Monocyte-Derived Dendritic Cells Induce Functionally Active Regulatory T Cells Upon Exposure to BCR-ABL Tyrosine Kinase Inhibitors.
Authors: Gutknecht Michael , Joas Simone , Güttler Lisa , Kanz Lothar , Salih Helmut R , Grünebach Frank , Rittig Susanne M 
Published: October 2019

Abstract
Abstract 2156
Multiple approaches for treatment of malignant disease presently aim to combine targeted therapy with tyrosine kinase inhibitors (TKI) with immunotherapy. Dendritic cells (DC) are frequently used in such strategies due to their unique ability to initiate potent T cell anti-tumor immunity. Unfortunately, DC may also activate suppressive CD25+FOXP3+ regulatory T cells (Treg), which depends on the stimuli that influence DC in immature state and/or during development from precursor cells. High frequencies of Treg have been described in several types of tumors within the tumor microenvironment, which is associated with poor prognosis and reduced survival. DC development and function are moreover governed by various tyrosine kinases of which some are also inhibited by clinically used TKI. TKI thus may cause immunoinhibitory side effects, and we previously demonstrated that exposure of monocyte-derived DC to the BCR-ABL inhibitor imatinib causes up-regulation of the immunosuppressive type I transmembrane glycoprotein osteoactivin (GPNMB, DC-HIL) and reduces expression of activating surface antigens as well as T cell-stimulatory capacity of DC in vitro (Schwarzbich et al., 2012). Other investigators reported that imatinib induces functionally Treg in CML patients, but the underlying mechanisms are so far unknown. (Bachy et al., 2011). On the other hand, TKI may inhibit proliferation and suppressive capacity of regulatory T cells in vitro (Chen et al., 2007). Here we tried to solve this apparent discrepancy by analyzing the influence of TKI on DC-Treg interaction. Monocyte-derived DC (moDC) were generated over 7 days by exposing blood monocytes to GM-CSF and IL-4. TNF was added on day 6 of culture in case of maturation, and imatinib or nilotinib (3μM each) were added to the culture medium every second day starting from the first day of culture. Induction and functionality of Treg was determined by FACS and so called effector T cell suppression assays upon culture of moDC with autologous PBMC. We found that exposure of moDC to imatinib or nilotinib only slightly increased the frequency of Treg as compared to controls. However, these Treg strongly inhibited autologous T cell proliferation as assessed by T cell suppression assays. This was mediated by direct cellular interaction, as culture supernatants of TKI-treated DC did not alter Treg function and also did not contain elevated levels of the immunosuppressive (and Treg inducing) cytokines TGF-β and IL-10. Thus, our data indicate that the seemingly contradictory results of the in vivo and in vitro studies described above may be explained by the effects caused by exposure of moDC to BCR-ABL TKI which results in the induction of functionally active Treg. These findings are of special importance for future combinatory approaches using TKI and DC-based immunotherapy.

Disclosures:
No relevant conflicts of interest to declare.