Inhibiting Metastatic Outgrowth of Dormant Tumor Cells Using Soluble Mediators of Resolution-Promoting Macrophages

Introduction: Breast cancer that recurs as metastatic disease many years after primary tumor resection and adjuvant therapy appears to arise from tumor cells that disseminated early in the course of the disease but did not develop into clinically apparent lesions. These long-term surviving, quiescent disseminated dormant tumor cells (QDTC) are resistant to conventional therapies that target actively dividing cells. The mechanisms responsible for maintaining the survival and outgrowth of QDTC remain largely unknown. Recently, we found that fibrotic-like microenvironment with extensive deposition of Type I collagen (Col-I) and fibronectin, established at the site of the residing QDTC, promoted the outbreak of QDTC. Therefore, we hypothesized that promoting resolution- the endogenous mechanism that terminates inflammation and fibrotic responses and actively directs tissue return to homeostasis- at the permissive site will ‘normalize’ it and prevent metastatic outbreak.

Materials and Methods: Conditioned media containing soluble mediators secreted by ex-vivo generated pro-resolving CD11b^low macrophages (CM-Mres) were utilized to test their impact on QDTC outgrowth along with resolution of inflammation in vivo and on dormant tumor cells co-cultured with myofibroblasts in vitro.

Results: CM-Mres promoted resolution of inflammation and inhibited the establishment of a fibrotic niche resulting in the inhibition of the metastatic outgrowth of QDTC. The inhibition of the fibrotic niche was due to inhibition of TGFβ1-induced differentiation of fibroblasts to myofibroblasts and myofibroblasts inactivation culminating in downregulation of Col-I expression and myofibroblasts apoptosis. Notably, the blockade of TGFβ1 induced fibroblast differentiation to myofibroblasts was mediated by p38 activation and inhibition of Wnt signaling rather than inhibition of the canonical signaling of TGFβ1.

Conclusions: Taken together our results demonstrate a pioneering conceptual approach to `normalize` the microenvironment that supports the outgrowth of QDTC by promoting the tissue resolution axis. This may represent a paradigm shift in current therapeutic practice that can serve as a future premise for the development of novel therapies.