The pro-metastatic effect and mechanism of immune cells in tumor metastasis

Tumor metastasis is the leading cause of cancer-related deaths, and its occurrence is not only determined by the invasive capacity of tumor cells themselves but is also profoundly influenced by the dynamic remodeling of the immune system. Recent studies over the past decade have shown that the immune mechanisms in tumor metastasis have expanded from the classical failure of immune surveillance to a systemic process involving immune cell functional remodeling, the construction of the pre-metastatic niche (PMN), and tumor immune editing. This review systematically reviews the tumor promoting mechanisms of various immune cells such as T cells, tumor-associated macrophages (TAMs), myeloid-derived suppressor cells (MDSCs), tumor-associated neutrophils (TANs), regulatory B cells (Bregs), tumor-associated natural killer cells (TANKs) and dendritic cells (DCs) in each stage of metastasis, and critically discusses the limitations, conceptual controversies, and future directions of current research. This review seeks to establish a systematic framework to enhance the understanding of immune cell mechanisms involved in tumor metastasis.