Upregulation of Osteoactivin Upon Exposure to Tyrosine Kinase Inhibitors Impairs the T Cell Stimulatory Capacity of Monocyte-Derived Dendritic Cells

Abstract 1112

Targeted therapies using tyrosine kinase (TK) inhibitors have significantly improved the treatment of cancer patients. Imatinib (Glivec, Gleevec, STI 571) was the first TK inhibitor (TKI) established for the treatment of cancer and efficiently blocks the activity of c-ABL, a non-receptor TK which is pathologically activated in philadelphia chromosome-positive (Ph⁺) chronic myelogenous leukemia (CML). Nilotinib (Tasigna) and dasatinib (Sprycel) are second-generation TKI that have shown efficacy in the treatment of Ph⁺ CML resistant or intolerant to imatinib. However, molecularly detectable disease persists in the majority of patients treated with TKI, causing relapse after discontinuation of TKI treatment in many cases. Thus, multiple approaches presently aim to combine TKI treatment with immunotherapy. As TKI, besides targeting their eponymous enzyme, influence multiple other signaling pathways involved in cellular functions, analysis of potential effects of TKI on immune effector cells may be key to develop successful combinatorial strategies. Due to their unique ability to initiate powerful anti-tumor T cell responses, dendritic cells (DC) are employed in many immunotherapeutic strategies aiming to eradicate the malignant cell population. Upon activation they change their expression pattern of cell surface molecules and secreted cytokines/chemokines, a process called DC maturation. Osteoactivin, also known as transmembrane glycoprotein NMB (GPNMB) and dendritic cell-associated transmembrane protein (DC-HIL), is a type I transmembrane glycoprotein that is detected abundantly in DC but not or substantially less in monocytes. Its expression can inhibit T cell activation by binding the type 1 transmembrane proteoglycan syndecan-4 (SD-4) on T cells. Here we extend our findings that the exposure of human peripheral blood monocytes to the immunosuppressive and anti-inflammatory cytokine IL-10 or to therapeutic concentrations of TKI during differentiation into monocyte-derived DC (moDC) leads to significant upregulation of osteoactivin at the transcript and protein level in vitro (Blood 2010 116: abstract 1733). We analyzed the expression of other inhibitory receptors, such as PD-L1, PD-L2, CD80, or CD86 and observed no significant differences of the expression under TKI treatment. Furthermore, we thoroughly examined the expression of osteoactivin in the presence of relevant maturation signals such as TLR ligands, IFN-γ or TNF. LPS, Poly I:C, Pam₃Cys or R848 nearly abolished osteoactivin expression compared to untreated control cells. In contrast, IFN-γ or TNF did not significantly reduce osteoactivin expression below the basal level. To evaluate the involvement of osteoactivin in TKI-triggered effects on moDC function we performed mixed lymphocyte reactions with allogenic T cells. Osteoactivin upregulation upon exposure to imatinib, dasatinib and nilotinib resulted in significantly reduced T cell stimulatory capacity of moDC. This was not due to IL-10 upregulation but rather due to direct inhibitory effects of osteoactivin on T cell proliferation which could be overcome by addition of blocking anti-osteoactivin antibody. Our data demonstrate that upregulation of osteoactivin upon exposure of immature moDC to TKI is critically involved in the inhibition of DC function. These findings indicate that inhibition of osteoactivin expression or function may serve as a novel strategy in combinatory approaches using TKI and DC-based immunotherapy and may enhance the efficacy of immunotherapeutic interventions in cancer patients.