Anti-CTLA-4 Antibody Is Not Able to Trigger Apoptosis in CTLA-4 Expressing Myeloid Tumor Cells

Abstract 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.