5054 Cytoxic T-lymphocyte-associated antigen-4 (CTLA-4) is a co-inhibitory molecule normally expressed on activated effector T cells and a subset of regulatory T cells. However, it has been reported also to be expressed on acute myeloid leukemia (AML) cells, CD34+ hematopoietic progenitor cells after stimulation as well as several solid tumor cell types. Furthermore, CTLA-4 engagement with recombinant CD80 and CD86 ligands has been shown to induce apoptosis in AML cells (Laurent et al. Br J Haematol 2007). In this study, we investigated CTLA-4 expression on different cell populations in bone marrow aspirates of myelodysplastic syndrome (MDS) patients, and explored the possibility of targeting CTLA-4 for apoptosis induction using the CTLA-4 antibody tremelimumab. Flow cytometry analysis showed CTLA-4 surface expression on immature CD34+, CD117+ and CD33+ myeloid cells, as well as CD14+ monocytic cells from MDS patients (n=11). In addition, CTLA-4 was expressed by both CD4+ and CD8+ T cells in bone marrow of both low and high risk MDS patients. In order to address whether anti-CTLA-4 antibody is able to induce apoptosis in myeloid leukemic progenitor cells, we incubated the CTLA-4-expressing myeloid cell lines HL-60 and KG1a in vitro for various time points (24h, 48h, 72h) with 25 microgram/ml tremelimumab. However, anti-CTLA-4 antibody alone as well as cross linking with a secondary antibody was unable to induce apoptosis, while serum-free conditions and irradiation induced high numbers of Annexin V and/or 7AAD positive cells. These data indicate that the anti-CTLA-4 antibody tremelimumab is unable to induce an apoptotic signal into CTLA-4-expressing myeloid tumor cells. Disclosures: No relevant conflicts of interest to declare.

Metastasis is the major cause of death in cancer patients and migration of cancer cells from the primary tumor to distant sites is the prerequisite of metastasis formation. Here we applied an imaging-based RNAi phenotypic cell migration screen uing two highly migratory basal breast cancer cell lines (Hs578T and MDA-MB-231 to provide a repository of candidate metastasis drug targets. We screened ~4,200 individual target genes covering most cell signaling components. We discovered 133 and 113 migratory modulators of Hs578T and MDA-MB-231, respectively, of which 45 genes were common denominators of cell migration. Interaction networks of candidate migratory modulators were in common with networks of different clinical breast cancer prognostic and metastasis classifiers. The splicing factors PRPF4B and BUD31 and the transcription factor BPTF were amplified in human primary breast tumors and the expression was associated with metastasis-free survival. Depletion of PRPF4B, BUD31 and BPTF caused primarily downregulation of genes involved in focal adhesion and ECM-interaction pathways. PRPF4B was essential for TNBC cell migration and critical for breast cancer metastasis formation in vivo, making PRPF4B a candidate for further drug development. Our systematic phenotypic screen provides an important repository for signaling determinants that functionally drive cancer cell migration. Citation Format: Bob van de Water, Erik Danen, Esmee Koedoot, Maria Rogkoti, Michiel Fokkelman, Sylvia Le Devedec, John Martens, Peter Stoilov, John Foekens. Uncovering the signaling landscape controlling breast cancer cell migration identifies splicing PRPF4B as a metastasis driver [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5179.