SPP1+ Macrophages and the Orchestration of Spatially Organized Immunosuppression in Cancer

This review describes the immunosuppressive effect of secreted phosphoprotein 1 (SPP1)+ tumor-associated macrophages (TAMs) in coordinating the tumor microenvironment (TME) as a functionally unique myeloid cell subgroup. SPP1+ TAMs transcend the traditional M1/M2 paradigm and represent a group of cells that are widely found in various cancer types. SPP1+ TAMs have the characteristics of high expression of SPP1 and promoting immune escape, matrix remodeling and metastasis. We clarify the dual developmental source of SPP1+ TAMs, and introduce the activation process of SPP1+ TAMs through recruitment, polarization and epigenetic locking. After SPP1+ TAMs are activated, they are strategically enriched in the tumor core and tumor marginal area to play their functions. Functionally, SPP1+ TAMs mainly promote the progression of tumors through three mechanisms: (1) Interacting with cancer-associated fibroblasts (CAFs): constructing an immunoexcluded fibrotic niche; (2) Multiple regulation of immune cells; (3) Promoting tumor metastasis and the construction of pre-metastatic niche (PMN). Overall, this review aims to provide a comprehensive overview of the mechanisms mediated by SPP1+ TAMs in the TME, and emphasize their unique role in cancer progression. At the same time, the treatment strategies targeting them are further explored, highlighting their potential as precise therapeutic targets for tumor treatment.