Abstract 394: A novel anti-a proliferation-inducing ligand hAPRIL.01A monoclonal antibody targets multiple myeloma cells in the bone marrow microenvironment (2015)

Abstract A proliferation-inducing ligand (APRIL), a close member of B-cell-activating factor (BAFF) belonging to the TNF superfamily, is mainly produced by bone marrow (BM) accessory cells to stimulates growth and survival of multiple myeloma (MM) cells. We here characterize molecular mechanisms regulating APRIL activation in the BM microenvironment and further determine whether a novel anti-APRIL monoclonal antibody hAPRIL.01A inhibits its functional sequelae in MM. First, in vitro culture showed that osteoclasts (OC) and macrophages secret significantly higher levels of APRIL than unstimulated CD14+ monocytes and BM stromal cells. All MM cell lines express cell surface BCMA in significantly higher level than TACI (p &lt; 9.06e-15). H929 MM cells (expressing only BCMA, but not TACI), and other MM cell lines were next stimulated with APRIL, in the presence or absence of hAPRIL.01 Ab followed by immunoblotting and TaqMan® Array assays on harvested protein lysates and mRNA. APRIL stimulation activated NF-κB, PI3K/AKT, and ERK1/2 signaling in MM cells. NF-κB-DNA binding activities of p65 and p50 (p52, to a less extent), were significantly upregulated as early as 15 minute after stimulation. Conversely, hAPRIL.01 Ab completely blocked these signaling cascades, consistent with decreased NF-κB-DNA binding activities in BCMA-knock-downed MM cells by shBCMA lentivirus transfection. APRIL further induced pro-survival proteins (Mcl1, Bcl2, BIRC3, XIAP) and MM cell growth-stimulating regulators (CCDN2, CDK4, CDK6, c-myc), which were completely inhibited by hAPRIL.01 Ab. These results correlated with blockade of hAPRIL.01 Ab in APRIL-promoted proliferation and survival of MM cells, in the presence of OCs or macrophages. APRIL also induces adhesion of MM cells to BMSC, which was blocked by hAPRIL.01 Ab. This concurred with hAPRIL.01 Ab-reduced adhesion molecules (CD44, ICAM-1) induced by APRIL. APRIL-increased VEGF-A and PECAM-1 in MM cells was also significantly reduced by this mAb. APRIL-upregulated chemotactic/osteoclast-activating factors (MIP-1α, MIP1β, SDF-1) were also inhibited by this Ab. Other angiogenesis and adhesion/chemoattractant factors were changed in a similar fashion, indicating specific blocking of hAPRIL.01 Ab to APRIL-induced downstream target genes. This mAb further inhibited APRIL-increased viability of plasmacytoid dendritic cells (pDC) and diminished MM cell viability protected by pDC in 3-d cocultures. Finally, hAPRIL.01 specifically overcame APRIL-, but not IL-6, induced protection in lenalidomide- or dexamethasone-treated MM1S and H929 MM cells. This Ab also blocks OC-induced MM cell growth and survival in ongoing SCID-hu model of MM. These studies confirm a constitutive APRIL activation via BCMA and TACI in promoting malignancies of myeloma cells, supporting a novel therapeutics of hAPRIL.01 Ab to target MM in the BM microenvironment. Note: This abstract was not presented at the meeting. Citation Format: Yu-Tzu Tai, Chirag Acharya, Gang An, Mike Y. Zhong, Xiaoyan Feng, Hua Jiang, Hans van Eenennaam, Andrea van Elsas, Nikhil C. Munshi, Kenneth C. Anderson. A novel anti-a proliferation-inducing ligand hAPRIL.01A monoclonal antibody targets multiple myeloma cells in the bone marrow microenvironment. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 394. doi:10.1158/1538-7445.AM2015-394