Targeting Dormant Disseminated Tumor Cells and their Permissive Niche by Pro-Resolving Mediators Derived from Resolution-Phase Macrophages

Introduction: Metastatic breast cancer (BC) can recur years after initial treatments and arises from quiescent disseminated tumor cells (QDTCs) that resist conventional therapies. To date, there are no treatments to specifically target QDTCs. Previously, the fibrotic-like niche (FLN) enriched with Type I collagen (Col-I) was shown to be required for the switch of QDTCs to overt metastases. Hence, we hypothesized that resolution of inflammation is aberrant at the metastatic niche, thus leading to the formation of a FLN that supports QDTC outgrowth. Therefore, we tested whether reinstating the resolution of inflammation by introducing pro-resolving mediators produced by ex vivo-generated pro-resolving macrophages (Mres) could hinder FLN establishment around residing QDTCs, thereby preventing their escape from dormancy.

Materials and Methods: We artificially reinstated the resolution of inflammation using soluble mediators secreted by ex vivo-generated pro-resolving CD11blow macrophages (CM-Mres). These mediators were tested for their ability to prevent FLN establishment and hinder QDTC outgrowth in vivo and in vitro. Immune silencing in vivo, fibroblast-to-myofibroblast differentiation, and myofibroblast apoptosis was assessed as part of the resolution process. We used a three-dimensional system that models tumor dormancy and outgrowth to evaluate the effect of CM-Mres on the viability of dormant and outgrowing breast cancer cells.

Results and Discussion: CM-Mres promoted immune silencing at the metastatic site as part of the resolution process and inhibited metastatic outgrowth and FLN formation. The latter occurred through the inhibition of fibroblast-to-myofibroblast differentiation independent of TGFβ1 canonical signaling and the abolishment of Col-I expression. Furthermore, CM-Mres eliminated myofibroblasts as part of the resolution process by inducing transient ROS generation via NADPH oxidase, leading to DNA damage and apoptosis. Dormant and outgrowing DTCs were also targeted by ROS-mediated apoptosis induced by CM-Mres.

Conclusions: These findings highlight the dual role of pro-resolving mediators in disrupting the QDTC-permissive niche and directly inducing apoptosis in QDTCs, offering a promising strategy to prevent BC recurrence. Hence, identifying the specific pro-resolving mediators responsible for these effects could provide a foundation for developing innovative therapies targeting both QDTCs and their metastatic niche.