Inflammatory signaling and myeloid cell polarization in the bone microenvironment of metastatic cancer

Cancer-related bone metastasis represents a dynamic and multifaceted process driven by reciprocal interactions between malignant cells and the specialized bone microenvironment. Within this setting, the immune system exerts a pivotal influence, modulating both the progression of metastasis and the remodeling of bone tissue. The metastatic bone niche is frequently characterized by a state of immune dysregulation, in which chronic inflammation and immunosuppressive signaling pathways coexist. This interplay results in an environment that impairs effective antitumor immunity while simultaneously facilitating tumor cell colonization, survival, and osteolytic activity. A defining feature of bone metastasis is its tendency to establish an immunologically cold microenvironment, typified by low infiltration of cytotoxic T cells and reduced expression of immune-activating cytokines. However, this does not imply an absence of immune activity. Rather, certain immune cell subsets, such as regulatory T cells, myeloid-derived suppressor cells (MDSCs), and alternatively activated macrophages, accumulate within the metastatic site, where they secrete factors that suppress adaptive immune responses and support tumor-associated osteoclast activation. Through these mechanisms, cancer cells effectively hijack immune regulatory pathways to evade immune surveillance and promote bone destruction. On the other hand, systemic inflammation and cytokine storms are central players in the complex network governing bone metastatic progression. They create a pro-tumor, osteolytic environment by recruiting immunosuppressive cells, fueling osteoclast activity, promoting tumor cell survival and dissemination, and destabilizing the bone matrix. These inflammatory processes are intertwined with immune evasion mechanisms, tissue remodeling, and dormancy-reactivation cycles, collectively accelerating the metastatic cascade in bone. Targeting these inflammatory pathways represents a critical strategy to alter the course of metastatic disease and improve therapeutic outcomes.