) and without unacceptable toxicities were eligible for further treatment cycles. Patients are enrolled in cohorts of 3 at each dose level, with DLT in the first cycle triggering cohort expansion. Results: To date 20 patients have been treated with BT062 at 7 dose levels ranging from 10 mg/m2 to 200 mg/m2. Maximum administered dose has not been defined to date with continued enrollment at 200 mg/m2 dose. None of the patients treated experienced serious hypersensitivity reactions or humoral responses (HAHA) against BT062. The most frequently reported adverse events to date cover primarily events expected for the underlying disease. Nevertheless, a few adverse events have also been observed involving skin and mucosa (tissues of epithelial origin with CD138 expressing cells). No grade 4 toxicity has been reported. Preliminary PK results indicate an unusual rapid clearance from plasma in the early elimination phase, followed by a generally normal terminal elimination phase at dose levels up to 120 mg/m2, whereas a more typical clearance profile was observed for all 3 patients at the 160 mg/m2 dose. Interestingly, even in phase I study decreased urine M-Protein or serum FLC levels have been observed in 2 patients. One of these patients showed a decrease in urine M-Protein by more than 50% after administration of 8 repeated low doses. At a high dose level another patient without detectable M-Protein levels, showed a decrease of serum FLC by more than 50% after having received the second dose of BT062. Furthermore, evidence of clinical benefit has been observed in at least 6 patients with early stabilization of M-protein levels (and light-chain burden) in serum and /or urine. Conclusion: Development of a monoclonal antibody in MM remains an important therapeutic option and BT062 is an exciting possibility. Preliminary data from this phase I study, demonstrate an acceptable toxicity profile of BT062 in the clinics. Even in phase I study, evidence of clinical activity is observed. These encouraging results and the unique PK observed support investigation of a more frequent dosing regimen for optimizing anti-MM responses. Updated data on safety, PK and efficacy of BT062 from this clinical trial will be presented at the meeting. Disclosures: Jagannath: Millennium: Honoraria, Membership on an entity's Board of Directors or advisory committees; Merck: Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees. Avigan:Genzyme: Consultancy, Research Funding; Celgene: Research Funding. Lutz:Immunogen, Inc.: Employment. Haeder:Biotest AG: Employment. Ruehle:Biotest AG: Employment. Uherek:Biotest AG: Employment. Wartenberg-Demand:Biotest AG: Employment. Munshi: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; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Anderson:Celgene: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Millennium: Consultancy, Research Funding; Biotest AG: Consultancy, Research Funding.

Description: Abstract 2883 Poster Board II-859 Background: IMGN901 (huN901-DM1/BB-10901) is a novel anticancer agent consisting of a potent cytotoxic maytansinoid, DM1, attached to a CD56-binding monoclonal antibody, huN901, using an engineered linker. Once bound to CD56 on a cancer cell, the conjugate is internalized and releases DM1. About 70% of multiple myeloma (MM) cases have surface expression of CD56. In preclinical settings, IMGN901 showed significant in vitro and in vivo anti-myeloma activity as a single agent and in combination with approved drugs such as lenalidomide. Objectives: To determine the maximum tolerated dose (MTD), pharmacokinetics (PK), and activity of IMGN901, used as monotherapy, in patients with MM. Methods: Patients with CD56+ relapsed or relapsed/refractory MM receive a single IV infusion of IMGN901 on 2 consecutive weeks every 3 weeks. Patients are enrolled into each dose level in cohorts of 3, with dose-limiting toxicity (DLT) triggering cohort expansion. The European Bone Marrow Transplant (EBMT) criteria were used for response assessment. Results: Twenty-three CD56+ MM patients have received IMGN901 at doses ranging from 40 to 140 mg/m2/week. Most of these 23 patients had been treated with 6 or more chemotherapy regimens prior to study entry. Two of 6 patients treated at the 140 mg/m2/week dose experienced DLT (grade 3 fatigue and grade 3 acute renal failure) and a lower dose has been defined as the MTD. Commonly reported adverse events that were at least possibly related to IMGN901 were fatigue, increased aspartate aminotransferase, increased uric acid, sensory neuropathy and headache. None of the patients experienced serious hypersensitivity reactions or demonstrated a humoral response against either the antibody or DM1 component of IMGN901. Sustained partial response (PR) was documented in 1 patient treated at 140 mg/m2/week and 3 minor responses (MR) were reported in 1 patient each at doses of 60, 90, and 112 mg/m2/week. Of the 23 patients receiving any dose level of IMGN901, 8 remained on IMGN901 treatment for at least 15 weeks. Five of these 8 patients continued treatment on IMGN901 for at least 24 weeks, and two of these 5 patients remained on IMGN901 for at least 50 weeks. Preliminary PK results indicate an approximately linear relationship between dose and observed maximal serum concentration. Conclusion: This is the first study of IMGN901 in patients with MM. The MTD of this agent in MM patients is now defined. Our experience with IMGN901 in this clinical trial demonstrates an overall favorable safety profile. Although the primary objective of this clinical trial was to determine the MTD of single agent IMGN901, exciting single agent activity was observed in heavily pretreated MM patients. This is particularly encouraging as the duration of treatment with IMGN901 in some patients was longer than duration of treatment with prior regimens of approved agents. Clinical observations noted here (including single agent efficacy and the favorable toxicity profile) as well as findings from preclinical combination studies warrant continued investigation of this novel agent in patients with MM especially in combination with approved anti-myeloma agents/regimens such as lenalidomide and dexamethasone. 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. Jagannath:Millennium: Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene: Honoraria; Merck: Honoraria. Miller:Celgene: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Millennium: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Guild:ImmunoGen, Inc: Employment. Zildjian:ImmunoGen, Inc: Employment. Qin:ImmunoGen, Inc.: Employment. O'Leary:ImmunoGen, Inc.: Employment.

Description: Cancer generates a state of immune suppression that contributes to tumor out-growth and the escape phase of the immune editing of nascent tumors. One hallmark of tumor mediated immune suppression is a decreased number of dendritic cells (DC) and an accumulation of immature myeloid cells (myeloid derived suppressor cells, MDSC) that are themselves directly immunosuppressive. Impairment of DC differentiation is mediated by numerous tumor derived factors (VEGF, GM-CSF, IL-6, and M-CSF) that activate Stat3. The mechanism by which Stat3 signaling subsequently inhibits DC differentiation has not been defined. Previous work in our lab has identified protein kinase C βII (PKC βII) as being essential in myeloid progenitor à DC differentiation and that knock down of PKC βII expression blocks DC differentiation. This leads us to hypothesize that tumor derived factor activation of Stat3 may inhibit DC differentiation by down regulating PKC βII expression. To test our hypothesis, we utilized the hematopoietic progenitor-like cell line KG1. In response to the phorbol ester PMA, KG1 differentiates into a DC-like cell (KG1-DC). KG1 cultured in media conditioned by a human (MCF-7) or murine (DA3) mammary cancer cell line had a 48% (DA3) and 51% (MCF-7) reduction in PKC βII protein levels. Additionally, tumor conditioned media treatment significantly decreased PKC βII mRNA transcript levels (38-fold reduction compared to untreated, p

Description: Multiple myeloma is a malignancy of long lived plasma cells. Like normal plasma cells, myeloma cells are dependent on the bone marrow microenvironment for survival. While the specific interactions and downstream signals mediated by the bone marrow stroma have yet to be fully characterized, drug resistance is linked to these pathways, and further understanding will uncover new therapeutic avenues for myeloma. CD28 and CD86 are best known for their role in T-cell activation; however they have recently been shown to play important roles in the generation and survival of normal long lived plasma cells. CD28 is the canonical costimulatory receptor known to activate the PI3K-Akt pathway in T-cells upon binding to CD80 or CD86 from an antigen-presenting cell. CD28 and CD86 are also expressed by normal plasma and myeloma cells, and we have previously shown that both CD28 and CD86 are necessary for myeloma cell survival. Silencing of either CD28 or CD86 results in cell death in 3 human myeloma cell lines (RPMI8226, MM.1s, and KMS18). Interestingly, in 2 cell lines, knockdown of CD86 results in higher levels of cell death than CD28. In these lines, silencing CD28 or addition of a soluble inhibitor CTLA4-Ig (Abatacept), results in an increase in CD86 expression. Taken together, these data suggest that CD28 and CD86 regulate the survival of myeloma cells through either cis or trans signals, and feedback signals from CD28 control CD86 expression. The data also suggest that signaling events result from ligation of either CD28 or CD86. To better define the nature of the survival signals emanating from CD28 and CD86, we performed RNA-Seq on myeloma cells where CD28 or CD86 expression had been silenced. For silencing of CD28 we found 1292, 1195, and 1697 transcripts that were significantly changed compared to vector control in KMS18, MM.1s and RPMI8226, and silencing CD86 results in a similar number of changes (1405, 1200 and 1866 transcripts respectively). Since CD28 and CD86 form a receptor-ligand pair, we focused on genes that were common to silencing of both. Of genes that had significant expression changes compared to vector control, we found 229, 221 and 399 transcripts that were commonly regulated by CD28 and CD86 in KMS18, MM.1s and RPMI8226 respectively. Most transcripts were either upregulated (45.4 to 57.5%) or downregulated (28.8 to 41.5%) by silencing of either CD28 or CD86. A subset of transcripts (13.1 to 14%) showed a pattern of expression similar to CD86 - silencing of CD28 and CD86 had opposite effects on expression. This subset of transcripts may represent genes that are regulated by CD86 signaling, and may explain the difference in sensitivity to CD28 vs. CD86 silencing. Curiously, in KMS18 where this difference was not observed, RNA-Seq indicates that these cells are homozygous for a CD86 SNP that is associated with increased cancer susceptibility and lower transplant rejection, and may represent a hypomorphic allele. Surprisingly, we did not observe any significant changes in either pro- or anti-apoptotic Bcl-2 genes in any cell line except for upregulation in minor transcripts of Bcl2L11 (Bim) in KMS18 cells. This change did not affect overall expression as confirmed by qRT-PCR. However, expression of several cell surface proteins associated with myeloma cell survival did change. Integrin-ß1 (ITGB1) and -ß7 (ITGB7) are surface molecules that facilitate both cell-matrix and cell-cell interactions, and have been implicated in myeloma growth, survival, and drug-resistance. Knockdown of CD28 or CD86 resulted in downregulation of ITGB7 that was confirmed by qRT-PCR. We also saw a reduction of ITGB7 at the cell surface with CD86 knockdown, but not with CD28. ITGB1 expression was reduced at the mRNA and cell surface levels with knockdown of CD86, but was induced with CD28 knockdown. Based on their patterns of expression, ITGB7 may be regulated by CD28 signaling, while ITGB1 may be downstream of CD86 signaling. These data indicate that CD28-86 signaling regulates the expression of integrins on the surface of myeloma cells. Because drug resistance has been linked to the myeloma cells’ interaction with the bone marrow stroma and its resident cells (CAM-DR), these surface molecules could be important mediators of CD28 and CD86 survival signaling. Taken together, our data indicate that targeting CD28-86 signaling is a promising therapeutic approach to CAM-DR, and may be a useful addition to current regimens against myeloma. Disclosures: Lonial: Onyx: Consultancy; Celgene: Consultancy; Millennium: Consultancy; Novartis: Consultancy; BMS: Consultancy; Sanofi: Consultancy; Onyx: Consultancy. Boise:Onyx Pharmaceuticals: Consultancy.