Inhibition of PI3K/Akt and Erk Signaling Pathways by BCR-ABL Tyrosine Kinase Inhibitors up-Regulates the Immunosuppressive Receptor Osteoactivin in Monocyte-Derived Dendritic Cells

Abstract 1050

Osteoactivin (GPNMB, DC-HIL) is a type I transmembrane glycoprotein that is expressed in dendritic cells (DC). Osteoactivin/syndecan-4 (SD-4) interaction was previously shown to inhibit T cell activation by antigen-presenting cells. We recently demonstrated that exposure of human peripheral blood monocytes to IL-10 or pharmacological levels of the BCR-ABL tyrosine kinase inhibitors (TKI) imatinib or nilotinib during differentiation into monocyte-derived DC (moDC) causes up-regulation of osteoactivin at the transcript and protein level in vitro (Schwarzbich et al., 2012). Here we aimed to elucidate the molecular mechanisms responsible for osteoactivin up-regulation in moDC upon exposure to IL-10 or TKI (imatinib, nilotinib). moDC were generated from blood monocytes by plastic adherence and exposure to GM-CSF and IL-4. Every second day, starting from the first day of culture, IL-10 or TKI were added to the culture medium. Alternatively, specific inhibitors of PI3K, Akt, Erk and/or C-Raf signal transduction pathways were added to the cell cultures. Cells were harvested on day 7 of culture for immunophenotyping and osteoactivin expression analysis using FACS, western-blotting and real-time qRT-PCR. The expression and activity of signal transduction molecules was monitored by western-blotting and pathway protein arrays. Analysis of moDC function was performed using mixed lymphocyte reactions (MLR). Our in vitro analysis revealed that IL-10 and BCR-ABL TKI inhibit the PI3K/Akt and, to a lesser extent, the Erk pathway in moDC. Specific inhibition of these signal transduction cascades resulted in profound up-regulation of osteoactivin expression in moDC confirming the involvement of PI3K/Akt and Erk in the regulation of osteoactivin expression. In line, TKI treatment and inhibition of PI3K/Akt and Erk comparably affected the phenotype of moDC. Moreover, the inhibition of these signalling cascades resulted in reduced stimulatory capacity of moDC in MLR with allogenic T cells, and this could be restored by addition of blocking osteoactivin antibody. Our data reveal that TKI exert immunosuppressive effects in moDC by interfering with pathways involved in IL-10 receptor signaling. Inhibition of osteoactivin expression or function may thus constitute a promising strategy in combinatory approaches using TKI and DC-based immunotherapy and may enhance the efficacy of immunotherapeutic interventions in cancer patients.