<i>In Vitro</i> interactions between TKIs and Leukemic-Derived Dendritic Cells As Potential Combination Therapy for Chronic Myeloid Leukemia

The development of tyrosine kinase inhibitors (TKIs) such as imatinib, nilotinib, dasatinib, and bosutinib has resulted in substantial improvement of clinical outcomes in the treatment of chronic myeloid leukemia (CML). However, TKI treatment needs to be given life-long, with less than 40% of patients being able to discontinue TKI treatment and reach durable treatment-free remission (TFR). Vididencel is a cellular immunotherapy based on irradiated, leukemic-derived dendritic cells from the DCOne cell line, which are administered via intradermal injection. In clinical studies in acute myeloid leukemia (AML), vididencel was proven to be safe and feasible, resulting in T-cell responses against common leukemic antigens, such as WT-1, PRAME and RHAMM. In AML patients in first complete remission, but with measurable residual disease, vididencel treatment was associated with durable clinical remissions. The positive data in AML and the presence of common shares leukemic antigens imply that vididencel may be applicable as an active immunotherapy in CML, in order to improve durable TFR rates. We therefore explored in vitro the interaction between vididencel with TKIs commonly used in CML. Clinically relevant concentrations of imatinib, dasatinib, nilotinib and bosutinib were tested to evaluate the direct effect of TKIs on DCOne mDC recovery and phenotype, as assessed by cell surface marker expression including CD80, CD83 and HLA molecules using flow cytometry. To mirror the clinical setting of long-term exposure to TKIs and the potential effect on immune cell functionality, peripheral blood mononuclear cells (PBMC) derived from heathy donors were pre-treated with TKIs for 2 days and subsequently co-cultured with DCOne mDC using a CFSE-based proliferation assay and flow cytometry markers for determination of T cell differentiation. Additionally, the influence of TKIs on capability of monocytes and different DCs within the PBMC population to endocytose DCOne mDC-derived antigenic material was explored. We observed that direct exposure of DCOne mDC to imatinib, dasatinib, nilotinib or bosutinib did not negatively influence the viability, recovery and phenotype of DCOne mDC. The potential of DCOne mDC to induce T cell proliferation and differentiation also remained unaffected by TKI exposure. TKI-exposed DCOne mDC were also effectively phagocytosed by antigen presenting cells present in healthy donor-derived PBMC, as compared to control DCOne mDC. Healthy donor-derived PBMC pretreated with TKIs for two days and cultured with DCOne mDC, demonstrated similar T cell proliferation and differentiation compared to control conditions. Co-cultures of PBMCs with DCOne mDC in the presence of imatinib showed similar DCOne mDC-induced proliferation and differentiation, whereas T cell proliferation was significantly reduced in the presence of dasatinib, nilotinib and bosutinib. Furthermore, dasatinib blocked differentiation of CD4 and CD8 T cells towards effector phenotype, resulting in increased frequency of naive T cells compared to control condition. This inhibition was also observed with nilotinib and bosutinib, although to a lesser extent compared to dasatinib. The ability of monocytes, dendritic cells and plasmacytoid DCs in PBMCs to endocytose DCOne mDC-derived cellular content was unaffected by imatinib, but decreased endocytosis of DCOne mDC was observed in the presence of dasatinib and, to a lesser extent, nilotinib. In conclusion, this in vitro study shows that the viability and phenotype of leukemic-derived dendritic cells is not affected by exposure to TKIs. Second generation TKIs, such as nilotinib, bosutinib and dasatinib impaired T-cell proliferation in MLR assay, with dasatinib having the strongest effect. Further preclinical experiments and ultimately clinical studies should provide more insights in the interaction between TKIs and vididencel in vivo and to explore its potential as an immunotherapy to improve TFR success in CML.